Presynaptic elements formed on polylysine-coated beads contain synaptic vesicle antigens

Presynapses contain distinct actin nanostructures

The architecture of the actin cytoskeleton that concentrates at presynapses remains poorly known, hindering our understanding of its roles in synaptic physiology. In this work, we measure and visualize presynaptic actin by diffraction-limited and super-resolution microscopy, thanks to a validated model of bead-induced presynapses in cultured neurons. We identify a major population of actin-enriched presynapses that concentrates more presynaptic components and shows higher synaptic vesicle cycling than their non-enriched counterparts. Pharmacological perturbations point to an optimal actin amount and the presence of distinct actin structures within presynapses. We directly visualize these nanostructures using Single Molecule Localization Microscopy (SMLM), defining three distinct types: an actin mesh at the active zone, actin rails between the active zone and deeper reserve pools, and actin corrals around the whole presynaptic compartment. Finally, CRISPR-tagging of endogenous actin allows us to validate our results in natural synapses between cultured neurons, confirming the role of actin enrichment and the presence of three types of presynaptic actin nanostructures.

Visualizing K48 Ubiquitination during Presynaptic Formation By Ubiquitination-Induced Fluorescence Complementation (UiFC)

In recent years, signaling through ubiquitin has been shown to be of great importance for normal brain development. Indeed, fluctuations in ubiquitin levels and spontaneous mutations in (de)ubiquitination enzymes greatly perturb synapse formation and neuronal transmission. In the brain, expression of lysine (K) 48-linked ubiquitin chains is higher at a developmental stage coincident with synaptogenesis. Nevertheless, no studies have so far delved into the involvement of this type of polyubiquitin chains in synapse formation. We have recently proposed a role for polyubiquitinated conjugates as triggering signals for presynaptic assembly. Herein, we aimed at characterizing the axonal distribution of K48 polyubiquitin and its dynamics throughout the course of presynaptic formation. To accomplish so, we used an ubiquitination-induced fluorescence complementation (UiFC) strategy for the visualization of K48 polyubiquitin in live hippocampal neurons. We first validated its use in neurons by analyzing changing levels of polyubiquitin. UiFC signal is diffusely distributed with distinct aggregates in somas, dendrites and axons, which perfectly colocalize with staining for a K48-specific antibody. Axonal UiFC aggregates are relatively stable and new aggregates are formed as an axon grows. Approximately 65% of UiFC aggregates colocalize with synaptic vesicle clusters and they preferentially appear in the axonal domains of axo-somatodendritic synapses when compared to isolated axons. We then evaluated axonal accumulation of K48 ubiquitinated signals in bead-induced synapses. We observed rapid accumulation of UiFC signal and endogenous K48 ubiquitin at the sites of newly formed presynapses. Lastly, we show by means of a microfluidic platform, for the isolation of axons, that presynaptic clustering on beads is dependent on E1-mediated ubiquitination at the axonal level. Altogether, these results indicate that enrichment of K48 polyubiquitin at the site of nascent presynaptic terminals is an important axon-intrinsic event for presynaptic differentiation.

β-Pix modulates actin-mediated recruitment of synaptic vesicles to synapses

Presynaptic compartments are formed through the recruitment of preassembled clusters of proteins to points of cell-cell contact, however, the molecular mechanism(s) underlying this process remains unclear. We demonstrate that clusters of polymerized actin can recruit and maintain synaptic vesicles to discrete sites along the axon, and that cadherin/β-catenin/scribble/β-pix complexes play an important role in this event. Previous work has demonstrated that β-catenin and scribble are important for the clustering of vesicles at synapses. We demonstrate that β-pix, a Rac/Cdc42 guanine nucleotide exchange factor (GEF), forms a complex with cadherin, β-catenin, and scribble at synapses and enhances localized actin polymerization in rat hippocampal neurons. In cells expressing β-pix siRNA or dominant-negative β-pix that lacks its GEF activity, actin polymerization at synapses is dramatically reduced, and synaptic vesicle localization is disrupted. This β-pix phenotype can be rescued by cortactin overexpression, suggesting that β-pix-mediated actin polymerization at synapses regulates vesicle localization.

Regenerating crayfish motor axons assimilate glial cells and sprout in cultured explants

Phasic and tonic motor nerves originating from crayfish abdominal ganglia, in 2-3-day-old cultured explants, display at their transected distal ends growth zones from which axonal sprouts arise. The subcellular morphology of this regenerative response was examined with thin serial-section electron microscopy and reveals two major remodeling features. First, the external sprouts that exit the nerve are a very small part of a much more massive sprouting response by individual axons comprising several orders of internal sprouts confined to the nerve. Both internal and external sprouts have a simple construction: a cytoskeleton of microtubules and populations of mitochondria, clear synaptic vesicles, membranous sacs, and extrasynaptic active zone dense bars, features reminiscent of motor nerve terminals. Close intermingling of the sprouts of several axons give rise to a neuropil-like arbor within the nerve. Thus, extensive sprouting is an intrinsic response of crayfish motor axons to transection. Second, an equally dramatic remodeling feature is the appearance of nuclei, which resemble those of adjacent glial cells, within the motor axons. These nuclei often appear where the adjoining membranes of the axon and glial cell are disrupted and where free-standing lengths of the double membrane are present. These images signify a breakdown of the dividing membranes and assimilation of the glial cell by the axon, the nucleus being the most visible sign of such assimilation. Thus, crayfish motor axons respond to transection by assimilating glial cells that may provide regulatory and trophic support for the sprouting response.

Influence of the target on distribution and functioning of the varicosities of Helix pomatia metacerebral cell C1 in dissociated cell culture

The serotonergic metacerebral giant cell (C1) of Helix pomatia was isolated with its bifurcate axon and plated in culture under five conditions: (i) with no target; (ii) with the appropriate target B2 near the stump of the bigger branch (CBC); (iii) with B2 near the stump of the smaller branch (CC); (iv) with a wrong target (C3) near the stump of the CBC branch and (v) with B2 and C3 positioned near the CBC and CC stump, respectively. The counting of anti-serotonin antibody-labelled varicosities of the C1 neuron showed that the presence of the appropriate target in either axonal domain both down-regulated the number of varicosities of the contralateral neuritic field, and increased their average size, whereas the wrong target induced an overall reduction of the number of C1 neuron varicosities, and inhibited the evoked transmitter release. The action potential-evoked calcium concentration increase in the neuritic terminals of the C1 neuron cultured alone, or in presence of the appropriate target, reached a value significantly higher than that reached in presence of the wrong target. These results provide evidence that the postsynaptic neuron regulates both morphological and functional development of presynaptic terminals.

Induction of presynaptic differentiation in cultured neurons by extracellular matrix components

Motoneurons reinnervating skeletal muscles form nerve terminals at sites of contact with a specialized basal lamina. To analyse the molecules and mechanisms that underly these responses, we introduce two systems in which basal lamina-derived components induce presynaptic differentiation of cultured neurons from chick ciliary ganglia in the absence of a postsynaptic cell. In one, ciliary neurites that contact substrates coated with a recombinant laminin beta2 fragment form varicosities that are rich in synaptic vesicle proteins, depleted of neurofilaments, and capable of depolarization-dependent exocytosis and endocytosis. Thus, a single molecule can trigger a complex, coordinated program of presynaptic differentiation. In a second system, neurites growing on cryostat sections of adult kidney form vesicle-rich, neurofilament-poor arbors on glomeruli. Glomerular basal lamina, like synaptic basal lamina, is rich in laminin beta2 and collagen (alpha3-5) IV. However, glomeruli from mutant mice lacking these proteins were capable of inducing differentiation, suggesting the glomerulus as a source of novel presynaptic organizing molecules.

Localization of the brain-specific high-affinity l-proline transporter in cultured hippocampal neurons: molecular heterogeneity of synaptic terminals

The expression of a brain-specific, high-affinity Na+-(and Cl--)dependent l-proline transporter in subpopulations of putative glutamatergic pathways in mammalian brain suggests a physiological role for this carrier in excitatory neurotransmission (Fremeau et al. , Neuron 8: 915-926, 1992). To assess further the cell-type and subcellular localization of PROT, we examined its distribution in low-density cultures of embryonic rat hippocampus. PROT immunoreactivity was detected beginning at 8 days in culture in a highly punctate pattern localizing to a subset of synaptic terminals. PROT was not detected at GABAergic terminals but was specifically localized to a subset of excitatory nerve terminals. PROT-labeled terminals showed partial apposition to AMPA-type and NMDA-type glutamate receptor clusters. Immunolabeling of isolated neurons grown in microisland cultures revealed that PROT was expressed by 60% of cultured hippocampal neurons. Individual microisland cultures were immunopositive for either PROT or glutamic acid decarboxylase, but never both. In the expressing pyramidal neurons, PROT was targeted to all presynaptic terminals. These findings indicate that PROT contributes to the molecular heterogeneity of glutamatergic terminals and suggest a novel presynaptic regulatory role for PROT in excitatory transmission at specific glutamatergic synapses.

Synapsin-like molecules in Aplysia punctata and Helix pomatia: identification and distribution in the nervous system and during the formation of synaptic contacts in vitro

The distribution and biochemical features of the synapsin-like peptides recognized in Aplysia and Helix by various antibodies directed against mammalian synapsins were studied. The peptides can be extracted at low pH and are digested by collagenase; further, they can be phosphorylated by both protein kinase A and Ca2+/calmodulin-dependent protein kinase II. In the ganglia of both snails, they are associated with the soma of most neurons and with the neuropil; punctate immunostaining is present along the neurites. Using cocultures of a Helix serotoninergic neuron and of its target cell, we analysed the redistribution of the synapsin-like peptides during the formation of active synaptic contacts. When the presynaptic neuron is plated in isolation, both synapsin and serotonin immunoreactivities are restricted to the distal axonal segments and to the growth cones; in the presence of the target, the formation of a chemical connection is accompanied by redistribution of the synapsin and serotonin immunoreactivities that concentrate in highly fluorescent round spots scattered along the newly grown neurites located close to the target cell. Almost every spot that is stained for serotonin is also positive for synapsin. In the presynaptic cell plated alone, the number of these varicosity-like structures is substantially stable throughout the whole period; by contrast, when the presynaptic cell synapses the target, their number increases progressively parallel to the increase in the mean amplitude of cumulative excitatory postsynaptic potentials recorded at the same times. The data indicate that mollusc synapsin-like peptides to some extent resemble their mammalian homologues, although they are not exclusively localized in nerve terminals and their expression strongly correlates with the formation of active synaptic contacts.

Localization of synapsin I in normal fibers and regenerating axonal sprouts of the rat sciatic nerve

The localization of synapsin I, a synaptic vesicle-associated protein, was investigated immunocyto-chemically in normal nerve fibers and regenerating axonal sprouts following crush-injuries to the rat sciatic nerve. In normal myelinated axons, weak synapsin I immunoreactivity was found in the axoplasmic/smooth endoplasmic domains, but not in the cytoskeletal domains comprising neurofilaments and microtubules. In non-myelinated axons without dense cytoskeletal structures, moderate immunoreactivity was distributed diffusely throughout the axoplasm. In the crush-injured nerves, intense synapsin I immunoreactivity was demonstrated by light microscopy in early regenerating sprouts emerging from nodes of Ranvier. These nodal sprouts subsequently elongated as regenerating axons through the space between the basal lamina and the myelin sheath (or Schwann cell plasma membrane). Intense synapsin I immunoreactivity was also found in the growth cones of such long regenerating axons. Electron microscopy revealed that synapsin I immunoreactivity was associated mainly with vesicular organelles in the nodal sprouts and growth cones of regenerating axons. Long regenerating axons exhibited no synapsin I immunoreactivity in the shaft, which contained an abundance of neurofilaments. However, vesicle accumulations remaining in the periphery of the shaft still exhibited intense synapsin I immunoreactivity. Thus, it can be concluded that synapsin I is localized at especially high density in the domains comprising vesicular organelles, which are characteristic of early nodal sprouts, as well as in growth cones of regenerating axons. These findings, together with the proposed functions of synapsin I investigated in other studies, suggest that synapsin I may play important roles in vesicular dynamics including the translocation of vesicles to the plasma membrane in sprouts and growth cones of regenerating axons.

Secretion of plasminogen activator and lysosomal enzymes from mouse skeletal muscle: effect of denervation

Levels of hydrolytic enzymes increase in skeletal muscle after denervation and their activities in the extracellular matrix appear to be important for interaction between muscle and nerve. Using enzymatic assays for beta-glucuronidase, beta-galactosidase, and plasminogen activator, we show that secretion of these enzymes from mouse skeletal muscle increases after denervation and that drugs interfering with the secretory pathway or the reuptake of enzymes modulate this release. Thus, brefeldin A inhibited secretion of plasminogen activator activity and mannan increased secreted amounts of beta-glucuronidase, but not of beta-galactosidase, in denervated muscle. In innervated muscle, brefeldin A decreased secreted activity of plasminogen activator, but mannan had no effect on secretion of either beta-glucuronidase or beta-galactosidase. Furthermore, secretion of plasminogen activator was temperature dependent. These observations, together with previous studies, suggest that secretion of hydrolytic enzymes from adult skeletal muscle may be of physiological significance in nerve-muscle communication.

Synaptophysin has a selective distribution in early endosomes of PC12 cells

We have studied the endocytic pathway in PC12 cells and localized synaptophysin to a subpopulation of early endosomes. Endocytosis was examined by electron microscopy using horseradish peroxidase as an endocytic tracer. Immediately following brief incubations with horseradish peroxidase, label was seen in small vesicles and tubules which appeared to be part of the tubular early endosomal network. A large vacuolar structure, containing a rim of horseradish peroxidase reaction product and an electron-lucent central region, was also labelled at the earliest time point examined. Within 5 min after the horseradish peroxidase pulse, reaction product was seen in multivesicular bodies. After prolonged chase periods, horseradish peroxidase label was lost from the early endosomal structures and accumulated in large dense vesicles containing lamellar stacks of membranes. The observed pattern of horseradish peroxidase distribution is consistent with delivery of the tracer into tubular and vacuolar early endosomes with subsequent movement of horseradish peroxidase out of these compartments and into lysosomes. Examination of synaptophysin distribution by EM immunocytochemistry following incubations with horseradish peroxidase revealed selective immunogold labelling of early endosomal structures. Notably, small vesicular and tubular profiles were frequently double-labelled while vacuolar early endosomes were only rarely labelled for synaptophysin. Immunocytochemical labelling was not observed in multivesicular bodies or large dense vesicles with lamellar stacks. Results of experiments in which endosomal structures were immunoprecipitated with antibody to synaptophysin were consistent with the immunocytochemical findings. Maximal recovery of endocytosed horseradish peroxidase activity was seen immediately following the horseradish peroxidase pulse, and a significant decrease was seen after brief chase periods. These results indicate the presence of synaptophysin in vesicles and tubules of the early endosomal compartment.

Increases in pericellular proteolysis at developing neuromuscular junctions in culture

To determine whether localized changes in pericellular proteolysis contribute to synapse formation, we examined the degradative actions of developing Xenopus laevis nerve and muscle cells on films of extracellular matrix proteins adsorbed to the glass surface of a tissue culture chamber. Skeletal myocytes, growing neurites, and fibroblasts all removed fluorescent fibronectin and laminin from the culture substratum at regions of close cell-surface contact. In addition, however, motor neurites also displayed a particularly enhanced rate of gelatin elimination at developing neuromuscular junctions. It has already been shown (a) that there is a similar remodeling of organized muscle basal lamina proteoglycan accumulations along the path of nerve-muscle contact and (b) that this is the earliest detectable biochemical change specific to developing neuromuscular junctions. Our observations thus suggest that the establishment of motoneuron-muscle contact leads to a further activation of pericellular proteinases along both the pre- and the postsynaptic surfaces of the developing junction. We therefore consider whether site-specific proteinase-activation cascades could contribute to the inductive signals that direct synaptic differentiation.

Synaptic differentiation can be evoked by polymer microbeads that mimic localized pericellular proteolysis by removing proteins from adjacent surfaces

Synaptic differentiation is normally "induced" by regulatory signals that are exchanged only at close contacts between neurites and their predetermined target cells. These signals can, however, be mimicked by contact of either cell with some kinds of polymer microbeads. To find what bead action is responsible for this mimicry, we compared the effects of active and inert microbeads on Xenopus muscle cells developing in culture and on glass-adsorbed films of laminin or fibronectin. Our results show that inductive bioactivity is a property of native polystyrene microbeads that (a) is not dependent merely on bead-muscle adhesion, (b) can be eliminated simply by exposing the beads to inert serum proteins, and (c) correlates closely with the ability of some beads to desorb proteins from adjacent surfaces. Quasi-synaptic differentiation of the muscle surface thus seems to be triggered by the focal removal of peripheral cell surface components, rather than by direct bead interactions with membrane receptors or ion channels or their gradual acquisition of endogenous regulatory substances. Since nerve-muscle interaction also causes an elimination of extracellular matrix proteins from the muscle surface, very early in synapse development, we consider the possibility that the extracellular degradation of peripheral surface components contributes to the transmission of inductive positional signals during synaptogenesis.

Redistribution of GAP-43 during growth cone development in vitro; immunocytochemical studies

The growth-associated protein GAP-43 (B-50, F1, pp46), has been found in elongating axons during development and regeneration, and has also been associated with synaptic plasticity in mature neurons. We have examined the loss of GAP-43 labelling from cerebellar granule cells with immunocytochemical localization of a polyclonal antibody to GAP-43. One day after plating, the plasma membrane of cell bodies, neurites and growth cones were all labelled with anti-GAP-43. By 10 days, most of the cell body labelling was lost, and by 20 days the neuritic and growth cone labelling was greatly reduced. Beginning at six days, anti-GAP-43 labelling of growth cones, which was initially uniform, became clustered. When growth cones were double-labelled with antibodies to GAP-43 and the synaptic vesicle protein, p65, inverse changes in the distribution of label was observed. While growth cone labelling with anti-p65 increased from three to 20 days in culture, GAP-43 label began to be lost from some growth cones by six days and showed continuing decline through 20 days. For individual growth cones, the loss of GAP-43 appeared to parallel the accumulation of p65, and first growth cones to lose GAP-43 appeared to be the first to accumulate p65 label. When cultures were grown on a substrate of basement membrane material, the time frames of neuritic outgrowth, loss of GAP-43 labelling, and increase in p65 labelling were all accelerated. At five days, labelling for GAP-43 was weak and labelling for p65 was strong, in a pattern comparable to that seen in older cultures on a polylysine substrate. These results suggest several conclusions concerning the expression and loss of GAP-43 in cultured cerebellar granule neurons. First, GAP-43 label is initially distributed in all parts of these cells. With increasing time in culture the label is first lost from cell bodies and later from neurites and growth cones. Second, the loss of GAP-43 label from growth cones is correlated with the appearance of the synaptic vesicle protein p65. Finally, in vitro developmental changes in the loss of GAP-43 can be altered by changing the growth substrate.

Transitional elements with characteristics of both growth cones and presynaptic terminals observed in cell cultures of cerebellar neurons

As growth cones interact with targets, they become presynaptic terminals by losing growth cone characteristics and acquiring presynaptic characteristics. Results presented here show that transitional elements can be identified in cell cultures of rat cerebellum, which have some characteristics of both growth cones and presynaptic terminals. During the first week in culture, slender growth cones have fine filopodia. Subsequently, many growth cones in contact with the polylysine substrate spontaneously enlarge and become non-motile. In transitional elements, the synaptic vesicle protein p65 extends into the peripheral domain and in some cases, extends into filopodia. Many of these transitional elements have active filopodia but show no movement over the substrate for periods of up to nine days. These transitional elements have lost the actin-rich peripheral domain of the growth cone but retain actin labelling in the filopodia. With electron microscopy, transitional elements were seen to contain accumulations of synaptic vesicles at the site of contact with the substrate. Electron microscopic immunocytochemistry showed these synaptic vesicles labelled for p65 with silver-developed gold particles. Thus, transitional elements have characteristics of both growth cones and presynaptic terminals, suggesting that they may also have functional attributes of both growth cones and presynaptic elements.

Development of calcitonin gene-related peptide (CGRP) immunoreactivity in relationship to the formation of neuromuscular junctions in Xenopus myotomal muscle

Immunoreactivity (IR) to calcitonin gene-related peptide (CGRP) has recently been found in chick motoneurons. Bath application of this peptide causes an increase in surface acetylcholine receptor (AChR) density and cAMP level in cultured chick muscle cells. These results suggest a role for this peptide in the formation of synaptic specializations. In this study, we examined the development of CGRP IR in larval Xenopus myotomal muscle in relation to synaptogenesis. Using an antiserum against CGRP, a monoclonal antibody against the p65 synaptic vesicle antigen and a fluorescence conjugate of alpha-bungarotoxin, we followed the development of synaptic specializations with fluorescence microscopy in whole-mount specimens. We found that the postsynaptic specialization in the form of AChR clusters was first detected in stage 22 (24 hour) embryos. The presynaptic specializations, including the synaptic vesicle clusters as evidenced by p65 antibody staining and CGRP IR, were first detected at stage 32 (40 hours). The appearance of these two presynaptic specializations followed the same time course. Subsequently, all three structures, the AChR clusters, CGRP IR, and synaptic vesicle clusters, were colocalized at the neuromuscular junction (NMJ). This shows that CGRP is unlikely to be involved in signaling the development of the postsynaptic apparatus. This premise is further examined in cultures of Xenopus myotomal muscle cells. CGRP at concentrations up to 1 microM did not affect the number of AChR clusters, nor did it interfere with the formation of clusters induced by polycation-coated beads. In contrast, an extract from the basement membrane of Torpedo electric organ promoted the formation of AChR clusters and interfered with the clustering activity of the beads. These results suggest that CGRP, an integral component of the presynaptic specialization, is not involved in signaling synaptogenesis at the NMJ.

Highly basic 30- and 32-kilodalton proteins associated with synapse formation on polylysine-coated beads in enriched neuronal cell cultures

Neuronal proteins involved in axonal outgrowth and synapse formation were examined in an enriched neuronal cell culture system of the cerebellum. In rat cerebellar cell cultures, 98.9% of the cells are neurons and the remaining 1.1% of the cells are flat nonneuronal cells. These enriched neuronal cultures, examined with two-dimensional gel electrophoresis, showed protein patterns similar to those of neonatal cerebellum, but very different patterns from glial enriched cultures. High levels of a neuronal membrane acidic 29-kilodalton (kD) protein were found. It has been shown previously that neuronal cultures incubated with polylysine-coated beads will develop numerous presynaptic elements on the bead surface. We report here that isolation of the beads from enriched neuronal cell cultures incubated with [35S]methionine showed, with two-dimensional nonequilibrium pH gradient gel electrophoresis (2D-NEPHGE), levels of a basic 32-kD protein (pI 8) note detected in cultures alone, and increased levels of a 30-kD protein (pI 10). When culture medium was examined with 2D-NEPHGE, three acidic proteins were identified that were secreted by the cultured neurons. In summary, a neuronal enriched cell culture system was used with isolated polylysine-coated beads to identify basic 30-kD and 32-kD proteins that may be involved in synapse formation.

Localization of calcitonin gene-related peptide (CGRP) at a neuronal nicotinic synapse

The localization of calcitonin gene-related peptide (CGRP) in the parasympathetic cardiac ganglion of the frog was investigated with an immunofluorescence method. By doubly staining the whole ganglion with antibodies against a synaptic-vesicle antigen and anti-CGRP antibodies, we found that all of the boutons at the nicotinic synapses on the postganglionic neurons contained immunoreactivity against this peptide. In contrast, neither the muscarinic nor the adrenergic terminals of the postganglionic fibers on the cardiac muscle contained CGRP immunoreactivity. This suggests that CGRP is involved in the functions of the neuronal nicotinic synapses.

Immunological characterization of a membrane glycoprotein of chromaffin granules: its presence in endocrine and exocrine tissues

A glycoprotein was isolated from detergent solubilized membranes of bovine chromaffin granules by high-performance liquid chromatography. Specific antisera raised against this glycoprotein reacted in one- and two-dimensional immunoblots with a heterogeneous component with a pI of 4.2-4.7 and Mr 100,000. The antiserum against bovine glycoprotein II cross-reacted with an analogous component in several species. The specific localization of glycoprotein II in chromaffin granules was established by density gradient centrifugation followed by immunoblotting. The antiserum, as shown by one- and two-dimensional immunoblotting, reacted with an analogous antigen in the posterior pituitary, in endocrine (anterior pituitary, parathyroid gland) and exocrine (parotid gland, pancreas) organs. In the pancreas the protein reacting with the antiserum was found in the membranes of zymogen granules. The results demonstrate for the first time that secretory vesicles of endocrine and exocrine tissues have at least one common antigen, i.e. the glycoprotein II. It seems likely that this protein is involved in a basic function common to all secretory vesicles.

Molecular events in synaptogenesis: nerve-muscle adhesion and postsynaptic differentiation

The clustering of acetylcholine receptors (AChR) in the postsynaptic membrane of newly innervated muscle fibers is one of the earliest events in the development of the vertebrate neuromuscular junction. Here, we describe two hypotheses that can account for AChR clustering in response to innervation. The "trophic factor" hypothesis proposes that the neuron releases a soluble factor that interacts with the muscle cell in a specific manner and that this interaction results in the local accumulation of AChR. The "contact and adhesion" hypothesis proposes that the binding of the nerve to the muscle cell surface is itself sufficient to induce AChR clustering, without the participation of soluble factors. We present a model for the molecular assembly of AChR clusters based on the contact and adhesion hypothesis. The model involves the sequential assembly of three distinct membrane domains. The first domain to form serves to attach microfilaments to the cytoplasmic surface of the muscle cell membrane at sites of muscle-nerve adhesion. The second domain to form is clathrin-coated membrane; it serves as a site of insertion of additional membrane elements, including AChR. Upon insertion of AChR into the cell surface, a membrane skeleton assembles by anchoring itself to the AChR. The skeleton, composed in part of actin and spectrin, binds and immobilizes significant numbers of AChR, thereby forming the third membrane domain of the AChR cluster. We make several predictions that should distinguish this model of AChR clustering from one that invokes soluble, trophic factors.

Presynaptic elements on artificial surfaces. A model for the study of development and regeneration of synapses

Recently a model has been developed to study the synapse formation in which the components of a synapse can be isolated and examined independently. The observation of neurites forming presynaptic elements on polylysine-coated surfaces is a model for which the formation of presynaptic elements can be studied independently of a cellular postsynaptic element. Studies with neurons from both cell cultures and the intact cerebellum have shown that beads coated with poly-basic proteins can serve as a "postsynaptic element." With use of this system, observation have shown that the presynaptic element can form quickly, within 3 h, and contain many of the characteristics of a mature presynaptic element, such as synaptic vesicle antigens. Additional studies have shown that astrocytes appear to be involved in the loss or removal of the presynaptic elements on beads. Thus, synaptogenesis may involve the development of inappropriate synaptic contacts, which are eliminated by astrocytes. The lack of regeneration in the central nervous system (CNS) also may involve the astrocyte's ability to remove immature and/or inappropriate presynaptic elements and growth cones as they attempt to cross the lesion site.