Molecular pathology of acute spinal cord injury in middle-aged mice
The median age of sustaining a spinal cord injury has steadily increased from 29 to 43 over the last several decades. Although more pre-clinical studies in aged rodents are being done to address this shift in demographics, there have not been comprehensive transcriptomic studies investigating SCI pathobiology in middle-aged mice. To address this gap in knowledge, we compared behavioral, histopathological, and transcriptional outcomes in young (2-4 months-old) and middle-aged (10-12 months-old) mice. In contrast to previous studies, open field tests showed no differences in locomotor recovery between the young and middle-aged mice over a one-month period. The injury site also demonstrated similar histopathology in terms of lesion size, numbers of macrophages and fibroblasts. Acutely after injury, proliferation of macrophages, fibroblasts, and astrocytes were also similar between the two age groups. In addition, spatial transcriptomics showed similar transcriptionally defined regions around the injury site at 3 days post injury. However, single cell RNA-sequencing of the cells at the injury site and surrounding spared tissue showed differences in select cell subpopulations.  Taken together, our results indicate that although young and middle-aged mice display similar locomotor recovery and histopathology after SCI, changes in cell subpopulations may underly a decline in repair mechanisms that are manifested after this age. Overall design: C57Bl/6J mouse contusive spinal cord injury sites were analyzed with single-cell sequencing (10X Chromium v3.1) at uninjured and 3 days-post injury conditions in middle-aged (10-12 months-old) and young (2-4 months-old) mice. 3dpi injury sites in aged and young mice were analyzed with spatial sequencing (10X Visium v1).