Cardiolipin Coordinates Lipopolysaccharide-induced Respiratory Complex II Disassembly & Degradation

Mack B. Reynolds and Mary X. O’Riordan

Macrophage metabolic plasticity is critical for initiation and resolution of inflammation. Control of mitochondrial respiratory complex (RC) activity, especially Respiratory Complex II (CII), is central to macrophage inflammatory programming. The mechanism governing CII activity, which links respiration with the tricarboxylic acid (TCA) cycle, remains to be defined in macrophages. Here we show that macrophage activation by lipopolysaccharide (LPS) triggers CII disassembly, followed by selective loss of the succinate dehydrogenase B (SDHB) subunit of CII. We observed sequestration of SDHB into puncta distinct from the mitochondrial network that were instead associated with the endolysosomal marker, LAMP1. Macrophages deficient in mitochondrial fission regulator Dynamin-related protein 1 (DRP1) still sequestered SDHB in response to LPS, but these puncta remained within the mitochondrial network. We hypothesized that the mitochondrial phospholipid cardiolipin (CL), which enables supramolecular organization of RCs, might facilitate programmed loss of CII during macrophage inflammatory activation. Knockdown (KD) of CL biosynthetic enzyme Cardiolipin Synthase (CRLS1) prevented LPS-induced inflammatory remodeling from respiratory to glycolytic metabolism, maintained pre-induction levels of SDHB and accumulated less succinate. Additionally, CRLS1 KD macrophages could not sequester SDHB within the mitochondrial network or decrease SDHB protein levels in response to LPS. CRLS1 KD resulted in a defect in LPS-induced pro-inflammatory cytokine production. In contrast, knockdown of CL remodeling enzyme Tafazzin in macrophages enhanced pro-inflammatory cytokine production, indicating a suppressive role for this CL remodeling enzyme in inflammatory signaling. Finally, CRLS1-dependent cytokine production could be partially rescued by pharmacological inhibition of CII. Taken together, we propose that CL acts as an organizing platform to direct CII disassembly and degradation to enable macrophage metabolic and inflammatory programming.