The effect of ISM absorption on stellar activity measurements and its relevance for exoplanet studies

Past ultraviolet and optical observations of stars hosting close-in Jupiter-mass planets have shown that some of these stars present an anomalously low chromospheric activity, significantly below the basal level. For the hot Jupiter planet host WASP-13, observations have shown that the apparent lack of activity is possibly caused by absorption from the intervening interstellar medium (ISM). Inspired by this result, we study the effect of ISM absorption on activity measurements (S and log R'HK indices) for main-sequence late-type stars. To this end, we employ synthetic stellar photospheric spectra combined with varying amounts of chromospheric emission and ISM absorption. We present the effect of ISM absorption on activity measurements by varying several instrumental (spectral resolution), stellar (projected rotational velocity, effective temperature, and chromospheric emission flux), and ISM parameters (relative velocity between stellar and ISM Ca II lines, broadening b-parameter, and Ca II column density). We find that for relative velocities between the stellar and ISM lines smaller than 30–40 km s−1 and for ISM Ca II column densities log NCaII ⪆ 12, the ISM absorption has a significant influence on activity measurements. Direct measurements and three dimensional maps of the Galactic ISM absorption indicate that an ISM Ca II column density of log NCaII = 12 is typically reached by a distance of about 100 pc along most sight lines. In particular, for a Sun-like star lying at a distance greater than 100 pc, we expect a depression (bias) in the log R'HK value larger than 0.05–0.1 dex, about the same size as the typical measurement and calibration uncertainties on this parameter. This work shows that the bias introduced by ISM absorption must always be considered when measuring activity for stars lying beyond 100 pc. We also consider the effect of multiple ISM absorption components. We discuss the relevance of this result for exoplanet studies and revise the latest results on stellar activity versus planet surface gravity correlation. We finally describe methods with which it would be possible to account for ISM absorption in activity measurements and provide a code to roughly estimate the magnitude of the bias. Correcting for the ISM absorption bias may allow one to identify the origin of the anomaly in the activity measured for some planet-hosting stars

N-acetylaspartate catabolism determines cytosolic acetyl-CoA levels and histone acetylation in brown adipocytes

Histone acetylation depends on the abundance of nucleo-cytoplasmic acetyl-CoA. Here, we present a novel route for cytoplasmic acetyl-CoA production in brown adipocytes. N-acetylaspartate (NAA) is a highly abundant brain metabolite catabolized by aspartoacylase yielding aspartate and acetate. The latter can be further used for acetyl-CoA production. Prior to this work, the presence of NAA has not been described in adipocytes. Here, we show that accumulation of NAA decreases the brown adipocyte phenotype. We increased intracellular NAA concentrations in brown adipocytes via media supplementation or knock-down of aspartoacylase and measured reduced lipolysis, thermogenic gene expression, and oxygen consumption. Combinations of approaches to increase intracellular NAA levels showed additive effects on lipolysis and gene repression, nearly abolishing the expression of Ucp1, Cidea, Prdm16, and Ppara. Transcriptome analyses of aspartoacylase knock-down cells indicate deficiencies in acetyl-CoA and lipid metabolism. Concordantly, cytoplasmic acetyl-CoA levels and global histone H3 acetylation were decreased. Further, activating histone marks (H3K27ac and H3K9ac) in promoters/enhancers of brown marker genes showed reduced acetylation status. Taken together, we present a novel route for cytoplasmic acetyl-CoA production in brown adipocytes. Thereby, we mechanistically connect the NAA pathway to the epigenomic regulation of gene expression, modulating the phenotype of brown adipocytes

α/β-Hydrolase Domain Containing Protein 15 (ABHD15) – an Adipogenic Protein Protecting from Apoptosis

<div><p>Our knowledge about adipocyte metabolism and development is steadily growing, yet many players are still undefined. Here, we show that α/β-hydrolase domain containing protein 15 (Abhd15) is a direct and functional target gene of peroxisome proliferator-activated receptor gamma (PPARγ), the master regulator of adipogenesis. In line, Abhd15 is mainly expressed in brown and white adipose tissue and strongly upregulated during adipogenesis in various murine and human cell lines. Stable knockdown of Abhd15 in 3T3-L1 cells evokes a striking differentiation defect, as evidenced by low lipid accumulation and decreased expression of adipocyte marker genes. In preconfluent cells, knockdown of Abhd15 leads to impaired proliferation, which is caused by apoptosis, as we see an increased SubG1 peak, caspase 3/7 activity, and BAX protein expression as well as a reduction in anti-apoptotic BCL-2 protein. Furthermore, apoptosis-inducing amounts of palmitic acid evoke a massive increase of Abhd15 expression, proposing an apoptosis-protecting role for ABHD15. On the other hand, in mature adipocytes physiological (i.e. non-apoptotic) concentrations of palmitic acid down-regulate Abhd15 expression. Accordingly, we found that the expression of Abhd15 in adipose tissue is reduced in physiological situations with high free fatty acid levels, like high-fat diet, fasting, and aging as well as in genetically obese mice. Collectively, our results position ABHD15 as an essential component in the development of adipocytes as well as in apoptosis, thereby connecting two substantial factors in the regulation of adipocyte number and size. Together with its intricate regulation by free fatty acids, ABHD15 might be an intriguing new target in obesity and diabetes research.</p> </div

Abhd15 is a direct and functional PPARγ target gene.

<p><b>A</b>. Genome organization around the Abhd15 transcription start side (TSS) of 3T3-L1 cells during differentiation with ChIP data of peroxisome proliferator-activated receptor gamma (PPARγ) (day 6 and day 10) and CCAAT-enhancer-binding protein alpha (C/EBPα) (day 10) binding, and Pparγ-Retinoid X receptor (RXRα) direct repeat motif analysis. The data suggest putative PPARγ-RXRα binding ~990 bp and ~440 bp upstream of the Abhd15 TSS. <b>B</b>-<b>D</b>. Abhd15 mRNA levels of 3T3-L1 cells upon PPARγ agonist rosiglitazone (Rosi) treatments. Cells were treated with 1 µM Rosi (<b>B</b>) during differentiation, (<b>C</b>) for 12 and 24 hours on day 7 of differentiation, and (<b>D</b>) for 6, 12, and 24 hours before induction of differentiation, all leading to increased Abhd15 expression. <b>E</b>. Abhd15 mRNA expression in Pparγ -/- and Pparγ +/- mouse embryonic fibroblasts (MEFs). Abhd15 is hardly expressed in Pparγ -/- MEFs and can only be further increased upon addition of Rosi (1 µM) in Pparγ +/- MEFs. <b>F</b>. Sequence map of the sequences containing either one (F2 and F3) or two (F1) of the putative PPARγ-RXRα binding sites, evaluated in figure A, used for the luciferase assay. <b>G</b>. The 3 regions of interest located upstream of the Abhd15 gene were cloned into luciferase reporter vectors (named pGL4.21-F1, pGL4.26-F2, pGL4.21-F3) and cotransfected with either Pparγ/Rxrα expressing vectors or an empty vector (pCMX) into Cos7 cells. The luciferase activity of pGL4.21-F1 and pGL4.21-F3, both containing the putative PPARγ-RXRα binding site ~440 bp upstream to the TSS, were significantly increased when compared to pCMX-transfected cells. Addition of Rosi to cells cotransfected with pGL4.21-F1 or pGL44.21-F3 and Pparγ/Rxrα, again significantly increased luciferase activity. Data is presented as mean ± SD from at least three independent experiments. Statistical significance was determined using the two-tailed Student’s t-test. *p<0.05, **p<0.01, ***p<0.001.</p

Abhd15 expression is required for adipogenesis.

<p><b>A</b>-<b>D</b>. 3T3-L1 cells were infected with lentiviral particles coding for Abhd15 shRNA (Abhd15_sil) or using a non-target shRNA as control (ntc), selected for puromycin resistance, expanded as a mixed population and differentiated. <b>A</b>. Silencing efficiency during adipogenesis of two knock-down lentiviruses against Abhd15, determined by qPCR assay. <b>B</b>. Protein was harvested at day 4 of differentiation of control (ntc) and Abhd15-silenced 3T3-L1 cells (Abhd15_sil1) and subjected to western blotting using the anti-Abhd15 antibody. β-actin served as loading control. Abhd15 protein expression is decreased in Abhd15-silenced 3T3-L1 cells compared to control cells. n=2 C. Silencing of Abhd15 impairs adipogenesis, indicated by the strongly decreased amount of neutral lipids on day 7 of differentiation, stained with Oil red O. <b>D</b>. Stable silencing of Abhd15 in 3T3-L1 cells showed high influences on the expression levels of various important adipogenic genes on day 5 of differentiation (Cebpα, Pparγ, fatty acid binding protein 4 (Fabp4), fatty acid synthase (Fasn)). <b>E</b>. Transient silencing of Abhd15 by electroporation of siRNAs on day 8 of differentiation did not show any effects onto the mRNA levels of adipogenic genes in fully differentiated 3T3-L1 cells (day 10). Data is presented as mean ± SD from at least three independent experiments if not otherwise stated. Statistical significance was determined using the two-tailed Student’s t-test. *p<0.05, **p<0.01, ***p<0.001.</p

Abhd15 expression is regulated during adipogenesis and decreased by elevated free fatty acid levels.

<p><b>A</b>-<b>B</b>. Abhd15 mRNA expression is increased during adipocyte differentiation of (<b>A</b>) OP 9 cells, mouse embryonic fibroblasts (MEFs), and (<b>B</b>) human Simpson-Golabi-Behmel syndrome (SGBS) cells. <b>C</b>. Abhd15 mRNA is highly expressed in brown and white adipose tissue (BAT and WAT), to a lower extent in liver (Liv), and hardly in skeletal (SM) and cardiac muscle (CM) of wild-type mice in the fed state. <b>D</b>. Abhd15 mRNA expression is decreased in WAT and BAT of genetically obese mice (ob/ob) compared to wild type (wt) mice. <b>E</b>. Mice fed a high fat diet (HFD, 60% calories in fat) show a decreased Abhd15 mRNA expression in WAT already after 3 days, but still after 15 weeks on this diet. Additionally, aging strongly decreases Abhd15 mRNA levels. <b>F</b>. Abhd15 mRNA expression is regulated depending on the nutritional status in mouse tissues. Upon fasting, the expression is decreased in both BAT and WAT. <b>G</b>. Simulated fasting of fully differentiated 3T3-L1 cells (day 7 of differentiation) with IBMX (0.5 mM) and isoproterenol (10 µM) for 2 hours resulted in reduced Abhd15 mRNA expression. <b>H</b>. Treatment of fully differentiated 3T3-L1 cells (day 7 of differentiation) with palmitic acid (100 µM) strongly reduces Abhd15 mRNA expression. Data is presented as mean ± SD from at least three independent experiments. Statistical significance was determined using the two-tailed Student’s t-test. *p<0.05, **p<0.01.</p

N-acetylaspartate availability is essential for juvenile survival on fat-free diet and determines metabolic health.

N-acetylaspartate (NAA) is synthesized by aspartate N-acetyltransferase (gene: Nat8l) from acetyl-coenzyme A and aspartate. In the brain, NAA is considered an important energy metabolite for lipid synthesis. However, the role of NAA in peripheral tissues remained elusive. Therefore, we characterized the metabolic phenotype of knockout (ko) and adipose tissue-specific (ako) Nat8l-ko mice as well as NAA-supplemented mice on various diets. We identified an important role of NAA availability in the brain during adolescence, as 75% of Nat8l-ko mice died on fat-free diet (FFD) after weaning but could be rescued by NAA supplementation. In adult life, NAA deficiency promotes a beneficial metabolic phenotype, as Nat8l-ko and Nat8l-ako mice showed reduced body weight, increased energy expenditure, and improved glucose tolerance on chow, high-fat, and FFDs. Furthermore, Nat8l-deficient adipocytes exhibited increased mitochondrial respiration, ATP synthesis, and an induction of browning. Conversely, NAA-treated wild-type mice showed reduced adipocyte respiration and lipolysis and increased de novo lipogenesis, culminating in reduced energy expenditure, glucose tolerance, and insulin sensitivity. Mechanistically, our data point to a possible role of NAA as modulator of pancreatic insulin secretion and suggest NAA as a critical energy metabolite for adipocyte and whole-body energy homeostasis.-Hofer, D. C., Zirkovits, G., Pelzmann, H. J., Huber, K., Pessentheiner, A. R., Xia, W., Uno, K., Miyazaki, T., Kon, K., Tsuneki, H., Pendl, T., Al Zoughbi, W., Madreiter-Sokolowski, C. T., Trausinger, G., Abdellatif, M., Schoiswohl, G., Schreiber, R., Eisenberg, T., Magnes, C., Sedej, S., Eckhardt, M., Sasahara, M., Sasaoka, T., Nitta, A., Hoefler, G., Graier, W. F., Kratky, D., Auwerx, J., Bogner-Strauss, J. G. N-acetylaspartate availability is essential for juvenile survival on fat-free diet and determines metabolic health