Intrinsic and extrinsic mechanisms of corticospinal axon regeneration

Growth factors are indispensable in the formation and maintenance of neuronal circuits during development. We investigated the ability of manipulating key developmental signaling cascades to recapitulate a growth state in lesioned corticospinal motor neurons (CSMNs). We hypothesized that stimulation of axotomized CSMNs with insulin-like growth factor I (IGF-I), which robustly promotes neonatal CSMN axon outgrowth, would induce regenerative growth in the adult. We found IGF-I to promote the survival of axotomized CSMNs but not to promote their regeneration following either a cervical spinal cord lesion or a lesion at the level of the internal capsule. As an alternative approach to solely providing a source of growth factor to axotomized CSMNs, we proposed that over-expression of a growth factor receptor would alter the intrinsic capacity of the cell and induce regeneration in response to its ligand. Utilizing viral transgene delivery, we over-expressed the high-affinity brain derived neurotrophic factor (BDNF) receptor trkB in layer V motor cortex. With subcortical axotomy and BDNF-secreting grafts, we observed regenerative axonal growth from identified CSMNs. We then attempted to refine a gene therapy technique for delivering a therapeutic transgene to the central nervous system via peripheral injection. We tested the ability of self-complementary adeno-associated virus (scAAV) serotypes 1-6 to retrogradely infect lower motor neurons following injection to either the neuromuscular junction or within the sciatic nerve. We found scAAV1 to be the most efficient for retrograde infection and anticipate exploiting this ability for delivering chemoattractive guidance cues to regenerating axon