Effect of Caloric Restriction on the in vivo Functional Properties of Aging Microglia

Throughout the lifespan, microglia, the primary innate immune cells of the brain, fulfill a plethora of homeostatic as well as active immune defense functions, and their aging-induced dysfunctionality is now considered as a key trigger of aging-related brain disorders. Recent evidence suggests that both organism's sex and age critically impact the functional state of microglia but in vivo determinants of such state(s) remain unclear. Therefore, we analyzed in vivo the sex-specific functional states of microglia in young adult, middle aged and old wild type mice by means of multicolor two-photon imaging, using the microglial Ca^{2 +} signaling and directed process motility as main readouts. Our data revealed the sex-specific differences in microglial Ca^{2 +} signaling at all ages tested, beginning with young adults. Furthermore, for both sexes it showed that during the lifespan the functional state of microglia changes at least twice. Already at middle age the cells are found in the reactive or immune alerted state, characterized by heightened Ca^{2 +} signaling but normal process motility whereas old mice harbor senescent microglia with decreased Ca^{2 +} signaling, and faster but disorganized directed movement of microglial processes. The 6-12 months long caloric restriction (70% of ad libitum food intake) counteracted these aging-induced changes shifting many but not all functional properties of microglia toward a younger phenotype. The improvement of Ca^{2 +} signaling was more pronounced in males. Importantly, even short-term (6-week-long) caloric restriction beginning at old age strongly improved microglial process motility and induced a significant albeit weaker improvement of microglial Ca^{2 +} signaling. Together, these data provide first sex-specific in vivo characterization of functional properties of microglia along the lifespan and identify caloric restriction as a potent, cost-effective, and clinically relevant tool for rejuvenation of microglia.