Both positive and negative factors regulate gene expression following chronic facial nerve resection

Cytokine and Growth Factor Activation In Vivo and In Vitro after Spinal Cord Injury

Spinal cord injury results in a life-disrupting series of deleterious interconnected mechanisms encompassed by the primary and secondary injury. These events are mediated by the upregulation of genes with roles in inflammation, transcription, and signaling proteins. In particular, cytokines and growth factors are signaling proteins that have important roles in the pathophysiology of SCI. The balance between the proinflammatory and anti-inflammatory effects of these molecules plays a critical role in the progression and outcome of the lesion. The excessive inflammatory Th1 and Th17 phenotypes observed after SCI tilt the scale towards a proinflammatory environment, which exacerbates the deleterious mechanisms present after the injury. These mechanisms include the disruption of the spinal cord blood barrier, edema and ion imbalance, in particular intracellular calcium and sodium concentrations, glutamate excitotoxicity, free radicals, and the inflammatory response contributing to the neurodegenerative process which is characterized by demyelination and apoptosis of neuronal tissue.

Modulation of satellite cells in rat facial muscle following denervation and delayed reinnervation

CONCLUSION

Long-term denervation-induced satellite cells (SCs) deficiency impairs facial muscle regenerative capacity. Delayed reinnervation can reactivate residual SCs to engage in muscle regeneration. However, the underlying mechanism remains to be elucidated.

OBJECTIVE

To evaluate the effects of denervation and delayed reinnervation on SCs in facial muscle.

METHODS

This was a prospective, randomized, controlled study in the rat facial nerve ligation and delayed decompression model. Animals were divided into denervation, 8-week-delay, and 16-week-delay reinnervation groups. Sham-operated animals served as a control group. Specific markers were used to investigate the differences in SC status, including quiescent (Pax7) and activated (myoD and myogenin) SCs and regenerative myofibers (embryonic myosin heavy chain, eMyHC). Quantitative assessment was performed by real-time PCR and Western blotting.

RESULTS

Activated SCs were detected 2-4 weeks after denervation and maintained for 4-8 weeks, accompanied by regenerating myofibers, whereas no SCs were detected beyond 20 weeks post-denervation. The myoD and myogenin up-regulation peaked 6-8 weeks after denervation and declined gradually to normal baseline 12 weeks after denervation. The 8-week-delay reinnervation group showed more activated SCs and regenerating myofibers than the 16-week-delay group, as well as greater up-regulation of myoD and myogenin (p < 0.05), suggesting reactivation of SCs for repair of adjacent fibers.

A guidance channel seeded with autologous Schwann cells for repair of cauda equina injury in a primate model

BACKGROUND/OBJECTIVE

To evaluate an implantable guidance channel (GC) seeded with autologous Schwann cells to promote regeneration of transected spinal nerve root axons in a primate model.

METHODS

Schwann cells were obtained from sural nerve segments of monkeys (Macaca fascicularis; cynomolgus). Cells were cultured, purified, and seeded into a PAN/PVC GC. Approximately 3 weeks later, monkeys underwent laminectomy and dural opening. Nerve roots of the L4 through L7 segments were identified visually. The threshold voltage needed to elicit hindlimb muscle electromyography (EMG) after stimulation of intact nerve roots was determined. Segments of 2 or 3 nerve roots (each approximately 8-15 mm in length) were excised. The GC containing Schwann cells was implanted between the proximal and distal stumps of these nerve roots and attached to the stumps with suture. Follow-up evaluation was conducted on 3 animals, with survival times of 9 to 14 months.

RESULTS

Upon reexposure of the implant site, subdural nerve root adhesions were noted in all 3 animals. Several of the implanted GC had collapsed and were characterized by thin strands of connective tissue attached to either end. In contrast, 3 of the 8 implanted GC were intact and had white, glossy cables entering and exiting the conduits. Electrical stimulation of the tissue cable in each of these 3 cases led to low-threshold evoked EMG responses, suggesting that muscles had been reinnervated by axons regenerating through the repair site and into the distal nerve stump. During harvesting of the GC implant, sharp transection led to spontaneous EMG in the same 3 roots showing a low threshold to electrical stimulation, whereas no EMG was seen when harvesting nerve roots with high thresholds to elicit EMG. Histology confirmed large numbers of myelinated axons at the midpoint of 2 GC judged to have reinnervated target muscles.

CONCLUSIONS

We found a modest rate of successful regeneration and muscle reinnervation after treatment of nerve root transection with a Schwann cell-seeded, implanted synthetic GC. Newer treatments, which include the use of absorbable polymers, neurotrophins, and antiscar agents, may further improve spinal nerve regeneration for repair of cauda equina injury.

Neuroprotection from diazinon-induced toxicity in differentiating murine N2a neuroblastoma cells

In previous work, the outgrowth of axon-like processes by differentiating mouse N2a neuroblastoma cells was shown to be inhibited by exposure to 10 microM diazinon. In the present work, N2a cells were induced to differentiate for 24 h in the presence and absence of 10 microM diazinon and 20% (v/v) conditioned medium derived from differentiating rat C6 glioma cells. Cells were then stained or lysed for morphological and biochemical analyses, respectively. The data showed that co-treatment with conditioned medium prevented the neurite inhibitory effect of diazinon. Furthermore, a significant recovery was also observed in the reduced levels of neurofilament heavy chain (NFH), heat shock protein-70 (HSP-70) and growth-associated protein-43 (GAP-43) observed as a result of diazinon treatment in the absence of conditioned medium, as seen by densitometric analysis of Western blots of cell lysates probed with monoclonal antibodies N52, BRM-22 and GAP-7B10. By contrast, no significant change was noted in the reactivity of cell lysates with antibodies against alpha- and beta-tubulin under any condition tested. After pre-incubation with a polyclonal anti-glial cell line-derived neurotrophic factor (GDNF) antibody, conditioned medium derived from rat C6 glioma cells lost its ability to protect N2a cells against the neurite inhibitory effects of diazinon. In conclusion, these data demonstrate that C6 conditioned medium protects N2a cells from the neurite inhibitory effects of diazinon by blocking molecular events leading to axon damage and that GDNF is implicated in these effects.

Local delivery of glial cell line-derived neurotrophic factor improves facial nerve regeneration after late repair

OBJECTIVES/HYPOTHESIS

Facial nerve regeneration is limited in some clinical situations: in long grafts, by aged patients, and when the delay between nerve lesion and repair is prolonged. This deficient regeneration is due to the limited number of regenerating nerve fibers, their immaturity and the unresponsiveness of Schwann cells after a long period of denervation. This study proposes to apply glial cell line-derived neurotrophic factor (GDNF) on facial nerve grafts via nerve guidance channels to improve the regeneration.

METHODS

Two situations were evaluated: immediate and delayed grafts (repair 7 months after the lesion). Each group contained three subgroups: a) graft without channel, b) graft with a channel without neurotrophic factor; and c) graft with a GDNF-releasing channel. A functional analysis was performed with clinical observation of facial nerve function, and nerve conduction study at 6 weeks. Histological analysis was performed with the count of number of myelinated fibers within the graft, and distally to the graft. Central evaluation was assessed with Fluoro-Ruby retrograde labeling and Nissl staining.

RESULTS

This study showed that GDNF allowed an increase in the number and the maturation of nerve fibers, as well as the number of retrogradely labeled neurons in delayed anastomoses. On the contrary, after immediate repair, the regenerated nerves in the presence of GDNF showed inferior results compared to the other groups.

CONCLUSIONS

GDNF is a potent neurotrophic factor to improve facial nerve regeneration in grafts performed several months after the nerve lesion. However, GDNF should not be used for immediate repair, as it possibly inhibits the nerve regeneration.

T cell memory in the injured facial motor nucleus: relation to functional recovery following facial nerve crush

T cells have the ability to mount a memory response to a previously encountered antigen such that re-exposure to the antigen results in a response that is greater in magnitude and function. Following facial nerve transection, T cells have been shown to traffic to injured motor neurons in the facial motor nucleus (FMN) and may have the ability to promote neuronal survival and functional recovery. Previously, we demonstrated that early exposure to neuronal injury on one side of the brain during young adulthood elicited a T cell response that was greater in magnitude following exposure to the same form of injury on the contralateral side later in adulthood. Whether the T cell memory response to neuronal injury influenced functional recovery following nerve crush injury was unknown. In the current study, we tested the hypotheses that (1) transection of the right facial nerve in sensitized mice would result in faster recovery of the whisker response when the contralateral facial nerve is crushed 10 weeks later, and (2) the early recovery would be associated with an increase in the magnitude of the T cell response in the contralateral FMN following crush injury in sensitized mice. The onset of modest recovery in sensitized mice occurred between 3 and 5 days following crush injury of the contralateral facial nerve, approximately 1.5 days earlier than naïve mice, and was associated with more than a two-fold increase in the magnitude of the T cell response in the contralateral FMN following crush injury. There was no difference between groups in the number of days to full recovery. Further study of how T cell memory influences neuroregeneration may have important implications for translational research.

Neuroregenerative effects of lentiviral vector-mediated GDNF expression in reimplanted ventral roots

Traumatic avulsion of spinal nerve roots causes complete paralysis of the affected limb. Reimplantation of avulsed roots results in only limited functional recovery in humans, specifically of distal targets. Therefore, root avulsion causes serious and permanent disability. Here, we show in a rat model that lentiviral vector-mediated overexpression of glial cell line-derived neurotrophic factor (GDNF) in reimplanted nerve roots completely prevents motoneuron atrophy after ventral root avulsion and stimulates regeneration of axons into reimplanted roots. However, over the course of 16 weeks neuroma-like structures are formed in the reimplanted roots, and regenerating axons are trapped at sites with high levels of GDNF expression. A high local concentration of GDNF therefore impairs long distance regeneration. These observations show the feasibility of combining neurosurgical repair of avulsed roots with gene-therapeutic approaches. Our data also point to the importance of developing viral vectors that allow regulated expression of neurotrophic factors.

Immunodeficiency impairs re-injury induced reversal of neuronal atrophy: relation to T cell subsets and microglia

Following facial nerve resection in the mouse, a substantial number of neurons reside in an atrophied state (characterized by cell shrinkage and decreased ability to uptake Nissl stain), which can be reversed by re-injury. The mechanisms mediating the reversal of neuronal atrophy remain unclear. Although T cells have been shown to prevent neuronal loss following peripheral nerve injury, it was unknown whether T cells play a role in mediating the reversal of axotomy-induced neuronal atrophy. Thus, we used a facial nerve re-injury model to test the hypothesis that the reversal of neuronal atrophy would be impaired in recombinase activating gene-2 knockout (RAG-2 KO) mice, which lack functional T and B cells. Measures of neuronal survival were compared in the injured facial motor nucleus (FMN) of RAG-2 KO and wild-type (WT) mice that received a resection of the right facial nerve followed by re-injury of the same nerve 10 weeks later ("chronic resection+re-injury") or a resection of the right facial nerve followed by sham re-injury of the same nerve 10 weeks later ("chronic resection+sham"). We recently demonstrated that prior exposure to neuronal injury elicited a marked increase in T cell trafficking indicative of a T cell memory response when the contralateral FMN was injured later in adulthood. We examined if such a T cell memory response would also occur in the current re-injury model. RAG-2 KO mice showed no reversal of neuronal atrophy whereas WT mice showed a robust response. The reversal of atrophy in WT mice was not accompanied by a T cell memory response. Although the number of CD4(+) and CD8(+) T cells in the injured FMN did not differ from each other, double-negative T cells appear to be recruited in response to neuronal injury. Re-injury did not result in increased expression of MHC2 by microglia. Our findings suggest that T cells may be involved in reversing the axotomy-induced atrophy of injured neurons.

PROLONGED TARGET DEPRIVATION REDUCES THE CAPACITY OF INJURED MOTONEURONS TO REGENERATE

OBJECTIVE

To investigate whether or not it is the frustrated growth state (no axon growth) that reduces regenerative capacity or the inability of axotomized motoneurons to remake muscle connections (axon growth-no muscle contact) that accounts for poor regenerative capacity of chronically axotomized motoneurons.

METHODS

We chronically axotomized rat femoral motoneurons for 2 months by cutting the nerve and either capping the proximal nerve to prevent axon regeneration (Group 1, no axon growth for 2 mo) or encouraging axon regeneration but not target reinnervation by suture to the distal stump of cut saphenous nerve (Group 2, axon growth with no muscle contact). In the control fresh axotomy group (axon growth with muscle contact), femoral nerve stumps were resutured immediately. Two months later, the femoral nerve was recut and sutured immediately to encourage regeneration in a freshly cut saphenous nerve stump for 6 weeks. Regenerating axons in the saphenous nerve were back-labeled with fluorogold for enumeration of the femoral motoneurons that regenerated their axons into the distal nerve stump.

RESULTS

We found that significantly fewer chronically axotomized motoneurons regenerated their axons than freshly axotomized motoneurons that regenerated their axons to reform nerve-muscle connections in the same length of time. The number of motoneurons that regenerated their axons was reduced in both the conditions of no axon growth and axon growth with no muscle contact; thus chronic axotomy for a 2-month period reduced regenerative success irrespective of whether the motoneurons were prevented from regenerating or encouraged to regenerate their axons in that same period of time.

CONCLUSION

Axonal regeneration does not protect motoneurons from the negative effects of prolonged axotomy on regenerative capacity. It is the period of chronic axotomy, in which motoneurons remain without target nerve-muscle connection, and not simply a state of frustrated growth that accounts for the reduced regenerative capacity of those neurons.

Neurotrophins and peripheral neuropathies

Peripheral neuropathy is a common disorder seen in general neurology and neuromuscular specialty clinics. Treatment options directed at the underlying cause can only be offered in a handful of conditions, such as those with possible autoimmune etiology. The remainder fall into the idiopathic or genetic category with no known treatment. This review surveys the evidence supporting the rationale for the therapeutic use of neurotrophins and other neurotrophic factors in these disorders in relationship to the underlying pathobiological process. Previous clinical trials are assessed, and increasingly better understood and appreciated therapeutic potential of neurotrophins is emphasized.

Prior facial motor neuron injury elicits endogenous T cell memory: relation to neuroregeneration

We tested the hypotheses that prior injury to the facial motor nucleus (FMN) would elicit a more robust T cell response in the opposite FMN when the contralateral facial nerve was injured later in life, and that this would result in improved neuroregeneration. Measures of T cell, neuronal and microglial status were compared in sensitized mice (right facial nerve transection followed by contralateral facial nerve transection 9.5 weeks later) and naïve mice (sham surgery of the right facial nerve followed by contralateral facial nerve transection 9.5 weeks later) following axotomy of the contralateral facial nerve. At day 14 post-axotomy, sensitized mice exhibited nearly a two-fold increase in T cells in the FMN compared to naïve mice. There were no differences between the groups in levels of dead neurons and NeuN expression by surviving motor neurons at day 14, or motor neuron survival and cell area at day 49 post-axotomy. Measures of microglial responsiveness did not differ between the groups. Further study is needed to delineate the role of endogenous T cell memory in neuronal injury and regeneration.