Precision of reinnervation of original postsynaptic sites in frog muscle after a nerve crush

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.

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.

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.

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.

Multiple roles for frequenin/NCS-1 in synaptic function and development

The calcium-binding protein frequenin (Frq), discovered in the fruit fly Drosophila, and its mammalian homologue neuronal calcium sensor 1 (NCS-1) have been reported to affect several aspects of synaptic transmission, including basal levels of neurotransmission and short- and long-term synaptic plasticities. However, discrepant reports leave doubts about the functional roles of these conserved proteins. In this review, we attempt to resolve some of these seemingly contradictory reports. We discuss how stimulation protocols, sources of calcium (voltage-gated channels versus internal stores), and expression patterns (presynaptic versus postsynaptic) of Frq may result in the activation of various protein targets, leading to different synaptic effects. In addition, the potential interactions of Frq's C-terminal and N-terminal domains with other proteins are discussed. Frq also has a role in regulating neurite outgrowth, axonal regeneration, and synaptic development. We examine whether the effects of Frq on neurotransmitter release and neurite outgrowth are distinct or interrelated through homeostatic mechanisms. Learning and memory are affected by manipulations of Frq probably through changes in synaptic transmission and neurite outgrowth, raising the possibility that Frq may be implicated in human pathological conditions, including schizophrenia, bipolar disorder, and X-linked mental retardation.

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.

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.

Ultrastructure of sites of cholinesterase activity on amphibian embryonic muscle cells cultured without nerve

Muscle cells, derived from 1-day-old embryos of Xenopus laevis, were cultured in the absence of nerve and then examined histochemically for cholinesterase activity. Virtually all of the cells exhibited one or more discrete patches of cholinesterase activity on their surface. The patches were located not only on the lower cell surface which was apposed to the floor of the culture dish but also on the lateral and upper surfaces which were not apposed to any solid substrate. At many of the cholinesterase patches the cell surface was invaginated and the invaginations also became filled with the histochemical reaction product. Surface sites of cholinesterase activity were often associated with a thickened plasma membrane and overlying basal lamina-like material. Regions of the cell surface without cholinesterase were typically smooth and ultrastructurally unspecialized. The cholinesterase patches and their associated ultrastructural features developed in the absence of electrical and contractile activity. The similarities between the muscle specializations at cholinesterase sites on cultured, noninnervated, Xenopus muscle cells and those at the neuromuscular and myotendinous junctions of normal skeletal muscle are discussed.

Polyneuronal innervation of single muscle fibers in cat eye muscle: inferior oblique

Single muscle fibers with multiple axonal endplates (multiply innervated fibers) are normally present in adult extraocular muscles (EOMs), while most other mammalian skeletal muscles contain fibers with a single myoneural junction. Recent findings by others led us to investigate for the presence of polyneuronal innervation (innervation of a single muscle fiber by >1 motoneuron) in the inferior oblique (IO) muscle of pentobarbital anesthetized cats. The IO muscle nerve branches, as they coursed through the orbit, were further divided for independent or simultaneous electrical stimulation with bipolar electrodes. Four of five established tests for polyneuronal innervation gave positive results. The sum of the twitch (1) and tetanic (2) tensions in response to individual nerve branch stimulation was greater than that for simultaneous (whole) nerve stimulation. The summed electromyographic (EMG) responses (3) gave a similar positive result. The result for crossed tetanic potentiation (4) was negative for polyneuronal innervation while the crossed fatigue (5) test was positive. These results are consistent with recent studies. That the EOMs exhibit polyneuronal innervation further explains the eye-movement system's functional integrity during some neuromuscular disorders as well as its ability to operate with precision after the loss of numerous motoneurons.

Biology and pathology of nonmyelinating Schwann cells

The CNS contains relatively few unmyelinated nerve fibers, and thus benefits from the advantages that are conferred by myelination, including faster conduction velocities, lower energy consumption for impulse transmission, and greater stability of point-to-point connectivity. In the PNS many fibers or regions of fibers the Schwann do not form myelin. Examples include C fibers nociceptors, postganglionic sympathetic fibers, and the Schwann cells associated with motor nerve terminals at neuromuscular junctions. These examples retain a degree of plasticity and a capacity to sprout collaterally that is unusual in myelinated fibers. Nonmyelin-forming Schwann cells, including those associated with uninjured fibers, have the capacity to act as the "first responders" to injury or disease in their neighborhoods.

Drosophila ankyrin 2 is required for synaptic stability

Synaptic connections are stabilized through transsynaptic adhesion complexes that are anchored in the underlying cytoskeleton. The Drosophila neuromuscular junction (NMJs) serves as a model system to unravel genes required for the structural remodeling of synapses. In a mutagenesis screen for regulators of synaptic stability, we recovered mutations in Drosophila ankyrin 2 (ank2) affecting two giant Ank2 isoforms that are specifically expressed in the nervous system and associate with the presynaptic membrane cytoskeleton. ank2 mutant larvae show severe deficits in the stability of NMJs, resulting in a reduction in overall terminal size, withdrawal of synaptic boutons, and disassembly of presynaptic active zones. In addition, lack of Ank2 leads to disintegration of the synaptic microtubule cytoskeleton. Microtubules and microtubule-associated proteins fail to extend into distant boutons. Interestingly, Ank2 functions downstream of spectrin in the anchorage of synaptic microtubules, providing the cytoskeletal scaffold that is essential for synaptic stability.

Retrograde signaling onto Ret during motor nerve terminal maturation

Establishment of the neuromuscular synapse requires bidirectional signaling between the nerve and muscle. Although much is known on nerve-released signals onto the muscle, less is known of signals important for presynaptic maturation of the nerve terminal. Our results suggest that the Ret tyrosine kinase receptor transmits a signal in motor neuron synapses that contribute to motor neuron survival and synapse maturation at postnatal stages. Ret is localized specifically to the presynaptic membrane with its ligands, GDNF (glial cell line-derived neurotrophic factor)/NTN (neurturin), expressed in skeletal muscle tissue. Lack of Ret conditionally in cranial motor neurons results in a developmental deficit of maturation and specialization of presynaptic neuromuscular terminals. Regeneration of Ret-deficient adult hypoglossal motor neurons is unperturbed, but despite contact with the unaffected postsynaptic specializations, presynaptic axon terminal maturation is severely compromised in the absence of Ret signaling. Thus, Ret transmits a signal in motor nerve terminals that participate in the organization and maturation of presynaptic specializations during development and during regeneration in the adult.

Blood vessels and desmin control the positioning of nuclei in skeletal muscle fibers

Skeletal muscle fibers contain hundreds to thousands of nuclei which lie immediately under the plasmalemma and are spaced out along the fiber, except for a small cluster of specialized nuclei at the neuromuscular junction. How the nuclei attain their positions along the fiber is not understood. Here we show that the nuclei are preferentially localized near blood vessels (BV), particularly in slow-twitch, oxidative fibers. Thus, in rat soleus muscle fibers, 81% of the nuclei appear next to BV. Lack of desmin markedly perturbs the distribution of nuclei along the fibers but does not prevent their close association with BV. Consistent with a role for desmin in the spacing of nuclei, we show that denervation affects the organization of desmin filaments as well as the distribution of nuclei. During chronic stimulation of denervated muscles, new BV form, along which muscle nuclei align themselves. We conclude that the positioning of nuclei along muscle fibers is plastic and that BV and desmin intermediate filaments each play a distinct role in the control of this positioning.

Characterization of matrix metalloproteinases in denervated muscle

In a nerve crush model of denervation, we examined muscle matrix metalloproteinase (MMP) expression, localization and activity. In normal muscle, MMP mRNA levels were low, and immunohistochemically MMPs were distributed around the muscle fibre with MMPs-3, -7 and -9 also staining at the neuromuscular junction. Seven days after nerve crush, muscle MMP immunoreactivity, especially MMP-12 and MMP-14, became irregularly distributed. At 20 days reinnervation of the muscle was observed, and some restitution of the normal pattern of immunoreactivity was noted concomitant with a higher level of MMP mRNA expression. In situ zymography showed that MMP activity was very weak in normal muscle whereas it was increased up to 40 days following denervation. Our results suggest that MMPs in muscle are involved in the tissue changes following denervation. Further experiments are required to test the hypothesis that MMP inhibition may be beneficial in protecting muscle from excessive remodelling following denervation and therefore improve reinnervation.

Characterisation of matrix metalloproteinases and the effects of a broad-spectrum inhibitor (BB-1101) in peripheral nerve regeneration

The effect of treatment with a broad-spectrum inhibitor (BB1101) of the matrix metalloproteinases (MMPs) on nerve regeneration and functional recovery after nerve crush was examined. Drug treatment had no effect on latency but from 63 days the compound muscle action potential was significantly increased and was no different to that in the sham-operated controls at 72 days. Levels of MMP mRNA expression, and the localisation and activity of MMP proteins, were examined in rats for a 2 month period following a nerve crush injury, and compared with sham-operated controls. The mRNA of all the MMPs studied was up-regulated by 5-10 days after nerve crush, and they remained up-regulated for 40-63 days, except for MMP-9 which was down-regulated by 10 days. MMP immunoreactivity was localised to Schwann cells, macrophages and endothelial cells, and with the exception of membrane type 1-MMP (MT1-MMP), it was more intense after nerve crush compared with sham-operated controls. Regenerating axons showed immunoreactivity for MMP-2 and MMP-3. In situ zymography confirmed that the activity of MMPs in the nerve was increased following crush but that the activity was greatly reduced in rats treated with BB-1101. Thus despite the inhibition of MMPs by BB-1101, the drug did not appear to essentially affect nerve degeneration or regeneration following nerve crush but that it could be beneficial in promoting the more effective reinnervation of muscles possibly by actions at the level of the muscles.

Peripheral and Spinal Motor Reorganization after Nerve Injury and Repair

Functional recovery after peripheral nerve injury depends on the amount as well as on the accuracy of reinnervation by regenerative axons. In this study, the rat sciatic nerve was subjected to crush injury or complete transection repaired by either (1) straight nerve suture, (2) crossed nerve suture of tibial and peroneal fascicles, or (3) silicone tubulization leaving a gap of 4 mm. The compound muscle action potentials (CMAP) of gastrocnemius, tibialis anterior and plantar muscles were recorded 90 days post operation to assess functional reinnervation and Fast Blue, Fluoro Gold and DiI were applied to the nerve branches projecting into these muscles to quantify morphological reinnervation. The CMAP amplitude achieved in gastrocnemius, tibialis anterior and plantar muscles was higher after nerve crush (86%, 82%, 65% of control) than after any surgical nerve repair (straight suture: 49%, 53%, 32%; crossed suture: 56%, 50%, 31%; silicone tube: 42%, 44%, 25%). The total number of labeled motoneurons, however, did not significantly differ between groups (control: 1238 +/- 82, crush: 1048 +/- 49, straight suture: 1175 +/- 106, crossed suture: 1085 +/- 84, silicone tube: 1250 +/- 182). The volume occupied by labeled motoneurons within the spinal cord was larger after surgical nerve repair than in crush or normal control animals, and fewer neurons showed abnormal multiple projections after crush (2.5%) or straight suture (2.2%) than following crossed suture (5%) or silicone tube (6%). In conclusion, nerve repair with a silicone tube leaving a short gap does not increase accuracy of reinnervation.

Dynactin is necessary for synapse stabilization

We present evidence that synapse retraction occurs during normal synaptic growth at the Drosophila neuromuscular junction (NMJ). An RNAi-based screen to identify the molecular mechanisms that regulate synapse retraction identified Arp-1/centractin, a subunit of the dynactin complex. Arp-1 dsRNA enhances synapse retraction, and this effect is phenocopied by a mutation in P150/Glued, also a dynactin component. The Glued protein is enriched within the presynaptic nerve terminal, and presynaptic expression of a dominant-negative Glued transgene enhances retraction. Retraction is associated with a local disruption of the synaptic microtubule cytoskeleton. Electrophysiological, ultrastructural, and immunohistochemical data support a model in which presynaptic retraction precedes disassembly of the postsynaptic apparatus. Our data suggests that dynactin functions locally within the presynaptic arbor to promote synapse stability.

The spatiotemporal relationship among Schwann cells, axons and postsynaptic acetylcholine receptor regions during muscle reinnervation in aged rats

To morphologically define the aging-related features during muscle reinnervation the spatiotemporal relationships among the major components of the neuromuscular junctions (NMJs) were investigated. A total of 64 rats, 30 adults (4 months old) and 34 aged adults (24 months old), were used. Between 1 and 12 weeks after sciatic nerve-crushing injury, cryosections of skeletal muscle were single or double labeled for S100, a marker of Schwann cells (SCs), for protein gene product 9.5, a neuronal marker, and for alpha-bungarotoxin (alpha-BT), a marker of the acetylcholine receptor site (AChR site), and then observed by confocal laser microscopy. The most obvious age changes were noted: (1) the regenerating SCs and axons were delayed in their arrival at the NMJ, (2) the dimensions of terminal SCs and AChR sites displayed a drastic and long-lasting drop (for terminal SCs, during 1-8 weeks; for AChR sites, during 1-12 weeks); (3) the degree of spatial overlap between AChR sites and terminal SCs was markedly low until 8 weeks post-crush; (4) damage and poor formation in the SCs, terminal axons and AChR sites, together with poor process extension from the terminal SC or terminal axon, were pronounced; (5) persistent aberrant changes, such as multiple innervation and terminal axon sprouting, together with poorly formed collateral innervation, nerve bundles, and NMJs, more frequently occurred in the later reinnervation period. Thus, with aging, regeneration is impaired during the period in which regenerating SC strands and axons extend into NMJs and the subsequent establishment of nerve-muscle contact is in progress. A complex set of morphological abnormalities between or among the TSCs, terminal axons, and AChR sites may be important in slowing of regeneration and reinnervation in aged motor endplates.

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

The perisynaptic Schwann cell (PSC) has gained recent attention with respect to its roles in synaptic function, remodeling, and regeneration at the vertebrate neuromuscular junction (NMJ). Here we test the hypothesis that, following nerve injury, processes extended by PSCs guide regenerating nerve terminals (NTs) in vivo, and that the extension of sprouts by PSCs is triggered by the arrival of regenerating NTs. Frog NMJs were double-stained with a fluorescent dye, FM4-64, for NTs, and fluorescein isothiocyanate (FITC)-tagged peanut agglutinin (PNA) for PSCs. Identified NMJs were imaged in vivo repeatedly for several months after nerve injury. PSCs sprouted profusely beginning 3-4 weeks after nerve transection and, as reinnervation progressed, regenerating NTs closely followed the preceding PSC sprouts, which could extend tens to hundreds of microns beyond the original synaptic site. The pattern of reinnervation was dictated by PSC sprouts, which could form novel routes joining neighboring junctions or develop into new myelinated axonal pathways. In contrast to mammals, profuse PSC sprouting in frog muscles was not seen in response to axotomy alone, and did not occur at chronically denervated NMJs. Instead, sprouting coincided with the arrival of regenerating NTs. Immunofluorescent staining revealed that in muscle undergoing reinnervation 4 weeks after axotomy, 91% of NMJs bore PSC sprouts, compared to only 6% of NMJs in muscle that was chronically denervated for 4 weeks. These results suggest that reciprocal interactions between regenerating NTs and PSCs govern the process of reinnervation at frog NMJs: regenerating NTs induce PSCs to sprout, and PSC sprouts, in turn, lead and guide the elaboration of NTs.

F-actin is concentrated in nonrelease domains at frog neuromuscular junctions

To gain insight into the role of F-actin in the organization of synaptic vesicles at release sites, we examined the synaptic distribution of F-actin by using a unique synaptic preparation of frog target-deprived nerve terminals. In this preparation, imaging of the synaptic site was unobstructed by the muscle fiber cytoskeleton, allowing for the examination of hundreds of synaptic sites in their entirety in whole mounts. At target-deprived synaptic sites F-actin was distributed in a ladder-like pattern and was colocalized with beta-fodrin. Surprisingly, F-actin stain, which we localized to the nerve terminal itself, did not overlap a synaptic vesicle marker, suggesting that it was concentrated in nonrelease domains of nerve terminals between clusters of synaptic vesicles. These findings suggest that the majority of the presynaptic F-actin is not involved in tethering synaptic vesicles. Instead, the strategic presynaptic positioning of this cytoskeletal meshwork in nonrelease domains of the nerve terminal suggests alternate functions such as restricting synaptic vesicles to release domains, recycling synaptic vesicles, or stabilizing the nerve terminal.

Rabbit facial nerve regeneration in autologous nerve grafts after antecedent injury

OBJECTIVE

The effect of incomplete antecedent injuries on subsequent facial nerve regeneration within cable graft repairs is not known. The purpose of this study is to compare facial nerve regeneration after an immediate and delayed neural cable graft repair.

METHOD

Rabbit facial nerve regeneration after complete transectional injuries of the buccal division was compared in two experimental models. In one, a 10-mm segment of the nerve was transected, rotated 180 degrees, and immediately repaired as a cable graft (N=8). In the second, a preliminary nerve crush was allowed to recover over a 4-week period and a 10-mm segment of nerve centered on the crush site was then transected, rotated 180 degrees, and delay repaired as a cable graft (N = 7). Data are presented as total numbers of regenerating myelinated axons that traverse the surgical repair to innervate the cable graft and distal nerve stumps, as well as the percentage of regenerating neurites compared with preoperative pooled and individual controls. Subpopulations of regenerating neurons are delineated to quantify the pattern of neural innervation.

RESULTS

Five weeks after cable graft repair both groups had similar myelinated outgrowth from the proximal nerve stump across the proximal anastomosis to innervate the cable graft (3995 +/- 1209 vs. 3284 +/- 651; P = .89). However, the delayed repair group had more intrafascicular regeneration within cable grafts (2261 +/- 931 vs. 1660 +/- 1169; P = .02) and distal nerve stump (1532 +/- 281 vs. 445 +/- 120; P = .004) than the immediate repair group. The immediate repair group had greater extrafascicular nerve regeneration in the cable graft (2335 +/- 1954 vs. 437 +/- 236; P = .001) and more myelin and axonal debris in pre-existing neural fascicles of the cable graft (P = .02) and distal nerve stump (463 +/- 187 vs. 103 +/- 87; P = .02).

CONCLUSIONS

Antecedent priming lesions do not enhance axonal survival as determined by regenerating myelinated axonal counts. However, antecedent injuries enhance the efficiency of neural innervation of the affected mimetic musculature by increasing the number of myelinated intrafascicular neural regenerants in the cable graft and distal nerve stump. This is accomplished by two factors: increased perineural fibrosis and decreased intrafascicular myelin and axonal debris.

Neuromuscular plasticity in the locust after permanent removal of an excitatory motoneuron of the extensor tibiae muscle

The capacity of the larval insect nervous system to compensate for the permanent loss of one of the two excitatory motoneurons innervating a leg muscle was investigated in the locust (Locusta migratoria). In the fourth instar, the fast extensor tibiae (FETi) motoneuron in the mesothoracic ganglion was permanently removed by photoinactivation with a helium-cadmium laser. Subsequently, the animals were allowed to develop into adulthood. When experimental animals were tested as adults after final ecdysis, fast-contracting fibers in the most proximal region of the corresponding extensor muscle, which are normally predominantly innervated by FETi only, uniformly responded to activity of the slow extensor tibiae (SETi) neuron. In adult operated animals, single pulses to SETi elicited large junctional responses in the fibers which resulted in twitch contractions of these fibers similar to the responses to FETi activity in control animals. The total number of muscle fibers, their properties as histochemically determined contractional types (fast and slow), and their distribution were not affected by photoinactivation of FETi. Possible mechanisms enabling the larval neuromuscular system to compensate for the loss of FETi through functionally similar innervation by a different motoneuron, i.e. SETi, are discussed.

gamma-AChR/epsilon-AChR switch at agrin-induced postsynaptic-like apparatus in skeletal muscle

We transfected the extrajunctional region of denervated soleus muscles in adult rats with neural agrin cDNA to induce myofibers to form postsynaptic-like apparatus containing acetylcholine receptor (AChR) aggregates. By 1 week approximately 30% of the AChR aggregates contained a mixture of epsilon-AChRs and gamma-AChRs while approximately 70% had only gamma-AChRs. If the transfected muscles were reinnervated in the original junctional region, the postsynaptic-like apparatus, despite the absence of apposed axon terminals, gradually came to have only epsilon-AChRs. We conclude that at the postsynaptic apparatus of ectopic neuromuscular junctions formed by a foreign nerve implanted into the extra-junctional region of denervated muscles, agrin secreted by the axon terminal plays a direct role in the gamma-AChR/epsilon-AChR switch that occurs as the apparatus reaches maturity. Our findings, together with results from other studies, indicate further that agrin and acetylcholine are the only nerve-derived factors required for this switch.

Precision of reinnervation and synaptic remodeling observed in neuromuscular junctions of living frogs

Repeated in vivo observations were used to study regenerated nerve terminals in neuromuscular junctions of the adult frog Rana pipiens. Sartorius junctions in living animals were stained with the fluorescent vital dye RH414 and viewed with video fluorescence microscopy. Each junction was observed in the intact muscle and then again 7, 10, and 13 weeks after nerve crush. At 13 weeks, junctions were determined to be mono- or polyneuronally innervated using intracellular recording. Between 7 and 13 weeks, most identified junctions were reinnervated less precisely and completely than described previously. Although some of the original synaptic gutters were reoccupied by regenerated terminal branches, other gutters were only partially occupied, and many appeared abandoned. Junctions showing precise recapitulation of original terminal arborizations comprised a small number of the total examined, as did those where reinnervation was very imprecise. Striking differences in the precision of reinnervation were found within the muscle such that distal terminals regenerated more precisely and completely than did proximal terminals. Terminals in reinnervated muscles were more dynamic than terminals in unoperated muscles over equivalent times. In singly innervated junctions, terminal growth was favored over regression. In doubly innervated junctions, regressive events were more common. Imprecise reinnervation is explained in terms of multisite innervation of muscle fibers and the activity dependence of synaptic stability. We hypothesize that when axons reinnervate the second or third junctions on a fiber, they do so less precisely, because the activity restored by reinnervation of the first junction renders later sites less attractive or less stable.

Native chick laminin-4 containing the beta 2 chain (s-laminin) promotes motor axon growth

After denervation of muscle, motor axons reinnervate original synaptic sites. A recombinant fragment of the synapse specific laminin beta 2 chain (s-laminin) was reported to inhibit motor axon growth. Consequently, a specific sequence (leucine-arginine-glutamate, LRE) of the laminin beta 2 chain was proposed to act as a stop signal and to mediate specific reinnervation at the neuromuscular junction (Porter, B.E., J. Weis, and J.R. Sanes. 1995. Neuron. 14:549-559). We demonstrate here that native chick laminin-4, which contains the beta 2 chain and is present in the synaptic basement membrane, does not inhibit but rather promotes motor axon growth. In native heterotrimeric laminin, the LRE sequence of the beta 2 chain is found in a triple coiled-coil region that is formed by all three subunits. We show here that the effect of LRE depends on the structural context. Whereas a recombinant randomly coiled LRE peptide indeed inhibited outgrowth by chick motoneurons, a small recombinant triple coiled-coil protein containing this sequence did not.

Specificity and development of innervation pattern of Xenopus pectoralis muscle

Multielectrode recordings were used to identify and measure the axonal inputs to each end plate on contiguous surface fibers covering about 25% of the Xenopus pectoralis muscle in mature and developing animals. The mature innervation pattern was remarkably precise. Individual axons tended to innervate fibers of similar input resistance (R(in)) in compact motor units restricted to only a portion of the region studied. Motor units comprising fibers of similar R(in) overlapped mainly near their borders. Most fibers had two end plates. In more than 80% of these fibers, both end plates received input from the same axon. In 57%, this was the only input to both end plates. This implies a powerful mechanism for excluding or eliminating inputs from other axons. About 16% of the mature junctions showed focal polyneuronal innervation, with the weaker end plate potential component often less than 1 mV in noncurarized preparation. However, we have no evidence that the weaker inputs were being eliminated. During development, motor units became more compact, which was associated with synapse elimination; but from the earliest times studied, soon after metamorphosis when many fibers were adding second end plates, a majority of those that had two end plates were innervated at both sites by the same axon.

Regeneration of specific innervation in Xenopus pectoralis muscle

We investigated the motor unit organization and precision of reinnervation in the Xenopus pectoralis muscle following different manipulations, including crush or section of the posterior pectoralis nerve, foreign nerve innervation, and crush coupled with activity modulation or block. Most fibers have two neuromuscular junctions, and multielectrode recordings were used to identify the axonal origin of all inputs to both junctions on most or all fibers covering about 25% of the muscle surface. Following simple nerve crush, a highly organized innervation pattern was restored, indistinguishable from the normal pattern, including selective innervation of fibers of similar input resistance (R(in)), compact motor unit organization, and high incidence of exclusive innervation of both end plates on each fiber by the same axon (distributed mononeuronal innervation, or a/a pattern). Initial reinnervation was equally precise when nerve conduction in the regenerating nerve was blocked by tetrodotoxin. More distant or repeated nerve crush or nerve section delayed and reduced the precision of reinnervation, but the majority of fibers still received input to both end plates by the same axon, often in combination with others. A foreign nerve, the pectoralis sternalis, which in its own muscle forms only single end plates, showed less precise reinnervation, but still had an incidence of a/a innervation far above chance. These data imply the expression and recognition of remarkably precise chemospecific cues even in mature animals, superimposed on which is a further refinement by synapse elimination, probably based on an activity-dependent process.

Anti-agrin staining is absent at abandoned synaptic sites of frog neuromuscular junctions

The neuromuscular junction is a plastic structure and is constantly undergoing changes as the nerve terminals that innervate the muscle fiber extend and retract their processes. In vivo observations on developing mouse neuromuscular junctions revealed that prior to the retraction of a nerve terminal the acetylcholine receptors (AChRs) under that nerve terminal disperse. Agrin is a protein released by nerve terminals that binds to synaptic basal lamina and directs the aggregation of AChRs and acetylcholinesterase (AChE) in and on the surface of the myotube. Thus, if the AChRs under a nerve terminal disperse, then the cellular signaling mechanism by which agrin maintains the aggregation of those AChRs, must have been disrupted. Two possibilities that could lead to the disruption of the agrin induced aggregation are that agrin is present at the synaptic basal lamina but is unable to direct the aggregation of AChRs, or that agrin has been removed from the synaptic basal lamina. Thus, if agrin were blocked, one would expect to see anti-agrin staining at abandoned synaptic sites; whereas if agrin were removed, anti-agrin staining would be absent at abandoned synaptic sites. We find that anti-agrin staining and alpha-bungarotoxin staining are absent at abandoned synaptic sites. Further, in vivo observations of retracting nerve terminals confirm that agrin is removed from the synaptic basal lamina within 7 days. Thus, while agrin will remain bound to synaptic basal lamina for months following denervation, it is removed within days following synaptic retraction.

The receptor tyrosine kinase MuSK is required for neuromuscular junction formation in vivo

Formation of neuromuscular synapses requires a series of inductive interactions between growing motor axons and differentiating muscle cells, culminating in the precise juxtaposition of a highly specialized nerve terminal with a complex molecular structure on the postsynaptic muscle surface. The receptors and signaling pathways mediating these inductive interactions are not known. We have generated mice with a targeted disruption of the gene encoding MuSK, a receptor tyrosine kinase selectively localized to the postsynaptic muscle surface. Neuromuscular synapses do not form in these mice, suggesting a failure in the induction of synapse formation. Together with the results of an accompanying manuscript, our findings indicate that MuSK responds to a critical nerve-derived signal (agrin), and in turn activates signaling cascades responsible for all aspects of synapse formation, including organization of the postsynaptic membrane, synapse-specific transcription, and presynaptic differentiation.

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.

Multiple strategies for directed growth cone extension and navigation of peripheral neurons

Leeches have a diverse constellation of peripheral neural elements that are challenged to extend growth cones in highly specific ways in a constantly changing embryonic environment. Two major systems are reviewed here. In one, peripheral afferents extend growth cones toward the central nervous system (CNS), forming common pathways, and then segregate into particular tracts within the CNS. A majority of these afferents depend on CNS-derived guidance cues and projections from the CNS to guide their way. However, not all of the nerves are established this way and at least one of the peripheral nerves is likely to be pioneered by sensillar sensory afferents. The distribution of particular antigens (such as the lan3-2 antigen) suggests the identity of molecules involved in homophilic adhesion along common pathways, whereas others (such as the lan4-2 and 3-6 antigens) are candidates for mediating specific pathway choices. In the second system, the myo-organizing Comb cell (C cell) projects multiple growth cones simultaneously along oblique trajectories not influenced by segmental or midline boundaries. Its parallel growth cones exhibit space-filling as well as directional growth and are guided by local cues to extend in discrete phases that are coordinated with the development of the environment. Both systems exhibit highly directed outgrowth orchestrated by a hierarchy of cues, establish patterns of neurites used to direct later migrating cells, and seem to be regulated temporally and spatially by interactions with the embryonic environment. These systems illustrate the strengths of examining neural development in vivo across several levels of analysis.

Schwann cell processes guide regeneration of peripheral axons

Terminal Schwann cells overlying the neuromuscular junction sprout elaborate processes upon muscle denervation. We show here that motor axons use these processes as guides/substrates during regeneration; in so doing, they escape the confines of endplates and grow between endplates to generate polyneuronal innervation. We also show that Schwann cells in the nerve provide similar guidance. Axons extend from the cut end of a nerve in association with Schwann cell processes and appear to navigate along them. The processes extend from axotomized nerves at the same rate and in the same manner as they do from axon-containing nerves. The rate of process extension limits the rate at which axons regenerate. Thus, Schwann cell processes lead and guide peripheral regeneration.

Synaptic activity and connective tissue remodeling in denervated frog muscle

Denervation of skeletal muscle results in dramatic remodeling of the cellular and molecular composition of the muscle connective tissue. This remodeling is concentrated in muscle near neuromuscular junctions and involves the accumulation of interstitial cells and several extracellular matrix molecules. Given the role of extracellular matrix in neurite outgrowth and synaptogenesis, we predict that this remodeling of the junctional connective tissue directly influences the regeneration of the neuromuscular junction. As one step toward understanding the role of this denervation-induced remodeling in synapse formation, we have begun to look for the signals that are involved in initiating the junctional accumulations of interstitial cells and matrix molecules. Here, the role of muscle inactivity as a signal was examined. The distributions of interstitial cells, fibronectin, and tenascin were determined in muscles inactivated by presynaptic blockade of muscle activity with tetrodotoxin. We found that blockade of muscle activity for up to 4 wk produced neither the junctional accumulation of interstitial cells nor the junctional concentrations of tenascin and fibronectin normally present in denervated frog muscle. In contrast, the muscle inactivity induced the extrajunctional appearance of two synapse-specific molecules, the acetylcholine receptor and a muscle fiber antigen, mAb 3B6. These results demonstrate that the remodeling of the junctional connective tissue in response to nerve injury is a unique response of muscle to denervation in that it is initiated by a mechanism that is independent of muscle activity. Thus connective tissue remodeling in denervated skeletal muscle may be induced by signals released from or associated with the nerve other than the evoked release of neurotransmitter.

Factors causing different properties at neuromuscular junctions in fast and slow rat skeletal muscles

Neuromuscular junctions on fast and slow skeletal muscle fibers have different properties. Possible reasons for these differences were examined in adult rat soleus (SOL) muscle fibers reinnervated at new ectopic or old denervated sites by fast fibular (FIB) or slow SOL motoneurons. FIB motoneurons formed large ectopic junctions with a high density of nerve terminal varicosities (fast appearance), whereas SOL motoneurons formed small ectopic junctions with a low density of varicosities (slow appearance). Both FIB and SOL motoneurons formed small junctions with a slow appearance when reinnervating old SOL endplates. FIB nerves innervating ectopic sites and SOL nerves reinnervating old sites had the same appearance whether they contacted the SOL fibers alone (single innervation) or together (dual innervation). Continuous stimulation of the FIB nerve at 10 Hz for 3-4 months reduced the size of ectopic FIB and intact extensor digitorum longus (EDL) junctions and caused a modest reduction in density of terminal varicosities in EDL. Junction size and muscle fiber diameter were positively correlated, but the slope describing this relation was steeper for FIB junctions than for SOL junctions. It is concluded that in the present system (1) motoneuron type and not muscle fiber type determines the fast or slow character of the neuromuscular junction. (2) denervated endplates of one type place stable and severe constraints on the termination pattern of reinnervating axons of another type, (3) the appearance of fast EDL junctions undergoes a modest fast to slow transformation when exposed to long-term slow pattern stimulation, and (4) not only the size of the muscle fibers, but also the type and firing pattern of the motoneurons and the spatial constraints at preformed endplates influence the relation between junction size and muscle fiber diameter.

Retrograde signaling in the formation and maintenance of the neuromuscular junction

The neuromuscular junction is characterized by precise alignment between the nerve terminal and the postsynaptic apparatus formed by the muscle fiber. Organization of the neuromuscular junction during embryonic development, growth, and maintenance is coordinated by signals exchanged between motor neurons and their target muscle fibers. Identification of proteins such as agrin, likely to represent neuronal agents that direct the organization of the postsynaptic apparatus, has focused attention on characterization of proteins that mediate retrograde signals that regulate the organization and function of the nerve terminal. The results of these studies implicate a role for both adhesive and diffusible signals in coordinating the development, maturation, and maintenance of the motor nerve terminal. The diversity of molecules identified to date that appear to play a role in these processes implies a considerable level of redundancy in the transduction pathway. However, studies of early nerve-muscle interactions suggest that a common feature of many of these retrograde agents is activation of a protein kinase coupled with an increase in cytosolic Ca2+ concentration. While the molecular signals that regulate growth and maintenance of neuromuscular junctions are less well understood it seems likely that similar adhesive and diffusible factors will be involved.

Acetylcholinesterase density and turnover number at frog neuromuscular junctions, with modeling of their role in synaptic function

Acetylcholinesterase (AChE) density at the neuromuscular junction of frog cutaneous pectoris muscle was determined by electron microscope autoradiography and biochemistry to be approximately 600 sites micron-2 of postsynaptic area, approximately 4-fold lower than all previous reports (mouse), whereas the hydrolytic turnover number was 9,500 s-1, well within the range (2,000-16,000 s-1) for AChE from other species. Monte Carlo computer simulations of miniature endplate currents showed that for vertebrate neuromuscular junctions with different morphologies, an AChE density of only approximately 400 sites microns-2 and a turnover number of only approximately 1,000 s-1 are sufficient for normal quantal currents. Above these critical lower limits, miniature endplate currents were essentially insensitive to AChE density and turnover number values up to 5,000 sites microns-2 and 16,000 s-1, respectively.

N-cadherin expression in developing, adult and denervated chicken neuromuscular system: accumulations at both the neuromuscular junction and the node of Ranvier

N-cadherin, a member of the Ca(2+)-dependent cell adhesion molecule family plays essential roles in morphogenesis and histogenesis. N-cadherin has been shown in vitro to promote myoblast fusion and neurite outgrowth. We report here the cellular localization of N-cadherin during development and regeneration of the chick neuromuscular system. N-cadherin was uniformly expressed along the surface of myoblasts and myotubes of E6 limb muscles. Later, as synaptogenesis and secondary myogenesis proceeded, N-cadherin expression was down-regulated and restricted to some large-diameter fibres, then to the areas of contact between few myofibres and subsequently disappeared by embryonic day 17, suggesting that this cadherin may be implicated predominantly in fusion of primary myoblasts and, at lower degree, of secondary myoblasts. The presence of N-cadherin in muscle during the period of nerve trunk ingrowth and its down-regulation after synaptogenesis suggests that this molecule might be implicated in both processes. N-cadherin became accumulated at the neuromuscular junction only a few days after the first synaptic contacts were established and remained at the adult neuromuscular junction, suggesting a role of this molecule in the stabilization of the mature neuromuscular junction. In sciatic nerve, the level of N-cadherin expression remained unchanged from hatching to adult life. N-cadherin was widely distributed on the surface of myelinated fibres and on myelinating Schwann cells: in addition, it was concentrated at the node of Ranvier. At the ultrastructural level, the molecule was detected inside, at the surface and in the basal lamina of Schwann cells and also associated with endoneurial collagen. These observations suggest a role of N-cadherin in the structuring and stabilization of the myelin sheaths. After nerve injury, N-cadherin continued to be expressed by proliferating Schwann cells in the distal stump providing a substratum for regenerating axons. N-cadherin reappeared at the surface of denervated muscle fibres without disappearing from the former synaptic sites. It was detected not only in the sarcoplasm and on sarcolemma of denervated muscle fibres, but also in the basal lamina and in the extracellular matrix. The reexpression of N-cadherin at the surface of denervated muscle fibres suggests a role for this molecule in muscle reinnervation. The presence of N-cadherin in basal lamina and its association with collagen fibres raise questions about the release of N-cadherin in the extracellular space and the existence of a putative heterophilic ligand for N-cadherin.

Synaptic reorganization in developing and adult nervous systems

Evidence is accumulating that synapse reorganization already starts during development, soon after first synapses appear. Although remodeling continues throughout ontogenesis, there are apparently (critical) periods which are characterized by enhanced synaptic reorganization. In certain parts of the peripheral and central nervous system, synapses may undergo remodeling which leads to changes in their transmission efficiency or complete elimination of the synaptic junctions, even in adulthood. Synaptic reorganization includes progressive and regressive changes on branches of dendritic and/or axonal processes that accompany the formation and elimination of synapses. Three modes of elimination are presently known: Physiological cell death of synaptically connected neurons is involved, especially during certain developmental periods, during hormonally induced metamorphosis and in the olfactory bulb. Synaptic disconnection ("stripping") and lysosomal degradation predominantly of presynaptic elements occur under different conditions. In order to undergo plastic changes, neurons seem to respond to exogenous or intrinsic factors such as lesions (partial deafferentation and axotomy), long-lasting changes in neuronal activity (e.g. drug application and sensory deprivation), hormonal influences (e.g. sexual hormones) or learning conditions.

Scanning electron microscopic study of denervated and reinnervated intrafusal muscle fibers in rats

Isolated muscle spindles from lower lumbrical muscles of rats were used to study the 3-dimensional organization of intrafusal structures by scanning electron microscopy following (a) complete denervation, (b) reinnervation after a single crush lesion of the sciatic nerve, or (c) reinnervation after transection and immediate suture of this nerve. One week after complete denervation, previous sites of intrafusal motor endplates were transformed into sarcolemmal ovoid bulges. These bulges persisted in denervated muscle spindles up to 12 weeks. Regenerated motor nerve endings were detected on intrafusal muscle fibers 1 month, and thereafter following sciatic nerve crush injuries, and 3 months and later following transection and suture of the nerve. Furthermore, 3 different types of subsynaptic areas of motor nerve terminals were observed. The scanning electron microscopic technique also allowed visualization of splitting and fusion of intrafusal muscle fibers. The findings are discussed in view of their possible functional implications.

Cytotactin is involved in synaptogenesis during regeneration of the frog neuromuscular system

The expression of cytotactin, an extracellular matrix glycoprotein involved in morphogenesis and regeneration, was determined in the normal and regenerating neuromuscular system of the frog Rana temporaria. Cytotactin was expressed in adult brain and gut as two major components of Mr 190,000 and 200,000 and a minor form of higher molecular weight, but was almost undetectable in skeletal muscle extract. However, cytotactin was concentrated at the neuromuscular junctions as well as at the nodes of Ranvier. After nerve transection, cytotactin staining increased in the distal stump along the endoneurial tubes. In preparations of basal lamina sheaths of frog cutaneous pectoris muscle obtained by inducing the degeneration of both nerve and muscle fibers, cytotactin was found in dense accumulations at original synaptic sites. In order to define the role of cytotactin in axonal regeneration and muscle reinnervation, the effect of anti-cytotactin antibodies on the reinnervation of the basal lamina sheaths preparations was examined in vivo. In control preparations, regenerating nerve terminals preferentially reinnervate the original synaptic sites. In the presence of anti-cytotactin antibodies, axon regeneration occurred with normal fasciculation and branching but with altered preterminal nerve fibers pathways. Ultrastructural observations showed that synaptic basal laminae reinnervation was greatly delayed or inhibited. These results suggest that cytotactin plays a primordial role in synaptogenesis, at least during nerve regeneration and reinnervation in the adult neuromuscular system.

Acetylcholinesterase from the motor nerve terminal accumulates on the synaptic basal lamina of the myofiber

Acetylcholinesterase (AChE) in skeletal muscle is concentrated at neuromuscular junctions, where it is found in the synaptic cleft between muscle and nerve, associated with the synaptic portion of the myofiber basal lamina. This raises the question of whether the synaptic enzyme is produced by muscle, nerve, or both. Studies on denervated and regenerating muscles have shown that myofibers can produce synaptic AChE, and that the motor nerve may play an indirect role, inducing myofibers to produce synaptic AChE. The aim of this study was to determine whether some of the AChE which is known to be made and transported by the motor nerve contributes directly to AChE in the synaptic cleft. Frog muscles were surgically damaged in a way that caused degeneration and permanent removal of all myofibers from their basal lamina sheaths. Concomitantly, AChE activity was irreversibly blocked. Motor axons remained intact, and their terminals persisted at almost all the synaptic sites on the basal lamina in the absence of myofibers. 1 mo after the operation, the innervated sheaths were stained for AChE activity. Despite the absence of myofibers, new AChE appeared in an arborized pattern, characteristic of neuromuscular junctions, and its reaction product was concentrated adjacent to the nerve terminals, obscuring synaptic basal lamina. AChE activity did not appear in the absence of nerve terminals. We concluded therefore, that the newly formed AChE at the synaptic sites had been produced by the persisting axon terminals, indicating that the motor nerve is capable of producing some of the synaptic AChE at neuromuscular junctions. The newly formed AChE remained adherent to basal lamina sheaths after degeneration of the terminals, and was solubilized by collagenase, indicating that the AChE provided by nerve had become incorporated into the basal lamina as at normal neuromuscular junctions.

Biochemical, morphological, and functional changes during peripheral nerve regeneration

The success of axon regeneration after nerve injury should be judged by the extent to which the target organs regain their function. Recovery of muscle contraction involves axon regeneration, reestablishment of nerve-muscle connections, recovery of transmission, and muscle force. All these processes were investigated under the same experimental conditions and correlated in order to better understand their time-course and interdependence. The sciatic nerve of a rat was crushed in the thigh. The ingrowth of regenerating motor axons into the soleus (SOL) and extensor digitorum longus (EDL) muscles was monitored by measuring the activity of choline acetyltransferase (ChAT), a marker enzyme for cholinergic nerve terminals, in the muscles. The electron microscopic cytochemistry of acetylcholine esterase (AChE) was used to estimate the reestablishment of neuromuscular junctions in these two muscles. The recovery of muscle contraction was followed by measuring the force of isometric contraction in the triceps surae muscle in vivo. The pattern of ChAT recovery during reinnervation was similar in the EDL and SOL. The statistically significant increase of ChAT activity in these muscles, 14 d after the nerve crush, signified the entry of regenerating axons into the calf muscles. Electron microscopic cytochemistry revealed the first small nerve endings in contact with the denervated end plates 12 d after denervation. Subsequently, the number of reinnervated motor end plates and the surface area of the neuromusclar junctions steadily increased. The recovery of muscle force started between d 14 and 21 after the nerve crush. Thirty-five days after denervation, the difference between the muscle force of the reinnervated muscle and the control became statistically insignificant. Morphological normalization of the motor end plates was practically complete 33 d after denervation, concomitant with the normalization of the muscle force. At that time, however, ChAT activity in both muscles was still clearly subnormal (33.5% in EDL and 45% of the control in SOL) and therefore does not reflect the true extent of muscle force recovery. Yet, it seems that in spite of this, the regenerated nerve terminals contained sufficient amounts of acetylcholine (ACh) to trigger normal muscle contractions.

Electromyography and morphology during regeneration of muscle injury in rats

Healing of the partially ruptured rat gastrocnemius muscle was studied correlating electromyographical findings with morphological changes. Fibrillation potentials and positive sharp waves were seen both proximal and distal to the injury 7 days after the injury and disappeared in the proximal part by day 14 and in the distal part by day 21. Late components of motor action potential were observed from day 14 onwards. Denervation was mainly myogenic, i.e. caused either by rupture of myofibres, whence the abjunctional stump lost its contact with the neuromuscular junction on the adjunctional stump, or by necrosis of the segment of the ruptured myofibre lying underneath the neuromuscular junction. Lesser extent of denervation was neurogenic, i.e. caused by damage to intramuscular nerve fibres. The reinnervation occurs either by regeneration of the necrotized myofibres, by regeneration of the severed nerves, or by collateral innervation of new neuromuscular junctions in the abjunctional stumps. The present study indicates that electromyography may be useful in the diagnosis and follow-up of skeletal muscle injuries.

Macrophages direct process elongation from adult frog motorneurons in culture

Motorneurons and macrophages have been isolated and identified in primary cultures from adult frog (Rana pipiens) spinal cord. Time-lapse video microscopy revealed that during the first two weeks migrating macrophages contact the growth cones of motorneurons. As they continue to migrate, the motorneuron processes elongate in close association with the moving macrophages. Elongating motorneuron processes are thereby brought into contact with other motorneurons and networks are formed. At later stages, the macrophages die but the motorneurons and the networks survive for at least another two weeks. These experiments show that macrophages can promote a directed elongation of motorneuron processes and suggest that they play a similar role during regeneration in vivo.

Selective promotion of neuronal adhesion by target tissue-derived materials

Chick sympathetic nerves densely innervate expansor secundariorum muscle, but not skeletal muscle. By contrast, parasympathetic ciliary nerves innervate striated muscle, but no parasympathetic innervation occurs in expansor secundariorum muscle. The present study revealed accordingly that sympathetic ganglionic neurons from chick embryos adhered firmly to a dish precoated with expansor secundariorum muscle-conditioned medium (ECM), but not to one precoated with skeletal muscle-conditioned medium (SCM), while parasympathetic ciliary ganglionic neurons adhered to a dish precoated with SCM, but not to one precoated with ECM. These results indicate that there are certain materials which mediate specific adhesion between neurons and their target cells.

Inactivity-induced motor nerve terminal sprouting in amphibian skeletal muscles chronically blocked by alpha-bungarotoxin

There is a positive correlation between contractile inactivity and the initiation of motor neuron sprouting. However, the exact mechanism responsible for this neuronal growth remains obscure. In a previous study (M. M. Wines and M.S. Letinsky, 1988, J. Neurosci. 8: 3909-3919) we investigated this phenomenon by inducing chronic contractile inactivity of an amphibian muscle by exposure to formamide and found that motor neuron sprouting occurs in the presence of normal pressynaptic transmitter release and propagated muscle fiber action potentials. The present study investigates motor neuron sprouting in response to inactivity produced when neuromuscular transmission is blocked by chronic exposure to alpha-bungarotoxin (alpha-BTX). The alpha-BTX-induced muscle paralysis was maintained for 1-63 days by repetitive application of the toxin to the cutaneous pectoris muscle of adult Rana pipiens. During the chronic alpha-BTX treatment end-plate potentials were reduced below threshold, which therefore removed both muscle fiber action potentials and contractile activity. Our findings showed only terminal sprouting. Also, higher sprouting frequencies (up to 100% of the observed terminals) were observed after chronic alpha-BTX treatment, compared to the sprouting response induced by formamide treatment. In view of our earlier formamide results, these observations suggest that the inhibition of the postsynaptic acetylcholine response, and consequently inhibition of muscle fiber electrical and contractile activity, produces a stronger stimulus to motor neuron sprouting than the presence of contractile inactivity alone coupled with normal synaptic transmission and muscle electrical activity.

Elevated levels of polyneuronal innervation persist for as long as two years in reinnervated frog neuromuscular junctions

When the nerve to an adult frog sartorius muscle is crushed, and axons are allowed to regenerate, the level of polyneuronal innervation at reinnervated neuromuscular junctions is higher than normal. With time, much of this polyneuronal innervation is reduced by the process of synapse elimination (Werle and Herrera, 1988). Using intracellular recording, we estimated the level of polyneuronal innervation in adult frog (Rana pipiens) sartorius muscles 2 years (range: 1.7-2.4 years) after crushing the sartorius nerve. We found that 27% (S.E. = 1.4%) of the junctions in muscles 2 years after reinnervation were polyneuronally innervated, whereas only 10% (S.E. = 1.2%) of the junctions in normal frog muscles were polyneuronally innervated. Thus, the synapse elimination that occurs following reinnervation does not restore the normal level of polyneuronal innervation. Histological comparisons of junctional structure between muscles 2 years after reinnervation and normal muscles revealed substantial differences. Reinnervated junctions had a greater length of synaptic gutter apposed by nerve terminal processes, more axonal inputs, more empty synaptic gutter, more instances of single synaptic gutters innervated by more than one axon, and longer lengths of nerve terminal processes that connect synaptic gutters within a junction. On the basis of this physiological and anatomical evidence, we conclude that nerve injury causes persistent changes in the pattern of muscle innervation.

Effects of denervation on the distribution of peanut agglutinin binding molecules in frog muscles

The extracellular matrix has been shown to play an important role in the differentiation of neuromuscular junctions during reinnervation in frogs. Peanut agglutinin, a lectin, is known to specifically bind to some glycoconjugates in the extracellular matrix at the frog neuromuscular junction and myotendinous junction. In order to determine if innervation has any role in regulating the specific binding of peanut agglutinin at neuromuscular junctions and myotendinous junctions, the distribution of peanut agglutinin binding was examined in muscles chronically denervated for various periods. Short-term denervated muscles (less than or equal to 2 months) showed no changes in peanut binding agglutinin binding at neuromuscular junctions and no extrajunctional binding. In contrast, long-term denervation (greater than 2 months - 7.5 months) resulted in altered peanut agglutinin distribution and a substantial reduction or a total loss in its binding at denervated neuromuscular junctions; binding at myotendinous junctions was not affected. Results of electron microscopic studies suggest that the presence of Schwann cells at denervated endplates delays the loss of peanut agglutinin binding. Reinnervation restores normal peanut agglutinin binding at neuromuscular junctions following long-term denervation. This report demonstrates that although the distribution of peanut agglutinin binding molecules is unchanged short-term denervation, intact innervation is necessary for the long-term maintenance of these molecules at neuromuscular junctions.

Long-term survival and sprouting in culture by motoneurons isolated from the spinal cord of adult frogs

Motoneurons of adult frog spinal cord have been retrogradely labeled with the carbocyanine derivative diI. Spinal cords were then dissociated and the labeled motoneurons partially purified by centrifugation over a bovine serum albumin (BSA) density gradient. The resulting cell suspension was plated on a substrate of innervated muscle extracellular matrix (ECM) to which the motoneurons attached and extended processes. Labeled adult motoneurons survived for more than 4 weeks in a defined medium in the absence of added serum or growth factors. These cultures of adult motoneurons provide a favorable preparation for studying molecular factors that influence process outgrowth and synapse formation.