Influencing axon regeneration after spinal cord injury

Abstract

Spinal cord injury (SCI) results in a severing of axonal connections that leads to permanent sensorimotor deficits. In cases of incomplete injury, rehabilitation paradigms have been reported to be successful in improving behavioral outcome in humans with SCI. However, in cases of more anatomically complete injury, which leaves little or no spared axon projections below the injury, rehabilitation is ineffective. For decades, it has been realized that restoration of function will likely require the regrowth of severed axons through lesion sites. Unfortunately, axons in the mature mammalian central nervous system (CNS) fail to regenerate following injury due to a low intrinsic neuronal growth capacity as well as a non-supportive lesion environment. Although much progress has been made in the field of axon regeneration, current strategies produce only relatively little growth past SCI lesions. A more thorough understanding of the mechanisms governing growth of particular neuronal systems will be critical for developing therapeutic approaches to restore axon connectivity across lesion sites that may lead to behavioral improvement. This thesis is divided into three chapters. In chapter one, I provide a literature review of various reasons for axon regenerative failure and potential strategies to overcome these challenges. In chapter two, I describe two cell-grafting approaches that I investigated to bridge axons across SCI lesions. In the first approach, I replicated a promising cell grafting technique and performed a pilot experiment where we combined cell grafting with neuroprosthetic rehabilitation. While a subset of these rats had continuous bridges spanning the lesion site, we failed to observe an increase in behavioral recovery. This may have been the result of i) undirected and excessive growth of graft derived axons, or ii) cell masses which migrated to ectopic locations in the spinal cord, which may have compressed tissue. In chapter three, I describe the majority of my thesis work, in which I have been characterizing the requirements for propriospinal axon regeneration past severe SCI lesions in both mice and rats. We used a biomaterials approach to deliver protein growth factors which modulated the lesion environment into upregulating substantive amounts of growth supportive laminin. A chemotropic stimulatory cue in the form of glial derived neurotropic factor (GDNF) was delivered both within and caudal to SCI lesions, allowing some axons to grow into host tissue. In the absence of chemotrophic guidance cues, axons failed to grow out of SCI lesions. This further reinforces the idea that the absence of facilitating factors is a primary reason for the failure of CNS neurons to regenerate following injury. In order to enhance this growth, we tested viral manipulations which have been previously demonstrated to increase growth of other CNS neurons, and combined this with hydrogel depot delivery of growth factors. We found that downregulation of PTEN failed to increase growth of propriospinal neurons. In contrast, over expression of IGF1/CNTF/OPN resulted in robust growth of propriospinal axons into and through severe SCI lesions. Axons grew into and through the lesion site towards the chemotropic hydrogel depot, but did not grow past this. The results in this thesis are, to the best of our knowledge, the most extensive degree of host axon regeneration past anatomically complete SCI lesions to date.