Unveiling the role of PYGB in pancreatic cancer: a novel diagnostic biomarker and gene therapy target.

PURPOSE: Pancreatic cancer (PC) is a highly malignant tumor that poses a severe threat to human health. Brain glycogen phosphorylase (PYGB) breaks down glycogen and provides an energy source for tumor cells. Although PYGB has been reported in several tumors, its role in PC remains unclear. METHODS: We constructed a risk diagnostic model of PC-related genes by WGCNA and LASSO regression and found PYGB, an essential gene in PC. Then, we explored the pro-carcinogenic role of PYGB in PC by in vivo and in vitro experiments. RESULTS: We found that PYGB, SCL2A1, and SLC16A3 had a significant effect on the diagnosis and prognosis of PC, but PYGB had the most significant effect on the prognosis. Pan-cancer analysis showed that PYGB was highly expressed in most of the tumors but had the highest correlation with PC. In TCGA and GEO databases, we found that PYGB was highly expressed in PC tissues and correlated with PC's prognostic and pathological features. Through in vivo and in vitro experiments, we found that high expression of PYGB promoted the proliferation, invasion, and metastasis of PC cells. Through enrichment analysis, we found that PYGB is associated with several key cell biological processes and signaling pathways. In experiments, we validated that the MAPK/ERK pathway is involved in the pro-tumorigenic mechanism of PYGB in PC. CONCLUSION: Our results suggest that PYGB promotes PC cell proliferation, invasion, and metastasis, leading to poor patient prognosis. PYGB gene may be a novel diagnostic biomarker and gene therapy target for PC.

Glycogen phosphorylase B promotes ovarian cancer progression via Wnt/ß-catenin signaling and is regulated by miR-133a-3p.

BACKGROUND: Ovarian cancer is one of the most common gynecologic cancers with high morbidity and mortality in women. Glycogen metabolism plays a critical role in cancer development and glycogen phosphorylase B (PYGB) has reported to be involved in various tumors. Here, we explored the role of PYGB in ovarian cancer. METHODS: PYGB mRNA expression were examined in ovarian cancer tissue and also analyzed using the dataset from The Cancer Genome Atlas cohort. Correlations between PYGB expression and prognosis of ovarian cancer patients were analyzed. PYGB was silenced to evaluate the ovarian cell proliferation, invasion and migration in vitro and tumorigenesis in vivo. MiR-133a-3p targeting PYGB was identified using online tools and confirmed with luciferase reporter experiment. MiR-133a-3p overexpression using miRNA mimics was conducted to evaluate its function on ovarian cancer cells. RESULTS: We showed that PYGB was upregulated in ovarian cancer tissue and high level of PYGB expression is markedly correlated with poor prognosis of ovarian cancer patients. PYGB knockdown significantly suppressed ovarian cancer cell proliferation, invasion and migration. Xenograft tumor formation further demonstrated that knockdown PYGB inhibited ovarian tumor development. Bioinformatics analysis revealed that PYGB regulated Wnt/ß-catenin signaling pathway in ovarian cancer cells. Mechanistically, miR-133a-3p directly bound to 3'-untranslated region of PYGB and overexpression miR-133a-3p suppressed proliferation, invasion and migration in ovarian cancer cells. CONCLUSION: Our data suggest that miR-133a-3p/PYGB/Wnt-ß-catenin axis plays a critical role in human ovarian cancer, which might serve as a promising therapeutic target of ovarian cancer treatment in the future.

Silencing of PYGB suppresses growth and promotes the apoptosis of prostate cancer cells via the NF­κB/Nrf2 signaling pathway.

Brain­type glycogen phosphorylase (PYGB) is an enzyme that metabolizes glycogen, whose function is to provide energy for an organism in an emergency state. The present study purposed to investigate the role and mechanism of PYGB silencing on the growth and apoptosis of prostate cancer cells. A cell counting kit­8 assay and flow cytometry were performed to determine the cell viability, apoptosis and reactive oxygen species (ROS) content, respectively. Colorimetry was performed to analyze the activity of caspase­3. Western blotting and reverse transcription­quantitative polymerase chain reaction were used to evaluate the associated mRNA and protein expression levels. The results revealed that PYGB was upregulated in prostate cancer tissues and was associated with disease progression. In addition, PYGB silencing suppressed the cell viability of PC3 cells. PYGB silencing promoted apoptosis of PC3 cells via the regulation of the expression levels of cleaved­poly (adenosine diphosphate­ribose) polymerase, cleaved­caspase­3, B­cell lymphoma­2 (Bcl­2) and Bcl­2­associated X protein. PYGB silencing increased the ROS content in PC3 cells, and affected nuclear factor (NF)­κB/nuclear factor­erythroid 2­related factor 2 (Nrf2) signaling pathways in PC3 cells. In conclusion, PYGB silencing suppressed the growth and promoted the apoptosis of prostate cancer cells by affecting the NF­κB/Nrf2 signaling pathway. The present study provided evidence that may lead to the development of a potential therapeutic strategy for prostate cancer.

PYGB siRNA inhibits the cell proliferation of human osteosarcoma cell lines.

Osteosarcoma is the most common malignant bone carcinoma that primarily occurs between childhood to adolescence. It was suggested by recent research that the Brain type glycogen phosphorylase (PYGB) gene may serve an important role in various types of cancer. In the present study, the PYGB gene was knocked down in order to evaluate the cell viability, invasion and migration of the human osteosarcoma cell lines MG63 and HOS. The expression levels of PYGB in osteosarcoma and bone cyst tissue samples, as well as in the osteosarcoma cell lines were identified using reverse transcription­quantitative polymerase chain reaction and western blot assay. Subsequently, a Cell Counting kit 8 assay was employed to evaluate cell proliferation. Cell apoptosis rate and cell cycle distribution were measured by flow cytometry. In addition, cell invasion and migration were evaluated through a Transwell assay. The expression levels of the cell apoptosis and tumor metastasis associated proteins B­cell lymphoma 2 (Bcl­2), Bcl­2­associated X protein, E­cadherin, Twist, matrix metalloproteinase (MMP)­9 and MMP2 were measured via western blotting. PYGB exhibited a higher expression level in the osteosarcoma tissue samples, particularly in the human osteosarcoma cell lines MG63 and HOS. Knockdown of PYGB resulted in a decline in cell proliferation, invasion and migration, which was coupled with induced cell apoptosis and cell cycle arrest in MG63 and HOS cells. Furthermore, alterations in the expression of apoptosis and metastasis associated proteins indicated that small interfering (si)PYGB may have regulated cell viability by targeting the Bcl/Caspase and cyclin dependent kinase (CDK)­1 signaling pathway. In conclusion, PYGB siRNA exerted an inhibitory effect on the cell viability of the human osteosarcoma cells MG63 and HOS by blocking the Caspase/Bcl and CDK1 signaling pathway, highlighting novel potential therapeutic methods for treating osteosarcoma.

Glycogen metabolism in brain and neurons - astrocytes metabolic cooperation can be altered by pre- and neonatal lead (Pb) exposure.

Lead (Pb) is an environmental neurotoxin which particularly affects the developing brain but the molecular mechanism of its neurotoxicity still needs clarification. The aim of this paper was to examine whether pre- and neonatal exposure to Pb (concentration of Pb in rat offspring blood below the "threshold level") may affect the brain's energy metabolism in neurons and astrocytes via the amount of available glycogen. We investigated the glycogen concentration in the brain, as well as the expression of the key enzymes involved in glycogen metabolism in brain: glycogen synthase 1 (Gys1), glycogen phosphorylase (PYGM, an isoform active in astrocytes; and PYGB, an isoform active in neurons) and phosphorylase kinase ß (PHKB). Moreover, the expression of connexin 43 (Cx43) was evaluated to analyze whether Pb poisoning during the early phase of life may affect the neuron-astrocytes' metabolic cooperation. This work shows for the first time that exposure to Pb in early life can impair brain energy metabolism by reducing the amount of glycogen and decreasing the rate of its metabolism. This reduction in brain glycogen level was accompanied by a decrease in Gys1 expression. We noted a reduction in the immunoreactivity and the gene expression of both PYGB and PYGM isoform, as well as an increase in the expression of PHKB in Pb-treated rats. Moreover, exposure to Pb induced decrease in connexin 43 immunoexpression in all the brain structures analyzed, both in astrocytes as well as in neurons. Our data suggests that exposure to Pb in the pre- and neonatal periods results in a decrease in the level of brain glycogen and a reduction in the rate of its metabolism, thereby reducing glucose availability, which as a further consequence may lead to the impairment of brain energy metabolism and the metabolic cooperation between neurons and astrocytes.

Molecular Mechanisms of Allosteric Inhibition of Brain Glycogen Phosphorylase by Neurotoxic Dithiocarbamate Chemicals.

Dithiocarbamates (DTCs) are important industrial chemicals used extensively as pesticides and in a variety of therapeutic applications. However, they have also been associated with neurotoxic effects and in particular with the development of Parkinson-like neuropathy. Although different pathways and enzymes (such as ubiquitin ligases or the proteasome) have been identified as potential targets of DTCs in the brain, the molecular mechanisms underlying their neurotoxicity remain poorly understood. There is increasing evidence that alteration of glycogen metabolism in the brain contributes to neurodegenerative processes. Interestingly, recent studies with N,N-diethyldithiocarbamate suggest that brain glycogen phosphorylase (bGP) and glycogen metabolism could be altered by DTCs. Here, we provide molecular and mechanistic evidence that bGP is a target of DTCs. To examine this system, we first tested thiram, a DTC pesticide known to display neurotoxic effects, observing that it can react rapidly with bGP and readily inhibits its glycogenolytic activity (kinact = 1.4 × 105 m-1 s-1). Using cysteine chemical labeling, mass spectrometry, and site-directed mutagenesis approaches, we show that thiram (and certain of its metabolites) alters the activity of bGP through the formation of an intramolecular disulfide bond (Cys318-Cys326), known to act as a redox switch that precludes the allosteric activation of bGP by AMP. Given the key role of glycogen metabolism in brain functions and neurodegeneration, impairment of the glycogenolytic activity of bGP by DTCs such as thiram may be a new mechanism by which certain DTCs exert their neurotoxic effects.

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.

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.

Brain isoform glycogen phosphorylase as a novel hepatic progenitor cell marker.

An appropriate liver-specific progenitor cell marker is a stepping stone in liver regenerative medicine. Here, we report brain isoform glycogen phosphorylase (GPBB) as a novel liver progenitor cell marker. GPBB was identified in a protein complex precipitated by a monoclonal antibody Ligab generated from a rat liver progenitor cell line Lig-8. Immunoblotting results show that GPBB was expressed in two liver progenitor cell lines Lig-8 and WB-F344. The levels of GPBB expression decreased in the WB-F344 cells under sodium butyrate (SB)-induced cell differentiation, consistent with roles of GPBB as a liver progenitor cell marker. Short hairpin RNA (shRNA)-mediated GPBB knockdown followed by glucose deprivation test shows that GPBB aids in liver progenitor cell survival under low glucose conditions. Furthermore, shRNA-mediated GPBB knockdown followed by SB-induced cell differentiation shows that reducing GPBB expression delayed liver progenitor cell differentiation. We conclude that GPBB is a novel liver progenitor cell marker, which facilitates liver progenitor cell survival under low glucose conditions and cell differentiation.

Serotonin-induced brain glycogenolysis in rainbow trout (Oncorhynchus mykiss).

In this study, we evaluated the serotonin-mediated control of cerebral glycogen levels in the rainbow trout, Oncorhynchus mykiss. Intracerebroventricular (i.c.v.) administration of serotonin (5-HT) to normoglycemic trout (time and dose response) decreased glycogen levels in the brain and increased brain glycogen phosphorylase activity (time response). In hypoglycemic fish (that had been fasted for 5 and 10 days), there was a time-dependent decrease in brain glycogen levels; under these conditions, i.c.v. administration of 5-HT also reduced the brain glycogen content in fish that had been fasted for 5 days. In fish with local cerebral hypoglycemia (induced by 2-DG administration), the glycogen levels decreased and, as above, i.c.v. administration of 5-HT also lowered the glycogen content. In hyperglycemic fish, 5-HT did not affect glycogen levels. Administration of receptor agonists 5-HT1A (8-OH-DPAT), 5-HT1B (anpirtoline and CP93129) or 5-HT2 (α-m-5-HT) decreased the brain glycogen levels. This effect was antagonized by the administration of receptor antagonists 5-HT1A (WAY100135 and NAN190), 5-HT1B (NAS181) and 5-HT2B/C (SB206553). Administration of the receptor agonists (±)-DOI (5-HT2A/2C), m-CPP (5-HT2B/2C), BW723C86 (5-HT2B) and WAY 161503 (5-HT2C) led to decreases in the levels of brain glycogen. We found that 5-HT is involved in the modulation of brain glycogen homeostasis in the rainbow trout, causing a glycogenolytic effect when fish are in a normoglycemic or hypoglycemic state, but not when they are in a hyperglycemic state. 5-HT1A, 5-HT1B, 5HT2B and 5-HT2C-like receptors appeared to be involved in the glycogenolytic action of 5-HT, although the effect mediated by 5-HT1A or 5-HT1B was apparently stronger.

Prognostic information of glycogen phosphorylase isoenzyme BB in patients with suspected acute coronary syndrome.

Early and adequate risk stratification is essential in patients with suspected acute coronary syndrome (ACS). The aim of the present study was to investigate whether glycogen phosphorylase BB (GPBB) could add prognostic information in the context of contemporary sensitive troponin I determination and B-type natriuretic peptide (BNP). Patients with suspected ACS were consecutively enrolled at 3 German study centers from January 2007 through December 2008. Troponin I, GPBB, and BNP were determined at admission. Follow-up information on the combined end point of death, myocardial infarction, revascularization, and hospitalization owing to a cardiovascular cause was obtained 6 months after enrollment. In total 1,818 patients (66% men) were enrolled of whom 413 (23%) were diagnosed as having acute myocardial infarction and 240 (13%) as having unstable angina pectoris, whereas in 1,165 patients (64%) an ACS could be excluded. Follow-up information was available in 98% of patients; 203 events were registered. GPBB measured on admission predicted an unfavorable outcome with a hazard ratio of 1.24 (p <0.05) in an unadjusted Cox regression model and showed a tendency with a hazard ratio of 1.13 (p = 0.07) in a fully adjusted model. Kaplan-Meier analysis revealed a poorer outcome in patients with increased GPBB levels amendatory to the information provided by troponin I or BNP. In conclusion, GPBB measurement provides predictive information on midterm prognosis in patients with chest pain in addition to BNP and troponin I.

The N-terminus of glycogen phosphorylase b is not required for activation by adenosine 5'-monophosphate.

The, so far unsuccessful, search for selective effective inhibitors of glycogen phosphorylase for the treatment of type II diabetes has made phosphorylase an active target of research for the past 20 years. Many crystallographic structures of phosphorylase are currently available to aid in this research. However, those structures have been interpreted, at least in part, on the basis of work conducted with a proteolytically derived form of phosphorylase that lacked the N-terminus (phosphorylase b'). It has been reported that phosphorylase b' shows no allostery, neither homotropic nor heterotropic. The original report on phosphorylase b' examined the allosteric characteristics over very narrow ranges of effector and substrate concentrations and reported the presence of proteolytic cleavages in addition to the removal of the N-terminus. We have applied molecular biological techniques to generate a truncate lacking the N-terminus with known primary structure, and we have established conditions for fully quantifying the allosteric effect of AMP on glycogen phosphorylase b. We report here for the first time the full thermodynamic effect of AMP on phosphorylase b. Our findings with a truncate lacking the N-terminus show that the effect of AMP binding does not depend on the N-terminus.

Immunohistochemical localization of glycogen phosphorylase isozymes in the rat gastrointestinal muscle layers and enteric nervous system.

Glycogen represents the major brain energy reserve though its precise functions are still under debate. Glycogen has also been found in different cell types of the enteric nervous system (ENS), the largest and most complex component of the peripheral nervous system. In the present work we have demonstrated, by application of isozyme-specific antibodies, the presence of isozymes of glycogen phosphorylase (GP), one of the major control sites in glycogen metabolism, in the rat ENS. Immunohistochemistry revealed that isoform BB (brain) is the predominant isozyme expressed in enteric glial cells (EGC) and rare neurons of the myenteric and submucosal plexuses. Isoform MM (muscle) appears in cells which are, according to their location and morphology, probably interstitial cells of Cajal (ICC). In addition, both GP isoforms are expressed in longitudinal and circular intestinal smooth muscle layers. As GP BB is mainly regulated by the cellular AMP level, a special function of glycogen in the energy supply of neural gut functions is suggested.

A glycogen phosphorylase inhibitor selectively enhances local rates of glucose utilization in brain during sensory stimulation of conscious rats: implications for glycogen turnover.

Glycogen is degraded during brain activation but its role and contribution to functional energetics in normal activated brain have not been established. In the present study, glycogen utilization in brain of normal conscious rats during sensory stimulation was assessed by three approaches, change in concentration, release of (14)C from pre-labeled glycogen and compensatory increase in utilization of blood glucose (CMR(glc)) evoked by treatment with a glycogen phosphorylase inhibitor. Glycogen level fell in cortex, (14)C release increased in three structures and inhibitor treatment caused regionally selective compensatory increases in CMR(glc) over and above the activation-induced rise in vehicle-treated rats. The compensatory rise in CMR(glc) was highest in sensory-parietal cortex where it corresponded to about half of the stimulus-induced rise in CMR(glcf) in vehicle-treated rats; this response did not correlate with metabolic rate, stimulus-induced rise in CMR(glc) or sequential station in sensory pathway. Thus, glycogen is an active fuel for specific structures in normal activated brain, not simply an emergency fuel depot and flux-generated pyruvate greatly exceeded net accumulation of lactate or net consumption of glycogen during activation. The metabolic fate of glycogen is unknown, but adding glycogen to the fuel consumed during activation would contribute to a fall in CMR(O2)/CMR(glc) ratio.

Immunocytochemical analysis of rat vagus nerve by antibodies against glycogen phosphorylase isozymes.

Glycogen is an endogenous store of glucose equivalents for energy metabolism in many tissues. The brain contains a significant amount of glycogen the role of which as an energy reserve is currently under debate. Apparently little is known concerning a possible role of glycogen in peripheral nerves. We have demonstrated immunocytochemically the presence of glycogen phosphorylase (GP), a key enzyme in glycogen metabolism, in large and small axons of the rat vagus nerve, but not in Schwann cells. Furthermore, the isozyme-specific antibodies applied detected only the presence of the brain isoform BB of GP, but not the muscle isoform MM. This is in agreement with the occurrence of solely the BB isoform in the few brain and spinal cord neurons that contain GP. In contrast, astroglial cells in brain and spinal cord have previously been shown to contain both isoforms. Since GP isozymes are regulated differentially, the expression of isoform BB may provide hints to possible functions of glycogen in the vagus nerve.

Clinicopathological significance of BGP expression in non-small-cell lung carcinoma: relationship with histological type, microvessel density and patients' survival.

AIMS: Brain-type glycogen phosphorylase (BGP) is the major isoform of glycogen phosphorylase found in fetal and neoplastic tissues, and is generally thought to induce glucose supply during an ischaemic period. This study was performed to investigate BGP expression in non-small-cell lung carcinoma (NSCLC). METHODS: A total of 119 cases of NSCLC, including 63 squamous cell carcinomas (SqCCs) and 56 adenocarcinomas (ACs), were imunohistochemically evaluated for BGP expression, and its expression was correlated with clinicopathological parameters. RESULTS: In total, 76.5% were positive, while non-neoplastic bronchial epithelial cells were weakly positive and pneumocytes were negative. High BGP expression was noted in 78.6% of ACs and 36.5% of SqCCs (p=0.001). Microvessel density was higher in the low BGP expression tumours (29.6 +/- 16.9/mm(2)) than in the high expression tumours (22.8+/-13.8/mm(2)) (p=0.017). BGP expression did not correlate with patient age or tumour stage, but was more frequent in females than males. Kaplan-Meier analysis showed that high BGP expression was associated with poorer survival (p=0.032). CONCLUSIONS: BGP is expressed in NSCLC, particularly AC, and is an independent poor prognostic factor.

Glycogen metabolism in rat ependymal primary cultures: regulation by serotonin.

Ependymal primary cultures are a model for studying ependymal energy metabolism. Intracellular glycogen is built up in the cultures dependent on culture age and the presence of glucose and glutamate. This energy store is mobilized upon glucose withdrawal, stimulation with isoproterenol, forskolin or serotonin and after uncoupling of oxidative phosphorylation from ATP production. Serotonin regulates ependymal glycogen metabolism predominantly via 5-HT receptor (5-HTR) 7, which elicits an increase in the level of ependymal cyclic AMP. Although the most abundant mRNAs for serotonin receptors are those of 5-HTR 2B and 5-HTR 3A, ependymal cells in primary culture do not respond to serotonin with an increase in their concentration of cytosolic calcium ions. The mRNAs of 5-HTRs 1A, 6, 1B, 5B, 7, 1/2C and 5A are also detectable in order of decreasing abundance. The mRNAs for 5-HTRs 1D, 1F, 3B and 4 are absent from the cultured cells. The ability of serotonin to mobilize ependymal glycogen depends on the culture age and the time allowed for glycogen buildup. During glycogen buildup time, glutamate is consumed by the cells. An increased ability of 5-HT to mobilize ependymal glycogen stores is noticed after the depletion of glutamate from the glycogen buildup medium. In ependymal primary cultures, cilia are colocalized with glycogen phosphorylase isozyme BB, while the MM isoform is not expressed. It is known from the literature that an increase in the concentration of cytosolic cAMP in ependymal cells leads to a decrease in ciliary beat frequency. Therefore, the present data point towards a function for ependymal glycogen other than supplying energy for the movement of cilia.

Glycogen phosphorylase isozyme pattern in mammalian retinal Müller (glial) cells and in astrocytes of retina and optic nerve.

Müller cells, the radially oriented dominant macroglial cells of the retina, are known to contain abundant glycogen as well as the key enzyme for its degradation, glycogen phosphorylase (GP), but the expressed isozyme pattern is unknown. To elucidate the isoform expression pattern, specific antisera directed against the brain (BB) and muscle (MM) isoforms of GP were applied to retinal sections, isolated Müller cells, and sections of the optic nerve. We show that Müller cells of rat, rabbit, guinea pig, and mouse retina exclusively express the BB isoform. Astrocytes of rat and rabbit optic nerve, as well as retina express only the BB isoform. In contrast, astrocytes in the brain and spinal cord as well as the epithelial cells of the pars caeca and of the ciliary body express both the BB and MM isoform. This result may indicate some differences in the role of glycogen in retinal macroglia and brain astrocytes, reflecting a local specialization of macroglia in the retina proper.