Gpr126/Adgrg6 contributes to the terminal Schwann cell response at the neuromuscular junction following peripheral nerve injury

Gpr126/Adgrg6 is an adhesion G protein-coupled receptor essential for Schwann cell (SC) myelination with important contributions to repair after nerve crush injury. Despite critical functions in myelinating SCs, the role of Gpr126 within nonmyelinating terminal Schwann cells (tSCs) at the neuromuscular junction (NMJ), is not known. tSCs have important functions in synaptic maintenance and reinnervation, and after injury tSCs extend cytoplasmic processes to guide regenerating axons to the denervated NMJ. In this study, we show that Gpr126 is expressed in tSCs, and that absence of Gpr126 in SCs (SC-specific Gpr126 knockout, cGpr126) results in a NMJ maintenance defect in the hindlimbs of aged mice, but not in young adult mice. After nerve transection and repair, cGpr126 mice display delayed NMJ reinnervation, altered tSC morphology with decreased S100β expression, and reduced tSC cytoplasmic process extensions. The immune response promoting reinnervation at the NMJ following nerve injury is also altered with decreased macrophage infiltration, Tnfα, and anomalous cytokine expression compared to NMJs of control mice. In addition, Vegfa expression is decreased in muscle, suggesting that cGpr126 non-cell autonomously modulates angiogenesis after nerve injury. In sum, cGpr126 mice demonstrated delayed NMJ reinnervation and decreased muscle mass following nerve transection and repair compared to control littermates. The integral function of Gpr126 in tSCs at the NMJ provides the framework for new therapeutic targets for neuromuscular disease.     

Remodeling of motor nerve terminals in demyelinating axons of periaxin-null mice

Myelin formation around axons increases nerve conduction velocity and influences both the structure and function of the myelinated axon. In the peripheral nervous system, demyelinating forms of hereditary Charcot-Marie-Tooth (CMT) diseases cause reduced nerve conduction velocity initially and ultimately axonal degeneration. Several mouse models of CMT diseases have been generated, allowing the study of the consequences of disrupting Schwann cell function on peripheral nerve fibers. Nevertheless, the effect of demyelination at the level of the neuromuscular synapse has been largely overlooked. Here we show that in mice lacking functional Periaxin (Prx) genes, a model of a recessive type of CMT disease known as CMT4F, neuromuscular junctions (NMJs) develop profound morphological changes in the preterminal region of motor axons. These changes include extensive preterminal branches that originate in demyelinated regions of the nerve fiber and axonal swellings associated with residually-myelinated regions of the fiber. Using intracellular recording from muscle fibers we detected asynchronous failure of action potential transmission at high but not low stimulation frequencies, a phenomenon consistent with branch point failure. Taken together, our morphological and electrophysiological findings suggest that preterminal branching due to segmental demyelination near the neuromuscular synapse in Periaxin KO mice may underlie some characteristics of disabilities, including coordination deficits, present in this mouse model of CMT disease. These results reveal the importance of studying how demyelinating diseases might influence NMJ function and contribute to clinical disability.     

The regulation of synaptic strength within motor units of the frog cutaneous pectoris muscle

The physiological properties of frog neuromuscular junctions may vary widely in a single muscle. In order to understand the factors that contribute to this variation, we have studied populations of synapses belonging to individual motor units of the frog cutaneous pectoris muscle. Motor units in this muscle differ widely in twitch strength. A motor axon's synaptic contacts could be found throughout the muscle, at both singly and polyneuronally innervated endplates. Indeed, over 36% of the endplates contacted by each isolated motor axon were polyneuronally innervated. Comparisons of synapses on muscle fibers in large twitch motor units with those in small twitch motor units reveal that endplate potential amplitude, transmitter release, and muscle fiber diameter are positively correlated with the strength of the motor unit contraction. Large and small twitch motor units differ more in their transmitter release than in their nerve terminal length, indicating that larger twitch motor units have a higher release per unit length of terminal. Among motor units of roughly similar twitch tension, transmitter release at endplates receiving only one axonal input is remarkably constant, independent of postsynaptic muscle fiber input resistance, or, presumably, nerve terminal size. In cases where two different motor axons contribute to a single endplate, the synaptic strength of each input is again related to properties of the contributing motoneuron, although the individual synaptic inputs are markedly reduced in strength and size relative to singly innervated endplates. Additionally, the diameter of polyneuronally innervated muscle fibers appears related to properties of both innervating motoneurons. Thus, the pre- and postsynaptic characteristics of neuromuscular junctions may be determined both by the motoneuron and by peripheral interactions between motoneurons.     

Nerve terminal growth remodels neuromuscular synapses in mice following regeneration of the postsynaptic muscle fiber

Muscle fibers degenerate and regenerate in response to contractile damage, during aging, and in various muscle diseases that weaken the fibers. It is known that degeneration and regeneration of the segment of the postsynaptic fiber produces dramatic alterations in the neuromuscular junction (NMJ) that forms on the regenerated fiber, but the mechanisms here are incompletely understood. We have used a laser microbeam to damage the postsynaptic fibers at individual NMJs in the sternomastoid muscle of living young adult mice and then followed the synapses vitally over time using fluorescent proteins expressed in motor neurons and glial cells and staining of postsynaptic acetylcholine receptors. We find, in contrast to previous reports, that the mouse nerve terminal retains contact with the synaptic basal lamina marked by cholinesterase staining even in the absence of the target, showing that this terminal does not require a continuous supply of target-derived molecules for its maintenance. Thus, remodeling of the nerve terminal during the period of target absence does not explain the subsequent changes in the new NMJ. Rather, we see that the synapse becomes altered as the new fiber segment regenerates. Mechanisms for remodeling the synapse include failure of the regenerating muscle fiber to contact the old basal lamina and nerve terminal, growth of the nerve terminal and its glia toward the regenerating fiber, and remodeling of the initial contact as the nerve terminal becomes varicose.     

Persistence of Neuromuscular Junctions after Axotomy in Mice with Slow Wallerian Degeneration (C57BL/Wlds)

The present study was undertaken to examine the fate of neuromuscular junctions in C57BL/Wld^s mice (formerly known as OLA mice) after nerve injury. When a peripheral nerve is injured, the distal axons normally degenerate within 1-3 days. For motor axons, an early event is deterioration of motor nerve terminals at neuromuscular junctions. Previously, the vulnerability of motor terminals has been attributed either to a 'signal'originating at the site of nerve injury and transported rapidly to the terminals or to their continual requirement for essential maintenance factors synthesized in the motor neuron cell body and supplied to the terminals by fast axonal transport. Mice of the Wld^s strain have normal axoplasmic transport but show an abnormally slow rate of axon and myelin degeneration. Structure and function are retained in the axons of distal nerve stumps for several days or even weeks after nerve injury in these mice. The results of the present study show that Wld^s neuromuscular junctions are also preserved and continue to release neurotransmitter and recycle synaptic vesicle membrane for at least 3 days and in some cases up to 2 weeks after nerve injury. Varying the site of the nerve lesion delayed degeneration by -1-2 days per centimetre of distal nerve remaining. These findings suggest that the mechanisms of nerve terminal degeneration after injury are more complex than can be accounted for simply by the failure of motor neuron cell bodies to supply their terminals with essential maintenance factors. Rather, the data support the view that nerve section normally activates cellular components or processes already present, but latent, in motor nerve endings, and that in Wld^s mice either the trigger or the cellular response is abnormal.     

Pertussis toxin-sensitive G-protein and protein kinase C activity are involved in normal synapse elimination in the neonatal rat muscle

Individual skeletal muscle fibers in most new-born rodents are innervated at a single endplate by several motor axons. During the first postnatal weeks, the polyneuronal innervation decreases in a process of synaptic elimination. Previous studies showed that the naturally occurring serine-protease thrombin mediates the activity-dependent synapse reduction at the neuromuscular junction (NMJ) in vitro and that thrombin-receptor activation may modulate nerve terminal consolidation through a protein kinase mechanism. To test whether these mechanisms may be operating in vivo, we applied external thrombin and its inhibitor hirudin, and several substances affecting the G protein-protein kinase C system (GP-PKC) directly over the external surface of the neonatal rat Levator auris longus muscle. Muscles were processed for immunocytochemistry to simultaneously detect acetylcholine receptors (AChRs) and axons for counting the percentage of polyinnervated NMJ. We found that exogenous thrombin accelerated synapse loss and hirudin blocked axonal removal. Phorbol-12-myristate-13-acetate, a potent PKC activator, had a similar effect as thrombin, whereas the PKC inhibitors, calphostin C and staurosporine, prevented axonal removal. Pertussis toxin, an effective blocker of GP function, blocked synapse elimination. These findings suggest that the normal synapse elimination in the neonatal rat muscle may be modulated, at least in part, by the pertussis-sensitive G-protein and PKC activity and that thrombin could play a role in the postnatal synaptic maturation in vivo.     

Synergistic PXT3003 therapy uncouples neuromuscular function from dysmyelination in male Charcot–Marie–Tooth disease type 1A (CMT1A) rats

Charcot–Marie–Tooth disease 1 A (CMT1A) is caused by an intrachromosomal duplication of the gene encoding for PMP22 leading to peripheral nerve dysmyelination, axonal loss, and progressive muscle weakness. No therapy is available. PXT3003 is a low-dose combination of baclofen, naltrexone, and sorbitol which has been shown to improve disease symptoms in Pmp22 transgenic rats, a bona fide model of CMT1A disease. However, the superiority of PXT3003 over its single components or dual combinations have not been tested. Here, we show that in a dorsal root ganglion (DRG) co-culture system derived from transgenic rats, PXT3003 induced myelination when compared to its single and dual components. Applying a clinically relevant (“translational”) study design in adult male CMT1A rats for 3 months, PXT3003, but not its dual components, resulted in improved performance in behavioral motor and sensory endpoints when compared to placebo. Unexpectedly, we observed only a marginally increased number of myelinated axons in nerves from PXT3003-treated CMT1A rats. However, in electrophysiology, motor latencies correlated with increased grip strength indicating a possible effect of PXT3003 on neuromuscular junctions (NMJs) and muscle fiber pathology. Indeed, PXT3003-treated CMT1A rats displayed an increased perimeter of individual NMJs and a larger number of functional NMJs. Moreover, muscles of PXT3003 CMT1A rats displayed less neurogenic atrophy and a shift toward fast contracting muscle fibers. We suggest that ameliorated motor function in PXT3003-treated CMT1A rats result from restored NMJ function and muscle innervation, independent from myelination.     

Terminal Schwann cell structure is altered in diaphragm of mdx mice

The diaphragm muscle of the mdx mouse is a model system of Duchenne muscular dystrophy, since it completely lacks dystrophin and shows severe fiber necrosis and loss of specific muscle force by 4-6 weeks of age. Changes in neuromuscular junction structure also become apparent around 4 weeks including postsynaptic acetylcholine receptor declustering, loss of postsynaptic junctional folds, abnormally complex presynaptic nerve terminals, and muscle fiber denervation. Normally, terminal Schwann cells (TSCs) cap both nerve terminals and acetylcholine receptors at the neuromuscular junction, and play a crucial role in regeneration of motor axons following muscle denervation by guiding axons to grow from innervated junctions to nearby denervated junctions. However, their role in restoring innervation in dystrophic muscle is unknown. We now show that TSCs fail to cap fully the neuromuscular junction in dystrophic muscle; TSCs extend processes, but the organization of these extensions is abnormal. TSC processes of dystrophic muscle do not form bridges from denervated fibers to nearby innervated endplates, but appear to be directed away from these endplates. Adequate signaling for TSC reactivity is present, since significant muscle fiber denervation and acetylcholine receptor declustering are present. Thus, significant structural denervation is present in the diaphragm of mdx mice and the ability of TSCs to form bridges between adjacent endplates to guide reinnervation of muscle fibers is impaired, possibly attenuating the ability of dystrophic muscle to recover from denervation and ultimately leading to muscle weakness.     

Persistence of intact retinal ganglion cell terminals after axonal transport loss in the DBA/2J mouse model of glaucoma

Axonal transport defects are an early pathology occurring within the retinofugal projection of the DBA/2J mouse model of glaucoma. Retinal ganglion cell (RGC) axons and terminals are detectable after transport is affected, yet little is known about the condition of these structures. We examined the ultrastructure of the glaucomatous superior colliculus (SC) with three‐dimensional serial block‐face scanning electron microscopy to determine the distribution and morphology of retinal terminals in aged mice exhibiting varying levels of axonal transport integrity. After initial axonal transport failure, retinal terminal densities did not vary compared with either transport‐intact or control tissue. Although retinal terminals lacked overt signs of neurodegeneration, transport‐intact areas of glaucomatous SC exhibited larger retinal terminals and associated mitochondria. This likely indicates increased oxidative capacity and may be a compensatory response to the stressors that this projection is experiencing. Areas devoid of transported tracer label showed reduced mitochondrial volumes as well as decreased active zone number and surface area, suggesting that oxidative capacity and synapse strength are reduced as disease progresses but before degeneration of the synapse. Mitochondrial volume was a strong predictor of bouton size independent of pathology. These findings indicate that RGC axons retain connectivity after losing function early in the disease process, creating an important therapeutic opportunity for protection or restoration of vision in glaucoma. J. Comp. Neurol. 524:3503–3517, 2016. © 2016 Wiley Periodicals, Inc.     

Experimental strategies to promote functional recovery after peripheral nerve injuries

The capacity of Schwann cells (SCs) in the peripheral nervous system to support axonal regeneration, in contrast to the oligodendrocytes in the central nervous system, has led to the misconception that peripheral nerve regeneration always restores function. Here, we consider how prolonged periods of time that injured neurons remain without targets during axonal regeneration (chronic axotomy) and that SCs in the distal nerve stumps remain chronically denervated (chronic denervation) progressively reduce the number of motoneurons that regenerate their axons. We demonstrate the effectiveness of low-dose, brain-derived neurotrophic and glial-derived neurotrophic factors to counteract the effects of chronic axotomy in promoting axonal regeneration. High-dose brain-derived neurotrophic factor (BDNF) on the other hand, acting through the p75 receptor, inhibits axonal regeneration and may be a factor in stopping regenerating axons from forming neuromuscular connections in skeletal muscle. The immunophilin, FK506, is also effective in promoting axonal regeneration after chronic axotomy. Chronic denervation of SCs (>1 month) severely deters axonal regeneration, although the few motor axons that do regenerate to reinnervate muscles become myelinated and form enlarged motor units in the reinnervated muscles. We found that in vitro incubation of chronically denervated SCs with transforming growth factor-beta re-established their growth-supportive phenotype in vivo, consistent with the idea that the interaction between invading macrophages and denervated SCs during Wallerian degeneration is essential to sustain axonal regeneration by promoting the growth-supportive SC phenotype. Finally, we consider the effectiveness of a brief period of 20 Hz electrical stimulation in promoting the regeneration of axons across the surgical gap after nerve repair.     

Accurate reinnervation of motor end plates after disruption of sheath cells and muscle fibers

After injury, regenerating motor axons grow back to form neuromuscular junctions at the original synaptic sites on muscle fibers. The pathways they grow along consist of basement membrane, Schwann cells, and perineurium that remained after degeneration of the original axons. All the factors necessary for directing axons to the original synaptic sites persist in muscles even after disruption of myofibers. The aim of the present experiments was to determine whether structural integrity of nerve sheath cells is required for precise reinnervation in the presence and absence of muscle fiber targets. The region of innervation of the cutaneous pectoris muscle of the frog was briefly frozen to eliminate all living cells from neuromuscular junctions, intramuscular nerve bundles, and from a 1-3-mm length of the nerve trunk. Only extracellular matrices persisted within the frozen region of muscle and nerve. These consisted of the basement membrane sheaths of myofibers, of Schwann cells, and of perineurial cells and the small fragments of disrupted cells that were bound to them. In some preparations new muscle fibers developed within the basement membrane sheaths. Regenerating axons grew through the naked basement membrane sheaths of original Schwann cells, formed numerous branches, and contacted the myofibers precisely at the original synaptic sites. By 5 weeks 75% of the original synaptic sites became reinnervated; the terminals were indistinguishable from those at normal neuromuscular junctions. In contrast, preparations in which all muscle fibers were prevented from regenerating far fewer synaptic sites became reinnervated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Suppression by phenytoin of convulsant-induced afterdischarges at presynaptic nerve terminals

The mechanisms underlying the induction of afterdischarges at presynaptic nerve terminal by convulsant aminopyridines and their suppression by the anticonvulsant drug phenytoin were studied at the frog neuromusclar preparation. Addition of aminopyridine to the perfusing solution induced the appearance of afterdischarges in motor nerve fibres following their primary response to a single nerve stimulus. The afterdischarges seemed to originate at or near the nerve terminals and to propagate both antidromically and orthodromically. The latter resulted in repetitive activation of the neuromuscular synapse. Focal recordings of nerve terminal potentials suggested that aminopyridines may induce afterdischarges by slowing spike repolarization and thereby producing a prolonged depolarization of nerve terminals. Phenytoin suppressed the aminopyridine-induced afterdischarges and the resultant repetitive excitation of the postsynaptic muscle fibres. This effect of phenytoin was associated with a depression of the action potential at the motor nerve terminals but not at their parent axons. These results single the presynaptic nerve terminals as preferential sites for convulsant and anti-convulsant actions.     

MHCI promotes developmental synapse elimination and aging-related synapse loss at the vertebrate neuromuscular junction

Synapse elimination at the developing neuromuscular junction (NMJ) sculpts motor circuits, and synapse loss at the aging NMJ drives motor impairments that are a major cause of loss of independence in the elderly. Here we provide evidence that at the NMJ, both developmental synapse elimination and aging-related synapse loss are promoted by specific immune proteins, members of the major histocompatibility complex class I (MHCI). MHCI is expressed at the developing NMJ, and three different methods of reducing MHCI function all disrupt synapse elimination during the second postnatal week, leaving some muscle fibers multiply-innervated, despite otherwise outwardly normal synapse formation and maturation. Conversely, overexpressing MHCI modestly accelerates developmental synapse elimination. MHCI levels at the NMJ rise with aging, and reducing MHCI levels ameliorates muscle denervation in aged mice. These findings identify an unexpected role for MHCI in the elimination of neuromuscular synapses during development, and indicate that reducing MHCI levels can preserve youthful innervation of aging muscle.     

Ultrastructural abnormalities of muscle and neuromuscular junction differentiation in a bovine congenital neuromuscular disease

The syndrome of arthrogryposis and palatoschisis (SAP), an inherited syndrome of muscular hypotonia in Charolais cattle, was used as an experimental model to study neuromuscular differentiation. The ultrastructural development of muscle, peripheral nerve, and neuromuscular junctions was studied to determine the sequence of events preceding hypotrophic changes in the skeletal muscles of affected calves at birth. Dorsiflexion of the metatarsophalangeal joints in the hindlimbs occurred in fetuses older than 3 months of age, but hypotrophic changes in skeletal muscle, manifested as small fibers scattered among larger and occasional degenerating fibers, was not apparent until late in gestation, affecting 8-month-old fetuses and neonatal calves. Electron microscope and enzyme histochemistry studies disclosed differentiation of skeletal muscle into fiber types which is consistent with changes expected from disuse and does not indicate a primary myopathic abnormality. Abnormal differentiation of neuromuscular junctions (NMJ), composed of several separated axonal endings terminating in shallow synaptic gutters, indicated impaired maturation of the synapse. The earliest indication of abnormal NMJ was observed in a 5-month-old SAP fetus. The clinical signs and pathologic changes found in the neuromuscular junction and skeletal muscle of SAP fetuses are consistent with an embryologic defect occurring during development of the central nervous system (CNS) that affects the integrated function of the motor neurons to the limbs. However, diversification of myofibers by histochemistry and ultrastructural parameters is evidence that the intrinsic physiologic properties of spinal motor neurons were retained.     

The effect of nerve crush and botulinum toxin on lead uptake in motor axons

Summary After lead (Pb) is injected into striated muscle it binds to the sarcolemma of the neuromuscular junction (NMJ) and crosses into the terminal axons of motor neurons. To find out whether this intra-axonal accumulation of Pb is due to active transport or to diffusion down a concentration gradient, Pb uptake into motor axons of mice was studied at active and inactive NMJs. Twenty-four hours after sciatic nerve crush, 0.1 ml of 5% lead nitrate was injected into the tibialis anterior muscle and 30 min later the location of Pb was sought with electron microscopy and X-ray elemental analysis. A greatly reduced amount of Pb entered the axons after nerve crush compared to non-nerve crush animals, indicating that an active NMJ is required for intra-axonal Pb accumulation. To test if Pb could be entering the axon via recycling vesicles, botulinum toxin (BoTx) was injected into the muscle 24 h before Pb injection. There was no difference in intra-axonal Pb uptake in control and BoTx-injected animals, indicating that Pb is unlikely to use recycled vesicles to enter the axon.Supported by a grant from the ALS-Motor Neuron Disease Research Institute, New South Wales     

Morphological and electrophysiological study of distal motor nerve fiber degeneration and sprouting after irreversible cholinesterase inhibition

A single subcutaneous injection of a sublethal dose of the irreversible organophosphate sarin (0.08 mg/kg) in rats induced a non-Wallerian-type axonal degeneration of the neuromuscular synapse in the slow twitch, soleus muscle. These alterations of the endplate region were more obvious in the soleus than in the fast extensor digitorum longus muscle and were slowly reversible, complete recovery requiring about 10 days. Silver-cholinesterase staining and electrophysiological techniques were used to define the spatiotemporal evolution of prejunctional abnormalities. The non-Wallerian-type axonal degeneration of the neuromuscular synapse was characterized by bead or balloon-like varicosities of the focal, distal, and terminal nerve fibers and a retraction of terminal axons. Axonal degeneration was accompanied by junctional and extrajunctional membrane depolarization and was followed by nerve sprouting at focal, distal, and terminal nerve fibers. Transients similar to miniature endplate potentials were recorded along the muscle fiber at distances of 800-2500 microns away from the parent endplate. New ectopic endings, originating from the same endplate, were discovered adjacent to the terminal axon and also distant from the parent endplate. Very elaborate terminal arborization and occasional multibranching arose from a progressive growth sprout. The new sprouting may have served to compensate for the loss of synaptic contact caused by sarin. Thus the present study demonstrates a direct cytotoxic effect of sarin and indicates that this organophosphate agent may be an important neurotoxicological tool to understand the mechanisms involved in nerve sprouting.     

Nonmuscle myosins IIA and IIB are present in adult motor nerve terminals

Throughout life, neuromuscular junctions undergo dynamic changes, remodelling occurring through extension and withdrawal of motor nerve terminals in conjunction with changes in the distribution of acetylcholine receptors at the muscle endplate. However, relatively little is known about the fundamental processes by which nerve terminals are remodelled. These dynamic processes are likely to be driven by molecular motors. Previously, we have implicated myosins IIA and IIB as opposing motors influencing neuronal growth cone dynamics. Using confocal microscopy of neuromuscular junction preparations colabelled for myosin II isoforms and nerve terminal or muscle endplate markers, we demonstrate that both myosin IIA and myosin IIB are localized in nerve terminals. We propose roles for these motor proteins in junctional stabilization and destabilization.     

Morphometric analysis of neuromuscular topography in the serratus anterior muscle

Groups of neurons form ordered topographic maps on their targets, and defining the mechanisms that develop such maps, and re-connect them after disruption, has biological as well as clinical importance. The neuromuscular system is an accessible and well-studied model for defining the principles that guide map formation, both during its development and its reformation after motor nerve damage. We present evidence for the expression of this map at the level of nerve terminal morphology and muscle fiber type in the serratus anterior muscle. Morphometric analyses indicate, first, a rostrocaudal difference in nerve terminal size depending on the ventral root of origin of the axons. Second, motor endplates are larger on type IIB than type IIA muscle fibers. Third, whereas IIB muscle fibers are distributed rather evenly along the rostrocaudal axis of the muscle, the more rostral type IIB fibers are preferentially innervated by anteriorly derived (C6) motor neurons, and more caudal IIB fibers are preferentially innervated by posteriorly derived (C7) motor neurons. This inference is supported by analysis of the size of nerve terminals formed in each muscle sector by rostral and caudal roots, and by evidence that the larger terminals are on IIB fibers. These results demonstrate a subcellular expression of neuromuscular topography in the serratus anterior muscle (SA) muscle in the form of differences in nerve terminal size. These results provide deeper insights into the organization of a neuromuscular system. They also offer a rationale for a topographic map, that is, to allow spinal motor centers to activate selectively different compartments within a muscle.     

Glial modulation of synaptic transmission at the neuromuscular junction

The neuromuscular junction (NMJ) is a cholinergic synapse that controls muscle contraction. Glial cells, called perisynaptic Schwann cells, surround nerve terminals at the NMJ. Transmitter release induced by repetitive nerve stimulation, elicit a frequency-dependent activation of G-protein-coupled receptors on perisynaptic Schwann cells and the release of calcium from internal stores. In return, perisynaptic Schwann cells modulate synaptic activity during and following high-frequency stimulation through short-term plasticity. In the present review, we discuss evidence of glial involvement in the short-term plasticity at the NMJ and the potential impact of such modulation on synaptic efficacy.     

Axon and Schwann cell partnership during nerve regrowth

Regeneration of peripheral nerve involves an essential contribution by Schwann cells (SCs) in collaboration with regrowing axons. We examined such collaboration between new axons and Schwann cells destined to reform peripheral nerve trucks in a regeneration chamber bridging transected rat sciatic nerves. There was a highly intimate "dance" between axons that followed outgrowing and proliferating SCs. Axons without SCs only grew short distances and almost all axon processes had associated SC processes. When regeneration chambers were infused through an external access port with local mitomycin, a mitosis inhibitor, SC proliferation, migration and subsequent axon regrowth were dramatically reduced. Adding laminin to mitomycin did not reverse this regenerative lag and indicated that SCs provide more than laminin synthesis alone. Laminin infused alone supplemented endogenous laminin and facilitated first SC then axon regrowth. "Wrong way" misdirected axons were associated with misdirected SC processes and were more numerous in bridges exposed to mitomycin, but were fewer in laminin supplemented bridges. Later, by 21 days, there was myelinated axon repopulation of regenerative bridges but those exposed to mitomycin alone at early time points had substantial impairments in axon investment. Reforming peripheral nerve trucks involves a very close and intimate relationship between axons and SCs that must proliferate and migrate, facilitated by laminin.