Microglia respond to elevated intraocular pressure and synapse loss in the visual thalamus in a mouse model of glaucoma

Microglia are resident immune cells of the central nervous system and mediate a broad array of adaptations and responses during disease, injury, and development. Typically, microglia morphology is understood to provide a window into their functional state. However, it is apparent that they have the capacity to adopt a broad spectrum of functional phenotypes characterized by numerous morphological profiles and associated gene expression profiles. Glaucoma, which leads to blindness from retinal ganglion cell (RGC) degeneration, is commonly associated with elevated intraocular pressure and has been shown to trigger microglia responses within the retinal layers, at the optic nerve head, and in retinal projection targets in the brain. The goal of this study was to determine the relationship of microglia morphology to intraocular pressure and the loss of retinal ganglion cell output synapses in the dorsolateral geniculate nucleus (dLGN), a RGC projection target in the thalamus that conveys information to the primary visual cortex. We accomplished this by analyzing dLGN microglia morphologies in histological sections from DBA/2J mice, which develop a form of inherited glaucoma. Microglia morphology was analyzed using skeletonized Iba1-fluorescence images and fractal analyses of individually reconstructed microglia cells. We found that microglia adopted more simplified morphologies, characterized by fewer endpoints and less total process length per microglia cell. There was an age-dependent shift in microglia morphology in tissue from control mice (DBA/2J ^Gpnmb+ ) that was accelerated in DBA/2J mice. Microglia morphological measurements correlated with cumulative intraocular pressure, immunofluorescence labeling for the complement protein C1q, and density of vGlut2-labeled RGC axon terminals. Additionally, fractal analysis revealed a clear distinction between control and glaucoma dLGN, with microglia from ocular hypertensive DBA/2J dLGN tissue showing an elongated rod-like morphology. RNA-sequencing of dLGN tissue samples showed an upregulation of immune system-related gene expression and several specific genes associated with microglia activation and potential neuroprotective functions. These results suggest that microglia in the dLGN alter their physiology to respond to RGC degeneration in glaucoma, potentially contributing to CNS adaptations to neurodegenerative vision loss.