Structure and function of mitochondria rough-er contact sites in mouse liver

La biologie des sites de contact entre organelles est une nouvelle branche de la biologie cellulaire qui étudie comment les organelles coopèrent ensemble pour coordonner des activités cellulaires complexes. Le premier contact de ce type à être décrit de manière fonctionnelle est celui qui est formé par le réticulum endoplasmique lisse (RE) et la mitochondrie, la MAM (Mitochondria-Associated ER Membrane). La MAM coordonne en effet les échanges de phospholipides et de calcium entre ces deux organelles. Cependant, on ignore si les mitochondries peuvent également former des contacts fonctionnels avec d'autres types de RE. Pour combler ce manque de connaissances, j'ai contribué, dans ma thèse, à étudier pour la première fois l'organisation morphologique et la fonction du contact entre l'ER brut et les mitochondries. Grâce à l'utilisation de la microscopie électronique (EM) et de la tomographie 3D, j'ai contribué à découvrir l'existence d'un nouveau type d'ER brut, que nous avons appelé wrappER, qui enveloppe étroitement les mitochondries dans les hépatocytes hépatiques de la souris. Sa caractérisation biochimique a révélé la présence de protéines et de transcrits impliqués dans la biogenèse des lipoprotéines de très basse densité (VLDL). De plus, mes études ont permis d'établir que la protéine Rrbp1 sert à lier les deux organelles et de découvrir que sa dérégulation dans le foie de la souris est caractérisée par une augmentation de la distance entre la mitochondrie et le wrappER ainsi qu'une diminution du taux de sécrétion des VLDL. Sur la base de ces résultats, nous avons proposé que les contacts wrappER-mitochondrie participent à l'homéostasie des lipides en régulant le taux de sécrétion des VLDL.The biology of inter-organelle contact sites is a new branch of cell biology that studies how organelles cooperate together to coordinate complex cellular activities. The first contact of this type to be functionally described was that that is formed by the smooth endoplasmic reticulum (ER) and the mitochondrion, the MAM (Mitochondria-Associated ER Membrane). The MAM indeed coordinates phospholipid and calcium exchanges between these two organelles. However, whether mitochondria can also form functionally competent contacts with other types of ER remains unknown. To fill this gap of knowledge, in my thesis I contributed to investigate for the first time the morphological organization and the function of the contact between the rough-ER and the mitochondria. Through the use of electron microscopy (EM) and 3D tomography, I have contributed to discover the existence a new type of rough-ER, which we called wrappER, that closely wraps the mitochondria in the liver hepatocytes of the mouse. Its biochemical characterization revealed the presence of proteins and transcripts involved in the biogenesis of the very low-density lipoproteins (VLDL). Furthermore, my studies have helped establish that the Rrbp1 protein serves to tether between the two organelles and discovered that its down-regulation in the mouse liver is characterized by an increase in the distance between mitochondrion and wrappER as well as to a decrease in the secretion rate of VLDL. Based on these results, we have proposed that wrappER-mitochondrion contacts participate in lipid homeostasis by regulating the rate of VLDL secretion

Mitochondria-rough-ER contacts in the liver regulate systemic lipid homeostasis

Contacts between organelles create microdomains that play major roles in regulating key intracellular activities and signaling pathways, but whether they also regulate systemic functions remains unknown. Here, we report the ultrastructural organization and dynamics of the inter-organellar contact established by sheets of curved rough endoplasmic reticulum closely wrapped around the mitochondria (wrappER). To elucidate the in vivo function of this contact, mouse liver fractions enriched in wrappER-associated mitochondria are analyzed by transcriptomics, proteomics, and lipidomics. The biochemical signature of the wrappER points to a role in the biogenesis of very-low-density lipoproteins (VLDL). Altering wrappER-mitochondria contacts curtails VLDL secretion and increases hepatic fatty acids, lipid droplets, and neutral lipid content. Conversely, acute liver-specific ablation of Mttp, the most upstream regulator of VLDL biogenesis, recapitulates this hepatic dyslipidemia phenotype and promotes remodeling of the wrappER-mitochondria contact. The discovery that liver wrappER-mitochondria contacts participate in VLDL biology suggests an involvement of inter-organelle contacts in systemic lipid homeostasis.Fil: Anastasia, Irene. Laval University; Canadá. Brain Research Center; CanadáFil: Ilacqua, Nicolò. Laval University; Canadá. Brain Research Center; CanadáFil: Raimondi, Andrea. San Raffaele Scientific Institute; ItaliaFil: Lemieux, Philippe. Brain Research Center; CanadáFil: Ghandehari-Alavijeh, Rana. Brain Research Center; CanadáFil: Faure, Guilhem. Broad Institute of MIT and Harvard; Estados Unidos. National Center For Biotechnology Information; Estados UnidosFil: Mekhedov, Sergei L.. National Center For Biotechnology Information ; Estados UnidosFil: Williams, Kevin J.. University of California at Los Angeles. School of Medicine; Estados UnidosFil: Caicci, Federico. Università di Padova; ItaliaFil: Valle, Giorgio. Università di Padova; ItaliaFil: Giacomello, Marta. Università di Padova; ItaliaFil: Quiroga, Ariel Dario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Fisiología Experimental. Universidad Nacional de Rosario. Facultad de Ciencias Bioquímicas y Farmacéuticas. Instituto de Fisiología Experimental; Argentina. University of Alberta; CanadáFil: Lehner, Richard. University of Alberta; CanadáFil: Miksis, Michael J.. Northwestern University; Estados UnidosFil: Toth, Katalin. University of Ottawa; CanadáFil: de Aguiar Vallim, Thomas Q.. University of California at Los Angeles. School of Medicine; Estados UnidosFil: Koonin, Eugene V.. National Center For Biotechnology Information ; Estados UnidosFil: Scorrano, Luca. Università di Padova; ItaliaFil: Pellegrini, Luca. Laval University; Canad

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

Abstract 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