Neurotrophic survival molecules for motoneurons: an embarrassment of riches

Trophic factor expression in phrenic motor neurons

The function of a motor neuron and the muscle fibers it innervates (i.e., a motor unit) determines neuromotor output. Unlike other skeletal muscles, respiratory muscles (e.g., the diaphragm, DIAm) must function from birth onwards in sustaining ventilation. DIAm motor units are capable of both ventilatory and non-ventilatory behaviors, including expulsive behaviors important for airway clearance. There is significant diversity in motor unit properties across different types of motor units in the DIAm. The mechanisms underlying the development and maintenance of motor unit diversity in respiratory muscles (including the DIAm) are not well understood. Recent studies suggest that trophic factor influences contribute to this diversity. Remarkably little is known about the expression of trophic factors and their receptors in phrenic motor neurons. This review will focus on the contribution of trophic factors to the establishment and maintenance of motor unit diversity in the DIAm, during development and in response to injury or disease.

Characterization of chronic glutamate-mediated motor neuron toxicity in organotypic spinal cord culture prepared from ALS model mice

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease characterized by selective loss of motor neurons. Although organotypic spinal slice cultures (OSCs) exposed to inhibitors of glutamate uptake have been used as a model of ALS for screening of potentially therapeutic drugs, little development of such drugs has been achieved. In the present study we attempted to establish OSCs from G93A SOD1 transgenic mice (G93A) and to characterize the specific cell death pathway in motoneurons using glial cell line-derived neurotrophic factor (GDNF) in these mice. In the presence of GDNF, the number of surviving neurons in the OSCs was dramatically increased in both G93A and control mice. Exposure to threo-hydroxyaspartate (THA), a glutamate transport inhibitor, for 14 days induced loss of motoneurons in OSCs in G93A and control mice. In OSCs cultured with GDNF, THA-induced motoneuronal death was significantly inhibited in G93A mice, whereas that in control mice was not significantly affected. Moreover, the cleaved form of caspase-12 was increased after THA in the OSCs in G93A but not in control mice, and the activation of caspase-12 was attenuated by OSCs cultured with GDNF. These results suggest that the pathway responsible for motoneuronal death induced by THA in OSCs in G93A mice involves not only in excitotoxicity but also other mechanisms, and that the caspase-12-dependent ER stress pathway plays a role in spinal neuronal death in G93A mice. Moreover, OSCs prepared from the G93A mouse model of ALS may provide a suitable in vitro drug screening model for ALS.

Amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterised by progressive muscular paralysis reflecting degeneration of motor neurones in the primary motor cortex, corticospinal tracts, brainstem and spinal cord. Incidence (average 1.89 per 100,000/year) and prevalence (average 5.2 per 100,000) are relatively uniform in Western countries, although foci of higher frequency occur in the Western Pacific. The mean age of onset for sporadic ALS is about 60 years. Overall, there is a slight male prevalence (M:F ratio approximately 1.5:1). Approximately two thirds of patients with typical ALS have a spinal form of the disease (limb onset) and present with symptoms related to focal muscle weakness and wasting, where the symptoms may start either distally or proximally in the upper and lower limbs. Gradually, spasticity may develop in the weakened atrophic limbs, affecting manual dexterity and gait. Patients with bulbar onset ALS usually present with dysarthria and dysphagia for solid or liquids, and limbs symptoms can develop almost simultaneously with bulbar symptoms, and in the vast majority of cases will occur within 1-2 years. Paralysis is progressive and leads to death due to respiratory failure within 2-3 years for bulbar onset cases and 3-5 years for limb onset ALS cases. Most ALS cases are sporadic but 5-10% of cases are familial, and of these 20% have a mutation of the SOD1 gene and about 2-5% have mutations of the TARDBP (TDP-43) gene. Two percent of apparently sporadic patients have SOD1 mutations, and TARDBP mutations also occur in sporadic cases. The diagnosis is based on clinical history, examination, electromyography, and exclusion of 'ALS-mimics' (e.g. cervical spondylotic myelopathies, multifocal motor neuropathy, Kennedy's disease) by appropriate investigations. The pathological hallmarks comprise loss of motor neurones with intraneuronal ubiquitin-immunoreactive inclusions in upper motor neurones and TDP-43 immunoreactive inclusions in degenerating lower motor neurones. Signs of upper motor neurone and lower motor neurone damage not explained by any other disease process are suggestive of ALS. The management of ALS is supportive, palliative, and multidisciplinary. Non-invasive ventilation prolongs survival and improves quality of life. Riluzole is the only drug that has been shown to extend survival.

In vivo requirement of TGF-beta/GDNF cooperativity in mouse development: focus on the neurotrophic hypothesis

Neurotrophic factors are well-recognized extracellular signaling molecules that regulate neuron development including neurite growth, survival and maturation of neuronal phenotypes in the central and peripheral nervous system. Previous studies have suggested that TGF-beta plays a key role in the regulation of neuron survival and death and potentiates the neurotrophic activity of several neurotrophic factors, most strikingly of GDNF. To test the physiological relevance of this finding, TGF-beta2/GDNF double mutant (d-ko) mice were generated. Double mutant mice die at birth like single mutants due to kidney agenesis (GDNF-/-) and congential cyanosis (TGF-beta2-/-), respectively. To test for the in vivo relevance of TGF-beta2/GDNF cooperativity to regulate neuron survival, mesencephalic dopaminergic neurons, lumbar motoneurons, as well as neurons of the lumbar dorsal root ganglion and the superior cervical ganglion were investigated. No loss of mesencephalic dopaminergic neurons was observed in double mutant mice at E18.5. A partial reduction in neuron numbers was observed in lumbar motoneurons, sensory and sympathetic neurons in GDNF single mutants, which was further reduced in TGF-beta2/GDNF double mutant mice at E18.5. However, TGF-beta2 single mutant mice showed no loss of neurons. These data point towards a cooperative role of TGF-beta2 and GDNF with regard to promotion of survival within the peripheral motor and sensory systems investigated.

Deletion of the mouse RegIIIbeta (Reg2) gene disrupts ciliary neurotrophic factor signaling and delays myelination of mouse cranial motor neurons

A large number of cytokines and growth factors support the development and subsequent maintenance of postnatal motor neurons. RegIIIbeta, also known as Reg2 in rat and HIP/PAP1 in humans, is a member of a family of growth factors found in many areas of the body and previously shown to play an important role in both the development and regeneration of subsets of motor neurons. It has been suggested that RegIIIbeta expressed by motor neurons is both an obligatory intermediate in the downstream signaling of the leukemia inhibitory factor/ciliary neurotrophic factor (CNTF) family of cytokines, maintaining the integrity of motor neurons during development, as well as a powerful influence on Schwann cell growth during regeneration of the peripheral nerve. Here we report that in mice with a deletion of the RegIIIbeta gene, motor neuron survival was unaffected up to 28 weeks after birth. However, there was no CNTF-mediated rescue of neonatal facial motor neurons after axotomy in KO animals when compared with wild-type. In mice, RegIIIbeta positive motor neurons are concentrated in cranial motor nuclei that are involved in the patterning of swallowing and suckling. We found that suckling was impaired in RegIIIbeta KO mice and correlated this with a significant delay in myelination of the hypoglossal nerve. In summary, we propose that RegIIIbeta has an important role to play in the developmental fine-tuning of neonatal motor behaviors mediating the response to peripherally derived cytokines and growth factors and regulating the myelination of motor axons.

Effects of local release of hepatocyte growth factor on peripheral nerve regeneration in acellular nerve grafts

Options for reconstructing peripheral nerve gaps after trauma are limited. The acellular nerve is a new kind of biomaterial used to reconstruct the peripheral nerve defect, but its use could be improved upon. We aimed to investigate the effect of adenoviral transfection with hepatocyte growth factor (HGF) on the functional recovery of transected sciatic nerves repaired by acellular nerve grafting. 30 Rats were divided into three groups (10/group) for autografting and acellular grafting, as well as acellular grafting with adenovirus transfection of HGF (1 x 10(8) pfu) injected in muscles around the proximal and distal allograft coapation. Sciatic functional index (SFI) was evaluated every 4 weeks to week 16 by measuring rat footprints on walking-track testing. The three groups presented initial complete functional loss, followed by slow but steady recovery, with final similar SFIs. Weight of the gastrocnemius and soleus muscles, histologic and morphometric study and neovascularization in the nerve grafts were evaluated at week 16. Autografting gave the best functional recovery, but HGF-treated acellular grafting gave better recovery than acellular grafting alone. Neovascularization was greater with HGF-treated acellular grafting than with autografting and acellular grafting alone. Axonal regeneration distance of autografting on the 20th postoperative day was the longest in the three groups,while that of acellular grafting alone was the smallest. Acellular nerve grafting may be useful for functional peripheral nerve regeneration, and with human HGF gene transfection may improve on acellular grafting alone in functional recovery.

Characterization and use of the NSC-34 cell line for study of neurotrophin receptor trafficking

This study addressed the suitability of the NSC-34 cell line as a motor neuron-like model for investigating neurotrophin receptor trafficking and associated subcellular processes. Initially, culture conditions were optimized for the use of NSC-34 cells in confocal microscopy. Cell surface markers, as well as markers associated with the regulated endosomal pathway thought to be associated with neurotrophin receptor transport, were identified. The study revealed the presence of a number of molecules previously not described in the literature, including the tropomyosin-like receptor kinase C (TrkC), sortilin, the vesicular acetylcholine transporter (VAChT), and the lipid raft-associated ganglioside GT1b. The presence of both sortilin and Gt1b was of special interest, insofar as these markers have been implicated in direct relationships with the p75NTR receptor. Evidence is provided for neurotrophin-dependent internalization of p75NTR and TrkB. Both nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) increased the rate of internalization of p75NTR, with internalization dynamics comparable to those described for other cell lines. Thus, these studies not only describe components of the regulatory process governing the trafficking of this important receptor but also clearly demonstrate the value of NSC-34 cells as a suitable motor neuron model for the study of internalization and trafficking of cell surface molecules.

In vitro methods to prepare astrocyte and motoneuron cultures for the investigation of potential in vivo interactions

This protocol details methods to isolate and purify astrocytes and motoneurons (MNs) from the chick lumbar spinal cord. In addition, an approach to study the influences of astrocyte secreted factors on MNs is provided. Astrocytes are isolated between embryonic days 10 and 12 (E10-12), propagated in serum (2-3 h) and differentiated in chemically defined medium (3-4 h). When prepared according to this protocol, astrocyte cultures are more than 98% pure when assessed using the astrocyte-specific markers glial fibrillary acidic protein (GFAP) and S100beta. MNs are isolated between E5.5 and 6.0 (3-4 h) using a procedure that takes selective advantage of the large size of these cells. These cultures can be maintained using individual trophic factors, target-derived factors or astrocyte-derived factors, the preparation of which is also described (5-6 h). All or part of these techniques can be used to investigate a variety of processes that occur during nervous system development and disease or after injury.

Identification and characterization of cholest-4-en-3-one, oxime (TRO19622), a novel drug candidate for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive death of cortical and spinal motor neurons, for which there is no effective treatment. Using a cell-based assay for compounds capable of preventing motor neuron cell death in vitro, a collection of approximately 40,000 low-molecular-weight compounds was screened to identify potential small-molecule therapeutics. We report the identification of cholest-4-en-3-one, oxime (TRO19622) as a potential drug candidate for the treatment of ALS. In vitro, TRO19622 promoted motor neuron survival in the absence of trophic support in a dose-dependent manner. In vivo, TRO19622 rescued motor neurons from axotomy-induced cell death in neonatal rats and promoted nerve regeneration following sciatic nerve crush in mice. In SOD1(G93A) transgenic mice, a model of familial ALS, TRO19622 treatment improved motor performance, delayed the onset of the clinical disease, and extended survival. TRO19622 bound directly to two components of the mitochondrial permeability transition pore: the voltage-dependent anion channel and the translocator protein 18 kDa (or peripheral benzodiazepine receptor), suggesting a potential mechanism for its neuroprotective activity. TRO19622 may have therapeutic potential for ALS and other motor neuron and neurodegenerative diseases.

Survival and death of mature avian motoneurons in organotypic slice culture: trophic requirements for survival and different types of degeneration

We have developed an organotypic culture technique that uses slices of chick embryo spinal cord, in which trophic requirements for long-term survival of mature motoneurons (MNs) were studied. Slices were obtained from E16 chick embryos and maintained for up to 28 days in vitro (DIV) in a basal medium. Under these conditions, most MNs died. To promote MN survival, 14 different trophic factors were assayed. Among these 14, glial cell line-derived neurotrophic factor (GDNF) and vascular endothelial growth factor were the most effective. GDNF was able to promote MN survival for at least 28 DIV. K(+) depolarization or caspase inhibition prevented MN death but also induced degenerative-like changes in rescued MNs. Agents that elevate cAMP levels promoted the survival of a proportion of MNs for at least 7 DIV. Examination of dying MNs revealed that, in addition to cells exhibiting a caspase-3-dependent apoptotic pattern, some MNs died by a caspase-3-independent mechanism and displayed autophagic vacuoles, an extremely convoluted nucleus, and a close association with microglia. This organotypic spinal cord slice culture may provide a convenient model for testing conditions that promote survival of mature-like MNs that are affected in late-onset MN disease such as amyotrophic lateral sclerosis.

Role of sustained overexpression of central nervous system IGF-I in the age-dependent decline of mouse excitation-contraction coupling

We investigated the effects of exclusive and sustained transgenic overexpression of insulin-like growth factor (IGF)-I in the central nervous system (CNS) on the age-dependent decline in muscle strength, excitation-contraction coupling, muscle innervation and neuromuscular junction postterminal architecture. We found that (1) transgenic IGF-I overexpression in the CNS does not modify the decline in extensor digitorum longus (EDL) and soleus muscle weight with aging and (2) strength significantly decreases in transgenic (Tg) compared to wild-type mice. The latter finding is consistent with (3) the decreased absolute and specific force measured in the EDL muscle in vitro and (4) the decreased charge movement and peak intracellular Ca(2+) mobilization in individual muscle fibers from old IGF-I Tg mice compared to young wild-type mice, which also is associated with (5) decreased dihydropyridine receptor alpha(1)-subunit expression in old compared to young IGF-I Tg mice. (6) Tg IGF-I prevents a change in muscle fiber type that is associated with (7) improved muscle innervation and postterminal neuromuscular structure. (8) IGF-I is expressed extensively across the spinal cord gray matter and the lateral motor column. Our results raise questions about the timing and cell location of CNS IGF-I overexpression necessary to prevent or to ameliorate age-dependent alterations in the structure and function of skeletal muscle.

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.

Astrocyte and muscle-derived secreted factors differentially regulate motoneuron survival

During development, motoneurons (MNs) undergo a highly stereotyped, temporally and spatially defined period of programmed cell death (PCD), the result of which is the loss of 40-50% of the original neuronal population. Those MNs that survive are thought to reflect the successful acquisition of limiting amounts of trophic factors from the target. In contrast, maturation of MNs limits the need for target-derived trophic factors, because axotomy of these neurons in adulthood results in minimal neuronal loss. It is unclear whether MNs lose their need for trophic factors altogether or whether, instead, they come to rely on other cell types for nourishment. Astrocytes are known to supply trophic factors to a variety of neuronal populations and thus may nourish MNs in the absence of target-derived factors. We investigated the survival-promoting activities of muscle- and astrocyte-derived secreted factors and found that astrocyte-conditioned media (ACM) was able to save substantially more motoneurons in vitro than muscle-conditioned media (MCM). Our results indicate that both ACM and MCM are significant sources of MN trophic support in vitro and in ovo, but only ACM can rescue MNs after unilateral limb bud removal. Furthermore, we provide evidence suggesting that MCM facilitates the death of a subpopulation of MNs in a p75(NTR) - and caspase-dependent manner; however, maturation in ACM results in MN trophic independence and reduced vulnerability to this negative, pro-apoptotic influence from the target.

Rho-kinase inhibition enhances axonal regeneration after peripheral nerve injury

In injured adult neurons, the process of axonal regrowth and reestablishment of the neuronal function have to be activated. We assessed in this study whether RhoA, a key regulator of neurite elongation, is activated after injury to the peripheral nervous system. RhoA is activated in motoneurons but not in Schwann cells after mouse sciatic nerve injury. To examine whether the activation of RhoA and its effector, Rho-kinase, retards axon regeneration of injured motoneurons, we employed a Rho-kinase inhibitor, fasudil. Amplitudes of distally evoked compound muscle action potentials are increased significantly faster after axonal injury in mice treated with fasudil compared with controls. Histological analysis shows that fasudil treatment increases the number of regenerating axons with large diameter, suggesting that axon maturation is facilitated by Rho-kinase inhibition. In addition, fasudil does not suppress the myelination of regenerating axons. These findings suggest that RhoA/Rho-kinase may be a practical molecular target to enhance axonal regeneration in human peripheral neuropathies.

Neuronal Control of Skin Function: The Skin as a Neuroimmunoendocrine Organ

This review focuses on the role of the peripheral nervous system in cutaneous biology and disease. During the last few years, a modern concept of an interactive network between cutaneous nerves, the neuroendocrine axis, and the immune system has been established. We learned that neurocutaneous interactions influence a variety of physiological and pathophysiological functions, including cell growth, immunity, inflammation, pruritus, and wound healing. This interaction is mediated by primary afferent as well as autonomic nerves, which release neuromediators and activate specific receptors on many target cells in the skin. A dense network of sensory nerves releases neuropeptides, thereby modulating inflammation, cell growth, and the immune responses in the skin. Neurotrophic factors, in addition to regulating nerve growth, participate in many properties of skin function. The skin expresses a variety of neurohormone receptors coupled to heterotrimeric G proteins that are tightly involved in skin homeostasis and inflammation. This neurohormone-receptor interaction is modulated by endopeptidases, which are able to terminate neuropeptide-induced inflammatory or immune responses. Neuronal proteinase-activated receptors or transient receptor potential ion channels are recently described receptors that may have been important in regulating neurogenic inflammation, pain, and pruritus. Together, a close multidirectional interaction between neuromediators, high-affinity receptors, and regulatory proteases is critically involved to maintain tissue integrity and regulate inflammatory responses in the skin. A deeper understanding of cutaneous neuroimmunoendocrinology may help to develop new strategies for the treatment of several skin diseases.

Arginase 1 regulation of nitric oxide production is key to survival of trophic factor-deprived motor neurons

When deprived of trophic factors, the majority of cultured motor neurons undergo nitric oxide-dependent apoptosis. However, for reasons that have remained unclear, 30-50% of the motor neurons survive for several days without trophic factors. Here we hypothesize that the resistance of this motor neuron subpopulation to trophic factor deprivation can be attributed to diminished nitric oxide production resulting from the activity of the arginine-degrading enzyme arginase. When incubated with nor-N(G)-hydroxy-nor-L-arginine (NOHA), the normally resistant trophic factor-deprived motor neurons showed a drop in survival rates, whereas trophic factor-treated neurons did not. NOHA-induced motor neuron death was inhibited by blocking nitric oxide synthesis and the scavenging of superoxide and peroxynitrite, suggesting that peroxynitrite mediates NOHA toxicity. When we transfected arginase 1 into motor neurons to see whether it alone could abrogate trophic factor deprivation-induced death, we found that its forced expression did indeed do so. The protection afforded by arginase 1 expression is reversed when cells are incubated with NOHA or with low concentrations of nitric oxide. These results reveal that arginase acts as a central regulator of trophic factor-deprived motor neuron survival by suppressing nitric oxide production and the consequent peroxynitrite toxicity. They also suggest that the resistance of motor neuron subpopulations to trophic factor deprivation may result from increased arginase activity.

ADAPTIVE ROLES OF PROGRAMMED CELL DEATH DURING NERVOUS SYSTEM DEVELOPMENT

The programmed cell death (PCD) of developing cells is considered an essential adaptive process that evolved to serve diverse roles. We review the putative adaptive functions of PCD in the animal kingdom with a major focus on PCD in the developing nervous system. Considerable evidence is consistent with the role of PCD in events ranging from neurulation and synaptogenesis to the elimination of adult-generated CNS cells. The remarkable recent progress in our understanding of the genetic regulation of PCD has made it possible to perturb (inhibit) PCD and determine the possible repercussions for nervous system development and function. Although still in their infancy, these studies have so far revealed few striking behavioral or functional phenotypes.

Interactions between beta-neuregulin and neurotrophins in motor neuron apoptosis

Neuregulins play a major role in the formation and stabilization of neuromuscular junctions, and are produced by both motor neurons and muscle. Although the effects and mechanism of neuregulins on skeletal muscle (e.g. regulation of acetylcholine receptor expression) have been studied extensively, the effects of neuregulins on motor neurons remain unknown. We report that neuregulin-1beta (NRGbeta1) inhibited apoptosis of rat motor neurons for up to 7 days in culture by a phosphatidylinositol 3 kinase-dependent pathway and synergistically enhanced motor neuron survival promoted by glial-derived neurotrophic factor (GDNF). However, binding of neurotrophins, including brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), to the p75 neurotrophin receptor (p75NTR) abolished the neuregulin anti-apoptotic effect on motor neurons. Inhibitors of the c-Jun N-terminal kinase (JNK) mitogen-activated protein kinase prevented motor neuron death caused by co-incubation of NRGbeta1 and BDNF or NGF, as well as by trophic factor deprivation. Motor neuron apoptosis resulting from both trophic factor deprivation and exposure to NRGbeta1 plus neurotrophins required the induction of neuronal nitric oxide synthase and peroxynitrite formation. Because motor neurons express both p75NTR and neuregulin erbB receptors during the period of embryonic programmed cell death, motor neuron survival may be the result of complex interactions between trophic and death factors, which may be the same molecules acting in different combinations.

Heat shock protein 27 rescues motor neurons following nerve injury and preserves muscle function

Heat shock proteins (HSPs) are a family of ubiquitously expressed proteins that are up-regulated in response to a range of stresses and play an important role in cellular defence mechanisms. In previous studies, we demonstrated that overexpression of heat shock protein 27 (HSP27) in transgenic mice has significant cytoprotective properties in vivo, reducing caspase-3-mediated cell death in the hippocampus associated with limbic seizures and reducing infarct size in cardiac ischaemia. In motor neurons, HSP27 is also implicated as a survival promoting factor; however, it remains to be established whether HSP27 is able to exert long-term neuroprotective effects following neonatal nerve injury. We now show that, following neonatal nerve crush, HSP27 overexpression in vivo provides a substantial rescue of motor neurons 5-6 months following nerve injury. Furthermore, in vivo isometric tension recordings demonstrate that surviving motor neurons were able to regenerate, resulting in a 90% improvement (P < 0.0005) in motor unit number in HSP27 mice. Moreover, this increase in motor unit number was associated with improved muscle weight, muscle force, contractile speeds, and histochemical markers of muscle activity. These properties of HSP27 therefore have considerable potential for improving long-term muscle function in motor neuron disorders.

Peripheral nerve avulsion injuries as experimental models for adult motoneuron degeneration

We have used adult rat peripheral nerve avulsion models to evaluate the effects of neuroprotective molecules on motoneuron degeneration. The right facial nerves of adult Fischer 344 male rats were avulsed and adenoviral vectors encoding glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), transforming growth factor-beta2 (TGFbeta2), and growth inhibitory factor (GIF) were injected into the facial canal. The treatment with the vectors significantly prevented the loss of lesioned facial motoneurons, improved choline acetyltransferase (ChAT) immunoreactivity and suppressed the induction of nitric oxide synthase activity in these neurons. In separate experiments, animals were orally administered a solution of a neuroprotective compound T-588 after avulsion. Both free oral administration and oral tube administration of T-588 improved the survival of injured motoneurons and ameliorated their ChAT immunoreactivity. These results indicate that the gene transfer of GDNF, BDNF, TGFbeta2, and GIF and oral administration of T-588 may prevent the degeneration of motoneurons in adult humans with motoneuron injury and motor neuron diseases.

Subpopulations of motor and sensory neurons respond differently to brain-derived neurotrophic factor depending on the presence of the skeletal muscle

The aim of our study was to assess the ability of brain-derived neurotrophic factor (BDNF) to rescue motor and sensory neurons from programmed cell death. It is clearly demonstrated that the administration of a single injection of a putative neurotrophic factor to mouse embryos in utero on embryonic day (E) 14.5 is sufficient to significantly reduce the death of motor neurons when assessed on E18.5. However, the trophic requirements of somatic neurons have not been unequivocally determined in a mammalian species in vivo. Indeed, the unexpectedly high numbers of surviving neurons observed in neurotrophin and tyrosine kinase receptor knockout mice are probably the consequence of functional redundancy between the neurotrophins and their receptors. We studied spinal cord and facial motor nucleus neurons and proprioceptive neurons in the dorsal root ganglion and mesencephalic nucleus. The action of BDNF was assessed in wild-type fetuses to gain insight into its ability to rescue neurons from naturally occurring programmed cell death. In addition, we used Myf5(-/-):MyoD(-/-) embryos, which completely lack skeletal musculature, to assess the ability of BDNF to rescue neurons from excessively occurring programmed cell death. We found that BDNF differentially rescued neurons from naturally vs. excessively occurring cell death and that its ability to do so varied among neuronal subpopulations.

Mullerian inhibiting substance acts as a motor neuron survival factor in vitro

The survival of motor neurons is controlled by multiple factors that regulate different aspects of their physiology. The identification of these factors is important because of their relationship to motor neuron disease. We investigate here whether Mullerian Inhibiting Substance (MIS) is a motor neuron survival factor. We find that motor neurons from adult mice synthesize MIS and express its receptors, suggesting that mature motor neurons use MIS in an autocrine fashion or as a way to communicate with each other. MIS was observed to support the survival and differentiation of embryonic motor neurons in vitro. During development, male-specific MIS may have a hormone effect because the blood-brain barrier has yet to form, raising the possibility that MIS participates in generating sex-specific differences in motor neurons.

Abducens internuclear neurons depend on their target motoneurons for survival during early postnatal development

The highly specific projection of abducens internuclear neurons onto medial rectus motoneurons in the oculomotor nucleus is a good model to evaluate the dependence on target cells for survival during development and in the adult. Thus, the procedure we chose to selectively deprive abducens internuclear neurons of their natural target was the enucleation of postnatal day 1 rats to induce the death of medial rectus motoneurons. Two months later, we evaluated both the extent of reduction in target size, by immunocytochemistry against choline acetyltransferase (ChAT) and Nissl counting, and the percentage of abducens internuclear neurons surviving target loss, by calretinin immunostaining and horseradish peroxidase (HRP) retrograde tracing. Firstly, axotomized oculomotor motoneurons died in a high percentage ( approximately 80%) as visualized 2 months after lesion. In addition, we showed a transient (1 month) and reversible down-regulation of ChAT expression in extraocular motoneurons induced by injury. Secondly, 2 months after enucleation, 61.6% and 60.5% of the population of abducens internuclear neurons appeared stained by retrograde tracing and calretinin immunoreaction, respectively, indicating a significant extent of cell death after target loss (38.4% or 39.5%). By contrast, in the adult rat, neither extraocular motoneurons died in response to axotomy nor abducens internuclear neurons died due to the loss of their target motoneurons induced by the retrograde transport of toxic ricin injected in the medial rectus muscle. These results indicate that, during development, abducens internuclear neurons depend on their target motoneurons for survival, and that they lose this dependence with maturation.

Extracellular heat shock protein 70: a critical component for motoneuron survival

The dependence of developing spinal motoneuron survival on a soluble factor(s) from their target, muscle tissue is well established both in vivo and in vitro. Considering this apparent dependence, we examined whether a specific component of the stress response mediates motoneuron survival in trophic factor-deprived environments. We demonstrate that, although endogenous expression of heat shock protein 70 (HSP70) did not change during trophic factor deprivation, application of e-rhHsp70 (exogenous recombinant human Hsp70) promoted motoneuron survival. Conversely, depletion of HSP70 from chick muscle extract (MEx) potently reduces the survival-promoting activity of MEx. Additionally, exogenous treatment with or spinal cord overexpression of Hsp70 enhances motoneuron survival in vivo during the period of naturally occurring cell death [programmed cell death (PCD)]. Hindlimb muscle cells and lumbar spinal astrocytes readily secrete HSP70 in vitro, suggesting potential physiological sources of extracellular Hsp70 for motoneurons. However, in contrast to exogenous treatment with or overexpression of Hsp70 in vivo, muscle-targeted injections of this factor in an ex vivo preparation fail to attenuate motoneuron PCD. These data (1) suggest that motoneuron survival requirements may extend beyond classical trophic factors to include HSP70, (2) indicate that the source of this factor is instrumental in determining its trophic function, and (3) may therefore influence therapeutic strategies designed to increase motoneuron Hsp70 signaling during disease or injury.

Nonviral HVJ (hemagglutinating virus of Japan) liposome-mediated retrograde gene transfer of human hepatocyte growth factor into rat nervous system promotes functional and histological recovery of the crushed nerve

Hepatocyte growth factor (HGF) is well known to be involved in many biological functions, such as organ regeneration and angiogenesis, and to exert neurotrophic effects on motor, sensory, and parasympathetic neurons. In this study, we gave repeated intramuscular injections of the human HGF gene, using nonviral HVJ (hemagglutinating virus of Japan) liposome method, to examine whether transfection of the rat nervous system with this gene is able to exert neurotrophic effects facilitating recovery of a crushed nerve. The expression of HGF protein and HGF mRNA indicated that gene transfer into the nervous system did occur via retrograde axonal transport. At 4 weeks after crush, electrophysiological examination of the crushed nerve showed a significantly shorter mean latency and a significantly greater mean maximum M-wave amplitude with repeated injections of HGF gene. Furthermore, histological findings showed that the mean diameter of the axons, the axon number and the axon population were significantly larger in the group with repeated injections of HGF gene. The above results show that repeated human HGF gene transfer into the rat nervous system is able to promote crushed-nerve recovery, both electrophysiologically and histologically, and suggest that HGF gene transfer has potential for the treatment of crushed nerve.

Presence of neurotrophic factors in skeletal muscle correlates with survival of spinal cord motor neurons

To determine which combination of skeletal muscle-derived neurotrophic factors may be important for the survival of specific subpopulations of developing spinal cord motor neurons, we used Myf5 and MyoD (myogenic regulatory factors) knockouts, containing differentially committed myogenic precursor cells (MPCc) and immunohistochemistry against several muscle-secreted neurotrophic factors. At the peak of motor neuron cell death, skeletal muscle development is delayed in the back and body wall muscles of Myf5-/- embryos and in the limb muscles of MyoD-/- embryos. We hypothesized that, if the skeletal muscle was indeed an important source of survival factors for motor neurons, the back, the abdominal wall, and the forelimb MPCs of Myf5-/- or MyoD-/- embryos should produce at least some neurotrophic factors necessary for the survival of motor neurons. In this report, we demonstrate that (1) different MPCs lacking Myf5, MyoD, or Myf5/MyoD have different capabilities in providing factors potentially required for the survival of motor neurons and intramuscular nerve branching, (2) MPCs in double-mutant embryos do not contain neurotrophic factors in the absence of myogenic specification, and (3) different subpopulations of MPCs contain different combinations of neurotrophic factors potentially required for the survival of the specific subpopulations of innervating motor neurons.

Synaptic targeting of retrogradely transported trophic factors in motoneurons: comparison of glial cell line-derived neurotrophic factor, brain-derived neurotrophic factor, and cardiotrophin-1 with tetanus toxin

Glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), and cardiotrophin-1 (CT-1) are the most potent neurotrophic factors for motoneurons, but their fate after retrograde axonal transport is not known. Internalized trophic factors may be degraded, or they may be recycled and transferred to other neurons, similar to the known route of tetanus toxin. We tested whether neonatal rat hypoglossal motoneurons target retrogradely transported trophic factors to synaptic sites on their dendrites within the brainstem and subsequently transfer these trophins across the synaptic cleft to afferent synapses (transsynaptic transcytosis). Motoneurons retrogradely transport from the tongue radiolabeled GDNF, BDNF, and CT-1 as well as tetanus toxin. Quantitative autoradiographic electron microscopy showed that GDNF and BDNF were transported into motoneuron dendrites with labeling densities similar to those of tetanus toxin. Although tetanus toxin accumulated rapidly (within 8 h) at presynaptic sites, GDNF accumulated at synapses more slowly (within 15 h), and CT-1 never associated with synapses. Thus, some retrogradely transported neurotrophic factors are trafficked similarly but not identically to tetanus toxin. Both GDNF and BDNF accumulate at the external (limiting) membrane of multivesicular bodies within proximal dendrites. We conclude that tetanus toxin, GDNF, and BDNF are released from postsynaptic sites and are internalized by afferent presynaptic terminals, thus demonstrating transsynaptic transcytosis. CT-1, however, follows a strict degradation pathway after retrograde transport to the soma. Synaptic and transcytotic trafficking thus are restricted to particular neurotrophic factors such as GDNF and BDNF.

Decreases in phosphoinositide-3-kinase/Akt and extracellular signal-regulated kinase 1/2 signaling activate components of spinal motoneuron death

Motoneuron dependence on target-derived trophic factors during development is well established, with loss of trophic support leading to the death of these cells. A complete understanding of the intracellular signal transduction machinery associated with extracellular survival signals requires the examination of individual pathways in various cellular and environmental contexts. In cells deprived of trophic support, and hence compromised for survival, phosphoinositide-3-kinase (PI3K) is decreased when compared with healthy cells supplied with trophic support. Extracellular signal-regulated kinase 1/2 (ERK1/2) signaling is dramatically decreased in deprived cells. We have examined the role of these two pathways to understand how changes in their activity regulate motoneuron survival and death. Pharmacological inhibition of PI3K attenuated motoneuron survival and was important in the regulation of Bcl-2 serine phosphorylation, limited release of cytochrome c into the cytoplasm and caspase activation. Bax translocation from cytoplasm to mitochondria was not altered when PI3K was inhibited. High levels of ERK1/2 inhibition robustly attenuated motoneuron survival in cells supplied with trophic support, whereas moderate inhibition of ERK1/2 activation had little effect. ERK1/2 inhibition in these cells decreased Bcl-2 phosphorylation and resulted in release of cytochrome c from the mitochondria. Bax translocation and caspase activation were not affected by ERK1/2 inhibition. These data reveal that changes in PI3K and ERK1/2 signaling lead to individual and overlapping effects on the cell-death machinery. Characterizing the role of these pathways is critical for a fundamental understanding of the development and degeneration of specific neuronal populations.

Neural adhesion molecules L1 and CHL1 are survival factors for motoneurons

Many neurotrophic factors with survival activity for motoneurons in vivo were first identified using cultures of purified embryonic motoneurons. The L1 neural cell adhesion molecule has multiple roles in brain development. We showed by in situ hybridization and RT-PCR that L1 mRNA was expressed at significant levels in motoneurons of embryonic and postnatal spinal cord. We therefore cultured purified motoneurons from E14 rat embryos in the absence of trophic factors but with L1-Fc and CHL1-Fc fusion proteins. L1-Fc prevented the death of approximately half of the motoneurons that were saved by BDNF in a dose-dependent manner (EC50 = 10 pM). CHL1-Fc saved the same number of motoneurons as did L1-Fc, whereas P0-Fc had little neurotrophic activity at the same concentrations. Survival induced by L1 and CHL1 was completely inhibited by 20 microM LY294002 and PD98059, indicating that both MEK and PI3K pathways are required for signaling by these molecules. L1 can signal in other cell types through the FGF receptor FGFR1. In cultures of motoneurons, effects of suboptimal concentrations of L1 and suboptimal concentrations of FGF-2 were additive, but the effects of optimal concentrations of FGF-2 (50 ng/ml) were not further increased in the presence of L1-Fc. Thus, in this system, too, FGF and L1 may use similar signaling pathways.

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.

Developmental regulation of activated ERK expression in the spinal cord and dorsal root ganglion of the chick embryo

Mitogen-activated protein kinases (MAPKs) are involved in the intracellular pathways that respond to various extracellular signals. Extracellular signal-regulated kinase (ERK) is a member of MAPKs and has various functions in neural development. However, the in vivo distribution of the activated form of ERK (p-ERK) in the developing nervous system is not well understood. Here, we investigated the expression of p-ERK in the spinal cord and dorsal root ganglion (DRG) of chick embryos. In the spinal cord, p-ERK-positive cells appeared in the ventral ventricular zone on embryonic day 4 (E4). From E6 onward, they appeared in the gray matter and in the white matter, suggesting migration from the ventricular zone. A double labeling method revealed that these p-ERK-positive cells included oligodendrocyte precursors. In the dorsal horn, p-ERK-positive small cells appeared on E6. Subsequently, the positive cells in the dorsal horn increased transiently in number and then decreased markedly by E10. Motoneurons also expressed p-ERK transiently on E7. In the DRG, weak p-ERK immunoreaction appeared in the ventrolateral region on E5. From E6, the immunoreactivity became stronger and by E9 intense p-ERK-positive cells were observed throughout the DRG. These data provide a neuroanatomical framework to begin to examine the in vivo role of ERK in neural development.

Rapid axoglial signaling mediated by neuregulin and neurotrophic factors

During peripheral nervous system development, Schwann cells are precisely matched to the axons that they support. This is mediated by axonal neuregulins that are essential for Schwann cell survival and differentiation. Here, we show that sensory and motor axons rapidly release heparin-binding forms of neuregulin in response to Schwann cell-derived neurotrophic factors in a dose-dependent manner. Neuregulin release occurs within minutes, is saturable, and occurs from axons that were isolated using a newly designed chamber slide apparatus. Although NGF and glial cell line-derived neurotrophic factor (GDNF) were the most potent neurotrophic factors to release neuregulin from sensory neurons, GDNF and BDNF were most potent for motor neurons and were the predominant neuregulin-releasing neurotrophic factors produced by cultured Schwann cells. Comparable levels of neuregulin could be released at a similar rate from neurons after protein kinase C activation with the phorbol ester, phorbol 12-myristate 13-acetate, which has also been shown to promote the cleavage and release of neuregulin from its transmembrane precursor. The rapid release of neuregulin from axons in response to Schwann cell-derived neurotrophic factors may be part of a spatially restricted system of communication at the axoglial interface important for proper peripheral nerve development, function, and repair.

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.

Unraveling the mechanisms involved in motor neuron degeneration in ALS

Although Charcot described amyotrophic lateral sclerosis (ALS) more than 130 years ago, the mechanism underlying the characteristic selective degeneration and death of motor neurons in this common adult motor neuron disease has remained a mystery. There is no effective remedy for this progressive, fatal disorder. Modern genetics has now identified mutations in one gene [Cu/Zn superoxide dismutase (SOD1)] as a primary cause and implicated others [encoding neurofilaments, cytoplasmic dynein and its processivity factor dynactin, and vascular endothelial growth factor (VEGF)] as contributors to, or causes of, motor neuron diseases. These insights have enabled development of model systems to test hypotheses of disease mechanism and potential therapies. Along with errors in the handling of synaptic glutamate and the potential excitotoxic response this provokes, these model systems highlight the involvement of nonneuronal cells in disease progression and provide new therapeutic strategies.

Reinnervation of the rat musculocutaneous nerve stump after its direct reconnection with the C5 spinal cord segment by the nerve graft following avulsion of the ventral spinal roots: a comparison of intrathecal administration of brain-derived neurotrophic factor and Cerebrolysin

Experimental model based on the C5 ventral root avulsion was used to evaluate the efficacy of brain-derived neurotrophic factor (BDNF) and Cerebrolysin treatment on motor neuron maintenance and survival resulted in the functional reinnervation of the nerve stump. In contrast to vehicle, BDNF treatment reduced the loss and atrophy of motor neurons and enhanced the regrowth axon sprouts into the distal stump of musculocutaneous nerve. However, the axon diameter of the myelinated fibers was smaller than those of control rats. The morphometric results were related to a low score in behavioral test similar to vehicle-treated rats. Cerebrolysin treatment greatly protected the motor neurons against cell death. Moreover, morphometric features of myelinated axons were better than those of rats treated with vehicle or BDNF. The mean score of grooming test suggested better results of the functional motor reinnervation than after BDNF administration. The majority of rescued motor neurons regenerating their axons through nerve graft in both BDNF- and Cerebrolysin-treated rats expressed choline acetyltransferase immunostaining. The results demonstrate that BDNF has more modest effects in preventing the death of motor neurons and functional recovery of injured motor nerve after root avulsion than Cerebrolysin.

The function of neurotrophic factor receptors expressed by the developing adductor motor pool in vivo

We examined the spatio-temporal relationship between neurotrophic factor receptor (NTF-R) expression and motoneuron (MN) survival in the developing avian spinal cord and observed heterogeneity in the expression of NTF-Rs between, but not within, pools of MNs projecting to individual muscles. We then focused on the role of NTFs in regulating the survival of one motor pool of MNs, all of which innervate a pair of adductor muscles in the thigh and hence compete for survival during the period of programmed cell death (PCD). The complete NTF-R complement of these MNs was analyzed and found to include many, but not all, NTF-Rs. Treatment with exogenous individual NTFs rescued some, but not all, adductor MNs expressing appropriate NTF-Rs. In contrast, administration of multiple NTFs completely rescued adductor MNs from PCD. Additionally, adductor MNs were partially rescued from PCD by NTFs for which they failed to express receptors. NTF-Rs expressed by the nerve but not in the muscle target were capable of mediating survival signals to MNs in trans. Finally, the expression of some NTF-Rs by adductor MNs was not required for MN survival. These studies demonstrate the complexity in NTF regulation of a defined subset of competing MNs and suggest that properties other than NTF-R expression itself can play a role in mediating trophic responses to NTFs.

Expression of Trk receptors in the oculomotor system of the adult cat

We examined the expression of the three Trk receptors for neurotrophins (TrkA, TrkB, and TrkC) in the extraocular motor nuclei of the adult cat by using antibodies directed against the full-Trk proteins in combination with horseradish peroxidase retrograde tracing. The three receptors were present in all neuronal populations investigated, including abducens motoneurons and internuclear neurons, medial rectus motoneurons of the oculomotor nucleus, and trochlear motoneurons. They were also present in the vestibular and prepositus hypoglossi nuclei. TrkA, TrkB, and TrkC immunopositive cells were found in similar percentages in the oculomotor and in the trochlear nuclei. In the abducens nucleus, however, a significantly higher percentage of cells expressed TrkB than the other two receptors, among both motoneurons (81.8%) and internuclear neurons (88.4%). The percentages obtained for the three Trk receptors in identified neuronal populations pointed to the colocalization of two or three receptors in a large number of cells. We used confocal microscopy to elucidate the subcellular location of Trk receptors. In this case, abducens motoneurons and internuclear neurons were identified with antibodies against choline acetyltransferase and calretinin, respectively. We found a different pattern of staining for each neurotrophin receptor, suggesting the possibility that each receptor and its cognate ligand may use a different route for cellular signaling. Therefore, the expression of Trk receptors in oculomotor, trochlear, and abducens motoneurons, as well as abducens internuclear neurons, suggests that their associated neurotrophins may exert an influence on the normal operation of the oculomotor circuitry. The presence of multiple Trk receptors on individual cells indicates that they likely act in concert with each other to regulate distinct functions.

Changes in spinal cord architecture after brachial plexus injury in the newborn

Obstetric brachial plexus palsy is a devastating birth injury. While many children recover spontaneously, 20-25% are left with a permanent impairment of the affected limb. So far, concepts of pathology and recovery have focused on the injury of the peripheral nerve. Proximal nerve injury at birth, however, leads to massive injury-induced motoneuron loss in corresponding motoneuron pools and therefore limits the extent of functional recovery. In the present study, the role of spinal cord plasticity after injury and recovery from obstetric brachial plexus lesions was investigated. A selective injury to spinal roots C5 and C6 was induced in newborn Sprague-Dawley rats, leading to motoneuron loss in corresponding motoneuron pools. Recovery of extremity function was evaluated with different behavioural paradigms. Permanent changes of adjacent motoneuron pools were quantitatively evaluated by retrograde tracing and functional muscle testing. We report that the adjacent C7 motoneuron contribution to biceps muscle innervation increased four-fold after upper trunk lesions in newborns, thus compensating for the injury-induced motoneuron loss. These results indicate that, in obstetric brachial plexus palsy, changes in spinal cord architecture are an integral part not only of primary pathology but also of the subsequent recovery process. While present treatment is directed towards the restoration of neural continuity, future treatment strategies must recognize and take advantage of CNS participation in the injury and recovery process.

Macrophage-derived tumor necrosis factor alpha, an early developmental signal for motoneuron death

Mechanisms inducing neuronal death at defined times during embryogenesis remain enigmatic. We show in explants that a developmental switch occurs between embryonic day 12 (E12) and E13 in rats that is 72-48 hr before programmed cell death. Half the motoneurons isolated from peripheral tissues at E12 escape programmed cell death, whereas 90% of motoneurons isolated at E13 enter a death program. The surrounding somite commits E12 motoneurons to death. This effect requires macrophage cells, is mimicked by tumor necrosis factor alpha (TNFalpha), and is inhibited by anti-TNFalpha antibodies. In vivo, TNFalpha is detected within somite macrophages, and TNF receptor 1 (TNFR1) is detected within motoneurons precisely between E12 and E13. Although motoneuron cell death occurs normally in TNFalpha-/- mice, this process is significantly reduced in explants from TNFalpha-/- and TNFR1-/- mice. Thus, embryonic motoneurons acquire the competence to die, before the onset of programmed cell death, from extrinsic signals such as macrophage-derived TNFalpha

Critical role for DP5/Harakiri, a Bcl-2 homology domain 3-only Bcl-2 family member, in axotomy-induced neuronal cell death

The survival of neurons is maintained primarily by neurotrophic factors that suppress the apoptotic program. Axotomy or removal of peripheral targets causes neuronal cell death, but the mechanisms involved in the induction of this type of cell death remain poorly understood. Here, we show that DP5/Harakiri, a Bcl-2 homology domain 3-only member of the Bcl-2 family, is induced in motoneurons after transection of the hypoglossal nerve in mice and in sympathetic neurons after nerve growth factor (NGF) withdrawal. To assess the role of DP5 in neuronal cell death, mutant mice deficient in DP5 were generated by gene targeting. DP5-/- mice were viable and exhibited normal postnatal development. Notably, motoneurons from DP5-/- mice were highly protected from cell death induced by resection of the hypoglossal nerve compared with motoneurons from DP5+/+ littermate mice. In addition, deficiency of DP5 in superior cervical ganglia (SCG) neurons resulted in delayed neuronal cell death triggered by NGF withdrawal. Analysis of SCG neurons from DP5-/- mice revealed increased preservation of mitochondrial membrane potential and reduced activation of caspase-3 compared with neurons from wild-type mice. These results indicate that DP5 plays an important role in neuronal cell death induced by axotomy and NGF deprivation through the regulation of mitochondrial function and caspase-3 activation.

Distinct muscle targets do not vary in the developmental regulation of brain-derived neurotrophic factor. J Comp Neurol 470:317-329,2004

Developing neurons depend on many target-derived signals. One of these signals is the neurotrophin brain-derived neurotrophic factor (BDNF). Exogenous application of BDNF in vitro and in vivo rescues a population of lumbar motoneurons from programmed cell death. Given that BDNF does not rescue all motoneurons and that motoneurons differ in trophic factor receptor expression, subpopulations of motoneurons may have different sensitivities to the factor. These differences may be reflected in distinct target muscles specialized to produce different protein concentrations, or muscles may contain equal amounts of the factor and receptor expression determines motoneuron responsiveness. By using a sensitive electrochemiluminescent immunoassay (ECLIA), we measured normal developmental changes in BDNF protein concentration in anatomically and functionally distinct chick embryonic thigh muscles from E6 to E18. We found that there were no significant differences in BDNF protein concentration between muscles classified according to function (fast vs. slow) or anatomical position (flexor vs. extensor) and that the quantity of BDNF in the target did not appear to be activity dependent. These results suggest that, during development, the differences in the response of motoneurons to BDNF are not due to the anatomical or functional diversity of muscle targets. J. Comp. Neurol. 470:330-337, 2004.

Hepatoma-derived growth factor is a neurotrophic factor harbored in the nucleus

Hepatoma-derived growth factor (HDGF) is a heparin-binding proliferating factor originally isolated from conditioned medium of the hepatoma-derived cell line HuH-7. HDGF has greatest homology in an amino acid sequence with high mobility group 1 (HMG1), which has been characterized as a DNA-binding, inflammatory, and potent neurite outgrowth molecule. HDGF is reported to be widely expressed and act as a growth factor in many kinds of cells. However, it has not been investigated in the nervous system. Here, we show by Western blot analysis that HDGF is present in the mouse brain from the embryonic period until adulthood. In situ hybridization and immunohistochemical analyses revealed that HDGF was expressed mainly in neurons, and HDGF protein was localized to the nucleus. HDGF and high mobility group 1 were secreted under physiological conditions and released extracellularly in necrotic conditions. Furthermore, we showed that exogenously supplied HDGF had a neurotrophic effect and was able to partially prevent the cell death of neurons in which endogenous HDGF was suppressed. Therefore, we propose that HDGF is a novel type of neurotrophic factor, on account of its localization in the nucleus and its potential to function in an autocrine manner under both physiological and pathological conditions throughout life.

Growing and working with spinal motor neurons

The chick embryo has a long tradition as a model organism in developmental biology as well as embryology. A year-round supply of fertilized eggs, accessibility to all stages of development, and the ease of manipulation of the embryo all contribute to the advantages of investigations using chick embryos. A plethora of culture systems have been developed over the past century allowing to culture intact embryos from as early as 2 days of development. Other culture systems include whole embryo slices, organotypic cultures, tissue explants, and dissociated cultures. Studies utilizing the chick embryo, and in particular spinal motor neurons, were crucial for our present knowledge of the development but also adult physiology, injury, and disease of the nervous system. Extensive studies on spinal motor neurons revealed many molecular mechanisms underlying fundamental events, such as neural induction, axon guidance, programmed cell death, and neuron-target interaction. Cultures of dissociated spinal motor neurons represent one important experimental paradigm. This chapter describes two alternative procedures to establish dissociated spinal motor neuron cultures with virtually no contamination by nonneuronal cells.

Neurotrophic effect of hepatocyte growth factor on neonatal facial motor neurons

The neurotrophic effect of hepatocyte growth factor (HGF) on axotomized facial motor neurons was examined after local application of HGF to the proximal facial nerve stump of the neonatal rat on post-natal day one (P1). Motor neuron survival was expressed as the neuronal cell count on the injured side as a percentage of that on the noninjured side. Motor neuron survival of the control group was 76% on P3, 54% on P5 and 23% on P8, that of the HGF-treated group 78% on P3, 69% on P5 and 31% on P8, and that of the brain-derived neurotrophic factor (BDNF)-treated group 91% on P5 and 45% on P8. The motor neuron survival rates were then adjusted by deducting the facial motor neurons corresponding to the uninjured retroauricular branch (20%) of the facial nerve. The adjusted values were 70% (P3), 42% (P5) and 4% (P8) for the control group, 72% (P3), 61% (P5) and 14% (P8) for the HGF-treated group, and 88% (P5) and 32% (P8) for the BDNF-treated group. These findings demonstrate that HGF has a neuroprotective effect on injured facial motor neurons and suggest that HGF has neurotrophic properties distinct from those of BDNF.

Response of motoneurons to neonatal sciatic nerve axotomy in Bax-knockout mice

Neonatal motoneurons (MNs) die rapidly after axotomy, a response that is mediated by the pro-apoptotic gene Bax and is followed by a mitochondria-mediated apoptotic cascade. Although motoneurons in neonatal Bax-deficient mice fail to degenerate following axotomy, it has not been previously examined whether the rescued MNs can regenerate following injury. We report here that although spinal MNs in Bax-knockout (Bax-KO) mice survive indefinitely, they undergo severe atrophy by 14 days after axotomy. By 1 month following axotomy, MN regeneration was observed and cellular atrophy was partially reversed. Interestingly, we observed that all MNs, including those previously rescued from normal developmental cell death in the embryo by Bax deletion, exhibit a regenerative response to peripheral nerve injury. The regenerative response may be mediated by specific trophic factors because the expression of glial cell line-derived neurotrophic factor (GDNF) was greatly increased in the proximal stump of injured nerves and application of a GDNF-blocking antibody greatly reduced regeneration/regrowth of rescued MNs in Bax-KO mice. These results indicate that MNs rescued from developmental or injury-induced cell death by Bax deletion have the potential to regenerate or regrow in response to nerve-derived signals following neonatal axotomy.

Cardiotrophin-like cytokine/cytokine-like factor 1 is an essential trophic factor for lumbar and facial motoneurons in vivo

The ciliary neurotrophic factor alpha-receptor (CNTFRalpha) is required for motoneuron survival during development, but the relevant ligand(s) has not been determined. One candidate is the heterodimer formed by cardiotrophin-like cytokine (CLC) and cytokine-like factor 1 (CLF). CLC/CLF binds to CNTFRalpha and enhances the survival of developing motoneurons in vitro; whether this novel trophic factor plays a role in neural development in vivo has not been tested. We examined motor and sensory neurons in embryonic chicks treated with CLC and in mice with a targeted deletion of the clf gene. Treatment with CLC increased the number of lumbar spinal cord motoneurons that survived the cell death period in chicks. However, this effect was regionally specific, because brachial and thoracic motoneurons were unaffected. Similarly, newborn clf-/- mice exhibited a significant reduction in lumbar motoneurons, with no change in the brachial or thoracic cord. Clf deletion also affected brainstem motor nuclei in a regionally specific manner; the number of motoneurons in the facial but not hypoglossal nucleus was significantly reduced. Sensory neurons of the dorsal root ganglia were not affected by either CLC treatment or clf gene deletion. Finally, mRNA for both clc and clf was found in skeletal muscle fibers of embryonic mice during the motoneuron cell death period. These findings support the view that CLC/CLF is a target-derived factor required for the survival of specific pools of motoneurons. The in vivo actions of CLC and CLF can account for many of the effects of CNTFRalpha on developing motoneurons.

Neuronal defects in the hindbrain of Hoxa1, Hoxb1 and Hoxb2 mutants reflect regulatory interactions among these Hox genes

Hox genes are instrumental in assigning segmental identity in the developing hindbrain. Auto-, cross- and para-regulatory interactions help establish and maintain their expression. To understand to what extent such regulatory interactions shape neuronal patterning in the hindbrain, we analysed neurogenesis, neuronal differentiation and motoneuron migration in Hoxa1, Hoxb1 and Hoxb2 mutant mice. This comparison revealed that neurogenesis and differentiation of specific neuronal subpopulations in r4 was impaired in a similar fashion in all three mutants, but with different degrees of severity. In the Hoxb1 mutants, neurons derived from the presumptive r4 territory were re-specified towards an r2-like identity. Motoneurons derived from that territory resembled trigeminal motoneurons in both their migration patterns and the expression of molecular markers. Both migrating motoneurons and the resident territory underwent changes consistent with a switch from an r4 to r2 identity. Abnormally migrating motoneurons initially formed ectopic nuclei that were subsequently cleared. Their survival could be prolonged through the introduction of a block in the apoptotic pathway. The Hoxa1 mutant phenotype is consistent with a partial misspecification of the presumptive r4 territory that results from partial Hoxb1 activation. The Hoxb2 mutant phenotype is a hypomorph of the Hoxb1 mutant phenotype, consistent with the overlapping roles of these genes in facial motoneuron specification. Therefore, we have delineated the functional requirements in hindbrain neuronal patterning that follow the establishment of the genetic regulatory hierarchy between Hoxa1, Hoxb1 and Hoxb2.