Optogenetics

Abstract

Purpose of review: In this review, we will discuss the recent developments in optogenetics and their potential applications in ophthalmology to restore vision in retinal degenerative diseases.

Recent findings: In recent years, we have seen major advances in the field of optogenetics, providing us with novel opsins for potential applications in the retina. Microbial opsins with improved light sensitivity and red-shifted action spectra allow optogenetic stimulation at light levels well below the safety threshold in the human eye. In parallel, remarkable success in the development of highly efficient viral vectors for ocular gene therapy led to new strategies of using these novel optogenetic tools for vision restoration.

Summary: These recent findings show that novel optogenetic tools and viral vectors for ocular gene delivery are now available providing many opportunities to develop potential optogenetic strategies for vision restoration.

Figure 1

(A) Fundus image of a rd1 mouse retina expressing ChR2(H134R)-GFP under the control of a ON-bipolar specific promotor. (B) Live two-photon images of ON-bipolar cells types. Examples of different ON-bipolar cell types expressing ChR2(H134R)-GFP fusion: (left) ON cone bipolar cell type 7, (middle) ON cone bipolar cell type 9, and (right) rod bipolar cell. (C) Wiring diagram illustrates the classical rod-cone bipolar cell pathway mediated by AII amacrine cells: rod bipolar cells release glutamate onto AII amacrine cells upon depolarization. AII amacrine cells form electrical synapses with axon terminals of ON cone bipolar cells and glycine-ergic (sign-inverting) synapses with those of OFF cone bipolar cells. These cone bipolar cells form glutamate-ergic synapses with ganglion cells. This synaptic circuitry is the basis for channelrhodopsin expressing ON-cone/rod-bipolar cells (both shown in green) triggering spiking of ON ganglion cells at light increments and in OFF ganglion cells at light decrements. (Reproduced with permission from Mace et al., Mol Ther. 2015 Jan;23(1):7–16. [33]*).

Figure 2

(A) AAV mediated expression of a light-sensitive chloride pump halorhodopsin in persisting photoreceptors of a blind mouse (Cnga3^{−/ −}; Rho^{−/ −}) (B) Halorhodopsin induced light responses in photoreceptors of the blind mouse (Cnga3^{−/ −}; Rho^{−/ −}). (from Busskamp et al., Science. 2010 Jul 23;329(5990):413–7.).

Figure 3

Translational aspects of halorhodopsin mediated reactivation of photoreceptors. (A) Retinal slice from a human retinal explant (24 hours postmortem). Scale bar, 30 μm. (B) Fluorescent live image of a lentivirus-transfected area from a human retina after 7 days in culture and 2 days after lentiviral administration. Scale bar, 20 μm. (C) Action spectrum of a halorhodopsin expressing human photoreceptor stimulated with full-field light flashes ranging from 450 to 650 nm (top, current response; bottom, voltage response). Gray bars indicate the timing of the stimulus. (D) Representative OCT scan covering the foveal region of a healthy individual. (E) Magnified image. IS, inner segments; RPE, retinal pigment epithelium. Scale bar, 200 μm. (F) OCT from the left eye of a 40-year-old male patient with sporadic retinitis pigmentosa (loss of vision since the age of 15). Outer segments are undetectable. (Reproduced with permission from Busskamp et al., Science. 2010 Jul 23;329(5990):413–7. [34]).