Specificity of the binding of synapsin I to Src homology 3 domains

PLCγ1 in dopamine neurons critically regulates striatal dopamine release via VMAT2 and synapsin III

Dopamine neurons are essential for voluntary movement, reward learning, and motivation, and their dysfunction is closely linked to various psychological and neurodegenerative diseases. Hence, understanding the detailed signaling mechanisms that functionally modulate dopamine neurons is crucial for the development of better therapeutic strategies against dopamine-related disorders. Phospholipase Cγ1 (PLCγ1) is a key enzyme in intracellular signaling that regulates diverse neuronal functions in the brain. It was proposed that PLCγ1 is implicated in the development of dopaminergic neurons, while the physiological function of PLCγ1 remains to be determined. In this study, we investigated the physiological role of PLCγ1, one of the key effector enzymes in intracellular signaling, in regulating dopaminergic function in vivo. We found that cell type-specific deletion of PLCγ1 does not adversely affect the development and cellular morphology of midbrain dopamine neurons but does facilitate dopamine release from dopaminergic axon terminals in the striatum. The enhancement of dopamine release was accompanied by increased colocalization of vesicular monoamine transporter 2 (VMAT2) at dopaminergic axon terminals. Notably, dopamine neuron-specific knockout of PLCγ1 also led to heightened expression and colocalization of synapsin III, which controls the trafficking of synaptic vesicles. Furthermore, the knockdown of VMAT2 and synapsin III in dopamine neurons resulted in a significant attenuation of dopamine release, while this attenuation was less severe in PLCγ1 cKO mice. Our findings suggest that PLCγ1 in dopamine neurons could critically modulate dopamine release at axon terminals by directly or indirectly interacting with synaptic machinery, including VMAT2 and synapsin III.

Compartmentalization of soluble endocytic proteins in synaptic vesicle clusters by phase separation

Synaptic vesicle (SV) clusters, which reportedly result from synapsin's capacity to undergo liquid-liquid phase separation (LLPS), constitute the structural basis for neurotransmission. Although these clusters contain various endocytic accessory proteins, how endocytic proteins accumulate in SV clusters remains unknown. Here, we report that endophilin A1 (EndoA1), the endocytic scaffold protein, undergoes LLPS under physiologically relevant concentrations at presynaptic terminals. On heterologous expression, EndoA1 facilitates the formation of synapsin condensates and accumulates in SV-like vesicle clusters via synapsin. Moreover, EndoA1 condensates recruit endocytic proteins such as dynamin 1, amphiphysin, and intersectin 1, none of which are recruited in vesicle clusters by synapsin. In cultured neurons, like synapsin, EndoA1 is compartmentalized in SV clusters through LLPS, exhibiting activity-dependent dispersion/reassembly cycles. Thus, beyond its essential function in SV endocytosis, EndoA1 serves an additional structural function by undergoing LLPS, thereby accumulating various endocytic proteins in dynamic SV clusters in concert with synapsin.

Liquid-liquid Phase Separation of α-Synuclein: A New Mechanistic Insight for α-Synuclein Aggregation Associated with Parkinson's Disease Pathogenesis

Aberrant aggregation of the misfolded presynaptic protein, α-Synuclein (α-Syn) into Lewy body (LB) and Lewy neuritis (LN) is a major pathological hallmark of Parkinson's disease (PD) and other synucleinopathies. Numerous studies have suggested that prefibrillar and fibrillar species of the misfolded α-Syn aggregates are responsible for cell death in PD pathogenesis. However, the precise molecular events during α-Syn aggregation, especially in the early stages, remain elusive. Emerging evidence has demonstrated that liquid-liquid phase separation (LLPS) of α-Syn occurs in the nucleation step of α-Syn aggregation, which offers an alternate non-canonical aggregation pathway in the crowded microenvironment. The liquid-like α-Syn droplets gradually undergo an irreversible liquid-to-solid phase transition into amyloid-like hydrogel entrapping oligomers and fibrils. This new mechanism of α-Syn LLPS and gel formation might represent the molecular basis of cellular toxicity associated with PD. This review aims to demonstrate the recent development of α-Syn LLPS, the underlying mechanism along with the microscopic events of aberrant phase transition. This review further discusses how several intrinsic and extrinsic factors regulate the thermodynamics and kinetics of α-Syn LLPS and co-LLPS with other proteins, which might explain the pathophysiology of α-Syn in various neurodegenerative diseases.

Ca2+ binding to synapsin I regulates resting Ca2+ and recovery from synaptic depression in nerve terminals

Synapsin I (SynI) is a synaptic vesicle (SV)-associated phosphoprotein that modulates neurotransmission by controlling SV trafficking. The SynI C-domain contains a highly conserved ATP binding site mediating SynI oligomerization and SV clustering and an adjacent main Ca²⁺ binding site, whose physiological role is unexplored. Molecular dynamics simulations revealed that the E373K point mutation irreversibly deletes Ca²⁺ binding to SynI, still allowing ATP binding, but inducing a destabilization of the SynI oligomerization interface. Here, we analyzed the effects of this mutation on neurotransmitter release and short-term plasticity in excitatory and inhibitory synapses from primary hippocampal neurons. Patch-clamp recordings showed an increase in the frequency of miniature excitatory postsynaptic currents (EPSCs) that was totally occluded by exogenous Ca²⁺ chelators and associated with a constitutive increase in resting terminal Ca²⁺ concentrations. Evoked EPSC amplitude was also reduced, due to a decreased readily releasable pool (RRP) size. Moreover, in both excitatory and inhibitory synapses, we observed a marked impaired recovery from synaptic depression, associated with impaired RRP refilling and depletion of the recycling pool of SVs. Our study identifies SynI as a novel Ca²⁺ buffer in excitatory terminals. Blocking Ca²⁺ binding to SynI results in higher constitutive Ca²⁺ levels that increase the probability of spontaneous release and disperse SVs. This causes a decreased size of the RRP and an impaired recovery from depression due to the failure of SV reclustering after sustained high-frequency stimulation. The results indicate a physiological role of Ca²⁺ binding to SynI in the regulation of SV clustering and trafficking in nerve terminals.

Protonation states at different pH, conformational changes and impact of glycosylation in synapsin Ia

Synapsin I (SynI) is the most abundant brain phosphoprotein present at presynaptic terminals that regulates neurotransmitter release, clustering of synaptic vesicles (SVs) at active zones, and stimulates synaptogenesis and neurite outgrowth. Earlier studies have established that SynI displays pH-dependent tethering of SVs to actin filaments and exhibits a maximum binding around neutral pH, however, the effect of pH shift from acidic to basic on the conformational stability of SynI has not been explored yet. Another important aspect of SynIa is its O-GlcNAcylation (O-GlcNac) at the Thr87 position, which is responsible for the positive regulation of synaptic plasticity linked to learning and memory in mice. Furthermore, reduced levels of O-GlcNAc have been observed in Alzheimer's disease, suggesting a possible link to deficits in synaptic plasticity. In this study, the effect of pH and glycosylation on the structure and functional stability of SynIa is determined through molecular dynamics (MD) simulation approach. The 3D structure of SynIa was established via threading-based homology modeling methods. It was observed that the structure of SynIa adopts extended conformational changes as the pH shifts from acidic to basic, resulting in a compact conformation at pH 8.0. Moreover, the results obtained by comparing the glycosylated and unglycosylated protein indicated that the glycan moiety imparts stability to the protein by forming intramolecular hydrogen bond interactions with the protein residues. The results indicate that although O-GlcNAc moieties do not induce a significant change in SynIa structure they minimize protein dynamics, likely leading to enhanced protein stability.

An updated reappraisal of synapsins: structure, function and role in neurological and psychiatric disorders

Synapsins (Syns) are phosphoproteins strongly involved in neuronal development and neurotransmitter release. Three distinct genes SYN1, SYN2 and SYN3, with elevated evolutionary conservation, have been described to encode for Synapsin I, Synapsin II and Synapsin III, respectively. Syns display a series of common features, but also exhibit distinctive localization, expression pattern, post-translational modifications (PTM). These characteristics enable their interaction with other synaptic proteins, membranes and cytoskeletal components, which is essential for the proper execution of their multiple functions in neuronal cells. These include the control of synapse formation and growth, neuron maturation and renewal, as well as synaptic vesicle mobilization, docking, fusion, recycling. Perturbations in the balanced expression of Syns, alterations of their PTM, mutations and polymorphisms of their encoding genes induce severe dysregulations in brain networks functions leading to the onset of psychiatric or neurological disorders. This review presents what we have learned since the discovery of Syn I in 1977, providing the state of the art on Syns structure, function, physiology and involvement in central nervous system disorders.

A liquid phase of synapsin and lipid vesicles

Neurotransmitter-containing synaptic vesicles (SVs) form tight clusters at synapses. These clusters act as a reservoir from which SVs are drawn for exocytosis during sustained activity. Several components associated with SVs that are likely to help form such clusters have been reported, including synapsin. Here we found that synapsin can form a distinct liquid phase in an aqueous environment. Other scaffolding proteins could coassemble into this condensate but were not necessary for its formation. Importantly, the synapsin phase could capture small lipid vesicles. The synapsin phase rapidly disassembled upon phosphorylation by calcium/calmodulin-dependent protein kinase II, mimicking the dispersion of synapsin 1 that occurs at presynaptic sites upon stimulation. Thus, principles of liquid-liquid phase separation may apply to the clustering of SVs at synapses.

Intersectin associates with synapsin and regulates its nanoscale localization and function

Mutations in genes regulating neurotransmission in the brain are implicated in neurological disorders and neurodegeneration. Synapsin is a crucial regulator of neurotransmission and allows synapses to maintain a large reserve pool of synaptic vesicles. Human mutations in synapsin genes are linked to epilepsy and autism. How synapsin function is regulated to allow replenishment of synaptic vesicles and sustain neurotransmission is largely unknown. Here we identify a function for the endocytic scaffold protein intersectin, a protein overexpressed in patients with Down syndrome, as a regulator of synapsin nanoscale distribution and function that is controlled by a phosphorylation-dependent autoinhibitory switch. Our results unravel a hitherto unknown molecular connection between the machineries for synaptic vesicle reserve pool organization and endocytosis.

Neurotransmission is mediated by the exocytic release of neurotransmitters from readily releasable synaptic vesicles (SVs) at the active zone. To sustain neurotransmission during periods of elevated activity, release-ready vesicles need to be replenished from the reserve pool of SVs. The SV-associated synapsins are crucial for maintaining this reserve pool and regulate the mobilization of reserve pool SVs. How replenishment of release-ready SVs from the reserve pool is regulated and which other factors cooperate with synapsins in this process is unknown. Here we identify the endocytic multidomain scaffold protein intersectin as an important regulator of SV replenishment at hippocampal synapses. We found that intersectin directly associates with synapsin I through its Src-homology 3 A domain, and this association is regulated by an intramolecular switch within intersectin 1. Deletion of intersectin 1/2 in mice alters the presynaptic nanoscale distribution of synapsin I and causes defects in sustained neurotransmission due to defective SV replenishment. These phenotypes were rescued by wild-type intersectin 1 but not by a locked mutant of intersectin 1. Our data reveal intersectin as an autoinhibited scaffold that serves as a molecular linker between the synapsin-dependent reserve pool and the presynaptic endocytosis machinery.

Synaptic Vesicle Clusters at Synapses: A Distinct Liquid Phase?

Phase separation in the cytoplasm is emerging as a major principle in intracellular organization. In this process, sets of macromolecules assemble themselves into liquid compartments that are distinct from the surrounding medium but are not delimited by membrane boundaries. Here, we discuss how phase separation, in which a component of one of the two phases is vesicles rather than macromolecules, could underlie the formation of synaptic vesicle (SV) clusters in proximity to presynaptic sites. The organization of SVs into a liquid phase could explain how SVs remain tightly clustered without being stably bound to a scaffold so that they can be efficiently recruited to release site by active zone components.

Huntingtin-associated protein-1 is a synapsin I-binding protein regulating synaptic vesicle exocytosis and synapsin I trafficking

Huntingtin-associated protein-1 (HAP1) is involved in intracellular trafficking, vesicle transport, and membrane receptor endocytosis. However, despite such diverse functions, the role of HAP1 in the synaptic vesicle (SV) cycle in nerve terminals remains unclear. Here, we report that HAP1 functions in SV exocytosis, controls total SV turnover and the speed of vesicle fusion in nerve terminals and regulates glutamate release in cortical brain slices. We found that HAP1 interacts with synapsin I, an abundant neuronal phosphoprotein that associates with SVs during neurotransmitter release and regulates synaptic plasticity and neuronal development. The interaction between HAP1 with synapsin I was confirmed by reciprocal co-immunoprecipitation of the endogenous proteins. Furthermore, HAP1 co-localizes with synapsin I in cortical neurons as discrete puncta. Interestingly, we find that synapsin I localization is specifically altered in Hap1(-/-) cortical neurons without an effect on the localization of other SV proteins. This effect on synapsin I localization was not because of changes in the levels of synapsin I or its phosphorylation status in Hap1(-/-) brains. Furthermore, fluorescence recovery after photobleaching in transfected neurons expressing enhanced green fluorescent protein-synapsin Ia demonstrates that loss of HAP1 protein inhibits synapsin I transport. Thus, we demonstrate that HAP1 regulates SV exocytosis and may do so through binding to synapsin I. The Proposed mechanism of synapsin I transport mediated by HAP1 in neurons. HAP1 interacts with synapsin I, regulating the trafficking of synapsin I containing vesicles and/or transport packets, possibly through its engagement of microtubule motors. The absence of HAP1 reduces synapsin I transport and neuronal exocytosis. These findings provide insights into the processes of neuronal trafficking and synaptic signaling.

A Novel Topology of Proline-rich Transmembrane Protein 2 (PRRT2): HINTS FOR AN INTRACELLULAR FUNCTION AT THE SYNAPSE

Proline-rich transmembrane protein 2 (PRRT2) has been identified as the single causative gene for a group of paroxysmal syndromes of infancy, including epilepsy, paroxysmal movement disorders, and migraine. On the basis of topology predictions, PRRT2 has been assigned to the recently characterized family of Dispanins, whose members share the two-transmembrane domain topology with a large N terminus and short C terminus oriented toward the outside of the cell. Because PRRT2 plays a role at the synapse, it is important to confirm the exact orientation of its N and C termini with respect to the plasma membrane to get clues regarding its possible function. Using a combination of different experimental approaches, including live immunolabeling, immunogold electron microscopy, surface biotinylation and computational modeling, we demonstrate a novel topology for this protein. PRRT2 is a type II transmembrane protein in which only the second hydrophobic segment spans the plasma membrane, whereas the first one is associated with the internal surface of the membrane and forms a helix-loop-helix structure without crossing it. Most importantly, the large proline-rich N-terminal domain is not exposed to the extracellular space but is localized intracellularly, and only the short C terminus is extracellular (N_cyt/C_exo topology). Accordingly, we show that PRRT2 interacts with the Src homology 3 domain-bearing protein Intersectin 1, an intracellular protein involved in synaptic vesicle cycling. These findings will contribute to the clarification of the role of PRRT2 at the synapse and the understanding of pathogenic mechanisms on the basis of PRRT2-related neurological disorders.

An Endocytic Scaffolding Protein together with Synapsin Regulates Synaptic Vesicle Clustering in the Drosophila Neuromuscular Junction

Many endocytic proteins accumulate in the reserve pool of synaptic vesicles (SVs) in synapses and relocalize to the endocytic periactive zone during neurotransmitter release. Currently little is known about their functions outside the periactive zone. Here we show that in the Drosophila neuromuscular junction (NMJ), the endocytic scaffolding protein Dap160 colocalizes during the SV cycle and forms a functional complex with the SV-associated phosphoprotein synapsin, previously implicated in SV clustering. This direct interaction is strongly enhanced under phosphorylation-promoting conditions and is essential for proper localization of synapsin at NMJs. In a dap160 rescue mutant lacking the interaction between Dap160 and synapsin, perturbed reclustering of SVs during synaptic activity is observed. Our data indicate that in addition to the function in endocytosis, Dap160 is a component of a network of protein-protein interactions that serves for clustering of SVs in conjunction with synapsin. During the SV cycle, Dap160 interacts with synapsin dispersed from SVs and helps direct synapsin back to vesicles. The proteins function in synergy to achieve efficient clustering of SVs in the reserve pool.

SIGNIFICANCE STATEMENT

We provide the first evidence for the function of the SH3 domain interaction in synaptic vesicle (SV) organization at the synaptic active zone. Using Drosophila neuromuscular junction as a model synapse, we describe the molecular mechanism that enables the protein implicated in SV clustering, synapsin, to return to the pool of vesicles during neurotransmitter release. We also identify the endocytic scaffolding complex that includes Dap160 as a regulator of the events linking exocytosis and endocytosis in synapses.

Phosphorylation of synapsin I by cyclin-dependent kinase-5 sets the ratio between the resting and recycling pools of synaptic vesicles at hippocampal synapses

Cyclin-dependent kinase-5 (Cdk5) was reported to downscale neurotransmission by sequestering synaptic vesicles (SVs) in the release-reluctant resting pool, but the molecular targets mediating this activity remain unknown. Synapsin I (SynI), a major SV phosphoprotein involved in the regulation of SV trafficking and neurotransmitter release, is one of the presynaptic substrates of Cdk5, which phosphorylates it in its C-terminal region at Ser(549) (site 6) and Ser(551) (site 7). Here we demonstrate that Cdk5 phosphorylation of SynI fine tunes the recruitment of SVs to the active recycling pool and contributes to the Cdk5-mediated homeostatic responses. Phosphorylation of SynI by Cdk5 is physiologically regulated and enhances its binding to F-actin. The effects of Cdk5 inhibition on the size and depletion kinetics of the recycling pool, as well as on SV distribution within the nerve terminal, are virtually abolished in mouse SynI knock-out (KO) neurons or in KO neurons expressing the dephosphomimetic SynI mutants at sites 6,7 or site 7 only. The observation that the single site-7 mutant phenocopies the effects of the deletion of SynI identifies this site as the central switch in mediating the synaptic effects of Cdk5 and demonstrates that SynI is necessary and sufficient for achieving the effects of the kinase on SV trafficking. The phosphorylation state of SynI by Cdk5 at site 7 is regulated during chronic modification of neuronal activity and is an essential downstream effector for the Cdk5-mediated homeostatic scaling.

Involvement of synaptic genes in the pathogenesis of autism spectrum disorders: the case of synapsins

Autism spectrum disorders (ASDs) are heterogeneous neurodevelopmental disorders characterized by deficits in social interaction and social communication, restricted interests, and repetitive behaviors. Many synaptic protein genes are linked to the pathogenesis of ASDs, making them prototypical synaptopathies. An array of mutations in the synapsin (Syn) genes in humans has been recently associated with ASD and epilepsy, diseases that display a frequent comorbidity. Syns are pre-synaptic proteins regulating synaptic vesicle traffic, neurotransmitter release, and short-term synaptic plasticity. In doing so, Syn isoforms control the tone of activity of neural circuits and the balance between excitation and inhibition. As ASD pathogenesis is believed to result from dysfunctions in the balance between excitatory and inhibitory transmissions in neocortical areas, Syns are novel ASD candidate genes. Accordingly, deletion of single Syn genes in mice, in addition to epilepsy, causes core symptoms of ASD by affecting social behavior, social communication, and repetitive behaviors. Thus, Syn knockout mice represent a good experimental model to define synaptic alterations involved in the pathogenesis of ASD and epilepsy.

Phosphoproteomic differences in major depressive disorder postmortem brains indicate effects on synaptic function

There is still a lack in the molecular comprehension of major depressive disorder (MDD) although this condition affects approximately 10% of the world population. Protein phosphorylation is a posttranslational modification that regulates approximately one-third of the human proteins involved in a range of cellular and biological processes such as cellular signaling. Whereas phosphoproteome studies have been carried out extensively in cancer research, few such investigations have been carried out in studies of psychiatric disorders. Here, we present a comparative phosphoproteome analysis of postmortem dorsolateral prefrontal cortex tissues from 24 MDD patients and 12 control donors. Tissue extracts were analyzed using liquid chromatography mass spectrometry in a data-independent manner (LC-MS(E)). Our analyses resulted in the identification of 5,195 phosphopeptides, corresponding to 802 non-redundant proteins. Ninety of these proteins showed differential levels of phosphorylation in tissues from MDD subjects compared to controls, being 20 differentially phosphorylated in at least 2 peptides. The majority of these phosphorylated proteins were associated with synaptic transmission and cellular architecture not only pointing out potential biomarker candidates but mainly shedding light to the comprehension of MDD pathobiology.

Inhibition of exocytosis or endocytosis blocks activity-dependent redistribution of synapsin

The synaptic vesicle cycle encompasses the pre-synaptic events that drive neurotransmission. Influx of calcium leads to the fusion of synaptic vesicles with the plasma membrane and the release of neurotransmitter, closely followed by endocytosis. Vacated release sites are repopulated with vesicles which are then primed for release. When activity is intense, reserve vesicles may be mobilized to counteract an eventual decline in transmission. Recently, interplay between endocytosis and repopulation of the readily releasable pool of vesicles has been identified. In this study, we show that exo-endocytosis is necessary to enable detachment of synapsin from reserve pool vesicles during synaptic activity. We report that blockage of exocytosis in cultured mouse hippocampal neurons, either by tetanus toxin or by the deletion of munc13, inhibits the activity-dependent redistribution of synapsin from the pre-synaptic terminal into the axon. Likewise, perturbation of endocytosis with dynasore or by a dynamin dominant-negative mutant fully prevents synapsin redistribution. Such inhibition of synapsin redistribution occurred despite the efficient phosphorylation of synapsin at its protein kinase A/CaMKI site, indicating that disengagement of synapsin from the vesicles requires exocytosis and endocytosis in addition to phosphorylation. Our results therefore reveal hitherto unidentified feedback within the synaptic vesicle cycle involving the synapsin-managed reserve pool.

The synapsins: multitask modulators of neuronal development

Neurons are examples of specialized cells that evolved the extraordinary ability to transmit electrochemical information in complex networks of interconnected cells. During their development, neurons undergo precisely regulated processes that define their lineage, positioning, morphogenesis and pattern of activity. The events leading to the establishment of functional neuronal networks follow a number of key steps, including asymmetric cell division from neuronal precursors, migration, establishment of polarity, neurite outgrowth and synaptogenesis. Synapsins are a family of abundant neuronal phosphoproteins that have been extensively studied for their role in the regulation of neurotransmission in presynaptic terminals. Beside their implication in the homeostasis of adult cells, synapsins influence the development of young neurons, interacting with cytoskeletal and vesicular components and regulating their dynamics. Although the exact molecular mechanisms determining synapsin function in neuronal development are still largely unknown, in this review we summarize the most important literature on the subject, providing a conceptual framework for the progress of present and future research.

SYN1 loss-of-function mutations in autism and partial epilepsy cause impaired synaptic function

Several genes predisposing to autism spectrum disorders (ASDs) with or without epilepsy have been identified, many of which are implicated in synaptic function. Here we report a Q555X mutation in synapsin 1 (SYN1), an X-linked gene encoding for a neuron-specific phosphoprotein implicated in the regulation of neurotransmitter release and synaptogenesis. This nonsense mutation was found in all affected individuals from a large French-Canadian family segregating epilepsy and ASDs. Additional mutations in SYN1 (A51G, A550T and T567A) were found in 1.0 and 3.5% of French-Canadian individuals with autism and epilepsy, respectively. The majority of these SYN1 mutations were clustered in the proline-rich D-domain which is substrate of multiple protein kinases. When expressed in synapsin I (SynI) knockout (KO) neurons, all the D-domain mutants failed in rescuing the impairment in the size and trafficking of synaptic vesicle pools, whereas the wild-type human SynI fully reverted the KO phenotype. Moreover, the nonsense Q555X mutation had a dramatic impact on phosphorylation by MAPK/Erk and neurite outgrowth, whereas the missense A550T and T567A mutants displayed impaired targeting to nerve terminals. These results demonstrate that SYN1 is a novel predisposing gene to ASDs, in addition to epilepsy, and strengthen the hypothesis that a disturbance of synaptic homeostasis underlies the pathogenesis of both diseases.

The role of synapsins in neuronal development

The synapsins, the first identified synaptic vesicle-specific proteins, are phosphorylated on multiple sites by a number of protein kinases and are involved in neurite outgrowth and synapse formation as well as in synaptic transmission. In mammals, the synapsin family consists of at least 10 isoforms encoded by 3 distinct genes and composed by a mosaic of conserved and variable domains. The synapsins are highly conserved evolutionarily, and orthologues have been found in invertebrates and lower vertebrates. Within nerve terminals, synapsins are implicated in multiple interactions with presynaptic proteins and the actin cytoskeleton. Via these interactions, synapsins control several mechanisms important for neuronal homeostasis. In this review, we describe the main functional features of the synapsins, in relation to the complex role played by these phosphoproteins in neuronal development.

The synapsin cycle: a view from the synaptic endocytic zone

Although the synapsin phosphoproteins were discovered more than 30 years ago and are known to play important roles in neurotransmitter release and synaptogenesis, a complete picture of their functions within the nerve terminal is lacking. It has been shown that these proteins play an important role in the clustering of synaptic vesicles (SVs) at active zones and function as modulators of synaptic strength by acting at both pre- and postdocking levels. Recent studies have demonstrated that synapsins migrate to the endocytic zone of central synapses during neurotransmitter release, which suggests that there are additional functions for these proteins in SV recycling.

Synapsin phosphorylation by SRC tyrosine kinase enhances SRC activity in synaptic vesicles

Synapsins are synaptic vesicle-associated phosphoproteins implicated in the regulation of neurotransmitter release. Synapsin I is the major binding protein for the SH3 domain of the kinase c-Src in synaptic vesicles. Its binding leads to stimulation of synaptic vesicle-associated c-Src activity. We investigated the mechanism and role of Src activation by synapsins on synaptic vesicles. We found that synapsin is tyrosine phosphorylated by c-Src in vitro and on intact synaptic vesicles independently of its phosphorylation state on serine. Mass spectrometry revealed a single major phosphorylation site at Tyr(301), which is highly conserved in all synapsin isoforms and orthologues. Synapsin tyrosine phosphorylation triggered its binding to the SH2 domains of Src or Fyn. However, synapsin selectively activated and was phosphorylated by Src, consistent with the specific enrichment of c-Src in synaptic vesicles over Fyn or n-Src. The activity of Src on synaptic vesicles was controlled by the amount of vesicle-associated synapsin, which is in turn dependent on synapsin serine phosphorylation. Synaptic vesicles depleted of synapsin in vitro or derived from synapsin null mice exhibited greatly reduced Src activity and tyrosine phosphorylation of other synaptic vesicle proteins. Disruption of the Src-synapsin interaction by internalization of either the Src SH3 or SH2 domains into synaptosomes decreased synapsin tyrosine phosphorylation and concomitantly increased neurotransmitter release in response to Ca(2+)-ionophores. We conclude that synapsin is an endogenous substrate and activator of synaptic vesicle-associated c-Src and that regulation of Src activity on synaptic vesicles participates in the regulation of neurotransmitter release by synapsin.

Dysregulation of growth factor receptor-bound protein 2 and fascin in hippocampus of mice polytransgenic for chromosome 21 structures

Nonchimeric polytransgenic 152F7 mice encompassing four human chromosome 21 genes (DSCR3, DSCR5, TTC3, and DYRK1A) within the Down syndrome critical region present with learning and memory impairment. However, no abnormalities were shown by in vitro electrophysiological or neuroanatomical findings in hippocampus of 152F7 mice. To search for molecular changes that may be linked to cognitive impairment, we compared hippocampal protein levels between nontransgenic (WT) and 152F7 mice by a proteomic approach. Protein extracts were run on two-dimensional gel electrophoresis, protein spots were analyzed by mass spectrometry (MALDI-TOF-TOF) followed by quantification by specific software. Three hundred and nineteen different gene products were identified, and 48 proteins were assigned as signaling-related proteins. Stringent statistical analysis considering P < 0.005 as statistically significant based upon multiple testing revealed that growth factor receptor-bound protein 2 (Grb2) levels were decreased and an expression form of fascin 1 was increased in 152F7 mice when compared with WT. A series of proteins showed trends for increased and decreased hippocampal levels (P > 0.005 and P < 0.05). Only 2 out of 319 different gene products were dysregulated, pointing to the specificity of the analysis. Decreased Grb2 levels in the hippocampus of 152F7 mice may contribute to impaired cytoskeleton functions because dynamin 1 binds to Grb2 and involved in the formation of the endocytic process. Fascin dysregulation is of relevance for actin bundling in vesicle trafficking and may represent or lead to impaired neurotransmission that, in turn, may lead to the cognitive defect observed in this mouse model of Down syndrome.

Protein sorting in the synaptic vesicle life cycle

At early stages of differentiation neurons already contain many of the components necessary for synaptic transmission. However, in order to establish fully functional synapses, both the pre- and postsynaptic partners must undergo a process of maturation. At the presynaptic level, synaptic vesicles (SVs) must acquire the highly specialized complement of proteins, which make them competent for efficient neurotransmitter release. Although several of these proteins have been characterized and linked to precise functions in the regulation of the SV life cycle, a systematic and unifying view of the mechanisms underlying selective protein sorting during SV biogenesis remains elusive. Since SV components do not share common sorting motifs, their targeting to SVs likely relies on a complex network of protein-protein and protein-lipid interactions, as well as on post-translational modifications. Pleiomorphic carriers containing SV proteins travel and recycle along the axon in developing neurons. Nevertheless, SV components appear to eventually undertake separate trafficking routes including recycling through the neuronal endomembrane system and the plasmalemma. Importantly, SV biogenesis does not appear to be limited to a precise stage during neuronal differentiation, but it rather continues throughout the entire neuronal lifespan and within synapses. At nerve terminals, remodeling of the SV membrane results from the use of alternative exocytotic pathways and possible passage through as yet poorly characterized vacuolar/endosomal compartments. As a result of both processes, SVs with heterogeneous molecular make-up, and hence displaying variable competence for exocytosis, may be generated and coexist within the same nerve terminal.

FAK and PYK2 interact with SAP90/PSD-95-Associated Protein-3

Focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (PYK2) are two related non-receptor tyrosine kinases highly expressed in brain. Although they are both involved in synaptic plasticity, little is known about their specific neuronal partners. Using a yeast two-hybrid screen and GST pull-down assays we show that SAPAP3 (SAP90/PSD-95-Associated Protein-3) interacts with FAK (residues 676-840) and PYK2. The three proteins partly co-distribute in the same sucrose gradient fractions as the post-synaptic density protein PSD-95 and Src. Our results suggest that SAPAP3 is an anchoring protein for FAK and PYK2 in post-synaptic densities and may contribute to the synaptic function of these tyrosine kinases.

Amphiphysin is a component of clathrin coats formed during synaptic vesicle recycling at the lamprey giant synapse

Amphiphysin is a protein enriched at mammalian synapses thought to function as a clathrin accessory factor in synaptic vesicle endocytosis. Here we examine the involvement of amphiphysin in synaptic vesicle recycling at the giant synapse in the lamprey. We show that amphiphysin resides in the synaptic vesicle cluster at rest and relocates to sites of endocytosis during synaptic activity. It accumulates at coated pits where its SH3 domain, but not its central clathrin/AP-2-binding (CLAP) region, is accessible for antibody binding. Microinjection of antibodies specifically directed against the CLAP region inhibited recycling of synaptic vesicles and caused accumulation of clathrin-coated intermediates with distorted morphology, including flat patches of coated presynaptic membrane. Our data provide evidence for an activity-dependent redistribution of amphiphysin in intact nerve terminals and show that amphiphysin is a component of presynaptic clathrin-coated intermediates formed during synaptic vesicle recycling.

Synapsin Is a Novel Rab3 Effector Protein on Small Synaptic Vesicles

Synapsins, a family of neuron-specific phosphoproteins, have been demonstrated to regulate the availability of synaptic vesicles for exocytosis by binding to both synaptic vesicles and the actin cytoskeleton in a phosphorylation-dependent manner. Although the above-mentioned observations strongly support a pre-docking role of the synapsins in the assembly and maintenance of a reserve pool of synaptic vesicles, recent results suggest that the synapsins may also be involved in some later step of exocytosis. In order to investigate additional interactions of the synapsins with nerve terminal proteins, we have employed phage display library technology to select peptide sequences binding with high affinity to synapsin I. Antibodies raised against the peptide YQYIETSMQ (syn21) specifically recognized Rab3A, a synaptic vesicle-specific small G protein implicated in multiple steps of exocytosis. The interaction between synapsin I and Rab3A was confirmed by photoaffinity labeling experiments on purified synaptic vesicles and by the formation of a chemically cross-linked complex between synapsin I and Rab3A in intact nerve terminals. Synapsin I could be effectively co-precipitated from synaptosomal extracts by immobilized recombinant Rab3A in a GTP-dependent fashion. In vitro binding assays using purified proteins confirmed the binding preference of synapsin I for Rab3A-GTP and revealed that the COOH-terminal regions of synapsin I and the Rab3A effector domain are required for the interaction with Rab3A to occur. The data indicate that synapsin I is a novel Rab3 interactor on synaptic vesicles and suggest that the synapsin-Rab3 interaction may participate in the regulation of synaptic vesicle trafficking within the nerve terminals.

Molecular determinants of synapsin targeting to presynaptic terminals

Although synapsins are abundant synaptic vesicle proteins that are widely used as markers of presynaptic terminals, the mechanisms that target synapsins to presynaptic terminals have not been elucidated. We have addressed this question by imaging the targeting of green fluorescent protein-tagged synapsins in cultured hippocampal neurons. Whereas all synapsin isoforms targeted robustly to presynaptic terminals in wild-type neurons, synapsin Ib scarcely targeted in neurons in which all synapsins were knocked-out. Coexpression of other synapsin isoforms significantly strengthened the targeting of synapsin Ib in knock-out neurons, indicating that heterodimerization is required for synapsin Ib to target. Truncation mutagenesis revealed that synapsin Ia targets via distributed binding sites that include domains B, C, and E. Although domain A was not necessary for targeting, its presence enhanced targeting. Domain D inhibited targeting, but this inhibition was overcome by domain E. Thus, multiple intermolecular and intramolecular interactions are required for synapsins to target to presynaptic terminals.

Protein-lipid interactions and phosphoinositide metabolism in membrane traffic: insights from vesicle recycling in nerve terminals

Great progress has been made in the elucidation of the function of proteins in membrane traffic. Less is known about the regulatory role of lipids in membrane dynamics. Studies of nerve terminals, compartments highly specialized for the recycling of synaptic vesicles, have converged with studies from other systems to reveal mechanisms in protein-lipid interactions that affect membrane shape as well as the fusion and fission of vesicles. Phosphoinositides have emerged as major regulators of the binding of cytosolic proteins to the bilayer. Phosphorylation on different positions of the inositol ring generates different isomers that are heterogeneously distributed on cell membranes and that together with membrane proteins generate a "dual keys" code for the recruitment of cytosolic proteins. This code helps controlling vectoriality of membrane transport. Powerful methods for the detection of lipids are rapidly advancing this field, thus complementing the broad range of information about biological systems that can be obtained from genomic and proteomic approaches.

Cytoskeletal interactions of synapsin I in non-neuronal cells

Synapsin I is a neuronal phosphoprotein involved in the localization and stabilization of synaptic vesicles. Recently, synapsin I has been detected in several non-neuronal cell lines, but its function in these cells is unclear. To determine the localization of synapsin I in non-neuronal cells, it was transiently expressed in HeLa and NIH/3T3 cells as an enhanced green fluorescent protein fusion protein. Synapsin I-enhanced green fluorescent protein colocalized with F-actin in both cell lines, particularly with microspikes and membrane ruffles. It did not colocalize with microtubules or vimentin and it did not cause major alterations in cytoskeletal organization. Synapsin Ia-enhanced green fluorescent protein colocalized with microtubule bundles in taxol-treated HeLa cells and with F-actin spots at the plasma membrane in cells treated with cytochalasin B. It did not noticeably affect F-actin reassembly following drug removal. Synapsin Ia-enhanced green fluorescent protein remained colocalized with F-actin in cells treated with nocodazole, and it did not affect reassembly of microtubules following drug removal. These results demonstrate that synapsin I interacts with F-actin in non-neuronal cells and suggest that synapsin I may have a role in regions where actin is highly dynamic.

Role of Amphiphysin II in Somatostatin Receptor Trafficking in Neuroendocrine Cells

Amphiphysins are SH3 domain-containing proteins thought to function in clathrin-mediated endocytosis. To investigate the potential role of amphiphysin II in cellular trafficking of G protein-coupled somatostatin (SRIF) receptors, we generated an AtT-20 cell line stably overexpressing amphiphysin IIb, a splice variant that does not bind clathrin. Endocytosis of (125)I-[d-Trp(8)]SRIF was not affected by amphiphysin IIb overexpression. However, the maximal binding capacity (B(max)) of the ligand on intact cells was significantly lower in amphiphysin IIb overexpressing than in non-transfected cells. This difference was no longer apparent when the experiments were performed on crude cell homogenates, suggesting that amphiphysin IIb overexpression interferes with SRIF receptor targeting to the cell surface and not with receptor synthesis. Accordingly, immunofluorescence experiments demonstrated that, in amphiphysin overexpressing cells, sst(2A) and sst(5) receptors were segregated in a juxtanuclear compartment identified as the trans-Golgi network. Amphiphysin IIb overexpression had no effect on corticotrophin-releasing factor 41-stimulated adrenocorticotropic hormone secretion, suggesting that it is not involved in the regulated secretory pathway. Taken together, these results suggest that amphiphysin II is not necessary for SRIF receptor endocytosis but is critical for its constitutive targeting to the plasma membrane. Therefore, amphiphysin IIb may be an important component of the constitutive secretory pathway.

C-terminal hydrophobic interactions play a critical role in oligomeric assembly of the P22 tailspike trimer

The tailspike protein from the bacteriophage P22 is a well characterized model system for folding and assembly of multimeric proteins. Folding intermediates from both the in vivo and in vitro pathways have been identified, and both the initial folding steps and the protrimer-to-trimer transition have been well studied. In contrast, there has been little experimental evidence to describe the assembly of the protrimer. Previous results indicated that the C terminus plays a critical role in the overall stability of the P22 tailspike protein. Here, we present evidence that the C terminus is also the critical assembly point for trimer assembly. Three truncations of the full-length tailspike protein, TSPDeltaN, TSPDeltaC, and TSPDeltaNC, were generated and tested for their ability to form mixed trimer species. TSPDeltaN forms mixed trimers with full-length P22 tailspike, but TSPDeltaC and TSPDeltaNC are incapable of forming similar mixed trimer species. In addition, mutations in the hydrophobic core of the C terminus were unable to form trimer in vivo. Finally, the hydrophobic-binding dye ANS inhibits the formation of trimer by inhibiting progression through the folding pathway. Taken together, these results suggest that hydrophobic interactions between C-terminal regions of P22 tailspike monomers play a critical role in the assembly of the P22 tailspike trimer.

Src regulates Golgi structure and KDEL receptor-dependent retrograde transport to the endoplasmic reticulum

The tyrosine kinase Src is present on the Golgi membranes. Its role, however, in the overall function and organization of the Golgi apparatus is unclear. We have found that in a cell line called SYF, which lacks the three ubiquitous Src-like kinases (Src, Yes, and Fyn), the organization of the Golgi apparatus is perturbed. The Golgi apparatus is composed of collapsed stacks and bloated cisternae in these cells. Expression of an activated form of Src relocated the KDEL receptor (KDEL-R) from the Golgi apparatus to the endoplasmic reticulum. Other Golgi-specific marker proteins were not affected under these conditions. Because of the specific effect of Src on the location of KDEL-R, we tested whether protein transport between ER and the Golgi apparatus involves Src. Transport of Pseudomonas exotoxin, which is transported to the ER by binding to the KDEL-R is accelerated by inhibition or genetic ablation of Src. Protein transport from ER to the Golgi apparatus however, is unaffected by Src deletion or inhibition. We propose that Src has an appreciable role in the organization of the Golgi apparatus, which may be linked to its involvement in protein transport from the Golgi apparatus to the endoplasmic reticulum.

Synapsin I-associated phosphatidylinositol 3-kinase mediates synaptic vesicle delivery to the readily releasable pool

Maintaining synaptic transmission requires replenishment of docked synaptic vesicles within the readily releasable pool (RRP) from synaptic vesicle clusters in the synapsin-bound reserve pool. We show that synapsin forms a complex with phosphatidylinositol 3-kinase (PI 3-kinase) in intact nerve terminals and that synapsin-associated kinase activity increases on depolarization. Disruption of either PI 3-kinase activity or its interaction with synapsin inhibited replenishment of the RRP, but did not affect exocytosis from the RRP. Thus we conclude that a synapsin-associated PI 3-kinase activity plays a role in synaptic vesicle delivery to the RRP. This also suggests that PI 3-kinase contributes to the maintenance of synaptic transmission during periods of high activity, indicating a possible role in synaptic plasticity.

Colocalization of synapsin and actin during synaptic vesicle recycling

It has been hypothesized that in the mature nerve terminal, interactions between synapsin and actin regulate the clustering of synaptic vesicles and the availability of vesicles for release during synaptic activity. Here, we have used immunogold electron microscopy to examine the subcellular localization of actin and synapsin in the giant synapse in lamprey at different states of synaptic activity. In agreement with earlier observations, in synapses at rest, synapsin immunoreactivity was preferentially localized to a portion of the vesicle cluster distal to the active zone. During synaptic activity, however, synapsin was detected in the pool of vesicles proximal to the active zone. In addition, actin and synapsin were found colocalized in a dynamic filamentous cytomatrix at the sites of synaptic vesicle recycling, endocytic zones. Synapsin immunolabeling was not associated with clathrin-coated intermediates but was found on vesicles that appeared to be recycling back to the cluster. Disruption of synapsin function by microinjection of antisynapsin antibodies resulted in a prominent reduction of the cytomatrix at endocytic zones of active synapses. Our data suggest that in addition to its known function in clustering of vesicles in the reserve pool, synapsin migrates from the synaptic vesicle cluster and participates in the organization of the actin-rich cytomatrix in the endocytic zone during synaptic activity.

Identification of p115 as a PLCgamma1-binding protein and the role of Src homology domains of PLCgamma1 in the vesicular transport

In order to gain further insight into the function(s) of PLCgamma1, we tried to identify the binding partners that can interact with the SH223 domains of PLCgamma1. Immunoscreening was performed with the purified antisera that are specific to SH223-binding proteins. Several immunoreactive clones were identified as the putative binding proteins and one of them was identified as p115. p115 was reported to be required for transcytotic fusion and subsequent binding of the vesicles to the target membrane. The interaction between PLCgamma1 and p115 was specific to carboxyl-terminal SH2 domain and SH3 domain of PLCgamma1, and also confirmed by biochemical approaches such as co-immunoprecipitation, pull-down assay, and glycerol gradient fractionation. To further characterize the role of SH domains of PLCgamma1 in the vesicle transport pathway, secreted form of alkaline phosphatase (SEAP) reporter assay was carried out. When the SH2 and/or SH3 domains of PLCgamma1 were deleted, the secretion of SEAP was significantly reduced. These findings indicate that the SH2 and SH3 domains of PLCgamma1 may play a role(s) in the process of the vesicle transport via interaction with other vesicle-associated proteins such as p115.

Specific interactions of neuronal focal adhesion kinase isoforms with Src kinases and amphiphysin

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that activates Src family kinases via SH2- and SH3-mediated interactions. Specific FAK isoforms (FAK+), responsive to depolarization and neurotransmitters, are enriched in neurons. We analyzed the interactions of endogenous FAK+ and recombinant FAK+ isoforms containing amino acid insertions (boxes 6,7,28) with an array of SH3 domains and the c-Src SH2/SH3 domain tandem. Endogenous FAK+ bound specifically to the SH3 domains of c-Src (but not n-Src), Fyn, Yes, phosphtidylinositol-3 kinase, amphiphysin II, amphiphysin I, phospholipase Cgamma and NH2-terminal Grb2. The inclusion of boxes 6,7 was associated with a significant decrease in the binding of FAK+ to the c-Src and Fyn SH3 domains, and a significant increase in the binding to the Src SH2 domain, as a consequence of the higher phosphorylation of Tyr-397. The novel interaction with the amphiphysin SH3 domain, involving the COOH-terminal proline-rich region of FAK, was confirmed by coimmunoprecipitation of the two proteins and a closely similar response to stimuli affecting the actin cytoskeleton. Moreover, an impairment of endocytosis was observed in synaptosomes after internalization of a proline-rich peptide corresponding to the site of interaction. The data account for the different subcellular distribution of FAK and Src kinases and the specific regulation of the transduction pathways linked to FAK activation in the brain and implicate FAK in the regulation of membrane trafficking in nerve terminals.

Phospholipase C-gamma is required for agonist-induced Ca2+ entry

We report here that PLC-gamma isoforms are required for agonist-induced Ca2+ entry (ACE). Overexpressed wild-type PLC-gamma1 or a lipase-inactive mutant PLC-gamma1 each augmented ACE in PC12 cells, while a deletion mutant lacking the region containing the SH3 domain of PLC-gamma1 was ineffective. RNA interference to deplete either PLC-gamma1 or PLC-gamma2 in PC12 and A7r5 cells inhibited ACE. In DT40 B lymphocytes expressing only PLC-gamma2, overexpressed muscarinic M5 receptors (M5R) activated ACE. Using DT40 PLC-gamma2 knockout cells, M5R stimulation of ER Ca2+ store release was unaffected, but ACE was abolished. Normal ACE was restored by transient expression of PLC-gamma2 or a lipase-inactive PLC-gamma2 mutant. The results indicate a lipase-independent role of PLC-gamma in the physiological agonist-induced activation of Ca2+ entry.

Protein-protein interactions: mechanisms and modification by drugs

Protein-protein interactions form the proteinaceous network, which plays a central role in numerous processes in the cell. This review highlights the main structures, properties of contact surfaces, and forces involved in protein-protein interactions. The properties of protein contact surfaces depend on their functions. The characteristics of contact surfaces of short-lived protein complexes share some similarities with the active sites of enzymes. The contact surfaces of permanent complexes resemble domain contacts or the protein core. It is reasonable to consider protein-protein complex formation as a continuation of protein folding. The contact surfaces of the protein complexes have unique structure and properties, so they represent prospective targets for a new generation of drugs. During the last decade, numerous investigations have been undertaken to find or design small molecules that block protein dimerization or protein(peptide)-receptor interaction, or on the other hand, induce protein dimerization.

Opposing changes in phosphorylation of specific sites in synapsin I during Ca2+-dependent glutamate release in isolated nerve terminals

Synapsins are major neuronal phosphoproteins involved in regulation of neurotransmitter release. Synapsins are well established targets for multiple protein kinases within the nerve terminal, yet little is known about dephosphorylation processes involved in regulation of synapsin function. Here, we observed a reciprocal relationship in the phosphorylation-dephosphorylation of the established phosphorylation sites on synapsin I. We demonstrate that, in vitro, phosphorylation sites 1, 2, and 3 of synapsin I (P-site 1 phosphorylated by cAMP-dependent protein kinase; P-sites 2 and 3 phosphorylated by Ca(2+)-calmodulin-dependent protein kinase II) were excellent substrates for protein phosphatase 2A, whereas P-sites 4, 5, and 6 (phosphorylated by mitogen-activated protein kinase) were efficiently dephosphorylated only by Ca(2+)-calmodulin-dependent protein phosphatase 2B-calcineurin. In isolated nerve terminals, rapid changes in synapsin I phosphorylation were observed after Ca(2+) entry, namely, a Ca(2+)-dependent phosphorylation of P-sites 1, 2, and 3 and a Ca(2+)-dependent dephosphorylation of P-sites 4, 5, and 6. Inhibition of calcineurin activity by cyclosporin A resulted in a complete block of Ca(2+)-dependent dephosphorylation of P-sites 4, 5, and 6 and correlated with a prominent increase in ionomycin-evoked glutamate release. These two opposing, rapid, Ca(2+)-dependent processes may play a crucial role in the modulation of synaptic vesicle trafficking within the presynaptic terminal.

Synapsin I is expressed in epithelial cells: localization to a unique trans-Golgi compartment

Synapsin I is abundant in neural tissues. Its phosphorylation is thought to regulate synaptic vesicle exocytosis in the pre-synaptic terminal by mediating vesicle tethering to the cytoskeleton. Using anti-synapsin antibodies, we detected an 85 kDa protein in liver cells and identified it as synapsin I. Like brain synapsin I, non-neuronal synapsin I is phosphorylated in vitro by protein kinase A and yields identical 32P-peptide maps after limited proteolysis. We also detected synapsin I mRNA in liver by northern blot analysis. These results indicate that the expression of synapsin I is more widespread than previously thought. Immunofluorescence analysis of several non-neuronal cell lines localizes synapsin I to a vesicular compartment adjacent to trans-elements of the Golgi complex, which is also labeled with antibodies against myosin II; no sub-plasma membrane synapsin I is evident. We conclude that synapsin I is present in epithelial cells and is associated with a trans-Golgi network-derived compartment; this localization suggests that it plays a role in modulating post-TGN trafficking pathways.

Src, Fyn, and Yes are not required for neuromuscular synapse formation but are necessary for stabilization of agrin-induced clusters of acetylcholine receptors

Mice deficient in src and fyn or src and yes move and breathe poorly and die perinatally, consistent with defects in neuromuscular function. Src and Fyn are associated with acetylcholine receptors (AChRs) in muscle cells, and Src and Yes can act downstream of ErbB2, suggesting roles for Src family kinases in signaling pathways regulating neuromuscular synapse formation. We studied neuromuscular synapses in src(-/-); fyn(-/-) and src(-/-); yes(-/-) mutant mice and found that muscle development, motor axon pathfinding, clustering of postsynaptic proteins, and synapse-specific transcription are normal in these double mutants, showing that these pairs of kinases are not required for early steps in synapse formation. We generated muscle cell lines lacking src and fyn and found that neural agrin and laminin-1 induced normal clustering of AChRs and that agrin induced normal tyrosine phosphorylation of the AChR beta subunit in the absence of Src and Fyn. Another Src family member, most likely Yes, was associated with AChRs and phosphorylated by agrin in myotubes lacking Src and Fyn, indicating that Yes may compensate for the loss of Src and Fyn. Nevertheless, PP1 and PP2, inhibitors of Src-class kinases, did not inhibit agrin signaling, suggesting that Src class kinase activity is dispensable for agrin-induced clustering and tyrosine phosphorylation of AChRs. AChR clusters, however, were less stable in myotubes lacking Src and Fyn but not in PP1- or PP2-treated wild-type cells. These data show that the stabilization of agrin-induced AChR clusters requires Src and Fyn and suggest that the adaptor activities, rather than the kinase activities, of these kinases are essential for this stabilization.