Promoting axon regeneration in the adult CNS by modulation of the melanopsin/GPCR signaling

Abstract

Cell-type-specific G protein-coupled receptor (GPCR) signaling regulates distinct neuronal responses to various stimuli and is essential for axon guidance and targeting during development. However, its function in axonal regeneration in the mature CNS remains elusive. We found that subtypes of intrinsically photosensitive retinal ganglion cells (ipRGCs) in mice maintained high mammalian target of rapamycin (mTOR) levels after axotomy and that the light-sensitive GPCR melanopsin mediated this sustained expression. Melanopsin overexpression in the RGCs stimulated axonal regeneration after optic nerve crush by up-regulating mTOR complex 1 (mTORC1). The extent of the regeneration was comparable to that observed after phosphatase and tensin homolog (Pten) knockdown. Both the axon regeneration and mTOR activity that were enhanced by melanopsin required light stimulation and Gq/11 signaling. Specifically, activating Gq in RGCs elevated mTOR activation and promoted axonal regeneration. Melanopsin overexpression in RGCs enhanced the amplitude and duration of their light response, and silencing them with Kir2.1 significantly suppressed the increased mTOR signaling and axon regeneration that were induced by melanopsin. Thus, our results provide a strategy to promote axon regeneration after CNS injury by modulating neuronal activity through GPCR signaling.axon regeneration | neuronal activity | melanopsin | GPCR | mTOR S evered axons in the adult mammalian CNS do not spontaneously regenerate to restore lost functions. The failure of axons to regenerate is mainly attributed to the diminished growth capacity of neurons as well as an inhibitory environment (1-6). Optic nerves have been extensively studied for mechanisms regulating axon regeneration in CNS. When presented with permissive substrates such as a sciatic nerve graft, only axons of small populations of retinal ganglion cells (RGCs) regrow into the graft (7). When the intrinsic growth program is boosted, distinct subtypes of RGCs regenerate their axons (8). These findings indicate that the differential responses of RGCs to axotomy and growth stimulation are related to their intrinsic properties. One of the critical determinants of the intrinsic regenerative abilities of adult RGCs is neuronal mammalian target of rapamycin (mTOR) activity (9). In retinal axons, the loss of the potential to regrow is accompanied by down-regulation of mTOR activity in RGCs with maturation, and further reduction after axotomy. However, a small percentage of RGCs maintain high mTOR activation levels after optic nerve crush (9, 10). One can ask whether specific subsets of RGCs differ in their ability to maintain mTOR activation. Deciphering the physiological mechanism behind the mTOR maintenance could help elucidate the differential responses of neurons to injury signals and develop strategies to promote axon regeneration.Type 1 melanopsin expressing intrinsically photosensitive retinal ganglion cells (M1 ipRGCs) and αRGCs are resistant to axotomy-induced cell death (8, 11). M1 ipRGCs mainl...

Mobile zinc increases rapidly in the retina after optic nerve injury and regulates ganglion cell survival and optic nerve regeneration

“…Optic nerve injury leads to numerous pathological changes in RGCs and reversing some of these changes improves cell survival, although these effects are often transitory and for the most part promote little or no axon regeneration (1)(2)(3)(4)(5)(6)(7)(8)(9)(10). Regeneration per se can be induced by intraocular inflammation combined with elevated cAMP (11,12), counteracting cell-intrinsic (13)(14)(15) or cellextrinsic (16,17) suppressors of axon growth, oncomodulin and other growth factors (18)(19)(20)(21)(22), or elevated physiological activity (23,24). Some of these treatments act synergistically and enable a modest number of RGCs to reestablish connections with appropriate target areas in the brain (25)(26)(27).…”

Gliotransmission orchestrates neuronal type-specific axon regeneration

“…Exogenously applied neuronal activation has been shown to promote regeneration and facilitate functional recovery (Beibei Feng et al 2016;Li et al, 2016;Lim et al, 2016;Udina et al, 2008). However, in most cases, intrinsic activities of individual neurons have not been determined.…”

Regulation of Melanopsin Signaling: Key Interactions of the Nonvisual Photopigment

“…In light‐exposed mouse RGCs, melanopsin (Opn4‐L) enhanced mTOR signaling by activating Gq/11 and in consequence Ca 2+ influx leading to the induction of mTOR complex 1 (mTORC1) and increasing the axon regeneration after optic nerve crush. Interestingly, injured M1‐ to M3‐ipRGCs expressed high level of melanopsin . It should be stressed that melanopsin‐triggered signaling also affects expression of Fos.…”