Reinnervation of original synaptic sites on muscle fiber basement membrane after disruption of the muscle cells

Physiologic signaling and viability of the muscle cuff regenerative peripheral nerve interface (MC-RPNI) for intact peripheral nerves

Background. Robotic exoskeleton devices have become a promising modality for restoration of extremity function in individuals with limb loss or functional weakness. However, there exists no consistent or reliable way to record efferent motor action potentials from intact peripheral nerves to control device movement. Peripheral nerve motor action potentials are similar in amplitude to that of background noise, producing an unfavorable signal-to-noise ratio (SNR) that makes these signals difficult to detect and interpret. To address this issue, we have developed the muscle cuff regenerative peripheral nerve interface (MC-RPNI), a construct consisting of a free skeletal muscle graft wrapped circumferentially around an intact peripheral nerve. Over time, the muscle graft regenerates, and the intact nerve undergoes collateral axonal sprouting to reinnervate the muscle. The MC-RPNI amplifies efferent motor action potentials by several magnitudes, thereby increasing the SNR, allowing for higher fidelity signaling and detection of motor intention. The goal of this study was to characterize the signaling capabilities and viability of the MC-RPNI over time.Methods. Thirty-seven rats were randomly assigned to one of five experimental groups (Groups A-E). For MC-RPNI animals, their contralateral extensor digitorum longus (EDL) muscle was harvested and trimmed to either 8 mm (Group A) or 13 mm (Group B) in length, wrapped circumferentially around the intact ipsilateral common peroneal (CP) nerve, secured, and allowed to heal for 3 months. Additionally, one 8 mm (Group C) and one 13 mm (Group D) length group had an epineurial window created in the CP nerve immediately preceding MC-RPNI creation. Group E consisted of sham surgery animals. At 3 months, electrophysiologic analyses were conducted to determine the signaling capabilities of the MC-RPNI. Additionally, electromyography and isometric force analyses were performed on the CP-innervated EDL to determine the effects of the MC-RPNI on end organ function. Following evaluation, the CP nerve, MC-RPNI, and ipsilateral EDL muscle were harvested for histomorphometric analysis.Results. Study endpoint analysis was performed at 3 months post-surgery. All rats displayed visible muscle contractions in both the MC-RPNI and EDL following proximal CP nerve stimulation. Compound muscle action potentials were recorded from the MC-RPNI following proximal CP nerve stimulation and ranged from 3.67 ± 0.58 mV to 6.04 ± 1.01 mV, providing efferent motor action potential amplification of 10-20 times that of a normal physiologic nerve action potential. Maximum tetanic isometric force (F_o) testing of the distally-innervated EDL muscle in MC-RPNI groups producedF_o(2341 ± 114 mN-2832 ± 102 mN) similar to controls (2497 ± 122 mN), thus demonstrating that creation of MC-RPNIs did not adversely impact the function of the distally-innervated EDL muscle. Overall, comparison between all MC-RPNI sub-groups did not reveal any statistically significant differences in signaling capabilities or negative effects on distal-innervated muscle function as compared to the control group.Conclusions. MC-RPNIs have the capability to provide efferent motor action potential amplification from intact nerves without adversely impacting distal muscle function. Neither the size of the muscle graft nor the presence of an epineurial window in the nerve had any significant impact on the ability of the MC-RPNI to amplify efferent motor action potentials from intact nerves. These results support the potential for the MC-RPNI to serve as a biologic nerve interface to control advanced exoskeleton devices.

Myosin heavy chains in extraocular muscle fibres: Distribution, regulation and function

This review examines kinetic properties and distribution of the 11 isoforms of myosin heavy chain (MyHC) expressed in extraocular muscle (EOM) fibre types and the regulation and function of these MyHCs. Although recruitment and discharge characteristics of ocular motoneurons during fixation and eye movements are well documented, work directly linking these properties with motor unit contractile speed and MyHC composition is lacking. Recruitment of motor units according to Henneman's size principle has some support in EOMs but needs consolidation. Both neurogenic and myogenic mechanisms regulate MyHC expression as in other muscle allotypes. Developmentally, multiply-innervated (MIFs) and singly-innervated fibres (SIFs) are derived presumably from distinct myoblast lineages, ending up expressing MyHCs in the slow and fast ends of the kinetic spectrum respectively. They modulate the synaptic inputs of their motoneurons through different retrogradely transported neurotrophins, thereby specifying their tonic and phasic impulse patterns. Immunohistochemical analyses of EOMs regenerating in situ and in limb muscle beds suggest that the very impulse patterns driving various ocular movements equip effectors with appropriate MyHC compositions and speeds to accomplish their tasks. These experiments also suggest that satellite cells of SIFs and MIFs are distinct lineages expressing different MyHCs during regeneration. MyHC compositions and functional characteristics of orbital fibres show longitudinal variations that facilitate linear ocular rotation during saccades. Palisade endings on global MIFs are postulated to respond to active and passive tensions by triggering axon reflexes that play important roles during fixation, saccades and vergence. How EOMs implement Listings law during ocular rotation is discussed.

Myasthenia Gravis: Pathogenic Effects of Autoantibodies on Neuromuscular Architecture

Myasthenia gravis (MG) is an autoimmune disease of the neuromuscular junction (NMJ). Autoantibodies target key molecules at the NMJ, such as the nicotinic acetylcholine receptor (AChR), muscle-specific kinase (MuSK), and low-density lipoprotein receptor-related protein 4 (Lrp4), that lead by a range of different pathogenic mechanisms to altered tissue architecture and reduced densities or functionality of AChRs, reduced neuromuscular transmission, and therefore a severe fatigable skeletal muscle weakness. In this review, we give an overview of the history and clinical aspects of MG, with a focus on the structure and function of myasthenic autoantigens at the NMJ and how they are affected by the autoantibodies' pathogenic mechanisms. Furthermore, we give a short overview of the cells that are implicated in the production of the autoantibodies and briefly discuss diagnostic challenges and treatment strategies.

Aberrant information transfer interferes with functional axon regeneration

Functional axon regeneration requires regenerating neurons to restore appropriate synaptic connectivity and circuit function. To model this process, we developed an assay in Caenorhabditis elegans that links axon and synapse regeneration of a single neuron to recovery of behavior. After axon injury and regeneration of the DA9 neuron, synapses reform at their pre-injury location. However, these regenerated synapses often lack key molecular components. Further, synaptic vesicles accumulate in the dendrite in response to axon injury. Dendritic vesicle release results in information misrouting that suppresses behavioral recovery. Dendritic synapse formation depends on dynein and jnk-1. But even when information transfer is corrected, axonal synapses fail to adequately transmit information. Our study reveals unexpected plasticity during functional regeneration. Regeneration of the axon is not sufficient for the reformation of correct neuronal circuits after injury. Rather, synapse reformation and function are also key variables, and manipulation of circuit reformation improves behavioral recovery.

Fundamental Molecules and Mechanisms for Forming and Maintaining Neuromuscular Synapses

The neuromuscular synapse is a relatively large synapse with hundreds of active zones in presynaptic motor nerve terminals and more than ten million acetylcholine receptors (AChRs) in the postsynaptic membrane. The enrichment of proteins in presynaptic and postsynaptic membranes ensures a rapid, robust, and reliable synaptic transmission. Over fifty years ago, classic studies of the neuromuscular synapse led to a comprehensive understanding of how a synapse looks and works, but these landmark studies did not reveal the molecular mechanisms responsible for building and maintaining a synapse. During the past two-dozen years, the critical molecular players, responsible for assembling the specialized postsynaptic membrane and regulating nerve terminal differentiation, have begun to be identified and their mechanism of action better understood. Here, we describe and discuss five of these key molecular players, paying heed to their discovery as well as describing their currently understood mechanisms of action. In addition, we discuss the important gaps that remain to better understand how these proteins act to control synaptic differentiation and maintenance.

The Plastic Glial-Synaptic Dynamics within the Neuropil: A Self-Organizing System Composed of Polyelectrolytes in Phase Transition

Several explanations have been proposed to account for the mechanisms of neuroglial interactions involved in neural plasticity. We review experimental results addressing plastic nonlinear interactions between glial membranes and synaptic terminals. These results indicate the necessity of elaborating on a model based on the dynamics of hydroionic waves within the neuropil. These waves have been detected in a small scale experimental model of the central nervous system, the in vitro retina. We suggest that the brain, as the heart and kidney, is a system for which the state of water is functional. The use of nonlinear thermodynamics supports experiments at convenient biological spatiotemporal scales, while an understanding of the properties of ions and their interactions with water requires explanations based on quantum theories. In our approach, neural plasticity is seen as part of a larger process that encompasses higher brain functions; in this regard, hydroionic waves within the neuropil are considered to carry both physiological and cognitive functions.

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.

Role of extracellular matrix proteins and their receptors in the development of the vertebrate neuromuscular junction

The vertebrate neuromuscular junction (NMJ) remains the best-studied model for understanding the mechanisms involved in synaptogenesis, due to its relatively large size, its simplicity of patterning, and its unparalleled experimental accessibility. During neuromuscular development, each skeletal myofiber secretes and deposits around its extracellular surface an assemblage of extracellular matrix (ECM) proteins that ultimately form a basal lamina. This is also the case at the NMJ, where the motor nerve contributes additional factors. Before most of the current molecular components were known, it was clear that the synaptic ECM of adult skeletal muscles was unique in composition and contained factors sufficient to induce the differentiation of both pre- and postsynaptic membranes. Biochemical, genetic, and microscopy studies have confirmed that agrin, laminin (221, 421, and 521), collagen IV (α3-α6), collagen XIII, perlecan, and the ColQ-bound form of acetylcholinesterase are all synaptic ECM proteins with important roles in neuromuscular development. The roles of their many potential receptors and/or binding proteins have been more difficult to assess at the genetic level due to the complexity of membrane interactions with these large proteins, but roles for MuSK-LRP4 in agrin signaling and for integrins, dystroglycan, and voltage-gated calcium channels in laminin-dependent phenotypes have been identified. Synaptic ECM proteins and their receptors are involved in almost all aspects of synaptic development, including synaptic initiation, topography, ultrastructure, maturation, stability, and transmission.

Biglycan is an extracellular MuSK binding protein important for synapse stability

The receptor tyrosine kinase MuSK is indispensable for nerve-muscle synapse formation and maintenance. MuSK is necessary for prepatterning of the endplate zone anlage and as a signaling receptor for agrin-mediated postsynaptic differentiation. MuSK-associated proteins such as Dok7, LRP4, and Wnt11r are involved in these early events in neuromuscular junction formation. However, the mechanisms regulating synapse stability are poorly understood. Here we examine a novel role for the extracellular matrix protein biglycan in synapse stability. Synaptic development in fetal and early postnatal biglycan null (bgn(-/o)) muscle is indistinguishable from wild-type controls. However, by 5 weeks after birth, nerve-muscle synapses in bgn(-/o) mice are abnormal as judged by the presence of perijunctional folds, increased segmentation, and focal misalignment of acetylcholinesterase and AChRs. These observations indicate that previously occupied presynaptic and postsynaptic territory has been vacated. Biglycan binds MuSK and the levels of this receptor tyrosine kinase are selectively reduced at bgn(-/o) synapses. In bgn(-/o) myotubes, the initial stages of agrin-induced MuSK phosphorylation and AChR clustering are normal, but the AChR clusters are unstable. This stability defect can be substantially rescued by the addition of purified biglycan. Together, these results indicate that biglycan is an extracellular ligand for MuSK that is important for synapse stability.

Glycosylated synaptomatrix regulation of trans-synaptic signaling

Synapse formation is driven by precisely orchestrated intercellular communication between the presynaptic and the postsynaptic cell, involving a cascade of anterograde and retrograde signals. At the neuromuscular junction (NMJ), both neuron and muscle secrete signals into the heavily glycosylated synaptic cleft matrix sandwiched between the two synapsing cells. These signals must necessarily traverse and interact with the extracellular environment, for the ligand-receptor interactions mediating communication to occur. This complex synaptomatrix, rich in glycoproteins and proteoglycans, comprises heterogeneous, compartmentalized domains where specialized glycans modulate trans-synaptic signaling during synaptogenesis and subsequent synapse modulation. The general importance of glycans during development, homeostasis and disease is well established, but this important molecular class has received less study in the nervous system. Glycan modifications are now understood to play functional and modulatory roles as ligands and co-receptors in numerous tissues; however, roles at the synapse are relatively unexplored. We highlight here properties of synaptomatrix glycans and glycan-interacting proteins with key roles in synaptogenesis, with a particular focus on recent advances made in the Drosophila NMJ genetic system. We discuss open questions and interesting new findings driving this investigation of complex, diverse, and largely understudied glycan mechanisms at the synapse.

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.

Atypical embryonic synapses fail to regenerate in adulthood

Functional recovery following central nervous system (CNS) injury in adult animals may depend on the reestablishment of the precise pattern of connections made during development. When the nervous system is injured during embryonic development, functional recovery may involve the formation of atypical connections. Can such atypical synapses regenerate in adults, particularly in a nervous system known for its capacity for repair? When the S interneuron in one segmental ganglion of the leech Hirudo is killed during development, two neighboring S cells extend their axons into the ganglion and restore function by making electrical synapses with the usual synaptic targets of the killed S cell. Although adult S-cell axons reliably regenerated their usual synaptic connections, the novel synapses induced following embryonic injury failed to regenerate in adults. In these preparations severed S-cell axons did not reach the denervated ganglion but grew close to it, independent of the distance required to grow. Thus, the developmental changes that permit aberrant but functional connections in embryos do not lead to a similar change in the capacity for axon growth and subsequent synapse regeneration in adults.

Dynamic remodelling of synapses can occur in the absence of the parent cell body

Background

Retraction of nerve terminals is a characteristic feature of development, injury and insult and may herald many neurodegenerative diseases. Although morphological events have been well characterized, we know relatively little about the nature of the underlying cellular machinery. Evidence suggests a strong local component in determining which neuronal branches and synapses are lost, but a greater understanding of this basic neurological process is required. Here we test the hypothesis that nerve terminals are semi-autonomous and able to rapidly respond to local stimuli in the absence of communication with their parent cell body.

Results

We used an isolated preparation consisting of distal peripheral nerve stumps, associated nerve terminals and post-synaptic muscle fibres, maintained in-vitro for up to 3 hrs. In this system synapses are intact but the presynaptic nerve terminal is disconnected from its cell soma. In control preparations synapses were stable for extended periods and did not undergo Wallerian degneration. In contrast, addition of purines triggers rapid changes at synapses. Using fluorescence and electron microscopy we observe ultrastructural and gross morphological events consistent with nerve terminal retraction. We find no evidence of Wallerian or Wallerian-like degeneration in these preparations. Pharmacological experiments implicate pre-synaptic P2X7 receptor subunits as key mediators of these events.

Conclusion

The data presented suggest; first that isolated nerve terminals are able to regulate connectivity independent of signals from the cell body, second that synapses exist in a dynamic state, poised to shift from stability to loss by activating intrinsic mechanisms and molecules, and third that local purines acting at purinergic receptors can trigger these events. A role for ATP receptors in this is not surprising since they are frequently activated during cellular injury, when adenosine tri-phosphate is released from damaged cells. Local control demands that the elements necessary to drive retraction are constitutively present. We hypothesize that pre-existing scaffolds of molecular motors and cytoskeletal proteins could provide the dynamism required to drive such structural changes in nerve terminals in the absence of the cell body.

Ultrasonically implanted PLA suture anchors are stable in osteopenic bone

We compared the in vitro pullout strengths of a metallic screw-type suture anchor (5-mm Corkscrew) and a new ultrasonically implanted absorbable pin from cadaveric humeral heads with different bone densities. They were assessed quantitatively using microcomputed tomography. Pullout tests were done at four standardized sites and then correlated with the local bone density. The mean pullout strengths for the 5-mm Corkscrew and the 3.5-mm polylactic pin were similar in weak bone (76 +/- 24 N versus 104 +/- 46 N), intermediate quality bone (194 +/- 81 N versus 218 +/- 76 N), and strong bone (349 +/- 127 N versus 325 +/- 100 N). Pullout strength correlated with bone density for both implants (r = 0.76 and r = 0.86 for the Corkscrew and polylactic pin, respectively). The ultrasonically implanted absorbable 3.5-mm polylactic pin achieved equal pullout strength in weak bone as the larger 5-mm titanium Corkscrew. Bone density is highly variable in humeral heads and influenced the pullout strength of both implants.

Structural and functional organization of synaptic acetylcholinesterase

The expression of the synaptic asymmetric form of the enzyme acetylcholinesterase (AChE) depends of two different genes: the gene that encodes for the catalytic subunit and the gene that encodes for the collagenic tail, ColQ. Asymmetric AChE is specifically localized to the basal lamina at the neuromuscular junction (NMJ). This highly organized distribution pattern suggests the existence of one or more specific binding sites in ColQ required for its anchorage to the synaptic basal lamina. Recent evidence support this notion: first, the presence of two heparin-binding domains in ColQ that interact with heparan sulfate proteoglycans (HSPGs) at the synaptic basal lamina; and second, a knockout mouse for perlecan, a HSPG concentrated in nerve-muscle contact, in which absence of asymmetric AChE at the NMJ is observed. The physiological importance of collagen-tailed AChE form in skeletal muscle has been illustrated by the identification of several mutations in the ColQ gene. These mutations determine end-plate acetylcholinesterase deficiency and induce one type of synaptic functional disorders observed in Congenital Myasthenic Syndromes (CMSs).

Fibre-type specific concentration of focal adhesion kinase at the sarcolemma: influence of fibre innervation and regeneration

In skeletal muscles, focal adhesion complexes (FACs) form part of the costamere, a sarcolemmal protein complex that enables lateral transfer of forces and ensures the stability of the sarcolemma. The present investigation tested whether localisation of a major assembly factor of FACs, focal adhesion kinase (FAK), to the sarcolemma parallels the known modulation of FACs by fibre type (innervation pattern) and fibre regeneration. Immunohistochemical experiments indicated that FAK is preferentially associated with the sarcolemma in a high proportion (>74 %) of the (slow-twitch) type I and(fast-twitch) type IIA fibres in normal rat soleus (N-SOL) muscle and of the type IIA fibres in extensor digitorum longus (N-EDL) muscle. In contrast, a low proportion (<15 %) of fast-twitch type IIB and type I fibres in N-EDL showed sarcolemmal FAK immunoreactivity. Cross-reinnervation of slow-twitch rat SOL muscle with the fast EDL nerve induced slow-to-fast fibre transformation and led to a significant reduction in sarcolemmal FAK immunoreactivity in type I and type IIA fibres. Transplantation of the fast EDL into the slow SOL bed with regeneration and reinnervation of the muscle by the SOL nerve (T-EDL) caused a significant increase in sarcolemmal FAK immunoreactivity in new type I and hybrid I/II fibres and a corresponding reduction in sarcolemmal FAK immunoreactivity in `normal' IIA and IIB fibres. Conversely, sarcolemmal FAK immunoreactivity in small IIB fibres of T-EDL muscle was increased. Correspondingly, the transplanted and regenerated SOL(reinnervated by the fast EDL nerve) maintained the percentage of FAK-positive sarcolemma in the (regenerated) type I and IIA fibres. Thus, the expression and association of FAK with the sarcolemma are regulated (i) by factors that determine the fibre type and (ii) during fibre regeneration. Our data suggest that the integrity of sarcolemmal FACs is dependent on the fibre type and that FAC turnover is increased during regeneration of muscle fibres.

Retrograde signaling in the development and modification of synapses

Retrograde signaling from the postsynaptic cell to the presynaptic neuron is essential for the development, maintenance, and activity-dependent modification of synaptic connections. This review covers various forms of retrograde interactions at developing and mature synapses. First, we discuss evidence for early retrograde inductive events during synaptogenesis and how maturation of presynaptic structure and function is affected by signals from the postsynaptic cell. Second, we review the evidence that retrograde interactions are involved in activity-dependent synapse competition and elimination in developing nervous systems and in long-term potentiation and depression at mature synapses. Third, we review evidence for various forms of retrograde signaling via membrane-permeant factors, secreted factors, and membrane-bound factors. Finally, we discuss the evidence and physiological implications of the long-range propagation of retrograde signals to the cell body and other parts of the presynaptic neuron.

Nerve terminal withdrawal from rat neuromuscular junctions induced by neuregulin and Schwann cells

Schwann cells (SCs) that cap neuromuscular junctions (nmjs) play roles in guiding nerve terminal growth in paralyzed and partially denervated muscles; however, the role of these cells in the day-to-day maintenance of this synapse is obscure. Neuregulins, alternatively spliced ligands for several erbB receptor tyrosine kinases, are thought to play important roles in cell-cell communication at the nmj, affecting synapse-specific gene expression in muscle fibers and the survival of terminal SCs during development. Here we show that application of a soluble neuregulin isoform, glial growth factor II (GGF2), to developing rat muscles alters terminal SCs, nerve terminals, and muscle fibers. SCs extend processes and migrate from the synapse. Nerve terminals retract from acetylcholine receptor-rich synaptic sites, and their axons grow, in association with SCs, to the ends of the muscle. These axons make effective synapses only after withdrawal of GGF2. These synaptic alterations appear to be induced by the actions of neuregulin on SCs, because SC transplants growing into contact with synaptic sites also caused withdrawal of nerve terminal branches. These results show that SCs can alter synaptic structure at the nmj and implicate these cells in the maintenance of this synapse.

Nerve terminal withdrawal from rat neuromuscular junctions induced by neuregulin and Schwann cells

Schwann cells (SCs) that cap neuromuscular junctions (nmjs) play roles in guiding nerve terminal growth in paralyzed and partially denervated muscles; however, the role of these cells in the day-to-day maintenance of this synapse is obscure. Neuregulins, alternatively spliced ligands for several erbB receptor tyrosine kinases, are thought to play important roles in cell-cell communication at the nmj, affecting synapse-specific gene expression in muscle fibers and the survival of terminal SCs during development. Here we show that application of a soluble neuregulin isoform, glial growth factor II (GGF2), to developing rat muscles alters terminal SCs, nerve terminals, and muscle fibers. SCs extend processes and migrate from the synapse. Nerve terminals retract from acetylcholine receptor-rich synaptic sites, and their axons grow, in association with SCs, to the ends of the muscle. These axons make effective synapses only after withdrawal of GGF2. These synaptic alterations appear to be induced by the actions of neuregulin on SCs, because SC transplants growing into contact with synaptic sites also caused withdrawal of nerve terminal branches. These results show that SCs can alter synaptic structure at the nmj and implicate these cells in the maintenance of this synapse.

S-laminin and N-acetylgalactosamine located at the synaptic basal lamina of skeletal muscle are involved in synaptic recognition by growing neurites

The purpose of the work reported here is to identify molecular components of the synaptic basal lamina of skeletal muscle fibres which allow recognition of original synaptic sites by regenerating motor axons. We focused on s-laminin and components recognized by the lectin Dolichos biflorus agglutinin previously shown to be specifically located at the synaptic basal lamina. We used a cryoculture bioassay in which chick ciliary ganglion neurons grow on rat skeletal muscle cryostat sections. In control cultures, neurites extended over the muscle sections in close association with the muscle cell surface. It was observed that most of the neurites that extended towards the endplate zone and reached an area of 40 microns around the neuromuscular junction ceased to grow when they contacted the synaptic site. Masking either lectin receptors or some s-laminin molecule epitopes prior to the culture of neurons alters the behaviour of growing neurites. On sections treated either with Dolichos biflorus agglutinin or anti s-laminin monoclonal antibodies (D5 and C4) most of the neurites did not stop their growth at the synaptic regions. Moreover, treating muscle sections with Dolichos biflorus agglutinin removed the gradient of substratum affinity around the endplate. These results indicate that the s-laminin and Dolichos biflorus agglutinin receptors present on muscle cell surfaces may play a functional role in the interaction of growing neurites with original synaptic sites in the process of neuromuscular regeneration.

ARIA is concentrated in the synaptic basal lamina of the developing chick neuromuscular junction

ARIA is a member of a family of polypeptide growth and differentiation factors that also includes glial growth factor (GGF), neu differentiation factor, and heregulin. ARIA mRNA is expressed in all cholinergic neurons of the central nervous systems of rats and chicks, including spinal cord motor neurons. In vitro, ARIA elevates the rate of acetylcholine receptor incorporation into the plasma membrane of primary cultures of chick myotubes. To study whether ARIA may regulate the synthesis of junctional synaptic acetylcholine receptors in chick embryos, we have developed riboprobes and polyclonal antibody reagents that recognize isoforms of ARIA that include an amino-terminal immunoglobulin C2 domain and examined the expression and distribution of ARIA in motor neurons and at the neuromuscular junction. We detected significant ARIA mRNA expression in motor neurons as early as embryonic day 5, around the time that motor axons are making initial synaptic contacts with their target muscle cells. In older embryos and postnatal animals, we found ARIA protein concentrated in the synaptic cleft at neuromuscular junctions, consistent with transport down motor axons and release at nerve terminals. At high resolution using immunoelectron microscopy, we detected ARIA immunoreactivity exclusively in the synaptic basal lamina in a pattern consistent with binding to synapse specific components on the presynaptic side of the basal lamina. These results support a role for ARIA as a trophic factor released by motor neuron terminals that may regulate the formation of mature neuromuscular synapses.

Sixth Annual Stuart Reiner Memorial Lecture: embryonic development of nerve and muscle

This article provides a basic scheme of sequential anatomic and some physiologic events occurring during the course of embryonic development of motor neurons and muscles, leading to the establishment of mature nerve-muscle relationships. Motor neurons and muscles begin their development independently and during embryogenesis they become dependent on each other for further development and survival. Aspects of development which occur independently and those requiring mutual interactions are identified. The development of motor neurons is discussed with respect to their production, projection, neuromuscular transmission, myelination, sprouting, survival, and death. The development of muscles is discussed with respect to the origin, differentiation, and muscle fiber types. Discussion on the development of neuromuscular junction includes differentiation of presynaptic nerve terminal, postsynaptic components, and elimination of multiple axons.

Polyclonal antibodies against NCAM and tenascin delay endplate reinnervation

Experiments were performed to block molecules with antibodies which are upregulated in nerve and muscle following denervation. The delay in endplate reinnervation was taken as a measure for their involvement in regeneration. Gluteus maximus muscles of 86 male CBA/J mice were hemidenervated by freezing the caudal gluteal nerve at a defined position. The degree of reinnervation was evaluated in identified endplates by repeated vital staining of ACh receptors with rhodaminated alpha-bungarotoxin and of axons with 4Di-2ASP. Normally, endplates were completely reinnervated by 13-14 days (108 endplates in seven muscles). After daily application of polyclonal antibodies against NCAM or tenascin, reinnervation was significantly delayed. Preimmune serum, rabbit immunoglobulins or saline did not show this effect. Several monoclonal antibodies against NCAM (H-28) and tenascin (576, 578, 630, 633) showed a tendency but no significant effect. It is concluded that both NCAM and tenascin, upregulated after denervation, are involved in axon guidance and/or endplate reinnervation.

Extracellular matrix regulates smooth muscle responses to substance P

Little is known about the extracellular factors that determine a cell's responsiveness to neurotransmitters. This is a particularly important issue for pharmacologically diverse cell types such as neurons and smooth muscle. This report demonstrates that the contractile responses of amniotic smooth muscle to a specific neuropeptide, substance P, is controlled by a molecule(s) intimately associated with the extracellular basement membrane. This molecule(s) normally represses the expression of substance P responsiveness in this tissue. When the amniotic smooth muscle is separated from the basement membrane by dissociation, normally unresponsive cells exhibit a progressive increase in responsiveness to substance P, beginning within the first 24 hr in culture. The induction of substance P responses was completely inhibited when the cells were plated onto isolated amniotic basement membrane rather than onto polyornithine or collagen I. Similar changes in the responsiveness to another agonist, histamine, did not occur. The data demonstrate that extracellular matrix exerts a major instructive influence in determining the responsiveness of avian amniotic smooth muscle to specific ligands. We suggest that similar regulatory mechanisms may operate in other tissues.

Synaptic basal lamina contains a signal for synapse-specific transcription

Nuclei in the synaptic region of multinucleated skeletal myofibers are transcriptionally distinct, since acetylcholine receptor genes are transcribed at a high rate by these nuclei, but not by nuclei elsewhere in the myofiber. Although this spatially restricted transcription pattern is presumably imposed by the motor nerve, the continuous presence of the nerve is not required, since synapse-specific transcription persists after denervation. These results suggest either that a transcriptional signal persists at synaptic sites after nerve terminals have degenerated, or that a transcriptional pattern in the myofiber, once established, is stable in the absence of a nerve-derived signal. To distinguish between these possibilities, we denervated muscle and damaged the myofibers and specialized cells located near synaptic sites, and then studied transcription of an acetylcholine receptor gene in myofibers that regenerated in their original basal lamina sheaths, but remained denervated. We show that synapse-specific transcription is re-induced in these regenerated myofibers, and we conclude that a signal for synapse-specific transcription is stably maintained in the synaptic basal lamina.

Terminal Schwann cells elaborate extensive processes following denervation of the motor endplate

Terminal Schwann cells, when stained for S100 (a calcium binding protein), can be seen to cap motor axons at the neuromuscular junction. Within days of denervation the Schwann cells begin to stain for the low affinity nerve growth factor receptor, but remain Thy-1 negative, and elaborate fine processes. These processes become longer and more disorganized over weeks, and cells positive for S100 and nerve growth factor receptor migrate into the perisynaptic area. Reinnervation results in a withdrawal of the processes. The morphology and location of terminal Schwann cells seems to depend on axonal contact. The spread of Schwann cells and their processes away from the synaptic zone following denervation, implies that these cells do not target axons directly to the endplate.

Synaptic dynamics at the neuromuscular junction: mechanisms and models

During development, the neuromuscular junction passes through a stage of extensive polyinnervation followed by a period of wholesale synapse elimination. In this report we discuss mechanisms and interactions that could mediate many of the key aspects of these important developmental events. Our emphasis is on (1) establishing an overall conceptual framework within which the role of many distinct cellular interactions and molecular factors can be evaluated, and (2) generating computer simulations that systematically test the adequacy of different models in accounting for a wide range of biological data. Our analysis indicates that several relatively simple mechanisms are each capable of explaining a variety of experimental observations. On the other hand, no one mechanism can account for the full spectrum of experimental results. Thus, it is important to consider models that are based on interactions among multiple mechanisms. A potentially powerful combination is one based on (1) a scaffold within the basal lamina or in the postsynaptic membrane which is induced by nerve terminals and which serves to stabilize terminals by a positive feedback mechanism; (2) a sprouting factor whose release by muscle fibers is down-regulated by activity and perhaps other factors; and (3) an intrinsic tendency of motor neurons to withdraw some connections while allowing others to grow.

Denervated single myofibers: neurite interactions and synaptic molecules

As a first step in defining the molecular cues that may be important for reinnervation of long-term denervated muscle, single adult rat muscle fibers that had been denervated from 2 to 24 months in vivo were maintained in culture for 5 days. Embryonic ventral spinal cord explants were added to some of these cultures. Interactions of neurites with individual short-term (up to 5 months) and long-term (17-24 months) denervated muscle fibers were compared with neurite interactions in cultures of young adult muscle fibers (from 3 to 5-month-old rats) or aged muscle fibers (from 17 to 26-month-old rats). We found the following. (1) Three molecules that are found at the neuromuscular junction (NMJ)--acetylcholinesterase (AChE), acetylcholine receptors (AChRs), and gelasmin (an acetylcholine receptor clustering factor that is found enriched at NMJ of adult muscle)--were reduced with increasing periods of denervation but not with aging. (2) The number of neurite contacts at junctional regions of muscle fibers that were formed and maintained on cultured muscle fibers depended on denervation time of the muscle in vivo; very few contacts were made or maintained on long-term denervated fibers. (3) Gelasmin, but not AChE or AChRs, was found at points of neurite contact on all muscle fibers examined, raising the possibility that it may serve as a cue for reinnervation and that its loss from long-term denervated muscle may be, at least in part, involved in the failure of neurite contacts to be made or maintained in culture and possibly in vivo.

Distribution and role in regeneration of N-CAM in the basal laminae of muscle and Schwann cells

The neural cell adhesion molecule (N-CAM) is a membrane glycoprotein involved in neuron-neuron and neuron-muscle adhesion. It can be synthesized in various forms by both nerve and muscle and it becomes concentrated at the motor endplate. Biochemical analysis of a frog muscle extract enriched in basal lamina revealed the presence of a polydisperse, polysialylated form of N-CAM with an average Mr of approximately 160,000 as determined by SDS-PAGE, which was converted to a form of 125,000 Mr by treatment with neuraminidase. To define further the role of N-CAM in neuromuscular junction organization, we studied the distribution of N-CAM in an in vivo preparation of frog basal lamina sheaths obtained by inducing the degeneration of both nerve and muscle fibers. Immunoreactive material could be readily detected by anti-N-CAM antibodies in such basal lamina sheaths. Ultrastructural analysis using immunogold techniques revealed N-CAM in close association with the basal lamina sheaths, present in dense accumulation at places that presumably correspond to synaptic regions. N-CAM epitopes were also associated with collagen fibrils in the extracellular matrix. The ability of anti-N-CAM antibodies to perturb nerve regeneration and reinnervation of the remaining basal lamina sheaths was then examined. In control animals, myelinating Schwann cells wrapped around the regenerated axon and reinnervation occurred only at the old synaptic areas; new contacts between nerve and basal lamina had a terminal Schwann cell capping the nerve terminal. In the presence of anti-N-CAM antibodies, three major abnormalities were observed in the regeneration and reinnervation processes: (a) regenerated axons in nerve trunks that had grown back into the old Schwann cell basal lamina were rarely associated with myelinating Schwann cell processes, (b) ectopic synapses were often present, and (c) many of the axon terminals lacked a terminal Schwann cell capping the nerve-basal lamina contact area. These results suggest that N-CAM may play an important role not only in the determination of synaptic areas but also in Schwann cell-axon interactions during nerve regeneration.

Visualization of collagenase-sensitive acetylcholinesterase in isolated cardiomyocytes and in heart tissue

Previous studies have indicated that the asymmetric form of acetylcholinesterase (collagen-tailed) is localized in the basal lamina of the neuromuscular junction of skeletal muscle. The present study shows localization of the asymmetric acetylcholinesterase in the heart of the rat. Antiserum to 14 + 18 S acetylcholinesterase of the electric eel was raised in rabbits. The purified antibody did not react with collagen type I or laminin. Collagenase reduced the immunoreactivity of the enzyme with the purified antibody. Isolated cardiomyocytes and frozen sections of the heart were stained for acetylcholinesterase with the antibody. Diffuse immunofluorescence appeared over the surface of the cardiomyocytes. In the frozen sections, the immunofluorescence was most intense at the cell boundaries. These data suggest that collagenase-sensitive acetylcholinesterase in the heart is present in the myocytes and occurs in the vicinity of the basal lamina.

In vitro reinnervation of adult rat muscle fibres by foreign neurons and transformed chromaffin PC12 cells

Adult rat muscle fibres were dissociated by using collagenase and maintained in culture. One to nine days later, neurons obtained from stages 22-30 Xenopus laevis embryos, or neonatal spinal cord, or pheochromocytoma (PC12) cells treated with nerve growth factor were added. Subsequently, the co-cultures were maintained for up to eight days. Functional synapses were formed with variable efficiency: 12% in rat-Xenopus nerve-muscle co-cultures, 23% in rat-rat and 33% in PC12 co-cultures. Miniature endplate potentials (MEPPs) and currents (MEPCs) were recorded, at frequencies ranging from 0.01 to 0.9 Hz. Their mean amplitude was smaller than in normal mammalian muscles. The rise time and time-constant of decay of MEPCs was about seven to ten times longer than that found in the original muscle, resembling immature synapses. (+)-Tubocurarine abolished the MEPPs in the rat-PC12 neuromuscular junctions. It is concluded that dissociated adult rat muscle fibres retain their ability of being reinnervated, and can form functional synapses with foreign neurons and transformed chromaffin cells.

Development of postsynaptic-like specializations of the neuromuscular synapse in the absence of motor nerve

It was previously reported that the acetylcholine receptor clusters and acetylcholinesterase appear on embryonic superior oblique muscle cells developing in vivo without motor nerve contacts. The objective of this study was to examine whether some other components of neuromuscular junction also form on muscle cells developing in vivo in the absence of motor neurons. In the present study, postsynaptic specializations such as junctional folds, postsynaptic density and basal lamina were studied in normal and aneural muscles. The superior oblique muscle of duck embryos was made aneural by permanent destruction of trochlear motor neurons by cauterizing midbrain on embryonic day 7; 3 days before the motor neurons normally project their axons into the muscle. Normal and aneural muscles from embryonic days 10 to 25 were processed for electron microscopy. The results indicate that morphological specializations such as junction-like folds, postsynaptic-like density, and basal lamina also develop in the absence of motor neuron contacts. Whether the differentiation of specialized synaptic basal lamina is dependent on the presence of motor neurons was examined by utilizing a monoclonal antibody against heparan sulfate proteoglycan. Immunohistochemical studies indicate that specialized synaptic basal lamina differentiates in the absence of motor neurons. Thus, the mechanism of development of postsynaptic components of neuromuscular junction in this muscle is not dependent on motor neuron contacts. These results also suggest that the postsynaptic cell plays a more active role in synapse formation than previously realized. The results are discussed in relation to the control of synapse numbers by the postsynaptic cell.

Agrin-like molecules at synaptic sites in normal, denervated, and damaged skeletal muscles

Several lines of evidence have led to the hypothesis that agrin, a protein extracted from the electric organ of Torpedo, is similar to the molecules in the synaptic cleft basal lamina at the neuromuscular junction that direct the formation of acetylcholine receptor and acetylcholinesterase aggregates on regenerating myofibers. One such finding is that monoclonal antibodies against agrin stain molecules concentrated in the synaptic cleft of neuromuscular junctions in rays. In the studies described here we made additional monoclonal antibodies against agrin and used them to extend our knowledge of agrin-like molecules at the neuromuscular junction. We found that anti-agrin antibodies intensely stained the synaptic cleft of frog and chicken as well as that of rays, that denervation of frog muscle resulted in a reduction in staining at the neuromuscular junction, and that the synaptic basal lamina in frog could be stained weeks after degeneration of all cellular components of the neuromuscular junction. We also describe anti-agrin staining in nonjunctional regions of muscle. We conclude the following: (a) agrin-like molecules are likely to be common to all vertebrate neuromuscular junctions; (b) the long-term maintenance of such molecules at the junction is nerve dependent; (c) the molecules are, indeed, a component of the synaptic basal lamina; and (d) they, like the molecules that direct the formation of receptor and esterase aggregates on regenerating myofibers, remain associated with the synaptic basal lamina after muscle damage.

De novo neuromuscular junction formation on human muscle fibres cultured in monolayer and innervated by foetal rat spinal cord: ultrastructural and ultrastructural--cytochemical studies

Ultrastructural features of neuromuscular junction formation and transverse tubule development were studied utilizing a newly developed model in which human muscle fibres cultured in monolayer are innervated by foetal rat spinal cord with dorsal root ganglia attached. At early innervation (7-10 days), when distinct 'boutons' are contacting muscle fibres, the contacts of nerve terminals with the muscle fibres are, ultrastructurally, superficial and unorganized, and there is no basal lamina-like material between nerve terminals and muscle fibres. A bouton consists, ultrastructurally, of a cluster of small nerve terminals contacting the muscle fibre. At 2-3 weeks of innervation, shallow 'beds' are formed on the muscle fibre just beneath nerve terminals, and occasionally there are irregular and miniscule fragments of basal lamina-like material in the cleft. There is no Schwann cell apposing the nerve terminal at this stage of innervation. After 4-5 weeks of innervation there is more definite basal lamina material in the cleft and suggestive postsynaptic plasmalemmal densities and invaginations. However, there is no Schwann cell apposing the nerve terminal at this stage. At 6-8 weeks of innervation, deep postsynaptic folds are present, a Schwann cell apposes the nerve terminal, and basal lamina surrounds the entire muscle fibre. At all four stages of innervation examined, ultrastructural cytochemistry of alpha-bungarotoxin binding reveals that nicotinic ACh receptors are located exclusively at the neuromuscular junctions. After 1-2 weeks of innervation, very few lanthanum-positive transverse tubules are observed and only in close proximity to the surface membrane. After 3 weeks of innervation, more lanthanum-positive tubules are present, and they are located deeper within the muscle fibre. Five weeks after innervation, somewhat more elaborated tubules (but no lateral sacs) appear, and honeycomb structures are often present. After 6-7 weeks of innervation the tubular system is very elaborate and lateral sacs are present. Hence, this study describes consecutive stages of the formation of neuromuscular junctions and transverse tubules in innervated cultured human muscle, and provides an important basis to which similar studies related to the diseased human muscle can be compared.

Cell accumulation in the junctional region of denervated muscle

If skeletal muscles are denervated, the number of mononucleated cells in the connective tissue between muscle fibers increases. Since interstitial cells might remodel extracellular matrix, and since extracellular matrix in nerve and muscle plays a direct role in reinnervation of the sites of the original neuromuscular junctions, we sought to determine whether interstitial cell accumulation differs between junctional and extrajunctional regions of denervated muscle. We found in muscles from frog and rat that the increase in interstitial cell number was severalfold (14-fold for frog, sevenfold for rat) greater in the vicinity of junctional sites than in extrajunctional regions. Characteristics of the response at the junctional sites of frog muscles are as follows. During chronic denervation, the accumulation of interstitial cells begins within 1 wk and it is maximal by 3 wk. Reinnervation 1-2 wk after nerve damage prevents the maximal accumulation. Processes of the cells form a multilayered veil around muscle fibers but make little, if any, contact with the muscle cell or its basal lamina sheath. The results of additional experiments indicate that the accumulated cells do not originate from terminal Schwann cells or from muscle satellite cells. Most likely the cells are derived from fibroblasts that normally occupy the space between muscle fibers and are known to make and degrade extracellular matrix components.

Expression of muscle-gene-specific isozymes of phosphorylase and creatine kinase in innervated cultured human muscle

Isozymes of creatine kinase and glycogen phosphorylase are excellent markers of skeletal muscle maturation. In adult innervated muscle only the muscle-gene-specific isozymes are present, whereas aneurally cultured human muscle has predominantly the fetal pattern of isozymes. We have studied the isozyme pattern of human muscle cultured in monolayer and innervated by rat embryo spinal cord explants for 20-42 d. In this culture system, large groups of innervated muscle fibers close to the ventral part of the spinal cord explant continuously contracted. The contractions were reversibly blocked by 1 mM d-tubocurarine. In those innervated fibers, the total activity and the muscle-gene-specific isozymes of both enzymes increased significantly. The amount of muscle-gene-specific isozymes directly correlated with the duration of innervation. Control noninnervated muscle fibers from the same dishes as the innervated fibers remained biochemically immature. This study demonstrated that de novo innervation of human muscle cultured in monolayer exerts a time-related maturational influence that is not mediated by a diffusable neural factor.

Thickness of the basal lamina at the frog neuromuscular junction

The Schwann cell basal lamina at the frog neuromuscular junction is much thicker than previously supposed. Its thickness has been demonstrated by modifying the fixation: the nerve muscle preparation is fixed in situ without prior exposure to Ringer solution and the osmolarity of the glutaraldehyde fixative is increased to 285 mOsm. With this procedure the basal lamina is seen to be up to 1 micron thick around the Schwann cells of the nerve terminal and the preterminal axon. The thickness of the basal lamina surrounding the Schwann cells of the myelinated portion of the axons within the perineural sheath (20 nm) is not changed by this modification of fixation. The present results indicate that in addition to the ability to form myelin, other properties of the Schwann cells differ inside and outside the perineurial sheath.

Rapid adhesion of nerve cells to muscle fibers from adult rats is mediated by a sialic acid-binding receptor

Single viable muscle fibers isolated from adult rats by collagenase digestion rapidly bind dissociated spinal neurons or PC-12 cells but not a variety of other cells tested. The adhesion process is calcium-independent, temperature-sensitive, and is not blocked by pretreating cells with inhibitors of energy metabolism or actin polymerization. Adhesion is mediated by a carbohydrate-binding protein and can be inhibited by N-acetylneuraminic acid or mucin, a glycoprotein with high sialic acids content. The hapten inhibitors do not dissociate cells if added after aggregation has occurred. Experiments to block adhesion by pretreatment of cells with either neuraminidase or mucin show that the sialic acids-rich moiety is on the nerve cells, while its receptor is on the muscle fibers.

The clustering of acetylcholine receptors and formation of neuromuscular junctions in regenerating mammalian muscle grafts

The present investigation was undertaken to study the relationship between acetylcholine receptor (AchR) clustering and endplate formation within regenerating skeletal muscle grafts. Silver staining of nerves was combined with rhodamine-alpha-bungarotoxin labeling of AchR clusters in heterotopic grafts of the rat soleus muscle. Two major graft procedures were used: whole muscle grafts and grafts which lacked the zone of original motor endplates (MEP-less grafts). These categories were subdivided into standard grafts, where subsequent innervation was allowed, and noninnervated grafts, which were experimentally deprived of innervation. Grafting brought about the death and removal of muscle fibers, followed by regeneration of myotubes within surviving basal lamina sheaths. A transient population of small extra-junctional AchR clusters spontaneously appears shortly after myotube formation in all four muscle graft types. Early myotubes of whole muscle grafts (both innervated and standard grafts, prior to the time of innervation) also develop presumptive secondary synaptic clefts and large, organized aggregations of AchRs at original synaptic sites. At later times, nerves regenerating into standard whole muscle and MEP-less grafts lead to the formation of numerous ectopic endplates. In whole muscle grafts, endplates may also form at original synaptic sites. Functional graft innervation is achieved in whole muscle and MEP-less grafts as early as 20 days postgrafting. The results of this study support the existence of still-unknown factors associated with the original synaptic site which can direct postsynaptic differentiation independent of innervation. They also demonstrate that functional endplates may form in mammalian muscle grafts at both original synaptic sites and ectopic locations, thus indicating that the zone of original synaptic sites is not necessary for the establishment of numerous functional and morphologically well-differentiated endplates.

Implantation of autologous cells in minced and devitalized rat skeletal muscles

Triceps surae of 3 week-old female Wistar rats were minced and orthotopically autografted. Thirty days later, the regenerates developed a force of 314 +/- 58 mN during an isometric maximal tetanus and the maximal area of the muscle tissue was 3.2 +/- 0.5 mm2 (n = 9) in a cross-section. At 60 days, their mean maximal isometric force was increased (1159 +/- 367 mN) as also were their maximal muscle cross-section area (6.8 +/- 0.7 mm2;n = 6). If the minces were devitalized by a freeze and thaw procedure, the regenerates neither contracted under electrical stimulation nor contained any muscle fibre. If devitalized minces were seeded with myogenic cells isolated from the contralateral triceps surae, orthotopically autografted and allowed to regenerate for 30 days, the regeneration was weak: 46 +/- 14 mN of force, 1.2 +/- 0.3 mm2 of muscle area in minces devitalized by cold (n = 7) and 23 +/- 8 mN of force, 0.2 +/- 0.1 mm2 of muscle area in minces devitalized by cold supplemented by heat (n = 8). If viable minces were seeded with myogenic cells, there was no improvement in the extent of regeneration: 275 +/- 83 mN of force, 2.0 +/- 0.6 mm2 of muscle area at 30 days (n = 7) and 738 +/- 191 mN of force, 5.6 +/- 0.9 mm2 of muscle area at 60 days (n = 5). Consequently, although it is possible to induce regeneration by grafting myogenic cells into a devitalized mince, this procedure has no effect when applied to a viable mince.

Formation of the vertebrate neuromuscular junction

The sequence of events leading to the formation of the NMJ based on the data presented in this chapter from rat, chick, and Xenopus muscle can be divided into three developmental stages, as shown in Table I. The essential components of the NMJ are acquired early. Acetylcholine is present and can be released from the growing nerve. Acetylcholine receptors are present in the muscle membrane and are functional even at the earliest times. These components of the junction--ACh release and functional ACh receptors--can develop independently of each other; i.e., cell culture studies have shown that nerve cells are capable of releasing ACh before their growing tips have come into contact with the postsynaptic muscle membrane. Conversely, muscle cells grown without nerve synthesize and incorporate in their membranes functional ACh receptors. This situation ensures that functional (table; see text) contacts can occur even at the earliest times. Local accumulation of ACh receptors is also detected at the earliest times of junction formation. Although cell culture studies have demonstrated that receptors can aggregate in the absence of nerve, it would appear that the nerve plays an important role in directing where the highest density of receptors will be localized. Acetylcholinesterase, identified both histochemically and electrophysiologically, occurs at the presumptive NMJ shortly after synaptic transmission and receptor clustering have begun, suggesting that these events may play a role in localizing cholinesterase. Although the studies on rat and chick muscle support this view, development of AChE on Xenopus muscle does not require prior exposure to nerve or muscle activity. The ultrastructural features characteristic of the adult NMJ also do not become apparent until after synaptic transmission and receptor clustering have been seen. However, detection of small regions of specialization could be easily overlooked at the ultrastructural level, particularly if the tissue has not been serially sectioned. The young tissue is more fragile (Gordon et al., 1974) and may be more susceptible to mechanical damage or alterations from the fixation procedures (Kullberg et al., 1977). For these reasons, results pertaining to when the ultrastructural specializations occur are difficult to interpret and must await identification of these structures by other means. A number of other changes occur at the NMJ late in development: (1) ACh receptors become metabolically more stable, (2) there is a conversion in the kinetics of the ACh receptor channel, and (3) junctional folds become apparent. The extent to which these changes occur varies among the different organisms discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Retrograde effect of muscle on forms of acetylcholinesterase in peripheral nerves

In the peripheral nerves of birds and mammals, acetylcholinesterase (AChE) exists in four main molecular forms (G1, G2, G4, and A12). The two heaviest forms (G4 and A12) are carried by rapid axoplasmic transport, whereas the two lightest forms (G1 and G2) are probably much more slowly transported. Here we report that nerves innervating fast-twitch (F nerves) and slow-twitch (S nerves) muscles of the rabbit differ both in their AChE molecular form patterns and in their anterograde and retrograde axonal transport parameters. Since we had previously shown a selective regulation of this enzyme in fast and slow parts of rabbit semimembranosus muscle, we wondered whether the differences observed in the nerve could be affected by the twitch properties of muscle. The results reported here show that in F nerves that reinnervate slow-twitch muscles, both the AChE molecular form patterns and axonal transport parameters turn into those of the S nerve. These data suggest the existence of a retrograde specific effect exerted by the muscles on their respective motoneurons.

Basal lamina directs acetylcholinesterase accumulation at synaptic sites in regenerating muscle

In skeletal muscles that have been damaged in ways which spare the basal lamina sheaths of the muscle fibers, new myofibers develop within the sheaths and neuromuscular junctions form at the original synaptic sites on them. At the regenerated neuromuscular junctions, as at the original ones, the muscle fibers are characterized by junctional folds and accumulations of acetylcholine receptors and acetylcholinesterase (AChE). The formation of junctional folds and the accumulation of acetylcholine receptors is known to be directed by components of the synaptic portion of the myofiber basal lamina. The aim of this study was to determine whether or not the synaptic basal lamina contains molecules that direct the accumulation of AChE. We crushed frog muscles in a way that caused disintegration and phagocytosis of all cells at the neuromuscular junction, and at the same time, we irreversibly blocked AChE activity. New muscle fibers were allowed to regenerate within the basal lamina sheaths of the original muscle fibers but reinnervation of the muscles was deliberately prevented. We then stained for AChE activity and searched the surface of the new muscle fibers for deposits of enzyme they had produced. Despite the absence of innervation, AChE preferentially accumulated at points where the plasma membrane of the new muscle fibers was apposed to the regions of the basal lamina that had occupied the synaptic cleft at the neuromuscular junctions. We therefore conclude that molecules stably attached to the synaptic portion of myofiber basal lamina direct the accumulation of AChE at the original synaptic sites in regenerating muscle. Additional studies revealed that the AChE was solubilized by collagenase and that it remained adherent to basal lamina sheaths after degeneration of the new myofibers, indicating that it had become incorporated into the basal lamina, as at normal neuromuscular junctions.

Presynaptic neurones may contribute a unique glycoprotein to the extracellular matrix at the synapse

As the extracellular matrix at the original site of a neuromuscular junction seems to play a major part in the specificity of synaptic regeneration, considerable attention has been paid to unique molecules localized to this region. Here we describe an extracellular matrix glycoprotein of the elasmobranch electric organ that is localized near the nerve endings. By immunological criteria, it is synthesized in the cell bodies, transported down the axons and is related to a glycoprotein in the synaptic vesicles of the neurones that innervate the electric organ. It is apparently specific for these neurones, as it cannot be detected elsewhere in the nervous system of the fish. Therefore, neurones seem to contribute unique extracellular matrix glycoproteins to the synaptic region. Synaptic vesicles could be involved in transporting these glycoproteins to or from the nerve terminal surface.