The skeletal muscle-specific glycogen-targeted protein phosphatase 1 plays a major role in the regulation of glycogen metabolism by adrenaline in vivo

Glycogen synthase activity in Candida albicans is partly controlled by the functional ortholog of Saccharomyces cerevisiae Gac1p

To adapt to various host microenvironments, the human fungal pathogen Candida albicans possesses the capacity to accumulate and store glycogen as an internal carbohydrate source. In the model yeast Saccharomyces cerevisiae, ScGlc7p and ScGac1p are the serine/threonine type 1 protein phosphatase catalytic and regulatory subunits that control glycogen synthesis by altering the phosphorylation state of the glycogen synthase Gsy2p. Despite recent delineation of the glycogen synthesis pathway in C. albicans, the molecular events driving synthase activation are currently undefined. In this study, using a combination of microbiologic and genetic techniques, we determined that the protein encoded by uncharacterized gene C1_01140C, and not the currently annotated C. albicans Gac1p, is the major regulatory subunit involved in glycogen synthesis. C1_01140Cp contains a conserved GVNK motif observed across multiple starch/glycogen-binding proteins in various species, and alanine substitution of each residue in this motif significantly impaired glycogen accumulation in C. albicans. Fluorescent protein tagging and microscopy indicated that C1_01140Cp-GFPy colocalized with CaGlc7p-tdTomato and CaGsy1p-tdTomato accordingly. Co-immunoprecipitation assays further confirmed that C1_01140Cp associates with CaGlc7p and CaGsy1p during glycogen synthesis. Lastly, c1_01140cΔ/Δ exhibited colonization defects in a murine model of vulvovaginal candidiasis. Collectively, our data indicate that uncharacterized C1_01140Cp is the functional ortholog of the PPP1R subunit ScGac1p in C. albicans.IMPORTANCEThe capacity to synthesize glycogen offers microbes metabolic flexibility, including the fungal pathogen Candida albicans. In Saccharomyces cerevisiae, dephosphorylation of glycogen synthase by the ScGlc7p-containing phosphatase is a critical rate-limiting step in glycogen synthesis. Subunits, including ScGac1p, target ScGlc7p to α-1,4-glucosyl primers for efficient ScGsy2p synthase activation. However, this process in C. albicans had not been delineated. Here, we show that the C. albicans genome encodes for two homologous phosphatase-binding subunits, annotated CaGac1p and uncharacterized C1_01140Cp, both containing a GVNK motif required for polysaccharide affinity. Surprisingly, loss of CaGac1p only moderately reduced glycogen accumulation, whereas loss of C1_01140Cp ablated it. Fluorescence microscopy and co-immunoprecipitation approaches revealed that C1_01140Cp associates with CaGlc7p and CaGsy1p during glycogen synthesis. Moreover, C1_01140Cp contributed to fungal fitness at the vaginal mucosa during murine vaginitis. Therefore, this work demonstrates that glycogen synthase regulation is conserved in C. albicans and C1_01140Cp is the functional ortholog of ScGac1p.

Biomarkers and Target-Specific Small-Molecule Drugs in Alzheimer's Diagnostic and Therapeutic Research: From Amyloidosis to Tauopathy

Alzheimer's disease (AD) is the most common type of human dementia and is responsible for over 60% of diagnosed dementia cases worldwide. Abnormal deposition of β-amyloid and the accumulation of neurofibrillary tangles have been recognised as the two pathological hallmarks targeted by AD diagnostic imaging as well as therapeutics. With the progression of pathological studies, the two hallmarks and their related pathways have remained the focus of researchers who seek for AD diagnostic and therapeutic strategies in the past decades. In this work, we reviewed the development of the AD biomarkers and their corresponding target-specific small molecule drugs for both diagnostic and therapeutic applications, underlining their success, failure, and future possibilities.

Variability in porcine microRNA genes and its association with mRNA expression and lipid phenotypes

BACKGROUND

Mature microRNAs (miRNAs) play an important role in repressing the expression of a wide range of mRNAs. The presence of polymorphic sites in miRNA genes and their corresponding 3'UTR binding sites can disrupt canonical conserved miRNA-mRNA pairings, and thus modify gene expression patterns. However, to date such polymorphic sites in miRNA genes and their association with gene expression phenotypes and complex traits are poorly characterized in pigs.

RESULTS

By analyzing whole-genome sequences from 120 pigs and wild boars from Europe and Asia, we identified 285 single nucleotide polymorphisms (SNPs) that map to miRNA loci, and 109,724 SNPs that are located in predicted 7mer-m8 miRNA binding sites within porcine 3'UTR. In porcine miRNA genes, SNP density is reduced compared with their flanking non-miRNA regions. By sequencing the genomes of five Duroc boars, we identified 12 miRNA SNPs that were subsequently genotyped in their offspring (N = 345, Lipgen population). Association analyses of miRNA SNPs with 38 lipid-related traits and hepatic and muscle microarray expression phenotypes recorded in the Lipgen population were performed. The most relevant detected association was between the genotype of the rs319154814 (G/A) SNP located in the apical loop of the ssc-miR-326 hairpin precursor and PPP1CC mRNA levels in the liver (q-value = 0.058). This result was subsequently confirmed by qPCR (P-value = 0.027). The rs319154814 (G/A) genotype was also associated with several fatty acid composition traits.

CONCLUSIONS

Our findings show a reduced variability of porcine miRNA genes, which is consistent with strong purifying selection, particularly in the seed region that plays a critical role in miRNA binding. Although it is generally assumed that SNPs mapping to the seed region are those with the most pronounced consequences on mRNA expression, we show that a SNP mapping to the apical region of ssc-miR-326 is significantly associated with hepatic mRNA levels of the PPP1CC gene, one of its predicted targets. Although experimental confirmation of such an interaction is reported in humans but not in pigs, this result highlights the need to further investigate the functional effects of miRNA polymorphisms that are located outside the seed region on gene expression in pigs.

Dok3-protein phosphatase 1 interaction attenuates Card9 signaling and neutrophil-dependent antifungal immunity

Invasive fungal infection is a serious health threat with high morbidity and mortality. Current antifungal drugs only demonstrate partial success in improving prognosis. Furthermore, mechanisms regulating host defense against fungal pathogens remain elusive. Here, we report that the downstream of kinase 3 (Dok3) adaptor negatively regulates antifungal immunity in neutrophils. Our data revealed that Dok3 deficiency increased phagocytosis, proinflammatory cytokine production, and netosis in neutrophils, thereby enhancing mutant mouse survival against systemic infection with a lethal dose of the pathogenic fungus Candida albicans. Biochemically, Dok3 recruited protein phosphatase 1 (PP1) to dephosphorylate Card9, an essential player in innate antifungal defense, to dampen downstream NF-κB and JNK activation and immune responses. Thus, Dok3 suppresses Card9 signaling, and disrupting Dok3-Card9 interaction or inhibiting PP1 activity represents therapeutic opportunities to develop drugs to combat candidaemia.

Functions and therapeutic potential of protein phosphatase 1: Insights from mouse genetics

Protein phosphatase 1 (PP1) catalyzes more than half of all phosphoserine/threonine dephosphorylation reactions in mammalian cells. In vivo PP1 does not exist as a free catalytic subunit but is always associated with at least one regulatory PP1-interacting protein (PIP) to generate a large set of distinct holoenzymes. Each PP1 complex controls the dephosphorylation of only a small subset of PP1 substrates. We screened the literature for genetically engineered mouse models and identified models for all PP1 isoforms and 104 PIPs. PP1 itself and at least 49 PIPs were connected to human disease-associated phenotypes. Additionally, phenotypes related to 17 PIPs were clearly linked to altered PP1 function, while such information was lacking for 32 other PIPs. We propose structural reverse genetics, which combines structural characterization of proteins with mouse genetics, to identify new PP1-related therapeutic targets. The available mouse models confirm the pleiotropic action of PP1 in health and diseases.

In vivo effects of diabetes, insulin and oleanolic acid on enzymes of glycogen metabolism in the skin of streptozotocin-induced diabetic male Sprague-Dawley rats

The skin is the largest organ in the body and diabetes induces pathologic changes on the skin that affect glucose homeostasis. Changes in skin glycogen and glucose levels can mirror serum glucose levels and thus the skin might contribute to whole body glucose metabolism. This study investigated the in vivo effects of diabetes, insulin and oleanolic acid (OA) on enzymes of glycogen metabolism in skin of type 1 diabetic rats. Diabetic and non-diabetic adult male Sprague-Dawley rats were treated with a single daily dose of insulin (4 IU/kg body weight), OA (80 mg/kg body weight) and a combination of OA + insulin for 14 days. Glycogen phosphorylase (GP) expression; and GP, glycogen synthase (GS) and hexokinase activities as well glycogen levels were evaluated. The results suggest that diabetes lowers hexokinase activity, GP activity and GP expression with no change in GS activity whilst the treatments increased GP expression and the activities of hexokinase, GP and GS except for the GS activity in OA treated rats. Glycogen levels were increased slightly by diabetes as well as OA treatment. In conclusion diabetes, OA and insulin can lead to changes in GS and GP activities in skin without significantly altering the glycogen content. We suggest that the skin may contribute to whole body glucose homeostasis particularly in disease states.

Effects of oleanolic acid on the insulin signaling pathway in skeletal muscle of streptozotocin-induced diabetic male Sprague-Dawley rats

BACKGROUND

The pant-derived triterpene oleanolic acid (OA) has been shown to have antidiabetic effects, but its action on the insulin signaling cascade has not been fully elucidated. The aim of the present study was to investigate the effects of OA on aspects of the phosphatidylinositol 3-kinase/Akt insulin signaling cascade in skeletal muscle of streptozotocin-induced type 1 diabetic male Sprague-Dawley rats.

METHODS

Diabetic and non-diabetic rats were treated with insulin (4 IU/kg), OA (80 mg/kg), and the combination of OA + insulin in acute 60-min and sub-chronic 14-day studies. Single and daily doses were administered in the acute and sub-chronic studies, respectively. In acute studies, phosphorylated (p-) Akt and p-glycogen synthase (GS) expression was evaluated. In sub-chronic studies, GS and glycogen phosphorylase (GP) expression and activity were evaluated, as were glycogen levels.

RESULTS

The findings show that OA enhances insulin-stimulated hypoglycemic effects in diabetic rats. In the acute study, OA increased levels of p-Akt and decreased levels of p-GS. In the sub-chronic study, OA increased both GS and GP activity, whereas OA + insulin increased GS and decreased GP activity. Treatment of rats with OA and OA + insulin increased GS expression in the skeletal muscle of diabetic rats and decreased GP expression. Glycogen levels were increased by OA but decreased OA + insulin treatment.

CONCLUSION

Oleanolic acid in synergy with insulin can enhance activation of the insulin signaling pathway. Furthermore, the present study provides evidence of OA activation of insulin signaling enzymes independent of insulin.

Epinephrine-stimulated glycogen breakdown activates glycogen synthase and increases insulin-stimulated glucose uptake in epitrochlearis muscles

Epinephrine increases glycogen synthase (GS) phosphorylation and decreases GS activity but also stimulates glycogen breakdown, and low glycogen content normally activates GS. To test the hypothesis that glycogen content directly regulates GS phosphorylation, glycogen breakdown was stimulated in condition with decreased GS activation. Saline or epinephrine (0.02 mg/100 g rat) was injected subcutaneously in Wistar rats (∼130 g) with low (24-h-fasted), normal (normal diet), and high glycogen content (fasted-refed), and epitrochlearis muscles were removed after 3 h and incubated ex vivo, eliminating epinephrine action. Epinephrine injection reduced glycogen content in epitrochlearis muscles with high (120.7 ± 17.8 vs. 204.6 ± 14.5 mmol/kg, P < 0.01) and normal glycogen (89.5 ± 7.6 vs. 152 ± 8.1 mmol/kg, P < 0.01), but not significantly in muscles with low glycogen (90.0 ± 5.0 vs. 102.8 ± 7.8 mmol/kg, P = 0.17). In saline-injected rats, GS phosphorylation at sites 2+2a, 3a+3b, and 1b was higher and GS activity lower in muscles with high compared with low glycogen. GS sites 2+2a and 3a+3b phosphorylation decreased and GS activity increased in muscles where epinephrine decreased glycogen content; these parameters were unchanged in epitrochlearis from fasted rats where epinephrine injection did not decrease glycogen content. Incubation with insulin decreased GS site 3a+3b phosphorylation independently of glycogen content. Insulin-stimulated glucose uptake was increased in muscles where epinephrine injection decreased glycogen content. In conclusion, epinephrine stimulates glycogenolysis in epitrochlearis muscles with normal and high, but not low, glycogen content. Epinephrine-stimulated glycogenolysis decreased GS phosphorylation and increased GS activity. These data for the first time document direct regulation of GS phosphorylation by glycogen content.

AMP-activated protein kinase (AMPK)α2 plays a role in determining the cellular fate of glucose in insulin-resistant mouse skeletal muscle

AIMS/HYPOTHESIS

We determined whether: (1) an acute lipid infusion impairs skeletal muscle AMP-activated protein kinase (AMPK)α2 activity, increases inducible nitric oxide synthase (iNOS) and causes peripheral insulin resistance in conscious, unstressed, lean mice; and (2) restoration of AMPKα2 activity during the lipid infusion attenuates the increase in iNOS and reverses the defect in insulin sensitivity in vivo.

METHODS

Chow-fed, 18-week-old C57BL/6J male mice were surgically catheterised. After 5 days they received: (1) a 5 h infusion of 5 ml kg(-1) h(-1) Intralipid + 6 U/h heparin (Lipid treatment) or saline (Control); (2) Lipid treatment or Control, followed by a 2 h hyperinsulinaemic-euglycaemic clamp (insulin clamp; 4 mU kg(-1) min(-1)); and (3) infusion of the AMPK activator, 5-aminoimidazole-4-carboxamide 1-β-D-ribofuranoside (AICAR) (1 mg kg(-1) min(-1)), or saline during Lipid treatment, followed by a 2 h insulin clamp. In a separate protocol, mice producing a muscle-specific kinase-dead AMPKα2 subunit (α2-KD) underwent an insulin clamp to determine the role of AMPKα2 in insulin-mediated muscle glucose metabolism.

RESULTS

Lipid treatment decreased AMPKα2 activity, increased iNOS abundance/activation and reduced whole-body insulin sensitivity in vivo. AICAR increased AMPKα2 activity twofold; this did not suppress iNOS or improve whole-body or tissue-specific rates of glucose uptake during Lipid treatment. AICAR caused a marked increase in insulin-mediated glycogen synthesis in skeletal muscle. Consistent with this latter result, lean α2-KD mice exhibited impaired insulin-stimulated glycogen synthesis even though muscle glucose uptake was not affected.

CONCLUSIONS/INTERPRETATION

Acute induction of insulin resistance via lipid infusion in healthy mice impairs AMPKα2, increases iNOS and causes insulin resistance in vivo. However, these changes do not appear to be interrelated. Rather, a functionally active AMPKα2 subunit is required for insulin-stimulated muscle glycogen synthesis.

Effects of adrenaline on whole-body glucose metabolism and insulin-mediated regulation of glycogen synthase and PKB phosphorylation in human skeletal muscle

In the present study, we investigated the effect of adrenaline on insulin-mediated regulation of glucose and fat metabolism with focus on regulation of skeletal muscle PKB, GSK-3, and glycogen synthase (GS) phosphorylation. Ten healthy subjects (5 men and 5 women) received a 240-minute intravenous infusion of adrenaline (0.05 μg/[kg min]) or saline; after 120 minutes, a hyperinsulinemic-euglycemic clamp was added. Adrenaline infusion increased blood glucose concentration by approximately 50%, but the hyperinsulinemic clamp normalized blood glucose within 30 minutes. Glucose infusion rate during the last hour was approximately 60% lower during adrenaline infusion compared with saline (4.3 ± 0.5 vs 11.2 ± 0.6 mg/kg lean body mass per minute). Insulin increased PKB Ser⁴⁷³, PKB Thr³⁰⁸, and GSK-3β Ser⁹ phosphorylation in skeletal muscles; coinfusion of adrenaline did not influence insulin-stimulated PKB and GSK-3 phosphorylation. Adrenaline alone did not influence phosphorylation of PKB and GSK-3β. Insulin increased GS fractional activity and decreased GS Ser⁶⁴¹ and Ser⁶⁴⁵,⁶⁴⁹,⁶⁵³,⁶⁵⁷ phosphorylation. In the presence of adrenaline, insulin did neither activate GS nor dephosphorylate GS Ser⁶⁴¹. Surprisingly, GS Ser⁷ phosphorylation was not influenced by adrenaline. Adrenaline increased plasma lactate concentration; and muscle glycogen content was reduced in skeletal muscle the day after adrenaline infusion, supporting that insulin does not stimulate glycogen synthesis in skeletal muscles when adrenaline is present. In conclusion, adrenaline did not influence basal or insulin-stimulated PKB and GSK-3β phosphorylation in muscles, but completely blocked insulin-mediated GS activation and Ser⁶⁴¹ dephosphorylation. Still, insulin normalized adrenaline-mediated hyperglycemia.

A limited role for PI(3,4,5)P3 regulation in controlling skeletal muscle mass in response to resistance exercise

Background

Since activation of the PI3K/(protein kinase B; PKB/akt) pathway has been shown to alter muscle mass and growth, the aim of this study was to determine whether resistance exercise increased insulin like growth factor (IGF) I/phosphoinositide 3-kinase (PI3K) signalling and whether altering PI(3,4,5)P₃ metabolism genetically would increase load induced muscle growth.

Methodology/Principal Findings

Acute and chronic resistance exercise in wild type and muscle specific PTEN knockout mice were used to address the role of PI(3,4,5)P₃ regulation in the development of skeletal muscle hypertrophy. Acute resistance exercise did not increase either IGF-1 receptor phosphorylation or IRS1/2 associated p85. Since insulin/IGF signalling to PI3K was unchanged, we next sought to determine whether inactivation of PTEN played a role in load-induced muscle growth. Muscle specific knockout of PTEN resulted in small but significant increases in heart (PTEN^+/+  = 5.00±0.02 mg/g, PTEN^−/−  = 5.50±0.09 mg/g), and TA (PTEN^+/+  = 1.74±0.04 mg/g, PTEN^−/−  = 1.89 ±0.03) muscle mass, while the GTN, SOL, EDL and PLN remain unchanged. Following ablation, hypertrophy of the PLN, SOL or EDL muscles was similar between PTEN^−/− and PTEN^+/+ animals. Even though there were some changes in overload-induced PKB and S6K1 phosphorylation, 1 hr following acute resistance exercise there was no difference in the phosphorylation state of S6K1 Thr389 between genotypes.

Conclusions/Significance

These data suggest that physiological loading does not lead to the enhanced activation of the PI3K/PKB/mTORC1 axis and that neither PI3K activation nor PTEN, and by extension PI(3,4,5)P₃ levels, play a significant role in adult skeletal muscle growth.

The alpha,alpha-difluorinated phosphonate L-pSer-analogue: an accessible chemical tool for studying kinase-dependent signal transduction

This overview focuses on the (alpha,alpha-difluoromethylene)phosphonate mimic of phosphoserine (pCF(2)Ser) and its application to the study of kinase-mediated signal transduction-pathways of great interest to drug development. The most versatile modes of access to these chemical biological tools are discussed, organized by method of PCF(2)-C bond formation. The pCF(2)-Ser mimic may be site-specifically incorporated into peptides (SPPS) and proteins (expressed protein ligation). This isopolar, dianionic pSer mimic results in a "constitutive phosphorylation" phenotype and is seen to support native protein-protein interactions that depend on serine phosphorylation. Signal transduction pathways studied with this chemical biological approach include the regulation of p53 tumor suppressor protein activity and of melatonin production. Given these successes, the future is bright for the use of such "teflon phospho-amino acid mimics" to map kinase-based signaling pathways.

Regulation of muscle glycogen synthase phosphorylation and kinetic properties by insulin, exercise, adrenaline and role in insulin resistance

In mammals, excess carbohydrate is stored as glycogen and glycogen synthase is the enzyme that incorporates glucose units into the glycogen particle. Glycogen synthase activity is regulated by phosphorylation and allosterically activated by glucose 6-phosphate. Phosphorylation of nine serines by different kinases regulates glycogen synthase affinity for glucose 6-phosphate and its substrate UDP-glucose. Glucose 6-phosphate increases both enzyme activity and substrate affinity. Insulin and exercise increase glycogen synthase affinity for glucose 6-phosphate and activity whereas high glycogen content and adrenaline decrease affinity for glucose 6-phosphate and activity. However, insulin, exercise and adrenaline also regulate intracellular concentration of glucose 6-phosphate which will influence in vivo glycogen synthase activity. Importantly, type 2 diabetes is associated with reduced insulin-stimulated glycogen synthase activation. The nine phosphorylation sites theoretically allow 512 combinations of phosphorylation configurations of glycogen synthase with different kinetic properties. However, due to hierarchal phosphorylation, the number of configurations in vivo is most likely much lower. Unfortunately, many studies only report data on glycogen synthase activity measured with high concentration of UDP-glucose which holds back information about changes in substrate affinity. In this paper we discuss the physiological regulation of glycogen synthase phosphorylation and how the phosphorylation pattern regulates glycogen synthase kinetic properties.

Insulin promotes glycogen synthesis in the absence of GSK3 phosphorylation in skeletal muscle

Insulin promotes dephosphorylation and activation of glycogen synthase (GS) by inactivating glycogen synthase kinase (GSK) 3 through phosphorylation. Insulin also promotes glucose uptake and glucose 6-phosphate (G-6-P) production, which allosterically activates GS. The relative importance of these two regulatory mechanisms in the activation of GS in vivo is unknown. The aim of this study was to investigate if dephosphorylation of GS mediated via GSK3 is required for normal glycogen synthesis in skeletal muscle with insulin. We employed GSK3 knockin mice in which wild-type GSK3 alpha and -beta genes are replaced with mutant forms (GSK3 alpha/beta S21A/S21A/S9A/S9A), which are nonresponsive to insulin. Although insulin failed to promote dephosphorylation and activation of GS in GSK3 alpha/beta S21A/S21A/S9A/S9A mice, glycogen content in different muscles from these mice was similar compared with wild-type mice. Basal and epinephrine-stimulated activity of muscle glycogen phosphorylase was comparable between wild-type and GSK3 knockin mice. Incubation of isolated soleus muscle in Krebs buffer containing 5.5 mM glucose in the presence or absence of insulin revealed that the levels of G-6-P, the rate of [14C]glucose incorporation into glycogen, and an increase in total glycogen content were similar between wild-type and GSK3 knockin mice. Injection of glucose containing 2-deoxy-[3H]glucose and [14C]glucose also resulted in similar rates of muscle glucose uptake and glycogen synthesis in vivo between wild-type and GSK3 knockin mice. These results suggest that insulin-mediated inhibition of GSK3 is not a rate-limiting step in muscle glycogen synthesis in mice. This suggests that allosteric regulation of GS by G-6-P may play a key role in insulin-stimulated muscle glycogen synthesis in vivo.

Glycogen content and contraction regulate glycogen synthase phosphorylation and affinity for UDP-glucose in rat skeletal muscles

Glycogen content and contraction strongly regulate glycogen synthase (GS) activity, and the aim of the present study was to explore their effects and interaction on GS phosphorylation and kinetic properties. Glycogen content in rat epitrochlearis muscles was manipulated in vivo. After manipulation, incubated muscles with normal glycogen [NG; 210.9 +/- 7.1 mmol/kg dry weight (dw)], low glycogen (LG; 108.1 +/- 4.5 mmol/ kg dw), and high glycogen (HG; 482.7 +/- 42.1 mmol/kg dw) were contracted or rested before the studies of GS kinetic properties and GS phosphorylation (using phospho-specific antibodies). LG decreased and HG increased GS K(m) for UDP-glucose (LG: 0.27 +/- 0.02 < NG: 0.71 +/- 0.06 < HG: 1.11 +/- 0.12 mM; P < 0.001). In addition, GS fractional activity inversely correlated with glycogen content (R = -0.70; P < 0.001; n = 44). Contraction decreased K(m) for UDP-glucose (LG: 0.14 +/- 0.01 = NG: 0.16 +/- 0.01 < HG: 0.33 +/- 0.03 mM; P < 0.001) and increased GS fractional activity, and these effects were observed independently of glycogen content. In rested muscles, GS Ser(641) and Ser(7) phosphorylation was decreased in LG and increased in HG compared with NG. GSK-3beta Ser(9) and AMPKalpha Thr(172) phosphorylation was not modulated by glycogen content in rested muscles. Contraction decreased phosphorylation of GS Ser(641) at all glycogen contents. However, contraction increased GS Ser(7) phosphorylation even though GS was strongly activated. In conclusion, glycogen content regulates GS affinity for UDP-glucose and low affinity for UDP-glucose in muscles with high glycogen content may reduce glycogen accumulation. Contraction increases GS affinity for UDP-glucose independently of glycogen content and creates a unique phosphorylation pattern.