Thesis defense Marie Palard

Equipe

Team IV - Cardiometabolic diseases

Directeur de thèse

Xavier Prieur

Rapporteurs

Dominique Langin, MD, PhD, PU-PH, Inserm UMR1297, I2MC, Toulouse
Marthe Moldes, PhD, Chargée de recherche,Inserm U938, CRSA, Paris

Examinateurs

Jérôme Gilleron, PhD, Chargé de recherche,Inserm U1065, C3M, Nice
Francesca Giordano, PhD, Directrice de recherche, CNRS I2BC, Gif Sur Yvette

Abstract

Obesity is a major risk factor for cardiometabolic diseases, with adipocyte dysfunction playing a central role. In individuals living with obesity, adipose tissue (AT) enters a state of metabolic inertia, reducing its capacity to store excess lipids and promoting ectopic lipid accumulation in non-adipose tissues—thereby contributing to cardiometabolic complications. Understanding the mechanisms that regulate lipid storage and mobilization in adipocytes—and how these are disrupted in obesity—is critical for addressing these complications. Generalized lipodystrophy, the most severe form of primary adipocyte dysfunction, is caused in approximately 50% of cases by mutations in the BSCL2 gene encoding Seipin, an endoplasmic reticulum (ER) protein essential for lipid droplet (LD) biogenesis and maintenance. Seipin also localizes to ER/mitochondria contact sites (MAM), where it regulates calcium exchange and mitochondrial function. This study aimed to determine whether Seipin’s recruitment to MAM and ER/LD contact sites overlaps and to assess the consequences of Seipin dysfunction on membrane contact site (MCS) dynamics and adipocyte metabolism. Using proximity ligation assays (PLA) and transmission electron microscopy (TEM), we observed altered MCS involving the ER, LDs, and mitochondria in Seipin-deficient models. Functional assays revealed that Seipin knockdown impairs triglyceride transfer to LDs, an effect that was rescued by the MAM-reinforcing synthetic peptide, the Linker-5. Further, we investigated how MCS remodeling influences adipocyte metabolic flexibility. Using TEM and PLA in both mouse AT and 3T3-L1 adipocytes, here, we show that lipid loading increases contacts involving the lipid droplet (LD), specifically ER/LD and mitochondria/LD (Mi/LD) contacts. However, lipid loading exerts opposite effects on MAM subtypes: oleic acid increases the MAM involving mitochondria in close contact with the LD, the MAM-LD, while decreasing the MAM involving cytosolic mitochondria, the “classical” MAM. -CM contacts.  Notably, this adaptive MCS remodeling was blunted in the AT of diet-induced obese mice. Genetic disruption of MCS in 3T3-L1 adipocytes led to altered lipid flux, impaired lipolysis, and reduced insulin signaling. Collectively, our findings demonstrate that MAM-LD contacts are central to adipocyte metabolic flexibility and lipid handling, and that their dysregulation in obesity may underlie the metabolic inflexibility characteristic of this condition.