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Schwann Cells: Dynamic Roles at the Neuromuscular Junction Rita J

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1054 Dispatch Schwann cells: Dynamic roles at the neuromuscular junction Rita J. Balice-Gordon Recent work shows that the non-myelinating ‘terminal’ for neurotransmitter molecules released from the nerve Schwann cells that cap neuromuscular junctions play an terminal. In the case of the ‘simplest’ synapse of them all, important role in synaptic maintenance and repair. the neuromuscular junction, recent work suggests that this view needs to be revised to include an additional, and pre- Address: Department of Neuroscience, University of Pennsylvania School of Medicine, 215 Stemmler Hall, Philadelphia, Pennsylvania viously under-appreciated, component — the Schwann 19104-6074, USA. cells that cap motor nerve terminals like frosting on a deceptively simple two-layer cake. It turns out that these Current Biology 1996, Vol 6 No 9:1054–1056 ‘terminal’ Schwann cells are much more than decorative, © Current Biology Ltd ISSN 0960-9822 they are an essential ingredient in neuromuscular synaptic maintenance and repair. Neuroscientists generally assume that synapses have two components: a presynaptic nerve terminal apposed to a Schwann cells, the glia of the peripheral nervous system, postsynaptic specialization — a specialized region of the come in two basic varieties: those that enwrap large nerve postsynaptic membrane with a high density of receptors axons and are induced to form an insulating sheath of Figure 1 Motor neurons in the ventral region of the Spinal cord Motor neuron cell bodies Myelinated motor axon spinal cord send axons out toward the periphery. At the muscle, individual axons leave the nerve bundle and branch widely; Muscle fiber each muscle fiber typically has a single synaptic site innervated by a single motor axon branch, and each motor axon innervates Motor axons many muscle fibers. At the synaptic site, Neuromuscular motor axons form nerve terminal branches; Nerve junction several non-myelinating, ‘terminal’ Schwann trunk cells are located over these nerve terminal branches and make processes that are closely apposed to them. Terminal Schwann cells, motor nerve terminal branches and the postsynaptic specializations together form the neuromuscular junction. Muscle nerve Endplate Skeletal band muscle Myelinating Schwann cell Motor axon Non-myelinating terminal Schwann cells Motor nerve terminals ( ) Postsynaptic specializations ( ) © 1996 Current Biology Dispatch 1055 Figure 2 structural and biochemical changes. When neuromuscular junctions are denervated by damaging motor axons, termi- nal Schwann cells extend processes which engulf degener- Degenerating ating nerve terminals, taking over their former position motor axons overlying postsynaptic specializations and even releasing neurotransmitter molecules [2]. Similar structural changes in terminal Schwann cells have been shown to occur when postsynaptic muscle fibers are damaged [3]. More recently, Reynolds and Woolf [4] used immunohistochem- istry to show that terminal Schwann cells are dramatically transformed following denervation, rapidly elaborating a profuse network of processes within the region of the muscle containing junctions; many of these processes were found to have extended branches hundreds of microns along muscle fibers. These observations showed that terminal Schwann cells can respond by changing their structure when motor ter- minals or muscle fibers are damaged. These cells are also exquisitely sensitive to the level of activity within their neuromuscular junction. On the one hand, neurotransmit- © 1996 Current Biology ter release by motor nerve terminals has been shown to lead to increased intracellular Ca2+ levels in overlying ter- When axons are damaged, nerve terminals and axons degenerate. minal Schwann cells [5]. On the other hand, blockade of Terminal Schwann cells form dense networks of processes, which neuromuscular transmission has been shown to change Thompson and colleagues [7,8] have shown play a role in inducing rapidly the repertoire of proteins expressed by terminal axons to form sprouts as well as in guiding axons back to denervated Schwann cells [6]. Taken together, this work establishes junctions. As discussed in the text, the terminal Schwann cells thus that terminal Schwann cells are far from static, but rather appear to play critically important roles in synapse formation, maintenance and repair. are highly dynamic cells worthy of closer inspection. Son and Thompson [7,8] recently determined whether the myelin, and those that enwrap smaller axons but do not processes extended by terminal Schwann cells upon den- form myelin. These varieties can be distinguished on the ervation influenced the regrowth of motor axons to the basis of the repertoire of proteins that they express. muscle and their guidance back to denervated neuromus- During axon outgrowth, in development as well as follow- cular junctions. One aspect of this process is the extension ing nerve damage, this repertoire is influenced by the type of profuse sprouts by regenerating motor axons, which can of axon that a Schwann cell encounters. Several lines of interconnect nearby junctions and in some cases result in a evidence suggest that Schwann cells reciprocate by influ- single junction transiently receiving more than one synap- encing the outgrowth of axons in vitro as well as in vivo tic input [9]. Similar multiple innervation also occurs tran- (reviewed in [1]). siently during normal neuromuscular development. Immunohistochemical markers specific for Schwann cells, Non-myelinating Schwann cells are also found in skeletal motor axons and synapses were used to demonstrate that, muscle, where they sit astride motor nerve terminals and during their return to muscle, motor axons seem to navi- extend processes which cover terminal branches (Fig. 1). gate along the processes extended by Schwann cells from In this location they are commonly referred to as ‘termi- the damaged nerve end [7]. Once there, motor axons grow nal’ Schwann cells and for many years have been consid- along the profuse network of processes extended by the ered to be a static sort of junctional exoskeleton, at best terminal Schwann cells that occupy denervated junctions passive onlookers of the myriad dynamic (and intensively (Fig. 2), and thus seem to be guided back to the synapse. studied) interactions between motor nerve terminals and postsynaptic muscle fibers. Attention has been sharply Son and Thompson [7] found that the regenerated motor focused on terminal Schwann cells, however, by recent axons often extended sprouts beyond the confines of the work which has established that they are intimately junction, along the processes of terminal Schwann cells involved in interactions at the neuromuscular junction. that interconnected adjacent junctions. They suggested that these sprout–process interactions lead to the tran- Terminal Schwann cells are known to respond to the sient reinnervation of a single neuromuscular junction by state of innervation of their neuromuscular junction with the processes of more than one motor neuron. Other 1056 Current Biology 1996, Vol 6 No 9 manipulations, such as botulinum intoxication to block in the survival of terminal Schwann cells following neona- nerve-evoked activity in muscle, or partial denervation, tal nerve damage. This work suggests that the trophic were found to cause robust motor axon sprouting in support of terminal Schwann cells by motor neurons muscle. Such sprouts were always observed to be associ- during early development may be a key factor in the estab- ated with the processes of terminal Schwann cells [8]. lishment of neuromuscular synaptic connections. These compelling experiments remind us that trophic interac- In partially denervated muscles, in which only a portion of tions at the neuromuscular junction are multi-directional neuromuscular junctions are denervated and the remain- and involve all three junctional cell types. der normally innervated, processes of terminal Schwann cells emanating from denervated junctions were found to Terminal Schwann cells might also be involved in the induce sprouting from normally innervated junctions [8]. reshaping of neuromuscular synaptic connections that That Schwann cell processes not only guide motor axon occurs during development and reinnervation, when sprouts but in fact induce them to occur was further several motor neurons transiently establish functional demonstrated in experiments in which a denervated nerve synapses with the same muscle fiber. Within such multiply fragment containing only Schwann cells was implanted innervated neuromuscular junctions, motor neurons into a normally innervated muscle [8]. In this case, sprouts compete with one another for sole innervation of the were induced to occur from intact neuromuscular junc- muscle fiber. Competition is known to be modulated by tions, and these extended along the terminal Schwann cell the relative activity patterns among the motor neurons processes emanating from the cut nerve. These experi- innervating the same muscle fiber. Because Schwann cells ments demonstrate that Schwann cells in nerve and in sense nerve terminal activity, engulf degenerating termi- muscle play a role in the induction and guidance of motor nals and induce terminal sprouts, they may be an integral axon regrowth in muscle. link in the chain of events that culminates in the physical withdrawal of motor nerve terminals from a muscle fiber. The integral role of Schwann cells in the reinnervation of Further study of the interactions among terminal Schwann neuromuscular
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