Lack of de novo phosphatidylinositol synthesis leads to endoplasmic reticulum stress and hepatic steatosis in cdipt-deficient zebrafish

Abstract

Hepatic steatosis is the initial stage of non-alcoholic fatty liver disease (NAFLD) and may predispose to more severe hepatic disease, including hepatocellular carcinoma. Endoplasmic reticulum (ER) stress has been recently implicated as a novel mechanism that may lead to NAFLD, although the genetic factors invoking ER stress are largely unknown. During a screen for liver defects from a zebrafish insertional mutant library, we isolated the mutant cdipthi559Tg/+ (hi559). CDIPT is known to play an indispensable role in phosphatidylinositol (PtdIns) synthesis. Here we show that cdipt is expressed in the developing liver and its disruption in hi559 mutants abrogates de novo PtdIns synthesis, resulting in hepatomegaly at 5-dpf. The hi559 hepatocytes display features of NAFLD, including macrovesicular steatosis, ballooning, and necroapoptosis. Gene set enrichment of microarray profiling revealed significant enrichment of ER stress response (ERSR) genes in hi559 mutants. ER stress markers, including atf6, hspa5, calr, xbp1, are selectively upregulated in the mutant liver. The hi559 expression profile showed significant overlap with that of mammalian hepatic ER stress and NAFLD. Ultrastructurally, the hi559 hepatocytes display marked disruption of ER architecture with hallmarks of chronic unresolved ER stress. Induction of ER stress by tunicamycin in wild-type larvae results in a fatty liver similar to hi559, suggesting that ER stress could be a fundamental mechanism contributing to hepatic steatosis. Conclusion: Cdipt-deficient zebrafish exhibit hepatic ER stress and NAFLD pathologies, implicating a novel link between PtdIns, ER stress, and steatosis. The tractability of hi559 mutant provides a valuable tool to dissect ERSR components, their contribution to molecular pathogenesis and evaluation of novel therapeutics of NAFLD.

Phosphoinositides: Tiny Lipids With Giant Impact on Cell Regulation

“…There appears to be no other gene for PtdIns synthesis, so the association of the enzyme activity with PM fractions may be related to their contamination with ER or with the lighter mobile PIS membrane compartment (796). Two studies reported defects in PIS activity in zebrafish, one describing ER stress and hepatic steatosis (1547), while the other observed lens structural defects and reduced number of photoreceptors (1098).…”

A monocarboxylate transporter required for hepatocyte secretion of ketone bodies during fasting

“…This conservation of function and the absence of fasting steatosis in larvae that have never been fed (Schlegel and Stainier 2006) allowed us to screen for mutations causing increased hepatic lipid mass. Performing the screen in animals carrying three transgenes that encode fluorescent protein labels of the Islet of Langerhans b cells, the liver hepatocytes, and the pancreatic acinar cells (Anderson et al 2009) helped us to avoid mutants that displayed changes in liver or pancreas morphology (Sadler et al 2005;Akimitsu et al 2008;Matthews et al 2009;Thakur et al 2011). Defects in yolk lipid transport were avoided by restricting our analysis to only larvae that consumed all their yolk by 6 d post-fertilization (dpf) (Schlegel and Stainier 2006;Raldú a et al 2008;Walters et al 2009).…”

Developing ‘integrative’ zebrafish models of behavioral and metabolic disorders

“…CDP-diacylglycerol-inositol 3-phosphatidyltransferase (CDIPT) deficiency increases endoplasmic retuculim stress, leading to hepatic steatosis Gastro-intestinal HI559, CDIPT [136] After 1-week fasting, zebrafish show unaltered leptin-a levels but reduced leptin-b [107]. In goldfish, a different teleost fish species, leptin injections suppress appetite via inhibiting the NPY signaling pathways [108].…”