High‐Fat Diet Impairs Translocation of Phosphatidylserine From Mitochondria‐Associated Membranes ( <scp>MAM</scp> ) Into Mitochondria by the Conservative Mechanism From Fish to Tetrapod

High-fat diet (HFD)-induced hepatic steatosis represents a significant global health challenge, yet the precise role of phospholipid (PLs) dysregulation in its pathogenesis remains unclear. Although the critical role of mitochondria-associated membranes (MAMs) in maintaining PL homeostasis is increasingly recognized, the molecular mechanisms underlying MAM-mediated inter-organelle PLs translocation and the resulting functional consequences remain poorly understood. Additionally, whether MAM-mediated PLs homeostasis represents a conserved mechanism across vertebrates has yet to be determined. To address these knowledge gaps, this study investigated the mechanistic and evolutionary basis of MAM-mediated PLs translocation in hepatic lipid metabolism using an in vivo model of yellow catfish (Pelteobagrus fulvidraco) fed either a control or HFD for 8 weeks, followed by comparative validation in tetrapods (frogs, chickens, mice and human beings) in vitro. The key findings include: (1) HFD primarily disrupts hepatic phosphatidylserine (PS) homeostasis by impairing ER-to-mitochondria translocation via MAMs, rather than by affecting PS biosynthesis; (2) The Ip3r-Grp75-Vdac/Mcu axis coordinates MAM structural integrity and Ca²⁺ signaling to mediate PS transfer, where dual grp75/mcu overexpression restores PS trafficking under lipotoxic stress; (3) MAM-dependent PS translocation is essential for maintaining mitochondrial dynamics and β-oxidation capacity, while its disruption under HFD promotes hepatic steatosis. Significantly, this study reveals that MAM-mediated PS homeostasis via the Ip3r-Grp75-Vdac/Mcu axis represents a conserved mechanism in tested species, offering new insights into vertebrate adaptations to lipid overload. These findings highlight MAMs as therapeutic targets for metabolic liver diseases through their role in linking PLs metabolism to mitochondrial function.