Loss of PINK1 Function Promotes Mitophagy through Effects on Oxidative Stress and Mitochondrial Fission

Mitochondrial dysregulation is strongly implicated in Parkinson disease. Mutations in PTEN-induced kinase 1 (PINK1) are associated with familial parkinsonism and neuropsychiatric disorders. Although overexpressed PINK1 is neuroprotective, less is known about neuronal responses to loss of PINK1 function. We found that stable knockdown of PINK1 induced mitochondrial fragmentation and autophagy in SH-SY5Y cells, which was reversed by the reintroduction of an RNA interference (RNAi)-resistant plasmid for PINK1. Moreover, stable or transient overexpression of wild-type PINK1 increased mitochondrial interconnectivity and suppressed toxin-induced autophagy/mitophagy. Mitochondrial oxidant production played an essential role in triggering mitochondrial fragmentation and autophagy in PINK1 shRNA lines. Autophagy/mitophagy served a protective role in limiting cell death, and overexpressing Parkin further enhanced this protective mitophagic response. The dominant negative Drp1 mutant inhibited both fission and mitophagy in PINK1-deficient cells. Interestingly, RNAi knockdown of autophagy proteins Atg7 and LC3/Atg8 also decreased mitochondrial fragmentation without affecting oxidative stress, suggesting active involvement of autophagy in morphologic remodeling of mitochondria for clearance. To summarize, loss of PINK1 function elicits oxidative stress and mitochondrial turnover coordinated by the autophagic and fission/fusion machineries. Furthermore, PINK1 and Parkin may cooperate through different mechanisms to maintain mitochondrial homeostasis.Parkinson disease is an age-related neurodegenerative disease that affects ∼1% of the population worldwide. The causes of sporadic cases are unknown, although mitochondrial or oxidative toxins such as 1-methyl-4-phenylpyridinium, 6-hydroxydopamine (6-OHDA),³ and rotenone reproduce features of the disease in animal and cell culture models (1). Abnormalities in mitochondrial respiration and increased oxidative stress are observed in cells and tissues from parkinsonian patients (2, 3), which also exhibit increased mitochondrial autophagy (4). Furthermore, mutations in parkinsonian genes affect oxidative stress response pathways and mitochondrial homeostasis (5). Thus, disruption of mitochondrial homeostasis represents a major factor implicated in the pathogenesis of sporadic and inherited parkinsonian disorders (PD).The PARK6 locus involved in autosomal recessive and early-onset PD encodes for PTEN-induced kinase 1 (PINK1) (6, 7). PINK1 is a cytosolic and mitochondrially localized 581-amino acid serine/threonine kinase that possesses an N-terminal mitochondrial targeting sequence (6, 8). The primary sequence also includes a putative transmembrane domain important for orientation of the PINK1 domain (8), a conserved kinase domain homologous to calcium calmodulin kinases, and a C-terminal domain that regulates autophosphorylation activity (9, 10). Overexpression of wild-type PINK1, but not its PD-associated mutants, protects against several toxic insults in neuronal cells (6, 11, 12). Mitochondrial targeting is necessary for some (13) but not all of the neuroprotective effects of PINK1 (14), implicating involvement of cytoplasmic targets that modulate mitochondrial pathobiology (8). PINK1 catalytic activity is necessary for its neuroprotective role, because a kinase-deficient K219M substitution in the ATP binding pocket of PINK1 abrogates its ability to protect neurons (14). Although PINK1 mutations do not seem to impair mitochondrial targeting, PD-associated mutations differentially destabilize the protein, resulting in loss of neuroprotective activities (13, 15).Recent studies indicate that PINK1 and Parkin interact genetically (3, 16-18) to prevent oxidative stress (19, 20) and regulate mitochondrial morphology (21). Primary cells derived from PINK1 mutant patients exhibit mitochondrial fragmentation with disorganized cristae, recapitulated by RNA interference studies in HeLa cells (3).Mitochondria are degraded by macroautophagy, a process involving sequestration of cytoplasmic cargo into membranous autophagic vacuoles (AVs) for delivery to lysosomes (22, 23). Interestingly, mitochondrial fission accompanies autophagic neurodegeneration elicited by the PD neurotoxin 6-OHDA (24, 25). Moreover, mitochondrial fragmentation and increased autophagy are observed in neurodegenerative diseases including Alzheimer and Parkinson diseases (4, 26-28). Although inclusion of mitochondria in autophagosomes was once believed to be a random process, as observed during starvation, studies involving hypoxia, mitochondrial damage, apoptotic stimuli, or limiting amounts of aerobic substrates in facultative anaerobes support the concept of selective mitochondrial autophagy (mitophagy) (29, 30). In particular, mitochondrially localized kinases may play an important role in models involving oxidative mitochondrial injury (25, 31, 32).Autophagy is involved in the clearance of protein aggregates (33-35) and normal regulation of axonal-synaptic morphology (36). Chronic disruption of lysosomal function results in accumulation of subtly impaired mitochondria with decreased calcium buffering capacity (37), implicating an important role for autophagy in mitochondrial homeostasis (37, 38). Recently, Parkin, which complements the effects of PINK1 deficiency on mitochondrial morphology (3), was found to promote autophagy of depolarized mitochondria (39). Conversely, Beclin 1-independent autophagy/mitophagy contributes to cell death elicited by the PD toxins 1-methyl-4-phenylpyridinium and 6-OHDA (25, 28, 31, 32), causing neurite retraction in cells expressing a PD-linked mutation in leucine-rich repeat kinase 2 (40). Whereas properly regulated autophagy plays a homeostatic and neuroprotective role, excessive or incomplete autophagy creates a condition of “autophagic stress” that can contribute to neurodegeneration (28).As mitochondrial fragmentation (3) and increased mitochondrial autophagy (4) have been described in human cells or tissues of PD patients, we investigated whether or not the engineered loss of PINK1 function could recapitulate these observations in human neuronal cells (SH-SY5Y). Stable knockdown of endogenous PINK1 gave rise to mitochondrial fragmentation and increased autophagy and mitophagy, whereas stable or transient overexpression of PINK1 had the opposite effect. Autophagy/mitophagy was dependent upon increased mitochondrial oxidant production and activation of fission. The data indicate that PINK1 is important for the maintenance of mitochondrial networks, suggesting that coordinated regulation of mitochondrial dynamics and autophagy limits cell death associated with loss of PINK1 function.     

Glutathione Participates in the Regulation of Mitophagy in Yeast

The antioxidant N-acetyl-l-cysteine prevented the autophagy-dependent delivery of mitochondria to the vacuoles, as examined by fluorescence microscopy of mitochondria-targeted green fluorescent protein, transmission electron microscopy, and Western blot analysis of mitochondrial proteins. The effect of N-acetyl-l-cysteine was specific to mitochondrial autophagy (mitophagy). Indeed, autophagy-dependent activation of alkaline phosphatase and the presence of hallmarks of non-selective microautophagy were not altered by N-acetyl-l-cysteine. The effect of N-acetyl-l-cysteine was not related to its scavenging properties, but rather to its fueling effect of the glutathione pool. As a matter of fact, the decrease of the glutathione pool induced by chemical or genetical manipulation did stimulate mitophagy but not general autophagy. Conversely, the addition of a cell-permeable form of glutathione inhibited mitophagy. Inhibition of glutathione synthesis had no effect in the strain Δuth1, which is deficient in selective mitochondrial degradation. These data show that mitophagy can be regulated independently of general autophagy, and that its implementation may depend on the cellular redox status.Autophagy is a major pathway for the lysosomal/vacuolar delivery of long-lived proteins and organelles, where they are degraded and recycled. Autophagy plays a crucial role in differentiation and cellular response to stress and is conserved in eukaryotic cells from yeast to mammals (1, 2). The main form of autophagy, macroautophagy, involves the non-selective sequestration of large portions of the cytoplasm into double-membrane structures termed autophagosomes, and their delivery to the vacuole/lysosome for degradation. Another process, microautophagy, involves the direct sequestration of parts of the cytoplasm by vacuole/lysosomes. The two processes coexist in yeast cells but their extent may depend on different factors including metabolic state: for example, we have observed that nitrogen-starved lactate-grown yeast cells develop microautophagy, whereas nitrogen-starved glucose-grown cells preferentially develop macroautophagy (3).Both macroautophagy and microautophagy are essentially non-selective, in the way that autophagosomes and vacuole invaginations do not appear to discriminate the sequestered material. However, selective forms of autophagy have been observed (4) that target namely peroxisomes (5, 6), chromatin (7, 8), endoplasmic reticulum (9), ribosomes (10), and mitochondria (3, 11–13). Although non-selective autophagy plays an essential role in survival by nitrogen starvation, by providing amino acids to the cell, selective autophagy is more likely to have a function in the maintenance of cellular structures, both under normal conditions as a “housecleaning” process, and under stress conditions by eliminating altered organelles and macromolecular structures (14–16). Selective autophagy targeting mitochondria, termed mitophagy, may be particularly relevant to stress conditions. The mitochondrial respiratory chain is both the main site and target of ROS⁴ production (17). Consequently, the maintenance of a pool of healthy mitochondria is a crucial challenge for the cells. The progressive accumulation of altered mitochondria (18) caused by the loss of efficiency of the maintenance process (degradation/biogenesis de novo) is often considered as a major cause of cellular aging (19–23). In mammalian cells, autophagic removal of mitochondria has been shown to be triggered following induction/blockade of apoptosis (23), suggesting that autophagy of mitochondria was required for cell survival following mitochondria injury (14). Consistent with this idea, a direct alteration of mitochondrial permeability properties has been shown to induce mitochondrial autophagy (13, 24, 25). Furthermore, inactivation of catalase induced the autophagic elimination of altered mitochondria (26). In the yeast Saccharomyces cerevisiae, the alteration of F₀F₁-ATPase biogenesis in a conditional mutant has been shown to trigger autophagy (27). Alterations of mitochondrial ion homeostasis caused by the inactivation of the K⁺/H⁺ exchanger was shown to cause both autophagy and mitophagy (28). We have reported that treatment of cells with rapamycin induced early ROS production and mitochondrial lipid oxidation that could be inhibited by the hydrophobic antioxidant resveratrol (29). Furthermore, resveratrol treatment impaired autophagic degradation of both cytosolic and mitochondrial proteins and delayed rapamycin-induced cell death, suggesting that mitochondrial oxidation events may play a crucial role in the regulation of autophagy. This existence of regulation of autophagy by ROS has received molecular support in HeLa cells (30): these authors showed that starvation stimulated ROS production, namely H₂O₂, which was essential for autophagy. Furthermore, they identified the cysteine protease hsAtg4 as a direct target for oxidation by H₂O₂. This provided a possible connection between the mitochondrial status and regulation of autophagy.Investigations of mitochondrial autophagy in nitrogen-starved lactate-grown yeast cells have established the existence of two distinct processes: the first one occurring very early, is selective for mitochondria and is dependent on the presence of the mitochondrial protein Uth1p; the second one occurring later, is not selective for mitochondria, is not dependent on Uth1p, and is a form of bulk microautophagy (3). The absence of the selective process in the Δuth1 mutant strongly delays and decreases mitochondrial protein degradation (3, 12). The putative protein phosphatase Aup1p has been also shown to be essential in inducing mitophagy (31). Additionally several Atg proteins were shown to be involved in vacuolar sequestration of mitochondrial GFP (3, 12, 32, 33). Recently, the protein Atg11p, which had been already identified as an essential protein for selective autophagy has also been reported as being essential for mitophagy (33).The question remains as to identify of the signals that trigger selective mitophagy. It is particularly intriguing that selective mitophagy is activated very early after the shift to a nitrogen-deprived medium (3). Furthermore, selective mitophagy is very active on lactate-grown cells (with fully differentiated mitochondria) but is nearly absent in glucose-grown cells (3). In the present paper, we investigated the relationships between the redox status of the cells and selective mitophagy, namely by manipulating glutathione. Our results support the view that redox imbalance is a trigger for the selective elimination of mitochondria.     

The Neuromediator Glutamate, through Specific Substrate Interactions, Enhances Mitochondrial ATP Production and Reactive Oxygen Species Generation in Nonsynaptic Brain Mitochondria

The finding that upon neuronal activation glutamate is transported postsynaptically from synaptic clefts and increased lactate availability for neurons suggest that brain mitochondria (BM) utilize a mixture of substrates, namely pyruvate, glutamate, and the tricarboxylic acid cycle metabolites. We studied how glutamate affected oxidative phosphorylation and reactive oxygen species (ROS) production in rat BM oxidizing pyruvate + malate or succinate. Simultaneous oxidation of glutamate + pyruvate + malate increased state 3 and uncoupled respiration by 52 and 71%, respectively. The state 4 ROS generation increased 100% over BM oxidizing pyruvate + malate and 900% over that of BM oxidizing glutamate + malate. Up to 70% of ROS generation was associated with reverse electron transport. These effects of pyruvate + glutamate + malate were observed only with BM and not with liver or heart mitochondria. The effects of glutamate + pyruvate on succinate-supported respiration and ROS generation were not organ-specific and depended only on whether mitochondria were isolated with or without bovine serum albumin. With the non-bovine serum albumin brain and heart mitochondria oxidizing succinate, the addition of pyruvate and glutamate abrogated inhibition of Complex II by oxaloacetate. We conclude that (i) during neuronal activation, simultaneous oxidation of glutamate + pyruvate temporarily enhances neuronal mitochondrial ATP production, and (ii) intrinsic inhibition of Complex II by oxaloacetate is an inherent mechanism that protects against ROS generation during reverse electron transport.Recently, it has emerged that mitochondrial dysfunctions play an important role in the pathogenesis of degenerative diseases of the central nervous system (1–3). The processes underlying neuronal degeneration are complex, and some authors suggest that several genetic alterations are involved (4). However, another level of complexity may be derived from the fact that virtually all cellular activities depend upon energy metabolism in the cell (5). Alterations in energy metabolism processes within cells may also contribute to pathogenic mechanisms underlying neurodegenerative disease.A large body of evidence suggests that increased oxidative stress is an important pathogenic mechanism that promotes neurodegeneration (6). Because neurons have a long life span, and most neurodegenerative diseases have a clear association with age (7), it is important to understand mechanisms underlying reactive oxygen species (ROS)² production in neurons. Recently, Kudin et al. (8) analyzed the contribution of mitochondria to the total ROS production in brain tissue. They concluded that mitochondria are the major source of ROS and that at least 50% of ROS generated by brain mitochondria was associated with succinate-supported reverse electron transport (RET). Under conditions of normoxia, about 1% of the respiratory chain electron flow was redirected to form superoxide (8).Recently, we suggested that the organization of the respiratory chain complexes into supercomplexes that occurs in brain mitochondria (BM) (9) may represent one of the intrinsic mechanisms to prevent excessive ROS generation (10). In this paper, we put forward the hypothesis that inhibition of Complex II by oxaloacetate (OAA) represents another important intrinsic mechanism to prevent oxidative stress. We provide evidence that glutamate and pyruvate specifically exert control over the production of ROS at the level of Complex II. Below we present a brief account of published theoretical and experimental evidence that underlie our hypothesis.The neural processing of information is metabolically expensive (11). More than 80% of energy is spent postsynaptically to restore the ionic composition of neurons (11). When neurons are activated, reuptake of glutamate stimulates aerobic glycolysis in astroglial cells (12), thereby making lactate the major substrate for neuronal mitochondria (4, 13). However, rapid conversion of lactate to pyruvate in neurons requires activation of the malate-aspartate shuttle (MAS). The shuttle is the major pathway for cytosolic reducing equivalents from NADH to enter the mitochondria and be oxidized (14, 15). The key component of MAS is the mitochondrial aspartate/glutamate carrier (AGC) (16), and recent data suggest that the AGC is expressed mainly in neurons (14). Absence of the AGC from astrocytes in the brain implies a compartmentation of intermediary metabolism, with glycolysis taking place in astrocytes and lactate oxidation in neurons (13, 14, 17). Active operation of MAS requires that a certain amount of glutamate must be transported from synaptic clefts into activated neurons. In isolated BM, it has been shown that besides pyruvate, glutamate is also a good respiratory substrate (5, 18). In the presynaptic elements, the concentration of cytosolic glutamate is ∼10 mm at all times (19). Yudkoff et al. (18) have shown that synaptosomal mitochondria utilize glutamate and pyruvate as mitochondrial respiratory substrates. Glutamate is also oxidized by the astroglial mitochondria (13).Until recently, it was generally accepted that most of the glutamate is rapidly removed from the synaptic cleft by glutamate transporters EAAT1 and EAAT2 located on presynaptic termini and glial cells (20–24). However, recent data show that a significant fraction of glutamate is rapidly bound and transported by the glutamate transporter isoform, EAAT4, located juxtasynaptically in the membranes of spines and dendrites (20, 25–28). At the climbing fiber to Purkinje cell synapses in the cerebellum, about 17% (28) or more than 50% (29) of synaptically released glutamate may be removed by postsynaptic transporters. Besides the cerebellum, EAAT4 protein was found to be omnipresent throughout the fore- and midbrain regions (30). Moreover, it was shown that although most of the EAAT2 protein is astroglial, around 15% is distributed in nerve terminals and axons in hippocampal slices and that this protein may be responsible for more than half of the total uptake of glutamate from synaptic clefts (24). These data suggest that postsynaptic transport of glutamate into nerve terminals where mitochondria are located (31) may occur in all brain regions. According to calculations of Brasnjo and Otis (28), in a single synapse, EAAT4 (excitatory amino acid transporter 4) binds and transports postsynaptically about 1.3 ± 0.1 × 10⁶ glutamate molecules. In the brain, on average, 1 mm³ of tissue contains 1 × 10⁸ synapses (32, 33). Because of the high density of synaptic contacts, the neuronal cells may be exposed to mediators released from hundreds of firing synapses. Thus, in a narrow space of spines and dendrites, several million glutamate molecules postsynaptically transported from synaptic boutons may create local cytosolic concentration of glutamate in the low millimolar range. Consequently, neuronal mitochondria, particularly those located at the axonal or dendritic synaptic junctions, may, in addition to metabolizing pyruvate, temporarily metabolize glutamate and succinate formed during mitochondrial catabolism of γ-aminobutyric acid in postsynaptic cells (34).The purpose of this study was to examine how the neuromediator glutamate affects respiratory activity and ROS generation in nonsynaptic BM when combined with pyruvate and the tricarboxylic acid cycle intermediates succinate and malate. We show that with pyruvate + glutamate + malate, the rate of oxidative phosphorylation increased more than 50%, and in resting mitochondria the rate of ROS generation associated with the reverse electron transport increased severalfold. These effects were observed only with brain and spinal cord mitochondria, not with liver or heart mitochondria, suggesting that they may be restricted to neuronal cells.Taken together, the data presented support the hypothesis that in activated neurons, the neuromediator glutamate stimulates mitochondrial ATP production when energy demand is increased. However, in the absence of energy consumption, glutamate + pyruvate may increase the generation of ROS severalfold. We suggest that intrinsic inhibition of Complex II by oxaloacetate is an important natural protective mechanism against ROS associated with reverse electron transport.     

Generation of Cubic Membranes by Controlled Homotypic Interaction of Membrane Proteins in the Endoplasmic Reticulum

Cell membranes predominantly consist of lamellar lipid bilayers. When studied in vitro, however, many membrane lipids can exhibit non-lamellar morphologies, often with cubic symmetries. An open issue is how lipid polymorphisms influence organelle and cell shape. Here, we used controlled dimerization of artificial membrane proteins in mammalian tissue culture cells to induce an expansion of the endoplasmic reticulum (ER) with cubic symmetry. Although this observation emphasizes ER architectural plasticity, we found that the changed ER membrane became sequestered into large autophagic vacuoles, positive for the autophagy protein LC3. Autophagy may be targeting irregular membrane shapes and/or aggregated protein. We suggest that membrane morphology can be controlled in cells.The observation that simple mixtures of amphiphilic (polar) lipids and water yield a rich flora of phase structures has opened a long-standing debate as to whether such membrane polymorphisms are relevant for living organisms (1–7). Lipid bilayers with planar geometry, termed lamellar symmetry, dominate the membrane structure of cells. However, this architecture comprises only a fraction of the structures seen with in vitro lipid-water systems (7–11). The propensity to form lamellar bilayers (a property exclusive to cylindrically shaped lipids) is flanked by a continuum of lipid structures that occur in a number of exotic and probably non-physiological non-bilayer configurations (3, 12). However, certain lipids, particularly those with smaller head groups and more bulky hydrocarbon chains, can adopt bilayered non-lamellar phases called cubic phases. Here the bilayer is curved everywhere in the form of saddle shapes corresponding to an energetically favorable minimal surface of zero mean curvature (1, 7). Because a substantial number of the lipids present in biological membranes, when studied as individual pure lipids, form cubic phases (13), cubic membranes have received particular interest in cell biology.Since the application of electron microscopy (EM)³ to the study of cell ultrastructure, unusual membrane morphologies have been reported for virtually every organelle (14, 15). However, interpretation of three-dimensional structures from two-dimensional electron micrographs is not easy (16). In seminal work, Landh (17) developed the method of direct template correlative matching, a technique that unequivocally assesses the presence of cubic membranes in biological specimens (16). Cubic phases adopt mathematically well defined three-dimensional configurations whose two-dimensional analogs have been derived (4, 17). In direct template correlative matching, electron micrographs are matched to these analogs. Cubic cell membrane geometries and in vitro cubic phases of purified lipid mixtures do differ in their lattice parameters; however, such deviations are thought to relate to differences in water activity and lipid to protein ratios (10, 14, 18). Direct template correlative matching has revealed thousands of examples of cellular cubic membranes in a broad survey of electron micrographs ranging from protozoa to human cells (14, 17) and, more recently, in the mitochondria of amoeba (19) and in subcellular membrane compartments associated with severe acute respiratory syndrome virus (20). Analysis of cellular cubic membranes has also been furthered by the development of EM tomography that confirmed the presence of cubic bilayers in the mitochondrial membranes of amoeba (21, 22).Although it is now clear that cubic membranes can exist in living cells, the generation of such architecture would appear tightly regulated, as evidenced by the dominance of lamellar bilayers in biology. In this light, we examined the capability and implications of generating cubic membranes in the endoplasmic reticulum (ER) of mammalian tissue culture cells. The ER is a spatially interconnected complex consisting of two domains, the nuclear envelope and the peripheral ER (23–26). The nuclear envelope surrounds the nucleus and is composed of two continuous sheets of membranes, an inner and outer nuclear membrane connected to each other at nuclear pores. The peripheral ER constitutes a network of branching trijunctional tubules that are continuous with membrane sheet regions that occur in closer proximity to the nucleus. Recently it has been suggested that the classical morphological definition of rough ER (ribosome-studded) and smooth ER (ribosome-free) may correspond to sheet-like and tubular ER domains, respectively (27). The ER has a strong potential for cubic architectures, as demonstrated by the fact that the majority of cubic cell membranes in the EM record come from ER-derived structures (14, 17). Furthermore, ER cubic symmetries are an inducible class of organized smooth ER (OSER), a definition collectively referring to ordered smooth ER membranes (=stacked cisternae on the outer nuclear membrane, also called Karmelle (28–30), packed sinusoidal ER (31), concentric membrane whorls (30, 32–34), and arrays of crystalloid ER (35–37)). Specifically, weak homotypic interactions between membrane proteins produce both a whorled and a sinusoidal OSER phenotype (38), the latter exhibiting a cubic symmetry (16, 39).We were able to produce OSER with cubic membrane morphology via induction of homo-dimerization of artificial membrane proteins. Interestingly, the resultant cubic membrane architecture was removed from the ER system by incorporation into large autophagic vacuoles. To assess whether these cubic symmetries were favored in the absence of cellular energy, we depleted ATP. To our surprise, the cells responded by forming large domains of tubulated membrane, suggesting that a cubic symmetry was not the preferred conformation of the system. Our results suggest that whereas the endoplasmic reticulum is capable of adopting cubic symmetries, both the inherent properties of the ER system and active cellular mechanisms, such as autophagy, can tightly control their appearance.     

Specific Activation of mTORC1 by Rheb G-protein in Vitro Involves Enhanced Recruitment of Its Substrate Protein

Rheb G-protein plays critical roles in the TSC/Rheb/mTOR signaling pathway by activating mTORC1. The activation of mTORC1 by Rheb can be faithfully reproduced in vitro by using mTORC1 immunoprecipitated by the use of anti-raptor antibody from mammalian cells starved for nutrients. The low in vitro kinase activity against 4E-BP1 of this mTORC1 preparation is dramatically increased by the addition of recombinant Rheb. On the other hand, the addition of Rheb does not activate mTORC2 immunoprecipitated from mammalian cells by the use of anti-rictor antibody. The activation of mTORC1 is specific to Rheb, because other G-proteins such as KRas, RalA/B, and Cdc42 did not activate mTORC1. Both Rheb1 and Rheb2 activate mTORC1. In addition, the activation is dependent on the presence of bound GTP. We also find that the effector domain of Rheb is required for the mTORC1 activation. FKBP38, a recently proposed mediator of Rheb action, appears not to be involved in the Rheb-dependent activation of mTORC1 in vitro, because the preparation of mTORC1 that is devoid of FKBP38 is still activated by Rheb. The addition of Rheb results in a significant increase of binding of the substrate protein 4E-BP1 to mTORC1. PRAS40, a TOR signaling (TOS) motif-containing protein that competes with the binding of 4EBP1 to mTORC1, inhibits Rheb-induced activation of mTORC1. A preparation of mTORC1 that is devoid of raptor is not activated by Rheb. Rheb does not induce autophosphorylation of mTOR. These results suggest that Rheb induces alteration in the binding of 4E-BP1 with mTORC1 to regulate mTORC1 activation.Rheb defines a unique member of the Ras superfamily G-proteins (1). We have shown that Rheb proteins are conserved and are found from yeast to human (2). Although yeast and fruit fly have one Rheb, mouse and human have two Rheb proteins termed Rheb1 (or simply Rheb) and Rheb2 (RhebL1) (2). Structurally, these proteins contain G1-G5 boxes, short stretches of amino acids that define the function of the Ras superfamily G-proteins including guanine nucleotide binding (1, 3, 4). Rheb proteins have a conserved arginine at residue 15 that corresponds to residue 12 of Ras (1). The effector domain required for the binding with downstream effectors encompasses the G2 box and its adjacent sequences (1, 5). Structural analysis by x-ray crystallography further shows that the effector domain is exposed to solvent, is located close to the phosphates of GTP especially at residues 35–38, and undergoes conformational change during GTP/GDP exchange (6). In addition, all Rheb proteins end with the CAAX (C is cysteine, A is an aliphatic amino acid, and X is the C-terminal amino acid) motif that signals farnesylation. In fact, we as well as others have shown that these proteins are farnesylated (7–9).Rheb plays critical roles in the TSC/Rheb/mTOR signaling, a signaling pathway that plays central roles in regulating protein synthesis and growth in response to nutrient, energy, and growth conditions (10–14). Rheb is down-regulated by a TSC1·TSC2 complex that acts as a GTPase-activating protein for Rheb (15–19). Recent studies established that the GAP domain of TSC2 defines the functional domain for the down-regulation of Rheb (20). Mutations in the Tsc1 or Tsc2 gene lead to tuberous sclerosis whose symptoms include the appearance of benign tumors called hamartomas at different parts of the body as well as neurological symptoms (21, 22). Overexpression of Rheb results in constitutive activation of mTOR even in the absence of nutrients (15, 16). Two mTOR complexes, mTORC1 and mTORC2, have been identified (23, 24). Whereas mTORC1 is involved in protein synthesis activation mediated by S6K and 4EBP1, mTORC2 is involved in the phosphorylation of Akt in response to insulin. It has been suggested that Rheb is involved in the activation of mTORC1 but not mTORC2 (25).Although Rheb is clearly involved in the activation of mTOR, the mechanism of activation has not been established. We as well as others have suggested a model that involves the interaction of Rheb with the TOR complex (26–28). Rheb activation of mTOR kinase activity using immunoprecipitated mTORC1 was reported (29). Rheb has been shown to interact with mTOR (27, 30), and this may involve direct interaction of Rheb with the kinase domain of mTOR (27). However, this Rheb/mTOR interaction is a weak interaction and is not dependent on the presence of GTP bound to Rheb (27, 28). Recently, a different model proposing that FKBP38 (FK506-binding protein 38) mediates the activation of mTORC1 by Rheb was proposed (31, 32). In this model, FKBP38 binds mTOR and negatively regulates mTOR activity, and this negative regulation is blocked by the binding of Rheb to FKBP38. However, recent reports dispute this idea (33).To further characterize Rheb activation of mTOR, we have utilized an in vitro system that reproduces activation of mTORC1 by the addition of recombinant Rheb. We used mTORC1 immunoprecipitated from nutrient-starved cells using anti-raptor antibody and have shown that its kinase activity against 4E-BP1 is dramatically increased by the addition of recombinant Rheb. Importantly, the activation of mTORC1 is specific to Rheb and is dependent on the presence of bound GTP as well as an intact effector domain. FKBP38 is not detected in our preparation and further investigation suggests that FKBP38 is not an essential component for the activation of mTORC1 by Rheb. Our study revealed that Rheb enhances the binding of a substrate 4E-BP1 with mTORC1 rather than increasing the kinase activity of mTOR.     

Mechanism of Sustained Activation of Ribosomal S6 Kinase (RSK) and ERK by Kaposi Sarcoma-associated Herpesvirus ORF45: MULTIPROTEIN COMPLEXES RETAIN ACTIVE PHOSPHORYLATED ERK AND RSK AND PROTECT THEM FROM DEPHOSPHORYLATION*

As obligate intracellular parasites, viruses exploit diverse cellular signaling machineries, including the mitogen-activated protein-kinase pathway, during their infections. We have demonstrated previously that the open reading frame 45 (ORF45) of Kaposi sarcoma-associated herpesvirus interacts with p90 ribosomal S6 kinases (RSKs) and strongly stimulates their kinase activities (Kuang, E., Tang, Q., Maul, G. G., and Zhu, F. (2008) J. Virol. 82 ,1838 -1850). Here, we define the mechanism by which ORF45 activates RSKs. We demonstrated that binding of ORF45 to RSK increases the association of extracellular signal-regulated kinase (ERK) with RSK, such that ORF45, RSK, and ERK formed high molecular mass protein complexes. We further demonstrated that the complexes shielded active pERK and pRSK from dephosphorylation. As a result, the complex-associated RSK and ERK were activated and sustained at high levels. Finally, we provide evidence that this mechanism contributes to the sustained activation of ERK and RSK in Kaposi sarcoma-associated herpesvirus lytic replication.The extracellular signal-regulated kinase (ERK)² mitogen-activated protein kinase (MAPK) signaling pathway has been implicated in diverse cellular physiological processes including proliferation, survival, growth, differentiation, and motility (1-4) and is also exploited by a variety of viruses such as Kaposi sarcoma-associated herpesvirus (KSHV), human cytomegalovirus, human immunodeficiency virus, respiratory syncytial virus, hepatitis B virus, coxsackie, vaccinia, coronavirus, and influenza virus (5-17). The MAPK kinases relay the extracellular signaling through sequential phosphorylation to an array of cytoplasmic and nuclear substrates to elicit specific responses (1, 2, 18). Phosphorylation of MAPK is reversible. The kinetics of deactivation or duration of signaling dictates diverse biological outcomes (19, 20). For example, sustained but not transient activation of ERK signaling induces the differentiation of PC12 cells into sympathetic-like neurons and transformation of NIH3T3 cells (20-22). During viral infection, a unique biphasic ERK activation has been observed for some viruses (an early transient activation triggered by viral binding or entry and a late sustained activation correlated with viral gene expression), but the responsible viral factors and underlying mechanism for the sustained ERK activation remain largely unknown (5, 8, 13, 23).The p90 ribosomal S6 kinases (RSKs) are a family of serine/threonine kinases that lie at the terminus of the ERK pathway (1, 24-26). In mammals, four isoforms are known, RSK1 to RSK4. Each one has two catalytically functional kinase domains, the N-terminal kinase domain (NTKD) and C-terminal kinase domain (CTKD) as well as a linker region between the two. The NTKD is responsible for phosphorylation of exogenous substrates, and the CTKD and linker region regulate RSK activation (1, 24, 25). In quiescent cells ERK binds to the docking site in the C terminus of RSK (27-29). Upon mitogen stimulation, ERK is activated by its upstream MAPK/ERK kinase (MEK). The active ERK phosphorylates Thr-359/Ser-363 of RSK in the linker region (amino acid numbers refer to human RSK1) and Thr-573 in the CTKD activation loop. The activated CTKD then phosphorylates Ser-380 in the linker region, creating a docking site for 3-phosphoinositide-dependent protein kinase-1. The 3-phosphoinositide-dependent protein kinase-1 phosphorylates Ser-221 of RSK in the activation loop and activates the NTKD. The activated NTKD autophosphorylates the serine residue near the ERK docking site, causing a transient dissociation of active ERK from RSK (25, 26, 28). The stimulation of quiescent cells by a mitogen such as epidermal growth factor or a phorbol ester such as 12-O-tetradecanoylphorbol-13-acetate (TPA) usually results in a transient RSK activation that lasts less than 30 min. RSKs have been implicated in regulating cell survival, growth, and proliferation. Mutation or aberrant expression of RSK has been implicated in several human diseases including Coffin-Lowry syndrome and prostate and breast cancers (1, 24, 25, 30-32).KSHV is a human DNA tumor virus etiologically linked to Kaposi sarcoma, primary effusion lymphoma, and a subset of multicentric Castleman disease (33, 34). Infection and reactivation of KSHV activate multiple MAPK pathways (6, 12, 35). Noticeably, the ERK/RSK activation is sustained late during KSHV primary infection and reactivation from latency (5, 6, 12, 23), but the mechanism of the sustained ERK/RSK activation is unclear. Recently, we demonstrated that ORF45, an immediate early and also virion tegument protein of KSHV, interacts with RSK1 and RSK2 and strongly stimulates their kinase activities (23). We also demonstrated that the activation of RSK plays an essential role in KSHV lytic replication (23). In the present study we determined the mechanism of ORF45-induced sustained ERK/RSK activation. We found that ORF45 increases the association of RSK with ERK and protects them from dephosphorylation, causing sustained activation of both ERK and RSK.     

Bortezomib Overcomes Tumor Necrosis Factor-related Apoptosis-inducing Ligand Resistance in Hepatocellular Carcinoma Cells in Part through the Inhibition of the Phosphatidylinositol 3-Kinase/Akt Pathway

Hepatocellular carcinoma (HCC) is one of the most common and aggressive human malignancies. Recombinant tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anti-tumor agent. However, many HCC cells show resistance to TRAIL-induced apoptosis. In this study, we showed that bortezomib, a proteasome inhibitor, overcame TRAIL resistance in HCC cells, including Huh-7, Hep3B, and Sk-Hep1. The combination of bortezomib and TRAIL restored the sensitivity of HCC cells to TRAIL-induced apoptosis. Comparing the molecular change in HCC cells treated with these agents, we found that down-regulation of phospho-Akt (P-Akt) played a key role in mediating TRAIL sensitization of bortezomib. The first evidence was that bortezomib down-regulated P-Akt in a dose- and time-dependent manner in TRAIL-treated HCC cells. Second, {"type":"entrez-nucleotide","attrs":{"text":"LY294002","term_id":"1257998346","term_text":"LY294002"}}LY294002, a PI3K inhibitor, also sensitized resistant HCC cells to TRAIL-induced apoptosis. Third, knocking down Akt1 by small interference RNA also enhanced TRAIL-induced apoptosis in Huh-7 cells. Finally, ectopic expression of mutant Akt (constitutive active) in HCC cells abolished TRAIL sensitization effect of bortezomib. Moreover, okadaic acid, a protein phosphatase 2A (PP2A) inhibitor, reversed down-regulation of P-Akt in bortezomib-treated cells, and PP2A knockdown by small interference RNA also reduced apoptosis induced by the combination of TRAIL and bortezomib, indicating that PP2A may be important in mediating the effect of bortezomib on TRAIL sensitization. Together, bortezomib overcame TRAIL resistance at clinically achievable concentrations in hepatocellular carcinoma cells, and this effect is mediated at least partly via inhibition of the PI3K/Akt pathway.Hepatocellular carcinoma (HCC)² is currently the fifth most common solid tumor worldwide and the fourth leading cause of cancer-related death. To date, surgery is still the only curative treatment but is only feasible in a small portion of patients (1). Drug treatment is the major therapy for patients with advanced stage disease. Unfortunately, the response rate to traditional chemotherapy for HCC patients is unsatisfactory (1). Novel pharmacological therapy is urgently needed for patients with advanced HCC. In this regard, the approval of sorafenib might open a new era of molecularly targeted therapy in the treatment of HCC patients.Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), a type II transmembrane protein and a member of the TNF family, is a promising anti-tumor agent under clinical investigation (2). TRAIL functions by engaging its receptors expressed on the surface of target cells. Five receptors specific for TRAIL have been identified, including DR4/TRAIL-R1, DR5/TRAIL-R2, DcR1, DcR2, and osteoprotegerin. Among TRAIL receptors, only DR4 and DR5 contain an effective death domain that is essential to formation of death-inducing signaling complex (DISC), a critical step for TRAIL-induced apoptosis. Notably, the trimerization of the death domains recruits an adaptor molecule, Fas-associated protein with death domain (FADD), which subsequently recruits and activates caspase-8. In type I cells, activation of caspase-8 is sufficient to activate caspase-3 to induce apoptosis; however, in another type of cells (type II), the intrinsic mitochondrial pathway is essential for apoptosis characterized by cleavage of Bid and release of cytochrome c from mitochondria, which subsequently activates caspase-9 and caspase-3 (3).Although TRAIL induces apoptosis in malignant cells but sparing normal cells, some tumor cells are resistant to TRAIL-induced apoptosis. Mechanisms responsible for the resistance include receptors and intracellular resistance. Although the cell surface expression of DR4 or DR5 is absolutely required for TRAIL-induced apoptosis, tumor cells expressing these death receptors are not always sensitive to TRAIL due to intracellular mechanisms. For example, the cellular FLICE-inhibitory protein (c-FLIP), a homologue to caspase-8 but without protease activity, has been linked to TRAIL resistance in several studies (4, 5). In addition, inactivation of Bax, a proapoptotic Bcl-2 family protein, resulted in resistance to TRAIL in MMR-deficient tumors (6, 7), and reintroduction of Bax into Bax-deficient cells restored TRAIL sensitivity (8), indicating that the Bcl-2 family plays a critical role in intracellular mechanisms for resistance of TRAIL.Bortezomib, a proteasome inhibitor approved clinically for multiple myeloma and mantle cell lymphoma, has been investigated intensively for many types of cancer (9). Accumulating studies indicate that the combination of bortezomib and TRAIL overcomes the resistance to TRAIL in various types of cancer, including acute myeloid leukemia (4), lymphoma (10–13), prostate (14–17), colon (15, 18, 19), bladder (14, 16), renal cell carcinoma (20), thyroid (21), ovary (22), non-small cell lung (23, 24), sarcoma (25), and HCC (26, 27). Molecular targets responsible for the sensitizing effect of bortezomib on TRAIL-induced cell death include DR4 (14, 27), DR5 (14, 20, 22–23, 28), c-FLIP (4, 11, 21–23, 29), NF-κB (12, 24, 30), p21 (16, 21, 25), and p27 (25). In addition, Bcl-2 family also plays a role in the combinational effect of bortezomib and TRAIL, including Bcl-2 (10, 21), Bax (13, 22), Bak (27), Bcl-xL (21), Bik (18), and Bim (15).Recently, we have reported that Akt signaling is a major molecular determinant in bortezomib-induced apoptosis in HCC cells (31). In this study, we demonstrated that bortezomib overcame TRAIL resistance in HCC cells through inhibition of the PI3K/Akt pathway.     

Analysis of a Critical Interaction within the Archaeal Box C/D Small Ribonucleoprotein Complex

In archaea and eukarya, box C/D ribonucleoprotein (RNP) complexes are responsible for 2′-O-methylation of tRNAs and rRNAs. The archaeal box C/D small RNP complex requires a small RNA component (sRNA) possessing Watson-Crick complementarity to the target RNA along with three proteins: L7Ae, Nop5p, and fibrillarin. Transfer of a methyl group from S-adenosylmethionine to the target RNA is performed by fibrillarin, which by itself has no affinity for the sRNA-target duplex. Instead, it is targeted to the site of methylation through association with Nop5p, which in turn binds to the L7Ae-sRNA complex. To understand how Nop5p serves as a bridge between the targeting and catalytic functions of the box C/D small RNP complex, we have employed alanine scanning to evaluate the interaction between the Pyrococcus horikoshii Nop5p domain and an L7Ae box C/D RNA complex. From these data, we were able to construct an isolated RNA-binding domain (Nop-RBD) that folds correctly as demonstrated by x-ray crystallography and binds to the L7Ae box C/D RNA complex with near wild type affinity. These data demonstrate that the Nop-RBD is an autonomously folding and functional module important for protein assembly in a number of complexes centered on the L7Ae-kinkturn RNP.Many biological RNAs require extensive modification to attain full functionality in the cell (1). Currently there are over 100 known RNA modification types ranging from small functional group substitutions to the addition of large multi-cyclic ring structures (2). Transfer RNA, one of many functional RNAs targeted for modification (3-6), possesses the greatest modification type diversity, many of which are important for proper biological function (7). Ribosomal RNA, on the other hand, contains predominantly two types of modified nucleotides: pseudouridine and 2′-O-methylribose (8). The crystal structures of the ribosome suggest that these modifications are important for proper folding (9, 10) and structural stabilization (11) in vivo as evidenced by their strong tendency to localize to regions associated with function (8, 12, 13). These roles have been verified biochemically in a number of cases (14), whereas newly emerging functional modifications are continually being investigated.Box C/D ribonucleoprotein (RNP)³ complexes serve as RNA-guided site-specific 2′-O-methyltransferases in both archaea and eukaryotes (15, 16) where they are referred to as small RNP complexes and small nucleolar RNPs, respectively. Target RNA pairs with the sRNA guide sequence and is methylated at the 2′-hydroxyl group of the nucleotide five bases upstream of either the D or D′ box motif of the sRNA (Fig. 1, star) (17, 18). In archaea, the internal C′ and D′ motifs generally conform to a box C/D consensus sequence (19), and each sRNA contains two guide regions ∼12 nucleotides in length (20). The bipartite architecture of the RNP potentially enables the complex to methylate two distinct RNA targets (21) and has been shown to be essential for site-specific methylation (22).Open in a separate windowFIGURE 1.Organization of the archaeal box C/D complex. The protein components of this RNP are L7Ae, Nop5p, and fibrillarin, which together bind a box C/D sRNA. The regions of the Box C/D sRNA corresponding to the conserved C, D, C′, and D′ boxes are labeled. The target RNA binds the sRNA through Watson-Crick pairing and is methylated by fibrillarin at the fifth nucleotide from the D/D′ boxes (star).In addition to the sRNA, the archaeal box C/D complex requires three proteins for activity (23): the ribosomal protein L7Ae (24, 25), fibrillarin, and the Nop56/Nop58 homolog Nop5p (Fig. 1). L7Ae binds to both box C/D and the C′/D′ motifs (26), which respectively comprise kink-turn (27) or k-loop structures (28), to initiate the assembly of the RNP (29, 30). Fibrillarin performs the methyl group transfer from the cofactor S-adenosylmethionine to the target RNA (31-33). For this to occur, the active site of fibrillarin must be positioned precisely over the specific 2′-hydroxyl group to be methylated. Although fibrillarin methylates this functional group in the context of a Watson-Crick base-paired helix (guide/target), it has little to no binding affinity for double-stranded RNA or for the L7Ae-sRNA complex (22, 26, 33, 34). Nop5p serves as an intermediary protein bringing fibrillarin to the complex through its association with both the L7Ae-sRNA complex and fibrillarin (22). Along with its role as an intermediary between fibrillarin and the L7Ae-sRNA complex, Nop5p possesses other functions not yet fully understood. For example, Nop5p self-dimerizes through a coiled-coil domain (35) that in most archaea and eukaryotic homologs includes a small insertion sequence of unknown function (36, 37). However, dimerization and fibrillarin binding have been shown to be mutually exclusive in Methanocaldococcus jannaschii Nop5p, potentially because of the presence of this insertion sequence (36). Thus, whether Nop5p is a monomer or a dimer in the active RNP is still under debate.In this study, we focus our attention on the Nop5p protein to investigate its interaction with a L7Ae box C/D RNA complex because both the fibrillarin-Nop5p and the L7Ae box C/D RNA interfaces are known from crystal structures (29, 35, 38). Individual residues on the surface of a monomeric form of Nop5p (referred to as mNop5p) (22) were mutated to alanine, and the effect on binding affinity for a L7Ae box C/D motif RNA complex was assessed through the use of electrophoretic mobility shift assays. These data reveal that residues important for binding cluster within the highly conserved NOP domain (39, 40). To demonstrate that this domain is solely responsible for the affinity of Nop5p for the preassembled L7Ae box C/D RNA complex, we expressed and purified it in isolation from the full Nop5p protein. The isolated Nop-RBD domain binds to the L7Ae box C/D RNA complex with nearly wild type affinity, demonstrating that the Nop-RBD is truly an autonomously folding and functional module. Comparison of our data with the crystal structure of the homologous spliceosomal hPrp31-15.5K protein-U4 snRNA complex (41) suggests the adoption of a similar mode of binding, further supporting a crucial role for the NOP domain in RNP complex assembly.     

Intersectin Regulates Dendritic Spine Development and Somatodendritic Endocytosis but Not Synaptic Vesicle Recycling in Hippocampal Neurons

Intersectin-short (intersectin-s) is a multimodule scaffolding protein functioning in constitutive and regulated forms of endocytosis in non-neuronal cells and in synaptic vesicle (SV) recycling at the neuromuscular junction of Drosophila and Caenorhabditis elegans. In vertebrates, alternative splicing generates a second isoform, intersectin-long (intersectin-l), that contains additional modular domains providing a guanine nucleotide exchange factor activity for Cdc42. In mammals, intersectin-s is expressed in multiple tissues and cells, including glia, but excluded from neurons, whereas intersectin-l is a neuron-specific isoform. Thus, intersectin-I may regulate multiple forms of endocytosis in mammalian neurons, including SV endocytosis. We now report, however, that intersectin-l is localized to somatodendritic regions of cultured hippocampal neurons, with some juxtanuclear accumulation, but is excluded from synaptophysin-labeled axon terminals. Consistently, intersectin-l knockdown (KD) does not affect SV recycling. Instead intersectin-l co-localizes with clathrin heavy chain and adaptor protein 2 in the somatodendritic region of neurons, and its KD reduces the rate of transferrin endocytosis. The protein also co-localizes with F-actin at dendritic spines, and intersectin-l KD disrupts spine maturation during development. Our data indicate that intersectin-l is indeed an important regulator of constitutive endocytosis and neuronal development but that it is not a prominent player in the regulated endocytosis of SVs.Clathrin-mediated endocytosis (CME)⁴ is a major mechanism by which cells take up nutrients, control the surface levels of multiple proteins, including ion channels and transporters, and regulate the coupling of signaling receptors to downstream signaling cascades (1-5). In neurons, CME takes on additional specialized roles; it is an important process regulating synaptic vesicle (SV) availability through endocytosis and recycling of SV membranes (6, 7), it shapes synaptic plasticity (8-10), and it is crucial in maintaining synaptic membranes and membrane structure (11).Numerous endocytic accessory proteins participate in CME, interacting with each other and with core components of the endocytic machinery such as clathrin heavy chain (CHC) and adaptor protein-2 (AP-2) through specific modules and peptide motifs (12). One such module is the Eps15 homology domain that binds to proteins bearing NPF motifs (13, 14). Another is the Src homology 3 (SH3) domain, which binds to proline-rich domains in protein partners (15). Intersectin is a multimodule scaffolding protein that interacts with a wide range of proteins, including several involved in CME (16). Intersectin has two N-terminal Eps15 homology domains that are responsible for binding to epsin, SCAMP1, and numb (17-19), a central coil-coiled domain that interacts with Eps15 and SNAP-23 and -25 (17, 20, 21), and five SH3 domains in its C-terminal region that interact with multiple proline-rich domain proteins, including synaptojanin, dynamin, N-WASP, CdGAP, and mSOS (16, 22-25). The rich binding capability of intersectin has linked it to various functions from CME (17, 26, 27) and signaling (22, 28, 29) to mitogenesis (30, 31) and regulation of the actin cytoskeleton (23).Intersectin functions in SV recycling at the neuromuscular junction of Drosophila and C. elegans where it acts as a scaffold, regulating the synaptic levels of endocytic accessory proteins (21, 32-34). In vertebrates, the intersectin gene is subject to alternative splicing, and a longer isoform (intersectin-l) is generated that is expressed exclusively in neurons (26, 28, 35, 36). This isoform has all the binding modules of its short (intersectin-s) counterpart but also has additional domains: a DH and a PH domain that provide guanine nucleotide exchange factor (GEF) activity specific for Cdc42 (23, 37) and a C2 domain at the C terminus. Through its GEF activity and binding to actin regulatory proteins, including N-WASP, intersectin-l has been implicated in actin regulation and the development of dendritic spines (19, 23, 24). In addition, because the rest of the binding modules are shared between intersectin-s and -l, it is generally thought that the two intersectin isoforms have the same endocytic functions. In particular, given the well defined role for the invertebrate orthologs of intersectin-s in SV endocytosis, it is thought that intersectin-l performs this role in mammalian neurons, which lack intersectin-s. Defining the complement of intersectin functional activities in mammalian neurons is particularly relevant given that the protein is involved in the pathophysiology of Down syndrome (DS). Specifically, the intersectin gene is localized on chromosome 21q22.2 and is overexpressed in DS brains (38). Interestingly, alterations in endosomal pathways are a hallmark of DS neurons and neurons from the partial trisomy 16 mouse, Ts65Dn, a model for DS (39, 40). Thus, an endocytic trafficking defect may contribute to the DS disease process.Here, the functional roles of intersectin-l were studied in cultured hippocampal neurons. We find that intersectin-l is localized to the somatodendritic regions of neurons, where it co-localizes with CHC and AP-2 and regulates the uptake of transferrin. Intersectin-l also co-localizes with actin at dendritic spines and disrupting intersectin-l function alters dendritic spine development. In contrast, intersectin-l is absent from presynaptic terminals and has little or no role in SV recycling.     

Major Urinary Protein-1 Increases Energy Expenditure and Improves Glucose Intolerance through Enhancing Mitochondrial Function in Skeletal Muscle of Diabetic Mice

Major urinary protein-1 (MUP-1) is a low molecular weight secreted protein produced predominantly from the liver. Structurally it belongs to the lipocalin family, which carries small hydrophobic ligands such as pheromones. However, the physiological functions of MUP-1 remain poorly understood. Here we provide evidence demonstrating that MUP-1 is an important player in regulating energy expenditure and metabolism in mice. Both microarray and real-time PCR analysis demonstrated that the MUP-1 mRNA abundance in the liver of db/db obese mice was reduced by ∼30-fold compared with their lean littermates, whereas this change was partially reversed by treatment with the insulin-sensitizing drug rosiglitazone. In both dietary and genetic obese mice, the circulating concentrations of MUP-1 were markedly decreased compared with the lean controls. Chronic elevation of circulating MUP-1 in db/db mice, using an osmotic pump-based protein delivery system, increased energy expenditure and locomotor activity, raised core body temperature, and decreased glucose intolerance as well as insulin resistance. At the molecular level, MUP-1-mediated improvement in metabolic profiles was accompanied by increased expression of genes involved in mitochondrial biogenesis, elevated mitochondrial oxidative capacity, decreased triglyceride accumulation, and enhanced insulin-evoked Akt signaling in skeletal muscle but not in liver. Altogether, these findings raise the possibility that MUP-1 deficiency might contribute to the metabolic dysregulation in obese/diabetic mice, and suggest that the beneficial metabolic effects of MUP-1 are attributed in part to its ability in increasing mitochondrial function in skeletal muscle.The liver is the primary organ for carbohydrate and lipid metabolism, including gluconeogenesis, glycogenesis, cholesterol biosynthesis, and lipogenesis (1, 2). These metabolic events in the liver are tightly controlled by several pancreatic hormones including insulin and glucagon. In addition, the liver itself is one of the largest endocrine organs in the body, secreting numerous humoral factors involved in the regulation of systemic glucose and lipid homeostasis. The importance of the liver-derived humoral factors in maintaining glucose metabolism is highlighted by the observation that glucose uptake by skeletal muscle is severely impaired by surgical or pharmacological blockade of hepatic parasympathetic nerves (3). In the past several years, a number of liver-derived humoral metabolic factors, including bone morphogenetic protein-9 (BMP-9) (4), fibroblast growth factor 21 (FGF21) (5–7), retinol-binding protein 4 (RBP4) (8, 9), adropin (10), and angiopoietin-like proteins (Angptl) 3, 4, and 6 (11–13), have been identified, and their roles in glucose and lipid metabolism have been characterized in great detail. Noticeably, BMP-9, FGF21, and Angptl6 exhibit potent insulin-sensitizing and glucose-lowering effects in animal models, and they have been proposed as potential candidates for the treatment of insulin resistance and type II diabetes (4, 6, 7, 13).To search for novel liver-derived secretory factors involved in the regulation of glucose homeostasis, we used microarray analysis as a global screening for systematic identification of genes differentially expressed in the liver of C57BLKS db/db mice (a genetically inherited diabetic mouse model that is characterized by severe insulin resistance and hyperglycemia) and their lean littermates. We found that the mRNA level of mouse major urinary protein-1 (MUP-1)² was markedly down-regulated in db/db mice, and the change was largely normalized upon treatment with the PPARγ agonist rosiglitazone. MUP-1 is a small molecular weight secreted protein abundantly expressed in the liver (14). Its expression in the liver is enhanced by administration of the hepatotoxic agent dimethylnitrosamine (15) but is reduced by interleukin 6-induced acute phase response in mice (16). Like other members of the MUP family, MUP-1 has been proposed to act as a pheromone-binding protein in urine (17), thereby accelerating puberty and promoting aggressive behavior in male mice. However, the precise functions of MUPs have yet to be determined.MUP-1 belongs to the lipocalin superfamily, the members of which share a common tertiary structure with a cup-shaped hydrophobic ligand binding pocket surrounded by an eight-stranded β-barrel (18, 19). This structure confers upon lipocalins the ability to bind and transport a wide variety of small lipophilic substances, including fatty acids, cholesterols, prostaglandins, and pheromones. Noticeably, several members of the lipocalin family, including RBP4, lipocalin-2, and adipocyte fatty acid-binding protein (A-FABP), have recently been shown to be important mediators of obesity-related insulin resistance and glucose intolerance (8, 20–22). Unlike MUP-1, the expression of RBP4, lipocalin-2, and A-FABP are elevated in obesity and diabetes (9, 20, 23).In this study, we investigated the metabolic role of MUP-1 in mice. Our results demonstrated that MUP-1 was abundantly present in the circulation. In genetic and dietary obese mouse models, the serum and urine concentrations of MUP-1 were remarkably decreased. Replenishment of recombinant MUP-1 led to improved glucose tolerance and insulin sensitivity, as well as increased energy expenditure and locomotor activity in db/db diabetic mice. Our data suggest that MUP-1 not only serves as a circulating biomarker, negatively correlated with obesity-related metabolic disorders, but also plays an active role in regulating energy homeostasis and insulin sensitivity in mice.     

Analysis of Intracellular Substrates and Products of Thimet Oligopeptidase in Human Embryonic Kidney 293 Cells

Thimet oligopeptidase (EC 3.4.24.15; EP24.15) is an intracellular enzyme that has been proposed to metabolize peptides within cells, thereby affecting antigen presentation and G protein-coupled receptor signal transduction. However, only a small number of intracellular substrates of EP24.15 have been reported previously. Here we have identified over 100 peptides in human embryonic kidney 293 (HEK293) cells that are derived from intracellular proteins; many but not all of these peptides are substrates or products of EP24.15. First, cellular peptides were extracted from HEK293 cells and incubated in vitro with purified EP24.15. Then the peptides were labeled with isotopic tags and analyzed by mass spectrometry to obtain quantitative data on the extent of cleavage. A related series of experiments tested the effect of overexpression of EP24.15 on the cellular levels of peptides in HEK293 cells. Finally, synthetic peptides that corresponded to 10 of the cellular peptides were incubated with purified EP24.15 in vitro, and the cleavage was monitored by high pressure liquid chromatography and mass spectrometry. Many of the EP24.15 substrates identified by these approaches are 9–11 amino acids in length, supporting the proposal that EP24.15 can function in the degradation of peptides that could be used for antigen presentation. However, EP24.15 also converts some peptides into products that are 8–10 amino acids, thus contributing to the formation of peptides for antigen presentation. In addition, the intracellular peptides described here are potential candidates to regulate protein interactions within cells.Intracellular protein turnover is a crucial step for cell functioning, and if this process is impaired, the elevated levels of aged proteins usually lead to the formation of intracellular insoluble aggregates that can cause severe pathologies (1). In mammalian cells, most proteins destined for degradation are initially tagged with a polyubiquitin chain in an energy-dependent process and then digested to small peptides by the 26 S proteasome, a large proteolytic complex involved in the regulation of cell division, gene expression, and other key processes (2, 3). In eukaryotes, 30–90% of newly synthesized proteins may be degraded by proteasomes within minutes of synthesis (3, 4). In addition to proteasomes, other extralysosomal proteolytic systems have been reported (5, 6). The proteasome cleaves proteins into peptides that are typically 2–20 amino acids in length (7). In most cases, these peptides are thought to be rapidly hydrolyzed into amino acids by aminopeptidases (8–10). However, some intracellular peptides escape complete degradation and are imported into the endoplasmic reticulum where they associate with major histocompatibility complex class I (MHC-I)³ molecules and traffic to the cell surface for presentation to the immune system (10–12). Additionally, based on the fact that free peptides added to the intracellular milieu can regulate cellular functions mediated by protein interactions such as gene regulation, metabolism, cell signaling, and protein targeting (13, 14), intracellular peptides generated by proteasomes that escape degradation have been suggested to play a role in regulating protein interactions (15). Indeed, oligopeptides isolated from rat brain tissue using the catalytically inactive EP24.15 (EC 3.4.24.15) were introduced into Chinese hamster ovarian-S and HEK293 cells and were found capable of altering G protein-coupled receptor signal transduction (16). Moreover, EP24.15 overexpression itself changed both angiotensin II and isoproterenol signal transduction, suggesting a physiological function for its intracellular substrates/products (16).EP24.15 is a zinc-dependent peptidase of the metallopeptidase M3 family that contains the HEXXH motif (17). This enzyme was first described as a neuropeptide-degrading enzyme present in the soluble fraction of brain homogenates (18). Whereas EP24.15 can be secreted (19, 20), its predominant location in the cytosol and nucleus suggests that the primary function of this enzyme is not the extracellular degradation of neuropeptides and hormones (21, 22). EP24.15 was shown in vivo to participate in antigen presentation through MHC-I (23–25) and in vitro to bind (26) or degrade (27) some MHC-I associated peptides. EP24.15 has also been shown in vitro to degrade peptides containing 5–17 amino acids produced after proteasome digestion of β-casein (28). EP24.15 shows substrate size restriction to peptides containing from 5 to 17 amino acids because of its catalytic center that is located in a deep channel (29). Despite the size restriction, EP24.15 has a broad substrate specificity (30), probably because a significant portion of the enzyme-binding site is lined with potentially flexible loops that allow reorganization of the active site following substrate binding (29). Recently, it has also been suggested that certain substrates may be cleaved by an open form of EP24.15 (31). This characteristic is supported by the ability of EP24.15 to accommodate different amino acid residues at subsites S4 to S3′, which even includes the uncommon post-proline cleavage (30). Such biochemical and structural features make EP24.15 a versatile enzyme to degrade structurally unrelated oligopeptides.Previously, brain peptides that bound to catalytically inactive EP24.15 were isolated and identified using mass spectrometry (22). The majority of peptides captured by the inactive enzyme were intracellular protein fragments that efficiently interacted with EP24.15; the smallest peptide isolated in these assays contained 5 and the largest 17 amino acids (15, 16, 22, 32), which is within the size range previously reported for natural and synthetic substrates of EP24.15 (18, 30, 33, 34). Interestingly, the peptides released by the proteasome are in the same size range of EP24.15 competitive inhibitors/substrates (7, 35, 36). Taken altogether, these data suggest that in the intracellular environment EP24.15 could further cleave proteasome-generated peptides unrelated to MHC-I antigen presentation (15).Although the mutated inactive enzyme “capture” assay was successful in identifying several cellular protein fragments that were substrates for EP24.15, it also found some interacting peptides that were not substrates. In this study, we used several approaches to directly screen for cellular peptides that were cleaved by EP24.15. The first approach involved the extraction of cellular peptides from the HEK293 cell line, incubation in vitro with purified EP24.15, labeling with isotopic tags, and analysis by mass spectrometry to obtain quantitative data on the extent of cleavage. The second approach examined the effect of EP24.15 overexpression on the cellular levels of peptides in the HEK293 cell line. The third set of experiments tested synthetic peptides with purified EP24.15 in vitro, and examined cleavage by high pressure liquid chromatography and mass spectrometry. Collectively, these studies have identified a large number of intracellular peptides, including those that likely represent the endogenous substrates and products of EP24.15, and this original information contributes to a better understanding of the function of this enzyme in vivo.