Motoneuron programmed cell death in response to proBDNF

Motoneurons (MN) as well as most neuronal populations undergo a temporally and spatially specific period of programmed cell death (PCD). Several factors have been considered to regulate the survival of MNs during this period, including availability of muscle-derived trophic support and activity. The possibility that target-derived factors may also negatively regulate MN survival has been considered, but not pursued. Neurotrophin precursors, through their interaction with p75(NTR) and sortilin receptors have been shown to induce cell death during development and following injury in the CNS. In this study, we find that muscle cells produce and secrete proBDNF. ProBDNF through its interaction with p75(NTR) and sortilin, promotes a caspase-dependent death of MNs in culture. We also provide data to suggest that proBDNF regulates MN PCD during development in vivo.

Neurotrophic interactions in the development of spinal cord motoneurons

The final number of spinal cord motoneurons is attained by a two-step process involving the proliferation of precursor cells and the loss by cell death of a proportion (approximately 50%) of the post-mitotic neurons. Although the mechanisms responsible for the proliferation of stereotyped numbers of motoneurons are not understood, considerable evidence from in vitro as well as in vivo studies indicates that the second step in attaining population size (cell death) is controlled by the interaction of motoneurons with both their efferent targets and their afferent inputs. Target influences on motoneuron survival are thought to be regulated by muscular activity and by competition for limited amounts of neurotrophic factors derived from striated skeletal muscles. However, evidence that such putative neurotrophic factors actually modulate motoneuron survival in vivo has been lacking. Using crude and partially purified extracts from embryonic hindlimbs (Days 8-9) we have found that the treatment of chick embryos in ovo with these agents during the normal cell death period (Days 5-10) rescues a significant number of motoneurons from degeneration. Kidney or lung extracts and heat-inactivated hindlimb extracts were ineffective. The survival-inducing activity of partially purified extract was dose dependent and developmentally regulated. The survival of sensory, sympathetic and a population of cholinergic sympathetic preganglionic neurons was unaffected by treatment with hindlimb extract. The massive motoneuron death that occurs after early target (hindlimb) removal was partially ameliorated by daily treatment with the hindlimb extract. Survival-inducing activity of the extract is lost after trypsin treatment. Taken collectively these results indicate that a target-derived protein or polypeptide neurotrophic factor is involved in the regulation of motoneuron survival in vivo.

Muscle activity and motor neuron death in the spinal cord of the chick embryo

During embryonic development in vertebrates about half the spinal motor neurons degenerate naturally after an initial period of normal differentiation. Motor neuron survival during this period is regulated by influences associated with both afferent and target contacts. Target-associated influences are regulated, at least in part, by activity (i.e. neuromuscular transmission or muscle contraction). Pharmacological blockade of neuromuscular activity reduces or prevents normal cell death whereas induced hyperactivity of targets enhances the death of motor neurons. Information supporting these assertions is reviewed and evidence is presented from studies which attempt to elucidate the major site at which neuromuscular activity affects motor neuron survival and degeneration in the chick embryo. Finally, a model and some supporting evidence are described in which activity is thought to regulate the production or availability of a target-derived trophic factor required by motor neurons for their survival during certain critical phases of early development.

Distinct susceptibility of developing neurons to death following Bax overexpression in the chicken embryo

Bax is a proapoptotic protein that is required for programmed cell death (PCD) of many neuronal populations. Here we show that, during an early period of retinal PCD and in naturally occurring sensory and motor neuron (MN) death in the spinal cord, Bax delivery results in enhanced death of these neural populations. In contrast, Bax overexpression fails to enhance an early phase of MN death that occurs in the cervical spinal cord, although overexpressed Bax appears to be activated in dying MNs. Bax overexpression does not also affect the survival of immature neurons prior to the PCD period. Taken together, these data provide the first in vivo evidence suggesting that Bax appears to act selectively as an executioner only in neurons undergoing PCD. Furthermore, although Bax appears to mediate the execution pathway for PCD, the effect of Bax overexpression on susceptibility to death differs between different neuronal populations.

Cytokines promote motoneuron survival through the Janus kinase-dependent activation of the phosphatidylinositol 3-kinase pathway

To determine which intracellular pathways mediate the survival effects of ciliary neurotrophic factor and cardiotrophin-1 cytokines on motoneurons, we studied the activation of the Jak/STAT, the PI 3-kinase/Akt, and the ERK pathways. At shorter time points, cytokines induced the activation of STAT3 and ERK, but not PI 3-kinase. Jak3 inhibitor suppressed cytokine- and muscle extract-induced survival. In contrast, PD 98059, a MEK inhibitor, was not able to prevent cytokine-induced survival, demonstrating that ERK is not involved. Surprisingly, the PI 3-kinase inhibitor LY 294002 prevented the survival-promoting effects of cytokines. When assays of PI 3-kinase activity were performed at later stages following cytokine treatment a significant increase was observed compared to control cultures. This delayed increase of activity could be completely prevented by treatment with protein synthesis or Jak3 inhibitors. Collectively, these results demonstrate that cytokines induce motoneuron survival through a PI 3-kinase activation requiring de novo protein synthesis dependent on Jak pathway.

Roles of caspases in the programmed cell death of motoneurons in vivo

Cysteine proteases comprising the caspase family have been considered one of the major executioners of programmed cell death. However, detailed analyses of the programmed cell death of developing motoneurons in mice following the genetic deletion of two key caspases, casp-3 and casp-9, and in the chick embryo following treatment with caspase inhibitors, indicate that normal amounts of cell loss occur although the death process is delayed. Motoneurons undergoing programmed cell death without caspase activities exhibit a nonapoptotic morphology in which nuclear changes such as chromatin condensation are absent or reduced and which exhibit extensive cytoplasmic vacuolization such as is rarely observed in degenerating control neurons. These results suggest that caspases are involved in, but are not indispensable for, the developmental death of motoneurons, and that one function of caspases may be to facilitate the removal of cells that are destined to die. Possible alternative caspase-independent pathways for the programmed death of motoneurons are discussed.

Long-lasting aberrant tubulovesicular membrane inclusions accumulate in developing motoneurons after a sublethal excitotoxic insult: a possible model for neuronal pathology in neurodegenerative disease

We have previously shown that chronic treatment of chick embryos [from embryonic day 5 (E5) to E9] with NMDA rescues spinal cord motoneurons (MNs) from programmed cell death. In this situation, MNs exhibit a reduced vulnerability to acute excitotoxic lesions and downregulate NMDA and AMPA-kainate receptors. Here, we report that this treatment results in long-lasting sublethal structural changes in MNs. In Nissl-stained sections from the spinal cord of NMDA-treated embryos, MNs display an area adjacent to an eccentrically positioned nucleus in which basophilia is excluded. Ultrastructurally, MNs accumulate tubulovesicular structures surrounded by Golgi stacks. Thiamine pyrophosphatase but not acid phosphatase was detected inside the tubulovesicular structures, which are resistant to disruption by brefeldin A or monensin. Immunocytochemistry reveals changes in the content and distribution of calcitonin gene-related peptide, the KDEL receptor, the early endosomal marker EEA1, and the recycling endosome marker Rab11, indicating that a dysfunction in membrane trafficking and protein sorting occurs in these MNs. FM1-43, a marker of the endocytic pathway, strongly accumulates in MNs from isolated spinal cords after chronic NMDA treatment. Changes in the distribution of cystatin C and presenilin-1 and an accumulation of amyloid precursor protein and beta-amyloid product were also observed in NMDA-treated MNs. None of these alterations involve an interruption of MN-target (muscle) connections, as detected by the retrograde tracing of MNs with cholera toxin B subunit. These results demonstrate that chronic NMDA treatment induces severe changes in the motoneuronal endomembrane system that may be related to some neuropathological alterations described in human MN disease.

Caspase activity is involved in, but is dispensable for, early motoneuron death in the chick embryo cervical spinal cord

We examined the role of caspases in the early programmed cell death (PCD) of motoneurons (MNs) in the chick embryo cervical cord between embryonic day (E) 4 and E5. An increase in caspase-3-like activity in MNs was observed at E4.5. Treatment with an inhibitor of caspase-3-like activity, Ac-DEVD-CHO, for 12 h blocked this increase and revealed that caspase-3-like activity is mainly responsible for DNA fragmentation and the nuclear changes during PCD but not for degenerative changes in the cytoplasm. When a more broad-spectrum caspase inhibitor was used (bocaspartyl (OMe)-fluoromethyl ketone, BAF), the appearance of degenerative changes in the cytoplasm was delayed by at least 12 h. However, following treatment with either Ac-DEVD-CHO or BAF for 24 h, the number of surviving healthy MNs did not differ from controls, indicating a normal occurrence of PCD despite the inhibition of caspases. These results suggest that caspase cascades that occur upstream of and are independent of the activation of caspase-3-like activity are responsible for the degenerative changes in the cytoplasm of dying cervical MNs. These data also suggest that, although one function of caspases may be to facilitate the kinetics of PCD, caspases are nonetheless dispensable for at least some forms of normal neuronal PCD in vivo.

Programmed cell death of developing mammalian neurons after genetic deletion of caspases

An analysis of programmed cell death of several populations of developing postmitotic neurons after genetic deletion of two key members of the caspase family of pro-apoptotic proteases, caspase-3 and caspase-9, indicates that normal neuronal loss occurs. Although the amount of cell death is not altered, the death process may be delayed, and the cells appear to use a nonapoptotic pathway of degeneration. The neuronal populations examined include spinal interneurons and motor, sensory, and autonomic neurons. When examined at both the light and electron microscopic levels, the caspase-deficient neurons exhibit a nonapoptotic morphology in which nuclear changes such as chromatin condensation are absent or reduced; in addition, this morphology is characterized by extensive cytoplasmic vacuolization that is rarely observed in degenerating control neurons. There is also reduced terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling in dying caspase-deficient neurons. Despite the altered morphology and apparent temporal delay in cell death, the number of neurons that are ultimately lost is indistinguishable from that seen in control animals. In contrast to the striking perturbations in the morphology of the forebrain of caspase-deficient embryos, the spinal cord and brainstem appear normal. These results are consistent with the growing idea that the involvement of specific caspases and the occurrence of caspase-independent programmed cell death may be dependent on brain region, cell type, age, and species or may be the result of specific perturbations or pathology.

Caspase inhibitors promote the survival of avulsed spinal motoneurons in neonatal rats

Following ventral root avulsion in neonatal animals, the degeneration of spinal motoneurons occurs by an apoptotic-like morphological pathway. In adult animals, however, the mechanism of degeneration of injured motoneurons is still controversial. Because caspases are important mediators of apoptosis, we have investigated the effects of the caspase inhibitors, benzyloxycarbonyl-Asp(OMe)fluoromethylketone (Boc-D-FMK), and N-acetyl-Asp-Glu-Val-Asp aldehyde (Ac-DEVD-CHO) on the survival of neonatal and adult spinal motoneurons after root avulsion of the C7 spinal cord. In the control neonatal animals, virtually all motoneurons had degenerated by 7 days following root avulsion. Treatment with either 0.5 microg Boc-D-FMK or 1 microg Ac-DEVD-CHO enhanced the survival of motoneurons to 80% and 85% for up to 2 weeks post-injury. By 21 days post-injury, 70% of avulsed motoneurons were still present after Boc-D-FMK treatment, whereas all avulsed motoneurons died after treatment with Ac-DEVD-CHO. In adult animals, neither inhibitor was neuroprotective for motoneurons following root avulsion. In summary, the inhibition of caspases effectively rescued avulsed neonatal motoneurons which are died by apoptotic pathway. By contrast, because caspase inhibitors failed to rescue injured motoneurons in adult animals, their death may occur by a non-apoptotic pathway.

Characterization of the execution pathway of developing motoneurons deprived of trophic support

Avian spinal motoneurons have been well characterized with regard to developmental programmed cell death (PCD). Approximately 50% of the neurons originally generated undergo cell death as they innervate their target muscles, and target derived trophic support plays an important role in regulating survival of these neurons. To investigate events mediating motoneuron PCD, we have examined the role of Bcl-2 family proteins, cytochrome C, and caspase-9 in this process. We report that while protein levels of Bcl-2, Bcl-xL, and Bax do not change within motoneurons as they become committed to die, a translocation of Bax from the cytosol to organelle membranes and the nucleus occurs coincident with the time when motoneurons become committed to cell death. In addition, cytochrome C is released from mitochondria to the cytosol in dying cells prior to the activation of caspases. Consequently, an enhanced caspase-9-like activity was detected in dying cells, and this activity was upstream and necessary for the appearance of a caspase-3-like activity. These results allow us to further define some of the critical events that mediate the execution phase of motoneuron death following trophic factor deprivation.

Cardiotrophin-1, a muscle-derived cytokine, is required for the survival of subpopulations of developing motoneurons

Developing motoneurons require trophic support from their target, the skeletal muscle. Despite a large number of neurotrophic molecules with survival-promoting activity for isolated embryonic motoneurons, those factors that are required for motoneuron survival during development are still not known. Cytokines of the ciliary neurotrophic factor (CNTF)-leukemia inhibitory factor (LIF) family have been shown to play a role in motoneuron (MN) survival. Importantly, in mice lacking the LIFRbeta or the CNTFRalpha there is a significant loss of MNs during embryonic development. Because genetic deletion of either (or both) CNTF or LIF fails, by contrast, to perturb MN survival before birth, it was concluded that another ligand exists that is functionally inactivated in the receptor deleted mice, resulting in MN loss during development. One possible candidate for this ligand is the CNTF-LIF family member cardiotrophin-1 (CT-1). CT-1 is highly expressed in embryonic skeletal muscle, secreted by myotubes, and promotes the survival of cultured embryonic mouse and rat MNs. Here we show that ct-1 deficiency causes increased motoneuron cell death in spinal cord and brainstem nuclei of mice during a period between embryonic day 14 and the first postnatal week. Interestingly, no further loss was detectable during the subsequent postnatal period, and nerve lesion in young adult ct-1-deficient mice did not result in significant additional loss of motoneurons, as had been previously observed in mice lacking both CNTF and LIF. CT-1 is the first bona fide muscle-derived neurotrophic factor to be identified that is required for the survival of subgroups of developing motoneurons.

Reduction of endogenous transforming growth factors beta prevents ontogenetic neuron death

We show that following immunoneutralization of endogenous transforming growth factors beta (TGF-beta) in the chick embryo, ontogenetic neuron death of ciliary, dorsal root and spinal motor neurons was largely prevented, and neuron losses following limb bud ablation were greatly reduced. Likewise, preventing TGF-beta signaling by treatment with a TbetaR-II fusion protein during the period of ontogenetic cell death in the ciliary ganglion rescued all neurons that normally die. TUNEL staining revealed decreased numbers of apoptotic cells following antibody treatment. Exogenous TGF-beta rescued the TGF-beta-deprived phenotype. We conclude that TGF-beta is critical in regulating ontogenetic neuron death as well as cell death following neuronal target deprivation.

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

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

Effects of neurotrophic factors on motoneuron survival following axonal injury in newborn rats

Using two different lesion models, the spinal root avulsion and the distal nerve axotomy, the present study investigated effects of known neurotrophic factors on motoneuron survival in newborn rats. Results of the present study show that 100% of motoneurons in the lesioned spinal segment die at 1 week following root avulsion, and more than 80% of them die at 2 weeks following distal nerve axotomy. Local application of GDNF can rescue 92% of motoneurons up to 1 week from degeneration due to root avulsion and almost 100% of them up to 2 weeks from degeneration due to distal nerve axotomy. Local application of BDNF fails to prevent any motoneuron death in newborn rats following root avulsion, but it can rescue about 50% of motoneurons up to 2 weeks from degeneration due to distal nerve axotomy. CNTF and IGF-1 fail to prevent any motoneuron death following either distal nerve axotomy or root avulsion. Thus, comparing all the neurotrophic factors tested in this study, GDNF is most effective in preventing death of motoneurons following axonal injury in newborn rats.

Glial cell line-derived neurotrophic factor and developing mammalian motoneurons: regulation of programmed cell death among motoneuron subtypes

Because of discrepancies in previous reports regarding the role of glial cell line-derived neurotrophic factor (GDNF) in motoneuron (MN) development and survival, we have reexamined MNs in GDNF-deficient mice and in mice exposed to increased GDNF after in utero treatment or in transgenic animals overexpressing GDNF under the control of the muscle-specific promoter myogenin (myo-GDNF). With the exception of oculomotor and abducens MNs, the survival of all other populations of spinal and cranial MNs were reduced in GDNF-deficient embryos and increased in myo-GDNF and in utero treated animals. By contrast, the survival of spinal sensory neurons in the dorsal root ganglion and spinal interneurons were not affected by any of the perturbations of GDNF availability. In wild-type control embryos, all brachial and lumbar MNs appear to express the GDNF receptors c-ret and GFRalpha1 and the MN markers ChAT, islet-1, and islet-2, whereas only a small subset express GFRalpha2. GDNF-dependent MNs that are lost in GDNF-deficient animals express ret/GFRalpha1/islet-1, whereas many surviving GDNF-independent MNs express ret/GFRalpha1/GFRalpha2 and islet-1/islet-2. This indicates that many GDNF-independent MNs are characterized by the presence of GFRalpha2/islet-2. It seems likely that the GDNF-independent population represent MNs that require other GDNF family members (neurturin, persephin, artemin) for their survival. GDNF-dependent and -independent MNs may reflect subtypes with distinct synaptic targets and afferent inputs.

Hepatocyte growth factor/scatter factor is a neurotrophic survival factor for lumbar but not for other somatic motoneurons in the chick embryo

Hepatocyte growth factor/scatter factor (HGF/SF) is expressed in the developing limb muscles of the chick embryo during the period of spinal motoneuron (MN) programmed cell death, and its receptor c-met is expressed in lumbar MNs during this same period. Although cultured motoneurons from brachial, thoracic, and lumbar segments are all rescued from cell death by chick embryo muscle extract (CMX) as well as by other specific trophic agents, HGF/SF only promotes the survival of lumbar MNs. Similarly, treatment of embryos in ovo with exogenous HGF/SF rescues lumbar but not other somatic MNs from cell death. Blocking antibodies to HGF/SF (anti-HGF) reduce the effects of CMX on MN survival in vitro and decrease the number of lumbar MNs in vivo. The expression of c-met on MNs in vivo is regulated by a limb-derived trophic signal distinct from HGF/SF. HGF/SF is a potent, select, and physiologically relevant survival factor for a subpopulation of developing spinal MNs in the lumbar segments of the chick embryo.

Expression pattern of GDNF, c-ret, and GFRalphas suggests novel roles for GDNF ligands during early organogenesis in the chick embryo

We have cloned a partial cDNA of chicken glial cell line-derived neurotrophic factor (GDNF) and systematically examined its expression pattern as well as that of GDNF-binding components (GDNF family receptor alpha-1 and 2: GFRalpha-1 and 2) and a common signal transduction receptor (c-ret protooncogene: RET) during very early developmental stages. In addition, we also examined the expression pattern of an apparent avian-specific binding component, GFRalpha-4. The cloned chicken cDNA for GDNF had approximately 80% homology to mammalian counterparts. The expression of GDNF mRNA occurred in many spatially and temporally discrete regions such as the intermediate mesoderm, the floor plate of the spinal cord, pharyngeal endoderm contacting the epibranchial placodes, distal ganglia of cranial nerves, subpopulations of mesenchyme cells in the craniofacial region, and in the mesodermal wall of the digestive tract. Both a GDNF receptor signal transduction component (RET) and a binding component (GFRalpha-1 or GFRalpha-2) were independently expressed in nearby interacting tissues such as the somites, peripheral and central nervous system, and mesenchyme cells in the craniofacial region. These observations suggest that possible combinations of novel unidentified receptors acting with RET or with GFRalphas may mediate GDNF-derived signals and indicate that GDNF or other family members may have previously unidentified actions in early organogenesis in the chick embryo.

The spatial-temporal gradient of naturally occurring motoneuron death reflects the time of prior exit from the cell cycle and position within the lateral motor column

Embryonic lumbar spinal motoneurons (MNs) are characterized by a period of programmed cell death (PCD) that spans several days and occurs in a rostrocaudal gradient. The generation of these MNs also takes place in a temporal-spatial gradient, such that MNs within rostral lumbar segments exit the cell cycle earlier and MNs within progressively caudal regions are born later. In vitro studies have shown that the latest born spinal MNs, presumably through the possession of endogenous "survival properties," are also the last to acquire their trophic dependence. If the birth date and therefore spinal cord location of lumbar spinal MNs influence the spatial-temporal pattern of PCD, then earlier born MNs should die sooner and be located more rostrally than those generated later. Alternatively, if the time at which MNs die during development is unrelated to their prior exit from the cell cycle, those born at various phases should die throughout the period of PCD. We report here that lumbar MNs generated during the earliest part (embryonic day 2-3) of the proliferative period in the developing chick spinal cord tend to die during the earliest stages of the PCD period and that MNs born in successive 12-h intervals die at correspondingly later periods during PCD. Furthermore, the spatial progression of PCD of these subpopulations of MNs occurs in a rostrocaudal gradient. Finally, while MNs do appear to die in a mediolateral gradient during the period of MN PCD, this pattern is only partly accounted for by MNs born in consecutive intervals. These data support the notion that the timing and rostrocaudal location of MNs undergoing PCD reflect their time of exit from the cell cycle.

Opposing effects of excitatory amino acids on chick embryo spinal cord motoneurons: excitotoxic degeneration or prevention of programmed cell death

Acute administration of a single dose of NMDA on embryonic day (E) 7 or later induces a marked excitotoxic injury in the chick spinal cord, including massive necrotic motoneuron (MN) death. When the same treatment was performed before E7, little, if any, excitotoxic response was observed. Chronic treatment with NMDA starting on E5 prevents the excitotoxic response produced by a later "acute" administration of NMDA. Additionally, chronic NMDA treatment also prevents the later excitotoxic injury induced by non-NMDA glutamate receptor agonists, such as kainate or AMPA. Chronic NMDA treatment also reduces normal MN death when treatment is maintained during the period of naturally occurring programmed cell death (PCD) of MNs and rescues MNs from PCD induced by early peripheral target deprivation. The trophic action of chronic NMDA treatment appears to involve a downregulation of glutamate receptors as shown by both a reduction in the obligatory NR1 subunit protein of the NMDA receptor and a decrease in the kainate-induced Co(2+) uptake in MNs. Both tolerance to excitotoxicity and trophic effects of chronic NMDA treatment are prevented by the NMDA receptor antagonist MK-801. Additionally, administration of MK-801 alone results in an increase in MN PCD. These data indicate for the first time that early activation of NMDA receptors in developing avian MNs in vivo has a trophic, survival-promoting effect, inhibiting PCD by a target-independent mechanism that involves NMDA receptor downregulation.

Androgens rescue avian embryonic lumbar spinal motoneurons from injury-induced but not naturally occurring cell death

The regulation of survival of spinal motoneurons (MNs) has been shown to depend during development and after injury on a variety of neurotrophic molecules produced by skeletal muscle target tissue. Increasing evidence also suggests that other sources of trophic support prevent MNs from undergoing naturally occurring or injury-induced death. We have examined the role of endogenous and exogenous androgens on the survival of developing avian lumbar spinal MNs during their period of programmed cell death (PCD) between embryonic day (E)6 and E11 or after axotomy on E12. We found that although treatment with testosterone, dihydrotestosterone (DHT), or the androgen receptor antagonist flutamide (FL) failed to affect the number of these MNs during PCD, administration of DHT from E12 to E15 following axotomy on E12 significantly attenuated injury-induced MN death. This effect was inhibited by cotreatment with FL, whereas treatment with FL alone did not affect MN survival. Finally, we examined the spinal cord at various times during development and following axotomy on E12 for the expression of androgen receptor using the polyclonal PG-21 antibody. Our results suggest that exogenously applied androgens are capable of rescuing MNs from injury-induced cell death and that they act directly on these cells via an androgen receptor-mediated mechanism. By contrast, endogenous androgens do not appear to be involved in the regulation of normal PCD of developing avian MNs.

Neurotrophic factor regulation of developing avian oculomotor neurons: differential effects of BDNF and GDNF

Neurotrophic factors support the development of motoneurons by several possible mechanisms. Neurotrophins may act as target-derived factors or as afferent factors derived from the central nervous system (CNS) or sensory ganglia. We tested whether brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), neurotrophin 4 (NT-4), and glial cell line-derived neurotrophic factor (GDNF) may be target-derived factors for neurons in the oculomotor (MIII) or trochlear (MIV) nucleus in chick embryos. Radio-iodinated BDNF, NT-3, NT-4, and GDNF accumulated in oculomotor neurons via retrograde axonal transport when the trophic factors were applied to the target. Systemic GDNF rescued oculomotor neurons from developmental cell death, while BDNF and NT-3 had no effect. BDNF enhanced neurite outgrowth from explants of MIII and MIV nuclei (identified by retrograde labeling in ovo with the fluorescent tracer DiI), while GDNF, NT-3, and NT-4 had no effect. The oculomotor neurons were immunoreactive for BDNF and the BDNF receptors p75(NTR) and trkB. To determine whether BDNF may be derived from its target or may act as an autocrine or paracrine factor, in situ hybridization and deprivation studies were performed. BDNF mRNA expression was detected in eye muscles, but not in CNS sources of afferent innervation to MIII, or the oculomotor complex itself. Injection of trkB fusion proteins in the eye muscle reduced BDNF immunoreactivity in the innervating motoneurons. These data indicate that BDNF trophic support for the oculomotor neurons was derived from their target.

Long-term effects of a single dose of brain-derived neurotrophic factor on motoneuron survival following spinal root avulsion in the adult rat

The long-term effect of a single dose of Brain-derived neurotrophic factor (BDNF) treatment on adult motoneuron survival and on expression of nitric oxide synthase (NOS) following nerve injury (avulsion) was investigated and compared with that of continuous BDNF treatment. By 6 weeks post-injury, more than 80% of motoneurons survived in animals treated with either a single dose or continuous treatment of BDNF, while only 30% of motoneurons survived in control animals (avulsion only). There were no significant differences in motoneuron survival between animals receiving a single dose and those with continuous treatment of BDNF. Additionally, the expression of NOS in avulsed motoneurons was almost completely inhibited in all BDNF treatment groups regardless of the mode of administration (single vs. continuous). These data indicate that treatment with a single dose of BDNF at the time of injury can inhibit NOS expression and provide the first evidence that in this situation BDNF has a long-term rescue effect on adult motoneuron survival after root avulsion.

Modulation of early but not later stages of programmed cell death in embryonic avian spinal cord by sonic hedgehog

Sonic hedgehog (Shh) is a secreted glycoprotein expressed by the notochord and floor plate that is involved in the induction and specification of ventral phenotypes in the vertebrate neural tube. Recently, Shh has also been shown to promote the survival of cultured rat embryo ventral brain and spinal cord cells. We have examined whether Shh can promote the survival of chick embryo neurons in vivo or in vitro. In the chick, Shh is expressed in notochord, floor plate, and ventral neural tube/spinal cord at several stages at which programmed cell death (PCD) occurs. However, the administration of exogenous Shh to embryos in vivo or to motoneuron cultures at these stages failed to promote the survival of several different neuronal populations, including spinal motoneurons, spinal interneurons, sympathetic preganglionic neurons, sensory neurons, and neuronal precursor cells. Rather, at the earliest stage of PCD examined here (embryonic day 3) Shh selectively induced the death of ventral neuronal precursors and floor-plate cells, resulting in a net loss of cells in the neural tube. Altered concentrations of Shh induce aberrant phenotypes that are removed by PCD. Accordingly, normal PCD in the early neural tube may play a role in dorsal-ventral patterning.

Close spatial-temporal relationship between islet-1-expressing cells and growing primary afferent axons in the dorsal spinal cord of chick embryo

The relationship between the appearance of Islet-1-expressing cells and the longitudinal growth of primary afferent axons (PAAs) in the dorsal spinal cord of chick embryos was examined. Islet-1-expressing cells first appeared in the dorsal spinal cord at embryonic days (E) 3-3.5. These immunoreactive cells were aligned in a longitudinal column in close proximity to longitudinally elongating PAAs in the presumptive dorsal funiculus. By E8, when many PAAs invade the spinal gray matter, Islet-1-expressing cells had disappeared in the dorsal spinal cord. Following the dorsoventral rotation of the spinal cord in ovo before the invasion of PAAs, a close topographical relationship between Islet-1-expressing cells and PAAs was maintained. These results suggest that Islet-1-expressing cells may play a role in the longitudinal growth of PAAs in the dorsal funiculus.

Involvement of specific caspases in motoneuron cell death in vivo and in vitro following trophic factor deprivation

The caspases have been shown to be key components of programmed cell death (PCD) in various cell types, including neurons. Caspase-3 (CPP32) is the predominant caspase that appears to be involved in cell death in several systems. In embryonic motoneuron cultures, caspase-3 activity increases beginning at 20 h following deprivation of trophic support, as determined by the cleavage of its specific substrates. Inhibition of caspase-3 by peptide inhibitors prevents the PCD of motoneurons following trophic factor deprivation in vitro, as well as in vivo. We also investigated the cleavage of poly(ADP-ribose) polymerase (PARP) in motoneurons after trophic factor withdrawal. No PARP cleavage was detected in either viable or dying cells. These data suggest that some components of the cell death machinery such as the involvement of caspases may be conserved in different cell types undergoing PCD, whereas the activation and specific substrates of the caspases may differ from one cell type to another.

Mechanisms of insulin-like growth factor regulation of programmed cell death of developing avian motoneurons

During development of the avian neuromuscular system, lumbar spinal motoneurons (MNs) innervate their muscle targets in the hindlimb coincident with the onset and progression of MN programmed cell death (PCD). Paralysis (activity blockade) of embryos during this period rescues large numbers of MNs from PCD. Because activity blockade also results in enhanced axonal branching and increased numbers of neuromuscular synapses, it has been postulated that following activity blockade, increased numbers of MNs can gain access to muscle-derived trophic agents that prevent PCD. An assumption of the access hypothesis of MN PCD is the presence of an activity-dependent, muscle-derived sprouting or branching agent. Several previous studies of sprouting in the rodent neuromuscular system indicate that insulin-like growth factors (IGFs) are candidates for such a sprouting factor. Accordingly, in the present study we have begun to test whether the IGFs may play a similar role in the developing avian neuromuscular system. Evidence in support of this idea includes the following: (a) IGFs promote MN survival in vivo but not in vitro; (b) neutralizing antibodies against IGFs reduce MN survival in vivo; (c) both in vitro and in vivo, IGFs increase neurite growth, branching, and synapse formation; (d) activity blockade increases the expression of IGF-1 and IGF-2 mRNA in skeletal muscles in vivo; (e) in vivo treatment of paralyzed embryos with IGF binding proteins (IGF-BPs) that interfere with the actions of endogenous IGFs reduce MN survival, axon branching, and synapse formation; (f) treatment of control embryos in vivo with IGF-BPs also reduces synapse formation; and (g) treatment with IGF-1 prior to the major period of cell death (i.e., on embryonic day 6) increases subsequent synapse formation and MN survival and potentiates the survival-promoting actions of brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) administered during the subsequent 4- to 5-day period of PCD. Collectively, these data provide new evidence consistent with the role of the IGFs as activity-dependent, muscle-derived agents that play a role in regulating MN survival in the avian embryo.

A novel strategy for introducing exogenous bcl-2 into neuronal cells: the Cre/loxP system-mediated activation of bcl-2 for preventing programmed cell death using recombinant adenoviruses

We have established a novel strategy for introducing exogenous Bcl-2 into neuronal cells that is mediated by Cre/loxP recombination using recombinant adenoviral vectors. An on/off-switching cassette for Bcl-2 (CALNLbcl-2) was designed to express Bcl-2 by recombinase Cre-mediated excisional deletion of a spacer DNA flanked by a pair of loxP sites. Exogenous Bcl-2 was clearly induced in PC12 cell lines carrying CALNLbcl-2 after infection with recombinant adenovirus producing recombinase Cre (AxCANCre). Dual infection with both AxCANCre and a recombinant adenovirus bearing CALNLbcl-2 showed efficient delivery of exogenous Bcl-2 into a hybrid motoneuronal cell line and primary chicken spinal motoneurons. The delivery of foreign Bcl-2 promoted survival of motoneurons in medium either containing or lacking trophic support. Thus, this strategy for delivery of exogenous Bcl-2 will be useful for studying neuronal death as well as for introducing foreign genes into postmitotic neurons under the control of recombinase Cre.

CEP-1347/KT7515 prevents motor neuronal programmed cell death and injury-induced dedifferentiation in vivo

CEP-1347, also known as KT7515, a derivative of a natural product indolocarbazole, inhibited motor neuronal death in vitro, inhibited activation of the stress-activated kinase JNK1 (c-jun NH terminal kinase) in cultured spinal motor neurons, but had no effect on the mitogen-activated protein kinase ERK1 in these cells. Results reported here profile the functional activity of CEP-1347/KT7515 in vivo in models of motor neuronal death or dedifferentiation. Application of CEP-1347/KT7515 to the chorioallantoic membrane of embryonic chicks rescued 40% of the lumbar motor neurons that normally die during the developmental period assessed. Peripheral administration of low doses (0.5 and 1 mg/kg daily) of CEP-1347/KT7515 reduced death of motor neurons of the spinal nucleus of the bulbocavernosus in postnatal female rats, with efficacy comparable to testosterone. Strikingly, daily administration of CEP-1347/KT7515 during the 4-day postnatal window of motor neuronal death resulted in persistent long-term motor neuronal survival in adult animals that received no additional CEP-1347/KT7515. In a model of adult motor neuronal dedifferentiation following axotomy, local application of CEP-1347/KT7515 to the transected hypoglossal nerve substantially reduced the loss of choline acetyl transferase immunoreactivity observed 7 days postaxotomy compared to untreated animals. Results from these experiments demonstrate that a small organic molecule that inhibits a signaling pathway associated with stress and injury also reduces neuronal death and degeneration in vivo.

Increased production of amyloid precursor protein provides a substrate for caspase-3 in dying motoneurons

Biochemical and molecular mechanisms of neuronal cell death are currently an area of intense research. It is well documented that the lumbar spinal motoneurons of the chick embryo undergo a period of naturally occurring programmed cell death (PCD) requiring new gene expression and activation of caspases. To identify genes that exhibit changed expression levels in dying motoneurons, we used a PCR-based subtractive hybridization protocol to identify messages uniquely expressed in motoneurons deprived of trophic support as compared with their healthy counterparts. We report that one upregulated message in developing motoneurons undergoing cell death is the mRNA for amyloid precursor protein (APP). Increased levels of APP and beta-amyloid protein are also detected within dying motoneurons. The predicted peptide sequence of APP indicates two potential cleavage sites for caspase-3 (CPP-32), a caspase activated in dying motoneurons. When peptide inhibitors of caspase-3 are administered to motoneurons destined to undergo PCD, decreased levels of APP protein and greatly reduced beta-amyloid production are observed. Furthermore, we show that APP is cleaved by caspase-3. Our results suggest that differential gene expression results in increased levels of APP, providing a potential substrate for one of the cell death-activated caspases that may ultimately cause the demise of the cell. These results, combined with information on the toxic role of APP and its proteolytic by-product beta-amyloid, in the neurodegenerative disease Alzheimer's, suggest that events of developmental PCD may be reactivated in early stages of pathological neurodegeneration.

Characterization of spinal motoneuron degeneration following different types of peripheral nerve injury in neonatal and adult mice

Experimental lesions have been used widely to induce motoneuron (MN) degeneration as a model to test the ability of different trophic molecules to prevent lesion-induced alterations. However, the morphological mechanisms of spinal MN death following different types of lesions is not clear at the present time. In this study, we have characterized the morphological characteristics of MN cell death by examining DNA fragmentation and the ultrastructural and light microscopic morphological features of MNs following different types of spinal nerve injury (i.e., axotomy and avulsion) in the developing and adult mouse. In neonatal mice, axotomy induced cell death as well as the atrophy of MNs that survived the injury. DNA fragmentation could be detected by using the terminal deoxynucleotidyl transferase (TUNEL) method during the cell death process following neonatal axotomy, whereas TUNEL labeling was not observed following either neonatal or adult avulsion. However, with the exception of TUNEL labeling, the morphological characteristics of MN death following neonatal axotomy and avulsion were similar, and both resembled most closely the form of programmed cell death termed cytoplasmic or type 3B, which exhibits similarities as well as differences with currently accepted definitions of apoptosis. By contrast, adult avulsion resulted in a type of degeneration that resembled necrosis more closely. However, even there, the morphology was mixed, showing characteristics of both apoptosis and necrosis. These results indicate that the mode of MN degeneration is complex and is related to developmental age and type of lesion.

Muscle-specific cell ablation conditional upon Cre-mediated DNA recombination in transgenic mice leads to massive spinal and cranial motoneuron loss

We describe here a binary transgenic system based on Cre-mediated DNA recombination for genetic cell ablation in mice that enabled us to obtain skeletal muscle-deficient embryos by mating two phenotypically normal transgenic lines. In those embryos, skeletal muscles are eliminated as a consequence of the expression of the gene encoding the diphtheria toxin A fragment. Cell ablation occurs gradually beginning approximately on embryonic day (E) 12.5, and by E18-5 almost all skeletal muscle is absent. Analysis of the consequences of muscle cell ablation revealed that almost all spinal motoneurons are lost by E18.5, providing strong evidence that survival of spinal motoneurons during embryogenesis is dependent on signals from their target tissue, skeletal muscle, and that trophic signals produced by nonmuscle sources are sufficient to support survival of no more than 10% of embryonic spinal motoneurons in the absence of muscle-derived signals. There was also substantial loss of cranial (hypoglossal and facial) motoneurons in the muscle-deficient embryos, thus indicating that cranial motoneuron survival is also dependent on trophic signals produced by their target tissue. Although spinal motoneurons are a major target of spinal interneurons, the loss of motoneurons did not affect interneuron survival. Muscle-deficient embryos had a cleft palate and abnormalities of the lower jaw, raising the possibility that they might serve as a mouse model for the human disorder, Robin sequence. The data reported here demonstrate the utility of a binary transgenic system for obtaining mouse embryos in which a specific cell population has been ablated, so that its role in embryonic development can be studied.

Peripheral target regulation of the development and survival of spinal sensory and motor neurons in the chick embryo

Unilateral limb-bud removal (LBR) before the outgrowth of sensory or motor neurons to the leg of chick embryos was used to examine the role of limb (target)-derived signals in the development and survival of lumbar motoneurons and sensory neurons in the dorsal root ganglia (DRG). After LBR, motor and sensory neurons underwent normal initial histological differentiation, and cell growth in both populations was unaffected. Before their death, target-deprived motoneurons also expressed a cell-specific marker, the homeodomain protein islet-1. Proliferation of sensory and motor precursor cells was also unaffected by LBR, and the migration of neural crest cells to the DRG and of motoneurons into the ventral horn occurred normally. During the normal period of programmed cell death (PCD), increased numbers of both sensory and motor neurons degenerated after LBR. However, whereas motoneuron loss increased by 40-50% (90% total), only approximately 25% more sensory neurons degenerated after LBR. A significant number of the surviving sensory neurons projected to aberrant targets in the tail after LBR, and many of these were lost after ablation of both the limb and tail. Treatment with neurotrophic factors (or muscle extract) rescued sensory and motor neurons from cell death after LBR without affecting precursor proliferation of either population. Activity blockade with curare failed to rescue motoneurons after LBR, and combined treatment with curare plus muscle extract was no more effective than muscle extract alone. Treatment with the antioxidant N-acetylcysteine rescued motoneurons from normal cell death but not after LBR. Two specific inhibitors of the interleukin beta1 converting enzyme (ICE) family of cysteine proteases also failed to prevent motoneuron death after LBR. Taken together these data provide definitive evidence that the loss of spinal neurons after LBR cannot be attributed to altered proliferation, migration, or differentiation. Rather, in the absence of limb-derived trophic signals, the affected neurons fail to survive and undergo PCD. Although normal cell death and cell death after target deprivation share many features in common, the intracellular pathways of cell death in the two may be distinct.

Cloning and expression of the cDNA encoding rat caspase-2

We have isolated cDNA clones for rat caspase-2 (also called Nedd2/Ich-1), that encodes a protein similar to interleukin-1beta-converting enzyme (ICE) and the product of the nematode Caenorhabditis elegans cell death gene ced-3 both of which play an important role in programmed cell death (PCD). The rat caspase-2 cDNA clones have an open reading frame (ORF) of 452 amino acids (aa). The predicted aa sequence of rat caspase-2 is highly similar to that of mouse Nedd2 (97.3%) and human Ich-1L (91.3%). The aa sequence QACRG containing the active Cys residue, that is necessary for the proteolytic activity of ICE/Ced-3 (caspase) family of proteases, is also conserved in rat caspase-2. Rat caspase-2 also has several Asp residues in the amino and carboxyl cleavage regions similar to other caspase family proteins. We have developed PC12 cells carrying an on/off switching cassette of caspase-2 (named PC-Nd cells), which contains the neo gene flanked by a pair of loxP sites, the Cre-specific recognition sequence of 34 nucleotides (nt), that lies between the promoter and the caspase-2 cDNA. This expression cassette was designed to express the neo gene initially and to turn on the expression of caspase-2 by site-specific recombinase Cre-mediated excisional deletion of the neo gene. After infection with Cre-producing recombinant adenovirus (re-Ad), the expression of caspase-2 was highly induced in PC-Nd cells and presumptive actively processed fragments of caspase-2 were also observed. This gene activation strategy of caspase-2 will be useful for the study of the biological effects of caspase family proteins in PCD.

Effects of excitatory amino acids on neuromuscular development in the chick embryo

To investigate the presumptive role of excitatory amino acids (EAAs) in the regulation of normally occurring motoneuron (MN) death, chick embryos were treated with the glutamate receptor antagonists dizocilpine maleate and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium. Both failed to alter the number of surviving MNs at the end of the critical period of programmed cell death. However, treatment with 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, a competitive N-methyl-D-aspartic acid (NMDA) receptor antagonist, was able to rescue a significant number of MNs from death. Treatment with several EAA agonists induced extensive excitotoxic lesions in the spinal cord. MN degeneration induced by excitotoxins exhibited changes characteristic of necrosis rather than apoptosis. However, when either 0.5 or 1 mg of NMDA was applied acutely on embryonic day (E) 7, about 50% of treated embryos failed to exhibit NMDA-induced excitoxicity but rather showed a clear reduction in the number of pyknotic MNs. This apparent neuroprotective effect of NMDA was also observed in a subset of embryos chronically treated with NMDA, in which an excessive number of MNs was detected when examined on E9. Surprisingly, in the same experiment other embryos showed either normal or highly reduced MN numbers. Embryos with motoneuronal depletion induced by NMDA also showed a delayed impairment of later neuromuscular development with the appearance of degenerative changes in surviving MNs and apoptosis of skeletal muscle cells. Because some of the alterations reported here are similar to those described in MN diseases, our experimental model may be useful for gaining insights into the mechanisms that control both developmentally regulated and pathological MN death.

Neurotrophin-3 promotes the differentiation of muscle spindle afferents in the absence of peripheral targets

The neurons of the dorsal root ganglia (DRG) that supply muscle spindles require target-derived factors for survival. One necessary factor for these neurons is neurotrophin-3 (NT3). To determine whether NT3 can promote the survival of these neurons in the absence of other target-derived factors, we analyzed the effects of exogenous NT3 after early limb bud deletion in the chick. In control embryos, limb bud deletion eliminated approximately 90% of the trkC-positive (trkC+) neurons in lumbar DRG on the deleted side. In addition, the deletion led to a dramatic loss of collateral sensory projections to motoneurons. Exogenous NT3 restored a normal population of trkC+ neurons in lumbar DRG on the deleted side and increased the number of trkC+ neurons in DRG with normal targets (contralateral lumbar and thoracic). The effect was highly selective; NT3 increased the number of trkC+ neurons without significantly changing the number of either trkA+ or trkB+ neurons. The effect of NT3 was attributable to the rescue of DRG neurons from cell death, because exogenous NT3 reduced the number of pyknotic nuclei without significantly altering proliferation. Analysis of spinal projections showed further that many of the trkC+ neurons rescued by NT3 projected to the ventral spinal cord. These neurons thus had central projections characteristic of muscle spindle afferents. Together, our results indicate that NT3 signaling is both necessary and sufficient for the development of the proprioceptive phenotype, even in the absence of other signals from limb muscle.

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

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

Target contact regulates expression of synaptotagmin genes in spinal motor neurons in vivo

During neuromuscular development, neuronal contact with peripheral targets is associated with an increase in synaptic vesicle protein (SVP) gene expression, suggesting that target contact and upregulation of SVP genes are causally related. To test this idea, we analyzed the developmental expression pattern of synaptotagmin (syt) mRNAs in the chick lateral motor column (LMC) using in situ hybridization. Syt I mRNA in the LMC is upregulated from Embryonic Day 4.5 (E4.5) to E5.5, coincident with the time these neurons begin to make contact with their muscle targets. In contrast, levels of mRNA for neurofilament do not change during this time. Extirpation of the limb bud prior to motor axon outgrowth eliminates the increase in syt I mRNA ipsilaterally. Later in development, there is a switch in syt isoform abundance in the LMC, with syt II mRNA being upregulated between E15 and E20 and syt I mRNA being downregulated. Our results suggest that contact with targets upregulates syt I gene expression during neuromuscular synapse formation in vivo, and that a later stage of synaptic maturation involves changes in SVP isoform abundance.

Regulation of spinal motoneuron survival by GDNF during development and following injury

During normal development of many vertebrate species, substantial numbers of neurons in the central and peripheral nervous system undergo naturally occurring (or programmed) cell death. For example, approximately 50% of spinal motoneurons degenerate and die at a time when these cells are establishing synaptic connections with their target muscles in the chick, mouse, rat, and human. It is generally thought that the survival of developing motoneurons depends on access to trophic molecules. Motoneurons that survive the period of programmed cell death may also die following injury in the developing or adult animal. Increasing evidence suggests that glial-cell-line-derived neurotrophic factor (GDNF) plays a physiological and/or pharmacological role in the survival of various neuronal cell types, including motoneurons. In this paper, we review the survival and growth-promoting effects of GDNF on spinal motoneurons during the period of programmed cell death and following injury.

Altered development of spinal cord in the mouse mutant (Patch) lacking the PDGF receptor alpha-subunit gene

The platelet-derived growth factor receptor alpha subunit (PDGFR alpha) is expressed by glial precursors, glial cells, and some peripheral neurons during normal rodent development. Its ligands are expressed ubiquitously in neurons, including sensory and motor neurons. Thus, neuronally secreted PDGF-A may play a paracrine role in the development of both glial cells and peripheral neurons. The Patch (Ph) mutation, which is a deletion of the PDGFR alpha, is a homozygous embryonic lethal mutation in the mouse. Previously, several developmental abnormalities, including deficiencies in connective tissues in many organs, aberrant neural crest cell migration, and defects in non-neuronal derivatives of crest cells, have been shown to be associated with the Patch mutation. However, whether and the extent to which motor and sensory neurons are affected by the mutation are not known. Here, we have examined the survival and/or morphological differentiation of spinal motor and sensory (dorsal root ganglion) neurons during the period of naturally occurring cell death, i.e., between E14 and E18, in control and Ph/Ph mice. The results show a 65-70% decrease in motor and sensory neuron numbers in Ph/Ph mice, compared to controls, at all stages examined. Furthermore, motoneurons in Ph/Ph mice were significantly smaller than those in controls. Because of the bidirectional nature of neuron-glial cell interactions, these results suggest that PDGFR alpha plays an important role in glial cell development and, thus, indirectly in neuronal cell development or, alternatively, that PDGF and the PDGFR alpha are directly involved in peripheral neuron survival and development by an autocrine/paracrine mechanism.

Programmed cell death in the developing nervous system

Virtually all cell populations in the vertebrate nervous system undergo massive "naturally-occurring" or "programmed" cell death (PCD) early in development. Initially neurons and glia are overproduced followed by the demise of approximately one-half of the original cell population. In this review we highlight current hypotheses regarding how large-scale PCD contributes to the construction of the developing nervous system. More germane to the theme of this symposium, we emphasize that the survival of cells during PCD depends critically on their ability to access "trophic" molecular signals derived primarily from interactions with other cells. Here we review the cell-cell interactions and molecular mechanisms that control neuronal and glial cell survival during PCD, and how the inability of such signals to suppress PCD may contribute to cell death in some diseases such as spinal muscular atrophy. Finally, by using neurotrophic factors (e.g. CNTF, GDNF) and genes that control the cell death cascade (e.g. Bcl-2) as examples, we underscore the importance of studying the mechanisms that control neuronal and glial cell survival during normal development as a means of identifying molecules that prevent pathology-induced cell death. Ultimately this line of investigation could reveal effective strategies for arresting neuronal and glial cell death induced by injury, disease, and/or aging in humans.

The concept of uptake and retrograde transport of neurotrophic molecules during development: history and present status

In the present review honoring Hans Thoenen's contributions to the concept of uptake and retrograde transport of trophic molecules, I have attempted to identify the major historical pathways that had to converge before this concept could be accepted as a fundamental principle in neurobiology. Some of the critical events in this history which are discussed here include: neuron-target interactions, bidirectional trophic signals, axoplasmic transport, receptor-mediated endocytosis, transneuronal trophic signals, the discovery of NGF, the retrograde transport of NGF, and the production of NGF by target tissues. Only when all of these diverse pieces of the puzzle were in place was the concept finally confirmed as being the mechanism that mediates the many phenomena attributed to the regulation and maintenance of neurons by their targets.

Schwann cell apoptosis during normal development and after axonal degeneration induced by neurotoxins in the chick embryo

In the present work, we show that chick embryo Schwann cells die by apoptosis both during normal development and after axonal degeneration induced by neurotoxin treatment. Schwann cell apoptosis during development takes place during a period roughly coincidental with normally occurring motoneuron death. Administration of NMDA to chick embryos on embryonic day 7 induces extensive excitotoxic motoneuronal damage in the spinal cord without any apparent effects on neurons in the dorsal root ganglia (DRG). The death of Schwann cells in ventral nerve roots after NMDA treatment causes degenerative changes that display ultrastructural features of apoptosis and exhibit in situ detectable DNA fragmentation. By contrast, NMDA treatment does not increase the death of Schwann cells in dorsal nerve roots. In situ detection of DNA fragmentation in combination with the avian Schwann cell marker 1E8 antibody demonstrates that dying cells in ventral nerve roots are in the Schwann cell lineage. Administration of cycloheximide does not prevent the toxic effects of NMDA on motoneurons, but dramatically reduces the number of pyknotic Schwann cells and DNA fragmentation profiles in the ventral nerve roots. In ovo administration of various tissue extracts (muscle, brain, and spinal cord) from the chick embryo or of the motoneuron conditioned medium fails to prevent Schwann cell apoptosis in NMDA-treated embryos. Intramuscular administration of the snake toxin beta-bungarotoxin produces a massive death of both lateral motor column motoneurons and DRG neurons, resulting in a substantial increase in the number of pyknotic Schwann cells in both ventral and dorsal nerve roots. It is concluded that during development, axonal-derived trophic signals are involved in the regulation of Schwann cell survival in peripheral nerves.

The paralysé mouse mutant: a new animal model of anterior horn motor neuron degeneration

The survival and morphometric characteristics of lumbar spinal motoneurons were examined in the paralysé mouse mutant. Affected (par/par) mice can be first recognized at approximately postnatal day (PN) 7 to 8 and are characterized by their smaller-than-normal body size, a progressive generalized muscle weakness, and lack of coordination. Mutant mice die by PN16-18, when they have become almost completely paralyzed. Previously, we have shown that this mutation involves alteration of several developmental aspects of the neuromuscular system. However, whether ventral (or anterior) horn motoneurons degenerate and die during the course of the disease was unknown. We report here that at the time the mutant phenotype can be first identified (i.e. approximately PN8), lumbar motoneuron numbers in the lateral motor column of the spinal cord of paralysé mice were not significantly different from those of control littermates. In contrast, by PN14, there was a significant (30 to 35%) decrease in motoneuron numbers in mutant compared to control mice. Furthermore, motoneuron (nuclear and soma) sizes were significantly decreased in the mutants at both stages examined, i.e. PN8 and PN14. These results show that the paralysé mutation involves atrophy and subsequent death of anterior horn motoneurons. Together with the rapid progression and the severity of the disease, these results suggest that the paralysé mouse may represent a good animal model for studying early-onset human motor neuron diseases such as spinal muscular atrophy.

A novel type of programmed neuronal death in the cervical spinal cord of the chick embryo

We examined the massive early cell death that occurs in the ventral horn of the cervical spinal cord of the chick embryo between embryonic days 4 and 5 (E4 and E5). Studies with immunohistochemical, in situ hybridization, and retrograde-tracing methods revealed that many dying cells express Islet proteins and Lim-3 mRNA (motoneuron markers) and send their axons to the somatic region of the embryo before cell death. Together, these data strongly suggest that the dying cells are somatic motoneurons. Cervical motoneurons die by apoptosis and can be rescued by treatment with cycloheximide and actinomycin D. Counts by motoneuron numbers between E3.5 and E10 revealed that, in addition to cell death between E4 and E5, motoneuron death also occur between E6 and E10 in the cervical cord. Studies with [3H]thymidine autoradiography and morphological techniques revealed that in the early cell-death phase (E4-E5), genesis of motoneurons, axonal elongation, and innervation of muscles is still ongoing. However, studies with [3H]thymidine autoradiography also revealed that the cells dying between E4 and E5 become postmitotic before E3.5. Increased size of peripheral targets, treatment with neuromuscular blockade, and treatment with partially purified muscle or brain extracts and defined neurotropic agents, such as NGF, BDNF, neurotrophin-3, CNTF, bFGF, PDGF, S100-beta, activin, cholinergic differentiation factor/leukemia inhibitory factor, bone morphogenetic protein-2, IGF-I, interleukin-6, and TGF-beta 1, were all ineffective in rescuing motoneurons dying between E4 and E5. By contrast, motoneurons that undergo programmed cell death at later stages (E6-E10) in the cervical cord are target-dependent and respond to activity blockade and trophic factors. Experimental approaches revealed that early cell death also occurs in a notochord-induced ectopic supernumerary motoneuron column in the cervical cord. Transplantation of the cervical neural tube to other segmental regions failed to alter the early death of motoneurons, whereas transplantation of other segments to the cervical region failed to induce early motoneuron death. These results suggest that the mechanisms that regulate motoneuron death in the cervical spinal cord between E4 and E5 are independent of interactions with targets. Rather, this novel type of cell death seems to be determined by signals that either are cell-autonomous or are derived from other cells within the cervical neural tube.