Expression of Synj2bp in mouse liver regulates the extent of wrappER-mitochondria contact to maintain hepatic lipid homeostasis.

Background: In mouse liver hepatocytes, nearly half of the surface area of every mitochondrion is covered by wrappER, a wrapping-type of ER that is rich in fatty acids and synthesizes lipoproteins (VLDL) (Anastasia et al. in Cell Rep 34:108873, 2021; Hurtley in Science (80- ) 372:142-143, 2021; Ilacqua et al. in J Cell Sci 135:1-11, 2021). A disruption of the ultrastructure of the wrappER-mitochondria contact results in altered fatty acid flux, leading to hepatic dyslipidemia (Anastasia et al. 2021). The molecular mechanism that regulates the extent of wrappER-mitochondria contacts is unknown.

Methods: We evaluated the expression level of the mitochondrial protein Synj2bp in the liver of normal and obese (ob/ob) mice. In addition, we silenced its expression in the liver using an AAV8 vector. We coupled quantitative EM morphometric analysis to proteomics and lipid analyses on these livers.

Results: The expression level of Synj2bp in the liver positively correlates with the extent of wrappER-mitochondria contacts. A 50% reduction in wrappER-mitochondria contacts causes hepatic dyslipidemia, characterized by a gross accumulation of lipid droplets in the liver, an increased hepatic secretion of VLDL and triglycerides, a curtailed ApoE expression, and an increased capacity of mitochondrial fatty acid respiration.

Conclusion: Synj2bp regulates the extent of wrappER-mitochondria contacts in the liver, thus contributing to the control of hepatic lipid flux.