Overexpression of protein targeting to glycogen (PTG) in rat hepatocytes causes profound activation of glycogen synthesis independent of normal hormone- and substrate-mediated regulatory mechanisms

Active Glycogen Synthase in the Liver Prevents High-Fat Diet-Induced Glucose Intolerance, Decreases Food Intake, and Lowers Body Weight

Many lines of evidence demonstrate a correlation between liver glycogen content and food intake. We previously demonstrated that mice overexpressing protein targeting to glycogen (PTG) specifically in the liver-which have increased glycogen content in this organ-are protected from high-fat diet (HFD)-induced obesity by reduced food intake. However, the use of PTG to increase liver glycogen implies certain limitations. PTG stimulates glycogen synthesis but also inhibits the enzyme responsible for glycogen degradation. Furthermore, as PTG is a regulatory subunit of protein phosphatase 1 (PP1), which regulates many cellular functions, its overexpression could have side effects beyond the regulation of glycogen metabolism. Therefore, it is necessary to determine whether the direct activation of glycogen synthesis, without affecting its degradation or other cellular functions, has the same effects. To this end, we generated mice overexpressing a non-inactivatable form of glycogen synthase (GS) specifically in the liver (9A-MGS^Alb mice). Control and 9a-MGS^Alb mice were fed a standard diet (SD) or HFD for 16 weeks. Glucose tolerance and feeding behavior were analyzed. 9A-MGS^Alb mice showed an increase in hepatic glycogen in fed and fasting conditions. When fed an HFD, these animals preserved their hepatic energy state, had a reduced food intake, and presented a lower body weight and fat mass than control animals, without changes in energy expenditure. Furthermore, 9A-MGS^Alb animals showed improved glucose tolerance when fed an SD or HFD. Moreover, liver triacylglycerol levels that were increased after HFD feeding were lower in these mice. These results confirm that increased liver glycogen stores contribute to decreased appetite and improve glucose tolerance in mice fed an HFD. On the basis of our findings, strategies to preserve hepatic glycogen stores emerge as potential treatments for obesity and hyperglycemia.

Protein Targeting to Glycogen (PTG): A Promising Player in Glucose and Lipid Metabolism

Protein phosphorylation and dephosphorylation are widely considered to be the key regulatory factors of cell function, and are often referred to as "molecular switches" in the regulation of cell metabolic processes. A large number of studies have shown that the phosphorylation/dephosphorylation of related signal molecules plays a key role in the regulation of liver glucose and lipid metabolism. As a new therapeutic strategy for metabolic diseases, the potential of using inhibitor-based therapies to fight diabetes has gained scientific momentum. PTG, a protein phosphatase, also known as glycogen targeting protein, is a member of the protein phosphatase 1 (PP1) family. It can play a role by catalyzing the dephosphorylation of phosphorylated protein molecules, especially regulating many aspects of glucose and lipid metabolism. In this review, we briefly summarize the role of PTG in glucose and lipid metabolism, and update its role in metabolic regulation, with special attention to glucose homeostasis and lipid metabolism.

Insulin signaling in health and disease

The molecular mechanisms of cellular insulin action have been the focus of much investigation since the discovery of the hormone 100 years ago. Insulin action is impaired in metabolic syndrome, a condition known as insulin resistance. The actions of the hormone are initiated by binding to its receptor on the surface of target cells. The receptor is an α2β2 heterodimer that binds to insulin with high affinity, resulting in the activation of its tyrosine kinase activity. Once activated, the receptor can phosphorylate a number of intracellular substrates that initiate discrete signaling pathways. The tyrosine phosphorylation of some substrates activates phosphatidylinositol-3-kinase (PI3K), which produces polyphosphoinositides that interact with protein kinases, leading to activation of the kinase Akt. Phosphorylation of Shc leads to activation of the Ras/MAP kinase pathway. Phosphorylation of SH2B2 and of Cbl initiates activation of G proteins such as TC10. Activation of Akt and other protein kinases produces phosphorylation of a variety of substrates, including transcription factors, GTPase-activating proteins, and other kinases that control key metabolic events. Among the cellular processes controlled by insulin are vesicle trafficking, activities of metabolic enzymes, transcriptional factors, and degradation of insulin itself. Together these complex processes are coordinated to ensure glucose homeostasis.

Increased liver glycogen levels enhance exercise capacity in mice

Muscle glycogen depletion has been proposed as one of the main causes of fatigue during exercise. However, few studies have addressed the contribution of liver glycogen to exercise performance. Using a low-intensity running protocol, here, we analyzed exercise capacity in mice overexpressing protein targeting to glycogen (PTG) specifically in the liver (PTG^OE mice), which show a high concentration of glycogen in this organ. PTG^OE mice showed improved exercise capacity, as determined by the distance covered and time ran in an extenuating endurance exercise, compared with control mice. Moreover, fasting decreased exercise capacity in control mice but not in PTG^OE mice. After exercise, liver glycogen stores were totally depleted in control mice, but PTG^OE mice maintained significant glycogen levels even in fasting conditions. In addition, PTG^OE mice displayed an increased hepatic energy state after exercise compared with control mice. Exercise caused a reduction in the blood glucose concentration in control mice that was less pronounced in PTG^OE mice. No changes were found in the levels of blood lactate, plasma free fatty acids, or β-hydroxybutyrate. Plasma glucagon was elevated after exercise in control mice, but not in PTG^OE mice. Exercise-induced changes in skeletal muscle were similar in both genotypes. These results identify hepatic glycogen as a key regulator of endurance capacity in mice, an effect that may be exerted through the maintenance of blood glucose levels.

Discovery and Development of Small-Molecule Inhibitors of Glycogen Synthase

The overaccumulation of glycogen appears as a hallmark in various glycogen storage diseases (GSDs), including Pompe, Cori, Andersen, and Lafora disease. Accumulating evidence suggests that suppression of glycogen accumulation represents a potential therapeutic approach for treating these GSDs. Using a fluorescence polarization assay designed to screen for inhibitors of the key glycogen synthetic enzyme, glycogen synthase (GS), we identified a substituted imidazole, (rac)-2-methoxy-4-(1-(2-(1-methylpyrrolidin-2-yl)ethyl)-4-phenyl-1H-imidazol-5-yl)phenol (H23), as a first-in-class inhibitor for yeast GS 2 (yGsy2p). Data from X-ray crystallography at 2.85 Å, as well as kinetic data, revealed that H23 bound within the uridine diphosphate glucose binding pocket of yGsy2p. The high conservation of residues between human and yeast GS in direct contact with H23 informed the development of around 500 H23 analogs. These analogs produced a structure-activity relationship profile that led to the identification of a substituted pyrazole, 4-(4-(4-hydroxyphenyl)-3-(trifluoromethyl)-1H-pyrazol-5-yl)pyrogallol, with a 300-fold improved potency against human GS. These substituted pyrazoles possess a promising scaffold for drug development efforts targeting GS activity in GSDs associated with excess glycogen accumulation.

PPP1R3C mediates metformin-inhibited hepatic gluconeogenesis

BACKGROUND

Metformin has been widely used to alleviate hyperglycemia in patients with type 2 diabetes mainly via suppressing hepatic gluconeogenesis. However, the underlying mechanism remains incompletely clear. Here, we aimed to explore the role of PPP1R3C in metformin-mediated inhibition of hepatic gluconeogenesis.

METHODS

The differentially expressed genes in primary mouse hepatocytes incubated with 8-Br-cAMP and metformin were analyzed by microarrays. Hepatic glucose production and gluconeogenic gene expressions were detected after adenovirus-mediated overexpression or silence of PPP1R3C in vitro and in vivo. The phosphorylation level and location of transducer of regulated CREB activity 2 (TORC2) were determined by Western blot and immunofluorescence.

RESULTS

Metformin and adenovirus-mediated activation of AMPK suppressed 8-Br-cAMP-stimulated Ppp1r3c mRNA expression in primary mouse hepatocytes. Overexpression of PPP1R3C in primary mouse hepatocytes or the livers of wild-type mice promoted hepatic glucose production and gluconeogenic gene expressions. On the contrary, adenovirus-mediated knockdown of PPP1R3C in primary mouse hepatocytes decreased hepatic gluconeogenesis, with the suppression of cAMP-stimulated gluconeogenic gene expressions and TORC2 dephosphorylation. Notably, Ppp1r3c expression was increased in the liver of db/db mice. After PPP1R3C silence in the livers of wild-type and db/db mice, blood glucose levels and hepatic glucose production were markedly lowered, with decreased expressions of key gluconeogenic enzymes and transcript factors as well as liver glycogen content.

CONCLUSION

Metformin-activated AMPK decreases hepatic PPP1R3C expression, leading to the suppression of hepatic gluconeogenesis through blocking cAMP-stimulated TORC2 dephosphorylation. Hepatic specific silence of PPP1R3C provides a promising therapeutic strategy for type 2 diabetes.

Cats and Carbohydrates: The Carnivore Fantasy?

The domestic cat’s wild ancestors are obligate carnivores that consume prey containing only minimal amounts of carbohydrates. Evolutionary events adapted the cat’s metabolism and physiology to this diet strictly composed of animal tissues and led to unique digestive and metabolic peculiarities of carbohydrate metabolism. The domestic cat still closely resembles its wild ancestor. Although the carnivore connection of domestic cats is well recognised, little is known about the precise nutrient profile to which the digestive physiology and metabolism of the cat have adapted throughout evolution. Moreover, studies show that domestic cats balance macronutrient intake by selecting low-carbohydrate foods. The fact that cats evolved consuming low-carbohydrate prey has led to speculations that high-carbohydrate diets could be detrimental for a cat’s health. More specifically, it has been suggested that excess carbohydrates could lead to feline obesity and diabetes mellitus. Additionally, the chances for remission of diabetes mellitus are higher in cats that consume a low-carbohydrate diet. This literature review will summarise current carbohydrate knowledge pertaining to digestion, absorption and metabolism of carbohydrates, food selection and macronutrient balancing in healthy, obese and diabetic cats, as well as the role of carbohydrates in prevention and treatment of obesity and diabetes mellitus.

Protein targeting to glycogen is a master regulator of glycogen synthesis in astrocytes

The storage and use of glycogen, the main energy reserve in the brain, is a metabolic feature of astrocytes. Glycogen synthesis is regulated by Protein Targeting to Glycogen (PTG), a member of specific glycogen-binding subunits of protein phosphatase-1 (PPP1). It positively regulates glycogen synthesis through de-phosphorylation of both glycogen synthase (activation) and glycogen phosphorylase (inactivation). In cultured astrocytes, PTG mRNA levels were previously shown to be enhanced by the neurotransmitter noradrenaline. To achieve further insight into the role of PTG in the regulation of astrocytic glycogen, its levels of expression were manipulated in primary cultures of mouse cortical astrocytes using adenovirus-mediated overexpression of tagged-PTG or siRNA to downregulate its expression. Infection of astrocytes with adenovirus led to a strong increase in PTG expression and was associated with massive glycogen accumulation (>100 fold), demonstrating that increased PTG expression is sufficient to induce glycogen synthesis and accumulation. In contrast, siRNA-mediated downregulation of PTG resulted in a 2-fold decrease in glycogen levels. Interestingly, PTG downregulation strongly impaired long-term astrocytic glycogen synthesis induced by insulin or noradrenaline. Finally, these effects of PTG downregulation on glycogen metabolism could also be observed in cultured astrocytes isolated from PTG-KO mice. Collectively, these observations point to a major role of PTG in the regulation of glycogen synthesis in astrocytes and indicate that conditions leading to changes in PTG expression will directly impact glycogen levels in this cell type.

Genetic models rule out a major role of beta cell glycogen in the control of glucose homeostasis

AIMS/HYPOTHESIS

Glycogen accumulation occurs in beta cells of diabetic patients and has been proposed to partly mediate glucotoxicity-induced beta cell dysfunction. However, the role of glycogen metabolism in beta cell function and its contribution to diabetes pathophysiology remain poorly understood. We investigated the function of beta cell glycogen by studying glucose homeostasis in mice with (1) defective glycogen synthesis in the pancreas; and (2) excessive glycogen accumulation in beta cells.

METHODS

Conditional deletion of the Gys1 gene and overexpression of protein targeting to glycogen (PTG) was accomplished by Cre-lox recombination using pancreas-specific Cre lines. Glucose homeostasis was assessed by determining fasting glycaemia, insulinaemia and glucose tolerance. Beta cell mass was determined by morphometry. Glycogen was detected histologically by periodic acid-Schiff's reagent staining. Isolated islets were used for the determination of glycogen and insulin content, insulin secretion, immunoblots and gene expression assays.

RESULTS

Gys1 knockout (Gys1 (KO)) mice did not exhibit differences in glucose tolerance or basal glycaemia and insulinaemia relative to controls. Insulin secretion and gene expression in isolated islets was also indistinguishable between Gys1 (KO) and controls. Conversely, despite effective glycogen overaccumulation in islets, mice with PTG overexpression (PTG(OE)) presented similar glucose tolerance to controls. However, under fasting conditions they exhibited lower glycaemia and higher insulinaemia. Importantly, neither young nor aged PTG(OE) mice showed differences in beta cell mass relative to age-matched controls. Finally, a high-fat diet did not reveal a beta cell-autonomous phenotype in either model.

CONCLUSIONS/INTERPRETATION

Glycogen metabolism is not required for the maintenance of beta cell function. Glycogen accumulation in beta cells alone is not sufficient to trigger the dysfunction or loss of these cells, or progression to diabetes.

Insulin Signaling in the Control of Glucose and Lipid Homeostasis

A continuous supply of glucose is necessary to ensure proper function and survival of all organs. Plasma glucose levels are thus maintained in a narrow range around 5 mM, which is considered the physiological set point. Glucose homeostasis is controlled primarily by the liver, fat, and skeletal muscle. Following a meal, most glucose disposals occur in the skeletal muscle, whereas fasting plasma glucose levels are determined primarily by glucose output from the liver. The balance between the utilization and production of glucose is primarily maintained at equilibrium by two opposing hormones, insulin and glucagon. In response to an elevation in plasma glucose and amino acids (after consumption of a meal), insulin is released from the beta cells of the islets of Langerhans in the pancreas. When plasma glucose falls (during fasting or exercise), glucagon is secreted by α cells, which surround the beta cells in the pancreas. Both cell types are extremely sensitive to glucose concentrations, can regulate hormone synthesis, and are released in response to small changes in plasma glucose levels. At the same time, insulin serves as the major physiological anabolic agent, promoting the synthesis and storage of glucose, lipids, and proteins and inhibiting their degradation and release back into the circulation. This chapter will focus mainly on signal transduction mechanisms by which insulin exerts its plethora of effects in liver, muscle, and fat cells, focusing on those pathways that are crucial in the control of glucose and lipid homeostasis.

Role of glycogen phosphorylase in liver glycogen metabolism

Liver glycogen is synthesized after a meal in response to an increase in blood glucose concentration in the portal vein and endocrine and neuroendocrine signals, and is degraded to glucose between meals to maintain blood glucose homeostasis. Glycogen degradation and synthesis during the diurnal cycle are mediated by changes in the activities of phosphorylase and glycogen synthase. Phosphorylase is regulated by phosphorylation of serine-14. Only the phosphorylated form of liver phosphorylase (GPa) is catalytically active. Interconversion between GPa and GPb (unphosphorylated) is dependent on the activities of phosphorylase kinase and of phosphorylase phosphatase. The latter comprises protein phosphatase-1 in conjunction with a glycogen-targeting protein (G-subunit) of the PPP1R3 family. At least two of six G-subunits (GL and PTG) expressed in liver are involved in GPa dephosphorylation. GPa to GPb interconversion is dependent on the conformational state of phosphorylase which can be relaxed (R) or tense (T) depending on the concentrations of allosteric effectors such as glucose, glucose 6-phosphate and adenine nucleotides and on the acetylation state of lysine residues. The G-subunit, GL, encoded by PPP1R3B gene is expressed at high levels in liver and can function as a phosphorylase phosphatase and a synthase phosphatase and has an allosteric binding site for GPa at the C-terminus which inhibits synthase phosphatase activity. GPa to GPb conversion is a major upstream event in the regulation of glycogen synthesis by glucose, its downstream metabolites and extracellular signals such as insulin and neurotransmitters.

Loss of protein targeting to glycogen sensitizes human hepatocellular carcinoma cells towards glucose deprivation mediated oxidative stress and cell death

PTG is a protein that is critical for glycogen accumulation in various tissues such as the liver. Our present study shows that its loss sensitizes liver cancer cells towards metabolic and oxidative stress.

Protein targeting to glycogen (PTG) is a ubiquitously expressed scaffolding protein that critically regulates glycogen levels in many tissues, including the liver, muscle and brain. However, its importance in transformed cells has yet to be explored in detail. Since recent studies have demonstrated an important role for glycogen metabolism in cancer cells, we decided to assess the effect of PTG levels on the ability of human hepatocellular carcinoma (HepG2) cells to respond to metabolic stress. Although PTG expression did not significantly affect the proliferation of HepG2 cells under normal culture conditions, we determined that PTG plays an important role during glucose deprivation. Overexpression of PTG protected cells from cell death in the absence of glucose, whereas knocking down PTG further promoted cytotoxicity, as measured by the release of lactate dehydrogenase (LDH) into the media. Additionally, we demonstrated that PTG attenuates glucose deprivation induced haeme oxygenase-1 (HO-1) expression, suggesting that PTG protects against glucose deprivation-induced oxidative stress. Indeed, treating cells with the antioxidant N-acetyl cysteine (NAC) rescued cells from cytotoxicity caused by glucose deprivation. Finally, we showed that loss of PTG resulted in enhanced autophagy. In control cells, glucose deprivation suppressed autophagy as determined by the increase in the levels of p62, an autophagy substrate. However, in knockdown cells, this suppression was relieved. Blockade of autophagy also attenuated cytotoxicity from glucose deprivation in PTG knockdown cells. Taken together, our findings identify a novel role for PTG in protecting hepatocellular carcinoma cells from metabolic stress, in part by regulating oxidative stress and autophagy.

Liver glycogen reduces food intake and attenuates obesity in a high-fat diet-fed mouse model

We generated mice that overexpress protein targeting to glycogen (PTG) in the liver (PTG(OE)), which results in an increase in liver glycogen. When fed a high-fat diet (HFD), these animals reduced their food intake. The resulting effect was a lower body weight, decreased fat mass, and reduced leptin levels. Furthermore, PTG overexpression reversed the glucose intolerance and hyperinsulinemia caused by the HFD and protected against HFD-induced hepatic steatosis. Of note, when fed an HFD, PTG(OE) mice did not show the decrease in hepatic ATP content observed in control animals and had lower expression of neuropeptide Y and higher expression of proopiomelanocortin in the hypothalamus. Additionally, after an overnight fast, PTG(OE) animals presented high liver glycogen content, lower liver triacylglycerol content, and lower serum concentrations of fatty acids and β-hydroxybutyrate than control mice, regardless of whether they were fed an HFD or a standard diet. In conclusion, liver glycogen accumulation caused a reduced food intake, protected against the deleterious effects of an HFD, and diminished the metabolic impact of fasting. Therefore, we propose that hepatic glycogen content be considered a potential target for the pharmacological manipulation of diabetes and obesity.

Glycogen metabolism and the homeostatic regulation of sleep

In 1995 Benington and Heller formulated an energy hypothesis of sleep centered on a key role of glycogen. It was postulated that a major function of sleep is to replenish glycogen stores in the brain that have been depleted during wakefulness which is associated to an increased energy demand. Astrocytic glycogen depletion participates to an increase of extracellular adenosine release which influences sleep homeostasis. Here, we will review some evidence obtained by studies addressing the question of a key role played by glycogen metabolism in sleep regulation as proposed by this hypothesis or by an alternative hypothesis named "glycogenetic" hypothesis as well as the importance of the confounding effect of glucocorticoïds. Even though actual collected data argue in favor of a role of sleep in brain energy balance-homeostasis, they do not support a critical and direct involvement of glycogen metabolism on sleep regulation. For instance, glycogen levels during the sleep-wake cycle are driven by different physiological signals and therefore appear more as a marker-integrator of brain energy status than a direct regulator of sleep homeostasis. In support of this we provide evidence that blockade of glycogen mobilization does not induce more sleep episodes during the active period while locomotor activity is reduced. These observations do not invalidate the energy hypothesis of sleep but indicate that underlying cellular mechanisms are more complex than postulated by Benington and Heller.

Altered glutamine metabolism and therapeutic opportunities for lung cancer

Disordered cancer metabolism was described almost a century ago as an abnormal adaptation of cancer cells to glucose utilization especially in hypoxic conditions; the so-called Warburg effect. Greater research interest in this area in the past two decades has led to the recognition of the critical coupling of specific malignant phenotypes such as increased proliferation and resistance to programmed cell death (apoptosis) with altered metabolic handling of key molecules that are essential for normal cellular metabolism. The altered glucose metabolism frequently encountered in cancer cells has already been exploited for cancer diagnosis and treatment. The role of other glycolytic pathway intermediates and alternative pathways for energy generation and macromolecular synthesis in cancer cells has only become recognized more recently. Especially, the important role of altered glutamine metabolism in the malignant behavior of cancer cells and the potential exploitation of this cellular adaptation for therapeutic targeting has now emerged as an important area of cancer research. Expectedly, attempts to exploit this understanding for diagnostic and therapeutic ends are running apace with the elucidation of the complex metabolic alterations that accompany neoplastic transformation. Because lung cancer is a leading cause of cancer death with limited curative therapy options, careful elucidation of the mechanism and consequences of disordered cancer metabolism in lung cancer is warranted. This review provides a concise, systematic overview of the current understanding of the role of altered glutamine metabolism in cancer, and how these findings intersect with current and future approaches to lung cancer management.

Normal glucose metabolism in carnivores overlaps with diabetes pathology in non-carnivores

Carnivores, such as the dolphin and the domestic cat, have numerous adaptations that befit consumption of diets with high protein and fat content, with little carbohydrate content. Consequently, nutrient metabolism in carnivorous species differs substantially from that of non-carnivores. Important metabolic pathways known to differ between carnivores and non-carnivores are implicated in the development of diabetes and insulin resistance in non-carnivores: (1) the hepatic glucokinase (GCK) pathway is absent in healthy carnivores yet GCK deficiency may result in diabetes in rodents and humans, (2) healthy dolphins and cats are prone to periods of fasting hyperglycemia and exhibit insulin resistance, both of which are risk factors for diabetes in non-carnivores. Similarly, carnivores develop naturally occurring diseases such as hemochromatosis, fatty liver, obesity, and diabetes that have strong parallels with the same disorders in humans. Understanding how evolution, environment, diet, and domestication may play a role with nutrient metabolism in the dolphin and cat may also be relevant to human diabetes.

Glucose-6-phosphate-mediated activation of liver glycogen synthase plays a key role in hepatic glycogen synthesis

The liver responds to an increase in blood glucose levels in the postprandial state by uptake of glucose and conversion to glycogen. Liver glycogen synthase (GYS2), a key enzyme in glycogen synthesis, is controlled by a complex interplay between the allosteric activator glucose-6-phosphate (G6P) and reversible phosphorylation through glycogen synthase kinase-3 and the glycogen-associated form of protein phosphatase 1. Here, we initially performed mutagenesis analysis and identified a key residue (Arg(582)) required for activation of GYS2 by G6P. We then used GYS2 Arg(582)Ala knockin (+/R582A) mice in which G6P-mediated GYS2 activation had been profoundly impaired (60-70%), while sparing regulation through reversible phosphorylation. R582A mutant-expressing hepatocytes showed significantly reduced glycogen synthesis with glucose and insulin or glucokinase activator, which resulted in channeling glucose/G6P toward glycolysis and lipid synthesis. GYS2(+/R582A) mice were modestly glucose intolerant and displayed significantly reduced glycogen accumulation with feeding or glucose load in vivo. These data show that G6P-mediated activation of GYS2 plays a key role in controlling glycogen synthesis and hepatic glucose-G6P flux control and thus whole-body glucose homeostasis.

Glucose induces protein targeting to glycogen in hepatocytes by fructose 2,6-bisphosphate-mediated recruitment of MondoA to the promoter

In the liver, a high glucose concentration activates transcription of genes encoding glucose 6-phosphatase and enzymes for glycolysis and lipogenesis by elevation in phosphorylated intermediates and recruitment of the transcription factor ChREBP (carbohydrate response element binding protein) and its partner, Mlx, to gene promoters. A proposed function for this mechanism is intracellular phosphate homeostasis. In extrahepatic tissues, MondoA, the paralog of ChREBP, partners with Mlx in transcriptional induction by glucose. We tested for glucose induction of regulatory proteins of the glycogenic pathway in hepatocytes and identified the glycogen-targeting proteins, G(L) and PTG (protein targeting to glycogen), as being encoded by Mlx-dependent glucose-inducible genes. PTG induction by glucose was MondoA dependent but ChREBP independent and was enhanced by forced elevation of fructose 2,6-bisphosphate and by additional xylitol-derived metabolites. It was counteracted by selective depletion of fructose 2,6-bisphosphate with a bisphosphatase-active kinase-deficient variant of phosphofructokinase 2/fructosebisphosphatase 2, which prevented translocation of MondoA to the nucleus and recruitment to the PTG promoter. We identify a novel role for MondoA in the liver and demonstrate that elevated fructose 2,6-bisphosphate is essential for recruitment of MondoA to the PTG promoter. Phosphometabolite activation of MondoA and ChREBP and their recruitment to target genes is consistent with a mechanism for gene regulation to maintain intracellular phosphate homeostasis.

A PTG variant contributes to a milder phenotype in Lafora disease

Lafora disease is an autosomal recessive form of progressive myoclonus epilepsy with no effective therapy. Although the outcome is always unfavorable, onset of symptoms and progression of the disease may vary. We aimed to identify modifier genes that may contribute to the clinical course of Lafora disease patients with EPM2A or EPM2B mutations. We established a list of 43 genes coding for proteins related to laforin/malin function and/or glycogen metabolism and tested common polymorphisms for possible associations with phenotypic differences using a collection of Lafora disease families. Genotype and haplotype analysis showed that PPP1R3C may be associated with a slow progression of the disease. The PPP1R3C gene encodes protein targeting to glycogen (PTG). Glycogen targeting subunits play a major role in recruiting type 1 protein phosphatase (PP1) to glycogen-enriched cell compartments and in increasing the specific activity of PP1 toward specific glycogenic substrates (glycogen synthase and glycogen phosphorylase). Here, we report a new mutation (c.746A>G, N249S) in the PPP1R3C gene that results in a decreased capacity to induce glycogen synthesis and a reduced interaction with glycogen phosphorylase and laforin, supporting a key role of this mutation in the glycogenic activity of PTG. This variant was found in one of two affected siblings of a Lafora disease family characterized by a remarkable mild course. Our findings suggest that variations in PTG may condition the course of Lafora disease and establish PTG as a potential target for pharmacogenetic and therapeutic approaches.

Generation of a dominant-negative glycogen targeting subunit for protein phosphatase-1

Modulation of the expression of the protein phosphatase-1 (PP1) glycogen-targeting subunit PTG exerts profound effects on cellular glycogen metabolism in vitro and in vivo. PTG contains three distinct binding domains for glycogen, PP1, and a common site for glycogen synthase and phosphorylase. The impact of disrupting the PP1-binding domain on PTG function was examined in 3T3-L1 adipocytes. A full-length PTG mutant was generated as an adenoviral construct in which the valine and phenylalanine residues in the conserved PP1-binding domain were mutated to alanine (PTG-VF). Infection of fully differentiated 3T3-L1 adipocytes with the PTG-VF adenovirus reduced glycogen stores by over 50%. In vitro, PTG-VF competitively interfered with wild-type PTG action, suggesting that the mutant construct acted as a dominant-negative molecule. The reduction in cellular glycogen storage was due to a significantly increased rate of glycogen turnover. Interestingly, acute basal and insulin-stimulated glucose uptake and glycogen synthesis rates were enhanced in PTG-VF expressing cells vs. control 3T3-L1 adipocytes, likely as a compensatory response to the loss of glycogen stores. These results indicate that the mutation of the PP1-binding domain on PTG resulted in the generation of a dominant-negative molecule that impeded endogenous PTG action and reduced cellular glycogen levels, through enhancement of glycogenolysis rather than impairment of glycogen synthesis.

Hepatic overexpression of a constitutively active form of liver glycogen synthase improves glucose homeostasis

In this study, we tested the efficacy of increasing liver glycogen synthase to improve blood glucose homeostasis. The overexpression of wild-type liver glycogen synthase in rats had no effect on blood glucose homeostasis in either the fed or the fasted state. In contrast, the expression of a constitutively active mutant form of the enzyme caused a significant lowering of blood glucose in the former but not the latter state. Moreover, it markedly enhanced the clearance of blood glucose when fasted rats were challenged with a glucose load. Hepatic glycogen stores in rats overexpressing the activated mutant form of liver glycogen synthase were enhanced in the fed state and in response to an oral glucose load but showed a net decline during fasting. In order to test whether these effects were maintained during long term activation of liver glycogen synthase, we generated liver-specific transgenic mice expressing the constitutively active LGS form. These mice also showed an enhanced capacity to store glycogen in the fed state and an improved glucose tolerance when challenged with a glucose load. Thus, we conclude that the activation of liver glycogen synthase improves glucose tolerance in the fed state without compromising glycogenolysis in the postabsorptive state. On the basis of these findings, we propose that the activation of liver glycogen synthase may provide a potential strategy for improvement of glucose tolerance in the postprandial state.

Metabolic response of the cerebral cortex following gentle sleep deprivation and modafinil administration

STUDY OBJECTIVES

The main energy reserve of the brain is glycogen, which is almost exclusively localized in astrocytes. We previously reported that cerebral expression of certain genes related to glycogen metabolism changed following instrumental sleep deprivation in mice. Here, we extended our investigations to another set of genes related to glycogen and glucose metabolism. We also compared the effect of instrumentally and pharmacologically induced prolonged wakefulness, followed (or not) by 3 hours of sleep recovery, on the expression of genes related to brain energy metabolism.

DESIGN

Sleep deprivation for 6-7 hours.

SETTING

Animal sleep research laboratory.

PARTICIPANTS

Adults OF1 mice.

INTERVENTIONS

Wakefulness was maintained by "gentle sleep deprivation" method (GSD) or by administration of the wakefulness-promoting drug modafinil (MOD) (200 mg/kg i.p.).

MEASUREMENTS AND RESULTS

Levels of mRNAs encoding proteins related to energy metabolism were measured by quantitative real-time PCR in the cerebral cortex. The mRNAs encoding protein targeting to glycogen (PTG) and the glial glucose transporter were significantly increased following both procedures used to prolong wakefulness. Glycogenin mRNA levels were increased only after GSD, while neuronal glucose transporter mRNA only after MOD. These effects were reversed after sleep recovery. A significant enhancement of glycogen synthase activity without any changes in glycogen levels was observed in both conditions.

CONCLUSIONS

These results indicate the existence of a metabolic adaptation of astrocytes aimed at maintaining brain energy homeostasis during the sleep-wake cycle.

Glucokinase and molecular aspects of liver glycogen metabolism

Conversion of glucose into glycogen is a major pathway that contributes to the removal of glucose from the portal vein by the liver in the postprandial state. It is regulated in part by the increase in blood-glucose concentration in the portal vein, which activates glucokinase, the first enzyme in the pathway, causing an increase in the concentration of glucose 6-P (glucose 6-phosphate), which modulates the phosphorylation state of downstream enzymes by acting synergistically with other allosteric effectors. Glucokinase is regulated by a hierarchy of transcriptional and post-transcriptional mechanisms that are only partially understood. In the fasted state, glucokinase is in part sequestered in the nucleus in an inactive state, complexed to a specific regulatory protein, GKRP (glucokinase regulatory protein). This reserve pool is rapidly mobilized to the cytoplasm in the postprandial state in response to an elevated concentration of glucose. The translocation of glucokinase between the nucleus and cytoplasm is modulated by various metabolic and hormonal conditions. The elevated glucose 6-P concentration, consequent to glucokinase activation, has a synergistic effect with glucose in promoting dephosphorylation (inactivation) of glycogen phosphorylase and inducing dephosphorylation (activation) of glycogen synthase. The latter involves both a direct ligand-induced conformational change and depletion of the phosphorylated form of glycogen phosphorylase, which is a potent allosteric inhibitor of glycogen synthase phosphatase activity associated with the glycogen-targeting protein, GL [hepatic glycogen-targeting subunit of PP-1 (protein phosphatase-1) encoded by PPP1R3B]. Defects in both the activation of glucokinase and in the dephosphorylation of glycogen phosphorylase are potential contributing factors to the dysregulation of hepatic glucose metabolism in Type 2 diabetes.

Hepatic glycogen synthesis in the absence of glucokinase: the case of embryonic liver

Glucokinase (GK, hexokinase type IV) is required for the accumulation of glycogen in adult liver and hepatoma cells. Paradoxically, mammalian embryonic livers store glycogen successfully in the absence of GK. Here we address how mammalian embryonic livers, but not adult livers or hepatoma cells, manage to accumulate glycogen in the absence of this enzyme. Hexokinase type I or II (HKI, HKII) substitutes for GK in hepatomas and in embryonic livers. We engineered FTO2B cells, a hepatoma cell line in which GK is not expressed, to unveil the modifications required to allow them to accumulate glycogen. In the light of these results, we then examined glycogen metabolism in embryonic liver. Glycogen accumulation in FTO2B cells can be triggered through elevated expression of HKI or either of the protein phosphatase 1 regulatory subunits, namely PTG or G L. Between these two strategies to activate glycogen deposition in the absence of GK, embryonic livers choose to express massive levels of HKI and HKII. We conclude that although the GK/liver glycogen synthase tandem is ideally suited to store glycogen in liver when blood glucose is high, the substitution of HKI for GK in embryonic livers allows the HKI/liver glycogen synthase tandem to make glycogen independently of the glucose concentration in blood, although it requires huge levels of HK. Moreover, the physiological consequence of the HK isoform switch is that the embryonic liver safeguards its glycogen deposits, required as the main source of energy at birth, from maternal starvation.

Mechanism suppressing glycogen synthesis in neurons and its demise in progressive myoclonus epilepsy

Glycogen synthesis is normally absent in neurons. However, inclusion bodies resembling abnormal glycogen accumulate in several neurological diseases, particularly in progressive myoclonus epilepsy or Lafora disease. We show here that mouse neurons have the enzymatic machinery for synthesizing glycogen, but that it is suppressed by retention of muscle glycogen synthase (MGS) in the phosphorylated, inactive state. This suppression was further ensured by a complex of laforin and malin, which are the two proteins whose mutations cause Lafora disease. The laforin-malin complex caused proteasome-dependent degradation both of the adaptor protein targeting to glycogen, PTG, which brings protein phosphatase 1 to MGS for activation, and of MGS itself. Enforced expression of PTG led to glycogen deposition in neurons and caused apoptosis. Therefore, the malin-laforin complex ensures a blockade of neuronal glycogen synthesis even under intense glycogenic conditions. Here we explain the formation of polyglucosan inclusions in Lafora disease by demonstrating a crucial role for laforin and malin in glycogen synthesis.

Metabolic impact of overexpression of liver glycogen synthase with serine-to-alanine substitutions in rat primary hepatocytes

To investigate the effect of elevation of liver glycogen synthase (GYS2) activity on glucose and glycogen metabolism, we performed adenoviral overexpression of the mutant GYS2 with six serine-to-alanine substitutions in rat primary hepatocytes. Cell-free assays demonstrated that the serine-to-alanine substitutions caused constitutive activity and electrophoretic mobility shift. In rat primary hepatocytes, overexpression of the mutant GYS2 significantly reduced glucose production by 40% and dramatically induced glycogen synthesis via the indirect pathway rather than the direct pathway. Thus, we conclude that elevation of glycogen synthase activity has an inhibitory effect on glucose production in hepatocytes by shunting gluconeogenic precursors into glycogen. In addition, although intracellular compartmentation of glucose-6-phosphate (G6P) remains unclear in hepatocytes, our results imply that there are at least two G6P pools via gluconeogenesis and due to glucose phosphorylation, and that G6P via gluconeogenesis is preferentially used for glycogen synthesis in hepatocytes.

Contributions of glucokinase and phosphofructokinase-2/fructose bisphosphatase-2 to the elevated glycolysis in hepatocytes from Zucker fa/fa rats

The insulin-resistant Zucker fa/fa rat has elevated hepatic glycolysis and activities of glucokinase and phosphofructokinase-2/fructose bisphosphatase-2 (PFK2). The latter catalyzes the formation and degradation of fructose-2,6-bisphosphate (fructose-2,6-P2) and is a glucokinase-binding protein. The contributions of glucokinase and PFK2 to the elevated glycolysis in fa/fa hepatocytes were determined by overexpressing these enzymes individually or in combination. Metabolic control analysis was used to determine enzyme coefficients on glycolysis and metabolite concentrations. Glucokinase had a high control coefficient on glycolysis in all hormonal conditions tested, whereas PFK2 had significant control only in the presence of glucagon, which phosphorylates PFK2 and suppresses glycolysis. Despite the high control strength of glucokinase, the elevated glycolysis in fa/fa hepatocytes could not be explained by the elevated glucokinase activity alone. In hepatocytes from fa/fa rats, glucokinase translocation between the nucleus and the cytoplasm was refractory to glucose but responsive to glucagon. Expression of a kinase-active PFK2 variant reversed the glucagon effect on glucokinase translocation and glucose phosphorylation, confirming the role for PFK2 in sequestering glucokinase in the cytoplasm. Glucokinase had a high control on glucose-6-phosphate content; however, like PFK2, it had a relative modest effect on the fructose-2,6-P2 content. However, combined overexpression of glucokinase and PFK2 had a synergistic effect on fructose-2,6-P2 levels, suggesting that interaction of these enzymes may be a prerequisite for formation of fructose-2,6-P2. Cumulatively, this study provides support for coordinate roles for glucokinase and PFK2 in the elevated hepatic glycolysis in fa/fa rats.

Transgenic overexpression of protein targeting to glycogen markedly increases adipocytic glycogen storage in mice

Adipocytes express the rate-limiting enzymes required for glycogen metabolism and increase glycogen synthesis in response to insulin. However, the physiological function of adipocytic glycogen in vivo is unclear, due in part to the low absolute levels and the apparent biophysical constraints of adipocyte morphology on glycogen accumulation. To further study the regulation of glycogen metabolism in adipose tissue, transgenic mice were generated that overexpressed the protein phosphatase-1 (PP1) glycogen-targeting subunit (PTG) driven by the adipocyte fatty acid binding protein (aP2) promoter. Exogenous PTG was detected in gonadal, perirenal, and brown fat depots, but it was not detected in any other tissue examined. PTG overexpression resulted in a modest redistribution of PP1 to glycogen particles, corresponding to a threefold increase in the glycogen synthase activity ratio. Glycogen synthase protein levels were also increased twofold, resulting in a combined greater than sixfold enhancement of basal glycogen synthase specific activity. Adipocytic glycogen levels were increased 200- to 400-fold in transgenic animals, and this increase was maintained to 1 yr of age. In contrast, lipid metabolism in transgenic adipose tissue was not significantly altered, as assessed by lipogenic rates, weight gain on normal or high-fat diets, or circulating free fatty acid levels after a fast. However, circulating and adipocytic leptin levels were doubled in transgenic animals, whereas adiponectin expression was unchanged. Cumulatively, these data indicate that murine adipocytes are capable of storing far higher levels of glycogen than previously reported. Furthermore, these results were obtained by overexpression of an endogenous adipocytic protein, suggesting that mechanisms may exist in vivo to maintain adipocytic glycogen storage at a physiological set point.

Regulation of the mouse protein targeting to glycogen (PTG) promoter by the FoxA2 forkhead protein and by 3',5'-cyclic adenosine 5'-monophosphate in H4IIE hepatoma cells

The scaffolding protein, protein targeting to glycogen (PTG), orchestrates the signaling of several metabolic enzymes involved in glycogen synthesis. However, little is known concerning the regulation of PTG itself. In this study, we have cloned and characterized the mouse promoter of PTG. We identified multiple FoxA2 binding sites within this region. FoxA2 is a member of the forkhead family of transcription factors that has recently been implicated in the cAMP-dependent regulation of several genes involved in liver metabolism. Using luciferase reporter constructs, we demonstrate that FoxA2 transactivates the PTG promoter in H4IIE hepatoma cells. Nuclear extracts prepared from mouse liver and H4IIE cells were able to bind a FoxA2-specific probe derived within the PTG promoter region. Chromatin immunoprecipitation experiments further demonstrate that FoxA2 binds to the PTG promoter in vivo. Finally, we show that treatment with cAMP analogs activates the PTG promoter and significantly increases PTG levels in H4IIE cells. Our results provide a framework to investigate how additional transcription factors may regulate PTG expression in other cell types.

Increased sensitivity of glycogen synthesis to phosphorylase-a and impaired expression of the glycogen-targeting protein R6 in hepatocytes from insulin-resistant Zucker fa/fa rats

Hepatic insulin resistance in the leptin-receptor defective Zucker fa/fa rat is associated with impaired glycogen synthesis and increased activity of phosphorylase-a. We investigated the coupling between phosphorylase-a and glycogen synthesis in hepatocytes from fa/fa rats by modulating the concentration of phosphorylase-a. Treatment of hepatocytes from fa/fa rats and Fa/? controls with a selective phosphorylase inhibitor caused depletion of phosphorylase-a, activation of glycogen synthase and stimulation of glycogen synthesis. The flux-control coefficient of phosphorylase on glycogen synthesis was glucose dependent and at 10 mm glucose was higher in fa/fa than Fa/? hepatocytes. There was an inverse correlation between the activities of glycogen synthase and phosphorylase-a in both fa/fa and Fa/? hepatocytes. However, fa/fa hepatocytes had a higher activity of phosphorylase-a, for a corresponding activity of glycogen synthase. This defect was, in part, normalized by expression of the glycogen-targeting protein, PTG. Hepatocytes from fa/fa rats had normal expression of the glycogen-targeting proteins G(L) and PTG but markedly reduced expression of R6. Expression of R6 protein was increased in hepatocytes from Wistar rats after incubation with leptin and insulin. Diminished hepatic R6 expression in the leptin-receptor defective fa/fa rat may be a contributing factor to the elevated phosphorylase activity and/or its high control strength on glycogen synthesis.

Muscle-specific deletion of the Glut4 glucose transporter alters multiple regulatory steps in glycogen metabolism

Mice with muscle-specific knockout of the Glut4 glucose transporter (muscle-G4KO) are insulin resistant and mildly diabetic. Here we show that despite markedly reduced glucose transport in muscle, muscle glycogen content in the fasted state is increased. We sought to determine the mechanism(s). Basal glycogen synthase activity is increased by 34% and glycogen phosphorylase activity is decreased by 17% (P < 0.05) in muscle of muscle-G4KO mice. Contraction-induced glycogen breakdown is normal. The increased glycogen synthase activity occurs in spite of decreased signaling through the insulin receptor substrate 1 (IRS-1)-phosphoinositide (PI) 3-kinase-Akt pathway and increased glycogen synthase kinase 3beta (GSK3beta) activity in the basal state. Hexokinase II is increased, leading to an approximately twofold increase in glucose-6-phosphate levels. In addition, the levels of two scaffolding proteins that are glycogen-targeting subunits of protein phosphatase 1 (PP1), the muscle-specific regulatory subunit (RGL) and the protein targeting to glycogen (PTG), are strikingly increased by 3.2- to 4.2-fold in muscle of muscle-G4KO mice compared to wild-type mice. The catalytic activity of PP1, which dephosphorylates and activates glycogen synthase, is also increased. This dominates over the GSK3 effects, since glycogen synthase phosphorylation on the GSK3-regulated site is decreased. Thus, the markedly reduced glucose transport in muscle results in increased glycogen synthase activity due to increased hexokinase II, glucose-6-phosphate, and RGL and PTG levels and enhanced PP1 activity. This, combined with decreased glycogen phosphorylase activity, results in increased glycogen content in muscle in the fasted state when glucose transport is reduced.

The Glycogenic Action of Protein Targeting to Glycogen in Hepatocytes Involves Multiple Mechanisms Including Phosphorylase Inactivation and Glycogen Synthase Translocation

Expression of the glycogen-targeting protein PTG promotes glycogen synthase activation and glycogen storage in various cell types. In this study, we tested the contribution of phosphorylase inactivation to the glycogenic action of PTG in hepatocytes by using a selective inhibitor of phosphorylase (CP-91149) that causes dephosphorylation of phosphorylase a and sequential activation of glycogen synthase. Similar to CP-91194, graded expression of PTG caused a concentration-dependent inactivation of phosphorylase and activation of glycogen synthase. The latter was partially counter-acted by the expression of muscle phosphorylase and was not additive with the activation by CP-91149, indicating that it is in part secondary to the inactivation of phosphorylase. PTG expression caused greater stimulation of glycogen synthesis and translocation of glycogen synthase than CP-91149, and the translocation of synthase could not be explained by accumulation of glycogen, supporting an additional role for glycogen synthase translocation in the glycogenic action of PTG. The effects of PTG expression on glycogen synthase and glycogen synthesis were additive with the effects of glucokinase expression, confirming the complementary roles of depletion of phosphorylase a (a negative modulator) and elevated glucose 6-phosphate (a positive modulator) in potentiating the activation of glycogen synthase. PTG expression mimicked the inactivation of phosphorylase caused by high glucose and counteracted the activation caused by glucagon. The latter suggests a possible additional role for PTG on phosphorylase kinase inactivation.

Regulation of glycogen metabolism in cultured human muscles by the glycogen phosphorylase inhibitor CP-91149

Pharmacological inhibition of liver GP (glycogen phosphorylase), which is currently being studied as a treatment for Type II (non-insulin-dependent) diabetes, may affect muscle glycogen metabolism. In the present study, we analysed the effects of the GP inhibitor CP-91149 on non-engineered or GP-overexpressing cultured human muscle cells. We found that CP-91149 treatment decreased muscle GP activity by (1) converting the phosphorylated AMP-independent a form into the dephosphorylated AMP-dependent b form and (2) inhibiting GP a activity and AMP-mediated GP b activation. Dephosphorylation of GP was exerted, irrespective of incubation of the cells with glucose, whereas inhibition of its activity was synergic with glucose. As expected, CP-91149 impaired the glycogenolysis induced by glucose deprivation. CP-91149 also promoted the dephosphorylation and activation of GS (glycogen synthase) in non-engineered or GP-overexpressing cultured human muscle cells, but exclusively in glucose-deprived cells. However, this inhibitor did not activate GS in glucose-deprived but glycogen-replete cells overexpressing PTG (protein targeting to glycogen), thus suggesting that glycogen inhibits the CP-91149-mediated activation of GS. Consistently, CP-91149 promoted glycogen resynthesis, but not its overaccumulation. Hence, treatment with CP-91149 impairs muscle glycogen breakdown, but enhances its recovery, which may be useful for the treatment of Type II (insulin-dependent) diabetes.

Glucocorticoids modulate neurotransmitter-induced glycogen metabolism in cultured cortical astrocytes

Glucocorticoids (GC) are considered as key modulators of glycogen homeostasis in peripheral tissues, but their role in the central nervous system has only partially been characterized. Exposure of primary cultures of cortical astrocytes to dexamethasone (DEX), a synthetic glucocorticoid, results in the reduction of noradrenaline (NA)-induced glycogen synthesis in a concentration-dependent manner with a IC50 of 4.88 nm and a maximum inhibition of 51%. Such an effect is mediated via glucocorticoid receptors (GRs), since it is mimicked by the glucocorticoid analogue RU28362 (100 nm) and prevented by the GR antagonist RU38486 (1 micro m). DEX does not act through alteration of signal transduction mechanisms, as cAMP formation induced by noradrenergic stimulation was unchanged. Moreover, glycogen synthesis was inhibited to the same extent when DEX was applied either together or only after a brief NA application. Neither [3H]2-deoxyglucose uptake nor lactate release was altered by DEX in the presence of NA, demonstrating that inhibition of glycogen synthesis is not a consequence of reduced glucose utilization or availability. Interestingly, enhancement of glycogen synthase activity induced by NA was reduced in the presence of DEX (-27%). These results suggest that GC could have a significant influence on neuroenergetics as they could modulate activity-related changes in brain glycogen metabolism.

Glycogen in the brain of Drosophila melanogaster: diurnal rhythm and the effect of rest deprivation

One function of sleep is thought to be the restoration of energy stores in the brain depleted during wakefulness. One such energy store found in mammalian brains is glycogen. Many of the genes involved in glycogen regulation in mammals have also been found in Drosophila melanogaster and rest behavior in Drosophila has recently been shown to have the characteristics of sleep. We therefore examined, in the fly, variation in the glycogen contents of the brain, the whole head and the body throughout the rest/activity cycle and after rest deprivation. Glycogen in the brain varies significantly throughout the day (p=0.001) and is highest during rest and lowest while flies are active. Glycogen levels in the whole head and body do not show diurnal variation. Brain glycogen drops significantly when flies are rest deprived for 3 h (p=0.034) but no significant differences are observed after 6 h of rest deprivation. In contrast, glycogen is significantly depleted in the body after both 3 and 6 h of rest deprivation (p<0.0001 and p<0.0001, respectively). Glycogen in the fly brain changes in relationship to rest and activity and demonstrates a biphasic response to rest deprivation similar to that observed in mammalian astrocytes in culture.

Regulation and function of the muscle glycogen-targeting subunit of protein phosphatase 1 (GM) in human muscle cells depends on the COOH-terminal region and glycogen content

G(M), the muscle-specific glycogen-targeting subunit of protein phosphatase 1 (PP1) targeted to the sarcoplasmic reticulum, was proposed to regulate recovery of glycogen in exercised muscle, whereas mutation truncation of its COOH-terminal domain is known to be associated with type 2 diabetes. Here, we demonstrate differential effects of G(M) overexpression in human muscle cells according to glycogen concentration. Adenovirus-mediated delivery of G(M) slightly activated glycogen synthase (GS) and inactivated glycogen phosphorylase (GP) in glycogen-replete cells, causing an overaccumulation of glycogen and impairment of glycogenolysis after glucose deprivation. Differently, in glycogen-depleted cells, G(M) strongly increased GS activation with no further enhancement of early glycogen resynthesis and without affecting GP. Effects of G(M) on GS and GP were abrogated by treatment with dibutyryl cyclic AMP. Expression of a COOH-terminal deleted-mutant (G(M) Delta C), lacking the membrane binding sequence to sarcoplasmic reticulum, failed to activate GS in glycogen-depleted cells, while behaving similar to native G(M) in glycogen-replete cells. This is explained by loss of stability of the G(M) Delta C protein following glycogen-depletion. In summary, G(M) promotes glycogen storage and inversely regulates GS and GP activities, while, specifically, synthase phosphatase activity of G(M)-PP1 is inhibited by glycogen. The conditional loss of function of the COOH-terminal deleted G(M) construct may help to explain the reported association of truncation mutation of G(M) with insulin resistance in human subjects.

Inactivation of phosphorylase is a major component of the mechanism by which insulin stimulates hepatic glycogen synthesis

Multiple signalling pathways are involved in the mechanism by which insulin stimulates hepatic glycogen synthesis. In this study we used selective inhibitors of glycogen synthase kinase-3 (GSK-3) and an allosteric inhibitor of phosphorylase (CP-91149) that causes dephosphorylation of phosphorylase a, to determine the relative contributions of inactivation of GSK-3 and dephosphorylation of phosphorylase a as alternative pathways in the stimulation of glycogen synthesis by insulin in hepatocytes. GSK-3 inhibitors (SB-216763 and Li+) caused a greater activation of glycogen synthase than insulin (90% vs. 40%) but a smaller stimulation of glycogen synthesis (30% vs. 150%). The contribution of GSK-3 inactivation to insulin stimulation of glycogen synthesis was estimated to be less than 20%. Dephosphorylation of phosphorylase a with CP-91149 caused activation of glycogen synthase and translocation of the protein from a soluble to a particulate fraction and mimicked the stimulation of glycogen synthesis by insulin. The stimulation of glycogen synthesis by phosphorylase inactivation cannot be explained by either inhibition of glycogen degradation or activation of glycogen synthase alone and suggests an additional role for translocation of synthase. Titrations with the phosphorylase inactivator showed that stimulation of glycogen synthesis by insulin can be largely accounted for by inactivation of phosphorylase over a wide range of activities of phosphorylase a. We conclude that a signalling pathway involving dephosphorylation of phosphorylase a leading to both activation and translocation of glycogen synthase is a critical component of the mechanism by which insulin stimulates hepatic glycogen synthesis. Selective inactivation of phosphorylase can mimic insulin stimulation of hepatic glycogen synthesis.

Hepatic expression of a targeting subunit of protein phosphatase-1 in streptozotocin-diabetic rats reverses hyperglycemia and hyperphagia despite depressed glucokinase expression

Glycogen-targeting subunits of protein phosphatase-1 (PP-1) are scaffolding proteins that facilitate the regulation of key enzymes of glycogen metabolism by PP-1. In the current study, we have tested the effects of hepatic expression of GMDeltaC, a truncated version of the muscle-targeting subunit isoform, in rats rendered insulin-deficient via injection of a single moderate dose of streptozotocin (STZ). Three key findings emerged. First, GMDeltaC expression in liver was sufficient to fully normalize blood glucose levels (from 335 +/- 31 mg/dl prior to viral injection to 109 +/- 28 mg/dl 6 days after injection) and liver glycogen content in STZ-injected rats. Second, this normalization occurred despite very low levels of liver glucokinase expression in the insulin-deficient STZ-injected rats. Finally, the hyperphagia induced by STZ injection was completely reversed by GMDeltaC expression in liver. In contrast to these findings with GMDeltaC, overexpression of another targeting subunit, GL, in STZ-injected rats caused a large increase in liver glycogen stores but only a transient decrease in food intake and blood glucose levels. The surprising demonstration of a glucose-lowering effect of GMDeltaC in the background of depressed hepatic glucokinase expression suggests that controlled stimulation of liver glycogen storage may be an effective mechanism for improving glucose homeostasis, even when normal pathways of glucose disposal are impaired.

PTG gene deletion causes impaired glycogen synthesis and developmental insulin resistance

Protein targeting to glycogen (PTG) is a scaffolding protein that targets protein phosphatase 1alpha (PP1alpha) to glycogen, and links it to enzymes involved in glycogen synthesis and degradation. We generated mice that possess a heterozygous deletion of the PTG gene. These mice have reduced glycogen stores in adipose tissue, liver, heart, and skeletal muscle, corresponding with decreased glycogen synthase activity and glycogen synthesis rate. Although young PTG heterozygous mice initially demonstrate normal glucose tolerance, progressive glucose intolerance, hyperinsulinemia, and insulin resistance develop with aging. Insulin resistance in older PTG heterozygous mice correlates with a significant increase in muscle triglyceride content, with a corresponding attenuation of insulin receptor signaling. These data suggest that PTG plays a critical role in glycogen synthesis and is necessary to maintain the appropriate metabolic balance for the partitioning of fuel substrates between glycogen and lipid.

A2B receptor activation promotes glycogen synthesis in astrocytes through modulation of gene expression

Adenosine has been proposed as a key factor regulating the metabolic balance between energy supply and demand in the central nervous system. Because astrocytes represent an important cellular element in the control of brain energy metabolism, we investigated whether adenosine could induce long-term changes of glycogen levels in primary cultures of mouse cortical astrocytes. We observed that adenosine increased glycogen content, up to 300%, in a time- (maximum at 8 h) and concentration-dependent manner with an EC(50) of 9.69 microM. Pharmacological experiments using the broad-spectrum agonist 5'-(N-ethylcarboxamido)adenosine (NECA) and specific agonists for the A(1), A(2A), and A(3) receptors [N(6)-cyclopentyladenosine (CPA), CGS-21680, and IB-MECA, respectively] suggest that the effect of adenosine is mediated through activation of the low-affinity A(2B) adenosine receptor subtype. Interestingly, adenosine induces in parallel the expression of the protein targeting to glycogen (PTG), one of the protein phosphatase-1 glycogen-targeting subunits that has been implicated in the control of glycogen levels in various tissues. These results indicate that adenosine can exert long-term control over glycogen levels in astrocytes and might therefore play a significant role in physiological and/or pathological processes involving long-term modulation of brain energy metabolism.

Sleep deprivation modulates brain mRNAs encoding genes of glycogen metabolism

Replenishment of brain glycogen stores depleted during waking has been suggested to constitute one of the functions of sleep [Benington, J. H. & Heller H. C. (1995) Prog. Neurobiol., 45, 347]. We have tested the hypothesis that the level of expression of enzymes involved in glycogen metabolism could undergo variations throughout the sleep-waking or rest-activity cycle, and after 6 h of 'gentle' total sleep deprivation in mice. Specifically, we determined the variations in mRNAs coding for protein targeting to glycogen (PTG), glycogen synthase and glycogen phosphorylase, all considered as key regulators of glycogen metabolism. Glycogen synthase and glycogen phosphorylase mRNAs exhibited significant variations throughout the light-dark cycle with a maximum at the middle of the light period and a minimum at the middle of the dark period. Following sleep deprivation, a two-fold increase in PTG mRNA and a decrease of mRNAs encoding glycogen synthase and glycogen phosphorylase were observed. These transcriptional events have functional consequences as the activity of glycogen synthase was increased 2.5-fold indicating a stimulating effect of sleep deprivation on glycogen synthesis. These results indicate that (i) expression of genes related to brain glycogen metabolism exhibit variations throughout the sleep-waking or rest-activity cycle and (ii) given the almost selective localization of glycogen to astrocytes, these cells might participate in the regulation of sleep.

Effects of modulation of glycerol kinase expression on lipid and carbohydrate metabolism in human muscle cells

Glycerol is taken up by human muscle in vivo and incorporated into lipids, but little is known about regulation of glycerol metabolism in this tissue. In this study, we have analyzed the role of glycerol kinase (GlK) in the regulation of glycerol metabolism in primary cultured human muscle cells. Isolated human muscle cells exhibited lower GlK activity than fresh muscle explants, but the activity in cultured cells was increased by exposure to insulin. [U-(14)C]Glycerol was incorporated into cellular phospholipids and triacylglycerides (TAGs), but little or no increase in TAG content or lactate release was observed in response to changes in the medium glycerol concentration. Adenovirus-mediated delivery of the Escherichia coli GlK gene (AdCMV-GlK) into muscle cells caused a 30-fold increase in GlK activity, which was associated with a marked rise in the labeling of phospholipid or TAG from [U-(14)C]glycerol compared with controls. Moreover, GlK overexpression caused [U-(14)C]glycerol to be incorporated into glycogen, which was dependent on the activation of glycogen synthase. Co-incubation of AdCMV-GlK-treated muscle cells with glycerol and oleate resulted in a large accumulation of TAG and an increase in lactate production. We conclude that GlK is the limiting step in muscle cell glycerol metabolism. Glycerol 3-phosphate is readily used for TAG synthesis but can also be diverted to form glycolytic intermediates that are in turn converted to glycogen or lactate. Given the high levels of glycerol in muscle interstitial fluid, these finding suggest that changes in GlK activity in muscle can exert important influences on fuel deposition in this tissue.

Glycogen-targeting subunits and glucokinase differentially affect pathways of glycogen metabolism and their regulation in hepatocytes

Overexpression of the glucose-phosphorylating enzyme glucokinase (GK) or members of the family of glycogen-targeting subunits of protein phosphatase-1 increases hepatic glucose disposal and glycogen synthesis. This study was undertaken to evaluate the functional properties of a novel, truncated glycogen-targeting subunit derived from the skeletal muscle isoform G(M)/R(Gl) and to compare pathways of glycogen metabolism and their regulation in cells with overexpressed targeting subunits and GK. When overexpressed in hepatocytes, truncated G(M)/R(Gl) (G(M)DeltaC) was approximately twice as potent as full-length G(M)/R(Gl) in stimulation of glycogen synthesis, but clearly less potent than GK or two other native glycogen-targeting subunits, G(L) and PTG. We also found that cells with overexpressed G(M)DeltaC are unique in that glycogen was efficiently degraded in response to lowering of media glucose concentrations, stimulation with forskolin, or a combination of both maneuvers, whereas cells with overexpressed G(L), PTG, or GK exhibited impairment in one or both of these glycogenolytic signaling pathways. (2)H NMR analysis of purified glycogen revealed that hepatocytes with overexpressed GK synthesized a larger portion of their glycogen from triose phosphates and a smaller portion from tricarboxylic acid cycle intermediates than cells with overexpressed glycogen-targeting subunits. Additional evidence for activation of distinct pathways of glycogen synthesis by GK and targeting subunits is provided by the additive effect of co-overexpression of the two types of proteins upon glycogen synthesis and a much larger stimulation of glucose utilization, glucose transport, and lactate production elicited by GK. We conclude that overexpression of the novel targeting subunit G(M)DeltaC confers unique regulation of glycogen metabolism. Furthermore, targeting subunits and GK stimulate glycogen synthesis by distinct pathways.

The level of the glycogen targetting regulatory subunit R5 of protein phosphatase 1 is decreased in the livers of insulin-dependent diabetic rats and starved rats

Hepatic glycogen synthesis is impaired in insulin-dependent diabetic rats owing to defective activation of glycogen synthase by glycogen-bound protein phosphatase 1 (PP1). The identification of three glycogen-targetting subunits in liver, G(L), R5/PTG and R6, which form complexes with the catalytic subunit of PP1 (PP1c), raises the question of whether some or all of these PP1c complexes are subject to regulation by insulin. In liver lysates of control rats, R5 and R6 complexes with PP1c were found to contribute significantly (16 and 21% respectively) to the phosphorylase phosphatase activity associated with the glycogen-targetting subunits, G(L)-PP1c accounting for the remainder (63%). In liver lysates of insulin-dependent diabetic and of starved rats, the phosphorylase phosphatase activities of the R5 and G(L) complexes with PP1c were shown by specific immunoadsorption assays to be substantially decreased, and the levels of R5 and G(L) were shown by immunoblotting to be much lower than those in control extracts. The phosphorylase phosphatase activity of R6-PP1c and the concentration of R6 protein were unaffected by these treatments. Insulin administration to diabetic rats restored the levels of R5 and G(L) and their associated activities. The regulation of R5 protein levels by insulin was shown to correspond to changes in the level of the mRNA, as has been found for G(L). The in vitro glycogen synthase phosphatase/phosphorylase phosphatase activity ratio of R5-PP1c was lower than that of G(L)-PP1c, suggesting that R5-PP1c may function as a hepatic phosphorylase phosphatase, whereas G(L)-PP1c may be the major hepatic glycogen synthase phosphatase. In hepatic lysates, more than half the R6 was present in the glycogen-free supernatant, suggesting that R6 may have lower affinity for glycogen than R5 and G(L)

The muscle-specific protein phosphatase PP1G/R(GL)(G(M))is essential for activation of glycogen synthase by exercise

In skeletal muscle both insulin and contractile activity are physiological stimuli for glycogen synthesis, which is thought to result in part from the dephosphorylation and activation of glycogen synthase (GS). PP1G/R(GL)(G(M)) is a glycogen/sarcoplasmic reticulum-associated type 1 phosphatase that was originally postulated to mediate insulin control of glycogen metabolism. However, we recently showed (Suzuki, Y., Lanner, C., Kim, J.-H., Vilardo, P. G., Zhang, H., Jie Yang, J., Cooper, L. D., Steele, M., Kennedy, A., Bock, C., Scrimgeour, A., Lawrence, J. C. Jr., L., and DePaoli-Roach, A. A. (2001) Mol. Cell. Biol. 21, 2683-2694) that insulin activates GS in muscle of R(GL)(G(M)) knockout (KO) mice similarly to the wild type (WT). To determine whether PP1G is involved in glycogen metabolism during muscle contractions, R(GL) KO and overexpressors (OE) were subjected to two models of contraction, in vivo treadmill running and in situ electrical stimulation. Both procedures resulted in a 2-fold increase in the GS -/+ glucose-6-P activity ratio in WT mice, but this response was completely absent in the KO mice. The KO mice, which also have a reduced GS activity associated with significantly reduced basal glycogen levels, exhibited impaired maximal exercise capacity, but contraction-induced activation of glucose transport was unaffected. The R(GL) OE mice are characterized by enhanced GS activity ratio and an approximately 3-4-fold increase in glycogen content in skeletal muscle. These animals were able to tolerate exercise normally. Stimulation of GS and glucose uptake following muscle contraction was not significantly different as compared with WT littermates. These results indicate that although PP1G/R(GL) is not necessary for activation of GS by insulin, it is essential for regulation of glycogen metabolism under basal conditions and in response to contractile activity, and may explain the reduced muscle glycogen content in the R(GL) KO mice, despite the normal insulin activation of GS.

Hepatic glucose uptake, gluconeogenesis and the regulation of glycogen synthesis

Hepatic glycogen is replenished during the absorptive period postprandially. This repletion is prompted partly by an increased hepatic uptake of glucose by the liver, partly by metabolite and hormonal signals in the portal vein, and partly by an increased gluconeogenic flux to glycogen (glyconeogenesis). There is some evidence that the direct formation of glycogen from glucose and that formed by gluconeogenic pathways is linked. This includes: (i) the inhibition of all glycogen synthesis, in vivo, when gluconeogenic flux is blocked by inhibitors; (ii) a dual relationship between glucose concentrations, lactate uptake by the liver and glycogen synthesis (by both pathways) which indicates that glucose sets the maximal rates of glycogen synthesis while lactate uptake determines the actual flux rate to glycogen; (iii) the decrease of both gluconeogenesis and glycogen synthesis by the biguanide, metformin; and (iv) correlations between increased gluconeogenesis and liver glycogen in obese patients and animal models. The degree to which the liver extracts portal glucose is not entirely agreed upon although a preponderance of evidence points to about a 5% extraction rate, following meals, which is dependent on a stimulation of glucokinase. This enzyme may be linked to the expression of other enzymes in the gluconeogenic pathway. Perivenous cells in the liver may induce additional gluconeogenesis in the periportal cells by increasing glycolytically produced lactate. A number of potential mechanisms therefore exist which could link glycogen synthesis from glucose and gluconeogenic substrate.

Insulin control of glycogen metabolism in knockout mice lacking the muscle-specific protein phosphatase PP1G/RGL

The regulatory-targeting subunit (RGL), also called GM) of the muscle-specific glycogen-associated protein phosphatase PP1G targets the enzyme to glycogen where it modulates the activity of glycogen-metabolizing enzymes. PP1G/RGL has been postulated to play a central role in epinephrine and insulin control of glycogen metabolism via phosphorylation of RGL. To investigate the function of the phosphatase, RGL knockout mice were generated. Animals lacking RGL show no obvious defects. The RGL protein is absent from the skeletal and cardiac muscle of null mutants and present at approximately 50% of the wild-type level in heterozygotes. Both the level and activity of C1 protein are also decreased by approximately 50% in the RGL-deficient mice. In skeletal muscle, the glycogen synthase (GS) activity ratio in the absence and presence of glucose-6-phosphate is reduced from 0.3 in the wild type to 0.1 in the null mutant RGL mice, whereas the phosphorylase activity ratio in the absence and presence of AMP is increased from 0.4 to 0.7. Glycogen accumulation is decreased by approximately 90%. Despite impaired glycogen accumulation in muscle, the animals remain normoglycemic. Glucose tolerance and insulin responsiveness are identical in wild-type and knockout mice, as are basal and insulin-stimulated glucose uptakes in skeletal muscle. Most importantly, insulin activated GS in both wild-type and RGL null mutant mice and stimulated a GS-specific protein phosphatase in both groups. These results demonstrate that RGL is genetically linked to glycogen metabolism, since its loss decreases PP1 and basal GS activities and glycogen accumulation. However, PP1G/RGL is not required for insulin activation of GS in skeletal muscle, and rather another GS-specific phosphatase appears to be involved.