Cellular and Molecular Dynamics of Retinal Microglial in the Context of Photoreceptor Degeneration

ABSTRACT While outer retinal degenerative diseases, including age-related macular degeneration (AMD) and certain inherited retinal degenerative diseases, continue to experience technological advances towards better clinical management, there is still a massive unmet medical need for millions of these patients worldwide. Pathophysiologically, dead and dying photoreceptors and their toxic byproducts can impede retinal pigment epithelial (RPE) phagocytosis of photoreceptor outer segments, negatively affect the recycling of visual pigments, contribute to changes in the metabolite flux patterns, and promote secondary death of photoreceptors and RPE. Our lab demonstrated direct evidence that microglia are central to the swift elimination of dead and dying photoreceptor debris; and, in so doing, these microglia mitigate secondary damage of photoreceptors and RPE associated with degeneration. Our single cell RNA-seq analysis of microglia from acute, inherited, and age-related photoreceptor degeneration models, guided our additional series of experiments enabling us to demonstrate that these protective microglia mediate their responses from the subretinal space. Taking advantage of human postmortem AMD eyes, we found parallels of these microglia at the transcriptome level, with protein markers, and tissue distribution, potentially suggesting the existence of a conserved protective response. Still, the specific elements of the photoreceptor debris which are eliminated by these microglia to protect the outer retina from secondary damage is not known. Also, given that phagocytosis is energetically demanding, how these microglia obtain the requisite fuel in a lactate rich environment to fulfill their role is not understood. Lastly, as microglia are highly plastic cells whose functions are influenced by the microenvironment, the retinal factors that instruct these protective microglia is also not fully known. To address these fundamental gaps in knowledge we will perform lipidomics to elucidate the toxic compounds eliminated by protective microglia. Separately, we will leverage genetic knockout mice established for perturbation of metabolic transporters to elucidate how these microglia are fueled. Lastly, we will perform RNA-seq to uncover how the cells of the retina shape the function of these protective microglia. With the expertise of key collaborators to help execute and interpret data generated by these incisive experiments, our project is successfully positioned to provide a new understanding of retinal resilience mechanisms in the modulation of microglia that modify the progression of outer retinal degeneration, ultimately providing new opportunities for the development of novel treatment strategies in preserving vision in patients with retinal degenerative diseases.