SummaryActivation of energy expenditure in thermogenic fat is a promising strategy to improve metabolic health, yet the dynamic processes that evoke this response are poorly understood. Here we show that synthesis of the mitochondrial phospholipid cardiolipin is indispensable for stimulating and sustaining thermogenic fat function. Cardiolipin biosynthesis is robustly induced in brown and beige adipose upon cold exposure. Mimicking this response through overexpression of cardiolipin synthase (Crls1) enhances energy consumption in mouse and human adipocytes. Crls1 deficiency in thermogenic adipocytes diminishes inducible mitochondrial uncoupling and elicits a nuclear transcriptional response through endoplasmic reticulum stress-mediated retrograde communication. Cardiolipin depletion in brown and beige fat abolishes adipose thermogenesis and glucose uptake, which renders animals insulin resistant. We further identify a rare human CRLS1 variant associated with insulin resistance and show that adipose CRLS1 levels positively correlate with insulin sensitivity. Thus, adipose cardiolipin has a powerful impact on organismal energy homeostasis through thermogenic fat bioenergetics.

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The mouse acyl-CoA-binding protein (ACBP) 5 /diazepam binding inhibitor is a 10-kDa intracellular protein consisting of 86 amino acids. It is highly conserved throughout evolution and expressed in all cell types in the eukaryotes investigated (1, 2). This, together with the characteristics of the ACBP promoter (3, 4), implies a housekeeping function of the gene. However, expression levels vary markedly between tissues (5) and in response to different metabolic stimuli (6 -9), thereby indicating that ACBP might perform more specialized functions in some cell types. The ACBP protein binds C 14 -C 22 acyl-CoA esters with high affinity and specificity (10, 11) and has very little or no affinity toward other ligands (11-13). From in vitro studies, ACBP is known to protect acyl-CoA esters from hydrolysis (14 -16) and to relieve acyl-CoA inhibition of a number of enzymes, including long chain acyl-CoA synthetase, acetyl-CoA carboxylase (ACC), adenine nucleotide translocase, fatty acid synthetase (FAS), carnitine palmitoyltransferase, and acyl-CoA:cholesterol acyltransferase (9, 16 -18). In addition, ACBP is known to donate acyl-CoA esters to phospholipid, glycerolipid, and cholesteryl ester (CE) synthesis (14, 18 -21). Finally, proteolytic products of secreted ACBP have been shown to have signaling functions in Dictyostelium as well as mammalian cells (22). Targeted disruption of the yeast ACBP gene (ACB1) revealed that ACBP deficiency results in increased levels of C18:0 acyl-CoA esters and a decrease in the amount of total C26:0 fatty acids, indicating that transport of FA toward elongation is impaired by lack of ACBP. Furthermore, sphingolipid and ceramide amounts were reduced, membrane structure was altered, and vesicular transport was compromised (23-25).The functions of ACBP in lipid metabolism have been further studied in different mammalian cell culture systems and animal models by both knockdown strategies and overexpression of the protein. It has been reported that knockdown of ACBP by small interfering RNA causes growth arrest and lethality in three different mammalian cell lines (26); however, data from our laboratory show that ACBP can be knocked down in many different cell systems without affecting growth and survival (27). 6 Recently, knockdown of ACBP in HepG2 cells was shown to suppress the expression of a number of genes involved in lipid biosynthesis and lead to decreased levels of saturated and monounsaturated fatty acids (28). In 3T3-L1 preadipocytes, knockdown of ACBP caused a mild impairment of adipocyte differentiation and accumulation of triacylglycerol (TAG) (27), whereas overexpression of ACBP in McA-RH7777 rat hepatoma cells resulted in increased intracellular TAG accumulation (29). Overexpression of ACBP in transgenic mice resulted in accumulation of different lipid classes, including TAG in the liver (30). These results suggest *

Acyl-CoA-binding protein (ACBP) is a ubiquitously expressed protein that binds intracellular acyl-CoA esters. Several studies have suggested that ACBP acts as an acyl-CoA pool former and regulates long-chain fatty acids (LCFA) metabolism in peripheral tissues. In the brain, ACBP is known as Diazepam-Binding Inhibitor, a secreted peptide acting as an allosteric modulator of the GABA A receptor. However, its role in central LCFA metabolism remains unknown. In the present study, we investigated ACBP cellular expression, ACBP regulation of LCFA intracellular metabolism, FA profile, and FA metabolism-related gene expression using ACBP-deficient and control mice. ACBP was mainly found in astrocytes with high expression levels in the mediobasal hypothalamus. We demonstrate that ACBP deficiency alters the central LCFACoA profile and impairs unsaturated (oleate, linolenate) but not saturated (palmitate, stearate) LCFA metabolic fluxes in hypothalamic slices and astrocyte cultures. In addition, lack of ACBP differently affects the expression of genes involved in FA metabolism in cortical versus hypothalamic astrocytes. Finally, ACBP deficiency increases FA content and impairs their release in response to palmitate in hypothalamic astrocytes. Collectively, these findings reveal for the first time that central ACBP acts as a regulator of LCFA intracellular metabolism in astrocytes.

ObjectiveCeramides are precursors of complex sphingolipids (SLs), which are important for normal functioning of both the developing and mature brain. Altered SL levels have been associated with many neurodegenerative disorders, including epilepsy, although few direct links have been identified between genes involved in SL metabolism and epilepsy.MethodsWe used quantitative real-time PCR, Western blotting, and enzymatic assays to determine the mRNA, protein, and activity levels of ceramide synthase 2 (CERS2) in fiibroblasts isolated from parental control subjects and from a patient diagnosed with progressive myoclonic epilepsy (PME). Mass spectrometry and fluorescence microscopy were used to examine the effects of reduced CERS2 activity on cellular lipid composition and plasma membrane functions.ResultsWe identify a novel 27 kb heterozygous deletion including the CERS2 gene in a proband diagnosed with PME. Compared to parental controls, levels of CERS2 mRNA, protein, and activity were reduced by ˜50% in fibroblasts isolated from this proband, resulting in significantly reduced levels of ceramides and sphingomyelins containing the very long-chain fatty acids C24:0 and C26:0. The change in SL composition was also reflected in a reduction in cholera toxin B immunofluorescence, indicating that membrane composition and function are altered.InterpretationWe propose that reduced levels of CERS2, and consequently diminished levels of ceramides and SLs containing very long-chain fatty acids, lead to development of PME.