Experimental approaches to promote functional recovery after severe peripheral nerve injuries

Abstract

INTRODUCTION: Reduced regenerative capacity of chronically axotomized neurons and reduced growth support by atrophic Schwann cells are key factors that account for the poor functional outcomes after proximal nerve injuries. In this study we examine two strategies aimed to circumvent deleterious effects of chronic axotomy and chronic denervation on axonal regeneration: (1) exogenous application of neurotrophic factors to chronically axotomized motoneurons to reverse time-related decline in regenerative capacity and (2) intracellular elevations of cyclic AMP (via exogenous application of forskolin and transforming growth factor β [TGF-β]) to induce atrophic dormant Schwann cells to proliferate and re-enter the nonmyelinating growth-supportive phenotype that normally supports axonal regeneration. METHODS: (1) Rat tibial motoneurons were chronically axotomized for 2 months and brain-derived neurotrophic factor (BDNF) and/or glia-derived neurotrophic factor (GDNF) supplied exogenously prior to enumeration of retrogradely labeled neurons that regenerate their axons. (2) Sciatic nerve Schwann cells were chronically denervated for 24 weeks prior to preparation and 48 h in vitro incubation of the nerve explants with saline or forskolin/TGF-β and reinsertion in a 1 cm long silastic sheath between cut tibial nerve stumps in vivo. Motoneurons that regenerated axons were enumerated 6 months later by retrograde labeling. RESULTS: (1) Low-dose but not high-dose BDNF and/or GDNF significantly increased the numbers of TIB motoneurons that regenerated axons after chronic but not immediate axotomy. The BDNF positive effect was mediated via trkB receptors, the inhibitory effects of high-dose BDNF being blocked by a p75-blocking antibody. (2) Treatment of chronically denervated Schwann cells with forskolin/TGF-β more than doubled the number of motoneurons that regenerated their axons through the treated Schwann cells. Regenerated axons were also well myelinated. DISCUSSION: Deleterious effects of chronic axotomy and Schwann cell denervation may be reversed by exogenous application of neurotrophic factors and by cytokines, respectively. Such experimental approaches may translate into strategies to promote functional recovery after nerve repair of the injured peripheral nervous system.