Glycogen, its transient occurrence in neurons of the rat CNS during normal postnatal development

STAT3 transcriptionally regulates the expression of genes related to glycogen metabolism in developing motor neurons

Motor neurons in the spinal cord are essential for movement. During the embryonic period, developing motor neurons store glycogen to protect against hypoglycemic and hypoxic stress. However, the mechanisms by which glycogen metabolism is regulated in motor neurons remain unclear. We herein investigated the transcriptional regulation of genes related to glycogen metabolism in the developing spinal cord. We focused on the regulatory mechanism of glycogen synthase (Gys1) and glycogen phosphorylase brain isoform (PygB), which play central roles in glycogen metabolism, and found that the transcription factor STAT3 regulated the expression of Gys1 and PygB via cis-regulatory promoter sequences in the developing spinal cord. These results suggest that STAT3 is important for the regulation of glycogen metabolism during motor neuron development.

Events Occurring in the Axotomized Facial Nucleus

Transection of the rat facial nerve leads to a variety of alterations not only in motoneurons, but also in glial cells and inhibitory neurons in the ipsilateral facial nucleus. In injured motoneurons, the levels of energy metabolism-related molecules are elevated, while those of neurofunction-related molecules are decreased. In tandem with these motoneuron changes, microglia are activated and start to proliferate around injured motoneurons, and astrocytes become activated for a long period without mitosis. Inhibitory GABAergic neurons reduce the levels of neurofunction-related molecules. These facts indicate that injured motoneurons somehow closely interact with glial cells and inhibitory neurons. At the same time, these events allow us to predict the occurrence of tissue remodeling in the axotomized facial nucleus. This review summarizes the events occurring in the axotomized facial nucleus and the cellular and molecular mechanisms associated with each event.

A thermodynamic function of glycogen in brain and muscle

Brain and muscle glycogen are generally thought to function as local glucose reserves, for use during transient mismatches between glucose supply and demand. However, quantitative measures show that glucose supply is likely never rate-limiting for energy metabolism in either brain or muscle under physiological conditions. These tissues nevertheless do utilize glycogen during increased energy demand, despite the availability of free glucose, and despite the ATP cost of cycling glucose through glycogen polymer. This seemingly wasteful process can be explained by considering the effect of glycogenolysis on the amount of energy obtained from ATP (ΔG'ATP). The amount of energy obtained from ATP is reduced by elevations in inorganic phosphate (Pi). Glycogen utilization sequesters Pi in the glycogen phosphorylase reaction and in downstream phosphorylated glycolytic intermediates, thereby buffering Pi elevations and maximizing energy yield at sites of rapid ATP consumption. This thermodynamic effect of glycogen may be particularly important in the narrow, spatially constrained astrocyte processes that ensheath neuronal synapses and in cells such as astrocytes and myocytes that release Pi from phosphocreatine during energy demand. The thermodynamic effect may also explain glycolytic super-compensation in brain when glycogen is not available, and aspects of exercise physiology in muscle glycogen phosphorylase deficiency (McArdle disease).

Neuronal GDPGP1 and glycogen metabolism: friend or foe?

The adult brain consumes glucose for energy needs and stores glucose as glycogen mainly in astrocytes. Schulz et al. (2020. J. Cell Biol.https://doi.org/10.1083/jcb.201807127) identify the stress-regulated metabolic enzyme GDPGP1 that promotes neuronal survival likely through glycogen reserves in mouse and C. elegans neurons.

Lack of Neuronal Glycogen Impairs Memory Formation and Learning-Dependent Synaptic Plasticity in Mice

Since brain glycogen is stored mainly in astrocytes, the role of this polysaccharide in neurons has been largely overlooked. To study the existence and relevance of an active neuronal glycogen metabolism in vivo, we generated a mouse model lacking glycogen synthase specifically in the Camk2a-expressing postnatal forebrain pyramidal neurons (GYS1^Camk2a–KO), which include the prefrontal cortex and the CA3 and CA1 cell layers of the hippocampus. The latter are involved in memory and learning processes and participate in the hippocampal CA3-CA1 synapse, the function of which can be analyzed electrophysiologically. Long-term potentiation evoked in the hippocampal CA3-CA1 synapse was decreased in alert behaving GYS1^Camk2a–KO mice. They also showed a significant deficiency in the acquisition of an instrumental learning task – a type of associative learning involving prefrontal and hippocampal circuits. Interestingly, GYS1^Camk2a–KO animals did not show the greater susceptibility to hippocampal seizures and myoclonus observed in animals completely depleted of glycogen in the whole CNS. These results unequivocally demonstrate the presence of an active glycogen metabolism in neurons in vivo and reveal a key role of neuronal glycogen in the proper acquisition of new motor and cognitive abilities, and in the changes in synaptic strength underlying such acquisition.

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.

Neurons have an active glycogen metabolism that contributes to tolerance to hypoxia

Glycogen is present in the brain, where it has been found mainly in glial cells but not in neurons. Therefore, all physiologic roles of brain glycogen have been attributed exclusively to astrocytic glycogen. Working with primary cultured neurons, as well as with genetically modified mice and flies, here we report that-against general belief-neurons contain a low but measurable amount of glycogen. Moreover, we also show that these cells express the brain isoform of glycogen phosphorylase, allowing glycogen to be fully metabolized. Most importantly, we show an active neuronal glycogen metabolism that protects cultured neurons from hypoxia-induced death and flies from hypoxia-induced stupor. Our findings change the current view of the role of glycogen in the brain and reveal that endogenous neuronal glycogen metabolism participates in the neuronal tolerance to hypoxic stress.

Glycogenolysis and purinergic signaling

Both ATP and glutamate are on one hand essential metabolites in brain and on the other serve a signaling function as transmitters. However, there is the major difference that the flux in the pathway producing transmitter glutamate is comparable to the rate of glucose metabolism in brain, whereas that producing transmitter ATP is orders of magnitude smaller than the metabolic turnover between ATP and ADP. Moreover, de novo glutamate production occurs exclusively in astrocytes, whereas transmitter ATP is produced both in neurons and astrocytes. This chapter deals only with ATP and exclusively with its formation and release in astrocytes, and it focuses on potential associations with glycogenolysis, which is known to be indispensable for the synthesis of glutamate. Glycogenolysis is dependent upon an increase in free intracellular Ca(2+) concentration (Ca(2+)]i). It can be further stimulated by cAMP, but in contrast to widespread beliefs, cAMP can on its own not induce glycogenolysis. Astrocytes generate ATP from accumulated adenosine, and this process does not seem to require glycogenolysis. A minor amount of the generated ATP is utilized as a transmitter, and its synthesis requires the presence of the mainly intracellular nucleoside transporter ENT3. Many transmitters as well as extracellular K(+) concentrations high enough to open the voltage-sensitive L-channels for Ca(2+) cause a release of transmitter ATP from astrocytes. Adenosine and ATP induce release of ATP by action at several different purinergic receptors. The release evoked by transmitters or elevated K(+) concentrations is abolished by DAB, an inhibitor of glycogenolysis.

Metabolic Flux and Compartmentation Analysis in the Brain In vivo

Through significant developments and progresses in the last two decades, in vivo localized nuclear magnetic resonance spectroscopy (MRS) became a method of choice to probe brain metabolic pathways in a non-invasive way. Beside the measurement of the total concentration of more than 20 metabolites, (1)H MRS can be used to quantify the dynamics of substrate transport across the blood-brain barrier by varying the plasma substrate level. On the other hand, (13)C MRS with the infusion of (13)C-enriched substrates enables the characterization of brain oxidative metabolism and neurotransmission by incorporation of (13)C in the different carbon positions of amino acid neurotransmitters. The quantitative determination of the biochemical reactions involved in these processes requires the use of appropriate metabolic models, whose level of details is strongly related to the amount of data accessible with in vivo MRS. In the present work, we present the different steps involved in the elaboration of a mathematical model of a given brain metabolic process and its application to the experimental data in order to extract quantitative brain metabolic rates. We review the recent advances in the localized measurement of brain glucose transport and compartmentalized brain energy metabolism, and how these reveal mechanistic details on glial support to glutamatergic and GABAergic neurons.

Lactate produced by glycogenolysis in astrocytes regulates memory processing

When administered either systemically or centrally, glucose is a potent enhancer of memory processes. Measures of glucose levels in extracellular fluid in the rat hippocampus during memory tests reveal that these levels are dynamic, decreasing in response to memory tasks and loads; exogenous glucose blocks these decreases and enhances memory. The present experiments test the hypothesis that glucose enhancement of memory is mediated by glycogen storage and then metabolism to lactate in astrocytes, which provide lactate to neurons as an energy substrate. Sensitive bioprobes were used to measure brain glucose and lactate levels in 1-sec samples. Extracellular glucose decreased and lactate increased while rats performed a spatial working memory task. Intrahippocampal infusions of lactate enhanced memory in this task. In addition, pharmacological inhibition of astrocytic glycogenolysis impaired memory and this impairment was reversed by administration of lactate or glucose, both of which can provide lactate to neurons in the absence of glycogenolysis. Pharmacological block of the monocarboxylate transporter responsible for lactate uptake into neurons also impaired memory and this impairment was not reversed by either glucose or lactate. These findings support the view that astrocytes regulate memory formation by controlling the provision of lactate to support neuronal functions.

Glycogen stores are impaired in hypothalamic nuclei of rats malnourished during early life

Perinatal nutrition has persistent influences on neural development and cognition. In humans and other animals, protein malnutrition during the perinatal period causes permanent changes, inducing to adulthood metabolic syndrome. Feeding is mainly modulated by neural and hormonal inputs to the hypothalamus. Hypothalamic glycogen stores are a source of glucose in high energetic demands, as during development of neural circuits. As some hypothalamic circuits are formed during lactation, we studied the effects of malnutrition, during the first 10 days of lactation, on glycogen stores in hypothalamic nuclei involved in the control of energy metabolism. Female pregnant rats were fed ad libitum with a normal protein diet (22% protein). After delivery, each dam was kept with 6 male pups. During the first 10 days of lactation, dams from the experimental group received a protein-free diet and the control group a normoprotein diet. By post-natal day 10 (P10), glycogen stores were very high in the arcuate nucleus and median eminence of control group. Glycogen stores decreased during development. In P20 control animals, glycogen stores were lower when compared to P10 control animals. Animals submitted to malnutrition presented a staining even lower than control ones. After P45, it was difficult to determine differences between control and diet groups because glycogen stores were reduced. We also showed that tanycytes were the cells presenting glycogen stores. Our data reinforce the concept that maternal nutritional state during lactation may be critical for neurodevelopment since it resulted in a low hypothalamic glycogen store, which may be critical for establishment of neuronal circuitry.

An astrocyte toxin influences the pattern of breathing and the ventilatory response to hypercapnia in neonatal rats

Recent in vitro data suggest that astrocytes may modulate respiration. To examine this question in vivo, we treated 5-day-old rat pups with methionine sulfoximine (MS), a compound that alters carbohydrate and glutamate metabolism in astrocytes, but not neurons. MS-treated pups displayed a reduced breathing frequency (f) in baseline conditions relative to saline-treated pups. Hypercapnia (5% CO(2)) increased f in both groups, but f still remained significantly lower in the MS-treated group. No differences between treatment groups in the responses to hypoxia (8% O(2)) were observed. Also, MS-treated rats showed an enhanced accumulation of glycogen in neurons of the facial nucleus, the nucleus ambiguus, and the hypoglossal nucleus, structures that regulate respiratory activity and airway patency. An altered transfer of nutrient molecules from astrocytes to neurons may underlie these effects of MS, although direct effects of MS upon neurons or upon peripheral structures that regulate respiration cannot be completely ruled out as an explanation.

Signaling by insulin-like growth factor 1 in brain

The homologous insulin and insulin-like growth factor (IGF) receptors are both expressed in the brain, in overlapping but distinct neuroanatomical patterns. In contrast to insulin, IGF1 is also highly expressed within the brain and is essential for normal brain development. IGF1 promotes projection neuron growth, dendritic arborization and synaptogenesis. IGF1 acts in an autocrine and/or paracrine manner to promote glucose utilization, using phosphatidylinositol 3 kinase (PI3K)/Akt, also known as protein kinase B (PKB)/glycogen synthase kinase 3beta (GSK3beta) pathways similar to insulin signaling in peripheral tissues. IGF1 promotes neuronal survival during normal brain development mainly in hippocampal and olfactory systems that depend on postnatal neurogenesis. IGF1's anabolic and neuroprotective roles may be coordinated by inhibition of GSK3beta. The identification of GSK3beta as a major target of brain IGF1 signaling provides a unifying pathway for IGF1's well-established anabolic and anti-apoptotic functions, with IGF1-induced inhibition of GSK3beta triggering multifaceted anabolic and neuroprotective effects.

Insulin-like growth factor 1 regulates developing brain glucose metabolism

The brain has enormous anabolic needs during early postnatal development. This study presents multiple lines of evidence showing that endogenous brain insulin-like growth factor 1 (Igf1) serves an essential, insulin-like role in promoting neuronal glucose utilization and growth during this period. Brain 2-deoxy-d- [1-(14)C]glucose uptake parallels Igf1 expression in wild-type mice and is profoundly reduced in Igf1-/- mice, particularly in those structures where Igf1 is normally most highly expressed. 2-Deoxy-d- [1-(14)C]glucose is significantly reduced in synaptosomes prepared from Igf1-/- brains, and the deficit is corrected by inclusion of Igf1 in the incubation medium. The serine/threonine kinase Akt/PKB is a major target of insulin-signaling in the regulation of glucose transport via the facilitative glucose transporter (GLUT4) and glycogen synthesis in peripheral tissues. Phosphorylation of Akt and GLUT4 expression are reduced in Igf1-/- neurons. Phosphorylation of glycogen synthase kinase 3beta and glycogen accumulation also are reduced in Igf1-/- neurons. These data support the hypothesis that endogenous brain Igf1 serves an anabolic, insulin-like role in developing brain metabolism.

The regional distribution of neuronal glycogen in the opossum brain, with special reference to hypothalamic systems

Neurons that accumulate glycogen have been identified in the opossum brain stem and diencephalon by a modified histochemical method using alcoholic solutions and fuchsin proper (pararosanilin) rather than the Schiff reagent (leucosulphite derivative). Several of the glycogen-positive cell groups such as the mesencephalic trigeminal nucleus and the brainstem somatic and special visceral efferent nuclei have been previously detected in the developing brain of small, common laboratory mammals. Scattered glycogen-containing neurons also appear in the dorsal thalamus and basal forebrain. A conspicuous, often Golgi-like accumulation of glycogen has been found in neurons of the magnocellular and parvocellular hypothalamic systems. Together with available data on the metabolic rate of marsupials, our results suggest that the patterns of glycogen deposition may be common to several vertebrates and may be a constant although not exclusive property of cells with axonal endings outside the blood-brain barrier.

Olfactory bulb glycogen metabolism: noradrenergic modulation in the young rat

The olfactory bulb exhibits high glycogen phosphorylase activity, the rate-limiting enzyme in the mobilization of glycogen. The bulb also receives dense noradrenergic innervation and noradrenaline is known to stimulate glycogen breakdown. We determined the levels of glycogen in the bulb over the course of development and then determined the ability of noradrenaline to mobilize bulb glycogen. At birth, olfactory bulbs have very high levels of glycogen, with levels declining as the pups develop. Picomolar levels of noradrenaline mobilize glycogen in the bulb,. Initially, beta-adrenergic receptors mediate teh glycogenolysis and subsequently, the alpha-noradrenergic receptors in the bulb stimulate the breakdown of glycogen. Carnosine is involved in the repletion of bulb glycogen levels. The stimulation of glycogen breakdown by noradrenaline may play a role in allowing the increased activity that accompanies early olfactory stimulation.

Glycogen phosphorylase activity and immunoreactivity during pre- and postnatal development of rat brain

Catalytic activity and immunoreactivity of glycogen phosphorylase were studied in pre- and postnatal rat brain. The catalytic activity was assayed in brain homogenates; immunoreactivity was investigated by immunoblot analysis using a monoclonal anti-bovine brain glycogen phosphorylase antibody. The cellular localization and intensity of immunoreactivity were analysed on paraffin-embedded sections utilizing the same monoclonal antibody. The catalytic activity increased 10-fold from embryonic day 16 to adult; immunoreactivity became detectable on embryonic day 16 and increased in intensity as the enzyme activity rose to adult values. The first cellular elements to be stained immunohistochemically were ependymal cells lining the ventricles, ependymal cells of the choroid plexus, meningeal cells and a selected population of neurons in the brain stem. The immunoreactivity of plexus cells and meningeal cells was reduced or absent in the adult rat brain. The earliest appearance of glycogen phosphorylase immunoreactivity in astroglial cells was seen at postnatal day 9 in the hippocampus. The staining pattern of the adult brain was reached at day 22 post partum. The developmental changes in glycogen deposition and in glycogen phophorylase activity and immunoreactivity may indicate a variable physiological role of glycogen metabolism for different cell types in the pre- and postnatal periods.

Sensory stimulation induces local cerebral glycogenolysis: demonstration by autoradiography

Brain glycogen stores are localized primarily to glia and undergo continuous utilization and resynthesis. To study the function of glycogen under normal conditions in brain, we developed an autoradiographic method of demonstrating local-glycogen utilization in the awake rat. The method employs labeling of brain glycogen with 14C(3,4)glucose, in situ microwave fixation of brain metabolism, and anhydrous tissue preparation. With this technique, tactile stimulation of the rat face and vibrissae was found to accelerate the utilization of labeled glycogen in brain regions known to receive sensory input from face and vibrissae: the contralateral somatosensory cortex and the ipsilateral trigeminal, sensory and motor nuclei. These findings demonstrate a link between neuronal activity and local glycogen utilization in mammalian brain and suggest that, like other tissues, brain may respond to sudden increases in energy demand in part by rapid glycolytic metabolism of glycogen. As cerebral glycogen is restricted primarily to glia, these observations also support a close coupling of glial energy metabolism with neuronal activity.

Astroglial differentiation in the opossum superior colliculus

Glial markers, namely, vimentin, glial fibrillary acidic protein (GFAP), and glycogen, as well as accumulation of axon-borne horseradish peroxidase (HRP), were used to visualize radial glial cells in the developing opossum superior colliculus (SC) and to follow changes in young astrocytes of the superficial layers. Vimentin, GFAP, and glycogen are relatively abundant in elements of the median ventricular formation (MVF), which persists at least as late as weaning time, i.e., postconception day 103, postnatal day 90 (PND90). Radial profiles and end-feet in the remaining collicular sectors (main radial system, MRS) are also vimentin-positive but show little or no glycogen or anti-GFAP staining. The numeric density of MRS profiles is very high at the final stages of neuronal migration (PND12) but falls to vestigial numbers by PND 56-60. Antivimentin staining and filling of MRS profiles by axon-borne HRP disappear in parallel. Before total regression of MRS profiles, young astrocytes of the superficial gray layer exhibit a transiently high GFAP expression that is not found in those of the subjacent layers. The results suggest that 1) radial glia at or near the collicular midline are well equipped for a mechanical supportive role, and their abundant glycogen accumulation may reflect their eventual transformation in cells with high glycolytic metabolism, including tanycytes; 2) in most collicular sectors, some radial glia cells persist for long periods after cessation of neuronal migration and may interact with afferent fibers coursing through the superficial neuropil; 3) radially oriented astrocytes of the superficial gray layer exhibit a transiently high GFAP expression that is temporally correlated with late transformations of the retinocollicular projections.

Abnormalities of central axons in a dysmyelinative rat mutant

The absence of normal myelin from the CNS of the dysmyelinative rat mutant, md, is associated with axonal abnormalities including organelle-poor and organelle-rich spheroids (OPS and ORS, respectively), wrinkling of the axolemma, persistence of glycogen aggregates and vacuoles in cerebellar mossy fiber terminals, and coalescence of synaptic vesicles in terminal boutons of the nucleus interpositus. OPS have a special predilection for medullary pyramid and the axons of Purkinje cells and further differ from ORS in their possession of nematosomes and in their lack of neurofilaments, microtubules, and degenerating mitochondria. Purkinje cells of md fail to increase in size after 30 days postnatal age and, unlike these neurons in normal neonatal rats, may have massed or dispersed granules of cytoplasmic glycogen which persist for at least 86 days postnatally. Morphometric study of axons of medullary pyramid and cervical corticospinal tract at 19-43 days of age shows a shift in frequency to axons of smaller size in md, as compared to age-matched controls, except that approximately 1% of md axons are larger than any encountered in controls. Finally, the pyramidal axons of md at 43 days of age have a significantly larger area of axoplasm occupied by mitochondria than obtains for the control condition. We conclude that the described abnormalities are secondary to the lack of a myelin investment and/or the loss of oligodendrocytes.

A morphometric analysis of the development of the fourth ventricle choroid plexus in the rat

The choroid plexus from the rat fourth ventricle was investigated at ages from 16 days of gestation to 30 days after birth. Choroid plexus weights were measured and light and electron micrographs were analysed by quantitative stereological techniques at 4 levels of magnification. There was a 10-fold increase in plexus weight between 19 days of gestation and 30 days after birth which was largely due to an increase in epithelial weight, with little change occurring in the connective tissue core. Before birth, epithelial cell height decreased whereas between 10 and 30 days after birth cross-sectional area increased, both events being accompanied by corresponding changes in cell volume. Large intracellular stores of glycogen were present around birth, when they occupied up to 20% of the cell volume. After birth there were significant increases in apical microvillus height, the number of microvilli per cell and in the size of the mitochondria, suggesting that a large increase in choroid plexus secretory function, or the commencement of a new function, occurs after birth in the rat.