Expression of naturally occurring variants in the muscle glycogen synthase gene

[Hyperbaric oxygen therapy of angiopathic changes in patients with inherited gene imbalance]

INTRODUCTION

Phenotype match inherited by genes is in most cases present in monozygotic twins. Their phenotypic resemblance is unfortunately characterized by strong susceptibility for the development of chronic non-infectious diseases. One of the most common non-infectious chronic diseases that are phenotipically represented in twins is diabetes mellitus. Genetic imbalance is, in most cases, placed in 2, 3, 7, 8, 11, 12, 19 and 20 chromosomal pair of the human genome.

CASE OUTLINE

This study describes a pair of monozygotic twins, aged 54, who were diagnosed for diabetes type 2 ten years earlier. The first patient had trophic changes of muscles and skin tissues of the lower limb, and a necrotic wound on his right leg tibial region with the claudication distance of 50 m. After arteriography, he was referred by a vascular surgeon for hyperbaric oxygen therapy (HBO). HBO protocol implied 70 min. application of 100% oxygen at 2.5 absolute atmospheres. After the first series of HBO therapies consisting of 20 HBO treatments, claudication was eliminated and the necrotic wound healed. Next, surgical aortofemoral bypass was done. During the second HBO treatment, his monozygotic twin brother presented with angiopathic changes due to diabetes. In both patients, biochemical parameters corresponded to the expected level for diabetes type 2 imbalance, and the localization of the chromosomal defect (placed on 3, 11 and 19 chromosomal pair) was also in accordance with the respective disorder. After they were included into next 10 HBO treatments, Doppler imaging of the major arteries of limbs revealed normal findings.

CONCLUSION

Identical genetic impairment in monozygotic twins can lead to identical somatic changes with resultant consequences. HBO treatment of such patients associated with other therapeutic procedures (conducted by diabetologist, vascular surgeon and physiatrist) can postpone or prevent irreversible changes occurring due to blood vessel disorders.

Gene expression profiling of mice with genetically modified muscle glycogen content

Glycogen, a branched polymer of glucose, forms an energy re-serve in numerous organisms. In mammals, the two largest glyco-gen stores are in skeletal muscle and liver, which express tissue-specific glycogen synthase isoforms. MGSKO mice, in which mGys1 (mouse glycogen synthase) is disrupted, are devoid of muscle glycogen [Pederson, Chen, Schroeder, Shou, DePaoli-Roach and Roach (2004) Mol. Cell. Biol. 24, 7179-7187]. The GSL30 mouse line hyper-accumulates glycogen in muscle [Manchester, Skurat, Roach, Hauschka and Lawrence (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 10707-10711]. We performed a microarray analysis of mRNA from the anterior tibialis, medial gastrocnemius and liver of MGSKO mice, and from the gastroc-nemius of GSL30 mice. In MGSKO mice, transcripts of 79 genes varied in their expression in the same direction in both the anterior tibialis and gastrocnemius. These included several genes encoding proteins proximally involved in glycogen metabolism. The Ppp1r1a [protein phosphatase 1 regulatory (inhibitor) sub-unit 1A] gene underwent the greatest amount of downregulation. In muscle, the downregulation of Pfkfb1 and Pfkfb3, encoding isoforms of 6-phosphofructo-2-kinase/fructose-2,6-bisphospha-tase, is consistent with decreased glycolysis. Pathways for branched-chain amino acid, and ketone body utilization appear to be downregulated, as is the capacity to form the gluconeogenic precursors alanine, lactate and glutamine. Expression changes among several members of the Wnt signalling pathway were identified, suggesting an as yet unexplained role in glycogen meta-bolism. In liver, the upregulation of Pfkfb1 and Pfkfb3 expression is consistent with increased glycolysis, perhaps as an adaptation to altered muscle metabolism. By comparing changes in muscle expression between MGSKO and GSL30 mice, we found a subset of 44 genes, the expression of which varied as a function of muscle glycogen content. These genes are candidates for regulation by glycogen levels. Particularly interesting is the observation that 11 of these genes encode cardiac or slow-twitch isoforms of muscle contractile proteins, and are upregulated in muscle that has a greater oxidative capacity in MGSKO mice.

Glucose metabolism in mice lacking muscle glycogen synthase

Glycogen is an important component of whole-body glucose metabolism. MGSKO mice lack skeletal muscle glycogen due to disruption of the GYS1 gene, which encodes muscle glycogen synthase. MGSKO mice were 5-10% smaller than wild-type littermates with less body fat. They have more oxidative muscle fibers and, based on the activation state of AMP-activated protein kinase, more capacity to oxidize fatty acids. Blood glucose in fed and fasted MGSKO mice was comparable to wild-type littermates. Serum insulin was lower in fed but not in fasted MGSKO animals. In a glucose tolerance test, MGSKO mice disposed of glucose more effectively than wild-type animals and had a more sustained elevation of serum insulin. This result was not explained by increased conversion to serum lactate or by enhanced storage of glucose in the liver. However, glucose infusion rate in a euglycemic-hyperinsulinemic clamp was normal in MGSKO mice despite diminished muscle glucose uptake. During the clamp, MGSKO animals accumulated significantly higher levels of liver glycogen as compared with wild-type littermates. Although disruption of the GYS1 gene negatively affects muscle glucose uptake, overall glucose tolerance is actually improved, possibly because of a role for GYS1 in tissues other than muscle.

Genetics of type 2 diabetes mellitus: status and perspectives

Throughout the last decade, molecular genetic studies of non-autoimmune diabetes mellitus have contributed significantly to our present understanding of this disease's complex aetiopathogenesis. Monogenic forms of diabetes (maturity-onset diabetes of the young, MODY) have been identified and classified into MODY1-6 according to the mutated genes that by being expressed in the pancreatic beta-cells confirm at the molecular level the clinical presentation of MODY as a predominantly insulin secretory deficient form of diabetes mellitus. Genomewide linkage studies of presumed polygenic type 2 diabetic populations indicate that loci on chromosomes 1q, 5q, 8p, 10q, 12q and 20q contain susceptibility genes. Yet, so far, the only susceptibility gene, calpain-10 (CAPN10), which has been identified using genomewide linkage studies, is located on chromosome 2q37. Mutation analyses of selected 'candidate' susceptibility genes in various populations have also identified the widespread Pro12Ala variant of the peroxisome proliferator-activated receptor-gamma and the common Glu23Lys variant of the ATP-sensitive potassium channel, Kir6.2 (KCNJ11). These variants may contribute significantly to the risk type 2 diabetes conferring insulin resistance of liver, muscle and fat (Pro12Ala) and a relative insulin secretory deficiency (Glu23Lys). It is likely that, in the near future, the recent more detailed knowledge of the human genome and insights into its haploblocks together with the developments of high-throughput and cheap genotyping will facilitate the discovery of many more type 2 diabetes gene variants in study materials, which are statistically powered and phenotypically well characterized. The results of these efforts are likely to be the platform for major progress in the development of personalized antidiabetic drugs with higher efficacy and few side effects.

Characterization of the human skeletal muscle glycogen synthase gene (GYS1) promoter

BACKGROUND

Impaired activation of the human skeletal muscle glycogen synthase by insulin is typical for type 2 diabetic patients. Regulation of glycogen synthase occurs mainly by phosphorylation/dephoshorylation but little is known whether there also is transcriptional regulation. Therefore we studied transcriptional regulation of the human skeletal muscle glycogen synthase gene (GYS1) and evaluated the effects of insulin and forskolin on the promoter activity.

METHODS

Seven promoter fragments were expressed in C2C12 myoblasts and myotubes and in HEK293 cells, and the luciferase assay was used to determine transcriptional activity.

RESULTS

The highest luciferase activity, 350-fold of the promoterless vector, was obtained with nucleotides -692 to +59 in myotubes (P < 0.001), while the nucleotides -250 to +59 provided the highest, 45-fold, activity in the HEK293 cells (P < 0.001). Longer promoter constructs (nucleotides -971, -1707 and -2158 to +59, respectively) had low promoter activity in both cell types. Forskolin treatment for 24 h resulted in approximately 30% decreased promoter activity in myotubes (P < 0.05). Insulin treatment for 0.5-3 h did not increase GYS1 promoter activity; instead the activity was slightly but significantly decreased after 24 h in myotubes (P < 0.005).

CONCLUSIONS

From our results we conclude that basal GYS1 promoter activity is obtained from the first 250 nucleotides of the promoter, while the nucleotides -692 to -544 seem to be responsible for muscle-specific expression, and nucleotides -971 to -692 for negative regulation. In myotubes, the GYS1 promoter was sensitive to negative regulation by forskolin, whereas insulin did not increase GYS1 transcription.

Association of muscle glycogen synthase polymorphism with insulin resistance in type 2 diabetic patients

The aim of the present study is to investigate whether Met416Val (M416V) polymorphism of glycogen synthase (GYS1) gene is associated with insulin resistance in type 2 diabetes. In 100 type 2 diabetic subjects (66 men and 34 women), the M416V polymorphism of GYS1 gene was analyzed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) as previously reported, and insulin resistance was assessed by euglycemic hyperinsulinemic clamp represented as M/I value, the mean of glucose infusion rate (M value) adjusted by steady state plasma insulin level. The means of age and body mass index (BMI) of the subjects were 53.1+/-11.6 (SD) years and 23.3+/-3.5 kg/m2. The allele frequencies of M416V polymorphism were 82.0% for MM, 16.0% for MV, and 2.0% for VV, and subjects were subsequently divided into V(+) group (n=18) and V(-) group (n=82) according to the presence or absence of V allele. There were no significant differences in age, BMI, blood pressure, fasting plasma glucose or insulin levels or glycosylated hemoglobin (HbA1c) levels between the V(+) and V(-) groups. No significant differences in either M or M/I value were found between the V(+) and V(-) groups (M value, 5.06+/-2.20 v 5.12+/-2.04 mg x kg(-1) x min(-1), P=.841; M/I value, 5.24+/-3.07 v 5.39+/-2.87 mg x kg(-1) x min(-1) x mU(-1) x L, P=.576). BMI showed the strongest independent contribution to M/I value, but the presence of V allele did not in multiple regression analysis. In conclusion, the M416V polymorphism of GYS1 gene is not associated with insulin resistance in type 2 diabetes.

Fat feeding impairs glycogen synthase activity in mice without effects on its gene expression

To examine whether the effects of high-fat feeding on glycogen synthase (GS) activity and mRNA levels differ between diabetes-prone (C57BL/6J) and diabetes-resistant mice (NMRI), we measured GS activity and mRNA levels in muscle from C57BL/6J and NMRI mice fed a high-fat or normal chow diet for 3, 6, and 15 months. As compared with chow feeding, fat feeding increased plasma insulin levels in C57BL/6J mice at 15 months (464 +/- 29 v 267 +/- 47 pmol/L, P =.005), which was associated with elevated plasma glucose levels at 15 months (5.3 +/- 0.3 v 3.8 +/- 0.2 mmol/L, P =.001). Fat feeding increased plasma insulin levels also in NMRI mice at 15 months (705 +/- 145 v 275 +/- 64 pmol/L, P =.01) without, however, a rise of plasma glucose levels. In parallel with increased insulin levels, decreased muscle GS fractional velocity (FV) was observed at 6 (49.0% +/- 2.6% v 69.1% +/- 7.3%, P =.04) and 15 (45.8% +/- 1.8% v 53.4% +/- 1.6 %, P <.01) months but not at 3 months in the fat-fed C57BL/6J mice. Similarly, there was a significant decrease in GS fractional activity at 3 (57.9% +/- 4.3% v 70.4% +/- 2.6 %, P <.03) and 15 (47.3% +/- 2.4% v 56.4% +/- 2.1%, P =.02) but not at 6 months in the fat-fed NMRI mice. The decrease in GS activity was not associated with changes in mRNA levels at any time points. We conclude that (1) fat feeding results in similar elevation of plasma insulin levels and impairs GS activity in C57BL/6J and NMRI mice, and (2) the changes in GS activity do not involve effects on gene expression.

The stimulation-induced increase in skeletal muscle glycogen synthase content is impaired in carriers of the glycogen synthase XbaI gene polymorphism

Associations between glycogen synthase gene (GYS1) polymorphism and states of insulin resistance and type 2 diabetes have been reported. The purpose of this study was to establish if the GYS1 genotype impacts on the content of glycogen synthase (GS) protein in muscle measured under basal and stimulated conditions. To examine this, GYS1 XbaI and Met416Val polymorphisms and thigh muscle GYS1 protein content were determined at rest, both before and after several weeks of neuromuscular electrical stimulation in carriers and noncarriers of the mutations. The allelic frequency was 0.086 for the XbaI mutation (A2) and 0.006 for the Met416Val in our cohort of French-Canadian subjects. When measured at rest, the GS protein content in muscle was similar among carriers and noncarriers of the XbaI variant. However, the stimulation-induced increase (23%) in the amount of GS muscle protein normally seen in wildtype individuals was impaired in those carrying the XbaI mutation. These data demonstrate that some individuals, because of their genetic background, are unable to stimulate the process of GS protein accumulation in skeletal muscle. These results could explain why some individuals appear to be genetically predisposed to developing skeletal muscle insulin resistance when exposed to unfavorable metabolic environments.

Impaired insulin-stimulated expression of the glycogen synthase gene in skeletal muscle of type 2 diabetic patients is acquired rather than inherited

To examine whether defective muscle glycogen synthase (GYS1) expression is associated with impaired glycogen synthesis in type 2 diabetes and whether the defect is inherited or acquired, we measured GYS1 gene expression and enzyme activity in muscle biopsies taken before and after an insulin clamp in 12 monozygotic twin pairs discordant for type 2 diabetes and in 12 matched control subjects. The effect of insulin on GYS1 fractional activity, when expressed as the increment over the basal values, was significantly impaired in diabetic (15.7 +/- 3.3%; P < 0.01), but not in nondiabetic (23.7 +/- 1.8%; P = NS) twins compared with that in control subjects (28.1 +/- 2.3%). Insulin increased GYS1 messenger ribonucleic acid (mRNA) expression in control subjects (from 0.14 +/- 0.02 to 1.74 +/- 0.10 relative units; P < 0.01) and in nondiabetic (from 0.24 +/- 0.05 to 1.81 +/- 0.16 relative units; P < 0.01) and diabetic (from 0.20 +/- 0.07 to 1.08 + 0.14 relative units; P < 0.01) twins. The effect of insulin on GYS1 expression was, however, significantly reduced in the diabetic (P < 0.003), but not in the nondiabetic, twins compared with that in control subjects. The postclamp GYS1 mRNA levels correlated strongly with the hemoglobin A1c levels (r = -0.61; P < 0.001). Despite the decrease in postclamp GYS1 mRNA levels, the GYS1 protein levels were not decreased in the diabetic twins compared with those in the control subjects (2.10 +/- 0.46 vs. 2.10 +/- 0.34 relative units; P = NS). We conclude that 1) insulin stimulates GYS1 mRNA expression; and 2) impaired stimulation of GYS1 gene expression by insulin in patients with type 2 diabetes is acquired and most likely is secondary to chronic hyperglycemia.