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The inability of the adult injured mammalian spinal cord to successfully regenerate is not well understood. Studies suggest that both extrinsic and intrinsic factors contribute to regeneration failure. In this review, we focus on intrinsic factors that impact regeneration, in particular integrin receptors and their downstream signaling pathways. We discuss studies that address the impact of integrins and integrin signaling pathways on growth cone guidance and motility and how lessons learned from these studies apply to spinal cord regeneration in vivo.

Extrinsic factors appear to contribute to the lack of regeneration in the injured adult spinal cord. It is likely that these extrinsic factors include a group of putative growth inhibitory molecules known as chondroitin sulfate proteoglycans (CSPGs). The aims of this study were to determine: (1) the consequences of spinal cord contusion injury on CSPG expression, (2) if CSPGs can be degraded in vivo by exogenous enzyme application, and (3) the effects of intraspinal transplantation on the expression of CSPGs. Chondroitin 6-sulfate proteoglycan immunoreactivity (CSPG-IR) dramatically increased following spinal cord contusion injury both at and adjacent to the injury site compared to normal controls (no surgical procedure) and laminectomy-only controls by 4 days postinjury. The dramatic increase in CSPG-IR persisted around the lesion and in the dorsal one-half to two-thirds of the spinal cord for at least 40 days postinjury. Glial fibrillary acidic protein (GFAP)-IR patterns were similarly intensified and spatially restricted as CSPG-IR patterns. These results suggest that: (1) CSPGs may contribute to the lack of regeneration following spinal cord injury and (2) astrocytes may contribute to the production of CSPGs. In addition, our results show that CSPGs could be cleaved in vivo with exogenous chondroitinase ABC application. This demonstration of cleavage may the basis for a model to directly assess CSPGs' role in growth inhibition in vivo (studies in progress) and hold potential as a therapeutic approach to enhance growth. Interestingly, the robust, injury-induced CSPG-IR patterns were not altered by intraspinal grafts of fetal spinal cord. The CSPG expression profile in the host spinal cord was similar to time-matched contusion-only animals. This was also true of GFAP-IR patterns. Furthermore, the fetal spinal cord tissue, which was generally CSPG negative at the time of transplantation, developed robust CSPG expression by 30 days posttransplantation. This increase in CSPG expression in the graft was paired with a moderate increase in GFAP-IR. CSPG-IR patterns suggest that these molecules may contribute to the limited regeneration seen following intraspinal transplantation. In addition, it suggests that the growth permissiveness of the graft may change overtime as CSPG expression develops within the graft. These correlations in the injured and transplanted spinal cord support CSPGs' putative growth inhibitory effect in the adult spinal cord.