The relationship of intramuscular nerve branching and synaptogenesis to motoneuron survival

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.

The NMJ as a model synapse: New perspectives on formation, synaptic transmission and maintenance: Acetylcholinesterase at the neuromuscular junction

Acetylcholinesterase (AChE) is an essential enzymatic component of the neuromuscular junction where it is responsible for terminating neurotransmission by the cholinergic motor neurons. The enzyme at the neuromuscular junction (NMJ) is contributed primarily by the skeletal muscle where it is produced at higher levels in the post-synaptic region of the fibers. The major form of AChE at the NMJ is a large asymmetric form consisting of three tetramers covalently attached to a three-stranded collagen-like tail which is responsible for anchoring it to the synaptic basal lamina. Its location and expression is regulated to a large extent by the motor neurons and occurs at the transcriptional, translational and post-translational levels. While its expression can be quite rapid in tissue cultured cells, its half-life in vivo appears to be quite long, about three weeks, although more rapidly turning over pools have been described. Finally the essential nature of this enzyme is underscored by the fact that no naturally occurring null mutations of the catalytic subunit have been described in higher organisms and the few dozen humans carrying mutations in the collagen tail responsible for anchoring the enzyme at the NMJ are severely affected.

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.

Preserving neuromuscular synapses in ALS by stimulating MuSK with a therapeutic agonist antibody

In amyotrophic lateral sclerosis (ALS) and animal models of ALS, including SOD1-G93A mice, disassembly of the neuromuscular synapse precedes motor neuron loss and is sufficient to cause a decline in motor function that culminates in lethal respiratory paralysis. We treated SOD1-G93A mice with an agonist antibody to MuSK, a receptor tyrosine kinase essential for maintaining neuromuscular synapses, to determine whether increasing muscle retrograde signaling would slow nerve terminal detachment from muscle. The agonist antibody, delivered after disease onset, slowed muscle denervation, promoting motor neuron survival, improving motor system output, and extending the lifespan of SOD1-G93A mice. These findings suggest a novel therapeutic strategy for ALS, using an antibody format with clinical precedence, which targets a pathway essential for maintaining attachment of nerve terminals to muscle.

Amyotrophic lateral sclerosis – often shortened to ALS – is a disease that starts with difficulties moving and progresses to paralysis of many muscles, including those used for breathing. The disease is usually lethal, with patients rarely surviving more than a few years after diagnosis. There is no cure or effective treatment for the disease. It begins with the breakdown of the connections, or synapses, between the muscles and the nerve cells that connect with them. After this, the nerve cell itself breaks down. Many therapeutic approaches have focused on attempts to prevent the nerve cells from dying, but few target the initial degeneration of the synapse.

Cantor et al. asked if intervening when the synapse has already begun to break down could slow the progression of the disease in mice with ALS. Their approach involved using an antibody to bind to a receptor protein called MuSK, which plays an important role in maintaining the synapse between muscle and nerve cell. The antibody boosted the receptor’s activity, helping to preserve synapses, including those that connect nerve cells to the diaphragm muscle.

The experiments showed that the antibody treatment led to fewer synapses breaking down, and kept more of the nerve cells alive. Healthier connections between the nervous system and the diaphragm improved the function of this muscle. As a result, the mice given the antibody treatment had a slightly extended lifespan, compared with those given no treatment.

The findings suggest a possible new way to develop treatments for ALS, which could be used in combination with other therapies, such as those aimed at improving the health of the nerve cells. Together, this could improve quality of life for the majority of patients with ALS. Similar strategies could be used to develop treatments to preserve synapses in other neurodegenerative diseases, such as Alzheimer’s, Parkinson’s and Huntington’s disease, as well as some kinds of dementia. Preserving synapses early on, before the significant loss of nerve cells, could help to slow the progression of these diseases, improve the patients' quality of life and extend their lifespans too.

Alterations in hypoglossal motor neurons due to GAD67 and VGAT deficiency in mice

There is an emerging body of evidence that glycinergic and GABAergic synaptic inputs onto motor neurons (MNs) help regulate the final number of MNs and axonal muscle innervation patterns. Using mutant glutamate decarboxylase 67 (GAD67) and vesicular inhibitory amino acid transporter (VGAT) deficient mice, we describe the effect that deficiencies of presynaptic GABAergic and/or glycinergic release have on the post-synaptic somato-dendritic structure of motor neurons, and the development of excitatory and inhibitory synaptic inputs to MNs. We use whole-cell patch clamp recording of synaptic currents in E18.5 hypoglossal MNs from brainstem slices, combined with dye-filling of these recorded cells with Neurobiotin™, high-resolution confocal imaging and 3-dimensional reconstructions. Hypoglossal MNs from GAD67- and VGAT-deficient mice display decreased inhibitory neurotransmission and increased excitatory synaptic inputs. These changes are associated with increased dendritic arbor length, increased complexity of dendritic branching, and increased density of spiny processes. Our results show that presynaptic release of inhibitory amino acid neurotransmitters are potent regulators of hypoglossal MN morphology and key regulators of synaptic inputs during this critical developmental time point.

Glycinergic Neurotransmission: A Potent Regulator of Embryonic Motor Neuron Dendritic Morphology and Synaptic Plasticity

Emerging evidence suggests that central synaptic inputs onto motor neurons (MNs) play an important role in developmental regulation of the final number of MNs and their muscle innervation for a particular motor pool. Here, we describe the effect of genetic deletion of glycinergic neurotransmission on single MN structure and on functional excitatory and inhibitory inputs to MNs. We measured synaptic currents in E18.5 hypoglossal MNs from brain slices using whole-cell patch-clamp recording, followed by dye-filling these same cells with Neurobiotin, to define their morphology by high-resolution confocal imaging and 3D reconstruction. We show that hypoglossal MNs of mice lacking gephyrin display increased dendritic arbor length and branching, increased spiny processes, decreased inhibitory neurotransmission, and increased excitatory neurotransmission. These findings suggest that central glycinergic synaptic activity plays a vital role in regulating MN morphology and glutamatergic central synaptic inputs during late embryonic development.

SIGNIFICANCE STATEMENT

MNs within the brainstem and spinal cord are responsible for integrating a diverse array of synaptic inputs into discrete contractions of skeletal muscle to achieve coordinated behaviors, such as breathing, vocalization, and locomotion. The last trimester in utero is critical in neuromotor development, as this is when central and peripheral synaptic connections are made onto and from MNs. At this time-point, using transgenic mice with negligible glycinergic postsynaptic responses, we show that this deficiency leads to abnormally high excitatory neurotransmission and alters the dendritic architecture responsible for coherently integrating these inputs. This study compliments the emerging concept that neurodevelopmental disorders (including autism, epilepsy, and amyotrophic lateral sclerosis) are underpinned by synaptic dysfunction and therefore will be useful to neuroscientists and neurologists alike.

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.

Axon and muscle spindle hyperplasia in the myostatin null mouse

Germline deletion of the myostatin gene results in hyperplasia and hypertrophy of the tension-generating (extrafusal) fibres in skeletal muscle. As this gene is expressed predominantly in myogenic tissues it offers an excellent model with which to investigate the quantitative relationship between muscle and axonal development. Here we show that skeletal muscle hyperplasia in myostatin null mouse is accompanied by an increase in nerve fibres in major nerves of both the fore- and hindlimbs. We show that axons within these nerves undergo hypertrophy. Furthermore, we provide evidence that the age-related neural atrophic process is delayed in the absence of myostatin. Finally, we show that skeletal muscle hyperplasia in the myostatin null mouse is accompanied by an increase in the number of muscle spindles (also called stretch receptors or proprioceptors). However, our work demonstrates that the mechanisms regulating intrafusal fibre hyperplasia and hypertrophy differ from those that control the aetiology of extrafusal fibres.

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.

Uncoupling nicotine mediated motoneuron axonal pathfinding errors and muscle degeneration in zebrafish

Zebrafish embryos offer a unique opportunity to investigate the mechanisms by which nicotine exposure impacts early vertebrate development. Embryos exposed to nicotine become functionally paralyzed by 42 hpf suggesting that the neuromuscular system is compromised in exposed embryos. We previously demonstrated that secondary spinal motoneurons in nicotine-exposed embryos were delayed in development and that their axons made pathfinding errors (Svoboda, K.R., Vijayaraghaven, S., Tanguay, R.L., 2002. Nicotinic receptors mediate changes in spinal motoneuron development and axonal pathfinding in embryonic zebrafish exposed to nicotine. J. Neurosci. 22, 10731-10741). In that study, we did not consider the potential role that altered skeletal muscle development caused by nicotine exposure could play in contributing to the errors in spinal motoneuron axon pathfinding. In this study, we show that an alteration in skeletal muscle development occurs in tandem with alterations in spinal motoneuron development upon exposure to nicotine. The alteration in the muscle involves the binding of nicotine to the muscle-specific AChRs. The nicotine-induced alteration in muscle development does not occur in the zebrafish mutant (sofa potato, [sop]), which lacks muscle-specific AChRs. Even though muscle development is unaffected by nicotine exposure in sop mutants, motoneuron axonal pathfinding errors still occur in these mutants, indicating a direct effect of nicotine exposure on nervous system development.

Slow- and fast-twitch rat hind limb skeletal muscle phenotypes 8 months after spinal cord transection and olfactory ensheathing glia transplantation

Paralysed skeletal muscle of rats with spinal cord injury (SCI) undergoes atrophy and a switch in gene expression pattern which leads to faster, more fatigable phenotypes. Olfactory ensheathing glia (OEG) transplants have been reported to promote axonal regeneration and to restore sensory-motor function in animals with SCI. We hypothesized that OEG transplants could attenuate skeletal muscle phenotypic deterioration and that this effect could underlie the functional recovery observed in behavioural tests. A variety of morphological, metabolic and molecular markers were assessed in soleus (SOL) and extensor digitorum longus (EDL) muscles of spinal cord transected (SCT), OEG-transplanted rats 8 months after the intervention and compared with non-transplanted SCT rats and sham-operated (without SCT) controls (C). A multivariate analysis encompassing all the parameters indicated that OEG-transplanted rats displayed skeletal muscle phenotypes intermediate between non-transplanted and sham-operated controls, but different from both. A high correlation was observed between behaviourally tested sensory-motor functional capacity and expression level of slow- and fast-twitch hind limb skeletal muscle phenotypic markers, particularly the histochemical glycerol-3-phosphate dehydrogenase activity (-0.843, P < 0.0001) and the fraction of variant 2s of the slow regulatory myosin light chain isoform (0.848, P < 0.0001) in SOL. Despite the mean overall effect of OEG transplants in patterning skeletal muscle protein expression towards normal, in 6 out of 9 animals they appeared insufficient to overcome fibre type switching and to support a consistent and generalized long-term maintenance of normal skeletal muscle characteristics. The interplay of OEG and exercise-mediated neurotrophic actions is a plausible mechanism underlying OEG transplantation effects on paralysed skeletal muscle.

Transcriptional regulation of vertebrate axon guidance and synapse formation

The establishment of functional neural connections requires the growth of axons to specific target areas and the formation of synapses with appropriate synaptic partners. Several molecules that regulate axon guidance and synapse formation have been identified in the past decade, but it is unclear how a relatively limited number of factors can specify a large number of connections. Recent evidence indicates that transcription factors make a crucial contribution to the specification of connections in the nervous system by coordinating the response of neurons to guidance molecules and neurotransmitters.

Neuromuscular development in the absence of programmed cell death: phenotypic alteration of motoneurons and muscle

The widespread, massive loss of developing neurons in the central and peripheral nervous system of birds and mammals is generally considered to be an evolutionary adaptation. However, until recently, models for testing both the immediate and long-term consequences of preventing this normal cell loss have not been available. We have taken advantage of several methods for preventing neuronal death in vivo to ask whether rescued neurons [e.g., motoneurons (MNs)] differentiate normally and become functionally incorporated into the nervous system. Although many aspects of MN differentiation occurred normally after the prevention of cell death (including the expression of several motoneuron-specific markers, axon projections into the ventral root and peripheral nerves, ultrastructure, dendritic arborization, and afferent axosomatic synapses), other features of the neuromuscular system (MNs and muscle) were abnormal. The cell bodies and axons of MNs were smaller than normal, many MN axons failed to become myelinated or to form functional synaptic contacts with target muscles, and a subpopulation of rescued cells were transformed from alpha- to gamma-like MNs. Additionally, after the rescue of MNs in myogenin glial cell line-derived neurotrophic factor (MyoGDNF) transgenic mice, myofiber differentiation of extrafusal skeletal muscle was transformed and muscle physiology and motor behaviors were abnormal. In contrast, extrafusal myofiber phenotype, muscle physiology, and (except for muscle strength tests) motor behaviors were all normal after the rescue of MNs by genetic deletion of the proapoptotic gene Bax. However, there was an increase in intrafusal muscle fibers (spindles) in Bax knock-out versus both wild-type and MyoGDNF mice. Together, these data indicate that after the prevention of MN death, the neuromuscular system becomes transformed in novel ways to compensate for the presence of the thousands of excess cells.

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.

Phosphorylation of c-Jun in avian and mammalian motoneurons in vivo during programmed cell death: an early reversible event in the apoptotic cascade

c-Jun is a transcription factor that is involved in various cellular events, including apoptotic cell death. For example, phosphorylation of c-Jun is one of the earliest biochemical changes detected in dying sympathetic neurons after NGF deprivation in vitro. However, currently, it is not known whether a similar molecular event is involved in the developmental programmed cell death (PCD) of neurons in vivo. We observed that only a subpopulation of motoneurons (MNs) exhibit c-Jun phosphorylation during the PCD period in chick [embryonic day 5 (E5)-E12] and mouse (E13-E18) embryos. Experimental perturbation of MN survival-promoting signals by limb bud removal (reduced signals) or by activity blockade (increased signals) in the chick embryo demonstrated that the presence of those signals is negatively correlated with the number of c-Jun-phosphorylated MNs. This suggests that insufficient survival signals (e.g., neurotrophic factors) may induce c-Jun phosphorylation of MNs in vivo. Consistent with the idea that c-Jun phosphorylation is a reversible event during normal PCD of MNs, we found that c-Jun phosphorylation was transiently observed in a subpopulation of mouse MNs rescued from PCD by deletion of the proapoptotic gene Bax. Inhibition of c-Jun signaling significantly reduced MN death in chick embryo, indicating that activation of c-Jun signaling is necessary for the PCD of MNs. Together, c-Jun phosphorylation appears to be required for the initiation of an early and reversible event in the intracellular PCD cascade in vivo after loss of survival-promoting signals such as neurotrophic factors.

Glycinergic and GABAergic synaptic activity differentially regulate motoneuron survival and skeletal muscle innervation

GABAergic and glycinergic synaptic transmission is proposed to promote the maturation and refinement of the developing CNS. Here we provide morphological and functional evidence that glycinergic and GABAergic synapses control motoneuron development in a region-specific manner during programmed cell death. In gephyrin-deficient mice that lack all postsynaptic glycine receptor and some GABA(A) receptor clusters, there was increased spontaneous respiratory motor activity, reduced respiratory motoneuron survival, and decreased innervation of the diaphragm. In contrast, limb-innervating motoneurons showed decreased spontaneous activity, increased survival, and increased innervation of their target muscles. Both GABA and glycine increased limb-innervating motoneuron activity and decreased respiratory motoneuron activity in wild-type mice, but only glycine responses were abolished in gephyrin-deficient mice. Our results provide genetic evidence that the development of glycinergic and GABAergic synaptic inputs onto motoneurons plays an important role in the survival, axonal branching, and spontaneous activity of motoneurons in developing mammalian embryos.

Target-determined expression of ?3 isoform of the Na+,K+-ATPase in the somatic nervous system of rat

Factors that determine the differential expression of isoforms of Na(+),K(+)-ATPase in the nervous system of vertebrates are not understood. To address this question we studied the expression of alpha(3) Na(+),K(+)-ATPase in the L5 dorsal root ganglia (DRG) of developing rat, the normal adult rat, and the adult rat after peripheral axotomy. During development, the first alpha(3) Na(+),K(+)-ATPase-positive DRG neurons appear by embryonic day 21. At birth, the L5 DRG have a full complement (14 +/- 2%) of these neurons. By 15 days after sciatic nerve transection in adult rat, the number of alpha(3) Na(+),K(+)-ATPase-positive DRG neurons and small myelinated L5 ventral root axons decreases to about 35% of control counts. These results combined with data from the literature suggest that the expression of alpha(3) Na(+),K(+)-ATPase by rat somatic neurons is determined by target-muscle spindle-derived factors.

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.

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.

Neuronal cell death, nerve growth factor and neurotrophic models: 50 years on

Viktor Hamburger has just died at the age of 100. It is 50 years since he and Rita Levi-Montalcini laid the foundations for the study of naturally occurring cell death and of neurotrophic factors in the nervous system. In a period of less than 10 years, from 1949 to 1958, Hamburger and Levi-Montalcini made the following seminal discoveries: that neuron cell death occurs in dorsal root ganglia, sympathetic ganglia and the cervical column of motoneurons; that the predictions arising from this observation, namely that survival is dependent on the supply of a trophic factor, could be substantiated by studying the effects of a sarcoma on the proliferation of ganglionic processes both in vivo and in vitro; and that the proliferation of these processes could be used as an assay system to isolate the factor. This work provides a short review mostly of the early history of this subject in the context of the Hamburger/Levi-Montalcini paradigm. This acts as an introduction to a consideration of models that have been proposed to account for how the different sources of growth factors provide for the survival of neurons during development. It is suggested that what has been called the 'social-control' model provides the most parsimonious quantitative description of the contribution of trophic factors to neuronal survival, a concept for which we are in debt to Viktor Hamburger and Rita Levi-Montalcini.

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.

Alterations in ciliary neurotrophic factor signaling in rapsyn deficient mice

Rapsyn is a key molecule involved in the formation of postsynaptic specializations at the neuromuscular junction, in its absence there are both pre- and post-synaptic deficits including failure to cluster acetylcholine receptors. Recently we have documented increases in both nerve-muscle branching and numbers of motoneurons, suggesting alterations in skeletal muscle derived trophic support for motoneurons. The aim of the present study was to evaluate the contribution of target derived trophic factors to increases in motoneuron branching and number, in rapsyn deficient mice that had their postsynaptic specializations disrupted. We have used reverse transcription-polymerase chain reaction and Western blot to document the expression of known trophic factors and their receptors in muscle, during the period of synapse formation in rapsyn deficient mouse embryos. We found that the mRNA levels for ciliary neurotrophic factor (CNTF) was decreased in the rapsyn deficient muscles compared with litter mate controls although those for NGF, BDNF, NT-3 and TGF-beta2 did not differ. We found that both the mRNA and the protein expression for suppressor of cytokine signaling 3 (SOCS3) decreased although janus kinase 2 (JAK2) did not change in the rapsyn deficient muscles compared with litter mate controls. These results suggest that failure to form postsynaptic specializations in rapsyn deficient mice has altered the CNTF cytokine signaling pathway within skeletal muscle, the target for motoneurons. This alteration may in part, account for the increased muscle nerve branching and motoneuron survival seen in rapsyn deficient mice.

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.

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

Spinal motoneurons (MNs) in the chick embryo undergo programmed cell death coincident with the establishment of nerve-muscle connections and the onset of synaptic transmission at the neuromuscular junction. Chronic treatment of embryos during this period with nicotinic acetylcholine receptor (nAChR)-blocking agents [e.g., curare or alpha-bungarotoxin (alpha-BTX)] prevents the death of MNs. Although this rescue effect has been attributed previously to a peripheral site of action of the nAChR-blocking agents at the neuromuscular junction (NMJ), because nAChRs are expressed in both muscle and spinal cord, it has been suggested that the rescue effect may, in fact, be mediated by a direct central action of nAChR antagonists. By using a variety of different nAChR-blocking agents that target specific muscle or neuronal nAChR subunits, we find that only those agents that act on muscle-type receptors block neuromuscular activity and rescue MNs. However, paralytic, muscular dysgenic mutant chick embryos also exhibit significant increases in MN survival that can be further enhanced by treatment with curare or alpha-BTX, suggesting that muscle paralysis may not be the sole factor involved in MN survival. Taken together, the data presented here support the argument that, in vivo, nAChR antagonists promote the survival of spinal MNs primarily by acting peripherally at the NMJ to inhibit synaptic transmission and reduce or block muscle activity. Although a central action of these agents involving direct perturbations of MN activity may also play a contributory role, further studies are needed to determine more precisely the relative roles of central versus peripheral sites of action in MN rescue.

Neuromuscular activity blockade induced by muscimol and d-tubocurarine differentially affects the survival of embryonic chick motoneurons

To understand better how spontaneous motoneuron activity and intramuscular nerve branching influence motoneuron survival, we chronically treated chicken embryos in ovo with either d-tubocurarine (dTC) or muscimol during the naturally occurring cell death period, assessing their effects on activity by in ovo motility measurement and muscle nerve recordings from isolated spinal cord preparations. Because muscimol, a GABA(A) agonist, blocked both spontaneous motoneuron bursting and that elicited by descending input but did not rescue motoneurons, we conclude that spontaneous bursting activity is not required for the process of normal motoneuron cell death. dTC, which rescues motoneurons and blocks neuromuscular transmission, blocked neither spontaneous nor descending input-elicited bursting and early in the cell death period actually increased burst amplitude. These changes in motoneuron activation could alter the uptake of trophic molecules or gene transcription via altered patterns of [Ca(2+)](i), which in turn could affect motoneuron survival directly or indirectly by altering intramuscular nerve branching. A good correlation was found between nerve branching and motoneuron survival under various experimental conditions: (1) dTC, but not muscimol, greatly increased branching; (2) the removal of PSA from NCAM partially reversed the effects of dTC on both branching and survival, indicating that branching is a critical variable influencing motoneuron survival; (3) muscimol, applied with dTC, prevented the effect of dTC on survival and motoneuron bursting and, to a large extent, its effect on branching. However, the central effects of dTC also appear to be important, because muscimol, which prevented motoneuron activity in the presence of dTC, also prevented the dTC-induced rescue of motoneurons.

Neuromuscular development in the avian paralytic mutant crooked neck dwarf (cn/cn): further evidence for the role of neuromuscular activity in motoneuron survival

Neuromuscular transmission and muscle activity during early stages of embryonic development are known to influence the differentiation and survival of motoneurons and to affect interactions with their muscle targets. We have examined neuromuscular development in an avian genetic mutant, crooked neck dwarf (cn/cn), in which a major phenotype is the chronic absence of the spontaneous, neurally mediated movements (motility) that are characteristic of avian and other vertebrate embryos and fetuses. The primary genetic defect in cn/cn embryos responsible for the absence of motility appears to be the lack of excitation-contraction coupling. Although motility in mutant embryos is absent from the onset of activity on embryonic days (E) 3-4, muscle differentiation appears histologically normal up to about E8. After E8, however, previously separate muscles fuse or coalesce secondarily, and myotubes exhibit a progressive series of histological and ultrastructural degenerative changes, including disarrayed myofibrils, dilated sarcoplasmic vesicles, nuclear membrane blebbing, mitochondrial swelling, nuclear inclusions, and absence of junctional end feet. Mutant muscle cells do not develop beyond the myotube stage, and by E18-E20 most muscles have almost completely degenerated. Prior to their breakdown and degeneration, mutant muscles are innervated and synaptic contacts are established. In fact, quantitative analysis indicates that, prior to the onset of muscle degeneration, mutant muscles are hyperinnervated. There is increased branching of motoneuron axons and an increased number of synaptic contacts in the mutant muscle on E8. Naturally occurring cell death of limb-innervating motoneurons is also significantly reduced in cn/cn embryos. Mutant embryos have 30-40% more motoneurons in the brachial and lumbar spinal cord by the end of the normal period of cell death. Electrophysiological recordings (electromyographic and direct records form muscle nerves) failed to detect any differences in the activity of control vs. mutant embryos despite the absence of muscular contractile activity in the mutant embryos. The alpha-ryanodine receptor that is genetically abnormal in homozygote cn/cn embryos is not normally expressed in the spinal cord. Taken together, these data argue against the possibility that the mutant phenotype described here is caused by the perturbation of a central nervous system (CNS)-expressed alpha-ryanodine receptor. The hyperinnervation of skeletal muscle and the reduction of motoneuron death that are observed in cn/cn embryos also occur in genetically paralyzed mouse embryos and in pharmacologically paralyzed avian and rat embryos. Because a primary common feature in all three of these models is the absence of muscle activity, it seems likely that the peripheral excitation of muscle by motoneurons during normal development is a major factor in regulating retrograde muscle-derived (or muscle-associated) signals that control motoneuron differentiation and survival.

The expression of trkB and p75 and the role of BDNF in the developing neuromuscular system of the chick embryo

The neurotrophin, brain-derived neurotrophic factor, prevents motoneuron cell death during the normal development of the chick embryo. Brain-derived neurotrophic factor is a ligand for the low-affinity NGF receptor, p75, and for the high-affinity neurotrophin receptor, trkB. If motoneurons respond directly to brain-derived neurotrophic factor then they must possess at least one, and possibly both, of these receptors during the period of naturally occurring cell death. Histological sections from the lumbar region of chick embryos were probed for the presence of trkB and p75 mRNA using digoxigenin-labeled anti-sense RNA probes. p75 mRNA was present in spinal cord motoneurons at stages of development that correlate with motoneuron cell death. Immunohistochemical localization also revealed that p75 protein was present in motoneurons, primarily along the ventral roots and developing intramuscular nerves. In contrast trkB mRNA was not present in chick motoneurons until after the process of cell death was underway. The timing of trkB expression suggested that some motoneurons, i.e., those that die prior to the onset of trkB expression, may be insensitive to brain-derived neurotrophic factor. This was confirmed by comparing the number of surviving motoneurons following different in vivo treatment paradigms. The evidence indicates that motoneurons undergo a temporal shift in sensitivity to brain-derived neurotrophic factor.

Muscle-derived neurotrophin-4 as an activity-dependent trophic signal for adult motor neurons

The production of neurotrophin-4 (NT-4) in rat skeletal muscle was found to depend on muscle activity. The amounts of NT-4 messenger RNA present decreased after blockade of neuromuscular transmission with alpha-bungarotoxin and increased during postnatal development and after electrical stimulation in a dose-dependent manner. NT-4 immunoreactivity was detected in slow, type I muscle fibers. Intramuscular administration of NT-4 induced sprouting of intact adult motor nerves. Thus, muscle-derived NT-4 acted as an activity-dependent neurotrophic signal for growth and remodeling of adult motor neuron innervation. NT-4 may thus be partly responsible for the effects of exercise and electrical stimulation on neuromuscular performance.

Molecular characterization of eukaryotic polysialyltransferase-1

Polysialic acid (PSA) is a dynamically regulated product of post-translational modification of the neural cell adhesion molecule, NCAM. Presence of the large anionic carbohydrate modulates NCAM binding properties and, by increasing the intercellular space, influences interactions between other cell surface molecules. PSA expression underlies cell type- and developmental-specific alterations and correlates with stages of cellular motility. In the adult, PSA becomes restricted to regions of permanent neural plasticity and regenerating neural and muscle tissues. Recent data implicate its important function in spatial learning and memory, and in tumour biology. Here we describe the molecular characterization of polysialyltransferase-1, the key enzyme of eukaryotic PSA synthesis. In reconstitution experiments, the newly cloned enzyme induces PSA synthesis in all NCAM-expressing cell lines. Our data therefore represent convincing evidence that the polycondensation of alpha-2,8-linked sialic acids in mammals is the result of a single enzymatic activity and provide a new basis for studying the functional role of PSA in neuro- and tumour biology.

Programmed cell death in development

Although cell death has long been recognized to be a significant element in the process of embryonic morphogenesis, its relationships to differentiation and its mechanisms are only now becoming apparent. This new appreciation has come about not only through advances in the understanding of cell death in parallel immunological and pathological situations, but also through progress in developmental genetics which has revealed the roles played by death in the cell lineages of invertebrate embryos. In this review, we discuss programmed cell death as it is understood in developmental situations, and its relationship to apoptosis. We describe the morphological and biochemical features of apoptosis, and some methods for its detection in tissues. The occurrence of programmed cell death during invertebrate development is reviewed, as well as selected examples in vertebrate development. In particular, we discuss cell death in the early vertebrate embryo, in limb development, and in the nervous system.

Immunolocalization of polysialic acid in the median eminence and neurointermediate hypophysial lobe of adult rats

Polysialic acid (PSA) is abundant on growing axons during brain development and down regulated on maturation. However, high amounts of this carbohydrate polymer have been found to persist in some regions of the adult rat brain including the mediobasal hypothalamus. In this study, confocal laser scanning microscopy combined with double fluorescence immunostaining was used to characterize the cellular localization of PSA throughout the median eminence and neurointermediate hypophysial lobe of adult rats. In these regions, polysialic acid-immunoreactivity (PSA-IR) generally appeared associated with fiber-like structures. Double immunostaining experiments demonstrated that, in addition to large axons of the neural lobe immunoreactive to vasopressin or oxytocin, PSA was constantly associated with fibers projecting into the intermediate hypophysial lobe immunoreactive to either gamma-aminobutyric acid (GABA) or tyrosine hydroxylase. Similarly, PSA-IR was detected on most, but not all the fibers immunoreactive to GABA or tyrosine hydroxylase dispersed throughout the neural lobe and the different layers of the median eminence. On the other hand, no PSA-IR was detected on axons immunoreactive to somatostatin or to corticotropin releasing hormone projecting throughout the median eminence, or on glial cell bodies and processes immunoreactive for glial fibrillary acidic protein (GFAP) or for vimentin dispersed throughout the median eminence and the neural lobe.

Effects of insulin-like growth factors on organotypic cocultures of embryonic rat brainstem slices and skeletal muscle fibers

Embryonic rat brainstem slices including the facial and hypoglossal motor nuclei were maintained in organotypic cocultures with explants of embryonic tongue or post-natal skeletal muscle for periods up to 3 weeks. Survival and differentiation of motoneurones were dependent both on the type of muscle explant and its position relative to the brainstem. Tongue explants induced a more important glial outgrowth, a motoneurone migration towards the muscle, earlier muscular contractions and a more complete neuronal and muscular differentiation. Since the foetal tongue contains IGF levels as high as foetal liver, these effects might be due in part to diffusion of IGF from the explanted muscle. Indeed, foetal liver explants or crude foetal liver extracts induced effects similar to those of tongue explants. These effects can be reproduced by addition of IGF-1 or IGF-2, or both, into the culture medium. Although IGF-1 and IGF-2 had similar effects, IGF-1 induced a more pronounced muscular differentiation and IGF-2 promoted neuronal differentiation. Our results suggest that IGFs are good candidates as muscle-derived neurotrophic factors promoting survival and differentiation of rat cranial motoneurones. These results also stress the importance of neuroglial trophic interactions and target development.

Adhesion molecules and structural plasticity of the adult hypothalamo-neurohypophysial system

The adult hypothalamo-neurohypophysial system, responsible for the secretion of the neurohormones, oxytocin, and vasopressin, undergoes reversible neuronal-glial and synaptic changes in response to stimulation (parturition, lactation, and osmotic stimulation). In the hypothalamus, these changes result in reduced astrocytic coverage of oxytocinergic somata and dendrites and concomitant increases in their GABAergic synapses; in the neurohypophysis, they lead to an enlarged neurovascular contact area. We discuss the possible role played by certain cell adhesion molecules, such as the highly sialylated isoform of the neural cell adhesion molecule, PSA-NCAM, the F3 glycoprotein, and the extracellular matrix molecule, tenascin, in such plasticity. The hypothalamo-neurohypophysial system continues to express high levels of these molecules during adulthood and they may serve as permissive factors to allow stimulus-induced structural remodelling to occur.

Interactions between spinal cord stimulation and activity blockade in the regulation of synaptogenesis and motoneuron survival in the chick embryo

The present study investigated the effects of spinal cord stimulation, neuromuscular blockade, or a combination of the two on neuromuscular development both during and after the period of naturally occurring motoneuron death in the chick embryo. Electrical stimulation of the spinal cord was without effect on motoneuron survival, synaptogenesis, or muscle properties. By contrast, activity blockade rescued motoneurons from cell death and altered synaptogenesis. A combination of spinal cord stimulation and activity blockade resulted in a marked increase in motoneuron death, and also altered synaptogenesis similar to that seen with activity blockade alone. Perturbation of normal nerve-muscle interactions by activity blockade may increase the vulnerability of developing motoneurons to excessive excitatory afferent input (spinal cord stimulation) resulting in excitotoxic-induced cell death.

Biological studies of a putative avian muscle-derived neurotrophic factor that prevents naturally occurring motoneuron death in vivo

A series of in vivo studies have been carried out using the chick embryo to address several critical questions concerning the biological, and to a lesser extent, the biochemical characteristics of a putative avian muscle-derived trophic agent that promotes motoneuron survival in vivo. A partially purified fraction of muscle extract was shown to be heat and trypsin sensitive and rescued motoneurons from naturally occurring cell death in a dose-dependent fashion. Muscle extract had no effect on mitotic activity in the spinal cord and did not alter cell number when administered either before or after the normal cell death period. The survival promoting activity in the muscle extract appears to be developmentally regulated. Treatment with muscle extract during the cell death period did not permanently rescue motoneurons. The motoneuron survival-promoting activity found in skeletal muscle was not present in extracts from a variety of other tissues, including liver, kidney, lung, heart, and smooth muscle. Survival activity was also found in extracts from fetal mouse, rat, and human skeletal muscle. Conditioned medium derived from avian myotube cultures also prevented motoneuron death when administered in vivo to chick embryos. Treatment of embryos in ovo with muscle extract had no effect on several properties of developing muscles. With the exception of cranial motoneurons, treatment with muscle extract did not promote the survival of several other populations of neurons in the central and peripheral nervous system that also exhibit naturally occurring cell death. Initial biochemical characterization suggests that the activity in skeletal muscle is an acidic protein between 10 and 30 kD. Examination of a number of previously characterized growth and trophic agents in our in vivo assay have identified several molecules that promote motoneuron survival to one degree or another. These include S100 beta, brain-derived neurotrophic factor (BDNF), neurotrophin 4/5 (NT-4/5), ciliary neurotrophic factor (CNTF), transforming growth factor beta (TGF beta), platelet-derived growth factor-AB (PDGF-AB), leukemia inhibitory factor (CDF/LIF), and insulin-like growth factors I and II (IGF). By contrast, the following agents were ineffective: nerve growth factor (NGF), neurotrophin-3 (NT3), epidermal growth factor (EGF), acidic and basic fibroblast growth factors (aFGF, bFGF), and the heparin-binding growth-associated molecule (HB-GAM).(ABSTRACT TRUNCATED AT 400 WORDS)