Avian and Mammalian Facilitative Glucose Transporters

Glycation resistance and life-history traits: lessons from non-conventional animal models

Glycation reactions play a key role in the senescence process and are involved in numerous age-related pathologies, such as diabetes complications or Alzheimer's disease. As a result, past studies on glycation have mostly focused on human and laboratory animal models for medical purposes. Very little is known about glycation and its link to senescence in wild animal species. Yet, despite feeding on high-sugar diets, several bat and bird species are long-lived and seem to escape the toxic effects of high glycaemia. The study of these models could open new avenues both for understanding the mechanisms that coevolved with glycation resistance and for treating the damaging effects of glycations in humans. Our understanding of glycaemia's correlation to proxies of animals' pace of life is emerging in few wild species; however, virtually nothing is known about their resistance to glycation, nor on the relationship between glycation, species' life-history traits and individual fitness. Our review summarizes the scarce current knowledge on the links between glycation and life-history traits in non-conventional animal models, highlighting the predominance of avian research. We also investigate some key molecular and physiological parameters involved in glycation regulation, which hold promise for future research on fitness and senescence of individuals.

Bridging the gap: glucose transporters, Alzheimer's, and future therapeutic prospects

Glucose is the major source of chemical energy for cell functions in living organisms. The aim of this mini-review is to provide a clearer and simpler picture of the fundamentals of glucose transporters as well as the relationship of these transporters to Alzheimer's disease. This study was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). Electronic databases (PubMed and ScienceDirect) were used to search for relevant studies mainly published during the period 2018-2023. This mini-review covers the two main types of glucose transporters, facilitated glucose transporters (GLUTs) and sodium-glucose linked transporters (SGLTs). The main difference between these two types is that the first type works through passive transport across the glucose concentration gradient. The second type works through active co-transportation to transport glucose against its chemical gradient. Fluctuation in glucose transporters translates into a disturbance of normal functioning, such as Alzheimer's disease, which may be caused by a significant downregulation of GLUTs most closely associated with insulin resistance in the brain. The first sign of Alzheimer's is a lack of GLUT4 translocation. The second sign is tau hyperphosphorylation, which is caused by GLUT1 and 3 being strongly upregulated. The current study focuses on the use of glucose transporters in treating diseases because of their proven therapeutic potential. Despite this, studies remain insufficient and inconclusive due to the complex and intertwined nature of glucose transport processes. This study recommends further understanding of the mechanisms related to these vectors for promising future therapies.

Regulations of myo-inositol homeostasis: Mechanisms, implications, and perspectives

Phosphorylation is the most common module of cellular signalling pathways. The dynamic nature of phosphorylation, which is conferred by the balancing acts of kinases and phosphatases, allows this modification to finely control crucial cellular events such as growth, differentiation, and cell cycle progression. Although most research to date has focussed on protein phosphorylation, non-protein phosphorylation substrates also play vital roles in signal transduction. The most well-established substrate of non-protein phosphorylation is inositol, whose phosphorylation generates many important signalling molecules such as the second messenger IP₃, a key factor in calcium signalling. A fundamental question to our understanding of inositol phosphorylation is how the levels of cellular inositol are controlled. While the availability of protein phosphorylation substrates is known to be readily controlled at the levels of transcription, translation, and/or protein degradation, the regulatory mechanisms that control the uptake, synthesis, and removal of inositol are underexplored. Potentially, such mechanisms serve as an important layer of regulation of cellular signal transduction pathways. There are two ways in which mammalian cells acquire inositol. The historic use of radioactive ³H-myo-inositol revealed that inositol is promptly imported from the extracellular environment by three specific symporters SMIT1/2, and HMIT, coupling sodium or proton entry, respectively. Inositol can also be synthesized de novo from glucose-6P, thanks to the enzymatic activity of ISYNA1. Intriguingly, emerging evidence suggests that in mammalian cells, de novo myo-inositol synthesis occurs irrespective of inositol availability in the environment, prompting the question of whether the two sources of inositol go through independent metabolic pathways, thus serving distinct functions. Furthermore, the metabolic stability of myo-inositol, coupled with the uptake and endogenous synthesis, determines that there must be exit pathways to remove this extraordinary sugar from the cells to maintain its homeostasis. This essay aims to review our current knowledge of myo-inositol homeostatic metabolism, since they are critical to the signalling events played by its phosphorylated forms.

The Mineralocorticoid Receptor Functions as a Key Glucose Regulator in the Skeletal Muscle of Zebrafish

Glucocorticoids (GCs) are essential for maintaining energy homeostasis as part of the adaptive stress response. Most work to date has characterized the metabolic role of GCs via the activation of the glucocorticoid receptor (nr3c1; GR), which is activated under high GC conditions. However, GCs also bind to the mineralocorticoid receptor (nr3c2; MR), a high-affinity corticosteroid receptor active under basal GC conditions. Despite the expression of MR in skeletal muscles, almost nothing is known about its physiological role. Here we tested the hypothesis that the MR promotes anabolic processes during resting cortisol levels and curtails the catabolic actions of the GR during high (stressed) levels of cortisol. To determine the effect of MR, a zebrafish line with a ubiquitous MR knockout (MRca402/ca402) was utilized. The GR was activated in the same group by chronically treating fish with exogenous cortisol. In the muscle, MR primarily promoted nutrient storage, and restricted energy substrate mobilization under resting conditions, whereas GR activation resulted in increased nutrient utilization. Interestingly, MR loss improved GR-driven metabolic flexibility, suggesting that the activation state of these receptors is a key determinant of skeletal muscle ability to switch fuel sources. To determine if the anabolism-promoting nature of MR was due to an interaction with insulin, fish were co-injected with insulin and the fluorescent glucose analogue 2-NBDG. A loss of MR abolished insulin-stimulated glucose uptake in the skeletal muscle. Taken together, we postulate that MR acts as a key modulator of glucose metabolism in the musculature during basal and stress conditions.

Revisiting glucose regulation in birds - A negative model of diabetes complications

Birds naturally have blood glucose concentrations that are nearly double levels measured for mammals of similar body size and studies have shown that birds are resistant to insulin-mediated glucose uptake into tissues. While a combination of high blood glucose and insulin resistance is associated with diabetes-related pathologies in mammals, birds do not develop such complications. Moreover, studies have shown that birds are resistant to oxidative stress and protein glycation and in fact, live longer than similar-sized mammals. This review seeks to explore how birds regulate blood glucose as well as various theories that might explain their apparent resistance to insulin-mediated glucose uptake and adaptations that enable them to thrive in a state of relative hyperglycemia.

Effects of maca (Lepidium meyenii) on nutrient digestibility and major nutrient transporters in rats fed a high-fat diet

SCOPE

This study was carried out to investigate the efficacy of a new combination of root extracts of the Lepidium meyenii (maca) plant, known for its nutritional and energizing features as well as its antioxidant properties, on nutrient digestibility and nutrient transporters expression.

METHODS AND RESULTS

A total of 28 Sprague-Dawley rats (8-week-old) were divided into four groups: (i) control, (ii) Lepidium m., (iii) high-fat diet (HFD), and (iv) HFD+Lepidium m. Maca was given to the rats as a powdered combination of the plant roots with a daily dose of 40 mg per kg BW. Maca administration significantly increased the digestibility of dry matter (DM), organic matter (OM), crude protein (CP), and ether extract (EE), and some nutrient transporter (Pept1/2, Fatp1, Glut1/2, and Sglt1)-expressions compared with non-treated control and HFD groups in the jejunum and ileum tissues (p < .0001).

CONCLUSIONS

Maca supplementation improved the digestibility of nutrients and expressions of nutrient transporters in the small intestine of the rats. These results indicate the positive communication between maca consumption and nutrient absorption in the small intestines of the animals.

Bird evolution by insulin resistance

Drift of oxygen concentrations in the atmosphere was one of the main drivers of the evolution of vertebrates. The drop in oxygen concentrations at the Permian-Triassic (PT) boundary may have been the biggest challenge to vertebrates. This hypoxic condition forced theropods to lose certain genes to maximize their efficiency of oxygen usage. Recent studies show that omentin and insulin-sensitive glucose transporter 4 (GLUT4) are missing in the bird genome. Since these gene products play essential roles in maintaining insulin sensitivity, this loss forced theropods to become insulin resistant. Insulin resistance may have been the key to allowing theropods to become hyperathletic under hypoxic conditions and to outcompete mammals during the Triassic period. A second challenge was the gradual increase in oxygen concentrations during the late Jurassic, Cretaceous, and Tertiary periods when reactive oxygen species (ROS) leakage from mitochondria became a problem. Since the simplest solution was the expansion of body size, some theropods became bigger to reduce ROS leakage per volume. Another solution was the development of a constitutively active countermeasure against ROS. A recent study shows that Neoaves have constitutively active nuclear factor erythroid 2-related factor 2 (NRF2) due to deletion of the C-terminal part of the KEAP1 protein, thus allowing Neoaves to express antioxidant enzymes to overcome ROS leakage.

Lung branching morphogenesis is accompanied by temporal metabolic changes towards a glycolytic preference

Background

Lung branching morphogenesis is characterized by epithelial-mesenchymal interactions that ultimately define the airway conducting system. Throughout this process, energy and structural macromolecules are necessary to sustain the high proliferative rates. The extensive knowledge of the molecular mechanisms underlying pulmonary development contrasts with the lack of data regarding the embryonic lung metabolic requirements. Here, we studied the metabolic profile associated with the early stages of chicken pulmonary branching.

Methods

In this study, we used an ex vivo lung explant culture system and analyzed the consumption/production of extracellular metabolic intermediates associated with glucose catabolism (alanine, lactate, and acetate) by ¹H-NMR spectroscopy in the culture medium. Then, we characterized the transcript levels of metabolite membrane transporters (glut1, glut3, glut8, mct1, mct3, mct4, and mct8) and glycolytic enzymes (hk1, hk2, pfk1, ldha, ldhb, pdha, and pdhb) by qPCR. ldha and ldhb mRNA spatial localization was determined by in situ hybridization. Proliferation was analyzed by directly assessing DNA synthesis using an EdU-based assay. Additionally, we performed western blot to analyze LDHA and LDHT protein levels. Finally, we used a Clark-Type Electrode to assess the lung explant's respiratory capacity.

Results

Glucose consumption decreases, whereas alanine, lactate, and acetate production progressively increase as branching morphogenesis proceeds. mRNA analysis revealed variations in the expression levels of key enzymes and transporters from the glycolytic pathway. ldha and ldhb displayed a compartment-specific expression pattern that resembles proximal–distal markers. In addition, high proliferation levels were detected at active branching sites. LDH protein expression levels suggest that LDHB may account for the progressive rise in lactate. Concurrently, there is a stable oxygen consumption rate throughout branching morphogenesis.

Conclusions

This report describes the temporal metabolic changes that accompany the early stages of chicken lung branching morphogenesis. Overall, the embryonic chicken lung seems to shift to a glycolytic lactate-based metabolism as pulmonary branching occurs. Moreover, this metabolic rewiring might play a crucial role during lung development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00654-w.

Impacts of increasing challenge with Eimeria maxima on the growth performance and gene expression of biomarkers associated with intestinal integrity and nutrient transporters

This study was conducted to investigate the impacts of graded severity of Eimeria maxima infection on the growth performance and intestine health of broiler chickens. Four different levels of E. maxima-challenged treatments were used, including a non-challenged control group, a low challenge (12 500 oocysts), a medium challenge (25 000 oocysts), and a high challenge dose (50 000 oocysts). There were eight replicate cages per treatment, with 12 birds in each cage, and chickens in the challenged groups orally received sporulated oocysts on day 14. Gastrointestinal permeability was measured by fluorescein isothiocyanate dextran at 5 days post-infection (dpi), whereas intestinal morphology and gene expression of nutrient transporters and tight junction proteins were determined at 6 dpi. The results demonstrate a linear reduction in growth performance, jejunal villus height, and jejunal integrity with graded challenge doses of E. maxima (P < 0.01). Moreover, linear regulation of nutrient transporters and tight junction proteins was a consequence of increasing Eimeria infection levels (P < 0.01). The linear increase of Claudin 1, cationic amino acid transporter, glucose transporter 1, and L-type amino acid transporter genes was associated with increased severity of coccidiosis (P < 0.01). Furthermore, expression of nutrient transporters located at the brush border membrane were down-regulated (P < 0.01) with increasing E. maxima inoculation dose. In conclusion, growth performance and key intestinal integrity biomarkers in broiler chickens were adversely influenced in a dose-dependent manner by E. maxima infection.

Reduced cellular glucose transport confers natural protection against dextrose-induced superoxide generation and endoplasmic reticulum stress in domestic hen

Normal blood glucose levels in avian species are two to fourfold higher than that in humans and the higher blood glucose levels in birds do not cause adverse effects. Endothelial cells isolated from the aorta of the domestic hen (Gallus gallus domesticus) and chicken aortic smooth muscle cells (CAOSMC) were compared to human coronary artery endothelial cells (HCAEC) and human primary aortic smooth muscle cells (HASMC). Superoxide (SO) generation was measured using a superoxide‐reactive probe. ER stress was measured using the placental alkaline phosphatase assay (ES‐TRAP). Glucose transport kinetics were determined using the ³H‐2‐deoxyglucose tracer. Dextrose‐induced SO generation and ER stress were significantly blunted in avian endothelial cells compared to human cells. The Vmax of glucose uptake (in nmoles/mg protein/min) in avian endothelial cells (0.0018 ± 0.0001) and smooth muscle cells (0.0015 ± 0.0007) was approximately 18–25 fold lower compared to the Vmax in HCAEC (0.033 ± 0.0025) and HASMC (0.038 ± 0.004) (all p < 0.0001). The Michaelis–Menten constant (Km) of transport was also significantly different (p < 0.0001) in avian species. The relative resistance of avian cells to dextrose‐induced oxidative stress and ER stress is mostly the result of reduced cellular dextrose transport.

A Role of Glucose Overload in Diabetic Cardiomyopathy in Nonhuman Primates

Type 2 diabetes (T2D) plays a major role in the development of heart failure. Patients with T2D have an increased risk to develop HF than healthy subjects, and they always have very poor outcomes and survival rates. However, the underlying mechanisms for this are still unclear. To help develop new therapeutic interventions, well-characterized animal models for preclinical and translational investigations in T2D and HF are urgently needed. Although studies in rodents are more often used, the research findings in rodents have often failed to be translated into humans due to the significant metabolic differences between rodents and humans. Nonhuman primates (NHPs) serve as valuable translational models between basic studies in rodent models and clinical studies in humans. NHPs can recapitulate the natural progress of these diseases in humans and study the underlying mechanism due to their genetic similarity and comparable spontaneous T2D rates to humans. In this review, we discuss the importance of using NHPs models in understanding diabetic cardiomyopathy (DCM) in humans with aspects of correlations between hyperglycemia and cardiac dysfunction progression, glucose overload, and altered glucose metabolism promoting cardiac oxidative stress and mitochondria dysfunction, glucose, and its effect on cardiac resynchronization therapy with defibrillator (CRT-d), the currently available diabetic NHPs models and the limitations involved in the use of NHP models.

Organ-specific effects on glycolysis by the dioxin-activated aryl hydrocarbon receptor

Activation of the aryl hydrocarbon receptor (AHR) by the environmental toxin dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD) causes diverse toxicities, including thymus atrophy and hepatosteatosis. The mechanisms by which AHR activation by TCDD leads to these toxicities are not fully understood. Here we studied the effects of TCDD on a major energy pathway, glycolysis, using the chick embryo close to hatching, a well-established model for studying dioxin toxicity. We showed that 24 hr of TCDD treatment causes changes in glycolysis in both thymus and liver. In thymus glands, TCDD decreased mRNAs for glycolytic genes and glucose transporters, glycolytic indices and levels of IL7 mRNA, phosphorylated AKT (pAKT) and HIF1A, stimulators of glycolysis and promoters of survival and proliferation of thymic lymphocytes. In contrast, in liver, TCDD increased mRNA levels for glycolytic genes and glucose transporters, glycolytic endpoints and pAKT levels. Similarly, increases by TCDD in mRNA levels for glycolytic genes and glucose transporters in human primary hepatocytes showed that effects in chick embryo liver pertain also to human cells. Treatment with the glycolytic inhibitor 2-deoxy-d-glucose exacerbated the effects on thymus atrophy by TCDD, supporting a role for decreased glycolysis in thymus atrophy by TCDD, but did not prevent hepatosteatosis. NAD⁺ precursors abolished TCDD effects on glycolytic endpoints in both thymus and liver. In summary, we report here that dioxin disrupts glycolysis mediated energy metabolism in both thymus and liver, and that it does so in opposite ways, decreasing it in the thymus and increasing it in the liver. Further, the findings support NAD⁺ boosting as a strategy against metabolic effects of environmental pollutants such as dioxins.

Glucose transporter expression and regulation following a fast in the ruby-throated hummingbird, Archilochus colubris

Hummingbirds, subsisting almost exclusively on nectar sugar, face extreme challenges to blood sugar regulation. The capacity for transmembrane sugar transport is mediated by the activity of facilitative glucose transporters (GLUTs) and their localisation to the plasma membrane (PM). In this study, we determined the relative protein abundance of GLUT1, GLUT2, GLUT3 and GLUT5 via immunoblot using custom-designed antibodies in whole-tissue homogenates and PM fractions of flight muscle, heart and liver of ruby-throated hummingbirds (Archilochus colubris). The GLUTs examined were detected in nearly all tissues tested. Hepatic GLUT1 was minimally present in whole-tissue homogenates and absent win PM fractions. GLUT5 was expressed in flight muscles at levels comparable to those of the liver, consistent with the hypothesised uniquely high fructose uptake and oxidation capacity of hummingbird flight muscles. To assess GLUT regulation, we fed ruby-throated hummingbirds 1 mol l-1 sucrose ad libitum for 24 h followed by either 1 h of fasting or continued feeding until sampling. We measured relative GLUT abundance and concentration of circulating sugars. Blood fructose concentration in fasted hummingbirds declined (∼5 mmol l-1 to ∼0.18 mmol l-1), while fructose-transporting GLUT2 and GLUT5 abundance did not change in PM fractions. Blood glucose concentrations remained elevated in fed and fasted hummingbirds (∼30 mmol l-1), while glucose-transporting GLUT1 and GLUT3 in flight muscle and liver PM fractions, respectively, declined in fasted birds. Our results suggest that glucose uptake capacity is dynamically reduced in response to fasting, allowing for maintenance of elevated blood glucose levels, while fructose uptake capacity remains constitutively elevated promoting depletion of blood total fructose within the first hour of a fast.

Dynamic changes of blood glucose, serum biochemical parameters and gene expression in response to exogenous insulin in Arbor Acres broilers and Silky fowls

Silky chicken is a breed of chickens with black skin and slow growth rate used in Chinese traditional medicine, whereas Arbor Acres broiler is a well-known commercial breed in the poultry industry, it is featured by a large size, rapid-growth rate, high feed-conversion rate and strong adaptability. The difference in their rate of growth may be primarily related to different mechanism for glucose metabolism. Here we compared the insulin sensitivity of the two breeds; we investigated the temporal changes (at 0 min, 120 min and 240 min) of serum insulin and other biochemical parameters and determined the spatio-temporal changes of gene mRNA abundance in response to exogenous insulin (80 μg/kg body weight). The results indicated that: (1) Silky chickens showed stronger blood glucose recovery than broilers in the insulin resistance test. (2) The serum urea level in Silky chickens was twice of broilers; exogenous insulin significantly up-regulated serum uric acid level in Silky fowls in a time-dependent manner and increased serum cholesterol content at 120 min. (3) Two breeds showed distinctly different temporal changed in serum insulin in response to exogenous insulin stimulation. The fasting serum insulin concentration of broilers was three-fold of Silky chickens at the basal state; it decreased significantly after insulin injection and the levels at 120 min and 240 min of broilers were only 23% (P < 0.01) and 14% (P < 0.01) of the basal state, respectively. Whereas the serum insulin content in Silky chickens showed stronger recovery, and the 240 min level was close to the 0 min level. (4) GLUT2, GLUT12, neuropeptide Y and insulin receptor (IR) were predominantly expressed in the liver, pectoralis major, olfactory bulb and pancreas, respectively, where these genes presented stronger insulin sensitivity. In addition, the IR mRNA level was strongly positively with the GLUT12 level. In conclusion, our findings suggested that Silky chickens have a stronger ability to regulate glucose homeostasis than broilers, owing to their higher IR levels in the basal state, stronger serum insulin homeostasis and candidate genes functioning primarily in their predominantly expressed tissue in response to exogenous insulin.

Delayed access to feed alters expression of genes associated with carbohydrate and amino acid utilization in newly hatched broiler chicks

Newly hatched chicks must transition from lipid-rich yolk to carbohydrate-rich feed as their primary nutrient source, and posthatch delays in access to feed can have long-term negative consequences on growth and metabolism. In this study, impacts of delayed access to feed at hatch on expression of genes related to nutrient uptake and utilization in two metabolically important tissues, liver and muscle, were determined in broiler (meat-type) chickens. Hatched chicks were given access to feed within 3 h (fed) or delayed access to feed for 48 h (delayed fed), and liver and breast muscle were collected from males at hatch and 4 h, 1 day, 2 days, 4 days, and 8 days posthatch for analysis of gene expression. Differential expression of carbohydrate response element-binding protein and peroxisome proliferator-activated receptor-γ in muscle and liver was observed, with results indicating a transitional delay from lipid to carbohydrate metabolism when hatched chicks were not given immediate access to feed. Extended upregulation of insulin receptor mRNA was observed in both tissues in delayed fed birds, suggesting increased sensitivity to circulating levels of the hormone. Developmental delays in expression patterns of cationic amino acid transporters 1 and 2 in both tissues and large neutral amino acid transporter 1 in muscle were also apparent when immediate feed access was prevented. These data suggest that delayed transition to carbohydrate use and altered nutrient transport and utilization within liver and breast muscle are key factors negatively affecting growth and metabolism following delayed feed access in broiler chickens.

Genome-wide identification and characterization of glucose transporter (glut) genes in spotted sea bass (Lateolabrax maculatus) and their regulated hepatic expression during short-term starvation

The glucose transporters (GLUTs) are well known for their essential roles in moving the key metabolites, glucose, galactose, fructose and a number of other important substrates in and out of cells. In this study, we identified a total of 21 glut genes in spotted sea bass (Lateolabrax maculatus) through extensive data mining of existing genomic and transcriptomic databases. Glut genes of spotted sea bass were classified into three subfamilies (Class I, Class II and Class III) according to the phylogenetic analysis. Glut genes of spotted sea bass were distributed in 15 out of 24 chromosomes. Deduced gene structure analysis including the secondary structure and the three-dimensional structures, as well as the syntenic analysis further supported their annotations and orthologies. Expression profile in healthy tissues indicated that 9 of 21 glut genes were expressed in liver of spotted sea bass. During short-term starvation, the mRNA expression levels of 3 glut genes (glut2, glut5, and glut10) were significantly up-regulated in liver (P < 0.05), indicating their potential roles in sugar transport and consumption. These findings in our study will facilitate the further evolutionary characterization of glut genes in fish species and provide a theoretical basis for their functional study.

PIMT/NCOA6IP Deletion in the Mouse Heart Causes Delayed Cardiomyopathy Attributable to Perturbation in Energy Metabolism

PIMT/NCOA6IP, a transcriptional coactivator PRIP/NCOA6 binding protein, enhances nuclear receptor transcriptional activity. Germline disruption of PIMT results in early embryonic lethality due to impairment of development around blastocyst and uterine implantation stages. We now generated mice with Cre-mediated cardiac-specific deletion of PIMT (csPIMT^−/−) in adult mice. These mice manifest enlargement of heart, with nearly 100% mortality by 7.5 months of age due to dilated cardiomyopathy. Significant reductions in the expression of genes (i) pertaining to mitochondrial respiratory chain complexes I to IV; (ii) calcium cycling cardiac muscle contraction (Atp2a1, Atp2a2, Ryr2); and (iii) nuclear receptor PPAR- regulated genes involved in glucose and fatty acid energy metabolism were found in csPIMT^−/− mouse heart. Elevated levels of Nppa and Nppb mRNAs were noted in csPIMT^−/− heart indicative of myocardial damage. These hearts revealed increased reparative fibrosis associated with enhanced expression of Tgfβ2 and Ctgf. Furthermore, cardiac-specific deletion of PIMT in adult mice, using tamoxifen-inducible Cre-approach (TmcsPIMT^−/−), results in the development of cardiomyopathy. Thus, cumulative evidence suggests that PIMT functions in cardiac energy metabolism by interacting with nuclear receptor coactivators and this property could be useful in the management of heart failure.

Glucose Transporters in Diabetic Kidney Disease-Friends or Foes?

Diabetic kidney disease (DKD) is a major microvascular complication of diabetes and a common cause of end-stage renal disease worldwide. DKD manifests as an increased urinary protein excretion (albuminuria). Multiple studies have shown that insulin resistance correlates with the development of albuminuria in non-diabetic and diabetic patients. There is also accumulating evidence that glomerular epithelial cells or podocytes are insulin sensitive and that insulin signaling in podocytes is essential for maintaining normal kidney function. At the cellular level, the mechanisms leading to the development of insulin resistance include mutations in the insulin receptor gene, impairments in the phosphoinositide 3-kinase (PI3K)/AKT signaling pathway, or perturbations in the trafficking of glucose transporters (GLUTs), which mediate the uptake of glucose into cells. Podocytes express several GLUTs, including GLUT1, GLUT2, GLUT3, GLUT4, and GLUT8. Of these, the most studied ones are GLUT1 and GLUT4, both shown to be insulin responsive in podocytes. In the basal state, GLUT4 is preferentially located in perinuclear and cytosolic vesicular structures and to a lesser extent at the plasma membrane. After insulin stimulation, GLUT4 is sorted into GLUT4-containing vesicles (GCVs) that translocate to the plasma membrane. GCV trafficking consists of several steps, including approaching of the GCVs to the plasma membrane, tethering, and docking, after which the lipid bilayers of the GCVs and the plasma membrane fuse, delivering GLUT4 to the cell surface for glucose uptake into the cell. Studies have revealed novel molecular regulators of the GLUT trafficking in podocytes and unraveled unexpected roles for GLUT1 and GLUT4 in the development of DKD, summarized in this review. These findings pave the way for better understanding of the mechanistic pathways associated with the development and progression of DKD and aid in the development of new treatments for this devastating disease.