Microglia Prevent the Loss of Visual Function in the Aging Retina by Maintaining the Health of Retinal Pigment Epithelial Cells.

Even in healthy individuals, aging leads to the progressive deterioration of visual function that coincides with increasing cellular dysfunction in retinal neurons and retinal pigment epithelial (RPE) cells. Interestingly, aging also coincides with the migration of microglia from the inner to the outer retina where they reside in the subretinal space between photoreceptor outer segments and the apical surface of RPE. Healthy young microglia act as the immune sentinels of the retina, and thus are protective and maintain retinal homeostasis. However, age alters cellular functions and the role of aging subretinal microglia is not known. Here we investigated whether aged microglia residing in the subretinal space are either preventing or promoting loss of visual function. Microglia were depleted in 33-month-old C57/BL6 mice fed for 6 weeks with a chow containing PLX5622, a small molecule inhibitor of colony-stimulating factor-1 receptor (Csf1r) which is expressed by microglia and is required for their proliferation, differentiation, and survival. As a negative control, age-matched mice received a control diet. Following microglia depletion, the optomotor reflex (OMR) was used to assess visual acuity and contrast sensitivity and electroretinogram (ERG) assessed retinal function. Morphological analysis was determined by immunohistochemical staining and TEM. To distinguish microglia from infiltrating macrophages, retinas from old mice were stained with P2RY12, a microglia-specific marker. The vast majority of cells residing between the photoreceptor outer segments and RPE layer in old mice were P2RY12+ microglia. These cells displayed a highly amoeboid and activated morphology and were filled with autofluorescence droplets similar to lipofuscin. PLX5622 treatment depleted up to 90% of the microglia. Microglia preserved visual function, and in the absence of microglia, there was a significant loss of contrast sensitivity and ERG c-wave, as compared with untreated controls. Loss of microglia coincided with a loss of RPE cells and increased RPE swelling indicating that microglia support RPE in aged animals. TEM indicates that subretinal microglia actively phagocytize shed photoreceptor outer segments, thereby compensating for the known age-related decline of RPE phagocytosis. We conclude that microglia in aging mice migrate to the subretinal space of the outer retina to support and maintain RPE cell functions that preserves vision.