During a spinal cord injury (SCI), irreversible traumatic damage is caused to the axon tracts that travel in the white matter of the spinal cord for connection between the brain and the rest of the body, including the corticospinal tract (CST), a tract that is essential to voluntary fine motor control in humans. Like other neurons in the central nervous system, CST neurons cannotspontaneously regenerate, as intrinsic, and extrinsic factors closely regulate them. Recent studies from our lab demonstrate robust regeneration of the CST when a neural progenitor cell (NPC) graft is placed into the injury site. However, our preliminary data shows that after mid-cervical SCI, robust regeneration only occurs for hindlimb CST (H-CST) but not forelimb CST (F-CST). To promote forelimb CST regeneration, we combined both an extrinsic stimulus, such as the provision of an NPC graft, and intrinsic manipulation, such as the co-deletion of two protooncogenes, phosphatase and tensin homologue (PTEN) and suppressor of cytokine signaling 3 (SOCS3) specifically in F-CST neurons, to amplify injury-induced signals for regeneration in a mouse model. The combinatory approach promoted robust F-CST axon regeneration compared to the graft-only treated mice. Additionally, the PTEN/SOCS3 co-deletion with NPC grafting showed robust axon regeneration across the whole injury site to reach the caudal border of the graft compared to the graft-only mice, a feature required to make functional neuronal relays crossing the injury site. Indeed, co-deletion of PTEN/SOCS3 enabled graft-derived neurons to function as descending relay neurons to transmit supraspinal signals to host spinal cord neurons below the injury site. Thus, the robust regeneration of F-CST reveals the importance of intrinsic regulation factors, such as the manipulation of genes crucial for regeneration in combination with the extrinsic support of an NPC graft to promote regeneration after SCI.

Although our previous studies demonstrate robust regeneration of corticospinal tract (CST) into neural progenitor cell (NPC) grafts placed into spinal cord injury (SCI) site, our recent preliminary data unexpectedly found that only hindlimb CST regenerated and forelimb CST rarely regenerated after mid-cervical SCI. This limited regeneration may be due to small proportional injury of forelimb CST and remaining synaptic connections above SCI, which is consistent with a recent new hypothesis, synaptic suppression of axon regeneration. In order to promote forelimb CST regeneration, we targeted to silence synaptic activity transiently by administration of Gabapentin, a pharmacological agent that blocks the Alpha2delta2 subunit of voltage-gated calcium channels involved in regulation of synaptic activity. Adult rats received mid-cervical SCI and NPC graft-expressing green fluorescent protein (GFP) followed by Gabapentin administration for two weeks. The forelimb-CST were specifically traced by injection of adeno-associated virus (AAV) expressing red fluorescent protein (RFP) and animals survived additional 3 weeks. The experimental group receiving Gabapentin did not show a dramatic increase of forelimb-CST regeneration as hindlimb CST, but a trend toward more regeneration compared to the control group. Further experiment with higher dosage and more frequent administration needs to confirm whether Gabapentin can elicit forelimb CST regeneration and has potential for treatment of SCI.