Subretinal electrical stimulation reveals intact network activity in the blind mouse retina.

Retinal degeneration (rd) leads to progressive photoreceptor cell death, resulting in vision loss. Stimulation of the inner-retinal neurons by neuroprosthetic implants is one of the clinically approved vision-restoration strategies, providing basic visual percepts to blind patients. However, little is understood as to what degree the degenerating retinal circuitry and the resulting aberrant hyperactivity may prevent the stimulation of physiological electrical activity. Therefore, we electrically stimulated ex vivo retinas from wild-type (wt; C57BL/6J) and blind (rd10 and rd1) mice using an implantable subretinal microchip and simultaneously recorded and analyzed the retinal ganglion cell (RGC) output with a flexible microelectrode array. We found that subretinal anodal stimulation of the rd10 retina and wt retina evoked similar spatiotemporal RGC-spiking patterns. In both retinas, electrically stimulated ON and a small percentage of OFF RGC responses were detected. The spatial selectivity of the retinal network to electrical stimuli reveals an intact underlying network with a median receptive-field center of 350 µm in both retinas. An antagonistic surround is activated by stimulation with large electrode fields. However, in rd10 and to a higher percentage, in rd1 retinas, rhythmic and spatially unconfined RGC patterns were evoked by anodal or by cathodal electrical stimuli. Our findings demonstrate that the surviving retinal circuitry in photoreceptor-degenerated retinas is preserved in a way allowing for the stimulation of temporally diverse and spatially confined RGC activity. Future vision restoration strategies can build on these results but need to avoid evoking the easily inducible rhythmic activity in some retinal circuits.

Daylight vision repair by cell transplantation.

Human daylight vision depends on cone photoreceptors and their degeneration results in visual impairment and blindness as observed in several eye diseases including age-related macular degeneration, cone-rod dystrophies, or late stage retinitis pigmentosa, with no cure available. Preclinical cell replacement approaches in mouse retina have been focusing on rod dystrophies, due to the availability of sufficient donor material from the rod-dominated mouse retina, leaving the development of treatment options for cone degenerations not well studied. Thus, an abundant and traceable source for donor cone-like photoreceptors was generated by crossing neural retina leucine zipper-deficient (Nrl(-/-) ) mice with an ubiquitous green fluorescent protein (GFP) reporter line resulting in double transgenic tg(Nrl(-/-); aGFP) mice. In Nrl(-/-) retinas, all rods are converted into cone-like photoreceptors that express CD73 allowing their enrichment by CD73-based magnetic activated cell sorting prior transplantation into the subretinal space of adult wild-type, cone-only (Nrl(-/-)), or cone photoreceptor function loss 1 (Cpfl1) mice. Donor cells correctly integrated into host retinas, acquired mature photoreceptor morphology, expressed cone-specific markers, and survived for up to 6 months, with significantly increased integration rates in the cone-only Nrl(-/-) retina. Individual retinal ganglion cell recordings demonstrated the restoration of photopic responses in cone degeneration mice following transplantation suggesting, for the first time, the feasibility of daylight vision repair by cell replacement in the adult mammalian retina.

Inflammatory stimulation preserves physiological properties of retinal ganglion cells after optic nerve injury.

Axonal injury in the optic nerve is associated with retinal ganglion cell (RGC) degeneration and irreversible loss of vision. However, inflammatory stimulation (IS) by intravitreal injection of Pam3Cys transforms RGCs into an active regenerative state enabling these neurons to survive injury and to regenerate axons into the injured optic nerve. Although morphological changes have been well studied, the functional correlates of RGCs transformed either into a de- or regenerating state at a sub-cellular level remain unclear. In the current study, we investigated the signal propagation in single intraretinal axons as well as characteristic activity features of RGCs in a naive, a degenerative or a regenerative state in ex vivo retinae 1 week after either optic nerve cut alone (ONC) or additional IS (ONC + IS). Recordings of single RGCs using high-density microelectrode arrays demonstrate that the mean intraretinal axonal conduction velocity significantly decreased within the first week after ONC. In contrast, when ONC was accompanied by regenerative Pam3Cys treatment the mean intraretinal velocity was undistinguishable from control RGCs, indicating a protective effect on the proximal axon. Spontaneous RGC activity decreased for the two most numerous RGC types (ON- and OFF-sustained cells) within one post-operative week, but did not significantly increase in RGCs after IS. The analysis of light-induced activity revealed that RGCs in ONC animals respond on average later and with fewer spikes than control RGCs. IS significantly improved the responsiveness of the two studied RGC types. These results show that the transformation into a regenerative state by IS preserves, at least transiently, the physiological functional properties of injured RGCs.