Alterations in transmission, vesicle dynamics, and transmitter release machinery at NCAM-deficient neuromuscular junctions

Frequency-dependent modes of synaptic vesicle endocytosis and exocytosis at adult mouse neuromuscular junctions

During locomotion, adult rodent lumbar motoneurons fire in high-frequency (80-100 Hz) 1-2 s bursts every several seconds, releasing between 10,000 and 20,000 vesicles per burst. The estimated total vesicle pool size indicates that all vesicles would be used within 30 s; thus, a mechanism for rapid endocytosis and vesicle recycling is necessary to maintain effective transmission and motor behavior. However, whether such rapid recycling exists at mouse neuromuscular junctions (NMJs) or how it is regulated has been unclear. Here, we show that much less FM1-43 dye is lost per stimulus with 100 Hz stimulation than with 10 Hz stimulation even when the same number of vesicles undergo exocytosis. Electrophysiological data using folimycin show this lesser amount of dye loss is caused in part by the rapid reuse of vesicles. We showed previously that a myosin light chain kinase (MLCK)-myosin II pathway was required for effective transmission at 100 Hz. Here, we confirm the activation of MLCK, based on increased nerve terminal phospho-MLC immunostaining, with 100 Hz but not with 10 Hz stimulation. We further demonstrate that activation of MLCK, by increased extracellular Ca(2+), by PKC (protein kinase C) activation, or by a MLCK agonist peptide, reduces the amount of dye lost even with 10 Hz stimulation. MLCK activation at 10 Hz also resulted in more vesicles being rapidly reused. Thus, MLCK activation by 100 Hz stimulation switches the mechanism of vesicle cycling to a rapid-reuse mode and is required to sustain effective transmission in adult mouse NMJs.

Cell adhesion, the backbone of the synapse: "vertebrate" and "invertebrate" perspectives

Synapses are asymmetric intercellular junctions that mediate neuronal communication. The number, type, and connectivity patterns of synapses determine the formation, maintenance, and function of neural circuitries. The complexity and specificity of synaptogenesis relies upon modulation of adhesive properties, which regulate contact initiation, synapse formation, maturation, and functional plasticity. Disruption of adhesion may result in structural and functional imbalance that may lead to neurodevelopmental diseases, such as autism, or neurodegeneration, such as Alzheimer's disease. Therefore, understanding the roles of different adhesion protein families in synapse formation is crucial for unraveling the biology of neuronal circuit formation, as well as the pathogenesis of some brain disorders. The present review summarizes some of the knowledge that has been acquired in vertebrate and invertebrate genetic model organisms.

Acetylcholine negatively regulates development of the neuromuscular junction through distinct cellular mechanisms

Emerging evidence suggests that the neurotransmitter acetylcholine (ACh) negatively regulates the development of the neuromuscular junction, but it is not clear if ACh exerts its effects exclusively through muscle ACh receptors (AChRs). Here, we used genetic methods to remove AChRs selectively from muscle. Similar to the effects of blocking ACh biosynthesis, eliminating postsynaptic AChRs increased motor axon branching and expanded innervation territory, suggesting that ACh negatively regulates synaptic growth through postsynaptic AChRs. However, in contrast to the effects of blocking ACh biosynthesis, eliminating postsynaptic AChRs in agrin-deficient mice failed to restore deficits in pre- and postsynaptic differentiation, suggesting that ACh negatively regulates synaptic differentiation through nonpostsynaptic receptors. Consistent with this idea, the ACh agonist carbachol inhibited presynaptic specialization of motorneurons in vitro. Together, these data suggest that ACh negatively regulates axon growth and presynaptic specialization at the neuromuscular junction through distinct cellular mechanisms.

Protogenin defines a transition stage during embryonic neurogenesis and prevents precocious neuronal differentiation

Many Ig superfamily members are expressed in the developing nervous system, but the functions of these molecules during neurogenesis are not all clear. Here, we explore the expression and function of one of members of this superfamily, protogenin (PRTG), in the developing nervous system. Expression of PRTG protein is strong in the neural tube of mouse embryos between embryonic days 7.75 and 9.5 but disappears after embryonic day 10.5 when the neural progenitor marker nestin expresses prominently. Perturbation of PRTG activity in P19 embryonal carcinoma cells and in chick embryos, by either RNA interference or a dominant-negative PRTG mutant, increases neuronal differentiation. Using yeast two-hybrid screening and an in situ binding assay, we were able to identify ERdj3 (a stress-inducible endoplasmic reticulum DnaJ homolog) as a putative PRTG ligand. Addition of purified ERdj3 protein into the P19 differentiation assay reduced neurogenesis. This effect was blocked by addition of either a neutralizing antibody against PRTG or purified PRTG ectodomain protein, indicating that the effect of ERdj3 on neurogenesis is mediated through PRTG. Forced expression of ERdj3 in the chick neural tube also impairs neuronal differentiation. Together, these results suggest that expression of PRTG defines a stage between pluripotent epiblasts and committed neural progenitors, and its signaling plays a critical role in suppressing premature neuronal differentiation during early neural development.

Neural cell adhesion molecule is required for stability of reinnervated neuromuscular junctions

Studies examining the etiology of motoneuron diseases usually focus on motoneuron death as the defining pathophysiology of the disease. However, impaired neuromuscular transmission and synapse withdrawal often precede cell death, raising the possibility that abnormalities in synaptic function contribute to disease onset. Although little is known about the mechanisms maintaining the synaptic integrity of neuromuscular junctions (NMJs), Drosophila studies suggest that Fasciclin II plays an important role. Inspired by these studies we used a reinnervation model of synaptogenesis to analyze neuromuscular function in mice lacking neural cell adhesion molecule (NCAM), the Fasciclin II vertebrate homolog. Our results showed that the recovery of contractile force was the same in wild-type and NCAM-/- mice at 1 month after nerve injury, indicating that endplates were appropriately reformed. This normality was only transient because the contractile force and myofiber number decreased at 3 months after injury in NCAM-/- mice. Both declined further 3 months later. Myofibers degenerated, not because motoneurons died but because synapses were withdrawn. Although neurotransmission was initially normal at reinnervated NCAM-/- NMJs, it was significantly compromised 3 months later. Interestingly, the selective ablation of NCAM from motoneurons, or muscle fibers, did not mimic the deficits observed in reinnervated NCAM-/- mice. Taken together, these results indicate that NCAM is required to maintain normal synaptic function at reinnervated NMJs, although its loss pre-synaptically or post-synaptically is not sufficient to induce synaptic destabilization. Consideration is given to the role of NCAM in terminal Schwann cells for maintaining synaptic integrity and how NCAM dysfunction may contribute to motoneuron disorders.

Activity of nAChRs containing alpha9 subunits modulates synapse stabilization via bidirectional signaling programs

Although the synaptogenic program for cholinergic synapses of the neuromuscular junction is well known, little is known of the identity or dynamic expression patterns of proteins involved in non-neuromuscular nicotinic synapse development. We have previously demonstrated abnormal presynaptic terminal morphology following loss of nicotinic acetylcholine receptor (nAChR) alpha9 subunit expression in adult cochleae. However, the molecular mechanisms underlying these changes have remained obscure. To better understand synapse formation and the role of cholinergic activity in the synaptogenesis of the inner ear, we exploit the nAChR alpha9 subunit null mouse. In this mouse, functional acetylcholine (ACh) neurotransmission to the hair cells is completely silenced. Results demonstrate a premature, effusive innervation to the synaptic pole of the outer hair cells in alpha9 null mice coinciding with delayed expression of cell adhesion proteins during the period of effusive contact. Collapse of the ectopic innervation coincides with an age-related hyperexpression pattern in the null mice. In addition, we document changes in expression of presynaptic vesicle recycling/trafficking machinery in the alpha9 null mice that suggests a bidirectional information flow between the target of the neural innervation (the hair cells) and the presynaptic terminal that is modified by hair cell nAChR activity. Loss of nAChR activity may alter transcriptional activity, as CREB binding protein expression is decreased coincident with the increased expression of N-Cadherin in the adult alpha9 null mice. Finally, by using mice expressing the nondesensitizing alpha9 L9'T point mutant nAChR subunit, we show that increased nAChR activity drives synaptic hyperinnervation.

Neural cell adhesion molecule modulates dopaminergic signaling and behavior by regulating dopamine D2 receptor internalization

The dopaminergic system plays an important role in the etiology of schizophrenia, and most antipsychotic drugs exert their functions by blocking dopamine D(2) receptors (D(2)Rs). Since the signaling strength mediated by D(2)Rs is regulated by internalization and degradation processes, it is crucial to identify molecules that modulate D(2)R localization at the cell surface. Here, we show that the neural cell adhesion molecule (NCAM) promotes D(2)R internalization/desensitization and subsequent degradation via direct interaction with a short peptide in the third intracellular loop of the D(2)R. NCAM deficiency in mice leads to increased numbers of D(2)Rs at the cell surface and augmented D(2)R signaling as a result of impaired D(2)R internalization. Furthermore, NCAM-deficient mice show higher sensitivity to the psychostimulant apomorphine and exaggerated activity of dopamine-related locomotor behavior. These results demonstrate that, in addition to its classical function in cell adhesion, NCAM is involved in regulating the trafficking of the neurotransmitter receptor D(2)R as well as receptor-mediated signaling and behavior, thus implicating NCAM as modulator of the dopaminergic system and a potential pharmacological target for dopamine-related neurological and psychiatric disorders.

Impaired insulin exocytosis in neural cell adhesion molecule-/- mice due to defective reorganization of the submembrane F-actin network

The neural cell adhesion molecule (NCAM) is required for cell type segregation during pancreatic islet organogenesis. We have investigated the functional consequences of ablating NCAM on pancreatic beta-cell function. In vivo, NCAM(-/-) mice exhibit impaired glucose tolerance and basal hyperinsulinemia. Insulin secretion from isolated NCAM(-/-) islets is enhanced at glucose concentrations below 15 mM but inhibited at higher concentrations. Glucagon secretion from pancreatic alpha-cells evoked by low glucose was also severely impaired in NCAM(-/-) islets. The diminution of insulin secretion is not attributable to defective glucose metabolism or glucose sensing (documented as glucose-induced changes in intracellular Ca(2+) and K(ATP)-channel activity). Resting K(ATP) conductance was lower in NCAM(-/-) beta-cells than wild-type cells, and this difference was abolished when F-actin was disrupted by cytochalasin D (1 muM). In wild-type beta-cells, the submembrane actin network disassembles within 10 min during glucose stimulation (30 mM), an effect not seen in NCAM(-/-) beta-cells. Cytochalasin D eliminated this difference and normalized insulin and glucagon secretion in NCAM(-/-) islets. Capacitance measurements of exocytosis indicate that replenishment of the readily releasable granule pool is suppressed in NCAM(-/-) alpha- and beta-cells. Our data suggest that remodeling of the submembrane actin network is critical to normal glucose regulation of both insulin and glucagon secretion.

An activity-dependent retrograde signal induces the expression of the high-affinity choline transporter in cholinergic neurons

A well-accepted view of developing circuits is that synapses must be active to mature and persist, whereas inactive synapses remain immature and are eventually eliminated. We question this long-standing view by investigating nonfunctional cholinergic nicotinic synapses in the superior cervical ganglia (SCG) of mice with a disruption in the alpha3 nicotinic receptor (nAChR) subunit gene, a gene essential for fast synaptic transmission in sympathetic ganglia. Using imaging and electrophysiology, we show that synapses persist for at least 2-3 months without postsynaptic activity; however, the presynaptic terminals lack high-affinity choline transporters (CHTs), and as a result, they are quickly depleted of transmitter. Moreover, we demonstrate with rescue experiments that CHT is induced by signals downstream of postsynaptic activity, converting immature terminals to mature terminals capable of sustaining transmitter release in response to high-frequency or continuous firing. Importantly, postsynaptic neurons must be continually active to maintain CHT in presynaptic terminals.

Synaptic vesicle protein trafficking at the glutamate synapse

Expression of the integral and associated proteins of synaptic vesicles is subject to regulation over time, by region, and in response to activity. The process by which changes in protein levels and isoforms result in different properties of neurotransmitter release involves protein trafficking to the synaptic vesicle. How newly synthesized proteins are incorporated into synaptic vesicles at the presynaptic bouton is poorly understood. During synaptogenesis, synaptic vesicle proteins sort through the secretory pathway and are transported down the axon in precursor vesicles that undergo maturation to form synaptic vesicles. Changes in protein content of synaptic vesicles could involve the formation of new vesicles that either mix with the previous complement of vesicles or replace them, presumably by their degradation or inactivation. Alternatively, new proteins could individually incorporate into existing synaptic vesicles, changing their functional properties. Glutamatergic vesicles likely express many of the same integral membrane proteins and share certain common mechanisms of biogenesis, recycling, and degradation with other synaptic vesicles. However, glutamatergic vesicles are defined by their ability to package glutamate for release, a property conferred by the expression of a vesicular glutamate transporter (VGLUT). VGLUTs are subject to regional, developmental, and activity-dependent changes in expression. In addition, VGLUT isoforms differ in their trafficking, which may target them to different pathways during biogenesis or after recycling, which may in turn sort them to different vesicle pools. Emerging data indicate that differences in the association of VGLUTs and other synaptic vesicle proteins with endocytic adaptors may influence their trafficking. These observations indicate that independent regulation of synaptic vesicle protein trafficking has the potential to influence synaptic vesicle protein composition, the maintenance of synaptic vesicle pools, and the release of glutamate in response to changing physiological requirements.

Cell adhesion molecules in the central nervous system

Cell-cell adhesion molecules play key roles at the intercellular junctions of a wide variety of cells, including interneuronal synapses and neuron-glia contacts. Functional studies suggest that adhesion molecules are implicated in many aspects of neural network formation, such as axon-guidance, synapse formation, regulation of synaptic structure and astrocyte-synapse contacts. Some basic cell biological aspects of the assembly of junctional complexes of neurons and glial cells resemble those of epithelial cells. However, the neuron specific junctional machineries are required to exert neuronal functions, such as synaptic transmission and plasticity. In this review, we describe the distribution and function of cell adhesion molecules at synapses and at contacts between synapses and astrocytes.

Molecular mechanisms of presynaptic differentiation

Information processing in the nervous system relies on properly localized and organized synaptic structures at the correct locations. The formation of synapses is a long and intricate process involving multiple interrelated steps. Decades of research have identified a large number of molecular components of the presynaptic compartment. In addition to neurotransmitter-containing synaptic vesicles, presynaptic terminals are defined by cytoskeletal and membrane specializations that allow highly regulated exo- and endocytosis of synaptic vesicles and that maintain precise registration with postsynaptic targets. Functional studies at multiple levels have revealed complex interactions between the transport of vesicular intermediates, the presynaptic cytoskeleton, growth cone navigation, and synaptic targets. With the advent of finer anatomical, physiological, and molecular tools, great insights have been gained toward the mechanistic dissection of functionally redundant processes controlling the specificity and dynamics of synapses. This review highlights the recent findings pertaining to the cellular and molecular regulation of presynaptic differentiation.

The effect of long-term lactobacilli (lactic acid bacteria) enteral treatment on the central nervous system of growing rats

The aim of this study was to explore the relationship between consumption of large doses of lactic acid bacteria (LAB) and the behaviour and brain morphobiochemistry of normal growing rats. Four groups of rats were treated with LAB cultures twice daily for 6 months. The control group received 1 ml of saline per treatment, while two experimental groups received 1 ml of living bacteria (Lactobacillus plantarum and Lactobacillus fermentum, respectively) and the remaining group received a heat-treated (inactivated) L. fermentum culture. After 2 and 6 months of treatment, respectively, eight animals from each group were sacrificed, and specimens were taken for further analyses. The behaviour of the rats was evaluated five times in an open-field test at monthly intervals throughout the study. Lactobacilli treatment for 2 months induced changes in the motoric behaviour of the rats. The concentration of the astrocytesoluble and filament glial fibrillary acidic protein (GFAP) decreased in the posterior part of the hemispheres, including the thalamus, hippocampus and cortex of the rats treated with L. fermentum. A greater decrease in filament GFAP (up to 50%) was shown in the group receiving the live form of L. fermentum. In contrast, the GFAP in the live L. plantarum-treated group increased, showing elevated levels of the soluble and filament forms of GFAP in the posterior part of the hemispheres. A 60-66% decrease in the amount of the astrocyte-specific Ca-binding protein S-100b was shown in the posterior parts of the hemispheres and in the hindbrain of rats given LAB for 2 months. Prolonged feeding with LAB for 4 months up to full adulthood led to a further decrease in astrocyte reaction, reflected as an additional decrease in the amount of soluble GFAP and locomotor activity in all experimental groups. The changes in filament GFAP and S-100b appeared to disappear after prolonged feeding (total of 6 months) with LAB. In summary, LAB dietary treatment affected the ontogenetic development of the astrocytes, with the highest intensity observed in the early stages of rat development. It can be postulated that LAB treatment may play a preventive role in neurological diseases by decreasing astrocyte reaction and, consequently, lowering locomotor activity.

Reduced SMN protein impairs maturation of the neuromuscular junctions in mouse models of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a common pediatric neuromuscular disorder caused by insufficient levels of the survival of motor neuron (SMN) protein. Studies involving SMA patients and animal models expressing the human SMN2 gene have yielded relatively little information about the earliest cellular consequences of reduced SMN protein. In this study, we have used severe- and mild-SMN2 expressing mouse models of SMA as well as material from human patients to understand the initial stages of neurodegeneration in the human disease. We show that the earliest structural defects appear distally and involve the neuromuscular synapse. Insufficient SMN protein arrests the post-natal development of the neuromuscular junction (NMJ), impairing the maturation of acetylcholine receptor (AChR) clusters into 'pretzels'. Pre-synaptic defects include poor terminal arborization and intermediate filament aggregates which may serve as a useful biomarker of the disease. These defects are reflected in functional deficits at the NMJ characterized by intermittent neurotransmission failures. We suggest that SMA might best be described as a NMJ synaptopathy and that one promising means of treating it could involve maintaining function at the NMJ.

Calcium channels, neuromuscular synaptic transmission and neurological diseases

Voltage-dependent calcium channels are essential in neuronal signaling and synaptic transmission, and their functional alterations underlie numerous human disorders whether monogenic (e.g., ataxia, migraine, etc.) or autoimmune. We review recent work on Ca(V)2.1 or P/Q channelopathies, mostly using neuromuscular junction preparations, and focus specially on the functional hierarchy among the calcium channels recruited to mediate neurotransmitter release when Ca(V)2.1 channels are mutated or depleted. In either case, synaptic transmission is greatly compromised; evidently, none of the reported functional replacements with other calcium channels compensates fully.

Developmental regulation of GABAergic interneuron branching and synaptic development in the prefrontal cortex by soluble neural cell adhesion molecule

Neural cell adhesion molecule, NCAM, is an important regulator of neuronal process outgrowth and synaptic plasticity. Transgenic mice that overexpress the soluble NCAM extracellular domain (NCAM-EC) have reduced GABAergic inhibitory and excitatory synapses, and altered behavioral phenotypes. Here, we examined the role of dysregulated NCAM shedding, modeled by overexpression of NCAM-EC, on development of GABAergic basket interneurons in the prefrontal cortex. NCAM-EC overexpression disrupted arborization of basket cells during the major period of axon/dendrite growth, resulting in decreased numbers of GAD65- and synaptophysin-positive perisomatic synapses. NCAM-EC transgenic protein interfered with interneuron branching during early postnatal stages when endogenous polysialylated (PSA) NCAM was converted to non-PSA isoforms. In cortical neuron cultures, soluble NCAM-EC acted as a dominant inhibitor of NCAM-dependent neurite branching and outgrowth. These findings suggested that excess soluble NCAM-EC reduces perisomatic innervation of cortical neurons by perturbing axonal/dendritic branching during cortical development.

Large-scale screening for novel low-affinity extracellular protein interactions

Extracellular protein-protein interactions are essential for both intercellular communication and cohesion within multicellular organisms. Approximately a fifth of human genes encode membrane-tethered or secreted proteins, but they are largely absent from recent large-scale protein interaction datasets, making current interaction networks biased and incomplete. This discrepancy is due to the unsuitability of popular high-throughput methods to detect extracellular interactions because of the biochemical intractability of membrane proteins and their interactions. For example, cell surface proteins contain insoluble hydrophobic transmembrane regions, and their extracellular interactions are often highly transient, having half-lives of less than a second. To detect transient extracellular interactions on a large scale, we developed AVEXIS (avidity-based extracellular interaction screen), a high-throughput assay that overcomes these technical issues and can detect very transient interactions (half-lives <or= 0.1 sec) with a low false-positive rate. We used it to systematically screen for receptor-ligand pairs within the zebrafish immunoglobulin superfamily and identified novel ligands for both well-known and orphan receptors. Genes encoding receptor-ligand pairs were often clustered phylogenetically and expressed in the same or adjacent tissues, immediately implying their involvement in similar biological processes. Using AVEXIS, we have determined the first systematic low-affinity extracellular protein interaction network, supported by independent biological data. This technique will now allow large-scale extracellular protein interaction mapping in a broad range of experimental contexts.

Heterogeneity in synaptic vesicle release at neuromuscular synapses of mice expressing synaptopHluorin

Mammalian neuromuscular junctions are useful model synapses to study the relationship between synaptic structure and function, although these have rarely been studied together at the same synapses. To do this, we generated transgenic lines of mice in which the thy1.2 promoter drives expression of synaptopHluorin (spH) as a means of optically measuring synaptic vesicle distribution and release. SpH is colocalized with other synaptic vesicle proteins in presynaptic terminals and does not alter normal synaptic function. Nerve stimulation leads to readily detectable and reproducible fluorescence changes in motor axon terminals that vary with stimulus frequency and, when compared with electrophysiological recordings, are reliable indicators of neurotransmitter release. Measurements of fluorescence intensity changes reveal a surprising amount of heterogeneity in synaptic vesicle release throughout individual presynaptic motor axon terminals. Some discrete terminal regions consistently displayed a greater rate and extent of release than others, regardless of stimulation frequency. The amount of release at a particular site is highly correlated to the relative abundance of synaptic vesicles there, indicating that a relatively constant fraction of the total vesicular pool, approximately 30%, is released in response to activity. These studies reveal previously unknown relationships between synaptic structure and function at mammalian neuromuscular junctions and demonstrate the usefulness of spH expressing mice as a tool for studying neuromuscular synapses in adults, as well as during development and diseases that affect neuromuscular synaptic function.

The pool of fast releasing vesicles is augmented by myosin light chain kinase inhibition at the calyx of Held synapse

Synaptic strength is determined by release probability and the size of the readily releasable pool of docked vesicles. Here we describe the effects of blocking myosin light chain kinase (MLCK), a cytoskeletal regulatory protein thought to be involved in myosin-mediated vesicle transport, on synaptic transmission at the mouse calyx of Held synapse. Application of three different MLCK inhibitors increased the amplitude of the early excitatory postsynaptic currents (EPSCs) in a stimulus train, without affecting the late steady-state EPSCs. A presynaptic locus of action for MLCK inhibitors was confirmed by an increase in the frequency of miniature EPSCs that left their average amplitude unchanged. MLCK inhibition did not affect presynaptic Ca(2+) currents or action potential waveform. Moreover, Ca(2+) imaging experiments showed that [Ca(2+)](i) transients elicited by 100-Hz stimulus trains were not altered by MLCK inhibition. Studies using high-frequency stimulus trains indicated that MLCK inhibitors increase vesicle pool size, but do not significantly alter release probability. Accordingly, when AMPA-receptor desensitization was minimized, EPSC paired-pulse ratios were unaltered by MLCK inhibition, suggesting that release probability remains unaltered. MLCK inhibition potentiated EPSCs even when presynaptic Ca(2+) buffering was greatly enhanced by treating slices with EGTA-AM. In addition, MLCK inhibition did not affect the rate of recovery from short-term depression. Finally, developmental studies revealed that EPSC potentiation by MLCK inhibition starts at postnatal day 5 (P5) and remains strong during synaptic maturation up to P18. Overall, our data suggest that MLCK plays a crucial role in determining the size of the pool of synaptic vesicles that undergo fast release at a CNS synapse.

Selective targeting of different neural cell adhesion molecule isoforms during motoneuron myotube synapse formation in culture and the switch from an immature to mature form of synaptic vesicle cycling

Characterization of neuromuscular junction formation and function in mice lacking all neural cell adhesion molecule (NCAM) isoforms or only the 180 isoform demonstrated that the 180 isoform was required at adult synapses to maintain effective transmission with repetitive stimulation whereas the 140 and/or 120 isoform(s) were sufficient to mediate the downregulation of synaptic vesicle cycling along the axon after synapse formation. However, the expression and targeting of each isoform and its relationship to distinct forms of synaptic vesicle cycling before and after synapse formation was previously unknown. By transfecting chick motoneurons with fluorescently tagged mouse 180, 140 and 120 isoforms, we show that before myotube contact the 180 and 140 isoforms are expressed in distinct puncta along the axon which are sites of an immature form (Brefeldin A sensitive, L-type Ca2+ channel mediated) of vesicle cycling. After myotube contact the 140 and 180 isoforms are downregulated from the axon and selectively targeted to the presynaptic terminal. This coincided with the downregulation of vesicle cycling along the axon and the expression of the mature form (BFA insensitive, P/Q type Ca2+ channel mediated) of vesicle cycling at the terminal. The synaptic targeting of exogenously expressed 180 and 140 isoforms also occurred when chick motoneurons contacted +/+ mouse myotubes; however only the 180 but not the 140 isoform was targeted on contact with NCAM-/- myotubes. These observations indicate that postsynaptic NCAM is required for the synaptic targeting of presynaptic 140 NCAM but that the localization of presynaptic 180 NCAM occurs via a different mechanism.

Transsynaptic modulation of the synaptic vesicle cycle by cell-adhesion molecules

Delicate control of the synaptic vesicle cycle is required to meet the demands imposed on synaptic transmission by the brain's complex information processing. In addition to intensively analyzed intrinsic regulation, extrinsic modulation of the vesicle cycle by the postsynaptic target neuron has become evident. Recent studies have demonstrated that several families of synaptic cell-adhesion molecules play a significant role in transsynaptic retrograde signaling. Different adhesion systems appear to specifically target distinct steps of the synaptic vesicle cycle. Signaling via classical cadherins regulates the recruitment of synaptic vesicles to the active zone. The neurexin/neuroligin system has been shown to modulate presynaptic release probability. In addition, reverse signaling via the EphB/ephrinB system plays an important role in the activity-dependent induction of long-term potentiation of presynaptic transmitter release. Moreover, the first hints of involvement of cell-adhesion molecules in vesicle endocytosis have been published. A general hypothesis is that specific adhesion systems might use different but parallel transsynaptic signaling pathways able to selectively modulate each step of the synaptic vesicle cycle in a tightly coordinated manner.

Increased asynchronous release and aberrant calcium channel activation in amyloid precursor protein deficient neuromuscular synapses

Despite the critical roles of the amyloid precursor protein (APP) in Alzheimer's disease pathogenesis, its physiological function remains poorly established. Our previous studies implicated a structural and functional activity of the APP family of proteins in the developing neuromuscular junction (NMJ). Here we performed comprehensive analyses of neurotransmission in mature neuromuscular synapse of APP deficient mice. We found that APP deletion led to reduced paired-pulse facilitation and increased depression of synaptic transmission with repetitive stimulation. Readily releasable pool size and total releasable vesicles were not affected, but probability of release was significantly increased. Strikingly, the amount of asynchronous release, a measure sensitive to presynaptic calcium concentration, was dramatically increased, and pharmacological studies revealed that it was attributed to aberrant activation of N- and L-type Ca(2+) channels. We propose that APP modulates synaptic transmission at the NMJ by ensuring proper Ca(2+) channel function.

Polysialic acid–neural cell adhesion molecule in brain plasticity: From synapses to integration of new neurons

Isoforms of the neuronal cell adhesion molecule (NCAM) carrying the linear homopolymer of alpha 2,8-linked sialic acid (polysialic acid, PSA) have emerged as particularly attractive candidates for promoting plasticity in the nervous system. The large negatively charged PSA chain of NCAM is postulated to be a spacer that reduces adhesion forces between cells allowing dynamic changes in membrane contacts. Accumulating evidence also suggests that PSA-NCAM-mediated interactions lead to activation of intracellular signaling cascades that are fundamental to the biological functions of the molecule. An important role of PSA-NCAM appears to be during development, when its expression level is high and where it contributes to the regulation of cell shape, growth or migration. However, PSA-NCAM does persist in adult brain structures such as the hippocampus that display a high degree of plasticity where it is involved in activity-induced synaptic plasticity. Recent advances in the field of PSA-NCAM research have not only consolidated the importance of this molecule in plasticity processes but also suggest a role for PSA-NCAM in the regulation of higher cognitive functions and psychiatric disorders. In this review, we discuss the role and mode of actions of PSA-NCAM in structural plasticity as well as its potential link to cognitive processes.

The Ig cell adhesion molecule Basigin controls compartmentalization and vesicle release at Drosophila melanogaster synapses

Synapses can undergo rapid changes in size as well as in their vesicle release function during both plasticity processes and development. This fundamental property of neuronal cells requires the coordinated rearrangement of synaptic membranes and their associated cytoskeleton, yet remarkably little is known of how this coupling is achieved. In a GFP exon-trap screen, we identified Drosophila melanogaster Basigin (Bsg) as an immunoglobulin domain-containing transmembrane protein accumulating at periactive zones of neuromuscular junctions. Bsg is required pre- and postsynaptically to restrict synaptic bouton size, its juxtamembrane cytoplasmic residues being important for that function. Bsg controls different aspects of synaptic structure, including distribution of synaptic vesicles and organization of the presynaptic cortical actin cytoskeleton. Strikingly, bsg function is also required specifically within the presynaptic terminal to inhibit nonsynchronized evoked vesicle release. We thus propose that Bsg is part of a transsynaptic complex regulating synaptic compartmentalization and strength, and coordinating plasma membrane and cortical organization.

Do different endocytic pathways make different synaptic vesicles?

At a wide range of synapses, synaptic vesicles reside in distinct pools that respond to different stimuli. The recycling pool supplies the vesicles required for release in response to modest stimulation, whereas the reserve pool is mobilized only by strong stimulation. Multiple pathways have been proposed for the recycling of synaptic vesicles after exocytosis, but the relationship of these pathways to the different synaptic vesicle pools has remained unclear. Synaptic vesicle proteins have also been assumed to undergo recycling as a unit. However, emerging data indicate that differences in the association with distinct endocytic adaptors such as the heterotetrameric adaptor AP3 influence the trafficking of individual synaptic vesicle proteins, affecting the composition of synaptic vesicles and hence their functional characteristics. These observations might begin to account for differences in the properties of different vesicle pools.

Neural recognition molecules of the immunoglobulin superfamily: signaling transducers of axon guidance and neuronal migration

Recognition molecules of the immunoglobulin superfamily have important roles in neuronal interactions during ontogeny, including migration, survival, axon guidance and synaptic targeting. Their downstream signal transduction events specify whether a cell changes its place of residence or projects axons and dendrites to targets in the brain, allowing the construction of a dynamic neural network. A wealth of recent discoveries shows that cell adhesion molecules interact with attractant and repellent guidance receptors to control growth cone and cell motility in a coordinate fashion. We focus on the best-studied subclasses, the neural cell adhesion molecule NCAM and the L1 family of adhesion molecules, which share important structural and functional features. We have chosen these paradigmatic molecules and their interactions with other recognition molecules as instructive for elucidating the mechanisms by which other recognition molecules may guide cell interactions during development or modify their function as a result of injury, learning and memory.

Embryonic stem cell-derived motor neurons preserve muscle after peripheral nerve injury

BACKGROUND

The potential of motor neuron progenitor cell transplants to preserve muscle tissue after denervation was studied in in vivo and in vitro adult mammalian model of peripheral nerve injury.

METHODS

Embryonic stem cells were differentiated to induce cholinergic motor neuron progenitors. Flourescent-labeled progenitor cells were injected into the gastrocnemius muscle of Sprague-Dawley rats (n = 10) after denervation by ipilateral sciatic nerve transection. Control rats received injections of either a phosphate-buffered saline solution only (n = 12), murine embryonic fibroblast (STO) cells (n= 6), or undifferentiated embryonic stem cells (n= 6). Muscles were weighed and analyzed at 7 and 21 days using histology, histomorphometry, and immunostaining.

RESULTS

Seven days after progenitor cell transplant, both muscle mass and myocyte cross-sectional area were preserved, compared with control muscles, which demonstrated muscle mass reduction to 70 percent and reduction of cross-sectional area to 72 percent of normal. Fluorescent microscopy of transplanted muscles confirmed the presence of motor neuron progenitors. Presynaptic neuronal staining of the transplants overlapped with alpha-bungarotoxin-labeled muscle fibers, revealing the presence of new neuromuscular junctions. By 21 days, muscle atrophy in the experimental muscles was equal to that of controls and no transplanted cells were observed. Co-culture of the motor neuron progenitor cells and myocytes also demonstrated new neuromuscular junctions by immunofluorescence.

CONCLUSIONS

Transplanted motor neuron progenitors prevent muscle atrophy after denervation for a brief time. These progenitor cell transplants appear to form new neuromuscular junctions with denervated muscle fibers in vivo and with myocytes in vitro.

GDNF and GFRalpha1 promote formation of neuronal synapses by ligand-induced cell adhesion

The establishment of synaptic connections requires precise alignment of pre- and postsynaptic terminals. The glial cell line-derived neurotrophic factor (GDNF) receptor GFRalpha1 is enriched at pre- and postsynaptic compartments in hippocampal neurons, suggesting that it has a function in synapse formation. GDNF triggered trans-homophilic binding between GFRalpha1 molecules and cell adhesion between GFRalpha1-expressing cells. This represents the first example of a cell-cell interaction being mediated by a ligand-induced cell adhesion molecule (LICAM). In the presence of GDNF, ectopic GFRalpha1 induced localized presynaptic differentiation in hippocampal neurons, as visualized by clustering of vesicular proteins and neurotransmitter transporters, and by activity-dependent vesicle recycling. Presynaptic differentiation induced by GDNF was markedly reduced in neurons lacking GFRalpha1. Gdnf mutant mice showed reduced synaptic localization of presynaptic proteins and a marked decrease in the density of presynaptic puncta, indicating a role for GDNF signaling in hippocampal synaptogenesis in vivo. We propose that GFRalpha1 functions as a LICAM to establish precise synaptic contacts and induce presynaptic differentiation.

Important contribution of alpha-neurexins to Ca2+-triggered exocytosis of secretory granules

Alpha-neurexins constitute a family of neuronal cell surface molecules that are essential for efficient neurotransmission, because mice lacking two or all three alpha-neurexin genes show a severe reduction of synaptic release. Although analyses of alpha-neurexin knock-outs and transgenic rescue animals suggested an involvement of voltage-dependent Ca2+ channels, it remained unclear whether alpha-neurexins have a general role in Ca2+-dependent exocytosis and how they may affect Ca2+ channels. Here we show by membrane capacitance measurements from melanotrophs in acute pituitary gland slices that release from endocrine cells is diminished by >50% in adult alpha-neurexin double knock-out and newborn triple knock-out mice. There is a reduction of the cell volume in mutant melanotrophs; however, no ultrastructural changes in size or intracellular distribution of the secretory granules were observed. Recordings of Ca2+ currents from melanotrophs, transfected human embryonic kidney cells, and brainstem neurons reveal that alpha-neurexins do not affect the activation or inactivation properties of Ca2+ channels directly but may be responsible for coupling them to release-ready vesicles and metabotropic receptors. Our data support a general and essential role for alpha-neurexins in Ca2+-triggered exocytosis that is similarly important for secretion from neurons and endocrine cells.

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.

Distinct endocytic pathways control the rate and extent of synaptic vesicle protein recycling

Synaptic vesicles have been proposed to form through two mechanisms: one directly from the plasma membrane involving clathrin-dependent endocytosis and the adaptor protein AP2, and the other from an endosomal intermediate mediated by the adaptor AP3. However, the relative role of these two mechanisms in synaptic vesicle recycling has remained unclear. We now find that vesicular glutamate transporter VGLUT1 interacts directly with endophilin, a component of the clathrin-dependent endocytic machinery. In the absence of its interaction with endophilin, VGLUT1 recycles more slowly during prolonged, high-frequency stimulation. Inhibition of the AP3 pathway with brefeldin A rescues the rate of recycling, suggesting a competition between AP2 and -3 pathways, with endophilin recruiting VGLUT1 toward the faster AP2 pathway. After stimulation, however, inhibition of the AP3 pathway prevents the full recovery of VGLUT1 by endocytosis, implicating the AP3 pathway specifically in compensatory endocytosis.

Comparative Proteomics in Neurodegenerative and Non-neurodegenerative Diseases Suggest Nodal Point Proteins in Regulatory Networking

Neurodegenerative disorders (ND) encompass clinically and genetically heterogeneous diseases with considerable overlap of their clinical, neuropathological and molecular phenotype. Various causes of neurodegeneration in disease may affect eventually the same proteins within protein networks. To identify common changes in ND, we compared brain protein changes detected by 2-D electrophoresis in four mouse models for ND: (i) Parkinson's disease, (ii) Huntington's disease, (iii) prion disease Scrapie, and (iv) a model for impaired synaptic transmission. To determine specificity of these changes for ND, we extended the scope of our investigation to three neurological conditions that do not result in neurodegeneration (non-ND). We detected 12 to 216 consistent qualitative or quantitative protein changes in individual ND and non-ND models when compared to controls. Up to 36% of these proteins were found to be altered in multiple disease states (at least three) and were therefore termed nodal point proteins. Alterations in alpha B-Crystallin and splicing factor 3b (subunit 4) occurred in at least three ND but not in non-ND. In contrast, alterations in peroxiredoxin 1 and 3, astrocytic phosphoprotein PEA15, complexin 2 and aminoacylase 1 were common to both ND and non-ND. Finally, we investigated the expression pattern of the nodal point proteins in three inbred mouse strains and found different protein abundance (expression polymorphisms) in all cases. Nodal point proteins showing expression polymorphisms may be candidate proteins for disease associated modifiers.

The resilient synapse: insights from genetic interference of synaptic cell adhesion molecules

Synaptic cell adhesion molecules (SCAMs) are mostly membrane-anchored molecules with extracellular domains that extend into the synaptic cleft. Prototypical SCAMs interact with homologous or heterologous molecules on the surface of adjacent cells, ensuring the precise apposition of pre- and postsynaptic elements. More recent definitions of SCAMs often include molecules involved in axon pathfinding, cell recognition and synaptic differentiation events, making SCAMs functionally and molecularly a highly diverse group. In this review, we summarize the proposed in vivo functions of a large variety of SCAMs. We mainly focus on results obtained from analyses of genetic model organisms, mostly mouse knockout mutants, lacking expression of the respective candidate genes. In contrast to the substantial effect yielded by some knockouts of molecules involved in synaptic vesicle release, no SCAM mutant has been reported thus far that shows a prominently altered structure of the majority of synapses or even lacks synapses altogether. This surprising resilience of synaptic structure might be explained by a high redundancy between different SCAMs, by the assumption that the crucial molecular players in synapse structure have yet to be discovered or by a grand variability in the mechanisms of synapse formation that underlies the diversity of synapses. Whatever the final answer turns out to be, the genetic dissection of the SCAM superfamilies has led to a much better understanding of the different steps required to form, differentiate and modify a synapse.

CD24 is expressed by myofiber synaptic nuclei and regulates synaptic transmission

The genes encoding several synaptic proteins, including acetylcholine receptors, acetylcholinesterase, and the muscle-specific kinase, MuSK, are expressed selectively by a small number of myofiber nuclei positioned near the synaptic site. Genetic analysis of mutant mice suggests that additional genes, expressed selectively by synaptic nuclei, might encode muscle-derived retrograde signals that regulate the differentiation of motor axon terminals. To identify candidate retrograde signals, we used a microarray screen to identify genes that are preferentially expressed in the synaptic region of muscle, and we analyzed one such gene, CD24, further. We show that CD24, which encodes a small, variably and highly glycosylated, glycosylphosphatidylinositol (GPI)-linked protein, is expressed preferentially by myofiber synaptic nuclei in embryonic and adult muscle, and that CD24 expression is restricted to the central region of muscle independent of innervation. Moreover, we show that CD24 has a role in presynaptic differentiation, because synaptic transmission is depressed and fails entirely, in a cyclical manner, after repetitive stimulation of motor axons in CD24 mutant mice. These deficits in synaptic transmission, which are accompanied by aberrant stimulus-dependent uptake of AM1-43 from axons, indicate that CD24 is required for normal presynaptic maturation and function. Because CD24 is also expressed in some neurons, additional experiments will be required to determine whether pre- or postsynaptic CD24 mediates these effects on presynaptic development and function.

Myosin light chain kinase regulates synaptic plasticity and fear learning in the lateral amygdala

Learning and memory depend on signaling molecules that affect synaptic efficacy. The cytoskeleton has been implicated in regulating synaptic transmission but its role in learning and memory is poorly understood. Fear learning depends on plasticity in the lateral nucleus of the amygdala. We therefore examined whether the cytoskeletal-regulatory protein, myosin light chain kinase, might contribute to fear learning in the rat lateral amygdala. Microinjection of ML-7, a specific inhibitor of myosin light chain kinase, into the lateral nucleus of the amygdala before fear conditioning, but not immediately afterward, enhanced both short-term memory and long-term memory, suggesting that myosin light chain kinase is involved specifically in memory acquisition rather than in posttraining consolidation of memory. Myosin light chain kinase inhibitor had no effect on memory retrieval. Furthermore, ML-7 had no effect on behavior when the training stimuli were presented in a non-associative manner. Anatomical studies showed that myosin light chain kinase is present in cells throughout lateral nucleus of the amygdala and is localized to dendritic shafts and spines that are postsynaptic to the projections from the auditory thalamus to lateral nucleus of the amygdala, a pathway specifically implicated in fear learning. Inhibition of myosin light chain kinase enhanced long-term potentiation, a physiological model of learning, in the auditory thalamic pathway to the lateral nucleus of the amygdala. When ML-7 was applied without associative tetanic stimulation it had no effect on synaptic responses in lateral nucleus of the amygdala. Thus, myosin light chain kinase activity in lateral nucleus of the amygdala appears to normally suppress synaptic plasticity in the circuits underlying fear learning, suggesting that myosin light chain kinase may help prevent the acquisition of irrelevant fears. Impairment of this mechanism could contribute to pathological fear learning.

Divergence between motoneurons: gene expression profiling provides a molecular characterization of functionally discrete somatic and autonomic motoneurons

Studies in the developing spinal cord suggest that different motoneuron (MN) cell types express very different genetic programs, but the degree to which adult programs differ is unknown. To compare genetic programs between adult MN columnar cell types, we used laser capture microdissection (LCM) and Affymetrix microarrays to create expression profiles for three columnar cell types: lateral and medial MNs from lumbar segments and sympathetic preganglionic motoneurons located in the thoracic intermediolateral nucleus. A comparison of the three expression profiles indicated that approximately 7% (813/11,552) of the genes showed significant differences in their expression levels. The largest differences were observed between sympathetic preganglionic MNs and the lateral motor column, with 6% (706/11,552) of the genes being differentially expressed. Significant differences in expression were observed for 1.8% (207/11,552) of the genes when comparing sympathetic preganglionic MNs with the medial motor column. Lateral and medial MNs showed the least divergence, with 1.3% (150/11,552) of the genes being differentially expressed. These data indicate that the amount of divergence in expression profiles between identified columnar MNs does not strictly correlate with divergence of function as defined by innervation patterns (somatic/muscle vs. autonomic/viscera). Classification of the differentially expressed genes with regard to function showed that they underpin all fundamental cell systems and processes, although most differentially expressed genes encode proteins involved in signal transduction. Mining the expression profiles to examine transcription factors essential for MN development suggested that many of the same transcription factors participate in combinatorial codes in embryonic and adult neurons, but patterns of expression change significantly.

Synaptic mitochondria are critical for mobilization of reserve pool vesicles at Drosophila neuromuscular junctions

In a forward screen for genes affecting neurotransmission in Drosophila, we identified mutations in dynamin-related protein (drp1). DRP1 is required for proper cellular distribution of mitochondria, and in mutant neurons, mitochondria are largely absent from synapses, thus providing a genetic tool to assess the role of mitochondria at synapses. Although resting Ca2+ is elevated at drp1 NMJs, basal synaptic properties are barely affected. However, during intense stimulation, mutants fail to maintain normal neurotransmission. Surprisingly, FM1-43 labeling indicates normal exo- and endocytosis, but a specific inability to mobilize reserve pool vesicles, which is partially rescued by exogenous ATP. Using a variety of drugs, we provide evidence that reserve pool recruitment depends on mitochondrial ATP production downstream of PKA signaling and that mitochondrial ATP limits myosin-propelled mobilization of reserve pool vesicles. Our data suggest a specific role for mitochondria in regulating synaptic strength.

NCAM 180 acting via a conserved C-terminal domain and MLCK is essential for effective transmission with repetitive stimulation

NCAM 180 isoform null neuromuscular junctions are unable to effectively mobilize and exocytose synaptic vesicles and thus exhibit periods of total transmission failure during high-frequency repetitive stimulation. We have identified a highly conserved C-terminal (KENESKA) domain on NCAM that is required to maintain effective transmission and demonstrate that it acts via a pathway involving MLCK and probably myosin light chain (MLC) and myosin II. By perfecting a method of introducing peptides into adult NMJs, we tested the hypothesized role of proteins in this pathway by competitive disruption of protein-protein interactions. The effects of KENESKA and other peptides on MLCK and MLC activation and on failures in both wild-type and NCAM 180 null junctions supported this pathway, and serine phosphorylation of KENESKA was critical. We propose that this pathway is required to replenish synaptic vesicles utilized during high levels of exocytosis by facilitating myosin-driven delivery of synaptic vesicles to active zones or their subsequent exocytosis.

Adrenal Chromaffin Cells Exhibit Impaired Granule Trafficking in NCAM Knockout Mice

Neural cell adhesion molecule (NCAM) plays several critical roles in neuron path-finding and intercellular communication during development. In the clinical setting, serum NCAM levels are altered in both schizophrenic and autistic patients. NCAM knockout mice have been shown to exhibit deficits in neuronal functions including impaired hippocampal long term potentiation and motor coordination. Recent studies in NCAM null mice have indicated that synaptic vesicle trafficking and active zone targeting are impaired, resulting in periodic synaptic transmission failure under repetitive physiological stimulation. In this study, we tested whether NCAM plays a role in vesicle trafficking that is limited to the neuromuscular junction or whether it may also play a more general role in transmitter release from other cell systems. We tested catecholamine release from neuroendocrine chromaffin cells in the mouse adrenal tissue slice preparation. We utilize electrophysiological and electrochemical measures to assay granule recruitment and targeting in wild-type and NCAM −/− mice. Our data show that NCAM −/− mice exhibit deficits in normal granule trafficking between the readily releasable pool and the highly release-competent immediately releasable pool. This defect results in a decreased rate of granule fusion and thus catecholamine release under physiological stimulation. Our data indicate that NCAM plays a basic role in the transmitter release mechanism in neuroendocrine cells through mediation of granule recruitment and is not limited to the neuromuscular junction and central synapse active zones.

Rabies virus receptors

There is convincing in vitro evidence that the muscular form of the nicotinic acetylcholine receptor (nAChR), the neuronal cell adhesion molecule (NCAM), and the p75 neurotrophin receptor (p75NTR) bind rabies virus and/or facilitate rabies virus entry into cells. Other components of the cell membrane, such as gangliosides, may also participate in the entry of rabies virus. However, little is known of the role of these molecules in vivo. This review proposes a speculative model that accounts for the role of these different molecules in entry and trafficking of rabies virus into the nervous system.

Selective capability of SynCAM and neuroligin for functional synapse assembly

Synaptic cell adhesion is central for synapse formation and function. Recently, the synaptic cell adhesion molecules neuroligin 1 (NL1) and SynCAM were shown to induce presynaptic differentiation in cocultured neurons when expressed in a non-neuronal cell. However, it is uncertain how similar the resulting artificial synapses are to regular synapses. Are these molecules isofunctional, or do all neuronal cell adhesion molecules nonspecifically activate synapse formation? To address these questions, we analyzed the properties of artificial synapses induced by NL1 and SynCAM, compared the actions of these molecules with those of other neuronal cell adhesion molecules, and examined the functional effects of NL1 and SynCAM overexpression in neurons. We found that only NL1 and SynCAM specifically induced presynaptic differentiation in cocultured neurons. The induced nerve terminals were capable of both spontaneous and evoked neurotransmitter release, suggesting that a full secretory apparatus was assembled. By all measures, SynCAM- and NL1-induced artificial synapses were identical. Overexpression in neurons demonstrated that only SynCAM, but not NL1, increased synaptic function in immature developing excitatory neurons after 8 d in vitro. Tests of chimeric molecules revealed that the dominant-positive effect of SynCAM on synaptic function in developing neurons was mediated by its intracellular cytoplasmic tail. Interestingly, morphological analysis of neurons overexpressing SynCAM or NL1 showed the opposite of the predictions from electrophysiological results. In this case, only NL1 increased the synapse number, suggesting a role for NL1 in morphological synapse induction. These results suggest that both NL1 and SynCAM act similarly and specifically in artificial synapse induction but that this process does not reflect a shared physiological function of these molecules.

Axon fasciculation defects and retinal dysplasias in mice lacking the immunoglobulin superfamily adhesion molecule BEN/ALCAM/SC1

The immunoglobulin superfamily adhesion molecule BEN (other names include ALCAM, SC1, DM-GRASP, neurolin, and CD166) has been implicated in the control of numerous developmental and pathological processes, including the guidance of retinal and motor axons to their targets. To test hypotheses about BEN function, we disrupted its gene via homologous recombination and analyzed the resulting mutant mice. Mice lacking BEN are viable and fertile, and display no external morphological defects. Despite grossly normal trajectories, both motor and retinal ganglion cell axons fasciculated poorly and were occasionally misdirected. In addition, BEN mutant retinae exhibited evaginated or invaginated regions with photoreceptor ectopias that resembled the "retinal folds" observed in some human retinopathies. Together, these results demonstrate that BEN promotes fasciculation of multiple axonal populations and uncover an unexpected function for BEN in retinal histogenesis.

Role of Aplysia cell adhesion molecules during 5-HT-induced long-term functional and structural changes

We previously reported that five repeated pulses of 5-HT lead to down-regulation of the TM-apCAM isoform at the surface of Aplysia sensory neurons (SNs). We here examined whether apCAM down-regulation is required for 5-HT-induced long-term facilitation. We also analyzed the role of the cytoplasmic and extracellular domains by overexpressing various apCAM mutants by DNA microinjection. When TM-apCAM was up-regulated in SNs by DNA microinjection, five pulses of 5-HT failed to produce either synaptic facilitation or an enhancement of synaptic growth, suggesting that down-regulation of apCAM is required for 5-HT-induced EPSP enhancement and new varicosity formation. However, disrupting the extracellular domain function of overexpressed apCAM with a specific antibody restored 5-HT-induced excitatory postsynaptic potential increase but not synaptic growth. The overexpression of the MAP Kinase mutant of TM-apCAM, which is not internalized by 5-HT, inhibited new varicosity formation, but did not inhibit excitatory postsynaptic potential increase. Deletion mutants containing only the cytoplasmic portion of apCAM blocked 5-HT-induced synaptic growth but not excitatory postsynaptic potential increase. Thus, our data suggest that TM-apCAM may act as a suppressor of both synaptic-strength enhancement in pre-existing synapses and of new synaptic varicosity formation in the nonsynaptic region, via different mechanisms.