Author:

Abstract:

Severe spinal cord injury in adults leads to irreversible paralysis below the lesion. However, adult rodents that received a complete thoracic lesion just after birth demonstrate proficient hindlimb locomotion without input from the brain. How the spinal cord achieves such striking plasticity remains unknown. In this study, we pursued two main objectives to address these questions. First, we performed anatomical analyses to examine how spinal circuits reorganize after injury, focusing on age-dependent differences in relation to functional recovery. Second, we conducted transcriptomic analyses using single-nucleus RNA sequencing to profile the gene expression patterns of spinal circuits in neonatal injured and adult injured mice. Through this dual approach, we aimed to compare how injury-induced circuit reorganization and gene expression differ across age groups. Our anatomical investigations revealed two opposing synaptic connectivity profiles of excitatory interneurons to motorneurons. We found that adult spinal cord injury prompts neurotransmitter switching of spatially defined excitatory interneurons to an inhibitory phenotype, promoting inhibition at synapses contacting motor neurons. In contrast, neonatal spinal cord injury maintains the excitatory phenotype of glutamatergic interneurons and causes synaptic sprouting to facilitate excitation. Furthermore, genetic manipulation to mimic the inhibitory phenotype observed in excitatory interneurons after adult spinal cord injury abrogates autonomous locomotor functionality in neonatally injured mice. In comparison, attenuating this inhibitory phenotype improves locomotor capacity after adult injury. Together, these data demonstrate that neurotransmitter phenotype of defined excitatory interneurons steers locomotor recovery after spinal cord injury. Furthermore, the transcriptomic analysis highlighted additional differences between neonatal and adult responses to injury. Neonatal spinal cords exhibited minimal transcriptomic divergence from intact spinal cords, with a low level of gene signatures suggestive of circuit reorganization. Conversely, adult spinal circuits showed widespread downregulation of genes involved in essential cellular processes, potentially limiting their capacity for recovery. Our findings highlight significant age-dependent differences in both structural plasticity and molecular responses following spinal cord injury. These insights could guide future therapeutic strategies aimed at promoting recovery in adult spinal cord injury by targeting age-specific molecular pathways and enhancing intrinsic spinal cord circuit potential.