Adipose triglyceride lipase affects triacylglycerol metabolism at brain barriers

Currently, little is known about the role of intracellular triacylglycerol (TAG) lipases in the brain. Adipose triglyceride lipase (ATGL) is encoded by the PNPLA2 gene and catalyzes the rate-limiting step of lipolysis. In this study, we investigated the effects of ATGL deficiency on brain lipid metabolism in vivo using an established knock-out mouse model (ATGL-ko). A moderate decrease in TAG hydrolase activity detected in ATGL-ko versus wild-type brain tissue was accompanied by a 14-fold increase in TAG levels and an altered composition of TAG-associated fatty acids in ATGL-ko brains. Oil Red O staining revealed a severe accumulation of neutral lipids associated to cerebrovascular cells and in distinct brain regions namely the ependymal cell layer and the choroid plexus along the ventricular system. In situ hybridization histochemistry identified ATGL mRNA expression in ependymal cells, the choroid plexus, pyramidal cells of the hippocampus, and the dentate gyrus. Our findings imply that ATGL is involved in brain fatty acid metabolism, particularly in regions mediating transport and exchange processes: the brain-CSF interface, the blood-CSF barrier, and the blood-brain barrier.     

Mapping the regioisomeric distribution of fatty acids in triacylglycerols by hybrid mass spectrometry

This study describes the use of hybrid mass spectrometry for the mapping, identification, and semi-quantitation of triacylglycerol regioisomers in fats and oils. The identification was performed based on the accurate mass and fragmentation pattern obtained by data-dependent fragmentation. Quantitation was based on the high-resolution ion chromatograms, and relative proportion of sn-1(3)/sn-2 regioisomers was calculated based on generalized fragmentation models and the relative intensities observed in the product ion spectra. The key performance features of the developed method are inter-batch mass accuracy < 1 ppm (n = 10); lower limit of detection (triggering threshold) 0.1 μg/ml (equivalent to 0.2 weight % in oil); lower limit of quantitation 0.2 μg/ml (equivalent to 0.4 weight % in oil); peak area precision 6.5% at 2 μg/ml concentration and 15% at 0.2 μM concentration; inter-batch precision of fragment intensities < 1% (n = 10) independent of the investigated concentration; and averaged accuracy using the generic calibration 3.8% in the 1–10 μg/ml range and varies between 1–23% depending on analytes. Inter-esterified fat, beef tallow, pork lard, and butter fat samples were used to show how well regioisomeric distribution of palmitic acid can be captured by this method.     

Tsc2, a positional candidate gene underlying a quantitative trait locus for hepatic steatosis

Nonalchoholic fatty liver disease (NAFLD) is the most common cause of liver dysfunction and is associated with metabolic diseases, including obesity, insulin resistance, and type 2 diabetes. We mapped a quantitative trait locus (QTL) for NAFLD to chromosome 17 in a cross between C57BL/6 (B6) and BTBR mouse strains made genetically obese with the Lep(ob/ob) mutation. We identified Tsc2 as a gene underlying the chromosome 17 NAFLD QTL. Tsc2 functions as an inhibitor of mammalian target of rapamycin, which is involved in many physiological processes, including cell growth, proliferation, and metabolism. We found that Tsc2(+/-) mice have increased lipogenic gene expression in the liver in an insulin-dependent manner. The coding single nucleotide polymorphism between the B6 and BTBR strains leads to a change in the ability to inhibit the expression of lipogenic genes and de novo lipogenesis in AML12 cells and to promote the proliferation of Ins1 cells. This difference is due to a different affinity of binding to Tsc1, which affects the stability of Tsc2.     

Lipid droplet formation on opposing sides of the endoplasmic reticulum

In animal cells, the primary repositories of esterified fatty acids and alcohols (neutral lipids) are lipid droplets that form on the lumenal and/or cytoplasmic side of the endoplasmic reticulum (ER) membrane. A monolayer of amphipathic lipids, intermeshed with key proteins, serves to solubilize neutral lipids as they are synthesized and desorbed. In specialized cells, mobilization of the lipid cargo for delivery to other tissues occurs by secretion of lipoproteins into the plasma compartment. Serum lipoprotein assembly requires an obligate structural protein anchor (apolipoprotein B) and a dedicated chaperone, microsomal triglyceride transfer protein. By contrast, lipid droplets that form on the cytoplasmic face of the ER lack an obligate protein scaffold or any required chaperone/lipid transfer protein. Mobilization of neutral lipids from the cytosol requires regulated hydrolysis followed by transfer of the products to different organelles or export from cells. Several proteins play a key role in controlling droplet number, stability, and catabolism; however, it is our premise that their formation initiates spontaneously, solely as a consequence of neutral lipid synthesis. This default pathway directs droplets into the cytoplasm where they accumulate in many lipid disorders.     

Molecular genetics of human lipoprotein lipase deficiency

Lipoprotein lipase (LPL) hydrolysis the triglyceride core of circulating chylomicrons and very-low-density lipoprotein, and modulates the levels and lipid composition of low and high density lipoproteins. Worldwide, more than 20 mutations in the LPL gene have been identified in patients with familial LPL deficiency. Most of these mutations are clustered in the region encoded by exons 4, 5 and 6 which forms the proposed catalytic domain of LPL. In French Canadians who have the highest reported frequency for LPL deficiency, three common mutations in the LPL gene have been identified which account for approximately 97% of mutant genes in this group. Simple DNA-based tests for the detection of all these mutations have been developed for the screening for carriers of LPL deficiency. This will facilitate further studies of phenotypic expression in heterozygous carriers and assessment of the risk of atherosclerosis in these individuals.