SEIPIN Regulates Lipid Droplet Expansion and Adipocyte Development by Modulating the Activity of Glycerol-3-phosphate Acyltransferase

Abstract

SUMMARY Berardinelli-Seip congenital lipodystrophy 2 (BSCL2) is caused by loss-of-function mutations in SEIPIN, a protein implicated in both adipogenesis and lipid droplet expansion but whose molecular function remains obscure. Here, we identify physical and functional interactions between SEIPIN and microsomal isoforms of glycerol-3-phosphate acyltransferase (GPAT) in multiple organisms. Compared to controls, GPAT activity was elevated in SEIPIN-deficient cells and tissues and GPAT kinetic values were altered. Increased GPAT activity appears to underpin the block in adipogenesis and abnormal lipid droplet morphology associated with SEIPIN loss. Over-expression of Gpat3 blocked adipogenesis, and Gpat3 knockdown in SEIPIN-deficient preadipocytes partially restored differentiation. GPAT overexpression in yeast, preadipocytes, and fly salivary glands also formed supersized lipid droplets. Finally, pharmacological inhibition of GPAT in Seipin−/− mouse preadipocytes partially restored adipogenesis. These data identify SEIPIN as an evolutionarily conserved regulator of microsomal GPAT and suggest that GPAT inhibitors might be useful for the treatment of human BSCL2 patients.

ER Membrane Phospholipids and Surface Tension Control Cellular Lipid Droplet Formation

“…These conditions correspond, for example, to the accumulation of negatively curved lipids such as diacylglycerols, PA, or cholesterol ( Figure 1G tensions ( Figure 4G). Our prediction is consistent with reported regulations of GPAT4 trafficking from ER to LDs: in increased PA levels in the ER membrane of various cell types lacking Seipin protein, a massive transfer of GPAT4 from the ER to almost all LDs is observed in comparison with normal cells (Fei et al, 2011;Pagac et al, 2016;Szymanski et al, 2007;Wang et al, 2016); likewise, under conditions where GPAT4 should exclusively remain in the ER membrane of fly cells (Wilfling et al, 2014b), the induction of cholesterol accumulation in the membrane restored GPAT4 targeting from ER to LDs (Wilfling et al, 2014b). Consequently, ER tension and phospholipid composition seem to be critical for determining the protein composition of LD during formation.…”

Lipid droplet growth: regulation of a dynamic organelle

“…SEIPIN acts in ER-LD contact sites to enable nascent LDs to acquire more lipids from the ER and grow to form mature LDs [12,15]. SEIPIN also regulates the metabolism of phosphatidic acid (PA) at the LD-ER contact, acting as a scaffolding protein recruiting PA-metabolism enzymes such as LIPIN1 and AGPAT2 [16,17] as well as controlling PA levels by inhibiting GPAT [18]. In addition to being the biosynthetic precursor of phospholipids and TAGs, PA is a cone-shaped phospholipid that facilitates LD fusion [19,20] and could be accommodated in the regions of negative membrane curvature associated to LD budding and LD-ER connections [21].…”

The collaborative work of droplet assembly

“…Besides yeast Ldb16p, six other seipin-binding partners have been identified, three of which catalyze successive steps in glycerolipid synthesis: glycerophosphate acyltransferase (GPAT) [17], AGPAT2 [18], and the phosphatidate phosphatase, lipin [19]. Binding of seipin to both lipin and AGPAT is not mutually exclusive [18].…”

“…GPAT, the rate limiting step in TAG synthesis, was recently described as a seipin-binding protein in yeast, Drosophila, and mammalian cells [17]. The absence of seipin resulted in higher GPAT activity but not protein levels, indicating that seipin negatively regulates its activity.…”