Structure-function study of the glucose-6-phosphate transporter, an eukaryotic antiporter deficient in glycogen storage disease type Ib

Genetics of enzymatic dysfunctions in metabolic disorders and cancer

Inherited metabolic disorders arise from mutations in genes involved in the biogenesis, assembly, or activity of metabolic enzymes, leading to enzymatic deficiency and severe metabolic impairments. Metabolic enzymes are essential for the normal functioning of cells and are involved in the production of amino acids, fatty acids and nucleotides, which are essential for cell growth, division and survival. When the activity of metabolic enzymes is disrupted due to mutations or changes in expression levels, it can result in various metabolic disorders that have also been linked to cancer development. However, there remains much to learn regarding the relationship between the dysregulation of metabolic enzymes and metabolic adaptations in cancer cells. In this review, we explore how dysregulated metabolism due to the alteration or change of metabolic enzymes in cancer cells plays a crucial role in tumor development, progression, metastasis and drug resistance. In addition, these changes in metabolism provide cancer cells with a number of advantages, including increased proliferation, resistance to apoptosis and the ability to evade the immune system. The tumor microenvironment, genetic context, and different signaling pathways further influence this interplay between cancer and metabolism. This review aims to explore how the dysregulation of metabolic enzymes in specific pathways, including the urea cycle, glycogen storage, lysosome storage, fatty acid oxidation, and mitochondrial respiration, contributes to the development of metabolic disorders and cancer. Additionally, the review seeks to shed light on why these enzymes represent crucial potential therapeutic targets and biomarkers in various cancer types.

Novel mutation of SLC37A4 in a glycogen storage disease type Ib patient with neutropenia, horseshoe kidney, and arteriovenous malformation: a case report

Glycogen storage disease type Ib (GSDIb) is an autosomal recessive disorder caused by mutations of SLC37A4 gene, which encodes glucose 6-phosphate translocase (G6PT). Malfunction of G6PT leads to excessive fat and glycogen in liver, kidney, and intestinal mucosa. The clinical manifestations of GSD1b include hepatomegaly, renomegaly, neutropenia, hypoglycemia, and lactic acidosis. Furthermore, the disorder may result in severe complications in long-term including inflammatory bowel disease (IBD), hepatocellular adenomas (HCA), short stature, and autoimmune disorders, which stem from neutropenia and neutrophil dysfunction. Here, we represent a novel mutation of SLC37A4 in a 5-month girl who has a history of hospitalizations several times due to recurrent infection and her early presentations were failure to thrive and tachypnea. Further investigations revealed mild atrial septal defect, mild arteriovenous malformation from left lung, esophageal reflux, Horseshoe kidney, and urinary reflux in this patient. Moreover, the lab tests showed neutropenia, immunoglobulin (Ig) G and IgA deficiency, as well as thrombocytosis. Whole exome sequencing revealed c.1245G > A P.W415 homozygous mutation in SLC37A4 gene and c.580G > A p.V1941 heterozygous mutation in PIK3CD gene. This study shows that manifestations of GSD1b may not be limited to what was previously known and it should be considered in a wider range of patients.

Metabolic Efficacy of Phosphate Prodrugs and the Remdesivir Paradigm

Drugs that contain phosphates (and phosphonates or phosphinates) have intrinsic absorption issues and are therefore often delivered in prodrug forms to promote their uptake. Effective prodrug forms distribute their payload to the site of the intended target and release it efficiently with minimal byproduct toxicity. The ability to balance unwanted payload release during transit with desired release at the site of action is critical to prodrug efficacy. Despite decades of research on prodrug forms, choosing the ideal prodrug form remains a challenge which is often solved empirically. The recent emergency use authorization of the antiviral remdesivir for COVID-19 exemplifies a new approach for delivery of phosphate prodrugs by parenteral dosing, which minimizes payload release during transit and maximizes tissue payload distribution. This review focuses on the role of metabolic activation in efficacy during oral and parenteral dosing of phosphate, phosphonate, and phosphinate prodrugs. Through examining prior structure–activity studies on prodrug forms and the choices that led to development of remdesivir and other clinical drugs and drug candidates, a better understanding of their ability to distribute to the planned site of action, such as the liver, plasma, PBMCs, or peripheral tissues, can be gained. The structure–activity relationships described here will facilitate the rational design of future prodrugs.

SLC37A2, a phosphorus-related molecule, increases in smooth muscle cells in the calcified aorta

Vascular calcification is major source of cardiovascular disease in patients with chronic kidney disease. Hyperphosphatemia leads to increased intracellular phosphorus influx, which leads to an increase in osteoblast-like cells in vascular smooth muscle cell. PiT-1 transports phosphate in vascular smooth muscle cell. However, the mechanism of vascular calcification is not completely understood. This study investigated candidate phosphorus-related molecules other than PiT-1. We hypothesized that phosphorus-related molecules belonging to the solute-carrier (SLC) superfamily would be involved in vascular calcification. As a result of DNA microarray analysis, we focused on SLC37A2 and showed that mRNA expression of these cells increased on calcified aotic smooth muscle cells (AoSMC). SLC37A2 has been reported to transport both glucose-6-phosphate/phosphate and phosphate/phosphate exchanges. In vitro analysis showed that SLC37A2 expression was not affected by inflammation on AoSMC. The expression of SLC37A2 mRNA and protein increased in calcified AoSMC. In vivo analysis showed that SLC37A2 mRNA expression in the aorta of chronic kidney disease rats was correlated with osteogenic marker genes. Furthermore, SLC37A2 was expressed at the vascular calcification area in chronic kidney disease rats. As a result, we showed that SLC37A2 is one of the molecules that increase with vascular calcification in vitro and in vivo.

The Physiopathological Role of the Exchangers Belonging to the SLC37 Family

The human SLC37 gene family includes four proteins SLC37A1-4, localized in the endoplasmic reticulum (ER) membrane. They have been grouped into the SLC37 family due to their sequence homology to the bacterial organophosphate/phosphate (Pi) antiporter. SLC37A1-3 are the less characterized isoforms. SLC37A1 and SLC37A2 are Pi-linked glucose-6-phosphate (G6P) antiporters, catalyzing both homologous (Pi/Pi) and heterologous (G6P/Pi) exchanges, whereas SLC37A3 transport properties remain to be clarified. Furthermore, SLC37A1 is highly homologous to the bacterial glycerol 3-phosphate permeases, so it is supposed to transport also glycerol-3-phosphate. The physiological role of SLC37A1-3 is yet to be further investigated. SLC37A1 seems to be required for lipid biosynthesis in cancer cell lines, SLC37A2 has been proposed as a vitamin D and a phospho-progesterone receptor target gene, while mutations in the SLC37A3 gene appear to be associated with congenital hyperinsulinism of infancy. SLC37A4, also known as glucose-6-phosphate translocase (G6PT), transports G6P from the cytoplasm into the ER lumen, working in complex with either glucose-6-phosphatase-α (G6Pase-α) or G6Pase-β to hydrolyze intraluminal G6P to Pi and glucose. G6PT and G6Pase-β are ubiquitously expressed, whereas G6Pase-α is specifically expressed in the liver, kidney and intestine. G6PT/G6Pase-α complex activity regulates fasting blood glucose levels, whereas G6PT/G6Pase-β is required for neutrophil functions. G6PT deficiency is responsible for glycogen storage disease type Ib (GSD-Ib), an autosomal recessive disorder associated with both defective metabolic and myeloid phenotypes. Several kinds of mutations have been identified in the SLC37A4 gene, affecting G6PT function. An increased autoimmunity risk for GSD-Ib patients has also been reported, moreover, SLC37A4 seems to be involved in autophagy.

Muscle transcriptome analysis reveals molecular pathways and biomarkers involved in extreme ultimate pH and meat defect occurrence in chicken

The processing ability and sensory quality of chicken breast meat are highly related to its ultimate pH (pHu), which is mainly determined by the amount of glycogen in the muscle at death. To unravel the molecular mechanisms underlying glycogen and meat pHu variations and to identify predictive biomarkers of these traits, a transcriptome profiling analysis was performed using an Agilent custom chicken 8 × 60 K microarray. The breast muscle gene expression patterns were studied in two chicken lines experimentally selected for high (pHu+) and low (pHu-) pHu values of the breast meat. Across the 1,436 differentially expressed (DE) genes found between the two lines, many were involved in biological processes related to muscle development and remodelling and carbohydrate and energy metabolism. The functional analysis showed an intensive use of carbohydrate metabolism to produce energy in the pHu- line, while alternative catabolic pathways were solicited in the muscle of the pHu+ broilers, compromising their muscle development and integrity. After a validation step on a population of 278 broilers using microfluidic RT-qPCR, 20 genes were identified by partial least squares regression as good predictors of the pHu, opening new perspectives of screening broilers likely to present meat quality defects.

Fine selection of up-regulated genes during ovulation by in vivo induction of oocyte maturation and ovulation in zebrafish

BACKGROUND

Two essential processes, oocyte maturation and ovulation, are independently induced, but proceed cooperatively as the final step in oogenesis before oocytes become fertilizable. Although these two processes are induced by the same maturation-inducing steroid, 17α, 20β-dihydroxy-4-pregnen-3-one (17, 20β-DHP), in zebrafish, it has been suggested that the receptor, and thus the signal transduction pathway is different for each process. Although much progress has been made in understanding the molecular mechanisms underlying the induction of oocyte maturation, the mechanisms for inducing ovulation remain under investigation. In the present study, in vivo induction techniques that permit the induction of oocyte maturation and ovulation in living zebrafish (in vivo assays) were used to select highly up-regulated genes (genes associated with ovulation). Using an in vivo assay, ovarian tissues that induced only oocyte maturation could be obtained. This made it possible for the first time to distinguish maturation-inducing genes from ovulation-inducing genes. Using a genome-wide microarray of zebrafish sequences, the gene expression levels were compared among an ethanol (EtOH)-treated group (non-activated group), a diethylstilbestrol (DES)- or testosterone (Tes)-treated group (maturation-induced group), and a 17, 20β-DHP-treated group (maturation- and ovulation-induced group). Ovulation-specific up-regulated genes were selected. The mRNA expression levels of the selected genes were measured by quantitative polymerase chain reaction (qPCR).

RESULTS

Among 34 genes identified, three that showed ovulation-specific increases were selected as candidates potentially associated with ovulation. The ovulation-specific up-regulation of three candidates, slc37a4a, zgc:65811 and zgc:92184 was confirmed by qPCR.

CONCLUSION

Our in vivo assay provides a new approach to precisely select genes associated with ovulation.

Identification of glucose-6-phosphate transporter as a key regulator functioning at the autophagy initiation step

Autophagy is a catabolic process involving autophagosome formation via lysosome. However, the initiation step of autophagy is largely unknown. We found an interaction between ULK1 and ATG9 in mammalian cells and utilized the interaction to identify novel regulators of autophagy upstream of ULK1. We established a cell-based screening assay employing bimolecular fluorescence complementation. By performing gain-of-function screening, we identified G6PT as an autophagy activator. G6PT enhanced the interaction between N-terminal Venus-tagged ULK1 and C-terminal Venus-tagged ATG9, and increased autophagic flux independent of its transport activity. G6PT negatively regulated mTORC1 activity, demonstrating that G6PT functions upstream of mTORC1 in stimulating autophagy.

The SLC37 family of sugar-phosphate/phosphate exchangers

The SLC37 family members are endoplasmic reticulum (ER)-associated sugar-phosphate/phosphate (P(i)) exchangers. Three of the four members, SLC37A1, SLC37A2, and SLC37A4, function as Pi-linked glucose-6-phosphate (G6P) antiporters catalyzing G6P:P(i) and P(i):P(i) exchanges. The activity of SLC37A3 is unknown. SLC37A4, better known as the G6P transporter (G6PT), has been extensively characterized, functionally and structurally, and is the best characterized family member. G6PT contains 10 transmembrane helices with both N and C termini facing the cytoplasm. The primary in vivo function of the G6PT protein is to translocate G6P from the cytoplasm into the ER lumen where it couples with either the liver/kidney/intestine-restricted glucose-6-phosphatase-α (G6Pase-α or G6PC) or the ubiquitously expressed G6Pase-β (or G6PC3) to hydrolyze G6P to glucose and P(i). The G6PT/G6Pase-α complex maintains interprandial glucose homeostasis, and the G6PT/G6Pase-β complex maintains neutrophil energy homeostasis and functionality. G6PT is highly selective for G6P and is competitively inhibited by cholorogenic acid and its derivatives. Neither SLC37A1 nor SLC37A2 can couple functionally with G6Pase-α or G6Pase-β, and the antiporter activities of SLC37A1 or SLC37A2 are not inhibited by cholorogenic acid. Deficiencies in G6PT cause glycogen storage disease type Ib (GSD-Ib), a metabolic and immune disorder. To date, 91 separate SLC37A4 mutations, including 39 missense mutations, have been identified in GSD-Ib patients. Characterization of missense mutations has yielded valuable information on functionally important residues in the G6PT protein. The biological roles of the other SLC37 proteins remain to be determined and deficiencies have not yet been correlated to diseases.

The SLC37 family of phosphate-linked sugar phosphate antiporters

The SLC37 family consists of four sugar-phosphate exchangers, A1, A2, A3, and A4, which are anchored in the endoplasmic reticulum (ER) membrane. The best characterized family member is SLC37A4, better known as the glucose-6-phosphate (G6P) transporter (G6PT). SLC37A1, SLC37A2, and G6PT function as phosphate (Pi)-linked G6P antiporters catalyzing G6P:Pi and Pi:Pi exchanges. The activity of SLC37A3 is unknown. G6PT translocates G6P from the cytoplasm into the lumen of the ER where it couples with either glucose-6-phosphatase-α (G6Pase-α) or G6Pase-β to hydrolyze intraluminal G6P to glucose and Pi. The functional coupling of G6PT with G6Pase-α maintains interprandial glucose homeostasis and the functional coupling of G6PT with G6Pase-β maintains neutrophil energy homeostasis and functionality. A deficiency in G6PT causes glycogen storage disease type Ib, an autosomal recessive disorder characterized by impaired glucose homeostasis, neutropenia, and neutrophil dysfunction. Neither SLC37A1 nor SLC37A2 can functionally couple with G6Pase-α or G6Pase-β, and there are no known disease associations for them or SLC37A3. Since only G6PT matches the characteristics of the physiological ER G6P transporter involved in blood glucose homeostasis and neutrophil energy metabolism, the biological roles for the other SLC37 proteins remain to be determined.

Inter-strain heterogeneity in rat hepatic transcriptomic responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

The biochemical and toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have been the subject of intense study for decades. It is now clear that essentially all TCDD-induced toxicities are mediated by DNA-protein interactions involving the Aryl Hydrocarbon Receptor (AHR). Nevertheless, it remains unknown which AHR target genes cause TCDD toxicities. Several groups, including our own, have developed rodent model systems to probe these questions. mRNA expression profiling of these model systems has revealed significant inter-species heterogeneity in rodent hepatic responses to TCDD. It has remained unclear if this variability also exists within a species, amongst rodent strains. To resolve this question, we profiled the hepatic transcriptomic response to TCDD of diverse rat strains (L-E, H/W, F344 and Wistar rats) and two lines derived from L-E×H/W crosses, at consistent age, sex, and dosing (100 μg/kg TCDD for 19 h). Using this uniquely consistent dataset, we show that the majority of TCDD-induced alterations in mRNA abundance are strain/line-specific: only 11 genes were affected by TCDD across all strains, including well-known dioxin-responsive genes such as Cyp1a1 and Nqo1. Our analysis identified two novel universally dioxin-responsive genes as well as 4 genes induced by TCDD in dioxin-sensitive rats only. These 6 genes are strong candidates to explain TCDD-related toxicities, so we validated them using 152 animals in time-course (0 to 384 h) and dose-response (0 to 3000 μg/kg) experiments. This study reveals that different rat strains exhibit dramatic transcriptional heterogeneity in their hepatic responses to TCDD and that inter-strain comparisons can help identify candidate toxicity-related genes.

Folding and stability of membrane transport proteins in vitro

Transmembrane transporters are responsible for maintaining a correct internal cellular environment. The inherent flexibility of transporters together with their hydrophobic environment means that they are challenging to study in vitro, but recently significant progress been made. This review will focus on in vitro stability and folding studies of transmembrane alpha helical transporters, including reversible folding systems and thermal denaturation. The successful re-assembly of a small number of ATP binding cassette transporters is also described as this is a significant step forward in terms of understanding the folding and assembly of these more complex, multi-subunit proteins. The studies on transporters discussed here represent substantial advances for membrane protein studies as well as for research into protein folding. The work demonstrates that large flexible hydrophobic proteins are within reach of in vitro folding studies, thus holding promise for furthering knowledge on the structure, function and biogenesis of ubiquitous membrane transporter families. This article is part of a Special Issue entitled: Protein Folding in Membranes.

SLC37A1 and SLC37A2 are phosphate-linked, glucose-6-phosphate antiporters

Blood glucose homeostasis between meals depends upon production of glucose within the endoplasmic reticulum (ER) of the liver and kidney by hydrolysis of glucose-6-phosphate (G6P) into glucose and phosphate (P_i). This reaction depends on coupling the G6P transporter (G6PT) with glucose-6-phosphatase-α (G6Pase-α). Only one G6PT, also known as SLC37A4, has been characterized, and it acts as a P_i-linked G6P antiporter. The other three SLC37 family members, predicted to be sugar-phosphate:P_i exchangers, have not been characterized functionally. Using reconstituted proteoliposomes, we examine the antiporter activity of the other SLC37 members along with their ability to couple with G6Pase-α. G6PT- and mock-proteoliposomes are used as positive and negative controls, respectively. We show that SLC37A1 and SLC37A2 are ER-associated, P_i-linked antiporters, that can transport G6P. Unlike G6PT, neither is sensitive to chlorogenic acid, a competitive inhibitor of physiological ER G6P transport, and neither couples to G6Pase-α. We conclude that three of the four SLC37 family members are functional sugar-phosphate antiporters. However, only G6PT/SLC37A4 matches the characteristics of the physiological ER G6P transporter, suggesting the other SLC37 proteins have roles independent of blood glucose homeostasis.

Neutropenia in type Ib glycogen storage disease. Curr Opin Hematol. 2010;17:36-42. [PMID: 19741523 DOI: 10.1097/moh.0b013e328331df85]

PURPOSE OF REVIEW

Glycogen storage disease type Ib, characterized by disturbed glucose homeostasis, neutropenia, and neutrophil dysfunction, is caused by a deficiency in a ubiquitously expressed glucose-6-phosphate transporter (G6PT). G6PT translocates glucose-6-phosphate (G6P) from the cytoplasm into the lumen of the endoplasmic reticulum, in which it is hydrolyzed to glucose either by a liver/kidney/intestine-restricted glucose-6-phosphatase-alpha (G6Pase-alpha) or by a ubiquitously expressed G6Pase-beta. The role of the G6PT/G6Pase-alpha complex is well established and readily explains why G6PT disruptions disturb interprandial blood glucose homeostasis. However, the basis for neutropenia and neutrophil dysfunction in glycogen storage disease type Ib is poorly understood. Recent studies that are now starting to unveil the mechanisms are presented in this review.

RECENT FINDINGS

Characterization of G6Pase-beta and generation of mice lacking either G6PT or G6Pase-beta have shown that neutrophils express the G6PT/G6Pase-beta complex capable of producing endogenous glucose. Loss of G6PT activity leads to enhanced endoplasmic reticulum stress, oxidative stress, and apoptosis that underlie neutropenia and neutrophil dysfunction in glycogen storage disease type Ib.

SUMMARY

Neutrophil function is intimately linked to the regulation of glucose and G6P metabolism by the G6PT/G6Pase-beta complex. Understanding the molecular mechanisms that govern energy homeostasis in neutrophils has revealed a previously unrecognized pathway of intracellular G6P metabolism in neutrophils.