The Na+/glucose cotransporters: from genes to therapy

Genetic Variants in Carbohydrate Digestive Enzyme and Transport Genes Associated with Risk of Irritable Bowel Syndrome

Irritable Bowel Syndrome (IBS) is characterized by abdominal pain and alterations in bowel pattern, such as constipation (IBS-C), diarrhea (IBS-D), or mixed (IBS-M). Since malabsorption of ingested carbohydrates (CHO) can cause abdominal symptoms that closely mimic those of IBS, identifying genetic mutations in CHO digestive enzymes associated with IBS symptoms is critical to ascertain IBS pathophysiology. Through candidate gene association studies, we identify several common variants in TREH, SI, SLC5A1 and SLC2A5 that are associated with IBS symptoms. By investigating rare recessive Mendelian or oligogenic inheritance patterns, we identify case-exclusive rare deleterious variation in known disease genes (SI, LCT, ALDOB, and SLC5A1) as well as candidate disease genes (MGAM and SLC5A2), providing potential evidence of monogenic or oligogenic inheritance in a subset of IBS cases. Finally, our data highlight that moderate to severe IBS-associated gastrointestinal symptoms are often observed in IBS cases carrying one or more of deleterious rare variants.

Heat stress modulates the disruptive effects of Eimeria maxima infection on the ileum nutrient digestibility, molecular transporters, and tissue morphology in meat-type chickens

Eimeria (E.) maxima is one of the most pathogenic Eimeria spp persistently invading the middle jejunum and ileum, damaging the intestinal mucosa of chickens. Heat stress (HS) is a common stressor and equally contributes to inflammation and oxidative stress. We investigated the effect of E. maxima infection and HS on ileal digestibility, mRNA expression of nutrient transporters, and ileal tissue morphology in broiler chickens. There were four treatment groups: thermoneutral control (TNc), thermoneutral infected (TNi), heat stress control (HSc), and heat stress infected (HSi), 6 replicates each of 10 birds per treatment. Chickens were fed a diet containing 0.2% TiO₂. At 6-day-post infection, ileal content and tissue were collected to quantify ileal digestibility of crude protein and fat, mRNA levels of nutrient transporters and histopathology. Growth and feed intake were reduced in all treatment groups, compared with the TNc. Contrary to expectation, the combination of two major stressors (E. maxima and HS) in the TNi group exhibited almost normal digestibility while only the TNi birds expressed severe digestibility depression, compared with the TNc group. The TNi group showed the lowest mRNA expression of the transporters: SGLT1, GLUT2-5-8-10-12, FABP1-2-6, and PEPT1 compared with the other treatment groups. The expression of the absorptive enterocytes’ gene markers (ACSL5, IAP, and SGLT1) supported by the ileal tissue morphology indicated that the TNi group had the highest enterocytic destruction. The expression of oxidative genes (iNOS and CYBB) dramatically increased only in the TNi group compared with the other treatment groups. Our results showed that exposing broiler chickens to HS can mitigate the disruptive effect of E. maxima on the ileal digestibility and absorption by limiting the parasite-induced tissue injury and suppressing the enterocytic inducible oxidative damage.

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.

Congenital Glucose-Galactose Malabsorption: A Case With a Novel SLC5A1 Mutation in a Saudi Infant

While only a few hundred cases have been reported in pediatrics, congenital glucose-galactose malabsorption (GGM) is an extremely rare autosomal-recessive metabolic disorder that is characterized by intractable diarrhea and severe dehydration, which can be life-threatening if not treated appropriately. Due to the rarity of the disease, it is challenging to consider GGM as an initial diagnosis for most clinicians. We report the clinical and diagnostic course of a seven-month-old Saudi infant who presented with severe recurrent episodes of watery diarrhea and failure to thrive in early infancy despite standard treatment. Molecular testing identified that our patient had a compound heterozygous variant in SLC5A1. Fructose-based formulae have been proven to be effective in treating GGM. This case highlights the importance of early diagnosis and timely management to prevent serious complications of undiagnosed GGM.

A Role for SGLT-2 Inhibitors in Treating Non-diabetic Chronic Kidney Disease

In recent years, inhibitors of the sodium-glucose co-transporter 2 (SGLT2 inhibitors) have been shown to have significant protective effects on the kidney and the cardiovascular system in patients with diabetes. This effect is also manifested in chronic kidney disease (CKD) patients and is minimally due to improved glycaemic control. Starting from these positive findings, SGLT2 inhibitors have also been tested in patients with non-diabetic CKD or heart failure with reduced ejection fraction. Recently, the DAPA-CKD trial showed a significantly lower risk of CKD progression or death from renal or cardiovascular causes in a mixed population of patients with diabetic and non-diabetic CKD receiving dapagliflozin in comparison with placebo. In patients with heart failure and reduced ejection fraction, two trials (EMPEROR-Reduced and DAPA-HF) also found a significantly lower risk of reaching the secondary renal endpoint in those treated with an SGLT2 inhibitor in comparison with placebo. This also applied to patients with CKD. Apart from their direct mechanism of action, SGLT2 inhibitors have additional effects that could be of particular interest for patients with non-diabetic CKD. Among these, SGLT2 inhibitors reduce blood pressure and serum acid uric levels and can increase hemoglobin levels. Some safety issues should be further explored in the CKD population. SGLT2 inhibitors can minimally increase potassium levels, but this has not been shown by the CREDENCE trial. They also increase magnesium and phosphate reabsorption. These effects could become more significant in patients with advanced CKD and will need monitoring when these agents are used more extensively in the CKD population. Conversely, they do not seem to increase the risk of acute kidney injury.

The Sniffing Kidney: Roles for Renal Olfactory Receptors in Health and Disease

Olfactory receptors (ORs) represent the largest gene family in the human genome. Despite their name, functions exist for these receptors outside of the nose. Among the tissues known to take advantage of OR signaling is the kidney. From mouse to man, the list of renal ORs continues to expand, and they have now been linked to a variety of processes involved in the maintenance of renal homeostasis, including the modulation of blood pressure, response to acidemia, and the development of diabetes. In this review, we highlight the recent progress made on the growing appreciation for renal ORs in physiology and pathophysiology.

Sweet Taste Is Complex: Signaling Cascades and Circuits Involved in Sweet Sensation

Sweetness is the preferred taste of humans and many animals, likely because sugars are a primary source of energy. In many mammals, sweet compounds are sensed in the tongue by the gustatory organ, the taste buds. Here, a group of taste bud cells expresses a canonical sweet taste receptor, whose activation induces Ca²⁺ rise, cell depolarization and ATP release to communicate with afferent gustatory nerves. The discovery of the sweet taste receptor, 20 years ago, was a milestone in the understanding of sweet signal transduction and is described here from a historical perspective. Our review briefly summarizes the major findings of the canonical sweet taste pathway, and then focuses on molecular details, about the related downstream signaling, that are still elusive or have been neglected. In this context, we discuss evidence supporting the existence of an alternative pathway, independent of the sweet taste receptor, to sense sugars and its proposed role in glucose homeostasis. Further, given that sweet taste receptor expression has been reported in many other organs, the physiological role of these extraoral receptors is addressed. Finally, and along these lines, we expand on the multiple direct and indirect effects of sugars on the brain. In summary, the review tries to stimulate a comprehensive understanding of how sweet compounds signal to the brain upon taste bud cells activation, and how this gustatory process is integrated with gastro-intestinal sugar sensing to create a hedonic and metabolic representation of sugars, which finally drives our behavior. Understanding of this is indeed a crucial step in developing new strategies to prevent obesity and associated diseases.

Role of Modulation of Hippocampal Glucose Following Pilocarpine-Induced Status Epilepticus

Status epilepticus (SE) is defined as continuous and self-sustaining seizures, which trigger hippocampal neurodegeneration, mitochondrial dysfunction, oxidative stress, and energy failure. During SE, the neurons become overexcited, increasing energy consumption. Glucose uptake is increased via the sodium glucose cotransporter 1 (SGLT1) in the hippocampus under epileptic conditions. In addition, modulation of glucose can prevent neuronal damage caused by SE. Here, we evaluated the effect of increased glucose availability in behavior of limbic seizures, memory dysfunction, neurodegeneration process, neuronal activity, and SGLT1 expression. Vehicle (VEH, saline 0.9%, 1 μL) or glucose (GLU; 1, 2 or 3 mM, 1 μL) were administered into hippocampus of male Wistar rats (Rattus norvegicus) before or after pilocarpine to induce SE. Behavioral analysis of seizures was performed for 90 min during SE. The memory and learning processes were analyzed by the inhibitory avoidance test. After 24 h of SE, neurodegeneration process, neuronal activity, and SGLT1 expression were evaluated in hippocampal and extrahippocampal regions. Modulation of hippocampal glucose did not protect memory dysfunction followed by SE. Our results showed that the administration of glucose after pilocarpine reduced the severity of seizures, as well as the number of limbic seizures. Similarly, glucose after SE reduced cell death and neuronal activity in hippocampus, subiculum, thalamus, amygdala, and cortical areas. Finally, glucose infusion elevated the SGLT1 expression in hippocampus. Taken together our data suggest that possibly the administration of intrahippocampal glucose protects brain in the earlier stage of epileptogenic processes via an important support of SGLT1.

Expression of glucose transporters in human neurodegenerative diseases

The central nervous system (CNS) plays an important role in the human body. It is involved in the receive, store and participation in information retrieval. It can use several substrates as a source of energy, however, the main source of energy is glucose. Cells of the central nervous system need a continuous supply of energy, therefore, transport of glucose into these cells is very important. There are three distinct families of glucose transporters: sodium-independent glucose transporters (GLUTs), sodium-dependent glucose cotransporters (SGLTs), and uniporter, SWEET protein. In the human brain only GLUTs and SGLTs were detected. In neurodegenerative diseases was observed hypometabolism of glucose due to decreased expression of glucose transporters, in particular GLUT1 and GLUT3. On the other hand, animal studies revealed, that increased levels of these glucose transporters, due to for example by the increased copy number of SLC2A genes, may have a beneficial effect and may be a targeted therapy in the treatment of patients with AD, HD and PD.

Pathophysiology of SARS-CoV-2 in Lung of Diabetic Patients

Novel coronavirus disease (COVID-19) is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Its impact on patients with comorbidities is clearly related to fatality cases, and diabetes has been linked to one of the most important causes of severity and mortality in SARS-CoV-2 infected patients. Substantial research progress has been made on COVID-19 therapeutics; however, effective treatments remain unsatisfactory. This unmet clinical need is robustly associated with the complexity of pathophysiological mechanisms described for COVID-19. Several key lung pathophysiological mechanisms promoted by SARS-CoV-2 have driven the response in normoglycemic and hyperglycemic subjects. There is sufficient evidence that glucose metabolism pathways in the lung are closely tied to bacterial proliferation, inflammation, oxidative stress, and pro-thrombotic responses, which lead to severe clinical outcomes. It is also likely that SARS-CoV-2 proliferation is affected by glucose metabolism of type I and type II cells. This review summarizes the current understanding of pathophysiology of SARS-CoV-2 in the lung of diabetic patients and highlights the changes in clinical outcomes of COVID-19 in normoglycemic and hyperglycemic conditions.

Literature review on congenital glucose-galactose malabsorption from 2001 to 2019

AIM

Congenital glucose-galactose malabsorption (CGGM) is a rare disease characterised by severe diarrhoea, dehydration and weight loss. To better understand CGGM, we investigated all the case reports and series of CGGM from 2001 to 2019.

METHODS

A review of reports of CGGM published from 2001 to 2019 was undertaken, using PubMed, Ovid Medline, Springer, Wanfang Database, CBMD database and CKNI database. The clinical features, diagnosis, treatment and prognosis of CGGM in these reports were obtained and analysed.

RESULTS

We reviewed 107 cases for this study. Out of 55 cases from Saudi Arabia and Turkey, 43 cases (78.2%) were from consanguineous marriage. Forty-nine cases (73.1%) were infants. Dehydration, diarrhoea and weight loss occurred in almost all cases. Half of the cases presented hypernatremia and abdominal distension. Vomiting, polyuria/haematuria and fever were reported in 11, 7 and 3 cases, respectively. Twenty cases (18.7%) showed hypercalcaemia or nephrolithiasis. Stool pH was tested in 43 cases (40.2%). Fifty-five cases (51.4%) were diagnosed for more than 1 month after the onset of symptoms. Two cases (1.9%) died, one needed amputation, and the other 104 cases (97.2%) recovered with fructose formula. Seventy-three cases (68.2%) underwent gene testing, 30 SLC5A1 gene mutations were detected, with 23 cases homozygous, and seven heterozygous mutation.

CONCLUSION

The clinical characteristics of CGGM are nonspecific, and the diagnosis method is not conventionally applied. Fasting and gene testing are the two most important diagnostic methods. The best treatment of CGGM is supplementation with fructose-based formula.

Modulation of Glucose Availability and Effects of Hypo- and Hyperglycemia on Status Epilepticus: What We Do Not Know Yet?

Status epilepticus (SE) can lead to serious neuronal damage and act as an initial trigger for epileptogenic processes that may lead to temporal lobe epilepsy (TLE). Besides promoting neurodegeneration, neuroinflammation, and abnormal neurogenesis, SE can generate an extensive hypometabolism in several brain areas and, consequently, reduce intracellular energy supply, such as adenosine triphosphate (ATP) molecules. Although some antiepileptic drugs show efficiency to terminate or reduce epileptic seizures, approximately 30% of TLE patients are refractory to regular antiepileptic drugs (AEDs). Modulation of glucose availability may provide a novel and robust alternative for treating seizures and neuronal damage that occurs during epileptogenesis; however, more detailed information remains unknown, especially under hypo- and hyperglycemic conditions. Here, we review several pathways of glucose metabolism activated during and after SE, as well as the effects of hypo- and hyperglycemia in the generation of self-sustained limbic seizures. Furthermore, this study suggests the control of glucose availability as a potential therapeutic tool for SE.

Role of Glucose Transporters in Drug Membrane Transport

BACKGROUND

Glucose is the main energy component of cellular activities. However, as a polar molecule, glucose cannot freely pass through the phospholipid bilayer structure of the cell membrane. Thus, glucose must rely on specific transporters in the membrane. Drugs with a similar chemical structure to glucose may also be transported through this pathway.

METHODS

This review describes the structure, distribution, action mechanism and influencing factors of glucose transporters and introduces the natural drugs mediated by these transporters and drug design strategies on the basis of this pathway.

RESULTS

The glucose transporters involved in glucose transport are of two major types, namely, Na+-dependent and Na+-independent transporters. Glucose transporters can help some glycoside drugs cross the biological membrane. The transmembrane potential is influenced by the chemical structure of drugs. Glucose can be used to modify drugs and improve their ability to cross biological barriers.

CONCLUSION

The membrane transport mechanism of some glycoside drugs may be related to glucose transporters. Glucose modification may improve the oral bioavailability of drugs or achieve targeted drug delivery.

Functional Expression of Sodium-Dependent Glucose Transporter in Amelogenesis

Glucose is an essential source of energy for mammalian cells and is transported into the cells by glucose transporters. There are 2 types of glucose transporters: one is a passive glucose transporter, GLUT (SLC2A), and the other is a sodium-dependent active glucose transporter, SGLT (SLC5A). We previously reported that the expression of GLUTs during tooth development is precisely and spatiotemporally controlled and that the glucose uptake mediated by GLUT1 plays a crucial role in early tooth morphogenesis and tooth size determination. This study aimed to clarify the localization and roles of SGLT1 and SGLT2 in murine ameloblast differentiation by using immunohistochemistry, immunoelectron microscopy, an in vitro tooth organ culture experiment, and in vivo administration of an inhibitor of SGLT1/2, phloridzin. SGLT1, which has high affinity with glucose, was immunolocalized in the early secretory ameloblasts and the ruffle-ended ameloblasts in the maturation stage. However, SGLT2, which has high glucose transport capacity, was observed in the stratum intermedium, papillary layer, and ameloblasts at the maturation stage and colocalized with Na⁺-K⁺-ATPase. The inhibition of SGLT1/2 by phloridzin in the tooth germs induced the disturbance of ameloblast differentiation and enamel matrix formation both in vitro (organ culture) and in vivo (mouse model). The expression of SGLT1 and SGLT2 was significantly upregulated in hypoxic conditions in the ameloblast-lineage cells. These findings suggest that the active glucose uptake mediated by SGLT1 and SGLT2 is strictly regulated and dependent on the intra- and extracellular microenvironments during tooth morphogenesis and that the appropriate passive and active glucose transport is an essential event in amelogenesis.

Jejunal Insulin Signalling Is Increased in Morbidly Obese Subjects with High Insulin Resistance and Is Regulated by Insulin and Leptin

Little is known about the jejunal insulin signalling pathways in insulin resistance/diabetes states and their possible regulation by insulin/leptin. We study in jejunum the relation between insulin signalling and insulin resistance in morbidly obese subjects with low (MO-low-IR) or with high insulin resistance (MO-high-IR), and with type 2 diabetes treated with metformin (MO-metf-T2DM)), and the effect of insulin/leptin on intestinal epithelial cells (IEC). Insulin receptor substrate-1 (IRS1) and the catalytic p110β subunit (p110β) of phosphatidylinositol 3-kinase (PI3K) were higher in MO-high-IR than in MO-low-IR. The regulatory p85α subunit of PI3K (p85α)/p110β ratio was lower in MO-high-IR and MO-metf-T2DM than in MO-low-IR. Akt-phosphorylation in Ser473 was reduced in MO-high-IR compared with MO-low-IR. IRS1 and p110-β were associated with insulin and leptin levels. The improvement of body mass index (BMI) and HOMA-IR (homeostasis model assessment of insulin resistance index) after bariatric surgery was associated with a higher IRS1 and a lower p85α/p110β ratio. IEC (intestinal epithelial cells) incubation with a high glucose + insulin dose produced an increase of p85α and p110β. High dose of leptin produced an increase of IRS1, p85α and p110β. In conclusion, despite the existence of insulin resistance, the jejunal expression of genes involved in insulin signalling was increased in MO-high-IR. Their expressions were regulated mainly by leptin. IRS1 and p85α/p110β ratio was associated with the evolution of insulin resistance after bariatric surgery.

Mechanisms of Protective Effects of SGLT2 Inhibitors in Cardiovascular Disease and Renal Dysfunction

Type 2 diabetes mellitus is one of the most common forms of the disease worldwide. Hyperglycemia and insulin resistance play key roles in type 2 diabetes mellitus. Renal glucose reabsorption is an essential feature in glycaemic control. Kidneys filter 160 g of glucose daily in healthy subjects under euglycaemic conditions. The expanding epidemic of diabetes leads to a prevalence of diabetes-related cardiovascular disorders, in particular, heart failure and renal dysfunction. Cellular glucose uptake is a fundamental process for homeostasis, growth, and metabolism. In humans, three families of glucose transporters have been identified, including the glucose facilitators GLUTs, the sodium-glucose cotransporter SGLTs, and the recently identified SWEETs. Structures of the major isoforms of all three families were studied. Sodium-glucose cotransporter (SGLT2) provides most of the capacity for renal glucose reabsorption in the early proximal tubule. A number of cardiovascular outcome trials in patients with type 2 diabetes have been studied with SGLT2 inhibitors reducing cardiovascular morbidity and mortality. The current review article summarises these aspects and discusses possible mechanisms with SGLT2 inhibitors in protecting heart failure and renal dysfunction in diabetic patients. Through glucosuria, SGLT2 inhibitors reduce body weight and body fat, and shift substrate utilisation from carbohydrates to lipids and, possibly, ketone bodies. These pleiotropic effects of SGLT2 inhibitors are likely to have contributed to the results of the EMPA-REG OUTCOME trial in which the SGLT2 inhibitor, empagliflozin, slowed down the progression of chronic kidney disease and reduced major adverse cardiovascular events in high-risk individuals with type 2 diabetes. This review discusses the role of SGLT2 in the physiology and pathophysiology of renal glucose reabsorption and outlines the unexpected logic of inhibiting SGLT2 in the diabetic kidney.

Nutrition management of congenital glucose-galactose malabsorption: Case report of a Chinese infant

RATIONALE

Congenital glucose-galactose malabsorption (CGGM) is a rare, autosomal recessive, hereditary disease that usuallypresents in newborns. CGGM manifests as severe diarrhea, hyperosmolar dehydration, and malnutrition. It does not respond to routine treatment and often is life-threatening.

PATIENT CONCERNS

We described a Chinese infant girl with refractory diarrhea, who suffered from severe dehydration and malnutrition even if with fluid replacement therapy and fed with several special formulas.

DIAGNOSES

The genetic analysis identified CGGM with SLC5A1 mutations. c.1436G > C (p.R479T) was a novel mutation.

INTERVENTIONS

The patient was managed by free-glucose and galactose formula, and then special low-carbohydrate dietary therapy.

OUTCOMES

The patient improved immediately after starting a free-glucose and galactose formula, and kept healthy with special low-carbohydrate diet. She had been followed up with nutritional management for 20 months.

LESSONS

This report highlights the importance of differential diagnosis of congenital diarrhea and enteropathies. For CGGM, free-glucose and galactose milk powder was the most effective treatment. Low-carbohydrate diet gradually introduced was still a great challenge that requires continuing guidance from child nutritionists and dietitians. Long-term nutrition management was extremely important to ensure the normal growth and development of children.

A Fluorescent Glucose Transport Assay for Screening SGLT2 Inhibitors in Endogenous SGLT2-Expressing HK-2 Cells

The sodium-dependent glucose transporters 2 (SGLT2) plays important role in renal reabsorption of urinal glucose back to plasma for maintaining glucose homeostasis. The approval of SGLT2 inhibitors for treatment of type 2 diabetes highlights the SGLT2 as a feasible and promising drug target in recent years. Current methods for screening SGLT2 inhibitors are complex, expensive and labor intensive. Particularly, these methods cannot directly measure nonradioactive glucose uptake in endogenous SGLT2-expressing kidney cells. In present work, human kidney cells, HK-2, was incubated with a fluorescent D-glucose derivant 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino]-2-deoxy-D-glucose (2-NBDG) and the fluorescent intensity of 2-NBDG was employed to measure the amount of glucose uptake into the cells. By optimizing the passages of HK-2 cells, 2-NBDG concentration and incubation time, and by measuring glucose uptake treated by Dapagliflozin, a clinical drug of SGLT2 inhibitors, we successfully developed a new assay for measuring glucose uptake through SGLT2. The nonradioactive microplate and microscope-based high-throughput screening assay for measuring glucose can be a new method for screening of SGLT2 inhibitors and implied for other cell assays for glucose measurement extensively.

[Potential of the Cerebral Sodium-Glucose Transporter as a Novel Therapeutic Target in Cerebral Ischemia]

　Cerebral ischemic stress often induces a hyperglycemic condition. This postischemic hyperglycemia exacerbates the development of cerebral ischemic neuronal damage, although the mechanism of this exacerbation remains to be clarified. We previously discovered that the cerebral sodium-glucose transporter (SGLT) was closely involved in the development of cerebral ischemic neuronal damage. SGLT is a member of the glucose transporter family and moves glucose together with sodium ions. SGLT-1, -3, -4, and -6 are distributed in the brain. We conducted further experiments to elucidate the detailed mechanism of the exacerbation of cerebral ischemia by cerebral SGLT. The results clarified: 1) the relationship between cerebral SGLT and postischemic hyperglycemia; 2) the involvement of cerebral SGLT-1 (a cerebral SGLT isoform) in cerebral ischemic neuronal damage; and 3) the effects of sodium influx through cerebral SGLT on the development of cerebral ischemic neuronal damage. This paper presents our data on the involvement of cerebral SGLT in the exacerbation of cerebral ischemic neuronal damage.

Dual Regulation of Gluconeogenesis by Insulin and Glucose in the Proximal Tubules of the Kidney

Growing attention has been focused on the roles of the proximal tubules (PTs) of the kidney in glucose metabolism, including the mechanism of regulation of gluconeogenesis. In this study, we found that PT-specific insulin receptor substrate 1/2 double-knockout mice, established by using the newly generated sodium-glucose cotransporter 2 (SGLT2)-Cre transgenic mice, exhibited impaired insulin signaling and upregulated gluconeogenic gene expression and renal gluconeogenesis, resulting in systemic insulin resistance. In contrast, in streptozotocin-treated mice, although insulin action was impaired in the PTs, the gluconeogenic gene expression was unexpectedly downregulated in the renal cortex, which was restored by administration of an SGLT1/2 inhibitor. In the HK-2 cells, the gluconeogenic gene expression was suppressed by insulin, accompanied by phosphorylation and inactivation of forkhead box transcription factor 1 (FoxO1). In contrast, glucose deacetylated peroxisome proliferator-activated receptor γ coactivator 1-α (PGC1α), a coactivator of FoxO1, via sirtuin 1, suppressing the gluconeogenic gene expression, which was reversed by inhibition of glucose reabsorption. These data suggest that both insulin signaling and glucose reabsorption suppress the gluconeogenic gene expression by inactivation of FoxO1 and PGC1α, respectively, providing insight into novel mechanisms underlying the regulation of gluconeogenesis in the PTs.

Distribution of glucose transporters in renal diseases

Kidneys play an important role in glucose homeostasis. Renal gluconeogenesis prevents hypoglycemia by releasing glucose into the blood stream. Glucose homeostasis is also due, in part, to reabsorption and excretion of hexose in the kidney.Lipid bilayer of plasma membrane is impermeable for glucose, which is hydrophilic and soluble in water. Therefore, transport of glucose across the plasma membrane depends on carrier proteins expressed in the plasma membrane. In humans, there are three families of glucose transporters: GLUT proteins, sodium-dependent glucose transporters (SGLTs) and SWEET. In kidney, only GLUTs and SGLTs protein are expressed. Mutations within genes that code these proteins lead to different renal disorders and diseases. However, diseases, not only renal, such as diabetes, may damage expression and function of renal glucose transporters.

Congenital Glucose-Galactose Malabsorption: A Case Report

Congenital glucose-galactose malabsorption (CGGM) is a rare cause of intractable infantile diarrhea, with only a few hundred cases recognized worldwide. This life-threatening disorder must be considered in the differential diagnosis of an infant who presents with diarrhea and dehydration that fails to respond to standard therapy. The clinical and diagnostic course of an infant with recurrent episodes of watery diarrhea and hypernatremic dehydration found to be homozygous for a rare variant in the SLC5A1 gene, c.187C>T (p.R63X) is described.

Saving the sweetness: renal glucose handling in health and disease

Glucose homeostasis is highly controlled, and the function of the kidney plays an integral role in this process. The exquisite control of blood glucose relies, in part, on renal glucose filtration, renal glucose reabsorption, and renal gluconeogenesis. Particularly critical to maintaining glucose homeostasis is the renal reabsorption of glucose; with ~162 g of glucose filtered by the kidney per day, it is imperative that the kidney have the ability to efficiently reabsorb nearly 100% of this glucose back in the bloodstream. In this review, we focus on this central process, highlighting the renal transporters and regulators involved in both the physiology and pathophysiology of glucose reabsorption.

Inhibition of sodium glucose cotransporters following status epilepticus induced by intrahippocampal pilocarpine affects neurodegeneration process in hippocampus

Temporal lobe epilepsy (TLE) is characterized by spontaneous recurrent seizures, starting from secondary functional disorders due to several insults, including self-sustaining continuous seizures identified as status epilepticus (SE). Although hypoglycemia has been associated with SE, the effect of inhibition of the Na(+)/glucose cotransporters (SGLTs) on hippocampus during SE is still unknown. Here we evaluated the functional role of SGLT in the pattern of limbic seizures and neurodegeneration process after pilocarpine (PILO)-induced SE. Vehicle (VEH, 1μL) or phlorizin, a specific SGLT inhibitor (PZN, 1μL, 50μg/μL), was administered in the hippocampus of rats 30min before PILO (VEH+PILO or PZN+PILO, respectively). The limbic seizures were classified using the Racine's scale, and the amount of wet dog shakes (WDS) was quantified before and during SE. Neurodegeneration process was evaluated by Fluoro-Jade C (FJ-C), and FJ-C-positive neurons (FJ-C+) were counted 24h and 15days after SE. The PZN-treated rats showed higher (p<0.05) number of WDS when compared with VEH+PILO. There was no difference in seizure severity between PZN+PILO and VEH+PILO groups. However, the pattern of limbic seizures significantly changed in PZN+PILO. Indeed, the class 5 seizures repeated themselves more times (p<0.05) than the other classes in the PZN group at 50min after SE induction. The PZN+PILO animals had a higher (p<0.05) number of FJ-C+ cells in the dentate gyrus (DG), hilus, and CA3 and CA1 of hippocampus, when compared with VEH+PILO. The PZN+PILO animals had a decreased number (p<0.05) of FJ-C+ cells in CA1 compared with VEH+PILO 15days after SE induction. Taken together, our data suggest that SGLT inhibition with PZN increased the severity of limbic seizures during SE and increased neurodegeneration in hippocampus 24h after SE, suggesting that SGLT1 and SGLT2 could participate in the modulation of earlier stages of epileptogenic processes.

Pharmacogenomics in diabetes mellitus: insights into drug action and drug discovery

Genomic studies have greatly advanced our understanding of the multifactorial aetiology of type 2 diabetes mellitus (T2DM) as well as the multiple subtypes of monogenic diabetes mellitus. In this Review, we discuss the existing pharmacogenetic evidence in both monogenic diabetes mellitus and T2DM. We highlight mechanistic insights from the study of adverse effects and the efficacy of antidiabetic drugs. The identification of extreme sulfonylurea sensitivity in patients with diabetes mellitus owing to heterozygous mutations in HNF1A represents a clear example of how pharmacogenetics can direct patient care. However, pharmacogenomic studies of response to antidiabetic drugs in T2DM has yet to be translated into clinical practice, although some moderate genetic effects have now been described that merit follow-up in trials in which patients are selected according to genotype. We also discuss how future pharmacogenomic findings could provide insights into treatment response in diabetes mellitus that, in addition to other areas of human genetics, facilitates drug discovery and drug development for T2DM.

Sperm glucose transport and metabolism in diabetic individuals

Individuals with diabetes mellitus (DM) present marked reduction in sperm quality and higher DNA damage in spermatozoa, evidencing that this metabolic disorder impairs male fertility. These effects are related to defective testicular metabolic pathways and signaling, resulting in altered sperm metabolism. Spermatozoa metabolize several substrates to ensure energy supplies and any alteration in this feature compromise sperm quality. For ATP production, spermatozoa require substrate availability and the involvement of specific hexose membrane carriers. DM is known to modulate the spermatozoa substrate consumption and/or production due to altered glycolytic behavior. In fact, glucose uptake and metabolism is highly deregulated in diabetic individuals. Herein, we present an overview of the implications of DM in sperm glucose uptake and metabolism. The understanding of these processes is essential to identify key mechanisms associated with DM-related male (in)fertility. Moreover, it may contribute to the development of therapeutics to counteract the dysfunction induced by DM in sperm metabolism.

Sodium-glucose transporter 2 inhibitors in type 2 diabetes mellitus: navigating between Scylla and Charybdis

Despite the extensive pharmacopeia for type 2 diabetes mellitus treatment, long-term glycemic control is far from being optimal, and morbimortality has increased. This demonstrates the importance of developing drugs with new mechanisms of actions, such as sodium-glucose transporter (SGLT) inhibitors (Charybdis). Since the beginning of 2000, numerous SGLT2-inhibitors have been developed and have started to be tested (Scylla) by the pharmaceutical companies that are engaged in this race. Although reductions in hemoglobin A1c have been shown in clinical trials, several issues related to the use of SGLT2 inhibitors deserve further investigation, rendering some aspects of their true safety still uncertain.

Genome-wide association study of meat quality traits in a White Duroc×Erhualian F2 intercross and Chinese Sutai pigs

Thousands of QTLs for meat quality traits have been identified by linkage mapping studies, but most of them lack precise position or replication between populations, which hinder their application in pig breeding programs. To localize QTLs for meat quality traits to precise genomic regions, we performed a genome-wide association (GWA) study using the Illumina PorcineSNP60K Beadchip in two swine populations: 434 Sutai pigs and 933 F2 pigs from a White Duroc×Erhualian intercross. Meat quality traits, including pH, color, drip loss, moisture content, protein content and intramuscular fat content (IMF), marbling and firmness scores in the M. longissimus (LM) and M. semimembranosus (SM) muscles, were recorded on the two populations. In total, 127 chromosome-wide significant SNPs for these traits were identified. Among them, 11 SNPs reached genome-wise significance level, including 1 on SSC3 for pH, 1 on SSC3 and 3 on SSC15 for drip loss, 3 (unmapped) for color a*, and 2 for IMF each on SSC9 and SSCX. Except for 11 unmapped SNPs, 116 significant SNPs fell into 28 genomic regions of approximately 10 Mb or less. Most of these regions corresponded to previously reported QTL regions and spanned smaller intervals than before. The loci on SSC3 and SSC7 appeared to have pleiotropic effects on several related traits. Besides them, a few QTL signals were replicated between the two populations. Further, we identified thirteen new candidate genes for IMF, marbling and firmness, on the basis of their positions, functional annotations and reported expression patterns. The findings will contribute to further identification of the causal mutation underlying these QTLs and future marker-assisted selection in pigs.

EGFR-mediated stimulation of sodium/glucose cotransport promotes survival of irradiated human A549 lung adenocarcinoma cells

BACKGROUND AND PURPOSE

Solid tumor cells may adapt to an ischemic microenvironment by upregulation of sodium/glucose cotransport (SGLT) in the plasma membrane which supplies the tumor cell with glucose even at very low extracellular glucose concentration. Since SGLT activity has been shown to depend on the epithelial growth factor receptor (EGFR) and EGFR reportedly is activated by ionizing radiation, we tested for irradiation-induced SGLT activity.

MATERIALS AND METHODS

A549 lung adenocarcinoma and FaDu head and neck squamous cancer cells were irradiated with 0 and 4 Gy X-ray and electrogenic SGLT transport activity was recorded by patch clamp current clamp in the presence and absence of extracellular glucose (5mM), the SGLT inhibitor phlorizin (500 μM), and the inhibitor of the EGFR tyrosine kinase activity erlotinib (1 μM). In addition, the effect of phlorizin and erlotinib on glucose uptake and clonogenic survival was tested in irradiated and control cells by tracer flux and colony formation assays, respectively.

RESULTS

Irradiated A549 cells exhibited a significantly lower membrane potential 3h after irradiation than the control cells. Phlorizin, erlotinib or removal of extracellular glucose, hyperpolarized the irradiated A549 cells to a significantly higher extent than the control cells. Similarly, but less pronounced, glucose removal hyperpolarized irradiated FaDu cells. In addition, irradiated A549 cells exhibited a highly increased (3)H-glucose uptake which was sensitive to phlorizin. Finally, phlorizin radiosensitized the A549 and FaDu cells as evident from the colony formation assays.

CONCLUSIONS

Taken together, these data suggest an irradiation-stimulated and EGFR-mediated increase in SGLT-generated glucose uptake which is required for the survival of the genotoxically stressed tumor cells.

Red carpeting the newer antidiabetics

The rapidly increasing prevalence of diabetes on a global scale beseeches an urgent need for newer and better treatment options. Our better understanding of the pathophysiology of diabetes has enabled a continual churn out of newer antidiabetic agents with varying modes of action. Sodium-Glucose Transport Proteins-2 inhibitors, dipeptidyl peptidase IV inhibitors, glucagon-like peptide analogues, glucokinase activators, dual peroxisome proliferator-activated receptor agonists, monoclonal antibodies, and dopamine-2 receptor agonists either as monotherapy or combination therapy with the existing oral hypoglycemic agents compound our fight against diabetes. A review of the newer drugs targeting various aspects in the management of diabetes is presented.

Clinical potential of sodium-glucose cotransporter 2 inhibitors in the management of type 2 diabetes

BACKGROUND

The kidney plays an important role in glucose metabolism, and has been considered a target for therapeutic intervention. The sodium-glucose cotransporter type 2 (SGLT2) mediates most of the glucose reabsorption from the proximal renal tubule. Inhibition of SGLT2 leads to glucosuria and provides a unique mechanism to lower elevated blood glucose levels in diabetes. The purpose of this review is to explore the physiology of SGLT2 and discuss several SGLT2 inhibitors which have clinical data in patients with type 2 diabetes.

METHODS

We performed a PubMed search using the terms "SGLT2" and "SGLT2 inhibitor" through April 10, 2012. Published articles, press releases, and abstracts presented at national and international meetings were considered.

RESULTS

SGLT2 inhibitors correct a novel pathophysiological defect, have an insulin-independent action, are efficacious with glycosylated hemoglobin reduction ranging from 0.5% to 1.5%, promote weight loss, have a low incidence of hypoglycemia, complement the action of other antidiabetic agents, and can be used at any stage of diabetes. They are generally well tolerated. However, due to side effects, such as repeated urinary tract and genital infections, increased hematocrit, and decreased blood pressure, appropriate patient selection for drug initiation and close monitoring after initiation will be important. Results of ongoing clinical studies of the effect of SGLT2 inhibitors on diabetic complications and cardiovascular safety are crucial to determine the risk-benefit ratio. A recent decision by the Committee for Medicinal Products for Human Use of the European Medicines Agency has recommended approval of dapagliflozin for the treatment of type 2 diabetes as an adjunct to diet and exercise, in combination with other glucose-lowering medicinal products, including insulin, and as a monotherapy for metformin-intolerant patients. Clinical research also remains to be carried out on the long-term effects of glucosuria and other potential effects of SGLT2 inhibitors, especially in view of the observed increase in the incidence of bladder and breast cancer. SGLT2 inhibitors represent a promising approach for the treatment of diabetes, and could potentially be an addition to existing therapies.

Teaching the renal tubular reabsorption of glucose using two classic papers by Shannon et al

Most of the transport along the nephron uses membrane proteins and exhibits the three characteristics of mediated transport: saturation, specificity, and competition. Glucose reabsorption in the nephron is an excellent example of the consequences of saturation. Two classic papers by James A. Shannon and colleagues clearly show the ability of the kidney in transporting glucose and its saturation process, providing students with examples of the handling of glucose by the kidney. In addition, these articles demonstrate how stable and reproducible is the transport maximum of glucose in the proximal tubule under different experimental conditions. One key figure from each classic paper can be used to give students insight into how glucose transport becomes saturated, resulting in the excretion of glucose in urine, and will also give students a clear example of how careful experimentation and a clear interest in renal physiology led Shannon and colleagues to advance the field.