Institution: University of California Berkeley
Poster Title: A Non-Human Primate Model of Photoreceptor Loss for Assessing Cell Replacement Therapy
Abstract: Retinal degenerations such as age-related macular degeneration and retinitis pigmentosa result in debilitating vision impairment due to irreversible photoreceptor loss. One therapeutic strategy being developed is cell replacement therapy, in which stem cell-derived photoreceptors are transplanted into the retina to replace lost photoreceptors. The success of this approach hinges upon donor cell survival and the formation of appropriate synaptic connections with the residual inner retinal circuitry. Although studies in non-primate retinas have shown some promise for cell replacement therapy, we know very little about how primate retinal circuits respond to photoreceptor lossparticularly the foveal circuits dedicated to high acuity central visionor whether stem cell-derived photoreceptors can integrate into a primate retina. Here, we used immunohistochemistry and confocal imaging to determine how deafferentationfrom a laser ablation model of photoreceptor degenerationimpacts macular bipolar cell (BC) and horizontal cell (HC) survival and synaptic integrity. We also assessed whether transplanted human pluripotent stem cell-derived photoreceptor precursors (hPSC-PRPs) showed signs of synaptic integration into the damaged areas. Focal photoreceptor ablations, between ~1 to 20 degrees from the foveal center, were made with a femtosecond-pulsed laser (730nm, 27-33 J/cm2) with an adaptive optics scanning laser ophthalmoscope. Tissues were processed for histological assessment at time points ranging from < 1 week post-ablation to >3 months post-ablation. Cell type-specific markers and synaptic protein markers were used to determine the impact of photoreceptor loss on the survival and synaptic integrity of BCs and HCs. In deafferented areas, we found the densities of rod BCs, On-BCs, and Off-midget BCs to be unchanged compared to control areas at all time points. However, deafferentation led to the loss of dendritic glutamate receptors on On- and Off-BCs. Moreover, we found a loss of both H1 and H2 HCs in deafferented areas as early as 3 weeks post-ablation. HCs show signs of cell death at 4 and 8 days post ablationtime points when the photoreceptors have been cleared in the lesioned areassuggesting HC loss occurs secondary to photoreceptor loss. To determine whether BCs can form synaptic connections with donor stem cell-derived photoreceptors, a scaffold seeded with hPSC-PRPs was transplanted into the subretinal space after photoreceptor ablation. Histological assessment showed both On and Off BC dendrites in close proximity to transplanted PRPs with expression of synaptic proteins at putative contact sites. The survival of BCs in deafferented areas and the extension of BC dendrites to transplanted stem cell-derived photoreceptors show promise for the development of cell replacement therapy for photoreceptor degenerations.