Intersubunit communication in glycogen phosphorylase influences substrate recognition at the catalytic sites

Glycogen phosphorylase (GP) is biologically active as a dimer of identical subunits, each activated by phosphorylation of the serine-14 residue. GP exists in three interconvertible forms, namely GPa (di-phosphorylated form), GPab (mono-phosphorylated form), and GPb (non-phosphorylated form); however, information on GPab remains scarce. Given the prevailing view that the two GP subunits collaboratively determine their catalytic characteristics, it is essential to conduct GPab characterization to gain a comprehensive understanding of glycogenolysis regulation. Thus, in the present study, we prepared rabbit muscle GPab from GPb, using phosphorylase kinase as the catalyst, and identified it using a nonradioactive phosphate-affinity gel electrophoresis method. Compared with the half-half GPa/GPb mixture, the as-prepared GPab showed a unique AMP-binding affinity. To further investigate the intersubunit communication in GP, its catalytic site was probed using pyridylaminated-maltohexaose (a maltooligosaccharide-based substrate comprising the essential dextrin structure for GP; abbreviated as PA-0) and a series of specifically modified PA-0 derivatives (substrate analogs lacking part of the essential dextrin structure). By comparing the initial reaction rates toward the PA-0 derivative (Vderivative) and PA-0 (VPA-0), we demonstrated that the Vderivative/VPA-0 ratio for GPab was significantly different from that for the half-half GPa/GPb mixture. This result indicates that the interaction between the two GP subunits significantly influences substrate recognition at the catalytic sites, thereby providing GPab its unique substrate recognition profile.

Identification of glycogen phosphorylase L as a potential target for lung cancer

Traditional Chinese medicine (TCM) has been widely used for cancer treatment. Identification of anti-cancer targets of TCM is the first and principal step in discovering molecular mechanisms of TCM as well as obtaining novel targets for cancer therapy. In this study, glycogen phosphorylase L (PYGL) was identified as one of the targeted proteins for several TCMs and was upregulated in various cancer types. The expression level of PYGL was positively correlated with the stage of lung cancer and the poor prognosis of patients. Meanwhile, knockdown of PYGL significantly inhibited proliferation and migration in lung cancer cells. In addition, PYGL was associated with spindle, kinetochore, and microtubule, the cellular components that are closely related to mitosis, in lung cancer. Moreover, PYGL was more susceptible to be upregulated by 144 mutated genes. Taken together, PYGL is a potential target for lung cancer treatment and its molecular mechanism probably influences the mitotic function of cells by regulating energy metabolism.

Sex Dimorphic Glucose Transporter-2 Regulation of Hypothalamic Astrocyte Glucose and Energy Sensor Expression and Glycogen Metabolism

The plasma membrane glucose transporter-2 (GLUT2) monitors brain cell uptake of the critical nutrient glucose, and functions within astrocytes of as-yet-unknown location to control glucose counter-regulation. Hypothalamic astrocyte-neuron metabolic coupling provides vital cues to the neural glucostatic network. Current research utilized an established hypothalamic primary astrocyte culture model along with gene knockdown tools to investigate whether GLUT2 imposes sex-specific regulation of glucose/energy sensor function and glycogen metabolism in this cell population. Data show that GLUT2 stimulates or inhibits glucokinase (GCK) expression in glucose-supplied versus -deprived male astrocytes, but does not control this protein in female. Astrocyte 5'-AMP-activated protein kinaseα1/2 (AMPK) protein is augmented by GLUT2 in each sex, but phosphoAMPKα1/2 is coincidently up- (male) or down- (female) regulated. GLUT2 effects on glycogen synthase (GS) diverges in the two sexes, but direction of this control is reversed by glucoprivation in each sex. GLUT2 increases (male) or decreases (female) glycogen phosphorylase-brain type (GPbb) protein during glucoprivation, yet simultaneously inhibits (male) or stimulates (female) GP-muscle type (GPmm) expression. Astrocyte glycogen accumulation is restrained by GLUT2 when glucose is present (male) or absent (both sexes). Outcomes disclose sex-dependent GLUT2 control of the astrocyte glycolytic pathway sensor GCK. Data show that glucose status determines GLUT2 regulation of GS (both sexes), GPbb (female), and GPmm (male), and that GLUT2 imposes opposite control of GS, GPbb, and GPmm profiles between sexes during glucoprivation. Ongoing studies aim to investigate molecular mechanisms underlying sex-dimorphic GLUT2 regulation of hypothalamic astrocyte metabolic-sensory and glycogen metabolic proteins, and to characterize effects of sex-specific astrocyte target protein responses to GLUT2 on glucose regulation.

Molecular mechanisms of glycogen particle assembly in Escherichia coli

It has been reported that glycogen in Escherichia coli has two structural states, that is, fragility and stability, which alters dynamically. However, molecular mechanisms behind the structural alterations are not fully understood. In this study, we focused on the potential roles of two important glycogen degradation enzymes, glycogen phosphorylase (glgP) and glycogen debranching enzyme (glgX), in glycogen structural alterations. The fine molecular structure of glycogen particles in Escherichia coli and three mutants (ΔglgP, ΔglgX and ΔglgP/ΔglgX) were examined, which showed that glycogen in E. coli ΔglgP and E. coli ΔglgP/ΔglgX were consistently fragile while being consistently stable in E. coli ΔglgX, indicating the dominant role of GP in glycogen structural stability control. In sum, our study concludes that glycogen phosphorylase is essential in glycogen structural stability, leading to molecular insights into structural assembly of glycogen particles in E. coli.

Metabolic alterations and mitochondrial dysfunction underlie hepatocellular carcinoma cell death induced by a glycogen metabolic inhibitor

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related deaths. There is an urgent need for new targets to treat HCC due to limited treatment options and drug resistance. Many cancer cells are known to have high amount of glycogen than their tissue of origin and inhibition of glycogen catabolism induces cancer cell death by apoptosis. To further understand the role of glycogen in HCC and target it for pharmacotherapy, we studied metabolic adaptations and mitochondrial function in HepG2 cells after pharmacological inhibition of glycogen phosphorylase (GP) by CP-91149 (CP). GP inhibition increased the glycogen levels in HepG2 cells without affecting overall glucose uptake. Glycolytic capacity and importantly glycolytic reserve decreased significantly. Electron microscopy revealed that CP treatment altered mitochondrial morphology leading to mitochondrial swelling with less defined cristae. A concomitant decrease in mitochondrial oxygen consumption and mitochondria-linked ATP generation was observed. Metabolomics and enzyme activity / expression studies showed a decrease in the pentose phosphate pathway. In addition, CP treatment decreased the growth of HepG2 3D tumor spheroids in a dose- and time-dependent manner. Taken together, our study provides insights into metabolic alterations and mitochondrial dysfunction accompanying apoptosis in HepG2 cells upon GP inhibition. Our study can aid in the understanding of the mechanism and development of metabolic inhibitors to treat HCC.

Glycogen phosphorylase isoform regulation of glucose and energy sensor expression in male versus female rat hypothalamic astrocyte primary cultures

Astrocyte glycogen constitutes the primary energy fuel reserve in the brain. Current research investigated the novel premise that glycogen turnover governs astrocyte responsiveness to critical metabolic and neurotransmitter (norepinephrine) regulatory signals in a sex-dimorphic manner. Here, rat hypothalamic astrocyte glycogen phosphorylase (GP) gene expression was silenced by short-interfering RNA (siRNA) to investigate how glycogen metabolism controlled by GP-brain type (GPbb) or GP-muscle type (GPmm) activity affects glucose [glucose transporter-2 (GLUT2)] and energy [5'-AMP-activated protein kinase (AMPK)] sensor and adrenergic receptor (AR) proteins in each sex. Results show that in the presence of glucose, glycogen turnover is regulated by GPbb in the male or by GPmm in the female, yet in the absence of glucose, glycogen breakdown is controlled by GPbb in each sex. GLUT2 expression is governed by GPmm-mediated glycogen breakdown in glucose-supplied astrocytes of each sex, but glycogenolysis controls glucoprivic GLUT2 up-regulation in male only. GPbb-mediated glycogen disassembly causes divergent changes in total AMPK versus phosphoAMPK profiles in male. During glucoprivation, glycogenolysis up-regulates AMPK content in male astrocytes by GPbb- and GPmm-dependent mechanisms, whereas GPbb-mediated glycogen breakdown inhibits phosphoAMPK expression in female. GPbb and GPmm activity governs alpha₂-AR and beta₁-AR protein levels in male, but has no effect on these profiles in the female. Outcomes provide novel evidence for sex-specific glycogen regulation of glucose- and energy-sensory protein expression in hypothalamic astrocytes, and identify GP isoforms that mediate such control in each sex. Results also show that glycogen regulation of hypothalamic astrocyte receptivity to norepinephrine is male-specific. Further studies are needed to characterize the molecular mechanisms that underlie sex differences in glycogen control of astrocyte protein expression.

Linc-RAM promotes muscle cell differentiation via regulating glycogen phosphorylase activity

Long non-coding RNAs (lncRNAs) are important regulators of diverse biological processes, especially skeletal muscle cell differentiation. Most of the lncRNAs identified to date are localized in the nucleus and play regulatory roles in gene expression. The cytoplasmic lncRNAs are less well understood. We previously identified a long intergenic non-coding RNA (linc-RNA) activator of myogenesis (Linc-RAM) that directly binds MyoD in the nucleus to enhance muscle cell differentiation. Here, we report that a substantial fraction of Linc-RAM is localized in the cytoplasm of muscle cells. To explore the molecular functions of cytoplasmic Linc-RAM, we sought to identify Linc-RAM-binding proteins. We report here that Linc-RAM physically interacts with glycogen phosphorylase (PYGM) in the cytoplasm. Knockdown of PYGM significantly attenuates the function of Linc-RAM in promoting muscle cell differentiation. Loss-of-function and gain-of function assays demonstrated that PYGM enhances muscle cell differentiation in an enzymatic activity-dependent manner. Finally, we show that the interaction between Linc-RAM and PYGM positively regulates the enzymatic activity of PYGM in muscle cells. Collectively, our findings unveil a molecular mechanism through which cytoplasmic Linc-RAM contributes to muscle cell differentiation by regulating PYGM activity. Our findings establish that there is crosstalk between lncRNAs and cellular metabolism during myogenic cell differentiation.

Formation and physicochemical properties of glycogen phosphorylase in complex with a cationic polyelectrolyte

The accumulation of rabbit muscle glycogen phosphorylase b (RMGPb) in electrostatic complexes with the cationic polyelectrolyte poly 2-(dimethylamino) ethyl methacrylate in its quenched form (QPDMAEMA) was studied in two buffer solutions. In the N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES) buffer, large complexes of RMGPb-QPDMAEMA were formed which adopted smaller sizes as QPDMAEMA concentration increased. However, in N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid) (HEPES) buffer, the hydrodynamic radius of the formed complexes gradually increased as the polymer concentration increased. Zeta potential measurements (ζ_p) showed that RMGPb significantly changed the ζ_p of the QPDMAEMA aggregates. Fluorescence studies showed that the interaction between RMGPb and QPDMAEAMA was enhanced as polymer concentration increased. Specifically, 8-anilinonaphthalene-1-sulfonic acid (ANS) fluorescence indicated that in the BES buffer the aggregates became denser as more QPDMAEMA was added, while in the HEPES buffer the density of the formed structures decreased. RMGPb's secondary structure was examined by Attenuated Total Reflection - Fourier Transform Infrared (ATR-FTIR) and Circular Dichroism (CD) showing that QPDMAEMA interaction with RMGPb does not induce any changes to the secondary structure of the enzyme. These observations suggest that cationic polyelectrolytes may be utilized for the formulation of RMGPb in multifunctional nanostructures and be further exploited in innovative biotechnology applications and bioinspired materials development.

Outliers in SAR and QSAR: 4. effects of allosteric protein-ligand interactions on the classical quantitative structure-activity relationships

Effects of allosteric interactions on the classical structure-activity relationship (SAR) and quantitative SAR (QSAR) have been investigated. Apprehending the outliers in SAR and QSAR studies can improve the quality, predictability, and use of QSAR in designing unknown compounds in drug discovery research. We explored allosteric protein-ligand interactions as a possible source of outliers in SAR/QSAR. We used glycogen phosphorylase as an example of a protein that has an allosteric site. Examination of the ligand-bound x-ray crystal structures of glycogen phosphorylase revealed that many inhibitors bound at more than one binding site. The results of QSAR analyses of the inhibitors included a QSAR that recognized an outlier bound at a distinctive allosteric binding site. The case provided an example of constructive use of QSAR identifying outliers with alternative binding modes. Other allosteric QSARs that captured our attention were the inverted parabola/bilinear QSARs. The x-ray crystal structures and the QSAR analyses indicated that the inverted parabola QSARs could be associated with the conformational changes in the allosteric interactions. Our results showed that the normal parabola, as well as the inverted parabola QSARs, can describe the allosteric interactions. Examination of the ligand-bound X-ray crystal structures of glycogen phosphorylase revealed that many inhibitors bound at more than one binding site. The results of QSAR analyses of the inhibitors included a QSAR that recognized an outlier bound at a distinctive allosteric binding site.

Denaturation manner of sarcoplasmic proteins in Pale, Soft and Exudative meat determines their positive impacts on myofibrillar water-holding capacity

The aim of the study was to investigate the denaturation manner of sarcoplasmic proteins (SP) under PSE condition to explain their positive impacts on water-holding compacity. We found that the SP precipitation under PSE-like condition (pH 5.5, 40 °C) and heating conditions (pH 5.5, 7.0, 8.0, 55 °C) were similar, but the myofibrillar water-holding capacity was improved only under PSE-like condition (pH 5.5, 40 °C). To understand the denaturation mechanism of SP, their physicochemical properties were examined. Results demonstrated that PSE-denaturation and heat-denaturation of SP were two different processes. At pH 7.0 and 8.0, the unfolding of SP due to temperature elevation did not alter the overall net surface negative charges but only increased hydrophobicity, whereas at pH 5.5, the net surface positive charges and hydrophobicity increased dramatically. We hypothesized that in PSE meat, denatured SP became highly positively charged and hydrophobic and easier to bind to the negatively charged MF, which is related to the improvement on water-holding capacity.

Molecular docking analysis of glycogen phosphorylase with inhibitors from Cissampelos pareira Linn

Cissampelos pareira Linn. is a climbing herb known in Indian traditional medicine as laghupatha. It belongs to the Menispermaceae family. The enzyme glycogen phosphorylase (GP) is a promising target for the treatment of type-2 diabetes (T2DM). A variety of natural product inhibitors with both pharmaceutical and nutraceutical potential have been reported in the search for powerful, selective and drug-like GP inhibitors that could lead to hypoglycemic medicines. Therefore, it is of interest to document the molecular docking analysis data of glycogen phosphorylase with compounds from Cissampelos pareira Linn. We report the optimal binding features of 4 compounds namely Trans-N-feruloyltyramine, Coclaurine, Magnoflorine, and Curine with the target protein for further consideration in the context of T2DM.

Hindbrain catecholamine regulation of ventromedial hypothalamic nucleus glycogen metabolism during acute versus recurring insulin-induced hypoglycemia in male versus female rat

Ventromedial hypothalamic nucleus (VMN) glycogen metabolism affects local glucoregulatory signaling. The hindbrain metabolic-sensitive catecholamine (CA) neurotransmitter norepinephrine controls VMN glycogen phosphorylase (GP)-muscle (GPmm) and -brain (GPbb) type expression in male rats. Present studies addressed the premise that CA regulation of hypoglycemic patterns of VMN glycogen metabolic enzyme protein expression is sex-dimorphic, and that this signal is responsible for sex differences in acclimation of these profiles to recurrent insulin-induced hypoglycemia (RIIH). VMN tissue was acquired by micropunch-dissection from male and female rats pretreated by caudal fourth ventricular administration of the CA neurotoxin 6-hydroxydopamine (6OHDA) before single or serial insulin injection. 6-OHDA averted acute hypoglycemic inhibition of VMN glycogen synthase (GS) and augmentation of GPmm and GPbb protein expression in males, and prevented GPmm and -bb down-regulation in females. Males recovered from antecedent hypoglycemia (AH) exhibited neurotoxin-preventable diminution of baseline GS profiles, whereas acclimated GPmm and -bb expression in females occurred irrespective of pretreatment. RIIH did not alter VMN GS, GPmm, and GPbb expression in vehicle- or 6-OHDA-pretreated animals of either sex. VMN glycogen content was correspondingly unchanged or increased in males versus females following AH; 6-OHDA augmented glycogen mass in AH-exposed animals of both sexes. RIIH did not alter VMN glycogen accumulation in vehicle-pretreated rats of either sex, but diminished glycogen in neurotoxin-pretreated animals. AH suppresses baseline GS (CA-dependent) or GPmm/GPbb (CA-independent) expression in male and female rats, respectively, which corresponds with unaltered or augmented VMN glycogen content in those sexes. AH-associated loss of sex-distinctive CA-mediated enzyme protein sensitivity to hypoglycemia (male: GS, GPmm, GPbb; female: GPmm, Gpbb) may reflect, in part, VMN target desensitization to noradrenergic input.

GLYCOGEN ACCUMULATION IN TRICHOMONAS IS DRIVEN BY THE AVAILABILITY OF EXTRACELLULAR GLUCOSE

The parasitic protist Trichomonas vaginalis is the causative agent of trichomoniasis, a highly prevalent sexually transmitted infection. The organism is known to accumulate substantial deposits of the polysaccharide glycogen, which is believed to serve as a store of carbon and energy that can be tapped during periods of nutrient limitation. Such nutrient limitation is likely to occur when T. vaginalis is transmitted between hosts, implying that glycogen may play an important role in the lifecycle of the parasite. Both T. vaginalis glycogen synthase and glycogen phosphorylase, key enzymes of glycogen synthesis and degradation, respectively, have been cloned and characterized, and neither enzyme is subject to the post-translational controls found in other, well-characterized eukaryotic systems. Thus, it is unclear how glycogen metabolism is regulated in this organism. Here we use a glucose limitation/re-feeding protocol to show that the activities of key enzymes of glycogen synthesis do not increase during re-feeding when glycogen synthesis is stimulated. Rather, a simple model appears to operate with glycogen storage being driven by the extracellular glucose concentration.

Central Type II Glucocorticoid Receptor Regulation of Ventromedial Hypothalamic Nucleus Glycogen Metabolic Enzyme and Glucoregulatory Neurotransmitter Marker Protein Expression in the Male Rat

The ventromedial hypothalamic nucleus (VMN) glucoregulatory neurotransmitters γ-aminobutyric acid (GABA) and nitric oxide (NO) signal adjustments in glycogen mobilization. Glucocorticoids control astrocyte glycogen metabolism in vitro. The classical (type II) glucocorticoid receptor (GR) is expressed in key brain structures that govern glucostasis, including the VMN. Current research addressed the hypothesis that forebrain GR regulation of VMN glycogen synthase (GS) and phosphorylase (GP) protein expression correlates with control of glucoregulatory transmission. Groups of male rats were pretreated by intracerebroventricular (icv) delivery of the GR antagonist RU486 or vehicle prior to insulin-induced hypoglycemia (IIH), or were pretreated icv with dexamethasone (DEX) or vehicle before subcutaneous insulin diluent injection. DEX increased VMN GS and norepinephrine-sensitive GP-muscle type (GPmm), but did not alter metabolic deficit-sensitive GP-brain type (GPbb) expression. RU486 enhanced GS and GPbb profiles during IIH. VMN astrocyte (MCT1) and neuronal (MCT2) monocarboxylate transporter profiles were up-regulated in euglycemic and hypoglycemic animals by DEX or RU486, respectively. Glutamate decarboxylase65/67 and neuronal nitric oxide synthase (nNOS) proteins were both increased by DEX, yet RU486 augmented hypoglycemic nNOS expression patterns. Results show that GR exert divergent effects on VMN GS, MCT1/2, and nNOS proteins during eu- (stimulatory) versus hypoglycemia (inhibitory); these findings imply that up-regulated NO transmission may reflect, in part, augmented glucose incorporation into glycogen and/or increased tissue lactate requirements. Data also provide novel evidence for metabolic state-dependent GR regulation of VMN GPmm and GPbb profiles; thus, GABA signaling of metabolic stability may reflect, in part, stimulus-specific glycogen breakdown during eu- versus hypoglycemia.

Sex differences in glucoprivic regulation of glycogen metabolism in hypothalamic primary astrocyte cultures: Role of estrogen receptor signaling

Hypoglycemia causes sex-reliant changes in hypothalamic astrocyte glycogen metabolism in vivo. The role of nuclear versus membrane astrocyte estrogen receptors (ER) in glucoprivic regulation of glycogen is unclear. Here, primary hypothalamic astrocyte cultures were treated with selective ER antagonists during glucoprivation to investigate the hypothesis that ER mediate sex-specific glycogen responses to glucoprivation. Results show that glucoprivic down-regulation of glycogen synthase expression is mediated by transmembrane G protein-coupled ER-1 (GPER) signaling in each sex and estrogen receptor (ER)-beta (ERβ) activity in females. Glucoprivic inhibition of glycogen phosphorylase involves GPER and ERβ in females, but ER-independent mechanisms in males. GPER, ERβ, and ER-alpha (ERα) inhibit or stimulate AMPK protein expression in male versus female astrocytes, respectively. Glucoprivic augmentation of phospho-AMPK profiles in male glia was opposed by GPER activation, whereas GPER and ERβ suppress this protein in females. Astrocyte ERα and GPER content was down-regulated in each sex during glucose deficiency, whereas ERβ levels was unaltered (males) or increased (females). Glucoprivation correspondingly elevated or diminished male versus female astrocyte glycogen content; ER antagonism reversed this response in males, but not females. Results identify distinctive ER variants involved in sex-similar versus sex-specific astrocyte protein responses to withdrawal of this substrate fuel. Notably, glucoprivation elicits a directional switch or gain-of-effect of GPER and ERβ on specific glial protein profiles. Outcomes infer that ERs are crucial for glucoprivic regulation of astrocyte glycogen accumulation in males. Alternatively, estradiol may act independently of ER signaling to disassemble this reserve in females.

Dynamic Variations in Brain Glycogen are Involved in Modulating Isoflurane Anesthesia in Mice

General anesthesia severely affects the metabolites in the brain. Glycogen, principally stored in astrocytes and providing the short-term delivery of substrates to neurons, has been implicated as an affected molecule. However, whether glycogen plays a pivotal role in modulating anesthesia–arousal remains unclear. Here, we demonstrated that isoflurane-anesthetized mice exhibited dynamic changes in the glycogen levels in various brain regions. Glycogen synthase (GS) and glycogen phosphorylase (GP), key enzymes of glycogen metabolism, showed increased activity after isoflurane exposure. Upon blocking glycogenolysis with 1,4-dideoxy-1,4-imino-D-arabinitol (DAB), a GP antagonist, we found a prolonged time of emergence from anesthesia and an enhanced δ frequency in the EEG (electroencephalogram). In addition, augmented expression of glycogenolysis genes in glycogen phosphorylase, brain (Pygb) knock-in (Pygb^H11/H11) mice resulted in delayed induction of anesthesia, a shortened emergence time, and a lower ratio of EEG-δ. Our findings revealed a role of brain glycogen in regulating anesthesia–arousal, providing a potential target for modulating anesthesia.

Kinetic modeling of glucose central metabolism in hepatocytes and hepatoma cells

BACKGROUND

Kinetic modeling and control analysis of a metabolic pathway may identify the steps with the highest control in tumor cells, and low control in normal cells, which can be proposed as the best therapeutic targets.

METHODS

Enzyme kinetic characterization, pathway kinetic modeling and control analysis of the glucose central metabolism were carried out in rat (hepatoma AS-30D) and human (cervix HeLa) cancer cells and normal rat hepatocytes.

RESULTS

The glycogen metabolism enzymes in AS-30D, HeLa cells and hepatocytes showed similar kinetic properties, except for higher AS-30D glycogen phosphorylase (GP) sensitivity to AMP. Pathway modeling indicated that fluxes of glycogen degradation and PPP were mainly controlled by GP and NADPH consumption, respectively, in both hepatocytes and cancer cells. Likewise, hexose-6-phosphate isomerase (HPI) and phosphoglucomutase (PGM) exerted significant control on glycolysis and glycogen synthesis fluxes in cancer cells but not in hepatocytes. Modeling also indicated that glycolytic and glycogen synthesis fluxes could be strongly decreased when HPI and PGM were simultaneously inhibited in AS-30D cells but not in hepatocytes. Experimental assessment of these predictions showed that both the glycolytic and glycogen synthesis fluxes of AS-30D cells, but not of hepatocytes, were inhibited by oxamate, by inducing increased Fru1,6BP levels, a competitive inhibitor of HPI and PGM.

CONCLUSION

HPI and PGM seem suitable targets for decreasing glycolytic and glycogen synthesis fluxes in AS-30D cells but not in hepatocytes.

GENERAL SIGNIFICANCE

The present study identified new therapeutic targets within glucose central metabolism in the analyzed cancer cells, with no effects on non-cancer cells.

Effects of phosphorylation on the activity of glycogen phosphorylase in mutton during incubation at 4 °C in vitro

The aim of this study was to assess the effects of the phosphorylation levels of glycogen phosphorylase on its activity in mutton sarcoplasmic protein samples during incubation at 4 °C. Samples of sarcoplasmic proteins from mutton longissimus thoracis muscles were prepared and separated into three treatment groups to obtain glycogen phosphorylase with different phosphorylation levels, which were (1) treated with protein kinase A, (2) treated with alkaline phosphatase, and (3) left untreated (control). Glycogen phosphorylase phosphorylation levels and activity as well as the levels of related endogenous substances were assessed. The results showed that phosphorylation of glycogen phosphorylase in mutton promoted its activity during incubation at 4 °C. The activity of glycogen phosphorylase was also influenced by other factors (glycogen, glucose, glucose 6-phosphate, ATP, etc.) in vitro. The combined effects of phosphorylation and endogenous substances on glycogen phosphorylase activity varied at different incubation times.

Metformin and Berberine suppress glycogenolysis by inhibiting glycogen phosphorylase and stabilizing the molecular structure of glycogen in db/db mice

Glycogen is a branched glucose polymer involved in sustaining blood glucose homeostasis. Liver glycogen comprises α particles (up to 300 nm in diameter) made of joined β particles (∼20 nm in diameter). Glycogen α particles in a mouse model for diabetes are molecularly fragile, breaking down into smaller β particles more readily than in healthy mice. Glycogen phosphorylase (GP), a rate-limiting enzyme in glycogen degradation, is overexpressed in diabetic mice. This study shows that Metformin and Berberine, two common drugs, two common drugs used to treat diabetes, are able to revert the liver glycogen of diabetic mice to the stable structure seen in non-diabetic mice. It is also shown that these drugs reduce the GP level via the cAMP/PKA signaling pathway in diabetic livers and decrease the affinity of GP with the glycogen of db/db mice. These effects of these drugs may slow down the degradation of liver glycogen and improve glucose homeostasis.

Effects of low salinity on gill and liver glycogen metabolism of great blue-spotted mudskippers (Boleophthalmus pectinirostris)

This study investigated the effects of low salinity exposure on glycogen and its metabolism biomarkers, glycogen synthase (GS) and glycogen phosphorylase (GP), representing glycogen synthesis and catabolism, respectively, in the gills and liver of great blue-spotted mudskippers (Boleophthalmus pectinirostris). The fish were accumulated at 10‰ salinity seawater for 1 week, then 270 healthy great blue-spotted mudskippers with similar size were randomly transferred to 10‰ (control group) or 3‰ (low salinity group) seawater for 72-hour stress experiment. Fish significantly elevated their blood glucose levels 12 h after low salinity challenge. At the end of experiments, a decrease in liver glycogen contents was observed in both the control and low salinity groups, the latter showing a pronounced decrease, while the gill glycogen contents were not changed for either group. The mRNA abundance and enzyme activity of GS and GP were both elevated in gill tissues, showing a rising glycogen synthesis and catabolism, probably resulting in the unchanging gill glycogen content. While in liver tissues, the mRNA abundance and enzyme activity were decreased for GS and increased for GP, showing a net increase for breaking down glycogen in liver, probably for supplying a sufficient glucose level for gills and other tissues/organs involved in the response to salinity changes.

Sex-specific estrogen regulation of hypothalamic astrocyte estrogen receptor expression and glycogen metabolism in rats

Brain astrocytes are implicated in estrogenic neuroprotection against bio-energetic insults, which may involve their glycogen energy reserve. Forebrain estrogen receptors (ER)-alpha (ERα) and -beta (ERβ) exert differential control of glycogen metabolic enzyme [glycogen synthase (GS); phosphorylase (GP)] expression in hypoglycemic male versus female rats. Studies were conducted using a rat hypothalamic astrocyte primary culture model along with selective ER agonists to investigate the premise that estradiol (E₂) exerts sex-dimorphic control over astrocyte glycogen mass and metabolism. Female astrocyte GS and GP profiles are more sensitive to E₂ stimulation than the male. E₂ did not regulate expression of phospho-GS (inactive enzyme form) in either sex. Data also show that transmembrane G protein-coupled ER-1 (GPER) signaling is implicated in E₂ control of GS profiles in each sex and alongside ERα, GP expression in females. E₂ increases total 5'-AMP-activated protein kinase (AMPK) protein in female astrocytes, but stimulated pAMPK (active form) expression with equivalent potency via GPER in females and ERα in males. In female astrocytes, ERα protein was up-regulated at a lower E₂ concentration and over a broader dosage range compared to males, whereas ERβ was increased after exposure to 1-10 nM versus 100 pM E2 levels in females and males, respectively. GPER profiles were stimulated by E₂ in female, but not male astrocytes. E₂ increased astrocyte glycogen content in female, but not male astrocytes; selective ERβ or ERα stimulation elevated glycogen levels in the female and male, respectively. Outcomes imply that dimorphic astrocyte ER and glycogen metabolic responses to E₂ may reflect, in part, differential steroid induction of ER variant expression and/or regulation of post-receptor signaling in each sex.

Role of phosphorylation on characteristics of glycogen phosphorylase in lamb with different glycolytic rates post-mortem

The relationship between glycogen phosphorylase activity and phosphorylation levels in the longissimus thoracis muscle post-mortem was studied. Sixty lamb samples were collected at 0.5 h, 2 h, 6 h, 12 h, 24 h, 48 h, and 72 h post-mortem and divided into three groups (n = 6) with different glycolytic rates (fast, intermediate, and slow) according to the pH at 6 h post-mortem. The phosphorylation level and activity and expression of glycogen phosphorylase were determined. The results showed that the phosphorylation level and activity of glycogen phosphorylase in the slow pH decline group was lower than that in the fast pH decline group during 24 h post-mortem (P < .05). There was a significant positive correlation between the glycogen phosphorylase activity and the phosphorylation level. In conclusion, these data demonstrated that the glycogen phosphorylase activity in lambs was affected by phosphorylation levels and postmortem duration.

Inhibition of glycogen catabolism induces intrinsic apoptosis and augments multikinase inhibitors in hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is one of the leading cancers in the world in incidence and mortality. Current pharmacotherapy of HCC is limited in the number and efficacy of anticancer agents. Metabolic reprogramming is a prominent feature of many cancers and has rekindled interest in targeting metabolic proteins for cancer therapy. Glycogen is a storage form of glucose, and the levels of glycogen have been found to correlate with biological processes in reprogrammed cancer cells. However, the contribution of glycogen metabolism to carcinogenesis, cancer cell growth, metastasis, and chemoresistance is poorly understood. Thus, we studied the processes involved in the inhibition of glycogen metabolism in HCC cells. Pharmacological inhibition of glycogen phosphorylase (GP), a rate-limiting enzyme in glycogen catabolism, by CP-91149 led to a decrease in HCC cell viability. GP inhibition induced cancer cell death through the intrinsic apoptotic pathway. Mitochondrial dysfunction and autophagic adaptations accompanied this apoptosis process whereas endoplasmic reticulum stress, necrosis, and necroptosis were not major components of the cell death. In addition, GP inhibition potentiated the effects of multikinase inhibitors sorafenib and regorafenib, which are key drugs in advanced-stage HCC therapy. Our study provides mechanistic insights into cell death by perturbation of glycogen metabolism and identifies GP inhibition as a potential HCC pharmacotherapy target.

Studies on the reversible enzyme reaction of rabbit muscle glycogen phosphorylase b using isothermal titration calorimetry

Glycogen phosphorylase enzymes (GP) catalyse reversible reactions; the glucose transfer from glycogen to inorganic phosphate (P_i, phosphorolysis) or the reverse glucose transfer from glucose-1-phosphate (G-1-P) to glycogen (synthesis). Rabbit muscle GPb (rmGPb) was used as a model enzyme to study the reversible enzyme reaction. To follow both directions of this reversible reaction, we have developed a novel isothermal titration calorimetry (ITC) method for the determination of the direct reaction rate. The preference of forward or reverse reaction was ensured by the 0.1 or 10 concentration ratios of G-1-P/P_i, respectively. Substrate specificity was studied using different maltooligosaccharides and glycogen. Based on the K_M values, glycogen and 2-chloro-4-nitrophenyl maltoheptaoside (CNP-G7) were found to be analogous substrates, which allowed to optimize the method by taking advantage of the CNP chromophore being detectable in HPLC. In case of CNP-G7, substrate inhibition was observed and characterised by K_i of 23 ± 7 mM. Inhibition of human GP is a promising strategy for the treatment of diabetes. Our ITC measurements have confirmed that caffeine and glucopyranosylidene-spiro-thiohydantoin (GTH), as known GPb inhibitors, inhibit the rmGPb-catalysed reversible reaction in both directions. K_i values obtained in the direction of synthesis (1.92 ± 0.14 mM for caffeine and 11.5 ± 2.0 μM for GTH) have been shown to be in good agreement with the K_i values obtained in the direction of phosphorolysis (4.05 ± 0.26 mM for caffeine and 13.8 ± 1.6 μM for GTH). The higher difference between the inhibition constants of caffeine was explained by the non-competitive mechanism. The described ITC method using the developed experimental design and reaction conditions is suitable for activity measurements of different phosphorylase enzymes on various substrates and is applicable for inhibition studies as well.

Norepinephrine control of ventromedial hypothalamic nucleus glucoregulatory neurotransmitter expression in the female rat: Role of monocarboxylate transporter function

The ventromedial hypothalamic nucleus (VMN) is a critical component of the neural circuitry that regulates glucostasis. Astrocyte glycogen is a vital reserve of glucose and its oxidizable metabolite L-lactate. In hypoglycemic female rats, estradiol-dependent augmentation of VMN glycogen phosphorylase (GP) protein requires hindbrain catecholamine input. Research here investigated the premise that norepinephrine (NE) regulation of VMN astrocyte metabolism shapes local glucoregulatory neurotransmitter signaling in this sex. Estradiol-implanted ovariectomized rats were pretreated by intra-VMN administration of the monocarboxylate transporter inhibitor alpha-cyano-4-hydroxy-cinnamic acid (4CIN) or vehicle before NE delivery to that site. NE caused 4CIN-reversible reduction or augmentation of VMN glycogen synthase and phosphorylase expression. 4CIN prevented NE stimulation of gluco-inhibitory (glutamate decarboxylase65/67) and suppression of gluco-stimulatory (neuronal nitric oxide synthase) neuron marker proteins. These outcomes imply that effects of noradrenergic stimulation of VMN astrocyte glycogen depletion on glucoregulatory transmitter signaling may be mediated, in part, by glycogen-derived substrate fuel provision. NE control of astrocyte glycogen metabolism may involve down-regulated adrenoreceptor (AR), e.g. alpha1 and alpha2, alongside amplified beta1 AR and estrogen receptor-beta signaling. Noradrenergic hypoglycemia was refractory to 4CIN, implying that additional NE-sensitive VMN glucoregulatory neurochemicals may be insensitive to monocarboxylate uptake. Augmentation of circulating free fatty acids by combinatory NE and 4CIN, but not NE alone implies that acute hypoglycemia induced here is an insufficient stimulus for mobilization of these fuels, but is adequate when paired with diminished brain monocarboxylate fuel availability.

The effect of Psidium guajava aqueous leaf extract on liver glycogen enzymes, hormone sensitive lipase and serum lipid profile in diabetic rats

Diabetes mellitus is characterized by hyperglycaemia that results from defects in insulin secretion or insulin action and is accompanied by general disturbances metabolism. Psidium guajava (PG) leaf is known to have antidiabetic effects that include lowering of blood glucose. The aim of the study was to investigate the effect of PG leaf extract on tissue activity of glycogen synthase (GS) and glycogen phosphorylase (GP); tissue activity of hormone sensitive lipase (HSL); serum lipid profile; and serum enzyme biomarkers of tissue damage. Diabetes was induced in male Sprague-Dawley rats with a single dose of 40 mg/kg body weight streptozotocin. The aqueous extract of PG leaves was used to treat both normal and diabetic animals (400 mg/kg body weight) for 2 weeks while control animals were treated with the vehicle. At the end of the treatment period, blood, liver and adipose tissue samples were collected from the euthanized animals. The results show that PG extract significantly decreased (P < 0.05) HSL activity in adipose tissue and liver of diabetic animals which was accompanied by increased glycogen levels, reduced serum triglycerides, total cholesterol, LDL-cholesterol and increased HDL-cholesterol. This study demonstrates that P. guajava has significant anti-diabetic effects that include increased glycogen storage and reduced HSL activity in the liver and adipose tissue with an improved serum lipid profile.

Changes in glycogen concentration and gene expression levels of glycogen-metabolizing enzymes in muscle and liver of developing masu salmon

Glycogen, as an intracellular deposit of polysaccharide, takes important roles in energy balance of many animals. In fish, however, the role of glycogen during development is poorly understood. In the present study, we assessed changes in glycogen concentration and gene expression patterns of glycogen-metabolizing enzymes in developing masu salmon (Oncorhynchus masou masou), a salmonid species inhabiting west side of North Pacific Ocean. As we measured glycogen levels in the bodies and yolk sacs containing the liver separately, the glycogen concentration increased in both parts as the fish developed, whereas it transiently decreased in the yolk sac after hatching, implying glycogen synthesis and breakdown in these tissues. Immunofluorescence staining using anti-glycogen monoclonal antibody revealed localization of glycogen in the liver, muscle and yolk syncytial layer of the pre-hatching embryos and hatched larvae. In order to estimate glycogen metabolism in the fish, the genes encoding homologs of glycogen synthase (gys1 and gys2) and glycogen phosphorylase (pygma, pygmb and pygl) were cloned, and their expression patterns were assessed by quantitative PCR and in situ hybridization. In the fish, gys1 and gys2 were robustly expressed in the muscle and liver, respectively. Also, expression of pyg isoforms was found in muscle, liver and yolk syncytial layer during hatching. With changes in glycogen concentration and expression patterns of relevant genes, our results suggest, for the first time, possible involvement of glycogen in energy balance of fish embryos, especially during hatching.

A new interpretation of sulfate activation of rabbit muscle glycogen phosphorylase

It is widely known that sulfate ion at high concentration serves like an allosteric activator of glycogen phosphorylase (GP). Based on the crystallographic studies on GP, it has been assumed that the sulfate ion is bound close to the phosphorylatable Ser¹⁴ site of nonactivated GP, causing a conformational change to catalytically-active GP. However, there are also reports that sulfate ion inhibits allosterically-activated GP by preventing the phosphate substrate from attaching to the catalytic site. In the present study, using a high concentration of sulfate ion, significant enhancement of GP activity was observed when macromolecular glycogen was used as substrate but not when smaller maltohexaose was used. In glycogen solution, nonreducing-end glucose residues are localized on the surface of glycogen and are not distributed homogenously in the solution. Using cyclodextrin-immobilized column chromatography, we found that sulfate at high concentration promoted GP-dextrin binding through the dextrin-binding site (DBS) located away from the catalytic site. This result is consistent with the properties of the DBSs found in glycogen-debranching enzyme and β-amylase. Therefore, we propose a new interpretation of the sulfate activation of GP, wherein sulfate ions at high concentration promote glycogen-binding to the DBS directly, and glycogen-binding to the catalytic site indirectly. Our findings were successfully applied to the affinity purification of porcine brain GP.

Anti-diabetic activity of quercetin extracted from Phyllanthus emblica L. fruit: In silico and in vivo approaches

In this study, molecular interactions of the ligands, quercetin, gallic acid, and metformin with various diabetes mellitus-related protein targets, such as glycogen phosphorylase and peroxisome proliferator-activated receptor gamma, were assessed. It was revealed that quercetin possesses good binding affinity to both targets. Quercetin is a major constituent of methanolic extracts of Phyllanthus emblica fruit. The antihyperglycemic effect of quercetin in streptozotocin (STZ)-induced diabetic rats was examined. The isolated quercetin administered at a dose of 75 mg/kg body weight produced a maximum decrease of 14.78% in blood glucose levels in the diabetic rats after 7 days of treatment. Furthermore, quercetin doses of 50 and 75 mg/kg were shown to significantly improve the profiles of triglycerides, high-density lipoprotein, very-low-density lipoprotein, low-density lipoprotein, and total cholesterol at the end of the study in STZ-induced diabetic rats. The administration of quercetin (25, 50, and 75 mg/kg body weight) daily for 28 days in STZ-induced diabetic rats resulted in a significant decrease in blood glucose and urine sugar levels, with a considerable rise in plasma insulin and hemoglobin levels. Therefore, quercetin is a potential drug with antidiabetic and antihyperglycemic action mediated by changes in the levels of glucose, cholesterol, and triglycerides as indicated by in silico and in vivo studies.

Immunochemical Study of the Effect of F2Glc on Glycogen Synthase Translocation and Glycogen Synthesis in Isolated Rat Hepatocytes

The compound 2-deoxy-2-fluoro-α-D-glucopyranosyl fluoride (F₂Glc), which is a nonmetabolized superior glucose analogue, is a potent inhibitor of glycogen phosphorylase and pharmacological properties are reported. Glycogen phosphorylase (GP) and glycogen synthase (GS) are responsible of the degradation and synthesis, respectively, of glycogen which is a polymer of glucose units that provides a readily available source of energy in mammals. GP and GS are two key enzymes that modulate cellular glucose and glycogen levels; therefore, these proteins are suggested as potential targets for the treatment of diseases related to glycogen metabolism disorders. We studied by Western Blot technique that F₂Glc decreased GP activity, and we also showed that F₂Glc did not affect GS activity and its translocation from a uniform cytosolic distribution to the hepatocyte periphery, which is crucial for glycogen synthesis, using immunoblotting and immunofluorescence labeling techniques. F₂Glc specifically inhibits glycogenolysis pathway and permits a greater deposition of glycogen. These observations open up the possibility of further develop drugs that act specifically on GP. The ability to selectively inhibit GP, which is a key enzyme for the release of glucose from the hepatic glycogen reserve, may represent a new approach for the treatment of hyperglycemia in type 2 diabetes.

An Improved Comparative Docking Approach for Developing Specific Glycogen Phosphorylase Inhibitors Using Pentacyclic Triterpenes

BACKGROUND

Mitigation of unwanted effects is a major issue to be addressed in drug discovery. The human genome shares a considerable degree of sequence identity and structural homology whereby multiple binding pockets of similar composition is seen across different family of proteins. This distribution of similar binding pockets in different proteins is a major reason for cross reactivity upon ligand binding. Therefore, identifying such off-targets on a case-to-case basis in the early stages of drug discovery pipeline is very much essential. Especially for developing specific ligands against isozymes such as glycogen phosphorylase it is a pre-requisite owing to its implications. The inhibitors of glycogen phosphorylase (GPi) control the blood glucose level effectively by inhibiting the hepatic GP (hGP). But due to its high degree of sequence identity with the muscle form (mGP) and similar binding pocket, these ligands cross react resulting in impaired muscle function due to prolonged relaxation. Therefore, molecular targeting or optimizing the lead to a specific target (hGP) is very essential. In the current work, a comparative docking approach was adopted wherein suitable ligands were designed based on the chemical descriptors present in lig18 binding site of mGP and hGP followed by virtual docking.

RESULTS

The docking results showed that lig18 optimized compound (lig18_7) had better affinity to hGP and mGP respectively. lig18_7 interacted with residues Arg171 and Asn172 that are present specifically in hGP indicating specificity. Using this improved comparative docking approach isoform- specific ligands can be effectively developed.

Probing the catalytic site of rabbit muscle glycogen phosphorylase using a series of specifically modified maltohexaose derivatives

Glycogen phosphorylase (GP) is an allosteric enzyme whose catalytic site comprises six subsites (SG₁, SG_-1, SG_-2, SG_-3, SG_-4, and SP) that are complementary to tandem five glucose residues and one inorganic phosphate molecule, respectively. In the catalysis of GP, the nonreducing-end glucose (Glc) of the maltooligosaccharide substrate binds to SG₁ and is then phosphorolyzed to yield glucose 1-phosphate. In this study, we probed the catalytic site of rabbit muscle GP using pyridylaminated-maltohexaose (Glcα1-4Glcα1-4Glcα1-4Glcα1-4Glcα1-4GlcPA, where GlcPA = 1-deoxy-1-[(2-pyridyl)amino]-D-glucitol]; abbreviated as PA-0) and a series of specifically modified PA-0 derivatives (Glc _m -AltNAc-Glc _n -GlcPA, where m + n = 4 and AltNAc is 3-acetoamido-3-deoxy-D-altrose). PA-0 served as an efficient substrate for GP, whereas the other PA-0 derivatives were not as good as the PA-0, indicating that substrate recognition by all the SG₁ -SG_-4 subsites was important for the catalysis of GP. By comparing the initial reaction rate toward the PA-0 derivatives (V _derivative) with that toward PA-0 (V _PA-0), we found that the value of V _derivative/V _PA-0 decreased significantly as the level of allosteric activation of GP increased. These results suggest that some conformational changes have taken place in the maltooligosaccharide-binding region of the GP catalytic site during allosteric regulation.

The regulatory α and β subunits of phosphorylase kinase directly interact with its substrate, glycogen phosphorylase

The selective phosphorylation of glycogen phosphorylase (GP) by its only known kinase, phosphorylase kinase (PhK), keeps glycogen catabolism tightly regulated. In addition to the obligatory interaction between the catalytic γ subunit of PhK and the phosphorylatable region of GP, previous studies have suggested additional sites of interaction between this kinase and its protein substrate. Using short chemical crosslinkers, we have identified direct interactions of GP with the large regulatory α and β subunits of PhK. These newfound interactions were found to be sensitive to ligands that bind PhK.

Intermittent fasting reduces body fat but exacerbates hepatic insulin resistance in young rats regardless of high protein and fat diets

Intermittent fasting (IMF) is a relatively new dietary approach to weight management, although the efficacy and adverse effects have not been full elucidated and the optimal diets for IMF are unknown. We tested the hypothesis that a one-meal-per-day intermittent fasting with high fat (HF) or protein (HP) diets can modify energy, lipid, and glucose metabolism in normal young male Sprague-Dawley rats with diet-induced obesity or overweight. Male rats aged 5 weeks received either HF (40% fat) or HP (26% protein) diets ad libitum (AL) or for 3 h at the beginning of the dark cycle (IMF) for 5 weeks. Epidydimal fat pads and fat deposits in the leg and abdomen were lower with HP and IMF. Energy expenditure at the beginning of the dark cycle, especially from fat oxidation, was higher with IMF than AL, possibly due to greater activity levels. Brown fat content was higher with IMF. Serum ghrelin levels were higher in HP-IMF than other groups, and accordingly, cumulative food intake was also higher in HP-IMF than HF-IMF. HF-IMF exhibited higher area under the curve (AUC) of serum glucose at the first part (0-40 min) during oral glucose tolerance test, whereas AUC of serum insulin levels in both parts were higher in IMF and HF. During intraperitoneal insulin tolerance test, serum glucose levels were higher with IMF than AL. Consistently, hepatic insulin signaling (GLUT2, pAkt) was attenuated and PEPCK expression was higher with IMF and HF than other groups, and HOMA-IR revealed significantly impaired attenuated insulin sensitivity in the IMF groups. However, surprisingly, hepatic and skeletal muscle glycogen storage was higher in IMF groups than AL. The higher glycogen storage in the IMF groups was associated with the lower expression of glycogen phosphorylase than the AL groups. In conclusion, IMF especially with HF increased insulin resistance, possibly by attenuating hepatic insulin signaling, and lowered glycogen phosphorylase expression despite decreased fat mass in young male rats. These results suggest that caution may be warranted when recommending intermittent fasting, especially one-meal-per-day fasting, for people with compromised glucose metabolism.

Glycogen metabolic reprogramming in hepatocellular carcinoma: An update

Hepatocellular carcinoma (HCC) is a highly prevalent malignant tumor. Abnormal glycogen metabolism, an important metabolism process in HCC, mainly results from the variant structure, function, and expression levels of the corresponding enzymes and proteins. This variation, also called metabolic reprogramming, can regulate glycogen metabolic pathway to promote HCC tumorigenesis. This review aims to summarize glycogen metabolism-regulated factors (including glucose transporters, glycogen synthase kinase 3β, and glycogen phosphorylase) involved in glycogen metabolic reprogramming in HCC

Enzymatic regulation of seasonal glycogen cycling in the freeze-tolerant wood frog, Rana sylvatica

Liver glycogen is an important energy store in vertebrates, and in the freeze-tolerant wood frog, Rana sylvatica, this carbohydrate also serves as a major source of the cryoprotectant glucose. We investigated how variation in the levels of the catalytic subunit of protein kinase A (PKAc), glycogen phosphorylase (GP), and glycogen synthase (GS) relates to seasonal glycogen cycling in a temperate (Ohioan) and subarctic (Alaskan) populations of this species. In spring, Ohioan frogs had reduced potential for glycogen synthesis, as evidenced by low GS activity and high PKAc protein levels. In addition, glycogen levels in spring were the lowest of four seasonal samples, as energy input was likely directed towards metabolism and somatic growth during this period. Near-maximal glycogen levels were reached by mid-summer, and remained unchanged in fall and winter, suggesting that glycogenesis was curtailed during this period. Ohioan frogs had a high potential for glycogenolysis and glycogenesis in winter, as evidenced by large glycogen reserves, high levels of GP and GS proteins, and high GS activity, which likely allows for rapid mobilization of cryoprotectant during freezing and replenishing of glycogen reserves during thawing. Alaskan frogs also achieved a near-maximal liver glycogen concentration by summer and displayed high glycogenic and glycogenolytic potential in winter, but, unlike Ohioan frogs, started replenishing their energy reserves early in spring. We conclude that variation in levels of both glycogenolytic and glycogenic enzymes likely happens in response to seasonal changes in energetic strategies and demands, with winter survival being a key component to understanding the regulation of glycogen cycling in this species.

Glycogen metabolism in humans

In the human body, glycogen is a branched polymer of glucose stored mainly in the liver and the skeletal muscle that supplies glucose to the blood stream during fasting periods and to the muscle cells during muscle contraction. Glycogen has been identified in other tissues such as brain, heart, kidney, adipose tissue, and erythrocytes, but glycogen function in these tissues is mostly unknown. Glycogen synthesis requires a series of reactions that include glucose entrance into the cell through transporters, phosphorylation of glucose to glucose 6-phosphate, isomerization to glucose 1-phosphate, and formation of uridine 5'-diphosphate-glucose, which is the direct glucose donor for glycogen synthesis. Glycogenin catalyzes the formation of a short glucose polymer that is extended by the action of glycogen synthase. Glycogen branching enzyme introduces branch points in the glycogen particle at even intervals. Laforin and malin are proteins involved in glycogen assembly but their specific function remains elusive in humans. Glycogen is accumulated in the liver primarily during the postprandial period and in the skeletal muscle predominantly after exercise. In the cytosol, glycogen breakdown or glycogenolysis is carried out by two enzymes, glycogen phosphorylase which releases glucose 1-phosphate from the linear chains of glycogen, and glycogen debranching enzyme which untangles the branch points. In the lysosomes, glycogen degradation is catalyzed by α-glucosidase. The glucose 6-phosphatase system catalyzes the dephosphorylation of glucose 6-phosphate to glucose, a necessary step for free glucose to leave the cell. Mutations in the genes encoding the enzymes involved in glycogen metabolism cause glycogen storage diseases.

Differential Muscle Involvement in Mice and Humans Affected by McArdle Disease

McArdle disease (muscle glycogenosis type V) is caused by myophosphorylase deficiency, which leads to impaired glycogen breakdown. We investigated how myophosphorylase deficiency affects muscle physiology, morphology, and glucose metabolism in 20-week-old McArdle mice and compared the findings to those in McArdle disease patients. Muscle contractions in the McArdle mice were affected by structural degeneration due to glycogen accumulation, and glycolytic muscles fatigued prematurely, as occurs in the muscles of McArdle disease patients. Homozygous McArdle mice showed muscle fiber disarray, variations in fiber size, vacuoles, and some internal nuclei associated with cytosolic glycogen accumulation and ongoing regeneration; structural damage was seen only in a minority of human patients. Neither liver nor brain isoforms of glycogen phosphorylase were upregulated in muscles, thus providing no substitution for the missing muscle isoform. In the mice, the tibialis anterior (TA) muscles were invariably more damaged than the quadriceps muscles. This may relate to a 7-fold higher level of myophosphorylase in TA compared to quadriceps in wild-type mice and suggests higher glucose turnover in the TA. Thus, despite differences, the mouse model of McArdle disease shares fundamental physiological and clinical features with the human disease and could be used for studies of pathogenesis and development of therapies.

Regional variation of gene regulation associated with storage lipid metabolism in American glass eels (Anguilla rostrata)

Variation in gene regulation may be involved in the differences observed for life history traits within species. American eel (Anguilla rostrata) is well known to harbor distinct ecotypes within a single panmictic population. We examined the expression of genes involved in the regulation of appetite as well as lipid and glycogen among glass eels migrating to different locations on the Canadian east coast and captured at two different periods of upstream migration. Gene expression levels of three reference and five candidate genes were analyzed by real-time PCR with Taqman probes in recently captured wild glass eels. All gene transcripts were detected in glass eels. Of the five candidate genes, bile salt activated and triacylglycerol lipases were respectively 7.65 and 3.25 times more expressed in glass eels from the St. Lawrence estuary than in those from Nova Scotia, and there was no effect related to the two-week difference in capture date. These two genes explained 82.41% of the dissimilarity between the two rivers. In contrast, glycogen phosphorylase, ghrelin, and leptin receptor genes showed no significant differences in gene transcription. These results confirmed at the molecular level an observation that was recently made at the phenotypic level that glass eels from the St. Lawrence estuary have a greater capacity to use lipid reserves to sustain their metabolic needs. These observations add to the body of evidence supporting the hypothesis that regional phenotypic variation observed in American eel is determined early in life and that part of this variation is likely under genetic control.

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.

An evaluation of indirubin analogues as phosphorylase kinase inhibitors

Phosphorylase kinase (PhK) has been linked with a number of conditions such as glycogen storage diseases, psoriasis, type 2 diabetes and more recently, cancer (Camus et al., 2012 [6]). However, with few reported structural studies on PhK inhibitors, this hinders a structure based drug design approach. In this study, the inhibitory potential of 38 indirubin analogues have been investigated. 11 of these ligands had IC50 values in the range 0.170-0.360μM, with indirubin-3'-acetoxime (1c) the most potent. 7-Bromoindirubin-3'-oxime (13b), an antitumor compound which induces caspase-independent cell-death (Ribas et al., 2006 [20]) is revealed as a specific inhibitor of PhK (IC50=1.8μM). Binding assay experiments performed using both PhK-holo and PhK-γtrnc confirmed the inhibitory effects to arise from binding at the kinase domain (γ subunit). High level computations using QM/MM-PBSA binding free energy calculations were in good agreement with experimental binding data, as determined using statistical analysis, and support binding at the ATP-binding site. The value of a QM description for the binding of halogenated ligands exhibiting σ-hole effects is highlighted. A new statistical metric, the 'sum of the modified logarithm of ranks' (SMLR), has been defined which measures performance of a model for both the "early recognition" (ranking earlier/higher) of active compounds and their relative ordering by potency. Through a detailed structure activity relationship analysis considering other kinases (CDK2, CDK5 and GSK-3α/β), 6'(Z) and 7(L) indirubin substitutions have been identified to achieve selective PhK inhibition. The key PhK binding site residues involved can also be targeted using other ligand scaffolds in future work.

Sodium valproate increases the brain isoform of glycogen phosphorylase: looking for a compensation mechanism in McArdle disease using a mouse primary skeletal-muscle culture in vitro

McArdle disease, also termed ‘glycogen storage disease type V’, is a disorder of skeletal muscle carbohydrate metabolism caused by inherited deficiency of the muscle-specific isoform of glycogen phosphorylase (GP-MM). It is an autosomic recessive disorder that is caused by mutations in the PYGM gene and typically presents with exercise intolerance, i.e. episodes of early exertional fatigue frequently accompanied by rhabdomyolysis and myoglobinuria. Muscle biopsies from affected individuals contain subsarcolemmal deposits of glycogen. Besides GP-MM, two other GP isoforms have been described: the liver (GP-LL) and brain (GP-BB) isoforms, which are encoded by the PYGL and PYGB genes, respectively; GP-BB is the main GP isoform found in human and rat foetal tissues, including the muscle, although its postnatal expression is dramatically reduced in the vast majority of differentiated tissues with the exception of brain and heart, where it remains as the major isoform. We developed a cell culture model from knock-in McArdle mice that mimics the glycogen accumulation and GP-MM deficiency observed in skeletal muscle from individuals with McArdle disease. We treated mouse primary skeletal muscle cultures in vitro with sodium valproate (VPA), a histone deacetylase inhibitor. After VPA treatment, myotubes expressed GP-BB and a dose-dependent decrease in glycogen accumulation was also observed. Thus, this in vitro model could be useful for high-throughput screening of new drugs to treat this disease. The immortalization of these primary skeletal muscle cultures could provide a never-ending source of cells for this experimental model. Furthermore, VPA could be considered as a gene-expression modulator, allowing compensatory expression of GP-BB and decreased glycogen accumulation in skeletal muscle of individuals with McArdle disease.

Summary: Use of this in vitro model showed that sodium valproate (VPA) can reverse the muscle phenotype from a McArdle-like to a normal histological and biochemical profile.

Cod liver oil ameliorates sodium nitrite-induced insulin resistance and degradation of rat hepatic glycogen through inhibition of cAMP/PKA pathway

AIMS

Sodium nitrite is used to inhibit the growth of microorganisms and is responsible for the desirable red color of meat; however, it can be toxic in high quantities for humans and other animals. Moreover, glycogen, a branched polysaccharide, efficiently stores and releases glucose monosaccharides to be accessible for metabolic and synthetic requirements of the cell. Therefore, we examined the impact of dietary sodium nitrite and cod liver oil on liver glycogen.

MAIN METHODS

Thirty-two Sprague-Dawley rats were treated daily with sodium nitrite (80 mg/kg) in the presence/absence of cod liver oil (5 ml/kg). Liver sections were stained with Periodic acid-Schiff. Hepatic homogenates were used for measurements of glycogen, cyclic adenosine monophosphate (cAMP), protein kinase A (PKA), glycogen synthase, glycogen synthase kinase, pyruvate carboxylase, fructose 1,6-diphosphatase, glucose 6-phosphatase, phosphodiesterase and glycogen phosphorylase. Glucose, pyruvate tolerances and HOMA insulin resistance were also determined.

KEY FINDINGS

Sodium nitrite significantly increased plasma glucose and insulin resistance. Moreover, sodium nitrite significantly reduced hepatic glycogen content as well as activities of glycogen synthase, glycogen synthase kinase-3, and phosphodiesterase. Sodium nitrite elevated hepatic cAMP, PKA, pyruvate carboxylase, fructose 1,6-diphosphatase, glucose 6-phosphatase and phosphorylase. Cod liver oil significantly blocked all of these except pyruvate carboxylase, fructose 1,6-diphosphatase and glucose 6-phosphatase.

SIGNIFICANCE

Sodium nitrite inhibited liver glycogenesis and enhanced liver glycogenolysis and gluconeogenesis, which is accompanied by hyperglycemia and insulin resistance through the activation of cAMP/PKA and the inhibition of phosphodiesterase. Cod liver oil blocked the sodium nitrite effects on glycogenesis and glycogenolysis without affecting gluconeogenesis.

Investigating sodium valproate as a treatment for McArdle disease in sheep

McArdle disease is due to an absence of the enzyme muscle glycogen phosphorylase and results in significant physical impairment in humans. We hypothesised that sodium valproate, an HDAC inhibitor, might have the ability to up-regulate the enzyme. We treated McArdle sheep with sodium valproate given enterically at 20-60 mg/kg body wt. Compared with untreated control animals, there was increased expression of phosphorylase in muscle fibres. The response was dose dependent and reached a maximum 2 hours after the application and increased with repeated applications. Improvement in mobility could not be demonstrated. These findings suggest that sodium valproate is a potential therapeutic treatment for McArdle disease.

Sex-specific basal and hypoglycemic patterns of in vivo caudal dorsal vagal complex astrocyte glycogen metabolic enzyme protein expression

Astrocytes contribute to neurometabolic stability through uptake, catabolism, and storage of glucose. These cells maintain the major brain glycogen reservoir, which is a critical fuel supply to neurons during glucose deficiency and increased brain activity. We used a combinatory approach incorporating immunocytochemistry, laser microdissection, and Western blotting to investigate the hypothesis of divergent expression of key enzymes regulating glycogen metabolism and glycolysis during in vivo normo- and/or hypoglycemia in male versus female hindbrain astrocytes. Glycogen synthase (GS) and glycogen phosphorylase (GP) levels were both enhanced in dorsal vagal complex astrocytes from vehicle-injected female versus male controls, with incremental increase in GS exceeding GP. Insulin-induced hypoglycemia (IIH) diminished GS and increased glycogen synthase kinase-3-beta (GSK3β) expression in both sexes, but decreased phosphoprotein phosphatase-1 (PP1) levels only in males. Astrocyte GP content was elevated by IIH in male, but not female rats. Data reveal sex-dependent sensitivity of these enzyme proteins to lactate as caudal hindbrain repletion of this energy substrate fully or incompletely reversed hypoglycemic inhibition of GS and prevented hypoglycemic augmentation of GSK3β and GP in females and males, respectively. Sex dimorphic patterns of glycogen branching and debranching enzyme protein expression were also observed. Levels of the rate-limiting glycolytic enzyme, phosphofructokinase, were unaffected by IIH with or without lactate repletion. Current data demonstrating sex-dependent basal and hypoglycemic patterns of hindbrain astrocyte glycogen metabolic enzyme expression imply that glycogen volume and turnover during glucose sufficiency and shortage may vary accordingly.

Tadalafil inhibits the cAMP stimulated glucose output in the rat liver

The purpose of the present work was to verify if tadalafil affects hepatic glucose output, one of the primary targets of cAMP, in the isolated perfused rat liver. No effects on glycogen catabolism and oxygen uptake were found under basal conditions for tadalafil concentrations in the range between 0.25 and 10 μM. However, tadalafil had a clear and time-dependent inhibitory effect on the cAMP- and glucagon-stimulated glucose release. Constant infusion of tadalafil in the range between 0.25 and 10 μM eventually abolished 100% of the stimulatory action of those effectors. The tadalafil concentrations producing half-maximal rates of inhibition of the cAMP and glucagon stimulated glycogenolysis were 0.46±0.04 and 1.07±0.16 μM, respectively. These concentrations are close to the plasma peak concentrations in patients after ingestion of 20 mg tadalafil. The drug also diminished the activity of glycogen phosphorylase a and increased the activities of glucose 6-phosphatase, glucokinase, pyruvate kinase and glucose 6-phosphate dehydrogenase. These actions occurred only in the cellular environment. Tadalafil did not affect binding of cAMP to protein kinase A. Diminution of cAMP-stimulated glucose output is the opposite of what can be expected from a phosphodiesterase inhibition, the most common effect attributed to tadalafil. Diminution of glucose output by tadalafil can be attributed (a) to an interference with glycogen phosphorylase stimulation and (b) to an increased futile cycling of glucose 6-phosphate and glucose with a concomitant increased flow of hexose units into cellular metabolic pathways. The effects described in the present work may prove to represent important side effects of tadalafil.

Differential effects of temperature and glucose on glycogenolytic enzymes in tissues of rainbow trout (Oncorhynchus mykiss)

The pathways and regulatory mechanisms of glycogenolysis remain relatively unexplored in non-mammalian vertebrates, especially poikilotherms. We studied the temperature sensitivity and inhibition of glycogenolytic enzymes in liver, ventricle, and white muscle of rainbow trout acclimated to 14 °C. Glycogen phosphorylase (GP) and acid α-glucosidase (GAA) activities were measured in homogenates of tissues at physiological temperatures (4, 14, and 24 °C), and in the presence of allosteric inhibitor, glucose. Higher GP versus GAA activity in all three tissues suggested a predominance of phosphorolytic glycogenolysis over the lysosomal glucosidic pathway. GP activities at 14 °C were ~2-fold higher in the ventricle and white muscle versus the liver and selectively increased by AMP in striated muscle. Conversely, the activities of GAA and lysosomal marker acid phosphatase were 8- to 10-fold higher in the liver compared with the ventricle and white muscle. Thermal sensitivity (Q10) was increased for GP in all tissues below 14 °C and decreased in striated muscle in the absence of AMP above 14 °C. GAA had lower Q10 values than GP below 14 °C, and, unlike GP, Q10s for GAA were not different between tissues or affected by temperature. Both GP (in the absence of AMP) and GAA were inhibited by glucose in a dose-dependent manner, with the lowest IC50 values observed in the white muscle (1.4 and 6.3 mM, respectively). In conclusion, despite comparatively low kinetic potential, lysosomal GAA might facilitate glycogenolysis at colder body temperatures in striated muscle and intracellular glucose could limit phosphorolytic and glucosidic glycogenolysis in multiple tissues of the rainbow trout.

Activity of glycogen synthase and glycogen phosphorylase in normal and cirrhotic rat liver during glycogen synthesis from glucose or fructose

Cirrhotic patients often demonstrate glucose intolerance, one of the possible causes being a decreased glycogen-synthesizing capacity of the liver. At the same time, information about the rates of glycogen synthesis in the cirrhotic liver is scanty and contradictory. We studied the dynamics of glycogen accumulation and the activity of glycogen synthase (GS) and glycogen phosphorylase (GP) in the course of 120min after per os administration of glucose or fructose to fasted rats with CCl4-cirrhosis or fasted normal rats. Blood serum and liver pieces were sampled for examinations. In the normal rat liver administration of glucose/fructose initiated a fast accumulation of glycogen, while in the cirrhotic liver glycogen was accumulated with a 20min delay and at a lower rate. In the normal liver GS activity rose sharply and GPa activity dropped in the beginning of glycogen synthesis, but 60min later a high synthesis rate was sustained at the background of a high GS and GPa activity. Contrariwise, in the cirrhotic liver glycogen was accumulated at the background of a decreased GS activity and a low GPa activity. Refeeding with fructose resulted in a faster increase in the GS activity in both the normal and the cirrhotic liver than refeeding with glucose. To conclude, the rate of glycogen synthesis in the cirrhotic liver is lower than in the normal one, the difference being probably associated with a low GS activity.

Identification and characterization of granule bound starch synthase I (GBSSI) gene of tartary buckwheat (Fagopyrum tataricum Gaertn.)

Tartary buckwheat (Fagopyrum tataricum Gaertn.) is increasingly considered as an important functional food material because of its rich nutraceutical compounds. Reserve starch is the major component of tartary buckwheat seed. However, the gene sequences and the molecular mechanism of tartary buckwheat starch synthesis are unknown so far. In this study, the complete genomic sequence and full-size cDNA coding tartary buckwheat granule-bound starch synthase I (FtGBSSI), which is responsible for amylose synthesis, were isolated and analyzed. The genomic sequence of the FtGBSSI contained 3947 nucleotides and was composed of 14 exons and 13 introns. The cDNA coding sequence of FtGBSSI shared 63.3%-75.1% identities with those of dicots and 56.6%-57.5% identities with monocots (Poaceae). In deduced amino acid sequence of FtGBSSI, eight motifs conserved among plant starch synthases were identified. A cleavage at the site IVC↓G of FtGBSSI protein produces the chloroplast transit sequence of 78 amino acids and the mature protein of 527 amino acids. The FtGBSSI mature protein showed an identity of 73.4%-77.8% with dicot plants, and 67.6%-70.4% with monocot plants (Poaceae). The mature protein was composed of 20 α-helixes and 16 β-strands, and folds into two main domains, N- and C-terminal domains. The critical residues which are involved in ADP and sugar binding were predicted. These results will be useful to modulate starch composition of buckwheat kernels with the aim to produce novel improved varieties in future breeding programs.

Phosphorylase re-expression, increase in the force of contraction and decreased fatigue following notexin-induced muscle damage and regeneration in the ovine model of McArdle disease

McArdle disease is caused by a deficiency of myophosphorylase and currently a satisfactory treatment is not available. The injection of notexin into, or the layering of notexin onto, the muscles of affected sheep resulted in necrosis followed by regeneration of muscle fibres with the expression of both non-muscle isoforms of phosphorylase within the fibres and a reduction of the amount of glycogen in the muscle with an increase in the strength of contraction and a decrease in fatiguability in the muscle fibres. The sustained re-expression of both the brain and liver isoforms of phosphorylase within the muscle fibres provides further emphasis that strategies to enhance the re-expression of these isoforms should be investigated as a possible treatment for McArdle disease.