Biosynthesis of proteoglycogen: modulation of glycogenin expression in the developing chicken

Structure and Regulation of Glycogen Synthase in the Brain

Brain glycogen synthesis is a regulated, multi-step process that begins with glucose transport across the blood brain barrier and culminates with the actions of glycogen synthase and the glycogen branching enzyme to elongate glucose chains and introduce branch points in a growing glycogen molecule. This review focuses on the synthesis of glycogen in the brain, with an emphasis on glycogen synthase, but draws on salient studies in mammalian muscle and liver as well as baker's yeast, with the goal of providing a more comprehensive view of glycogen synthesis and highlighting potential areas for further study in the brain. In addition, deficiencies in the glycogen biosynthetic enzymes which lead to glycogen storage diseases in humans are discussed, highlighting effects on the brain and discussing findings in genetically modified animal models that recapitulate these diseases. Finally, implications of glycogen synthesis in neurodegenerative and other diseases that impact the brain are presented.

Metabolic profiling of late-term turkey embryos by microarrays

The last stages of embryonic development are crucial for turkeys as their metabolism shifts to accommodate posthatch survival and growth. To better understand the metabolic change that occurs during the perinatal period, focused microarray methodology was used to identify changes in the expression of key genes that control metabolism of turkey embryos from 20 d of incubation (E) until hatch (E28). Gene expression patterns were evaluated in liver, pectoral muscle, and hatching muscle and were associated with measured embryonic growth and tissue glycogen concentration. Within the studied period, the expression of 60 genes significantly changed in liver, 53 in pectoral muscle, and 51 in hatching muscle. Genes related to lipid metabolism (enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, 3-hydroxymethylglutaryl-CoA reductase, acetyl-CoA carboxylase, lipoprotein lipase, and thyroxine deiodinase) had reduced expression between E22 and E26, corresponding to the period of expected limited oxygen supply. In contrast, genes related to opposing pathways in carbohydrate metabolism, such as glycolysis and gluconeogenesis (hexokinases, glucose-6 phosphatase, phosphofructokinases, glucose 1-6 phosphatase, pyruvate kinase, and phosphoenolpyruvate carboxykinase), or glycogenesis and glycogenolysis (glycogen synthase and glycogen phosphorylase) had rather static expression patterns between E22 and E26, indicating their enzymatic activity must be under posttranscriptional control. Metabolic survey by microarray methodology brings new insights into avian embryonic development and physiology.

Glycogenin protein and mRNA expression in response to changing glycogen concentration in exercise and recovery

Glycogenin (GN-1) is essential for the formation of a glycogen granule; however, rarely has it been studied when glycogen concentration changes in exercise and recovery. It is unclear whether GN-1 is degraded or is liberated and exists as apoprotein (apo)-GN-1 (unglycosylated). To examine this, we measured GN-1 protein and mRNA level at rest, at exhaustion (EXH), and during 5 h of recovery in which the rate of glycogen restoration was influenced by carbohydrate (CHO) provision. Ten males cycled (65% VO2 max) to volitional EXH (117.8 +/- 4.2 min) on two separate occasions. Subjects were administered carbohydrate (CHO; 1 g.kg(-1).h(-1) Gatorlode) or water [placebo (PL)] during 5 h of recovery. Muscle biopsies were taken at rest, at EXH, and following 30, 60, 120, and 300 min of recovery. At EXH, total glycogen concentration was reduced (P < 0.05). However, GN-1 protein and mRNA content did not change. By 5 h of recovery, glycogen was resynthesized to approximately 60% of rest in the CHO trial and remained unchanged in the PL trial. GN-1 protein and mRNA level did not increase during recovery in either trial. We observed modest amounts of apo-GN-1 at EXH, suggesting complete degradation of some granules. These data suggest that GN-1 is conserved, possibly as very small, or nascent, granules when glycogen concentration is low. This would provide the ability to rapidly restore glycogen during early recovery.

GNN is a self-glucosylating protein involved in the initiation step of glycogen biosynthesis in Neurospora crassa

The initiation of glycogen synthesis requires the protein glycogenin, which incorporates glucose residues through a self-glucosylation reaction, and then acts as substrate for chain elongation by glycogen synthase and branching enzyme. Numerous sequences of glycogenin-like proteins are available in the databases but the enzymes from mammalian skeletal muscle and from Saccharomyces cerevisiae are the best characterized. We report the isolation of a cDNA from the fungus Neurospora crassa, which encodes a protein, GNN, which has properties characteristic of glycogenin. The protein is one of the largest glycogenins but shares several conserved domains common to other family members. Recombinant GNN produced in Escherichia coli was able to incorporate glucose in a self-glucosylation reaction, to trans-glucosylate exogenous substrates, and to act as substrate for chain elongation by glycogen synthase. Recombinant protein was sensitive to C-terminal proteolysis, leading to stable species of around 31kDa, which maintained all functional properties. The role of GNN as an initiator of glycogen metabolism was confirmed by its ability to complement the glycogen deficiency of a S. cerevisiae strain (glg1 glg2) lacking glycogenin and unable to accumulate glycogen. Disruption of the gnn gene of N. crassa by repeat induced point mutation (RIP) resulted in a strain that was unable to synthesize glycogen, even though the glycogen synthase activity was unchanged. Northern blot analysis showed that the gnn gene was induced during vegetative growth and was repressed upon carbon starvation.

C-chain-bound glycogenin is released from proteoglycogen by isoamylase and is able to autoglucosylate

Proteoglycogen glycogenin is linked to the glucose residue of the C-chain reducing end of glycogen. We describe for the first time the release by isoamylase and isolation of C-chain-bound glycogenin (C-glycogenin) from proteoglycogen. The treatment of proteoglycogen with alpha-amylase releases monoglucosylated and diglucosylated glycogenin (a-glycogenin) which is able to autoglucosylate. It had been described that isoamylase splits the glucose-glycogenin linkage of fully autoglucosylated glycogenin previously digested with trypsin, releasing the maltosaccharide moiety. It was also described that carbohydrate-free apo-glycogenin shows higher mobility in SDS-PAGE and twice the autoglucosylation capacity of partly glucosylated glycogenin. On the contrary, we found that the C-glycogenin released from proteoglycogen by isoamylolysis shows lower mobility in SDS-PAGE and about half the autoglucosylation acceptor capacity of the partly glucosylated a-glycogenin. This behavior is consistent with the release of maltosaccharide-bound glycogenin instead of apo-glycogenin. No label was split from auto-[14C]glucosylated C-glycogenin or fully auto-[14C]glucosylated a-glycogenin subjected to isoamylolysis without previous trypsinolysis, thus proving no hydrolysis of the maltosaccharide-tyrosine linkage. The ability of C-glycogenin for autoglucosylation would indicate that the size of the C-chain is lower than the average length of the other glycogen chains.

Differences between glycogen biogenesis in fast- and slow-twitch rabbit muscle

Skeletal muscle glycogen is an essential energy substrate for muscular activity. The biochemical properties of the enzymes involved in de novo synthesis of glycogen were analysed in two types of rabbit skeletal muscle fiber (fast- and slow-twitch). Glycogen concentration was higher in fast-twitch muscle than in slow-twitch muscle, but the latter contained many more small intermediate-acceptor molecules that could act as glycogen synthase substrates. The enzymes involved in de novo synthesis of glycogen in fast-twitch muscle were strongly stimulated by Glc-6-P, but those in slow-twitch muscle were not.

Inactivation and thermal stabilization of glycogenin by linked glycogen

Glycogen-free but not glycogen-bound glycogenin transglucosylates dodecyl-beta-maltoside. Furthermore, its sugar nucleotide-binding site can be photoaffinity labeled using [beta-(32)P]5-azido-UDP-glucose. Disruption with DMSO of the hydrogen bonds that stabilize the alpha-helical structure of glycogen restored the photoaffinity labeling of the glycogen-bound enzyme but not its transglucosylation activity. The larger size polysaccharide that linked to glycogenin allowed transglucosylation corresponding to that of PG-200, a proteoglycogen species of M(r) 200 kDa. PG-200 showed lower activity and increased activation energy than glycogen-free glycogenin. Heat denaturation of glycogen-free and glycogen-bound glycogenin occurred at 51 and 64 degrees C, respectively. Active glycogenin was recovered after the glycogen-bound form was heated at 60-70 degrees C and immediately cooled. Treatment at 60 degrees C of the glycogen-free enzyme resulted in inactivation. This is the first report describing the inactivation and thermal stabilization of an enzyme by linked polysaccharide.

No limiting role for glycogenin in determining maximal attainable glycogen levels in rat skeletal muscle

We examined whether the protein level and/or activity of glycogenin, the protein core upon which glycogen is synthesized, is limiting for maximal attainable glycogen levels in rat skeletal muscle. Glycogenin activity was 27.5 +/- 1.4, 34.7 +/- 1.7, and 39.7 +/- 1.3 mU/mg protein in white gastrocnemius, red gastrocnemius, and soleus muscles, respectively. A similar fiber type dependency of glycogenin protein levels was seen. Neither glycogenin protein level nor the activity of glycogenin correlated with previously determined maximal attainable glycogen levels, which were 69.3 +/- 5.8, 137.4 +/- 10.1, and 80.0 +/- 5.4 micromol/g wet wt in white gastrocnemius, red gastrocnemius, and soleus muscles, respectively. In additional experiments, rats were exercise trained by swimming, which resulted in a significant increase in the maximal attainable glycogen levels in soleus muscles ( approximately 25%). This increase in maximal glycogen levels was not accompanied by an increase in glycogenin protein level or activity. Furthermore, even in the presence of very high glycogen levels ( approximately 170 micromol/g wet wt), approximately 30% of the total glycogen pool continued to be present as unsaturated glycogen molecules (proglycogen). Therefore, it is concluded that glycogenin plays no limiting role for maximal attainable glycogen levels in rat skeletal muscle.

Glycogenin activity in human skeletal muscle is proportional to muscle glycogen concentration

The de novo biosynthesis of glycogen is catalyzed by glycogenin, a self-glucosylating protein primer. To date, the role of glycogenin in regulating glycogen metabolism and the attainment of maximal glycogen levels in skeletal muscle are unknown. We measured glycogenin activity after enzymatic removal of glucose by alpha-amylase, an indirect measure of glycogenin amount. Seven male subjects performed an exercise and dietary protocol that resulted in one high-carbohydrate leg (HL) and one low-carbohydrate leg (LL) before testing. Resting muscle biopsies were obtained and analyzed for total glycogen, proglycogen (PG), macroglycogen (MG), and glycogenin activity. Results showed differences (P < 0.05) between HL and LL for total glycogen (438.0 +/- 69.5 vs. 305.7 +/- 57.4 mmol glucosyl units/kg dry wt) and PG (311.4 +/- 38.1 vs. 227.3 +/- 33.1 mmol glucosyl units/kg dry wt). A positive correlation between total muscle glycogen content and glycogenin activity (r = 0.84, P < 0.001) was observed. Similar positive correlations (P < 0.05) were also evident between both PG and MG concentration and glycogenin activity (PG, r = 0.82; MG, r = 0.84). It can be concluded that glycogenin does display activity in human skeletal muscle and is proportional to glycogen concentration. Thus it must be considered as a potential regulator of glycogen synthesis in human skeletal muscle.

Characterization of mouse glycogenin-1 cDNA and promoter region

Glycogenin-1 is an autocatalytic, self-glucosylating protein which acts as the primer for glycogen synthesis in mammalian skeletal muscle. In this study, we have cloned the glycogenin-1 cDNA from mouse skeletal muscle. Mouse glycogenin-1 has a predicted molecular mass of 37¿ omitted¿399 Da, and the deduced amino acid sequence exhibited 87% homology with human glycogenin-1. Northern blot analysis specifically detected mouse glycogenin-1 transcript in skeletal muscle and heart, and to a lesser extent in kidney, lung and brain. 5'-RACE analysis revealed the major transcription start site to be localized 47 bp upstream of the initiation of translation codon. Sequence analysis of approximately 2 kb of the 5'-flanking region revealed potentially important regions of homology between the mouse and human glycogenin-1 promoters. Several conserved but putative elements, including a TATA box, Sp1 site, and a cyclic AMP responsive element, were observed proximal to the transcription start site. Significantly, Northern blot analysis revealed dibutyryl-cAMP treatment of cultured mouse C2C12 myotubes markedly reduced the levels of glycogenin-1 mRNA in a dose-dependent manner.

Characterization of the human glycogenin-1 gene: identification of a muscle-specific regulatory domain

The de-novo synthesis of glycogen is now known to involve a novel class of self-glucosylating protein primers. In mammalian skeletal muscle, glycogenin-1 is the protein responsible for this initiation step. Northern blot analysis revealed that glycogenin-1 gene transcription is differentially regulated in the C2C12 mouse muscle cell line. To define the regulatory elements that control expression of the glycogenin-1 gene, we have cloned and characterized the genomic structure of the human glycogenin-1 gene and its promoter region. This gene consists of seven exons and six introns, and spans over 13kb. Transcription of human glycogenin-1 is initiated at two major sites, 80 and 86bp upstream from the initiation of translation codon. Nucleotide sequence analysis of 2.1kb of the 5'-flanking region revealed the proximal promoter contains both a TATA box and two putative Sp1 binding sites located in a CpG island. There are numerous binding sites for developmental and cell-type-specific transcription factors, including AP-1, AP-2, GATA, and several potential Oct 1 binding domains. There are also nine consensus E-boxes that bind the basic helix-loop-helix family of muscle-specific transcription factors. The transcriptional activity of the glycogenin-1 gene was investigated by transient transfection of the 5'-flanking region in HepG2 cells and C2C12 myoblasts and myotubes. These results permitted the definition of a minimal 232bp promoter fragment that is responsible for basal level transcription in a cell-type-independent manner. Furthermore, we have identified a regulatory region located between -2076 and -1736 of the 5'-flanking region of the human glycogenin-1 gene that allows myotube-specific expression in C2C12 cells.