Molecular and cellular correlates of human nerve regeneration: ADCYAP1 encoding PACAP enhances sensory neuron outgrowth

We only have a rudimentary understanding of the molecular and cellular determinants of human nerve regeneration. Here, we use carpal tunnel syndrome (CTS) as a human model system to prospectively evaluate correlates of neural regeneration and their relationship with clinical recovery after decompression surgery. At 6 months post-surgery, we noted a significant improvement of median nerve neurophysiological and somatosensory function. Serial skin biopsies revealed a partial recovery of intraepidermal innervation, whose extent correlated with symptom improvement. In myelinated afferents, nodal length increased postoperatively. Transcriptional profiling of the skin revealed 23 differentially expressed genes following decompression, with ADCYAP1 (encoding PACAP) being the most strongly upregulated and showing an association with regeneration of intraepidermal nerve fibres. Using human induced pluripotent stem cell-derived sensory neurons, we confirmed that PACAP significantly enhances axon outgrowth in vivo. Since PACAP signals through G-protein receptors, this pathway provides an interesting therapeutic target for human sensory nerve regeneration.