Differences in regulatory properties between human and rat glucokinase regulatory protein

Effects of coding variants in the glucokinase regulatory protein gene on hepatic glucose and triglyceride metabolism suggest a gene regulatory function of glucokinase

BACKGROUND

Regulation of glucose metabolism after a meal is the major task of hepatic glucokinase (GCK). Inhibition and nuclear retention of glucokinase during fasting is achieved by glucokinase regulatory protein (GKRP). Compounds disrupting the GCK-GKRP interaction alter glucose but not triglyceride levels, whilst GKRP coding alleles lower glucose but elevate triglycerides. The aim of this study was to identify yet unknown functions of GKRP by examining human variants both rare (p.Q234P, p.H438Y) and common (p.P446L).

METHODS

Fluorescently labelled human GKRP variant and GCK proteins were expressed in hepatoma cells or primary mouse hepatocytes to investigate the subcellular localization of both proteins, cellular glucose uptake, and triglyceride levels. Mutational effects on GKRP protein structure were analyzed with PyMOL. Nuclear-to-cytoplasmic distribution of the GCK-GKRP complex was modeled in MATLAB.

RESULTS

Nuclear localization of the GKRP variants was decreased compared to wild-type. Only H438Y-GKRP still evoked WT-like GCK nuclear accumulation. Nuclear localization of Q234P-GKRP was most impaired and depended on the presence of GCK, which, supported by structural analyses, could stabilize its conformation. Nonetheless, inhibition of glucose uptake was least impaired with Q234P-GKRP. Triglyceride contents related to the glucose uptake of hepatoma cells were disproportionately high for cells expressing wild-type or H438Y-GKRP, the two variants that induced higher nuclear sequestration of GCK.

CONCLUSIONS

Our results, supported by a modeling approach, suggest that GKRP-mediated nuclear localization of GCK has a function in liver metabolism beyond GCK inhibition and sequestration. This needs further elucidation given that GKRP disruptors have been proposed for antihyperglycemic therapy.

The GCKR-P446L gene variant predisposes to raised blood cholesterol and lower blood glucose in the P446L mouse-a model for GCKR rs1260326

OBJECTIVES

The Glucokinase Regulatory Protein GKRP, encoded by GCKR, enables acute regulation of liver glucokinase to support metabolic demand. The common human GCKR rs1260326:Pro446 > Leu variant within a large linkage disequilibrium region associates with pleiotropic traits including lower Type 2 diabetes risk and raised blood triglycerides and cholesterol. Whether the GCKR-P446 > L substitution is causal to the raised lipids is unknown. We determined whether mouse GKRP phenocopies the human GKRP:P446 > L substitution and studied a GKRP:P446L knockin mouse to identify physiological consequences to P446 > L.

METHODS

GKRP-deficient hepatocytes were transfected with adenoviral vectors for human or mouse GKRP:446 P or 446 L for cellular comprehensive analysis including transcriptomics consequent to P446 > L. Physiological traits in the diet-challenged P446L mouse were compared with pleiotropic associations at the human rs1260326 locus. Transcriptomics was compared in P446L mouse liver with hepatocytes overexpressing glucokinase or GKRP:446 P/L.

RESULTS

1. P446 > L substitution in mouse or human GKRP similarly compromises protein expressivity of GKRP:446 L, nuclear sequestration of glucokinase and counter-regulation of gene expression. 2. The P446L knockin mouse has lower liver glucokinase and GKRP protein similar to human liver homozygous for rs1260326-446 L. 3. The diet-challenged P446L mouse has lower blood glucose, raised blood cholesterol and altered hepatic cholesterol homeostasis consistent with relative glucokinase-to-GKRP excess, but not raised blood triglycerides.

CONCLUSIONS

Mouse GKRP phenocopies the human GKRP:P446 > L substitution despite the higher affinity for glucokinase of human GKRP. The diet-challenged P446L mouse replicates several traits found in association with the rs1260326 locus on chromosome 2 including raised blood cholesterol, lower blood glucose and lower liver glucokinase and GKRP protein but not raised blood triglycerides.

Functional Variant in the GCKR Gene Affects Lactate Levels Differentially in the Fasting State and During Hyperglycemia

The rs780094 single nucleotide polymorphism (SNP; C/T) of glucokinase regulatory protein gene (GCKR) is a regulatory genetic variant that has been associated with lactate levels in the fasting state. However, the association of this locus with lactate during hyperglycemia, and the mechanisms underlying these associations remain unknown. We investigated the association of rs780094 with lactate levels in a frequently sampled oral glucose tolerance test in humans and evaluated the effect of increasing GCKR expression on lactate production in liver cells. The C allele of rs780094 was associated with lower lactate levels in fasting but increased lactate level during hyperglycemia independently of insulin levels. Increased expression of GKRP induced higher lactate level in HepG2 cells and in human primary hepatocytes (HPH) upon glucose stimulation by increasing the amount of GCK. Glucagon induced the expression of GCKR in HepG2 and HPH cells. Our results suggest that the association of rs780094 with lactate levels may involve differential GCKR expression between the carriers of the C and T alleles.

Functional characterization of MODY2 mutations in the nuclear export signal of glucokinase

Glucokinase (GCK) plays a key role in glucose homeostasis. Heterozygous inactivating mutations in the GCK gene cause the familial, mild fasting hyperglycaemia named MODY2. Besides its particular kinetic characteristics, glucokinase is regulated by subcellular compartmentation in hepatocytes. Glucokinase regulatory protein (GKRP) binds to GCK, leading to enzyme inhibition and import into the nucleus at fasting. When glucose concentration increases, GCK-GKRP dissociates and GCK is exported to the cytosol due to a nuclear export signal (NES). With the aim to characterize the GCK-NES, we have functionally analysed nine MODY2 mutations located within the NES sequence. Recombinant GCK mutants showed reduced catalytic activity and, in most cases, protein instability. Most of the mutants interact normally with GKRP, although mutations L306R and L309P impair GCK nuclear import in cotransfected cells. We demonstrated that GCK-NES function depends on exportin 1. We further showed that none of the mutations fully inactivate the NES, with the exception of mutation L304P, which likely destabilizes its α-helicoidal structure. Finally, we found that residue Glu300 negatively modulates the NES activity, whereas other residues have the opposite effect, thus suggesting that some of the NES spacer residues contribute to the low affinity of the NES for exportin 1, which is required for its proper functioning. In conclusion, our results have provided functional and structural insights regarding the GCK-NES and contributed to a better knowledge of the molecular mechanisms involved in the nucleo-cytoplasmic shuttling of glucokinase. Impairment of this regulatory mechanism by some MODY2 mutations might contribute to the hyperglycaemia in the patients.

Early gene-diet interaction between glucokinase regulatory protein (GCKR) polymorphism, vegetable and fish intakes in modulating triglyceride levels in healthy adolescents

BACKGROUND AND AIMS

The benefits of dietary vegetable and fish consumptions on improving glucose and lipid metabolism have been well established. Recently, the T-allele of a common genetic variant rs780094 at glucokinase regulatory protein (GCKR) was reported to be associated with elevated triglyceride (TG) levels but reduced fasting plasma glucose (FPG) and type 2 diabetes risk. However, the dietary modulation on genetic risk is not clearly understood.

METHODS AND RESULTS

A cohort of 2095 Chinese adolescents (mean age 15.6 ± 2.0 years, 45.3% male) recruited from a population-based school survey for cardiovascular risk factor assessment, with dietary data including weekly vegetable and fish consumptions as well as clinical data were genotyped for the GCKR rs780094 polymorphism. In the linear regression analysis with adjustment for sex, age, body mass index, and socioeconomic status (school banding, paternal and maternal education levels), the frequency of vegetable intake per week was inversely associated with FPG (P = 0.044). Individuals with low fish intake generally had elevated TG levels but reduced TC, HDL-C and LDL-C (0.006 < P < 0.029). We also observed significant associations of the minor T-allele of GCKR rs780094 with decreased FPG (P = 0.013) and increased TG levels (P = 2.7 × 10(-8)). There were significant gene-diet interactions between rs780094 and vegetable consumption (P(interaction) = 0.009), and between rs780094 and fish consumption (P(interaction) = 0.031) in modulating TG levels. The T-allele of GCKR locus was associated with higher TG levels amongst individuals with ≥7 vegetable meals per week (P = 6.4 × 10(-9)), and among individuals with <7 fish meals per week (P = 0.020 and 7.0 × 10(-7) for 4-6 and ≤3 meals per week, respectively). High intake of vegetable exerted a reduction in TG levels only among CC genotype carriers (Ptrend = 0.020), while high intake of fish was associated with reduced TG levels only among TT genotype carriers (Ptrend = 0.026).

CONCLUSIONS

In summary, our data indicated that the favorable associations of higher vegetable and fish intakes on TG levels are dependent on the genetic background of an individual. In particular, at-risk TT- genotype carriers of the GCKR variant may derive more benefits from a high fish intake, while the CC-genotype carriers may find further benefits from a high consumption of vegetable.

Glucokinase regulatory protein: complexity at the crossroads of triglyceride and glucose metabolism

PURPOSE OF REVIEW

Glucokinase regulator (GCKR) encodes glucokinase regulatory protein (GKRP), a hepatocyte-specific inhibitor of the glucose-metabolizing enzyme glucokinase (GCK). Genome-wide association studies have identified a common coding variant within GCKR associated with multiple metabolic traits. This review focuses on recent insights into the critical role of GKRP in hepatic glucose metabolism that have stemmed from the study of human genetics. This knowledge has improved our understanding of glucose and lipid physiology and informed the development of targeted molecular therapeutics for diabetes.

RECENT FINDINGS

Rare GCKR variants have effects on GKRP expression, localization, and activity. These variants are collectively associated with hypertriglyceridaemia but are not causal. Crystal structures of GKRP and the GCK-GKRP complex have been solved, providing greater insight into the molecular interactions between these proteins. Finally, small molecules have been identified that directly bind GKRP and reduce blood glucose levels in rodent models of diabetes.

SUMMARY

GCKR variants across the allelic spectrum have effects on glucose and lipid homeostasis. Functional analysis has highlighted numerous molecular mechanisms for GKRP dysfunction. Hepatocyte-specific GCK activation via small molecule GKRP inhibition may be a new avenue for type 2 diabetes treatment, particularly considering evidence indicating GKRP loss-of-function alone does not cause hypertriglyceridaemia.

Standardized extract of Ficus deltoidea stimulates insulin secretion and blocks hepatic glucose production by regulating the expression of glucose-metabolic genes in streptozitocin-induced diabetic rats

BACKGROUND

Recently, there has been increasing interest in Ficus deltoidea Jack. (Moraceae) due to its chemical composition and the potential health benefits. The present study was undertaken to investigate the effect of extracts of F. deltoidea leaves on diabetes.

METHODS

The petroleum ether, chloroform and methanol extracts of F. deltoidea were prepared and subjected to standardization using preliminary phytochemical and HPLC analysis. Dose selection was made on the basis of acute oral toxicity study (50-5000 mg/kg b. w.) as per OECD guidelines. Diabetes mellitus was induced with streptozotocin and rats found diabetic were orally administered with the extract (250, 500 and 1000 mg/kg) for 14 days. Levels of blood glucose and insulin were measured in control as well as diabetic rats on 0, 7 and 14th day. In addition, glucose metabolism regulating gene expression was assessed using RT-PCR.

RESULTS

HPLC analysis revealed that the methanol extract is enriched with C-glycosylflavones particularly, vitexin and isovitexin. In oral glucose tolerance test, oral administration of the methanol extract increased the glucose tolerance. The methanol extract showed significant (P < 0.01) antidiabetic activity. The extract treatment caused significant reduction (p < 0.01) in elevated fasting blood glucose level in streptozotocin-induced diabetic rats. The streptozotocin-related weight loss in rats was noticeably reversed by the extract treatment. Finally, RT-PCR analysis revealed a novel mechanisms for the anti-diabetic action of methanol extract of F. deltoidea. The extract exerted its effect via an increase of insulin secretion which impeded the hepatic glucose production, via down-regulation of phosphoenolpyruvate carboxykinase and glucose-6-phosphatase genes expression on one hand, and up-regulation of hepatic GK and PPARγ genes expression on the other hand. The extract caused an increased expression of GLUT-4 gene expression in skeletal muscles which leads to normalize the hyperglycemia. The extract also nullified the toxic effects of streptozitocin by blocking its entry into the islet β-cells through reducing the expression of GLUT-2 gene.

CONCLUSION

It can be concluded that, F. deltoidea could potentially inhibits the streptozitocin-induced hyperglycemia in rats. Further the herb can be utilized as useful remedy for alleviation of diabetes complications.

Analysis of the co-operative interaction between the allosterically regulated proteins GK and GKRP using tryptophan fluorescence

Hepatic glucose phosphorylation by GK (glucokinase) is regulated by GKRP (GK regulatory protein). GKRP forms a cytosolic complex with GK followed by nuclear import and storage, leading to inhibition of GK activity. This process is initiated by low glucose, but reversed nutritionally by high glucose and fructose or pharmacologically by GKAs (GK activators) and GKRPIs (GKRP inhibitors). To study the regulation of this process by glucose, fructose-phosphate esters and a GKA, we measured the TF (tryptophan fluorescence) of human WT (wild-type) and GKRP-P446L (a mutation associated with high serum triacylglycerol) in the presence of non-fluorescent GK with its tryptophan residues mutated. Titration of GKRP-WT by GK resulted in a sigmoidal increase in TF, suggesting co-operative PPIs (protein-protein interactions) perhaps due to the hysteretic nature of GK. The affinity of GK for GKRP was decreased and binding co-operativity increased by glucose, fructose 1-phosphate and GKA, reflecting disruption of the GK-GKRP complex. Similar studies with GKRP-P446L showed significantly different results compared with GKRP-WT, suggesting impairment of complex formation and nuclear storage. The results of the present TF-based biophysical analysis of PPIs between GK and GKRP suggest that hepatic glucose metabolism is regulated by a metabolite-sensitive drug-responsive co-operative molecular switch, involving complex formation between these two allosterically regulated proteins.

Glucotoxicity targets hepatic glucokinase in Zucker diabetic fatty rats, a model of type 2 diabetes associated with obesity

A loss of glucose effectiveness to suppress hepatic glucose production as well as increase hepatic glucose uptake and storage as glycogen is associated with a defective increase in glucose phosphorylation catalyzed by glucokinase (GK) in Zucker diabetic fatty (ZDF) rats. We extended these observations by investigating the role of persistent hyperglycemia (glucotoxicity) in the development of impaired hepatic GK activity in ZDF rats. We measured expression and localization of GK and GK regulatory protein (GKRP), translocation of GK, and hepatic glucose flux in response to a gastric mixed meal load (MMT) and hyperglycemic hyperinsulinemic clamp after 1 or 6 wk of treatment with the sodium-glucose transporter 2 inhibitor (canaglifrozin) that was used to correct the persistent hyperglycemia of ZDF rats. Defective augmentation of glucose phosphorylation in response to a rise in plasma glucose in ZDF rats was associated with the coresidency of GKRP with GK in the cytoplasm in the midstage of diabetes, which was followed by a decrease in GK protein levels due to impaired posttranscriptional processing in the late stage of diabetes. Correcting hyperglycemia from the middle diabetic stage normalized the rate of glucose phosphorylation by maintaining GK protein levels, restoring normal nuclear residency of GK and GKRP under basal conditions and normalizing translocation of GK from the nucleus to the cytoplasm, with GKRP remaining in the nucleus in response to a rise in plasma glucose. This improved the liver's metabolic ability to respond to hyperglycemic hyperinsulinemia. Glucotoxicity is responsible for loss of glucose effectiveness and is associated with altered GK regulation in the ZDF rat.

A panel of diverse assays to interrogate the interaction between glucokinase and glucokinase regulatory protein, two vital proteins in human disease

Recent genetic and clinical evidence has implicated glucokinase regulatory protein (GKRP) in the pathogenesis of type 2 diabetes and related traits. The primary role of GKRP is to bind and inhibit hepatic glucokinase (GCK), a critically important protein in human health and disease that exerts a significant degree of control over glucose metabolism. As activation of GCK has been associated with improved glucose tolerance, perturbation of the GCK-GKRP interaction represents a potential therapeutic target for pharmacological modulation. Recent structural and kinetic advances are beginning to provide insight into the interaction of these two proteins. However, tools to comprehensively assess the GCK-GKRP interaction, particularly in the context of small molecules, would be a valuable resource. We therefore developed three robust and miniaturized assays for assessing the interaction between recombinant human GCK and GKRP: an HTRF assay, a diaphorase-coupled assay, and a luciferase-coupled assay. The assays are complementary, featuring distinct mechanisms of detection (luminescence, fluorescence, FRET). Two assays rely on GCK enzyme activity modulation by GKRP while the FRET-based assay measures the GCK-GKRP protein-protein interaction independent of GCK enzymatic substrates and activity. All three assays are scalable to low volumes in 1536-well plate format, with robust Z’ factors (>0.7). Finally, as GKRP sequesters GCK in the hepatocyte nucleus at low glucose concentrations, we explored cellular models of GCK localization and translocation. Previous findings from freshly isolated rat hepatocytes were confirmed in cryopreserved rat hepatocytes, and we further extended this study to cryopreserved human hepatocytes. Consistent with previous reports, there were several key differences between the rat and human systems, with our results suggesting that human hepatocytes can be used to interrogate GCK translocation in response to small molecules. The assay panel developed here should help direct future investigation of the GCK-GKRP interaction in these or other physiologically relevant human systems.

Pharmacodynamic effects of the oral glucokinase activator AZD6370 after single doses in healthy volunteers assessed with euglycaemic clamp

AIMS

This study evaluated the safety, tolerability, pharmacokinetics and pharmacodynamic effects of the glucokinase activator (GKA) AZD6370 in non-diabetic subjects, using the euglycaemic clamp to avoid the risk of hypoglycaemia.

METHODS

Oral single ascending doses of AZD6370 10-650 mg or subcutaneous short-acting insulin 4 or 12 U were given to healthy fasting subjects. AZD6370 safety, tolerability and pharmacokinetics were assessed. Pharmacodynamic effects on serum (S)-insulin and glucose infusion rate (GIR) were investigated with euglycaemic clamp. AZD6370 10-20 mg was also assessed when taken with food without euglycaemic clamp.

RESULTS

AZD6370 was well tolerated and no safety concerns were raised. AZD6370 was rapidly absorbed and eliminated, and plasma concentration was proportional to dose. Both S-insulin and GIR increased following AZD6370 administration. The observed increase in GIR correlated with increasing AZD6370 area under the plasma concentration vs. time curve, demonstrating a dose-concentration-dependent pharmacodynamic effect. AZD6370 at doses of 50 and 80 mg had similar effects to short-acting insulin 4 U on peripheral S-insulin levels but greater effects on GIR, suggesting an effect beyond the increase of peripheral S-insulin levels at lower doses. In the food interaction part of the study, performed without euglycaemic clamp, dose escalation was stopped at a low dose (20 mg) because of hypoglycaemia.

CONCLUSION

The euglycaemic clamp was successfully used to avoid hypoglycaemia and to demonstrate pharmacodynamic effects, that is, markedly increased insulin secretion and glucose utilisation, following administration of AZD6370 in healthy fasting subjects. In addition to the effect on pancreatic insulin secretion, the data support an extra-pancreatic (hepatic) component of GKA action.

Identification and functional characterisation of novel glucokinase mutations causing maturity-onset diabetes of the young in Slovakia

Heterozygous glucokinase (GCK) mutations cause a subtype of maturity-onset diabetes of the young (GCK-MODY). Over 600 GCK mutations have been reported of which ∼65% are missense. In many cases co-segregation has not been established and despite the importance of functional studies in ascribing pathogenicity for missense variants these have only been performed for <10% of mutations. The aim of this study was to determine the minimum prevalence of GCK-MODY amongst diabetic subjects in Slovakia by sequencing GCK in 100 Slovakian probands with a phenotype consistent with GCK-MODY and to explore the pathogenicity of identified variants through family and functional studies. Twenty-two mutations were identified in 36 families (17 missense) of which 7 (I110N, V200A, N204D, G258R, F419S, c.580-2A>C, c.1113-1114delGC) were novel. Parental DNA was available for 22 probands (covering 14/22 mutations) and co-segregation established in all cases. Bioinformatic analysis predicted all missense mutations to be damaging. Nine (I110N, V200A, N204D, G223S, G258R, F419S, V244G, L315H, I436N) mutations were functionally evaluated. Basic kinetic analysis explained pathogenicity for 7 mutants which showed reduced glucokinase activity with relative activity indices (RAI) between 0.6 to <0.001 compared to wild-type GCK (1.0). For the remaining 2 mutants additional molecular mechanisms were investigated. Differences in glucokinase regulatory protein (GKRP) -mediated-inhibition of GCK were observed for both L315H & I436N when compared to wild type (IC(50) 14.6±0.1 mM & 20.3±1.6 mM vs.13.3±0.1 mM respectively [p<0.03]). Protein instability as assessed by thermal lability studies demonstrated that both L315H and I436N show marked thermal instability compared to wild-type GCK (RAI at 55°C 8.8±0.8% & 3.1±0.4% vs. 42.5±3.9% respectively [p<0.001]). The minimum prevalence of GCK-MODY amongst Slovakian patients with diabetes was 0.03%. In conclusion, we have identified 22 GCK mutations in 36 Slovakian probands and demonstrate that combining family, bioinformatic and functional studies can aid the interpretation of variants identified by molecular diagnostic screening.

Modulation of glucokinase by glucose, small-molecule activator and glucokinase regulatory protein: steady-state kinetic and cell-based analysis

GK (glucokinase) is an enzyme central to glucose metabolism that displays positive co-operativity to substrate glucose. Small-molecule GKAs (GK activators) modulate GK catalytic activity and glucose affinity and are currently being pursued as a treatment for Type 2 diabetes. GK progress curves monitoring product formation are linear up to 1 mM glucose, but biphasic at 5 mM, with the transition from the lower initial velocity to the higher steady-state velocity being described by the rate constant kact. In the presence of a liver-specific GKA (compound A), progress curves at 1 mM glucose are similar to those at 5 mM, reflecting activation of GK by compound A. We show that GKRP (GK regulatory protein) is a slow tight-binding inhibitor of GK. Analysis of progress curves indicate that this inhibition is time dependent, with apparent initial and final Ki values being 113 and 12.8 nM respectively. When GK is pre-incubated with glucose and compound A, the inhibition observed by GKRP is time dependent, but independent of GKRP concentration, reflecting the GKA-controlled transition between closed and open GK conformations. These data are supported by cell-based imaging data from primary rat hepatocytes. This work characterizes the modulation of GK by a novel GKA that may enable the design of new and improved GKAs.

Functional characterization of MODY2 mutations highlights the importance of the fine-tuning of glucokinase and its role in glucose sensing

Glucokinase (GK) acts as a glucose sensor in the pancreatic beta-cell and regulates insulin secretion. Heterozygous mutations in the human GK-encoding GCK gene that reduce the activity index increase the glucose-stimulated insulin secretion threshold and cause familial, mild fasting hyperglycaemia, also known as Maturity Onset Diabetes of the Young type 2 (MODY2). Here we describe the biochemical characterization of five missense GK mutations: p.Ile130Thr, p.Asp205His, p.Gly223Ser, p.His416Arg and p.Ala449Thr. The enzymatic analysis of the corresponding bacterially expressed GST-GK mutant proteins show that all of them impair the kinetic characteristics of the enzyme. In keeping with their position within the protein, mutations p.Ile130Thr, p.Asp205His, p.Gly223Ser, and p.His416Arg strongly decrease the activity index of GK, affecting to one or more kinetic parameters. In contrast, the p.Ala449Thr mutation, which is located in the allosteric activator site, does not affect significantly the activity index of GK, but dramatically modifies the main kinetic parameters responsible for the function of this enzyme as a glucose sensor. The reduced Kcat of the mutant (3.21±0.28 s(-1) vs 47.86±2.78 s(-1)) is balanced by an increased glucose affinity (S(0.5) = 1.33±0.08 mM vs 7.86±0.09 mM) and loss of cooperativity for this substrate. We further studied the mechanism by which this mutation impaired GK kinetics by measuring the differential effects of several competitive inhibitors and one allosteric activator on the mutant protein. Our results suggest that this mutation alters the equilibrium between the conformational states of glucokinase and highlights the importance of the fine-tuning of GK and its role in glucose sensing.

Cellular characterisation of the GCKR P446L variant associated with type 2 diabetes risk

AIMS/HYPOTHESIS

Translation of genetic association signals into molecular mechanisms for diabetes has been slow. The glucokinase regulatory protein (GKRP; gene symbol GCKR) P446L variant, associated with inverse modulation of glucose- and lipid-related traits, has been shown to alter the kinetics of glucokinase (GCK) inhibition. As GCK inhibition is associated with nuclear sequestration, we aimed to determine whether this variant also alters the direct interaction between GKRP and GCK and their intracellular localisation.

METHODS

Fluorescently tagged rat and human wild-type (WT)- or P446L-GCKR and GCK were transiently transfected into HeLa cells and mouse primary hepatocytes. Whole-cell and nuclear fluorescence was quantified in individual cells exposed to low- or high-glucose conditions (5.5 or 25 mmol/l glucose, respectively). Interaction between GCK and GKRP was measured by sensitised emission-based fluorescence resonance energy transfer (FRET) efficiency.

RESULTS

P446L-GKRP had a decreased degree of nuclear localisation, ability to sequester GCK and direct interaction with GCK as measured by FRET compared with WT-GKRP. Decreased interaction was observed between WT-GKRP and GCK at high compared with low glucose, but not between P446L-GKRP and GCK. Rat WT-GKRP and P446L-GKRP behaved quite differently: both variants responded to high glucose by diminished sequestration of GCK but showed no effect of the P446L variant on nuclear localisation or GCK sequestration.

CONCLUSIONS/INTERPRETATION

Our study suggests the common human P446L-GKRP variant protein results in elevated hepatic glucose uptake and disposal by increasing active cytosolic GCK. This would increase hepatic lipid biosynthesis but decrease fasting plasma glucose concentrations and provides a potential mechanism for the protective effect of this allele on type 2 diabetes risk.

Discovery of a novel site regulating glucokinase activity following characterization of a new mutation causing hyperinsulinemic hypoglycemia in humans

Type 2 diabetes is a global problem, and current ineffective therapeutic strategies pave the way for novel treatments like small molecular activators targeting glucokinase (GCK). GCK activity is fundamental to beta cell and hepatocyte glucose metabolism, and heterozygous activating and inactivating GCK mutations cause hyperinsulinemic hypoglycemia (HH) and maturity onset diabetes of the young (MODY) respectively. Over 600 naturally occurring inactivating mutations have been reported, whereas only 13 activating mutations are documented to date. We report two novel GCK HH mutations (V389L and T103S) at residues where MODY mutations also occur (V389D and T103I). Using recombinant proteins with in vitro assays, we demonstrated that both HH mutants had a greater relative activity index than wild type (6.0 for V389L, 8.4 for T103S, and 1.0 for wild type). This was driven by an increased affinity for glucose (S(0.5), 3.3 ± 0.1 and 3.5 ± 0.1 mm, respectively) versus wild type (7.5 ± 0.1 mm). Correspondingly, the V389D and T103I MODY mutants had markedly reduced relative activity indexes (<0.1). T103I had an altered affinity for glucose (S(0.5), 24.9 ± 0.6 mm), whereas V389D also exhibited a reduced affinity for ATP and decreased catalysis rate (S(0.5), 78.6 ± 4.5 mm; ATP(K(m)), 1.5 ± 0.1 mm; K(cat), 10.3 ± 1.1s(-1)) compared with wild type (ATP(K(m)), 0.4 ± <0.1; K(cat), 62.9 ± 1.2). Both Thr-103 mutants showed reduced inhibition by the endogenous hepatic inhibitor glucokinase regulatory protein. Molecular modeling demonstrated that Thr-103 maps to the allosteric activator site, whereas Val-389 is located remotely to this position and all other previously reported activating mutations, highlighting α-helix 11 as a novel region regulating GCK activity. Our data suggest that pharmacological manipulation of GCK activity at locations distal from the allosteric activator site is possible.

From genetic association to molecular mechanism

Over the past 3 years, there has been a dramatic increase in the number of confirmed type 2 diabetes (T2D) susceptibility loci, most arising through the implementation of genome-wide association studies (GWAS). However, progress toward the understanding of disease mechanisms has been slowed by modest effect sizes and the fact that most GWAS signals map away from coding sequence: the presumption is that their effects are mediated through regulation of nearby transcripts, but the identities of the genes concerned are often far from clear. In this review we describe the progress that has been made to date in translating association signals into molecular mechanisms with a focus on the most tractable signals (eg, KCNJ11/ABCC8, SLC30A8, GCKR) and those in which human, animal, and cellular models (FTO, TCF7L2, G6PC2) have provided insights into the role in T2D pathogenesis. Finally, the challenges for the field with the advent of genome-scale next-generation resequencing efforts are discussed.

Antibodies against the voltage-dependent anion channel (VDAC) and its protective ligand hexokinase-I in children with autism

Autistic children show elevated serum levels of autoantibodies to several proteins essential for the function of normal brains. The voltage-dependent anion channel (VDAC) and hexokinase-I, a VDAC protective ligand, were identified as targets of this autoimmunity in autistic children. These autoantibodies were purified using immunoaffinity chromatographic techniques. Both antibodies induce apoptosis of cultured human neuroblastoma cells. Because VDAC and hexokinase-I are essential for brain protection from ischemic damage, the presence of these autoantibodies suggests a possible causal role in the neurologic pathogenesis of autism.

Evolution of vertebrate glucokinase regulatory protein from a bacterial N-acetylmuramate 6-phosphate etherase

Mammalian GKRP [GK (glucokinase) regulatory protein], a fructose 6-phosphate and fructose 1-phosphate sensitive inhibitor of GK, appears to have resulted from the duplication of a gene similar to bacterial N-acetylmuramate 6-phosphate etherase MurQ. In the present study, we show that several genomes of primitive eukaryotes encode a GKRP-like protein with two MurQ repeats. Recombinant Haemophilus influenzae MurQ and the GKRP homologue of the amoeboflagellate Naegleria gruberi both behaved as excellent N-acetylmuramate 6-phosphate etherases, with Kcat values (83 and 20 s(-1)) at least as high as that reported for Escherichia coli MurQ. In contrast, rat and Xenopus GKRP displayed much lower etherase activities (Kcat=0.08 and 0.05 s(-1) respectively). The etherase activity of rat GKRP was inhibited by ligands (fructose 6-phosphate, fructose 1-phosphate and sorbitol 6-phosphate) known to regulate its interaction with GK and by mutations affecting the binding of these phosphate esters. This indicated that these phosphate esters all bind to a single regulatory site, which evolved from the original catalytic site. Sorbitol 6-phosphate and other phosphate esters also inhibited the etherase activity of Xenopus GKRP, but did not affect its ability to inhibit GK. Thus, unlike what was previously thought, Xenopus GKRP has a binding site for phosphate esters, but this site is uncoupled from the GK-binding site. Taken together, these data indicate that duplication of the murQ gene led to a eukaryotic-type etherase, which subsequently evolved to GKRP by acquiring a new binding specificity while losing most of its etherase activity.

The P446L variant in GCKR associated with fasting plasma glucose and triglyceride levels exerts its effect through increased glucokinase activity in liver

Genome-wide association studies have identified a number of signals for both Type 2 Diabetes and related quantitative traits. For the majority of loci, the transition from association signal to mutational mechanism has been difficult to establish. Glucokinase (GCK) regulates glucose storage and disposal in the liver where its activity is regulated by glucokinase regulatory protein (GKRP; gene name GCKR). Fructose-6 and fructose-1 phosphate (F6P and F1P) enhance or reduce GKRP-mediated inhibition, respectively. A common GCKR variant (P446L) is reproducibly associated with triglyceride and fasting plasma glucose levels in the general population. The aim of this study was to determine the mutational mechanism responsible for this genetic association. Recombinant human GCK and both human wild-type (WT) and P446L-GKRP proteins were generated. GCK kinetic activity was observed spectrophotometrically using an NADP⁺-coupled assay. WT and P446L-GKRP-mediated inhibition of GCK activity and subsequent regulation by phosphate esters were determined. Assays matched for GKRP activity demonstrated no difference in dose-dependent inhibition of GCK activity or F1P-mediated regulation. However, the response to physiologically relevant F6P levels was significantly attenuated with P446L-GKRP (n = 18; P ≤ 0.03). Experiments using equimolar concentrations of both regulatory proteins confirmed these findings (n = 9; P < 0.001). In conclusion, P446L-GKRP has reduced regulation by physiological concentrations of F6P, resulting indirectly in increased GCK activity. Altered GCK regulation in liver is predicted to enhance glycolytic flux, promoting hepatic glucose metabolism and elevating concentrations of malonyl-CoA, a substrate for de novo lipogenesis, providing a mutational mechanism for the reported association of this variant with raised triglycerides and lower glucose levels.

Common missense variant in the glucokinase regulatory protein gene is associated with increased plasma triglyceride and C-reactive protein but lower fasting glucose concentrations

OBJECTIVE

Using the genome-wide association approach, we recently identified the glucokinase regulatory protein gene (GCKR, rs780094) region as a novel quantitative trait locus for plasma triglyceride concentration in Europeans. Here, we sought to study the association of GCKR variants with metabolic phenotypes, including measures of glucose homeostasis, to evaluate the GCKR locus in samples of non-European ancestry and to fine- map across the associated genomic interval.

RESEARCH DESIGN AND METHODS

We performed association studies in 12 independent cohorts comprising >45,000 individuals representing several ancestral groups (whites from Northern and Southern Europe, whites from the U.S., African Americans from the U.S., Hispanics of Caribbean origin, and Chinese, Malays, and Asian Indians from Singapore). We conducted genetic fine-mapping across the approximately 417-kb region of linkage disequilibrium spanning GCKR and 16 other genes on chromosome 2p23 by imputing untyped HapMap single nucleotide polymorphisms (SNPs) and genotyping 104 SNPs across the associated genomic interval.

RESULTS

We provide comprehensive evidence that GCKR rs780094 is associated with opposite effects on fasting plasma triglyceride (P(meta) = 3 x 10(-56)) and glucose (P(meta) = 1 x 10(-13)) concentrations. In addition, we confirmed recent reports that the same SNP is associated with C-reactive protein (CRP) level (P = 5 x 10(-5)). Both fine-mapping approaches revealed a common missense GCKR variant (rs1260326, Pro446Leu, 34% frequency, r(2) = 0.93 with rs780094) as the strongest association signal in the region.

CONCLUSIONS

These findings point to a molecular mechanism in humans by which higher triglycerides and CRP can be coupled with lower plasma glucose concentrations and position GCKR in central pathways regulating both hepatic triglyceride and glucose metabolism.

Biophysical characterization of the interaction between hepatic glucokinase and its regulatory protein: impact of physiological and pharmacological effectors

Glucokinase (GK) is a key enzyme of glucose metabolism in liver and pancreatic beta-cells, and small molecule activators of GK (GKAs) are under evaluation for the treatment of type 2 diabetes. In liver, GK activity is controlled by the GK regulatory protein (GKRP), which forms an inhibitory complex with the enzyme. Here, we performed isothermal titration calorimetry and surface plasmon resonance experiments to characterize GK-GKRP binding and to study the influence that physiological and pharmacological effectors of GK have on the protein-protein interaction. In the presence of fructose-6-phosphate, GK-GKRP complex formation displayed a strong entropic driving force opposed by a large positive enthalpy; a negative change in heat capacity was observed (Kd = 45 nm, DeltaH = 15.6 kcal/mol, TDeltaS = 25.7 kcal/mol, DeltaCp = -354 cal mol(-1) K(-1)). With k(off) = 1.3 x 10(-2) s(-1), the complex dissociated quickly. The thermodynamic profile suggested a largely hydrophobic interaction. In addition, effects of pH and buffer demonstrated the coupled uptake of one proton and indicated an ionic contribution to binding. Glucose decreased the binding affinity between GK and GKRP. This decrease was potentiated by an ATP analogue. Prototypical GKAs of the amino-heteroaryl-amide type bound to GK in a glucose-dependent manner and impaired the association of GK with GKRP. This mechanism might contribute to the antidiabetic effects of GKAs.

Glucokinase and molecular aspects of liver glycogen metabolism

Conversion of glucose into glycogen is a major pathway that contributes to the removal of glucose from the portal vein by the liver in the postprandial state. It is regulated in part by the increase in blood-glucose concentration in the portal vein, which activates glucokinase, the first enzyme in the pathway, causing an increase in the concentration of glucose 6-P (glucose 6-phosphate), which modulates the phosphorylation state of downstream enzymes by acting synergistically with other allosteric effectors. Glucokinase is regulated by a hierarchy of transcriptional and post-transcriptional mechanisms that are only partially understood. In the fasted state, glucokinase is in part sequestered in the nucleus in an inactive state, complexed to a specific regulatory protein, GKRP (glucokinase regulatory protein). This reserve pool is rapidly mobilized to the cytoplasm in the postprandial state in response to an elevated concentration of glucose. The translocation of glucokinase between the nucleus and cytoplasm is modulated by various metabolic and hormonal conditions. The elevated glucose 6-P concentration, consequent to glucokinase activation, has a synergistic effect with glucose in promoting dephosphorylation (inactivation) of glycogen phosphorylase and inducing dephosphorylation (activation) of glycogen synthase. The latter involves both a direct ligand-induced conformational change and depletion of the phosphorylated form of glycogen phosphorylase, which is a potent allosteric inhibitor of glycogen synthase phosphatase activity associated with the glycogen-targeting protein, GL [hepatic glycogen-targeting subunit of PP-1 (protein phosphatase-1) encoded by PPP1R3B]. Defects in both the activation of glucokinase and in the dephosphorylation of glycogen phosphorylase are potential contributing factors to the dysregulation of hepatic glucose metabolism in Type 2 diabetes.

The common P446L polymorphism in GCKR inversely modulates fasting glucose and triglyceride levels and reduces type 2 diabetes risk in the DESIR prospective general French population

OBJECTIVE

Hepatic glucokinase (GCK) is a key regulator of glucose storage and disposal in the liver, where its activity is competitively modulated, with respect to glucose, by binding to glucokinase regulatory protein (GCKR) in the presence of fructose 6-phosphate. Genome-wide association studies for type 2 diabetes identified GCKR as a potential locus for modulating triglyceride levels. We evaluated, in a general French population, the contribution of the GCKR rs1260326-P446L polymorphism to quantitative metabolic parameters and to dyslipidemia and hyperglycemia risk.

RESEARCH DESIGN AND METHODS

Genotype effects of rs1260326 were studied in 4,833 participants from the prospective DESIR (Data from an Epidemiological Study on the Insulin Resistance syndrome) cohort both at inclusion and using the measurements at follow-up.

RESULTS

The minor T-allele of rs1260326 was strongly associated with lower fasting glucose (-1.43% per T-allele; P = 8 x 10(-13)) and fasting insulin levels (-4.23%; P = 3 x 10(-7)), lower homeostasis model assessment of insulin resistance index (-5.69%; P = 1 x 10(-8)), and, conversely, higher triglyceride levels (3.41%; P = 1 x 10(-4)) during the 9-year study. These effects relate to a lower risk of hyperglycemia (odds ratio [OR] 0.79 [95% CI 0.70-0.88]; P = 4 x 10(-5)) and of incident cases during the study (hazard ratio [HR] 0.83 [0.74-0.95]; P = 0.005). Moreover, an additive effect of GCKR rs1260326(T) and GCK (-30G) alleles conferred lower fasting glycemia (P = 1 x 10(-13)), insulinemia (P = 5 x 10(-6)), and hyperglycemia risk (P = 1 x 10(-6)).

CONCLUSIONS

GCKR-L446 carriers are protected against type 2 diabetes despite higher triglyceride levels and risk of dyslipidemia, which suggests a potential molecular mechanism by which these two components of the metabolic syndrome can be dissociated.

Succinate dehydrogenase flavoprotein subunit expression in Saccharomyces cerevisiae--involvement of the mitochondrial FAD transporter, Flx1p

The mitochondrial FAD transporter, Flx1p, is a member of the mitochondrial carrier family responsible for FAD transport in Saccharomyces cerevisiae. It has also been suggested that it has a role in maintaining the normal activity of mitochondrial FAD-binding enzymes, including lipoamide dehydrogenase and succinate dehydrogenase flavoprotein subunit Sdh1p. A decrease in the amount of Sdh1p in the flx1Delta mutant strain has been determined here to be due to a post-transcriptional control that involves regulatory sequences located upstream of the SDH1 coding sequence. The SDH1 coding sequence and the regulatory sequences located downstream of the SDH1 coding region, as well as protein import and cofactor attachment, seem to be not involved in the decrease in the amount of protein.

Glucokinase Thermolability and Hepatic Regulatory Protein Binding Are Essential Factors for Predicting the Blood Glucose Phenotype of Missense Mutations

To better understand how glucokinase (GK) missense mutations associated with human glycemic diseases perturb glucose homeostasis, we generated and characterized mice with either an activating (A456V) or inactivating (K414E) mutation in the gk gene. Animals with these mutations exhibited alterations in their blood glucose concentration that were inversely related to the relative activity index of GK. Moreover, the threshold for glucose-stimulated insulin secretion from islets with either the activating or inactivating mutation were left- or right-shifted, respectively. However, we were surprised to find that mice with the activating mutation had markedly reduced amounts of hepatic GK activity. Further studies of bacterially expressed mutant enzymes revealed that GK(A456V) is as stable as the wild type enzyme, whereas GK(K414E) is thermolabile. However, the ability of GK regulatory protein to inhibit GK(A456V) was found to be less than that of the wild type enzyme, a finding consistent with impaired hepatic nuclear localization. Taken together, this study indicates that it is necessary to have knowledge of both thermolability and the interactions of mutant GK enzymes with GK regulatory protein when attempting to predict in vivo glycemic phenotypes based on the measurement of enzyme kinetics.

A defect in glucose-induced dissociation of glucokinase from the regulatory protein in Zucker diabetic fatty rats in the early stage of diabetes

Effect of stimulation of glucokinase (GK) export from the nucleus by small amounts of sorbitol on hepatic glucose flux in response to elevated plasma glucose was examined in 6-h fasted Zucker diabetic fatty rats at 10 wk of age. Under basal conditions, plasma glucose, insulin, and glucagon were ∼8 mM, 2,000 pmol/l, and 60 ng/l, respectively. Endogenous glucose production (EGP) was 44 ± 4 μmol·kg−1·min−1. When plasma glucose was raised to ∼17 mM, GK was still predominantly localized with its inhibitory protein in the nucleus. EGP was not suppressed. When sorbitol was infused at 5.6 and 16.7 μmol·kg−1·min−1, along with the increase in plasma glucose, GK was exported to the cytoplasm. EGP (23 ± 19 and 12 ± 5 μmol·kg−1·min−1) was suppressed without a decrease in glucose 6-phosphatase flux (145 ± 23 and 126 ± 16 vs. 122 ± 10 μmol·kg−1·min−1without sorbitol) but increased in glucose phosphorylation as indicated by increases in glucose recycling (122 ± 17 and 114 ± 19 vs. 71 ± 11 μmol·kg−1·min−1), glucose-6-phosphate content (254 ± 32 and 260 ± 35 vs. 188 ± 20 nmol/g liver), fractional contribution of plasma glucose to uridine 5′-diphosphate-glucose flux (43 ± 8 and 42 ± 8 vs. 27 ± 6%), and glycogen synthesis from plasma glucose (20 ± 4 and 22 ± 5 vs. 9 ± 4 μmol glucose/g liver). The decreased glucose effectiveness to suppress EGP and stimulate hepatic glucose uptake may result from failure of the sugar to activate GK by stimulating the translocation of the enzyme.

Interaction of glucokinase with the liver regulatory protein is conferred by leucine-asparagine motifs of the enzyme

The glucokinase regulatory protein (GRP) plays a pivotal role in the regulation of metabolic flux in liver by the glucose-phosphorylating enzyme glucokinase. Random peptide phage display library screening for binding partners of GRP allowed the identification of an asparagine-leucine consensus motif. Asparagine-leucine motifs of glucokinase located in the hinge region, as well as in the large domain, were changed by site-directed mutagenesis. The L58R/N204Y and the L309R/N313Y glucokinase mutants showed a significantly reduced interaction with GRP. The L355R/N350Y mutant had a fivefold-higher binding affinity for GRP than wild-type glucokinase. Imaging of glucokinase and GRP fluorescence fusion proteins revealed that the L58R/N204Y glucokinase mutant lacked glucose-dependent translocation by GRP, whereas the L355R/N350Y glucokinase mutant was trapped in the nucleus due to high affinity for GRP. The results indicate that the L58/N204 motif in the hinge region confers binding to GRP, while the L355/N350 motif may modulate the binding affinity for GRP. This latter motif is part of the alpha10 helix of glucokinase and accessible to GRP in the free and complex conformation.