Fueling synapses

The transmission of information across neuronal synapses is an energetically taxing business. Sheng and colleagues monitored the localization of mitochondria following different levels of synaptic activation and discovered that these organelles change their distribution in interesting ways, stalling near synapses when neurons are activated and increasing their movement when neurons are silent (Li et al., 2004 [this issue of Cell]).     

Synaptic vesicle fraction devoid of adenosine triphosphatase activity from bovine caudatolenticular nuclei and thalamus.

1. As a part of studies on the mechanism by which catecholamines are released from the nerve terminals, the synaptic vesicle fraction was isolated from bovine caudatolenticular nuclei and thalamus by differential centrifugation essentially according to the method of Kadota and Kadota (17). 2. Further centrifugation on a sucrose density gradient of the synaptic vesicle fraction by the method of Whittaker et al. (1) yielded white materials on the upper portion of 0.4 M sucrose, which consisted of vesicles averaging 600-800 A in diameter, and did not show Mg2+-dependent ATpase activity. On the other hand, the denser materials centering on 0.6 M sucrose, consisting of a mixture of microsomes and synaptic vesicles of 400-500 A diameter, showed an ATpase activity activated by either Mg2+ or Ca2+ but not inhibited by ouabain. 3. The white materials on 0.4 M sucrose were almost free of mitochondria, but they contained a large amount of non-heme iron, as reported elsewhere (2). Furthermore, the protein components analyzed on SDS-polyacrylamide gels were similar to those already reported for purified synaptic vesicles (3). 4. Based on these results, the white materials were assumed to be synaptic vesicles devoid of Mg2+-dependent ATPase activity.     

Preparation and properties of mitochondria derived from synaptosomes.

A method has been developed whereby a fraction of rat brain mitochondria (synaptic mitochondria) was isolated from synaptosomes. This brain mitochondrial fraction was compared with the fraction of "free" brain mitochondria (non-synaptic) isolated by the method of Clark & Nicklas (1970). (J. Biol. Chem. 245, 4724-4731). Both mitochondrial fractions are shown to be relatively pure, metabolically active and well coupled. 2. The oxidation of a number of substrates by synaptic and non-synaptic mitochondria was studied and compared. Of the substrates studied, pyruvate plus malate was oxidized most rapidly by both mitochondrial populations. However, the non-synaptic mitochondria oxidized glutamate plus malate almost twice as rapidly as the synaptic mitochondria. 3. The activities of certain tricarboxylic acid-cycle and related enzymes in synaptic and non-synaptic mitochondria were determined. Citrate synthase (EC 4.1.3.7), isocitrate dehydrogenase (EC 1.1.1.41) and malate dehydrogenase (EC 1.1.1.37) activities were similar in both fractions, but pyruvate dehydrogenase (EC 1.2.4.1) activity in non-synaptic mitochondria was higher than in synaptic mitochondria and glutamate dehydrogenase (EC 1.4.1.3) activity in non-synaptic mitochondria was lower than that in synaptic mitochondria. 4. Comparison of synaptic and non-synaptic mitochondria by rate-zonal separation confirmed the distinct identity of the two mitochondrial populations. The non-synaptic mitochondria had higher buoyant density and evidence was obtained to suggest that the synaptic mitochondria might be heterogeneous. 5. The results are also discussed in the light of the suggested connection between the heterogeneity of brain mitochondria and metabolic compartmentation.     

Distribution of Six Synaptic Membrane Antigens in Subcellular Fractions of Rat Brain Cortex

Abstract: The subcellular distribution in rat brain cortex of six synaptic membrane antigens (56K, 58K, 62K, 63K, 64K, 66K) was studied by rocket immunoelectrophoresis, using antiserum to a highly purified synaptic plasma membrane fraction. Initial analysis of the insoluble portion of subcellular fractions showed that these antigens were also present in smooth microsomes, rough microsomes, and synaptic vesicles; that only traces were present in synaptic junctions; and that none was present in nuclei, mitochondria, and myelin. A trace amount of activity was also present in synaptic vesicle cytosol, but none in whole brain cytosol. Quantitative measurements of synaptic plasma membranes, smooth microsomes, and synaptic vesicles showed that all six antigens were present in synaptic plasma membranes and smooth microsomes, but that the 66K antigen was absent from synaptic vesicles. The 56K, 58K, 62K, 63K, and 64K antigens were present in highest concentration in synaptic plasma membranes, whereas the 66K antigen content was highest in smooth microsomes. Only the 58K, 62K, and 63K antigens were detectable in the membrane fraction of whole brain. Their enrichments in synaptic plasma membranes were 10.9, 5.4, and 5.9, respectively. We conclude that the 56K, 58K, 62K, 63K and 64K antigens are primary components of synaptic plasma membranes. The presence of synaptic plasma membrane antigens in smooth microsomes and synaptic vesicles probably represents material being actively transported, consistent with the hypothesis that proteins of synaptic plasma membranes and synaptic vesicles are transported via smooth endoplasmic reticulum.     

Localization and identification of covalently bound flavoproteins in rat liver mitochondria by prelabeling of their flavin moiety

The covalently bound flavoproteins in rat liver mitochondria were prelabeled by injecting [14C]riboflavin into a rat, then liver mitochondria were obtained and further labeled with [3H]pargyline, a suicide inhibitor of monoamine oxidase. When the mitochondria were subjected to osmotic lysis, two covalently bound flavoproteins having molecular weights of 110,000 and 94,000 were found in the supernatant. These proteins were identified as sarcosine dehydrogenases. Upon treatment of the membranous fraction with 1% Triton X-100, succinate dehydrogenase with a molecular weight of 70,000 was found in the soluble fraction, while two well-separated proteins doubly-labeled with 14C and 3H were found in the insoluble fraction. Their molecular weights were 61,000 and 57,000. By isoelectric focusing, two 3H peaks were observed with pI values of 8.3 and 8.4. The former corresponded to the 61,000-dalton protein, and the latter, to the 57,000 one. From the data obtained by using selective inhibitors, deprenyl and clorgyline, the [3H]pargyline-binding proteins with molecular weights of 61,000 and 57,000 were assigned to proteins of monoamine oxidases of type A and type B, respectively.     

Calcium mobilization from mitochondria in synaptic transmitter release

Mitochondria can rapidly accumulate and release Ca2+ upon cell stimulation. A paper by Yang and coworkers in this issue reports an unusual form of synaptic potentiation, dependent on Ca2+ release from mitochondria through the Na+/Ca2+ exchanger and triggered by Na+ entry through voltage-gated channels (Yang et al., 2003).     

Differential distribution of the enzymes glutamate dehydrogenase and aspartate aminotransferase in cortical synaptic mitochondria contributes to metabolic compartmentation in cortical synaptic terminals

There have been numerous studies on the activity and localization of aspartate aminotransferase (AAT) and glutamate dehydrogenase (GDH) in brain tissue. However, there is still a controversy as to the specific roles and relative importance of these enzymes in glutamate and glutamine metabolism in astrocytes and neurons or synaptic terminals. There are many reports documenting GDH activity in synaptic terminals, yet the misconception that it is a glial enzyme persists. Furthermore, there is evidence that this tightly regulated enzyme may have an increased role in synaptic metabolism in adverse conditions such as low glucose and hyperammonemia that could compromise synaptic function. In the present study, we report high activity of both AAT and GDH in mitochondrial subfractions from cortical synaptic terminals. The relative amount of GDH/AAT activity was higher in SM2 mitochondria, compared to SM1 mitochondria. Such a differential distribution of enzymes can contribute significantly to the compartmentation of metabolism. There is evidence that the metabolic capabilities of the SM1 and SM2 subfractions of synaptic mitochondria are compatible with the compartments A and B of neuronal metabolism proposed by Waagepetersen et al. (1998b. Dev. Neurosci. 20, 310-320).     

Lipid Composition in Synaptic and Nonsynaptic Mitochondria from Rat Brains and Effect of Aging

The cholesterol, phospholipid, and fatty acid compositions in synaptic and nonsynaptic mitochondria from rat brains and the effect of aging were studied. Both cholesterol and phospholipid contents were found to be significantly different in synaptic compared to nonsynaptic mitochondria. In both types of brain mitochondria, aging decreases the cholesterol content by 27% and the phospholipid content by approximately 12%. The difference between these decreases observed in the organelles causes decreases in the cholesterol/phospholipid molar ratios for synaptic and nonsynaptic mitochondria of 17 and 19%, respectively. Also, the phospholipid composition is significantly different in synaptic compared to nonsynaptic mitochondria. Among phospholipids, only the cardiolipin fraction showed a significant decrease (26%) in nonsynaptic mitochondria from the brains of aged rats. Instead, the fatty acid composition was not significantly different in synaptic compared to nonsynaptic mitochondria. The 21% aging decrease in linoleic acid (18:2), observed only in nonsynaptic mitochondria, may be related to a decrease in cardiolipin, which contains a large amount of this fatty acid.     

Mass spectrometric identification of a novel phosphorylation site in subunit NDUFA10 of bovine mitochondrial complex I

Mitochondrial Complex I (NADH:ubiquinone oxidoreductase) consists of at least 46 subunits. Phosphorylation of the 42-kDa subunit NDUFA10 was recently reported using a novel phosphoprotein stain [Schulenberg et al. (2003) Analysis of steady-state protein phosphorylation in mitochondria using a novel fluorescent phosphosensor dye. J. Biol. Chem. 278, 27251]. Two smaller Complex I phosphoproteins, ESSS and MWFE, and their sites of modification, have since been determined [Chen et al. (2004) The phosphorylation of subunits of complex I from bovine heart mitochondria. J. Biol. Chem. 279, 26036]. Here we identify the site of phosphorylation in NDUFA10 from bovine heart mitochondria by tandem mass spectrometry. A single phosphopeptide spanning residues 47-60 was identified and confirmed by synthesis to be (47)LITVDGNICSGKpSK(60), establishing serine-59 as the site of phosphorylation.     

Isolation and partial characterization of rat brain synaptic plasma membranes

Abstract— Synaptic plasma membranes from the cortices of adult rat brain were isolated from synaptosomes prepared by flotation of a washed mitochondrial pellet (P2) in a discontinuous Ficoll-sucrose gradient. Contamination of the synaptosome fraction by microsomes was estimated by enzymic and chemical analysis to be less than 15 per cent. (2) The purified synaptosome fraction was subjected to osmotic shock, subfractionated on a discontinuous sucrose gradient and the distribution of enzymic and chemical markers for synaptic plasma membranes, microsomal membranes and mitochondria was determined. (3) Comparison of synaptosome subfractions prepared in the presence and absence of 1 mM NaH₂ PO₄/0.1 mM EDTA buffer pH 7.5, indicated that the ionic composition of the isolation medium markedly affected the distribution and enzymic composition of the subfractions. (4) Synaptic plasma membranes prepared in the presence of PO₄/EDTA exhibited a 10-fold enrichment in [Na⁺+ K⁺] ATPase and were characterized by less than 15 and 10 per cent contamination by microsomes and mitochondria respectively. (5) The polypeptide composition of the purified synaptic plasma membranes was compared with the microsomes and mitochondria by polyacrylamide gel electrophoresis in sodium dodecyl sulphate. No differences between the protein and glycoprotein composition of the synaptic plasma membranes and microsomes were detected. The mitochondria, in contrast, possessed a unique protein composition.     

Relationship between solubility of grifolan, a fungal 1,3-beta-D-glucan, and production of tumor necrosis factor by macrophages in vitro.

Grifolan, GRN, is a fungal antitumor beta-glucan isolated from Grifola frondosa. Various studies suggested that the underlying mechanism of the antitumor activity of GRN is strongly related to immune modulation. In the previous publication (Adachi et al., 1994; Okazaki et al., 1995), we have shown that GRN activates macrophages to produce tumor necrosis factor (TNF) in vitro. In this study, the structural unit essential to produce TNF was examined by chemical modifications of GRN. GRN suspended in distilled water was treated at 150 degrees C for up to 3 h. Addition of the resulting turbid solution to the RAW 264.7 macrophage-like cell line produced TNF, and the relative activity was diminished in relation to the heat treatment period. The fractions with a heating period longer than 15 min did not show any activity. After centrifugation of the resulting solution, significant activity was shown by precipitate fractions, suggesting that the insoluble form of GRN is important for TNF production. Interestingly, the precipitate fraction obtained from 9 min of treatment also had significant activity. In addition, admixing the soluble fraction with the particles significantly inhibited the TNF production. In contrast to these observations, the high-molecular-mass subfraction of the soluble fraction prepared by ultrafiltration produced significant amounts of TNF. Similar phenomena were shown with sodium hydroxide treatment and dimethylsulfoxide treatment. These facts strongly suggested that insoluble as well as a high molecular mass soluble form of GRN are required for TNF production by macrophages.     

Phosphorylation of Synaptic Membrane Glycoproteins: The Effects of Ca2+ and Calmodulin

Synaptic membranes were incubated with [gamma-32P]ATP, and glycoproteins were isolated by affinity chromatography on concanavalin A agarose. Glycoproteins accounted for 1.5-2.5% of the total 32P incorporated into synaptic membrane proteins. Ca2+ and calmodulin enhanced the phosphorylation of synaptic membrane glycoproteins approximately threefold. In the presence of Ca2+ and calmodulin, the rate of glycoprotein dephosphorylation was also increased three- to four-fold. Gel electrophoretic analysis identified several synaptic membrane glycoproteins that incorporated 32P, with the most highly labeled glycoprotein under basal phosphorylating conditions having an apparent Mr of 205,000 (gpiii). Ca2+ and calmodulin produced a marked increase in the phosphorylation of a glycoprotein with an apparent Mr of 180,000 (gpiv) and lesser increases in the labeling of three other glycoproteins. Membranes that had been labeled with [gamma-32P]ATP were extracted with Triton X-100 under conditions that yield a detergent-insoluble residue enriched in postsynaptic structures. The Triton X-100 insoluble residue accounted for 20-25% of the 32P associated with synaptic membrane glycoproteins. Gpiv and other glycoproteins, the phosphorylation of which was stimulated by calmodulin, were located exclusively in the Triton X-100 insoluble residue, whereas gpiii and other calmodulin-insensitive glycoproteins partitioned predominantly into the Triton X-100-soluble fraction. Phosphopeptide maps and phosphoamino acid analysis of gpiv isolated from synaptic membranes and a postsynaptic glycoprotein of apparent Mr of 180,000 (gp180) isolated from synaptic junctions indicated that the former protein was identical to the previously identified postsynaptic-specific gp180. In addition to phosphoserine and phosphothreonine, gpiv also contained phosphotyrosine, identifying it as a substrate for tyrosine-protein kinase as well as for Ca2+/calmodulin-dependent protein kinase.     

Calcium trafficking integrates endoplasmic reticulum function with mitochondrial bioenergetics

Calcium homeostasis is central to all cellular functions and has been studied for decades. Calcium acts as a critical second messenger for both extracellular and intracellular signaling and is fundamental in cell life and death decisions (Berridge et al., 2000) [1]. The calcium gradient in the cell is coupled with an inherent ability of the divalent cation to reversibly bind multiple target biological molecules to generate an extremely versatile signaling system [2]. Calcium signals are used by the cell to control diverse processes such as development, neurotransmitter release, muscle contraction, metabolism, autophagy and cell death. “Cellular calcium overload” is detrimental to cellular health, resulting in massive activation of proteases and phospholipases leading to cell death (Pinton et al., 2008) [3]. Historically, cell death associated with calcium ion perturbations has been primarily recognized as necrosis. Recent evidence clearly associates changes in calcium ion concentrations with more sophisticated forms of cellular demise, including apoptosis (Kruman et al., 1998; Tombal et al., 1999; Lynch et al., 2000; Orrenius et al., 2003) , ,  and . Although the endoplasmic reticulum (ER) serves as the primary calcium store in the metazoan cell, dynamic calcium release to the cytosol, mitochondria, nuclei and other organelles orchestrate diverse coordinated responses. Most evidence supports that calcium transport from the ER to mitochondria plays a significant role in regulating cellular bioenergetics, production of reactive oxygen species, induction of autophagy and apoptosis. Recently, molecular identities that mediate calcium traffic between the ER and mitochondria have been discovered (Mallilankaraman et al., 2012a; Mallilankaraman et al., 2012b; Sancak et al., 2013)[8–10]. The next questions are how they are regulated for exquisite tight control of ER–mitochondrial calcium dynamics. This review attempts to summarize recent advances in the role of calcium in regulation of ER and mitochondrial function. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.     

Studies on the control of 4-aminobutyrate metabolism in ''synaptosomal'' and free rat brain mitochondria.

1. The specific activities of 4-aminobutyrate aminotransferase (EC 2.6.1.19) and succinate semialdehyde dehydrogenase (EC 1.2.1.16) were significantly higher in brain mitochondria of non-synaptic origin (fraction M) than those derived from the lysis of synaptosomes (fraction SM2). 2. The metabolisms of 4-aminobutyrate in both 'free' (non-synaptic, fraction M) and 'synaptic' (fraction SM2) rat brain mitochondria was studied under various conditions. 3. It is proposed that 4-aminobutyrate enters both types of brain mitochondria by a non-carrier-mediated process. 4. The rate of 4-aminobutyrate metabolism was in all cases higher in the 'free' (fraction M) brain mitochondria than in the synaptic (fraction SM2) mitochondria, paralleling the differences in the specific activities of the 4-aminobutyrate-shunt enzymes. 5. The intramitochondrial concentration of 2-oxoglutarate appears to be an important controlling parameter in the rate of 4-aminobutyrate metabolism, since, although 2-oxoglutarate is required, high concentrations (2.5 mM) of extramitochondrial 2-oxoglutarate inhibit the formation of aspartate via the glutamate-oxaloacetate transaminase. 6. The redox state of the intramitochondrial NAD pool is also important in the control of 4-aminobutyrate metabolism; NADH exhibits competitive inhibition of 4-aminobutyrate metabolism by both mitochondrial populations with an apparent Ki of 102 muM. 7. Increased potassium concentrations stimulate 4-aminobutyrate metabolsim in the synaptic mitochondria but not in 'free' brain mitochondria. This is discussed with respect to the putative transmitter role of 4-aminobutyrate.     

Synaptic mitochondria are more susceptible to Ca2+overload than nonsynaptic mitochondria

Mitochondria in nerve terminals are subjected to extensive Ca2+ fluxes and high energy demands, but the extent to which the synaptic mitochondria buffer Ca2+ is unclear. In this study, we identified a difference in the Ca2+ clearance ability of nonsynaptic versus synaptic mitochondrial populations enriched from rat cerebral cortex. Mitochondria were isolated using Percoll discontinuous gradients in combination with high pressure nitrogen cell disruption. Mitochondria in the nonsynaptic fraction originate from neurons and other cell types including glia, whereas mitochondria enriched from a synaptosomal fraction are predominantly neuronal and presynaptic in origin. There were no differences in respiration or initial Ca2+ loads between nonsynaptic and synaptic mitochondrial populations. Following both bolus and infusion Ca2+ addition, nonsynaptic mitochondria were able to accumulate significantly more exogenously added Ca2+ than the synaptic mitochondria before undergoing mitochondrial permeability transition, observed as a loss in mitochondrial membrane potential and decreased Ca2+ uptake. The limited ability of synaptic mitochondria to accumulate Ca2+ could result from several factors including a primary function of ATP production to support the high energy demand of presynaptic terminals, their relative isolation in comparison with the threads or clusters of mitochondria found in the soma of neurons and glia, or the older age and increased exposure to oxidative damage of synaptic versus nonsynaptic mitochondria. By more readily undergoing permeability transition, synaptic mitochondria may initiate neuron death in response to insults that elevate synaptic levels of intracellular Ca2+, consistent with the early degeneration of distal axon segments in neurodegenerative disorders.     

Isolation of small agranular synaptic vesicles of rat brain by gel filtration chromatography

I attempted to isolate synaptic vesicles by gel filtration. The rat brain synaptic vesicles in a synaptosomal lysate were collected by ammonium sulfate salting-out and fractionated on a Sephacryl S-500 with a mean exclusion size of 200 nm. Peak I at the void volume contained large vesicular membranes and coated vesicles besides synaptic vesicles; Peak II consisted almost entirely of small agranular synaptic vesicles of 40-50 nm diameter; and Peak III comprised soluble proteins. Western blotting revealed that components of 72 kDa in peaks I and II reacted with an anti-H(+)-ATPase A-subunit antibody [Moriyama et al. (1995) FEBS Lett. 367, 233-236]. When examined for Mg(2+)-ATPase activity, peak I showed specific activity of 4.52 ( micromol ATP hydrolyzed/mg protein/30 min), while that of peak II was as low as 0.22. As estimated from the inhibition by bafilomycin A(1) [Bowman et al. (1988) PROC: Natl. Acad. Sci. USA 85, 7972-7976], the percentage of H(+)-ATPase as to total Mg(2+)-ATPase, 18-22%, was unchanged, indicating no accumulation of the H(+)-ATPase in peak II even on the chromatography. In brief, the small agranular synaptic vesicles in peak II showed little or no Mg(2+)-ATPase activity, although they reacted with the H(+)-ATPase antibody. The reason for this is obscure. Mg(2+)-ATPase might not be a constituent of small agranular synaptic vesicles of rat brain.     

Type A and type B monoamine oxidase activities in rat brain and liver mitochondria: a comparison of their properties using the non-ionic detergent Triton X-100

1. The effects of the non-ionic detergent Triton X-100 on the heterogeneity of monoamine oxidase activities were studied and compared in synaptic (fractions SM and SM2) and non-synaptic (fraction M) brain mitochondria and liver mitochondria. 2. Triton X-100 inhibited type A and type B monoamine oxidase activities in all four mitochondrial fractions in a concentration-dependent manner. Liver mitochondrial enzymatic activities were much more sensitive to this inhibition than those of brain mitochondria. The activities in the SM fraction of synaptic brain mitochondria were the least susceptible. 3. In all four mitochondrial fractions, type A activities were more sensitive to inhibition than type B activities. 4. These results suggest that the membrane micro-environment around the enzyme molecules in situ may be important in the functional expression of the activity of the enzyme.     

Mitochondrial trafficking and anchoring in neurons: New insight and implications

Mitochondria are essential organelles for neuronal growth, survival, and function. Neurons use specialized mechanisms to drive mitochondria transport and to anchor them in axons and at synapses. Stationary mitochondria buffer intracellular Ca²⁺ and serve as a local energy source by supplying ATP. The balance between motile and stationary mitochondria responds quickly to changes in axonal and synaptic physiology. Defects in mitochondrial transport are implicated in the pathogenesis of several major neurological disorders. Recent work has provided new insight in the regulation of microtubule-based mitochondrial trafficking and anchoring, and on how mitochondrial motility influences neuron growth, synaptic function, and mitophagy.

Introduction

Mitochondria are cellular power plants that convert chemicals into ATP through coupled electron transport chain and oxidative phosphorylation. A cortical neuron in human brain utilizes up to ∼4.7 billion ATP molecules per second to power various biological functions (Zhu et al., 2012). Constant ATP supply is essential for nerve cell survival and function (Nicholls and Budd, 2000). Mitochondrial ATP production supports synapse assembly (Lee and Peng, 2008), generation of action potentials (Attwell and Laughlin, 2001), and synaptic transmission (Verstreken et al., 2005). Synaptic mitochondria maintain and regulate neurotransmission by buffering Ca²⁺ (Medler and Gleason, 2002; David and Barrett, 2003). In addition, mitochondria sequester presynaptic Ca²⁺ transients elicited by trains of action potentials and release Ca²⁺ after stimulation, thus inducing certain forms of short-term synaptic plasticity (Werth and Thayer, 1994; Tang and Zucker, 1997; Billups and Forsythe, 2002; Levy et al., 2003; Kang et al., 2008). Removing mitochondria from axon terminals results in aberrant synaptic transmission likely due to insufficient ATP supply or altered Ca²⁺ transients (Stowers et al., 2002; Guo et al., 2005; Ma et al., 2009).Neurons are polarized cells consisting of a relatively small cell body, dendrites with multiple branches and elaborate arbors, and a thin axon that can extend up to a meter long in some peripheral nerves. Due to these extremely varied morphological features, neurons face exceptional challenges to maintain energy homeostasis. Neurons require specialized mechanisms to efficiently distribute mitochondria to far distal areas where energy is in high demand, such as synaptic terminals, active growth cones, and axonal branches (Fig. 1; Morris and Hollenbeck, 1993; Ruthel and Hollenbeck, 2003). Axonal branches and synapses undergo dynamic remodeling during neuronal development and in response to synaptic activity, thereby changing mitochondrial trafficking and distribution. Neurons are postmitotic cells surviving for the lifetime of the organism. A mitochondrion needs to be removed when it becomes aged or dysfunctional. Mitochondria also alter their motility and distribution under certain stress conditions or when their integrity is impaired (Miller and Sheetz, 2004; Chang and Reynolds, 2006; Cai et al., 2012). Therefore, efficient regulation of mitochondrial trafficking and anchoring is essential to: (1) recruit and redistribute mitochondria to meet altered metabolic requirements; and (2) remove aged and damaged mitochondria and replenish healthy ones at distal terminals. Research into neuronal regulation of mitochondrial trafficking and anchoring is thus a very important frontier in neurobiology. This review article focuses on new mechanistic insight into the regulation of microtubule (MT)-based mitochondrial trafficking and anchoring and provides an updated overview of how mitochondrial motility influences neuronal growth, synaptic function, and mitochondrial quality control. Additional insight and overviews from different perspectives can be found in other in-depth reviews (Frederick and Shaw, 2007; Morfini et al., 2009; Hirokawa et al., 2010; MacAskill and Kittler, 2010; Schon and Przedborski, 2011; Court and Coleman, 2012; Saxton and Hollenbeck, 2012; Sheng and Cai, 2012; Birsa et al., 2013; Lovas and Wang, 2013; Schwarz, 2013).Open in a separate windowFigure 1.Mitochondrial trafficking and anchoring in neurons. Due to complex structural features, neurons require specialized mechanisms trafficking mitochondria to their distal destinations and anchoring them in regions where metabolic and calcium homeostatic capacity is in a high demand. The figure highlights transporting mitochondria to a presynaptic bouton (A) and an axonal terminal (B). MT-based long-distance mitochondrial transport relies on MT polarity. In axons, the MT’s plus ends (+) are oriented toward axonal terminals whereas minus ends (−) are directed toward the soma. Thus, KIF5 motors are responsible for anterograde transport to distal synaptic terminals whereas dynein motors return mitochondria to the soma. The motor adaptor Trak proteins can mediate both KIF5- and dynein-driven bi-directional transport of axonal mitochondria (van Spronsen et al., 2013). Myosin motors likely drive short-range mitochondrial movement at presynaptic terminals where enriched actin filaments constitute cytoskeletal architecture. Motile mitochondria can be recruited into stationary pools via dynamic anchoring interactions between syntaphilin and MTs (Kang et al., 2008) or via an unidentified actin-based anchoring receptor (Chada and Hollenbeck, 2004). Such anchoring mechanisms ensure neuronal mitochondria are adequately distributed along axons and at synapses, where constant energy supply is crucial (figure courtesy of Qian Cai, Department of Cell Biology and Neuroscience, Rutgers University, Piscataway, NJ).

Neuronal mitochondria display complex motility patterns

The complex motility patterns of mitochondrial transport in neurons can be visualized by applying time-lapse imaging in live primary neurons. Mitochondria are labeled by expressing DsRed-Mito, a mitochondria-targeted fluorescent protein, or by loading fluorescent dye MitoTracker Green or MitoTracker Red CMXRos, which stains mitochondria in live cells dependent upon membrane potential. Mitochondria move bi-directionally over long distances along processes, pause briefly, and move again, frequently changing direction. Motile mitochondria can become stationary, and stationary ones can be remobilized and redistributed in response to changes in metabolic status and synaptic activity. Given frequent pauses and bi-directional movements, the mean velocities of neuronal mitochondrial transport are highly variable, ranging from 0.32 to 0.91 µm/s (Morris and Hollenbeck, 1995; MacAskill et al., 2009a). In mature neurons, ∼20–30% of axonal mitochondria are motile, some of which pass by presynaptic terminals while the remaining two thirds are stationary (Kang et al., 2008). Using dual-channel imaging of mitochondria and synaptic terminals in hippocampal neurons, a recent study identified five motility patterns of axonal mitochondria including: stationary mitochondria sitting out of synapses (54.07 ± 2.53%) or docking at synapses (16.29 ± 1.66%); motile mitochondria passing through synapses (14.77 ± 1.58%); or pausing at synapses for a short (<200 s, 7.01 ± 1.29%) or a longer period of time (>200 s, 8.30 ± 1.52%; Sun et al., 2013). These findings are consistent with a previous study in primary cortical neurons (Chang et al., 2006).Altered mitochondrial transport is one of the pathogenic changes in major adult-onset neurodegenerative diseases (Sheng and Cai, 2012). Although in vitro live imaging in primary neurons prepared from embryonic or newborn mice provides an important tool to study mechanisms regulating mitochondrial transport, it has obvious limitations for investigating adult-onset disease mechanisms. For example, reduced transport of axonal mitochondria was consistently reported in amyotrophic lateral sclerosis–linked hSOD1^G93A mice (De Vos et al., 2007; Magrané and Manfredi, 2009; Bilsland et al., 2010; Zhu and Sheng, 2011; Marinkovic et al., 2012; Song et al., 2013). Therefore, it is critical to establish spinal motor neuron cultures directly isolated from adult mice, which will provide more reliable models to investigate mitochondrial transport defects underlying adult-onset pathogenesis in motor neurons. Given that the morphology and conditions of neurons grown in culture dishes differ from that of neurons in vivo, an in vivo transport study would better reflect physiological environments. An ex vivo study of mitochondrial transport in acute nerve explants was developed in transgenic mice, in which neuronal mitochondria are labeled with mito-CFP or mito-YFP (Misgeld et al., 2007).

Moving mitochondria along MTs

Mitochondrial transport between the soma and distal processes or synapses depends upon MT-based motors, which drive their cargos via mechanisms requiring ATP hydrolysis (Martin et al., 1999; Hirokawa et al., 2010). MTs are uniformly arranged in axons: their plus ends are oriented distally and minus ends are directed toward the soma. Such a uniform polarity has made axons particularly useful for exploring mechanisms regulating bi-directional transport: dynein motors drive retrograde movement, whereas kinesin motors mediate anterograde transport. Of the 45 kinesin motor genes identified, the kinesin-1 family (KIF5) is the key motor driving mitochondrial transport in neurons (Hurd and Saxton, 1996; Tanaka et al., 1998; Górska-Andrzejak et al., 2003; Cai et al., 2005; Pilling et al., 2006). KIF5 heavy chain (KHC) contains a motor domain with ATPase at the N terminus and a C-terminal tail for binding cargo directly or indirectly via a cargo adaptor. Three isoforms (KIF5A, KIF5B, and KIF5C) of the KIF5 family are found in mammals, of which KIF5B is expressed ubiquitously whereas KIF5A and KIF5C are expressed selectively in neurons (Kanai et al., 2000).KIF5 motors attach to mitochondria through adaptor proteins. The motor–adaptor coupling ensures targeted trafficking and effective regulation of mitochondrial transport. Drosophila protein Milton acts as a KIF5 motor adaptor by binding the KIF5 cargo-binding domain and mitochondrial outer membrane receptor Miro (Stowers et al., 2002; Glater et al., 2006). The Drosophila mutation in Milton reduces mitochondrial trafficking to synapses. Milton appears specific for mitochondrial trafficking because the trafficking of other cargoes such as synaptic vesicles is not affected. Two Milton orthologues, Trak1 and Trak2, are found in mammals and are required for mitochondrial trafficking (Smith et al., 2006; MacAskill et al., 2009b; Koutsopoulos et al., 2010). Elevated Trak2 expression in hippocampal neurons robustly enhances axonal mitochondrial motility (Chen and Sheng, 2013). Conversely, depleting Trak1 or expressing its mutants results in impaired mitochondrial motility in axons (Brickley and Stephenson, 2011). A recent study revealed that mammalian Trak1 and 2 contain one N-terminal KIF5B-binding site and two dynein/dynactin-binding sites, one at the N-terminal domain and the other at the C-terminal domain (van Spronsen et al., 2013). Thus, the Trak proteins can mediate both KIF5- and dynein-driving bi-directional transport of axonal mitochondria (Fig. 1). In contrast to the specific role of Drosophila Milton in mitochondrial transport, mammalian Trak proteins are also recruited to endosomal compartments and associate with GABAA (γ-aminobutyric acid A) receptors and K⁺ channels, highlighting their role in neuronal cargo transport (Grishin et al., 2006; Kirk et al., 2006; Webber et al., 2008).The mitochondria outer membrane receptor Miro is a Rho-GTPase with two Ca²⁺-binding EF-hand motifs and two GTPase domains (Frederick et al., 2004; Fransson et al., 2006). Mutation of the Drosophila miro gene impairs mitochondrial anterograde transport, thus depleting mitochondria at distal synaptic terminals (Guo et al., 2005). The first crystal structures of Drosophila melanogaster Miro comprising the tandem EF-hands and GTPase domains were recently resolved (Klosowiak et al., 2013). From these structures, two additional hidden EF-hands were identified, each one pairing with a canonical EF-hand. These structures will help our understanding of the conformational changes in Miro upon Ca²⁺ binding and nucleotide sensing in regulating mitochondrial motility, dynamics, and function. There are two mammalian Miro isoforms, Miro1 and Miro2. The Miro1–Trak2 adaptor complex regulates mitochondrial transport in hippocampal neurons (MacAskill et al., 2009b). Elevated Miro1 expression increases mitochondrial transport, likely by recruiting more Trak2 and motors to the mitochondria (Chen and Sheng, 2013). Thus, KIF5, Milton (Trak), and Miro assemble into the transport machinery that drives mitochondrial anterograde movement (Fig. 2 A). An alternative KIF5 adaptor for neuronal mitochondria is syntabulin (Fig. 2 B; Cai et al., 2005). Depleting syntabulin or blocking the KIF5–syntabulin coupling reduces mitochondrial anterograde transport in axons. Several other proteins are also suggested as candidate KIF5 motor adaptors for mitochondrial transport, including FEZ1 (fasciculation and elongation protein zeta-1) and RanBP2 (Ran-binding protein 2; Cho et al., 2007; Fujita et al., 2007), although their role in neuronal mitochondrial transport requires further investigation. The existence of multiple motor adaptors may highlight the complex regulation of mitochondrial motility in response to various physiological and pathological signals.Open in a separate windowFigure 2.Activity-dependent regulation of mitochondrial transport. (A) The Miro–Milton (or Miro–Trak) adaptor complex mediates KIF5-driven mitochondrial transport. (B) Syntabulin, FEZ1, and RanBP2 serve as an alternative KIF5 motor adaptor in driving mitochondrial anterograde transport. (C and D) Miro-Ca²⁺ models in regulating mitochondrial motility. Miro contains Ca²⁺-binding EF-hand motifs. The C-terminal cargo-binding domain of KIF5 motors binds to the Miro–Trak adaptor complex. (C) Ca²⁺ binding to Miro’s EF-hands induces the motor domain to disconnect with MTs and thus prevents motor–MT engagement (Wang and Schwarz, 2009). (D) Alternatively, Ca²⁺ binding releases KIF5 motors from mitochondria (MacAskill et al., 2009a). Thus, Ca²⁺ influx after synaptic activity arrests motile mitochondria at activated synapses. (E) Syntaphilin-mediated “engine-switch and brake” model. A Miro-Ca²⁺–sensing pathway triggers the binding switch of KIF5 motors from the Miro–Trak adaptor complex to anchoring protein syntaphilin, which immobilizes axonal mitochondria via inhibiting motor ATPase activity. Thus, syntaphilin turns off the “KIF5 motor engine” by sensing a “stop sign” (elevated Ca²⁺) and putting a brake on mitochondria, thereby anchoring them in place on MTs. When in their stationary status, KIF5 motor–loaded mitochondria remain associated with the MT tracks while KIF5 ATPase is in an inactive state (Chen and Sheng, 2013; Figure courtesy of Qian Cai).Dynein motors are composed of multiple chains including heavy chains (DHC) as the motor domain, intermediate (DIC), light intermediate (DLIC), and light chains (DLC), which function by associating with cargos and regulating motility. Dynein associates with Drosophila mitochondria, and mutations of DHC alter velocity and run length of mitochondrial retrograde transport in axons (Pilling et al., 2006). Miro may serve as an adaptor for both KIF5 and dynein motors in Drosophila (Guo et al., 2005; Russo et al., 2009), as loss of dMiro impairs both kinesin- and dynein-driven transport and overexpressing dMiro also alters mitochondrial transport in both directions. These findings support an attractive model in which the relative activity of the opposite-moving motors is regulated by their cargo–adaptor proteins.

Mitochondrial motility and membrane dynamics

Mitochondrial trafficking, anchoring, and membrane dynamics are coordinated, thus controlling their shape, integrity, and distribution in neurons. It is expected that the size of a mitochondrion may have a direct impact on its mobility. Indeed, interconnected mitochondrial tubules found in fission-deficient neurons are less efficiently transported to distal synapses. Hippocampal neurons expressing defective Drp1, a mitochondrial fission protein, display accumulated mitochondria within the soma and reduced mitochondrial density in dendrites (Li et al., 2004). Drosophila Drp1 is required for delivering mitochondria to neuromuscular junctions (Verstreken et al., 2005). Conversely, mutation of Miro results in mitochondrial fragmentation in addition to impaired motility, whereas overexpressing Miro not only enhances mitochondrial transport but also induces their interconnection (Fransson et al., 2006; Saotome et al., 2008; Liu and Hajnóczky, 2009; MacAskill et al., 2009b). Additionally, yeast Miro, Gem1p, regulates mitochondrial morphology; cells lacking Gem1p contain collapsed, globular, or grape-like mitochondria (Frederick et al., 2004).Mitochondrial fusion events require two organelles to move closer. A “kiss-and-run” interplay is proposed for driving mitochondrial fusion (Liu et al., 2009). Thus, motile mitochondria, particularly in distal axons and at synapses, undergo fusion events at a higher rate than anchored stationary mitochondria. Recent studies highlight the cross talk between the mitochondrial fusion protein Mfn2 and a motor adaptor complex. Mfn2 associates with Miro2 and regulates axonal mitochondrial transport by affecting kinesin motor processivity or by coordinating the motor switch between kinesin and dynein (Misko et al., 2010). Neurons defective in Mfn2 display reduced mitochondrial motility and altered mitochondrial distribution (Chen et al., 2003, 2007; Baloh et al., 2007). Therefore, regulating mitochondrial trafficking directly affects their morphology. Conversely, changing mitochondrial fusion–fission dynamics also impacts their transport and distribution.

Anchoring mitochondria at MTs

Proper axonal and synaptic function requires stationary mitochondria docking in regions where energy is in high demand. The diffusion capacity of intracellular ATP is rather limited (Hubley et al., 1996), particularly within long axonal processes (Sun et al., 2013). Therefore, docked mitochondria ideally serve as stationary power plants for stable and continuous ATP supply necessary to maintain the activity of Na⁺–K⁺ ATPase, fast spike propagation, and synaptic transmission. In addition, mitochondria maintain calcium homeostasis at synapses by sequestering intracellular [Ca²⁺] (Billups and Forsythe, 2002; Kang et al., 2008). Given the fact that two thirds of the mitochondria are stationary for an extended period along axons, neurons require mechanisms dissociating mitochondria from motor proteins or anchoring mitochondria to the cytoskeleton. One intriguing mitochondrial-anchoring protein is syntaphilin, which is a “static anchor” for immobilizing mitochondria specifically in axons (Kang et al., 2008; Chen et al., 2009). Syntaphilin targets axonal mitochondria through its C-terminal mitochondria-targeting domain and axon-sorting sequence. Syntaphilin immobilizes axonal mitochondria by anchoring to MTs. Deleting syntaphilin results in a robust increase of axonal mitochondria in motile pools. Conversely, overexpressing syntaphilin abolishes axonal mitochondrial transport. These findings provide new mechanistic insight that motile mitochondria can be recruited into stationary pools via anchoring interactions between mitochondria-targeted syntaphilin and MTs. Thus, syntaphilin serves as an attractive molecular target for investigations into mechanisms recruiting motile mitochondria into activated synapses. The syntaphilin knockout mouse is an ideal genetic model to examine the impact of enhanced motility of axonal mitochondria on presynaptic function and mitochondrial quality control in distal axons.

Synaptic activity–dependent regulation

Mitochondrial transport and distribution in axons and at synapses are correlated with synaptic activity. Mitochondria are recruited to synapses in response to elevated intracellular Ca²⁺ during sustained synaptic activity. Elevated Ca²⁺ influx, either by activating voltage-dependent calcium channels or NMDA receptors, arrests mitochondrial movement (Rintoul et al., 2003; Yi et al., 2004; Chang et al., 2006; Szabadkai et al., 2006; Ohno et al., 2011). These studies raise a fundamental question: how are mitochondria recruited to synapses in response to Ca²⁺ influx? Recent work from three groups identified Miro as a Ca²⁺ sensor in regulating mitochondrial motility (Saotome et al., 2008; MacAskill et al., 2009a; Wang and Schwarz, 2009). Miro has four Ca²⁺-binding EF-hands (Klosowiak et al., 2013). Elevated cytosolic Ca²⁺ levels, by firing action potentials or activating glutamate receptors, arrest mitochondrial transport through a Miro-Ca²⁺–sensing pathway that inactivates or disassembles the KIF5–Miro–Trak transport machineries. By this mechanism, mitochondria are recruited to activated synapses. A Miro-Ca²⁺–sensing model was proposed in which KIF5 is loaded onto mitochondria through physical coupling via the motor adaptor complex Miro–Milton (or Miro–Trak) at resting Ca²⁺ levels. When motile mitochondria pass by an active synaptic terminal, the local elevated intracellular Ca²⁺ disrupts or inactivates the Miro–Trak–kinesin transport complex by binding to the EF-hand and changing Miro’s conformation; thus, mitochondria are immobilized at activated synapses (Fig. 2, C and D; MacAskill et al., 2009a; Wang and Schwarz, 2009). The two studies disagreed, however, as to whether kinesin remained associated with stationary mitochondria or whether the motor was completely decoupled from the organelle upon immobilization. In addition, Miro-Ca²⁺–dependent regulation affects both anterograde and retrograde transport. When KIF5 transport machinery is disrupted by the Miro-Ca²⁺–sensing mechanism, dynein-driven retrograde transport does not necessarily take over. It is unclear how the Miro-Ca²⁺–sensing pathway inactivates dynein transport machineries. Thus, the role of the Miro-Ca²⁺–sensing pathway in immobilizing mitochondria likely occurs through an anchoring mechanism.A recent study provides evidence to support the above-described hypothesis. Activating the Miro-Ca²⁺ pathway fails to arrest axonal mitochondria in syntaphilin-null hippocampal neurons: axonal mitochondria keep moving during neuronal firing while dendritic mitochondria are effectively immobilized (Chen and Sheng, 2013). This study suggests that syntaphilin-mediated anchoring plays a central role in the activity-dependent immobilization of axonal mitochondria. Syntaphilin competes with Trak2 for binding KIF5 in cells and inhibits the KIF5 motor ATPase activity in vitro, thus suggesting that syntaphilin functions to prevent KIF5 from moving along MTs. Synaptic activity and elevated calcium levels favor the anchoring interaction between syntaphilin and KIF5 by disrupting the Miro–Trak–kinesin transport complex. In addition, sustained neuronal activity induces syntaphilin translocation to axonal mitochondria, although the underlying mechanisms remain elusive. Expressing the syntaphilin mutant lacking this axon-sorting domain results in its distribution into all mitochondria, including those in the soma and dendrites (Kang et al., 2008). These studies collectively suggest a possibility that syntaphilin is delivered to axons before its translocation into mitochondria. One interesting question is how syntaphilin-anchored mitochondria are remobilized. Given the fact that syntaphilin is a cellular target of the E3 ubiquitin ligase Cullin 1 (Yen and Elledge, 2008), syntaphilin may be degraded by ubiquitin-mediated pathways. It is necessary to further investigate the mechanisms underlying activity-induced syntaphilin translocation and its subsequent removal from mitochondria.A new “engine-switch and brake” model (Fig. 2 E) was recently proposed (Chen and Sheng 2013). In response to a “stop” sign (elevated Ca²⁺) at active synapses, syntaphilin switches off the kinesin motor and puts a brake on mitochondria, thereby anchoring them in place on MTs. Thus, KIF5-loaded mitochondria remain associated with the axonal MT track while in their stationary phase. This model may help reconcile the standing disagreements regarding how Miro-Ca²⁺ sensing immobilizes mitochondria in neurons. MacAskill et al. (2009a) propose that Ca²⁺ binding to Miro disconnects KIF5 motors from mitochondria in dendrites by activating glutamate receptors (Fig. 2 D), whereas Wang and Schwarz (2009) suggest that KIF5 remains associated with immobilized mitochondria within axons during elevated Ca²⁺ conditions (Fig. 2 C). These discrepant results are likely due to their selective observations of mitochondrial motility in dendrites versus axons. Upon Miro-Ca²⁺ sensing, KIF5 motors remain associated with axonal mitochondria by binding syntaphilin, which is absent from dendritic mitochondria (Fig. 2 E). This difference may help axonal mitochondria be more responsive to changes in synaptic activity. If the Ca²⁺ signal is removed, the cargo-loaded motor proteins can be quickly reactivated to move mitochondria to new active synapses. This model also suggests that motor loading on mitochondria is insufficient for transport. The release of the anchoring interaction is also required. The latter is supported by a study that both KIF5 and dynein motors remain bound on prion protein vesicles regardless of whether they are motile or stationary (Encalada et al., 2011). A newly reported Alex3 protein, organized in a genomic Armcx cluster encoding mitochondria-targeted proteins, regulates mitochondrial trafficking in neurons by interacting with the KIF5–Miro–Trak complex in a Ca²⁺-dependent manner (López-Doménech et al., 2012). Although the underlying mechanism is unknown, the study highlights the complexity in controlling mitochondrial motility in response to various physiological signals in neurons.

Mitochondrial motility and synaptic variability

Mitochondria are commonly found within synaptic terminals (Shepherd and Harris, 1998), where they power neurotransmission by supplying ATP. A stationary mitochondrion within presynaptic terminals provides a stable and continuous ATP supply and maintains energy homeostasis at synapses. Conversely, a motile mitochondrion passing by presynaptic boutons dynamically alters local ATP levels, thus influencing various ATP-dependent functions at synapses such as: (1) establishing the proton gradient necessary for neurotransmitter loading; (2) removing Ca²⁺ from nerve terminals; and (3) driving synaptic vesicle transport from reserve pools to release sites. For example, Drosophila photoreceptors expressing mutant Milton display impaired synaptic transmission due to reduced mitochondrial trafficking to synapses (Stowers et al., 2002). Mutation of Drosophila Drp1 reduces synaptic localization of mitochondria and results in faster depletion of synaptic vesicles during prolonged stimulation (Verstreken et al., 2005). Addition of ATP to terminals partially rescues these defects, indicating that mitochondrial ATP production is required for maintaining sustained synaptic transmission. Syntabulin loss of function reduces mitochondrial density in axonal terminals, which is accompanied by accelerated synaptic depression and slowed synapse recovery during high-frequency firing (Ma et al., 2009).One of the most notable characteristics of synaptic transmission is the wide variation in synaptic strength in response to repeated stimulations. The effects of synaptic variability on neuronal circuit activity are increasingly recognized. Some degree of synaptic variability may be necessary for signal processing in flexible or adaptive systems (Murthy et al., 1997). A long-standing fundamental question is how the variation in synaptic strength arises. There is general agreement that some structural and molecular stochastic alternations at synapses are the basis for heterogeneity in synaptic transmission from neuron to neuron or from synapse to synapse (Atwood and Karunanithi, 2002; Stein et al., 2005; Marder and Goaillard, 2006; Branco and Staras, 2009; Ribrault et al., 2011). However, it is unknown whether mitochondrial motility at axonal terminals contributes to pulse-to-pulse variability of presynaptic strength at single-bouton levels. This is particularly relevant in hippocampal neurons, where approximately one third of axonal mitochondria are highly motile, some of which dynamically pass through presynaptic boutons.We recently solved this puzzle by combining dual-channel imaging in live neurons and electrophysiological analysis in syntaphilin knockout hippocampal neurons and acute hippocampal slices in which axonal mitochondrial motility is selectively manipulated (Sun et al., 2013). Our study revealed that the motility of axonal mitochondria is one of the primary mechanisms underlying the variability of presynaptic strength. Enhanced motility of axonal mitochondria increases the pulse-to-pulse variability of EPSC amplitudes, whereas immobilizing axonal mitochondria effectively diminishes the variability. The data raise two assumptions: (1) EPSC amplitudes are averaged through summation from multiple synapses; and (2) release at individual boutons changes over time when mitochondrial distribution and motility are dynamically altered. Thus, at any given time these boutons display different patterns of mitochondrial distribution and motility; mitochondria move either in or out of boutons or pass through boutons during stimulation, thus contributing to the pulse-to-pulse variability. By applying dual-channel live imaging of mitochondria and synaptic vesicle release at single-bouton levels, the study further shows that mitochondrial movement, either into or out of presynaptic boutons, significantly influences synaptic vesicle release due to fluctuation of synaptic ATP levels. In the absence of a stationary mitochondrion within an axonal terminal, there is no constant on-site ATP supply. A motile mitochondrion passing through could temporally supply ATP, thus changing synaptic energy levels and influencing ATP-dependent processes including synaptic vesicle cycling, mobilization, and replenishment (Fig. 3). This is further supported by a recent study applying a new quantitative optical presynaptic ATP reporter. Synaptic activity drives large ATP consumption at nerve terminals and synaptic vesicle cycling consumes most presynaptic ATP (Rangaraju et al., 2014). It is possible that all of these ATP-dependent processes collectively contribute to presynaptic variability. Therefore, the fluctuation of synaptic ATP levels resulting from mitochondrial movement is one of the primary sources for the wide variability of EPSC amplitudes. This study revealed, for the first time, that the dynamic movement of axonal mitochondria is one of the primary mechanisms underlying the presynaptic variation in the CNS, thus providing new insight into the fundamental properties of the CNS to ensure the plasticity and reliability of synaptic transmission (Sun et al., 2013).Open in a separate windowFigure 3.Motile mitochondria passing by synapses contribute to presynaptic variation. A stationary mitochondrion within presynaptic terminals powers neurotransmission by stable and continuous ATP supply (left). Conversely, in the absence of a mitochondrion within a presynaptic bouton (right), there is no stable on-site ATP supply; a motile mitochondrion passing through this bouton temporally supplies ATP, thus changing synaptic energy levels and influencing ATP-dependent synaptic functions over time when mitochondrial distribution and motility are altered. Therefore, the fluctuation of synaptic ATP levels resulting from mitochondrial movement is one of the primary sources for the wide variability of synaptic vesicle release and amplitudes of excitatory postsynaptic currents (EPSCs; Sun et al., 2013).

Mitochondrial motility, neuronal morphology, and energy consumption

Proper mitochondrial transport into neurites in developing neurons is tightly regulated to ensure that metabolically active areas are adequately supplied with ATP (Morris and Hollenbeck, 1993). A recent study showed the differential functions of two mitochondrial motor adaptors, Trak1 and Trak2, in driving polarized mitochondrial transport (van Spronsen et al., 2013). Trak1 is prominently localized in axons and is required for axonal mitochondria transport by binding to both kinesin-1 and dynein/dynactin, whereas Trak2 is present in dendrites and mainly transports mitochondria into dendrites by interacting with dynein/dynactin. The folded configuration of Trak2 preferentially binds dynein motors rather than kinesin-1. Consistently, depleting Trak1 inhibits axonal outgrowth, whereas suppressing Trak2 impairs dendrite morphology. Thus, this study established a new role of Trak proteins in polarized mitochondrial targeting necessary to maintain neuronal morphology.Anchored stationary mitochondria ideally serve as local energy power plants. Increased intracellular [ADP]_i slows down mitochondrial movement and recruits mitochondria to subcellular regions correlated with local [ATP]_i depletion (Mironov, 2007). However, the underlying mechanisms coordinating mitochondria transport and energy consumption remain elusive. The balance between motile and stationary mitochondria responds quickly to changes in axonal growth status. Two recent studies demonstrated that generating and maintaining axonal branching depend upon stable and continuous ATP supply by stationary mitochondria captured at branching points. Courchet et al. (2013) revealed that LKB1, the serine/threonine liver kinase B1, regulates terminal axon branching of cortical neurons in both in vitro and in vivo systems through activating downstream kinase NUAK1, an AMPK-like kinase. AMPK is an AMP-activated protein kinase, which is a master regulator of cellular energy homeostasis and is activated in response to stresses that deplete cellular ATP supplies. Deleting LKB1 or NUAK1 results in a threefold decrease in stationary pools of axonal mitochondria, whereas overexpressing LKB1 or NUAK1 conversely increases the proportion of immobilized mitochondria along axons, thus establishing a causal correlation between stationary mitochondria and axonal branching. To test the hypothesis, Courchet et al. (2013) altered the motility of axonal mitochondria by changing expression levels of syntaphilin. Intriguingly, suppressing syntaphilin expression in cortical progenitors at E15.5 enhances axonal mitochondrial motility accompanied by decreased axon branching. Conversely, overexpressing syntaphilin immobilizes axonal mitochondria and increases axon branching, thus highlighting a critical role for syntaphilin-mediated mitochondrial anchoring in AMPK-induced axonal branching. It is possible that LKB1 and NUAK1-mediated signaling pathways regulate axon branch formation and stabilization by recruiting mitochondria to branch points. However, this study raises one mechanistic question: does syntaphilin or the mitochondrial transport motor–adaptor complex serve as a downstream effector of this signaling pathway? Syntaphilin is strictly developmentally regulated in mouse brains, with low expression before postnatal day 7 and peaking two weeks after birth; thus, its expression is hardly detectable in early embryonic stages (Kang et al., 2008). It remains unclear how siRNA application at E15.5 significantly impacts endogenous syntaphilin levels in the brain. Furthermore, it is unknown whether local ATP homeostasis maintained by stationary but not motile mitochondria contributes to the formation and stabilization of axonal branching.As a cellular energy sensor for detecting increased intracellular [AMP]_i or ATP consumption, AMPK activation positively regulates signaling pathways that replenish cellular ATP supplies. Tao et al. (2013) provide further evidence that sustained neuronal depolarization for 20 min depletes cellular ATP and activates AMPK. Activation of AMPK increases anterograde flux of mitochondria into axons and induces axonal branching in regions where mitochondria are docked in an ATP-dependent manner. The mitochondrial uncoupler FCCP blocks axonal branching, even when mitochondria are localized nearby. These studies provide evidence for how the AMPK signaling pathway regulates axon branch formation and stabilization. These studies collectively highlight a critical role for syntaphilin-mediated mitochondrial anchoring in axon branching. One possible mechanism underlying mitochondrial recruiting and anchoring in support of axon branching is through localized intra-axonal protein synthesis (Spillane et al., 2013). Inhibiting mitochondrial respiration and protein synthesis in axons impairs axonal branching. Thus, stationary mitochondria coordinate maturation of axonal filopodia into axon branching likely by supporting active mRNA translation in axonal hot spots. Activity-induced mitochondrial recruitment in axons is further supported by an in vivo study in mouse saphenous nerve axons. Sustained electrical stimulation (50 Hz) enhances anterograde flux of mitochondria into myelinated axons and redistributes them at the peripheral terminals (Sajic et al., 2013).

Mitochondrial motility and mitophagy

Cumulative evidence indicates mitochondrial dysfunction, impaired transport, altered dynamics, and perturbation of mitochondrial turnover are associated with the pathology of major neurodegenerative disorders (Chen and Chan, 2009; Sheng and Cai, 2012). Defective mitochondrial transport accompanied by mitochondrial pathology is one of the most notable pathological changes in several aging-associated neurodegenerative diseases, including Alzheimer’s disease (Rui et al., 2006; Du et al., 2010; Calkins et al., 2011) and amyotrophic lateral sclerosis (De Vos et al., 2007; Magrané and Manfredi, 2009; Bilsland et al., 2010; Shi et al., 2010; Martin, 2011; Zhu and Sheng, 2011; Marinkovic et al., 2012). Dysfunctional mitochondria not only supply less ATP and maintain Ca²⁺ buffering capacity less efficiently, but also release harmful reactive oxygen species (ROS). Defects in mitochondrial transport reduce the delivery of healthy mitochondria to distal processes and also impair removal of damaged mitochondria from synapses, thus causing energy depletion and altered Ca²⁺ transient at synapses. In addition, toxic ROS may further trigger synaptic stress, thereby contributing to neurodegeneration. Mitochondrial quality control involves multiple levels of surveillance to ensure mitochondrial integrity, including repairing dysfunctional mitochondria by fusion with healthy ones (Chen and Chan 2009; Westermann, 2010) and segregating irreversibly damaged mitochondria for mitophagy, a cargo-specific autophagy to remove damaged mitochondria. Effective sequestration of damaged mitochondria into autophagosomes and subsequent clearance within the autophagy–lysosomal system constitute a key cellular pathway in mitochondrial quality control mechanisms. Recent studies indicate that PINK1/Parkin-mediated pathways ensure mitochondrial integrity and function (Clark et al., 2006; Gautier et al., 2008; Narendra et al., 2008). Loss-of-function mutations in PINK1 and Parkin are associated with recessive forms of Parkinson’s disease. When mitochondria are depolarized or their integrity is damaged, PINK1 accumulates on the outer mitochondrial membrane and recruits the E3 ubiquitin ligase Parkin from the cytosol. Parkin ubiquitinates mitochondrial proteins (Geisler et al., 2010; Poole et al., 2010; Ziviani et al., 2010; Yoshii et al., 2011). This process is essential for damaged mitochondria to be engulfed by isolation membranes that then fuse with lysosomes for degradation.Investigating those mechanisms coordinating mitochondrial motility and mitophagy represents an important emerging area for disease-oriented research. Mature acidic lysosomes are mainly located in the soma (Overly and Hollenbeck, 1996; Cai et al., 2010; Lee et al., 2011). Thus, a long-standing question remains: how are aged and damaged mitochondria at distal terminals efficiently eliminated via the autophagy–lysosomal pathway? Cai et al. (2012) recently reported the unique features of spatial and dynamic Parkin-mediated mitophagy in mature cortical neurons, in which mitochondria remain motile after prolonged and chronic dissipation of the mitochondrial membrane potential (Δψ_m) for 24 h. Given the fact that in vitro mitochondrial depolarization will quickly trigger neuron apoptosis, a high-quality culturing system is essential so that the neurons survive long enough to exhibit altered mitochondrial transport and Parkin-mediated mitophagy. Chronically dissipating Δψ_m by prolonged treatment of low concentrations of Δψ_m-uncoupling reagent CCCP (10 µM) results in accumulation of Parkin-targeted mitochondria in the soma and proximal regions. Such compartmental restriction for Parkin-targeted mitochondria is consistently observed in neurons treated with low concentrations of another Δψ_m-dissipating reagent, antimycin A (1 µM), or mitochondrial ATP synthase inhibitor oligomycin (1 µM). This distribution pattern is attributable to altered motility of chronically depolarized mitochondria with reduced anterograde and relatively enhanced retrograde transport, thus reducing anterograde flux of damaged mitochondria into distal processes. Intriguingly, when anchored by syntaphilin before the dissipation of Δψ_m, mitochondria in distal axons also recruit Parkin for mitophagy. Altered motility may be protective under chronic mitochondrial stress conditions; healthy mitochondria remain distally while aged and damaged ones return to the soma for degradation (Fig. 4). This spatial distribution allows neurons to efficiently remove dysfunctional mitochondria distally and then eliminate them via the autophagy–lysosomal pathway in the soma.Open in a separate windowFigure 4.Functional interplay between mitochondrial transport and mitophagy. Chronically dissipating mitochondrial membrane potential (Δψ_m) by prolonged treatment of low concentrations of Δψ_m-uncoupling reagents accumulates Parkin-targeted mitochondria in the soma and proximal regions. Such compartmental restriction is a result of altered motility of depolarized mitochondria with reduced anterograde and relatively enhanced retrograde transport, thus reducing anterograde flux of damaged mitochondria into distal processes (Cai et al., 2012). This spatial process allows neurons to efficiently remove dysfunctional mitochondria from distal axons via the autophagy–lysosomal pathway in the soma, where mature lysosomes are relatively enriched. Damaged mitochondria at axonal terminals can also recruit Parkin for mitophagy once they are anchored by syntaphilin (Cai et al., 2012) or immobilized by turnover of the motor adaptor Miro on the mitochondrial surface (Weihofen et al., 2009; Chan et al., 2011; Wang et al., 2011; Yoshii et al., 2011; Liu et al., 2012; Sarraf et al., 2013). Autophagosomes including those engulfing damaged mitochondria at axonal terminals transport predominantly to the soma for maturation and more efficient degradation of cargoes within acidic lysosomes (Maday et al., 2012).Reduced anterograde transport of depolarized mitochondria is consistent with recent studies showing Parkin-mediated degradation of Miro on the mitochondrial surface upon Δψ_m dissipation–induced mitophagy. In addition to binding to the KIF5–Trak motor complex, the mitochondrial outer membrane protein Miro also interacts with PINK1 and Parkin and is ubiquitinated by Parkin while mitochondria are depolarized (Weihofen et al., 2009; Chan et al., 2011; Wang et al., 2011; Liu et al., 2012; Sarraf et al., 2013). These studies support the current model that altered mitochondrial transport is a critical aspect of the mechanisms by which the PINK1/Parkin pathway governs mitochondrial quality control. Turnover of Miro on the mitochondrial surface may favor their retrograde transport to the soma or immobilize damaged organelles at distal regions for mitophagy (Fig. 4). A recent study reports the predominant retrograde transport of autophagosomes including those engulfing damaged mitochondria from distal axons to the soma (Maday et al., 2012). The retrograde movement is critical for autophagosome maturation and degradation within acidic lysosomes in the proximal regions of neurons. This study highlights the possibility that distal damaged mitochondria, which are either anchored by syntaphilin or immobilized by turning over Miro, may be also degraded in the soma after their retrograde transport in engulfed autophagosomes. Thus, a functional interplay is proposed between mitochondrial motility and mitophagy to ensure proper removal of aged and dysfunctional mitochondria from distal processes. The correlation between mitochondrial Δψ_m and motility is controversial, as demonstrated in two previous studies by acutely treating neurons with high concentrations of Δψ_m-dissipating reagents (Miller and Sheetz, 2004; Verburg and Hollenbeck, 2008). Instead, Cai et al. (2012) examined mitochondrial motility and mitophagy in mature cortical neurons after a 24-h treatment with a much lower concentration of CCCP (10 µM) or antimycin A (1 µM) combined with applying the pan-caspase inhibitor Z-VAD. The majority of neurons survive and mitochondria remain highly motile under such prolonged stress conditions, thus allowing detection of the altered transport event. Therefore, prolonged Δψ_m dissipation in cultured neurons better reflects chronic mitochondria stress under in vivo physiological or pathological conditions.

Moving forward

The recent discoveries of proteins involved in mitochondrial transport and anchoring boosted our understanding of the molecular mechanisms that regulate mitochondrial motility and distribution in response to physiological and pathological signals. However, there are still many mechanistic questions to be addressed in the near future. In particular, how does a Miro-Ca²⁺–sensing pathway arrest both anterograde and retrograde movement of mitochondria at active synapses? How do dynein and KIF5 motors physically or mechanistically interplay to coordinate mitochondrial bi-directional transport? How are stationary mitochondria remobilized, recruited, and redistributed (and vice versa) to sense, integrate, and respond to changes in mitochondrial membrane potential, cellular metabolic status, neuronal growth signals, and pathological stress? Investigating how mitochondrial motility coordinates their function and integrity in neurons represents an important emerging frontier in neurobiology. Mitochondria undergo dynamic membrane fission/fusion and mitophagy to maintain their quality during the neuronal lifetime. Mitochondrial pathology is one of the most notable hallmarks in several major aging-associated neurodegenerative diseases (Chen and Chan, 2009; Schon and Przedborski, 2011; Court and Coleman, 2012; Sheng and Cai, 2012; Itoh et al., 2013). The proper and efficient elimination of damaged mitochondria via mitophagy may serve as an early neuroprotective mechanism. Studying these dynamic processes in live neurons directly isolated from disease-stage adult mice will provide more reliable models for delineating defects in mitochondrial transport and mitophagy underlying adult-onset pathogenesis. Mechanistic insight into these fundamental processes in coordinating mitochondrial trafficking and anchoring, membrane dynamics, and mitophagy will advance our understanding of human neurodegenerative diseases.     

Malnutrition increases insoluble-to-soluble tubulin ratio and in vitro incorporation of ATP in rat cerebral cortex

Wistar rats were fed a normal protein (25% casein) or an isoenergetic low protein (8% casein) diet from the day of birth to weaning on day 21. Litters were killed at weaning and cerebral cortex was removed. Tubulin was prepared by centrifugation at 100,000 g, 4°C, as described by Shelansky et al. [Proc. Natn. Acad. Sci. U.S.A.70, 765–768 (1973)]. Cold-insoluble tubulin was recovered in the pellet (Pl) fraction and cold-soluble tubulin in the supernatant (Sl) fraction. Alpha and beta tubulin were quantified by electrophoretic and immunological methods in both fractions. Our results indicated that malnutrition enhanced the ratio of cold-insoluble-tubulin-to-cold-soluble-tubulin. Furthermore malnutrition induced an increased in vitro incorporation of ³²P into both soluble and insoluble tubulins. Although tubulin phosphorylation has been related to tubulin stability properties, we cannot unequivocally ascribe the increased insoluble/soluble tubulin ratio with malnutrition to increased in vitro incorporation of ³²P.     

Increased transglutaminase in the aortas of cholesterol-fed rabbits: occurrence of buffer soluble and insoluble forms and an inhibitor.

Rabbits were fed for 10-12 weeks on a normal pellet diet or on the same diet containing 1% cholesterol and 6% peanut oil. The animals were killed and the aortas divided into three layers which were homogenized and extracted. The extracts and the insoluble residues were assayed for transglutaminase activity and tissue transglutaminase antigen. When compared with normal aortas, the inner and middle layers of aortas with atherosclerotic lesions from cholesterol-fed rabbits showed higher transglutaminase activities in the buffer-soluble fraction without a corresponding increase in antigen. The buffer extracts showed two peaks (I and II) of activity and antigen on DE 52 chromatography; peak I was also found, together with lipid, in Triton X-100 extracts of the buffer-insoluble residue. The Triton X-100 insoluble fraction showed higher concentrations of both activity and antigen in the inner and middle layers of atherosclerotic aortas than in normal aortas, but the activity per nanogram of antigen was lower than in the buffer-soluble fraction. The activity in this insoluble residue was largely extracted, together with an inhibitor, by an NaCl-sucrose-dithiothreitol-Triton X-100 solution. DE 52 chromatography of this extract showed a third peak of activity and antigen (peak III) and an inhibitor peak that was distinct from the activity peaks.