Mitochondrial Transplantation Attenuates Brain Dysfunction in Sepsis by Driving Microglial M2 Polarization

Activation of microglia and mitochondrial dysfunction are two major contributors to the pathogenesis of sepsis-associated brain dysfunction. Mitochondrial dysfunction can alter the immunological profile of microglia favoring to a pro-inflammatory phenotype. Mitochondrial transplantation, as an emerging mitochondria-targeted therapy, possesses considerable therapeutic potential in various central nervous system injuries or diseases. However, the effects of mitochondrial transplantation on microglial polarization and neuroprotection after sepsis remain unclear. In this study, lipopolysaccharide (LPS)/interferon-gamma (IFN-gamma) and interleukin-4 (IL-4)/interleukin-13 (IL-13) were used to induce different phenotypes of BV2 microglial cells. We observed that mitochondrial content and function were enhanced in IL-4-/IL-13-stimulated microglia. In vitro, mitochondria treatment conferred neuroprotection by enhancing microglial polarization from the M1 phenotype to the M2 phenotype and suppressing microglial-derived inflammatory cytokine release. Furthermore, microglial phenotypes and behavior tests were assessed after mice were subjected to sepsis by cecal ligation and puncture (CLP) followed by intracerebroventricular injection of exogenous functional mitochondria. We found that mitochondrial transplantation induced microglial M2 rather than M1 response 24 h after sepsis. Mitochondrial transplantation improved behavioral deficits by increasing the latency time in inhibitory avoidance test and decreasing the number of crossing and rearing in the test session of open field test 10 days after CLP onset. These findings indicate that mitochondrial transplantation promotes the phenotypic conversion of microglia and improves cognitive impairment in sepsis survivors, supporting the potential use of exogenous mitochondrial transplantation therapy that may be a potential therapeutic opportunity for sepsis-associated brain dysfunction.