Intercellular mitochondria transfer rescues aerobic respiration in metabolically compromised macrophages in vitro and in vivo

Abstract Mitochondria evolved through an ancient endosymbiotic process and are essential for eukaryotic cellular metabolism. An emerging body of literature indicates that some cells export mitochondria for delivery to other cells, a process termed intercellular mitochondria transfer. Recently, we demonstrated that adipocytes transfer mitochondria to macrophages in vivo and that this process maintains white and brown adipose tissue homeostasis (Brestoff et al., Cell Metab, 2021; Rosina et al., Cell Metab, 2022; Borcherding/Jia et al., Cell Metab, 2022). It is believed that macrophages degrade exogenous mitochondria as a form of licensed mitophagy that supports adipocyte mitochondria quality control, however it is unclear whether externally-derived mitochondria are able to respire in macrophages once captured. We administered purified mitochondria to wildtype (WT) or NDUFS4 KO mice, which lack an enzyme required for mitochondrial Complex I activity. Although WT peritoneal macrophages quickly degraded exogenous mitochondria, KO peritoneal macrophages captured, retained, and utilized exogenous mitochondria to rescue cell-intrinsic defects in aerobic respiration. Similarly, healthy BV2 microglial cells quickly degraded captured mitochondria in vitro, but BV2 microglial cells exposed to rotenone and Antimycin A captured and utilized exogenous WT mitochondria to rescue the toxin-induced defects in aerobic respiration. Purified mitochondria from NDUFS4 KO mice failed to rescue aerobic respiration in toxin-stressed BV2 cells. These data suggest that macrophages with defective mitochondrial metabolism can obtain and utilize exogenous, cell-free mitochondria from their environment to overcome a metabolic crisis. Supported by a grant from the Burroughs Wellcome Fund (CAMS #1019648) to JRB.