Mitochondrial Fragmentation Promotes Inflammation Resolution Responses in Macrophages via Histone Lactylation

Abstract

During the inflammatory response, macrophage phenotypes can be broadly classified as pro-inflammatory/classically activated 'M1', or pro-resolving/alternatively 'M2' macrophages. Although the classification of macrophages is general and assumes there are distinct phenotypes, in reality macrophages exist across a spectrum and must transform from a pro-inflammatory state to a pro-resolving state following an inflammatory insult. To adapt to changing metabolic needs of the cell, mitochondria undergo fusion and fission, which have important implications for cell fate and function. We hypothesized that mitochondrial fission and fusion directly contribute to macrophage function during the pro-inflammatory and pro-resolving phases. We find that mitochondrial length directly contributes to macrophage phenotype, primarily during the transition from a pro-inflammatory to a pro-resolving state. Phenocopying the elongated mitochondrial network (by disabling the fission machinery using siRNA) leads to a baseline reduction in the inflammatory marker IL-1β, but a normal inflammatory response to LPS, similar to control macrophages. In contrast, in macrophages with a phenocopied fragmented phenotype (by disabling the fusion machinery using siRNA) there is a heightened inflammatory response to LPS and increased signaling through the ATF4/c-Jun transcriptional axis compared to control macrophages. Importantly, macrophages with a fragmented mitochondrial phenotype show increased expression of pro-resolving mediator Arginase 1 and increased phagocytic capacity. Promoting mitochondrial fragmentation caused an increase in cellular lactate, and an increase in histone lactylation which caused an increase in Arginase 1 expression. Furthermore phenocopied mitochondrial fragmentation via MYLS22 (Opa1 inhibitor) also led to an improved zymosan induced peritonitis resolution timeline in-vivo. These studies demonstrate that a fragmented mitochondrial phenotype is critical for the pro-resolving response in macrophages and specifically drive epigenetic changes via lactylation of histones following an inflammatory insult.