The role of Protocadherin gamma in adult sensory neurons and skin reinnervation.

The clustered protocadherins (cPcdhs) play a critical role in the patterning of several central nervous system (CNS) axon and dendritic arbors, through regulation of homophilic self and neighbouring interactions. While not explored, primary peripheral sensory afferents that innervate the epidermis may require similar constraints to convey spatial signals with appropriate fidelity. Here we show that members of the gamma-Pcdh (Pcdhγ) family are expressed in both adult sensory neuron axons and in neighbouring keratinocytes that have close interactions during skin reinnervation. Adult mice of both sexes were studied. Pcdhγ knockdown either through siRNA transduction or AAV-Cre recombinase transfection of adult mouse primary sensory neurons from floxed Pcdhγ mice was associated with a remarkable rise in neurite outgrowth and branching. Rises in outgrowth were abrogated by Rac1 inhibition. Moreover, AAV-Cre knockdown in Pcdhγ floxed neurons generated a rise in neurite self-intersections, and a robust rise in neighbour intersections or tiling, suggesting a role in sensory axon repulsion. Interestingly, preconditioned (3 day axotomy) neurons with enhanced growth had temporary declines in Pcdhγ and lessened outgrowth from Pcdhγ siRNA. In vivo , mice with local hindpaw skin Pcdhγ knockdown by siRNA had accelerated reinnervation by new epidermal axons with greater terminal branching and reduced intra-axonal spacing. Pcdhγ knockdown also had reciprocal impacts on keratinocyte density and nuclear size. Taken together, this work provides evidence for a role of Pcdhγ in attenuating outgrowth of sensory axons and their interactions, with implications in how new reinnervating axons following injury fare amidst skin keratinocytes that also express Pcdhγ. Significance Statement The molecular mechanisms and potential constraints that govern skin reinnervation and patterning by sensory axons are largely unexplored. Here we show that Pcdhγ may help to dictate interaction not only among axons but also between axons and keratinocytes as the former re-enter the skin during reinnervation. Pcdhγ neuronal knockdown enhances outgrowth in peripheral sensory neurons, involving the growth cone protein Rac1 whereas skin Pcdhγ knockdown generates rises in terminal epidermal axon growth and branching during re-innervation. Manipulation of sensory axon regrowth within the epidermis offers an opportunity to influence regenerative outcomes following nerve injury.