Ann-Britt Marcher

Disruption of the Acyl-CoA-binding Protein Gene Delays Hepatic Adaptation to Metabolic Changes at Weaning

The acyl-CoA-binding protein (ACBP)/diazepam binding inhibitor is an intracellular protein that binds C14–C22 acyl-CoA esters and is thought to act as an acyl-CoA transporter. In vitro analyses have indicated that ACBP can transport acyl-CoA esters between different enzymatic systems; however, little is known about the in vivo function in mammalian cells. We have generated mice with targeted disruption of ACBP (ACBP−/−). These mice are viable and fertile and develop normally. However, around weaning, the ACBP−/− mice go through a crisis with overall weakness and a slightly decreased growth rate. Using microarray analysis, we show that the liver of ACBP−/− mice displays a significantly delayed adaptation to weaning with late induction of target genes of the sterol regulatory element-binding protein (SREBP) family. As a result, hepatic de novo cholesterogenesis is decreased at weaning. The delayed induction of SREBP target genes around weaning is caused by a compromised processing and decreased expression of SREBP precursors, leading to reduced binding of SREBP to target sites in chromatin. In conclusion, lack of ACBP interferes with the normal metabolic adaptation to weaning and leads to delayed induction of the lipogenic gene program in the liver.

The acyl-CoA binding protein is required for normal epidermal barrier function in mice.

The acyl-CoA binding protein (ACBP) is a 10 kDa intracellular protein expressed in all eukaryotic species. Mice with targeted disruption of Acbp (ACBP−/− mice) are viable and fertile but present a visible skin and fur phenotype characterized by greasy fur and development of alopecia and scaling with age. Morphology and development of skin and appendages are normal in ACBP−/− mice; however, the stratum corneum display altered biophysical properties with reduced proton activity and decreased water content. Mass spectrometry analyses of lipids from epidermis and stratum corneum of ACBP+/+ and ACBP−/− mice showed very similar composition, except for a significant and specific decrease in the very long chain free fatty acids (VLC-FFA) in stratum corneum of ACBP−/− mice. This finding indicates that ACBP is critically involved in the processes that lead to production of stratum corneum VLC-FFAs via complex phospholipids in the lamellar bodies. Importantly, we show that ACBP−/− mice display a ∼50% increased transepidermal water loss compared with ACBP+/+ mice. Furthermore, skin and fur sebum monoalkyl diacylglycerol (MADAG) levels are significantly increased, suggesting that ACBP limits MADAG synthesis in sebaceous glands. In summary, our study shows that ACBP is required for production of VLC-FFA for stratum corneum and for maintaining normal epidermal barrier function.

Delayed Hepatic Adaptation to Weaning in ACBP−/− Mice Is Caused by Disruption of the Epidermal Barrier

We previously reported that mice deficient in acyl-CoA-binding protein (ACBP) display a delayed metabolic adaptation to weaning. This includes a delayed activation of the hepatic lipogenic gene program, which may result from hepatic accumulation of triacylglycerol and/or cholesteryl esters in the late suckling period. To further investigate the basis for this phenotype, we generated mice deficient in ACBP in hepatocytes (Alb-ACBP(-/-)) and keratinocytes (K14-ACBP(-/-)). Surprisingly, the delayed adaptation to weaning, including hepatic lipid accumulation, is caused by ACBP deficiency in the skin rather than in the liver. Similarly to ACBP(-/-) mice, K14-ACBP(-/-) mice exhibit an increased transepidermal water loss, and we show that the hepatic phenotype is caused specifically by the epidermal barrier defect, which leads to increased lipolysis in white adipose tissue. Our data demonstrate that an imperfect epidermal barrier leads to profound suppression of the hepatic SREBP gene program and lipid accumulation in the liver.

RNA-Seq and Mass-Spectrometry-Based Lipidomics Reveal Extensive Changes of Glycerolipid Pathways in Brown Adipose Tissue in Response to Cold

Cold exposure greatly alters brown adipose tissue (BAT) gene expression and metabolism to increase thermogenic capacity. Here, we used RNA sequencing and mass-spectrometry-based lipidomics to provide a comprehensive resource describing the molecular signature of cold adaptation at the level of the transcriptome and lipidome. We show that short-term (3-day) cold exposure leads to a robust increase in expression of several brown adipocyte genes related to thermogenesis as well as the gene encoding the hormone irisin. However, pathway analysis shows that the most significantly induced genes are those involved in glycerophospholipid synthesis and fatty acid elongation. This is accompanied by significant changes in the acyl chain composition of triacylglycerols (TAGs) as well as subspecies-selective changes of acyl chains in glycerophospholipids. These results indicate that cold adaptation of BAT is associated with significant and highly species-selective remodeling of both TAGs and glycerophospholipids.

Mice with targeted disruption of the acyl-CoA binding protein display attenuated urine concentrating ability and diminished renal aquaporin-3 abundance

The acyl-CoA binding protein (ACBP) is a small intracellular protein that specifically binds and transports medium to long-chain acyl-CoA esters. Previous studies have shown that ACBP is ubiquitously expressed but found at particularly high levels in lipogenic cell types as well as in many epithelial cells. Here we show that ACBP is widely expressed in human and mouse kidney epithelium, with the highest expression in the proximal convoluted tubules. To elucidate the role of ACBP in the renal epithelium, mice with targeted disruption of the ACBP gene (ACBP(-/-)) were used to study water and NaCl balance as well as urine concentrating ability in metabolic cages. Food intake and urinary excretion of Na(+) and K(+) did not differ between ACBP(-/-) and (+/+) mice. Interestingly, however, water intake and diuresis were significantly higher at baseline in ACBP(-/-) mice compared with that of (+/+) mice. Subsequent to 20-h water deprivation, ACBP(-/-) mice exhibited increased diuresis, reduced urine osmolality, elevated hematocrit, and higher relative weight loss compared with (+/+) mice. There were no significant differences in plasma concentrations of renin, corticosterone, and aldosterone between mice of the two genotypes. After water deprivation, renal medullary interstitial fluid osmolality and concentrations of Na(+), K(+), and urea did not differ between genotypes and cAMP excretion was similar. Renal aquaporin-1 (AQP1), -2, and -4 protein abundances did not differ between water-deprived (+/+) and ACBP(-/-) mice; however, ACBP(-/-) mice displayed increased apical targeting of pS256-AQP2. AQP3 abundance was lower in ACBP(-/-) mice than in (+/+) control animals. Thus we conclude that ACBP is necessary for intact urine concentrating ability. Our data suggest that the deficiency in urine concentrating ability in the ACBP(-/-) may be caused by reduced AQP3, leading to impaired efflux over the basolateral membrane of the collecting duct.

DBI/ACBP loss-of-function does not affect anxiety-like behaviour but reduces anxiolytic responses to diazepam in mice.

Diazepam is well known for its anxiolytic properties, which are mediated via activation of the GABAA receptor. Diazepam Binding Inhibitor (DBI), also called acyl-CoA binding protein (ACBP), is a ubiquitously expressed protein originally identified based on its ability to displace diazepam from its binding site on the GABAA receptor. Central administration of ACBP or its cleaved fragment, commonly referred to as endozepines, induces proconflict and anxiety-like behaviour in rodents. For this reason, ACBP is known as an anxiogenic peptide. However, the role of endogenous ACBP in anxiety-like behaviour and anxiolytic responses to diazepam has not been investigated. To address this question, we assessed anxiety behaviour and anxiolytic responses to diazepam in two complementary loss-of-function mouse models including astrocyte-specific ACBP KO (ACBP(GFAP) KO) and whole-body KO (ACBP KO) mice. Male and female ACBP(GFAP) KO and ACBP KO mice do not show significant changes in anxiety-like behaviour compared to control littermates during elevated plus maze (EPM) and open field (OF) tests. Surprisingly, ACBP(GFAP) KO and ACBP KO mice were unresponsive to the anxiolytic effect of a low dose of diazepam during EPM tests. In conclusion, our experiments using genetic ACBP loss-of-function models suggest that endozepines deficiency does not affect anxiety-like behaviour in mice and impairs the anxiolytic action of diazepam.

Compromised epidermal barrier stimulates Harderian gland activity and hypertrophy in ACBP−/− mice

Acyl-CoA binding protein (ACBP) is a small, ubiquitously expressed intracellular protein that binds C14-C22 acyl-CoA esters with very high affinity and specificity. We have recently shown that targeted disruption of the Acbp gene leads to a compromised epidermal barrier and that this causes delayed adaptation to weaning, including the induction of the hepatic lipogenic and cholesterogenic gene programs. Here we show that ACBP is highly expressed in the Harderian gland, a gland that is located behind the eyeball of rodents and involved in the production of fur lipids and lipids used for lubrication of the eye lid. We show that disruption of the Acbp gene leads to a significant enlargement of this gland with hypertrophy of the acinar cells and increased de novo synthesis of monoalkyl diacylglycerol, the main lipid species produced by the gland. Mice with conditional targeting of the Acbp gene in the epidermis recapitulate this phenotype, whereas generation of an artificial epidermal barrier during gland development reverses the phenotype. Our findings indicate that the Harderian gland is activated by the compromised epidermal barrier as an adaptive and protective mechanism to overcome the barrier defect.