Cytological and cytochemical (RNA) studies on rubral neurons after unilateral rubrospinal tractotomy: the impact of GM1 ganglioside administration

The effects of Garcinia kola and curcumin on the dorsal root ganglion of the diabetic rat after peripheral nerve transection injury

OBJECTIVE

To test the protective effects of Garcinia kola and curcumin on the ganglion tissues of diabetic rats following the use of autologous vein graft in peripheral nerve transection injury.

METHODS

The sciatic nerve on the right side was transected, and anastomosis was performed between the proximal and distal ends using an autologous vein graft. Curcumin and Garcinia kola seed extract were administered daily by oral gavage. The ganglion tissues were harvested after a 90-day waiting period. Sensory neurons in the dorsal root ganglion at the L4 and L5 levels were used for stereological evaluations. Mean sensory neuron numbers were analyzed using a stereological technique. The size of the light and dark neurons was also estimated, and ultrastructural and immunohistochemical evaluations were performed.

RESULTS

A statistically significant difference in sensory neuron numbers was observed between the groups with and without Garcinia kola and curcumin applications. The immunohistochemical results showed that the s-100 protein is expressed selectively between cell types.

CONCLUSION

The results of this study show that curcumin and Garicinia kola prevented sensory neuron loss in diabetic rats following transection injury to the sciatic nerve.

Chromatolysis: Do injured axons regenerate poorly when ribonucleases attack rough endoplasmic reticulum, ribosomes and RNA?

After axonal injury, chromatolysis (fragmentation of Nissl substance) can occur in the soma. Electron microscopy shows that chromatolysis involves fission of the rough endoplasmic reticulum. In CNS neurons (which do not regenerate axons back to their original targets) or in motor neurons or dorsal root ganglion neurons denied axon regeneration (e.g., by transection and ligation), chromatolysis is often accompanied by degranulation (loss of ribosomes from rough endoplasmic reticulum), disaggregation of polyribosomes and degradation of monoribosomes into dust‐like particles. Ribosomes and rough endoplasmic reticulum may also be degraded in autophagic vacuoles by ribophagy and reticulophagy, respectively. In other words, chromatolysis is disruption of parts of the protein synthesis infrastructure. Whereas some neurons may show transient or no chromatolysis, severely injured neurons can remain chromatolytic and never again synthesize normal levels of protein; some may atrophy or die. Ribonuclease(s) might cause the following features of chromatolysis: fragmentation and degranulation of rough endoplasmic reticulum, disaggregation of polyribosomes and degradation of monoribosomes. For example, ribonucleases in the EndoU/PP11 family can modify rough endoplasmic reticulum; many ribonucleases can degrade mRNA causing polyribosomes to unchain and disperse, and they can disassemble monoribosomes; Ribonuclease 5 can control rRNA synthesis and degrade tRNA; Ribonuclease T2 can degrade ribosomes, endoplasmic reticulum and RNA within autophagic vacuoles; and Ribonuclease IRE1α acts as a stress sensor within the endoplasmic reticulum. Regeneration might be improved after axonal injury by protecting the protein synthesis machinery from catabolism; targeting ribonucleases using inhibitors can enhance neurite outgrowth and could be a profitable strategy in vivo. © 2018 Wiley Periodicals, Inc. Develop Neurobiol, 2018

Neuronal PTEN deletion in adult cortical neurons triggers progressive growth of cell bodies, dendrites, and axons

Deletion of the phosphatase and tensin (PTEN) gene in neonatal mice leads to enlargement of the cell bodies of cortical motoneurons (CMNs) in adulthood (Gutilla et al., 2016). Here, we assessed whether PTEN deletion in adult mice would trigger growth of mature neurons. PTEN was deleted by injecting AAV-Cre into the sensorimotor cortex of adult transgenic mice with a lox-P flanked exon 5 of the PTEN gene and Cre-dependent reporter gene tdTomato. PTEN-deleted CMN's identified by tdT expression and retrograde labeling with fluorogold (FG) were significantly enlarged four months following PTEN deletion, and continued to increase in size through the latest time intervals examined (12-15 months post-deletion). Sholl analyses of tdT-positive pyramidal neurons revealed increases in dendritic branches at 6 months following adult PTEN deletion, and greater increases at 12 months. 12 months after adult PTEN deletion, axons in the medullary pyramids were significantly larger and G-ratios were higher. Mice with PTEN deletion exhibited no overt neurological symptoms and no seizures. Assessment of motor function on the rotarod and cylinder test revealed slight impairment of coordination with unilateral deletion; however, mice with bilateral PTEN deletion in the motor cortex performed better than controls on the rotarod at 8 and 10 months post-deletion. Our findings demonstrate that robust neuronal growth can be induced in fully mature cortical neurons long after the developmental period has ended and that this continuous growth occurs without obvious functional impairments.

Rat motor neurons caudal to a rubrospinal tract (RST) transection remain viable

In the rat, the rubrospinal tract (RST) is a descending motor pathway involved in the production of skilled reaching movement. The RST originates in the red nucleus in the midbrain and runs down the spinal cord in the lateral most aspect of the dorsolateral funiculus (DLF). The RST makes monosynaptic contact with interneurons within the intermediate laminae of the cord, however a contingent of RST axons constitutes direct supraspinal input for spinal cord motor neurons. The current study investigated the effects of unilateral RST transection at cervical levels C3-4 on the population of motor neurons in both spinal segments C5-6 and L2-3. The total number of large, medium and small motor neurons in these segments was estimated with stereological techniques in both ventral horns at 1, 3, 7 and 14days post-injury. In both spinal cord segments under investigation, no change was detected in mean number of motor neurons over time, in either ventral horn. That the loss of direct supraspinal input resulting from the RST transection does not affect the viability of motor neurons caudal to the injury indicates that these neurons have the potential to be re-innervated, should the RST injury be repaired.

Interactions Between MPTP-Induced and Age-Related Neuronal Death in a Murine Model of Parkinson’s Disease

Abiotrophy is hypothesized to explain the onset and time course of deficits in Parkinson’s disease (PD) Abiotrophy includes: 1) exposure to agent(s) causing the death of dopaminergic nigrostriatal neurons (DNSns), 2) gradual death of DNSns with age, 3) summation of 1) and 2) until DNSn numbers fall below a threshold for detectable neurological deficits. Murine DNSn death following methyl-phenyl-tetrahydropyridine (MPTP) exposure occurs according to an exponential relationship while age-related death of DNSns occurs according to a second exponential relationship. Summing the two exponential losses overestimates experimental DNSn death showing a simple abiotrophic model is not sufficient. Aged murine DNSns greatly increase their dopamine synthesis and the density of their striatal axon terminals which may explain the above threshold. Murine DNSns die gradually after MPTP exposure and L-deprenyl treatment rescues MPTP-damaged DNSns by a previously undiscovered action, altering the abiotrophic interactions and possibly explaining the slowed progression of PD found with deprenyl treatment.

Cannabinoid CB2 receptor (CB2R) stimulation delays rubrospinal mitochondrial-dependent degeneration and improves functional recovery after spinal cord hemisection by ERK1/2 inactivation

Spinal cord injury (SCI) is a devastating condition of CNS that often results in severe functional impairments for which there are no restorative therapies. As in other CNS injuries, in addition to the effects that are related to the primary site of damage, these impairments are caused by degeneration of distal regions that are connected functionally to the primary lesion site. Modulation of the endocannabinoid system (ECS) counteracts this neurodegeneration, and pharmacological modulation of type-2 cannabinoid receptor (CB2R) is a promising therapeutic target for several CNS pathologies, including SCI. This study examined the effects of CB2R modulation on the fate of axotomized rubrospinal neurons (RSNs) and functional recovery in a model of spinal cord dorsal hemisection (SCH) at the cervical level in rats. SCH induced CB2R expression, severe atrophy, and cell death in contralateral RSNs. Furthermore, SCH affected molecular changes in the apoptotic cascade in RSNs – increased cytochrome c release, apoptosome formation, and caspase-3 activity. CB2R stimulation by its selective agonist JWH-015 significantly increased the bcl-2/bax ratio, reduced cytochrome c release, delayed atrophy and degeneration, and improved spontaneous functional recovery through ERK1/2 inactivation. These findings implicate the ECS, particularly CB2R, as part of the endogenous neuroprotective response that is triggered after SCI. Thus, CB2R modulation might represent a promising therapeutic target that lacks psychotropic effects and can be used to exploit ECS-based approaches to counteract neuronal degeneration.

Astrocyte transplantation for spinal cord injury: current status and perspective

Spinal cord injury (SCI) often causes incurable neurological dysfunction because axonal regeneration in adult spinal cord is rare. Astrocytes are gradually recognized as being necessary for the regeneration after SCI as they promote axonal growth under both physiological and pathophysiological conditions. Heterogeneous populations of astrocytes have been explored for structural and functional restoration. The results range from the early variable and modest effects of immature astrocyte transplantation to the later significant, but controversial, outcomes of glial-restricted precursor (GRP)-derived astrocyte (GDA) transplantation. However, the traditional neuron-centric view and the concerns about the inhibitory roles of astrocytes after SCI, along with the sporadic studies and the lack of a comprehensive review, have led to some confusion over the usefulness of astrocytes in SCI. It is the purpose of the review to discuss the current status of astrocyte transplantation for SCI based on a dialectical view of the context-dependent manner of astrocyte behavior and the time-associated characteristics of glial scarring. Critical issues are then analyzed to reveal the potential direction of future research.

Systemic administration of a deoxyribozyme to xylosyltransferase-1 mRNA promotes recovery after a spinal cord contusion injury

After spinal cord injury, proteoglycans with growth-inhibitory glycosaminoglycan (GAG-) side chains in scar tissue limit spontaneous axonal sprouting/regeneration. Interventions that reduce scar-related inhibition facilitate an axonal growth response and possibly plasticity-based spinal cord repair. Xylosyltransferase-1 (XT-1) is the enzyme that initiates GAG-chain formation. We investigated whether intravenous administration of a deoxyribozyme (DNA enzyme) to XT-1 mRNA (DNAXT-1as) would elicit plasticity after a clinically relevant contusion of the spinal cord in adult rats. Our data showed that systemic DNAXT-1as administration resulted in a significant increase in sensorimotor function and serotonergic axon presence caudal to the injury. DNAXT1as treatment did not cause pathological or toxicological side effects. Importantly, intravenous delivery of DNAXT-1as did not exacerbate contusion-induced neuropathic pain. Collectively, our data demonstrate that DNAXT-1as is a safe neurotherapeutic, which holds promise to become an integral component of therapies that aim to improve the quality of life of persons with spinal cord injury.

Delayed treatment with chondroitinase ABC reverses chronic atrophy of rubrospinal neurons following spinal cord injury

Degradation of extracellular matrix chondroitin sulphate proteoglycans (CSPGs) using Chondroitinase ABC (ChABC) is a promising strategy for the treatment of spinal cord injury, with potent effects on promoting functional recovery and anatomical repair in spinal injured animals. We have previously demonstrated that ChABC treatment prevents atrophy of corticospinal projection neurons following spinal injury in adult YFP-H mice. Here, we investigate whether ChABC-mediated repair of the cell body extends to rubrospinal projection neurons (RSNs), whether neuroprotective effects can be sustained long-term and importantly, whether delayed treatment with ChABC can reverse chronic atrophy. Adult YFP-H mice underwent unilateral rubrospinal tract transection and were treated with ChABC or a control enzyme, delivered either acutely post-injury or after a one month delay. Eight weeks following injury and control treatment, RSNs in the injured red nucleus, identified by YFP label and NeuN immunoreactivity, showed severe atrophy, with ~40% loss of mean cell area compared to uninjured neurons in the contralateral red nucleus. Both acute and delayed treatment with ChABC promoted a significant rescue of injured RSNs, restoring cell area to ~80% and ~70%, respectively, of that in uninjured neurons. Thus, we demonstrate for the first time that CSPG degradation in the injured spinal cord not only promotes sustained rescue of cell atrophy when delivered acutely but can also reverse chronic atrophy in descending projection neurons. Thus, modulation of the extracellular matrix can mediate neuroprotective effects both early and late after spinal cord injury.

Manipulating the glial scar: chondroitinase ABC as a therapy for spinal cord injury

Chondroitin sulphate proteoglycans (CSPGs) are potent inhibitors of growth in the adult CNS. Use of the enzyme chondroitinase ABC (ChABC) as a strategy to reduce CSPG inhibition in experimental models of spinal cord injury has led to observations of a remarkable capacity for repair. Here we review the evidence that treatment with ChABC, either as an individual therapy or in combination with other strategies, can have multiple beneficial effects on promoting repair following spinal cord injury. These include promoting regeneration of injured axons, plasticity of uninjured pathways and neuroprotection of injured projection neurons. More importantly, ChABC therapy has been demonstrated to promote significant recovery of function to spinal injured animals. Thus, there is robust pre-clinical evidence demonstrating beneficial effects of ChABC treatment following spinal cord injury. Furthermore, these effects have been replicated in a number of different injury models, with independent confirmation by different laboratories, providing an important validation of ChABC as a promising therapeutic strategy. We discuss putative mechanisms underlying ChABC-mediated repair as well as potential issues and considerations in translating ChABC treatment into a clinical therapy for spinal cord injury.

Survival of trauma-injured neurons in rat brain by treatment with proline-rich peptide (PRP-1): an immunohistochemical study

The objective of this immunohistochemical research was to reveal the distribution of a proline-rich peptide-1 (PRP-1) in various brain structures of intact and trauma-injured rats and to identify the mechanisms of promotion of neuronal recovery processes following PRP-1 treatment. PRP-1, produced by bovine hypothalamic magnocellular cells and consisting of 15 amino acid residues, is a fragment of neurophysin vasopressin associated glycoprotein isolated from bovine neurohypophysis neurosecretory granules. PRP-1-immunoreactivity (PRP-1-IR) was detected in the brain of intact rats in the neurons of paraventricular (PVN) and supraoptic (SON) nuclei in the hypothalamus, in almost all cell groups in the medulla oblongata, in Purkinje and some cerebellar nuclei cells, and in nerve fibers. At 3 weeks after hemisection of the spinal cord (SC) an asymmetry of PRP-1 localization in the PVN and SON was observed: no PRP-1-IR was exhibited at the affected sides of both nuclei. Daily intramuscular administration of PRP-1 for 3 weeks significantly increased the number of PRP-1-immunoreactive (PRP-1-Ir) varicose nerve fibers, and cells in PVN and SON and in cell groups of the limbic system and brain stem. Tanycytes in the median eminence and covering ependyma also demonstrated strong PRP-1-IR. PRP-1 treatment also activated neuropeptide Y-IR (NPY-IR) in nerve fibers and immunophilin fragment-IR (IphF-IR) in lymphocytes and nerve cells. A strong increase of PRP-1-IR was observed in the PVN and SON of SC-injured rats following the treatment with another PRP (PRP-3). Preliminary physiological data demonstrate that PRP-3 is more "aggressive" in the recovery processes than PRP-1. Based on the findings regarding PRP action on neurons survival, axons regeneration, and the number of IphF-Ir lymphocytes and NPY-Ir nerve fibers, PRP is suggested to act as a neuroprotector, functioning as a putative neurotransmitter and immunomodulator.

Adeno-associated viral vector-mediated gene transfer of brain-derived neurotrophic factor reverses atrophy of rubrospinal neurons following both acute and chronic spinal cord injury

Rubrospinal neurons (RSNs) undergo marked atrophy after cervical axotomy. This progressive atrophy may impair the regenerative capacity of RSNs in response to repair strategies that are targeted to promote rubrospinal tract regeneration. Here, we investigated whether we could achieve long-term rescue of RSNs from lesion-induced atrophy by adeno-associated viral (AAV) vector-mediated gene transfer of brain-derived neurotrophic factor (BDNF). We show for the first time that AAV vectors can be used for the persistent transduction of highly atrophic neurons in the red nucleus (RN) for up to 18 months after injury. Furthermore, BDNF gene transfer into the RN following spinal axotomy resulted in counteraction of atrophy in both the acute and chronic stage after injury. These novel findings demonstrate that a gene therapeutic approach can be used to reverse atrophy of lesioned CNS neurons for an extended period of time.

Glial fibrillary acidic protein expression in the hamster red nucleus: effects of axotomy and testosterone treatment

Testosterone propionate (TP) administration coincident with facial nerve axotomy in the hamster attenuates glial fibrillary acidic protein (GFAP) expression in the facial nucleus that is normally increased following axotomy alone. This ability of TP to modulate astrocyte activity has been linked to the ability of steroid hormones to enhance the regenerative response of injured motor neurons. In an ongoing study designed to examine the potential influences of steroid hormones on centrally projecting motoneurons, the astrocyte reaction in the red nucleus was examined. In the present study, in situ hybridization was used to assess changes in GFAP mRNA in the hamster red nucleus following spinal cord injury (SCI) and TP treatment. Castrated male hamsters were subjected to right rubrospinal tract (RST) transection at spinal cord level T1, with half the animals implanted subcutaneously with Silastic capsules containing 100% crystalline TP and the remainder sham implanted. The uninjured red nucleus served as an internal control. Postoperative survival times were 1, 2, 7, and 14 days. Qualitative-quantitative analyses of emulsion autoradiograms were accomplished. Axotomy alone resulted in a significant but transient increase in GFAP mRNA levels at 2 days postoperative in the injured red nucleus compared with the contralateral uninjured red nucleus. However, in TP-treated animals, GFAP mRNA levels were no different than control levels at 2 dpo but were significantly increased at 7 dpo relative to contralateral control. Additionally, the increase in GFAP mRNA levels following TP treatment was significantly smaller than following axotomy alone. These data suggest that testosterone both delays and reduces the astrocytic reaction in the red nucleus following rubrospinal tract axotomy, and confirms a difference between peripheral and central glial responses to axotomy and steroid administration.

Ribosomal RNA transcriptional activation and processing in hamster rubrospinal motoneurons: effects of axotomy and testosterone treatment

Rubrospinal motoneurons (RSMN) represent a population of androgen receptor-expressing central motoneurons with limited regenerative potential relative to their peripheral counterparts. A key determinant of regenerative capability lies in the nucleolar reaction of injured neurons. To date, characterization of the nucleolar reaction in injured central motoneurons has not been accomplished. Furthermore, it has been documented that testosterone propionate (TP) augments peripheral motoneuron regeneration through regulation of the nucleolar reaction to injury. In this study, the effects of injury alone, or in conjunction with TP, on the nucleolar response of injured RSMN were examined using in situ hybridization (ISH) techniques. Castrated adult male hamsters were subjected to right spinal cord hemisection at the C7/T1 vertebral level. Half the animals were subcutaneously implanted with one Silastic TP capsule, with the other half sham implanted. ISH for precursor 45S and mature 28S rRNA was accomplished with a (3)H-labeled ribosomal DNA probe specific to the external transcribed spacer region or to the 28S region of the ribosomal gene, respectively. Postoperative times of 2, 6, and 24 hours were selected for examination of precursor 45S rRNA (i.e., rRNA transcriptional activation) levels and 0.25, 2, 4, and 14 days for examination of mature rRNA (i.e., ribosome) levels. Transcriptional activation of the rRNA gene was rapidly and transiently increased in injured RSMN, analogously to previously documented effects of injury on rRNA transcription in peripheral motoneurons, but, in contrast, this did not translate into an increase in mature ribosomes. TP administration failed to affect positively the nucleolar response of injured RSMN at all. From this study, a key component underlying inherent differences in the regenerative capacity of peripheral vs. central motoneurons has been identified, which can be targeted in future experiments designed to enhance the regenerative potential of selective neuronal populations.

Identification of regenerative tissue cables using in vivo MRI after spinal cord hemisection and schwann cell bridging transplantation

The purpose of this study was to examine the feasibility of a non-invasive in vivo magnetic resonance imaging (MRI) procedure, performed at 1.5 T, to detect regenerative tissue cables in a rat spinal cord hemisection and Schwann cell (SC) bridging transplantation paradigm. Two months after implantation of a SC-seeded guidance channel (1.25 mm in diameter and 3.0 mm in length) into a T8 spinal cord hemisection-gap lesion, axial fast-spin echo (FSE) T2-weighted MR imaging (T2WI) was performed. Axial T2WI through the graft identified a circular area of low intensity surrounded by high-intensity signal within the guidance channel lumen. Correlative histological assessments of Toluidine blue-stained sections confirmed that the low-intensity signal represented a tissue cable, which, in most cases, contained a substantial number of myelinated axons oriented along the rostro-caudal axis of the spinal cord. The percentage of guidance channel cross-sectional area occupied by the tissue cable, expressed as the tissue cable index (TCI), was also determined from histological sections. Linear regression analysis of the TCI plotted relative to the number of myelinated axons revealed a strong positive correlation (r(2) = 0.85) between these two outcome measures. In addition, the sensitivity of MRI to detect regenerative tissue cables within guidance channels was 86%. These results demonstrate that (1). 1.5 T MR imaging performed 2 months after spinal cord hemisection and SC bridging transplantation is sensitive in detecting low-intensity regenerative tissue cables, and (2). the TCI strongly correlates with the extent of axonal regeneration into implanted SC-seeded guidance channels.

Axonally transported peripheral signals regulate alpha-internexin expression in regenerating motoneurons

The class IV neuronal intermediate filament (IF) family proteins includes the neurofilament (NF) triplet proteins NF-L, NF-M, and NF-H and also the more recently characterized alpha-internexin-NF66. It is well established that NF-L, -M, and -H protein and mRNA are downregulated after peripheral nerve injury. We examined alpha-internexin protein expression after three facial nerve lesion paradigms: crush, transection, and resection. Alpha-internexin immunoreactivity was absent in the perikarya of uninjured facial motoneurons but increased dramatically in all three injury paradigms, with maximum immunoreactivity observed at 7 d after injury. Twenty-eight days after nerve crush or transection, there was a dramatic decrease in the number of alpha-internexin-positive cells. In contrast, alpha-internexin remained elevated 28 d after nerve resection, an injury that hinders regeneration and target reinnervation. In situ hybridization studies showed an increase in alpha-internexin mRNA expression in the facial nucleus at 7 and 14 d after injury. Retrograde transport of fluorogold from the whisker pads to the facial nucleus was seen only in motoneurons that lacked alpha-internexin immunoreactivity, supporting the idea that target reinnervation and inhibitory signals from the periphery regulate the expression of alpha-internexin. Blockage of axonal transport through local colchicine application induced strong immunoreactivity in motoneurons. Alpha-internexin expression was also examined after central axotomy of rubrospinal neurons, which constitutively show alpha-internexin immunoreactivity. After rubrospinal tractotomy, alpha-internexin immunoreactivity transiently increased by 7 d after injury but returned to control levels by 14 d. We conclude that alpha-internexin upregulation in injured motoneurons suggests a role for this IF protein in neuronal regeneration.

Delayed glial cell death following wallerian degeneration in white matter tracts after spinal cord dorsal column cordotomy in adult rats

The devastating consequences of spinal cord injury (SCI) result primarily from damage to long tracts in the spinal white matter. To elucidate the secondary injury processes occurring after SCI, we investigated the relationship between apoptosis and Wallerian degeneration in spinal white matter tracts. In the rat spinal cord, the corticospinal tract (CST) and the dorsal ascending tract (DAT) are separated from each other in the dorsal column and relay information in opposite directions. A dorsal column cordotomy at the eighth thoracic (T8) level simultaneously induces Wallerian degeneration in the CST caudal to and in the DAT rostral to the injury. Using the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) method, we demonstrate that apoptosis occurred in areas of Wallerian degeneration in both tracts throughout the length of the cord segments studied (from T3 to T12). This delayed cell death, more apparent in the DAT, began at 7 days after injury and peaked at 14 days for the DAT and 28 days for the CST. Although a few TUNEL+ cells, slightly above the noninjury control level, were found in intact areas of both tracts, statistically significant differences in the number of TUNEL+ cells were found between the intact and the lesioned tract segments (CST, F < 0.01; DAT, F < 0.001). Within a particular spinal segment, a mean number of 64 and 939 TUNEL+ cells in the degenerating CST and DAT, respectively, were estimated stereologically at 14 days postinjury. TUNEL+ cells in degenerating tracts outnumber their intact counterparts by 3.8:1 in the CST and 4.1:1 in the DAT, although a statistically significant difference between the two was only found in the DAT at this time point (P < 0.05). Finally, we demonstrated that oligodendrocytes, the myelin-forming cells in the central nervous system, constitute at least a portion of the cells undergoing apoptosis within areas of Wallerian degeneration.

Survival effects of BDNF and NT-3 on axotomized rubrospinal neurons depend on the temporal pattern of neurotrophin administration

This study shows that both BDNF and NT-3 can prevent cell death in axotomized adult rat rubrospinal neurons (RSNs), but that the efficacy of neuroprotection depends on the temporal pattern of treatment. At 8 weeks after cervical spinal cord injury, 51% of the RSNs had died. Subarachnoidal BDNF infusion into the cisterna magna for 4 weeks resulted in neuronal hypertrophy and 71% survival. Continuous infusion for 8 weeks into the lumbar subarachnoidal space with either BDNF or NT-3 gave similar survival rates, while a combination of BDNF and NT-3 resulted in 96% survival, although the cells were atrophic. When administration of either BDNF or NT-3 was delayed and performed during postoperative weeks 5-8, the number of surviving neurons was increased compared to early treatment. Delayed treatment with a combination of BDNF and NT-3 resulted in complete survival and a reduction in neuronal atrophy. A decreased expression of TrkB receptors and microtubule-associated protein-2 in the RSNs after axotomy was counteracted by BDNF and NT-3. Microglial activity remained increased even when complete cell survival was achieved. Thus, the combination of neurotrophins as well as the temporal pattern of treatment need to be adequately defined to optimize survival of injured spinal tract neurons.

Survival of chronically-injured neurons can be prolonged by treatment with neurotrophic factors

Axonal regeneration by chronically-injured supraspinal neurons can be enhanced by neurotrophic factor treatment at the site of injury, although the number of regenerating neurons decreases as the interval between spinal cord injury and treatment increases. This study investigated whether this decline in regenerative response could be due to continued loss of neurons during the post-injury period. Adult rats received a cervical hemisection lesion and axotomized neurons were labeled by retrograde transport of True Blue from the lesion site. Animals were killed one, four or eight weeks after injury and surviving neurons (True Blue-labeled) were counted in the red nucleus and lateral vestibular nucleus. The neuron number in the lateral vestibular nucleus was stable for eight weeks after spinal cord injury, while survival in the red nucleus decreased by 25% between four and eight weeks. To test how neurons respond to a second injury with or without trophic factor treatment, at four, eight, 14 or 22 weeks after injury the lesion cavity was enlarged by 0.5 mm in a rostral direction. Gel foam saturated with ciliary neurotrophic factor, brain-derived neurotrophic factor or basic fibroblast growth factor was placed into the cavity. Animals were killed four weeks later. Re-injury of the spinal cord caused a significant decrease in neuron survival in both the red nucleus and lateral vestibular nucleus, the effects of which were lessened by treatment with ciliary neurotrophic factor or brain-derived neurotrophic factor for the red nucleus and with ciliary neurotrophic factor for the lateral vestibular nucleus, when re-injured at four or eight weeks. Basic fibroblast growth factor did not affect neuron survival at any time post-injury. Ciliary neurotrophic factor was not effective with longer delays (14 or 22 weeks) between the initial injury and re-injury. These results indicate a delayed pattern of secondary neuronal cell loss after spinal cord injury that is exaggerated by re-injury, but which can be ameliorated by treatment with neurotrophic factors.

Androgenic regulation of the central glia response following nerve damage

Current research on the effects of gonadal steroids on the brain and spinal cord indicates that these agents have profound trophic effects on many aspects of neuronal functioning, including cell survival, growth and metabolism, elaboration of processes, synaptogenesis, and neurotransmission (Jones et al., 1985; Luine, 1985; Nordeen et al., 1985; Matsumoto et al., 1988a,b; Gould et al., 1990). Since many of the aspects of normal neuronal functioning altered by gonadal steroids are affected by injury to the nervous system, we initiated a series of experiments designed to exploit the trophic capabilities of steroids as therapeutic agents in neuronal injury and repair (Kujawa et al., 1989, 1991; Kujawa and Jones, 1990). Three steroid-sensitive model systems were used for these studies: the hamster facial motoneuron, the rat sciatic motoneuron, and the hamster rubrospinal motoneuron. The results of our initial series of experiments suggest that androgens, and possibly estrogens, act either directly or indirectly on the injured motoneuron and enhance elements of the neuronal reparative response that are critical to successful recovery of function. Recently, we discovered that gonadal steroids may also modulate the central glia response to nerve damage. In this review, a summary of our data identifying a therapeutic role for androgens in enhancing the reparative response of motoneurons to injury is presented. This is followed by a discussion of the effects of androgens on the glial response to injury.

Effect of graded spinal cord compression on cardiovascular neurons in the rostro-ventro-lateral medulla

In patients with spinal cord injury, cardiovascular disturbances such as hypotension, bradycardia and autonomic dysreflexia can be directly linked to abnormalities of central autonomic control. To date, the changes in bulbospinal innervation of sympathetic preganglionic neurons after compressive spinal cord injury have not been investigated. Thus, we examined the effect of varying severity of compressive spinal cord injury on neurons of the rostro-ventro-lateral medulla, a nucleus of key importance in cardiovascular control. Adult rats with 20 g, 35 g and 50 g clip compression injuries (n= 18) of the cord at T1 and uninjured controls (n=13) were studied. Neurons in the rostro-ventro-lateral medulla with preserved spinal connections eight weeks after spinal cord injury were identified by retrograde labelling with 4% FluoroGold introduced into the cord at T6. Bulbospinal neurons in the rostro-ventro-lateral medulla were also examined immunocytochemically for the adrenaline-synthesizing enzyme phenylethanolamine-N-methyltransferase. In control rats an average of 451+/-12 rostro-ventrolateral medulla neurons were phenylethanolamine-N-methyltransferase positive. Of these, 213+/-6 projected to the T6 spinal cord. The number of rostro-ventro-lateral medulla neurons retrogradely labelled by FluoroGold decreased as a linear function of severity of spinal cord injury (r= -0.95; P<0.0001). After 50g spinal cord injury at T1, only 7+/-1 rostro-ventro-lateral medulla neurons were labelled by FluoroGold, of which 6+/-1 were phenylethanolamine-N-methyltransferase positive. Moreover, the number of phenylethanolamine-N-methyltransferase positive rostro-ventro-lateral medulla neurons decreased to 361+/-16 after 50 g spinal cord injury. We conclude that compressive spinal cord injury results in disconnection of rostro-ventro-lateral medulla neurons, which project to the thoracic spinal cord, and that these changes vary with the severity of injury. The majority of these axotomized rostro-ventro-lateral medulla neurons maintain their immunopositivity for the adrenaline-synthesizing enzyme phenylethanolamine-N-methyltransferase.

Regrowth of axons into the distal spinal cord through a Schwann-cell-seeded mini-channel implanted into hemisected adult rat spinal cord

Schwann cells (SCs) have been shown to be a key element in promoting axonal regeneration after being grafted into the central nervous system (CNS). In the present study, SC-supported axonal regrowth was tested in an adult rat spinal cord implantation model. This model is characterized by a right spinal cord hemisection at the eighth thoracic segment, implantation of a SC-containing mini-channel and restoration of cerebrospinal fluid circulation by suturing the dura. We demonstrate that a tissue cable containing grafted SCs formed an effective bridge between the two stumps of the hemicord 1 month after transplantation. Approximately 10 000 myelinated and unmyelinated axons (1 : 9) per cable were found at its midpoint. In addition to propriospinal axons and axons of peripheral nervous system (PNS) origin, axons from as many as 19 brainstem regions also grew into the graft without additional treatments. Most significantly, some regenerating axons in the SC grafts were able to penetrate through the distal graft-host interface to re-enter the host environment, as demonstrated by anterograde axonal labelling. These axons coursed toward, and then entered the grey matter where terminal bouton-like structures were observed. In channels containing no SCs, limited axonal growth was seen within the graft and no axons penetrated the distal interface. These findings further support the notion that SCs are strong promotors of axonal regeneration and that the mini-channel model may be appropriate for further investigation of axonal re-entry, synaptic reconnection and functional recovery following spinal cord injury.

Differential regulation of alpha- and beta-CGRP mRNAs within oculomotor, trochlear, abducens, and trigeminal motoneurons in response to axotomy

Spinal and cranial motoneurons express alpha- and beta-calcitonin gene-related peptide (CGRP) mRNAs constitutively at variable ratios, and these two mRNAs are differentially regulated following axotomy in spinal, facial, and hypoglossal motoneurons. The purpose of this study was to investigate the change in CGRP mRNA expression following nerve injury in oculomotor, trochlear, abducens, and trigeminal motor nuclei in which beta-CGRP mRNA is predominantly expressed under normal conditions. Using male Sprague-Dawley rats, either the left eyeball and the orbital contents including the bulbar muscles were removed, or the left masseter nerve was ligated and transected. The rats were allowed to survive for 1, 3, 7, 14, 28, 56 days following these procedures. The levels of mRNAs for alpha- and beta-CGRP and growth-associated protein (GAP)-43 were analyzed by in situ hybridization histochemistry using 35S-labeled oligonucleotide probes. Following nerve injury, the expression of alpha-CGRP mRNA rapidly increased on the directly-injured side in all of these nuclei. Thereafter, it gradually decreased and returned to about the control level at postoperative day 56 within oculomotor, trochlear, and abducens motoneurons, but it sustained at a high level within trigeminal motoneurons. The expression of beta-CGRP was quite variable among these nuclei, and significant changes were also seen on the side contralateral to the directly-injured side. These data indicate that the up-regulation of alpha-CGRP mRNA may be a common response of cranial motor neurons following axotomy even if the constitutive expression of beta-CGRP mRNA exceeds that of alpha-CGRP mRNA in these neurons.

Fetal spinal cord transplants and exogenous neurotrophic support enhance c-Jun expression in mature axotomized neurons after spinal cord injury

The responses of the central (CNS) and peripheral (PNS) nervous system to axotomy differ in a number of ways; these differences can be observed in both the cell body responses to injury and in the extent of regeneration that occurs in each system. The cell body responses to injury in the PNS involves the upregulation of genes that are not upregulated following comparable injuries to CNS neurons. The expression of particular genes following injury may be essential for regeneration to occur. In the present study, we have evaluated the hypothesis that expression of the inducible transcription factor c-Jun is associated with regrowth of axotomized CNS neurons. In these experiments, we compared c-Jun expression in axotomized brainstem neurons after thoracic spinal cord hemisection alone (a condition in which no regrowth occurs) and in groups of animals where hemisections were combined with treatments such as transplants of fetal spinal cord tissue and/or application of neurotrophic factors to the lesion site. The latter conditions enhance the capacity of the CNS for regrowth. We have demonstrated that hemisections alone do not upregulate expression of c-Jun, indicating that this particular cell body response is not a direct result of axotomy. However, c-Jun expression is upregulated in animals that received application of transplants and neurotrophins. Because these interventions also promote sprouting and regrowth of CNS axons after spinal cord lesions, we suggest that transplants and exogenous neurotrophic factor application activate a cell body response consistent with a role for c-Jun in axonal growth.

BDNF and NT-4/5 prevent atrophy of rat rubrospinal neurons after cervical axotomy, stimulate GAP-43 and Talpha1-tubulin mRNA expression, and promote axonal regeneration

Rubrospinal neurons (RSNs) undergo a marked atrophy in the second week after cervical axotomy. This delayed atrophy is accompanied by a decline in the expression of regeneration-associated genes such as GAP-43 and Talpha1-tubulin, which are initially elevated after injury. These responses may reflect a deficiency in the trophic support of axotomized RSNs. To test this hypothesis, we first analyzed the expression of mRNAs encoding the trk family of neurotrophin receptors. In situ hybridization revealed expression of full-length trkB receptors in virtually all RSNs, which declined 7 d after axotomy. Full-length trkC mRNA was expressed at low levels. Using RT-PCR, we found that mRNAs encoding trkC isoforms with kinase domain inserts were present at levels comparable to that for the unmodified receptor. TrkA mRNA expression was not detected in RSNs, and the expression of p75 was restricted to a small subpopulation of axotomized cells. In agreement with the pattern of trk receptor expression, infusion of recombinant human BDNF or NT-4/5 into the vicinity of the axotomized RSNs, between days 7 and 14 after axotomy, fully prevented their atrophy. This effect was still evident 2 weeks after the termination of BDNF treatment. Moreover, BDNF or NT-4/5 treatment stimulated the expression of GAP-43 and Talpha1-tubulin mRNA and maintained the level of trkB expression. Vehicle, NGF, or NT-3 treatment had no significant effect on cell size or GAP-43 and Talpha1-tubulin expression. In a separate experiment, infusion of BDNF also was found to increase the number of axotomized RSNs that regenerated into a peripheral nerve graft. Thus, in BDNF-treated animals, the prevention of neuronal atrophy and the stimulation GAP-43 and Talpha1-tubulin expression is correlated with an increased regenerative capacity of axotomized RSNs.

Transplants and neurotrophic factors prevent atrophy of mature CNS neurons after spinal cord injury

Axotomy of mature rubrospinal neurons leads to a substantial atrophy of these neurons within days after surgery. In addition, these neurons do not successfully regenerate following axotomy. The relationship of atrophy to regenerative failure is not clear, and the signals which regulate these events have not been identified. However, it is possible that the atrophy of these neurons plays a role in preventing regeneration. In the present study, we evaluated the hypothesis that interventions which have been shown to promote growth of axotomized CNS neurons are also capable of reversing the axotomy-induced atrophy. To test this hypothesis, adults rats received thoracic spinal cord hemisection alone or in combination with transplants of fetal spinal cord tissue and/or neurotrophic factor support. Our data indicate that application of either transplants or neurotrophic factors partially reverse the axotomy-induced atrophy in rubrospinal neurons, but that both interventions together reverse the atrophy completely. These results suggest that the same pathways that are activated to enhance growth of rubrospinal neurons after axotomy may also be involved in the maintenance of cell morphology.

Axotomized rubrospinal neurons rescued by fetal spinal cord transplants maintain axon collaterals to rostral CNS targets

Neurons that maintain extensive axon collaterals proximal to the site of axotomy may be better able to survive injury. Early lesions of the rubrospinal tract lead to retrograde cell death of the majority of axotomized immature neurons. Transplants of fetal spinal cord tissue rescue axotomized rubrospinal neurons and promote their axonal regeneration. Rubrospinal neurons develop many of their axon collaterals postnatally. The present study tests the hypothesis that the axotomized rubrospinal neurons that are rescued by transplants and regenerate their axons are those neurons that have established axon collaterals to targets rostral to the lesion. Neonatal rats received a transplant of fetal spinal cord tissue placed into a midthoracic spinal cord hemisection. One month after transplantation, the retrogradely transported fluorescent tracers fast blue (FB) and diamidino yellow (DY) were used to identify rubrospinal neurons with collaterals to particular targets. FB was injected either into the interpositus nucleus of the cerebellum or into the gray matter of the cervical enlargement to identify collaterals to these targets, and DY was injected into the spinal cord approximately 5 mm caudal to the transplant and lesion site to label retrogradely the neurons that regenerated their axons. Double labeling was observed in the axotomized neurons of the red nucleus after tracer injections into the cervical spinal cord but not after injections into the cerebellum. This labeling pattern indicates that axotomized rubrospinal neurons that are rescued and regenerate axons caudal to the transplant maintain axon collaterals at cervical spinal cord levels. Cerebellar collaterals do not appear to play a role in the survival and regrowth of axotomized rubrospinal neurons.

Expression of beta-calcitonin gene-related peptide in axotomized rubrospinal neurons and the effect of brain derived neurotrophic factor

The mRNA levels for alpha- and beta-calcitonin gene-related peptide (CGRP) in rat rubrospinal neurons were studied by in situ hybridization 3, 7, 14, 28 and 56 days following cervical spinal hemisection. CGRP-like immunoreactivity (LI) in the rubrospinal neurons and the rubrospinal tract in cervical spinal cords were examined using immunohistochemistry. There was almost no signal for alpha- and beta-CGRP mRNAs and undetectable level of CGRP-LI in the rubrospinal neurons ipsilateral to cervical spinal hemisection (control side). Fourteen days after spinal hemisection, the rubrospinal neurons contralateral to cervical hemisection (axotomized side) showed CGRP-LI in their cell bodies, and CGRP containing fibers were observed in the lateral funiculi just proximal, but not distal, to the injury sites. In situ hybridization showed upregulation of beta-CGRP mRNA in a subpopulation of the rubrospinal neurons on the axotomized side. The proportion of beta-CGRP mRNA-expressing neurons reached its maximum (approximately 19%) 4 days following axotomy and slowly decreased to about 5% 56 days after axotomy. The percentage of alpha-CGRP mRNA-expressing neurons was much lower than that of beta-CGRP mRNA (maximum about 2.6% 4 days after axotomy) and not significantly different from the control side throughout the time period studied. These data indicate that axotomy induces de novo synthesis of the CGRP beta-subtype in rubrospinal neurons and that the beta-CGRP is transported to the injury site through the rubrospinal tract. In addition, we studied the effect of the intracerebral injections of brain derived neurotrophic factor (BDNF). BDNF treatment fully reversed the severe cell atrophy that followed axotomy and increased the number of neurons labeled for beta-CGRP mRNA, but did not increase the percentage of rubrospinal neurons expressing beta-CGRP mRNA. Thus, topical application of BDNF does not have direct modulatory effect on CGRP induction in axotomized neurons in the red nucleus.

The c-Jun transcription factor--bipotential mediator of neuronal death, survival and regeneration

Axon interruption elicits a complex neuronal response that leaves neurons poised precariously between death and regeneration. The signals underlying this dichotomy are not fully understood. The transcription factor c-Jun is one of the earliest and most consistent markers for neurons that respond to nerve-fiber transection, and its expression can be related to both degeneration and survival including target re-innervation. In vitro experiments have demonstrated that expression of c-Jun can kill neonatal neurons but, in the adult nervous system, c-Jun might also be involved in neuroprotection and regeneration. The functional characteristics of c-Jun offer a model for the ability of a single molecule to serve as pivotal regulator for death or survival, not only in the response of the cell body to axonal lesions but also following neurodegenerative disorders. In this model, the fate of neurons is determined by a novel transcriptional network comprising c-Jun, ATF-2 (activating transcription factor-2) and JNKs (c-Jun N-terminal kinases).

Injury induced expression of growth-associated protein-43 in adult mouse retinal ganglion cells in vitro

In optic fibers, as in most axons of the central nervous system, the axonal growth-associated protein, GAP-43, is abundant during development but absent in adults. Since optic fibers can be induced to regenerate in culture, we examined whether this was associated with an increased expression of GAP-43 in adult mouse optic fibers that were regenerating from organotypic retinal explants on to laminin substrates. We found that simply placing adult mouse retina in culture under serum-free conditions was sufficient to induce GAP-43, which was detectable after about four to five days in vitro, coincident with the initiation of neurite outgrowth. In explants taken from animals in which the optic nerve was crushed in the orbit eight days prior to culturing, GAP-43 was observed within one day, as was neurite outgrowth. This priming effect was also seen in vivo as an increased level of GAP-43 reactivity in retinal ganglion cells and optic fibers in histological sections taken eight days after nerve crush. Reactivity in the adult fibers in culture was comparable to that observed in optic neurites growing from embryonic retinal explants and could be maintained for at least four weeks in culture. In the adult neurites, especially with longer times in culture, GAP-43 tended to be concentrated into varicosities that were often found in terminal-like arbors that formed in culture. Placing adult retina in culture under serum-free conditions in sufficient to induce re-expression of GAP-43 for an indefinite period of time. This suggests that GAP-43 expression and the propensity for growth in vivo may be repressed by a factor that is absent in vitro.

Persistence of rubrospinal projections following spinal cord injury in the rat

Recent neurophysiological and magnetic resonance imaging studies of clinically "complete" human spinal cord injuries indicate that some patients have considerable subclinical function and substantial morphological integrity of descending spinal tracts. In contrast, extensive histopathological changes, including cell death, have been described in central nervous system nuclei projecting to the cord following experimental transection or hemisection of the spinal cord in animal models. We have used a rodent model of severe compression cord injury that more closely resembles the clinical pathophysiology to investigate the extent of the persistence of the rubrospinal projection. Counts of red nucleus neurons retrogradely labelled with Fluorogold demonstrate that in contrast to the results obtained with transection models, compression injuries of the spinal cord do not result in massive loss of rubrospinal projections, at least up to 8 weeks postinjury. The results also suggest that many of the axons persist distal to the lesion site and that they are functionally intact with respect to retrograde transport capabilities.

The response of rubrospinal neurons to axotomy at different stages of development in the North American opossum

Rubral axons can grow around a lesion of their pathway in the thoracic spinal cord of developing opossums and a critical period exists for that plasticity. The critical period probably begins when rubral axons first grow into the thoracic cord, and it extends until approximately postnatal day 30. We previously noted that most rubrospinal neurons die after transection of their axon during the critical period, suggesting that plasticity results primarily from growth of axons not damaged by the lesion (Xu and Martin, J. Comp. Neurol. 279, 368-381, 1989). That observation led us to study the response of rubrospinal neurons to axotomy in more detail and at additional stages of development, using a prelabeling paradigm. We first injected fast blue (FB) into the caudal thoracic or rostral lumbar spinal cord in animals ranging from estimated postnatal day 9 to 50 and, about 4 days later, lesioned the rubrospinal tract several segments rostral to the injection. Approximately 30 days later, the animals were killed so that the red nucleus could be searched for labeled neurons. During the critical period for plasticity, rubrospinal neurons showed signs of degeneration 1 week after their axon was cut. When animals were killed 2-3 weeks after lesioning, there was an obvious decrease in axotomized neurons within the red nucleus, and by 4 weeks, more than 75% of them had degenerated. The marked susceptibility of rubrospinal neurons to axotomy during the critical period for plasticity is consistent with the hypothesis that developmental plasticity of the rubrospinal tract results primarily from growth of axons that were not damaged by the lesion. Our results also suggest that survival of axotomized rubrospinal neurons increases with age.

Perineuronal glial responses after axotomy of central and peripheral axons. A comparison

In each of 6 mature rats, unilateral rubrospinal tractotomy and hypoglossal neurectomy were done at one operative sitting. Paired operated animals were killed by formaldehyde and ethanol-acetic acid perfusion 3, 14 and 28 days later. One pair of unoperated control rats was perfused also. All rats were injected i.p. with [3H]thymidine 24 h before death. Immunohistochemical methods were applied to paraffin sections to visualize glial fibrillary acidic protein (GFAP) and transferrin in astrocytes and oligodendroglia, respectively. Microglia were demonstrated by both lectin-binding and histoautoradiographic methods. Neuroglia and nerve cells were counted in hematoxylin-eosin and azure B stains. Cell areas and the RNA concentration of hypoglossal neurons were determined by the Zeiss Image Scan System. Three days after hypoglossal neurectomy, increased astroglial staining (GFAP) and microglial hyperplasia (radiolabeled nuclei) were evident in the ipsilateral hypoglossal nucleus (HN). Microglial hyperplasia waned rapidly after 3 days and microglial numbers decreased. However, astroglial hypertrophy, demonstrable by GFAP staining, persisted 4 weeks postoperatively when astroglial processes were concentrated in a perineuronal position. Oligodendroglia were unaltered. In contrast to the HN, the axotomized red nucleus (RN) contained few radiolabeled microglia while a slight increase in GFAP-positive astroglial processes was seen only in animals killed 28 days postoperatively. Again, oligodendroglia were unchanged. In neither HN nor RN did axotomy cause nerve cell death. Although axotomized rubral neurons atrophy and become depleted of RNA, no statistically significant changes in somal size and RNA content of axotomized hypoglossal neurons occurred. The apparent absence of a neuroglial response of putatively supportive nature in the environs of axotomized rubral neurons may relate to their failure to regenerate. The neuroglial response likely is originated by the axotomized neuron and its absence may be an innate defect in the reaction of intrinsic neurons to axonic severance. Somas of axotomized peripherally projecting nerve cells appear to have the capacity to summon a neuroglial response.