ENU mutagenesis identifies mice with cardiac fibrosis and hepatic steatosis caused by a mutation in the mitochondrial trifunctional protein beta-subunit

Mitochondrial bioenergetics and cardiolipin remodeling abnormalities in mitochondrial trifunctional protein deficiency

Mitochondrial trifunctional protein (TFP) deficiency is an inherited metabolic disorder leading to a block in long-chain fatty acid β-oxidation. Mutations in HADHA and HADHB, which encode the TFP α and β subunits, respectively, usually result in combined TFP deficiency. A single common mutation, HADHA c.1528G>C (p.E510Q), leads to isolated 3-hydroxyacyl-CoA dehydrogenase deficiency. TFP also catalyzes a step in the remodeling of cardiolipin (CL), a phospholipid critical to mitochondrial membrane stability and function. We explored the effect of mutations in TFP subunits on CL and other phospholipid content and composition and the consequences of these changes on mitochondrial bioenergetics in patient-derived fibroblasts. Abnormalities in these parameters varied extensively among different fibroblasts, and some cells were able to maintain basal oxygen consumption rates similar to controls. Although CL reduction was universally identified, a simultaneous increase in monolysocardiolipins was discrepant among cells. A similar profile was seen in liver mitochondria isolates from a TFP-deficient mouse model. Response to new potential drugs targeting CL metabolism might be dependent on patient genotype.

A review of fatty acid oxidation disorder mouse models

Fatty acid oxidation disorders (FAODs) are a family of rare, genetic disorders that affect any part of the fatty acid oxidation pathway. Patients present with severe phenotypes, such as hypoketotic hypoglycemia, cardiomyopathy, and rhabdomyolysis, and currently manage these symptoms by the avoidance of fasting and maintaining a low-fat, high-carbohydrate diet. Because knowledge about FAODs is limited due to the small number of patients, rodent models have been crucial in learning more about these disorders, particularly in studying the molecular mechanisms involved in different phenotypes and in evaluating treatments for patients. The purpose of this review is to present the different FAOD mouse models and highlight the benefits and limitations of using these models. Specifically, we discuss the phenotypes of the available FAOD mouse models, the potential molecular causes of prominent FAOD phenotypes that have been studied using FAOD mouse models, and how FAOD mouse models have been used to evaluate treatments for patients.

How mass spectrometry revolutionized newborn screening

This article offers a personal account of a remarkable journey spanning over 30 years of applied mass spectrometry in a clinical setting. It begins with the author's inspiration from a clinician's story of rescuing a child from near death with a revolutionary therapeutic intervention. Motivated by this experience, the author delved into the field of chemistry and mass spectrometry to solve an analytical challenge. The breakthrough came with the development of the first front-line diagnostic test performed by MS/MS, which focused on analyzing acylcarnitines to detect and diagnose inherited disorders related to fatty acid and branched-chain amino acid catabolism. Building upon this success, the author expanded the application of the method to dried blood spots, incorporating additional analytical components such as essential amino acids. The result was a groundbreaking multiplex assay capable of screening newborns for more than 30 inherited metabolic conditions with just one test. This novel approach laid the foundation for a targeted metabolomics platform that facilitated the identification of new animal models of metabolic disease through screening the offspring of genetically modified adults. The development and utilization of MS/MS with UPLC has led to the creation of new assays for biomarkers of metabolic disease, benefiting both the diagnosis and therapeutic monitoring of these conditions. The article provides compelling examples from the author's laboratory, highlighting the value and vast applications of these methods in the field of metabolic disease research.

Betaine Supplementation Causes an Increase in Fatty Acid Oxidation and Carbohydrate Metabolism in Livers of Mice Fed a High-Fat Diet: A Proteomic Analysis

Betaine, a common methyl donor whose methylation is involved in the biosynthesis of carnitine and phospholipids in animals, serves as food and animal feed additive. The present study used liquid chromatography-mass spectrometry (LC-MS) to analyze the liver protein profile of mice on a high fat (HF) diet to investigate the mechanism by which betaine affects hepatic metabolism. Although betaine supplementation had no significant effect on body weight, a total of 103 differentially expressed proteins were identified between HF diet + 1% betaine group (HFB) and HF diet group by LC-MS (fold change > 2, p < 0.05). The addition of 1% betaine had a significant enhancement of the expression of enzymes related to fatty acid oxidation metabolism, such as hydroxyacyl-Coenzyme A dehydrogenase (HADHA), enoyl Coenzyme A hydratase 1 (ECHS1) (p < 0.05) etc., and the expression of apolipoprotein A-II (APOA2) protein was significantly reduced (p < 0.01). Meanwhile, the protein expression of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and succinate-CoA ligase (SUCLG1) were highly significant (p < 0.01). Pathway enrichment using the Kyoto Encyclopedia of Genes and Genomes (KEGG) revealed that the functions of differential proteins involved fatty acid catabolism, carbohydrate metabolism, tricarboxylic acid cycle (TCA) and peroxisome proliferator-activated receptor alpha (PPARα) signaling pathway. Protein−protein interaction (PPI) analysis discovered that acetyl-Coenzyme A acetyltransferase 1 (ACAT1), HADHA and ECHS1 were central hubs of hepatic proteomic changes in the HFB group of mice. Betaine alleviates hepatic lipid accumulation by enhancing fatty acid oxidation and accelerating the TCA cycle and glycolytic process in the liver of mice on an HF diet.

Characterization and Duodenal Transcriptome Analysis of Chinese Beef Cattle With Divergent Feed Efficiency Using RNA-Seq

Residual feed intake (RFI) is an important measure of feed efficiency for agricultural animals. Factors associated with cattle RFI include physiology, dietary factors, and the environment. However, a precise genetic mechanism underlying cattle RFI variations in duodenal tissue is currently unavailable. The present study aimed to identify the key genes and functional pathways contributing to variance in cattle RFI phenotypes using RNA sequencing (RNA-seq). Six bulls with extremely high or low RFIs were selected for detecting differentially expressed genes (DEGs) by RNA-seq, followed by conducting GO, KEGG enrichment, protein-protein interaction (PPI), and co-expression network (WGCNA, n = 10) analysis. A total of 380 differentially expressed genes was obtained from high and low RFI groups, including genes related to energy metabolism (ALDOA, HADHB, INPPL1), mitochondrial function (NDUFS1, RFN4, CUL1), and feed intake behavior (CCK). Two key sub-networks and 26 key genes were detected using GO analysis of DEGs and PPI analysis, such as TPM1 and TPM2, which are involved in mitochondrial pathways and protein synthesis. Through WGCNA, a gene network was built, and genes were sorted into 27 modules, among which the blue (r = 0.72, p = 0.03) and salmon modules (r = -0.87, p = 0.002) were most closely related with RFI. DEGs and genes from the main sub-networks and closely related modules were largely involved in metabolism; oxidative phosphorylation; glucagon, ribosome, and N-glycan biosynthesis, and the MAPK and PI3K-Akt signaling pathways. Through WGCNA, five key genes, including FN1 and TPM2, associated with the biological regulation of oxidative processes and skeletal muscle development were identified. Taken together, our data suggest that the duodenum has specific biological functions in regulating feed intake. Our findings provide broad-scale perspectives for identifying potential pathways and key genes involved in the regulation of feed efficiency in beef cattle.

MTP deficiency caused by HADHB mutations: Pathophysiology and clinical manifestations

Mutations in the HADHB gene lead to Mitochondrial Trifunctional Protein (MTP) deficiency. MTP deficiency is a rare autosomal recessive disorder affecting long-chain fatty acid oxidation. Patients affected by MTP deficiency are unable to metabolize long-chain fatty-acids and suffer a variety of symptoms exacerbated during fasting. The three phenotypes associated with complete MTP deficiency are an early-onset cardiomyopathy and early death, an intermediate form with recurrent hypoketotic hypoglycemia and a sensorimotor neuropathy with episodic rhabdomyolysis with small amount of residual enzyme activities. This review aims to discuss the pathophysiological mechanisms and clinical manifestations of each phenotype, which appears different and linked to HADHB expression levels. Notably, the pathophysiology of the sensorimotor neuropathy is relatively unknown and we provide a hypothesis on the qualitative aspect of the role of acylcarnitine buildup in Schwann cells in MTP deficiency patients. We propose that acylcarnitine may exit the Schwann cell and alter membrane properties of nearby axons leading to axonal degeneration based on recent findings in different metabolic disorders.

Correlation of gene expression and clinical parameters identifies a set of genes reflecting LV systolic dysfunction and morphological alterations

To gain new insights into the complex pathophysiology of dilated cardiomyopathy (DCM) we performed a quantitative approach to identify genes with expression patterns that linearly correlate with parameters of cardiac morphology (left ventricular end-diastolic diameter indexed by body surface are (LVEDDI), systolic function [LV ejection fraction (LVEF)], and serum levels of cardiac peptide hormone NH2-terminal probrain natriuretic peptide (NT-proBNP) in human endomyocardial biopsies of 47 DCM patients and eight individuals with normal LVEF. A set of genes was identified as common heart failure markers characterized by correlation of their expression with cardiac morphology, systolic function, and NT-proBNP. Among them are already known genes encoding e.g., the natriuretic peptide hormones NPPA and NPPB and its converting enzyme corin, but also potential new heart failure markers like EP300 antisense RNA1 and dimethylarginine dimethylaminohydrolase 1 (DDAH1) along with other genes with so far unknown relation to heart function. In contrast, the expression of other genes including the Ca2+ flux regulating genes phospholamban (PLN), sarcoplasmic/endoplasmic reticulum calcium ATPase 2 (SERCA), and extracellular matrix proteins showed significant correlation with LVEF and LVEDDI only. Those genes seem to reflect more specifically pathological alterations of systolic function and morphology in DCM hearts.

Expression of miR-33 from an SREBP2 intron inhibits the expression of the fatty acid oxidation-regulatory genes CROT and HADHB in chicken liver

1. Limiting the growth of adipose tissue in chickens is a major issue in the poultry industry. In chickens, de novo synthesis of lipids occurs primarily in the liver. Thus, it is necessary to understand how fatty acid accumulation in the liver is controlled. The miR-33 is an intronic microRNA (miRNA) of the chicken sterol regulatory element binding transcription factor 2 (SREBF2), which is a master switch in activating many genes involved in the uptake and synthesis of cholesterol, triglycerides, fatty acids and phospholipids. 2. In the current study, the genes CROT and HADHB known to encode enzymes critical for fatty acid oxidation were predicted to be potential targets of miR-33 in chickens via the miRNA target prediction programs 'miRanda' and 'TargetScan'. Co-transfection and dual-luciferase reporter assays showed that the expression of luciferase reporter gene linked to the 3'-untranslated region (3'UTR) of the chicken CROT and HADHB mRNA was down-regulated by overexpression of the chicken miR-33 (P < 0.05). This down-regulation was completely abolished when the predicted miR-33 target sites in the CROT and HADHB 3'UTR were mutated. 3. Transfecting miR-33 mimics into the LMH cells led to a decrease in the mRNA expression of CROT and HADHB (P < 0.01), and this transfection had a similar effect on the proteins (P < 0.05). In contrast, the expression of CROT in primary chicken hepatocytes was up-regulated after transfection with the miR-33 inhibitor LNA-anti-miR-33 (P < 0.05). 4. Using quantitative RT-PCR, it was shown that the expression of miR-33 was increased in the chicken liver from day 0 to day 49 of age, whereas the CROT and HADHB mRNA levels decreased during the same period. 5. These findings support the conclusion that miR-33 might play an important role in lipid metabolism in the chicken liver by negatively regulating the expression of the CROT and HADHB genes, which encode enzymes critical for lipid oxidation.

Remodeling adipose tissue through in silico modulation of fat storage for the prevention of type 2 diabetes

BACKGROUND

Type 2 diabetes is one of the leading non-infectious diseases worldwide and closely relates to excess adipose tissue accumulation as seen in obesity. Specifically, hypertrophic expansion of adipose tissues is related to increased cardiometabolic risk leading to type 2 diabetes. Studying mechanisms underlying adipocyte hypertrophy could lead to the identification of potential targets for the treatment of these conditions.

RESULTS

We present iTC1390adip, a highly curated metabolic network of the human adipocyte presenting various improvements over the previously published iAdipocytes1809. iTC1390adip contains 1390 genes, 4519 reactions and 3664 metabolites. We validated the network obtaining 92.6% accuracy by comparing experimental gene essentiality in various cell lines to our predictions of biomass production. Using flux balance analysis under various test conditions, we predict the effect of gene deletion on both lipid droplet and biomass production, resulting in the identification of 27 genes that could reduce adipocyte hypertrophy. We also used expression data from visceral and subcutaneous adipose tissues to compare the effect of single gene deletions between adipocytes from each compartment.

CONCLUSIONS

We generated a highly curated metabolic network of the human adipose tissue and used it to identify potential targets for adipose tissue metabolic dysfunction leading to the development of type 2 diabetes.

Lung fibrosis: drug screening and disease biomarker identification with a lung slice culture model and subtracted cDNA Library

Pulmonary fibrosis is a progressive and irreversible disorder with no appropriate cure. A practical and effective experimental model that recapitulates the disease will greatly benefit the research community and, ultimately, patients. In this study, we tested the lung slice culture (LSC) system for its potential use in drug screening and disease biomarker identification. Fibrosis was induced by treating rat lung slices with 1ng/ml TGF-β1 and 2.5μM CdCl2, quantified by measuring the content of hydroxyproline, and confirmed by detecting the expression of collagen type III alpha 1 (Col3α1) and connective tissue growth factor (CTGF) genes. The anti-fibrotic effects of pirfenidone, spironolactone and eplerenone were assessed by their capability to reduce hydroxyproline content. A subtractive hybridisation technique was used to create two cDNA libraries (subtracted and unsubtracted) from lung slices. The housekeeping gene glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was employed to assess the subtraction efficiency of the subtracted cDNA library. Clones from the two libraries were sequenced and the genes were identified by performing a BLAST search on the NCBI GenBank database. Furthermore, the relevance of the genes to fibrosis formation was verified. The results presented here show that fibrosis was effectively induced in cultured lung slices, which exhibited significantly elevated levels of hydroxyproline and Col3α1/CTGF gene expression. Several inhibitors have demonstrated their anti-fibrotic effects by significantly reducing hydroxyproline content. The subtracted cDNA library, which was enriched for differentially expressed genes, was used to successfully identify genes associated with fibrosis. Collectively, the results indicate that our LSC system is an effective model for the screening of drug candidates and for disease biomarker identification.

Long-chain Acylcarnitines Reduce Lung Function by Inhibiting Pulmonary Surfactant

The role of mitochondrial energy metabolism in maintaining lung function is not understood. We previously observed reduced lung function in mice lacking the fatty acid oxidation enzyme long-chain acyl-CoA dehydrogenase (LCAD). Here, we demonstrate that long-chain acylcarnitines, a class of lipids secreted by mitochondria when metabolism is inhibited, accumulate at the air-fluid interface in LCAD(-/-) lungs. Acylcarnitine accumulation is exacerbated by stress such as influenza infection or by dietary supplementation with l-carnitine. Long-chain acylcarnitines co-localize with pulmonary surfactant, a unique film of phospholipids and proteins that reduces surface tension and prevents alveolar collapse during breathing. In vitro, the long-chain species palmitoylcarnitine directly inhibits the surface adsorption of pulmonary surfactant as well as its ability to reduce surface tension. Treatment of LCAD(-/-) mice with mildronate, a drug that inhibits carnitine synthesis, eliminates acylcarnitines and improves lung function. Finally, acylcarnitines are detectable in normal human lavage fluid. Thus, long-chain acylcarnitines may represent a risk factor for lung injury in humans with dysfunctional fatty acid oxidation.

Heart failure and mitochondrial dysfunction: the role of mitochondrial fission/fusion abnormalities and new therapeutic strategies

The treatment of heart failure (HF) has evolved during the past 30 years with the recognition of neurohormonal activation and the effectiveness of its inhibition in improving the quality of life and survival. Over the past 20 years, there has been a revolution in the investigation of the mitochondrion with the development of new techniques and the finding that mitochondria are connected in networks and undergo constant division (fission) and fusion, even in cardiac myocytes. This has led to new molecular and cellular discoveries in HF, which offer the potential for the development of new molecular-based therapies. Reactive oxygen species are an important cause of mitochondrial and cellular injury in HF, but there are other abnormalities, such as depressed mitochondrial fusion, that may eventually become the targets of at least episodic treatment. The overall need for mitochondrial fission/fusion balance may preclude sustained change in either fission or fusion. In this review, we will discuss the current HF therapy and its impact on the mitochondria. In addition, we will review some of the new drug targets under development. There is potential for effective, novel therapies for HF to arise from new molecular understanding.

Quantitative proteomic study of myocardial mitochondria in urea transporter B knockout mice

In previous research, we showed that 16-week-old urea transporter B (UT-B) null mice have an atrial-ventricular conduction block, and hypothesized myocardial mitochondrial dysfunction. To investigate the mechanism of this block, we examined the proteomic differences in the myocardial mitochondria of UT-B null and wild-type mice with nanoscale LC-MS/MS. Of 26 proteins clearly downregulated in the UT-B null mice, 15 are involved in complexes I, III, IV, and V of the respiratory chain, which would strongly reduce the activity of the electron transport chain. Excess electrons from complexes I and III pass directly to O2 to generate ROS and deplete ROS-scavenging enzymes. Myocardial intracellular ROS were significantly higher in UT-B null mice than in wild-type mice (p < 0.01), constituting an important cause of oxidative stress injury in the myocardia of UT-B null mice. The mitochondrial membrane potential (ΔΨm) was also lower in UT-B null mice than in wild-type mice (p < 0.05), causing oxidative phosphorylation dysfunction of complex V and insufficient ATP in the myocardial cells of UT-B null mice. HADHA (a trifunctional protein) and HSP60 were also downregulated in the UT-B null myocardial mitochondria. These results confirm that mitochondrial dysfunction underlies the pathogenesis of the atrial-ventricular conduction block in UT-B null mice.

The molecular pathogenic role of inflammatory stress in dysregulation of lipid homeostasis and hepatic steatosis

Non-alcoholic Fatty Liver Disease (NAFLD) is becoming the leading cause of chronic liver injury in developed countries and China. Chronic systemic inflammation plays a decisive role and is fundamental in the progression of NAFLD from simple steatosis (SS) toward higher risk nonalcoholic steatohepatitis (NASH) states. However, the exact mechanisms by which inflammation leading to NASH are incompletely understood. In this review, we focus the role of the cross talk between inflammation and lipid homeostasis on the progression of NAFLD.

The stress response resolution assay. I. Quantitative assessment of environmental agent/condition effects on cellular stress resolution outcomes in epithelium

The events or factors that lead from normal cell function to conditions and diseases such as aging or cancer reflect complex interactions between cells and their environment. Cellular stress responses, a group of processes involved in homeostasis and adaptation to environmental change, contribute to cell survival under stress and can be resolved with damage avoidance or damage tolerance outcomes. To investigate the impact of environmental agents/conditions upon cellular stress response outcomes in epithelium, a novel quantitative assay, the "stress response resolution" (SRR) assay, was developed. The SRR assay consists of pretreatment with a test agent or vehicle followed later by a calibrated stress conditions exposure step (here, using 6-thioguanine). Pilot studies conducted with a spontaneously-immortalized murine mammary epithelial cell line pretreated with vehicle or 20 µg N-ethyl-N-nitrososurea/ml medium for 1 hr, or two hTERT-immortalized human bronchial epithelial cell lines pretreated with vehicle or 100 µM zidovudine/lamivudine for 12 days, found minimal alterations in cell morphology, survival, or cell function through 2 weeks post-exposure. However, when these pretreatments were followed 2 weeks later by exposure to calibrated stress conditions of limited duration (for 4 days), significant alterations in stress resolution were observed in pretreated cells compared with vehicle-treated control cells, with decreased damage avoidance survival outcomes in all cell lines and increased damage tolerance outcomes in two of three cell lines. These pilot study results suggest that sub-cytotoxic pretreatments with chemical mutagens have long-term adverse impact upon the ability of cells to resolve subsequent exposure to environmental stressors.

A comprehensive survey of copy number variation in 18 diverse pig populations and identification of candidate copy number variable genes associated with complex traits

Background

Copy number variation (CNV) is a major source of structural variants and has been commonly identified in mammalian genome. It is associated with gene expression and may present a major genetic component of phenotypic diversity. Unlike many other mammalian genomes where CNVs have been well annotated, studies of porcine CNV in diverse breeds are still limited.

Result

Here we used Porcine SNP60 BeadChip and PennCNV algorithm to identify 1,315 putative CNVs belonging to 565 CNV regions (CNVRs) in 1,693 pigs from 18 diverse populations. Total 538 out of 683 CNVs identified in a White Duroc × Erhualian F₂ population fit Mendelian transmission and 6 out of 7 randomly selected CNVRs were confirmed by quantitative real time PCR. CNVRs were non-randomly distributed in the pig genome. Several CNV hotspots were found on pig chromosomes 6, 11, 13, 14 and 17. CNV numbers differ greatly among different pig populations. The Duroc pigs were identified to have the most number of CNVs per individual. Among 1,765 transcripts located within the CNVRs, 634 genes have been reported to be copy number variable genes in the human genome. By integrating analysis of QTL mapping, CNVRs and the description of phenotypes in knockout mice, we identified 7 copy number variable genes as candidate genes for phenotypes related to carcass length, backfat thickness, abdominal fat weight, length of scapular, intermuscle fat content of logissimus muscle, body weight at 240 day, glycolytic potential of logissimus muscle, mean corpuscular hemoglobin, mean corpuscular volume and humerus diameter.

Conclusion

We revealed the distribution of the unprecedented number of 565 CNVRs in pig genome and investigated copy number variable genes as the possible candidate genes for phenotypic traits. These findings give novel insights into porcine CNVs and provide resources to facilitate the identification of trait-related CNVs.

A medium-chain fatty acid as an alternative energy source in mouse preimplantation development

To further optimize the culturing of preimplantation embryos, we undertook metabolomic analysis of relevant culture media using capillary electrophoresis time-of-flight mass spectrometry (CE-TOFMS). We detected 28 metabolites: 23 embryo-excreted metabolites including 16 amino acids and 5 media-derived metabolites (e.g., octanoate, a medium-chain fatty acid (MCFA)). Due to the lack of information on MCFAs in mammalian preimplantation development, this study examined octanoate as a potential alternative energy source for preimplantation embryo cultures. No embryos survived in culture media lacking FAs, pyruvate, and glucose, but supplementation of octanoate rescued the embryonic development. Immunoblotting showed significant expression of acyl-CoA dehydrogenase and hydroxyacyl-CoA dehydrogenase, important enzymes for ß-oxidation of MCFAs, in preimplantation embryo. Furthermore, CE-TOFMS traced [1-¹³C₈] octanoate added to the culture media into intermediate metabolites of the TCA cycle via ß-oxidation in mitochondria. These results are the first demonstration that octanoate could provide an efficient alternative energy source throughout preimplantation development.

Extraction and analysis of carnitine and acylcarnitines by electrospray ionization tandem mass spectrometry directly from dried blood and plasma spots using a novel autosampler

RATIONALE

Acylcarnitines are routinely analyzed by electrospray ionization tandem mass spectrometry (ESI-MS/MS) both in clinical diagnostic and public health newborn screening laboratories from plasma and dried whole blood spots (DBS) on filter paper. The use of DBS as a convenient method of collecting and storing samples for subsequent analysis of various biomolecules is increasing, thus prompting the development of new devices to recover and quantify such analytes in an automated manner.

METHODS

Acylcarnitines were extracted directly from DBS using a novel autosampler that sequentially loads DBS cards into a pneumatic clamp and then pumps a fixed volume of solvent containing appropriate internal standards through a section of the DBS card directly into a triple quadrupole mass spectrometer via ESI. Plasma was first spiked with internal standard then spotted onto filter paper for analysis.

RESULTS

Acylcarnitines were analyzed in DBS, and both free and total carnitine were assayed in dried plasma spots (DPS). Results using the new autosampling technique were of equal quality to those obtained by punching a 3-mm diameter disk from a DBS or DPS, then extracting and analyzing the target analytes from conventional 96-well microtiter plates, with far reduced time per sample. Recovery for most analytes was >60% and reproducibility was generally within 20% (CV).

CONCLUSIONS

The simplicity and robustness of the DBS autosampler make it an attractive alternative to conventional methods of analyzing DBS specimens, thus saving time and labor costs, especially in high-throughput applications. Although the method as described is for direct infusion analysis, the autosampler is easily coupled to column hardware for applications requiring liquid chromatography/mass spectrometry (LC/MS).

Methyl donor deficiency impairs fatty acid oxidation through PGC-1α hypomethylation and decreased ER-α, ERR-α, and HNF-4α in the rat liver

BACKGROUND & AIMS

Folate and cobalamin are methyl donors needed for the synthesis of methionine, which is the precursor of S-adenosylmethionine, the substrate of methylation in epigenetic, and epigenomic pathways. Methyl donor deficiency produces liver steatosis and predisposes to metabolic syndrome. Whether impaired fatty acid oxidation contributes to this steatosis remains unknown.

METHODS

We evaluated the consequences of methyl donor deficient diet in liver of pups from dams subjected to deficiency during gestation and lactation.

RESULTS

The deprived rats had microvesicular steatosis, with increased triglycerides, decreased methionine synthase activity, S-adenosylmethionine, and S-adenosylmethionine/S-adenosylhomocysteine ratio. We observed no change in apoptosis markers, oxidant and reticulum stresses, and carnityl-palmitoyl transferase 1 activity, and a decreased expression of SREBP-1c. Impaired beta-oxidation of fatty acids and carnitine deficit were the predominant changes, with decreased free and total carnitines, increased C14:1/C16 acylcarnitine ratio, decrease of oxidation rate of palmitoyl-CoA and palmitoyl-L-carnitine and decrease of expression of novel organic cation transporter 1, acylCoA-dehydrogenase and trifunctional enzyme subunit alpha and decreased activity of complexes I and II. These changes were related to lower protein expression of ER-α, ERR-α and HNF-4α, and hypomethylation of PGC-1α co-activator that reduced its binding with PPAR-α, ERR-α, and HNF-4α.

CONCLUSIONS

The liver steatosis resulted predominantly from hypomethylation of PGC1-α, decreased binding with its partners and subsequent impaired mitochondrial fatty acid oxidation. This link between methyl donor deficiency and epigenomic deregulations of energy metabolism opens new insights into the pathogenesis of fatty liver disease, in particular, in relation to the fetal programming hypothesis.

Metabolomics as a tool for cardiac research

Metabolomics represents a paradigm shift in metabolic research, away from approaches that focus on a limited number of enzymatic reactions or single pathways, to approaches that attempt to capture the complexity of metabolic networks. Additionally, the high-throughput nature of metabolomics makes it ideal to perform biomarker screens for diseases or follow drug efficacy. In this Review, we explore the role of metabolomics in gaining mechanistic insight into cardiac disease processes, and in the search for novel biomarkers. High-resolution NMR spectroscopy and mass spectrometry are both highly discriminatory for a range of pathological processes affecting the heart, including cardiac ischemia, myocardial infarction, and heart failure. We also discuss the position of metabolomics in the range of functional-genomic approaches, being complementary to proteomic and transcriptomic studies, and having subdivisions such as lipidomics (the study of intact lipid species). In addition to techniques that monitor changes in the total sizes of pools of metabolites in the heart and biofluids, the role of stable-isotope methods for monitoring fluxes through pathways is examined. The use of these novel functional-genomic tools to study metabolism provides a unique insight into cardiac disease progression.

Alternative splicing in Acad8 resulting a mitochondrial defect and progressive hepatic steatosis in mice

Using a combination of N-ethyl-N-nitrosourea-mediated mutagenesis and metabolomics-guided screening, we identified mice with elevated blood levels of short-chain C4-acylcarnitine and increased urine isobutyryl-glycine. Genome-wide homozygosity screening, followed by fine mapping, located the disease gene to 15-25 Mb of mouse chromosome 9 where a candidate gene, Acad8, encoding mitochondrial isobutyryl-CoA dehydrogenase was located. Genomic DNA sequencing revealed a single-nucleotide mutation at -17 of the first intron of Acad8 in affected mice. cDNA sequencing revealed an intronic 28-bp insertion at the site of the mutation, which caused a frame shift with a premature stop codon. In vitro splicing assay confirmed that the mutation was sufficient to activate an upstream, aberrant 3' splice site. There was a reduction in the expression of Acad8 at both the mRNA and protein levels. The mutant mice grew normally but demonstrated cold intolerance at young age with a progressive hepatic steatosis. Homozygous mutant mice hepatocytes had abnormal mitochondria with crystalline inclusions, suggestive of mitochondriopathy. This mouse model of isobutyryl-CoA dehydrogenase deficiency could provide us a better understanding of the possible role of IBD deficiency in mitochondriopathy and fatty liver.

Acylcarnitines: analysis in plasma and whole blood using tandem mass spectrometry

The acylcarnitine profile is a diagnostic test for inherited disorders of fatty acid and branched-chain amino acid catabolism. Patients with this type of metabolic disorder accumulate disease-specific acylcarnitines that correlate with the acyl coenzyme A compounds in the affected mitochondrial metabolic pathways. For example, propionylcarnitine accumulates in patients with both propionic and methylmalonic acidemias. The test identifies and quantifies the species of acylcarnitines in the whole blood or blood plasma of patients at risk for or suspected of having such a disorder. The acylcarnitines are analyzed using electrospray ionization-tandem mass spectrometry. The instrument is used in the precursor ion scan mode to record the molecular species giving rise to fragment ions at m/z 99, derived specifically from the methylated acylcarnitines within the specimen. Quantification is based on the principle of stable isotope dilution, whereby concentrations are derived from the response ratio of each acylcarnitine species to that of a deuterium-labeled acylcarnitine standard. Interpretation of the acylcarnitine profile requires recognition of abnormal concentrations of specific analytes or patterns of analytes and knowledge of their metabolic origin.

Expression of miR-33 from an SREBP2 intron inhibits cholesterol export and fatty acid oxidation

The regulation of synthesis, degradation, and distribution of lipids is crucial for homeostasis of organisms and cells. The sterol regulatory element-binding protein (SREBP) transcription factor family is post-translationally activated in situations of reduced lipid abundance and activates numerous genes involved in cholesterol, fatty acid, and phospholipid synthesis. In this study, we provide evidence that the primary transcript of SREBP2 contains an intronic miRNA (miR-33) that reduces cellular cholesterol export via inhibition of translation of the cholesterol export pump ABCA1. Notably, miR-33 also inhibits translation of several transcripts encoding proteins involved in fatty acid β-oxidation including CPT1A, HADHB, and CROT, thereby reducing fatty acid degradation. The genetic locus encoding SREBP2 and miR-33 therefore contains a protein that increases lipid synthesis and a miRNA that prevents export and degradation of newly synthesized lipids. These results add an additional layer of complexity to our understanding of lipid homeostasis and might open possibilities for future therapeutic intervention.

Systems level studies of mammalian metabolomes: the roles of mass spectrometry and nuclear magnetic resonance spectroscopy

The study of biological systems in a holistic manner (systems biology) is increasingly being viewed as a necessity to provide qualitative and quantitative descriptions of the emergent properties of the complete system. Systems biology performs studies focussed on the complex interactions of system components; emphasising the whole system rather than the individual parts. Many perturbations to mammalian systems (diet, disease, drugs) are multi-factorial and the study of small parts of the system is insufficient to understand the complete phenotypic changes induced. Metabolomics is one functional level tool being employed to investigate the complex interactions of metabolites with other metabolites (metabolism) but also the regulatory role metabolites provide through interaction with genes, transcripts and proteins (e.g. allosteric regulation). Technological developments are the driving force behind advances in scientific knowledge. Recent advances in the two analytical platforms of mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy have driven forward the discipline of metabolomics. In this critical review, an introduction to metabolites, metabolomes, metabolomics and the role of MS and NMR spectroscopy will be provided. The applications of metabolomics in mammalian systems biology for the study of the health-disease continuum, drug efficacy and toxicity and dietary effects on mammalian health will be reviewed. The current limitations and future goals of metabolomics in systems biology will also be discussed (374 references).

Mice with alopecia, osteoporosis, and systemic amyloidosis due to mutation in Zdhhc13, a gene coding for palmitoyl acyltransferase

Protein palmitoylation has emerged as an important mechanism for regulating protein trafficking, stability, and protein-protein interactions; however, its relevance to disease processes is not clear. Using a genome-wide, phenotype driven N-ethyl-N-nitrosourea-mediated mutagenesis screen, we identified mice with failure to thrive, shortened life span, skin and hair abnormalities including alopecia, severe osteoporosis, and systemic amyloidosis (both AA and AL amyloids depositions). Whole-genome homozygosity mapping with 295 SNP markers and fine mapping with an additional 50 SNPs localized the disease gene to chromosome 7 between 53.9 and 56.3 Mb. A nonsense mutation (c.1273A>T) was located in exon 12 of the Zdhhc13 gene (Zinc finger, DHHC domain containing 13), a gene coding for palmitoyl transferase. The mutation predicted a truncated protein (R425X), and real-time PCR showed markedly reduced Zdhhc13 mRNA. A second gene trap allele of Zdhhc13 has the same phenotypes, suggesting that this is a loss of function allele. This is the first report that palmitoyl transferase deficiency causes a severe phenotype, and it establishes a direct link between protein palmitoylation and regulation of diverse physiologic functions where its absence can result in profound disease pathology. This mouse model can be used to investigate mechanisms where improper palmitoylation leads to disease processes and to understand molecular mechanisms underlying human alopecia, osteoporosis, and amyloidosis and many other neurodegenerative diseases caused by protein misfolding and amyloidosis.

[Recent development of metabonomics and its applications in clinical research]

In the post-genomic era, systems biology is central to the biological sciences. Functional genomics such as transcriptomics and proteomics can simultaneous determine massive gene or protein expression changes following drug treatment or other intervention. However, these changes can't be coupled directly to changes in biological function. As a result, metabonomics and its many pseudonyms (metabolomics, metabolic profiling, etc.) have exploded onto the scientific scene in the past several years. Metabonomics is a rapidly growing research area and a system approach for comprehensive and quantitative analysis of the global metabolites in a biological matrix. Analytical chemistry approach is necessary for the development of comprehensive metabonomics investigations. Fundamentally, there are two types of metabonomics approaches: mass-spectrometry (MS) based and nuclear magnetic resonance (NMR) methodologies. Metabonomics measurements provide a wealth of data information and interpretation of these data relies mainly on chemometrics approaches to perform large-scale data analysis and data visualization, such as principal and independent component analysis, multidimensional scaling, a variety of clustering techniques, and discriminant function analysis, among many others. In this review, the recent development of analytical and statistical techniques used in metabonomics is summarized. Major applications of metabonomics relevant to clinical and preclinical study are then reviewed. The applications of metabonomics in study of liver diseases, cancers and other diseases have proved useful both as an experimental tool for pathogenesis mechanism re-search and ultimately a tool for diagnosis and monitoring treatment response of these diseases. Next, the applications of metabonomics in preclinical toxicology are discussed and the role that metabonomics might do in pharmaceutical research and development is explained with special reference to the aims and achievements of the Consortium for Metabonomic Toxicology (COMET), and the concept of pharmacometabonomics as a way of predicting an individual's response to treatment is highlighted. Finally, the role of metabonomics in elucidating the function of the unknown or novel enzyme is mentioned.

The tight junction protein CAR regulates cardiac conduction and cell-cell communication

The Coxsackievirus-adenovirus receptor (CAR) is known for its role in virus uptake and as a protein of the tight junction. It is predominantly expressed in the developing brain and heart and reinduced upon cardiac remodeling in heart disease. So far, the physiological functions of CAR in the adult heart are largely unknown. We have generated a heart-specific inducible CAR knockout (KO) and found impaired electrical conduction between atrium and ventricle that increased with progressive loss of CAR. The underlying mechanism relates to the cross talk of tight and gap junctions with altered expression and localization of connexins that affect communication between CAR KO cardiomyocytes. Our results indicate that CAR is not only relevant for virus uptake and cardiac remodeling but also has a previously unknown function in the propagation of excitation from the atrium to the ventricle that could explain the association of arrhythmia and Coxsackievirus infection of the heart.