Ectopic expression of multiple microbial rhodopsins restores ON and OFF light responses in retinas with photoreceptor degeneration.

By expressing channel rhodopsin-2 (ChR2) in inner retinal neurons, previous studies have demonstrated restoration of ON responses in the retina after the death of rod and cone photoreceptors. In this study, we report that the expression of halorhodopsin (HaloR), a light-driven chloride pump, can effectively restore OFF responses in inner retinal neurons of mice with retinal degeneration. We show that HaloR-expressing retinal ganglion cells respond to light with rapid hyperpolarization and suppression of spike activity. After termination of the light stimulus, their membrane potential exhibits a rapid rebound overshoot with robust sustained or transient spike firing. Furthermore, we show that coexpression of ChR2/HaloR in retinal ganglion cells can produce ON, OFF, and even ON-OFF responses, depending on the wavelength of the light stimulus. Our results suggest that the expression of multiple microbial rhodopsins such as ChR2 and HaloR is a possible strategy to restore both ON and OFF light responses in the retina after the death of rod and cone photoreceptors.