Cytosolic biosynthesis of GTP and ATP in normal rat pancreatic islets

Emerging Roles of Small GTPases in Islet β-Cell Function

Several small guanosine triphosphatases (GTPases) from the Ras protein superfamily regulate glucose-stimulated insulin secretion in the pancreatic islet β-cell. The Rho family GTPases Cdc42 and Rac1 are primarily involved in relaying key signals in several cellular functions, including vesicle trafficking, plasma membrane homeostasis, and cytoskeletal dynamics. They orchestrate specific changes at each spatiotemporal region within the β-cell by coordinating with signal transducers, guanine nucleotide exchange factors (GEFs), GTPase-activating factors (GAPs), and their effectors. The Arf family of small GTPases is involved in vesicular trafficking (exocytosis and endocytosis) and actin cytoskeletal dynamics. Rab-GTPases regulate pre-exocytotic and late endocytic membrane trafficking events in β-cells. Several additional functions for small GTPases include regulating transcription factor activity and mitochondrial dynamics. Importantly, defects in several of these GTPases have been found associated with type 2 diabetes (T2D) etiology. The purpose of this review is to systematically denote the identities and molecular mechanistic steps in the glucose-stimulated insulin secretion pathway that leads to the normal release of insulin. We will also note newly identified defects in these GTPases and their corresponding regulatory factors (e.g., GDP dissociation inhibitors (GDIs), GEFs, and GAPs) in the pancreatic β-cells, which contribute to the dysregulation of metabolism and the development of T2D.

Effects of immunosuppression on alpha and beta cell renewal in transplanted mouse islets

AIMS/HYPOTHESIS

Immunosuppressive drugs used in human islet transplantation interfere with the balance between beta cell renewal and death, and thus may contribute to progressive graft dysfunction. We analysed the influence of immunosuppressants on the proliferation of transplanted alpha and beta cells after syngeneic islet transplantation in streptozotocin-induced diabetic mice.

METHODS

C57BL/6 diabetic mice were transplanted with syngeneic islets in the liver and simultaneously abdominally implanted with a mini-osmotic pump delivering BrdU alone or together with an immunosuppressant (tacrolimus, sirolimus, everolimus or mycophenolate mofetil [MMF]). Glycaemic control was assessed for 4 weeks. The area and proliferation of transplanted alpha and beta cells were subsequently quantified.

RESULTS

After 4 weeks, glycaemia was significantly higher in treated mice than in controls. Insulinaemia was significantly lower in mice treated with everolimus, tacrolimus and sirolimus. MMF was the only immunosuppressant that did not significantly reduce beta cell area or proliferation, albeit its levels were in a lower range than those used in clinical settings.

CONCLUSIONS/INTERPRETATION

After transplantation in diabetic mice, syngeneic beta cells have a strong capacity for self-renewal. In contrast to other immunosuppressants, MMF neither impaired beta cell proliferation nor adversely affected the fractional beta cell area. Although human beta cells are less prone to proliferate compared with rodent beta cells, the use of MMF may improve the long-term outcome of islet transplantation.

Folate-mediated one-carbon metabolism

Tetrahydrofolate (THF) polyglutamates are a family of cofactors that carry and chemically activate one-carbon units for biosynthesis. THF-mediated one-carbon metabolism is a metabolic network of interdependent biosynthetic pathways that is compartmentalized in the cytoplasm, mitochondria, and nucleus. One-carbon metabolism in the cytoplasm is required for the synthesis of purines and thymidylate and the remethylation of homocysteine to methionine. One-carbon metabolism in the mitochondria is required for the synthesis of formylated methionyl-tRNA; the catabolism of choline, purines, and histidine; and the interconversion of serine and glycine. Mitochondria are also the primary source of one-carbon units for cytoplasmic metabolism. Increasing evidence indicates that folate-dependent de novo thymidylate biosynthesis occurs in the nucleus of certain cell types. Disruption of folate-mediated one-carbon metabolism is associated with many pathologies and developmental anomalies, yet the biochemical mechanisms and causal metabolic pathways responsible for the initiation and/or progression of folate-associated pathologies have yet to be established. This chapter focuses on our current understanding of mammalian folate-mediated one-carbon metabolism, its cellular compartmentation, and knowledge gaps that limit our understanding of one-carbon metabolism and its regulation.

Investigations into the electrooxidation of guanosine-5'-triphosphate at the pyrolytic graphite electrode

The electrochemical oxidation of guanosine-5'-triphosphate has been investigated in phosphate-containing electrolytes in the pH range 1.5-10.9 at a pyrolytic graphite electrode by cyclic sweep voltammetry, spectral studies, bulk electrolysis and related techniques. In this pH range, the oxidation occurred in a single well-defined peak (Ia). The peak potential of oxidation peaks (Ep) was found to be dependent on pH, concentration and sweep rate. The kinetics of the UV-absorbing intermediates was followed spectrophotometrically and the decay of the intermediate occurred in a pseudo-first-order reaction. The first-order rate constants for the disappearance of the UV-absorbing intermediate have also been calculated. The products of the electrode reaction were characterized by HPLC and GC/MS. A tentative mechanism for the formation of the products has also been suggested.

Down syndrome: advances in molecular biology and the neurosciences

The entire DNA sequence for human chromosome 21 is now complete, and it is predicted to contain only about 225 genes, which is approximately three-fold fewer than the number initially predicted just 10 years ago. Despite this remarkable achievement, very little is known about the mechanism(s) whereby increased gene copy number (gene dosage) results in the characteristic phenotype of Down syndrome. Although many of the phenotypic traits show large individual variation, neuromotor dysfunction and cognitive and language impairment are observed in virtually all individuals. Currently, there are no efficacious biomedical treatments for these central nervous system-associated impairments. To develop novel therapeutic strategies, the effects of gene dosage imbalance need to be understood within the framework of those critical biological events that regulate brain organization and function.

Modeling ischemia in vitro: selective depletion of adenine and guanine nucleotide pools

Intracellular ATP depletion is a hallmark event in ischemic injury. It has been extensively characterized in models of chemical anoxia in vitro. In contrast, the fate of GTP during ischemia remains unknown. We used LLC-PK proximal tubular cells to measure GTP and ATP changes during anoxia. In 45 min, antimycin A decreased ATP and GTP to 8% and 2% of controls, respectively. Ischemia in vivo resulted in comparable reductions in GTP and ATP. After 2 h of recovery, GTP levels in LLC-PK cells increased to 65% while ATP increased to 29%. We also investigated steady-state models of selective ATP or GTP depletion. Combinations of antimycin A and mycophenolic acid selectively reduced GTP to 51% or 25% of control. Similarly, alanosine selectively reduced ATP to 61% or 26% of control. Selective GTP depletion resulted in significant apoptosis. Selective ATP depletion caused mostly necrosis. These models of ATP or GTP depletion can prove useful in dissecting the relative contribution of the two nucleotides to the ischemic phenotype.

The human GARS-AIRS-GART gene encodes two proteins which are differentially expressed during human brain development and temporally overexpressed in cerebellum of individuals with Down syndrome

Purines are critical for energy metabolism, cell signalling and cell reproduction. Nevertheless, little is known about the regulation of this essential biochemical pathway during mammalian development. In humans, the second, third and fifth steps of de novo purine biosynthesis are catalyzed by a trifunctional protein with glycinamide ribonucleotide synthetase (GARS), aminoimidazole ribonucleotide synthetase (AIRS) and glycinamide ribonucleotide formyltransferase (GART) enzymatic activities. The gene encoding this trifunctional protein is located on chromosome 21. The enzyme catalyzing the intervening fourth step of de novo purine biosynthesis, phosphoribosylformylglycineamide amidotransferase (FGARAT), is encoded by a separate gene on chromosome 17. To investigate the regulation of these proteins, we have generated monoclonal and/or polyclonal antibodies specific to each of these enzymatic domains. Using these antibodies on western blots of Chinese hamster ovary (CHO) cells transfected with the human GARS-AIRS-GART gene, we show that this gene encodes not only the trifunctional protein of 110 kDa, but also a monofunctional GARS protein of 50 kDa. This carboxy-truncated human GARS protein is produced by alternative splicing resulting in the use of a polyadenylation site in the intron between the terminal GARS and the first AIRS exons. The expression of both the GARS and GARS-AIRS-GART proteins are regulated during development of the human cerebellum, while the expression of FGARAT appears to be constitutive. All three proteins are expressed at high levels during normal prenatal cerebellum development while the GARS and GARS-AIRS-GART proteins become undetectable in this tissue shortly after birth. In contrast, the GARS and GARS-AIRS-GART proteins continue to be expressed during the postnatal development of the cerebellum in individuals with Down syndrome.

Inhibition of calcium-induced insulin secretion from intact HIT-T15 or INS-1 beta cells by GTP depletion

Using intact rat islets, we previously observed that GTP depletion (achieved through the use of mycophenolic acid or other synthesis inhibitors) impedes nutrient- but not K+-induced insulin secretion. It was concluded that a proximal nutrient-dependent step in stimulus-secretion coupling (but not the process of Ca2+-induced exocytosis itself) is modulated by ambient GTP levels. To examine Ca2+-dependent steps further in intact beta cells, INS-1 cells (which synthesize GTP and ATP similarly to rat islets) and HIT-T15 cells (whose synthesis of purine nucleotides is different) were studied following cell culture for 1-18 hr in various concentrations of mycophenolic acid (MPA) or mizoribine (MZ). Both agents profoundly reduced GTP content (mean: -78%) and lowered the GTP/GDP ratio by an average of -73%; concomitantly, MPA or MZ reduced insulin secretion induced by 10 mM glucose, 30 or 40 mM KCl, or 100 microM tolbutamide, independent of any changes in cell viability, insulin content, ATP content, the ATP/ADP ratio, or cytosolic free Ca2+ concentrations. In INS-1 cells (which appear to have normal nucleobase transport and "salvage" pathway activities), guanine (but not adenine) restored GTP content, the GTP/GDP ratio, and Ca2+-induced secretion. In HIT cells, the phosphoribosylation of exogenous guanine or hypoxanthine is defective; however, provision of 500 microM guanosine (but not adenosine) reversed the effects of MPA. We conclude that, at least in certain situations, a requisite role for GTP in the distal step(s) of exocytosis can be demonstrated.

Tight links between adenine and guanine nucleotide pools in mouse pancreatic islets: a study with mycophenolic acid

Glucose metabolism in pancreatic B-cells leads to an increase in the ATP/ADP ratio that might participate in the regulation of insulin secretion. Good correlations have also been observed between guanine nucleotide levels in isolated pancreatic islets and insulin secretion. To assess whether guanine nucleotides have a specific role in stimulus-secretion coupling, their concentration should be modified selectively. This was attempted by culturing mouse islets overnight in the presence of mycophenolic acid (MPA), an inhibitor of GMP synthesis at the level of IMP dehydrogenase. The drug (25-50 microg/ml) did not affect the insulin content but decreased the GTP content of the islets and inhibited insulin secretion during subsequent incubation in the presence of 15 mM glucose. However, MPA also decreased the ATP/ADP ratio in the islets. The addition of guanine to the culture medium (to stimulate the salvage pathway of GTP synthesis) restored normal GTP levels, corrected the ATP/ADP ratio and partly prevented the inhibition of insulin release. In contrast, attempts to stimulate ATP synthesis specifically (by provision of adenine or adenosine) failed to reverse any of the effects of MPA. It is concluded that guanine and adenine nucleotide pools are tightly linked and cannot be specifically affected by MPA in pancreatic islet cells, probably because of the activity of nucleoside diphosphate kinase and because of the role of GTP in several reactions leading to adenine nucleotide generation. Contrary to previous claims, MPA is not an adequate tool for evaluating a specific role of guanine nucleotides in the control of insulin secretion.