Reduction of neuromuscular activity is required for the rescue of motoneurons from naturally occurring cell death by nicotinic-blocking agents

Structure and Function of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) comprises a presynaptic terminal, a postsynaptic receptor region on the muscle fiber (endplate), and the perisynaptic (terminal) Schwann cell. As with any synapse, the purpose of the NMJ is to transmit signals from the nervous system to muscle fibers. This neural control of muscle fibers is organized as motor units, which display distinct structural and functional phenotypes including differences in pre- and postsynaptic elements of NMJs. Motor units vary considerably in the frequency of their activation (both motor neuron discharge rate and duration/duty cycle), force generation, and susceptibility to fatigue. For earlier and more frequently recruited motor units, the structure and function of the activated NMJs must have high fidelity to ensure consistent activation and continued contractile response to sustain vital motor behaviors (e.g., breathing and postural balance). Similarly, for higher force less frequent behaviors (e.g., coughing and jumping), the structure and function of recruited NMJs must ensure short-term reliable activation but not activation sustained for a prolonged period in which fatigue may occur. The NMJ is highly plastic, changing structurally and functionally throughout the life span from embryonic development to old age. The NMJ also changes under pathological conditions including acute and chronic disease. Such neuroplasticity often varies across motor unit types. © 2022 American Physiological Society. Compr Physiol 12:1-36, 2022.

Motoneuron-specific loss of VAChT mimics neuromuscular defects seen in congenital myasthenic syndrome

Motoneurons (MNs) control muscle activity by releasing the neurotransmitter acetylcholine (ACh) at the level of neuromuscular junctions. ACh is packaged into synaptic vesicles by the vesicular ACh transporter (VAChT), and disruptions in its release can impair muscle contraction, as seen in congenital myasthenic syndromes (CMS). Recently, VAChT gene mutations were identified in humans displaying varying degrees of myasthenia. Moreover, mice with a global deficiency in VAChT expression display several characteristics of CMS. Despite these findings, little is known about how a long-term decrease in VAChT expression in vivo affects MNs structure and function. Using Cre-loxP technology, we generated a mouse model where VAChT is deleted in select groups of MNs (mnVAChT-KD). Molecular analysis revealed that the VAChT deletion was specific to MNs and affected approximately 50% of its population in the brainstem and spinal cord, with alpha-MNs primarily targeted (70% in spinal cord). Within each animal, the cell body area of VAChT-deleted MNs was significantly smaller compared to MNs with VAChT preserved. Likewise, muscles innervated by VAChT-deleted MNs showed atrophy while muscles innervated by VAChT-containing neurons appeared normal. In addition, mnVAChT KD mice had decreased muscle strength, were hypoactive, leaner and exhibited kyphosis. This neuromuscular dysfunction was evident at 2 months of age and became progressively worse by 6 months. Treatment of mutants with a cholinesterase inhibitor was able to improve some of the motor deficits. As these observations mimic what is seen in CMS, this new line could be valuable for assessing the efficacy of potential CMS drugs.

Roles of Collagen XXV and Its Putative Receptors PTPσ/δ in Intramuscular Motor Innervation and Congenital Cranial Dysinnervation Disorder

Intramuscular motor innervation is an essential process in neuromuscular development. Recently, mutations in COL25A1, encoding CLAC-P/collagen XXV, have been linked to the development of a congenital cranial dysinnervation disorder (CCDD). Yet the molecular mechanisms of intramuscular innervation and the etiology of CCDD related to COL25A1 have remained elusive. Here, we report that muscle-derived collagen XXV is indispensable for intramuscular innervation. In developing skeletal muscles, Col25a1 expression is tightly regulated by muscle excitation. In vitro and cell-based assays reveal a direct interaction between collagen XXV and receptor protein tyrosine phosphatases (PTPs) σ and δ. Motor explant assays show that expression of collagen XXV in target cells attracts motor axons, but this is inhibited by exogenous PTPσ/δ. CCDD mutations attenuate motor axon attraction by reducing collagen XXV-PTPσ/δ interaction. Overall, our study identifies PTPσ/δ as putative receptors for collagen XXV, implicating collagen XXV and PTPσ/δ in intramuscular innervation and a developmental ocular motor disorder.

Blocking skeletal muscle DHPRs/Ryr1 prevents neuromuscular synapse loss in mutant mice deficient in type III Neuregulin 1 (CRD-Nrg1)

Schwann cells are integral components of vertebrate neuromuscular synapses; in their absence, pre-synaptic nerve terminals withdraw from post-synaptic muscles, leading to muscle denervation and synapse loss at the developing neuromuscular junction (NMJ). Here, we report a rescue of muscle denervation and neuromuscular synapses loss in type III Neuregulin 1 mutant mice (CRD-Nrg1-/-), which lack Schwann cells. We found that muscle denervation and neuromuscular synapse loss were prevented in CRD-Nrg1-/-mice when presynaptic activity was blocked by ablating a specific gene, such as Snap25 (synaptosomal-associated 25 kDa protein) or Chat (choline acetyltransferase). Further, these effects were mediated by a pathway that requires postsynaptic acetylcholine receptors (AChRs), because ablating Chrna1 (acetylcholine receptor α1 subunit), which encodes muscle-specific AChRs in CRD-Nrg1-/-mice also rescued muscle denervation. Moreover, genetically ablating muscle dihydropyridine receptor (DHPR) β1 subunit (Cacnb1) or ryanodine receptor 1 (Ryr1) also rescued muscle denervation and neuromuscular synapse loss in CRD-Nrg1-/-mice. Thus, these genetic manipulations follow a pathway-from presynaptic to postsynaptic, and, ultimately to muscle activity mediated by DHPRs and Ryr1. Importantly, electrophysiological analyses reveal robust synaptic activity in the rescued, Schwann-cell deficient NMJs in CRD-Nrg1-/-Cacnb1-/-or CRD-Nrg1-/-Ryr1-/-mutant mice. Thus, a blockade of synaptic activity, although sufficient, is not necessary to preserve NMJs that lack Schwann cells. Instead, a blockade of muscle activity mediated by DHRPs and Ryr1 is both necessary and sufficient for preserving NMJs that lack Schwann cells. These findings suggest that muscle activity mediated by DHPRs/Ryr1 may destabilize developing NMJs and that Schwann cells play crucial roles in counteracting such a destabilizing activity to preserve neuromuscular synapses during development.

Differences in lumbar motor neuron pruning in an animal model of early onset spasticity

Motor neuron (MN) development in early onset spasticity is poorly understood. For example, spastic cerebral palsy (sCP), the most common motor disability of childhood, is poorly predicted by brain imaging, yet research remains focused on the brain. By contrast, MNs, via the motor unit and neurotransmitter signaling, are the target of most therapeutic spasticity treatments and are the final common output of motor control. MN development in sCP is a critical knowledge gap, because the late embryonic and postnatal periods are not only when the supposed brain injury occurs but also are critical times for spinal cord neuromotor development. Using an animal model of early onset spasticity [ spa mouse (B6.Cg- Glrb^spa/J) with a glycine (Gly) receptor mutation], we hypothesized that removal of effective glycinergic neurotransmitter inputs to MNs during development will influence MN pruning (including primary dendrites) and MN size. Spa (Glrb-/-) and wild-type (Glrb+/+) mice, ages 4-9 wk, underwent unilateral retrograde labeling of the tibialis anterior muscle MNs via peroneal nerve dip in tetramethylrhodamine. After 3 days, mice were euthanized and perfused with 4% paraformaldehyde, and the spinal cord was excised and processed for confocal imaging. Spa mice had ~61% fewer lumbar tibialis anterior MNs ( P < 0.01), disproportionately affecting larger MNs. Additionally, a ~23% reduction in tibialis anterior MN somal surface area ( P < 0.01) and a 12% increase in primary dendrites ( P = 0.046) were observed. Thus MN pruning and MN somal surface area are abnormal in early onset spasticity. Fewer and smaller MNs may contribute to the spastic phenotype. NEW & NOTEWORTHY Motor neuron (MN) development in early onset spasticity is poorly understood. In an animal model of early onset spasticity, spa mice, we found ~61% fewer lumbar tibialis anterior MNs compared with controls. This MN loss disproportionately affected larger MNs. Thus number and heterogeneity of the MN pool are decreased in spa mice, likely contributing to the spastic phenotype.

Acetylcholine receptors from human muscle as pharmacological targets for ALS therapy

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease affecting motor neurons that leads to progressive paralysis of skeletal muscle. Studies of ALS have revealed defects in expression of acetylcholine receptors (AChRs) in skeletal muscle that occur even in the absence of motor neuron anomalies. The endocannabinoid palmitoylethanolamide (PEA) modified the clinical conditions in one ALS patient, improving muscle force and respiratory efficacy. By microtransplanting muscle membranes from selected ALS patients into Xenopus oocytes, we show that PEA reduces the desensitization of acetylcholine-evoked currents after repetitive neurotransmitter application (i.e., rundown). The same effect was observed using muscle samples from denervated (non-ALS) control patients. The expression of human recombinant α1β1γδ (γ-AChRs) and α1β1εδ AChRs (ε-AChRs) in Xenopus oocytes revealed that PEA selectively affected the rundown of ACh currents in ε-AChRs. A clear up-regulation of the α1 subunit in muscle from ALS patients compared with that from non-ALS patients was found by quantitative PCR, but no differential expression was found for other subunits. Clinically, ALS patients treated with PEA showed a lower decrease in their forced vital capacity (FVC) over time as compared with untreated ALS patients, suggesting that PEA can enhance pulmonary function in ALS. In the present work, data were collected from a cohort of 76 ALS patients and 17 denervated patients. Our results strengthen the evidence for the role of skeletal muscle in ALS pathogenesis and pave the way for the development of new drugs to hamper the clinical effects of the disease.

Thioredoxin-2 Modulates Neuronal Programmed Cell Death in the Embryonic Chick Spinal Cord in Basal and Target-Deprived Conditions

Thioredoxin-2 (Trx2) is a mitochondrial protein using a dithiol active site to reduce protein disulfides. In addition to the cytoprotective function of this enzyme, several studies have highlighted the implication of Trx2 in cellular signaling events. In particular, growing evidence points to such roles of redox enzymes in developmental processes taking place in the central nervous system. Here, we investigate the potential implication of Trx2 in embryonic development of chick spinal cord. To this end, we first studied the distribution of the enzyme in this tissue and report strong expression of Trx2 in chick embryo post-mitotic neurons at E4.5 and in motor neurons at E6.5. Using in ovo electroporation, we go on to highlight a cytoprotective effect of Trx2 on the programmed cell death (PCD) of neurons during spinal cord development and in a novel cultured spinal cord explant model. These findings suggest an implication of Trx2 in the modulation of developmental PCD of neurons during embryonic development of the spinal cord, possibly through redox regulation mechanisms.

Genetic absence of the vesicular inhibitory amino acid transporter differentially regulates respiratory and locomotor motor neuron development

During mid to late embryonic development (E13 to birth in mice), the neuromotor system is refined by reducing motor neuron (MN) numbers and establishing nascent synaptic connections onto and by MNs. Concurrently, the response to GABAergic and glycinergic synaptic activity switches from postsynaptic excitation to inhibition. Our previous studies on mutant mice lacking glycinergic transmission or deficient in GABA suggests that altered MN activity levels during this developmental period differentially regulates MN survival and muscle innervation for respiratory and non-respiratory motor pools. To determine if combined loss of GABAergic and glycinergic transmission plays a similar or exaggerated role, we quantified MN number and muscle innervation in two respiratory (hypoglossal and phrenic) and two locomotor (brachial and lumbar) motor pools, in mice lacking vesicular inhibitory amino acid transporter, which display absent or severely impaired GABAergic and glycinergic neurotransmission. For respiratory MNs, we observed significant decreases in MN number (-20 % hypoglossal and -36 % phrenic) and diaphragm axonal branching (-60 %). By contrast, for non-respiratory brachial and lumbar MNs, we observed increases in MN number (+62 % brachial and +84 % lumbar) and axonal branching for innervated muscles (+123 % latissimus dorsi for brachial and +61 % gluteal for lumbar). These results show that combined absence of GABAergic and glycinergic neurotransmission causes distinct regional changes in MN number and muscle innervation, which are dependent upon the motor function of the specific motor pool.

Genetic deficiency of GABA differentially regulates respiratory and non-respiratory motor neuron development

Central nervous system GABAergic and glycinergic synaptic activity switches from postsynaptic excitation to inhibition during the stage when motor neuron numbers are being reduced, and when synaptic connections are being established onto and by motor neurons. In mice this occurs between embryonic (E) day 13 and birth (postnatal day 0). Our previous work on mice lacking glycinergic transmission suggested that altered motor neuron activity levels correspondingly regulated motor neuron survival and muscle innervation for all respiratory and non respiratory motor neuron pools, during this period of development [1]. To determine if GABAergic transmission plays a similar role, we quantified motor neuron number and the extent of muscle innervation in four distinct regions of the brain stem and spinal cord; hypoglossal, phrenic, brachial and lumbar motor pools, in mice lacking the enzyme GAD67. These mice display a 90% drop in CNS GABA levels ( [2]; this study). For respiratory-based motor neurons (hypoglossal and phrenic motor pools), we have observed significant drops in motor neuron number (17% decline for hypoglossal and 23% decline for phrenic) and muscle innervations (55% decrease). By contrast for non-respiratory motor neurons of the brachial lateral motor column, we have observed an increase in motor neuron number (43% increase) and muscle innervations (99% increase); however for more caudally located motor neurons within the lumbar lateral motor column, we observed no change in either neuron number or muscle innervation. These results show in mice lacking physiological levels of GABA, there are distinct regional changes in motor neuron number and muscle innervation, which appear to be linked to their physiological function and to their rostral-caudal position within the developing spinal cord. Our results also suggest that for more caudal (lumbar) regions of the spinal cord, the effect of GABA is less influential on motor neuron development compared to that of glycine.

Differential effects of RET and TRKB on axonal branching and survival of parasympathetic neurons

Interactions between neurons and their targets of innervation influence many aspects of neural development. To examine how synaptic activity interacts with neurotrophic signaling, we determined the effects of blocking neuromuscular transmission on survival and axonal outgrowth of ciliary neurons from the embryonic chicken ciliary ganglion. Ciliary neurons undergo a period of cell loss due to programmed cell death between embryonic Days (E) 8 and 14 and they innervate the striated muscle of the iris. The nicotinic antagonist d-tubocurarine (dTC) induces an increase in branching measured by counting neurofilament-positive voxels (NF-VU) in the iris between E14-17 while reducing ciliary neuron survival. Blocking ganglionic transmission with dihyro-β-erythroidin and α-methyllycacontine does not mimic dTC. At E8, many trophic factors stimulate neurite outgrowth and branching of neurons placed in cell culture; however, at E13, only GDNF stimulates branching selectively in cultured ciliary neurons. The GDNF-induced branching at E13 could be inhibited by BDNF. Blocking ret signaling in vivo with a dominant negative (dn)ret decreases survival of ciliary and choroid neurons at E14 and prevents dTC induced increases in NF-VU in the iris at E17. Blocking TRKB signaling with dn TRKB increases NF-VU in the iris at E17 and decreases neuronal survival at E17, but not at E14. Thus, RET promotes survival during programmed cell death in the ciliary ganglion and contributes to promoting branching when synaptic transmission is blocked while TRKB inhibits branching and promotes maintenance of neuronal survival. These studies highlight the multifunctional nature of trophic molecule function during neuronal development.

Riluzole blocks human muscle acetylcholine receptors

Riluzole, the only drug available against amyotrophic lateral sclerosis (ALS), has recently been shown to block muscle ACh receptors (AChRs), raising concerns about possible negative side-effects on neuromuscular transmission in treated patients. In this work we studied riluzole's impact on the function of muscle AChRs in vitro and on neuromuscular transmission in ALS patients, using electrophysiological techniques. Human recombinant AChRs composed of α(1)β(1)δ subunits plus the γ or ε subunit (γ- or ε-AChR) were expressed in HEK cells or Xenopus oocytes. In both preparations, riluzole at 0.5 μm, a clinically relevant concentration, reversibly reduced the amplitude and accelerated the decay of ACh-evoked current if applied before coapplication with ACh. The action on γ-AChRs was more potent and faster than on ε-AChRs. In HEK outside-out patches, riluzole-induced block of macroscopic ACh-evoked current gradually developed during the initial milliseconds of ACh presence. Single channel recordings in HEK cells and in human myotubes from ALS patients showed that riluzole prolongs channel closed time, but has no effect on channel conductance and open duration. Finally, compound muscle action potentials (CMAPs) evoked by nerve stimulation in ALS patients remained unaltered after a 1 week suspension of riluzole treatment. These data indicate that riluzole, while apparently safe with regard to synaptic transmission, may affect the function of AChRs expressed in denervated muscle fibres of ALS patients, with biological consequences that remain to be investigated.

Novel mouse model reveals distinct activity-dependent and -independent contributions to synapse development

The balanced action of both pre- and postsynaptic organizers regulates the formation of neuromuscular junctions (NMJ). The precise mechanisms that control the regional specialization of acetylcholine receptor (AChR) aggregation, guide ingrowing axons and contribute to correct synaptic patterning are unknown. Synaptic activity is of central importance and to understand synaptogenesis, it is necessary to distinguish between activity-dependent and activity-independent processes. By engineering a mutated fetal AChR subunit, we used homologous recombination to develop a mouse line that expresses AChR with massively reduced open probability during embryonic development. Through histological and immunochemical methods as well as electrophysiological techniques, we observed that endplate anatomy and distribution are severely aberrant and innervation patterns are completely disrupted. Nonetheless, in the absence of activity AChRs form postsynaptic specializations attracting motor axons and permitting generation of multiple nerve/muscle contacts on individual fibers. This process is not restricted to a specialized central zone of the diaphragm and proceeds throughout embryonic development. Phenotypes can be attributed to separate activity-dependent and -independent pathways. The correct patterning of synaptic connections, prevention of multiple contacts and control of nerve growth require AChR-mediated activity. In contrast, myotube survival and acetylcholine-mediated dispersal of AChRs are maintained even in the absence of AChR-mediated activity. Because mouse models in which acetylcholine is entirely absent do not display similar effects, we conclude that acetylcholine binding to the AChR initiates activity-dependent and activity-independent pathways whereby the AChR modulates formation of the NMJ.

Glial cell line-derived neurotrophic factor protein content in rat skeletal muscle is altered by increased physical activity in vivo and in vitro

Current evidence suggests that exercise and glial cell line-derived neurotrophic factor (GDNF) independently cause significant morphological changes in the neuromuscular system. The aim of the current study was to determine if increased physical activity regulates GDNF protein content in rat skeletal muscle. Extensor Digitorum Longus (EDL) and Soleus (SOL) hind limb skeletal muscles were analyzed following 2 weeks of involuntary exercise and 4 h of field stimulation or stretch in muscle bath preparations. GDNF protein content was measured via enzyme-linked immunosorbent assay (ELISA). Two weeks of exercise increased GDNF protein content in SOL as compared to sedentary controls (4.4±0.3 pg GDNF/mg tissue and 3.1±0.6 pg GDNF/mg tissue, respectively) and decreased GDNF protein content in EDL as compared to controls (1.0±0.1 pg GDNF/mg tissue and 2.3±0.7 pg GDNF/mg tissue, respectively). GDNF protein content in the EDL decreased following both field stimulation (56%±18% decrease from controls) and stretch (66%±10% decrease from controls). SOL responded to field stimulation with a 38%±7% increase from controls in GDNF protein content, but showed no change following stretch. Pre-treatment with α-bungarotoxin abolished the effects of field stimulation in both muscles and blocked the effect of stretch in EDL. α-bungarotoxin pre-treatment and stretch increased GDNF protein content to 240%±10% of controls in the SOL. Exposure to carbamylcholine decreased GDNF protein content to 51%±28% of controls in the EDL but not SOL. These results suggest that GDNF protein content in skeletal muscle may be controlled by stretch, where it may increase GDNF protein content, and membrane depolarization/acetylcholine (ACh) which acts to decrease GDNF protein content.

Iron in neuronal function and dysfunction

Iron (Fe) is an essential element for many metabolic processes, serving as a cofactor for heme and nonheme proteins. Cellular iron deficiency arrests cell growth and leads to cell death; however, like most transition metals, an excess of intracellular iron is toxic. The ability of Fe to accept and donate electrons can lead to the formation of reactive nitrogen and oxygen species, and oxidative damage to tissue components; contributing to disease and, perhaps, aging itself. It has also been suggested that iron-induced oxidative stress can play a key role in the pathogenesis of several neurodegenerative diseases. Iron progressively accumulates in the brain both during normal aging and neurodegenerative processes. However, iron accumulation occurs without the concomitant increase in tissue ferritin, which could increase the risk of oxidative stress. Moreover, high iron concentrations in the brain have been consistently observed in Alzheimer's disease (AD) and Parkinson's disease (PD). In this regard, metalloneurobiology has become extremely important in understanding the role of iron in the onset and progression of neurodegenerative diseases. Neurons have developed several protective mechanisms against oxidative stress, among them the activation of cellular signaling pathways. The final response will depend on the identity, intensity, and persistence of the oxidative insult. The characterization of the mechanisms involved in high iron induced in neuronal dysfunction and death is central to understanding the pathology of a number of neurodegenerative disorders.

The vesicular acetylcholine transporter is required for neuromuscular development and function

The vesicular acetylcholine (ACh) transporter (VAChT) mediates ACh storage by synaptic vesicles. However, the VAChT-independent release of ACh is believed to be important during development. Here we generated VAChT knockout mice and tested the physiological relevance of the VAChT-independent release of ACh. Homozygous VAChT knockout mice died shortly after birth, indicating that VAChT-mediated storage of ACh is essential for life. Indeed, synaptosomes obtained from brains of homozygous knockouts were incapable of releasing ACh in response to depolarization. Surprisingly, electrophysiological recordings at the skeletal-neuromuscular junction show that VAChT knockout mice present spontaneous miniature end-plate potentials with reduced amplitude and frequency, which are likely the result of a passive transport of ACh into synaptic vesicles. Interestingly, VAChT knockouts exhibit substantial increases in amounts of choline acetyltransferase, high-affinity choline transporter, and ACh. However, the development of the neuromuscular junction in these mice is severely affected. Mutant VAChT mice show increases in motoneuron and nerve terminal numbers. End plates are large, nerves exhibit abnormal sprouting, and muscle is necrotic. The abnormalities are similar to those of mice that cannot synthesize ACh due to a lack of choline acetyltransferase. Our results indicate that VAChT is essential to the normal development of motor neurons and the release of ACh.

The rescue of developing avian motoneurons from programmed cell death by a selective inhibitor of the fetal muscle-specific nicotinic acetylcholine receptor

In an attempt to determine whether the rescue of developing motoneurons (MNS) from programmed cell death (PCD) in the chick embryo following reductions in neuromuscular function involves muscle or neuronal nicotinic acetylcholine receptors (nAChRs), we have employed a novel cone snail toxin alphaA-OIVA that acts selectively to antagonize the embryonic/fetal form of muscle nAChRs. The results demonstrate that alphaA-OIVA is nearly as effective as curare or alpha-bungarotoxin (alpha-BTX) in reducing neuromuscular function and is equally effective in increasing MN survival and intramuscular axon branching. Together with previous reports, we also provide evidence consistent with a transition between the embryonic/fetal form to the adult form of muscle nAChRs in chicken that involves the loss of the gamma subunit in the adult receptor. We conclude that selective inhibition of the embryonic/fetal form of the chicken muscle nAChR is sufficient to rescue MNs from PCD without any involvement of neuronal nAChRs.

In utero model for pharmacologically investigating spontaneous activity during early ontogeny

We describe an in utero model in which it is possible to investigate the involvement of supraspinal and spinal neurons in the genesis of spontaneous motor activity, a feature of early fetal life. To date almost all studies of the circuits that give rise to spontaneous motor activity during early ontogeny, and the neurotransmitters involved, have been carried out with in vitro models. Limitations of in vitro models include the relatively short viability of the preparation and the need to stimulate the nervous system either pharmacologically or electrically to produce the activity to be studied, in contrast to the activity that spontaneously occurs normally in utero. Our model uses fetal sheep, chronically instrumented with electromyogram electrodes and a catheter placed either intrathecally at the spinal level or in the peritoneal cavity. Motor activity can be studied over lengthy periods of fetal life and it is possible to examine the effects of infusing agonists and antagonists of central neurotransmitters on spontaneous motor activity. The use of our new model in parallel with the pre-existing in vitro models has the potential to add substantially to our understanding of the mechanisms behind changes in spontaneous activity that occur throughout fetal life.

Essential roles of the acetylcholine receptor gamma-subunit in neuromuscular synaptic patterning

Formation of the vertebrate neuromuscular junction (NMJ) takes place in a stereotypic pattern in which nerves terminate at select sarcolemmal sites often localized to the central region of the muscle fibers. Several lines of evidence indicate that the muscle fibers may initiate postsynaptic differentiation independent of the ingrowing nerves. For example, nascent acetylcholine receptors (AChRs) are pre-patterned at select regions of the muscle during the initial stage of neuromuscular synaptogenesis. It is not clear how these pre-patterned AChR clusters are assembled, and to what extent they contribute to pre- and post-synaptic differentiation during development. Here, we show that genetic deletion of the AChR gamma-subunit gene in mice leads to an absence of pre-patterned AChR clusters during initial stages of neuromuscular synaptogenesis. The absence of pre-patterned AChR clusters was associated with excessive nerve branching, increased motoneuron survival, as well as aberrant distribution of acetylcholinesterase (AChE) and rapsyn. However, clustering of muscle specific kinase (MuSK) proceeded normally in the gamma-null muscles. AChR clusters emerged at later stages owing to the expression of the AChR epsilon-subunit, but these delayed AChR clusters were broadly distributed and appeared at lower level compared with the wild-type muscles. Interestingly, despite the abnormal pattern, synaptic vesicle proteins were progressively accumulated at individual nerve terminals, and neuromuscular synapses were ultimately established in gamma-null muscles. These results demonstrate that the gamma-subunit is required for the formation of pre-patterned AChR clusters, which in turn play an essential role in determining the subsequent pattern of neuromuscular synaptogenesis.

Relevance of motoneuron specification and programmed cell death in embryos to therapy of ALS

The molecular cues that generate spinal motoneurons in early embryonic development are well defined. Motoneurons are generated in excess and consequently undergo a natural period of programmed cell death. Although it is not known exactly how motoneurons compete for survival in embryonic development, it is hypothesized that they rely on the ability to access limited amounts of trophic factors from peripheral tissues, a process that is tightly regulated by skeletal muscle activity. Attempts to elucidate the molecular mechanisms that underlie motoneuron generation and programmed cell death in embryos have led to various effective strategies for treating injury and disease in animal models. Such studies provide great hope for the amelioration of human amyotrophic lateral sclerosis (ALS), a devastating progressive motoneuron degenerative disease. Here we review the clinical relevance of studying motoneuron specification and death during embryonic development.

A-current expression is regulated by activity but not by target tissues in developing lumbar motoneurons of the chick embryo

The functional expression of A-type K+ channels (IA) was examined in chick lumbar motoneurons (LMNs) at embryonic days 6 and 11 (E6 and E11). We observed a threefold increase in IA density between E6 and E11 in spinal cord slices and acutely dissociated LMNs. There was no change in current density, kinetics, or voltage dependence of IA in E11 homozygous limbless mutants or in E11 embryos in which hindlimbs were surgically removed at E6. Moreover, chronic in ovo administration of D-tubocurarine, which causes an increase in motoneuron branching on the surface of target muscles, had no effect on IA. Electrical activity played an important role in IA regulation in LMNs in vitro and in ovo. Blocking spontaneous electrical activity of LMNs by chronic in ovo application of mecamylamine or muscimol reduced IA by 80%. LMNs cultured in the presence of TTX also failed to express normal densities of IA, even when the cultures also contained target tissues. The portion of IA that remained after in ovo or in vitro blockade of activity inactivated more quickly than the IA of LMNs that were allowed to discharge spikes. The developmental expression of LMN IA increases significantly during development, and this increase is activity dependent but does not require interactions with target tissues. Ongoing activity also seems to regulate the kinetics of IA inactivation.

Exposure to low concentrations of nicotine during cranial nerve development inhibits apoptosis and causes cellular hypertrophy in the ventral oculomotor nuclei of the chick embryo

Maternal cigarette use during pregnancy is associated with increased incidence of neural impairments in offspring, but nicotine's unique contribution to any neuropathology remains unclear, and nicotine's neurodevelopmental effects assessed in animal models vary with concentration. During ontogenesis, the chick oculomotor complex (OMN) is regulated by central nervous system (CNS) afferent-derived and target-derived trophic factors, allowing assessment of nicotine's potential interference in receptor-mediated CNS trophic phenomena, unconfounded by myriad other compounds in cigarette smoke. In the current study, 100 ng nicotine applied daily in ovo to yolk during embryonic days (E) 1-7 mimicked maternal plasma nicotine concentrations during fetal cranial nerve development. Nicotine-treated embryos exhibited a 15% decrease in whole body weight and 7% decrease in brain weight at E16. However, at E16, nicotine-treated embryos had 37% and 15% increases in the combined ventromedial+lateral (v) OMN motoneuron density and soma area, respectively, effects not observed in the optic tectum, in which nicotine cholinergic receptor expression is delayed until E8-12. Incorporation of tritiated thymidine into whole brain DNA demonstrated that the nicotine treatment did not cause increased rates of whole brain mitosis, suggesting that the dosage regimen did not elicit a cytotoxic, wound-healing, response of differentiating cells. As determined by DNA fragment-labeling assay during the normal period of cell death, vOMN apoptosis occurs maximally on E11 during a normal period of declining cell density, and a dose-response study demonstrated 78% E11 vOMN apoptotic suppression at approximately 0.30 microM cumulative yolk nicotine with an inhibition threshold between 0.10 and 0.20 microM. These results suggest that plasma nicotine concentrations resulting from tobacco use or nicotine replacement therapy (NRT) are sufficient to inhibit motoneuron apoptosis and enhance neuronal growth.

Neuromuscular development after the prevention of naturally occurring neuronal death by Bax deletion

The removal of excess neurons by programmed cell death (PCD) is believed to be critical for the proper development and function of the nervous system. A major role of this neuronal loss is to attain quantitative matching of neurons with their targets and afferents. Because motoneurons (MNs) in Bax knock-out (Bax KO) mice fail to undergo PCD in the face of normal target muscle development, we asked whether the excess rescued neurons in Bax KO mice can develop normally. We observed many small atrophied MNs in postnatal Bax KO mice, and these failed to innervate limb muscle targets. When examined embryonically during the PCD period, however, these excess MNs had initiated target innervation. To examine whether a limitation in trophic factor availability is responsible for postnatal MN atrophy and loss of innervation, we applied glial cell line-derived neurotrophic factor (GDNF) to neonatal mice. GDNF injection for 7-14 d induced the regrowth and reinnervation of muscle targets by atrophic MNs in Bax KO mice and prevented the normal postnatal death of MNs in wild-type mice. These results indicate that, although initially all of the MNs, including those rescued by Bax deletion, are able to project to and innervate targets, because of limited target-derived signals required for maintaining innervation and growth, only a subpopulation can grow and retain target contacts postnatally. Although sensory neurons in the dorsal root ganglia are also rescued from PCD by Bax deletion, their subsequent development is less affected than that of MNs.

Neuromuscular synapses mediate motor axon branching and motoneuron survival during the embryonic period of programmed cell death

The embryonic period of motoneuron programmed cell death (PCD) is marked by transient motor axon branching, but the role of neuromuscular synapses in regulating motoneuron number and axonal branching is not known. Here, we test whether neuromuscular synapses are required for the quantitative association between reduced skeletal muscle contraction, increased motor neurite branching, and increased motoneuron survival. We achieved this by comparing agrin and rapsyn mutant mice that lack acetylcholine receptor (AChR) clusters. There were significant reductions in nerve-evoked skeletal muscle contraction, increases in intramuscular axonal branching, and increases in spinal motoneuron survival in agrin and rapsyn mutant mice compared with their wild-type littermates at embryonic day 18.5 (E18.5). The maximum nerve-evoked skeletal muscle contraction was reduced a further 17% in agrin mutants than in rapsyn mutants. This correlated to an increase in motor axon branch extension and number that was 38% more in agrin mutants than in rapsyn mutants. This suggests that specializations of the neuromuscular synapse that ensure efficient synaptic transmission and muscle contraction are also vital mediators of motor axon branching. However, these increases in motor axon branching did not correlate with increases in motoneuron survival when comparing agrin and rapsyn mutants. Thus, agrin-induced synaptic specializations are required for skeletal muscle to effectively control motoneuron numbers during embryonic development.

Rescue of developing spinal motoneurons from programmed cell death by the GABA(A) agonist muscimol acts by blockade of neuromuscular activity and increased intramuscular nerve branching

Blockade of neuromuscular activity in the chick embryo during the period of programmed cell death of motoneurons results in a complete rescue of these cells. Understanding the cellular mechanisms that mediate this counterintuitive effect is of considerable interest with respect to the regulation of motoneuron survival during development as well as for understanding why motoneurons die pathologically. Although considerable evidence supports the role of a peripheral site of action at the neuromuscular junction in mediating the rescue of motoneurons following activity blockade, some evidence also supports a role for central nervous system (CNS) neurons. For example, the rescue of motoneurons by curare has been reported to be blocked by the GABA(A) agonist muscimol via its actions on CNS neurons. We have carried out a series of studies to further investigate this interesting observation. Surprisingly, we find that: (1) muscimol blocks activity and rescues MNs in a dose-dependent manner, similar to curare; (2) muscimol's effects on MN survival appear to be mediated by its action on intramuscular nerve branching, similar to curare; and (3) although muscimol acts centrally, the effects of muscimol on MN survival and axon branching are mediated peripherally at the neuromuscular junction, similar to curare. Because muscimol reduces MN depolarization these data also suggest that the depolarization of MNs by afferents is not required for promoting MN survival. Taken together, these data provide further evidence in support of a peripheral site of action of activity blockade in rescuing motoneurons from developmental cell death.

Activation of JNK by vanadate induces a Fas-associated death domain (FADD)-dependent death of cerebellar granule progenitors in vitro

Apoptosis is a highly regulated process that plays a critical role in neuronal development as well as the homeostasis of the adult nervous system. Vanadate, an environmental toxicant, causes developmental defects in the central nervous system. Here, we demonstrated that vanadate induced apoptosis in cultured cerebellar granule progenitors (CGPs). Treatment of cultured CGPs with vanadate activated ERKs and JNKs but not p38 MAPK and also induced c-Jun phosphorylation. In addition, vanadate induced FasL production, Fas (CD95) aggregation, and its association with the Fas-associated death domain (FADD), as well as the activation of caspase-8. Furthermore, vanadate generated reactive oxygen species (ROS) in CGPs; however, ROS was not involved in vanadate-mediated MAPK activation. Vanadate-induced FasL expression was ROS-dependent but JNK-independent. In contrast, vanadate-elicited Fas aggregation and Fas-FADD association, as well as caspase-8 activation, were dependent on JNK activation but were minimally regulated by ROS generation. The hydrogen peroxide scavenger, catalase, blocked vanadate-induced FasL expression and partially mitigated vanadate-induced cell death. On the other hand, dominant negative FADD and caspase-8 inhibitor completely eliminated vanadate-induced apoptosis. Thus, JNK signaling plays a major role in vanadate-mediated activation of the Fas-FADD-caspase-8 pathway that accounts for vanadate-induced apoptosis of CGPs.

Aberrant patterning of neuromuscular synapses in choline acetyltransferase-deficient mice

In this study we examined the developmental roles of acetylcholine (ACh) by establishing and analyzing mice lacking choline acetyltransferase (ChAT), the biosynthetic enzyme for ACh. As predicted, ChAT-deficient embryos lack both spontaneous and nerve-evoked postsynaptic potentials in muscle and die at birth. In mutant embryos, abnormally increased nerve branching occurs on contact with muscle, and hyperinnervation continues throughout subsequent prenatal development. Postsynaptically, ACh receptor clusters are markedly increased in number and occupy a broader muscle territory in the mutants. Concomitantly, the mutants have significantly more motor neurons than normal. At an ultrastructural level, nerve terminals are smaller in mutant neuromuscular junctions, and they make fewer synaptic contacts to the postsynaptic muscle membrane, although all of the typical synaptic components are present in the mutant. These results indicate that ChAT is uniquely essential for the patterning and formation of mammalian neuromuscular synapses.

Developmental expression of nicotinic receptors in the chick and human spinal cord

Naturally occurring programmed cell death of lumbar motor neurons in the chick spinal cord occurs between embryonic day (E) 6 and E12; whereas, a peak of motor neuron degeneration in the human spinal cord occurs between 12 and 16 weeks gestation. One of the major neurotransmitters, acetylcholine, is released from the embryonic motor neuron early in development and is thought to be responsible for early muscle activity that serves as a signal for regulating motor neuron survival. The effects of acetylcholine are mediated by two functionally distinct classes of receptors; namely, muscarinic and nicotinic with nicotinic receptors being used at the neuromuscular synapse. In this study, we determined the developmental expression profile of nicotinic acetylcholine receptor subunits in the chick and human lumbar motor neurons and skeletal muscle using reverse transcription polymerase chain reaction, immunoblots, and immunocytochemistry. Our results show that, in the chick, nicotinic receptor subunits alpha1, alpha4, alpha7, alpha8, and beta2 appear to be regulated during the process of naturally occurring motor neuron cell death in the spinal cord. A new finding was the expression of alpha8 mRNA and protein from E4.5 through E7 in chick motor neurons. Interestingly, we also found that, at E14, alpha8 protein was localized only in sensory dorsal horn neurons. In the developing human spinal cord, we determined that nicotinic receptor subunits alpha1, alpha2, alpha3, alpha4, alpha7, beta2, and beta3 were expressed before the programmed cell death period, and alpha2, alpha4, alpha7, beta2, beta3, and beta4 were expressed during the programmed cell death period. Our data demonstrate that neuronal and muscle nicotinic receptor mRNAs and proteins are expressed during important embryonic periods. This finding raises the possibility that nicotinic receptors play an important role in the spinal cord and skeletal muscle during early development.

Nicotinic receptors mediate changes in spinal motoneuron development and axonal pathfinding in embryonic zebrafish exposed to nicotine

We show that transient exposure of embryonic zebrafish to nicotine delays the development of secondary spinal motoneurons. Furthermore, there is a long-lasting alteration in axonal pathfinding in secondary motoneurons that is not ameliorated by drug withdrawal. These effects of nicotine were reversed by mammalian nicotinic receptor antagonists. Coupled with these changes is a long-term alteration in swimming behavior. Our results show that transient embryonic exposure to nicotine leads to long-lasting effects on the vertebrate nervous system. These results also demonstrate that the zebrafish is a useful model to examine the effects of nicotine specifically, and drugs of abuse in general, on the development of the CNS in vertebrates.

Developmental cell death in vivo: rescue of neurons independently of changes at target tissues

Programmed cell death is a prominent feature of neural development that is regulated by a variety of cell-cell interactions. We used the avian ciliary ganglion to dissect the relative contributions of target tissues vs. ganglionic inputs in regulating cell death. The two populations of the ciliary ganglion innervate different targets: choroid neurons innervate vasculature, whereas ciliary neurons innervate the iris and ciliary body. By counting after labeling all neurons with Islet-1 and choroid neurons with anti-somatostatin, we determined that alpha-bungarotoxin (alpha-btx) at 12.5 microg/day rescued only ciliary neurons, whereas 75 microg/day rescued both ciliary and choroid neurons. It is unlikely that alpha-btx acted by blocking nerve transmission at both targets because the choroid vasculature lacked transcripts for alpha-btx binding molecules. In addition, no inherent trophic activity could be ascribed to alpha-btx, and survival could not be attributed to differences in total trophic activity of eyes from saline vs. alpha-btx-treated embryos. In contrast, the alpha7 antagonist alpha-methyllycaconitine (MLA) rescued ciliary neurons at 2.6 microg/day, whereas 26 microg/day rescued choroid neurons. Nerve terminals of ciliary neurons rescued with alpha-btx were significantly larger; however, differences in nerve terminal size or branching of axons were not observed in ciliary neurons rescued with MLA or choroid neurons rescued by either MLA or alpha-btx. Our results suggest that neuronal survival can be promoted independently of changes at the target tissues when orthograde signals acting by means of neuronal alpha7 nicotinic receptors are blocked.

GDNF is regulated in an activity-dependent manner in rat skeletal muscle

Glial cell line-derived neurotrophic factor (GDNF) is produced by skeletal muscle and affects peripheral motor neurons. Elevated expression of GDNF in skeletal muscle leads to hyperinnervation of neuromuscular junctions, whereas postnatal administration of GDNF causes synaptic remodeling at the neuromuscular junction. Studies have demonstrated that altered physical activity causes changes in the neuromuscular junction. However, the role played by GDNF in this process in not known. The objective of this study was to determine whether changes in neuromuscular activity cause altered GDNF content in rat skeletal muscle. Following 4 weeks of walk-training on a treadmill, or 2 weeks of hindlimb unloading, soleus, gastrocnemius, and pectoralis major were removed and analyzed for GDNF content by enzyme-linked immunosorbant assay. Results indicated that walk-training is associated with increased GDNF content. Skeletal muscle from hindlimb-unloaded animals showed a decrease in GDNF in soleus and gastrocnemius, and an increase in pectoralis major. The altered production of GDNF may be responsible for activity-dependent remodeling of the neuromuscular junction and may aid in recovery from injury and disease.

Neuregulin expression at neuromuscular synapses is modulated by synaptic activity and neurotrophic factors

The proper formation of neuromuscular synapses requires ongoing synaptic activity that is translated into complex structural changes to produce functional synapses. One mechanism by which activity could be converted into these structural changes is through the regulated expression of specific synaptic regulatory factors. Here we demonstrate that blocking synaptic activity with curare reduces synaptic neuregulin expression in a dose-dependent manner yet has little effect on synaptic agrin or a muscle-derived heparan sulfate proteoglycan. These changes are associated with a fourfold increase in number and a twofold reduction in average size of synaptic acetylcholine receptor clusters that appears to be caused by excessive axonal sprouting with the formation of new, smaller acetylcholine receptor clusters. Activity blockade also leads to threefold reductions in brain-derived neurotrophic factor and neurotrophin 3 expression in muscle without appreciably changing the expression of these same factors in spinal cord. Adding back these or other neurotrophic factors restores synaptic neuregulin expression and maintains normal end plate band architecture in the presence of activity blockade. The expression of neuregulin protein at synapses is independent of spinal cord and muscle neuregulin mRNA levels, suggesting that neuregulin accumulation at synapses is independent of transcription. These findings suggest a local, positive feedback loop between synaptic regulatory factors that translates activity into structural changes at neuromuscular synapses.

Activity- and target-dependent regulation of large-conductance Ca2+-activated K+ channels in developing chick lumbar motoneurons

The functional expression of large-conductance (BK-type) Ca2+-activated K+ (K(Ca)) channels was examined in developing chick lumbar motoneurons (LMNs) between embryonic day 6 (E6) and E13 using patch-clamp recording techniques. The macroscopic K(Ca) current of E13 LMNs is inhibited by iberiotoxin and resistant to apamin. The average macroscopic K(Ca) density was low before E8 and increased 3.3-fold by E11, with an additional 1.8-fold increase occurring by E13. BK-type K(Ca) channels could not be detected in inside-out patches from E8 LMNs but were readily detected at E11. The density of voltage-activated Ca2+ currents did not change between E8 and E11. Surgical ablation of target tissues at E5 caused a significant reduction in average K(Ca) density in LMNs measured at E11. Conversely, chronic in ovo administration of d-tubocurarine, which causes an increase in motoneuron branching on the surface of the muscle target tissue, evoked a 1.8-fold increase in average LMN K(Ca) density measured at E11. Electrical activity also contributed to developmental regulation of LMN K(Ca) density. A significant reduction in E11 K(Ca) density was found after chronic in ovo treatment with the neuronal nicotinic antagonist mecamylamine or the GABA receptor agonist muscimol, agents that reduce activation of LMNs in ovo. Moreover, 3 d exposure to depolarizing concentrations of external K+ to LMNs cultured at E8 caused an increase in K(Ca) expression. Conversely, tetrodotoxin caused a decrease in K(Ca) expression in cultured E8 LMNs developing for 3 d in the presence of neurotrophic factors that promote neuronal survival in the absence of target tissues.

Target-derived cardiotrophin-1 and insulin-like growth factor-I promote neurite growth and survival of developing oculomotor neurons

Several trophic factors support the survival of developing motoneurons, but it is not known whether these factors act in a retrograde fashion from the motoneuron target muscle or are derived from other sources. Cardiotrophin-1 (CT-1) and the insulin-like growth factors (IGFs) are candidate target-derived motoneuron survival factors as both are expressed in muscle during naturally occurring motoneuron death and, applied systemically, support the survival of developing motoneurons. By using the embryonic chick oculomotor system, we show that CT-1 and IGF-I promote neurite outgrowth from E13-derived oculomotor explants and are retrogradely transported from muscle to nerve cell body in vivo, and injection of CT-1 or IGF-I into eye muscles increases motoneuron survival by 20 and 30%, respectively, as evidenced by calibrated stereological counting techniques. Pharmacological depletion of endogenous target-derived IGF-I in vivo reduces oculomotor neuron survival by up to 30% in a dose-dependent manner. These results significantly extend previous studies using systemic administration of trophic factors and are the first to demonstrate a target-derived retrograde mechanism of developing motoneuron survival factors.

Activity regulates programmed cell death of zebrafish Rohon-Beard neurons

Programmed cell death is a normal aspect of neuronal development. Typically, twice as many neurons are generated than survive. In extreme cases, all neurons within a population disappear during embryogenesis or by early stages of postnatal development. Examples of transient neuronal populations include Cajal-Retzius cells of the cerebral cortex and Rohon-Beard cells of the spinal cord. The novel mechanisms that lead to such massive cell death have not yet been identified.

We provide evidence that electrical activity regulates the cell death program of zebrafish Rohon-Beard cells. Activity was inhibited by reducing Na+ current in Rohon-Beard cells either genetically (the macho mutation) or pharmacologically (tricaine). We examined the effects of activity block on three different reporters of cell death: DNA fragmentation, cytoskeletal rearrangements and cell body loss. Both the mao mutation and pharmacological blockade of Na+ current reduced these signatures of the cell death program. Moreover, the mao mutation and pharmacological blockade of Na+ current produced similar reductions in Rohon-Beard cell death. The results indicate that electrical activity provides signals that are required for the normal elimination of Rohon-Beard cells.

Repeated injury to the sciatic nerve in immature rats causes motoneuron death and impairs muscle recovery

Injury to the sciatic nerve of newborn rats causes motoneuron death, while the same insult inflicted 5 days later does not. In this study the effects of prolonging the period of target deprivation and axonal regeneration were investigated by inflicting a second nerve crush 6 days after the first, just before reinnervation of the muscle occurred. Two to 4 months later the number of motoneurons supplying soleus, tibialis anterior, and extensor digitorum longus muscles was established by retrograde labeling with horseradish peroxidase injected into the muscle. After nerve injury at 5 days there was no significant loss of motoneurons to any muscle. However, when the injury was repeated, the number of labeled motoneurons was reduced, suggesting that a significant proportion had died. Motoneurons to soleus were affected more than those to the fast muscles, reflecting their lesser maturity. Moreover, motoneurons to soleus that survived both injuries to their axon failed to grow to their full size. The relative impairment of recovery of the muscles, indicated by weight and maximal tetanic tension, mirrored the loss of motoneurons in each case. Previous studies have suggested that repeated nerve injuries in adult animals can enhance reinnervation. However, the present results along with those of other recent studies suggest that immature motoneurons that are repeatedly induced to support growth of their axons are at greater risk of death and can result in poorer reinnervation of the muscles.

Development of cholinergic terminals around rat spinal motor neurons and their potential relationship to developmental cell death

Neuron death seems to be regulated mainly by postsynaptic target cells. In chicks, nicotinic antagonists such as alpha-bungarotoxin (alphaBT) can prevent normal cell death of somatic motor neurons (SMNs). For this effect, however, alphaBT could be acting at peripheral neuromuscular junctions and/or central cholinergic synapses. To investigate this issue, we first studied the development of cholinergic terminals in the rat spinal cord by using vesicular acetylcholine transporter immunocytochemistry. Labeled terminals were seen in the ventral horn as early as embryonic day 15 (E15), the beginning of the cell death period. Thus, central cholinergic synapses form at the correct time and place to be able to influence SMN death. We next added alphaBT to organotypic, spinal slice cultures made at E15. After 5 days in vitro, the number of SMNs in treated cultures was substantially greater than in control cultures, indicating that alphaBT can reduce SMN cell death in rats as it does in chicks. Moreover, peripheral target removal led to extensive loss of SMNs, and such a loss occurred even in the presence of alphaBT, indicating the necessity of peripheral target for the alphaBT effect. Finally, to determine whether central cholinergic terminals also may be involved in SMN death, we delayed the alphaBT treatment until after central cholinergic terminals had disappeared from the slice cultures. The increased number of surviving SMNs, even in the absence of central terminals, argued that alphaBT acts at peripheral, not central, cholinergic synapses to rescue SMNs from developmental cell death.

Promotion of motoneuron survival and branching in rapsyn-deficient mice

Inhibition of programmed cell death of motoneurons during embryonic development requires the presence of their target muscle and coincides with the initial stages of synaptogenesis. To evaluate the role of synapse formation on motoneuron survival during embryonic development, we counted the number of motoneurons in rapsyn-deficient mice. Rapsyn is a 43 kDa protein needed for the formation of postsynaptic specialisations at vertebrate neuromuscular synapses. Here we show that the rapsyn-deficient mice have a significant increase in the number of motoneurons in the brachial lateral motor column during the period of naturally occurring programmed cell death compared to their wild-type littermates. In addition, we observed an increase in intramuscular axonal branching in the rapsyn-deficient diaphragms compared to their wild-type littermates at embryonic day 18.5. These results suggest that deficits in the formation of the postsynaptic specialisation at the neuromuscular synapse, brought about by the absence of rapsyn, are sufficient to induce increases in both axonal branching and the survival of the innervating motoneuron. Moreover, these results support the idea that skeletal muscle activity through effective synaptic transmission and intramuscular axonal branching are major mechanisms that regulate motoneuron survival during development.