A new strategy for vision restoration based on melanopsin transduction mechanisms

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PROJECT SUMMARY A pressing challenge in the treatment of retinal diseases is to restore vision in the retina with irreversible photoreceptor degeneration. In theory, even in the absence of rods and cones, such diseased retina should still be able to sense light through intrinsically photosensitive retinal ganglion cells (ipRGCs) using a photopigment named melanopsin. However, the kinetics of melanopsin-mediated light response is slow, thus giving only very poor temporal resolution and therefore largely limiting its ability to provide image-forming information. For the same reason, a vision restoration approach previously proposed – which made use of virally expressed melanopsin to endow light sensitivity to conventional retinal ganglion cells (RGCs) – is also limited by the slow response kinetics of melanopsin. Our recent study has firmly established that melanopsin-mediated phototransduction is rate-limited by its downstream components and that it can be accelerated by virally expressed ion channels with faster kinetics. We propose to develop a novel vision restoration strategy by accelerating melanopsin signaling via manipulating downstream transduction components in photoreceptor degenerated mouse models. We have two major aims. In Aim 1, we shall focus on enhancing the light response of endogenous melanopsin in M4- and M5-subtypes of ipRGCs because they have been shown to project to image-forming brain centers, allowing them to contribute to the image-forming vision. We have discovered that M4-cells respond to melanopsin photoactivation by elevating intracellular levels of cyclic nucleotides (cNMP) and subsequent opening of cNMP-sensitive HCN channels. Our new preliminary data suggest that M5 cells also use the HCN pathway. We have further shown that the response of the HCN-dependent signaling pathway can be sped up by introducing a member of the cyclic nucleotide-gated channel family, CNGA2, resulting in faster response kinetics and larger amplitudes, proving the concept of our strategy. In Aim 2, we shall restore light response in conventional retinal ganglion cells using virally expressed melanopsin together with faster signaling molecules. Our multielectrode array results show that simultaneously expressing melanopsin and CNGA2 in cRGCs provides higher light sensitivities and faster rising phases than expressing melanopsin alone. Using water-based vision-guided maze tests, we have further found that exogenous melanopsin and CNGA2 not only restore light sensitivity but also confer pattern vision in photoreceptor-degenerated animals. Together, these exciting preliminary observations raise the prospect of using CNG channels and/or modifications to other phototransduction components of melanopsin as a therapeutic modality to restore vision following photoreceptor degeneration. Innovation. All optogenetic approaches proposed thus far focus on finding a better light-sensing protein with higher expression, better light sensitivity, and faster response kinetics. We are the first to exploit downstream components and novel signaling molecules of melanopsin for vision restoration purposes. This represents a conceptual innovation and an out-of-the-box strategy for vision restoration.