Conditional Sox9 ablation improves locomotor recovery after spinal cord injury by increasing reactive sprouting

The absence of axonal regeneration after spinal cord injury (SCI) has been attributed to the up-regulation of axon-repelling molecules, such as chondroitin sulfate proteoglycans (CSPGs) present in the glial scar that forms post-SCI. We previously identified the transcription factor SOX9 as a key up-regulator of CSPG production and also demonstrated that conditional Sox9 ablation leads to decreased CSPG levels and improved recovery of hind limb function after SCI. We herein demonstrate increased neural input onto spinal neurons caudal to the lesion in spinal cord injured Sox9 conditional knock out mice as indicated by increased levels of the presynaptic markers synaptophysin and vesicular glutamate transporter 1 (VGLUT1) compared to controls. Axonal sparing, long-range axonal regeneration and reactive sprouting were investigated as possible explanations for the increase in neural inputs caudal to the lesion and for the improved locomotor outcomes in spinal cord-injured Sox9 conditional knock out mice. Whereas retrograde tract-tracing studies failed to reveal any evidence for increased axonal sparing or for long-range regeneration in the Sox9 conditional knock out mice, anterograde tract-tracing experiments demonstrated increased reactive sprouting caudal to the lesion after SCI. Finally we demonstrate that application of a broad spectrum MMP inhibitor to reduce CSPG degradation in Sox9 conditional knock out mice prevents the improvements in locomotor recovery observed in untreated Sox9 conditional knock out mice. These results suggest that improved recovery of locomotor function in Sox9 conditional knock out mice after SCI is due to increased reactive sprouting secondary to reduced CSPG levels distal to the lesion.