Glucose and glycogen in the diabetic kidney: Heroes or villains?

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.

Glycogen β-particles surface characterized by a combination of size exclusion chromatography and pyrene excimer fluorescence before and after β-amylolysis

Size exclusion chromatography (SEC) and pyrene excimer formation (PEF) experiments were conducted to characterize the local density profile inside a glycogen sample before (Glycogen) and after (Gly-β-LD) treatment with β-amylase. These experiments were conducted to assess whether the density at the periphery of the glycogen particles was very high to limit access to proteins involved in the metabolism of glycogen as predicted by the Tier model or low as suggested by the Gilbert model. SEC analysis indicated that the density inside the Glycogen and Gly-β-LD samples remained constant with particle size and was not affected by β-amylolysis. Analysis of the PEF experiments conducted on the Glycogen and Gly-β-LD samples labeled with 1-pyrenebutyric acid showed that the particles have a dense interior and loose corona. The conclusions reached by the SEC and PEF experiments agree with the Gilbert model and have implications for the association of glycogen β-particles into larger α-particles.

Effect of Dapagliflozin on Renal and Hepatic Glucose Kinetics in T2D and NGT Subjects

Acute and chronic sodium-glucose cotransporter 2 (SGLT-2) inhibition increases endogenous glucose production (EGP). However, the organ-liver versus kidney-responsible for the increase in EGP has not been identified. In this study, 20 subjects with type 2 diabetes (T2D) and 12 subjects with normal glucose tolerance (NGT) received [3-3H]glucose infusion (to measure total EGP) combined with arterial and renal vein catheterization and para-aminohippuric acid infusion for determination of renal blood flow. Total EGP, net renal arteriovenous balance, and renal glucose production were measured before and 4 h after dapagliflozin (DAPA) and placebo administration. Following DAPA, EGP increased in both T2D and NGT from baseline to 240 min, while there was a significant time-related decrease after placebo in T2D. Renal glucose production at baseline was <5% of basal EGP in both groups and did not change significantly following DAPA in NGT or T2D. Renal glucose uptake (sum of tissue glucose uptake plus glucosuria) increased in both T2D and NGT following DAPA (P < 0.05 vs. placebo). The increase in renal glucose uptake was entirely explained by the increase in glucosuria. A single dose of DAPA significantly increased EGP, which primarily is explained by an increase in hepatic glucose production, establishing the existence of a novel renal-hepatic axis.

ARTICLE HIGHLIGHTS

Antidiabetic and hepatoprotection effect of butterfly pea flower (Clitoria ternatea L.) through antioxidant, anti-inflammatory, lower LDH, ACP, AST, and ALT on diabetes mellitus and dyslipidemia rat

This study explores the antidiabetic and hepatoprotective potential of Butterfly pea flower extract (Clitoria ternatea L.) (CTE) in diabetic and dyslipidemia rat models. Diabetes Mellitus (DM) is a chronic metabolic disorder marked by high levels of blood glucose, which can cause dyslipidemia and liver damage as a result of oxidative stress. CTE, a natural substance, is recognized for its positive attributes, such as anti-inflammatory, antioxidant, anti-diabetic, anti-dyslipidemia, antibiotic, and liver tissue protection capabilities. Dyslipidemia was induced in rats using a high-fat diet (HFD) and propylthiouracil (PTU) for 28 days. DM was induced using streptozotocin (STZ) and nicotinamide (NA). Rats were treated with varying doses of CTE for 28 days, along with glibenclamide and simvastatin. The research showed that CTE raised the levels of SOD, CAT, and liver proteins while lowering the levels of MDA, LDH, ACP, AST, ALT, IL-1β, and CRP in rats with DM and dyslipidemia. This suggests that CTE might be useful for treating DM.

Effect of experimental pulmonary arterial hypertension on renal and bone parameters of rats submitted to resistance exercise training

Pulmonary arterial hypertension (PAH) is characterized by right ventricular failure and diminished cardiac output, potentially leading to renal and bone impairments. In contrast, resistance exercise training (RT) offers cardiovascular and bone health benefits. This study aimed to assess the impacts of stable PAH induced by monocrotaline (MCT) and RT on renal morphometry, as well as bone morphometry and biomechanical properties in male Wistar rats. Four experimental groups, untrained control (UC, n=7), trained control (TC, n=7), untrained hypertensive (UH, n=7), trained hypertensive (TH, n=7), were defined. After the first MCT or saline injection (20 mg/kg), trained rats were submitted to a RT program (i.e., Ladder climbing), 5 times/week. Seven days later the rats received the second MCT or saline dose. After euthanasia, renal and femoral histomorphometry and femoral biomechanical properties were assessed. PAH reduced renal glomerular area and volume, which was prevented by the RT. While PAH did not harm the femoral morphometry, structural and mechanical properties, RT improved the femoral parameters (e.g., length, percentage of trabeculae and bone marrow, ultimte and yield loads). Experimental stable PAH promotes renal but not bone damages, whereas RT prevents renal deteriorations and improves the femoral morphological and biomechanical properties.

Hyperglycemia - A culprit of podocyte pathology in the context of glycogen metabolism

Prolonged disruption in the balance of glucose can result in metabolic disorders. The kidneys play a significant role in regulating blood glucose levels. However, when exposed to chronic hyperglycemia, the kidneys' ability to handle glucose metabolism may be impaired, leading to an accumulation of glycogen. Earlier studies have shown that there can be a significant increase in glucose storage in the form of glycogen in the kidneys in diabetes. Podocytes play a crucial role in maintaining the integrity of filtration barrier. In diabetes, exposure to elevated glucose levels can lead to significant metabolic and structural changes in podocytes, contributing to kidney damage and the development of diabetic kidney disease. The accumulation of glycogen in podocytes is not a well-established phenomenon. However, a recent study has demonstrated the presence of glycogen granules in podocytes. This review delves into the intricate connections between hyperglycemia and glycogen metabolism within the context of the kidney, with special emphasis on podocytes. The aberrant storage of glycogen has the potential to detrimentally impact podocyte functionality and perturb their structural integrity. This review provides a comprehensive analysis of the alterations in cellular signaling pathways that may potentially lead to glycogen overproduction in podocytes.

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.

Tea seed saponin‑reduced extract ameliorates palmitic acid‑induced insulin resistance in HepG2 cells

Tea (Camellia sinensis) seed cake is a potential resource that contains a wealth of bioactive compounds. However, the high toxicity of tea saponins in tea seed cake restricts its applications. The present study aimed to i) develop a method of extracting bioactive compounds and reducing tea saponins during the process of tea seed cake extraction and ii) investigate the anti‑insulin resistance effect of tea seed saponin‑reduced extract (TSSRE) in a palmitic acid (PA)‑induced insulin resistance HepG2‑cell model. The concentration of tea saponins in TSSRE was ~10‑fold lower than that in tea seed crude extract (TSCE) after the saponin‑reduction process. In addition, TSSRE cytotoxicity was significantly lower than that of TSCE in HepG2 cells. TSSRE treatment improved glucose consumption as well as glucose transporter (GLUT) 2 and GLUT4 expression levels in PA‑stimulated HepG2 cells. Moreover, TSSRE enhanced the phosphorylation of the insulin receptor substrate 1/protein kinase B/forkhead box protein O1/glycogen synthase kinase 3β and inhibited the elevated expression of phosphoenolpyruvate carboxykinase in PA‑exposed HepG2 cells. The effect of TSSRE on the mediation of the insulin signaling pathway was attributed to the inhibition of PA‑induced mitogen‑activated protein kinase activation. The findings of the present study indicated that TSSRE ameliorates hepatic insulin resistance by ameliorating insulin signaling and inhibiting inflammation-related pathways.

Hexokinase-linked glycolytic overload and unscheduled glycolysis in hyperglycemia-induced pathogenesis of insulin resistance, beta-cell glucotoxicity, and diabetic vascular complications

Hyperglycemia is a risk factor for the development of insulin resistance, beta-cell glucotoxicity, and vascular complications of diabetes. We propose the hypothesis, hexokinase-linked glycolytic overload and unscheduled glycolysis, in explanation. Hexokinases (HKs) catalyze the first step of glucose metabolism. Increased flux of glucose metabolism through glycolysis gated by HKs, when occurring without concomitant increased activity of glycolytic enzymes-unscheduled glycolysis-produces increased levels of glycolytic intermediates with overspill into effector pathways of cell dysfunction and pathogenesis. HK1 is saturated with glucose in euglycemia and, where it is the major HK, provides for basal glycolytic flux without glycolytic overload. HK2 has similar saturation characteristics, except that, in persistent hyperglycemia, it is stabilized to proteolysis by high intracellular glucose concentration, increasing HK activity and initiating glycolytic overload and unscheduled glycolysis. This drives the development of vascular complications of diabetes. Similar HK2-linked unscheduled glycolysis in skeletal muscle and adipose tissue in impaired fasting glucose drives the development of peripheral insulin resistance. Glucokinase (GCK or HK4)-linked glycolytic overload and unscheduled glycolysis occurs in persistent hyperglycemia in hepatocytes and beta-cells, contributing to hepatic insulin resistance and beta-cell glucotoxicity, leading to the development of type 2 diabetes. Downstream effector pathways of HK-linked unscheduled glycolysis are mitochondrial dysfunction and increased reactive oxygen species (ROS) formation; activation of hexosamine, protein kinase c, and dicarbonyl stress pathways; and increased Mlx/Mondo A signaling. Mitochondrial dysfunction and increased ROS was proposed as the initiator of metabolic dysfunction in hyperglycemia, but it is rather one of the multiple downstream effector pathways. Correction of HK2 dysregulation is proposed as a novel therapeutic target. Pharmacotherapy addressing it corrected insulin resistance in overweight and obese subjects in clinical trial. Overall, the damaging effects of hyperglycemia are a consequence of HK-gated increased flux of glucose metabolism without increased glycolytic enzyme activities to accommodate it.

A fingerprint of 2-[18F]FDG radiometabolites - How tissue-specific metabolism beyond 2-[18F]FDG-6-P could affect tracer accumulation

Studies indicate that the radiotracer 2-[18F]fluoro-2-deoxy-D-glucose (2-[18F]FDG) can be metabolized beyond 2-[18F]FDG-6-phosphate (2-[18F]FDG-6-P), but its metabolism is incompletely understood. Most importantly, it remains unclear whether downstream metabolism affects tracer accumulation in vivo. Here we present a fingerprint of 2-[18F]FDG radiometabolites over time in cancer cells, corresponding tumor xenografts and murine organs. Strikingly, radiometabolites representing glycogen metabolism or the oxPPP correlated inversely with tracer accumulation across all examined tissues. Recent studies suggest that not only hexokinase, but also hexose-6-phosphate dehydrogenase (H6PD), an enzyme of the oxidative pentose phosphate pathway (oxPPP), determines 2-[18F]FDG accumulation. However, little is known about the corresponding enzyme glucose-6-phosphate dehydrogenase (G6PD). Our mechanistic in vitro experiments on the role of the oxPPP propose that 2-[18F]FDG can be metabolized via both G6PD and H6PD, but data from separate enzyme knockdown suggest diverging roles in downstream tracer metabolism. Overall, we propose that tissue-specific metabolism beyond 2-[18F]FDG-6-P could matter for imaging.

The SGLT2 inhibitor dapagliflozin improves kidney function in glycogen storage disease XI

Glycogen storage disease XI, also known as Fanconi-Bickel syndrome (FBS), is a rare autosomal recessive disorder caused by mutations in the SLC2A2 gene that encodes the glucose-facilitated transporter type 2 (GLUT2). Patients develop a life-threatening renal proximal tubule dysfunction for which no treatment is available apart from electrolyte replacement. To investigate the renal pathogenesis of FBS, SLC2A2 expression was ablated in mouse kidney and HK-2 proximal tubule cells. GLUT2Pax8Cre+ mice developed time-dependent glycogen accumulation in proximal tubule cells and recapitulated the renal Fanconi phenotype seen in patients. In vitro suppression of GLUT2 impaired lysosomal autophagy as shown by transcriptomic and biochemical analysis. However, this effect was reversed by exposure to a low glucose concentration, suggesting that GLUT2 facilitates the homeostasis of key cellular pathways in proximal tubule cells by preventing glucose toxicity. To investigate whether targeting proximal tubule glucose influx can limit glycogen accumulation and correct symptoms in vivo, we treated mice with the selective SGLT2 inhibitor dapagliflozin. Dapagliflozin reduced glycogen accumulation and improved metabolic acidosis and phosphaturia in the animals by normalizing the expression of Napi2a and NHE3 transporters. In addition, in a patient with FBS, dapagliflozin was safe, improved serum potassium and phosphate concentrations, and reduced glycogen content in urinary shed cells. Overall, this study provides proof of concept for dapagliflozin as a potentially suitable therapy for FBS.

A Role for Sodium-Glucose Cotransporter 2 Inhibitors in the Treatment of Chronic Kidney Disease: A Mini Review

BACKGROUND

Sodium-glucose cotransport protein 2 (SGLT2) inhibitors, a new type of glucose-lowering drug, have been well proved in several clinical studies for their glucose-lowering and nephroprotective effects, and the nephroprotective effects include both indirect effects of metabolic improvement and direct effects, independent of glucose-lowering effects.

SUMMARY

In patients with diabetic kidney disease (DKD), several studies have demonstrated the potential nephroprotective mechanisms of SGLT2 inhibitors, and evidence of nephroprotective mechanisms in the non-DKD population is accumulating. Although the nephroprotective mechanism of SGLT2 inhibitors has not been fully elucidated, several laboratory studies have illustrated the mechanism underlying the effects of SGLT2 inhibitors at various aspects.

KEY MESSAGES

The purpose of this article is to review the mechanism of nephroprotective effect of SGLT2 inhibitors and to look forward to promising research in the future.

Analysis of Cell Glycogen with Quantitation and Determination of Branching Using Liquid Chromatography-Mass Spectrometry

Glycogen is a highly branched biomacromolecule that functions as a glucose buffer. It is involved in multiple diseases such as glycogen storage disorders, diabetes, and even liver cancer, where the imbalance between biosynthetic and catabolic enzymes results in structural alterations and abnormal accumulation of glycogen that can be toxic to cells. Accurate and sensitive glycogen quantification and structural determination are prerequisites for understanding the phenotypes and biological functions of glycogen under these conditions. In this research, we furthered cell glycogen characterization by presenting a highly sensitive method to measure the glycogen content and degree of branching. The method employed a novel fructose density gradient as an alternative to the traditional sucrose gradient to fractionate glycogen from cell mixtures using ultracentrifugation. Fructose was used to avoid the large glucose background, allowing the method to be highly quantitative. The glycogen content was determined by quantifying 1-phenyl-3-methyl-5-pyrazolone (PMP)-derivatized glucose residues obtained from acid-hydrolyzed glycogen using ultra-high-performance liquid chromatography/triple quadrupole mass spectrometry (UHPLC/QqQ-MS). The degree of branching was determined through linkage analysis where the glycogen underwent permethylation, hydrolysis, PMP derivatization, and UHPLC/QqQ-MS analysis. The new approach was used to study the effect of insulin on the glycogen phenotypes of human hepatocellular carcinoma (Hep G2) cells. We observed that cells produced greater amounts of glycogen with less branching under increasing insulin levels before reaching the cell's insulin-resistant state, where the trend reversed and the cells produced less but higher-branched glycogen. The advantage of this method lies in its high sensitivity in characterizing both the glycogen level and the structure of biological samples.

Gluconeogenesis in the kidney: in health and in chronic kidney disease

Chronic kidney disease (CKD) is a global health issue with increasing prevalence. Despite large improvements in current therapies, slowing CKD progression remains a challenge. A better understanding of renal pathophysiology is needed to offer new therapeutic targets. The role of metabolism alterations and mitochondrial dysfunction in tubular cells is increasingly recognized in CKD progression. In proximal tubular cells, CKD progression is associated with a switch from fatty acid oxidation to glycolysis. Glucose synthesis through gluconeogenesis is one of the principal physiological functions of the kidney. Loss of tubular gluconeogenesis in a stage-dependent manner is a key feature of CKD and contributes to systemic and possibly local metabolic complications. The local consequences observed may be related to an accumulation of precursors, such as glycogen, but also to the various physiological functions of the gluconeogenesis enzymes. The basic features of metabolism in proximal tubular cells and their modifications during CKD will be reviewed. The metabolic modifications and their influence on kidney disease will be described, as well as the local and systemic consequences. Finally, therapeutic interventions will be discussed.

Whole Grain Proso Millet (Panicum miliaceum L.) Attenuates Hyperglycemia in Type 2 Diabetic Mice: Involvement of miRNA Profile

This work aimed to investigate the hypoglycemic effects and underlying mechanism of whole grain proso millet (Panicum miliaceum L.; WPM) on type 2 diabetes mellitus (T2DM). The results showed that WPM supplementation significantly reduced fasting blood glucose (FBG) and serum lipid levels in T2DM mice induced by a high-fat diet (HFD) combined with streptozotocin (STZ), with improved glucose tolerance, liver and kidney injury, and insulin resistance. In addition, WPM significantly inhibited the expression of gluconeogenesis-related genes G6pase, Pepck, Foxo1, and Pgc-1α. Further study by miRNA high-throughput sequencing revealed that WPM supplementation mainly altered the liver miRNA expression profile of T2DM mice by increasing the expression of miR-144-3p_R-1 and miR-423-5p, reducing the expression of miR-22-5p_R-1 and miR-30a-3p. GO and KEGG analyses showed that the target genes of these miRNAs were mainly enriched in the PI3K/AKT signaling pathway. WPM supplementation significantly increased the level of PI3K, p-AKT, and GSK3β in the liver of T2DM mice. Taken together, WPM exerts antidiabetic effects by improving the miRNA profile and activating the PI3K/AKT signaling pathway to inhibit gluconeogenesis. This study implies that PM can act as a dietary supplement to attenuate T2DM.

High-intensity interval training induces renal injury and fibrosis in type 2 diabetic mice

AIMS

Previous studies showed that high-intensity interval training (HIIT) improved fasting blood glucose and insulin resistance in type 2 diabetes mellitus (T2DM) mice. However, the effect of HIIT on the kidneys of mice with T2DM has not been examined. This study aimed to investigate the impact of HIIT on the kidneys of T2DM mice.

MATERIALS AND METHODS

T2DM mice were induced with a high-fat diet (HFD) and one-time 100 mg/kg streptozotocin intraperitoneal injection, and then T2DM mice were treated with 8 weeks of HIIT. Renal function and glycogen deposition were observed by serum creatinine levels and PAS staining, respectively. Sirius red staining, hematoxylin-eosin staining, and Oil red O staining were used to detect fibrosis and lipid deposition. Western blotting was performed to detect the protein levels.

KEY FINDINGS

HIIT significantly ameliorated the body composition, fasting blood glucose, and serum insulin of the T2DM mice. HIIT also improved glucose tolerance, insulin tolerance, and renal lipid deposition of T2DM mice. However, we found that HIIT increased serum creatinine and glycogen accumulation in the kidneys of T2DM mice. Western blot analysis showed that the PI3K/AKT/mTOR signaling pathway was activated after HIIT. The expression of fibrosis-related proteins (TGF-β1, CTGF, collagen-III, α-SMA) increased, while the expression of klotho (sklotho) and MMP13 decreased in the kidneys of HIIT mice.

SIGNIFICANCE

This study concluded that HIIT induced renal injury and fibrosis, although it also improved glucose homeostasis in T2DM mice. The current study reminds us that patients with T2DM should be cautious when participating in HIIT.

Phloretamide Prevent Hepatic and Pancreatic Damage in Diabetic Male Rats by Modulating Nrf2 and NF-κB

This study examined the effect of phloretamide, a metabolite of phloretin, on liver damage and steatosis in streptozotocin-induced diabetes mellitus (DM) in rats. Adult male rats were divided into two groups: control (nondiabetic) and STZ-treated rats, each of which was further treated orally with the vehicle phloretamide 100 mg or 200 mg. Treatments were conducted for 12 weeks. Phloretamide, at both doses, significantly attenuated STZ-mediated pancreatic β-cell damage, reduced fasting glucose, and stimulated fasting insulin levels in STZ-treated rats. It also increased the levels of hexokinase, which coincided with a significant reduction in glucose-6 phosphatase (G-6-Pase), and fructose-1,6-bisphosphatase 1 (PBP1) in the livers of these diabetic rats. Concomitantly, both doses of phloretamide reduced hepatic and serum levels of triglycerides (TGs) and cholesterol (CHOL), serum levels of low-density lipoprotein cholesterol (LDL-c), and hepatic ballooning. Furthermore, they reduced levels of lipid peroxidation, tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), mRNA, and total and nuclear levels of NF-κB p65, but increased mRNA levels, total and nuclear levels of Nrf2, as well as levels of reduced glutathione (GSH), superoxide dismutase (SOD-1), catalase (CAT), and heme-oxygenase-1 (HO-1) in the livers of diabetic rats. All of these effects were dose-dependent. In conclusion, phloretamide is a novel drug that could ameliorate DM-associated hepatic steatosis via its powerful antioxidant and anti-inflammatory effects. Mechanisms of protection involve improving the β-cell structure and hepatic insulin action, suppressing hepatic NF-κB, and stimulating hepatic Nrf2.

Deuterium body array for the simultaneous measurement of hepatic and renal glucose metabolism and gastric emptying with dynamic 3D deuterium metabolic imaging at 7 T

Deuterium metabolic imaging (DMI) is a novel noninvasive method to assess tissue metabolism and organ (patho)physiology in vivo using deuterated substrates, such as [6,6'-² H₂ ]-glucose. The liver and kidneys play a central role in whole-body glucose homeostasis, and in type 2 diabetes, both hepatic and renal glucose metabolism are dysregulated. Diabetes is also associated with gastric emptying abnormalities. In this study, we developed a four-channel ² H transmit/receive body array coil for DMI in the human abdomen at 7 T and assessed its performance. In addition, the feasibility of simultaneously measuring gastric emptying, and hepatic and renal glucose uptake and metabolism with dynamic 3D DMI upon administration of deuterated glucose, was investigated. Simulated and measured B₁ ⁺ patterns were in good agreement. The intrasession variability of the natural abundance deuterated water signal in the liver and right kidney, measured in nine healthy volunteers, was 5.6% ± 0.9% and 4.9% ± 0.7%, respectively. Dynamic 3D DMI scans with oral administration of [6,6'-² H₂ ]-glucose showed similar kinetics of deuterated glucose appearance and disappearance in the liver and kidney. The measured gastric emptying half time was 80 ± 10 min, which is in good agreement with scintigraphy measurements. In conclusion, DMI with oral administration of [6,6'-² H₂ ]-glucose enables simultaneous assessment of gastric emptying and liver and kidney glucose uptake and metabolism. When applied in patients with diabetes, this approach may advance our understanding of the interplay between disturbances in liver and kidney glucose uptake and metabolism and gastric emptying, at a detail that cannot be achieved by any other method.

The Mechanism of Hyperglycemia-Induced Renal Cell Injury in Diabetic Nephropathy Disease: An Update

Diabetic Nephropathy (DN) is a serious complication of type I and II diabetes. It develops from the initial microproteinuria to end-stage renal failure. The main initiator for DN is chronic hyperglycemia. Hyperglycemia (HG) can stimulate the resident and non-resident renal cells to produce humoral mediators and cytokines that can lead to functional and phenotypic changes in renal cells and tissues, interference with cell growth, interacting proteins, advanced glycation end products (AGEs), etc., ultimately resulting in glomerular and tubular damage and the onset of kidney disease. Therefore, poor blood glucose control is a particularly important risk factor for the development of DN. In this paper, the types and mechanisms of DN cell damage are classified and summarized by reviewing the related literature concerning the effect of hyperglycemia on the development of DN. At the cellular level, we summarize the mechanisms and effects of renal damage by hyperglycemia. This is expected to provide therapeutic ideas and inspiration for further studies on the treatment of patients with DN.

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.

Kidney Proximal Tubule GLUT2-More than Meets the Eye

Tubulopathy plays a central role in the pathophysiology of diabetic kidney disease (DKD). Under diabetic conditions, the kidney proximal tubule cells (KPTCs) are exposed to an extensive amount of nutrients, most notably glucose; these nutrients deteriorate KPTCs function and promote the development and progression of DKD. Recently, the facilitative glucose transporter 2 (GLUT2) in KPTCs has emerged as a central regulator in the pathogenesis of DKD. This has been demonstrated by identifying its specific role in enhancing glucose reabsorption and glucotoxicity, and by deciphering its effect in regulating the expression of the sodium-glucose transporter 2 (SGLT2) in KPTCs. Moreover, reduction/deletion of KPTC-GLUT2 has been recently found to ameliorate DKD, raising the plausible idea of considering it as a therapeutic target against DKD. However, the underlying molecular mechanisms by which GLUT2 exerts its deleterious effects in KPTCs remain vague. Herein, we review the current findings on the proximal tubule GLUT2 biology and function under physiologic conditions, and its involvement in the pathophysiology of DKD. Furthermore, we shed new light on its cellular regulation during diabetic conditions.

Abietic acid ameliorates nephropathy progression via mitigating renal oxidative stress, inflammation, fibrosis and apoptosis in high fat diet and low dose streptozotocin-induced diabetic rats

BACKGROUND

Abietic acid (AA) has been reported to exhibit anti-inflammatory activity, however its protective effect against inflammation and its trigger factor i.e., oxidative stress and the related sequelae i.e., apoptosis and fibrosis in the kidney in diabetes mellitus (DM) is unknown.

PURPOSE

To identify the ability of AA to mitigate the inflammatory and inflammation-related insults to the kidney in DM.

METHODS & STUDY DESIGN

Adult male rats were induced type-2 DM by feeding with a high-fat diet for twelve weeks followed by injection with a single dose of streptozotocin (STZ) (30 mg/kg/bw) intraperitoneally at twelve weeks. Following DM confirmation, AA (10 and 20 mg/kg/day) was given orally for another four weeks. Then the fasting blood glucose (FBG) and renal profile were determined and oral glucose tolerance test (OGTT) and insulin tolerance test (ITT) tests were performed. A day after the last treatment, rats were sacrificed and kidneys were harvested and subjected for histopathological and molecular biological analysis.

RESULTS

AA treatment was found to reduce the FBG, serum urea and creatinine levels (p < 0.05) while improving the OGTT and ITT (p < 0.05) in diabetic rats. Besides, AA treatment also mitigated kidney histopathological changes, reduces kidney oxidative stress as reflected by reduced levels of RAGE and Keap1 but increased levels of kidney antioxidants Nrf2, SOD, CAT, GPX, HO-1 & NQO-1 (p < 0.05). Additionally, AA treatment also decreases kidney inflammation (NF-kB p65, IL-1β, IL-6, TNF-α and iNOS) and fibrosis (TGF-β1 and GSK-3β) (p < 0/05). Kidney apoptosis decreased as reflected by decreased levels of Bax, caspase-3 and caspase-9 while its anti-apoptosis Bcl-2 protein levels increased (p < 0.05).

CONCLUSION

AA helps to mitigate nephropathy development in DM via counteracting oxidative stress, inflammation and apoptosis.

Application of FTIR Spectroscopy to Detect Changes in Skeletal Muscle Composition Due to Obesity with Insulin Resistance and STZ-Induced Diabetes

Age, obesity, and diabetes mellitus are pathophysiologically interconnected factors that significantly contribute to the global burden of non-communicable diseases. These metabolic conditions are associated with impaired insulin function, which disrupts the metabolism of carbohydrates, lipids, and proteins and can lead to structural and functional changes in skeletal muscle. Therefore, the alterations in the macromolecular composition of skeletal muscle may provide an indication of the underlying mechanisms of insulin-related disorders. The aim of this study was to investigate the potential of Fourier transform infrared (FTIR) spectroscopy to reveal the changes in macromolecular composition in weight-bearing and non-weight-bearing muscles of old, obese, insulin-resistant, and young streptozotocin (STZ)-induced diabetic mice. The efficiency of FTIR spectroscopy was evaluated by comparison with the results of gold-standard histochemical techniques. The differences in biomolecular phenotypes and the alterations in muscle composition in relation to their functional properties observed from FTIR spectra suggest that FTIR spectroscopy can detect most of the changes observed in muscle tissue by histochemical analyses and more. Therefore, it could be used as an effective alternative because it allows for the complete characterization of macromolecular composition in a single, relatively simple experiment, avoiding some obvious drawbacks of histochemical methods.

Dapagliflozin Prevents Kidney Glycogen Accumulation and Improves Renal Proximal Tubule Cell Functions in a Mouse Model of Glycogen Storage Disease Type 1b

BACKGROUND

Mutations in SLC37A4, which encodes the intracellular glucose transporter G6PT, cause the rare glycogen storage disease type 1b (GSD1b). A long-term consequence of GSD1b is kidney failure, which requires KRT. The main protein markers of proximal tubule function, including NaPi2A, NHE3, SGLT2, GLUT2, and AQP1, are downregulated as part of the disease phenotype.

METHODS

We utilized an inducible mouse model of GSD1b, TM-G6PT-/-, to show that glycogen accumulation plays a crucial role in altering proximal tubule morphology and function. To limit glucose entry into proximal tubule cells and thus to prevent glycogen accumulation, we administered an SGLT2-inhibitor, dapagliflozin, to TM-G6PT-/- mice.

RESULTS

In proximal tubule cells, G6PT suppression stimulates the upregulation and activity of hexokinase-I, which increases availability of the reabsorbed glucose for intracellular metabolism. Dapagliflozin prevented glycogen accumulation and improved kidney morphology by promoting a metabolic switch from glycogen synthesis toward lysis and by restoring expression levels of the main proximal tubule functional markers.

CONCLUSION

We provide proof of concept for the efficacy of dapagliflozin in preserving kidney function in GSD1b mice. Our findings could represent the basis for repurposing this drug to treat patients with GSD1b.

Evaluation of Diabetes Effects of Selenium Nanoparticles Synthesized from a Mixture of Luteolin and Diosmin on Streptozotocin-Induced Type 2 Diabetes in Mice

The absence of a treatment efficient in the control of type 2 diabetes mellitus requires more functional products to assist treatment. Luteolin (LU) and diosmin (DIO) have been known as bioactive molecules with potential for the treatment of diabetes. This work aimed to establish the role that a combination of LU and DIO in selenium nanoparticles (SeNPs) played in streptozotocin (STZ)- induced diabetes mice. Green synthesis of Se NPs was performed by mixing luteolin and diosmin with the solution of Na₂SeO₃ under continuous stirring conditions resulting in the flavonoids conjugated with SeNPs. The existence of flavonoids on the surface of SeNPs was confirmed by UV-Vis spectra, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) images, and DLS graphs via Zetasizer. The average diameter of GA/LU/DIO-SeNPs was 47.84 nm with a PDI of −0.208, a zeta potential value of −17.6, a Se content of 21.5% with an encapsulation efficiency of flavonoids of 86.1%, and can be stabilized by gum Arabic for approximately 175 days without any aggregation and precipitation observed at this time. Furthermore, The C57BL/6 mice were treated with STZ induced-diabetes and were exposed to LU/DIO, SeNPs, and GA/LU/DIO-SeNPs for six weeks. The treatment by nanospheres (GA/LU/DIO-SeNPs) in the mice with diabetes for a period of 6 weeks restored their blood glucose, lipid profile, glycogen, glycosylated hemoglobin, and insulin levels. At the same time, there were significant changes in body weight, food intake, and water intake compared with the STZ- untreated induced diabetic mice. Moreover, the GA/LU/DIO-SeNPs showed good antioxidant activity examined by catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx) in liver and kidney and can prevent the damage in the liver evaluated by aspartate aminotransferase (AST), alanine aminotransferase (ALT), and alkaline phosphatase (ALP) activities. The nanospheres exhibited a significant anti-diabetic activity with a synergistic effect between the selenium and flavonoids. This investigation provides novel SeNPs nanospheres prepared by a high-efficiency strategy for incorporating luteolin and diosmin to improve the efficiency in type 2 diabetes.

Measuring and estimating insulin resistance in clinical and research settings

The article discusses how to measure insulin resistance in muscle, liver, and adipose tissue in human participants. The most frequently used methodologies to evaluate insulin resistance are described in detail starting from the gold standard, that is, the euglycemic hyperinsulinemic clamp, to the intravenous glucose tolerance test, surrogate indices based on fasting measurements, or dynamic tests (such as oral glucose or mixed meal tolerance tests). The accuracy, precision, and reproducibility of the tests as well as cutoff values are reported.

Identification of the pathogenic effects of missense variants causing PRKAG2 cardiomyopathy

BACKGROUND

Pathogenic missense variants in PRKAG2, the gene for the gamma 2 regulatory subunit of adenosine monophosphate-activated protein kinase (AMPK), cause severe progressive cardiac disease and sudden cardiac death, named PRKAG2 cardiomyopathy. In our previous study, we reported a E506K variant in the PRKAG2 gene that was associated with this disease. This study aimed to functionally characterize the three missense variants (E506K, E506Q, and R531G) of PRKAG2 and determine the possible effects on AMPK activity.

METHODS

The proband was clinically monitored for eight years. To investigate the functional effects of three missense variants of PRKAG2, in vitro mutagenesis experiments using HEK293 cells with wild and mutant transcripts and proteins were comparatively analyzed using quantitative RT-PCR, immunofluorescence staining, and enzyme-linked immunosorbent assay.

RESULTS

In the long-term follow-up, the proband was deceased due to progressive heart failure. In the in vitro experimental studies, PRKAG2 was overexpressed after 48 h of transfection in three mutated cells, after which the expression levels of PRKAG2 were regressed to the level of wild-type cells in 3-weeks stably transformed cells, except for the cells with E506K variant. E506K, E506Q, and R531G variants had caused a reduction in the AMPK activity and resulted in the formation of cytoplasmic glycogen deposits.

CONCLUSION

Three missense variants that alter AMPK activity affect a residue in the CBS4 domain associated with ATP/AMP-binding. Detailed information on the influence of PRKAG2 pathogenic variants on AMPK activity would be helpful to improve the treatment and management of patients with metabolic cardiomyopathy.

Taurine Ameliorates Streptozotocin-Induced Diabetes by Modulating Hepatic Glucose Metabolism and Oxidative Stress in Mice

Taurine is a sulfated amino acid derivative that plays an important role in maintaining the cell function of the living body. Although taurine has been shown to ameliorate diabetes, its mechanism of action has not yet been fully elucidated. The present study investigated the effects of taurine on diabetes focusing on glucose metabolism and oxidative stress. Type 1 diabetes was induced by the administration of streptozotocin (STZ) to male C57BL/6J mice. Taurine was dissolved in drinking water at 3% (w/v) and allowed to be freely ingested by diabetic mice. The weight and blood glucose levels were measured weekly. After nine weeks, mice were sacrificed and their serum, liver, and kidney were removed and used for biochemical and histological analyses. A microarray analysis was also performed in normal mice. Taurine alleviated STZ-induced hyperglycemia and hyperketonemia, accompanied by the suppression of the decrease in hepatic glycogen and upregulation of the mRNA expression of hepatic glucose transporter GLUT-2. Furthermore, STZ-induced elevation of oxidative stress in the liver and kidney was suppressed by taurine treatment. These results showed that taurine ameliorated diabetes and diabetic complications by improving hepatic glucose metabolism and reducing oxidative stress.

Endoplasmic reticulum stress-dependent activation of TRB3-FoxO1 signaling pathway exacerbates hyperglycemic nephrotoxicity: Protection accorded by naringenin

Endoplasmic reticulum (ER) dysfunction contributes greatly to the pathophysiology of hyperglycemic nephrotoxicity. This study unravels the critical role of Tribbles 3 (TRB3)-Forkhead box O1 (FoxO1) signaling pathway during hyperglycemic renal toxicity. It also uncovers the novel role of Naringenin, a flavanone, in regulating ER stress in proximal tubular cells, NRK 52E, and kidneys of streptozotocin/nicotinamide induced experimental diabetic Wistar rats. Results demonstrate that expression of ER stress marker proteins including phosphorylated protein kinase ER like kinase (p-PERK), phosphorylated eukaryotic Initiation Factor 2α (p-eIF2α), X Box Binding Protein 1 spliced (XBP1s), Activating Transcription Factor 4 (ATF4) and C/EBP Homologous Protein (CHOP) were upregulated in diabetic kidneys indicating the activation of ER stress response due to nephrotoxicity. Treatment with Naringenin reduced the expression of TRB3, an ER stress-inducible pseudokinase, both in vitro and in vivo. Gene silencing of TRB3 enhanced Akt and FoxO1 phosphorylation and alleviated FoxO1 mediated apoptosis during hyperglycemic nephrotoxicity. Notably, TRB3 gene silencing effects were comparable to the response with Naringenin treatment. Prevention of nuclear colocalization of ATF4 and CHOP in Naringenin treated cells was evident. Naringenin also reduced insulin resistance, apoptosis and glycogen accumulation along with enhancement of glucose tolerance in diabetic rats. Prevention of ultrastructural aberrations in the ER of hyperglycemic renal cells by Naringenin confirmed its anti-ER stress effects. These findings affirm that activation of TRB3-FoxO1 signaling is critical in the pathogenesis of hyperglycemia-induced renal toxicity and protective effect of Naringenin via modulation of ER stress may be exploited as a novel approach for its management.

San-Huang-Yi-Shen Capsule Ameliorates Diabetic Kidney Disease through Inducing PINK1/Parkin-Mediated Mitophagy and Inhibiting the Activation of NLRP3 Signaling Pathway

San-Huang-Yi-Shen capsule (SHYS) has been used in the treatment of diabetic kidney disease (DKD) in clinics. However, the mechanism of SHYS on DKD remains unclear. In this study, we used a high-fat diet combined with streptozocin (STZ) injection to establish a rat model of DKD, and different doses of SHYS were given by oral gavage to determine the therapeutic effects of SHYS on DKD. Then, we studied the effects of SHYS on PINK1/Parkin-mediated mitophagy and the activation of NLRP3 inflammasome to study the possible mechanisms of SHYS on DKD. Our result showed that SHYS could alleviate DKD through reducing the body weight loss, decreasing the levels of fasting blood glucose (FBG), and improving the renal function, insulin resistance (IR), and inhibiting inflammatory response and oxidative stress in the kidney. Moreover, transmission electron microscopy showed SHYS treatment improved the morphology of mitochondria in the kidney. In addition, western blot and immunoflourescence staining showed that SHYS treatment induced the PINK1/Parkin-mediated mitophagy and inhibited the activation of NLRP3 signaling pathway. In conclusion, our study demonstrated the therapeutic effects of SHYS on DKD. Additionally, our results indicated that SHYS promoted PINK1/Parkin-mediated mitophagy and inhibited NLRP3 inflammasome activation to improve mitochondrial injury and inflammatory responses.

Understanding the Mechanism of Dysglycemia in a Fanconi-Bickel Syndrome Patient

Fanconi-Bickel Syndrome (FBS) is a rare disorder of carbohydrate metabolism that is characterized mainly by the accumulation of glycogen in the liver and kidney. It is inherited as an autosomal recessive disorder caused by mutations in the SLC2A2 gene, which encodes for GLUT2. Patients with FBS have dysglycemia but the molecular mechanisms of dysglycemia are still not clearly understood. Therefore, we aimed to understand the underlying molecular mechanisms of dysglycemia in a patient with FBS. Genomic DNA was isolated from a peripheral blood sample and analyzed by whole genome and Sanger sequencing. CRISPR-Cas9 was used to introduce a mutation that mimics the patient's mutation in a human kidney cell line expressing GLUT2 (HEK293T). Mutant cells were used for molecular analysis to investigate the effects of the mutation on the expression and function of GLUT2, as well as the expression of other genes implicated in dysglycemia. The patient was found to have a homozygous nonsense mutation (c.901C>T, R301X) in the SLC2A2 gene. CRISPR-Cas9 successfully mimicked the patient's mutation in HEK293T cells. The mutant cells showed overexpression of a dysfunctional GLUT2 protein, resulting in reduced glucose release activity and enhanced intracellular glucose accumulation. In addition, other glucose transporters (SGLT1 and SGLT2 in the kidney) were found to be induced in the mutant cells. These findings suggest the last loops (loops 9-12) of GLUT2 are essential for glucose transport activity and indicate that GLUT2 dysfunction is associated with dysglycemia in FBS.

Renal Tubular Handling of Glucose and Fructose in Health and Disease

The proximal tubule of the kidney is programmed to reabsorb all filtered glucose and fructose. Glucose is taken up by apical sodium-glucose cotransporters SGLT2 and SGLT1 whereas SGLT5 and potentially SGLT4 and GLUT5 have been implicated in apical fructose uptake. The glucose taken up by the proximal tubule is typically not metabolized but leaves via the basolateral facilitative glucose transporter GLUT2 and is returned to the systemic circulation or used as an energy source by distal tubular segments after basolateral uptake via GLUT1. The proximal tubule generates new glucose in metabolic acidosis and the postabsorptive phase, and fructose serves as an important substrate. In fact, under physiological conditions and intake, fructose taken up by proximal tubules is primarily utilized for gluconeogenesis. In the diabetic kidney, glucose is retained and gluconeogenesis enhanced, the latter in part driven by fructose. This is maladaptive as it sustains hyperglycemia. Moreover, renal glucose retention is coupled to sodium retention through SGLT2 and SGLT1, which induces secondary deleterious effects. SGLT2 inhibitors are new anti-hyperglycemic drugs that can protect the kidneys and heart from failing independent of kidney function and diabetes. Dietary excess of fructose also induces tubular injury. This can be magnified by kidney formation of fructose under pathological conditions. Fructose metabolism is linked to urate formation, which partially accounts for fructose-induced tubular injury, inflammation, and hemodynamic alterations. Fructose metabolism favors glycolysis over mitochondrial respiration as urate suppresses aconitase in the tricarboxylic acid cycle, and has been linked to potentially detrimental aerobic glycolysis (Warburg effect). © 2022 American Physiological Society. Compr Physiol 12:2995-3044, 2022.

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.

Glucose Metabolism in Acute Kidney Injury and Kidney Repair

The kidneys play an indispensable role in glucose homeostasis via glucose reabsorption, production, and utilization. Conversely, aberrant glucose metabolism is involved in the onset, progression, and prognosis of kidney diseases, including acute kidney injury (AKI). In this review, we describe the regulation of glucose homeostasis and related molecular factors in kidneys under normal physiological conditions. Furthermore, we summarize recent investigations about the relationship between glucose metabolism and different types of AKI. We also analyze the involvement of glucose metabolism in kidney repair after injury, including renal fibrosis. Further research on glucose metabolism in kidney injury and repair may lead to the identification of novel therapeutic targets for the prevention and treatment of kidney diseases.

Human Glucose Transporters in Renal Glucose Homeostasis

The kidney plays an important role in glucose homeostasis by releasing glucose into the blood stream to prevent hypoglycemia. It is also responsible for the filtration and subsequent reabsorption or excretion of glucose. As glucose is hydrophilic and soluble in water, it is unable to pass through the lipid bilayer on its own; therefore, transport takes place using carrier proteins localized to the plasma membrane. Both sodium-independent glucose transporters (GLUT proteins) and sodium-dependent glucose transporters (SGLT proteins) are expressed in kidney tissue, and mutations of the genes coding for these glucose transporters lead to renal disorders and diseases, including renal cancers. In addition, several diseases may disturb the expression and/or function of renal glucose transporters. The aim of this review is to describe the role of the kidney in glucose homeostasis and the contribution of glucose transporters in renal physiology and renal diseases.

Human hair follicles operate an internal Cori cycle and modulate their growth via glycogen phosphorylase

Hair follicles (HFs) are unique, multi-compartment, mini-organs that cycle through phases of active hair growth and pigmentation (anagen), apoptosis-driven regression (catagen) and relative quiescence (telogen). Anagen HFs have high demands for energy and biosynthesis precursors mainly fulfilled by aerobic glycolysis. Histochemistry reports the outer root sheath (ORS) contains high levels of glycogen. To investigate a functional role for glycogen in the HF we quantified glycogen by Periodic-Acid Schiff (PAS) histomorphometry and colorimetric quantitative assay showing ORS of anagen VI HFs contained high levels of glycogen that decreased in catagen. qPCR and immunofluorescence microscopy showed the ORS expressed all enzymes for glycogen synthesis and metabolism. Using human ORS keratinocytes (ORS-KC) and ex vivo human HF organ culture we showed active glycogen metabolism by nutrient starvation and use of a specific glycogen phosphorylase (PYGL) inhibitor. Glycogen in ORS-KC was significantly increased by incubation with lactate demonstrating a functional Cori cycle. Inhibition of PYGL significantly stimulated the ex vivo growth of HFs and delayed onset of catagen. This study defines translationally relevant and therapeutically targetable new features of HF metabolism showing that human scalp HFs operate an internal Cori cycle, synthesize glycogen in the presence of lactate and modulate their growth via PYGL activity.

Expression and localization of apelin and its receptor in the testes of diabetic mice and its possible role in steroidogenesis

Type 1 diabetes mellitus (T1DM) is a metabolic disorder with severe hyperglycemia, one of the complications of which is testicular dysfunctions, androgen deficiency and decreased male fertility. In the diabetic testes, the expression and signaling pathways of leptin and a number of other adipokines are significantly changed. However, there is no information on the localization and expression of adipokine, apelin and its receptor (APJ) in the diabetic testes, although there is information on the involvement of apelin in the regulation of reproductive functions. The aim of this study was to investigate the expression and localization of apelin and APJ in the testes of mice with streptozotocin-induced T1DM and to estimate the effects of agonist (apelin-13) and antagonist (ML221) of APJ on the testosterone production by diabetic testis explants in the in vitro conditions. We first detected the expression of apelin and its receptor in the mouse testes, and showed an increased intratesticular expression of apelin and APJ along with the reduced testosterone secretion in T1DM. Using imunohistochemical approach, we showed that apelin and APJ are localized in the Leydig and germ cells, and in diabetes, the amount of these proteins was significantly higher than in the control mice. The diabetic testes had a decrease in germ cell proliferation (the reduced PCNA and GCNA levels) and an increase in apoptosis (the increased active caspase-3 and decreased BCL2 levels). These results suggest an involvement of apelin and APJ in T1DM-induced testicular pathogenesis. Treatment of the cultured testis explants with ML221 significantly increased the testosterone secretion, whereas apelin-13 was ineffective. Thus, hyperapelinemia in the testes can significantly contribute to testicular pathogenesis in T1DM, and pharmacological inhibition of apelin receptors can improve testicular steroidogenesis.

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.

Non-negative Least Squares Approach to Quantification of 1H Nuclear Magnetic Resonance Spectra of Human Urine

Because of its quantitative character and capability for high-throughput screening, ¹H nuclear magnetic resonance (NMR) spectroscopy is used extensively in the profiling of biofluids such as urine and blood plasma. However, the narrow frequency bandwidth of ¹H NMR spectroscopy leads to a severe overlap of the spectra of components present in the complex mixtures such as biofluids. Therefore, ¹H NMR-based metabolomics analysis is focused on targeted studies related to concentrations of the small number of metabolites. Here, we propose a library-based approach to quantify proportions of overlapping metabolites from ¹H NMR mixture spectra. The method boils down to the linear non-negative least squares (NNLS) problem, whereas proportions of the pure components contained in the library stand for the unknowns. The method is validated on an estimation of the proportions of (i) the 78 pure spectra, presumably related to type 2 diabetes mellitus (T2DM), from their synthetic linear mixture; (ii) metabolites present in 62 ¹H NMR spectra of urine of subjects with T2DM and 62 ¹H NMR spectra of urine of control subjects. In both cases, the in-house library of 210 pure component ¹H NMR spectra represented the design matrix in the related NNLS problem. The proposed method pinpoints 63 metabolites that in a statistically significant way discriminate the T2DM group from the control group and 46 metabolites discriminating control from the T2DM group. For several T2DM-discriminative metabolites, we prove their presence by independent analytical determination or by pointing out the corresponding findings in the published literature.

Role of Glucose Metabolism and Mitochondrial Function in Diabetic Kidney Disease

PURPOSE OF REVIEW

Diabetic kidney disease (DKD) continues to be the primary cause of chronic kidney disease in the USA and around the world. The numbers of people with DKD also continue to rise despite current treatments. Certain newer hypoglycemic drugs offer a promise of slowing progression, but it remains to be seen how effective these will be over time. Thus, continued exploration of the mechanisms underlying the development and progression of DKD is essential in order to discover new treatments. Hyperglycemia is the main cause of the cellular damage seen in DKD. But, exactly how hyperglycemia leads to the activation of processes that are ultimately deleterious is incompletely understood.

RECENT FINDINGS

Studies primarily over the past 10 years have provided novel insights into the interplay of hyperglycemia, glucose metabolic pathways, mitochondrial function, and the potential importance of what has been called the Warburg effect on the development and progression of DKD. This review will provide a brief overview of glucose metabolism and the hypotheses concerning the pathogenesis of DKD and then discuss in more detail the supporting data that indicate a role for the interplay of glucose metabolic pathways and mitochondrial function.

The controversial role of glucose in the diabetic kidney

The kidneys play an important role in maintaining glucose homeostasis being the main mechanisms, the gluconeogenesis, renal glucose consumption and glucose reabsorption in the proximal tubules. In this review, we present the main research into the role of glycogen-the stored form of glucose, and how it accumulates in the cells, providing new information on the link between diabetes and diabetic kidney disease. In the last 10 years, research under the scope of renal insulin handling, glucose transport in the proximal tubules, renal gluconeogenesis and renal insulin resistance, made possible to relate the roles of glucose and glycogen in the kidney with other several organs, like the liver. On the one hand, insulin positively regulates kidney uptake and degradation, and there is probably a specific action and resistance to insulin at the renal site. Moreover, insulin regulates the bioavailability of the sodium-glucose co-transporters-SGLT2 inhibitor, and inhibits renal gluconeogenesis. Only the liver and kidneys can supply glucose to the circulation through the process of gluconeogenesis, which involves the synthesis of glucose again from non-glycemic substrates; and the decomposition of stored glycogen. In the mind of nephrologists, diabetologists and scientists, glucose metabolism in the kidney is the focus, with the relevant success of inhibitors in reducing kidney and cardiovascular diseases in individuals with diabetes. However, these new data led to the intriguing paradigm that many of the beneficial effects on the renal and cardiovascular system appear to be independent of the systemic glucose-lowering actions of these agents. The goal of this work puts in context a highly relevant research area for renal glucose metabolism, of glycogen accumulation and metabolism in the diabetic kidney.

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.

•

Diabetes involves uncontrolled blood glucose levels

•

Liver glycogen acts as a blood glucose buffer

•

Diabetes can lead to molecularly fragile liver glycogen particles

•

Molecularly fragile liver glycogen may exacerbate poor blood glucose control

Revisiting Glycogen in Cancer: A Conspicuous and Targetable Enabler of Malignant Transformation

Once thought to be exclusively a storage hub for glucose, glycogen is now known to be essential in a range of physiological processes and pathological conditions. Glycogen lies at the nexus of diverse processes that promote malignancy, including proliferation, migration, invasion, and chemoresistance of cancer cells. It is also implicated in processes associated with the tumor microenvironment such as immune cell effector function and crosstalk with cancer-associated fibroblasts to promote metastasis. The enzymes of glycogen metabolism are dysregulated in a wide variety of malignancies, including cancers of the kidney, ovary, lung, bladder, liver, blood, and breast. Understanding and targeting glycogen metabolism in cancer presents a promising but under-explored therapeutic avenue. In this review, we summarize the current literature on the role of glycogen in cancer progression and discuss its potential as a therapeutic target for cancer treatment.

High-sucrose diet potentiates hyperaldosteronism and renal injury induced by stress in young adult rats

Analyze the effect of stress and high-sucrose diet on serum aldosterone levels and the morphometric characteristics of the kidney in young adult rats. Wistar male rats aged 21 days old weaned were randomly assigned into four groups: control (C), stressed (St), high-sucrose diet (S30), and chronic restraint stress plus a 30% sucrose diet (St + S30). Rats were fed with a standard chow and tap water ad libitum (C group) or 30% sucrose diluted in water (S30 group) during eight weeks. The St and St + S30 groups were subject to restraint stress (1-hour daily in a plastic cylinder, 5 days per week), four weeks before euthanasia. At 81 days old, all animals were killed and blood samples and kidneys were collected. Stressed rats had an increase in the serum aldosterone and renal triacylglycerol, a decrease in the area of the renal corpuscle, glomeruli, proximal tubules, and aquaporin 2 expressions with loss of glomeruli. For its part, the high-sucrose diet decreased the area of the renal corpuscle, glomeruli, and aquaporin 2 expressions in the cortex. The combination of stress and high- sucrose diet maintained similar effects on the kidney as the stress alone, although it induced an increase in the creatinine levels and renal glycogen. Our results showed that chronic stress induces hyperaldosteronism and kidney injury. The intake of a high-sucrose diet may potentiate the renal injury promoted by stress.

Mutation of regulatory phosphorylation sites in PFKFB2 worsens renal fibrosis

Fatty acid oxidation is the major energy pathway used by the kidney, although glycolysis becomes more important in the low oxygen environment of the medulla. Fatty acid oxidation appears to be reduced in renal fibrosis, and drugs that reverse this improve fibrosis. Expression of glycolytic genes is more variable, but some studies have shown that inhibiting glycolysis reduces renal fibrosis. To address the role of glycolysis in renal fibrosis, we have used a genetic approach. The crucial control point in the rate of glycolysis is 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase. Phosphorylation of the kidney isoform, PFKFB2, on residues Ser⁴⁶⁸ and Ser⁴⁸⁵ stimulates glycolysis and is the most important mechanism regulating glycolysis. We generated transgenic mice with inactivating mutations of Ser⁴⁶⁸ and Ser⁴⁸⁵ in PFKFB2 (PFKFB2 KI mice). These mutations were associated with a reduced ability to increase glycolysis in primary cultures of renal tubular cells from PFKFB2 KI mice compared to WT cells. This was associated in PFKFB2 KI mice with increased renal fibrosis, which was more severe in the unilaternal ureteric obstruction (UUO) model compared with the folic acid nephropathy (FAN) model. These studies show that phosphorylation of PFKFB2 is important in limiting renal fibrosis after injury, indicating that the ability to regulate and maintain adequate glycolysis in the kidney is crucial for renal homeostasis. The changes were most marked in the UUO model, probably reflecting a greater effect on distal renal tubules and the greater importance of glycolysis in the distal nephron.

Incidence and risk factors associated with hypoglycemia among patients with chronic kidney disease: A systematic review

Hypoglycemia is a common complication in patients with chronic kidney disease (CKD), more so if they have diabetes as well. The occurrence of hypoglycemia in CKD is associated with considerable morbidity and mortality, both of which are treatable and preventable. This review summarizes the incidence and risk factors associated with hypoglycemia among patients with CKD. The meta-analysis was performed as per the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement. A search was done on PubMed, EMBASE, SCOPUS, Cochrane Library, Google Scholar, and Cumulative Index to Nursing and Allied Health Literature for cohort studies in English published between January 2000 and August 2019 using search terms related to hypoglycemia (low blood sugar), chronic kidney disease (chronic renal failure OR renal failure), and incidence (risk OR epidemiology OR risk factors). Summary measures were calculated using random-effects model. A total of 5 studies involving 311,817 persons were included in the meta-analysis. The pooled incidence of hypoglycemia in patients with CKD was 0.188 (confidence interval [CI] = 0.097-0.287). The incidence of hypoglycemia was significantly higher in patients with CKD than in patients without CKD (Relative risk [RR] = 1.89, 95% CI = 1.86-1.92, P < 0.0001). No heterogeneity was reported between the studies (I² = 0%, P > 0.05), and publication bias was also found. Females, patients who had diabetes mellitus of long duration, and those on antidiabetic drugs such as insulin and sulfonylureas were at risk of developing hypoglycemia in CKD as per narrative review. The incidence of hypoglycemia in patients with CKD is high. Therefore, there is need to closely monitor affected individuals so that appropriate management protocols could be set up. Further probing of various risk factors for hypoglycemia in CKD patients is necessary for early detection and initiation of timely preventive and curative measures.

Fanconi-Bickel Syndrome: A Review of the Mechanisms That Lead to Dysglycaemia

Accumulation of glycogen in the kidney and liver is the main feature of Fanconi-Bickel Syndrome (FBS), a rare disorder of carbohydrate metabolism inherited in an autosomal recessive manner due to SLC2A2 gene mutations. Missense, nonsense, frame-shift (fs), in-frame indels, splice site, and compound heterozygous variants have all been identified in SLC2A2 gene of FBS cases. Approximately 144 FBS cases with 70 different SLC2A2 gene variants have been reported so far. SLC2A2 encodes for glucose transporter 2 (GLUT2) a low affinity facilitative transporter of glucose mainly expressed in tissues playing important roles in glucose homeostasis, such as renal tubular cells, enterocytes, pancreatic β-cells, hepatocytes and discrete regions of the brain. Dysfunctional mutations and decreased GLUT2 expression leads to dysglycaemia (fasting hypoglycemia, postprandial hyperglycemia, glucose intolerance, and rarely diabetes mellitus), hepatomegaly, galactose intolerance, rickets, and poor growth. The molecular mechanisms of dysglycaemia in FBS are still not clearly understood. In this review, we discuss the physiological roles of GLUT2 and the pathophysiology of mutants, highlight all of the previously reported SLC2A2 mutations associated with dysglycaemia, and review the potential molecular mechanisms leading to dysglycaemia and diabetes mellitus in FBS patients.

Lipotoxicity and Diabetic Nephropathy: Novel Mechanistic Insights and Therapeutic Opportunities

Lipotoxicity is characterized by the ectopic accumulation of lipids in organs different from adipose tissue. Lipotoxicity is mainly associated with dysfunctional signaling and insulin resistance response in non-adipose tissue such as myocardium, pancreas, skeletal muscle, liver, and kidney. Serum lipid abnormalities and renal ectopic lipid accumulation have been associated with the development of kidney diseases, in particular diabetic nephropathy. Chronic hyperinsulinemia, often seen in type 2 diabetes, plays a crucial role in blood and liver lipid metabolism abnormalities, thus resulting in increased non-esterified fatty acids (NEFA). Excessive lipid accumulation alters cellular homeostasis and activates lipogenic and glycogenic cell-signaling pathways. Recent evidences indicate that both quantity and quality of lipids are involved in renal damage associated to lipotoxicity by activating inflammation, oxidative stress, mitochondrial dysfunction, and cell-death. The pathological effects of lipotoxicity have been observed in renal cells, thus promoting podocyte injury, tubular damage, mesangial proliferation, endothelial activation, and formation of macrophage-derived foam cells. Therefore, this review examines the recent preclinical and clinical research about the potentially harmful effects of lipids in the kidney, metabolic markers associated with these mechanisms, major signaling pathways affected, the causes of excessive lipid accumulation, and the types of lipids involved, as well as offers a comprehensive update of therapeutic strategies targeting lipotoxicity.