OPTOGENETIC EVALUATION OF FUNCTIONAL SYNAPTIC RECONNECTION BY CORTICOSPINAL TRACT AXONS IN THE INJURED SPINAL CORD
Spinal cord injury results in partial or complete loss of neural communication across the site of injury. To restore function, axons must extend into denervated territory, and, critically, must form functional synapses with appropriate targets. We have previously shown that forced overexpression of transcription factors KLF7 or Sox11 promotes axon regeneration in corticospinal tract (CST) neurons, but that behavioral outcomes are modest or even negative. It is therefore unclear whether the newly sprouted axons are able to form functional synapses, or alternatively, whether functional synapses are formed in a mistargeted fashion. Here we use an optogenetic strategy to assess the ability of Sox11-stimulated axons to form functional synapses. A pyramidotomy was performed in adult mice to deprive the left side of the spinal cord of CST input, and the right CST was treated with AAV-Sox11 or EBFP control, along with viral channelrhodopsin (rAAV9/CamKII-ChR2-EYFP). As we have shown previously, Sox11 treatment caused robust midline crossing of CST axons into the previously denervated left spinal cord. We then paired optogenetic stimulation of newly sprouted CST axon terminals in the left spinal cord with extracellular recordings of post-synaptic spinal neurons. We observed clear post-synaptic responses driven by optogenetic activation of Sox11-treated axons, demonstrating the ability to form functional synapses with target cells. Further, the use of multiunit electrodes enabled mapping of the pattern of spinal activity following light stimulation in different regions of the spinal cord. The presence of functional synapses without significant behavioral benefit suggests that new connections may be mistargeted, motivating ongoing experiments that incorporate rehabilitative training to refine the newly formed synapses. Overall these data illustrate the utility of an optogenetic approach to monitor and optimize functional reconnection by newly sprouted axons in the injured spinal cord.
*Naveen JAYAPRAKASH, B. HOEYNCK, N. KRUEGER, Z. WANG, Murray BLACKMORE; Marquette Univ., Milwaukee, WI
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