A Flexible Circuit Design that Restores Locomotion after Injury

A huge barrier for restoring locomotion after injury in mammals, including humans, is that the descending brain inputs needed to initiate and regulate downstream spinal circuits are unable to regrow across the damaged spinal cord. Unfortunately, biologically ‘tricking’ neurons to reconnect with former targets has been met with limited success. Might there be another way of activating locomotor spinal circuitry below the site of injury? Our newest experiments in the medicinal leech indicate that the remodeling of the stretch-sensitive neurons, activated during the elongation and shortening phases of crawling behavior, are likely key to the understanding of how locomotor circuits are once again turned on and contribute to coordinated locomotion. Our work provides a rare and unique opportunity to explore how a novel sensory-based circuit design replaces a centrally-based governing system for the initiation and coordination of locomotion. Importantly, we can examine the cellular basis of this switch in the same individuals over time. This flexibility in design should have significant ramifications for the creation of locomoting robotic systems, especially those that are soft-bodied like the leech. Our research is highly interdisciplinary and will involve the use of innovative molecular and anatomical approaches to understanding neuronal plasticity; we also aim to include citizen-scientists and other volunteers around the globe in our research. The PI and Co-PI will continue to develop ways to support college students and scientists in their labs, and through the development of new initiatives through their respective scientific organizations. Our interdisciplinary team will explore how novel neural circuits, in the medicinal leech, emerge and orchestrate a recovery of locomotion after the nerve cord and brain become separated. When descending inputs from the brain neuron R3b-1 are removed, crawling is completely lost, yet surprisingly returns after about 2 weeks. Our goal is to study the underlying circuit design that accounts for this restored crawling. Key to crawl recovery are the proprioceptive stretch receptors (SRs) that pepper each body-wall segment. These proprioceptors, although not injured themselves, sprout new centrally-located (and intersegmental) output branches that are predicted to target crawl-related circuitry, especially in the ‘lead’ ganglion directly below the site of nerve cord injury. We will use electrophysiological recording, voltage-sensitive dye imaging, and electron microscopy to interrogate the role of the lead ganglion and SRs in establishing crawl recovery. To study changes in gene expression across neurons in the lead and adjacent ganglia, and in the SRs, we will use spatial transcriptomics, whereby genome-wide expression analysis will be mapped back to precise locations in specific ganglia and SR processes obtained from histological sections. Our methods and instrumentation, at our respective institutions, will allow for subcellular localization of mRNA molecules in somata, neuronal processes (intrinsic and extrinsic), and synaptic terminals. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.