Reciprocal interactions between perisynaptic Schwann cells and regenerating nerve terminals at the frog neuromuscular junction

Purinergic Tuning of the Tripartite Neuromuscular Synapse

The vertebrate neuromuscular junction (NMJ) is a specialised chemical synapse involved in the transmission of bioelectric signals between a motor neuron and a skeletal muscle fiber, leading to muscle contraction. Typically, the NMJ is a tripartite synapse comprising (a) a presynaptic region represented by the motor nerve ending, (b) a postsynaptic skeletal motor endplate area, and (c) perisynaptic Schwann cells (PSCs) that shield the motor nerve terminal. Increasing evidence points towards the role of PSCs in the maintenance and control of neuromuscular integrity, transmission, and plasticity. Acetylcholine (ACh) is the main neurotransmitter at the vertebrate skeletal NMJ, and its role is fine-tuned by co-released purinergic neuromodulators, like adenosine 5'-triphosphate (ATP) and its metabolite adenosine (ADO). Adenine nucleotides modulate transmitter release and expression of postsynaptic ACh receptors at motor synapses via the activation of P2Y and P2X receptors. Endogenously generated ADO modulates ACh release by acting via co-localised inhibitory A₁ and facilitatory A_2A receptors on motor nerve terminals, whose tonic activation depends on the neuronal firing pattern and their interplay with cholinergic receptors and neuropeptides. Thus, the concerted action of adenine nucleotides, ADO, and ACh/neuropeptide co-transmitters is paramount to adapting the neuromuscular transmission to the working load under pathological conditions, like Myasthenia gravis. Unravelling these functional complexities prompted us to review our knowledge about the way purines orchestrate neuromuscular transmission and plasticity in light of the tripartite synapse concept, emphasising the often-forgotten role of PSCs in this context.

Macrophage-Derived Vascular Endothelial Growth Factor-A Is Integral to Neuromuscular Junction Reinnervation after Nerve Injury

Functional recovery in the end target muscle is a determinant of outcome after peripheral nerve injury. The neuromuscular junction (NMJ) provides the interface between nerve and muscle and includes non-myelinating terminal Schwann cells (tSCs). After nerve injury, tSCs extend cytoplasmic processes between NMJs to guide axon growth and NMJ reinnervation. The mechanisms related to NMJ reinnervation are not known. We used multiple mouse models to investigate the mechanisms of NMJ reinnervation in both sexes, specifically whether macrophage-derived vascular endothelial growth factor-A (Vegf-A) is crucial to establishing NMJ reinnervation at the end target muscle. Both macrophage number and Vegf-A expression increased in end target muscles after nerve injury and repair. In mice with impaired recruitment of macrophages and monocytes (Ccr2-/- mice), the absence of CD68+ cells (macrophages) in the muscle resulted in diminished muscle function. Using a Vegf-receptor 2 (VegfR2) inhibitor (cabozantinib; CBZ) via oral gavage in wild-type (WT) mice resulted in reduced tSC cytoplasmic process extension and decreased NMJ reinnervation compared with saline controls. Mice with Vegf-A conditionally knocked out in macrophages (Vegf-Afl/fl; LysMCre mice) demonstrated a more prolonged detrimental effect on NMJ reinnervation and worse functional muscle recovery. Together, these results show that contributions of the immune system are integral for NMJ reinnervation and functional muscle recovery after nerve injury.SIGNIFICANCE STATEMENT This work demonstrates beneficial contributions of a macrophage-mediated response for neuromuscular junction (NMJ) reinnervation following nerve injury and repair. Macrophage recruitment occurred at the NMJ, distant from the nerve injury site, to support functional recovery at the muscle. We have shown hindered terminal Schwann cell (tSC) injury response and NMJ recovery with inhibition of: (1) macrophage recruitment after injury; (2) vascular endothelial growth factor receptor 2 (VegfR2) signaling; and (3) Vegf secretion from macrophages. We conclude that macrophage-derived Vegf is a key component of NMJ recovery after injury. Determining the mechanisms active at the end target muscle after motor nerve injury reveals new therapeutic targets that may translate to improve motor recovery following nerve injury.

Specific labeling of synaptic schwann cells reveals unique cellular and molecular features

Perisynaptic Schwann cells (PSCs) are specialized, non-myelinating, synaptic glia of the neuromuscular junction (NMJ), that participate in synapse development, function, maintenance, and repair. The study of PSCs has relied on an anatomy-based approach, as the identities of cell-specific PSC molecular markers have remained elusive. This limited approach has precluded our ability to isolate and genetically manipulate PSCs in a cell specific manner. We have identified neuron-glia antigen 2 (NG2) as a unique molecular marker of S100β+ PSCs in skeletal muscle. NG2 is expressed in Schwann cells already associated with the NMJ, indicating that it is a marker of differentiated PSCs. Using a newly generated transgenic mouse in which PSCs are specifically labeled, we show that PSCs have a unique molecular signature that includes genes known to play critical roles in PSCs and synapses. These findings will serve as a springboard for revealing drivers of PSC differentiation and function.

The neuromuscular junction of Xenopus tadpoles: Revisiting a classical model of early synaptogenesis and regeneration

The frog neuromuscular junction (NMJ) has been extensively used as a model system to dissect the mechanisms involved in synapse formation, maturation, maintenance, regeneration, and function. Early NMJ synaptogenesis relies on a combination of cell-autonomous and interdependent pre/postsynaptic communication processes. Due to their transparency, comparatively easy manipulation, and remarkable regenerative abilities, frog tadpoles constitute an excellent model to study NMJ formation and regeneration. Here, we aimed to contribute new aspects on the characterization of the ontogeny of NMJ formation in Xenopus embryos and to explore the morphological changes occurring at the NMJ after spinal cord injury. Following analyses of X. tropicalis tadpoles during development we found that the early pathfinding of rostral motor axons is likely helped by previously formed postsynaptic specializations, whereas NMJ formation in recently differentiated ventral muscles in caudal segments seems to rely on presynaptic inputs. After spinal cord injury of X. laevis tadpoles our results suggest that rostral motor axon projections help caudal NMJ re-innervation before spinal cord connectivity is repaired.

Ephrin-A3 promotes and maintains slow muscle fiber identity during postnatal development and reinnervation

Each adult mammalian skeletal muscle has a unique complement of fast and slow myofibers, reflecting patterns established during development and reinforced via their innervation by fast and slow motor neurons. Existing data support a model of postnatal "matching" whereby predetermined myofiber type identity promotes pruning of inappropriate motor axons, but no molecular mechanism has yet been identified. We present evidence that fiber type-specific repulsive interactions inhibit innervation of slow myofibers by fast motor axons during both postnatal maturation of the neuromuscular junction and myofiber reinnervation after injury. The repulsive guidance ligand ephrin-A3 is expressed only on slow myofibers, whereas its candidate receptor, EphA8, localizes exclusively to fast motor endplates. Adult mice lacking ephrin-A3 have dramatically fewer slow myofibers in fast and mixed muscles, and misexpression of ephrin-A3 on fast myofibers followed by denervation/reinnervation promotes their respecification to a slow phenotype. We therefore conclude that Eph/ephrin interactions guide the fiber type specificity of neuromuscular interactions during development and adult life.

Perisynaptic Schwann Cells at the Neuromuscular Synapse: Adaptable, Multitasking Glial Cells

The neuromuscular junction (NMJ) is engineered to be a highly reliable synapse to carry the control of the motor commands of the nervous system over the muscles. Its development, organization, and synaptic properties are highly structured and regulated to support such reliability and efficacy. Yet, the NMJ is also highly plastic, able to react to injury and adapt to changes. This balance between structural stability and synaptic efficacy on one hand and structural plasticity and repair on another hand is made possible by the intricate regulation of perisynaptic Schwann cells, glial cells at this synapse. They regulate both the efficacy and structural plasticity of the NMJ in a dynamic, bidirectional manner owing to their ability to decode synaptic transmission and by their interactions via trophic-related factors.

Brain-derived neurotrophic factor inhibits neuromuscular junction maturation in a cAMP-PKA-dependent way

The development of neuromuscular junction (NMJ) is initiated by motor axon's contact with the skeletal muscle cell that is followed by synaptic maturation. Previous studies showed that brain-derived neurotrophic factor (BDNF) enhanced motoneurons' survival and growth but significantly inhibited synaptogenesis. Here, we report that chronic application of BDNF resulted in inhibition in the maturation process both physiologically and morphologically. The response to BDNF was mediated by its cognate receptor TrkB as the effects were abolished by Trk receptor inhibitor K252a. Protein kinase A (PKA) inhibitor reversed the effects of BDNF in inhibiting NMJ maturation. These results indicate that BDNF suppresses NMJ maturation through cAMP-PKA signaling pathway. Together with the previous studies, these results suggest that BDNF suppresses NMJ formation and maturation despite its effects in enhancing neuronal survival and growth.

Terminal Schwann cells participate in neuromuscular synapse remodeling during reinnervation following nerve injury

Schwann cells (SCs) at neuromuscular junctions (NMJs) play active roles in synaptic homeostasis and repair. We have studied how SCs contribute to reinnervation of NMJs using vital imaging of mice whose motor axons and SCs are transgenically labeled with different colors of fluorescent proteins. Motor axons most commonly regenerate to the original synaptic site by following SC-filled endoneurial tubes. During the period of denervation, SCs at the NMJ extend elaborate processes from the junction, as shown previously, but they also retract some processes from territory they previously occupied within the endplate. The degree of this retraction depends on the length of the period of denervation. We show that the topology of the remaining SC processes influences the branching pattern of regenerating axon terminals and the redistribution of acetylcholine receptors (AChRs). Upon arriving at the junction, regenerating axons follow existing SC processes within the old synaptic site. Some of the AChR loss that follows denervation is correlated with failure of portions of the old synaptic site that lack SC coverage to be reinnervated. New AChR clustering is also induced by axon terminals that follow SC processes extended during denervation. These observations show that SCs participate actively in the remodeling of neuromuscular synapses following nerve injury by their guidance of axonal reinnervation.

Axonal regeneration after sciatic nerve lesion is delayed but complete in GFAP- and vimentin-deficient mice

Peripheral axotomy of motoneurons triggers Wallerian degeneration of injured axons distal to the lesion, followed by axon regeneration. Centrally, axotomy induces loss of synapses (synaptic stripping) from the surface of lesioned motoneurons in the spinal cord. At the lesion site, reactive Schwann cells provide trophic support and guidance for outgrowing axons. The mechanisms of synaptic stripping remain elusive, but reactive astrocytes and microglia appear to be important in this process. We studied axonal regeneration and synaptic stripping of motoneurons after a sciatic nerve lesion in mice lacking the intermediate filament (nanofilament) proteins glial fibrillary acidic protein (GFAP) and vimentin, which are upregulated in reactive astrocytes and Schwann cells. Seven days after sciatic nerve transection, ultrastructural analysis of synaptic density on the somata of injured motoneurons revealed more remaining boutons covering injured somata in GFAP(-/-)Vim(-/-) mice. After sciatic nerve crush in GFAP(-/-)Vim(-/-) mice, the fraction of reinnervated motor endplates on muscle fibers of the gastrocnemius muscle was reduced 13 days after the injury, and axonal regeneration and functional recovery were delayed but complete. Thus, the absence of GFAP and vimentin in glial cells does not seem to affect the outcome after peripheral motoneuron injury but may have an important effect on the response dynamics.

Age affects reciprocal cellular interactions in neuromuscular synapses following peripheral nerve injury

Studies of the influence of age on regeneration and reinnervation in the peripheral nervous system (PNS) and neuromuscular junction (NMJ) are reviewed, with a particular focus on aged and denervated skeletal muscles. The morphological and functional features of incomplete regeneration and reinnervation are compared between adult and aged animals. In addition, some possible mechanisms of the age-related defects will be discussed. Increased fragmentation or damage in individual components of the NMJ (terminal Schwann cells (TSCs), axon terminals and acetylcholine receptor sites occurs during muscle reinnervation following PNS injury in the aged animals. The capacity to produce ultraterminal sprouting or multiple innervation secondary to PNS injury is maintained, but not the capacity to eliminate such anomalous axonal profiles. The frequency and accuracy of reoccupation of the synaptic sites by TSCs and axon terminals are impaired. Thus, despite the capability of extending neural processes, the rate at which regenerating nerve fibers grow, mature and precisely appose the postsynaptic muscle fiber is impaired, resulting in the failure of re-establishment of the normal single motor innervation in the NMJ. A complex set of cellular interactions in the NMJ are known to participate in the neurotrophism and neurotrophism to support growth of the regenerating and sprouting axons and their pathfinding to direct the target muscle fiber. Besides the capability of α-motoneurons, signaling originating from the TSCs and muscle may be impaired during aging.

Distinct muscarinic acetylcholine receptor subtypes contribute to stability and growth, but not compensatory plasticity, of neuromuscular synapses

Muscarinic acetylcholine receptors (mAChRs) modulate synaptic function, but whether they influence synaptic structure remains unknown. At neuromuscular junctions (NMJs), mAChRs have been implicated in compensatory sprouting of axon terminals in paralyzed or denervated muscles. Here we used pharmacological and genetic inhibition and localization studies of mAChR subtypes at mouse NMJs to demonstrate their roles in synaptic stability and growth but not in compensatory sprouting. M(2) mAChRs were present solely in motor neurons, whereas M(1), M(3), and M(5) mAChRs were associated with Schwann cells and/or muscle fibers. Blockade of all five mAChR subtypes with atropine evoked pronounced effects, including terminal sprouting, terminal withdrawal, and muscle fiber atrophy. In contrast, methoctramine, an M(2/4)-preferring antagonist, induced terminal sprouting and terminal withdrawal, but no muscle fiber atrophy. Consistent with this observation, M(2)(-/-) but no other mAChR mutant mice exhibited spontaneous sprouting accompanied by extensive loss of parental terminal arbors. Terminal sprouting, however, seemed not to be the causative defect because partial loss of terminal branches was common even in the M(2)(-/-) NMJs without sprouting. Moreover, compensatory sprouting after paralysis or partial denervation was normal in mice deficient in M(2) or other mAChR subtypes. We also found that many NMJs of M(5)(-/-) mice were exceptionally small and reduced in proportion to the size of parental muscle fibers. These findings show that axon terminals are unstable without M(2) and that muscle fiber growth is defective without M(5). Subtype-specific muscarinic signaling provides a novel means for coordinating activity-dependent development and maintenance of the tripartite synapse.

Presynaptic secretion of mind-the-gap organizes the synaptic extracellular matrix-integrin interface and postsynaptic environments

Mind-the-Gap (MTG) is required during synaptogenesis of the Drosophila glutamatergic neuromuscular junction (NMJ) to organize the postsynaptic domain. Here, we generate MTG::GFP transgenic animals to demonstrate MTG is synaptically targeted, secreted, and localized to punctate domains in the synaptic extracellular matrix (ECM). Drosophila NMJs form specialized ECM carbohydrate domains, with carbohydrate moieties and integrin ECM receptors occupying overlapping territories. Presynaptically secreted MTG recruits and reorganizes secreted carbohydrates, and acts to recruit synaptic integrins and ECM glycans. Transgenic MTG::GFP expression rescues hatching, movement, and synaptogenic defects in embryonic-lethal mtg null mutants. Targeted neuronal MTG expression rescues mutant synaptogenesis defects, and increases rescue of adult viability, supporting an essential neuronal function. These results indicate that presynaptically secreted MTG regulates the ECM-integrin interface, and drives an inductive mechanism for the functional differentiation of the postsynaptic domain of glutamatergic synapses. We suggest that MTG pioneers a novel protein family involved in ECM-dependent synaptic differentiation.

Schwann cell-derived factors modulate synaptic activities at developing neuromuscular synapses

Glial cells are active participants in the function, formation, and maintenance of the chemical synapse. To investigate the molecular basis of neuron-glia interactions at the peripheral synapse, we examined whether and how Schwann cell-derived factors modulate synaptic function at developing neuromuscular junctions (NMJs). Schwann cell-conditioned medium (SC-CM) from Xenopus Schwann cell cultures was collected and applied to Xenopus nerve-muscle cocultures. We found that SC-CM increased the frequency of spontaneous synaptic currents (SSCs) within 3-15 min by an average of approximately 150-fold at developing neuromuscular synapses. The increase in SSC frequency by SC-CM is a presynaptic effect independent of neuronal excitability and requires the influx of Ca2+. In contrast to its potentiating effect on spontaneous transmitter release, SC-CM suppressed the evoked transmitter release. The SC-CM effect required the presence of motoneuron soma but not protein synthesis. Using molecular weight cutoff filters and dialysis membranes, we found that the molecular weight of functional factor(s) in SC-CM was within 500 and 5000 Da. The SC-CM effect was not attributable to currently known factors that modulate synaptic efficacy, including neurotrophins, glutamate, and ATP. SC-CM also enhanced spontaneous synaptic release at developing NMJs in Xenopus tadpoles in situ. Our results suggest that Schwann cells release small molecules that enhance spontaneous synaptic activities acutely and potently at developing neuromuscular synapses, and the glial cell-enhanced spontaneous neurotransmission may contribute to synaptogenesis.

Ciliary neurotrophic factor is not required for terminal sprouting and compensatory reinnervation of neuromuscular synapses: re-evaluation of CNTF null mice

Loss of synaptic activity or innervation induces sprouting of intact motor nerve terminals that adds or restores nerve-muscle connectivity. Ciliary neurotrophic factor (CNTF) and terminal Schwann cells (tSCs) have been implicated as molecular and cellular mediators of the compensatory process. We wondered if the previously reported lack of terminal sprouting in CNTF null mice was due to abnormal reactivity of tSCs. To this end, we examined nerve terminal and tSC responses in CNTF null mice using experimental systems that elicited extensive sprouting in wildtype mice. Contrary to the previous report, we found that motor nerve terminals in the null mice sprout extensively in response to major sprouting-stimuli such as exogenously applied CNTF per se, botulinum toxin-elicited paralysis, and partial denervation by L4 spinal root transection. In addition, the number, length and growth patterns of terminal sprouts, and the extent of reinnervation by terminal or nodal sprouts, were similar in wildtype and null mice. tSCs in the null mice were also reactive to the sprouting-stimuli, elaborating cellular processes that accompanied terminal sprouts or guided reinnervation of denervated muscle fibers. Lastly, CNTF was absent in quiescent tSCs in intact, wildtype muscles and little if any was detected in reactive tSCs in denervated muscles. Thus, CNTF is not required for induction of nerve terminal sprouting, for reactivation of tSCs, and for compensatory reinnervation after nerve injury. We interpret these results to support the notion that compensatory sprouting in adult muscles is induced primarily by contact-mediated mechanisms, rather than by diffusible factors.

Activation of matrix metalloproteinase-3 is altered at the frog neuromuscular junction following changes in synaptic activity

The extracellular matrix surrounding the neuromuscular junction is a highly specialized and dynamic structure. Matrix Metalloproteinases are enzymes that sculpt the extracellular matrix. Since synaptic activity is critical to the structure and function of this synapse, we investigated whether changes in synaptic activity levels could alter the activity of Matrix Metalloproteinases at the neuromuscular junction. In particular, we focused on Matrix Metalloproteinase 3 (MMP3), since antibodies to MMP3 recognize molecules at the frog neuromuscular junction, and MMP3 cleaves a number of synaptic basal lamina molecules, including agrin. Here we show that the fluorogenic compound (M2300) can be used to perform in vivo proteolytic imaging of the frog neuromuscular junction to directly measure the activity state of MMP3. Application of this compound reveals that active MMP3 is concentrated at the normal frog neuromuscular junction, and is tightly associated with the terminal Schwann cell. Blocking presynaptic activity via denervation, or TTX nerve blockade, results in a decreased level of active MMP3 at the neuromuscular junction. The loss of active MMP3 at the neuromuscular junction in denervated muscles can result from decreased activation of pro-MMP3, or it could result from increased inhibition of MMP3. These results support the hypothesis that changes in synaptic activity can alter the level of active MMP3 at the neuromuscular junction.

Synergistic terminal motor end-to-side nerve graft repair: investigation in a non-human primate model

End-to-side nerve repair has re-emerged in the literature in recent years but clinical applications for this technique are not yet fully defined and clinical reports are rare and controversial. Hypothetically, there might be useful functional results performing peripheral end-to-side nerve graft repair using synergistic terminal branches with defined motor function. An end-to-side nerve graft repair bridging from the terminal motor branch of deep branch of the ulnar nerve to the thenar motor branch of the median nerve was performed in non-human primates. The results in this non-human primate model demonstrate the efficacy of end-to-side nerve graft repair at the level of peripheral terminal motor branches. End-to-side neurorrhaphy may present a viable alternative in conditions of unsuitable end-to-end coaptation and inappropriate nerve grafting procedures.

Synapse-glia interactions at the vertebrate neuromuscular junction

Glial cells are widely distributed throughout the nervous system, including at the chemical synapse. However, our knowledge of the role of glial cells at the synapse is rudimentary. Recent studies using a model synapse, the vertebrate neuromuscular junction (NMJ), have demonstrated that perisynaptic Schwann cells (PSCs), which are the glia juxtaposed to the nerve terminal at the NMJ, play active and essential roles in synaptic function, maintenance, and development. PSCs can respond to nerve activity by increasing intracellular calcium and are capable of modulating synaptic function in response to pharmacological manipulations. Studies using PSC ablation in vivo have shown that PSCs are essential for the long-term maintenance of synaptic structure and function at the adult NMJ. In vivo observations have also shown that PSCs guide presynaptic nerve terminal extension and dictate the pattern of innervation during synaptic regeneration and remodeling at adult NMJs. PSCs may also induce postsynaptic acetylcholine receptor aggregation. Furthermore, PSCs play an essential role in synaptic growth and maintenance during development of NMJs in vivo, and Schwann cell-derived factors can promote synaptogenesis and enhance synaptic transmission in tissue culture. These recent findings advance the emerging concept that glial cells help make bigger, stronger, and more stable synapses.

Fluorescent proteins expressed in mouse transgenic lines mark subsets of glia, neurons, macrophages, and dendritic cells for vital examination

To enable vital observation of glia at the neuromuscular junction, transgenic mice were generated that express proteins of the green fluorescent protein family under control of transcriptional regulatory sequences of the human S100B gene. Terminal Schwann cells were imaged repetitively in living animals of one of the transgenic lines to show that, except for extension and retraction of short processes, the glial coverings of the adult neuromuscular synapse are stable. In other lines, subsets of Schwann cells were labeled. The distribution of label suggests that Schwann cells at individual synapses are clonally related, a finding with implications for how these cells might be sorted during postnatal development. Other labeling patterns, some present in unique lines, included astrocytes, microglia, and subsets of cerebellar Bergmann glia, spinal motor neurons, macrophages, and dendritic cells. We show that lines with labeled macrophages can be used to follow the accumulation of these cells at sites of injury.

Glial cells maintain synaptic structure and function and promote development of the neuromuscular junction in vivo

To investigate the in vivo role of glial cells in synaptic function, maintenance, and development, we have developed an approach to selectively ablate perisynaptic Schwann cells (PSCs), the glial cells at the neuromuscular junction (NMJ), en masse from live frog muscles. In adults, following acute PSC ablation, synaptic structure and function were not altered. However, 1 week after PSC ablation, presynaptic function decreased by approximately half, while postsynaptic function was unchanged. Retraction of nerve terminals increased over 10-fold at PSC-ablated NMJs. Furthermore, nerve-evoked muscle twitch tension was reduced. In tadpoles, repeated in vivo observations revealed that PSC processes lead nerve terminal growth. In the absence of PSCs, growth and addition of synapses was dramatically reduced, and existing synapses underwent widespread retraction. Our findings provide in vivo evidence that glial cells maintain presynaptic structure and function at adult synapses and are vital for the growth and stability of developing synapses.

Glial Cells and Neurotransmission

Glial cells throughout the nervous system are closely associated with synapses. Accompanying these anatomical couplings are intriguing functional interactions, including the capacity of certain glial cells to respond to and modulate neurotransmission. Glial cells can also help establish, maintain, and reconstitute synapses. In this review, we discuss evidence indicating that glial cells make important contributions to synaptic function.

Differential effects of neurotrophins and schwann cell-derived signals on neuronal survival/growth and synaptogenesis

Recent studies have shown that the survival of mammalian motoneurons in vitro is promoted by neurotrophins (NTs) and cAMP. There is also evidence that neurotrophins enhance transmitter release. We thus investigated whether these agents also promote synaptogenesis. Cultured Xenopus spinal cord neurons were treated with a mixture of BDNF, glia-derived neurotrophic factor, NT-3, and NT-4, in addition to forskolin and IBMX or the cell-permeant form of cAMP, to elevate the cAMP level. The outgrowth and survival of neurons were dramatically increased by this trophic stimulation. However, when these neurons were cocultured with muscle cells, the trophic agents resulted in a failure of synaptogenesis. Specifically, the induction of ACh receptor (AChR) clustering in cultured muscle cells was inhibited at nerve-muscle contacts, in sharp contrast to control, untreated cocultures. Because AChR clustering induced by agrin or growth factor-coated beads in muscle cells was unaffected by trophic stimulation, its effect on synaptogenesis is presynaptic in origin. In the control, agrin was deposited along the neurite and at nerve-muscle contacts. This was significantly downregulated in cultures treated with trophic stimuli. Reverse transcriptase-PCR analyses showed that this decrease in agrin deposition was caused by an inhibition of agrin synthesis by trophic stimuli. Both agrin synthesis and induction of AChR clustering were restored under trophic stimulation when Schwann cell-conditioned medium was introduced. These results suggest that trophic stimulation maintains spinal neurons in the growth state, and Schwann cell-derived factors allow them to switch to the synaptogenic state.

Perisynaptic Schwann cells at the neuromuscular junction: nerve- and activity-dependent contributions to synaptic efficacy, plasticity, and reinnervation

Glial cells are increasingly recognized for their important contributions to CNS and PNS synaptic function. Perisynaptic Schwann cells, which are glial cells at the neuromuscular junction, have proven to be an exceptionally useful model for studying these roles. Recent studies have shown that they detect and reciprocally modulate synaptic efficacy in an activity-dependent manner in the short term. In addition, perisynaptic Schwann cells guide reinnervating nerve sprouts after deinnervation, and many important parameters of this are dependent on synapse activity. Thus, it is hypothesized that perisynaptic Schwann cells are key integrators in a continuum of synaptic efficacy, stability, and plasticity at the neuromuscular junction, which is important for maintaining and restoring synaptic efficacy.

Activity alters muscle reinnervation and terminal sprouting by reducing the number of Schwann cell pathways that grow to link synaptic sites

In partially denervated rodent muscle, terminal Schwann cells (TSCs) located at denervated end plates grow processes, some of which contact neighboring innervated end plates. Those processes that contact neighboring synapses (termed "bridges") appear to initiate nerve terminal sprouting and to guide the growth of the sprouts so that they reach and reinnervate denervated end plates. Studies conducted prior to knowledge of this potential involvement of Schwann cells showed that direct muscle stimulation inhibits terminal sprouting following partial denervation (Brown and Holland, 1979). We have investigated the possibility this inhibition results from an alteration in the growth of TSC processes. We find that stimulation of partially denervated rat soleus muscle does not alter the length or number of TSC processes but does reduce the number of TSC bridges. Stimulation also reduces the number of TSC bridges that form between end plates during reinnervation of a completely denervated muscle. The nerve processes ("escaped fibers") that normally grow onto TSC processes during reinnervation are also reduced in length. Therefore, stimulation alters at least two responses to denervation in muscles: (1) the ability of TSC processes to form or maintain bridges with innervated synaptic sites, and (2) the growth of axons along processes extended by TSCs.

Schwann cells express active agrin and enhance aggregation of acetylcholine receptors on muscle fibers

To explore novel roles of glial cells in synaptic function and formation, we examined the expression of agrin in frog Schwann cells and tested their role in the aggregation of acetylcholine receptors (AChRs). Using reverse transcription-PCR, we found that Schwann cells along nerve fibers in tadpoles expressed only the inactive agrin isoform B0 but began to also express active agrin isoforms B11 and B19 at approximately metamorphosis. During nerve regeneration in the adult, the expression of these active agrin isoforms in Schwann cells was upregulated, including the appearance of the most potent isoform, B8. This upregulation was induced by regenerating axons but not by nerve injury per se. In muscle cultures, the presence of adult Schwann cells enhanced the number and the total area of AChR aggregates 2.2- and 4.5-fold, respectively, and this enhancement was eliminated by heparin treatment. Furthermore, adult Schwann cells in culture expressed active agrin isoforms and produced agrin protein. Using a novel technique to selectively ablate perisynaptic Schwann cells (PSCs) at the neuromuscular junction, we found that PSCs also expressed active agrin isoforms B11 and B19, and these active isoforms were upregulated, including the appearance of B8, during reinnervation. Observation in vivo showed that extrajunctional AChR aggregates were associated with PSC sprouts after nerve injury and subsequent reinnervation. These results suggest that, contrary to the prevailing view that only neurons express active agrin, glial cells also express active agrin and play a role in the aggregation of AChRs both in vitro and in vivo.

Synapse-glia interactions at the mammalian neuromuscular junction

Perisynaptic Schwann cells (PSCs) play critical roles in regulating and stabilizing nerve terminals at the mammalian neuromuscular junction (NMJ). However, although these functions are likely regulated by the synaptic properties, the interactions of PSCs with the synaptic elements are not known. Therefore, our goal was to study the interactions between mammalian PSCs in situ and the presynaptic terminals using changes in intracellular Ca(2+) as an indicator of cell activity. Motor nerve stimulation induced an increase in intracellular Ca(2+) in PSCs, and this increase was greatly reduced when transmitter release was blocked. Furthermore, local application of acetylcholine induced Ca(2+) responses that were blocked by the muscarinic antagonist atropine and mimicked by the muscarinic agonist muscarine. The nicotinic antagonist alpha-bungarotoxin had no effect on Ca(2+) responses induced by acetylcholine. Local application of the cotransmitter ATP induced Ca(2+) responses that were unaffected by the P2 antagonist suramin, whereas local application of adenosine induced Ca(2+) responses that were greatly reduced by the A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT). However, the presence of the A1 antagonist in the perfusate did not block responses induced by ATP. Ca(2+) responses evoked by stimulation of the motor nerve were reduced in the presence of CPT, whereas atropine almost completely abolished them. Ca(2+) responses were further reduced when both antagonists were present simultaneously. Hence, PSCs at the mammalian NMJ respond to the release of neurotransmitter induced by stimulation of the motor nerve through the activation of muscarinic and adenosine A1 receptors.

The role of perisynaptic Schwann cells in development of neuromuscular junctions in the frog (Xenopus laevis)

Fluorescence microscopy was used to study the behavior of perisynaptic Schwann cells (PSCs) in relation to motor nerve terminals and postsynaptic clusters of acetylcholine receptors, during the development of the neuromuscular junction (NMJ) in the frog Xenopus laevis. Pectoral (supracoracoideus) muscles were labeled with monoclonal antibody 2A12 for Schwann cells, the dye FM4-64 for nerve terminals (NTs), alpha-bungarotoxin for acetylcholine receptors (AChRs), and Hoechst 33258 for cellular nuclei, in animals from tadpole stage 57 to fully grown adults. When muscle fibers first appeared in stage 57, NMJs consisted of tightly apposed NTs and AChRs and were only partially covered with PSCs or their processes. Within a few stages, PSCs fully occupied and overgrew the NMJs, extending fine sprouts between a few micrometers and hundreds of micrometers beyond the borders of the junction. Sprouts of PSCs were most abundant during the time when secondary myogenesis, synaptogenesis, and synaptic growth occurred at their highest rates. PSCs were recruited to NMJs during synaptic growth, at rates between 1.3 PSCs/100 microm junctional length early on and 0.4 PSCs/100 microm later. Shortly after metamorphosis, PSC sprouts disappeared and NMJs acquired the adult appearance, in which PSCs, NTs, and AChRs were mostly congruent. The results suggest that, although PSCs may not be required for initial nerve-muscle contacts, PSCs sprouts lead synaptic growth and play a role in the extension and maturation of developing NMJs.