Chemogenetic Attenuation of Acute Nociceptive Signaling Enhances Functional Outcomes Following Spinal Cord Injury and Dissecting Connectivity of Transplanted Neural Progenitor Cell Grafts in the Injured Spinal Cord

Abstract

Spinal cord injury (SCI) is a traumatic and debilitating injury that frequently results in permanent neurological dysfunction, including chronic neuropathic pain and paralysis. These consequences drastically reduce quality of life for SCI patients. While the study presented in chapter 2 aims to mitigate maladaptive dorsal root ganglion (DRG) nociceptor hyperactivity to prevent neuropathic pain, the study in chapter 3 underscores the importance of providing appropriate subtypes of neural progenitor cell (NPC) grafts to promote regeneration of forelimb motor circuits following SCI. We first hypothesized that silencing the activity of nociceptors early after injury will improve long-term functional outcomes. We utilized inhibitory Gi-DREADDs to selectively silence nociceptors during the acute phase of a clinically relevant thoracic contusion SCI model in adult female rats. Gi-DREADD expression was restricted to small-diameter nociceptors including CGRP+, substance P+, and IB4-binding neurons. Through analysis of behavioral outcomes, we observed significantly reduced thermal hyperalgesia, and greater hindlimb locomotor recovery in subjects that received acute nociceptor silencing, compared to controls. Histological assessments of spinal cord tissue suggest a trend showing reduced lesion volume, increased neuronal sparing and increased CGRP+ axon sprouting in Gi-DREADD treated subjects. Together, these findings suggest that nociceptor silencing early after SCI may promote beneficial plasticity in the acute phase of injury that can impact long-term functional outcomes. Next, we hypothesized that NPC grafts enriched for ventral spinal cord neuron subtypes will support more extensive synaptic connectivity with injured host CNS motor circuitry compared to standard whole cord grafts. We have transplanted either whole spinal cord embryonic NPCs or ventrally enriched NPCs into a C5 dorsal column lesion model of SCI, followed by transsynaptic rabies tracing to identify direct synaptic connections between the host and the graft. Through immunohistochemical methods of analysis we have observed that graft type does not influence survival and neuronal differentiation of transplanted NPCs. We have identified that rabies infection is present in higher numbers in ventrally enriched grafts compared to whole spinal cord NPC grafts. However, most of the RabV-GFP+ expression was found within grafts, with minimal expression in neurons outside the graft in the host CNS. We have also observed some graft RabV-GFP+ axons extending outside the graft ROI into host tissue. We will further characterize the specific graft neuron subpopulations that are extending axons and forming connections with the host. Together, our work highlights approaches to improve regeneration and functional recovery following spinal cord injury.