Differential effects of neurotrophic factors on motoneuron retrograde labeling in a murine model of motoneuron disease

Influence of altered serum and muscle concentrations of BDNF on electrophysiological properties of spinal motoneurons in wild-type and BDNF-knockout rats

The purpose of this study was to determine whether altered serum and/or muscle concentrations of brain-derived neurotrophic factor (BDNF) can modify the electrophysiological properties of spinal motoneurons (MNs). This study was conducted in wild-type and Bdnf heterozygous knockout rats (HET, SD-BDNF). Rats were divided into four groups: control, knockout, control trained, and knockout trained. The latter two groups underwent moderate-intensity endurance training to increase BDNF levels in serum and/or hindlimb muscles. BDNF and other neurotrophic factors (NFs), including glial cell-derived neurotrophic factor (GDNF), neurotrophin-3 (NT-3), nerve growth factor (NGF), and neurotrophin-4 (NT-4) were assessed in serum and three hindlimb muscles: the tibialis anterior (TA), medial gastrocnemius (MG), and soleus (Sol). The concentrations of tropomyosin kinase receptor B (Trk-B), interleukin-15 (IL-15), and myoglobin (MYO/MB) were also evaluated in these muscles. The electrophysiological properties of lumbar MNs were studied in vivo using whole-cell current-clamp recordings. Bdnf knockout rats had reduced levels of all studied NFs in serum but not in hindlimb muscles. Interestingly, decreased serum NF levels did not influence the electrophysiological properties of spinal MNs. Additionally, endurance training did not change the serum concentrations of any of the NFs tested but significantly increased BDNF and GDNF levels in the TA and MG muscles in both trained groups. Furthermore, the excitability of fast MNs was reduced in both groups of trained rats. Thus, changes in muscle (but not serum) concentrations of BDNF and GDNF may be critical factors that modify the excitability of spinal MNs after intense physical activity.

Fast Blue and Cholera Toxin-B Survival Guide for Alpha-Motoneurons Labeling: Less Is Better in Young B6SJL Mice, but More Is Better in Aged C57Bl/J Mice

Fast Blue (FB) and Cholera Toxin-B (CTB) are two retrograde tracers extensively used to label alpha-motoneurons (α-MNs). The overall goals of the present study were to (1) assess the effectiveness of different FB and CTB protocols in labeling α-MNs, (2) compare the labeling quality of these tracers at standard concentrations reported in the literature (FB 2% and CTB 0.1%) versus lower concentrations to overcome tracer leakage, and (3) determine an optimal protocol for labeling α-MNs in young B6SJL and aged C57Bl/J mice (when axonal transport is disrupted by aging). Hindlimb muscles of young B6SJL and aged C57Bl/J mice were intramuscularly injected with different FB or CTB concentrations and then euthanized at either 3 or 5 days after injection. Measurements were performed to assess labeling quality via seven different parameters. Our results show that tracer protocols of lower concentration and shorter labeling durations were generally better in labeling young α-MNs, whereas tracer protocols of higher tracer concentration and longer labeling durations were generally better in labeling aged α-MNs. A 0.2%, 3-day FB protocol provided optimal labeling of young α-MNs without tracer leakage, whereas a 2%, 5-day FB protocol or 0.1% CTB protocol provided optimal labeling of aged α-MNs. These results inform future studies on the selection of optimal FB and CTB protocols for α-MNs labeling in normal, aging, and neurodegenerative disease conditions.

Muscle-to-Brain Signaling Via Myokines and Myometabolites

Skeletal muscle health and function are important determinants of systemic metabolic homeostasis and organism-wide responses, including disease outcome. While it is well known that exercise protects the central nervous system (CNS) from aging and disease, only recently this has been found to depend on the endocrine capacity of skeletal muscle. Here, we review muscle-secreted growth factors and cytokines (myokines), metabolites (myometabolites), and other unconventional signals (e.g. bioactive lipid species, enzymes, and exosomes) that mediate muscle-brain and muscle-retina communication and neuroprotection in response to exercise and associated processes, such as the muscle unfolded protein response and metabolic stress. In addition to impacting proteostasis, neurogenesis, and cognitive functions, muscle-brain signaling influences complex brain-dependent behaviors, such as depression, sleeping patterns, and biosynthesis of neurotransmitters. Moreover, myokine signaling adapts feeding behavior to meet the energy demands of skeletal muscle. Contrary to protective myokines induced by exercise and associated signaling pathways, inactivity and muscle wasting may derange myokine expression and secretion and in turn compromise CNS function. We propose that tailoring muscle-to-CNS signaling by modulating myokines and myometabolites may combat age-related neurodegeneration and brain diseases that are influenced by systemic signals.

Pegylated Insulin-Like Growth Factor 1 attenuates Hair Cell Loss and promotes Presynaptic Maintenance of Medial Olivocochlear Cholinergic Fibers in the Cochlea of the Progressive Motor Neuropathy Mouse

The progressive motor neuropathy (PMN) mouse is a model of an inherited motor neuropathy disease with progressive neurodegeneration. Axon degeneration associates with homozygous mutations of the TBCE gene encoding the tubulin chaperone E protein. TBCE is responsible for the correct dimerization of alpha and beta-tubulin. Strikingly, the PMN mouse also develops a progressive hearing loss after normal hearing onset, characterized by degeneration of the auditory nerve and outer hair cell (OHC) loss. However, the development of this neuronal and cochlear pathology is not fully understood yet. Previous studies with pegylated insulin-like growth factor 1 (peg-IGF-1) treatment in this mouse model have been shown to expand lifespan, weight, muscle strength, and motor coordination. Accordingly, peg-IGF-1 was evaluated for an otoprotective effect. We investigated the effect of peg-IGF-1 on the auditory system by treatment starting at postnatal day 15 (p15). Histological analysis revealed positive effects on OHC synapses of medial olivocochlear (MOC) neuronal fibers and a short-term attenuation of OHC loss. Peg-IGF-1 was able to conditionally restore the disorganization of OHC synapses and maintain the provision of cholinergic acetyltransferase in presynapses. To assess auditory function, frequency-specific auditory brainstem responses and distortion product otoacoustic emissions were recorded in animals on p21 and p28. However, despite the positive effect on MOC fibers and OHC, no restoration of hearing could be achieved. The present work demonstrates that the synaptic pathology of efferent MOC fibers in PMN mice represents a particular form of “efferent auditory neuropathy.” Peg-IGF-1 showed an otoprotective effect by preventing the degeneration of OHCs and efferent synapses. However, enhanced efforts are needed to optimize the treatment to obtain detectable improvements in hearing performances.

Muscle Contraction Regulates BDNF/TrkB Signaling to Modulate Synaptic Function through Presynaptic cPKCα and cPKCβI

The neurotrophin brain-derived neurotrophic factor (BDNF) acts via tropomyosin-related kinase B receptor (TrkB) to regulate synapse maintenance and function in the neuromuscular system. The potentiation of acetylcholine (ACh) release by BDNF requires TrkB phosphorylation and Protein Kinase C (PKC) activation. BDNF is secreted in an activity-dependent manner but it is not known if pre- and/or postsynaptic activities enhance BDNF expression in vivo at the neuromuscular junction (NMJ). Here, we investigated whether nerve and muscle cell activities regulate presynaptic conventional PKC (cPKCα and βI) via BDNF/TrkB signaling to modulate synaptic strength at the NMJ. To differentiate the effects of presynaptic activity from that of muscle contraction, we stimulated the phrenic nerve of rat diaphragms (1 Hz, 30 min) with or without contraction (abolished by μ-conotoxin GIIIB). Then, we performed ELISA, Western blotting, qRT-PCR, immunofluorescence and electrophysiological techniques. We found that nerve-induced muscle contraction: (1) increases the levels of mature BDNF protein without affecting pro-BDNF protein or BDNF mRNA levels; (2) downregulates TrkB.T1 without affecting TrkB.FL or p75 neurotrophin receptor (p75) levels; (3) increases presynaptic cPKCα and cPKCβI protein level through TrkB signaling; and (4) enhances phosphorylation of cPKCα and cPKCβI. Furthermore, we demonstrate that cPKCβI, which is exclusively located in the motor nerve terminals, increases activity-induced acetylcholine release. Together, these results show that nerve-induced muscle contraction is a key regulator of BDNF/TrkB signaling pathway, retrogradely activating presynaptic cPKC isoforms (in particular cPKCβI) to modulate synaptic function. These results indicate that a decrease in neuromuscular activity, as occurs in several neuromuscular disorders, could affect the BDNF/TrkB/PKC pathway that links pre- and postsynaptic activity to maintain neuromuscular function.

Development of Precision Small-Molecule Proneurotrophic Therapies for Neurodegenerative Diseases

Age-related neurodegenerative diseases, such as Alzheimer's disease, will represent one of the largest future burdens on worldwide healthcare systems due to the increasing proportion of elderly in our society. As deficiencies in neurotrophins are implicated in the pathogenesis of many age-related neurodegenerative disorders, it is reasonable to consider that global neurotrophin resistance may also become a major healthcare threat. Central nervous system networks are effectively maintained through aging by neuroprotective and neuroplasticity signaling mechanisms which are predominantly controlled by neurotrophin receptor signaling. Neurotrophin receptors are single pass receptor tyrosine kinases that form dimeric structures upon ligand binding to initiate cellular signaling events that control many protective and plasticity-related pathways. Declining functionality of the neurotrophin ligand-receptor system is considered one of the hallmarks of neuropathological aging. Therefore, it is imperative to develop effective therapeutic strategies to contend with this significant issue. While the therapeutic applications of cognate ligands for neurotrophin receptors are limited, the development of nonpeptidergic, small-molecule ligands can overcome these limitations, and productively regulate this important receptor system with beneficial effects. Using our advanced knowledge of the high-dimensionality complexity of receptor systems, the future generation of precision medicines targeting these systems will be an attainable goal.

Emerging Trends in Retrograde Signaling

Retrograde signaling is defined as the signaling events leading from the plastids to the nucleus in plants and across the chemical synapse, from the postsynaptic neuron to the presynaptic neuron in animals. The discovery of various retrograde messengers has opened many avenues and clouds of thoughts as to the role of retrograde signaling. They have been implicated particularly in long-term potentiation (LTP) and synaptic plasticity. But the basic assumptions about retrograde signaling have not been studied upon for many years. This review focuses on established facts and hypothesis put forward in retrograde signaling.

Androgen-dependent loss of muscle BDNF mRNA in two mouse models of SBMA

Transgenic expression of neurotrophic factors in skeletal muscle has been found to protect mice from neuromuscular disease, including spinal bulbar muscular atrophy (SBMA), triggering renewed interest in neurotrophic factors as therapeutic agents for treating neuromuscular disease. Because SBMA is an androgen-dependent disease, and brain-derived neurotrophic factor (BDNF) mediates effects of androgens on neuromuscular systems, we asked whether BDNF expression is impaired in two different transgenic (Tg) mouse models of SBMA, the so called "97Q" and "myogenic" SBMA models. The 97Q model globally overexpresses a full length human AR with 97 glutamine repeats whereas the myogenic model of SBMA overexpresses a wild-type rat androgen receptor (AR) only in skeletal muscle fibers. Using quantitative PCR, we find that muscle BDNF mRNA declines in an androgen-dependent manner in both models, paralleling changes in motor function, with robust deficits (6-8 fold) in both fast and slow twitch muscles of impaired Tg males. Castration rescues or reverses disease-related deficits in muscle BDNF mRNA in both models, paralleling its effect on motor function. Moreover, when disease is acutely induced in Tg females, both motor function and muscle BDNF mRNA expression plummet, with the deficit in muscle BDNF emerging before overt motor dysfunction. That androgen-dependent motor dysfunction is tightly associated with a robust and early down-regulation of muscle BDNF mRNA suggests that BDNF delivered to skeletal muscle may have therapeutic value for SBMA.

Transplanted modified muscle progenitor cells expressing a mixture of neurotrophic factors delay disease onset and enhance survival in the SOD1 mouse model of ALS

Neurotrophic factors (NTFs) are essential growth factor proteins that support the development, survival, and proper function of neurons. We have developed muscle progenitor cell (MPC) populations expressing brain-derived neurotrophic factor (BDNF), glial-derived neurotrophic factor (GDNF), vascular endothelial growth factor (VEGF), or insulin-like growth factor-1 (IGF-1). Transplantation of a mixture of such MPC populations (MPC-MIX) into the hind legs of SOD1 G93A transgenic mice (SOD1 mice), the commonly used model of ALS, delayed the onset of disease symptoms by 30 days and prolonged the average lifespan by 13 days. Treated mice also showed a decrease in the degeneration of neuromuscular junction and an increase in axonal survival. Cellular mechanism assays suggest a synergistic rescue effect of NTFs that involves the AKT and BAD signaling pathways. The results suggest that long-term delivery of a mixture of several NTFs by the transplantation of engineered MPC has a beneficial effect in the ALS mouse model.

Bone marrow transplantation in hindlimb muscles of motoneuron degenerative mice reduces neuronal death and improves motor function

Bone marrow has proved to be an adequate source of stem cells for the treatment of numerous disorders, including neurodegenerative diseases. Bone marrow can be easily and relatively painlessly extracted from a patient or allogenic donor and then transplanted into the degenerative area. Here, the grafted cells will activate a number of mechanisms in order to protect, repair, and/or regenerate the damaged tissue. These properties make the bone marrow a feasible source for cell therapy. In this work, we transplanted bone marrow cells into a mouse model of motoneuron degeneration, with the particularity of placing the cells in the hindlimb muscles rather than in the spinal cord where neuronal degeneration occurs. To this end, we analyze the possibility for the transplanted cells to increase the survival rate of the spinal cord motoneurons by axonal-guided retrograde neurotrophism. As a result, the mice significantly improved their motor functions. This coincided with an increased number of motoneurons innervating the treated muscle compared with the neurons innervating the non-treated contralateral symmetric muscle. In addition, we detected an increase in glial-derived neurotrophic factor in the spinal cord, a neurotrophic factor known to be involved in the rescue of degenerating motoneurons, exerting a neuroprotective effect. Thus, we have proved that bone marrow injected into the muscles is capable of rescuing these motoneurons from death, which may be a possible therapeutic approach for spinal cord motoneuron degenerative diseases, such as amyotrophic lateral sclerosis.

Local axonal function of STAT3 rescues axon degeneration in the pmn model of motoneuron disease

Axonal maintenance, plasticity, and regeneration are influenced by signals from neighboring cells, in particular Schwann cells of the peripheral nervous system. Schwann cells produce neurotrophic factors, but the mechanisms by which ciliary neurotrophic factor (CNTF) and other neurotrophic molecules modify the axonal cytoskeleton are not well understood. In this paper, we show that activated signal transducer and activator of transcription-3 (STAT3), an intracellular mediator of the effects of CNTF and other neurotrophic cytokines, acts locally in axons of motoneurons to modify the tubulin cytoskeleton. Specifically, we show that activated STAT3 interacted with stathmin and inhibited its microtubule-destabilizing activity. Thus, ectopic CNTF-mediated activation of STAT3 restored axon elongation and maintenance in motoneurons from progressive motor neuronopathy mutant mice, a mouse model of motoneuron disease. This mechanism could also be relevant for other neurodegenerative diseases and provide a target for new therapies for axonal degeneration.

Therapy development for spinal muscular atrophy in SMN independent targets

Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disorder, leading to progressive muscle weakness, atrophy, and sometimes premature death. SMA is caused by mutation or deletion of the survival motor neuron-1 (SMN1) gene. An effective treatment does not presently exist. Since the severity of the SMA phenotype is inversely correlated with expression levels of SMN, the SMN-encoded protein, SMN is the most important therapeutic target for development of an effective treatment for SMA. In recent years, numerous SMN independent targets and therapeutic strategies have been demonstrated to have potential roles in SMA treatment. For example, some neurotrophic, antiapoptotic, and myotrophic factors are able to promote survival of motor neurons or improve muscle strength shown in SMA mouse models or clinical trials. Plastin-3, cpg15, and a Rho-kinase inhibitor regulate axonal dynamics and might reduce the influences of SMN depletion in disarrangement of neuromuscular junction. Stem cell transplantation in SMA model mice resulted in improvement of motor behaviors and extension of survival, likely from trophic support. Although most therapies are still under investigation, these nonclassical treatments might provide an adjunctive method for future SMA therapy.

The recent understanding of the neurotrophin's role in skeletal muscle adaptation

This paper summarizes the various effects of neurotrophins in skeletal muscle and how these proteins act as potential regulators of the maintenance, function, and regeneration of skeletal muscle fibers. Increasing evidence suggests that this family of neurotrophic factors influence not only the survival and function of innervating motoneurons but also the development and differentiation of myoblasts and muscle fibers. Muscle contractions (e.g., exercise) produce BDNF mRNA and protein in skeletal muscle, and the BDNF seems to play a role in enhancing glucose metabolism and may act for myokine to improve various brain disorders (e.g., Alzheimer's disease and major depression). In adults with neuromuscular disorders, variations in neurotrophin expression are found, and the role of neurotrophins under such conditions is beginning to be elucidated. This paper provides a basis for a better understanding of the role of these factors under such pathological conditions and for treatment of human neuromuscular disease.

Impaired motoneuronal retrograde transport in two models of SBMA implicates two sites of androgen action

Spinal and bulbar muscular atrophy (SBMA) impairs motor function in men and is linked to a CAG repeat mutation in the androgen receptor (AR) gene. Defects in motoneuronal retrograde axonal transport may critically mediate motor dysfunction in SBMA, but the site(s) where AR disrupts transport is unknown. We find deficits in retrograde labeling of spinal motoneurons in both a knock-in (KI) and a myogenic transgenic (TG) mouse model of SBMA. Likewise, live imaging of endosomal trafficking in sciatic nerve axons reveals disease-induced deficits in the flux and run length of retrogradely transported endosomes in both KI and TG males, demonstrating that disease triggered in muscle can impair retrograde transport of cargo in motoneuron axons, possibly via defective retrograde signaling. Supporting the idea of impaired retrograde signaling, we find that vascular endothelial growth factor treatment of diseased muscles reverses the transport/trafficking deficit. Transport velocity is also affected in KI males, suggesting a neurogenic component. These results demonstrate that androgens could act via both cell autonomous and non-cell autonomous mechanisms to disrupt axonal transport in motoneurons affected by SBMA.

Motor neuron trophic factors: therapeutic use in ALS?

The modest effects of neurotrophic factor (NTF) treatment on lifespan in both animal models and clinical studies of Amyotropic Lateral Sclerosis (ALS) may result from any one or combination of the four following explanations: 1.) NTFs block cell death in some physiological contexts but not in ALS; 2.) NTFs do not rescue motoneurons (MNs) from death in any physiological context; 3.) NTFs block cell death in ALS but to no avail; and 4.) NTFs are physiologically effective but limited by pharmacokinetic constraints. The object of this review is to critically evaluate the role of both NTFs and the intracellular cell death pathway itself in regulating the survival of spinal and cranial (lower) MNs during development, after injury and in response to disease. Because the role of molecules mediating MN survival has been most clearly resolved by the in vivo analysis of genetically engineered mice, this review will focus on studies of such mice expressing reporter, null or other mutant alleles of NTFs, NTF receptors, cell death or ALS-associated genes.

Recovery of whisking function after manual stimulation of denervated vibrissal muscles requires brain-derived neurotrophic factor and its receptor tyrosine kinase B

Functional recovery following facial nerve injury is poor. Neuromuscular junctions (NMJs) are "bridged" by terminal Schwann cells and numerous regenerating axonal sprouts. We have shown that this poly-innervation of NMJs can be reduced by manual stimulation (MS) with restoration of whisking function. In addition, we have recently reported that insulin-like growth factor-1 (IGF-1) is required to mediate the beneficial effects of MS. Here we extend our findings to brain derived neurotrophic factor (BDNF). We then examined the effect of MS after facial-facial anastomosis (FFA) in heterozygous mice deficient in BDNF (BDNF(+/-)) or in its receptor TrkB (TrkB(+/-)). We quantified vibrissal motor performance and the percentage of NMJ bridged by S100-positive terminal Schwann cells. In intact BDNF(+/-) or TrkB(+/-) mice and their wild type (WT) littermates, there were no differences in vibrissal whisking nor in the percentage of bridged NMJ (0% in each genotype). After FFA and handling alone (i.e. no MS) in WT animals, vibrissal whisking amplitude was reduced (60% lower than intact) and the percentage of bridged NMJ increased (27% more than intact). MS improved both the amplitude of vibrissal whisking (not significantly different from intact) and the percentage of bridged NMJ (11% more than intact). After FFA and handling in BDNF(+/-) or TrkB(+/-) mice, whisking amplitude was again reduced (53% and 60% lower than intact) and proportion of bridged NMJ increased (24% and 29% more than intact). However, MS failed to improve outcome in both heterozygous strains (whisking amplitude 55% and 58% lower than intact; proportion of bridged NMJ 27% and 18% more than intact). We conclude that BDNF and TRkB are required to mediate the effects of MS on target muscle reinnervation and recovery of whisking function.

Repetetive hindlimb movement using intermittent adaptive neuromuscular electrical stimulation in an incomplete spinal cord injury rodent model

The long-term objective of this work is to understand the mechanisms by which electrical stimulation based movement therapies may harness neural plasticity to accelerate and enhance sensorimotor recovery after incomplete spinal cord injury (iSCI). An adaptive neuromuscular electrical stimulation (aNMES) paradigm was implemented in adult Long Evans rats with thoracic contusion injury (T8 vertebral level, 155+/-2 Kdyne). In lengthy sessions with lightly anesthetized animals, hip flexor and extensor muscles were stimulated using an aNMES control system in order to generate desired hip movements. The aNMES control system, which used a pattern generator/pattern shaper structure, adjusted pulse amplitude to modulate muscle force in order to control hip movement. An intermittent stimulation paradigm was used (5-cycles/set; 20-second rest between sets; 100 sets). In each cycle, hip rotation caused the foot plantar surface to contact a stationary brush for appropriately timed cutaneous input. Sessions were repeated over several days while the animals recovered from injury. Results indicated that aNMES automatically and reliably tracked the desired hip trajectory with low error and maintained range of motion with only gradual increase in stimulation during the long sessions. Intermittent aNMES thus accounted for the numerous factors that can influence the response to NMES: electrode stability, excitability of spinal neural circuitry, non-linear muscle recruitment, fatigue, spinal reflexes due to cutaneous input, and the endogenous recovery of the animals. This novel aNMES application in the iSCI rodent model can thus be used in chronic stimulation studies to investigate the mechanisms of neuroplasticity targeted by NMES-based repetitive movement therapy.

Wld(S), Nmnats and axon degeneration--progress in the past two decades

A chimeric protein called Wallerian degeneration slow (Wld(S)) was first discovered in a spontaneous mutant strain of mice that exhibited delayed Wallerian degeneration. This provides a useful tool in elucidating the mechanisms of axon degeneration. Over-expression of Wld(S) attenuates the axon degeneration that is associated with several neurodegenerative disease models, suggesting a new logic for developing a potential protective strategy. At molecular level, although Wld(S) is a fusion protein, the nicotinamide mononucleotide adenylyl transferase 1 (Nmnat1) is required and sufficient for the protective effects of Wld(S), indicating a critical role of NAD biosynthesis and perhaps energy metabolism in axon degeneration. These findings challenge the proposed model in which axon degeneration is operated by an active programmed process and thus may have important implication in understanding the mechanisms of neurodegeneration. In this review, we will summarize these recent findings and discuss their relevance to the mechanisms of axon degeneration.

Ciliary neurotrophic factor-induced sprouting preserves motor function in a mouse model of mild spinal muscular atrophy

Proximal spinal muscular atrophy (SMA) is caused by homozygous loss or mutation of the SMN1 gene on human chromosome 5. Depending on the levels of SMN protein produced from a second SMN gene (SMN2), different forms of the disease are distinguished. In patients with milder forms of the disease, type III or type IV SMA that normally reach adulthood, enlargement of motor units is regularly observed. However, the underlying mechanisms are not understood. Smn(+/-) mice, a mouse model of type III/IV SMA, reveal progressive loss of motor neurons and denervation of motor endplates starting at 4 weeks of age. Loss of spinal motor neurons between 1 month and 12 months reaches 40%, whereas muscle strength is not reduced. In these animals, amplitude of single motor unit action potentials in the gastrocnemic muscle is increased more than 2-fold. Confocal analysis reveals pronounced sprouting of innervating motor axons. As ciliary neurotrophic factor (CNTF) is highly expressed in Schwann cells, we investigated its role for a compensatory sprouting response and maintenance of muscle strength in this mouse model. Genetic ablation of CNTF results in reduced sprouting and decline of muscle strength in Smn(+/-) mice. These findings indicate that CNTF is necessary for a sprouting response and thus enhances the size of motor units in skeletal muscles of Smn(+/-) mice. This compensatory mechanism could guide the way to new therapies for this motor neuron disease.

Auditory development in progressive motor neuronopathy mouse mutants

The present study was performed to elucidate the hearing development in the progressive motor neuronopathy (pmn) mouse mutant. This mouse has been used as a model for human motoneuron disease. A missense mutation in the tubulin-specific chaperon E (Tbce) gene on mouse chromosome 13 was localized as the underlying genetic defect. The protein encoded by the Tbce gene is essential for the formation of primary tubulin complexes. Studies on motoneurons show disorganization in microtubules and disturbed axonal transport, followed by retrograde degeneration of the motoneurons. A similar pathomechanism is also possible for hearing disorders where disrupted microtubules could cause functional deficits in spiral ganglion neurons or in cochlear hair cells. Click auditory brainstem response (ABR) audiometry in homozygous pmn mutants showed a normal onset of hearing, but an increasing hearing threshold from postnatal day 26 (P26) on to death, compared to heterozygous mutants and wild-type mice. Histological sections of the cochlea at different ages showed a regular morphology. Additionally, spiral ganglion explants from mutant and wild-type mice were cultured. The neurite length from pmn mutants was shorter than in wild-type mice, and the neurite number/explant was significantly decreased in pmn mutants. We show that the pmn mouse mutant is a model for a progressive rapid hearing loss from P26 on, after initially normal hearing development. Heterozygous mice are not affected by this defect. With the knowledge of the well-known pathomechanism of this defect in motoneurons, a dysfunction of cellular mechanisms regulating tubulin assembling suggests that tubulin assembling plays an essential role in hearing function and maintenance.

Chemical communication between regenerating motor axons and Schwann cells in the growth pathway

There are receptors on denervated Schwann cells that may respond to the neurotransmitters that are released from growth cones of regenerating motor axons. In order to ascertain whether the interaction of the transmitters and their receptors plays a role during axon regeneration, we investigated whether pharmacological block of the interaction would reduce the number of motoneurons that regenerate their axons after nerve section and surgical repair. Peripheral nerves in the hindlimbs of rats and mice were cut and repaired, and various drugs were applied to the peripheral nerve stump either directly or via mini-osmotic pumps over a 2-4-week period to block the binding of acetylcholine to nicotinic and muscarinic acetylcholine receptors (AChRs: alpha-bungarotoxin, tubocurarine, atropine and, gallamine) and binding of ATP to P2Y receptors (suramin). In rats, the nicotinic AChR antagonistic drugs and suramin reduced the number of motoneurons that regenerated their axons through the distal nerve stump. In mice, suramin significantly reduced the upregulation of the carbohydrate HNK-1 on the Schwann cells in the distal nerve stump that normally occurs during motor axon regeneration. These data indicate that chemical communication between regenerating axons and Schwann cells during axon regeneration via released neurotransmitters and their receptors may play an important role in axon regeneration.

Grafted neural stem cells increase the life span and protect motoneurons in pmn mice

In this study, we have grafted neural stem cells (NSCs) into the lumbar spinal cord of a mouse mutant that has a specific loss of motoneurons (progressive motor neuronopathy/pmn). A small number of grafted cells ( approximately 3000) increased the life span of the mice by 56%. The improved survival was accompanied by a rescue of host motoneurons, a stabilization in the weight and an increase in the size of the muscle fibers. The grafted NSCs were small and round and exhibited no neural markers, suggesting that they remained in an undifferentiated state. Thus grafting of NSCs in a mouse model with motoneuron degeneration exerts a neuroprotective effect.

Mechanisms of axon degeneration: from development to disease

Axon degeneration is an active, tightly controlled and versatile process of axon segment self-destruction. Although not involving cell death, it resembles apoptosis in its logics. It involves three distinct steps: induction of competence in specific neurons, triggering of degeneration at defined axon segments of competent neurons, and rapid fragmentation and removal of the segments. The mechanisms that initiate degeneration are specific to individual settings, but the final pathway of pruning is shared; it involves microtubule disassembly, axon swellings, axon fragmentation, and removal of the remnants by locally recruited phagocytes. The tight regulatory properties of axon degeneration distinguish it from passive loss phenomena, and confer significance to processes that involve it. Axon degeneration has prominent roles in development, upon lesions and in disease. In development, it couples the progressive specification of neurons and circuits to the removal of defined axon branches. Competence might involve transcriptional switches, and local triggering can involve axon guidance molecules and synaptic activity patterns. Lesion-induced Wallerian degeneration is inhibited in the presence of Wld(S) fusion protein in neurons; it involves early local, and later, distal degeneration. It has recently become clear that like in other settings, axon degeneration in disease is a rapid and specific process, which should not be confused with a variety of disease-related pathologies. Elucidating the specific mechanisms that initiate axon degeneration should open up new avenues to investigate principles of circuit assembly and plasticity, to uncover mechanisms of disease progression, and to identify ways of protecting synapses and axons in disease.

Axonal involvement in the Wlds neuroprotective effect: analysis of pure motoneurons in a mouse model protected from motor neuron disease at a pre-symptomatic age

The identification of the Wlds gene that delays axonal degeneration in several models of neurodegenerative disease provides an interesting tool to study mechanisms of axonal loss. We showed that crossing a mouse mutant with a motoneuron disease (pmn for progressive motor neuronopathy) with mice that express the Wlds gene delayed axonal loss, increased the life span, partially rescued axonal transport deficit and prolonged the survival of the motoneuron cell bodies. To determine factors involved in the neuroprotective effect of Wlds, we combined laser capture microdissection and microarray analysis to identify genes that are differentially regulated at a pre-symptomatic age in motoneuron cell bodies in pmn/pmn,Wlds/Wlds mice as compared with pmn/pmn mice. Only 56 genes were de-regulated; none of the 'classical' genes implicated in apoptosis were de-regulated. Interestingly, a large proportion of these genes are related to axonal function and to retrograde and anterograde transport (i.e. members of the dynactin complex and kinesin family). These results were confirmed by real-time PCR, in situ hybridization and at protein level in sciatic nerves. Thus, genes related to axonal function and in particular to axonal transport may be involved at an early stage in the neuroprotective property of the Wlds gene and confirm the importance of axonal involvement in this model of motor neuron disease.

Progesterone modulates brain-derived neurotrophic factor and choline acetyltransferase in degenerating Wobbler motoneurons

Progesterone (PROG) shows neuroprotective effects in nervous system diseases. The Wobbler mouse, a model of motoneuron degeneration, suffers a mutation of the Vsp154 gene on chromosome 11 leading to motoneuron vacuolation and astrocytosis of the spinal cord. Previous work has demonstrated beneficial effects of PROG in the Wobbler mouse. As an extension of this work, we now studied steroid effects on neuronal brain-derived neurotrophic factor (BDNF) mRNA and protein, on choline acetyltransferase (ChAT) immunoreactivity (IR) and activity in the spinal cord, and on recovery of muscle atrophy. Wobbler mice received implants of PROG pellets (20 mg) at 6 and 10 weeks of age and were killed at 14 weeks. In situ hybridization for BDNF mRNA demonstrated that grain density in large (>600 microm2) and medium size (<600 microm2) ventral horn neurons was decreased in untreated Wobblers, whereas PROG treatment increased BDNF mRNA in both neuronal types. PROG also induced a subcellular redistribution of BDNF protein, which in controls and steroid-naive Wobblers showed a predominant perinuclear and nucleolar location, whereas after PROG treatment, it was detected in cytoplasmic aggregates. ChAT activity was reduced by 55.3% in muscles of untreated Wobbler mice, whereas a significant increment was obtained after PROG treatment. Wobblers also showed reduced number of ChAT positive motoneurons, but this number was restored to normal by PROG. Finally, the pronounced biceps atrophy of steroid-naive Wobbler mice was slightly but significantly increased by PROG-treatment. Considering the important role played by neurotrophins on neuronal function, changes in BDNF might be part of the PROG activated-pathways to provide neuroprotection and re-establish neurotransmission and neuromuscular function in this degeneration model.

Brain-derived neurotrophic factor facilitates in vivo internalization of tetanus neurotoxin C-terminal fragment fusion proteins in mature mouse motor nerve terminals

In a previous study it was reported that fusion proteins composed of the atoxic C-terminal fragment of tetanus toxin (TTC) and green fluorescent protein or beta-galactosidase (GFP-TTC and beta-gal-TTC, respectively) rapidly cluster at motor nerve terminals of the mouse neuromuscular junction (NMJ). Because this traffic involves presynaptic activity, probably via the secretion of active molecules, we examined whether it is affected by brain-derived neurotrophic factor (BDNF). Quantitative confocal microscopy and a fluorimetric assay for beta-gal activity revealed that co-injecting BDNF and the fusion proteins significantly increased the kinetics and amount of the proteins' localization at the NMJ and their internalization by motor nerve terminals. The observed increases were independent of synaptic vesicle recycling because BDNF did not affect spontaneous quantal acetylcholine release. In addition, injecting anti-BDNF antibody shortly before injecting GFP-TTC, and before co-injecting GFP-TTC and BDNF, significantly reduced the fusion protein's localization at the NMJ. Co-injecting GFP-TTC with neurotrophin-4 (NT-4) or glial-derived neurotrophic factor (GDNF), but not with nerve growth factor, neurotrophin-3 or ciliary neurotrophic factor, also significantly increased the fusion protein's localization at the NMJ. Thus, TTC probes may use for their neuronal internalization endocytic pathways normally stimulated by BDNF, NT-4 and GDNF binding. Different tyrosine kinase receptors with similar signalling pathways are activated by BDNF/NT-4 and GDNF binding. Thus, activated components of these signalling pathways may be involved in the TTC probes' internalization, perhaps by facilitating localization of receptors of TTC in specific membrane microdomains or by recruiting various factors needed for internalization of TTC.

Effect of conduit repair on aberrant motor axon growth within the nerve graft in rats

After peripheral nerve injury, minimizing axonal misdirection has been a matter of importance to obtain good functional outcomes. In general, it becomes more challenging as the nerve defect length is longer. As previous works suggested that a conduit repair leaving a short gap could induce some target-specific reinnervation, we expected that a distally placed conduit combined with nerve graft would enhance the specificity of reinnervation, especially in dealing with a long gap. To test this, a 14-mm-long gap was created in the rat sciatic nerves and repaired with either 1) whole nerve graft (WG), 2) interfascicular nerve grafts (FG), or 3) whole nerve graft combined with distally placed silicone tube leaving a 5-mm gap (TUBG). At the end of follow up, the extent of target specific reinnervation (measurement of the compound muscle action potentials evoked by stimulation of the sciatic nerve and its tibial and common peroneal fascicles) and the accuracy of motoneuronal projection (sequential retrograde labeling of the common peroneal motor pool) were assessed. Both assessments revealed that groups FG and TUBG had a similar selectivity, which was significantly higher than in group WG. Consistent with these results, the functional recovery as assessed by walking track analysis showed no significant difference between groups FG and TUBG, whereas those were significantly superior to group WG. In contrast, histomorphometric assessment of the regenerating axons and wet muscle weight showed no significant difference among the three groups. In conclusion, conduit repair would have some effects on reducing motor axonal misdirection, and it might be more effective when used in the management of a large defect in combination with nerve graft.

The neurotrophin receptor p75NTR in Schwann cells is implicated in remyelination and motor recovery after peripheral nerve injury

The function of the p75(NTR) neurotrophin receptor (p75(NTR)) in nervous system regeneration is still controversial. Part of that controversy may be due to the fact that p75(NTR) is expressed by both neuronal and glial cell types and may have very distinct and even contradictory roles in each population. In this study, to elucidate the in vivo function of p75(NTR) in Schwann cells during remyelination after peripheral nerve injury, we established a new animal model for p75(NTR)-deficient Schwann cell transplantation in nude mice. We performed quantitative assessments of the functional, histological, and electrophysiological recovery after sciatic nerve injury, and compared them with those of the p75(NTR)(+/+) Schwann cell transplanted animals. At 7-10 weeks after injury, the motor recovery in the p75(NTR)(-/-) Schwann cell transplanted animals was significantly impaired compared with that in the p75(NTR)(+/+) Schwann cell transplanted animals. The lower number of the retrogradely labeled motoneurons and the hypomyelination in the p75(NTR)(-/-) Schwann cell transplanted animals were evident at 6 and 10 weeks after injury. At 10 weeks after injury, the radial growth in the axon caliber was also impaired in the p75(NTR)(-/-) Schwann cell transplanted animals. Measurement of the amount of myelin proteins and the nerve conduction velocity at 10 weeks after injury reflected these results. In summary, the p75(NTR) expression in Schwann cells is important for remyelination process, and the motor recovery after injury is impaired due to impaired axonal growth, remyelination, and radial growth in the axon calibers.

Myelin-associated glycoprotein reduces axonal branching and enhances functional recovery after sciatic nerve transection in rats

The mature peripheral nervous system (PNS) generally shows better regeneration of injured axons as opposed to the central nervous system (CNS). However, complete functional recovery is rarely achieved even in the PNS although morphologically good axonal regeneration often occurs. This mainly results from aberrant reinnervation due to extensive branching of cut axons with consequent failure of synchronized movements of the muscles. Myelin-associated glycoprotein (MAG), a well-characterized molecule existing both in the CNS and PNS myelin, is considered to be a potent inhibitor of axonal regeneration especially in the CNS. In the present study, we investigated whether MAG has any effects not only on axonal elongation, but also on axonal branching. We show herein that MAG minimized branching of the peripheral axons both in vitro and in vivo via activation of RhoA. Furthermore, after sciatic nerve transection in rats, focal and temporary application of MAG to the lesion dramatically enhanced the functional recovery. Using double retrograde labeling and preoperative/postoperative labeling of spinal neurons, reduced hyperinnervation and improved accuracy of target reinnervation was confirmed, respectively. In conclusion, as MAG significantly improves the quality of axonal regeneration, it can be used as a new therapeutic approach for peripheral nerve repair with possible focal and temporary application.

p21Cip1/WAF1 regulates radial axon growth and enhances motor functional recovery in the injured peripheral nervous system

Recent studies have provided evidence that p21Cip1/WAF1 has not only cell cycle-associated activities but also other biological activities like neurite elongation. To investigate the role of p21Cip1/WAF1 in the in vivo axonal regeneration in the peripheral nervous system, we developed a p21Cip1/WAF1 knockout (KO) mice sciatic nerve injury model. We performed quantitative assessments of the functional, histological, and electrophysiological recoveries after sciatic nerve injury in p21Cip1/WAF1 KO mice and compared the results with those of the wild-type mice. p21Cip1/WAF1 KO mice showed a significant delay of the motor functional recovery between 21 and 42 days after sciatic nerve injury. The values of motor conduction velocity in p21Cip1/WAF1 KO mice were significantly lower than those in the wild-type mice on postoperative day 28. The mean percent neural tissue and the mean nerve axon width of p21Cip1/WAF1 KO mice were significantly less than those of the wild-type mice, which was caused by hyperphosphorylation of neurofilaments. Therefore, p21Cip1/WAF1 was considered to be involved in radial axon growth and to be essential for the motor functional recovery following peripheral nervous system injury.

Selective vulnerability and pruning of phasic motoneuron axons in motoneuron disease alleviated by CNTF

Neurodegenerative diseases can have long preclinical phases and insidious progression patterns, but the mechanisms of disease progression are poorly understood. Because quantitative accounts of neuronal circuitry affected by disease have been lacking, it has remained unclear whether disease progression reflects processes of stochastic loss or temporally defined selective vulnerabilities of distinct synapses or axons. Here we derive a quantitative topographic map of muscle innervation in the hindlimb. We show that in two mouse models of motoneuron disease (G93A SOD1 and G85R SOD1), axons of fast-fatiguable motoneurons are affected synchronously, long before symptoms appear. Fast-fatigue-resistant motoneuron axons are affected at symptom-onset, whereas axons of slow motoneurons are resistant. Axonal vulnerability leads to synaptic vesicle stalling and accumulation of BC12a1-a, an anti-apoptotic protein. It is alleviated by ciliary neurotrophic factor and triggers proteasome-dependent pruning of peripheral axon branches. Thus, motoneuron disease involves predictable, selective vulnerability patterns by physiological subtypes of axons, episodes of abrupt pruning in the target region and compensation by resistant axons.

Glaucoma: thinking in new ways-a rôle for autonomous axonal self-destruction and other compartmentalised processes?

Glaucoma is a common neurodegenerative disease that affects retinal ganglion cells (RGCs). Substantial effort is being expended to determine how RGCs die in glaucoma. As in other neurodegenerative diseases, the majority of effort focuses on characterising apoptotic self-destruct pathways. However, apoptosis is not the only self-destruct mechanism that may be activated in neurons. It is now known that neurons have distinct classes of self-destruct programme that are spatially compartmentalised. In addition to the well-described intracellular suicide machinery in the neuronal soma, responsible for apoptosis, there is another, molecularly distinct, self-destruct programme localised in the axon. Evidence also supports the existence of compartmentalised degeneration programmes in synapses and dendrites. RGCs are no exception to this. Recent data, from in vitro studies and from an inherited mouse model of glaucoma, suggest that molecularly distinct degenerative pathways underlie the destruction of RGC somata and RGC axons. In various neurodegenerative diseases, axons, dendrites and synapses often degenerate well before the cells die, and there is increasing evidence that this is important for the production of clinical symptoms and signs. We hypothesise that such compartmentalised and autonomous programmes are of critical importance in the pathophysiology of glaucoma, and we suggest that studies of these processes are essential for a complete understanding of this complex disease.

The BDNF-Val66Met polymorphism: Implications for susceptibility to multiple sclerosis and severity of disease

Neurodegeneration following inflammatory injury is considered to be a pathological correlate of irreversible disability in patients with multiple sclerosis. The availability of neurotrophins could influence the probability or rate of disease progression and the time of onset. The BDNF-Val66Met-polymorphism leads to altered intracellular transport and secretion of BDNF, and is thus a logical candidate for a gene that influences susceptibility and, more specifically, the clinical course of multiple sclerosis. In order to test this hypothesis we genotyped the polymorphism in 951 UK multiple sclerosis trio families, but found no evidence for association before (p=0.63) or after stratification for clinical course (p=0.73).

Treatment with trkC agonist antibodies delays disease progression in neuromuscular degeneration (nmd) mice

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a fatal autosomal recessive disorder seen in infants. It is characterized by lower motor neuron degeneration, progressive muscle paralysis and respiratory failure, for which no effective treatment exists. The phenotype of neuromuscular degeneration (nmd) mice closely resembles the human SMARD1. The identification of the mutated mouse gene in nmd mice, Ighmbp2, led to the discovery of mutations of the homologous gene in humans with SMARD1. We have studied the nmd mouse model with in vivo electrophysiological techniques and evaluated the efficacy of Mab2256, a monoclonal antibody with agonist effect on the tyrosine kinase receptor C, trkC, on disease progression in nmd mice. Treatment with Mab2256 resulted in a significant but transient improvement of muscle strength in nmd mice, as well as normalization of the neuromuscular depression during high-frequency nerve stimulation. These results suggest the potential of using monoclonal agonist antibodies for neurotrophin receptors in lower motor neuron diseases such as SMARD1.

Mutant superoxide dismutase disrupts cytoplasmic dynein in motor neurons

Cytoplasmic dynein and dynactin drive retrograde axonal transport in neurons, and mutations in dynein/dynactin cause motor neuron degeneration. To test whether defects in dynein/dynactin function are involved in the neurodegenerative disease amyotrophic lateral sclerosis, we examined neurotracer transport from muscle to motor neuron in a transgenic mouse model of amyotrophic lateral sclerosis. Significant inhibition was observed, which was temporally correlated with declines in muscle strength. No decrease in dynein/dynactin expression was observed, but immunohistochemistry suggests that dynein associates with aggregates of mutant Cu/Zn superoxide dismutase 1. Expression of mutant Cu/Zn superoxide dismutase 1 in primary motor neurons altered the cellular localization of dynein, suggesting an inhibition of dynein/dynactin function. Thus, inhibition of dynein/dynactin function may have a role in motor neuron degeneration in amyotrophic lateral sclerosis.

GFRalpha1 expression in cells lacking RET is dispensable for organogenesis and nerve regeneration

The GDNF family ligands signal through a receptor complex composed of a ligand binding subunit, GFRalpha, and a signaling subunit, the RET tyrosine kinase. GFRalphas are expressed not only in RET-expressing cells, but also in cells lacking RET. A body of evidence suggests that RET-independent GFRalphas are important for (1) modulation of RET signaling in a non-cell-autonomous fashion (trans-signaling) and (2) regulation of NCAM function. To address the physiological significance of these roles, we generated mice specifically lacking RET-independent GFRalpha1. These mice exhibited no deficits in regions where trans-signaling has been implicated in vitro, including enteric neurons, motor neurons, kidney, and regenerating nerves. Furthermore, no abnormalities were found in the olfactory bulb, which requires proper NCAM function for its formation and is putatively a site of GDNF-GFRalpha-NCAM signaling. Thus RET-independent GFRalpha1 is dispensable for organogenesis and nerve regeneration in vivo, indicating that trans-signaling and GFRalpha-dependent NCAM signaling play a minor role physiologically.

Quantification of neurotrophin mRNA expression in PMN mouse: modulation by xaliproden

Compounds possessing neurotrophic properties may represent a possible treatment for neurodegenerative disorders such as amyotrophic lateral sclerosis. Xaliproden (SR57746A), an orally-active non-peptide compound, which has been found to exhibit neurotrophic effects in vitro and in vivo, increased the lifespan and delayed the progression of the motor neuron degeneration in PMN mice. We have used a quantitative reverse transcription/polymerase chain reaction amplification technique to study the regulation of neurotrophin mRNA and trk mRNA expression in PMN mice. NGF and NT-3 mRNA are downregulated in PMN mice. These deficiencies can be overcome by a treatment with xaliproden. Such an effect could contribute to neurotrophic effects of xaliproden in vivo and in vitro.

Overexpression of glial cell line-derived neurotrophic factor in the CNS rescues motoneurons from programmed cell death and promotes their long-term survival following axotomy

To study the role of one of the most potent motoneuron (MN) survival factors, glial cell line-derived neurotrophic factor (GDNF) derived from the CNS, we generated transgenic animals overexpressing GDNF under the control of an astrocyte-specific GFAP promoter. In situ hybridization revealed that GDNF was expressed at high levels in astrocytes throughout the brain and spinal cord. We analyzed the effects of CNS-derived GDNF on MN survival during the period of programmed cell death (PCD) and after nerve axotomy. In GFAP-GDNF mice at E15, E18, and P1, the survival of brachial MNs was increased on average by 30%, lumbar MNs by 20%, and thoracic MNs at P1 by 33%. GDNF also prevented MN PCD in several cranial motor nuclei. We demonstrated for the first time that the number of MNs in the mouse abducens nucleus was also increased by 40%, thus extending known MN populations that are responsive to GDNF. Next, we tested if GDNF could support complete and relatively long-term survival of MNs following neonatal facial nerve axotomy. We found that virtually all MNs (91%) in GFAP-GDNF mice survived for up to 18 weeks post-axotomy. This is the longest GDNF-mediated survival of neonatal MNs reported following axotomy. Most of surviving MNs were not atrophic, and MN-specific ChAT and neurofilament immunoreactivity (IR) were preserved. Furthermore, GDNF attenuated axotomy-induced astroglial activation. These data demonstrate that overexpression of GDNF in the CNS has very profound effects on MN survival both during the PCD period and after neuronal injury. GFAP-GDNF mice will be valuable to study the effects of CNS-derived GDNF in mouse models of MN degenerative diseases and axonal regeneration in vivo.

VEGF delivery with retrogradely transported lentivector prolongs survival in a mouse ALS model

Amyotrophic lateral sclerosis (ALS) causes adult-onset, progressive motor neuron degeneration in the brain and spinal cord, resulting in paralysis and death three to five years after onset in most patients. ALS is still incurable, in part because its complex aetiology remains insufficiently understood. Recent reports have indicated that reduced levels of vascular endothelial growth factor (VEGF), which is essential in angiogenesis and has also been implicated in neuroprotection, predispose mice and humans to ALS. However, the therapeutic potential of VEGF for the treatment of ALS has not previously been assessed. Here we report that a single injection of a VEGF-expressing lentiviral vector into various muscles delayed onset and slowed progression of ALS in mice engineered to overexpress the gene coding for the mutated G93A form of the superoxide dismutase-1 (SOD1(G93A)) (refs 7-10), even when treatment was only initiated at the onset of paralysis. VEGF treatment increased the life expectancy of ALS mice by 30 per cent without causing toxic side effects, thereby achieving one of the most effective therapies reported in the field so far.

Axonal defects in mouse models of motoneuron disease

Human motoneuron disease is characterized by loss of motor endplates, axonal degeneration, and cell death of motoneurons. The identification of the underlying gene defects for familial ALS, spinal muscular atrophy (SMA), and spinal muscular atrophy with respiratory distress (SMARD) has pointed to distinct pathophysiological mechanisms that are responsible for the various forms of the disease. Accumulating evidence from mouse models suggests that enhanced vulnerability and sensitivity to proapoptotic stimuli is only responsible for some but not all forms of motoneuron disease. Mechanisms that modulate microtubule assembly and the axonal transport machinery are defective in several spontaneous and ENU (ethylnitrososurea) mutagenized mouse models but also in patients with mutations in the p150 subunit of dynactin. Recent evidence suggests that axonal growth defects contribute significantly to the pathophysiology of spinal muscular atrophy. Reduced levels of the survival motoneuron protein that are responsible for SMA lead to disturbed RNA processing in motoneurons. This could also affect axonal transport of mRNAs for beta-actin and other proteins that play an essential role in axon growth and synaptic function. The local translation of specific proteins might be affected, because developing motoneurons contain ribosome-like structures in distal axons and growth cones. Altogether, the evidence from these mouse models and the new genetic data from patients suggest that axon growth and maintenance involves a variety of mechanisms, including microtubule assembly and axonal transport of proteins and ribonucleoproteins (RNPs). Thus, defects in axon maintenance could play a leading role in the development of several forms of human motoneuron disease.

Progressive and selective degeneration of motoneurons in a mouse model of SMA

Spinal muscular atrophy (SMA), an autosomal recessive disorder characterized by the degeneration of motoneurons of the spinal cord and brainstem, results from loss-of-function mutations in the survival motor neuron gene (smn). The goal of these experiments was to analyse axons and cell bodies of motoneurons in different regions of the CNS during disease progression in a mouse model of SMA carrying a deletion of the exon 7 directed to neurons. These experiments demonstrate a progressive loss of motor axons and of motoneurons in the CNS. This is the first study that describes a selective neurodegeneration in this line of mice and underlines the importance of exon 7 in some populations of motoneurons for survival in vivo.

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.