Restoration of Direct Corticospinal Communication Across Sites of Spinal Injury

Abstract

Injury to the spinal cord often disrupts long-distance axon tracts that link the brain and spinal cord, causing permanent disability. Axon regeneration is then prevented by a combination of inhibitory signals that emerge at the injury site and by a low capacity for regeneration within injured neurons. The corticospinal tract (CST) is essential for fine motor control but has proven refractory to many attempted pro-regenerative treatments. Although strategies are emerging to create relay or detour circuits that reroute cortical motor commands through spared circuits, these have only partially met the challenge of restoring motor control. Here, using a murine model of spinal injury, we elevated the intrinsic regenerative ability of CST neurons by supplying a pro-regenerative transcription factor, KLF6, while simultaneously supplying injured CST axons with a growth-permissive graft of neural progenitor cells (NPCs) transplanted into a site of spinal injury. The combined treatment produced robust CST regeneration directly through the grafts and into distal spinal cord. Moreover, selective optogenetic stimulation of regenerated CST axons and single-unit electrophysiology revealed extensive synaptic integration by CST axons with spinal neurons beyond the injury site. Finally, when KLF6 was delivered to injured neurons with a highly effective retrograde vector, combined KLF6/NPC treatment yielded significant improvements in forelimb function. These findings highlight the utility of retrograde gene therapy as a strategy to treat CNS injury and establish conditions that restore functional CST communication across a site of spinal injury. Significance StatementDamage to the spinal cord results in incurable paralysis because axons that carry descending motor commands are unable to regenerate. Here we deployed a two-pronged strategy in a rodent model of spinal injury to promote regeneration by the corticospinal tract, a critical mediator of fine motor control. Delivering pro-regenerative KLF6 to injured neurons while simultaneously transplanting neural progenitor cells to injury sites resulted in robust regeneration directly through sites of spinal injury, accompanied by extensive synapse formation with spinal neurons. In addition, when KLF6 was delivered with improved retrograde gene therapy vectors, the combined treatment significantly improved forelimb function in injured animals. This work represents important progress toward restoring regeneration and motor function after spinal injury.

Gabapentinoid treatment promotes corticospinal plasticity and regeneration following murine spinal cord injury

“…Next, we tested whether corticospinal reorganization promotes recovery of forelimb function after cervical SCI. We first confirmed that corticospinal function is required during forelimb skilled walking (4). To test this, we injected AAV particles expressing the designer receptor exclusively activated by designer drug (DREADD) hM4Di (27) into the right sensory-motor cortex to transiently silence the corticospinal pathway on 1 side of adult naive mice when clozapine N-oxide (CNO) is administered (Supplemental Figure 7A).…”

“…Recovery of spinal cord function in adults might be attained by promoting axon sprouting, regeneration, and de novo formation of neural circuits (2)(3)(4). Among the descending pathways important for SCI repair, the corticospinal tract exerts the sensory and motor control that is necessary for accurate limb placement and voluntary movements (5).…”

Optimization of adeno-associated viral vector-mediated transduction of the corticospinal tract: comparison of four promoters

“…AAV6 [48] and a directed evolved variant of AAV2 named 'AAV2-Retro' [49] have the capability for retrograde transduction in the CNS. AAV2-retro has been applied for robust transduction of the CST by injection into the spinal cord of adult mice [50,51].…”

Strategies for Oligodendrocyte and Myelin Repair in Traumatic CNS Injury

“…While these barriers to remyelination and regeneration have been a source of consternation for scientists and clinicians alike, technological advances in the last decade have greatly improved prospects. One preprint from the Blackmore lab utilizing cell transplantation paired with pro-regenerative gene delivery and growth permissive grafting showed enhanced axon regeneration and functional recovery in a murine SCI model (Jayaprakash et al, 2019 ). Such combination therapies hold considerable promise in the treatment of TBI/SCI.…”

“…While there is strong therapeutic potential for CRT itself, combinatorial approaches continue to yield the best results in TBI/SCI. Cell transplantation has been paired with RNAi, electrical stimulation, as well as a variety of small molecules to promote neurophysiological recovery (Karimi-Abdolrezaee et al, 2006 ; Zhang et al, 2012 ; Jayaprakash et al, 2019 ). The use of combination therapies has been especially beneficial in developing treatments for chronic SCI.…”