Diabetes-Related Ankyrin Repeat Protein (DARP/Ankrd23) Modifies Glucose Homeostasis by Modulating AMPK Activity in Skeletal Muscle

The titin N2B and N2A regions: biomechanical and metabolic signaling hubs in cross-striated muscles

Muscle specific signaling has been shown to originate from myofilaments and their associated cellular structures, including the sarcomeres, costameres or the cardiac intercalated disc. Two signaling hubs that play important biomechanical roles for cardiac and/or skeletal muscle physiology are the N2B and N2A regions in the giant protein titin. Prominent proteins associated with these regions in titin are chaperones Hsp90 and αB-crystallin, members of the four-and-a-half LIM (FHL) and muscle ankyrin repeat protein (Ankrd) families, as well as thin filament-associated proteins, such as myopalladin. This review highlights biological roles and properties of the titin N2B and N2A regions in health and disease. Special emphasis is placed on functions of Ankrd and FHL proteins as mechanosensors that modulate muscle-specific signaling and muscle growth. This region of the sarcomere also emerged as a hotspot for the modulation of passive muscle mechanics through altered titin phosphorylation and splicing, as well as tethering mechanisms that link titin to the thin filament system.

Molecular Profiling of DNA Methylation and Alternative Splicing of Genes in Skeletal Muscle of Obese Rabbits

DNA methylation and the alternative splicing of precursor messenger RNAs (pre-mRNAs) are two important genetic modification mechanisms. However, both are currently uncharacterized in the muscle metabolism of rabbits. Thus, we constructed the Tianfu black rabbit obesity model (obese rabbits fed with a 10% high-fat diet and control rabbits from 35 days to 70 days) and collected the skeletal muscle samples from the two groups for Genome methylation sequencing and RNA sequencing. DNA methylation data showed that the promoter regions of 599 genes and gene body region of 2522 genes had significantly differential methylation rates between the two groups, of which 288 genes had differential methylation rates in promoter and gene body regions. Analysis of alternative splicing showed 555 genes involved in exon skipping (ES) patterns, and 15 genes existed in differential methylation regions. Network analysis showed that 20 hub genes were associated with ubiquitinated protein degradation, muscle development pathways, and skeletal muscle energy metabolism. Our findings suggest that the two types of genetic modification have potential regulatory effects on skeletal muscle development and provide a basis for further mechanistic studies in the rabbit.

Muscle ankyrin repeat protein 1 (MARP1) locks titin to the sarcomeric thin filament and is a passive force regulator

Muscle ankyrin repeat protein 1 (MARP1) is frequently up-regulated in stressed muscle, but its effect on skeletal muscle function is poorly understood. Here, we focused on its interaction with the titin–N2A element, found in titin’s molecular spring region. We show that MARP1 binds to F-actin, and that this interaction is stronger when MARP1 forms a complex with titin–N2A. Mechanics and super-resolution microscopy revealed that MARP1 “locks” titin–N2A to the sarcomeric thin filament, causing increased extension of titin’s elastic PEVK element and, importantly, increased passive force. In support of this mechanism, removal of thin filaments abolished the effect of MARP1 on passive force. The clinical relevance of this mechanism was established in diaphragm myofibers of mechanically ventilated rats and of critically ill patients. Thus, MARP1 regulates passive force by locking titin to the thin filament. We propose that in stressed muscle, this mechanism protects the sarcomere from mechanical damage.

Persistent or Transient Human β Cell Dysfunction Induced by Metabolic Stress: Specific Signatures and Shared Gene Expression with Type 2 Diabetes

Pancreatic β cell failure is key to type 2 diabetes (T2D) onset and progression. Here, we assess whether human β cell dysfunction induced by metabolic stress is reversible, evaluate the molecular pathways underlying persistent or transient damage, and explore the relationships with T2D islet traits. Twenty-six islet preparations are exposed to several lipotoxic/glucotoxic conditions, some of which impair insulin release, depending on stressor type, concentration, and combination. The reversal of dysfunction occurs after washout for some, although not all, of the lipoglucotoxic insults. Islet transcriptomes assessed by RNA sequencing and expression quantitative trait loci (eQTL) analysis identify specific pathways underlying β cell failure and recovery. Comparison of a large number of human T2D islet transcriptomes with those of persistent or reversible β cell lipoglucotoxicity show shared gene expression signatures. The identification of mechanisms associated with human β cell dysfunction and recovery and their overlap with T2D islet traits provide insights into T2D pathogenesis, fostering the development of improved β cell-targeted therapeutic strategies.

Anti-hyperglycemic and hypolipidemic effects of black ginseng extract containing increased Rh4, Rg5, and Rk1 content in muscle and liver of type 2 diabetic db/db mice

Black ginseng (BG), which is produced by repeated steaming and drying of fresh ginseng, has various pharmacological and therapeutic properties. This study investigated the anti-hyperglycemic and hypolipidemic effects of BG ethanolic extract in type 2 diabetic db/db mice. The levels of fasting blood glucose, HbA1c, insulin levels and thiobarbituric acid reactive substances values were decreased in the groups fed BG extract (BG) (100 and 900 mg/kg BW/day), compared to the control group (CON). In the BG compared with the CON, hepatic steatosis in the liver and the size of adipocytes in muscle tissue were improved. The administration of BG regulated the glucose transporter type (GLUT) 4 and 2, and peroxisome proliferator-activated receptor (PPAR) α and γ in muscle and liver. Moreover, ginsenosides (Rh4, Rg5, and Rk1), which profiled by HPLC, regulated the markers for lipid metabolism and glucose metabolism; PPARs and GLUTs in muscle and C2C12 rather than liver cells and tissue. These findings suggested that ginsenosides (Rh4, Rg5, and Rk1) from BG extract can ameliorate type 2 diabetes through their anti-hyperglycemic and hypolipidemic effects.

Systems biology of the IMIDIA biobank from organ donors and pancreatectomised patients defines a novel transcriptomic signature of islets from individuals with type 2 diabetes

AIMS/HYPOTHESIS

Pancreatic islet beta cell failure causes type 2 diabetes in humans. To identify transcriptomic changes in type 2 diabetic islets, the Innovative Medicines Initiative for Diabetes: Improving beta-cell function and identification of diagnostic biomarkers for treatment monitoring in Diabetes (IMIDIA) consortium ( www.imidia.org ) established a comprehensive, unique multicentre biobank of human islets and pancreas tissues from organ donors and metabolically phenotyped pancreatectomised patients (PPP).

METHODS

Affymetrix microarrays were used to assess the islet transcriptome of islets isolated either by enzymatic digestion from 103 organ donors (OD), including 84 non-diabetic and 19 type 2 diabetic individuals, or by laser capture microdissection (LCM) from surgical specimens of 103 PPP, including 32 non-diabetic, 36 with type 2 diabetes, 15 with impaired glucose tolerance (IGT) and 20 with recent-onset diabetes (<1 year), conceivably secondary to the pancreatic disorder leading to surgery (type 3c diabetes). Bioinformatics tools were used to (1) compare the islet transcriptome of type 2 diabetic vs non-diabetic OD and PPP as well as vs IGT and type 3c diabetes within the PPP group; and (2) identify transcription factors driving gene co-expression modules correlated with insulin secretion ex vivo and glucose tolerance in vivo. Selected genes of interest were validated for their expression and function in beta cells.

RESULTS

Comparative transcriptomic analysis identified 19 genes differentially expressed (false discovery rate ≤0.05, fold change ≥1.5) in type 2 diabetic vs non-diabetic islets from OD and PPP. Nine out of these 19 dysregulated genes were not previously reported to be dysregulated in type 2 diabetic islets. Signature genes included TMEM37, which inhibited Ca²⁺-influx and insulin secretion in beta cells, and ARG2 and PPP1R1A, which promoted insulin secretion. Systems biology approaches identified HNF1A, PDX1 and REST as drivers of gene co-expression modules correlated with impaired insulin secretion or glucose tolerance, and 14 out of 19 differentially expressed type 2 diabetic islet signature genes were enriched in these modules. None of these signature genes was significantly dysregulated in islets of PPP with impaired glucose tolerance or type 3c diabetes.

CONCLUSIONS/INTERPRETATION

These studies enabled the stringent definition of a novel transcriptomic signature of type 2 diabetic islets, regardless of islet source and isolation procedure. Lack of this signature in islets from PPP with IGT or type 3c diabetes indicates differences possibly due to peculiarities of these hyperglycaemic conditions and/or a role for duration and severity of hyperglycaemia. Alternatively, these transcriptomic changes capture, but may not precede, beta cell failure.

Long-lasting effect of obesity on skeletal muscle transcriptome

BACKGROUND

Reduced physical activity and increased intake of calorically-dense diets are the main risk factors for obesity, glucose intolerance, and type 2 diabetes. Chronic overnutrition and hyperglycemia can alter gene expression, contributing to long-term obesity complications. While caloric restriction can reduce obesity and glucose intolerance, it is currently unknown whether it can effectively reprogram transcriptome to a pre-obesity level. The present study addressed this question by the preliminary examination of the transcriptional dynamics in skeletal muscle after exposure to overnutrition and following caloric restriction.

RESULTS

Six male rhesus macaques of 12-13 years of age consumed a high-fat western-style diet for 6 months and then were calorically restricted for 4 months without exercise. Skeletal muscle biopsies were subjected to longitudinal gene expression analysis using next-generation whole-genome RNA sequencing. In spite of significant weight loss and normalized insulin sensitivity, the majority of WSD-induced (n = 457) and WSD-suppressed (n = 47) genes remained significantly dysregulated after caloric restriction (FDR ≤0.05). The Metacore^TM pathway analysis reveals that western-style diet induced the sustained activation of the transforming growth factor-β gene network, associated with extracellular matrix remodeling, and the downregulation of genes involved in muscle structure development and nutritional processes.

CONCLUSIONS

Western-style diet, in the absence of exercise, induced skeletal muscle transcriptional programing, which persisted even after insulin resistance and glucose intolerance were completely reversed with caloric restriction.

Black ginseng extract exerts anti-hyperglycemic effect via modulation of glucose metabolism in liver and muscle

ETHNOPHARMACOLOGICAL RELEVANCE

Ginseng (Panax ginseng C. A. Meyer, Araliaceae) has been used as a traditional medicine for thousands of years for the treatment of a wide variety of diseases, including diabetes. Processed ginseng named Black ginseng exhibits more potent biological activities than white and red ginseng. The aim of this study was to investigate the effects of black ginseng extract (GBG05-FF) on hyperglycemia and glucose tolerance in streptozotocin (STZ)-induced diabetic mice.

MATERIALS AND METHODS

Black ginseng was produced by a repeated steaming and drying process, subsequent extraction with 70% ethanol, filtration, and lyophilization. The effect of GBG05-FF on glucose uptake and related protein expression and phosphorylation were determined in C2C12 cells. Furthermore, we evaluated the anti-diabetic effects of GBG05-FF in STZ-induced diabetic mice.

RESULTS

GBG05-FF significantly (p<0.05) increased glucose uptake in C2C12 myotubes via AMPK, Sirt1 and PI3-K pathway. In addition, GBG05-FF improved the fasting blood glucose levels and glucose tolerance in STZ-induced diabetic mice. GBG05-FF decreased blood parameters such as glycated hemoglobin, triglyceride and total cholesterol. Quantitative RT-PCR assay revealed that in the STZ-induced diabetic mice treated with GBG05-FF, the expression of hepatic genes involved in gluconeogenesis (phosphoenolpyruvate carboxykinase (PEPCK), glucose 6-phosphatase (G6Pase)), glycogenolysis (liver glycogen phosphorylase (LGP)) and glycogenesis (glycogen synthase (GS)) was suppressed, while the expression of the genes involved in glucose uptake (glucose transporter (GLUT) 1, GLUT4) and β-oxidation (acyl-CoA oxidase (ACO), carnitine palmitoyl transferase 1a (CPT1a), mitochondrial medium chain acyl-CoA dehydrogenase (MCAD)) in muscle were increased. GBG05-FF delayed diabetes-associated muscle atrophy by activating mTOR. The major bioactive compounds including ginsenoside Rg1, Rg3(S), Rg3(R), Rg5, Rk1 and Rh4 were evaluated for glucose uptake effect in C2C12 myotubes; the data indicated that Rh4 significantly (p<0.05) increased glucose uptake.

CONCLUSION

Collectively, the results suggested that GBG05-FF is a potentially useful agent for treatment of diabetes by increasing glucose uptake.