The proteome of cytosolic lipid droplets isolated from differentiated Caco-2/TC7 enterocytes reveals cell-specific characteristics.

BACKGROUND INFORMATION: Intestinal absorption of alimentary lipids is a complex process ensured by enterocytes and leading to TRL [TAG (triacylglycerol)-rich lipoprotein] assembly and secretion. The accumulation of circulating intestine-derived TRL is associated with atherosclerosis, stressing the importance of the control of postprandial hypertriglyceridaemia. During the postprandial period, TAGs are also transiently stored as CLDs (cytosolic lipid droplets) in enterocytes. As a first step for determining whether CLDs could play a role in the control of enterocyte TRL secretion, we analysed the protein endowment of CLDs isolated by sucrose-gradient centrifugation from differentiated Caco-2/TC7 enterocytes, the only human model able to secrete TRL in culture and to store transiently TAGs as CLDs when supplied with lipids. Cells were analysed after a 24 h incubation with lipid micelles and thus in a state of CLD-associated TAG mobilization. RESULTS: Among the 105 proteins identified in the CLD fraction by LC-MS/MS (liquid chromatography coupled with tandem MS), 27 were directly involved in lipid metabolism pathways potentially relevant to enterocyte-specific functions. The transient feature of CLDs was consistent with the presence of proteins necessary for fatty acid activation (acyl-CoA synthetases) and for TAG hydrolysis. In differentiated Caco-2/TC7 enterocytes, we identified for the first time LPCAT2 (lysophosphatidylcholine acyltransferase 2), involved in PC (phosphatidylcholine) synthesis, and 3BHS1 (3-ß-hydroxysteroid dehydrogenase 1), involved in steroid metabolism, and confirmed their partial CLD localization by immunofluorescence. In enterocytes, LPCAT2 may provide an economical source of PC, necessary for membrane synthesis and lipoprotein assembly, from the lysoPC present in the intestinal lumen. We also identified proteins involved in lipoprotein metabolism, such as ApoA-IV (apolipoprotein A-IV), which is specifically expressed by enterocytes and has been proposed to play many functions in vivo, including the formation of lipoproteins and the control of their size. The association of ApoA-IV with CLD was confirmed by confocal and immunoelectron microscopy and validated in vivo in the jejunum of mice fed with a high-fat diet. CONCLUSIONS: We report for the first time the protein endowment of Caco-2/TC7 enterocyte CLDs. Our results suggest that their formation and mobilization may participate in the control of enterocyte TRL secretion in a cell-specific manner.

Mutations in KEOPS-complex genes cause nephrotic syndrome with primary microcephaly.

Galloway-Mowat syndrome (GAMOS) is an autosomal-recessive disease characterized by the combination of early-onset nephrotic syndrome (SRNS) and microcephaly with brain anomalies. Here we identified recessive mutations in OSGEP, TP53RK, TPRKB, and LAGE3, genes encoding the four subunits of the KEOPS complex, in 37 individuals from 32 families with GAMOS. CRISPR-Cas9 knockout in zebrafish and mice recapitulated the human phenotype of primary microcephaly and resulted in early lethality. Knockdown of OSGEP, TP53RK, or TPRKB inhibited cell proliferation, which human mutations did not rescue. Furthermore, knockdown of these genes impaired protein translation, caused endoplasmic reticulum stress, activated DNA-damage-response signaling, and ultimately induced apoptosis. Knockdown of OSGEP or TP53RK induced defects in the actin cytoskeleton and decreased the migration rate of human podocytes, an established intermediate phenotype of SRNS. We thus identified four new monogenic causes of GAMOS, describe a link between KEOPS function and human disease, and delineate potential pathogenic mechanisms.