Axonal autophagosomes recruit dynein for retrograde transport through fusion with late endosomes

Efficient degradation of autophagic vacuoles (AVs) via lysosomes is an important cellular homeostatic process. This is particularly challenging for neurons because mature acidic lysosomes are relatively enriched in the soma. Although dynein-driven retrograde transport of AVs was suggested, a fundamental question remains how autophagosomes generated at distal axons acquire dynein motors for retrograde transport toward the soma. In this paper, we demonstrate that late endosome (LE)–loaded dynein–snapin complexes drive AV retrograde transport in axons upon fusion of autophagosomes with LEs into amphisomes. Blocking the fusion with syntaxin17 knockdown reduced recruitment of dynein motors to AVs, thus immobilizing them in axons. Deficiency in dynein–snapin coupling impaired AV transport, resulting in AV accumulation in neurites and synaptic terminals. Altogether, our study provides the first evidence that autophagosomes recruit dynein through fusion with LEs and reveals a new motor–adaptor sharing mechanism by which neurons may remove distal AVs engulfing aggregated proteins and dysfunctional organelles for efficient degradation in the soma.     

Defective retrograde transport impairs autophagic clearance in Alzheimer disease neurons

Macroautophagy/autophagy plays a key role in cellular quality control by eliminating protein aggregates and damaged organelles, which is essential for the maintenance of neuronal homeostasis. Defective autophagy has been implicated in the pathogenesis of Alzheimer disease (AD). In AD brains, autophagic vacuoles (AVs) accumulate massively within dystrophic neurites. This raises a fundamental question as to whether impaired autophagic clearance contributes to AD-associated autophagic stress. We recently revealed that AD neurons display defective retrograde transport and accumulation of amphisomes predominantly in axons and presynaptic terminals. Amyloid β (Aβ) oligomers are enriched in axons and interact with dynein motors. This interaction interferes with the coupling of the dynein motor with its adaptor SNAPIN. Such deficits disrupt dynein-driven retrograde transport of amphisomes, thus trapping them in distal axons and impairing their degradation in the soma. Therefore, our study provides new mechanistic insights into AD-linked autophagic pathology, and builds a foundation for developing potential AD therapeutic strategies by rescuing retrograde transport of amphisomes.     

Primary lysosomal dysfunction causes cargo-specific deficits of axonal transport leading to Alzheimer-like neuritic dystrophy

Abnormally swollen regions of axons and dendrites (neurites) filled mainly with autophagy-related organelles represent the highly characteristic and widespread form of "neuritic dystrophy" in Alzheimer disease (AD), which implies dysfunction of autophagy and axonal transport. In this punctum, we discuss our recent findings that autophagic/lysosomal degradation is critical to proper axonal transport of autophagic vacuoles (AVs) and lysosomes. We showed that lysosomal protease inhibition induces defective axonal transport of specific cargoes, causing these cargoes to accumulate in axonal swellings that biochemically and morphologically resemble the dystrophic neurites in AD. Our findings suggest that a cargo-specific failure of axonal transport promotes neuritic dystrophy in AD, which involves a mechanism distinct from the global axonal transport deficits seen in some other neurodegenerative diseases.     

Primary lysosomal dysfunction causes cargo-specific deficits of axonal transport leading to Alzheimer-like neuritic dystrophy

Abnormally swollen regions of axons and dendrites (neurites) filled mainly with autophagy-related organelles represent the highly characteristic and widespread form of “neuritic dystrophy” in Alzheimer disease (AD), which implies dysfunction of autophagy and axonal transport. In this punctum, we discuss our recent findings that autophagic/lysosomal degradation is critical to proper axonal transport of autophagic vacuoles (AVs) and lysosomes. We showed that lysosomal protease inhibition induces defective axonal transport of specific cargoes, causing these cargoes to accumulate in axonal swellings that biochemically and morphologically resemble the dystrophic neurites in AD. Our findings suggest that a cargo-specific failure of axonal transport promotes neuritic dystrophy in AD, which involves a mechanism distinct from the global axonal transport deficits seen in some other neurodegenerative diseases.     

Ultrastructural and Immunocytochemical Characterization of Autophagic Vacuoles in Isolated Hepatocytes: Effects of Vinblastine and Asparagine on Vacuole Distributions

The interactions between the autophagic and the endocytic degradation pathways were investigated by means of immunogold labeling of autophagic vacuoles (AVs) in ultrathin frozen sections from isolated rat hepatocytes. AVs were identified by their autophagocytosed contents of the degradation-resistant cytosolic enzyme CuZn-superoxide dismutase (SOD). Another cytosolic enzyme, carbonic anhydrase (CAIII), was rapidly degraded in the lysosomes, making the vacuolar CAIII/SOD ratio useful as a rough indicator of the progress of autophagic-lysosomal degradation. Lysosomes could be recognized by the presence of the lysosomal membrane glycoprotein lgp120, which was absent from hepatocytic endosomes. Endocytic inputs into the AVs were detected by the presence of gold-conjugated bovine serum albumin (BSA-gold), taken up by fluid-phase endocytosis. All vacuoles recognized morphologically as AVs were SOD-positive, as were essentially all of the lysosomes (96%). The majority (72%) of the lysosomes also labeled positively for BSA within 2 h of endocytosis. The data are thus compatible with the notion that all lysosomes can engage in both autophagic and endocytic degradation. Lgp120 appeared to distinguish well between lysosomes and nonlysosomal AVs: the lgp120-negative AVs (nonlysosomes) had a CAIII/SOD ratio identical to that of the cytosol, indicating that no degradation had occurred, In the lgp120-positive AVs (lysosomes), the ratio was only 43% of the cytosolic value, consistent with substantial CAIII degradation. Among the nonlysosomal AVs (about one-third of all AVs), one-half were BSA-positive, suggesting that early AVs (autophagasomes) and intermediary AVs (amphisomes) that had fused with endosomes were equally abundant. These morphological data thus support previous biochemical evidence for a prelysosomal meeting of the autophagic and endocytic pathways. The microtubule inhibitor vinblastine inhibited the autophagic influx to the lysosomes, causing an accumulation of autophagosomes and a reduction in average lysosomal size. Vinblastine also inhibited the endocytic flux, thereby precluding the formation of amphisomes and of BSA-positive lysosomes. High concentrations (20 mM) of asparagine induced swelling of amphisomes and of BSA-positive lysosomes, probably reflecting an acidotropic effect of ammonia generated by asparagine deamination. Asparagine also caused an accumulation of autophagosomes, amphisomes, and BSA-negative lysosomes, presumably as a result of impaired fusion with the swollen BSA-positive lysosomes. The two agents thus appear to perturb the autophagic-endocytic-lysosomal vacuole dynamics by different mechanisms, making them useful in the further study of these complex organelle interactions.     

Progressive endolysosomal deficits impair autophagic clearance beginning at early asymptomatic stages in fALS mice

Autophagy is an important homeostatic process that functions by eliminating defective organelles and aggregated proteins over a neuron''s lifetime. One pathological hallmark in amyotrophic lateral sclerosis (ALS)-linked motor neurons (MNs) is axonal accumulation of autophagic vacuoles (AVs), thus raising a fundamental question as to whether reduced autophagic clearance due to an impaired lysosomal system contributes to autophagic stress and axonal degeneration. We recently revealed progressive lysosomal deficits in spinal MNs beginning at early asymptomatic stages in fALS-linked mice expressing the human (Hs) SOD1^G93A protein. Such deficits impair the degradation of AVs engulfing damaged mitochondria from distal axons. These early pathological changes are attributable to mutant HsSOD1, which interferes with dynein-driven endolysosomal trafficking. Elucidation of this pathological mechanism is broadly relevant, because autophagy-lysosomal deficits are associated with several major neurodegenerative diseases. Therefore, enhancing autophagic clearance by rescuing endolysosomal trafficking may be a potential therapeutic strategy for ALS and perhaps other neurodegenerative diseases.     

Mesenchymal stem cells enhance autophagy and increase β-amyloid clearance in Alzheimer disease models

Current evidence suggests a central role for autophagy in Alzheimer disease (AD), and dysfunction in the autophagic system may lead to amyloid-β (Aβ) accumulation. Using in vitro and in vivo AD models, the present study investigated whether mesenchymal stem cells (MSCs) could enhance autophagy and thus exert a neuroprotective effect through modulation of Aβ clearance In Aβ-treated neuronal cells, MSCs increased cellular viability and enhanced LC3-II expression compared with cells treated with Aβ only. Immunofluorescence revealed that MSC coculture in Aβ-treated neuronal cells increased the number of LC3-II-positive autophagosomes that were colocalized with a lysosomal marker. Ultrastructural analysis revealed that most autophagic vacuoles (AVs) in Aβ-treated cells were not fused with lysosomes, whereas a large portion of autophagosomes were conjoined with lysosomes in MSCs cocultured with Aβ-treated neuronal cells. Furthermore, MSC coculture markedly increased Aβ immunoreactivity colocalized within lysosomes and decreased intracellular Aβ levels compared with Aβ-treated cells. In Aβ-treated animals, MSC administration significantly increased autophagosome induction, final maturation of late AVs, and fusion with lysosomes. Moreover, MSC administration significantly reduced the level of Aβ in the hippocampus, which was elevated in Aβ-treated mice, concomitant with increased survival of hippocampal neurons. Finally, MSC coculture upregulated BECN1/Beclin 1 expression in AD models. These results suggest that MSCs significantly enhance autolysosome formation and clearance of Aβ in AD models, which may lead to increased neuronal survival against Aβ toxicity. Modulation of the autophagy pathway to repair the damaged AD brain using MSCs would have a significant impact on future strategies for AD treatment.     

Novel Quantitative Autophagy Analysis by Organelle Flow Cytometry after Cell Sonication

Autophagy is a dynamic process of bulk degradation of cellular proteins and organelles in lysosomes. Current methods of autophagy measurement include microscopy-based counting of autophagic vacuoles (AVs) in cells. We have developed a novel method to quantitatively analyze individual AVs using flow cytometry. This method, OFACS (organelle flow after cell sonication), takes advantage of efficient cell disruption with a brief sonication, generating cell homogenates with fluorescently labeled AVs that retain their integrity as confirmed with light and electron microscopy analysis. These AVs could be detected directly in the sonicated cell homogenates on a flow cytometer as a distinct population of expected organelle size on a cytometry plot. Treatment of cells with inhibitors of autophagic flux, such as chloroquine or lysosomal protease inhibitors, increased the number of particles in this population under autophagy inducing conditions, while inhibition of autophagy induction with 3-methyladenine or knockdown of ATG proteins prevented this accumulation. This assay can be easily performed in a high-throughput format and opens up previously unexplored avenues for autophagy analysis.     

Snapin-regulated late endosomal transport is critical for efficient autophagy-lysosomal function in neurons

Neuron maintenance and survival require late endocytic transport from distal processes to the soma where lysosomes are predominantly localized. Here, we report a role for Snapin in attaching dynein to late endosomes through its intermediate chain (DIC). snapin(-/-) neurons exhibit aberrant accumulation of immature lysosomes, clustering and impaired retrograde transport of late endosomes along processes, reduced lysosomal proteolysis due to impaired delivery of internalized proteins and hydrolase precursors from late endosomes to lysosomes, and impaired clearance of autolysosomes, combined with reduced neuron viability and neurodegeneration. The phenotypes are rescued by expressing the snapin transgene, but not the DIC-binding-defective Snapin-L99K mutant. Snapin overexpression in wild-type neurons enhances late endocytic transport and lysosomal function, whereas expressing the mutant defective in Snapin-DIC coupling shows a dominant-negative effect. Altogether, our study highlights new mechanistic insights into how Snapin-DIC coordinates retrograde transport and late endosomal-lysosomal trafficking critical for autophagy-lysosomal function, and thus neuronal homeostasis.     

Nudel Promotes Axonal Lysosome Clearance and Endo-lysosome Formation via Dynein-Mediated Transport

Axonal transport is critical for neuronal function and survival. Cytoplasmic dynein and its accessory complex dynactin form a microtubule minus end-directed motor in charge of retrograde transport. In this study, we show that Nudel, a dynein regulator, was highly expressed in dorsal root ganglion (DRG) neurons. Microinjection of anti-Nudel antibody into cultured DRG neurons abolished retrograde transport of membranous organelles in the axon and led to dispersions of Golgi cisternae in the soma. As a result, lysosomes, which are normally enriched in the soma, moved persistently into and thus accumulated in axons. Endo-lysosome formation was also markedly delayed. As anterograde motility of mitochondria was not inhibited, the antibody apparently did not abolish retrograde transport by destructing axonal microtubule tracks. Similar results were obtained by microinjecting N-terminal Nudel, anti-dynein antibody or a p150^Glued mutant capable of abrogating the dynein–dynactin association. These results indicate a critical role of Nudel in dynein-mediated axonal transport. Moreover, the effects of dynein on endolysosome formation and regional sequestration of lysosomes may contribute to defects in the endocytic pathway seen in neurons of patients or animals with malfunction of dynein.     

Macroautophagy--a novel Beta-amyloid peptide-generating pathway activated in Alzheimer's disease

Macroautophagy, which is a lysosomal pathway for the turnover of organelles and long-lived proteins, is a key determinant of cell survival and longevity. In this study, we show that neuronal macroautophagy is induced early in Alzheimer's disease (AD) and before beta-amyloid (Abeta) deposits extracellularly in the presenilin (PS) 1/Abeta precursor protein (APP) mouse model of beta-amyloidosis. Subsequently, autophagosomes and late autophagic vacuoles (AVs) accumulate markedly in dystrophic dendrites, implying an impaired maturation of AVs to lysosomes. Immunolabeling identifies AVs in the brain as a major reservoir of intracellular Abeta. Purified AVs contain APP and beta-cleaved APP and are highly enriched in PS1, nicastrin, and PS-dependent gamma-secretase activity. Inducing or inhibiting macroautophagy in neuronal and nonneuronal cells by modulating mammalian target of rapamycin kinase elicits parallel changes in AV proliferation and Abeta production. Our results, therefore, link beta-amyloidogenic and cell survival pathways through macroautophagy, which is activated and is abnormal in AD.     

Research Article: Mesenchymal stem cells enhance autophagy and increase β-amyloid clearance in Alzheimer disease models

Current evidence suggests a central role for autophagy in Alzheimer disease (AD), and dysfunction in the autophagic system may lead to amyloid-β (Aβ) accumulation. Using in vitro and in vivo AD models, the present study investigated whether mesenchymal stem cells (MSCs) could enhance autophagy and thus exert a neuroprotective effect through modulation of Aβ clearance In Aβ-treated neuronal cells, MSCs increased cellular viability and enhanced LC3-II expression compared with cells treated with Aβ only. Immunofluorescence revealed that MSC coculture in Aβ-treated neuronal cells increased the number of LC3-II-positive autophagosomes that were colocalized with a lysosomal marker. Ultrastructural analysis revealed that most autophagic vacuoles (AVs) in Aβ-treated cells were not fused with lysosomes, whereas a large portion of autophagosomes were conjoined with lysosomes in MSCs cocultured with Aβ-treated neuronal cells. Furthermore, MSC coculture markedly increased Aβ immunoreactivity colocalized within lysosomes and decreased intracellular Aβ levels compared with Aβ-treated cells. In Aβ-treated animals, MSC administration significantly increased autophagosome induction, final maturation of late AVs, and fusion with lysosomes. Moreover, MSC administration significantly reduced the level of Aβ in the hippocampus, which was elevated in Aβ-treated mice, concomitant with increased survival of hippocampal neurons. Finally, MSC coculture upregulated BECN1/Beclin 1 expression in AD models. These results suggest that MSCs significantly enhance autolysosome formation and clearance of Aβ in AD models, which may lead to increased neuronal survival against Aβ toxicity. Modulation of the autophagy pathway to repair the damaged AD brain using MSCs would have a significant impact on future strategies for AD treatment.     

Dynamics and distribution of endosomes and lysosomes in dendrites

All cells are filled with membrane-bound organelles which are responsible for the synthesis and transport as well as degradation of membrane proteins. The localization of these organelles inside cells is highly regulated. The regulation of organelle positioning has been widely studied in many cell types. In neurons, organelle positioning and its regulation is of particular interest because of the enormous size of neurons and the high spatial heterogeneity of different functional domains, such as axons, proximal and distal portions of dendrites, and synapses. We will discuss new discoveries with regard to the dynamic positioning of endosomes and lysosomes between soma and along dendrites. Just as the “how” of dynamic endosome/lysosome positioning is still being investigated, the “why” is also being explored. An exciting possibility is that synaptic activity influences organelle behaviors. We will discuss what is currently known about the how and the why of endosome/lysosome dynamics in dendrites.     

Characterization of the proteolytic compartment in rat hepatocyte nodules

Persistent liver nodules (hepatocyte nodules, neoplastic nodules) were produced in rat liver by intermittent feeding with 2-acetylaminofluorene. Dense bodies (secondary lysosomes) were purified and characterized from the nodules. The purity of the dense body fraction was 90%. The levels of various lysosomal enzyme activities were lower in these dense bodies in comparison with dense bodies from control liver. Similarly, protein degradation was 50% lower in dense bodies from liver nodules than in control liver. The number of autophagic vacuoles (AVs) in the nodular tissue increased considerably after 3 h vinblastine treatment. We have taken advantage of this expansion in an effort to isolate these organelles from liver nodules. Autophagic vacuoles have been isolated recently from liver and kidney but not from putatively premalignant liver nodules. Fraction purity of AVs from liver nodules was 95%. As with dense bodies, AVs from nodular tissue displayed lower activities of proteinases and lower rates of protein degradation when compared with their counterparts from normal liver tissue. Accordingly, the lower rate of overall protein degradation in liver nodules can be ascribed to a decrease in lysosomal activity. A diminished autophagic sequestration capacity is the most plausible explanation for the decreased rate of proteolysis in cells. This could conceivably give these nodular cells a growth advantage and assist in their selective outgrowth as well as in their transformation from neoplastic into true cancer cells.     

Uncovering the role of Snapin in regulating autophagy-lysosomal function

The autophagy-lysosomal system is the major degradation pathway essential for the maintenance and survival of neurons. This process requires efficient late endocytic transport from distal processes to the soma, in which lysosomes are predominantly localized. However, it is not clear how late endocytic transport has an impact upon neuronal autophagy-lysosomal function. We recently revealed that Snapin acts as a dynein motor adaptor and coordinates retrograde transport and late endosomal-lysosomal trafficking, thus maintaining efficient autophagy-lysosomal function in neurons. Snapin(-/-) neurons display impaired retrograde transport and clustering of late endosomes along neuronal processes, aberrant accumulation of immature lysosomes, and impaired clearance of autolysosomes. Snapin deficiency leads to reduced neuron viability, neurodegeneration, and developmental defects in the central nervous system. Reintroducing the snapin transgene rescues these phenotypes by enhancing the delivery of endosomal cargos to lysosomes and by facilitating autophagy-lysosomal function. Our study suggests that Snapin is a candidate molecular target for autophagy-lysosomal regulation.     

Mitophagy of damaged mitochondria occurs locally in distal neuronal axons and requires PINK1 and Parkin

To minimize oxidative damage to the cell, malfunctioning mitochondria need to be removed by mitophagy. In neuronal axons, mitochondrial damage may occur in distal regions, far from the soma where most lysosomal degradation is thought to occur. In this paper, we report that PINK1 and Parkin, two Parkinson’s disease–associated proteins, mediate local mitophagy of dysfunctional mitochondria in neuronal axons. To reduce cytotoxicity and mimic physiological levels of mitochondrial damage, we selectively damaged a subset of mitochondria in hippocampal axons. Parkin was rapidly recruited to damaged mitochondria in axons followed by formation of LC3-positive autophagosomes and LAMP1-positive lysosomes. In PINK1^−/− axons, damaged mitochondria did not accumulate Parkin and failed to be engulfed in autophagosomes. Similarly, initiation of mitophagy was blocked in Parkin^−/− axons. Our findings demonstrate that the PINK1–Parkin-mediated pathway is required for local mitophagy in distal axons in response to focal damage. Local mitophagy likely provides rapid neuroprotection against oxidative stress without a requirement for retrograde transport to the soma.     

A Voltage-Gated Calcium Channel Regulates Lysosomal Fusion with Endosomes and Autophagosomes and Is Required for Neuronal Homeostasis

Autophagy helps deliver sequestered intracellular cargo to lysosomes for proteolytic degradation and thereby maintains cellular homeostasis by preventing accumulation of toxic substances in cells. In a forward mosaic screen in Drosophila designed to identify genes required for neuronal function and maintenance, we identified multiple cacophony (cac) mutant alleles. They exhibit an age-dependent accumulation of autophagic vacuoles (AVs) in photoreceptor terminals and eventually a degeneration of the terminals and surrounding glia. cac encodes an α1 subunit of a Drosophila voltage-gated calcium channel (VGCC) that is required for synaptic vesicle fusion with the plasma membrane and neurotransmitter release. Here, we show that cac mutant photoreceptor terminals accumulate AV-lysosomal fusion intermediates, suggesting that Cac is necessary for the fusion of AVs with lysosomes, a poorly defined process. Loss of another subunit of the VGCC, α2δ or straightjacket (stj), causes phenotypes very similar to those caused by the loss of cac, indicating that the VGCC is required for AV-lysosomal fusion. The role of VGCC in AV-lysosomal fusion is evolutionarily conserved, as the loss of the mouse homologues, Cacna1a and Cacna2d2, also leads to autophagic defects in mice. Moreover, we find that CACNA1A is localized to the lysosomes and that loss of lysosomal Cacna1a in cerebellar cultured neurons leads to a failure of lysosomes to fuse with endosomes and autophagosomes. Finally, we show that the lysosomal CACNA1A but not the plasma-membrane resident CACNA1A is required for lysosomal fusion. In summary, we present a model in which the VGCC plays a role in autophagy by regulating the fusion of AVs with lysosomes through its calcium channel activity and hence functions in maintaining neuronal homeostasis.     

Synapses have autophagy under control

Regulation of autophagy in neurons remains unclear. In this issue, Kulkarni et al. (2021. J. Cell Biol. https://doi.org/10.1083/jcb.202002084) show with elegant live imaging that in dendrites, but not in axons, autophagosome motility and function is regulated by synaptic activity.

Macroautophagy is a type of autophagy that refers to the capacity to form double membrane compartments called autophagosomes that engulf large protein aggregates and defective organelles. Autophagosomes fuse with lysosomes, forming degradative autolysosomes (1). Autophagosome formation depends on the conjugation of LC3-I (cytosolic) to phosphatidylethanolamine, generating LC3-II, which remains bound to autolysosomes (1). In neurons, inactivation of autophagy genes impacts neurodevelopment, axon growth and guidance, synapse formation and pruning, ultimately leading to neurodegeneration. Particularly, in motor neurons and cerebellum Purkinje cells, autophagy gene knockout leads to the accumulation of intracellular protein aggregates and degeneration, impacting movement coordination (1). Interestingly, stimulation of memory up-regulates autophagy, and while reducing autophagy reduces memory, activating it has the opposite effect on memory (2). What triggers macroautophagy in neurons remains unclear. In this issue, Kulkarni et al. test whether synaptic activity regulates autophagy and detail the impact of synaptic activity on autophagosome motility (3).Kulkarni et al. used multiple strategies to manipulate synaptic activity. They stimulated synaptic activity by depolarizing neurons with high potassium, treating them with a cocktail of antagonists of voltage-gated potassium channels and inhibitory gamma-aminobutyric A receptors, and using uncaging of the excitatory neurotransmitter glutamate. To inhibit synaptic activity, the researchers treated neurons with antagonists of excitatory α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate receptors (4). To image autophagosomes and autolysosomes (here globally termed autophagic vacuoles [AVs]) in live neurons, the authors expressed LC3 tagged with fluorescent proteins. They elegantly imaged the same neuronal compartment before and after depolarization, or under basal, increased, or reduced synaptic activity, and used kymograph analysis (via Kymoanalyser; 5) to quantify the mean speeds of AVs in both dendrites and axons. An increase in intracellular calcium measured with a genetically encoded calcium sensor, GCAMP3, indicated synaptic activity. Kulkarni et al. observed that, in dendrites, AVs stop with synaptic activity and move with synaptic inhibition (Fig. 1). This AV movement change was swift and unaltered by co-culture with astrocytes, and reversible. One key finding is that this change in AV movement occurred in dendrites, but not in axons. Interestingly, AVs stopped at or near synapses, which were identified with PSD-95-GFP.Open in a separate windowFigure 1.In dendrites, AVs stop at synapses upon synaptic activity.The authors further characterized the AVs in terms of acidity (lysotracker labelling of acidic organelles) and of degradative capacity (DQ-BSA fluorescence accumulation upon lysosomal degradation). Lysotracker motility changed similarly with synaptic activity. Interestingly, the lysotracker density increased with synaptic stimulation. The higher number of acidic organelles (likely autolysosomes) indicated increased autophagy or acidification with synaptic activity, which could underlie increased degradative activity. Indeed, about half of the LC3-positive AVs were degradative in dendrites, while in axons there was virtually no degradative AV, supporting the requirement for transport to the soma for degradation of autophagic cargo (6). Finally, Kulkarni et al. show that degradative AVs increase with synaptic activity, correlating with the reduced motility of LC3-positive AVs.An intriguing observation is that the autophagic vacuoles identified by LC3-mCherry were virtually all positive for LAMP1, a marker of late endosomes and lysosomes, indicating that dendrites mainly contain autolysosomes and no or very few autophagosomes (LC3-positive and LAMP1-negative) and late endosomes/lysosomes (LC3-negative and LAMP1-positive). One is left wondering if it results from LC3 overexpression and overflooding to interconnected organelles. An alternative possibility is that LC3 may not always label autophagosomes, in which case complementary electron microscopy is necessary for confirmation. Where are dendritic autolysosomes formed? In axons, a fraction of the LC3 autophagic vacuoles was LAMP1 negative, and the formation of autophagosomes at axon terminals has been well documented (7). Thus, do autophagosomes form in axons, fuse with LAMP1-positive late endosomes/lysosomes, and only after are they transported to dendrites? Alternatively, autophagosomes may form in dendrites and fuse with late endosomes/lysosomes, preventing their detection unless fusion is inhibited (8).Another interesting observation concerns the similar change in the motility of early endosomes, identified by Rab5, an early endosome GTPase, with synaptic activity. Other organelles, post-ER vesicles (4), and proteasomes (9) similarly display a change in motility in dendrites upon synaptic activity. In contrast, mitochondria stop moving in axons with synaptic activity (10). The significance of this arrest of several dendritic organelles with synaptic activity is an attractive area for research.Neuronal autophagy dysfunction is implicated in many neurodegenerative diseases (1). At least early in the disease, increasing autophagy improves neuronal function and synapse activity (1). Genetic risk factors include lysosomal proteins, whose defective function leads to the accumulation of nondegraded autophagic vacuoles and recapitulate neurodegenerative phenotypes (11). Lysosomal dysfunction is a mechanism of cellular aging. Moreover, synapses become dysfunctional with aging and lost in neurodegenerative diseases (12). Based on this study, synapse dysfunction and thus reduced synaptic activity could increase AV motility and reduce acidification and the degradative capacity of autolysosomes. Similarly, neuronal overexcitability, as in early Alzheimer''s disease patients with seizures, could cause excessive AV motility and degradative activity.What is the mechanism that stops AV movement? Do early endosomes, secretory vesicles, or proteasomes change motility using similar mechanisms? For post-ER vesicles, the CAMKII dependent phosphorylation of the microtubule motor Kif17 was sufficient to arrest movement (4). Alternatively, could it be the actin cytoskeleton that forms patches in the dendritic shaft at the base of postsynaptic glutamatergic synapses to halt microtubule-dependent transport of organelles (13)? More work is needed to tackle these questions and define the cell biological mechanisms by which synaptic activity controls AV function and dynamics in different neuronal compartments. Understanding the mechanisms underlying the regulation of autophagy and autophagosome maturation and degradation provides an exciting opportunity for therapeutic development in neurodegenerative diseases.     

Isolation of autophagic vacuoles from rat liver: morphological and biochemical characterization

The induction of autophagy caused by vinblastine (VBL) has been found to be concomitant with a stimulation of proteolysis in a mitochondrial- lysosomal (ML) fraction from the rat liver (Marzella and Glaumann, 1980, Lab. Invest., 42: 8-17. Marzella and Glaumann, 1980, Lab. Invest., 42:18-27). In this fraction the enhanced proteolysis is associated with a threefold increase in the relative fractional volume of autophagic vacuoles (AVs). In an attempt to isolate the AVs, we subfractionated the ML suspension at different intervals after the induction of autophagy by VBL by centrifugation on a discontinuous Metrizamide gradient ranging from 50% to 15%. The material banding at the 24 to 20% and the 20 to 15% interphases was collected. Morphological analysis reveals that 3 h after induction of autophagy these fractions consist predominantly (approximately 90%) of intact autophagic vacuoles. These autophagic vacuoles contain cytosol, mitochondria, portions of endoplasmic reticulum, and occasional very low density lipoprotein, particles either free or in Golgi apparatus derivatives, in particular secretory granules. The sequestered materials show ultrastructural signs of ongoing degradation. In addition to containing typical autophagic vacuoles, the isolated fractions consist of lysosomes lacking morphologically recognizable cellular components. Contamination from nonlysosomal material is only a few percent as judged from morphometric analysis. Typical lysosomal "marker" enzymes are enriched 15-fold, whereas the proteolytic activity is enriched 10- to 20-fold in the isolated AV fraction as compared to the homogenate. Initially, the yield of nonlysosomal mitochondrial and microsomal enzyme activities increases in parallel with the induction of autophagy but, later on, decreases with advanced degradation of the sequestered cell organelles. Therefore, in the case of AVs the presence of nonlysosomal marker enzymes cannot be used for calculation of fraction purity, since newly sequestered organelles are enzymatically active. Isolated autophagic vacuoles show proteolytic activity when incubated in vitro. The comparatively high phospholipid/protein ratio (0.5) of the AV fraction suggests that phospholipids are degraded more slow than proteins. Is it concluded that AVs can be isolated into a pure fraction and are the subcellular site of enhanced protein degradation in the rat liver after induction of autophagy.