Aging Converts Microglial Repopulation into Maladaptive Reprogramming that Exacerbates Cognitive Deficits

Background and Purpose Aging has been associated with neuroinflammation and cognitive decline. Microglial repopulation after pharmacological depletion has been proposed as a strategy to alleviate microglia-driven neuropathology. However, the effects of microglial repopulation on age-related cognitive decline remain largely unexplored. In the present study, we examined how microglial repopulation affects the transcriptomic profiles of cortical microglia and the decline in prefrontal cortex-dependent cognitive function in aged mice. Experimental Approach Young and aged male C57BL/6J mice were used in this study. Microglial depletion was induced by treatment with PLX3397, a CSF1R inhibitor, followed by microglial repopulation after drug withdrawal. Microglia isolated from the entire cerebral cortex were subjected to bulk RNA-sequencing analysis. The visual discrimination test followed by the response direction test was conducted to assess sensory learning and attentional set shifting abilities, respectively. Key Results Repopulated cortical microglia in aged, but not young, mice exhibited aberrant gene expression patterns, including reduced expression of microglial identity genes, reprogramming of innate and adaptive immune-related gene expression, and derepression of neuronal gene expression. Furthermore, microglial repopulation selectively impaired visual discrimination learning and attentional set shifting in aged mice. Conclusion and Implications These findings demonstrate that aging converts microglial repopulation into maladaptive reprogramming that exacerbates cognitive decline, possibly through aberrant gene expression programs. Therefore, the therapeutic potential of this approach must be carefully evaluated with respect to specific behavioral domains and disease contexts, including aging.