Ectopic expression of melanopsin promotes axonal regeneration in retinal ganglion cells via cAMP and TRPC signaling

In mammals, retinal ganglion cells (RGCs) show very minor axonal regeneration following axotomy. Despite genetic and pharmacological efforts to promote this regeneration, progress remains slow and limited, especially in overcoming potentially cell-type-specific barriers. In this paper, we virally expressed melanopsin (OPN4), which is the native photopigment in intrinsically-photosensitive RGCs (ipRGCs), broadly in RGCs. IpRGCs are known to show relative resistance to degeneration and axonal loss after injury. We correlated the exogenous-melanopsin-mediated intrinsic light responses of the successfully infected RGCs (referred to as ^+OPN4 RGCs) with their ability to show axonal regeneration after an optic-nerve crush. Specifically, we found ^+OPN4 RGCs to show functional rhabdomeric and ciliary motifs of phototransduction as we previously found in ipRGCs. ^+OPN4 αRGCs (the largest RGCs in the retina) exhibited light-triggered fast (via TRPC signaling) and slow (via cAMP signaling) response components, whereas ^+OPN4 non-αRGC responses comprised predominantly just a fast component. Genetic ablation of TRPC channels (TRPC6/7) or group II adenylyl cyclase (AC2/4/7) to abolish the fast or the slow response component, respectively, largely attenuated the axonal regeneration of ^+OPN4 αRGCs in the optic-nerve-crush model. At the same time, administration of cell-permeant cAMP not only rescued the AC2/4/7-knock-out phenotype in ^+OPN4 αRGCs but also potentiated axonal regeneration in some ^+OPN4 non-αRGCs, thus, possibly overcoming the potential subtype-specific barrier to axonal regeneration. Our results reveal an important role of joint TRPC- and cAMP-signaling in ^+OPN4 RGC axonal regeneration.