Axotomy-induced neuronal death during development

The invulnerability of adult neurons: a critical role for p73

Here, we investigated the intracellular mechanisms that underlie the relative invulnerability of adult versus developing dorsal root ganglion (DRG) sensory neurons. In culture, adult neurons were resistant to stimuli that caused apoptosis of their neonatal counterparts. In both adult and neonatal neurons, death stimuli induced the apoptotic c-Jun N-terminal protein kinase (JNK) pathway, but JNK activation only caused death of neonatal neurons, indicating that adult neurons have a downstream block to apoptosis. Expression of the dominant-inhibitory p53 family member, DeltaNp73, rescued JNK-induced apoptosis of neonatal neurons, suggesting that it might participate in the downstream apoptotic block in adult neurons. To test this possibility, we examined adult DRG neurons cultured from p73+/- mice. Adult p73+/- DRG neurons were more vulnerable to apoptotic stimuli than their p73+/+ counterparts, and invulnerability could be restored to the p73+/- neurons by increased expression of DeltaNp73. Moreover, although DRG neuron development was normal in p73+/- animals in vivo, axotomy caused death of adult p73+/- but not p73+/+ DRG neurons. Thus, one way adult neurons become invulnerable is to enhance endogenous survival pathways, and one critical component of these survival pathways is p73.

Human neural stem cell-derived cholinergic neurons innervate muscle in motoneuron deficient adult rats

Motoneuron damage occurs in spinal cord injury and amyotrophic lateral sclerosis. Current advances offer hope that human embryonic stem cells [Science 282 (1998) 1145] or neural stem cells (NSC) [Exp Neurol 161 (2000) 67; Exp Neurol 158 (1999) 265; J Neurosci Methods 85 (1998) 141; Proc Natl Acad Sci USA 97 (2000) 14720; Exp Neurol 156 (1999) 156 ] may be donors to replace lost motoneurons. Previously, we developed a priming procedure that produced cholinergic cells that resemble motoneurons from human NSCs grafted into adult rat spinal cord [Nat Neurosci 5 (2002a) 1271]. However, effective replacement therapy will ultimately rely on successful connection of new motoneurons with their muscle targets. In this study, we examined the potential of human fetal NSC transplantation to replace lost motoneurons in an animal model of chronic motoneuron deficiency (newborn sciatic axotomy) [J Comp Neurol 224 (1984) 252; J Neurobiol 23 (1992) 1231]. We found, for the first time, that human neural stem cell-derived motoneurons send axons that pass through ventral root and sciatic nerve to form neuromuscular junctions with their peripheral muscle targets. Furthermore, this new cholinergic innervation correlates with partial improvement of motor function.

Administration of Hsp70 in vivo inhibits motor and sensory neuron degeneration

The induction of heat shock proteins (Hsps) serves not only as a marker for cellular stress but also as a promoter of cell survival, which is especially important in the nervous system. We examined the regulation of the constitutive and stress-induced 70-kD Hsps (Hsc70 and Hsp70, respectively) after sciatic nerve (SN) axotomy in the neonatal mouse. Additionally, the prevention of axotomy-induced SN cell death by administration of several preparations of exogenous Hsc70 and Hsp70 was tested. Immunohistochemistry and Western blot analyses showed that endogenous levels of Hsc70 and Hsp70 did not increase significantly in lumbar motor neurons or dorsal root ganglion sensory neurons up to 24 hours after axotomy. When a variety of Hsc70 and Hsp70 preparations at doses ranging from 5 to 75 microg were applied to the SN stump after axotomy, the survival of both motor and sensory neurons was significantly improved. Thus, it appears that motor and sensory neurons in the neonatal mouse do not initiate a typical Hsp70 response after traumatic injury and that administration of exogenous Hsc/Hsp70 can remedy that deficit and reduce the subsequent loss of neurons by apoptosis.

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

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

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.

Cortical interneurons become activated by deafferentation and instruct the apoptosis of pyramidal neurons

Unlike peripheral nervous system neurons and certain groups of nerve cells in the CNS, cortical projection neurons are tolerant of axonal lesions. This resistance is incongruent with the massive death of pyramidal neurons in age-associated neurodegenerative diseases that proceed along corticocortical connections. Some insights have emerged from our previous work showing that pyramidal cells in piriform cortex undergo classical apoptosis within 24 h after bulbectomy via transsynaptic, but not retrograde, signaling. These findings allow the investigation of cellular and molecular changes that take place in the context of experimental cortical degeneration. In the present study, we show that the transsynaptic death of pyramidal neurons in piriform cortex is a nitric oxide-mediated event signaled by activated interneurons in layer I. Thus, we demonstrate that cortical interneurons play an essential role in transducing injury to apoptotic signaling that selectively targets pyramidal neurons. We propose that this mechanism may be generic to cortical degenerations and amenable to therapeutic interventions.

Adult neuron survival strategies--slamming on the brakes

Developing neurons are programmed to die by an apoptotic pathway unless they are rescued by extrinsic growth factors that generate an anti-apoptotic response. By contrast, adult neurons need to survive for the lifetime of the organism, and their premature death can cause irreversible functional deficits. The default apoptotic pathway is shut down when development is complete, and consequently growth factors are no longer required to prevent death. To protect against accidental apoptotic cell death, anti-apoptotic mechanisms are activated in mature neurons in response to stress. Loss or reduced activity of these intrinsic anti-apoptotic 'brakes' might contribute to or accelerate neurodegeneration, whereas their activation might rescue neurons from injury or genetic abnormalities.

Comparative analysis of genomic HSV vectors for gene delivery to motor neurons following peripheral inoculation in vivo

The use of viral vectors for gene delivery to motor neurons in vivo has been hampered by the need to perform invasive surgery to inject directly the vector into the anterior horn of the spinal cord. Here, we have characterized the features of herpes simplex virus-1 (HSV)-derived vectors, in terms of gene mutations and promoter constructs, that are required to allow efficient transduction of motor neurons following a relatively noninvasive peripheral administration via sciatic nerve or footpad injection. Owing to the wide variety of animal models used to study neurodegenerative diseases of motor neurons, we analysed the effectiveness of these vectors in adult mice and adult and neonatal rats. We tested viruses with differing degrees of disablement based on the 1764 backbone (deleted for ICP34.5 and an insertional inactivation in VP16) rendered completely replication incompetent by the deletion of the essential immediate-early genes ICP27 and/or ICP4. In the adult mouse, prolonged gene expression in motor neurons was obtained after sciatic nerve inoculation with a vector defective in ICP4 and ICP27. In the adult rat, both the vector defective in ICP4 and the vector defective in ICP4 and ICP27 were capable of transducing motor neurons for extended periods of time during viral latency. This study demonstrates the feasibility of using HSV vectors for persistent transgene expression in motor neurons in a safe and nontoxic manner following peripheral administration. These vectors are potentially useful tools to investigate the functions of genes involved in motor neuronal survival and regeneration in models of motor neuron diseases in vivo.

T-588 protects motor neuron death following axotomy

R(-)-1-(benzo [b] thiophen-5-yl)-2-[2-(N,N-diethylamino)ethoxy] ethanol hydrochloride) (T-588) enhances acetylcholine release. This compound slows the motor deterioration of wobbler mouse motor neuron disease and enhances neurite outgrowth and choline acetyltransferase activity in cultured rat spinal motor neurons. We examined the ability of T-588 on axotomized spinal motor neuron death in the rat spinal cord. After the postnatal unilateral section of sciatic nerve, there was approximately a 50% survival of motor neurons in the fourth lumbar segment. In comparison with vehicle, intraperitoneal injection of T-588 for 14 consecutive days rescued spinal motor neuron death. Our results showing in vivo neurotrophic activity of T-588 for motor neurons support the applicability of T-588 for the treatment of motor neuron diseases, such as amyotrophic lateral sclerosis and motor neuropathies.

Regeneration of supraspinal axons after complete transection of the thoracic spinal cord in neonatal opossums (Monodelphis domestica)

These studies define the time table and origin of supraspinal axons regenerating across a complete spinal transection in postnatal Monodelphis domestica. After lumbar (L1) spinal cord injection of fluorophore-dextran amine conjugate on postnatal (P) day 4, a consistent number of neurons could be labeled. The numbers of labeled neurons remained stable for several weeks, but subsequently declined by P60 in control animals and by P35 in animals with complete spinal transection (T4-T6) performed at P7. In control animals, 25-40% of neurons labeled with a fluorophore injected (L1) at P4 could also be double-labeled by a second fluorophore injected (T8-T10) at different older ages. In spinally transected animals, total numbers of neurons labeled with the second marker were initially lower compared with age-matched controls, but were not significantly different by 3 weeks after injury. The proportion of double-labeled neurons in spinally transected animals increased from approximately 2% 1 week after injury (P14) to approximately 50% by P60, indicating that a substantial proportion of neurons with axons transected at P7 is able to regenerate and persist into adulthood. However, the proportion of axons originating from regenerating neurons made only a small contribution at older ages to total numbers of fibers growing through the injury site, because much of development of the spinal cord occurs after P7. Evidence was obtained that degenerating neurons with both apoptotic and necrotic morphologies were present in brainstem nuclei; the number of neurons with necrotic morphology was much greater in the brainstem of animals with spinal cords transected at P7.

Regenerative process of the facial nerve: rate of regeneration of fibers and their bifurcations

After the main trunk of the mouse facial nerve was injured by crushing, a fiber tracing method was used to quantify the facial motor neurons that extended regenerating nerve fibers to the specific site of the facial nerve branch. The total number of motor neurons retrogradely labeled with a fluorescent tracer, Fluoro-Gold (FG), were 0 on postsurgical days (PSDs) 1 and 2, 75+/-25 on PSD3, 264+/-21 on PSD4, 378+/-19 on PSD6, 428+/-19 on PSD8, 491+/-13 on PSD12 and 532+/-15 on PSD16. Assuming that the FG-positive neurons (535+/-11) of the control mice represent 100%, the FG-labeled neurons accounted for 0, 14, 49, 71, 80, 92 and 99% on the corresponding days. Two different fluorescent tracers were applied to the different facial nerve branches 16 days after facial nerve injuries. Double-labeled neurons were consistently found in the nerve-crushed facial nucleus (3.2%), and their number increased in the nerve-transected facial nucleus (12.2%). The present study indicates that the regenerating facial nerve consists of heterogeneous nerve fibers with varying growth rates and that excessive axonal branching occurs more frequently in the nerve-transected than in the nerve-crushed injuries.

Great potentiality of neonatal facial motor neurons for neural plasticity as determined by functionally essential neuronal population

The present study was undertaken to determine the neuronal population essential for normal and minimal facial function of young adult rats that had received various degrees of crush injuries to the facial nerve in the neonatal period. Using a neuronal tracer, it was found in young adult rats receiving neonatal injuries that the minimum number of tracer-labeled facial motor neurons necessary for normal facial function corresponded to 13-14% of the neurons (2540+/-64) of the age-matched control animals, whereas the minimum number of neurons necessary for minimal facial function corresponded to 5%. On the other hand, the minimum numbers of tracer-labeled facial motor neurons necessary for normal and minimal facial function of young adult rats that received various degrees of crush injuries corresponded to 61 and 27-30%, respectively, of the neurons (2540+/-64) of the uninjured control animals. These results indicate that the facial function of animals with nerves crushed at the neonatal stage can be adequately maintained by a very small population of neurons, implying a great potential of neonatal neurons for neural plasticity.

Sciatic nerve axotomy in aged rats: response of motoneurons and the effect of GM1 ganglioside treatment

The number, size, and staining intensity of choline acetyltransferase (ChAT)-immunopositive cells in the retrodorsal lateral nucleus (RDLN) of the spinal cord were studied in young (3-5 months old) and aged (22-24 months old) rats following left sciatic nerve distal transection (axotomy) and treatment with GM1 ganglioside. The cell size and the ChAT immunostaining density were decreased in the RDLN of non-manipulated as well as in the contralateral intact side of axotomized aged rats. Axotomy had no effect on the number of RDLN motoneurons in both aged and young rats. In the young rats, there was a decrease in the size of motoneurons 7 days post-axotomy and a partial spontaneous recovery occurred by 21 days. Axotomy did not reduce further the size of aged motoneurons, however. The ChAT staining intensity of the axotomized RDLN declined in both age groups after 7 days, and there was spontaneous near normal recovery by 21 days. In the aged rats, GM1 administration for 7 days corrected the cell size and ChAT immunoreactivity of the contralateral intact RDLN. With regard to axotomized RDLN neurons, 7 days of GM1 restored the cell size but not the ChAT immunostaining in young animals. The same treatment schedule, however, corrected both cell size and staining in aged rats. Administration of GM1 for 21 days had no further effect on the morphometric parameters of the axotomized motoneurons in aged rats, but slightly enhanced the recovery of ChAT immunostaining in young rats. Thus, it appears that GM1 facilitates the phenotypic recovery of RDLN motoneurons during aging and after axotomy.

Regeneration of periodontal Ruffini endings in adults and neonates

We reviewed the regeneration of periodontal Ruffini endings, primary mechanoreceptors in the periodontal ligament, following injury to the inferior alveolar nerve (IAN) in adult and neonatal rats. Morphologically, mature Ruffini endings are characterized by an extensive arborization of axonal terminals and association with specialized Schwann cells, called lamellar or terminal Schwann cells. Following injury to IAN in the adult, the periodontal Ruffini endings of the rat lower incisor ligament regenerate more rapidly than Ruffini endings in other tissues. During regeneration, terminal Schwann cells migrate into regions where they are never found under normal conditions. The development of periodontal Ruffini endings of the rat incisor is closely associated with the eruption of the teeth; the morphology and distribution of the terminal Schwann cells became almost identical to those in adults during postnatal days 15-18 (PN 15-18d) when the first molars appear in the oral cavity, while the axonal elements showed extensive ramification around PN 28d when the functional occlusion commences. When the IAN was injured in neonates, the regeneration of periodontal Ruffini endings was delayed compared with the adults. The migration of terminal Schwann cells is also observed following IAN injury, after which the distribution of terminal Schwann cells became almost identical to that of the adults, i.e., PN 14d. Since the interaction between axon and Schwann cell is important during regeneration and development, further studies are required to elucidate its molecular mechanism during the regeneration as well as the development of the periodontal Ruffini endings.

Expression of nitric oxide synthase and 27-kD heat shock protein in motor neurons of ventral root-avulsed rats

Root avulsion of adult spinal nerves causes the subacute cell loss of motor neurons. To explore the mechanisms of the elimination of motor neurons, we investigated the expression of two molecules--neuronal nitric oxide synthase (nNOS) as a cytotoxity marker and a 27-kD heat shock protein (HSP27) as a cytoprotection marker--in rat spinal motor neurons after ventral root avulsion, using immunofluorescent labeling technique for confocal laser microscopy. A drastic cell loss of motor neurons occurred during the first week following the avulsion, and the surviving motor neurons fell to approximately 60% of the control value at one week. Subsequent cell loss proceeded slowly, as the surviving motor neurons decreased to 35% at nine weeks. HSP27 immunohistochemistry showed that normal spinal motor neurons consisted of two types of motor neurons: HSP27-negative small motor neurons (< 500 micrometer2 ) (about 30%), and HSP27-positive large motor neurons (> 500 micrometer2) (about 70%). At one week, all of the HSP27-negative small motor neurons had died and only HSP27-positive large motor neurons survived. This event was followed by the induction of nNOS in the surviving large motor neurons, which showed a significant upregulation of HSP27. HSP27-negative small motor neurons were thus found to be more vulnerable to avulsion than HSP27-positive large motor neurons, suggesting that HSP27 may have protected the avulsed motor neurons from cell death. In addition, NO was involved in the gradual cell death of large motor neurons. The persistent upregulation of HSP27 and its colocalization with nNOS in surviving motor neurons may imply a keen competition in motor neuron survival between cytotoxic and cytoprotective systems.

Constitutive expression of the low-affinity neurotrophin receptor and changes during axotomy-induced death of sensory neurones in the neonatal rat dorsal root ganglion

Sensory neurones in the dorsal root ganglion (DRG) of the neonatal rat express the 75-kDa low-affinity neurotrophin receptor (p75NTR) and these neurones degenerate rapidly after axotomy. p75NTR belongs to the tumour necrosis factor superfamily, several members of which have a role in cell death and it is constitutively expressed within a subpopulation of DRG neurones. p75NTR has been implicated in mediating the degeneration of these neurones after axotomy. In this study, we characterize the expression of p75NTR in sensory neurones of the newborn rat DRG using immunohistochemistry. Furthermore, we investigate the change in constitutive expression pattern of p75NTR in these neurones following axotomy. In the C7 and C8 DRG of the newborn rat, p75NTR is expressed in approximately 70% of DRG neurones. Those expressing p75NTR can be classified into subpopulations with moderate or intense p75NTR expression, each present in approximately equal proportions. Whilst p75NTR expression is observed in neurones throughout the entire neuronal diameter range, a correlation exists between neuronal diameter and p75NTR expression intensity. We also found that the most vulnerable population following axotomy were those sensory neurones which constitutively express the highest levels of p75NTR, i.e. the large-diameter neurones.

Neuroprotective actions of FK506 and cyclosporin A on motor neuron survival following neonatal axotomy

We show that nonimmunosuppressive analogues of the immunosuppressive drugs FK506 and cyclosporin A (CsA) rescue axotomized neonatal motor neuron death. Unilateral sciatic nerve was transected in neonatal rats. Animals were then treated daily with different doses of FK506 and CsA for 14 days with intraperitoneal injection. Control rats received phosphate buffer saline (PBS) in the same fashion. After treatment, the number of spinal motor neurons was determined at L4 level. In comparison with vehicle, both FK506 (5.0 mg kg(-1)) and CsA (10.0 mg kg(-1)) rescued motor neuron death in a similar way. These results indicate therapeutic relevance in the treatment of damaged motor neuron disorders, such as motor neuropathy or amyotrophic lateral sclerosis.

Developmental alteration of nerve injury induced glial cell line-derived neurotrophic factor (GDNF) receptor expression is crucial for the determination of injured motoneuron fate

Axotomy-induced neuronal death occurs in neonatal motoneurons, but not in adult rat. Here we demonstrated that during the course of postnatal development, nerve injury induced down-regulation of the glial cell line-derived neurotrophic factor (GDNF) receptor GFRalpha1 in axotomized hypoglossal motoneurons of rat are gradually converted to the adult up-regulation pattern of response. The compensatory expression of GFRalpha1 specifically in the injured motoneurons of neonates by adenovirus succeeded in rescuing the injured neurons without an application of growth factors. To the contrary, the nuclear antisense RNA for GFRalpha1 expression accelerates the axotomy-induced neuronal death in pups. These findings suggest that the receptor expression response after nerve injury is critical for the determination of injured motoneuron fate.

On the use of fast blue, fluoro-gold and diamidino yellow for retrograde tracing after peripheral nerve injury: uptake, fading, dye interactions, and toxicity

The usefulness of three retrograde fluorescent dyes for tracing injured peripheral axons was investigated. The rat sciatic was transected bilaterally and the proximal end briefly exposed to either Fast Blue (FB), Fluoro-Gold (FG) or to Diamidino Yellow (DY) on the right side, and to saline on the left side, respectively. The nerves were then resutured and allowed to regenerate. Electrophysiological tests 3 months later showed similar latencies and amplitudes of evoked muscle and nerve action potentials between tracer groups. The nerves were then cut distal to the original injury and exposed to a second (different) dye. Five days later, retrogradely labelled neurones were counted in the dorsal root ganglia (DRGs) and spinal cord ventral horn. The number of neurones labelled by the first tracer was similar for all three dyes in the DRG and ventral horn except for FG, which labelled fewer motoneurones. When used as second tracer, DY labelled fewer neurones than FG and FB in some experimental situations. The total number of neurones labelled by the first and/or second tracer was reduced by about 30% compared with controls. The contributions of cell death as well as different optional tracer combinations for studies of nerve regeneration are discussed.

There is no loss of motor neurons in the rat spinal cord during postnatal maturation

Motor neurons are lost during embryonic development, but it remains controversial whether motor neuron cell death occurs during postnatal life. In this study we investigated the effect of postnatal maturation on the number of intact spinal motor neurons in the rat using retrograde labelling with model-based counting, and an unbiased stereological counting technique. To determine the number of motor neurons innervating a specific forelimb muscle in rats of different postnatal ages FluoroGold was injected into the flexor carpi radialis. Before postnatal day 21 there were higher numbers of retrogradely labelled motor neurons than in adult rats, suggesting a 'loss' with postnatal maturation. This loss may be attributed to tracer diffusion to adjacent muscles and to the permeability of the muscle spindle capsule in younger animals. To obtain an unbiased estimate of the number of motor neurons in the C7 and C8 segments of the postnatal rat cervical spinal cord the fractionator/optical disector counting technique was used. This method did not show a loss of spinal motor neurons between birth and adulthood. The main conclusion from this study is that there is no loss of spinal motor neurons during postnatal maturation.

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.

Reinnervation of the biceps brachii muscle following cotransplantation of fetal spinal cord and autologous peripheral nerve into the injured cervical spinal cord of the adult rat

In order to compensate the loss of motoneurons resulting from severe spinal cord injury and to reestablish peripheral motor connectivity, solid pieces of fetal spinal cord, taken from embryonic day 14 rat embryos, were transplanted into unilateral aspiration lesions of the cervical spinal cord of adult rats. Concomitantly, one end of a 3.5-cm autologous peripheral nerve graft was put in close contact with the embryonic graft; the other end was sutured to the distal stump of the musculocutaneous nerve which innervate the biceps brachii muscle. The animals were examined 3 and 6 months after surgery. Following intramuscular injection of horseradish peroxidase, retrograde axonal labeling studies indicated that both transplanted and host spinal neurons were able to extend axons all the way through the peripheral nerve graft and nerve stump, up to the reconnected muscles. The labeled cells in the transplant were generally observed close to the intraspinal tip of the peripheral nerve graft. Retrograde axonal tracing, as well as electrophysiological and histological data, demonstrated the sensory and motor reinnervation of the reconnected muscles. This muscular reinnervation was able to reverse the atrophic changes observed in the denervated muscle. In control experiments, the extraspinal end of the peripheral nerve graft was ligatured in order to compare the differentiation of the transplanted neurons and the survival of their growing axons with or without their muscular targets. Six months after both types of surgery, large-size grafted neurons, identified as motoneurons by immunocytochemistry for peripherine and calcitonin gene-related peptide, were only observed in fetal spinal cord transplants which were connected to denervated muscles, thus demonstrating the trophic influence of the muscle target on the survival and differentiation of the transplanted neurons and on the maintenance of the axons they had grown into the peripheral nerve graft.

Regeneration of descending spinal axons after transection of the thoracic spinal cord during early development in the North American opossum, Didelphis virginiana

Opossums are born in an immature, fetal-like state, making it possible to lesion their spinal cord early in development without intrauterine surgery. When the thoracic spinal cord of the North American opossum, Didelphis virginiana, is transected on postnatal day 5, and injections of Fast Blue (FB) are made caudal to the lesion site 30-40 days or 6 months later, neurons are labeled in all of the spinal and supraspinal areas that are labeled after comparable injections in age-matched, unlesioned controls. Double-labeling studies document that regeneration of cut axons contributes to growth of axons through the lesion site and behavioral studies show that animals lesioned on postnatal day 5 use their hindlimbs in normal appearing locomotion as adults. The critical period for developmental plasticity of descending spinal axons extends to postnatal day 26, although axons which grow through the lesion site become fewer in number and more restricted as to origin with increasing age. Animals lesioned between postnatal day 12 and 26 use the hindlimbs better than animals lesioned as adults, but hindlimb function is markedly abnormal and uncoordinated with that of the forelimbs. We conclude that restoration of anatomical continuity occurs after transection of the spinal cord in developing opossums, that descending axons grow through the lesion site, that regeneration of cut axons contributes to such growth, and that animals lesioned early enough in development have relatively normal motor function as adults.

Soluble TNF receptors partially protect injured motoneurons in the postnatal CNS

There is accumulating evidence that cytokines are involved in the functioning of the brain and the spinal cord. However, it has been controversial whether they exert a neurotoxic or a neuroprotective effect. To address this question in vivo, we have examined the survival of injured motoneurons in a line of transgenic mice that overexpress the soluble form of tumour necrosis factor receptor-1 (sTNFR1). In these animals, all of the circulating TNF and lymphotoxin-alpha are neutralized by the continuous expression of the soluble receptor. Following axotomy of the facial nerve in 7-day-old control mice, we observed a loss of approximately 90% of the motoneurons at two weeks survival. In the transgenic mice under the same conditions, the percentage of motoneuron survival was increased two-fold (515 vs. 224) and varied as a function of the level of the circulating receptor. These results indicate that neutralization of endogenous TNF and lymphotoxin-alpha by means of overexpression of the soluble receptor can decrease cell death of injured motoneurons and suggest that these cytokines may play an important role in neuronal degeneration in the CNS following a lesion.

Nerve terminals form but fail to mature when postsynaptic differentiation is blocked: in vivo analysis using mammalian nerve-muscle chimeras

To better understand the role of the postsynaptic cell in the differentiation of presynaptic terminals, we transplanted muscles that lacked postsynaptic differentiation from mutant mice into normal adult immunocompatible hosts and attached the host nerve to the grafts. Host motor axons innervated wild-type grafted muscle fibers and established normal appearing chimeric neuromuscular junctions. By repeated in vivo imaging, we found that these synapses were stably maintained. Results were different when nerves entered transplanted muscles derived from mice lacking muscle-specific receptor tyrosine kinase (MuSK) or rapsyn, muscle-specific components required for postsynaptic differentiation. Initial steps in presynaptic differentiation (e.g., formation of rudimentary arbors and vesicle clustering at terminals) occurred when wild-type neurites contacted MuSK- or rapsyn deficient muscle fibers, either in vivo or in vitro. However, wild-type terminals contacting MuSK or rapsyn mutant muscle fibers were unable to mature, even when the chimeras were maintained for up to 7 months. Moreover, in contrast to the stability of wild-type synapses, wild-type nerve terminals in mutant muscles underwent continuous remodeling. These results suggest that postsynaptic cells supply two types of signals to motor axons: ones that initiate presynaptic differentiation and others that stabilize the immature contacts so that they can mature. Normal postsynaptic differentiation appears to be dispensable for initial stages of presynaptic differentiation but required for presynaptic maturation.

Complete and long-term rescue of lesioned adult motoneurons by lentiviral-mediated expression of glial cell line-derived neurotrophic factor in the facial nucleus

To date, delivery of neurotrophic factors has only allowed to transiently protect axotomized facial motoneurons against cell death. In the present report, long-term protection of these neurons was evaluated by continuously expressing the neurotrophic factor glial cell line-derived neurotrophic factor (GDNF) within the facial nucleus using a lentiviral vector system. The viral vector was injected unilaterally into the facial nucleus of 4-month-old Balb/C mice. In contrast to axotomy in other adult rodents, facial nerve lesion in these animals leads to a progressive and sustained loss and/or atrophy of >50% of the motoneurons. This model thus represents an attractive model to evaluate potential protective effects of neurotrophic factors for adult-onset motoneuron diseases, such as amyotrophic lateral sclerosis. One month after unilateral lentiviral vector injection, the facial nerve was sectioned, and the animals were killed 3 months later. Viral delivery of the GDNF gene led to long-term expression and extensive diffusion of GDNF within the brainstem. In addition, axotomized motoneurons were completely protected against cell death, because 95% of the motoneurons were present as demonstrated by both Nissl staining and choline acetyltransferase immunoreactivity. Furthermore, GDNF prevented lesion-induced neuronal atrophy and maintained proximal motoneuron axons, despite the absence of target cell reinnervation. This is the first evidence that viral-mediated delivery of GDNF close to the motoneuron cell bodies of the facial nucleus of adult mice can lead to complete and long-term protection against lesion-induced cell death.

Induction of reactive astrocytosis and prevention of motoneuron cell death by the I(2)-imidazoline receptor ligand LSL 60101

I(2)-imidazoline receptors are mainly expressed on glial cells in the rat brain. This study was designed to test the effect of treatment with the I(2)-imidazoline selective receptor ligand LSL 60101 [2-(2-benzofuranyl)imidazole] on the morphology of astrocytes in the neonate and adult rat brain, and to explore the putative neuroprotective effects of this glial response. Short-term (3 days) or chronic (7-10 days) treatment with LSL 60101 (1 mg kg(-1), i.p. every 12 h) enhanced the area covered by astroglial cells in sections of facial motor nucleus from neonate rats processed for glial fibrillary acidic protein (GFAP) immunostaining. Facial motoneurons surrounded by positive glial cell processes were frequently observed in sections of LSL 60101-treated rats. A similar glial response was observed in the parietal cortex of adult rats after chronic (10 days) treatment with LSL 60101 (10 mg kg(-1), i.p. every 12 h). Western-blot detection of the specific astroglial glutamate transporter GLT-1, indicated increased immunoreactivity after LSL 60101 treatment in the pons of neonate and in the parietoccipital cortex of adult rats. In the facial motor nucleus of neonate rats, the glial response after LSL 60101 treatment was associated to a redistribution of the immunofluorescence of the basic fibroblast growth factor (FGF-2) from the perinuclear area of motoneurons to cover most of their cytoplasm, suggesting a translocation of this mitogenic and neurotrophic factor towards secretion pathways. The neuroprotective potential of the above effects of LSL 60101 treatment was tested after neonatal axotomy of facial motor nucleus. Treatment with LSL 60101 (1 mg kg(-1), i.p. every 12 h from day 0 to day 10 after birth) significantly reduced (38%) motoneuron death rate 7 days after facial nerve axotomy performed on day 3 after birth. It is concluded that treatment with the I(2)-imidazoline selective receptor ligand LSL 60101 provokes morphological/biochemical changes in astroglia that are neuroprotective after neonatal axotomy.

Effects of neonatal injury of the inferior alveolar nerve on the development and regeneration of periodontal nerve fibers in the rat incisor

Our previous study showed that the migration of terminal Schwann cells occurred in the periodontal ligament of the rat lower incisor following transection of the inferior alveolar nerve (IAN) in the adult animals [Y. Atsumi, K. Matsumoto, M. Sakuda, T. Maeda, K. Kurisu, S. Wakisaka, Altered distribution of Schwann cells in the periodontal ligament of the rat incisor following resection of the inferior alveolar nerve: An immunohistochemical study on S-100 proteins, Brain Res. 849 (1999) 187-195]. The aim of the present study was to investigate the effects of neonatal transection of the IAN on the regeneration of axon elements and Schwann cells in the periodontal ligament of the rat lower incisor. Following transection of IAN at post-natal day 5 (PN 5d), when the numbers of both axon elements and the terminal Schwann cells were very small, regenerating nerve fibers appeared between post-injured days 7 (PO 7d) and PO 14d, and increased in number thereafter gradually. Although the terminal morphologies of regenerated Ruffini endings became identical to those of the adult animals by PO 54d, the number of regenerated PGP 9.5-IR nerve fibers did not recover the adult levels even by PO 56d. A small number of Schwann cells migrated into the shear zone, the border between the alveolus-related part (ARP) and the tooth-related part (TRP), but did not enter into the TRP. Following transection of the IAN at PN 14d or PN 28d, when clusters of apparent terminal Schwann cells could be recognized, axon regeneration started around PO 5d. Individual axon terminals of the regenerating Ruffini endings ramified and became identical to those of the adult animals around PO 28d, but the number of regenerated Ruffini endings was smaller than that of the adult animals. Similar to the adult animals, the migration of Schwann cells into the shear zone and TRP occurred, and disappeared prior to the completion of the axonal regeneration. The present results indicate that the migration of the Schwann cells into TRP during the regeneration of the periodontal nerve fibers following nerve injury to the IAN depends on the maturation of the terminal Schwann cells of the periodontal Ruffini endings, not on post-operative time.

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.

Comparative evaluation of cytokine profiles and reactive gliosis supports a critical role for interleukin-6 in neuron-glia signaling during regeneration

Using reverse transcription polymerase chain reaction (RT-PCR), we have studied the temporal expression of interleukin-1beta (IL-1beta), interleukin-6 (IL-6), transforming growth factor-beta 1 (TGF-beta 1), and tumor necrosis factor-alpha (TNF-alpha) mRNAs in three axotomy paradigms with distinct functional outcomes. Axotomy of adult rat facial motoneurons results in neuronal regeneration, axotomy of neonatal facial motoneurons results in neuronal apoptosis, and axotomy of rubrospinal neurons results in neuronal atrophy. Our RT-PCR findings show that a significant and sustained upregulation of IL-6 mRNA is associated uniquely with the regeneration of adult facial motoneurons. Histochemical studies using IL-6 immunohistochemistry show intense IL-6 immunoreactivity in axotomized adult facial motoneurons. Assessment of reactive glial changes with astroglial and microglial markers reveals that the reactive gliosis following adult facial nerve axotomy is more intense than that observed in either of the other two paradigms. Exposure of cultured microglial cells to IL-6 stimulates microglial proliferation in a dose-dependent manner. Cultured microglia also show expression of IL-6 receptor mRNA, as determined by RT-PCR. Our findings support the idea that reactive gliosis is required for neuron regeneration to occur, and more specifically, they suggest that neuron-derived IL-6 serves as a signalling molecule that induces microglial proliferation during motoneuron regeneration.

Similar poly(C)-sensitive RNA-binding complexes regulate the stability of the heavy and light neurofilament mRNAs

The potential role of RNA processing in regulating neurofilament (NF) subunit expression and in mediating the neuropathic effects of NF transgenes was explored by determining whether similar regulatory elements and cognate binding factors are present in NF mRNAs. Gel-shift studies were used to compare RNA-binding complexes that assemble on the 3'UTR of the heavy (NF-H), mid-sized (NF-M) and light (NF-L) NF mRNAs when radioactive RNA probes are incubated with high-speed supernatants (S100) of rat brain homogenates. RNA-binding complexes were characterized by their rate of migration in non-denaturing gels and by their ability to be competed with specific homoribopolymers. Similar RNA-binding complexes formed on probes to the 3'UTRs of NF-L and NF-H mRNAs. The complexes were competed with poly(C) and are referred to as poly(C)-sensitive complexes. Their binding sites were localized to a 36 nt sequence in the mid-distal region of the NF-H 3'UTR and to a 45 nt sequence at the proximal edge of the 3'UTR of the NF-L transcript. Although the binding sites showed limited sequence homology, the complexes were cross-competed with unlabeled probes and radioactivity in either probe was cross-linked to a 43 kDa protein. The 43 kDa protein also bound directly to NF-L and NF-H probes in Northwestern blots. Functional studies showed that deletion of the binding sites markedly increased expression of a luciferase reporter gene containing the 3'UTR of NF-L or NF-H by stabilizing the fusion transcripts. Point mutations in the NF-H binding site which prevented formation of the poly(C)-sensitive complex also stabilized the fusion mRNA. The findings reveal a common destabilizing element in the 3'UTR of NF-L and NF-H mRNAs that may be important in coordinating NF subunit expression and in mediating the neuropathic effects of the NF-L and NF-H transgenes in transgenic mice.

Inhibition of apoptotic signaling cascades causes loss of trophic factor dependence during neuronal maturation

During development, neurons are acutely dependent on target-derived trophic factors for survival. This dependence on trophic support decreases dramatically with maturation in several neuronal populations, including sympathetic neurons. Analyses of nerve growth factor deprivation in immature and mature sympathetic neurons indicate that maturation aborts the cell death pathway at a point that is mechanistically indistinguishable from Bax deletion. However, neither the mRNA nor protein level of BAX changes with neuronal maturation. Therefore, BAX must be regulated posttranslationally in mature neurons. Nerve growth factor deprivation in immature sympathetic neurons induces two parallel processes: (a) a protein synthesis-dependent, caspase-independent translocation of BAX from the cytosol to mitochondria, followed by mitochondrial membrane integration and loss of cytochrome c; and (b) the development of competence-to-die, which requires neither macromolecular synthesis nor BAX expression. Activation of both signaling pathways is required for caspase activation and apoptosis in immature sympathetic neurons. In contrast, nerve growth factor withdrawal in mature sympathetic neurons did not induce the translocation of either BAX or cytochrome c. Moreover, mature neurons did not develop competence-to-die with cytoplasmic accumulation of cytochrome c. Therefore, inhibition of both BAX-dependent cytochrome c release and the development of competence-to-die contributed to the loss of trophic factor dependence associated with neuronal maturation.

Akt/protein kinase B prevents injury-induced motoneuron death and accelerates axonal regeneration

Motoneurons require neurotrophic factors for their survival and axonal projection during development, as well as nerve regeneration. By using the axotomy-induced neuronal death paradigm and adenovirus-mediated gene transfer, we attempted to gain insight into the functional significances of major growth factor receptor downstream cascades, Ras-extracellular signal-regulated kinase (Ras-ERK) pathway and phosphatidylinositol-3 kinase-Akt (PI3K-Akt) pathway. After neonatal hypoglossal nerve transection, the constitutively active Akt-overexpressing neurons could survive as well as those overexpressing Bcl-2, whereas the constitutively active ERK kinase (MEK)-overexpressing ones failed to survive. A dominant negative Akt experiment demonstrated that inhibition of Akt pathway hastened axotomy-induced neuronal death in the neonate. In addition, the dominant active Akt-overexpressing adult hypoglossal neurons showed accelerated axonal regeneration after axotomy. These results suggest that Akt plays dual roles in motoneuronal survival and nerve regeneration in vivo and that PI3K-Akt pathway is probably more vital in neuronal survival after injury than Ras-ERK pathway.

Activating transcription factor 3 (ATF3) induction by axotomy in sensory and motoneurons: A novel neuronal marker of nerve injury

Activating transcription factor 3 (ATF3), a member of ATF/CREB family of transcription factors, is induced in a variety of stressed tissue. ATF3 regulates transcription by binding to DNA sites as a homodimer or heterodimer with Jun proteins. The purpose of this study was to examine the expression and regulation of ATF3 after axonal injury in neurons in dorsal root ganglia (DRG) and spinal cord. In naive rats, ATF3 was not expressed in the DRG and spinal cord. Following the cut of peripheral nerve, ATF3 was immediately induced in virtually all DRG neurons and motoneurons that were axotomized, and the time course of induction was dependent on the distance between the injury site and the cell body. Double labeling using immunohistochemistry revealed that the population of DRG neurons expressing ATF3 included those expressing c-jun, and in motoneurons ATF3 and c-jun were concurrently expressed after axotomy. In contrast to c-jun, ATF3 was not induced transsynaptically in spinal dorsal horn neurons. We conclude that ATF3 is specifically induced in sensory and motoneurons in the spinal cord following nerve injury and should be regarded as an unique neuronal marker of nerve injury in the nervous system.

Neurotrophin expression by spinal motoneurons in adult and developing rats

Expression of the neurotrophins NT-4, brain-derived neurotrophic factor (BDNF), and NT-3 in adult rat lumbosacral spinal cord motoneurons is reported. A sensitive in situ hybridization procedure demonstrates localization of the mRNA for each of these neurotrophins within spinal motoneurons of the adult and in early postnatal development. A majority of adult rat spinal cord lumbar motoneurons (approximately 63%) express NT-4 mRNA as assessed by counting motoneurons in the L4 and L5 segments of two adult rat spinal cords on adjacent cresyl violet-stained and in situ hybridization sections. Similarly, a majority of lumbar motoneurons (approximately 73%) express BDNF mRNA. Further analyses of adjacent lumbar spinal cord sections revealed that many, although not all motoneurons coexpress both NT-4 and BDNF mRNAs. At birth, the mRNA encoding NT-3 is expressed in motoneurons, but BDNF mRNA is not apparent until postnatal day 5 (P5) and NT-4 mRNA first appears at P9. The potential biological significance of neurotrophin mRNA expression in spinal motoneurons is supported by immunohistochemical localization of each neurotrophin protein in adult motoneurons. We discuss the potential role of spinal cord neurotrophins as autocrine or paracrine factors involved in modulating motoneuron synaptic function.

Rescue of axotomized rubrospinal neurons by brain-derived neurotrophic factor (BDNF) in the developing opossum, Didelphis virginiana

Many rubrospinal neurons die in developing opossums when their axon is cut at thoracic levels of the spinal cord and in the present study we asked whether they can be rescued by brain-derived neurotrophic factor (BDNF). Bilateral injections of Fast Blue (FB) were made into the rostral lumbar cord to prelabel rubrospinal neurons and 5 days later the rubrospinal tract was cut unilaterally by hemisecting the thoracic cord. Immediately after hemisection, BDNF-soaked gelfoam was placed into the lesion cavity. Since pilot data indicated that one application of BDNF was not sufficient to produce a rescue effect, a second application was made 7 days later. Seven days after the second application the pups were killed by an overdose of anesthetic so that the red nucleus contralateral and ipsilateral to the lesion site could be examined for labeled neurons. The rubrospinal tract is almost entirely crossed, so the red nucleus contralateral to the lesion contained many axotomized neurons, whereas the red nucleus ipsilateral to it did not. Age-matched controls were subjected to the same procedures, but the gelfoam applied to the lesion site in the experimental animals was soaked only in the vehicle used to deliver BDNF. In all cases, labeled neurons were fewer in number in the red nucleus contralateral to the lesion than ipsilateral to it. It was of particular interest, however, that labeled neurons contralateral to the lesion were more numerous in the animals treated with BDNF than in the controls. We conclude that BDNF rescues at least some rubrospinal neurons from axotomy-induced cell death in developing opossums suggesting that loss of access to BDNF, and perhaps other neurotrophins, contributes to failure of rubrospinal neurons to survive axotomy.

Prevention by insulin-like growth factor-I and riluzole in motor neuron death after neonatal axotomy

Transection of the sciatic nerve in neonatal rats results discernable loss of motor neurons in the spinal cord. This neuronal death could be due to lack of retrogradely transported target derived neurotrophic factors, since some of these factors have been shown to be effective in injury induced motor neuron death. Another hypothesis suggests that glutamate and its receptors has been implicated as possible mechanism for motor neuron death, because inhibitor of glutamate release and antagonists of glutamate receptors are effective in preventing axotomized motor neuron death. To investigate the effect of insulin-like growth factor-I (IGF-I) and riluzole, a drug that inhibits glutamate release, on axotomy induced motor neuron death. Newborn rats were anesthetized with hypothermia. Sciatic nerve was cut near the obturator tendon in the left thigh. Animals were then treated daily with different doses of IGF-I and riluzole for 14 days with intraperitoneal injections. Control rats received PBS in the same fashion. After the treatment, the number of surviving motor neurons and the motor neuron diameter in the L(4) was assessed. Both IGF-I (1.0 mg/kg) and riluzole (5.0 mg/kg) rescued motor neuron death in a similar way. Co-administration of IGF-I (1.0 mg/kg) and riluzole (5.0 mg/kg) was more effective than either agent alone and there was a statistically significant difference between co-administration and IGF-I alone. However there was no significant difference between simultaneous treatment and riluzole alone. As for diameter of motor neurons, riluzole (5.0 mg/kg) preserved the motor neuron diameter in the lesion side. Nonetheless, no further increase in motor neuron diameter was seen when riluzole (5 mg/kg) and IGF-I (1.0 mg/kg) were applied in combination. Both agents did not affect diameter of motor neurons in the non-axotomy side. Riluzole is available in amyotrophic lateral sclerosis (ALS) and the positive results of clinical trials with IGF-I suggests that combination treatment of IGF-I and riluzole in ALS remains to be determined.

Effect of sympathectomy on the expression of NMDA receptors in the spinal cord

The expression of NMDA receptors in the intermediolateral (IML) region of the upper thoracic spinal cord, was studied in 3 week old rats. The effect of section of the cervical sympathetic nerve on neuronal cell number and receptor expression was examined up to two weeks after the operation. Age-matched sham-operated and unoperated animals were used as controls. It was shown using quantitative autoradiography with the NMDA receptor antagonist [(3)H]MK-801 (dizocilpine maleate), that there was a marked downregulation of receptors in all groups of animals, beginning at approximately 4 weeks of age. However after sympathectomy, which resulted in the death of 44% of neurones in the IML by 7 days, there was a significant increase in receptor density per neurone compared to sham-operated controls. In the control animals there was a significant increase in the Kd value of the binding between 21 and 24 days after birth indicating an increased expression of a low affinity receptor, but no such increase was seen after axotomy. The results are consistent with two populations of NMDA receptors being transiently expressed in the IML in developing animals, and the higher affinity receptor being down-regulated between 4 and 5 weeks of age. The presence of the high affinity receptor subtype may predispose neurones to die after axotomy.

Mitral cell loss following lateral olfactory tract transection increases proliferation density in rat olfactory epithelium

Olfactory sensory neurons are replaced throughout the life of vertebrates by proliferation of basal cells and differentiation of the new cells into neurons. Removal of their target, the olfactory bulb, increases proliferation twofold because sensory neurons die prematurely, suggesting that the olfactory bulb provides a trophic substance required for survival. We asked whether mitral cells, a major postsynaptic target of olfactory sensory neurons, are involved in their survival. We report here that depletion of mitral cells increases proliferation and cell death in the olfactory sensory neuron population. Mitral cell loss was induced unilaterally by transection of their axons in the lateral olfactory tract in 18-day-old rats. At all time points after surgery (3 weeks, 7 weeks, 3 months, 14 months) there was a 29% mean reduction in the number of mitral cells ipsilateral to the transection. The surviving mitral cells were smaller than controls and had less rough endoplasmic reticulum. In the olfactory epithelium, proliferation density (BrdU-positive cells/mm epithelial length) in the progenitor basal cells was increased by an average of 20-25% at all time points, as was the number of TUNEL-positive dying cells. The results are consistent with the notion that mitral cells, or the synaptic sites on them, are a source of trophic factor required for maintenance of the lives of olfactory sensory cells. The target field of postsynaptic neurons remaining after lateral olfactory tract transection is insufficient to maintain normal survival of all existing olfactory neurons. In unperturbed animals the proliferation density declines in an age-dependent manner and interestingly the decline on the tractotomized side is parallel. This suggests that with age the sensory cells are less dependent on their targets.

Rapid loss of dorsal horn lectin binding after massive brachial plexus axotomy in young rats

Lectins are proteins with binding affinities for specific sugars in complex glycoconjugates, some of which have been implicated in limiting synaptic plasticity or modulating nerve growth and guidance. We studied the expression of the glycoconjugate recognized by the isolectin B4 of Griffonia simplicifolia (Gs-IB4) in spinal dorsal horns after massive axotomy of the brachial plexus in weanling rats. Gs-IB4+ binding sites in Rexed's lamina II were rapidly reduced after massive peripheral axotomy. This rapid loss suggests that multiple nerve lesions minimize the number of intact fibers that converge with lesioned fibers into the same cord segments and thus may prevent the plastic changes accompanying the lesion of single nerves.

Extensive neuronal cell death following intracranial transection of the facial nerve in the adult rat

The aim of the present study is to examine the neuronal degeneration and the glial response following intracranial transection of the facial nerve close to the brainstem and furthermore to compare the results with a distal nerve injury. The facial nerve was cut either intracranially in the posterior cranial fossa or further distally, where it passes the parotid gland, in adult rats. Intracranial axotomy caused a massive loss of neuronal profiles. Only 26.8+/-11.3% of facial motor neuronal profiles were found ipsilateral to the nerve injury when compared to the contralateral side, following intracranial axotomy. This was statistically significant in comparison to the distal injury (72.4+/-9.5%), 4 weeks post-lesion. Reactive microglial cells expressed ED1 immunoreactivity following the intracranial axotomy but not following the distal nerve injury. In conclusion, there was a large discrepancy in neuronal degeneration as well as presence of phagocytic (ED1 positive) microglia between the two lesions. The intracranial lesion model used in the present study generates a massive neuronal cell death and should therefore be a useful tool for studies on proximal cranial nerve injuries and in particular mechanisms causing cell death, which may occur following, for example, head trauma.

Discordant expression of c-Ret and glial cell line-derived neurotrophic factor receptor alpha-1 mRNAs in response to motor nerve injury in neonate rats

Adult motoneurons can survive following axotomy, whereas neonate motoneurons result in cell death. Following hypoglossal nerve axotomy in neonate rat, Glial cell line-Derived Neurotrophic Factor (GDNF) receptor alpha-1 (GFRalpha-1) mRNA expression was dramatically suppressed in the injured motoneurons, while a slight increase of c-Ret mRNA expression was observed. In adult, both GFRalpha-1 and c-Ret mRNAs increased substantially after axotomy. The present result suggests that the difference of motoneuron fate after axotomy may be partly due to the coordinate or discordant responses of GFRalpha-1 and c-Ret expression to nerve injury.

Time course and age dependence of motor neuron death following facial nerve crush injury: role of fibroblast growth factor

Peripheral nerve crush injury (PNCI) has been used for many years in adult animals to study central and peripheral changes related to regeneration across the injury site. While these adult animals experience full recovery with no neuronal cell loss following PNCI, it has been noted that the injury in perinatal animals is followed by retrograde neuronal cell death. The present study determines, in mice of different postnatal ages, the degree to which motor neurons are vulnerable to PNCI induced cell death and examines the rate of neuronal loss. Animals of 4 days of age and younger were found to be significantly more vulnerable to motor neuron cell death following PNCI. There also was a proportional relationship between age at injury and final motor neuronal survival and an inverse relationship between age at injury and rate of neuronal cell death following injury. In addition a proportional relationship was observed between the expression level of acidic fibroblast growth factor within motor neurons and the resistance to PNCI induced neuronal death. It was also found that PNCI in an environment that contained higher levels of FGFs (either in mice treated with acidic FGF or in transgenic mice that overexpress basic FGF) significantly decreases neuronal cell death following early postnatal injury.

A role for p75 receptor in neurotrophin-3 functioning during the development of limb proprioception

Neurotrophin-3 is indispensable for the development of limb proprioceptive neurons and their end organs, muscle spindles. To determine whether the low-affinity p75 receptor potentiates the actions of neurotrophin-3, we examined the development of the proprioceptive system in p75 null mutant mice that had either normal or decreased tissue levels of neurotrophin-3. Postnatal mice lacking both copies of the p75 gene had fewer sensory neurons in dorsal root ganglia, but normal complements of muscle spindles in fast hindlimb muscles, although the slow soleus muscle showed a 50% loss of spindles. However, compound mutants lacking both copies of the p75 gene as well as one copy of the neurotrophin-3 gene displayed a dystonic/ataxic phenotype similar to that observed previously in neurotrophin-3 null mutants devoid of proprioception. The compound mutants also exhibited a commensurate loss of parvalbumin-expressing (proprioceptive) neurons in dorsal root ganglia. The degree of deficiency of spindles (and presumably proprioceptive neurons) in the compound mutants exceeded the sum of deficits in single mutants lacking either both copies of p75 genes or one copy of neurotrophin-3 gene, suggesting a synergistic interaction between the p75 receptor and neurotrophin-3. Neuronal deficits in the compound mutants were present prior to embryonic day 14, indicating an early role for the p75 receptor in sensory neuronogenesis. Collectively, these data indicate that the p75 receptor is not essential for the survival and differentiation of most limb proprioceptive neurons when neurotrophin-3 is expressed at normal levels. However, the p75 receptor may act in synergy with neurotrophin-3 to enhance the survival of proprioceptive neurons when tissue levels of neurotrophin-3 are a limiting factor.

Insulin-like growth factor-II increases and IGF is required for postnatal rat spinal motoneuron survival following sciatic nerve axotomy

The prolonged disconnection of nerve from muscle results in the death of motoneurons and permanent paralysis. Because clinical nerve injuries generally involve postbirth motoneurons, there is interest in uncovering factors that may support their survival. A rich history of research dating back to the time of Santiago Ramon y Cajal and Viktor Hamburger supports the inference that there are soluble neurotrophic factors associated with nerve and muscle. However, the endogenous factors normally required for motoneuron survival following nerve injury have eluded identification. Two interrelated hypotheses were tested: (1) administration of insulin-like growth factor-II (IGF-II) can support the survival of postbirth motoneurons, and (2) endogenous IGFs are essential for motoneuron survival following nerve injury. We report that IGF-II locally administered close to the proximal nerve stump prevented the death of motoneurons (estimated by relative numbers of neuronal profiles) which ordinarily follows sciatic nerve transection in neonatal rats. By contrast, anti-IGF antiserum, as well as IGF binding proteins-4 and -6, significantly increased (P < 0.01) motoneuron death. This report shows that IGF-II can support survival, and contains the novel observation that endogenous IGF activity in or near nerves is required for motoneuron survival. Other studies have determined that IGF gene and protein expression are increased in nerve and muscle following sciatic nerve crush, and that IGFs are required for nerve regeneration. Taken together, these data show that IGFs are nerve- and muscle-derived soluble factors that support motoneuron survival as well as nerve regeneration.

Differential time course of neuronal and glial apoptosis in neonatal rat dorsal root ganglia after sciatic nerve axotomy

Sensory neurons in neonatal rat lumbar dorsal root ganglia die after sciatic nerve axotomy, and previous studies have estimated the total cell loss to be 40-95%. We have used the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labelling (TUNEL) technique, combined with immunohistochemistry, to investigate the contribution of apoptosis to the cell loss that occurs after unilaterally transecting the sciatic nerve of new-born rats. TUNEL-positive cells were detected 1 day post-lesion, and their number peaked 3 days after the injury. Combining TUNEL labelling with immunohistochemistry, for neuron-specific neurofilament 150 kDa, or glial-specific S-100beta, enabled us to identify dying neurons and dying glia. One day after axotomy, most of the TUNEL-positive cells (58%) were neurons, whereas 3 days post-injury, only a small number of dying cells (6%) were neuronal. This lower incidence was due to a decrease in neuronal death and an increase in glial death. The glia in the dorsal root ganglia therefore die subsequent to the neurons. The apoptotic nature of the cell death was confirmed by electron microscopy, with fine structural features of apoptotic cell death, e.g. chromatin compaction and membrane blebbing, being observed in both glia and neurons. Our results confirm that extensive apoptosis occurs in the neonatal lumbar dorsal root ganglia after sciatic nerve section, and show that neurons and glial cells die with different time-courses. The results suggest a neuron-glia trophic interdependence in the dorsal root ganglia.

SR57746A: a survival factor for motor neurons in vivo

SR57746A(1-[2-(naphth-2-yl)ethy]]-4-(3-trifluoromethyl phenyl)-1,2,5,6-tetrahydropyridine, hydrochloride) is a non-peptide compound which has been shown to exhibit a wide range of neurotrophic effects both in vitro and in vivo. Here we examine the ability of SR57746A on axotomized spinal motor neuron death in the developing rat spinal cord. After postnatal unilateral section of rat sciatic nerve, there was approximately a 50% survival of motor neurons in the fourth lumbar segment (L4). Intraperitoneal injection of SR57746A for consecutive 14 days rescued motor neuron death but did not preserve the motor neuron diameter both on axotomy and non-axotomy side. These results suggest that SR57746A is a survival factor for motor neurons in vivo and may serve as therapeutic agent for damaged motor neurons.

Differences in developmental cell death between somatic and autonomic motor neurons of rat spinal cord

Considerable knowledge concerning developmental cell death has come from the study of somatic motor neurons (SMNs), but a related set of spinal neurons, the autonomic motor neurons (AMNs), have been studied less extensively in this respect. In the present study, we used three different approaches to determine the amount of AMN cell death during normal development in the rat. First, target dependency was studied in organotypic slice cultures, and it was found that AMNs survived for at least 12 days after removal of their postsynaptic targets. No factors were added to the serum-free medium to substitute for the ablated targets, indicating that AMNs were able to survive without target-derived trophic factors. Such target-independent survival is not characteristic of neurons that undergo typical developmental cell death. Second, AMNs were counted in double-stained choline acetyltransferase immunocytochemical and NADPH diaphorase histochemical preparations at ages (postnatal days 4-22) encompassing the period when AMN postsynaptic target cells undergo developmental death. Neuron numbers were essentially identical at all ages examined, indicating that no AMN cell death occurred postnatally. Finally, from embryonic day 13 to postnatal day 22, animals were analyzed by using terminal transferase-mediated nick-end labeling to identify dying cells. Many fewer labeled cells were observed among AMNs than among SMNs. Thus, all three approaches indicated that there is a significant SMN/AMN difference in developmental cell death. The phenotypic trait(s) that underlies this difference may also be important in the relative resistance of AMNs to pathological conditions that induce death of SMNs, e.g., those involved in amyotrophic lateral sclerosis and excitotoxicity.

Degenerative phenomena and reactive modifications of the adult rat inferior olivary neurons following axotomy and disconnection from their targets

Adult olivocerebellar axons are capable of vigorous regeneration when provided with growth-permissive environmental conditions. To elucidate the contribution of intrinsic properties to the regenerative capabilities of inferior olivary neurons, we have examined the cellular modifications occurring in these neurons following axotomy and target deprivation in the absence of exogenous growth-promoting influences. Axotomized inferior olivary neurons undergo perikaryal shrinkage, dendritic atrophy and a loss of anti-calbindin immunoreactivity. A conspicuous cell death occurs during the first few weeks after lesion, but about 35% of the affected neurons survive up to 60 days. Coincidentally, a subset of the injured nerve cells become strongly reactive for NADPH diaphorase histochemistry, and this expression is correlated with survival in the medial accessory olive and in the principal olive. In addition, the affected neurons express or maintain the expression of several markers related to regenerative processes, including transcription factors c-Jun, JunD and Krox-24, the growth-associated protein GAP-43 and the developmentally regulated calcitonin gene-related peptide (CGRP). The expression of all these markers is sustained up to two months after lesion, the longest survival time examined. These results show that although adult axotomized inferior olivary neurons undergo severe regressive modifications leading to a conspicuous cell loss, at least a subset of them is resistant to the lesion. In addition, the long-lasting expression of several axon-growth associated markers expressed in these neurons in response to injury reveals that they are endowed with a strong intrinsic regenerative potential.