Damage to the spinal cord results in permanent loss of sensation and motor control below the level of injury, because the long axons that connect the spinal cord to the brain fail to regenerate. This contrasts starkly with the vigorous regrowth of axons in injured peripheral nerves, where successful regeneration is closely correlated with the re-induction of many genes ordinarily expressed during development. Spinal cord lesions often fail to activate those genes, and the injured cells are unable to regenerate their axons. Using transgenic mice, we have found that expression of just two of those genes - coding for the growth cone proteins growth associated protein (GAP)-43 and cytoskeleton associated protein (CAP)-23 -- triggers a 60-fold increase in the ability of adult neurons to support regeneration of spinal cord axons in vivo. The small number of proteins required to activate regeneration raises the possibility that viral-mediated expression or other forms of direct gene replacement in injured neurons could become one key component of future strategies to stimulate spinal axon repair. The proposed studies constitute a test in principle of this strategy. We first will determine which combination(s) of GAP-43, CAP-23, and the related protein myristoylated alanine-rich C kinase substrate (MARCKS) are most effective in triggering the intrinsic ability of neurons to support axon regeneration, and the conditions under which viral-mediated expression of these genes can activate this growth competence after spinal cord or dorsal root injury in normal (non-transgenic) adult animals.
The second aim i s to identify specific contexts in which expression of GAP-43, CAP-23 and/or MARCKS may overcome the effects of growth-inhibiting molecules in the spinal cord environment, and conversely to identify those inhibitory elements that continue to impair axon extension by neurons expressing GAP-43, CAP-23 and MARCKS. Together, the proposed studies outline the essential features of a new strategy for overcoming one key hurdle to central nervous system (CNS) axon repair, and provide a guide to additional treatments that would need to be combined with this gene therapy approach in order to obtain effective regeneration of spinal axons in vivo.
