Restoring the ON Switch in Blind Retinas: Opto-mGluR6, a Next-Generation, Cell-Tailored Optogenetic Tool

Abstract

Photoreceptor degeneration is one of the most prevalent causes of blindness. Despite photoreceptor loss, the inner retina and central visual pathways remain intact over an extended time period, which has led to creative optogenetic approaches to restore light sensitivity in the surviving inner retina. The major drawbacks of all optogenetic tools recently developed and tested in mouse models are their low light sensitivity and lack of physiological compatibility. Here we introduce a next-generation optogenetic tool, Opto-mGluR6, designed for retinal ON-bipolar cells, which overcomes these limitations. We show that Opto-mGluR6, a chimeric protein consisting of the intracellular domains of the ON-bipolar cell–specific metabotropic glutamate receptor mGluR6 and the light-sensing domains of melanopsin, reliably recovers vision at the retinal, cortical, and behavioral levels under moderate daylight illumination.

A New Promoter Allows Optogenetic Vision Restoration with Enhanced Sensitivity in Macaque Retina

“…Finally, for patients who still have an intact bipolar cell layer, new viral technologies might also grant access to the bipolar cells of the primate retina for circuit-specific insertion of G proteincoupled vertebrate opsins. [50][51][52] Our study provides evidence that the choice of an appropriate promoter is essential in obtaining high-level microbial opsin expression compatible with illumination safety. A promoter that can drive strong gene expression in designated cells is crucial in the clinical development of a strategy using a protein of foreign origin, which requires high-level expression for functional activation.…”

Long-term restoration of visual function in end-stage retinal degeneration using subretinal human melanopsin gene therapy

“…Furthermore, increased availability of chromophore (retinal) in the outer retina may be required for effective photon capture in the absence of specialized outer segment discs. Other studies have used AAV vectors containing a mouse bipolar-cell-specific enhancer to target a melanopsin.mGluR6 chimera (17) or rhodopsin (6) to bipolar cells via intravitreal injection. However, there is variation in anatomy between primates and mouse models (18), and this may render the intravitreal approach less effective in humans.…”

Optogenetic user s guide to Opto-GPCRs

“…Additionally, opsins profit from an integral signal amplification cascade inherent to GPCRs. In contrast to the traditional optogenetic tools, a single photon can activate multiple Gproteins, each in turn triggering large changes in second messengers, which renders animal opsins 2-3 log units more light sensitive than non-GCPR based optogenetic tools (9).…”

“…We recently engineered a functional Opto-GPCR between a class A and a class C GPCR (9). We based our engineering on vertebrate melanopsin, a bistable opsin and replaced IL2, IL3 and CT of melanopsin by those of the class C GPCR mGluR6, our target receptor.…”

“…Owing to the extreme cell-type and cell-state specific effects of various GPCRs, they represent one of the most sought after drug targets for the pharmaceutical industry (8). The potential benefit of a comprehensive understanding of specific GPCR signaling pathways in health and disease has fueled the development of a novel category of optogenetic tools, light-activatable GPCRs or Opto-GPCRs that can tap into cell-specific signaling pathways (9)(10)(11).…”