Dynamic changes in mitochondria support phenotypic flexibility of microglia

The ability of microglia to sense the environment and alter their cellular phenotype according to local neuron and tissue needs is a hallmark feature of these cells. Numerous receptors that comprise the microglial “sensome” have been identified, but how microglia interpret combined signaling from diverse receptors and adjust multiple cellular attributes in a coordinated fashion is not well understood. Mitochondria are increasingly recognized as essential signaling hubs, and these organelles can regulate coordinated remodeling of cell attributes in immune cells, including macrophages. Given these findings, surprisingly little is known about microglial mitochondria in vivo and how the state of these organelles may impact microglial attributes and functions. Here, we generated novel transgenic crosses for high resolution analysis of microglial mitochondria in both fixed tissue and acute brain sections. Fixed tissue analysis indicated that mitochondrial abundance was tightly linked to microglial morphological complexity and that regional differences in microglial phenotype were accompanied by regional differences in mitochondrial mass and number. Surprisingly, multiphoton imaging revealed that mitochondrial abundance was not correlated with microglial cell process remodeling or rapid cell process extension toward focal sites of tissue injury. FACS- and qPCR-based analyses revealed remodeling of microglial mitochondrial state within hours of systemic LPS injections. Moreover, microglial expression of inflammation-, trophic-, and phagocytosis-relevant genes was strongly correlated with expression levels of numerous mitochondrial-relevant genes. Finally, FACS and fixed tissue imaging revealed that region-specific responses of microglia to aging were tightly linked to remodeling of these organelles. Overall, this study provides foundational information about microglial mitochondria and their relationship to differences in cell phenotype that occur across brain region, during pathological insults, and during aging. Moreover, these data demonstrate mitochondria support microglial phenotypic flexibility. ### Competing Interest Statement The authors have declared no competing interest.