Inhibitory Role of Endophilin 3 in Receptor-mediated Endocytosis

Biology of endophilin and it's role in disease

Endophilin is an evolutionarily conserved family of protein that involves in a range of intracellular membrane dynamics. This family consists of five isoforms, which are distributed in various tissues. Recent studies have shown that Endophilin regulates diseases pathogenesis, including neurodegenerative diseases, tumors, cardiovascular diseases, and autoimmune diseases. In vivo, it regulates different biological functions such as vesicle endocytosis, mitochondrial morphological changes, apoptosis and autophagosome formation. Functional studies confirmed the role of Endophilin in development and progression of these diseases. In this study, we have comprehensively discussed the complex function of Endophilin and how the family contributes to diseases development. It is hoped that this study will provide new ideas for targeting Endophilin in diseases.

Protein Interactors and Trafficking Pathways That Regulate the Cannabinoid Type 1 Receptor (CB1R)

The endocannabinoid system (ECS) acts as a negative feedback mechanism to suppress synaptic transmission and plays a major role in a diverse range of brain functions including, for example, the regulation of mood, energy balance, and learning and memory. The function and dysfunction of the ECS are strongly implicated in multiple psychiatric, neurological, and neurodegenerative diseases. Cannabinoid type 1 receptor (CB1R) is the most abundant G protein-coupled receptor (GPCR) expressed in the brain and, as for any synaptic receptor, CB1R needs to be in the right place at the right time to respond appropriately to changing synaptic circumstances. While CB1R is found intracellularly throughout neurons, its surface expression is highly polarized to the axonal membrane, consistent with its functional expression at presynaptic sites. Surprisingly, despite the importance of CB1R, the interacting proteins and molecular mechanisms that regulate the highly polarized distribution and function of CB1R remain relatively poorly understood. Here we set out what is currently known about the trafficking pathways and protein interactions that underpin the surface expression and axonal polarity of CB1R, and highlight key questions that still need to be addressed.

Distinct Functions of Endophilin Isoforms in Synaptic Vesicle Endocytosis

Endophilin isoforms perform distinct characteristics in their interactions with N-type Ca²⁺ channels and dynamin. However, precise functional differences for the endophilin isoforms on synaptic vesicle (SV) endocytosis remain unknown. By coupling RNA interference and electrophysiological recording techniques in cultured rat hippocampal neurons, we investigated the functional differences of three isoforms of endophilin in SV endocytosis. The results showed that the amplitude of normalized evoked excitatory postsynaptic currents in endophilin1 knockdown neurons decreased significantly for both single train and multiple train stimulations. Similar results were found using endophilin2 knockdown neurons, whereas endophilin3 siRNA exhibited no change compared with control neurons. Endophilin1 and endophilin2 affected SV endocytosis, but the effect of endophilin1 and endophilin2 double knockdown was not different from that of either knockdown alone. This result suggested that endophilin1 and endophilin2 functioned together but not independently during SV endocytosis. Taken together, our results indicate that SV endocytosis is sustained by endophilin1 and endophilin2 isoforms, but not by endophilin3, in primary cultured hippocampal neurons.

Rheb activation disrupts spine synapse formation through accumulation of syntenin in tuberous sclerosis complex

Rheb is a small GTP-binding protein and its GTPase activity is activated by the complex of Tsc1 and Tsc2 whose mutations cause tuberous sclerosis complex (TSC). We previously reported that cultured TSC neurons showed impaired spine synapse morphogenesis in an mTORC1-independent manner. Here we show that the PDZ protein syntenin preferentially binds to the GDP-bound form of Rheb. The levels of syntenin are significantly higher in TSC neurons than in wild-type neurons because the Rheb-GDP-syntenin complex is prone to proteasomal degradation. Accumulated syntenin in TSC neurons disrupts spine synapse formation through inhibition of the association between syndecan-2 and calcium/calmodulin-dependent serine protein kinase. Instead, syntenin enhances excitatory shaft synapse formation on dendrites by interacting with ephrinB3. Downregulation of syntenin in TSC neurons restores both spine and shaft synapse densities. These findings suggest that Rheb-syntenin signalling may be a novel therapeutic target for abnormalities in spine and shaft synapses in TSC neurons.

Activation of Rheb, but not of mTORC1, impairs spine synapse morphogenesis in tuberous sclerosis complex

Mutations in the Tsc1 or Tsc2 genes cause tuberous sclerosis complex (TSC). Tsc1 and Tsc2 proteins form a complex that inhibits mammalian target of rapamycin complex 1 (mTORC1) signalling through Rheb-GTPase. We found that Tsc2^+/− neurons showed impaired spine synapse formation, which was resistant to an mTORC1 inhibitor. Knockdown of mTOR also failed to restore these abnormalities, suggesting mTORC may not participate in impaired spinogenesis in Tsc2^+/− neurons. To address whether Rheb activation impairs spine synapse formation, we expressed active and inactive forms of Rheb in WT and Tsc2^+/− neurons, respectively. Expression of active Rheb abolished dendritic spine formation in WT neurons, whereas inactive Rheb restored spine synapse formation in Tsc2^+/− neurons. Moreover, inactivation of Rheb with farnesyl transferase inhibitors recovered spine synapse morphogenesis in Tsc2^+/− neurons. In conclusion, dendritic spine abnormalities in TSC neurons may be caused through activation of Rheb, but not through of mTORC1.

Identification of two functionally distinct endosomal recycling pathways for dopamine D₂ receptor

Dopamine D₂ receptor (DRD2) is important for normal function of the brain reward circuit. Lower DRD2 function in the brain increases the risk for substance abuse, obesity, attention deficit/hyperactivity disorder, and depression. Moreover, DRD2 is the target of most antipsychotics currently in use. It is well known that dopamine-induced DRD2 endocytosis is important for its desensitization. However, it remains controversial whether DRD2 is recycled back to the plasma membrane or targeted for degradation following dopamine stimulation. Here, we used total internal reflection fluorescent microscopy (TIRFM) to image DRD2 with a superecliptic pHluorin tagged to its N terminus. With these technical advances, we were able to directly visualize vesicular insertion events of DRD2 in cultured mouse striatal medium spiny neurons. We showed that insertion of DRD2 occurs on neuronal somatic and dendritic surfaces. Lateral diffusion of DRD2 was observed following its insertion. Most importantly, using our new approach, we uncovered two functionally distinct recycling pathways for DRD2: a constitutive recycling pathway and a dopamine activity-dependent recycling pathway. We further demonstrated that Rab4 plays an important role in constitutive DRD2 recycling, while Rab11 is required for dopamine activity-dependent DRD2 recycling. Finally, we demonstrated that the two DRD2 recycling pathways play distinct roles in determining DRD2 function: the Rab4-sensitive constitutive DRD2 recycling pathway determines steady-state surface expression levels of DRD2, whereas the Rab11-sensitive dopamine activity-dependent DRD2 recycling pathway is important for functional resensitization of DRD2. Our findings underscore the significance of endosomal recycling in regulation of DRD2 function.

Characterization of the plasticity-related gene, Arc, in the frog brain

In mammals, expression of the immediate early gene Arc/Arg3.1 in the brain is induced by exposure to novel environments, reception of sensory stimuli, and production of learned behaviors, suggesting a potentially important role in neural and behavioral plasticity. To date, Arc has only been characterized in a few species of mammals and birds, which limits our ability to understand its role in modifying behavior. To begin to address this gap, we identified Arc in two frog species, Xenopus tropicalis and Physalaemus pustulosus, and characterized its expression in the brain of P. pustulosus. We found that the predicted protein for frog Arc shared 60% sequence similarity with Arc in other vertebrates, and we observed high Arc expression in the forebrain, but not the midbrain or hindbrain, of female túngara frogs sacrificed at breeding ponds. We also examined the time-course of Arc induction in the medial pallium, the homologue of the mammalian hippocampus, in response to a recording of a P. pustulosus mating chorus and found that accumulation of Arc mRNA peaked 0.75 h following stimulus onset. We found that the mating chorus also induced Arc expression in the lateral and ventral pallia and the medial septum, but not in the striatum, hypothalamus, or auditory midbrain. Finally, we examined acoustically induced Arc expression in response to different types of mating calls and found that Arc expression levels in the pallium and septum did not vary with the biological relevance or acoustic complexity of the signal.

The novel membrane protein TMEM59 modulates complex glycosylation, cell surface expression, and secretion of the amyloid precursor protein

Ectodomain shedding of the amyloid precursor protein (APP) by the two proteases alpha- and beta-secretase is a key regulatory event in the generation of the Alzheimer disease amyloid beta peptide (Abeta). At present, little is known about the cellular mechanisms that control APP shedding and Abeta generation. Here, we identified a novel protein, transmembrane protein 59 (TMEM59), as a new modulator of APP shedding. TMEM59 was found to be a ubiquitously expressed, Golgi-localized protein. TMEM59 transfection inhibited complex N- and O-glycosylation of APP in cultured cells. Additionally, TMEM59 induced APP retention in the Golgi and inhibited Abeta generation as well as APP cleavage by alpha- and beta-secretase cleavage, which occur at the plasma membrane and in the endosomes, respectively. Moreover, TMEM59 inhibited the complex N-glycosylation of the prion protein, suggesting a more general modulation of Golgi glycosylation reactions. Importantly, TMEM59 did not affect the secretion of soluble proteins or the alpha-secretase like shedding of tumor necrosis factor alpha, demonstrating that TMEM59 did not disturb the general Golgi function. The phenotype of TMEM59 transfection on APP glycosylation and shedding was similar to the one observed in cells lacking conserved oligomeric Golgi (COG) proteins COG1 and COG2. Both proteins are required for normal localization and activity of Golgi glycosylation enzymes. In summary, this study shows that TMEM59 expression modulates complex N- and O-glycosylation and suggests that TMEM59 affects APP shedding by reducing access of APP to the cellular compartments, where it is normally cleaved by alpha- and beta-secretase.

The ubiquitin-like protein monoclonal nonspecific suppressor factor beta conjugates to endophilin II and regulates phagocytosis

Monoclonal nonspecific suppressor factor beta (MNSFbeta) is a ubiquitously expressed member of the ubiquitin-like family that has been implicated in various biological functions. Previous studies have demonstrated that MNSFbeta covalently binds to the intracellular proapoptotic protein Bcl-G in cells of the macrophage cell line Raw264.7, suggesting involvement of this ubiquitin-like protein in apoptosis. In this study, we purified a 62 kDa MNSFbeta adduct from murine liver lysates by sequential chromatography on DEAE and anti-MNSFbeta IgG-conjugated Sepharose. MALDI-TOF MS fingerprinting revealed that this MNSFbeta adduct consists of an 8.5 kDa MNSFbeta and endophilin II, a member of the endophilin A family. MNSFbeta may conjugate to endophilin II with a linkage between the C-terminal Gly74 and Lys294. We confirmed this result by immunoprecipitation/western blot studies. Endophilin II was ubiquitously expressed in various tissues, although a truncated form was observed in liver. The 62 kDa MNSFbeta-endophilin II was specifically expressed in liver and macrophages. Small interfering RNA-mediated knockdown of endophilin II and/or MNSFbeta promoted phagocytosis of zymosan in Raw264.7 cells. Conversely, cotransfection of endophilin II and MNSFbeta cDNAs inhibited the phagocytosis of zymosan. Such inhibition was not observed in cells expressing a mutant of endophilin II in which Lys294 was replaced by arginine. These results suggest that the post-translational modification of endophilin II by MNSFbeta might be implicated in phagocytosis by macrophages.

Cellular and molecular characterization of oxidative stress in olfactory epithelium of Harlequin mutant mouse

Oxidative stress in the olfactory system is a major factor associated with age-related olfactory impairment, although the mechanisms by which this occurs are not completely understood. The Harlequin mutant mouse (Hq/Y), which carries an X-linked recessive mutation in the Aifm1 gene, is a model of progressive oxidative stress-induced neurodegeneration in the cerebellum and retina. To determine whether the Hq/Y mutant mouse is a suitable model of oxidative stress-associated olfactory aging, we investigated cellular and molecular changes in the olfactory epithelium (OE) and olfactory bulb (OB) of 6-month-old male Hq/Y mice compared to those in sex-matched littermate controls (+/Y) and in age- and sex-matched C57BL/6 mice. Immunoreactivity for apoptosis-inducing factor, the protein product of Aifm1, was localized in mature olfactory sensory neurons (mOSNs) in +/Y mice but was rarely detected in Hq/Y mice. Hq/Y mice also exhibited increased lipofuscin autofluorescence and increased immunoreactivity for an oxidative DNA/RNA damage marker in mOSNs and in mitral/tufted cells in the OB and an increased number of cleaved caspase-3 immunoreactive apoptotic cells in the OE. Microarray analysis demonstrated that Aifm1 expression was down-regulated by 80% in the OE of Hq/Y mice compared to that in +/Y mice. Most significantly, regulated genes were classified into functional categories of cell signaling/apoptosis/cell cycle, oxidative stress/aging, and cytoskeleton/extracellular matrix/transport-associated. Analysis with EASE software indicated that the functional categories significantly overrepresented in Hq/Y mice included up-regulated mitochondrial genes and down-regulated cytoskeletal organization- and neurogenesis-related genes. Our results strongly support the Hq/Y mutant mouse being a novel model for mechanistic studies of oxidative stress-associated olfactory aging.

Identification of endophilins 1 and 3 as selective binding partners for VGLUT1 and their co-localization in neocortical glutamatergic synapses: implications for vesicular glutamate transporter trafficking and excitatory vesicle formation

1. Selective protein-protein interactions between neurotransmitter transporters and their synaptic targets play important roles in regulating chemical neurotransmission. We screened a yeast two-hybrid library with bait containing the C-terminal amino acids of VGLUT1 and obtained clones that encode endophilin 1 and endophilin 3, proteins considered to play an integral role in glutamatergic vesicle formation. 2. Using a modified yeast plasmid vector to enable more cost-effective screens, we analyzed the selectivity and specificity of this interaction. Endophilins 1 and 3 selectively recognize only VGLUT1 as the C-terminus of VGLUT2 and VGLUT3 do not interact with either endophilin isoform. We mutagenized four conserved stretches of primary sequence in VGLUT1 that includes two polyproline motifs (Pro1, PPAPPP, and Pro2, PPRPPPP), found only in VGLUT1, and two conserved stretches (SEEK, SYGAT), found also in VGLUT2 and VGLUT3. The absence of the VGLUT conserved regions does not affect VGLUT1-endophilin association. Of the two polyproline stretches, only one (Pro2) is required for binding specificity to both endophilin 1 and endophilin 3. 3. We also show that endophilin 1 and endophilin 3 co-localize with VGLUT1 in synaptic terminals of differentiated rat neocortical neurons in primary culture. These results indicate that VGLUT1 and both endophilins are enriched in a class of excitatory synaptic terminals in cortical neurons and there, may interact to play an important role affecting the vesicular sequestration and synaptic release of glutamate.

Expression cloning screen for modifiers of amyloid precursor protein shedding

Ectodomain shedding of the amyloid precursor protein (APP) is a key regulatory step in the generation of the amyloid beta peptide (Abeta), which is thought to provoke the pathogenesis of Alzheimer's disease. To better understand the cellular processes that regulate ectodomain shedding of APP we used human embryonic kidney 293 cells and applied a sib-selection expression cloning approach. In addition to a known activator of APP shedding -- protein kinase A -- the following cDNAs were identified: the endocytic proteins endophilin A1 and A3, the metabotropic glutamate receptor 3 (mGluR3), palmitoyl-protein thioesterase 1 (PPT1), Numb-like and the kinase MEKK2. Endophilins A1 and A3, as well as mGluR3 activated APP shedding relatively specifically. They had little or no effect on the shedding of the unrelated membrane proteins TNF receptor 2 and P-selectin glycoprotein ligand-1. In contrast, MEKK2 and PKA also increased shedding of TNF receptor 2, suggesting that these kinases contribute to a general program regulating ectodomain shedding. The strongest activator of APP shedding, endophilin A3, reduced the rate of APP endocytosis and specifically increased APP shedding by the protease alpha-secretase, as measured in an antibody uptake assay and by immunoblot analysis. This suggests that endophilin A3 is a novel modulator of APP trafficking affecting access of APP to alpha-secretase. In summary, this study shows that expression cloning is a suitable way to identify proteins controlling ectodomain shedding of membrane proteins.

Proteomic identification of differentially expressed proteins in the aging murine olfactory system and transcriptional analysis of the associated genes

Decline in olfactory ability has been associated with aging as well as neurodegenerative disorders. The aim of this study was to gain fundamental insight into molecular events associated with the aging olfactory system. We report a comparative proteomic analysis of the olfactory epithelium (OE) and olfactory bulb (OB) of old (80-week old) and young (6-week old) mice with further analysis of age-related differences in differentially expressed proteins at the mRNA level using real-time RT-PCR. Nine proteins in the OE and 20 in the OB were differentially expressed in old and young mice; of these, aldolase 1, peptidyl prolyl isomerase A, mitochondrial aconitase 2, mitochondrial aldehyde dehydrogenase 2 and albumin 1 were identified in the OE; and ATP synthase isoform 1, enolase 1, ferritin heavy chain, malate dehydrogenase 1, tropomyosin alpha 3 chain and dynamin 1 were identified in the OB. At the transcriptional level, aconitase 2 in the OE and ferritin heavy chain 1 in the OB were differentially expressed with aging, in concordance with the proteomic data. Our results demonstrate an altered proteomic profile of the aged murine olfactory system. The identified proteins fall into three broadly defined functional categories: (i) metabolism, (ii) transport/motility and (iii) stress response. Our transcriptional analysis provides insight into possible mechanisms by which protein expression may be regulated in the OE and OB. The results are discussed in relation to the decrement in olfactory sensitivity with aging.

Direct interaction between metastasis-associated protein 1 and endophilin 3

The yeast two-hybrid system was used to search for partners of mouse metastasis-associated protein 1 (Mta1). Screening of a cDNA library prepared from mouse embryo yielded positive clones coding for endophilin 3. The site of interaction was suggested to be the SH-3-binding domain of Mta1 and SH-3 domain of endophilin 3. This interaction was confirmed by GST pull-down assay in vitro and immunoprecipitation in vivo. The Mta1 and endophilin 3 transcripts were highly expressed in testis and brain. But, Mta1 localized mainly in nucleus and to a lesser extent in cytoplasm while endophilin 3 localized mainly in cytoplasm. If Mta1 functions in cytoplasm, it might be involved in the regulation of endocytosis mediated by endophilin 3.

Src homology 3-domain growth factor receptor-bound 2-like (endophilin) interacting protein 1, a novel neuronal protein that regulates energy balance

To identify genes involved in the central regulation of energy balance, we compared hypothalamic mRNA from lean and obese Psammomys obesus, a polygenic model of obesity, using differential display PCR. One mRNA transcript was observed to be elevated in obese, and obese diabetic, P. obesus compared with lean animals and was subsequently found to be increased 4-fold in the hypothalamus of lethal yellow agouti (A(y)/a) mice, a murine model of obesity and diabetes. Intracerebroventricular infusion of antisense oligonucleotide targeted to this transcript selectively suppressed its hypothalamic mRNA levels and resulted in loss of body weight in both P. obesus and Sprague Dawley rats. Reductions in body weight were mediated by profoundly reduced food intake without a concomitant reduction in metabolic rate. Yeast two-hybrid screening, and confirmation in mammalian cells by bioluminescence resonance energy transfer analysis, demonstrated that the protein it encodes interacts with endophilins, mediators of synaptic vesicle recycling and receptor endocytosis in the brain. We therefore named this transcript Src homology 3-domain growth factor receptor-bound 2-like (endophilin) interacting protein 1 (SGIP1). SGIP1 encodes a large proline-rich protein that is expressed predominantly in the brain and is highly conserved between species. Together these data suggest that SGIP1 is an important and novel member of the group of neuronal molecules required for the regulation of energy homeostasis.

Crystal Structure of the Endophilin-A1 BAR Domain

Endophilin has been implicated in the retrieval of membrane via endocytosis of clathrin-coated vesicles, which is crucial for the maintenance of neurotransmitter exocytosis during stimulation; both exocytosis and endocytosis are regulated by intracellular calcium levels. Here, we present the 2.3 A crystal structure of the endophilin-A1 BAR domain, which has been suggested to function in inducing and sensing membrane curvature at the site of endocytosis. Endo-BAR folds into a crescent-shaped dimer composed of two elongated, three-helix bundles. Two additional domains of 30 residues each, inserted into helix 1 at the center of the concave side of the dimer, may interfere with the proposed mode of BAR domain membrane interaction. In addition, the dimer binds 11 divalent cadmium ions in the crystal mostly with typical Ca2+ co-ordination spheres. The endophilin-1A BAR domain thus constitutes a new variant of a BAR domain, and it may link endophilin-1A BAR function to calcium regulation of endocytosis.

Activation of EGF receptor endocytosis and ERK1/2 signaling by BPGAP1 requires direct interaction with EEN/endophilin II and a functional RhoGAP domain

Rho GTPases are important regulators for cell dynamics. They are activated by guanine nucleotide exchange factors and inactivated by GTPase-activating proteins (GAPs). We recently identified a novel RhoGAP, BPGAP1, that uses the BNIP-2 and Cdc42GAP homology (BCH) domain, RhoGAP domain and proline-rich region to regulate cell morphology and migration. To further explore its roles in intracellular signaling, we employed protein precipitations and matrix-assisted laser desorption/ionization mass-spectrometry and identified EEN/endophilin II as a novel partner of BPGAP1. EEN is a member of the endocytic endophilin family but its function in regulating endocytosis remains unclear. Pull-down and co-immunoprecipitation studies with deletion mutants confirmed that EEN interacted directly with BPGAP1 via its Src homology 3 (SH3) domain binding to the proline-rich region 182-PPPRPPLP-189 of BPGAP1, with prolines 184 and 186 being indispensable for this interaction. Overexpression of EEN or BPGAP1 alone induced EGF-stimulated receptor endocytosis and ERK1/2 phosphorylation. These processes were further enhanced when EEN was present together with the wildtype but not with the non-interactive proline mutant of BPGAP1. However, EEN lacking the SH3 domain served as a dominant negative mutant that completely inhibited these effects. Furthermore, BPGAP1 with a catalytically inactive GAP domain also blocked the effect of EEN and/or BPGAP1 in EGF receptor endocytosis and concomitantly reduced their level of augmentation for ERK1/2 phosphorylation. Our findings reveal a concomitant activation of endocytosis and ERK signaling by BPGAP1 via the coupling of its proline-rich region, which targets EEN and its functional GAP domain. BPGAP1 could therefore provide an important link between cytoskeletal network, endocytic trafficking and Ras/MAPK signaling.