Investigating Lipid Droplets biogenesis: the role of CtBP1-S/BARS.

Lipid droplets (LDs) are intracellular lipid-storing organelles surrounded by a phospholipid monolayer. LDs emerge as metabolically active organelles involved in many biological processes and linked to the pathogenesis of several dysfunctions including metabolic diseases (obesity, diabetes, atherosclerosis and hepatic steatosis). The mechanism underlying LD formation is still poorly understood and the molecular players that take part in this process have to be identified. Here, we provide evidence that a member of the C-terminal binding protein (CtBP) family, CtBP1-S/BARS (for brevity BARS), associates with LDs and is necessary for their formation. BARS is a fission-inducing protein that regulates several membrane trafficking events. Since the widely accepted model regarding LD biogenesis describes the formation of these organelles as fission products from specific endoplasmic reticulum microdomains, we wondered whether BARS could play a role in this process. We show that BARS is highly expressed during 3T3-L1 adipocytes cell differentiation. Interestingly, either the depletion or the inhibition of BARS strongly reduced the number of LD with an increased their size (in several tested cell types), indicating a central role of BARS in LD biogenesis. Furthermore, we found that BARS interacts and activates Acyl-CoA:lysocardiolipin acyltransferase (LCLAT1/AGPAT8) that catalyses the acylation of lysophosphatidilinositol (LPI) into phosphatidylinositol (PI), one of the main LD surface phospholipid component. Interestingly, we demonstrate that both BARS and AGPAT8 are recruited onto LDs where they interact and their specific LD membrane targeting is required for proper LD biogenesis. Our results shed new light on mechanisms underlying neutral lipid storage and highlight BARS and its binding partner AGPAT8 as key regulators in the biogenesis of lipid droplets.