Glycogen structure in type 1 diabetic mice: Towards understanding the origin of diabetic glycogen molecular fragility

Normal and abnormal glycogen structure - A review

Glycogen, a complex branched glucose polymer, is found in animals and bacteria, where it serves as an energy storage molecule. It has linear (1 → 4)-α glycosidic bonds between anhydroglucose monomer units, with branch points connected by (1 → 6)-α bonds. Individual glycogen molecules are referred to as β particles. In organs like the liver and heart, these β particles can bind into larger aggregate α particles, which exhibit a rosette-like morphology. The mechanisms and bonding underlying the aggregation process are not fully understood. For example, mammalian liver glycogen has been observed to be molecularly fragile under certain conditions, such as glycogen from diabetic livers fragmenting when exposed to dimethyl sulfoxide (DMSO), while glycogen from healthy livers is much less fragile; this indicates some difference, as yet unknown, in the bonding between β particles in healthy and diabetic glycogen. This fragility may have implications for blood sugar regulation, especially in pathological conditions such as diabetes.

Using molecular fine structure to identify optimal methods of extracting fungal glycogen

Glycogen is a highly branched glucose polymer that is an energy storage material in fungi and animals. Extraction of glycogen from its source in a way that minimizes its molecular degradation is essential to investigate its native structure. In this study, the following extraction methods were compared: sucrose gradient density ultracentrifugation, thermal alkali, hot alcohol and hot water extractions. Molecular-size and chain-length distributions of glycogen were measured by size-exclusion chromatography and fluorophore-assisted carbohydrate electrophoresis, respectively. These two fine-structure features are the most likely structural characteristics to be degraded during extraction. The results show that the thermal alkali, hot alcohol and hot water extractions degrade glycogen molecular size and/or chain-length distributions, and that sucrose gradient density ultracentrifugation with neither high temperature nor alkaline treatment is the most suitable method for fungal glycogen extraction.

Liver glycogen fragility in the presence of hydrogen-bond breakers

Glycogen, a complex branched glucose polymer, is responsible for sugar storage in blood glucose homeostasis. It comprises small β particles bound together into composite α particles. In diabetic livers, α particles are fragile, breaking apart into smaller particles in dimethyl sulfoxide, DMSO; they are however stable in glycogen from healthy animals. We postulate that the bond between β particles in α particles involves hydrogen bonding. Liver-glycogen fragility in normal and db/db mice (an animal model for diabetes) is compared using various hydrogen-bond breakers (DMSO, guanidine and urea) at different temperatures. The results showed different degrees of α-particle disruption. Disrupted glycogen showed changes in the mid-infra-red spectrum that are related to hydrogen bonds. While glycogen α-particles are only fragile under harsh, non-physiological conditions, these results nevertheless imply that the bonding between β particles in α particles is different in diabetic livers compared to healthy, and is probably associated with hydrogen bonding.

Structural abnormality of hepatic glycogen in rat liver with diethylnitrosamine-induced carcinogenic injury

Growing evidence confirms associations between glycogen metabolic re-wiring and the development of liver cancer. Previous studies showed that glycogen structure changes abnormally in liver diseases such as cystic fibrosis, diabetes, etc. However, few studies focus on glycogen molecular structural characteristics during liver cancer development, which is worthy of further exploration. In this study, a rat model with carcinogenic liver injury induced by diethylnitrosamine (DEN) was successfully constructed, and hepatic glycogen structure was characterized. Compared with glycogen structure in the healthy rat liver, glycogen chain length distribution (CLD) shifts towards a short region. In contrast, glycogen particles were mainly present in small-sized β particles in DEN-damaged carcinogenic rat liver. Comparative transcriptomic analysis revealed significant expression changes of genes and pathways involved in carcinogenic liver injury. A combination of transcriptomic analysis, RT-qPCR, and western blot showed that the two genes, Gsy1 encoding glycogen synthase and Gbe1 encoding glycogen branching enzyme, were significantly altered and might be responsible for the structural abnormality of hepatic glycogen in carcinogenic liver injury. Taken together, this study confirmed that carcinogenic liver injury led to structural abnormality of hepatic glycogen, which provided clues to the future development of novel drug targets for potential therapeutics of carcinogenic liver injury.

Targeting negative energy balance with calorie restriction and mitochondrial uncoupling in db/db mice

OBJECTIVE

Calorie restriction is a first-line treatment for overweight individuals with metabolic impairments. However, few patients can adhere to long-term calorie restriction. An alternative approach to calorie restriction that also causes negative energy balance is mitochondrial uncoupling, which decreases the amount of energy that can be extracted from food. Herein we compare the metabolic effects of calorie restriction with the mitochondrial uncoupler BAM15 in the db/db mouse model of severe hyperglycemia, obesity, hypertriglyceridemia, and fatty liver.

METHODS

Male db/db mice were treated with ∼50% calorie restriction, BAM15 at two doses of 0.1% and 0.2% (w/w) admixed in diet, or 0.2% BAM15 with time-restricted feeding from 5 weeks of age. Mice were metabolically phenotyped over 4 weeks with assessment of key readouts including body weight, glucose tolerance, and liver steatosis. At termination, liver tissues were analysed by metabolomics and qPCR.

RESULTS

Calorie restriction and high-dose 0.2% BAM15 decreased body weight to a similar extent, but mice treated with BAM15 had far better improvement in glucose control. High-dose BAM15 treatment completely normalized fasting glucose and glucose tolerance to levels similar to lean db/+ control mice. Low-dose 0.1% BAM15 did not affect body mass but partially improved glucose tolerance to a similar degree as 50% calorie restriction. Both calorie restriction and high-dose BAM15 significantly improved hyperglucagonemia and liver and serum triglyceride levels. Combining high-dose BAM15 with time-restricted feeding to match the time that calorie restricted mice were fed resulted in the best metabolic phenotype most similar to lean db/+ controls. BAM15-mediated improvements in glucose control were associated with decreased glucagon levels and decreased expression of enzymes involved in hepatic gluconeogenesis.

CONCLUSIONS

BAM15 and calorie restriction treatments improved most metabolic disease phenotypes in db/db mice. However, mice fed BAM15 had superior effects on glucose control compared to the calorie restricted group that consumed half as much food. Submaximal dosing with BAM15 demonstrated that its beneficial effects on glucose control are independent of weight loss. These data highlight the potential for mitochondrial uncoupler pharmacotherapies in the treatment of metabolic disease.

Fractional extraction and structural characterization of glycogen particles from the whole cultivated caterpillar fungus Ophiocordyceps sinensis

Ophiocordyceps sinensis (syn. Cordyceps sinensis) is a valuable medicinal fungus in traditional Chinese medicine, and one or more polysaccharides are the key constituents with important medical effects. Glycogen as a functional polysaccharide is widely identified in eukaryotes including fungi. However, there is no definitive report of glycogen presence in O. sinensis. In this study, we carefully fractionated polysaccharides from cultivated caterpillar fungus O. sinensis, which were then characterized via methods for glycogen analysis. According to the results, 1.03 ± 0.43 % of polysaccharides were quantified via amyloglucosidase digestion in the whole cultivated caterpillar fungus, which had a typical spherical shape under transmission electron microscope with an average peak radius of 37.63 ± 0.57 nm via size exclusion chromatography and an average chain length of 12.47 ± 0.94 degree of polymerization via fluorophore-assisted capillary electrophoresis. Taken together, this study confirmed that the polysaccharides extracted form O. sinensis were mostly glycogen.

Molecular mechanisms of glycogen particle assembly in Escherichia coli

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

Moderate Hyperglycemia-Preventive Effect and Mechanism of Action of Periplaneta americana Oligosaccharides in Streptozotocin-Induced Diabetic Mice

Periplaneta americana is a kind of medicinal and edible insect, and its oligosaccharides (PAOS) have been reported to exert anti-inflammatory effects by regulating immunity, reducing oxidative stress, and meliorating gut microbiota. We hypothesized PAOS might benefit experimental diabetes mellitus (DM), an inflammatory disease coordinated by both innate and adaptive immunity. This study aimed to evaluate the effect of PAOS on glycemia and its potential mechanisms. Mice model of diabetes was established, and then the potential effects of PAOS was tested in vivo. Here, we found that PAOS triggered a moderate hyperglycemia-preventive effect on DM mice, showing markedly alleviated symptoms of DM, reduced blood glucose, and meliorated functions of liver and pancreas β cell. Deciphering the underlying mechanism of PAOS-improving diabetes, the results revealed that PAOS downregulated the blood glucose level by activating PI3K/AKT/mTOR and Keap/Nrf2/HO-1 pathways, meanwhile inhibiting TLR4/MAPK/NF-κB, Beclin1/LC3, and NLRP3/caspase1 pathways in vivo. Furthermore, analyses of the microbial community intriguingly exhibited that PAOS promoted the communities of bacteria producing short-chain fatty acids (SCFAs), whereas attenuating lipopolysaccharides (LPS)-producing ones that favored inflammatory tolerance. Collectively, balancing the intestinal bacterial communities by PAOS, which favored anabolism but suppressed inflammatory responses, contributed substantially to the glycemia improvement of PAOS in DM mice. Accordingly, PAOS might function as complementary and alternative medicine for DM.

The fragility of liver glycogen from humans with type 2 diabetes: A pilot study

Liver glycogen is a highly branched glucose polymer found as β particles (~20 nm in diameter), which can bind together into larger composite α particles. Hepatic α particles have been shown to be structurally fragile (breaking up into smaller particles in certain solvents) in mouse models of diabetes; if occurring in vivo, the resulting small glycogen particles could exacerbate the poor blood-sugar homeostasis characteristic of the disease. Here we tested if this α-particle fragility also occurred in liver glycogen obtained from humans with diabetes. It was found that liver glycogen from diabetic humans was indeed more fragile than from non-diabetic humans, which was also seen in the mouse experiments we ran in parallel. Proteomic analysis revealed three candidate proteins from differentially expressed glycogen proteins (Diabetes/ Non-diabetes) in both human and mouse groups. Identifying these proteins may give clues to the binding mechanism that holds together α particles together, which, being different in diabetic glycogen, is relevant to diabetes prevention and management.

Liver fibrosis alters the molecular structures of hepatic glycogen

Liver fibrosis (LF) leads to liver failure and short survival. Liver glycogen is a hyperbranched glucose polymer, comprising individual β particles, which can bind together to form aggregated α particles. Glycogen functionality depends on its molecular structure. This study compared the molecular structure of liver glycogen from both LF and healthy rats, and explored underlying mechanisms for observed differences. Glycogen from both groups contained α and β particles; the LF group contained a higher proportion of β particles, with the glycogen containing fewer long chains than seen in the control group. Both glycogen branching enzyme and glycogen phosphorylase showed a significant decrease of activity in the LF group. Transcriptomics and proteomics revealed a functional deficiency of mitochondria in the LF group, which may lead to changes in glycogen structure. These results provide for the first time an understanding of how liver fibrosis affects liver glycogen metabolism and glycogen structure. HYPOTHESIS: We hypothesized that the molecular structure of liver glycogen from a rat model of liver fibrosis would be altered compared to the control group.

Combined treatment with niclosamide ethanolamine and artemether combination improves type 1 diabetes via the targeting of liver mitochondria

Type 1 diabetes (T1D) is characterized by dysregulated blood glucose and liver metabolism. In previous studies, niclosamide ethanolamine salt (NEN) and artemether (Art) displayed significant hypoglycemic effects. However, their combined therapeutic effects on the liver in T1D have remained elusive. In the present study, T1D mice were established and randomly allocated into groups. Following treatment, the physiological and metabolic parameters, including liver function, glycogen content, glucose-6-phosphatase (G6Pase) protein expression levels, mitochondrial biogenesis and mitochondrial metabolism were analyzed. Compared with the NEN or Art treatments alone, their combination improved glycometabolism and the symptoms of diabetes. Combined treatment with NEN and Art also significantly ameliorated liver injury and increased liver glycogen storage. Furthermore, combinatorial treatment significantly downregulated hepatic G6Pase protein expression levels and regulated mitochondrial biogenesis. NEN and Art increased the respiratory exchange rate and reduced mitochondrial phosphoenolpyruvate carboxykinase and branched-chain α-keto acid dehydrogenase complex protein expression levels, whereby the effects were obviously enhanced by their application as a combined treatment. In conclusion, the present study confirmed that combined treatment with NEN and Art improved glycometabolism and liver function in T1D mice and the therapeutic effects may be partially associated with the regulation of liver mitochondria.

The effect of high-amylose resistant starch on the glycogen structure of diabetic mice

Glycogen is a complex branched glucose polymer found in many tissues and acts as a blood-glucose buffer. In the liver, smaller β glycogen particles can bind into larger composite α particles. In mouse models of diabetes, these liver glycogen particles are molecularly fragile, breaking up into smaller particles in the presence of solvents such as dimethyl sulfoxide (DMSO). If this occurs in vivo, such a rapid enzymatic degradation of these smaller particles into glucose could exacerbate the poor blood-glucose control that is characteristic of the disease. High-amylose resistant starch (RS) can escape digestion in the small intestine and ferment in the large intestine, which elicits positive effects on glycemic response and type 2 diabetes. Here we postulate that RS would help attenuate diabetes-related liver glycogen fragility. Normal maize starch and two types of high-amylose starch were fed to diabetic and non-diabetic mice. Molecular size distributions and chain-length distributions of liver glycogen from both groups were characterized to test glycogen fragility before and after DMSO treatment. Consistent with the hypothesis that high blood glucose is associated with glycogen fragility, a high-amylose RS diet prevented the fragility of liver-glycogen α particles. The diets had no significant effect on the glycogen chain-length distributions.

Possible regulation of liver glycogen structure through the gut-liver axis by resistant starch: a review

Liver glycogen α particles in diabetic patients are fragile relative to those in healthy individuals, and restoring these fragile glycogen particles to a normal state shows potential to contribute to the remission of diabetes. Resistant starch (RS) is beneficial for diabetes management through its interactions with the gut microbiota. However, its effects on glycogen fragility are not fully understood. This review aims to summarize the recent understanding of the interactions between RS and the human gut microbiota and the possible connections to liver glycogen biosynthesis to elucidate its role in the development of glycogen fragility. RS might regulate glycogen fragility in diabetes by modulating the postprandial glycemic response and glycogen biosynthesis pathways. Before RS can be applied to repair fragile glycogen, more work should be done to better understand in vivo RS structures and identify the factor binding glycogen β particles together. This review contains important information on the connections between glycogen fragility and RS-gut microbiota interactions, which could help to better understand the health benefits of RS consumption.

Ethanol extract of mulberry leaves partially restores the composition of intestinal microbiota and strengthens liver glycogen fragility in type 2 diabetic rats

BACKGROUND

Mulberry leaf as a traditional Chinese medicine is able to treat obesity, diabetes, and dyslipidemia. It is well known that diabetes leads to intestinal microbiota dysbiosis. It is also recently discovered that liver glycogen structure is impaired in diabetic animals. Since mulberry leaves are able to improve the diabetic conditions through reducing blood glucose level, it would be interesting to investigate whether they have any positive effects on intestinal microbiota and liver glycogen structure.

METHODS

In this study, we first determined the bioactive components of ethanol extract of mulberry leaves via high-performance liquid chromatography (HPLC) and liquid chromatography/mass spectrometry (LC/MS). Murine animal models were divided into three groups, normal Sprague-Dawley (SD) rats, high-fat diet (HFD) and streptozotocin (STZ) induced type 2 diabetic rats, and HFD/STZ-induced rats administered with ethanol extract of mulberry leaves (200 mg/kg/day). Composition of intestinal microbiota was analyzed via metagenomics by sequencing the V3-V4 region of 16S rDNAs. Liver glycogen structure was characterized through size exclusion chromatography (SEC). Both Student's t-test and Tukey's test were used for statistical analysis.

RESULTS

A group of type 2 diabetic rat models were successfully established. Intestinal microbiota analysis showed that ethanol extract of mulberry leaves could partially change intestinal microbiota back to normal conditions. In addition, liver glycogen was restored from fragile state to stable state through administration of ethanol extract of mulberry leaves.

CONCLUSIONS

This study confirms that the ethanol extract of mulberry leaves (MLE) ameliorates intestinal microbiota dysbiosis and strengthens liver glycogen fragility in diabetic rats. These finding can be helpful in discovering the novel therapeutic targets with the help of further investigations.

From Prokaryotes to Eukaryotes: Insights Into the Molecular Structure of Glycogen Particles

Glycogen is a highly-branched polysaccharide that is widely distributed across the three life domains. It has versatile functions in physiological activities such as energy reserve, osmotic regulation, blood glucose homeostasis, and pH maintenance. Recent research also confirms that glycogen plays important roles in longevity and cognition. Intrinsically, glycogen function is determined by its structure that has been intensively studied for many years. The recent association of glycogen α-particle fragility with diabetic conditions further strengthens the importance of glycogen structure in its function. By using improved glycogen extraction procedures and a series of advanced analytical techniques, the fine molecular structure of glycogen particles in human beings and several model organisms such as Escherichia coli, Caenorhabditis elegans, Mus musculus, and Rat rattus have been characterized. However, there are still many unknowns about the assembly mechanisms of glycogen particles, the dynamic changes of glycogen structures, and the composition of glycogen associated proteins (glycogen proteome). In this review, we explored the recent progresses in glycogen studies with a focus on the structure of glycogen particles, which may not only provide insights into glycogen functions, but also facilitate the discovery of novel drug targets for the treatment of diabetes mellitus.

Optimization of liver glycogen extraction when considering the fine molecular structure

Liver glycogen is a branched glucose polymer that functions as a blood-sugar buffer in animals. Previous studies have shown that glycogen's molecular structure affects its properties. This makes it important to develop a technique that extracts and purifies a representative sample of glycogen. Here we aim to optimize the sucrose density gradient centrifugation method for preserving glycogen's molecular structure by varying the density of the sucrose solution. The preservation of glycogen's structure involves: 1) minimizing molecular damage and 2) obtaining a structurally representative sample of glycogen. The addition of a 10-minute boiling step was also tested as a means for denaturing any glycogen degrading enzymes. Lower sucrose concentrations and the introduction of the boiling step were shown to be beneficial in obtaining a more structurally representative sample, with the preservation of smaller glycogen particles and decreased glycogen chain degradation.

Increasing hepatic glycogen moderates the diabetic phenotype in insulin-deficient Akita mice

Hepatic glycogen metabolism is impaired in diabetes. We previously demonstrated that strategies to increase liver glycogen content in a high-fat-diet mouse model of obesity and insulin resistance led to a reduction in food intake and ameliorated obesity and glucose tolerance. These effects were accompanied by a decrease in insulin levels, but whether this decrease contributed to the phenotype observed in this animal was unclear. Here we sought to evaluate this aspect directly, by examining the long-term effects of increasing liver glycogen in an animal model of insulin-deficient and monogenic diabetes, namely the Akita mouse, which is characterized by reduced insulin production. We crossed Akita mice with animals overexpressing protein targeting to glycogen (PTG) in the liver to generate Akita mice with increased liver glycogen content (Akita-PTG^OE). Akita-PTG^OE animals showed lower glycemia, lower food intake, and decreased water consumption and urine output compared with Akita mice. Furthermore, Akita-PTG^OE mice showed a restoration of the hepatic energy state and a normalization of gluconeogenesis and glycolysis back to nondiabetic levels. Moreover, hepatic lipogenesis, which is reduced in Akita mice, was reverted in Akita-PTG^OE animals. These results demonstrate that strategies to increase liver glycogen content lead to the long-term reduction of the diabetic phenotype, independently of circulating insulin.

The dynamic changes of glycogen molecular structure in Escherichia coli BL21(DE3)

Diurnal alteration of glycogen molecular structure has been identified in healthy mice. Recently, both fragile (disintegration in dimethyl sulfoxide) and stable (not disintegrating in DMSO) glycogen particles were found in Escherichia coli. However, how glycogen structure changes dynamically in E. coli is not clear. The question examined here is whether fragile, stable glycogen α particles occur in bacteria, following a similar pattern as in mice. In this study, we examine the dynamic changes of glycogen molecular structure over 24-h in E. coli BL21(DE3), using transmission electron microscopy, size exclusion chromatography and fluorophore-assisted carbohydrate electrophoresis at representative time points. It was found that glycogen structure was mainly fragile at the synthesis stage and largely stable during the degradation stage. qRT-PCR results indicated that balance of anabolic and catabolic gene expression levels in glycogen metabolism could be a key factor affecting the fragility of glycogen α particles in bacteria.

The importance of glycogen molecular structure for blood glucose control

Type 2 diabetes incidence continues to increase rapidly. This disease is characterized by a breakdown in blood glucose homeostasis. The impairment of glycemic control is linked to the structure of glycogen, a highly branched glucose polymer. Liver glycogen, a major controller of blood sugar, comprises small β particles which can link together to form larger α particles. These degrade to glucose more slowly than β particles, enabling a controlled release of blood glucose. The α particles in diabetic mice are however easily broken down into β particles, which degrade more quickly. Because this may lead to higher blood glucose, understanding this diabetes-associated breakdown of α-particle molecular structure may help in the development of diabetes therapeutics. We review the extraction of liver glycogen, its molecular structure, and how this structure is affected by diabetes and then use this knowledge to make postulates to guide the development of strategies to help mitigate type 2 diabetes.

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Diabetes involves uncontrolled blood glucose levels

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Liver glycogen acts as a blood glucose buffer

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Diabetes can lead to molecularly fragile liver glycogen particles

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Molecularly fragile liver glycogen may exacerbate poor blood glucose control

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

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

Characterization of glycogen molecular structure in the worm Caenorhabditis elegans

Glycogen, a glucose homopolymer with many glucose chains, is the primary blood-sugar reservoir in many organisms. It comprises β particles (∼20 nm) which can bind together to form large α particles with a rosette morphology. When dimethyl sulfoxide (DMSO) is added to glycogen from diabetic livers, α particles break apart to β particles ('fragility'), possibly due to H-bond disruption; this is not seen in healthy livers. Glycogen α and β particles, and α-particle fragility, are observed in mammals and bacteria, and are examined here in the worm Caenorhabditis elegans, with glycogen from two C. elegans strains, cultured in normal and high-glucose conditions. There were mainly β particles, with some large α particles. Most particles were fragile in DMSO. Growing in a high-glucose medium results in more long chains and more fragility, consistent with previous observations in diabetic animal models. Why high glucose levels facilitate fragility is worthy of further investigation.

Effects of fasting on liver glycogen structure in rats with type 2 diabetes

Liver glycogen, a highly branched glucose polymer, is important for blood sugar homeostasis. It comprises α particles which are made of linked β particles; the molecular structure changes diurnally. In diabetic liver, the α particles are fragile, easily breaking apart into β particles in chaotropic agents such as dimethyl sulfoxide. We here use size-exclusion chromatography to study how fasting changes liver-glycogen structure in vivo for mice in which type-2 diabetes had previously been induced. Diabetic glycogen degraded enzymatically more quickly in the fasted animals than did glycogen without fasting, with fewer α particles, which however were still fragile. The glycogen had fewer long chains and more shorter chains after fasting. This study gives an overview of the in vivo dynamic changes in α-particles under starvation conditions in both normal and diabetic livers.

Dendrobium officinale polysaccharide ameliorates diabetic hepatic glucose metabolism via glucagon-mediated signaling pathways and modifying liver-glycogen structure

ETHNOPHARMACOLOGICAL RELEVANCE

Dendrobium officinale polysaccharide (DOP) is the main active ingredient of Dendrobium officinale Kimura & Migo, which is a precious traditional Chinese medicine and often used in treatment of hepatitis, diabetes, obesity and rheumatoid arthritis.

AIM OF THE STUDY

DOP exhibits significant hypoglycemic activity, while its mechanism remains unclear. The present study aims to investigate the hypoglycemic mechanisms of DOP based on the glucagon-mediated signaling pathways and the liver glycogen structure, which catalyze hepatic glucose metabolism, and provide new knowledge about the antidiabetic mechanism of DOP and further evidence for its clinical use for diabetes.

MATERIALS AND METHODS

DOP were obtained from the dry stems of Dendrobium officinale by water extraction and alcohol precipitation method. T2DM mice model was established by high-fat diet combined with streptozotocin. Liver histopathological changes were observed by H&E and PAS straining. Pancreatic histology was studied by H&E staining and immunofluorescence analysis. The levels of glucagon and insulin were detected by Elisa Kit and the hepatic glycogen content was detected by GOPOD. The expressions of the hepatic glycogen-related metabolism enzymes, hepatic gluconeogenesis enzymes, and the related protein in cAMP-PKA and Akt/FoxO1 signaling pathways were detected by western blots. Liver glycogen was extracted from the liver tissues by sucrose density gradient centrifugation, and size exclusion chromatography (SEC) was used to analyze the structure of liver glycogen.

RESULTS

DOP could significantly affect the glucagon-mediated signaling pathways, cAMP-PKA and Akt/FoxO1, to further promote hepatic glycogen synthesis, inhibit hepatic glycogen degradation and hepatic gluconeogenesis. Moreover, DOP could reverse the instability of the liver glycogen structure and thus probably suppressed glycogen degradation. Thus, DOP finally would ameliorate hepatic glucose metabolism via glucagon-mediated signaling pathways and modifying liver-glycogen structure in diabetic mice.

CONCLUSIONS

The hypoglycemic mechanism of DOP might be associated with the regulation of glucagon-mediated hepatic glycogen metabolism and gluconeogenesis, and of liver glycogen structure, contributing to improved hepatic glucose metabolism in diabetic mice.

Is liver glycogen fragility a possible drug target for diabetes?

Liver glycogen α particles are molecularly fragile in diabetic mice, and readily form smaller β particles, which degrade more rapidly to glucose. This effect is well associated with the loss of blood-glucose homeostasis in diabetes. The biological mechanism of such fragility is still unknown; therefore, there are perceived opportunities that could eventually lead to new means to manage type 2 diabetes. The hierarchical structures of glycogen particles are controlled by the underlying biosynthesis/degradation process that involves various enzymes, including, for example, glycogen synthase (GS) and glycogen-branching enzyme (GBE). Recent studies have shown that fragile glycogen α particles in diabetic mice have longer chains and a higher molecular density compared to wild-type mice, indicating an enhanced enzymatic activity ratio of GS to GBE in diabetes. Furthermore, it has been shown that with an improved blood glucose homeostasis, the glycogen fragility in diabetic mice can be restored by treatment with active ingredients from traditional Chinese medicine, yet the underlying mechanism is unknown. In this review, we summarize recent advances in understandings glycogen fragility from the perspectives of glycogen biosynthesis/degradation, glycogen hierarchical structures, and its relation to diabetes. Importantly, we for the first time set GS/GBE activity ratio as the therapeutic target for diabetes.

Some molecular structural features of glycogen in the kidneys of diabetic rats

Glycogen, a highly-branched glucose polymer, functions as a sugar reservoir in many organs and tissues. Liver glycogen comprises small β particles which can bind to form into large agglomerates (α particles) which readily degrade to β particles in diabetic livers. Muscle glycogen has only β particles, optimal for quick energy release. Healthy kidney contains negligible glycogen, but there is an abnormally high accumulation in diabetic kidneys. We here compare the molecular structure of glycogen in diabetic kidneys with that in liver and muscle, using a diabetic rat model. This involved exploring extraction techniques to minimize glycogen degradation. Using size exclusion chromatography and transmission electron microscopy, it was found that there were only β particles in diabetic kidneys. These are postulated to form during periods of abnormally high blood sugar, the driving force being the need to reduce blood sugar under such circumstances.