Mimp/Mtch2, an Obesity Susceptibility Gene, Induces Alteration of Fatty Acid Metabolism in Transgenic Mice

Differential expression of microRNAs in serum exosomes of obese and non-obese mice and analysis of their function

OBJECTIVE

To extract exosomes from obese and non-obese mice, screen specifically expressed microRNAs by high-throughput sequencing and explore their roles.

METHODS

An animal obesity model was constructed, and the successful construction of the obesity model was verified by HE staining, Western Blot and RT-qPCR. In addition, exosomes were extracted and verified by Western Blot. High-throughput sequencing was performed on the extracted serum exosomes to screen for differentially expressed microRNAs. fluorescence quantitative RT-PCR (RT-qPCR) was used to validate the differentially expressed miRNAs and explore their functions.

RESULTS

8 microRNAs were up-regulated and 11 microRNAs were down-regulated. mmu-miR-674-5p and X_28316 were significantly down-regulated and had the greatest impact on protein pathways. 8_13258 was significantly up-regulated and affected multiple protein pathways. GO enrichment analysis suggested that the differentially expressed microRNAs were mainly involved in the cleavage of microtubule activity, transferase activity/transferase pentameric acid. GO enrichment analysis suggested that differentially expressed microRNAs were mainly involved in the processes of cleavage microtubule activity, transferase activity/transfer pentamer, and threonine phosphatase/threonine kinase activity.KEGG pathway enrichment analysis showed that differentially expressed microRNAs were mainly involved in the processes of regulating the phosphorylation of TP53 activity, the G2/M DNA damage checkpoint, and the processing of the ends of DNA double-strand breaks. Protein interaction networks were enriched for Stat3, Fgr, Camk2b, Rac1, Asb6, and Ankfy1. Suggesting that they may be mediated by differential genes to participate in the process of insulin resistance. qRT-PCR results showed that the expression trend of mmu-miR-674-5p was consistent with the sequencing results. It suggests that it may be able to participate in the regulation of insulin resistance as a target gene.

CONCLUSION

microRNAs were differentially expressed in serum exosomes of obese and non-obese mice and might be involved in the specific regulation of insulin resistance. mmu-miR-674-5p was differentially expressed significantly and the validation trend was consistent with it, suggesting that it might be able to participate in the regulation of insulin resistance as a target gene.

MTCH2 in Metabolic Diseases, Neurodegenerative Diseases, Cancers, Embryonic Development and Reproduction

Mitochondrial carrier homolog 2 (MTCH2) is a member of the solute carrier 25 family, located on the outer mitochondrial membrane. MTCH2 was first identified in 2000. The development in MTCH2 research is rapidly increasing. The most well-known role of MTCH2 is linking to the pro-apoptosis BID to facilitate mitochondrial apoptosis. Genetic variants in MTCH2 have been investigated for their association with metabolic and neurodegenerative diseases, however, no intervention or therapeutic suggestions were provided. Recent studies revealed the physiological and pathological function of MTCH2 in metabolic diseases, neurodegenerative diseases, cancers, embryonic development and reproduction via regulating mitochondrial apoptosis, metabolic shift between glycolysis and oxidative phosphorylation, mitochondrial fusion/fission, epithelial-mesenchymal transition, etc. This review endeavors to assess a total of 131 published articles to summarise the structure and physiological/pathological role of MTCH2, which has not previously been conducted. This review concludes that MTCH2 plays a crucial role in metabolic diseases, neurodegenerative diseases, cancers, embryonic development and reproduction, and the predominant molecular mechanism is regulation of mitochondrial function. This review gives a comprehensive state of current knowledgement on MTCH2, which will promote the therapeutic research of MTCH2.

Effects of aerobic exercise on the regulation of mitochondrial carrier homolog-2 and its influence on the catabolic and anabolic activity of lipids in the mesenteric adipose tissue of obese mice

The aim was to understand the direct impact of aerobic short-term exercise on lipid metabolism, specifically in regulating the mitochondrial carrier homolog 2 (MTCH2) and how it interferes with lipid metabolism in mesenteric adipose tissue. Swiss mice were divided into three groups: control, sedentary obese, and exercised obese. The obese groups were induced into obesity for fourteen weeks of a high-fat diet, and the trained submitted to seven aerobic exercise sessions. The exercise proved the significant increase of the pPerilipin-1, a hormone-sensitive lipase gene, and modulates lipid metabolism by increasing the expression of Mtch2 and acetyl Co-A carboxylase, perhaps occurring as feedback to regulate lipid metabolism in adipose tissue. In conclusion, we demonstrate, for the first time, how aerobic physical exercise increases Mtch2 transcription in mesenteric adipose tissue. This increase was due to changes in energy demand caused by exercise, confirmed by observing the significant reduction in mesenteric adipose tissue mass in the exercised group. Also, we showed that physical exercise increased the phosphorylative capacity of PLIN1, a protein responsible for the degradation of fatty acids in the lipid droplet, providing acyl and glycerol for cellular metabolism. Although our findings demonstrate evidence of MTCH2 as a protein that regulates lipid homeostasis, scant knowledge exists concerning the signaling of the MTCH2 pathway in regulatingfatty acid metabolism. Therefore, unveiling the means of molecular signaling of MTCH2 demonstrates excellent potential for treating obesity.

The biology of mitochondrial carrier homolog 2

The mitochondrial carrier system is in charge of small molecule transport between the mitochondria and the cytoplasm as well as being an integral portion of the core mitochondrial function. One member of the mitochondrial carrier family of proteins, mitochondrial carrier homolog 2 (MTCH2), is characterized as a critical mitochondrial outer membrane protein insertase participating in mitochondrial homeostasis. Accumulating evidence demonstrate that MTCH2 is integrally linked to cell death and mitochondrial metabolism, and its genetic alterations cause a variety of disease phenotypes, ranging from obesity, Alzheimer's disease, and tumor. To provide a comprehensive insight into the current understanding of MTCH2, we present a detailed description of the physiopathological functions of MTCH2, ranging from apoptosis, mitochondrial dynamics, and metabolic homeostasis regulation. Moreover, we summarized the impact of MTCH2 in human diseases, and highlighted tumors, to assess the role of MTCH2 mutations or variable expression on pathogenesis and target therapeutic options.

MTCH2 cooperates with MFN2 and lysophosphatidic acid synthesis to sustain mitochondrial fusion

Fusion of the outer mitochondrial membrane (OMM) is regulated by mitofusin 1 (MFN1) and 2 (MFN2), yet the differential contribution of each of these proteins is less understood. Mitochondrial carrier homolog 2 (MTCH2) also plays a role in mitochondrial fusion, but its exact function remains unresolved. MTCH2 overexpression enforces MFN2-independent mitochondrial fusion, proposedly by modulating the phospholipid lysophosphatidic acid (LPA), which is synthesized by glycerol-phosphate acyl transferases (GPATs) in the endoplasmic reticulum (ER) and the OMM. Here we report that MTCH2 requires MFN1 to enforce mitochondrial fusion and that fragmentation caused by loss of MTCH2 can be specifically counterbalanced by overexpression of MFN2 but not MFN1, partially independent of its GTPase activity and mitochondrial localization. Pharmacological inhibition of GPATs (GPATi) or silencing ER-resident GPATs suppresses MFN2's ability to compensate for the loss of MTCH2. Loss of either MTCH2, MFN2, or GPATi does not impair stress-induced mitochondrial fusion, whereas the combined loss of MTCH2 and GPATi or the combined loss of MTCH2 and MFN2 does. Taken together, we unmask two cooperative mechanisms that sustain mitochondrial fusion.

Coexistence of Multiple Functional Variants and Genes Underlies Genetic Risk Locus 11p11.2 of Alzheimer's Disease

BACKGROUND

Genome-wide association studies have identified dozens of genetic risk loci for Alzheimer's disease (AD), yet the underlying causal variants and biological mechanisms remain elusive, especially for loci with complex linkage disequilibrium and regulation.

METHODS

To fully untangle the causal signal at a single locus, we performed a functional genomic study of 11p11.2 (the CELF1/SPI1 locus). Genome-wide association study signals at 11p11.2 were integrated with datasets of histone modification, open chromatin, and transcription factor binding to distill potentially functional variants (fVars). Their allelic regulatory activities were confirmed by allele imbalance, reporter assays, and base editing. Expressional quantitative trait loci and chromatin interaction data were incorporated to assign target genes to fVars. The relevance of these genes to AD was assessed by convergent functional genomics using bulk brain and single-cell transcriptomic, epigenomic, and proteomic datasets of patients with AD and control individuals, followed by cellular assays.

RESULTS

We found that 24 potential fVars, rather than a single variant, were responsible for the risk of 11p11.2. These fVars modulated transcription factor binding and regulated multiple genes by long-range chromatin interactions. Besides SPI1, convergent evidence indicated that 6 target genes (MTCH2, ACP2, NDUFS3, PSMC3, C1QTNF4, and MADD) of fVars were likely to be involved in AD development. Disruption of each gene led to cellular amyloid-β and phosphorylated tau changes, supporting the existence of multiple likely causal genes at 11p11.2.

CONCLUSIONS

Multiple variants and genes at 11p11.2 may contribute to AD risk. This finding provides new insights into the mechanistic and therapeutic challenges of AD.

Opposing effects of genetic variation in MTCH2 for obesity versus heart failure

Genetic variation in genes regulating metabolism may be advantageous in some settings but not others. The non-failing adult heart relies heavily on fatty acids as a fuel substrate and source of ATP. In contrast, the failing heart favors glucose as a fuel source. A bootstrap analysis for genes with deviant allele frequencies in cardiomyopathy cases versus controls identified the MTCH2 gene as having unusual variation. MTCH2 encodes an outer mitochondrial membrane protein, and prior genome-wide studies associated MTCH2 variants with body mass index, consistent with its role in metabolism. We identified the referent allele of rs1064608 (p.Pro290) as being overrepresented in cardiomyopathy cases compared to controls, and linkage disequilibrium analysis associated this variant with the MTCH2 cis eQTL rs10838738 and lower MTCH2 expression. To evaluate MTCH2, we knocked down Mtch in Drosophila heart tubes which produced a dilated and poorly functioning heart tube, reduced adiposity and shortened life span. Cardiac Mtch mutants generated more lactate at baseline, and they displayed impaired oxygen consumption in the presence of glucose but not palmitate. Treatment of cardiac Mtch mutants with dichloroacetate, a pyruvate dehydrogenase kinase inhibitor, reduced lactate and rescued lifespan. Deletion of MTCH2 in human cells similarly impaired oxygen consumption in the presence of glucose but not fatty acids. These data support a model in which MTCH2 reduction may be favorable when fatty acids are the major fuel source, favoring lean body mass. However, in settings like heart failure, where the heart shifts toward using more glucose, reduction of MTCH2 is maladaptive.

Evidence for correlations between BMI-associated SNPs and circRNAs

Circular RNAs (circRNAs) are regulators of processes like adipogenesis. Their expression can be modulated by SNPs. We analysed links between BMI-associated SNPs and circRNAs. First, we detected an enrichment of BMI-associated SNPs on circRNA genomic loci in comparison to non-significant variants. Analysis of sex-stratified GWAS data revealed that circRNA genomic loci encompassed more genome-wide significant BMI-SNPs in females than in males. To explore whether the enrichment is restricted to BMI, we investigated nine additional GWAS studies. We showed an enrichment of trait-associated SNPs in circRNAs for four analysed phenotypes (body height, chronic kidney disease, anorexia nervosa and autism spectrum disorder). To analyse the influence of BMI-affecting SNPs on circRNA levels in vitro, we examined rs4752856 located on hsa_circ_0022025. The analysis of heterozygous individuals revealed an increased level of circRNA derived from the BMI-increasing SNP allele. We conclude that genetic variation may affect the BMI partly through circRNAs.

Positively selected genes in the hoary bat (Lasiurus cinereus) lineage: prominence of thymus expression, immune and metabolic function, and regions of ancient synteny

Background

Bats of the genus Lasiurus occur throughout the Americas and have diversified into at least 20 species among three subgenera. The hoary bat (Lasiurus cinereus) is highly migratory and ranges farther across North America than any other wild mammal. Despite the ecological importance of this species as a major insect predator, and the particular susceptibility of lasiurine bats to wind turbine strikes, our understanding of hoary bat ecology, physiology, and behavior remains poor.

Methods

To better understand adaptive evolution in this lineage, we used whole-genome sequencing to identify protein-coding sequence and explore signatures of positive selection. Gene models were predicted with Maker and compared to seven well-annotated and phylogenetically representative species. Evolutionary rate analysis was performed with PAML.

Results

Of 9,447 single-copy orthologous groups that met evaluation criteria, 150 genes had a significant excess of nonsynonymous substitutions along the L. cinereus branch (P < 0.001 after manual review of alignments). Selected genes as a group had biased expression, most strongly in thymus tissue. We identified 23 selected genes with reported immune functions as well as a divergent paralog of Steep1 within suborder Yangochiroptera. Seventeen genes had roles in lipid and glucose metabolic pathways, partially overlapping with 15 mitochondrion-associated genes; these adaptations may reflect the metabolic challenges of hibernation, long-distance migration, and seasonal variation in prey abundance. The genomic distribution of positively selected genes differed significantly from background expectation by discrete Kolmogorov-Smirnov test (P < 0.001). Remarkably, the top three physical clusters all coincided with islands of conserved synteny predating Mammalia, the largest of which shares synteny with the human cat-eye critical region (CECR) on 22q11. This observation coupled with the expansion of a novel Tbx1-like gene family may indicate evolutionary innovation during pharyngeal arch development: both the CECR and Tbx1 cause dosage-dependent congenital abnormalities in thymus, heart, and head, and craniodysmorphy is associated with human orthologs of other positively selected genes as well.

The modified mitochondrial outer membrane carrier MTCH2 links mitochondrial fusion to lipogenesis

Mitochondrial function is integrated with cellular status through the regulation of opposing mitochondrial fusion and division events. Here we uncover a link between mitochondrial dynamics and lipid metabolism by examining the cellular role of mitochondrial carrier homologue 2 (MTCH2). MTCH2 is a modified outer mitochondrial membrane carrier protein implicated in intrinsic cell death and in the in vivo regulation of fatty acid metabolism. Our data indicate that MTCH2 is a selective effector of starvation-induced mitochondrial hyperfusion, a cytoprotective response to nutrient deprivation. We find that MTCH2 stimulates mitochondrial fusion in a manner dependent on the bioactive lipogenesis intermediate lysophosphatidic acid. We propose that MTCH2 monitors flux through the lipogenesis pathway and transmits this information to the mitochondrial fusion machinery to promote mitochondrial elongation, enhanced energy production, and cellular survival under homeostatic and starvation conditions. These findings will help resolve the roles of MTCH2 and mitochondria in tissue-specific lipid metabolism in animals.

Stop codon read-through of mammalian MTCH2 leading to an unstable isoform regulates mitochondrial membrane potential

Stop codon read-through (SCR) is a process of continuation of translation beyond a stop codon. This phenomenon, which occurs only in certain mRNAs under specific conditions, leads to a longer isoform with properties different from that of the canonical isoform. MTCH2, which encodes a mitochondrial protein that regulates mitochondrial metabolism, was selected as a potential read-through candidate based on evolutionary conservation observed in the proximal region of its 3' UTR. Here, we demonstrate translational read-through across two evolutionarily conserved, in-frame stop codons of MTCH2 using luminescence- and fluorescence-based assays, and by analyzing ribosome-profiling and mass spectrometry (MS) data. This phenomenon generates two isoforms, MTCH2x and MTCH2xx (single- and double-SCR products, respectively), in addition to the canonical isoform MTCH2, from the same mRNA. Our experiments revealed that a cis-acting 12-nucleotide sequence in the proximal 3' UTR of MTCH2 is the necessary signal for SCR. Functional characterization showed that MTCH2 and MTCH2x were localized to mitochondria with a long t1/2 (>36 h). However, MTCH2xx was found predominantly in the cytoplasm. This mislocalization and its unique C terminus led to increased degradation, as shown by greatly reduced t1/2 (<1 h). MTCH2 read-through-deficient cells, generated using CRISPR-Cas9, showed increased MTCH2 expression and, consistent with this, decreased mitochondrial membrane potential. Thus, double-SCR of MTCH2 regulates its own expression levels contributing toward the maintenance of normal mitochondrial membrane potential.

Computational analyses of obesity associated loci generated by genome-wide association studies

OBJECTIVES

Genome-wide association studies (GWASs) have discovered associations of numerous SNPs and genes with obesity. However, the underlying molecular mechanisms through which these SNPs and genes affect the predisposition to obesity remain not fully understood. Aims of our study are to comprehensively characterize obesity GWAS SNPs and genes through computational approaches.

METHODS

For obesity GWAS identified SNPs, functional annotation, effects on miRNAs binding and impact on protein phosphorylation were performed via RegulomeDB and 3DSNP, miRNASNP, and the PhosSNP 1.0 database, respectively. For obesity associated genes, protein-protein interaction network construction, gene ontology and pathway enrichment analyses were performed by STRING, PANTHER and STRING, respectively.

RESULTS

A total of 445 SNPs are significantly associated with obesity related phenotypes at threshold P < 5×10-8. A number of SNPs were eQTLs for obesity associated genes, some SNPs located at binding sites of obesity related transcription factors. SNPs that might affect miRNAs binding and protein phosphorylation were identified. Protein-protein interaction network analysis identified the highly-interconnected "hub" genes. Obesity associated genes mainly involved in metabolic process and catalytic activity, and significantly enriched in 15 signal pathways.

CONCLUSIONS

Our results provided the targets for follow-up experimental testing and further shed new light on obesity pathophysiology.

Non-apoptotic functions of BCL-2 family proteins

The BCL-2 family proteins are major regulators of the apoptosis process, but the mechanisms by which they regulate this process are only partially understood. It is now well documented that these proteins play additional non-apoptotic roles that are likely to be related to their apoptotic roles and to provide important clues to cracking their mechanisms of action. It seems that these non-apoptotic roles are largely related to the activation of cellular survival pathways designated to maintain or regain cellular survival, but, if unsuccessful, will switch over into a pro-apoptotic mode. These non-apoptotic roles span a wide range of processes that include the regulation of mitochondrial physiology (metabolism, electron transport chain, morphology, permeability transition), endoplasmic reticulum physiology (calcium homeostasis, unfolded protein response (UPR)), nuclear processes (cell cycle, DNA damage response (DDR)), whole-cell metabolism (glucose and lipid), and autophagy. Here we review all these different non-apoptotic roles, make an attempt to link them to the apoptotic roles, and present many open questions for future research directions in this fascinating field.

MTCH2 is a conserved regulator of lipid homeostasis

OBJECTIVE

More than one-third of U.S. adults have obesity, causing an alarming increase in obesity-related comorbidities such as type 2 diabetes. The functional role of mitochondrial carrier homolog 2 (MTCH2), a human obesity-associated gene, in lipid homeostasis was investigated in Caenorhabditis elegans, cell culture, and mice.

METHODS

In C. elegans, MTCH2/MTCH-1 was depleted, using RNAi and a genetic mutant, and overexpressed to assess its effect on lipid accumulation. In cells and mice, shRNAs against MTCH2 were used for knockdown and MTCH2 overexpression vectors were used for overexpression to study the role of this gene in fat accumulation.

RESULTS

MTCH2 knockdown reduced lipid accumulation in adipocyte-like cells in vitro and in C. elegans and mice in vivo. MTCH2 overexpression increased fat accumulation in cell culture, C. elegans, and mice. Acute MTCH2 inhibition reduced fat accumulation in animals subjected to a high-fat diet. Finally, MTCH2 influenced estrogen receptor 1 (ESR1) activity.

CONCLUSIONS

MTCH2 is a conserved regulator of lipid homeostasis. MTCH2 was found to be both required and sufficient for lipid homeostasis shifts, suggesting that pharmacological inhibition of MTCH2 could be therapeutic for treatment of obesity and related disorders. MTCH2 could influence lipid homeostasis through inhibition of ESR1 activity.