Ip3r-Grp75-Vdac and relevant Ca2+ signaling regulate dietary palmitic acid-induced de novo lipogenesis by mitochondria-associated ER membrane (MAM) recruiting Seipin in yellow catfish

The mitochondria-associated endoplasmic reticulum membrane membranes (MAM), is the central hub for endoplasmic reticulum and mitochondria functional communicate. It plays a crucial role in hepatic lipid homeostasis. However, even though MAM has been acknowledged to be enriched in enzymes that contribute to lipid biosynthesis, no study has yet investigated the exact role of MAM on hepatic neutral lipid synthesis. To address these gaps, this study investigated the systemic control mechanisms for MAM in neutral lipids synthesis by recruiting Seipin, focusing on the role of the Ip3r-Grp75-Vdac complex and their relevant Ca2+ signaling in this process. To this end, a model animal for lipid metabolism, yellow catfish (Pelteobagrus fulvidraco), were fed six different diets containing a range of palmitic acid (PA) concentrations from 0 g/kg to 150 g/kg in vivo for 10 weeks. In vitro experiments were also conducted to intercept the MAM-mediated Ca2+ signaling in isolated hepatocytes by transfecting them with si-mitochondrial calcium uniporter (mcu). Since mcu was resided in the inner mitochondrial membrane (IMM), si-mcu cannot disrupt MAM's structural integrity. 1. Hepatocellular MAM sub-proteome analysis indicated excessive dietary PA intake enhanced hepatic MAM structural join by activating Ip3r-Grp75-voltage-dependent anion channel (Vdac) complexes. 2. Dietary PA intake induced hepatic neutral lipid accumulation through MAM recruiting Seipin, which activated lipid droplet biogenesis. Our findings also revealed a previously unidentified mechanism whereby MAM recruited Seipin and controlled hepatic lipid homeostasis, depending on Ip3r-Grp75-Vdac-controlled Ca2+ signaling but not only MAM's structural integrity. These results offer a novel insight into the MAM-recruited Seipin in controlling hepatic lipid synthesis in a MAM structural integrity-dependent and Ca2+ signaling-dependent manner, highlighting the critical contribution of MAM in maintaining hepatic neutral lipid homeostasis.