Isoform-selective regulation of glycogen phosphorylase by energy deprivation and phosphorylation in astrocytes

Brain-Type Glycogen Phosphorylase (PYGB) in the Pathologies of Diseases: A Systematic Review

Glycogen metabolism is a form of crucial metabolic reprogramming in cells. PYGB, the brain-type glycogen phosphorylase (GP), serves as the rate-limiting enzyme of glycogen catabolism. Evidence is mounting for the association of PYGB with diverse human diseases. This review covers the advancements in PYGB research across a range of diseases, including cancer, cardiovascular diseases, metabolic diseases, nervous system diseases, and other diseases, providing a succinct overview of how PYGB functions as a critical factor in both physiological and pathological processes. We present the latest progress in PYGB in the diagnosis and treatment of various diseases and discuss the current limitations and future prospects of this novel and promising target.

Differential G Protein-Coupled Estrogen Receptor-1 Regulation of Counter-Regulatory Transmitter Marker and 5'-AMP-Activated Protein Kinase Expression in Ventrolateral versus Dorsomedial Ventromedial Hypothalamic Nucleus

INTRODUCTION

The ventromedial hypothalamic nucleus (VMN) is an estrogen receptor (ER)-rich structure that regulates glucostasis. The role of nuclear but not membrane G protein-coupled ER-1 (GPER) in that function has been studied.

METHODS

Gene silencing and laser-catapult microdissection/immunoblot tools were used to examine whether GPER regulates transmitter and energy sensor function in dorsomedial (VMNdm) and/or ventrolateral (VMNvl) VMN counter-regulatory nitrergic and γ-Aminobutyric acid (GABA) neurons.

RESULTS

Intra-VMN GPER siRNA administration to euglycemic animals did not affect VMNdm or -vl nitrergic neuron nitric oxide synthase (nNOS), but upregulated (VMNdm) or lacked influence on (VMNvl) GABA nerve cell glutamate decarboxylase65/67 (GAD) protein. Insulin-induced hypoglycemia (IIH) caused GPER knockdown-reversible augmentation of nNOS, 5'-AMP-activated protein kinase (AMPK), and phospho-AMPK proteins in nitrergic neurons in both divisions. IIH had dissimilar effects on VMNvl (unchanged) versus VMNdm (increased) GABAergic neuron GAD levels, yet GPER knockdown affected these profiles. GPER siRNA prevented hypoglycemic upregulation of VMNvl and -dm GABA neuron AMPK without altering pAMPK expression.

CONCLUSIONS

Outcomes infer that GPER exerts differential control of VMNdm versus -vl GABA transmission during glucostasis and is required for hypoglycemic upregulated nitrergic (VMNdm and -vl) and GABA (VMNdm) signaling. Glycogen metabolism is reported to regulate VMN nNOS and GAD proteins. Data show that GPER limits VMNvl glycogen phosphorylase (GP) protein expression and glycogen buildup during euglycemia but mediates hypoglycemic augmentation of VMNvl GP protein and glycogen content; VMNdm glycogen mass is refractory to GPER control. GPER regulation of VMNvl glycogen metabolism infers that this receptor may govern local counter-regulatory transmission in part by astrocyte metabolic coupling.

Glycogen phosphorylase isoenzyme GPbb versus GPmm regulation of ventromedial hypothalamic nucleus glucoregulatory neurotransmitter and counter-regulatory hormone profiles during hypoglycemia: Role of L-lactate and octadecaneuropeptide

Glucose accesses the brain primarily via the astrocyte cell compartment, where it passes through the glycogen shunt before catabolism to the oxidizable fuel L-lactate. Glycogen phosphorylase (GP) isoenzymes GPbb and GPmm impose distinctive control of ventromedial hypothalamic nucleus (VMN) glucose-regulatory neurotransmission during hypoglycemia, but lactate and/or gliotransmitter involvement in those actions is unknown. Lactate or the octadecaneuropeptide receptor antagonist cyclo(1-8)[DLeu5] OP (LV-1075) did not affect gene product down-regulation caused by GPbb or GPmm siRNA, but suppressed non-targeted GP variant expression in a VMN region-specific manner. Hypoglycemic up-regulation of neuronal nitric oxide synthase was enhanced in rostral and caudal VMN by GPbb knockdown, yet attenuated by GPMM siRNA in the middle VMN; lactate or LV-1075 reversed these silencing effects. Hypoglycemic inhibition of glutamate decarboxylase65/67 was magnified by GPbb (middle and caudal VMN) or GPmm (middle VMN) knockdown, responses that were negated by lactate or LV-1075. GPbb or GPmm siRNA enlarged hypoglycemic VMN glycogen profiles in rostral and middle VMN. Lactate and LV-1075 elicited progressive rostral VMN glycogen augmentation in GPbb knockdown rats, but stepwise-diminution of rostral and middle VMN glycogen after GPmm silencing. GPbb, not GPmm, knockdown caused lactate or LV-1075 - reversible amplification of hypoglycemic hyperglucagonemia and hypercorticosteronemia. Results show that lactate and octadecaneuropeptide exert opposing control of GPbb protein in distinct VMN regions, while the latter stimulates GPmm. During hypoglycemia, GPbb and GPmm may respectively diminish (rostral, caudal VMN) or enhance (middle VMN) nitrergic transmission and each oppose GABAergic signaling (middle VMN) by lactate- and octadecaneuropeptide-dependent mechanisms.

Effects of Ventromedial Hypothalamic Nucleus (VMN) Aromatase Gene Knockdown on VMN Glycogen Metabolism and Glucoregulatory Neurotransmission

The enzyme aromatase is expressed at high levels in the ventromedial hypothalamic nucleus (VMN), a principal component of the brain gluco-regulatory network. Current research utilized selective gene knockdown tools to investigate the premise that VMN neuroestradiol controls glucostasis. Intra-VMN aromatase siRNA administration decreased baseline aromatase protein expression and tissue estradiol concentrations and either reversed or attenuated the hypoglycemic regulation of these profiles in a VMN segment-specific manner. Aromatase gene repression down-regulated protein biomarkers for gluco-stimulatory (nitric oxide; NO) and -inhibitory (gamma-aminobutyric acid; GABA) neurochemical transmitters. Insulin-induced hypoglycemia (IIH) up- or down-regulated neuronal nitric oxide synthase (nNOS) and glutamate decarboxylase_65/67 (GAD), respectively, throughout the VMN. Interestingly, IIH caused divergent changes in tissue aromatase and estradiol levels in rostral (diminished) versus middle and caudal (elevated) VMN. Aromatase knockdown prevented hypoglycemic nNOS augmentation in VMN middle and caudal segments, but abolished the GAD inhibitory response to IIH throughout this nucleus. VMN nitrergic and GABAergic neurons monitor stimulus-specific glycogen breakdown. Here, glycogen synthase (GS) and phosphorylase brain- (GPbb; AMP-sensitive) and muscle- (GPmm; noradrenergic -responsive) type isoform responses to aromatase siRNA were evaluated. Aromatase repression reduced GPbb and GPmm content in euglycemic controls and prevented hypoglycemic regulation of GPmm but not GPbb expression while reversing glycogen accumulation. Aromatase siRNA elevated baseline glucagon and corticosterone secretion and abolished hypoglycemic hyperglucagonemia and hypercorticosteronemia. Outcomes document the involvement of VMN neuroestradiol signaling in brain control of glucose homeostasis. Aromatase regulation of VMN gluco-regulatory signaling of hypoglycemia-associated energy imbalance may entail, in part, control of GP variant-mediated glycogen disassembly.

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.

G protein-coupled lactate receptor GPR81 control of ventrolateral ventromedial hypothalamic nucleus glucoregulatory neurotransmitter and 5'-AMP-activated protein kinase expression

Astrocytes store glycogen as energy and promote neurometabolic stability through supply of oxidizable l-lactate. Whether lactate regulates ventromedial hypothalamic nucleus (VMN) glucostatic function as a metabolic volume transmitter is unknown. Current research investigated whether G protein-coupled lactate receptor GPR81 controls astrocyte glycogen metabolism and glucose-regulatory neurotransmission in the ventrolateral VMN (VMNvl), where glucose-regulatory neurons reside. Female rats were pretreated by intra-VMN GPR81 or scramble siRNA infusion before insulin or vehicle injection. VMNvl cell or tissue samples were acquired by laser-catapult- or micropunch microdissection for Western blot protein or uHPLC-electrospray ionization-mass spectrometric glycogen analyses. Data show that GPR81 regulates eu- and/or hypoglycemic patterns of VMNvl astrocyte glycogen metabolic enzyme and 5'-AMP-activated protein kinase (AMPK) protein expression according to VMNvl segment. GPR81 stimulates baseline rostral and caudal VMNvl glycogen accumulation but mediates glycogen breakdown in the former site during hypoglycemia. During euglycemia, GPR81 suppresses the transmitter marker neuronal nitric oxide synthase (nNOS) in rostral and caudal VMNvl nitrergic neurons, but stimulates (rostral VMNvl) or inhibits (caudal VMNvl) GABAergic neuron glutamate decarboxylase65/67 (GAD)protein. During hypoglycemia, GPR81 regulates AMPK activation in nitrergic and GABAergic neurons located in the rostral, but not caudal VMNvl. VMN GPR81 knockdown amplified hypoglycemic hypercorticosteronemia, but not hyperglucagonemia. Results provide novel evidence that VMNvl astrocyte and glucose-regulatory neurons express GPR81 protein. Data identify neuroanatomical subpopulations of VMNvl astrocytes and glucose-regulatory neurons that exhibit differential reactivity to GPR81 input. Heterogeneous GPR81 effects during eu- versus hypoglycemia infer that energy state may affect cellular sensitivity to or postreceptor processing of lactate transmitter signaling.

Proteomic characterization of spontaneously regrowing spinal cord following injury in the teleost fish Apteronotus leptorhynchus, a regeneration-competent vertebrate

In adult mammals, spontaneous repair after spinal cord injury (SCI) is severely limited. By contrast, teleost fish successfully regenerate injured axons and produce new neurons from adult neural stem cells after SCI. The molecular mechanisms underlying this high regenerative capacity are largely unknown. The present study addresses this gap by examining the temporal dynamics of proteome changes in response to SCI in the brown ghost knifefish (Apteronotus leptorhynchus). Two-dimensional difference gel electrophoresis (2D DIGE) was combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and tandem mass spectrometry (MS/MS) to collect data during early (1 day), mid (10 days), and late (30 days) phases of regeneration following caudal amputation SCI. Forty-two unique proteins with significant differences in abundance between injured and intact control samples were identified. Correlation analysis uncovered six clusters of spots with similar expression patterns over time and strong conditional dependences, typically within functional families or between isoforms. Significantly regulated proteins were associated with axon development and regeneration; proliferation and morphogenesis; neuronal differentiation and re-establishment of neural connections; promotion of neuroprotection, redox homeostasis, and membrane repair; and metabolism or energy supply. Notably, at all three time points examined, significant regulation of proteins involved in inflammatory responses was absent.

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.

Liver glycogen phosphorylase is upregulated in glioblastoma and provides a metabolic vulnerability to high dose radiation

Channelling of glucose via glycogen, known as the glycogen shunt, may play an important role in the metabolism of brain tumours, especially in hypoxic conditions. We aimed to dissect the role of glycogen degradation in glioblastoma (GBM) response to ionising radiation (IR). Knockdown of the glycogen phosphorylase liver isoform (PYGL), but not the brain isoform (PYGB), decreased clonogenic growth and survival of GBM cell lines and sensitised them to IR doses of 10-12 Gy. Two to five days after IR exposure of PYGL knockdown GBM cells, mitotic catastrophy and a giant multinucleated cell morphology with senescence-like phenotype developed. The basal levels of the lysosomal enzyme alpha-acid glucosidase (GAA), essential for autolysosomal glycogen degradation, and the lipidated forms of gamma-aminobutyric acid receptor-associated protein-like (GABARAPL1 and GABARAPL2) increased in shPYGL U87MG cells, suggesting a compensatory mechanism of glycogen degradation. In response to IR, dysregulation of autophagy was shown by accumulation of the p62 and the lipidated form of GABARAPL1 and GABARAPL2 in shPYGL U87MG cells. IR increased the mitochondrial mass and the colocalisation of mitochondria with lysosomes in shPYGL cells, thereby indicating reduced mitophagy. These changes coincided with increased phosphorylation of AMP-activated protein kinase and acetyl-CoA carboxylase 2, slower ATP generation in response to glucose loading and progressive loss of oxidative phosphorylation. The resulting metabolic deficiencies affected the availability of ATP required for mitosis, resulting in the mitotic catastrophy observed in shPYGL cells following IR. PYGL mRNA and protein levels were higher in human GBM than in normal human brain tissues and high PYGL mRNA expression in GBM correlated with poor patient survival. In conclusion, we show a major new role for glycogen metabolism in GBM cancer. Inhibition of glycogen degradation sensitises GBM cells to high-dose IR indicating that PYGL is a potential novel target for the treatment of GBMs.

Lactate Supply from Astrocytes to Neurons and its Role in Ischemic Stroke-induced Neurodegeneration

Glucose transported to the brain is metabolized to lactate in astrocytes and supplied to neuronal cells via a monocarboxylic acid transporter (MCT). Lactate is used in neuronal cells for various functions, including learning and memory formation. Furthermore, lactate can block stroke-induced neurodegeneration. We aimed to clarify the effect of astrocyte-produced lactate on stroke-induced neurodegeneration. Previously published in vivo and in vitro animal and cell studies, respectively, were searched in PubMed, ScienceDirect, and Web of Science. Under physiological conditions, lactate production and release by astrocytes are regulated by changes in lactate dehydrogenase (LDH) and MCT expression. Moreover, considering stroke, lactate production and supply are regulated through hypoxia-inducible factor (HIF)-1α expression, especially with hypoxic stimulation, which may promote neuronal apoptosis; contrastingly, neuronal survival may be promoted via HIF-1α. Stroke stimulation could prevent neurodegeneration through the strong enhancement of lactate production, as well as upregulation of MCT4 expression to accelerate lactate supply. However, studies using astrocytes derived from animal stroke models revealed significantly reduced lactate production and MCT expression. These findings suggest that the lack of lactate supply may strongly contribute to hypoxia-induced neurodegeneration. Furthermore, diminished lactate supply from astrocytes could facilitate stroke-induced neurodegeneration. Therefore, astrocyte-derived lactate may contribute to stroke prevention.

Glycogen Phosphorylase Isoform Regulation of Ventromedial Hypothalamic Nucleus Gluco-Regulatory Neuron 5'-AMP-Activated Protein Kinase and Transmitter Marker Protein Expression

Brain glycogen is remodeled during metabolic homeostasis and provides oxidizable L-lactate equivalents. Brain glycogen phosphorylase (GP)-brain (GPbb; AMP-sensitive) and -muscle (GPmm; norepinephrine-sensitive) type isoforms facilitate stimulus-specific control of glycogen disassembly. Here, a whole animal model involving stereotactic-targeted delivery of GPmm or GPbb siRNA to the ventromedial hypothalamic nucleus (VMN) was used to investigate the premise that these variants impose differential control of gluco-regulatory transmission. Intra-VMN GPmm or GPbb siRNA administration inhibited glutamate decarboxylate_65/67 (GAD), a protein marker for the gluco-inhibitory transmitter γ--aminobutyric acid (GABA), in the caudal VMN. GPbb knockdown, respectively overturned or exacerbated hypoglycemia-associated GAD suppression in rostral and caudal VMN. GPmm siRNA caused a segment-specific reversal of hypoglycemic augmentation of the gluco-stimulatory transmitter indicator, neuronal nitric oxide synthase (nNOS). In both cell types, GP siRNA down-regulated 5′-AMP-activated protein kinase (AMPK) during euglycemia, but hypoglycemic suppression of AMPK was reversed by GPmm targeting. GP knockdown elevated baseline GABA neuron phosphoAMPK (pAMKP) content, and amplified hypoglycemic augmentation of pAMPK expression in each neuron type. GPbb knockdown increased corticosterone secretion in eu- and hypoglycemic rats. Outcomes validate efficacy of GP siRNA delivery for manipulation of glycogen breakdown in discrete brain structures in vivo, and document VMN GPbb control of local GPmm expression. Results document GPmm and/or -bb regulation of GABAergic and nitrergic transmission in discrete rostro-caudal VMN segments. Contrary effects of glycogenolysis on metabolic-sensory AMPK protein during eu- versus hypoglycemia may reflect energy state-specific astrocyte signaling. Amplifying effects of GPbb knockdown on hypoglycemic stimulation of pAMPK infer that glycogen mobilization by GPbb limits neuronal energy instability during hypoglycemia.

Norepinephrine Regulation of Adrenergic Receptor Expression, 5' AMP-Activated Protein Kinase Activity, and Glycogen Metabolism and Mass in Male Versus Female Hypothalamic Primary Astrocyte Cultures

Norepinephrine (NE) control of hypothalamic gluco-regulation involves astrocyte-derived energy fuel supply. In male rats, exogenous NE regulates astrocyte glycogen metabolic enzyme expression in vivo through 5’-AMP-activated protein kinase (AMPK)-dependent mechanisms. Current research utilized a rat hypothalamic astrocyte primary culture model to investigate the premise that NE imposes sex-specific direct control of AMPK activity and glycogen mass and metabolism in these glia. In male rats, NE down-regulation of pAMPK correlates with decreased CaMMKB and increased PP1 expression, whereas noradrenergic augmentation of female astrocyte pAMPK may not involve these upstream regulators. NE concentration is a critical determinant of control of hypothalamic astrocyte glycogen enzyme expression, but efficacy varies between sexes. Data show sex variations in glycogen synthase expression and glycogen phosphorylase-brain and –muscle type dose-responsiveness to NE. Narrow dose-dependent NE augmentation of astrocyte glycogen content during energy homeostasis infers that NE maintains, over a broad exposure range, constancy of glycogen content despite possible changes in turnover. In male rats, beta₁- and beta₂-adrenergic receptor (AR) profiles displayed bi-directional responses to increasing NE doses; female astrocytes exhibited diminished beta₁-AR content at low dose exposure, but enhanced beta₂-AR expression at high NE dosages. Thus, graded variations in noradrenergic stimulation may modulate astrocyte receptivity to NE in vivo. Sex dimorphic NE regulation of hypothalamic astrocyte AMPK activation and glycogen metabolism may be mediated, in part, by one or more ARs characterized here by divergent sensitivity to this transmitter.

Haider A, Alshamrani A, Alhamyani A, Bheemanapally K, Ibrahim MM. 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.

The Motor Neuron-Like Cell Line NSC-34 and Its Parent Cell Line N18TG2 Have Glycogen that is Degraded Under Cellular Stress

Brain glycogen has a long and versatile history: Primarily regarded as an evolutionary remnant, it was then thought of as an unspecific emergency fuel store. A dynamic role for glycogen in normal brain function has been proposed later but exclusively attributed to astrocytes, its main storage site. Neuronal glycogen had long been neglected, but came into focus when sensitive technical methods allowed quantification of glycogen at low concentration range and the detection of glycogen metabolizing enzymes in cells and cell lysates. Recently, an active role of neuronal glycogen and even its contribution to neuronal survival could be demonstrated. We used the neuronal cell lines NSC-34 and N18TG2 and could demonstrate that they express the key-enzymes of glycogen metabolism, glycogen phosphorylase and glycogen synthase and contain glycogen which is mobilized on glucose deprivation and elevated potassium concentrations, but not by hormones stimulating cAMP formation. Conditions of metabolic stress, namely hypoxia, oxidative stress and pH lowering, induce glycogen degradation. Our studies revealed that glycogen can contribute to the energy supply of neuronal cell lines in situations of metabolic stress. These findings shed new light on the so far neglected role of neuronal glycogen. The key-enzyme in glycogen degradation is glycogen phosphorylase. Neurons express only the brain isoform of the enzyme that is supposed to be activated primarily by the allosteric activator AMP and less by covalent phosphorylation via the cAMP cascade. Our results indicate that neuronal glycogen is not degraded upon hormone action but by factors lowering the energy charge of the cells directly.

Norepinephrine Regulation of Ventromedial Hypothalamic Nucleus Astrocyte Glycogen Metabolism

The catecholamine norepinephrine (NE) links hindbrain metabolic-sensory neurons with key glucostatic control structures in the brain, including the ventromedial hypothalamic nucleus (VMN). In the brain, the glycogen reserve is maintained within the astrocyte cell compartment as an alternative energy source to blood-derived glucose. VMN astrocytes are direct targets for metabolic stimulus-driven noradrenergic signaling due to their adrenergic receptor expression (AR). The current review discusses recent affirmative evidence that neuro-metabolic stability in the VMN may be shaped by NE influence on astrocyte glycogen metabolism and glycogen-derived substrate fuel supply. Noradrenergic modulation of estrogen receptor (ER) control of VMN glycogen phosphorylase (GP) isoform expression supports the interaction of catecholamine and estradiol signals in shaping the physiological stimulus-specific control of astrocyte glycogen mobilization. Sex-dimorphic NE control of glycogen synthase and GP brain versus muscle type proteins may be due, in part, to the dissimilar noradrenergic governance of astrocyte AR and ER variant profiles in males versus females. Forthcoming advances in the understanding of the molecular mechanistic framework for catecholamine stimulus integration with other regulatory inputs to VMN astrocytes will undoubtedly reveal useful new molecular targets in each sex for glycogen mediated defense of neuronal metabolic equilibrium during neuro-glucopenia.

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-dimorphic neuroestradiol regulation of ventromedial hypothalamic nucleus glucoregulatory transmitter and glycogen metabolism enzyme protein expression in the rat

Background

Ventromedial hypothalamic nucleus (VMN) gluco-regulatory transmission is subject to sex-specific control by estradiol. The VMN is characterized by high levels of aromatase expression.

Methods

The aromatase inhibitor letrozole (LZ) was used with high-resolution microdissection/Western blot techniques to address the hypothesis that neuroestradiol exerts sex-dimorphic control of VMN neuronal nitric oxide synthase (nNOS) and glutamate decarboxylase_65/67 (GAD) protein expression. Glycogen metabolism impacts VMN nNOS and GAD profiles; here, LZ treatment effects on VMN glycogen synthase (GS) and phosphorylase brain- (GPbb; glucoprivic-sensitive) and muscle (GPmm; norepinephrine-sensitive) variant proteins were examined.

Results

VMN aromatase protein content was similar between sexes. Intracerebroventricular LZ infusion of testes-intact male and ovariectomized, estradiol-replaced female rats blocked insulin-induced hypoglycemic (IIH) up-regulation of this profile. LZ exerted sex-contingent effects on basal VMN nNOS and GAD expression, but blocked IIH-induced NO stimulation and GAD suppression in each sex. Sex-contingent LZ effects on basal and hypoglycemic patterns of GPbb and GPmm expression occurred at distinctive levels of the VMN. LZ correspondingly down- or up-regulated baseline pyruvate recycling pathway marker protein expression in males (glutaminase) and females (malic enzyme-1), and altered INS effects on those proteins.

Conclusions

Results infer that neuroestradiol is required in each sex for optimal VMN metabolic transmitter signaling of hypoglycemic energy deficiency. Sex differences in VMN GP variant protein levels and sensitivity to aromatase may correlate with sex-dimorphic glycogen mobilization during this metabolic stress. Neuroestradiol may also exert sex-specific effects on glucogenic amino acid energy yield by actions on distinctive enzyme targets in each sex.

Impact of caudal hindbrain glycogen metabolism on A2 noradrenergic neuron AMPK activation and ventromedial hypothalamic nucleus norepinephrine activity and glucoregulatory neurotransmitter marker protein expression

The brain glycogen reserve is a source of oxidizable substrate fuel. Lactoprivic-sensitive hindbrain A2 noradrenergic neurons provide crucial metabolic-sensory input to downstream hypothalamic glucose-regulatory structures. Current research examined whether hindbrain glycogen fuel supply impacts A2 energy stability and governance of ventromedial hypothalamic nucleus (VMN) metabolic transmitter signaling. Male rats were injected into the caudal fourth ventricle (CV4) with the glycogen phosphorylase inhibitor 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) prior to continuous intra-CV4 infusion of L-lactate or vehicle. Lactate reversed DAB suppression of A2 neuron AMPK protein and up-regulated phosphoAMPK profiles. A2 dopamine-β-hydroxylase expression was refractory to DAB, but elevated by DAB/lactate. Lactate normalized A2 estrogen receptor-alpha and GPER proteins and up-regulated estrogen receptor-beta levels in DAB-treated rats. VMN norepinephrine content was decreased by DAB, but partially restored by lactate. DAB caused lactate-reversible or -irreversible augmentation of VMN glycogen phosphorylase-brain (GPbb) and -muscle type (GPmm) variant profiles, and correspondingly up- or down-regulated VMN protein markers of glucose-stimulatory nitrergic and glucose-inhibitory γ-aminobutyric acid transmission. DAB did not alter plasma glucose, but suppressed or elevated circulating glucagon and corticosterone in that order. Results show that diminished hindbrain glycogen breakdown is communicated to the VMN, in part by NE signaling, to up-regulate VMN glycogen breakdown and trigger neurochemical signaling of energy imbalance in that site. DAB effects on GPmm, VMN glycogen content, and counter-regulatory hormone secretion were unabated by lactate infusion, suggesting that aside from substrate fuel provision rate, additional indicators of glycogen metabolism such as turnover rate may be monitored in the hindbrain.

Estrogen Receptor Involvement in Noradrenergic Regulation of Ventromedial Hypothalamic Nucleus Glucoregulatory Neurotransmitter and Stimulus-Specific Glycogen Phosphorylase Enzyme Isoform Expression

Norepinephrine (NE) directly regulates ventromedial hypothalamic nucleus (VMN) glucoregulatory neurons and also controls glycogen-derived fuel provision to those cells. VMN nitric oxide (NO) and γ-aminobutyric acid (GABA) neurons and astrocytes express estrogen receptor-alpha (ERα) and ER-beta (ERβ) proteins. Current research used selective ERα (1,3Bis(4-hydroxyphenyl)-4-methyl-5-[4-(2-piperidinylethoxy)phenol]-1H-pyrazole dihydrochloride) or ERβ (4-[2-phenyl-5,7-bis(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-3-yl]phenol) antagonists to address the premise that these ERs govern basal and/or NE-associated patterns of VMN metabolic neuron signaling and astrocyte glycogen metabolism. Both ERs stimulate expression of the enzyme marker protein neuronal nitric oxide synthase, not glutamate decarboxylase_65/67. NE inhibition or augmentation of neuronal nitric oxide synthase and glutamate decarboxylase_65/67 profiles was ER-independent or -dependent, respectively. In both neuron types, VMN ERβ activity inhibited baseline alpha1- (α₁-) and/or alpha2- (α₂-)adrenergic receptor (AR) expression, but ERα and -β signaling was paradoxically crucial for noradrenergic upregulation of α₂-AR. NE inhibited glycogen synthase expression and exerted opposite effects on VMN adenosine monophosphate-sensitive glycogen phosphorylase (GP)-brain type (stimulatory) versus NE-sensitive GP muscle (inhibitory) via ERα or -β activity. Results document unique ERα and ERβ actions on metabolic transmitter and AR protein expression in VMN nitrergic versus GABAergic neurons. ER effects varied in the presence versus absence of NE, indicating that both neuron types are substrates for estradiol and noradrenergic regulatory interaction. NE-dependent ER control of VMN GP variant expression implies that these signals also act on astrocytes to direct physiological stimulus-specific control of glycogen metabolism, which may in turn influence GABA transmission.

Hindbrain lactoprivic regulation of hypothalamic neuron transactivation and gluco-regulatory neurotransmitter expression: Impact of antecedent insulin-induced hypoglycemia

Hindbrain energy state shapes hypothalamic control of glucostasis. Dorsal vagal complex (DVC) L-lactate deficiency is a potent glucose-stimulatory signal that triggers neuronal transcriptional activation in key hypothalamic metabolic loci. The energy gauge AMPK is activated in DVC metabolic-sensory A2 noradrenergic neurons by hypoglycemia-associated lactoprivation, but sensor reactivity is diminished by antecedent hypoglycemia (AH). Current research addressed the premise that AH alters hindbrain lactoprivic regulation of hypothalamic metabolic transmitter function. AH did not modify reductions in A2 dopamine-beta-hydroxylase and monocarboxylate-2 (MCT2) protein expression elicited by caudal fourth ventricular delivery of the MCT inhibitor alpha-cyano-4-hydroxycinnamic acid (4CIN), but attenuated 4CIN activation of A2 AMPK. 4CIN constraint of hypothalamic norepinephrine (NE) activity was averted by AH in a site-specific manner. 4CIN induction of Fos immunolabeling in hypothalamic arcuate (ARH), ventromedial (VMN), dorsomedial (DMN) and paraventricular (PVN) nuclei and lateral hypothalamic area (LHA) was avoided by AH. AH affected reactivity of select hypothalamic metabolic neurotransmitter/enzyme marker proteins, e.g. ARH neuropeptide Y, VMN glutamate decarboxylase, DMN RFamide-related peptide-1 and -3, and LHA orexin-A profiles to 4CIN, but did not alleviate drug inhibition of ARH proopiomelanocortin. AH prevented 4CIN augmentation of circulating glucagon, but did not alter hyperglycemic or hypocorticosteronemic responses to that treatment. Results identify hindbrain lactate deficiency as a stimulus for glucagon secretion, and imply that habituation of this critical counter-regulatory hormone to recurring hypoglycemia may involve one or more hypothalamic neurotransmitters characterized here by acclimation to this critical sensory stimulus.

Aspects of astrocytic cAMP signaling with an emphasis on the putative power of compartmentalized signals in health and disease

This review discusses aspects of known and putative compartmentalized 3',5'-cyclic adenosine monophosphate (cAMP) signaling in astrocytes, a cell type that has turned out to be a key player in brain physiology and pathology. cAMP has attracted less attention than Ca²⁺ in recent years, but could turn out to rival Ca²⁺ in its potential to drive cellular functions and responses to intra- and extracellular cues. Further, Ca²⁺ and cAMP are known to engage in extensive crosstalk and cAMP signals often take place within subcellular compartments revolving around multi-protein signaling complexes; however, we know surprisingly little about this in astrocytes. Here, we review aspects of astrocytic cAMP signaling, provide arguments for an increased interest in this subject, suggest possible future research directions within the field, and discuss putative drug targets.

Methodological considerations for studies of brain glycogen

Glycogen stores in the brain have been recognized for decades, but the underlying physiological function of this energy reserve remains elusive. This uncertainty stems in part from several technical challenges inherent in the study of brain glycogen metabolism. These include low glycogen content in the brain, non-homogeneous labeling of glycogen by radiotracers, rapid glycogenolysis during postmortem tissue handling, and effects of the stress response on brain glycogen turnover. Here we briefly review the aspects of the glycogen structure and metabolism that bear on these technical challenges and present ways they can be addressed.

Does shuttling of glycogen-derived lactate from astrocytes to neurons take place during neurotransmission and memory consolidation?

Glycogen levels in resting brain and its utilization rates during brain activation are high, but the functions fulfilled by glycogenolysis in living brain are poorly understood. Studies in cultured astrocytes have identified glycogen as the preferred fuel to provide ATP for Na⁺ ,K⁺ -ATPase for the uptake of extracellular K⁺ and for Ca²⁺ -ATPase to pump Ca²⁺ into the endoplasmic reticulum. Studies in astrocyte-neuron co-cultures led to the suggestion that glycogen-derived lactate is shuttled to neurons as oxidative fuel to support glutamatergic neurotransmission. Furthermore, both knockout of brain glycogen synthase and inhibition of glycogenolysis prior to a memory-evoking event impair memory consolidation, and shuttling of glycogen-derived lactate as neuronal fuel was postulated to be required for memory. However, lactate shuttling has not been measured in any of these studies, and procedures to inhibit glycogenolysis and neuronal lactate uptake are not specific. Testable alternative mechanisms to explain the observed findings are proposed: (i) disruption of K⁺ and Ca²⁺ homeostasis, (ii) release of gliotransmitters, (iii) imposition of an energy crisis on astrocytes and neurons by inhibition of mitochondrial pyruvate transport by compounds used to block neuronal monocarboxylic acid transporters, and (iv) inhibition of astrocytic filopodial movements that secondarily interfere with glutamate and K⁺ uptake from the synaptic cleft. Evidence that most pyruvate/lactate derived from glycogen is not oxidized and does not accumulate suggests predominant glycolytic metabolism of glycogen to support astrocytic energy demands. Sparing of blood-borne glucose for use by neurons is a reasonable explanation for the requirement for glycogenolysis in neurotransmission and memory processing.

Epidermal growth factor receptor controls glycogen phosphorylase in T cells through small GTPases of the RAS family

We recently uncovered a regulatory pathway of the muscle isoform of glycogen phosphorylase (PYGM) that plays an important role in regulating immune function in T cells. Here, using various enzymatic, pulldown, and immunoprecipitation assays, we describe signaling cross-talk between the small GTPases RAS and RAP1A, member of RAS oncogene family (RAP1) in human Kit 225 lymphoid cells, which, in turn, is regulated by the epidermal growth factor receptor (EGFR). We found that this communication bridge is essential for glycogen phosphorylase (PYG) activation through the canonical pathway because this enzyme is inactive in the absence of adenylyl cyclase type 6 (ADCY6). PYG activation required stimulation of both exchange protein directly activated by cAMP 2 (EPAC2) and RAP1 via RAS and ADCY6 phosphorylation, with the latter being mediated by Raf-1 proto-oncogene, Ser/Thr kinase (RAF1). Consistent with this model, PYG activation was EGFR-dependent and may be initiated by the constitutively active form of RAS. Consequently, PYG activation in Kit 225 T cells could be blocked with specific inhibitors of RAS, EPAC, RAP1, RAF1, ADCY6, and cAMP-dependent protein kinase. Our results establish a new paradigm for the mechanism of PYG activation, which depends on the type of receptor involved.

Astrocytic glycogen metabolism in the healthy and diseased brain

The brain contains a fairly low amount of glycogen, mostly located in astrocytes, a fact that has prompted the suggestion that glycogen does not have a significant physiological role in the brain. However, glycogen metabolism in astrocytes is essential for several key physiological processes and is adversely affected in disease. For instance, diminished ability to break down glycogen impinges on learning, and epilepsy, Alzheimer's disease, and type 2 diabetes are all associated with abnormal astrocyte glycogen metabolism. Glycogen metabolism supports astrocytic K⁺ and neurotransmitter glutamate uptake and subsequent glutamine synthesis-three fundamental steps in excitatory signaling at most brain synapses. Thus, there is abundant evidence for a key role of glycogen in brain function. Here, we summarize the physiological brain functions that depend on glycogen, discuss glycogen metabolism in disease, and investigate how glycogen breakdown is regulated at the cellular and molecular levels.

The dynamic life of the glycogen granule

Glycogen, the primary storage form of glucose, is a rapid and accessible form of energy that can be supplied to tissues on demand. Each glycogen granule, or "glycosome," is considered an independent metabolic unit composed of a highly branched polysaccharide and various proteins involved in its metabolism. In this Minireview, we review the literature to follow the dynamic life of a glycogen granule in a multicompartmentalized system, i.e. the cell, and how and where glycogen granules appear and the factors governing its degradation. A better understanding of the importance of cellular compartmentalization as a regulator of glycogen metabolism is needed to unravel its role in brain energetics.

The regulation of glycogenolysis in the brain

The key regulatory enzymes of glycogenolysis are phosphorylase kinase, a hetero-oligomer with four different types of subunits, and glycogen phosphorylase, a homodimer. Both enzymes are activated by phosphorylation and small ligands, and both enzymes have distinct isoforms that are predominantly expressed in muscle, liver, or brain; however, whole-transcriptome high-throughput sequencing analyses show that in brain both of these enzymes are likely composed of subunit isoforms representing all three tissues. This Minireview examines the regulatory properties of the isoforms of these two enzymes expressed in the three tissues, focusing on their potential regulatory similarities and differences. Additionally, the activity, structure, and regulation of the remaining enzyme necessary for glycogenolysis, glycogen-debranching enzyme, are also reviewed.

The structure of brain glycogen phosphorylase-from allosteric regulation mechanisms to clinical perspectives

Glycogen phosphorylase (GP) is the key enzyme that regulates glycogen mobilization in cells. GP is a complex allosteric enzyme that comprises a family of three isozymes: muscle GP (mGP), liver GP (lGP), and brain GP (bGP). Although the three isozymes display high similarity and catalyze the same reaction, they differ in their sensitivity to the allosteric activator adenosine monophosphate (AMP). Moreover, inactivating mutations in mGP and lGP have been known to be associated with glycogen storage diseases (McArdle and Hers disease, respectively). The determination, decades ago, of the structure of mGP and lGP have allowed to better understand the allosteric regulation of these two isoforms and the development of specific inhibitors. Despite its important role in brain glycogen metabolism, the structure of the brain GP had remained elusive. Here, we provide an overview of the human brain GP structure and its relationship with the two other members of this key family of the metabolic enzymes. We also summarize how this structure provides valuable information to understand the regulation of bGP and to design specific ligands of potential pharmacological interest.

An Isozyme-specific Redox Switch in Human Brain Glycogen Phosphorylase Modulates Its Allosteric Activation by AMP

Brain glycogen and its metabolism are increasingly recognized as major players in brain functions. Moreover, alteration of glycogen metabolism in the brain contributes to neurodegenerative processes. In the brain, both muscle and brain glycogen phosphorylase isozymes regulate glycogen mobilization. However, given their distinct regulatory features, these two isozymes could confer distinct metabolic functions of glycogen in brain. Interestingly, recent proteomics studies have identified isozyme-specific reactive cysteine residues in brain glycogen phosphorylase (bGP). In this study, we show that the activity of human bGP is redox-regulated through the formation of a disulfide bond involving a highly reactive cysteine unique to the bGP isozyme. We found that this disulfide bond acts as a redox switch that precludes the allosteric activation of the enzyme by AMP without affecting its activation by phosphorylation. This unique regulatory feature of bGP sheds new light on the isoform-specific regulation of glycogen phosphorylase and glycogen metabolism.

Phosphoproteomic profiling of myofibrillar and sarcoplasmic proteins of muscle in response to salting

A phosphoproteomic profile of myofibrillar and sarcoplasmic proteins of muscle in response to salting was investigated. Myofibrillar and sarcoplasmic proteins extracted from salted meat with 0, 1, 2, 3, 4, and 5% salt for 0, 2, 4, 6, 8, and 16 h were analyzed by SDS-PAGE electrophoresis and fluorescence staining. The global phosphorylation of myofibrillar proteins in salted meat was lower than that in control muscle at 16 h of salting (p<0.05), and the global phosphorylation of myofibrillar proteins in 3% salt-treated group at 16 h was the lowest. However, salting showed no significant effect on phosphorylation of sarcoplasmic proteins. Four categories of phosphorylated protein were identified by LC-MS/MS, involved in stress response (heat shock protein), glycometabolism (glycogen phosphorylase, glyceraldehyde-3-phosphate dehydrogenase), oxidation or reduction (superoxide dismutase), and others (myoglobin), the phosphorylation of which was affected by salting. Thus, salting may influence meat quality through protein phosphorylation, which regulates protein degradation and glycolysis.

Insights into Brain Glycogen Metabolism: THE STRUCTURE OF HUMAN BRAIN GLYCOGEN PHOSPHORYLASE

Brain glycogen metabolism plays a critical role in major brain functions such as learning or memory consolidation. However, alteration of glycogen metabolism and glycogen accumulation in the brain contributes to neurodegeneration as observed in Lafora disease. Glycogen phosphorylase (GP), a key enzyme in glycogen metabolism, catalyzes the rate-limiting step of glycogen mobilization. Moreover, the allosteric regulation of the three GP isozymes (muscle, liver, and brain) by metabolites and phosphorylation, in response to hormonal signaling, fine-tunes glycogenolysis to fulfill energetic and metabolic requirements. Whereas the structures of muscle and liver GPs have been known for decades, the structure of brain GP (bGP) has remained elusive despite its critical role in brain glycogen metabolism. Here, we report the crystal structure of human bGP in complex with PEG 400 (2.5 Å) and in complex with its allosteric activator AMP (3.4 Å). These structures demonstrate that bGP has a closer structural relationship with muscle GP, which is also activated by AMP, contrary to liver GP, which is not. Importantly, despite the structural similarities between human bGP and the two other mammalian isozymes, the bGP structures reveal molecular features unique to the brain isozyme that provide a deeper understanding of the differences in the activation properties of these allosteric enzymes by the allosteric effector AMP. Overall, our study further supports that the distinct structural and regulatory properties of GP isozymes contribute to the different functions of muscle, liver, and brain glycogen.

Astroglial glutamate transporters coordinate excitatory signaling and brain energetics

In the mammalian brain, a family of sodium-dependent transporters maintains low extracellular glutamate and shapes excitatory signaling. The bulk of this activity is mediated by the astroglial glutamate transporters GLT-1 and GLAST (also called EAAT2 and EAAT1). In this review, we will discuss evidence that these transporters co-localize with, form physical (co-immunoprecipitable) interactions with, and functionally couple to various 'energy-generating' systems, including the Na(+)/K(+)-ATPase, the Na(+)/Ca(2+) exchanger, glycogen metabolizing enzymes, glycolytic enzymes, and mitochondria/mitochondrial proteins. This functional coupling is bi-directional with many of these systems both being regulated by glutamate transport and providing the 'fuel' to support glutamate uptake. Given the importance of glutamate uptake to maintaining synaptic signaling and preventing excitotoxicity, it should not be surprising that some of these systems appear to 'redundantly' support the energetic costs of glutamate uptake. Although the glutamate-glutamine cycle contributes to recycling of neurotransmitter pools of glutamate, this is an over-simplification. The ramifications of co-compartmentalization of glutamate transporters with mitochondria for glutamate metabolism are discussed. Energy consumption in the brain accounts for ∼20% of the basal metabolic rate and relies almost exclusively on glucose for the production of ATP. However, the brain does not possess substantial reserves of glucose or other fuels. To ensure adequate energetic supply, increases in neuronal activity are matched by increases in cerebral blood flow via a process known as 'neurovascular coupling'. While the mechanisms for this coupling are not completely resolved, it is generally agreed that astrocytes, with processes that extend to synapses and endfeet that surround blood vessels, mediate at least some of the signal that causes vasodilation. Several studies have shown that either genetic deletion or pharmacologic inhibition of glutamate transport impairs neurovascular coupling. Together these studies strongly suggest that glutamate transport not only coordinates excitatory signaling, but also plays a pivotal role in regulating brain energetics.

Altered Plasticity of Glycogen Phosphorylase in Forebrain Gliosomes Obtained from Insulinoma Patients

We investigated a control model of hypoglycemia-exposed brain tissues from a small series of patients with insulinoma, immediately dissect them, and perform a differential cold centrifugation to obtain gliosomes and examine alterations of glycogenolytic mechanisms. The BB as well as MM isoforms of glycogen phosphorylase enzymatic protein expression remained unaltered between insulinoma and control subjects within the gliosomes. However, the glycogen phosphorylase remained in a form that was potentially activated several folds on placing the gliosomes in a glucose-free medium. This was examined by its increased interaction with protein kinase A. Inhibitors of glycogen phosphorylase was used as controls. Furthermore, we demonstrated that glucose-depleted medium enhanced production of both ATP and lactate by the gliosomes. It is possible that a portion of glucose obtained from glycogen breakdown was circuited through glycolytic pathways to generate ATP. It has been reported earlier that ATP within gliosomes plays a major role in glutamate uptake, thus potentially preventing seizure during active bouts of hypoglycemia. Lactate shuttle from astrocytes is a potential mechanism to balance neuronal bioenergetics during events of hypoglycemia. Newer approaches to pharmacologically modulate glycogen phosphorylase may prove to be rational approach for neuroprotective therapy in this common clinical syndrome of hypoglycemia.

Role of the Astrocytic Na(+), K(+)-ATPase in K(+) Homeostasis in Brain: K(+) Uptake, Signaling Pathways and Substrate Utilization

This paper describes the roles of the astrocytic Na(+), K(+)-ATPase for K(+) homeostasis in brain. After neuronal excitation it alone mediates initial cellular re-accumulation of moderately increased extracellular K(+). At higher K(+) concentrations it is assisted by the Na(+), K(+), 2Cl(-) transporter NKCC1, which is Na(+), K(+)-ATPase-dependent, since it is driven by Na(+), K(+)-ATPase-created ion gradients. Besides stimulation by high K(+), NKCC1 is activated by extracellular hypertonicity. Intense excitation is followed by extracellular K(+) undershoot which is decreased by furosemide, an NKCC1 inhibitor. The powerful astrocytic Na(+), K(+)-ATPase accumulates excess extracellular K(+), since it is stimulated by above-normal extracellular K(+) concentrations. Subsequently K(+) is released via Kir4.1 channels (with no concomitant Na(+) transport) for re-uptake by the neuronal Na(+), K(+)-ATPase which is in-sensitive to increased extracellular K(+), but stimulated by intracellular Na(+) increase. Operation of the astrocytic Na(+), K(+)-ATPase depends upon Na(+), K(+)-ATPase/ouabain-mediated signaling and K(+)-stimulated glycogenolysis, needed in these non-excitable cells for passive uptake of extracellular Na(+), co-stimulating the intracellular Na(+)-sensitive site. A gradual, spatially dispersed release of astrocytically accumulated K(+) will therefore not re-activate the astrocytic Na(+), K(+)-ATPase. The extracellular K(+) undershoot is probably due to extracellular hypertonicity, created by a 3:2 ratio between Na(+), K(+)-ATPase-mediated Na(+) efflux and K(+) influx and subsequent NKCC1-mediated volume regulation. The astrocytic Na(+), K(+)-ATPase is also stimulated by β1-adrenergic signaling, which further stimulates hypertonicity-activation of NKCC1. Brain ischemia leads to massive extracellular K(+) increase and Ca(2+) decrease. A requirement of Na(+), K(+)-ATPase signaling for extracellular Ca(2+) makes K(+) uptake (and brain edema) selectively dependent upon β1-adrenergic signaling and inhibitable by its antagonists.