Multiplex polymerase chain reaction assay for the detection of minute virus of mice and mouse parvovirus infections in laboratory mice

Mouse parvoviruses are among the most prevalent infectious pathogens in contemporary mouse colonies. To improve the efficiency of routine screening for mouse parvovirus infections, a multiplex polymerase chain reaction (PCR) assay targeting the VP gene was developed. The assay detected minute virus of mice (MVM), mouse parvovirus (MPV) and a mouse housekeeping gene (α-actin) and was able to specifically detect MVM and MPV at levels as low as 50 copies. Co-infection with the two viruses with up to 200-fold differences in viral concentrations can easily be detected. The multiplex PCR assay developed here could be a useful tool for monitoring mouse health and the viral contamination of biological materials.

Angiotensin mediates renal fibrosis in the nephropathy of glycogen storage disease type Ia

Patients with glycogen storage disease type Ia (GSD-Ia) develop renal disease of unknown etiology despite intensive dietary therapies. This renal disease shares many clinical and pathological similarities to diabetic nephropathy. We studied the expression of angiotensinogen, angiotensin type 1 receptor, transforming growth factor-beta1, and connective tissue growth factor in mice with GSD-Ia and found them to be elevated compared to controls. While increased renal expression of angiotensinogen was evident in 2-week-old mice with GSD-Ia, the renal expression of transforming growth factor-beta and connective tissue growth factor did not increase for another week; consistent with upregulation of these factors by angiotensin II. The expression of fibronectin and collagens I, III, and IV was also elevated in the kidneys of mice with GSD-Ia, compared to controls. Renal fibrosis was characterized by a marked increase in the synthesis and deposition of extracellular matrix proteins in the renal cortex and histological abnormalities including tubular basement membrane thickening, tubular atrophy, tubular dilation, and multifocal interstitial fibrosis. Our results suggest that activation of the angiotensin system has an important role in the pathophysiology of renal disease in patients with GSD-Ia.

Glucose-6-phosphate transporter gene therapy corrects metabolic and myeloid abnormalities in glycogen storage disease type Ib mice

Glycogen storage disease type Ib (GSD-Ib) is caused by a deficiency in the glucose-6-phosphate transporter (G6PT), an endoplasmic reticulum-associated transmembrane protein that is ubiquitously expressed. GSD-Ib patients suffer from disturbed glucose homeostasis and myeloid dysfunctions. To evaluate the feasibility of gene replacement therapy for GSD-Ib, we have infused adenoviral (Ad) vector containing human G6PT (Ad-hG6PT) into G6PT-deficient (G6PT(-/-)) mice that manifest symptoms characteristics of the human disorder. Ad-hG6PT infusion restores significant levels of G6PT mRNA expression in the liver, bone marrow and spleen, and corrects metabolic as well as myeloid abnormalities in G6PT(-/-) mice. The G6PT(-/-) mice receiving gene therapy exhibit improved growth; normalized serum profiles for glucose, cholesterol, triglyceride, uric acid and lactic acid; and reduced hepatic glycogen deposition. The therapy also corrects neutropenia and lowers the elevated serum levels of granulocyte colony-stimulating factor. The development of bone and spleen in the infused G6PT(-/-) mice is improved and accompanied by increased cellularity and normalized myeloid progenitor cell frequencies in both tissues. This effective use of gene therapy to correct metabolic imbalances and myeloid dysfunctions in GSD-Ib mice holds promise for the future of gene therapy in humans.

Long-term correction of murine glycogen storage disease type Ia by recombinant adeno-associated virus-1-mediated gene transfer

Glycogen storage disease type Ia (GSD-Ia) is caused by a deficiency in glucose-6-phosphatase-alpha (G6Pase-alpha), a nine-transmembrane domain, endoplasmic reticulum-associated protein expressed primarily in the liver and kidney. Previously, we showed that infusion of an adeno-associated virus (AAV) serotype 2 vector carrying murine G6Pase-alpha (AAV2-G6Pase-alpha) into neonatal GSD-Ia mice failed to sustain their life beyond weaning. We now show that neonatal infusion of GSD-Ia mice with an AAV serotype 1-G6Pase-alpha (AAV1-G6Pase-alpha) or AAV serotype 8-G6Pase-alpha (AAV8-G6Pase-alpha) results in hepatic expression of the G6Pase-alpha transgene and markedly improves the survival of the mice. However, only AAV1-G6Pase-alpha can achieve significant renal transgene expression. A more effective strategy, in which a neonatal AAV1-G6Pase-alpha infusion is followed by a second infusion at age one week, provides sustained expression of a complete, functional, G6Pase-alpha system in both the liver and kidney and corrects the metabolic abnormalities in GSD-Ia mice for the 57 week length of the study. This effective use of gene therapy to correct metabolic imbalances and disease progression in GSD-Ia mice holds promise for the future of gene therapy in humans.

In islet-specific glucose-6-phosphatase-related protein, the beta cell antigenic sequence that is targeted in diabetes is not responsible for the loss of phosphohydrolase activity

AIMS/HYPOTHESIS

There are three members of the glucose-6-phosphatase (G6Pase) family: (1) the liver/kidney/intestine G6Pase-alpha (encoded by G6PC), which is a key enzyme in glucose homeostasis; (2) the ubiquitous G6Pase-beta (encoded by G6PC3); and (3) the islet-specific G6Pase-related protein (IGRP, encoded by /G6PC2). While G6Pase-alpha and G6Pase-beta are functional glucose-6-phosphate hydrolases, IGRP possesses almost no hydrolase activity. This was unexpected since G6Pase-alpha is more closely related to IGRP than G6Pase-beta. Recently, amino acids 206-214 in IGRP were identified as a beta cell antigen targeted by a prevalent population of pathogenic CD8+ T cells in autoimmune diabetes, suggesting that this peptide confers functional specificity to IGRP. We therefore investigated the molecular events that inactivate IGRP activity and the effects of the beta cell antigen sequence on the stability and enzymatic activity of G6Pase-alpha.

METHODS

Studies were performed using site-directed mutagenesis and transient expression assays. Protein stability was evaluated by Western blotting, proteasome inhibitor studies and in vitro transcription-translation.

RESULTS

We showed that the residues responsible for G6Pase activity are more extensive than previously recognised. Introducing the IGRP antigenic motif into G6Pase-alpha does not completely destroy activity, although it does destabilise the protein. The low hydrolytic activity in IGRP is due to the combination of multiple independent mutations.

CONCLUSIONS/INTERPRETATION

The loss of catalytic activity in IGRP arises from the sum of many sequence differences. G6Pase-alpha mutants containing the beta cell antigen sequence are preferentially degraded in cells, which prevents targeting by pathogenic CD8+ T cells. It is possible that IGRP levels in beta cells could dictate susceptibilities to diabetes.

Newborn liver gene transfer by an HIV-2-based lentiviral vector

Newborn gene therapy, because it can prevent the damage caused by the onset of a disease, deserves specific attention. To evaluate gene transfer in tissues of newborn mice, we used a human immunodeficiency virus (HIV)-2 based lentiviral vector pseudotyped with vesicular stomatitis virus G glycoprotein expressing the green fluorescent protein reporter gene under the control of the cytomegalovirus promoter. We found that very low doses of HIV-2 could infect and be expressed in newborn mice. Under these conditions, the virus was preferentially expressed in the liver and hepatocytes were the predominant target. The treatment was not toxic, the infected liver cells proliferated and the transduced gene was stably expressed. Adult mice could be infected by HIV-2, but the vector was detected in the liver only utilizing the sensitive method of polymerase chain reaction coupled with Southern blot. Direct comparison between newborn and adult recipients demonstrated a much greater efficiency of liver transduction in the newborn mouse. These results indicate that the combination of early intervention and low multiplicity of infection may be a strategy for preferentially and efficiently targeting newborn liver for gene therapy applications.

Immunohistochemical diagnosis of mouse hepatitis virus and mycoplasma pulmonis infection with murine antiserum

This study established a modified alkaline phosphatase-labelled avidin-biotin-complex (ABC-AP) method for diagnosis of mouse hepatitis virus (MHV) and Mycoplasma pulmonis infection from formalin-fixed, paraffin wax-embedded sections, murine antibody-positive serum being used as the primary reagent. With this method, MHV antigen in cAnNCrj.Cg-Foxn1(nu)/Foxn1(nu) mice and M. pulmonis antigen in Wistar rats were immunolabelled in tissue sections. MHV antigen was clearly detected in samples of liver, stomach, caecal and colonic mucosa, and spleen. M. pulmonis antigen was demonstrated on the luminal surface of bronchiolar epithelial cells. This method may prove useful in diagnosis when commercial antisera are unavailable or when immunosuppression prevents serological diagnosis.

Monitoring the correction of glycogen storage disease type 1a in a mouse model using [(18)F]FDG and a dedicated animal scanner

Monitoring gene therapy of glycogen storage disease type 1a in a mouse model was achieved using [(18)F]FDG and a dedicated animal scanner. The G6Pase knockout (KO) mice were compared to the same mice after infusion with a recombinant adenovirus containing the murine G6Pase gene (Ad-mG6Pase). Serial images of the same mouse before and after therapy were obtained and compared with wild-type (WT) mice of the same strain to determine the uptake and retention of [(18)F]FDG in the liver. Image data were acquired from heart, blood pool and liver for twenty minutes after injection of [(18)F]FDG. The retention of [(18)F]FDG was lower for the WT mice compared to the KO mice. The mice treated with adenovirus-mediated gene therapy had retention similar to that found in age-matched WT mice. These studies show that FDG can be used to monitor the G6Pase concentration in liver of WT mice as compared to G6Pase KO mice. In these mice, gene therapy returned the liver function to that found in age matched WT controls as measured by the FDG kinetics in the liver compared to that found in age matched wild type controls.

Discriminant responses of the catalytic unit and glucose 6-phosphate transporter components of the hepatic glucose-6-phosphatase system in Ehrlich ascites-tumor-bearing mice

The effect of Ehrlich ascites tumor cells, in vivo, on the hepatic glucose-6-phosphatase (G6Pase) system was examined. The V(max) for glucose 6-phosphate hydrolysis by G6Pase was reduced by 40% and a greater than 15-fold decrease in mRNA encoding the catalytic unit of the G6Pase system was observed 8 days after injection with tumor cells. Blood glucose concentration was decreased from 169 +/- 17 to 105 +/- 9 mg/dl in tumor-bearing mice. There was no change in the G6P transporter (G6PT) mRNA level. However, there was a significant decrease in G6P accumulation into hepatic microsomal vesicles derived from tumor-bearing mice. Decreased G6P accumulation was also associated with a decrease in G6Pase hydrolytic activity in the presence of vanadate, a potent catalytic-unit inhibitor. In addition, G6P accumulation was nearly abolished in microsomes treated with N-bromoacetylethanolamine phosphate, an irreversible inhibitor of the G6PT. These results demonstrate that the catalytic unit and G6PT components of the G6Pase system can be discriminantly regulated, and that microsomal glucose 6-phosphate uptake is dependent on catalytic unit activity as well as G6PT action.

Glucocorticoids activate transcription of the gene for the glucose-6-phosphate transporter, deficient in glycogen storage disease type 1b

Deficiencies in the glucose-6-phosphate transporter (G6PT) cause glycogen storage disease type 1b (GSD-1b), a heritable metabolic disorder. The G6PT protein translocates glucose-6-phosphate from the cytoplasm to the lumen of the endoplasmic reticulum, where glucose-6-phosphatase metabolizes it to glucose and phosphate. Therefore, G6PT and glucose-6-phosphatase work in concert to maintain glucose homeostasis. To delineate the control of G6PT gene expression, we first demonstrated that transcription of the gene requires hepatocyte nuclear factor 1alpha. Consequently, hepatocyte nuclear factor 1alpha-null mice manifest a G6PT deficiency like that of GSD-1b patients. In this study, we delineated the role of glucocorticoids in the transcription of the G6PT gene. We showed that the basal G6PT promoter is contained within nucleotides -369 to -1 upstream of the translation start site, which contains three activation elements. Further, we demonstrated that glucocorticoids activate G6PT transcription and that glucocorticoid action is mediated through a glucocorticoid response element within activation element-2 of the promoter. Taken together, the results suggest that glucocorticoids play a pivotal role in regulating the G6PT gene.

A molecular link between the common phenotypes of type 1 glycogen storage disease and HNF1alpha-null mice

The clinical manifestations of type 1 glycogen storage disease (GSD-1) in patients deficient in the glucose-6-phosphatase (G6Pase) system (e.g. growth retardation, hepatomegaly, hyperlipidemia, and renal dysfunction) are shared by Hnf1alpha(-/-) mice deficient of a transcriptional activator, hepatocyte nuclear factor 1alpha (HNF1alpha). However, the molecular mechanism is unknown. The G6Pase system, essential for the maintenance of glucose homeostasis, is comprised of glucose 6-phosphate transporter (G6PT) and G6Pase. G6PT translocates G6P from the cytoplasm to the lumen of the endoplasmic reticulum where it is metabolized by G6Pase to glucose and phosphate. Deficiencies in G6Pase and G6PT cause GSD-1a and GSD-1b, respectively. Hnf1alpha(-/-) mice also develop noninsulin-dependent diabetes mellitus caused by defective insulin secretion. In this study, we sought to determine whether there is a molecular link between HNF1alpha deficiency and function of the G6Pase system. Transactivation studies revealed that HNF1alpha is required for transcription of the G6PT gene. Hepatic G6PT mRNA levels and microsomal G6P transport activity are also markedly reduced in Hnf1alpha(-/-) mice as compared with Hnf1alpha(+/+) and Hnf1alpha(+/-) littermates. On the other hand, hepatic G6Pase mRNA expression and activity are up-regulated in Hnf1alpha(-/-) mice, consistent with observations that G6Pase expression is increased in diabetic animals. Taken together, the results strongly suggest that metabolic abnormalities in HNF1alpha-null mice are caused in part by G6PT deficiency and by perturbations of the G6Pase system.

Molecular characterization of murine pancreatic phospholipase A(2)

The pancreatic secretory phospholipase A(2) (sPLA(2)IB) is considered to be a digestive enzyme, although it has several important receptor-mediated functions. In this study, using the newly isolated murine sPLA(2)IB cDNA clone as a probe, we demonstrate that in addition to the pancreas, the sPLA(2)IB mRNA was expressed in extrapancreatic organs such as the liver, spleen, duodenum, colon, and lungs. We also demonstrate that sPLA(2)IB mRNA expression was detectable from the 17(th) day of gestation in the developing mouse fetus, coinciding with the time of completion of differentiation of the pancreas. Furthermore, the mRNA expression pattern of sPLA(2)IB was distinct from those of sPLA(2)IIA and cPLA(2) in various tissues examined. The murine sPLA(2)IB gene structure is well conserved, consistent with findings in other mammalian species, and this gene mapped to the region of mouse chromosome 5F1-G1.1. Taken together, our results suggest that sPLA(2)IB plays important roles both in the pancreas and in extrapancreatic tissues and that in the mouse, its expression is developmentally regulated.

The molecular basis of type 1 glycogen storage diseases

Glycogen storage disease type 1 (GSD-1), also known as von Gierke disease, is a group of autosomal recessive metabolic disorders caused by deficiencies in the activity of the glucose-6-phosphatase (G6Pase) system that consists of at least two membrane proteins, glucose-6-phosphate transporter (G6PT) and G6Pase. G6PT translocates glucose-6-phosphate (G6P) from cytoplasm to the lumen of the endoplasmic reticulum (ER) and G6Pase catalyzes the hydrolysis of G6P to produce glucose and phosphate. Therefore, G6PT and G6Pase work in concert to maintain glucose homeostasis. Deficiencies in G6Pase and G6PT cause GSD-1a and GSD-1b, respectively. Both manifest functional G6Pase deficiency characterized by growth retardation, hypoglycemia, hepatomegaly, kidney enlargement, hyperlipidemia, hyperuricemia, and lactic acidemia. GSD-1b patients also suffer from chronic neutropenia and functional deficiencies of neutrophils and monocytes, resulting in recurrent bacterial infections as well as ulceration of the oral and intestinal mucosa. The G6Pase gene maps to chromosome 17q21 and encodes a 36-kDa glycoprotein that is anchored to the ER by 9 transmembrane helices with its active site facing the lumen. Animal models of GSD-1a have been developed and are being exploited to delineate the disease more precisely and to develop new therapies. The G6PT gene maps to chromosome 11q23 and encodes a 37-kDa protein that is anchored to the ER by 10 transmembrane helices. A functional assay for the recombinant G6PT protein has been established, which showed that G6PT functions as a G6P transporter in the absence of G6Pase. However, microsomal G6P uptake activity was markedly enhanced in the simultaneous presence of G6PT and G6Pase. The cloning of the G6PT gene now permits animal models of GSD-1b to be generated. These recent developments are increasing our understanding of the GSD-l disorders and the G6Pase system, knowledge that will facilitate the development of novel therapeutic approaches for these disorders.

Structural requirements for the stability and microsomal transport activity of the human glucose 6-phosphate transporter

Deficiencies in glucose 6-phosphate (G6P) transporter (G6PT), a 10-helical endoplasmic reticulum transmembrane protein of 429 amino acids, cause glycogen storage disease type 1b. To date, only three missense mutations in G6PT have been shown to abolish microsomal G6P transport activity. Here, we report the results of structure-function studies on human G6PT and demonstrate that 15 missense mutations and a codon deletion (delta F93) mutation abolish microsomal G6P uptake activity and that two splicing mutations cause exon skipping. While most missense mutants support the synthesis of G6PT protein similar to that of the wild-type transporter, immunoblot analysis shows that G20D, delta F93, and I278N mutations, located in helix 1, 2, and 6, respectively, destabilize the G6PT. Further, we demonstrate that G6PT mutants lacking an intact helix 10 are misfolded and undergo degradation within cells. Moreover, amino acids 415-417 in the cytoplasmic tail of the carboxyl-domain, extending from helix 10, also play a critical role in the correct folding of the transporter. However, the last 12 amino acids of the cytoplasmic tail play no essential role(s) in functional integrity of the G6PT. Our results, for the first time, elucidate the structural requirements for the stability and transport activity of the G6PT protein.

Human variant glucose-6-phosphate transporter is active in microsomal transport

Glycogen storage disease type lb (GSD-lb) is caused by deficiencies in the glucose-6-phosphate transporter (G6PT), which works together with glucose-6-phosphatase to maintain glucose homeostasis. In humans, there are two alternatively spliced transcripts, G6PT and variant G6PT (vG6PT), differing by the inclusion of a 66-bp exon-7 sequence in vG6PT. We have previously shown that the G6PT protein functions as a microsomal glucose-6-phosphate (G6P) transporter, which is anchored to the endoplasmic reticulum by ten transmembrane helices. Here, we demonstrate that vG6PT is also active in microsomal G6P transport. The additional 22 amino acids in vG6PT is predicted to constitute a part of the luminal loop 4. Our data indicate that this loop plays no vital role in microsomal G6P transport. Further, we show that G6PT mRNA is expressed in all organs and tissues examined, but that the vG6PT transcript is expressed exclusively in the brain, heart, and skeletal muscle. These results raise the possibility that mutations in exon-7 of the G6PT gene, which would not perturb glucose homeostasis, might have other deleterious effects.

Cellular release of [18F]2-fluoro-2-deoxyglucose as a function of the glucose-6-phosphatase enzyme system

[(18)F]-2-Fluoro-2-deoxyglucose (FDG) is a glucose analog currently utilized for positron emission tomography imaging studies in humans. FDG taken up by the liver is rapidly released. This property is attributed to elevated glucose-6-phosphatase (Glc-6-Pase) activity. To characterize this issue we studied the relationship between Glc-6-Pase activity and FDG release kinetics in a cell culture system. We overexpressed the Glc-6-Pase catalytic unit in a Glc-6-Pase-deficient mouse hepatocyte (Ho-15) and in A431 tumor cell lines. Glc-6-Pase enzyme activity and FDG release rates were determined in cells transfected with the Glc-6-Pase gene (Ho-15-D3 and A431-AC3), in mock-transfected cells of both cell lines, and in wild-type mouse hepatocytes (WT10) as control. Although the highest level of Glc-6-Pase activity was measured in A431-AC3, Ho-15-D3 cells showed much faster FDG release rates. The faster FDG release correlated with the level of glucose 6-phosphate transporter (Glc-6-PT) mRNA, which was found to be expressed at higher levels in Ho-15 compared with A431 cells. Overexpression of Glc-6-PT in A431-AC3 produced a dramatic increase in FDG release compared with control cells. This study gives the first direct evidence that activity of the Glc-6-Pase complex can be quantified in vivo by measuring FDG release. Adequate levels of Glc-6-Pase catalytic unit and Glc-6-PT are required for this function. FDG-positron emission tomography may be utilized to evaluate functional status of the Glc-6-Pase complex.

Correction of glycogen storage disease type 1a in a mouse model by gene therapy

Glycogen storage disease type 1a (GSD-1a), characterized by hypoglycemia, liver and kidney enlargement, growth retardation, hyperlipidemia, and hyperuricemia, is caused by a deficiency in glucose-6-phosphatase (G6Pase), a key enzyme in glucose homeostasis. To evaluate the feasibility of gene replacement therapy for GSD-1a, we have infused adenoviral vector containing the murine G6Pase gene (Ad-mG6Pase) into G6Pase-deficient (G6Pase(-/-)) mice that manifest symptoms characteristic of human GSD-1a. Whereas <15% of G6Pase(-/-) mice under glucose therapy survived weaning, a 100% survival rate was achieved when G6Pase(-/-) mice were infused with Ad-mG6Pase, 90% of which lived to 3 months of age. Hepatic G6Pase activity in Ad-mG6Pase-infused mice was restored to 19% of that in G6Pase(+/+) mice at 7-14 days post-infusion; the activity persisted for at least 70 days. Ad-mG6Pase infusion also greatly improved growth of G6Pase(-/-) mice and normalized plasma glucose, cholesterol, triglyceride, and uric acid profiles. Furthermore, liver and kidney enlargement was less pronounced with near-normal levels of glycogen depositions in both organs. Our data demonstrate that a single administration of a recombinant adenoviral vector can alleviate the pathological manifestations of GSD-1a in mice, suggesting that this disorder in humans can potentially be corrected by gene therapy.

Molecular genetics of hepatic methionine adenosyltransferase deficiency

Hepatic methionine adenosyltransferase (MAT) deficiency is caused by mutations in the human MAT1A gene that abolish or reduce hepatic MAT activity that catalyzes the synthesis of S-adenosylmethionine from methionine and ATP. This genetic disorder is characterized by isolated persistent hypermethioninemia in the absence of cystathionine beta-synthase deficiency, tyrosinemia, or liver disease. Depending on the nature of the genetic defect, hepatic MAT deficiency can be transmitted either as an autosomal recessive or dominant trait. Genetic analyses have revealed that mutations identified in the MAT1A gene only partially inactivate enzymatic activity, which is consistent with the fact that most hepatic MAT-deficient individuals are clinically well. Two hypermethioninemic individuals with null MAT1A mutations have developed neurological problems, including brain demyelination, although this correlation is by no means absolute. Presently, it is recommended that a DNA-based diagnosis should be performed for isolated hypermethioninemic individuals with unusually high plasma methionine levels to assess if therapy aimed at the prevention of neurological manifestations is warranted.

Molecular characterization of Plasmodium falciparum S-adenosylmethionine synthetase

S-Adenosylmethionine (AdoMet) synthetase (SAMS: EC 2.5.1.6) catalyses the formation of AdoMet from methionine and ATP. We have cloned a gene for Plasmodium falciparum AdoMet synthetase (PfSAMS) (GenBank accession no. AF097923), consisting of 1209 base pairs with no introns. The gene encodes a polypeptide (PfSAMS) of 402 amino acids with a molecular mass of 44844 Da, and has an overall base composition of 67% A+T. PfSAMS is probably a single copy gene, and was mapped to chromosome 9. The PfSAMS protein is highly homologous to all other SAMS, including a conserved motif for the phosphate-binding P-loop, HGGGAFSGKD, and the signature hexapeptide, GAGDQG. All the active-site amino acids for the binding of ADP, P(i) and metal ions are similarly preserved, matching entirely those of human hepatic SAMS and Escherichia coli SAMS. Molecular modelling of PfSAMS guided by the X-ray crystal structure of E. coli SAMS indicates that PfSAMS binds ATP/Mg(2+) in a manner similar to that seen in the E. coli SAMS structure. However, the PfSAMS model shows that it can not form tetramers as does E. coli SAMS, and is probably a dimer instead. There was a differential sensitivity towards the inhibition by cycloleucine between the expressed PfSAMS and the human hepatic SAMS with K(i) values of 17 and 10 mM, respectively. Based on phylogenetic analysis using protein parsimony and neighbour-joining algorithms, the malarial PfSAMS is closely related to SAMS of other protozoans and plants.

Type-1c glycogen storage disease is not caused by mutations in the glucose-6-phosphate transporter gene

Glycogen storage disease type 1 (GSD-1) is a group of autosomal recessive disorders caused by deficiencies in glucose-6-phosphatase (G6Pase) and the associated substrate/product transporters. Molecular genetic studies have demonstrated that GSD-1a and GSD-1b are caused by mutations in the G6Pase enzyme and a glucose-6-phosphate transporter (G6PT), respectively. While kinetic studies of G6Pase catalysis predict that the index GSD-1c patient is deficient in a pyrophosphate/phosphate transporter, the existence of a separate locus for GSD-1c remains unclear. We have previously shown that the G6Pase gene of the index GSD-1c patient is intact; we now show that the G6PT gene of this patient is normal, strongly suggesting the existence of a distinct GSD-1c locus.

Transmembrane topology of human glucose 6-phosphate transporter

Glycogen storage disease type 1b is caused by a deficiency in a glucose 6-phosphate transporter (G6PT) that translocates glucose 6-phosphate from the cytoplasm to the endoplasmic reticulum lumen where the active site of glucose 6-phosphatase is situated. Using amino- and carboxyl-terminal tagged G6PT, we demonstrate that proteolytic digestion of intact microsomes resulted in the cleavage of both tags, indicating that both termini of G6PT face the cytoplasm. This is consistent with ten and twelve transmembrane domain models for G6PT predicted by hydropathy analyses. A region of G6PT corresponding to amino acid residues 50-71, which constitute a transmembrane segment in the twelve-domain model, are situated in a 51-residue luminal loop in the ten-domain model. To determine which of these two models is correct, we generated two G6PT mutants, T53N and S55N, that created a potential Asn-linked glycosylation site at residues 53-55 (N53SS) or 55-57 (N55QS), respectively. N53SS or N55QS would be glycosylated only if it is situated in a luminal loop larger than 33 residues as predicted by the ten-domain model. Whereas wild-type G6PT is not a glycoprotein, both T53N and S55N mutants are glycosylated, strongly supporting the ten-helical model for G6PT.

Pregnancy-specific glycoprotein gene expression in recurrent aborters: a potential correlation to interleukin-10 expression

PROBLEM

The expression of the pregnancy-specific glycoprotein (PSG) genes in the uterine endometrium of women experiencing recurrent first-trimester abortions, and potential correlations to cytokine expression were examined.

METHOD OF STUDY

Endometrial RNA, isolated from women with a history of either repetitive first-trimester pregnancy losses or uncomplicated pregnancies, was isolated and analyzed for PSG transcripts by the reverse transcriptase-polymerase chain reaction method. PSG genes showing different patterns of expression were expressed in baculovirus, and the purified proteins examined for their effects on cytokine expression.

RESULTS

The expression of PSG11 in the endometria of recurrent aborters was significantly lower than in that of controls (P < 0.01). When tested on monocytes, PSG11 stimulated secretion of interleukin (IL)-10.

CONCLUSIONS

The level of expression of the PSG11 gene in the uterine endometrium, during the peri-implantation period, correlates with the risk of pregnancy loss in some women experiencing recurrent spontaneous abortions. The ability of PSG11 to influence the secretion of IL-10 suggests that PSG11 may contribute to the local modulation of the inflammatory T helper-1 response seen in the endometrium of these women.

Inactivation of the glucose 6-phosphate transporter causes glycogen storage disease type 1b

Glycogen storage disease type 1b (GSD-1b) is proposed to be caused by a deficiency in microsomal glucose 6-phosphate (G6P) transport, causing a loss of glucose-6-phosphatase activity and glucose homeostasis. However, for decades, this disorder has defied molecular characterization. In this study, we characterize the structural organization of the G6P transporter gene and identify mutations in the gene that segregate with the GSD-1b disorder. We report the functional characterization of the recombinant G6P transporter and demonstrate that mutations uncovered in GSD-1b patients disrupt G6P transport. Our results, for the first time, define a molecular basis for functional deficiency in GSD-1b and raise the possibility that the defective G6P transporter contributes to neutropenia and neutrophil/monocyte dysfunctions characteristic of GSD-1b patients.

Cloning and characterization of cDNAs encoding a candidate glycogen storage disease type 1b protein in rodents

Glycogen storage disease type 1 (GSD-1) is a group of genetic disorders caused by a deficiency in the activity of the enzyme glucose-6-phosphatase. (G6Pase). GSD-1a and GSD-1b, the two major subgroups, have been confirmed at the molecular genetic level. The gene responsible for GSD-1b maps to human chromosome 11q23 and a candidate human GSD-1b cDNA that encodes a microsomal transmembrane protein has been identified. In this study, we show that this cDNA maps to chromosome 11q23; thus it is a strong candidate for GSD-1b. Furthermore, we isolated and characterized candidate murine and rat GSD-1b cDNAs. Both encode transmembrane proteins sharing 93-95% sequence homology to the human GSD-1b protein. The expression profiles of murine GSD-1b and G6Pase differ both in the liver and in the kidney; the GSD-1b transcript appears before the G6Pase mRNA during development. In addition to G6Pase deficiency, GSD-1b patients suffer neutropenia, neutrophil dysfunction, and recurrent bacterial infections. Interestingly, although the G6Pase mRNA is expressed primarily in the liver, kidney, and intestine, the GSD-1b mRNA is expressed in numerous tissues, including human neutrophils/monocytes.

Effect of glucocorticoids on the mouse methionine adenosyltransferase A1 gene expression, which is regulated by two promoters

The methionine adenosyltransferase A1 (MATA1) gene encodes the hepatic forms of the enzyme MAT I and III. To determine the molecular mechanisms that regulate MATA1 gene expression, we characterized promoters and the 5'-flanking sequence of MATA1. Transient expression assays demonstrated the presence of two promoters for the MATA1 gene. The p1 promoter is contained in the -57 to -2 nucleotide region, and gives rise to the P1 transcript initiated at +1. The p2 promoter is contained in the -248 to -146 nucleotide region. The -229 to -213 nucleotide region of the MATA1 gene, which contains an Ets-binding-site sequence, was necessary for p2 promoter activity. Sequence analysis of 5'-RACE products indicated that there was a transcript (P2) initiated at -156. The -107 to +145 nucleotide region is missing from the mature P2 transcript, which suggests that the -107 to +145 nucleotide sequence is an intron of the P2 transcript. The p2 promoter may give rise to the P2 transcript. The p1 promoter activity was increased by glucocorticoids, but the p2 promoter activity was not affected by glucocorticoids.

Hepatocyte nuclear factor 1alpha is an accessory factor required for activation of glucose-6-phosphatase gene transcription by glucocorticoids

Deficiency of glucose-6-phosphatase (G6Pase), a key enzyme in glucose homeostasis, causes glycogen storage disease type 1a (GSD-1a), also know as von Gierke disease. Expression of the G6Pase gene is regulated by multiple hormones, including glucocorticoids. The synthetic glucocorticoid dexamethasone increased G6Pase mRNA abundance and gene transcription in H4-IIE hepatoma cells. Transient transfection assays demonstrated that the G6Pase promoter was active in H4-IIE cells only in the presence of dexamethasone. The minimal G6Pase promoter was contained within nucleotides -234/+3, which has two putative glucocorticoid response elements (GREs) at nucleotides -178/-164 (site 1) and -154/-140 (site 2). Electromobility shift and transient transfection assays showed that only GRE site 1 was required for glucocorticoid-activated transcription from the G6Pase promoter. Deletion analysis demonstrated that the DNA elements absolutely essential for glucocorticoid-stimulated transcription from the G6Pase promoter were contained within nucleotides -234/-212, encompassing binding motifs for hepatocyte nuclear factors (HNFs) 1 (-226/-212) and 4 (-231/-220). Electromobility shift and cotransfection assays showed that HNF1alpha bound to its cognate site and mediated transcription activation of the G6Pase gene by glucocorticoids.

Ontogeny of the murine glucose-6-phosphatase system

A deficiency in microsomal glucose-6-phosphatase (G6Pase) activity causes glycogen storage disease type 1 (GSD-1), a clinically and biochemically heterogeneous group of diseases. It has been suggested that catalysis by G6Pase involves multiple components, with defects in the G6Pase catalytic unit causing GSD-1a and defects in the putative substrate and product translocases causing GSD-1b, 1c, and 1d. However, this model is open to debate. To elucidate the G6Pase system, we have examined G6Pase mRNA expression, G6Pase activity, and glucose 6-phosphate (G6P) transport activity in the murine liver and kidney during normal development. In the liver, G6Pase mRNA and enzymatic activity were detected at 18 days gestation and increased markedly at parturition, before leveling off to adult levels. In the kidney, G6Pase mRNA and enzymatic activity appeared at 19 days gestation and peaked at weaning, suggesting that kidney G6Pase may have a different metabolic role. In situ hybridization analysis demonstrated that, in addition to the liver and kidney, the intestine expressed G6Pase. Despite the expression of G6Pase in the embryonic liver, microsomal G6P transport activity was not detectable until birth, peaking at about age 4 weeks. Our study strongly supports the multicomponent model for the G6Pase system.

Asparagine-linked oligosaccharides are localized to a luminal hydrophilic loop in human glucose-6-phosphatase

Deficiency of glucose-6-phosphatase (G6Pase), an endoplasmic reticulum transmembrane glycoprotein, causes glycogen storage disease type 1a. We have recently shown that human G6Pase contains an odd number of transmembrane segments, supporting a nine-transmembrane helical model for this enzyme. Sequence analysis predicts the presence of three potential asparagine (N)-linked glycosylation sites, N96TS, N203AS, and N276SS, conserved among mammalian G6Pases. According to this model, Asn96, located in a 37-residue luminal loop, is a potential acceptor for oligosaccharides, whereas Asn203 and Asn276, located in a 12-residue cytoplasmic loop and helix 7, respectively, would not be utilized for this purpose. We therefore characterized mutant G6Pases lacking one, two, or all three potential N-linked glycosylation sites. Western blot and in vitro translation studies showed that G6Pase is glycosylated only at Asn96, further validating the nine-transmembrane topology model. Substituting Asn96 with an Ala (N96A) moderately reduced enzymatic activity and had no effect on G6Pase synthesis or degradation, suggesting that oligosaccharide chains do not play a major role in protecting the enzyme from proteolytic degradation. In contrast, mutation of Asn276 to an Ala (N276A) destabilized the enzyme and markedly reduced enzymatic activity. We present additional evidence suggesting that the integrity of transmembrane helices is essential for G6Pase stability and catalytic activity.

Establishment and characterization of a simian virus 40-transformed temperature-sensitive rat luteal cell line

The primary culture of rat luteal cells and their long-term maintenance have been difficult. Low cellular yields have limited the possibility for the study of gene regulation in luteal cells. The goal of this study was to develop a cell line to serve as a model by which to study the expression and regulation of various genes specific to luteal cells. We attempted to develop a luteal cell line by transformation of large luteal cells through infection with a temperature-sensitive simian virus (SV-40 tsA209) mutant that has a temperature-sensitive mutation required for the maintenance of cell transformation. We report here the successful establishment of such a cell line, designated GG-CL cells. Large luteal cells were purified to homogeneity by flow cytometry from corpora lutea of day 14 pregnant rats, cultured for 24 h, and then infected with the SV-40 tsA209 mutant virus. Transformed cells were maintained at the permissive temperature (33 C) until colonies were identified. Several colonies of transformed cells were isolated and passaged. They multiplied at 33 C and formed multilayers. At the nonpermissive temperature (40 C), cells reverted to the normal differentiated phenotype similar to the primary luteal cells in culture. To determine whether GG-CL cells express the genes found in normal luteal cells, messenger RNA (mRNA) expression was examined by either Northern analysis or RT-PCR with primers specific to each mRNA. GG-CL cells were found to express receptors for interleukin-6 and glucocorticoid, as well as the newly discovered estrogen receptor-beta (ER-beta) and the orphan nuclear receptor nur 77. No receptors for ER-alpha, progesterone, LH, or PRL could be detected. This cell line also expressed 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD), but not cholesterol side-chain cleavage cytochrome P450 (P450scc), 3beta-hydroxysteroid dehydrogenase, or aromatase cytochrome P450 (P450arom). Although the cells did not express the PRL receptor, they did express Janus kinase (JAK2) and signal transducers and activators of transcription (Stat5b), and, when transfected with the PRL receptor, they responded to PRL with a marked inhibition in 20alpha-HSD mRNA expression. In addition, estradiol enhanced ER-beta expression in a dose-dependent manner whereas cAMP stimulation caused a marked and rapid increase in the expression of the orphan receptor nur 77. In summary, a temperature-sensitive cell line was successfully established from the large luteal cells of rat corpora lutea. These cells express key genes encoding enzymes and receptors inherent to this defined luteal cell population and respond to stimulation by PRL, estradiol, and cAMP.

Immortalization and characterization of bone marrow stromal fibroblasts from a patient with a loss of function mutation in the estrogen receptor-alpha gene

A male patient with abnormal postpubertal bone elongation was shown earlier to have a mutation in both alleles of the estrogen receptor, resulting in a nonfunctional gene. Marrow stromal fibroblasts (MSFs) derived from this patient were called HERKOs (human estrogen receptor knock outs), and in order to obtain continuous HERKO cell lines, they were immortalized using a recombinant adenovirus-origin-minus SV40 virus. MSFs are unique cells because they support hematopoesis and contain a mixed population of precursor cells for bone, cartilage, and fat. Three established cell lines (HERKO2, HERKO4, and HERKO7) were characterized and compared with the heterogeneous population of nonimmortalized HERKOs for their osteogenic potential. We performed Northern analysis of matrix genes implicated in bone development and metabolism and an in vivo bone formation assay by transplanting the cells subcutaneously into immunodeficient mice. All three HERKO lines expressed high amounts of collagen 1A1, osteopontin, osteonectin, fibronectin, decorin, biglycan, and alkaline phosphatase. Except for osteopontin, expression of these genes was slightly lower compared with nonimmortalized HERKOs. In the in vivo bone formation assay, the heterogeneous population of nonimmortalized HERKOs formed bone with high efficiency, while the HERKO lines induced a high-density, bone-like matrix. Finally, all HERKO cell types secreted high levels of insulin-like growth factor I and interleukin-6 into the culture medium relative to cells of normal human subjects. In summary, these lines of HERKO cells retain several of the phenotypic traits of MSFs after immortalization, including matrix and cytokine production, and provide a valuable source of a unique human material for future studies involving estrogen action in bone and bone marrow metabolism.

Transmembrane topology of glucose-6-phosphatase

Deficiency of microsomal glucose-6-phosphatase (G6Pase), the key enzyme in glucose homeostasis, causes glycogen storage disease type 1a, an autosomal recessive disorder. Characterization of the transmembrane topology of G6Pase should facilitate the identification of amino acid residues contributing to the active site and broaden our understanding of the effects of mutations that cause glycogen storage disease type 1a. Using N- and C-terminal tagged G6Pase, we show that in intact microsomes, the N terminus is resistant to protease digestion, whereas the C terminus is sensitive to such treatment. Our results demonstrate that G6Pase possesses an odd number of transmembrane helices, with its N and C termini facing the endoplasmic reticulum lumen and the cytoplasm, respectively. During catalysis, a phosphoryl-enzyme intermediate is formed, and the phosphoryl acceptor in G6Pase is a His residue. Sequence alignment suggests that mammalian G6Pases, lipid phosphatases, acid phosphatases, and a vanadium-containing chloroperoxidase (whose tertiary structure is known) share a conserved phosphatase motif. Active-site alignment of the vanadium-containing chloroperoxidase and G6Pases predicts that Arg-83, His-119, and His-176 in G6Pase contribute to the active site and that His-176 is the residue that covalently binds the phosphoryl moiety during catalysis. This alignment also predicts that Arg-83, His-119, and His-176 reside on the same side of the endoplasmic reticulum membrane, which is supported by the recently predicted nine-transmembrane helical model for G6Pase. We have previously shown that Arg-83 is involved in positioning the phosphate during catalysis and that His-119 is essential for G6Pase activity. Here we demonstrate that substitution of His-176 with structurally similar or dissimilar amino acids inactivates the enzyme, suggesting that His-176 could be the phosphoryl acceptor in G6Pase during catalysis.

The gene for glycogen-storage disease type 1b maps to chromosome 11q23

Glycogen-storage disease type 1 (GSD-1), also known as "von Gierke disease," is caused by a deficiency in microsomal glucose-6-phosphatase (G6Pase) activity. There are four distinct subgroups of this autosomal recessive disorder: 1a, 1b, 1c, and 1d. All share the same clinical manifestations, which are caused by abnormalities in the metabolism of glucose-6-phosphate (G6P). However, only GSD-1b patients suffer infectious complications, which are due to both the heritable neutropenia and the functional deficiencies of neutrophils and monocytes. Whereas G6Pase deficiency in GSD-1a patients arises from mutations in the G6Pase gene, this gene is normal in GSD-1b patients, indicating a separate locus for the disorder in the 1b subgroup. We now report the linkage of the GSD-1b locus to genetic markers spanning a 3-cM region on chromosome 11q23. Eventual molecular characterization of this disease will provide new insights into the genetic bases of G6P metabolism and neutrophil-monocyte dysfunction.

Gene transfer into hepatocytes mediated by helper virus-free HSV/AAV hybrid vectors

BACKGROUND

Vectors based on herpes simplex virus type 1 (HSV-1) can efficiently transduce hepatocytes in the mouse liver, and vector genomes can persist for at least 2 months. However, 24 hr after gene transfer, the number of cells that express the transgene decreases rapidly and no transduced cells are detectable after 7 days. In this study, we examined the capability of a helper virus-free HSV/AAV hybrid amplicon vector to extend transgene expression in hepatocytes in vivo.

MATERIALS AND METHODS

HSV-1 amplicon or HSV/AAV hybrid amplicon vectors that express reporter genes from different transcriptional regulatory sequences were packaged into HSV-1 virions using a helper virus-free packaging system. To determine relative transduction efficiencies, vector stocks were titered on four different cell lines, including hamster kidney (BHK21) and human lung (Hs913T) fibroblasts, and mouse (G6Pase-/-) and human (NPLC) hepatocytes. After in vivo injection of vector stocks into mouse liver, tissue sections were examined for reporter gene expression and cellular inflammatory response. Blood samples were collected to measure serum transaminase levels as a biochemical index of liver toxicity.

RESULTS

Expression of a reporter gene from liver-specific promoter sequences was consistently more effective in hepatic cells compared with fibroblasts, whereas the opposite was true when using an HSV-1 immediate-early promoter. Expression in hepatocytes in vivo was markedly longer from HSV/AAV hybrid vector compared with traditional HSV-1 amplicon vector: the number of transduced cells (approximately 2% of all hepatocytes) remained stable over 7 days after injection of HSV/AAV hybrid vector, whereas no transduced cells were detected 7 days after gene transfer with standard HSV-1 amplicon vector. The rapid decline in reporter gene expression from standard amplicons was not solely caused by a B or T lymphocyte-mediated immune response, as it also occurred in RAG2-/- mice. Hepatocyte toxicity and cellular inflammatory effects associated with HSV/AAV hybrid vector-mediated gene transfer were minimal, and readministration of vector stock proved equally effective in naive mice and in animals that received a first vector dose 4 weeks earlier.

CONCLUSIONS

HSV/AAV hybrid amplicon vectors support gene expression in vivo for considerably longer than do traditional HSV-1 amplicon vectors. Moreover, expression from these vectors does not provoke an overt inflammatory or immune response, allowing efficacious expression following repeated in vivo dosing. These characteristics suggest that such vectors may hold future promise for hepatic gene replacement therapy.

Characterization and cellular localization of PSG in rat testis

In order to establish the rat testis as a model system for studying the human pregnancy-specific beta1-glycoprotein (PSG), expression and cellular distribution of PSG in rat testis were examined. Three partial PSG cDNAs, namely, rnCGM6, rnGCM7, and rnCGM8 were obtained when rat testis cDNA libraries were screened with a human placental PSG cDNA probe. Unlike the human PSGs, the rat PSGs show less nucleotide and amino acid sequence homology among family members. The rat PSGs also have multiple truncated leader sequences followed by immunoglobulin variable-like N domains while human PSGs have a single N domain. Examination of the testis, intestine, kidney, liver, lung, and muscle of male rats by reverse transcription-polymerase chain reaction (RT-PCR) with nested gene-specific primers showed that rnCGM6 was present only in the testis, while rnCGM8 was present in the testis, intestine and lung. On the other hand rnCMG7 was found in all tissues examined. Furthermore, rnCGM7 transcript was present in all somatic cells examined whereas rnCGM6 was predominantly in myoid cells and rnCMG8 in Leydig cells. These results suggest that there is cell-specificity in the expression of PSGs in the rat testis and that the rat testis is a good model for studying the biological activities of the PSGs.

The role of HNF1alpha, HNF3gamma, and cyclic AMP in glucose-6-phosphatase gene activation

The gene for glucose-6-phosphatase (G6Pase), the key enzyme in glucose homeostasis, is expressed in a tissue-specific manner in the liver and kidney. To understand the molecular mechanisms regulating liver-specific expression of the G6Pase gene, we characterized G6Pase promoter activity by transient expression assays. The G6Pase promoter is active in HepG2 hepatoma cells, but inactive in JEG3 choriocarcinoma or 3T3 cells. DNA elements essential for optimal and liver-specific expression of the G6Pase gene were contained within nucleotides -234 to +3. Deletion analysis revealed that the G6Pase promoter contained three activation elements (AEs) at nucleotides -234 to -212 (AE-I), -146 to -125 (AE-II), and -124 to -71 (AE-III). AE-I contains binding sites for hepatocyte nuclear factors (HNF) 1 and 4. Electromobility shift and cotransfection assays demonstrated that HNF1alpha, but not HNF4, bound to its cognate site and transactivated G6Pase gene expression. The G6Pase promoter contained five HNF3 motifs, 1 (-180/-174), 2 (-139/-133), 3 (-91/-85), 4 (-81/-75), and 5 (-72/-66), and all five sites bound HNF3gamma with high affinity. Transient expression and cotransfection assays showed that HNF3 site 1 is not required for basal promoter activity, but is essential for HNF3gamma-activated transcription from the G6Pase promoter. We further showed that HNF3 sites 3, 4, and 5 were essential for basal G6Pase promoter activity and transactivation by HNF3gamma. AE-II contains, in addition to a HNF3 motif, a cAMP-response element (CRE) and a C/EBP half-site. The G6Pase(-146/-116) DNA containing AE-II formed multiple protein-DNA complexes with HepG2 nuclear extracts, including HNF3gamma, CRE-binding protein (CREB), C/EBPalpha, and C/EBPbeta. We showed that AE-II mediated transcription activation of the G6Pase gene by cAMP.

Glycogen storage disease type 1a in Israel: biochemical, clinical, and mutational studies

Glycogen storage disease type 1a (von Gierke disease, GSD 1a) is caused by the deficiency of microsomal glucose-6-phosphatase (G6Pase) activity which catalyzes the final common step of glycogenolysis and gluconeogenesis. The recent cloning of the G6Pase cDNA and characterization of the human G6Pase gene enabled the characterization of the mutations causing GSD 1a. This, in turn, allows the introduction of a noninvasive DNA-based diagnosis that provides reliable carrier testing and prenatal diagnosis. In this study, we report the biochemical and clinical characteristics as well as mutational analyses of 12 Israeli GSD 1a patients of different families, who represent most GSD 1a patients in Israel. The mutations, G6Pase activity, and glycogen content of 7 of these patients were reported previously. The biochemical data and clinical findings of all patients were similar and compatible with those described in other reports. All 9 Jewish patients, as well as one Muslim Arab patient, presented the R83C mutation. Two Muslim Arab patients had the V166G mutation which was not found in other patients' populations. The V166G mutation, which was introduced into the G6Pase cDNA by site-directed mutagenesis following transient expression in COS-1 cells, was shown to cause complete inactivation of the G6Pase. The characterization of all GSD 1a mutations in the Israeli population lends itself to carrier testing in these families as well as to prenatal diagnosis, which was carried out in 2 families. Since all Ashkenzai Jewish patients harbor the same mutation, our study suggests that DNA-based diagnosis may be used as an initial diagnostic step in Ashkenazi Jews suspected of having GSD 1a, thereby avoiding liver biopsy.

Two new mutations in the glucose-6-phosphatase gene cause glycogen storage disease in Hungarian patients

Glycogen storage disease type 1a (von Gierke disease, GSD-1A) is caused by the deficiency of microsomal glucose-6-phosphatase (G6Pase) activity which catalyzes the final common step of glycogenolysis and gluconeogenesis. The cloning of the G6Pase cDNA and characterization of the human G6Pase gene enabled the identification of the mutations causing GSD-1a. This, in turn, allows the development of non-invasive DNA-based diagnosis that provides reliable carrier testing and prenatal diagnosis. Here we report on two new mutations E110Q and D38V causing GSD-1a in two Hungarian patients. The analyses of these mutations by site-directed mutagenesis followed by transient expression assays demonstrated that E110Q retains 17% of G6Pase enzymatic activity while the D38V abolishes the enzymatic activity. The patient with the E110Q has G222R as his other mutation. G222R was also shown to preserve about 4% of the G6Pase enzymatic activity. Nevertheless, the patient presented with the classical severe symptomatology of the GSD-1a.

Dominant inheritance of isolated hypermethioninemia is associated with a mutation in the human methionine adenosyltransferase 1A gene

Methionine adenosyltransferase (MAT) I/III deficiency, characterized by isolated persistent hypermethioninemia, is caused by mutations in the MAT1A gene encoding MAT(alpha)1, the subunit of major hepatic enzymes MAT I ([alpha1]4) and III([alpha1]2). We have characterized 10 MAT1A mutations in MAT I/III-deficient individuals and shown that the associated hypermethioninemic phenotype was inherited as an autosomal recessive trait. However, dominant inheritance of hypermethioninemia, also hypothesized to be caused by MAT I/III deficiency, has been reported in two families. Here we show that the only mutation uncovered in one of these families, G, is a G-->A transition at nt 791 in exon VII of one MAT1A allele that converts an arginine at position 264 to a histidine (R264H). This single allelic R264H mutation was subsequently identified in two hypermethioninemic individuals in an additional family, C. Family C members were also found to inherit hypermethioninemia in a dominant fashion, and the available affected members analyzed carried the single allelic R264H mutation. Substitution of R-264 with histidine (R264H, the naturally occurring mutant), leucine (R264L), aspartic acid (R264D), or glutamic acid (R264E) greatly reduced MAT activity and severely impaired the ability of the MAT(alpha)1 subunits to form homodimers essential for optimal catalytic activity. On the other hand, when lysine was substituted for R-264 (R264K), the mutant alpha1 subunit was able to form dimers that retain significant MAT activity, suggesting that amino acid 264 is involved in intersubunit salt-bridge formation. Cotransfection studies show that R264/R264H MAT(alpha)1 heterodimers are enzymatically inactive, thus providing an explanation for the R264H-mediated dominant inheritance of hypermethioninemia.

MK-801 augments pilocarpine-induced electrographic seizure but protects against brain damage in rats

1. The authors examined the anticonvulsant effects of MK-801 on the pilocarpine-induced seizure model. Intraperitoneal injection of pilocarpine (400 mg/kg) induced tonic and clonic seizure. Scopolamine (10 mg/kg) and pentobarbital (5 mg/kg) prevented development of pilocarpine-induced behavioral seizure but MK-801 (0.5 mg/kg) did not. 2. An electrical seizure measured with hippocampal EEG appeared in the pilocarpine-treated group. Scopolamine and pentobarbital blocked the pilocarpine-induced electrographic seizure, MK-801 treatment augmented the electrographic seizure induced by pilocarpine. 3. Brain damage was assessed by examining the hippocampus microscopically. Pilocarpine produced neuronal death in the hippocampus, which showed pyknotic changes. Pentobarbital, scopolamine and MK-801 protected the brain damage by pilocarpine, though in the MK-801-treated group, the pyramidal cells of hippocampus appeared darker than normal. In all treatments, granule cells of the dentate gyrus were not affected. 4. These results indicate that status epilepticus induced by pilocarpine is initiated by cholinergic overstimulation and propagated by glutamatergic transmission, the elevation of which may cause brain damage through an excitatory NMDA receptor-mediated mechanism.

Demyelination of the brain is associated with methionine adenosyltransferase I/III deficiency

Individuals deficient in hepatic methionine adenosyltransferase (MAT) activity (MAT I/III deficiency) have been demonstrated to contain mutations in the gene (MATA1) that encodes the major hepatic forms, MAT I and III. MAT I/III deficiency is characterized by isolated persistent hypermethioninemia and, in some cases, unusual breath odor. Most individuals with isolated hypermethioninemia have been free of major clinical difficulties. Therefore a definitive diagnosis of MAT I/III deficiency, which requires hepatic biopsy, is not routinely made. However, two individuals with isolated hypermethioninemia have developed abnormal neurological problems, including brain demyelination, suggesting that MAT I/III deficiency can be deleterious. In the present study we have examined the MATA1 gene of eight hypermethioninemic individuals, including the two with demyelination of the brain. Mutations that abolish or reduce the MAT activity were detected in the MATA1 gene of all eight individuals. Both patients with demyelination are homozygous for mutations that alter the reading frame of the encoded protein such that the predicted MATalpha1 subunits are truncated and enzymatically inactive. The product of MAT, S-adenosylmethionine (AdoMet), is the major methyl donor for a large number of biologically important compounds including the two major myelin phospholipids, phosphatidylcholine and sphingomyelin. Both are synthesized primarily in the liver. Our findings demonstrate that isolated persistent hypermethioninemia is a marker of MAT I/III deficiency, and that complete lack of MAT I/III activity can lead to neurological abnormalities. Therefore, a DNA-based diagnosis should be performed for individuals with isolated hypermethioninemia to assess if therapy aimed at the prevention of neurological manifestations is warranted.

Differential expression and butyrate response of human alkaline phosphatase genes are mediated by upstream DNA elements

Human placentas express high levels of the placental alkaline phosphatase (PLAP) gene and low levels of a highly related gene, germ cell AP (GCAP). Malignant transformation of the placenta is accompanied by a reversal of this pattern of expression. Three Sp1-binding GC-rich DNA elements (sites I-III) located within the first 156 base pairs upstream of the GCAP gene have been shown to direct optimal GCAP gene expression in choriocarcinoma cells. Here we show that the first 100 base pairs upstream of the GCAP gene, which contains sites I and II, constitutes a minimal GCAP promoter. The simultaneous presence of both sites I and II is necessary for GCAP expression and its induction by sodium butyrate. The PLAP promoter directs only a very low level of gene expression in choriocarcinoma cells; the expression does not respond to butyrate. The -100/-1 DNA regions between the GCAP and PLAP promoters differ by only eight base pairs. However, the GC-rich stretches in sites I and II of the GCAP promoter are disrupted in the corresponding PLAP promoter. This disruption blocks or markedly reduces the binding of choriocarcinoma nuclear factors to the PLAP promoter, leading to a reduction in expression and a loss of butyrate response. We further demonstrate that nucleotides -75 to -58 in both AP promoters, which bind a human Y-box binding protein, appear to down-regulate GCAP expression.

Glucose-6-phosphatase dependent substrate transport in the glycogen storage disease type-1a mouse

Glycogen storage disease type 1a (GSD-1a) is caused by a deficiency in microsomal glucose-6-phosphatase (G6Pase), the key enzyme in glucose homeostasis. A G6Pase knockout mouse which mimics the pathophysiology of human GSD-1a patients was created to understand the pathogenesis of this disorder, to delineate the mechanisms of G6Pase catalysis, and to develop future therapeutic approaches. By examining G6Pase in the liver and kidney, the primary gluconeogenic tissues, we demonstrate that glucose-6-P transport and hydrolysis are performed by separate proteins which are tightly coupled. We propose a modified translocase catalytic unit model for G6Pase catalysis.

A motif in PSG11s mediates binding to a receptor on the surface of the promonocyte cell line THP-1

The pregnancy-specific glycoproteins (PSG) form a large family of essential pregnancy proteins, but their biological function is unknown. We have investigated whether one function of the PSG is to interact with cells of the maternal immune system. Normal human peripheral blood mononuclear cells, activated with phorbol ester, are shown to bind to purified placental PSG. This binding activity can be mimicked using a chemically synthesized peptide ligand containing the Arg-Gly-Asp (RGD) motif present in the N-terminal domain of PSG11s. The PSG11s receptors are present on cells of the myeloid cell lineage but not of the T cell or B cell lineages. The binding is mediated in part by the RGD motif and can be competed against by appropriate RGD-containing, but not Arg-Ala-Asp (RAD)-containing, ligands. Ligand binding requires a functional cytoskeleton. By examining the U937 and THP-1 promonocyte cell lines, the presence of receptors with two different binding characteristics are demonstrated. The THP-1 receptor is identified by chemical cross-linking as a protein of 46 kilodaltons (kDa), and affinity chromatography demonstrates the presence of three protein species of 32 kDa, 16.8 kDa, and 15.9 kDa, suggesting the receptor has multiple subunits.

Molecular mechanisms of an inborn error of methionine pathway. Methionine adenosyltransferase deficiency

Methionine adenosyltransferase (MAT) is a key enzyme in transmethylation, transsulfuration, and the biosynthesis of polyamines. Genetic deficiency of alpha/beta-MAT causes isolated persistent hypermethioninemia and, in some cases, unusual breath odor or neural demyelination. However, the molecular mechanism(s) underlying this deficiency has not been clearly defined. In this study, we characterized the human alpha/beta-MAT transcription unit and identified several mutations in the gene of patients with enzymatically confirmed diagnosis of MAT deficiency. Site-directed mutagenesis and transient expression assays demonstrated that these mutations partially inactivate MAT activity. These results establish the molecular basis of this disorder and allow for the development of DNA-based methodologies to investigate and diagnose hypermethioninemic individuals suspected of having abnormalities at this locus.

Role of the transcription factor C/EBP beta in expression of a rat pregnancy-specific glycoprotein gene

Pregnancy-specific glycoproteins (PSGs), which are the major placental proteins, and the carcinoembryonic antigens comprise a subfamily within the immunoglobulin superfamily. To understand the molecular mechanisms underlying the control of PSG expression, we characterized the promoter elements of a rodent PSG gene, rnCGM3, and showed that DNA elements at nucleotides -326 to -185 (PI) relative to the translation start site of rnCGM3 function as a promoter. The rnCGM3 PI promoter contains two placental factor binding sites, PISI and PISII. Both are transcription activation elements. In the present report, we screened a placental expression cDNA library with a rnCGM3-PISII probe (nucleotides -263 to -233) encompassing two overlapping palindromes (TGTTGCTCAACATGTTG) and demonstrated that the PISII-binding factor is C/EBP beta, a leucine zipper family of transcription factor. Gel mobility-shift and transient expression analyses showed that C/EBP beta and C/EBP isoforms, C/EBP alpha and C/EBP delta, bind to the PISII element and trans-activate rnCGM3 gene expression. Deletion of PISII from the rnCGM3 PI promoter greatly reduced the basal as well as the C/EBP-activated rnCGM3 expression. Gel supershift assays demonstrated that C/EBP beta is the placental isoform that binds to the PISII site rnCGM3. Moreover, C/EBP beta is expressed in high levels in the placenta, ovary, liver, lung, heart, and spleen, in contrast to C/EBP alpha, which is expressed primarily in the liver and only low levels in the placenta. Our results demonstrate that C/EBP beta is one of the transcription factors that positively regulate rnCGM3 expression during pregnancy.

Structure-function analysis of human glucose-6-phosphatase, the enzyme deficient in glycogen storage disease type 1a

Glucose-6-phosphatase (G6Pase) is the enzyme deficient in glycogen storage disease type 1a, an autosomal recessive disorder. We have previously identified six mutations in the G6Pase gene of glycogen storage disease type 1a patients and demonstrated that these mutations abolished or greatly reduced enzymatic activity of G6Pase, a hydrophobic protein of 357 amino acids. Of these, four mutations (R83C, R295C, G222R, and Q347X) are missense and one (Q347X) generates a truncated G6Pase of 346 residues. To further understand the roles and structural requirements of amino acids 83, 222, 295, and those at the carboxyl terminus in G6Pase catalysis, we characterized mutant G6Pases generated by near-saturation mutagenesis of the aforementioned amino acids. Substitution of Arg-83 with amino acids of diverse structures including Lys, a conservative change, yielded mutant G6Pase with no enzymatic activity. On the other hand, substitution of Arg-295 with Lys (R295K) retained high activity, and R295N, R295S, and R295Q exhibited moderate activity. All other substitutions of Arg-295 had no G6Pase activity, suggesting that the role of Arg-295 is to stabilize the protein either by salt bridge or hydrogen-bond formation. Substitution of Gly-222, however, remained functional unless a basic (Arg or Lys), acidic (Asp), or large polar (Gln) residue was introduced, consistent with the hydrophobic requirement of codon 222, which is predicted to be in the fourth membrane-spanning domain. It is possible that Arg-83 is involved in stabilizing the enzyme (His)-phosphate intermediate formed during G6Pase catalysis. There exist 9 conserved His residues in human G6Pase. His-9, His-119, His-252, and His-353 reside on the same side of the endoplasmic reticulum membrane as Arg-83. Whereas H119A mutant G6Pase had no enzymatic activity, H9A, H252A, and H353A mutant G6Pases retained significant activity. Substitution of His-119 with amino acids of diverse structures also yielded mutant G6Pase with no activity, suggesting that His-119 is the phosphate acceptor in G6Pase catalysis. We also present data demonstrating that the carboxyl-terminal 8 residues in human G6Pase are not essential for G6Pase catalysis.

Development and characterization of a simian virus 40-transformed, temperature-sensitive rat antimesometrial decidual cell line

The rat decidual tissue is formed by two cell populations, which express different genes and play diverse roles in pregnancy. Cells that decidualize in the antimesometrial region secrete several hormones and serve as a true endocrine gland. Isolation and maintenance of these decidual cells in primary culture is difficult. The goal of these experiments was to develop a cell line to serve as a model to study the expression and regulation of various genes specific to the antimesometrial decidual cells. Decidualization was induced in pseudopregnant rats. The antimesometrial decidua was dissected out, and cells were enzymatically dissociated and were cultured for 18 h at 37 C in RPMI-1640 medium containing 10% fetal bovine serum. Cells were washed repeatedly and then infected with a temperature-sensitive mutant of the simian virus. Transformed cells were maintained at the permissive temperature (33 C) until colonies were identified and harvested. Whereas primary cells in culture did not divide, the cloned decidual cell lines demonstrated transformed features; they multiplied at 33 C and formed multilayers. At the nonpermissive temperature (39 C), cell replication decreased, and after 4 days of culture the cells lost their transformed phenotype and continued to grow as a monolayer similar to primary cells. Cells under these conditions also assumed morphological characteristics similar to antimesometrial cells: polynucleated, large, and having cytoplasm filled with lipid droplets. Interestingly, cells cultured at 39 C that were shifted back to 33 C resumed rapid growth. To determine whether these cells also express messenger RNAs (mRNAs) found in normal antimesometrial decidual cells, the presence of activin beta A mRNA was investigated by Northern analysis and reverse transcription polymerase chain reaction. A single 6.8-kilobase activin beta A transcript was expressed abundantly at both 33 and 39 C, indicating that even when cells are rapidly dividing they express activin beta A. Activin beta B mRNA was also expressed in these cells, although in lower abundance, as were two binding proteins for activin, activin receptor II and follistatin. The activin beta A and beta B genes were responsive to cAMP stimulation in these cells. Since the hallmark of the antimesometrial decidual cells is the secretion of PRL-related hormones, the expression of decidual PRL-related protein and PRL-like protein B was examined. Northern analysis revealed a major 1.2-kilobase transcript of PRL-like protein B expressed equally at both temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)