PPP2R5C Couples Hepatic Glucose and Lipid Homeostasis

Saturated fatty acids stimulate cytokine production in tanycytes via the PP2Ac-dependent signaling pathway

The hypothalamic tanycytes are crucial for free fatty acids (FFAs) detection, storage, and transport within the central nervous system. They have been shown to effectively respond to fluctuations in circulating FFAs, thereby regulating energy homeostasis. However, the precise molecular mechanisms by which tanycytes modulate lipid utilization remain unclear. Here, we report that the catalytic subunit of protein phosphatase 2 A (PP2Ac), a serine/threonine phosphatase, is expressed in tanycytes and its accumulation and activation occur in response to high-fat diet consumption. In vitro, tanycytic PP2Ac responds to palmitic acid (PA) exposure and accumulates and is activated at an early stage in an AMPK-dependent manner. Furthermore, activated PP2Ac boosts hypoxia-inducible factor-1α (HIF-1α) accumulation, resulting in upregulation of an array of cytokines. Pretreatment with a PP2Ac inhibitor, LB100, prevented the PA-induced elevation of vascular endothelial growth factor (VEGF), fibroblast growth factor 1 (FGF1), hepatocyte growth factor (HGF), and dipeptidyl peptidase IV (DPPIV or CD26). Our results disclose a mechanism of lipid metabolism in tanycytes that involves the activation of PP2Ac and highlight the physiological significance of PP2Ac in hypothalamic tanycytes in response to overnutrition and efficacious treatment of obesity.

A Novel Mouse Model of Combined Hepatocellular-Cholangiocarcinoma Induced by Diethylnitrosamine and Loss of Ppp2r5d

Primary liver cancer (PLC) can be classified in hepatocellular (HCC), cholangiocarcinoma (CCA), and combined hepatocellular-cholangiocarcinoma (cHCC-CCA). The molecular mechanisms involved in PLC development and phenotype decision are still not well understood. Complete deletion of Ppp2r5d, encoding the B56δ subunit of Protein Phosphatase 2A (PP2A), results in spontaneous HCC development in mice via a c-MYC-dependent mechanism. In the present study, we aimed to examine the role of Ppp2r5d in an independent mouse model of diethylnitrosamine (DEN)-induced hepatocarcinogenesis. Ppp2r5d deletion (heterozygous and homozygous) accelerated HCC development, corroborating its tumor-suppressive function in liver and suggesting Ppp2r5d may be haploinsufficient. Ppp2r5d-deficient HCCs stained positively for c-MYC, consistent with increased AKT activation in pre-malignant and tumor tissues of Ppp2r5d-deficient mice. We also found increased YAP activation in Ppp2r5d-deficient tumors. Remarkably, in older mice, Ppp2r5d deletion resulted in cHCC-CCA development in this model, with the CCA component showing increased expression of progenitor markers (SOX9 and EpCAM). Finally, we observed an upregulation of Ppp2r5d in tumors from wildtype and heterozygous mice, revealing a tumor-specific control mechanism of Ppp2r5d expression, and suggestive of the involvement of Ppp2r5d in a negative feedback regulation restricting tumor growth. Our study highlights the tumor-suppressive role of mouse PP2A-B56δ in both HCC and cHCC-CCA, which may have important implications for human PLC development and targeted treatment.

Protein Phosphatase 2A Regulates Phenotypic and Metabolic Alteration of Microglia Cells in HFD-Associated Vascular Dementia Mice via TNF-α/Arg-1 Axis

Protein phosphatase 2A (PP2A), the activity of which is dictated by the composition of its regulatory subunit, is strongly related to the progression of neurodegenerative disease. The potential role of PP2A on the phenotypic transition of microglial cells under obese conditions is poorly explored. An understanding of the role of PP2A and identification of regulatory subunits contributing to microglial phenotypic transitions in obese condition may serve as a therapeutic target for obesity-associated neurodegeneration. C57BL/6 mice were exposed to obese-associated vascular dementia conditions by performing unilateral common carotid artery occlusion on obese mice of microglial polarization and PP2A activity using flow cytometry, real-time PCR, western blotting, and immunoprecipitation enzymatic assay, followed identifications of PP2A regulatory subunits using LCMS and RT-PCR. Chronic HFD feeding significantly increased the populations of infiltrated macrophages, showing a high percentage of CD86⁺ in VaD mice, and the expression of pro-inflammatory cytokines, and we observed that PP2A modulates metabolic reprogramming of microglia by regulating OXPHOS/ECAR activity. Using Co-IP and LCMS, we identified the six specific regulatory subunits, namely PPP2R2A, PPP2R2D, PPP2R5B, PPP2R5C, PPP2R5D, and PPP2R5E, that are associated with microglial-activation during obesity-associated-VaD. Interestingly, pharmacological up-regulation of PP2A more significantly suppressed the expression of TNF-alpha than other pro-inflammatory-cytokines and increased the expression of Arginase-1, suggesting that PP2A modulates microglial-phenotypic transitions through TNF-α/Arg-1 axis. Our present findings demonstrate microglial polarization in HFD associated with VaD, and point towards a therapeutic target by providing specific PP2A regulatory-subunits implicated in microglial activation during obesity-related-vascular-dementia.

Analysis of the Association of Two SNPs in the Promoter Regions of the PPP2R5C and SLC39A5 Genes with Litter Size in Yunshang Black Goats

Screening for candidate genes and genetic variants associated with litter size is important for goat breeding. The aim of this study was to analyze the relationship between single nucleotide polymorphisms (SNPs) in PPP2R5C and SLC39A5 and litter size in Yunshang black goats. KASP genotyping was used to detect the SNP genetic markers in the PPP2R5C and SLC39A5 in a population of 569 Yunshang black goats. The results show that there were two SNPs in the PPP2R5C and SLC39A5 promoter regions. Association analysis revealed that the polymorphisms PPP2R5C g.65977743C>T and SLC39A5 g.50676693T>C were significantly associated with the litter size of the third parity of Yunshang black goats (p < 0.05). To further explore the regulatory mechanism of the two genes, the expression of different genotypes of PPP2R5C and SLC39A5 was validated by RT-qPCR and Western blotting. The expression of PPP2R5C was significantly higher in individuals with the TT genotype than in those with the TC and CC genotypes (p < 0.05). The expression of SLC39A5 was also significantly higher in individuals with the TT genotype than in TC and CC genotypes (p < 0.05). Dual luciferase reporter analysis showed that the luciferase activity of PPP2R5C-C variant was significantly higher than that of PPP2R5C-T variant (p < 0.05). The luciferase activity of SLC39A5-T variant was significantly higher than that of SLC39A5-C variant (p < 0.05). Software was used to predict the binding of transcription factors to the polymorphic sites, and the results show that SOX18, ZNF418, and ZNF667 and NKX2-4 and TBX6 might bind to PPP2R5C g.65977743C>T and SLC39A5 g.50676693T>C, respectively. These results provide new insights into the identification of candidate genes for marker-assisted selection (MAS) in goats.

Ser/Thr phosphatases: One of the key regulators of insulin signaling

Protein phosphorylation is an important post-translational modification that regulates several cellular processes including insulin signaling. The evidences so far have already portrayed the importance of balanced actions of kinases and phosphatases in regulating the insulin signaling cascade. Therefore, elucidating the role of both kinases and phosphatases are equally important. Unfortunately, the role of phosphatases is less studied as compared to kinases. Since brain responds to insulin and insulin signaling is reported to be crucial for many neuronal processes, it is important to understand the role of neuronal insulin signaling regulators. Ser/Thr phosphatases seem to play significant roles in regulating neuronal insulin signaling. Therefore, in this review, we discussed the involvement of Ser/Thr phosphatases in regulating insulin signaling and insulin resistance in neuronal system at the backdrop of the same phosphatases in peripheral insulin sensitive tissues.

PM2.5 deregulated microRNA and inflammatory microenvironment in lung injury

PM2.5 negatively affects human health, particularly lung injury. However, the role of PM2.5-regulated miRNAs in lung injury remains unknown. MiRNA array results showed mmu-miR-467c-5p regulated Prdx6 expression to adapt to lung injury condition, and deregulated miRNAs regulated macrophages to build a localized inflammatory microenvironment. In addition, miRNAs were transferred into adjacent alveolar epithelial cells, regulating the expressions of cell injury signaling pathway-targeted genes, and accelerating local lung tissue injury. NO and RAGE were increased in the coculture supernatant, and SPD was decreased. PM2.5 exposure induced local lung injury, promoted inflammation in local lung tissues, increased capillary permeability in the lung tissue, and rearranged the local lung tissue structure. We also confirmed in AECOPD patients TNF-α and IL-1β levels are obviously higher than healthy person. These findings provide new mechanistic insights regarding PM2.5 and targeted miRNAs in the inflammatory microenvironment, which increases our knowledge of PM2.5-lung injury interactions.

Hepatic IRF3 fuels dysglycemia in obesity through direct regulation of Ppp2r1b

Inflammation has profound but poorly understood effects on metabolism, especially in the context of obesity and nonalcoholic fatty liver disease (NAFLD). Here, we report that hepatic interferon regulatory factor 3 (IRF3) is a direct transcriptional regulator of glucose homeostasis through induction of Ppp2r1b, a component of serine/threonine phosphatase PP2A, and subsequent suppression of glucose production. Global ablation of IRF3 in mice on a high-fat diet protected against both steatosis and dysglycemia, whereas hepatocyte-specific loss of IRF3 affects only dysglycemia. Integration of the IRF3-dependent transcriptome and cistrome in mouse hepatocytes identifies Ppp2r1b as a direct IRF3 target responsible for mediating its metabolic actions on glucose homeostasis. IRF3-mediated induction of Ppp2r1b amplified PP2A activity, with subsequent dephosphorylation of AMPKα and AKT. Furthermore, suppression of hepatic Irf3 expression with antisense oligonucleotides reversed obesity-induced insulin resistance and restored glucose homeostasis in obese mice. Obese humans with NAFLD displayed enhanced activation of liver IRF3, with reversion after bariatric surgery. Hepatic PPP2R1B expression correlated with HgbA1C and was elevated in obese humans with impaired fasting glucose. We therefore identify the hepatic IRF3-PPP2R1B axis as a causal link between obesity-induced inflammation and dysglycemia and suggest an approach for limiting the metabolic dysfunction accompanying obesity-associated NAFLD.

Long-Chain Acylcarnitines Decrease the Phosphorylation of the Insulin Receptor at Tyr1151 Through a PTP1B-Dependent Mechanism

The accumulation of lipid intermediates may interfere with energy metabolism pathways and regulate cellular energy supplies. As increased levels of long-chain acylcarnitines have been linked to insulin resistance, we investigated the effects of long-chain acylcarnitines on key components of the insulin signalling pathway. We discovered that palmitoylcarnitine induces dephosphorylation of the insulin receptor (InsR) through increased activity of protein tyrosine phosphatase 1B (PTP1B). Palmitoylcarnitine suppresses protein kinase B (Akt) phosphorylation at Ser473, and this effect is not alleviated by the inhibition of PTP1B by the insulin sensitizer bis-(maltolato)-oxovanadium (IV). This result indicates that palmitoylcarnitine affects Akt activity independently of the InsR phosphorylation level. Inhibition of protein kinase C and protein phosphatase 2A does not affect the palmitoylcarnitine-mediated inhibition of Akt Ser473 phosphorylation. Additionally, palmitoylcarnitine markedly stimulates insulin release by suppressing Akt Ser473 phosphorylation in insulin-secreting RIN5F cells. In conclusion, long-chain acylcarnitines activate PTP1B and decrease InsR Tyr1151 phosphorylation and Akt Ser473 phosphorylation, thus limiting the cellular response to insulin stimulation.

Combined approaches to reveal genes associated with litter size in Yunshang black goats

Intensive artificial selection has been imposed in Yunshang black goats, the first black specialist mutton goat breed in China, with a breeding object of improving reproductive performance, which has contributed to reshaping of the genome including the characterization of SNP, ROH and haplotype. However, variation in reproductive ability exists in the present population. A WGS was implemented in two subpopulations (polytocous group, PG, and monotocous group, MG) with evident differences of litter size. Following the mapping to reference genome, and SNP calling and pruning, three approaches - GWAS, ROH analysis and detection of signatures of selection - were employed to unveil candidate genes responsible for litter size. Consequently, 12 candidate genes containing OSBPL8 with the minimum P-value were uncovered by GWAS. Differences were observed in the pattern of ROH between two subpopulations that shared similar low inbreeding coefficients. Two ROH hotspots and 12 corresponding genes emerged from ROH pool association analysis. Based on the nS_L statistic, 15 and 61 promising genes were disclosed under selection for MG and PG respectively. Of them, some promising genes participate in ovarian function (PPP2R5C, CDC25A, ESR1, RPS26 and SERPINBs), seasonal reproduction (DIO3, BTG1 and CRYM) and metabolism (OSBPL8, SLC39A5 and SERPINBs). Our study pinpointed some novel promising genes influencing litter size, provided a comprehensive insight into genetic makeup of litter size and might facilitate selective breeding in goats.

MiR-542-5p Inhibits Hyperglycemia and Hyperlipoidemia by Targeting FOXO1 in the Liver

PURPOSE

This research was designed to investigate how miR-542-5p regulates the progression of hyperglycemia and hyperlipoidemia.

MATERIALS AND METHODS

An in vivo model with diabetic db/db mice and an in vitro model with forskolin/dexamethasone (FSK/DEX)-induced primary hepatocytes and HepG2 cells were employed in the study. Bioinformatics analysis was conducted to identify the expression of candidate miRNAs in the liver tissues of diabetic and control mice. H&E staining revealed liver morphology in diabetic and control mice. Pyruvate tolerance tests, insulin tolerance tests, and intraperitoneal glucose tolerance test were utilized to assess insulin resistance. ELISA was conducted to evaluate blood glucose and insulin levels. Red oil O staining showed lipid deposition in liver tissues. Luciferase reporter assay was used to depict binding between miR-542-5p and forkhead box O1 (FOXO1).

RESULTS

MiR-542-5p expression was under-expressed in the livers of db/db mice. Further in vitro experiments revealed that FSK/DEX, which mimics the effects of glucagon and glucocorticoids, induced cellular glucose production in HepG2 cells and in primary hepatocytes cells. Notably, these changes were reversed by miR-542-5p. We found that transcription factor FOXO1 is a target of miR-542-5p. Further in vivo study indicated that miR-542-5p overexpression decreases FOXO1 expression, thereby reversing increases in blood glucose, blood lipids, and glucose-related enzymes in diabetic db/db mice. In contrast, anti-miR-542-5p exerted an adverse influence on blood glucose and blood lipid metabolism, and its stimulatory effects were significantly inhibited by sh-FOXO1 in normal control mice.

CONCLUSION

Collectively, our results indicated that miR-542-5p inhibits hyperglycemia and hyperlipoidemia by targeting FOXO1.

Reduced methylation of PP2Ac promotes ethanol-induced lipid accumulation through FOXO1 phosphorylation in vitro and in vivo

Although disturbance of the methionine cycle and sequent decrease in hepatic methylation capacity are known to be important factors in the development of alcoholic liver injury, the underlying mechanisms are not fully understood. Here, we investigated the importance of the methylation of protein phosphatase 2A (PP2A) in alcoholic liver disease (ALD). We found that the severity of ethanol-induced liver injury and the extent of demethylation of PP2A catalytic C subunit (PP2Ac) were reduced after treatment with betaine, a methyl donor involved in the methionine-homocysteine cycle. These results suggest that PP2Ac methylation is decreased due to a broad decrease in hepatic methylation capacity after exposure to ethanol. Moreover, we found that the reduction in PP2Ac methylation led to increased degradation of the regulatory Bα subunit, thus promoting the phosphorylation and nuclear exclusion of Forkhead box O1 (FOXO1) and reducing FOXO1 transcriptional activity. Ultimately, the reduced activity of FOXO1 led to increased expression of TXNIP, which caused hepatic lipid accumulation. Our findings suggest that the reduction of PP2A methylation, a result of decrease hepatic methylation capacity, played an important role in ethanol-induced lipid accumulation via down-regulation of PP2A/Bα and FOXO1 phosphorylation.

Genomic diversity and selection sweeps identified in Indian swamp buffaloes reveals it's uniqueness with riverine buffaloes

The present investigation was focused to study genomic diversity of Indian swamp buffalo populations through reduced representation approach (ddRAD). The heterozygosity (F_ST) among the swamp buffaloes was 0.11 between Assam and Manipuri; 0.20 between swamp (Manipuri) and riverine buffaloes; 0.30 between swamp (Manipuri) and cattle. The average observed and expected heterozygosity in swamp buffalo populations was 0.254 and 0.221 respectively. The Inbreeding coefficient (F_IS) value was 0.02 among the swamp buffaloes. PCA and structure analysis revealed Manipuri swamp buffalo was genetically distinct and closely related to Nagaland swamp buffalo and least to Assam swamp buffalo. Identification of selective sweeps revealed 1087 regions to have undergone selection related to immune response, adaptation and nervous system. A total of 3451 SSRs were identified in the genome of swamp buffaloes. The study evidenced the genomic diversity in the swamp buffalo populations and its uniqueness in comparison with riverine buffalo and cattle.

Comparison of visceral adipose tissue DNA methylation and gene expression profiles in female adolescents with obesity

BACKGROUND

Epigenetic changes in visceral adipose tissue (VAT) with obesity and their effects on gene expression are poorly understood, especially during emergent obesity in youth. The current study tested the hypothesis that methylation and gene expression profiles of key growth factor and inflammatory pathways are altered in VAT from obese compared to non-obese youth.

METHODS

VAT samples from adolescent females grouped as Lean (L; n = 15; age = 15 ± 3 years, BMI = 21.9 ± 3.0 kg/m2) or Obese (Ob; n = 15, age = 16 ± 2 years, BMI = 45.8 ± 9.8 kg/m2) were collected. Global methylation (n = 20) and gene expression (N = 30) patterns were profiled via microarray and interrogated for differences between groups by ANCOVA (p < 0.05), followed by biological pathway analyses.

RESULTS

Overlapping differences in methylation and gene expression in 317 genes were found in VAT from obese compared to lean groups. PI3K/AKT Signaling (p = 1.83 × 10-6; 11/121 molecules in dataset/pathway) was significantly overrepresented in Ob VAT according to pathway analysis. Upregulations in the PI3K/AKT signaling pathway mRNAs TFAM (p = 0.03; fold change = 1.8) and PPP2R5C (p = 0.03, FC = 2.6) were confirmed via qRT-PCR.

CONCLUSION

Our analyses show obesity-related differences in DNA methylation and gene expression in visceral adipose tissue of adolescent females. Specifically, we identified methylation site/gene expression pairs differentially regulated and mapped these differences to pathways including PI3K/AKT signaling, suggesting that PI3K/AKT signaling pathway dysfunction in obesity may be driven in part by changes in DNA methylation.

Genome-wide association study identifies loci influencing natural antibody titers in milk of Dutch Holstein-Friesian cattle

Natural antibodies (NAb) are produced without any antigenic stimulation as a part of the innate immune system and provide a first line of defense against pathogens. Hence, they may be a useful trait when estimating an animal's potential immune competence and in selection for disease resistance. The aim of this study was to identify genomic regions associated with different NAb traits in milk and potentially describe candidate genes. Milk samples from 1,695 first-lactation Holstein Friesian cows with titer measurements for keyhole limpet hemocyanin, lipopolysaccharide, lipoteichoic acid, and peptidoglycan-binding total NAb and isotypes IgG₁, IgM, and IgA were used. Genome-wide association study analyses were performed using imputed 777K SNP genotypes, accounting for relationships using pedigree information. Functional enrichment analysis was performed on the significantly associated genomic regions to look for candidate genes. For IgM NAb, significant associations (false discovery rate <0.05) were found on Bos taurus autosome (BTA) 17, 18, and 21 with candidate genes related to immunoglobulin structure and early B cell development. For IgG₁, associations were found on BTA3, and we confirmed a quantitative trait loci on BTA21 previously reported for IgG NAb in serum. Our results provide new insights into the regulation of milk NAb that will help unravel the complex relationship between milk immunoglobulins and disease resistance in dairy cattle.

BioID: A Method to Generate a History of Protein Associations

Proximity-dependent labeling methods for detecting candidate protein-protein interactions (PPIs) or mapping the protein constituency of subcellular domains have become increasingly utilized by the scientific community. One such method, BioID, allows for the identification of not only strong interactions but also weak and transient associations between a protein of interest (POI) or targeting motif and adjacent proteins. A promiscuous biotin ligase is fused to a POI or targeting motif, expressed in living cells, and induced to biotinylate proximal proteins during a defined labeling period by biotin supplementation. This generates a history of protein-protein associations that occurred with the POI or the protein constituency within a discrete subcellular domain during the labeling period. Biotinylated proteins are subsequently isolated, identified via mass spectrometry, and investigated as candidate interactors with the POI or as constituents within a subcellular domain. The BioID method has been utilized by numerous research groups and is continually being optimized, applied to new models, and modified for use in novel applications. Here we describe a protocol by which a BioID fusion protein can be validated and utilized for BioID pull-downs.

Towards improving proximity labeling by the biotin ligase BirA

The discovery and validation of protein-protein interactions provides a knowledge base that is critical for defining protein networks and how they underpin the biology of the cell. Identification of protein interactions that are highly transient, or sensitive to biochemical disruption, can be very difficult. This challenge has been met by proximity labeling methods which generate reactive species that chemically modify neighboring proteins. The most widely used proximity labeling method is BioID, which features a mutant biotin ligase BirA(Arg118Gly), termed BirA^*, fused to a protein of interest. Here, we explore how amino acid substitutions at Arg118 affect the biochemical properties of BirA. We found that relative to wild-type BirA, the Arg118Lys substitution both slightly reduced biotin affinity and increased the release of reactive biotinyl-5'-AMP. BioID using a BirA(Arg118Lys)-Lamin A fusion enabled identification of PCNA as a lamina-proximal protein in HEK293T cells, a finding that was validated by immunofluorescence microscopy. Our data expand on the concept that proximity labeling by BirA fused to proteins of interest can be modulated by amino acid substitutions that affect biotin affinity and the release of biotinyl-5'-AMP.

Newborn blood DNA epigenetic variations and signaling pathway genes associated with Tetralogy of Fallot (TOF)

Tetralogy of Fallot (TOF) is the most common Critical Congenital Heart Defect (CCHD). The etiology of TOF is unknown in most cases. Preliminary data from our group and others suggest that epigenetic changes may play an important role in CHD. Epidemiologically, a significant percentage of CHD including TOF fail to be diagnosed in the prenatal and early newborn period which can negatively affect health outcomes. We performed genome-wide methylation assay in newborn blood in 24 non-syndromic TOF cases and 24 unaffected matched controls using Illumina Infinium HumanMethylation450 BeadChips. We identified 64 significantly differentially methylated CpG sites in TOF cases, of which 25 CpG sites had high predictive accuracy for TOF, based on the area under the receiver operating characteristics curve (AUC ROC) ≥ 0.90). The CpG methylation difference between TOF and controls was ≥10% in 51 CpG targets suggesting biological significance. Gene ontology analysis identified significant biological processes and functions related to these differentially methylated genes, including: CHD development, cardiomyopathy, diabetes, immunological, inflammation and other plausible pathways in CHD development. Multiple genes known or plausibly linked to heart development and post-natal heart disease were found to be differentially methylated in the blood DNA of newborns with TOF including: ABCB1, PPP2R5C, TLR1, SELL, SCN3A, CREM, RUNX and LHX9. We generated novel and highly accurate putative molecular markers for TOF detection using leucocyte DNA and thus provided information on pathogenesis of TOF.

Parallel Exploration of Interaction Space by BioID and Affinity Purification Coupled to Mass Spectrometry

Complete understanding of cellular function requires knowledge of the composition and dynamics of protein interaction networks, the importance of which spans all molecular cell biology fields. Mass spectrometry-based proteomics approaches are instrumental in this process, with affinity purification coupled to mass spectrometry (AP-MS) now widely used for defining interaction landscapes. Traditional AP-MS methods are well suited to providing information regarding the temporal aspects of soluble protein-protein interactions, but the requirement to maintain protein-protein interactions during cell lysis and AP means that both weak-affinity interactions and spatial information is lost. A more recently developed method called BioID employs the expression of bait proteins fused to a nonspecific biotin ligase, BirA*, that induces in vivo biotinylation of proximal proteins. Coupling this method to biotin affinity enrichment and mass spectrometry negates many of the solubility and interaction strength issues inherent in traditional AP-MS methods, and provides unparalleled spatial context for protein interactions. Here we describe the parallel implementation of both BioID and FLAG AP-MS allowing simultaneous exploration of both spatial and temporal aspects of protein interaction networks.

Characterization of the mammalian DEAD-box protein DDX5 reveals functional conservation with S. cerevisiae ortholog Dbp2 in transcriptional control and glucose metabolism

DEAD-box proteins are a class of nonprocessive RNA helicases that dynamically modulate the structure of RNA and ribonucleoprotein complexes (RNPs). However, the precise roles of individual members are not well understood. Work from our laboratory revealed that the DEAD-box protein Dbp2 in Saccharomyces cerevisiae is an active RNA helicase in vitro that functions in transcription by promoting mRNP assembly, repressing cryptic transcription initiation, and regulating long noncoding RNA activity. Interestingly, Dbp2 is also linked to glucose sensing and hexose transporter gene expression. DDX5 is the mammalian ortholog of Dbp2 that has been implicated in cancer and metabolic syndrome, suggesting that the role of Dbp2 and DDX5 in glucose metabolic regulation is conserved. Herein, we present a refined biochemical and biological comparison of yeast Dbp2 and human DDX5 enzymes. We find that human DDX5 possesses a 10-fold higher unwinding activity than Dbp2, which is partially due to the presence of a mammalian/avian specific C-terminal extension. Interestingly, ectopic expression of DDX5 rescues the cold sensitivity, cryptic initiation defects, and impaired glucose import in dbp2Δ cells, suggesting functional conservation. Consistently, we show that DDX5 promotes glucose uptake and glycolysis in mouse AML12 hepatocyte cells, suggesting that mammalian DDX5 and S. cerevisiae Dbp2 share conserved roles in cellular metabolism.

Recent progress in genetics, epigenetics and metagenomics unveils the pathophysiology of human obesity

In high-, middle- and low-income countries, the rising prevalence of obesity is the underlying cause of numerous health complications and increased mortality. Being a complex and heritable disorder, obesity results from the interplay between genetic susceptibility, epigenetics, metagenomics and the environment. Attempts at understanding the genetic basis of obesity have identified numerous genes associated with syndromic monogenic, non-syndromic monogenic, oligogenic and polygenic obesity. The genetics of leanness are also considered relevant as it mirrors some of obesity's aetiologies. In this report, we summarize ten genetically elucidated obesity syndromes, some of which are involved in ciliary functioning. We comprehensively review 11 monogenic obesity genes identified to date and their role in energy maintenance as part of the leptin-melanocortin pathway. With the emergence of genome-wide association studies over the last decade, 227 genetic variants involved in different biological pathways (central nervous system, food sensing and digestion, adipocyte differentiation, insulin signalling, lipid metabolism, muscle and liver biology, gut microbiota) have been associated with polygenic obesity. Advances in obligatory and facilitated epigenetic variation, and gene-environment interaction studies have partly accounted for the missing heritability of obesity and provided additional insight into its aetiology. The role of gut microbiota in obesity pathophysiology, as well as the 12 genes associated with lipodystrophies is discussed. Furthermore, in an attempt to improve future studies and merge the gap between research and clinical practice, we provide suggestions on how high-throughput '-omic' data can be integrated in order to get closer to the new age of personalized medicine.

Autophagy and hepatic lipid metabolism

Autophagy is initially thought to be a non-selective process in which intracellular proteins or damaged organelles are degraded. It is activated when cells lack nutrients and energy. Autophagy degrades cytoplasmic components within lysosomes and reuses the energy of amino acids to promote cell survival and maintain the cytoplasmic content. Current evidence implicates autophagy in the regulation of lipid stores within the two main organs involved in maintaining lipid homeostasis, the liver and adipose tissue. Upregulation of autophagy may lead to conversion of white adipose tissue into brown adipose tissue, thus regulating energy expenditure and obesity. Discovering new therapeutic interventions to treat lipid and lipoprotein disorders is of great interest and the discovery of autophagy as a regulator of lipid metabolism has opened up a new avenue for this area. In the liver, autophagy can play a role in some common metabolic disorders, which needs further research.

Mitochondria-Associated Membranes Response to Nutrient Availability and Role in Metabolic Diseases

Metabolic diseases are associated with nutrient excess and metabolic inflexibility. Mitochondria and endoplasmic reticulum are important organelles and nutrient sensors, and their dysfunction has been extensively and independently implicated in metabolic diseases. Both organelles interact at sites known as mitochondria-associated membranes (MAMs), in order to exchange metabolites and calcium. Recent evidence indicates that MAM could be a hub of hepatic insulin signaling and nutrient sensing. In this review, we discuss the roles organelle function and communication play in the cell's adaptation to nutrient availability, in both physiology and metabolic diseases. We highlight how dynamic regulation of MAM affects mitochondria physiology and adaptation of cellular metabolism to nutrient availability, and how chronic MAM disruption participates in the metabolic inflexibility associated with metabolic disorders.

Filling the Void: Proximity-Based Labeling of Proteins in Living Cells

There are inherent limitations with traditional methods to study protein behavior or to determine the constituency of proteins in discrete subcellular compartments. In response to these limitations, several methods have recently been developed that use proximity-dependent labeling. By fusing proteins to enzymes that generate reactive molecules, most commonly biotin, proximate proteins are covalently labeled to enable their isolation and identification. In this review we describe current methods for proximity-dependent labeling in living cells and discuss their applications and future use in the study of protein behavior.

Meet the neighbors: Mapping local protein interactomes by proximity-dependent labeling with BioID

Proximity-dependent biotin identification (BioID) is a recently developed method that allows the identification of proteins in the close vicinity of a protein of interest in living cells. BioID relies on fusion of the protein of interest with a mutant form of the biotin ligase enzyme BirA (BirA*) that is capable of promiscuously biotinylating proximal proteins irrespective of whether these interact directly or indirectly with the fusion protein or are merely located in the same subcellular neighborhood. The covalent addition of biotin allows the labeled proteins to be purified from cell extracts on the basis of their affinity for streptavidin and identified by mass spectrometry. To date, BioID has been successfully applied to study a variety of proteins and processes in mammalian cells and unicellular eukaryotes and has been shown to be particularly suited to the study of insoluble or inaccessible cellular structures and for detecting weak or transient protein associations. Here, we provide an introduction to BioID, together with a detailed summary of where and how the method has been applied to date, and briefly discuss technical aspects involved in the planning and execution of a BioID study.

Haploinsufficiency of the Insulin Receptor in the Presence of a Splice-Site Mutation in Ppp2r2a Results in a Novel Digenic Mouse Model of Type 2 Diabetes

Insulin resistance in mice typically does not manifest as diabetes due to multiple compensatory mechanisms. Here, we present a novel digenic model of type 2 diabetes in mice heterozygous for a null allele of the insulin receptor and an N-ethyl-N-nitrosourea-induced alternative splice mutation in the regulatory protein phosphatase 2A (PP2A) subunit PPP2R2A. Inheritance of either allele independently results in insulin resistance but not overt diabetes. Doubly heterozygous mice exhibit progressive hyperglycemia, hyperinsulinemia, and impaired glucose tolerance from 12 weeks of age without significant increase in body weight. Alternative splicing of Ppp2r2a decreased PPP2R2A protein levels. This reduction in PPP2R2A containing PP2A phosphatase holoenzyme was associated with decreased serine/threonine protein kinase AKT protein levels. Ultimately, reduced insulin-stimulated phosphorylated AKT levels were observed, a result that was confirmed in Hepa1-6, C2C12, and differentiated 3T3-L1 cells knocked down using Ppp2r2a small interfering RNAs. Altered AKT signaling and expression of gluconeogenic genes in the fed state contributed to an insulin resistance and hyperglycemia phenotype. This model demonstrates how genetic changes with individually small phenotypic effects interact to cause diabetes and how differences in expression of hypomorphic alleles of PPP2R2A and potentially other regulatory proteins have deleterious effects and may therefore be relevant in determining diabetes risk.

Pancreatic β-Cell Dysfunction in Diet-Induced Obese Mice: Roles of AMP-Kinase, Protein Kinase Cε, Mitochondrial and Cholesterol Metabolism, and Alterations in Gene Expression

Diet induced obese (DIO) mice can be stratified according to their weight gain in response to high fat diet as low responders (LDR) and high responders (HDR). This allows the study of β-cell failure and the transitions to prediabetes (LDR) and early diabetes (HDR). C57BL/6N mice were fed for 8 weeks with a normal chow diet (ND) or a high fat diet and stratified as LDR and HDR. Freshly isolated islets from ND, LDR and HDR mice were studied ex-vivo for mitochondrial metabolism, AMPK activity and signalling, the expression and activity of key enzymes of energy metabolism, cholesterol synthesis, and mRNA profiling. Severely compromised glucose-induced insulin secretion in HDR islets, as compared to ND and LDR islets, was associated with suppressed AMP-kinase activity. HDR islets also showed reduced acetyl-CoA carboxylase activity and enhanced activity of 3-hydroxy-3-methylglutaryl-CoA reductase, which led respectively to elevated fatty acid oxidation and increased cholesterol biosynthesis. HDR islets also displayed mitochondrial membrane hyperpolarization and reduced ATP turnover in the presence of elevated glucose. Expression of protein kinase Cε, which reduces both lipolysis and production of signals for insulin secretion, was elevated in DIO islets. Genes whose expression increased or decreased by more than 1.2-fold were minor between LDR and ND islets (17 differentially expressed), but were prominent between HDR and ND islets (1508 differentially expressed). In HDR islets, particularly affected genes were related to cell cycle and proliferation, AMPK signaling, mitochondrial metabolism and cholesterol metabolism. In conclusion, chronically reduced AMPK activity, mitochondrial dysfunction, elevated cholesterol biosynthesis in islets, and substantial alterations in gene expression accompany β-cell failure in HDR islets. The β-cell compensation process in the prediabetic state (LDR) is largely independent of transcriptional adaptive changes, whereas the transition to early diabetes (HDR) is associated with major alterations in gene expression.

An improved smaller biotin ligase for BioID proximity labeling

A smaller promiscuous biotin ligase for proximity biotinylation called BioID2 enables more-selective targeting of fusion proteins, requires less biotin supplementation, exhibits enhanced labeling of proximate proteins, and demonstrates the use of a flexible linker to modulate the biotin-labeling radius.

The BioID method uses a promiscuous biotin ligase to detect protein–protein associations as well as proximate proteins in living cells. Here we report improvements to the BioID method centered on BioID2, a substantially smaller promiscuous biotin ligase. BioID2 enables more-selective targeting of fusion proteins, requires less biotin supplementation, and exhibits enhanced labeling of proximate proteins. Thus BioID2 improves the efficiency of screening for protein–protein associations. We also demonstrate that the biotinylation range of BioID2 can be considerably modulated using flexible linkers, thus enabling application-specific adjustment of the biotin-labeling radius.