Overlapping Yet Distinct Functions of Vacuole Membrane Protein 1 and Transmembrane Protein 41B in Modulating Hepatic Lipoprotein Secretion and Autophagy

BACKGROUND & AIMS: Transmembrane protein 41B (TMEM41B) and vacuolar membrane protein 1 (VMP1) are endoplasmic reticulum (ER) scramblases whose roles in hepatic lipoprotein secretion and autophagy in metabolic-associated steatotic liver disease (MASLD) remain unclear. METHODS: We undertook lipidomic and functional studies in liver- and hepatocyte-specific Tmem41b knockout (KO) mice; Tmem41b knock-in (KI) mice, Tmem41b/Vmp1 double KO (DKO); Tmem41b KO/Vmp1 KI, and Vmp1 KO/Tmem41b KI mice. RESULTS: TMEM41B protein levels decreased in the livers of human subjects with MASLD. Loss of hepatic Tmem41b impaired very low-density lipoprotein (VLDL) secretion, with steatosis, inflammation, and fibrosis, whereas hepatic TMEM41B overexpression mitigated these effects. Tmem41b/Vmp1 DKO mice showed further impairment in VLDL secretion compared with single Tmem41b KO. Lipidomic analysis revealed decreased phosphatidylcholine and phosphatidylethanolamine, with increased neutral lipids in Tmem41b KO mice. VMP1 and TMEM41B localize at the mitochondrial-associated membrane with reduced mitochondria-ER contact in Vmp1 and Tmem41b KO mice. Loss of hepatic VMP1 or TMEM41B increased levels of LC3B-II and p62/SQSTM1, which were not further changed in DKO mice. Restoring VMP1 in Tmem41b KO mice and TMEM41B in Vmp1 KO mice partially corrected defective VLDL secretion and hepatic steatosis in these single KO mice, respectively. Restoring VMP1 at a low but not a high dose corrected defective autophagy in Tmem41b KO mice, whereas overexpression of TMEM41B dose-dependently improved defective autophagy in Vmp1 KO mice. CONCLUSIONS: Loss of hepatic VMP1 or TMEM41B reduces VLDL secretion and promotes MASLD via overlapping but distinct mechanisms that regulate lipoprotein secretion and autophagy.