Sequence of rat mitochondrial glycerol-3-phosphate dehydrogenase cDNA. Evidence for EF-hand calcium-binding domains

Glycerol 3-phosphate dehydrogenases (1 and 2) in cancer and other diseases

The glycerol 3-phosphate shuttle (GPS) is composed of two different enzymes: cytosolic NAD+-linked glycerol 3-phosphate dehydrogenase 1 (GPD1) and mitochondrial FAD-linked glycerol 3-phosphate dehydrogenase 2 (GPD2). These two enzymes work together to act as an NADH shuttle for mitochondrial bioenergetics and function as an important bridge between glucose and lipid metabolism. Since these genes were discovered in the 1960s, their abnormal expression has been described in various metabolic diseases and tumors. Nevertheless, it took a long time until scientists could investigate the causal relationship of these enzymes in those pathophysiological conditions. To date, numerous studies have explored the involvement and mechanisms of GPD1 and GPD2 in cancer and other diseases, encompassing reports of controversial and non-conventional mechanisms. In this review, we summarize and update current knowledge regarding the functions and effects of GPS to provide an overview of how the enzymes influence disease conditions. The potential and challenges of developing therapeutic strategies targeting these enzymes are also discussed.

Effects of triethylamine on the expression patterns of two G3PDHs and lipid accumulation in Dunaliella tertiolecta

Glycerol-3-phosphate (G3P) is the important precursors for triacylglycerol synthesis, while glycerol-3-phosphate dehydrogenase (GPDH) determines the formation of G3P. In this study, two GDPH genes, Dtgdp1 and Dtgdp2 were isolated and identified from Dunaliella tertiolecta. The full-length Dtgdp1 and Dtgdp2 CDS were 2016 bp and 2094 bp, which encoded two putative protein sequences of 671 and 697 amino acids with predicted molecular weights of 73.64 kDa and 76.73 kDa, respectively. DtGDP1 and DtGDP2 both had a close relationship with those of algal and higher plants. DtGDP1 shared two conserved superfamily (A1 and A2) and four signature motifs (I-IV), and the DtGDP2 showed six signature domains (from motif I to VI) and DAO_C conserved family. Our previous work showed that the triethylamine intervention could greatly increase the triacylglycerol content (up to 80%) of D. tertiolecta. This study aims to investigate the effect of triethylamine on GPDH expression. Results showed that, when treated by triethylamine at 100 ppm and 150 ppm, the expression levels of Dtgdp1 and Dtgpd2 were increased to 5.121- and 56.964-fold compared with the control, respectively. Triethylamine seemed to enhance lipid metabolic flow by inducing the expressions of Dtgdp1 and Dtgdp2 to increase the lipid content, which provides a new insight into the desired pathway of lipid synthesis in algae through genetic engineering.

Regulation of mitochondrial calcium in plants versus animals

Ca(2+) acts as an important cellular second messenger in eukaryotes. In both plants and animals, a wide variety of environmental and developmental stimuli trigger Ca(2+) transients of a specific signature that can modulate gene expression and metabolism. In animals, mitochondrial energy metabolism has long been considered a hotspot of Ca(2+) regulation, with a range of pathophysiology linked to altered Ca(2+) control. Recently, several molecular players involved in mitochondrial Ca(2+) signalling have been identified, including those of the mitochondrial Ca(2+) uniporter. Despite strong evidence for sophisticated Ca(2+) regulation in plant mitochondria, the picture has remained much less clear. This is currently changing aided by live imaging and genetic approaches which allow dissection of subcellular Ca(2+) dynamics and identification of the proteins involved. We provide an update on our current understanding in the regulation of mitochondrial Ca(2+) and signalling by comparing work in plants and animals. The significance of mitochondrial Ca(2+) control is discussed in the light of the specific metabolic and energetic needs of plant and animal cells.

Mitochondrial malic enzyme 3 is important for insulin secretion in pancreatic β-cells

Pancreatic β-cells with severely knocked down cytosolic malic enzyme (ME1) and mitochondrial NAD(P) malic enzyme (ME2) show normal insulin secretion. The mitochondrial NADP malic enzyme (ME3) is very low in pancreatic β-cells, and ME3 was previously thought unimportant for insulin secretion. Using short hairpin RNAs that targeted one or more malic enzyme mRNAs in the same cell, we generated more than 25 stable INS-1 832/13-derived insulin cell lines expressing extremely low levels of ME1, ME2, and ME3 alone or low levels of two of these enzymes in the same cell line. We also used double targeting of the same Me gene to achieve even more severe reduction in Me1 and Me2 mRNAs and enzyme activities than we reported previously. Knockdown of ME3, but not ME1 or ME2 alone or together, inhibited insulin release stimulated by glucose, pyruvate or 2-aminobicyclo [2,2,1]heptane-2-carboxylic acid-plus-glutamine. The data suggest that ME3, far more than ME1 or ME2, is necessary for insulin release. Because ME3 enzyme activity is low in β-cells, its role in insulin secretion may involve a function other than its ME catalytic activity.

Mitochondrial production of reactive oxygen species

Numerous biochemical studies are aimed at elucidating the sources and mechanisms of formation of reactive oxygen species (ROS) because they are involved in cellular, organ-, and tissue-specific physiology. Mitochondria along with other cellular organelles of eukaryotes contribute significantly to ROS formation and utilization. This review is a critical account of the mitochondrial ROS production and methods for their registration. The physiological and pathophysiological significance of the mitochondrially produced ROS are discussed.

Metabolic control of redox and redox control of metabolism in plants

SIGNIFICANCE

Reduction-oxidation (Redox) status operates as a major integrator of subcellular and extracellular metabolism and is simultaneously itself regulated by metabolic processes. Redox status not only dominates cellular metabolism due to the prominence of NAD(H) and NADP(H) couples in myriad metabolic reactions but also acts as an effective signal that informs the cell of the prevailing environmental conditions. After relay of this information, the cell is able to appropriately respond via a range of mechanisms, including directly affecting cellular functioning and reprogramming nuclear gene expression.

RECENT ADVANCES

The facile accession of Arabidopsis knockout mutants alongside the adoption of broad-scale post-genomic approaches, which are able to provide transcriptomic-, proteomic-, and metabolomic-level information alongside traditional biochemical and emerging cell biological techniques, has dramatically advanced our understanding of redox status control. This review summarizes redox status control of metabolism and the metabolic control of redox status at both cellular and subcellular levels.

CRITICAL ISSUES

It is becoming apparent that plastid, mitochondria, and peroxisome functions influence a wide range of processes outside of the organelles themselves. While knowledge of the network of metabolic pathways and their intraorganellar redox status regulation has increased in the last years, little is known about the interorganellar redox signals coordinating these networks. A current challenge is, therefore, synthesizing our knowledge and planning experiments that tackle redox status regulation at both inter- and intracellular levels.

FUTURE DIRECTIONS

Emerging tools are enabling ever-increasing spatiotemporal resolution of metabolism and imaging of redox status components. Broader application of these tools will likely greatly enhance our understanding of the interplay of redox status and metabolism as well as elucidating and characterizing signaling features thereof. We propose that such information will enable us to dissect the regulatory hierarchies that mediate the strict coupling of metabolism and redox status which, ultimately, determine plant growth and development.

Novel inhibitors of mitochondrial sn-glycerol 3-phosphate dehydrogenase

Mitochondrial sn-glycerol 3-phosphate dehydrogenase (mGPDH) is a ubiquinone-linked enzyme in the mitochondrial inner membrane best characterized as part of the glycerol phosphate shuttle that transfers reducing equivalents from cytosolic NADH into the mitochondrial electron transport chain. Despite the widespread expression of mGPDH and the availability of mGPDH-null mice, the physiological role of this enzyme remains poorly defined in many tissues, likely because of compensatory pathways for cytosolic regeneration of NAD⁺ and mechanisms for glycerol phosphate metabolism. Here we describe a novel class of cell-permeant small-molecule inhibitors of mGPDH (iGP) discovered through small-molecule screening. Structure-activity analysis identified a core benzimidazole-phenyl-succinamide structure as being essential to inhibition of mGPDH while modifications to the benzimidazole ring system modulated both potency and off-target effects. Live-cell imaging provided evidence that iGPs penetrate cellular membranes. Two compounds (iGP-1 and iGP-5) were characterized further to determine potency and selectivity and found to be mixed inhibitors with IC₅₀ and K_i values between ∼1–15 µM. These novel mGPDH inhibitors are unique tools to investigate the role of glycerol 3-phosphate metabolism in both isolated and intact systems.

Knockdown of both mitochondrial isocitrate dehydrogenase enzymes in pancreatic beta cells inhibits insulin secretion

BACKGROUND

There are three isocitrate dehydrogenases (IDHs) in the pancreatic insulin cell; IDH1 (cytosolic) and IDH2 (mitochondrial) use NADP(H). IDH3 is mitochondrial, uses NAD(H) and was believed to be the IDH that supports the citric acid cycle.

METHODS

With shRNAs targeting mRNAs for these enzymes we generated cell lines from INS-1 832/13 cells with severe (80%-90%) knockdown of the mitochondrial IDHs separately and together in the same cell line.

RESULTS

With knockdown of both mitochondrial IDH's mRNA, enzyme activity and protein level, (but not with knockdown of only one mitochondrial IDH) glucose- and BCH (an allosteric activator of glutamate dehydrogenase)-plus-glutamine-stimulated insulin release were inhibited. Cellular levels of citrate, α-ketoglutarate, malate and ATP were altered in patterns consistent with blockage at the mitochondrial IDH reactions. We were able to generate only 50% knockdown of Idh1 mRNA in multiple cell lines (without inhibition of insulin release) possibly because greater knockdown of IDH1 was not compatible with cell line survival.

CONCLUSIONS

The mitochondrial IDHs are redundant for insulin secretion. When both enzymes are severely knocked down, their low activities (possibly assisted by transport of IDH products and other metabolic intermediates from the cytosol into mitochondria) are sufficient for cell growth, but inadequate for insulin secretion when the requirement for intermediates is certainly more rapid. The results also indicate that IDH2 can support the citric acid cycle.

GENERAL SIGNIFICANCE

As almost all mammalian cells possess substantial amounts of all three IDH enzymes, the biological principles suggested by these results are probably extrapolatable to many tissues.

When less is more: the forbidden fruits of gene repression in the adult β-cell

Outside of the biological arena the term 'repression' often has a negative connotation. However, in the pancreatic β-cell a small group of genes, which are abundantly expressed in most if not all other mammalian tissues, are highly selectively repressed, with likely functional consequences. The two 'founder' members of this group, lactate dehydrogenase A (Ldha) and monocarboxylate transporter-1 (MCT-1/Slc16a1), are inactivated by multiple mechanisms including histone modifications and microRNA-mediated silencing. Their inactivation ensures that pyruvate and lactate, derived from muscle during exercise, do not stimulate insulin release inappropriately. Correspondingly, activating mutations in the MCT-1 promoter underlie 'exercise-induced hyperinsulinism' (EIHI) in man, a condition mimicked by forced over-expression of MCT-1 in the β-cell in mice. Furthermore, LDHA expression in the β-cell is upregulated in both human type 2 diabetes and in rodent models of the disease. Recent work by us and by others has identified a further ∼60 genes which are selectively inactivated in the β-cell, a list which we refine here up to seven by detailed comparison of the two studies. These genes include key regulators of cell proliferation and stimulus-secretion coupling. The present, and our earlier results, thus highlight the probable importance of shutting down a subset of 'disallowed' genes for the differentiated function of β-cells, and implicate previously unsuspected signalling pathways in the control of β-cell expansion and insulin secretion. Targeting of deregulated 'disallowed' genes in these cells may thus, in the future, provide new therapeutic avenues for type 2 diabetes.

A refined analysis of superoxide production by mitochondrial sn-glycerol 3-phosphate dehydrogenase

The oxidation of sn-glycerol 3-phosphate by mitochondrial sn-glycerol 3-phosphate dehydrogenase (mGPDH) is a major pathway for transfer of cytosolic reducing equivalents to the mitochondrial electron transport chain. It is known to generate H(2)O(2) at a range of rates and from multiple sites within the chain. The rates and sites depend upon tissue source, concentrations of glycerol 3-phosphate and calcium, and the presence of different electron transport chain inhibitors. We report a detailed examination of H(2)O(2) production during glycerol 3-phosphate oxidation by skeletal muscle, brown fat, brain, and heart mitochondria with an emphasis on conditions under which mGPDH itself is the source of superoxide and H(2)O(2). Importantly, we demonstrate that a substantial portion of H(2)O(2) production commonly attributed to mGPDH originates instead from electron flow through the ubiquinone pool into complex II. When complex II is inhibited and mGPDH is the sole superoxide producer, the rate of superoxide production depends on the concentrations of glycerol 3-phosphate and calcium and correlates positively with the predicted reduction state of the ubiquinone pool. mGPDH-specific superoxide production plateaus at a rate comparable with the other major sites of superoxide production in mitochondria, the superoxide-producing center shows no sign of being overreducible, and the maximum superoxide production rate correlates with mGPDH activity in four different tissues. mGPDH produces superoxide approximately equally toward each side of the mitochondrial inner membrane, suggesting that the Q-binding pocket of mGPDH is the major site of superoxide generation. These results clarify the maximum rate and mechanism of superoxide production by mGPDH.

Group VIB Phospholipase A(2) promotes proliferation of INS-1 insulinoma cells and attenuates lipid peroxidation and apoptosis induced by inflammatory cytokines and oxidant agents

Group VIB Phospholipase A(2) (iPLA(2)γ) is distributed in membranous organelles in which β-oxidation occurs, that is, mitochondria and peroxisomes, and is expressed by insulin-secreting pancreatic islet β-cells and INS-1 insulinoma cells, which can be injured by inflammatory cytokines, for example, IL-1β and IFN-γ, and by oxidants, for example, streptozotocin (STZ) or t-butyl-hydroperoxide (TBHP), via processes pertinent to mechanisms of β-cell loss in types 1 and 2 diabetes mellitus. We find that incubating INS-1 cells with IL-1β and IFN-γ, with STZ, or with TBHP causes increased expression of iPLA(2)γ mRNA and protein. We prepared INS-1 knockdown (KD) cell lines with reduced iPLA(2)γ expression, and they proliferate more slowly than control INS-1 cells and undergo increased membrane peroxidation in response to cytokines or oxidants. Accumulation of oxidized phospholipid molecular species in STZ-treated INS-1 cells was demonstrated by LC/MS/MS scanning, and the levels in iPLA(2)γ-KD cells exceeded those in control cells. iPLA(2)γ-KD INS-1 cells also exhibited higher levels of apoptosis than control cells when incubated with STZ or with IL-1β and IFN-γ. These findings suggest that iPLA(2)γ promotes β-cell proliferation and that its expression is increased during inflammation or oxidative stress as a mechanism to mitigate membrane injury that may enhance β-cell survival.

Group VIA PLA2 (iPLA2β) is activated upstream of p38 mitogen-activated protein kinase (MAPK) in pancreatic islet β-cell signaling

Group VIA phospholipase A(2) (iPLA(2)β) in pancreatic islet β-cells participates in glucose-stimulated insulin secretion and sarco(endo)plasmic reticulum ATPase (SERCA) inhibitor-induced apoptosis, and both are attenuated by pharmacologic or genetic reductions in iPLA(2)β activity and amplified by iPLA(2)β overexpression. While exploring signaling events that occur downstream of iPLA(2)β activation, we found that p38 MAPK is activated by phosphorylation in INS-1 insulinoma cells and mouse pancreatic islets, that this increases with iPLA(2)β expression level, and that it is stimulated by the iPLA(2)β reaction product arachidonic acid. The insulin secretagogue D-glucose also stimulates β-cell p38 MAPK phosphorylation, and this is prevented by the iPLA(2)β inhibitor bromoenol lactone. Insulin secretion induced by d-glucose and forskolin is amplified by overexpressing iPLA(2)β in INS-1 cells and in mouse islets, and the p38 MAPK inhibitor PD169316 prevents both responses. The SERCA inhibitor thapsigargin also stimulates phosphorylation of both β-cell MAPK kinase isoforms and p38 MAPK, and bromoenol lactone prevents both events. Others have reported that iPLA(2)β products activate Rho family G-proteins that promote MAPK kinase activation via a mechanism inhibited by Clostridium difficile toxin B, which we find to inhibit thapsigargin-induced β-cell p38 MAPK phosphorylation. Thapsigargin-induced β-cell apoptosis and ceramide generation are also prevented by the p38 MAPK inhibitor PD169316. These observations indicate that p38 MAPK is activated downstream of iPLA(2)β in β-cells incubated with insulin secretagogues or thapsigargin, that this requires prior iPLA(2)β activation, and that p38 MAPK is involved in the β-cell functional responses of insulin secretion and apoptosis in which iPLA(2)β participates.

Molecular analysis, tissue profiles, and seasonal patterns of cytosolic and mitochondrial GPDH in freeze-resistant rainbow smelt (Osmerus mordax)

Rainbow smelt (Osmerus mordax) is an anadromous teleost that, beginning in late fall, accumulates plasma glycerol in excess of 200 mM, which subsequently decreases in the spring. The activity of cytosolic glycerol-3-phosphate dehydrogenase (cGPDH) is higher (i) in liver of smelt than in that of Atlantic salmon and capelin (nonglycerol accumulators), (ii) in liver of smelt maintained at 1°C than in that of smelt held at 8°-10°C, and (iii) in smelt liver than in smelt muscle, heart, brain, or kidney. In addition, transcript levels of cGPDH in liver peak in December during the onset of glycerol production and then decline over the remainder of the season. There are four cGPDH protein isoforms in smelt liver that are present regardless of glycerol production status. A minimum of four cGPDH gene copies identified by Southern blotting provide adequate genetic potential to yield multiple protein isoforms. A full-length cDNA for smelt mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) was cloned and characterized. The 2,790-bp cDNA contains a 109-bp 5'UTR, a 2,193-bp open reading frame, and a 488-bp 3'UTR; transcripts are ubiquitously expressed in both warm- and cold-acclimated smelt tissues. Smelt mGPDH encodes a 730-aa protein that clusters with that of zebrafish and frog and contains several common structural motifs. mGPDH transcript levels generally increase late in the seasonal glycerol cycle, and mGPDH enzyme activity increases significantly during the glycerol decrease phase. Taken together, these findings suggest that liver cGPDH and mGPDH play a key role in the glycerol accumulation and decrease phases, respectively.

Inactivation of the mitochondrial carrier SLC25A25 (ATP-Mg2+/Pi transporter) reduces physical endurance and metabolic efficiency in mice

An ATP-Mg(2+/)P(i) inner mitochondrial membrane solute transporter (SLC25A25), which is induced during adaptation to cold stress in the skeletal muscle of mice with defective UCP1/brown adipose tissue thermogenesis, has been evaluated for its role in metabolic efficiency. SLC25A25 is thought to control ATP homeostasis by functioning as a Ca(2+)-regulated shuttle of ATP-Mg(2+) and P(i) across the inner mitochondrial membrane. Mice with an inactivated Slc25a25 gene have reduced metabolic efficiency as evidenced by enhanced resistance to diet-induced obesity and impaired exercise performance on a treadmill. Mouse embryo fibroblasts from Slc25a25(-/-) mice have reduced Ca(2+) flux across the endoplasmic reticulum, basal mitochondrial respiration, and ATP content. Although Slc25a25(-/-) mice are metabolically inefficient, the source of the inefficiency is not from a primary function in thermogenesis, because Slc25a25(-/-) mice maintain body temperature upon acute exposure to the cold (4 °C). Rather, the role of SLC25A25 in metabolic efficiency is most likely linked to muscle function as evidenced from the physical endurance test of mutant mice on a treadmill. Consequently, in the absence of SLC25A25 the efficiency of ATP production required for skeletal muscle function is diminished with secondary effects on adiposity. However, in the absence of UCP1-based thermogenesis, induction of Slc25a25 in mice with an intact gene may contribute to an alternative thermogenic pathway for the maintenance of body temperature during cold stress.

Chronic reduction of the cytosolic or mitochondrial NAD(P)-malic enzyme does not affect insulin secretion in a rat insulinoma cell line

The cytosolic malic enzyme (ME1) has been suggested to augment insulin secretion via the malate-pyruvate and/or citrate-pyruvate shuttles, through the production of NADPH or other metabolites. We used selectable vectors expressing short hairpin RNA (shRNA) to stably decrease Me1 mRNA levels by 80-86% and ME1 enzyme activity by 78-86% with either of two shRNAs in the INS-1 832/13 insulinoma cell line. Contrary to published short term ME1 knockdown experiments, our long term targeted cells showed normal insulin secretion in response to glucose or to glutamine plus 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid. We found no increase in the mRNAs and enzyme activities of the cytosolic isocitrate dehydrogenase or glucose-6-phosphate dehydrogenase, which also produce cytosolic NADPH. There was no compensatory induction of the mRNAs for the mitochondrial malic enzymes Me2 or Me3. Interferon pathway genes induced in preliminary small interfering RNA experiments were not induced in the long term shRNA experiments. We repeated our study with an improved vector containing Tol2 transposition sequences to produce a higher rate of stable transferents and shortened time to testing, but this did not alter the results. We similarly used stably expressed shRNA to reduce mitochondrial NAD(P)-malic enzyme (Me2) mRNA by up to 95%, with severely decreased ME2 protein and a 90% decrease in enzyme activity. Insulin release to glucose or glutamine plus 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid remained normal. The maintenance of robust insulin secretion after lowering expression of either one of these malic enzymes is consistent with the redundancy of pathways of pyruvate cycling and/or cytosolic NADPH production in insulinoma cells.

Hypoxia triggers subcellular compartmental redox signaling in vascular smooth muscle cells

RATIONALE

Recent studies have implicated mitochondrial reactive oxygen species (ROS) in regulating hypoxic pulmonary vasoconstriction (HPV), but controversy exists regarding whether hypoxia increases or decreases ROS generation.

OBJECTIVE

This study tested the hypothesis that hypoxia induces redox changes that differ among subcellular compartments in pulmonary (PASMCs) and systemic (SASMCs) smooth muscle cells.

METHODS AND RESULTS

We used a novel, redox-sensitive, ratiometric fluorescent protein sensor (RoGFP) to assess the effects of hypoxia on redox signaling in cultured PASMCs and SASMCs. Using genetic targeting sequences, RoGFP was expressed in the cytosol (Cyto-RoGFP), the mitochondrial matrix (Mito-RoGFP), or the mitochondrial intermembrane space (IMS-RoGFP), allowing assessment of oxidant signaling in distinct intracellular compartments. Superfusion of PASMCs or SASMCs with hypoxic media increased oxidation of both Cyto-RoGFP and IMS-RoGFP. However, hypoxia decreased oxidation of Mito-RoGFP in both cell types. The hypoxia-induced oxidation of Cyto-RoGFP was attenuated through the overexpression of cytosolic catalase in PASMCs.

CONCLUSIONS

These results indicate that hypoxia causes a decrease in nonspecific ROS generation in the matrix compartment, whereas it increases regulated ROS production in the IMS, which diffuses to the cytosol of both PASMCs and SASMCs.

The role of Ca(2+) signaling in the coordination of mitochondrial ATP production with cardiac work

The heart is capable of balancing the rate of mitochondrial ATP production with utilization continuously over a wide range of activity. This results in a constant phosphorylation potential despite a large change in metabolite turnover. The molecular mechanisms responsible for generating this energy homeostasis are poorly understood. The best candidate for a cytosolic signaling molecule reflecting ATP hydrolysis is Ca(2+). Since Ca(2+) initiates and powers muscle contraction as well as serves as the primary substrate for SERCA, Ca(2+) is an ideal feed-forward signal for priming ATP production. With the sarcoplasmic reticulum to cytosolic Ca(2+) gradient near equilibrium with the free energy of ATP, cytosolic Ca(2+) release is exquisitely sensitive to the cellular energy state providing a feedback signal. Thus, Ca(2+) can serve as a feed-forward and feedback regulator of ATP production. Consistent with this notion is the correlation of cytosolic and mitochondrial Ca(2+) with work in numerous preparations as well as the localization of mitochondria near Ca(2+) release sites. How cytosolic Ca(2+) signaling might regulate oxidative phosphorylation is a focus of this review. The relevant Ca(2+) sensitive sites include several dehydrogenases and substrate transporters together with a post-translational modification of F1-FO-ATPase and cytochrome oxidase. Thus, Ca(2+) apparently activates both the generation of the mitochondrial membrane potential as well as utilization to produce ATP. This balanced activation extends the energy homeostasis observed in the cytosol into the mitochondria matrix in the never resting heart.

Regulation of mitochondrial dehydrogenases by calcium ions

Studies in Bristol in the 1960s and 1970s, led to the recognition that four mitochondrial dehydrogenases are activated by calcium ions. These are FAD-glycerol phosphate dehydrogenase, pyruvate dehydrogenase, NAD-isocitrate dehydrogenase and oxoglutarate dehydrogenase. FAD-glycerol phosphate dehydrogenase is located on the outer surface of the inner mitochondrial membrane and is influenced by changes in cytoplasmic calcium ion concentration. The other three enzymes are located within mitochondria and are regulated by changes in mitochondrial matrix calcium ion concentration. These and subsequent studies on purified enzymes, mitochondria and intact cell preparations have led to the widely accepted view that the activation of these enzymes is important in the stimulation of the respiratory chain and hence ATP supply under conditions of increased ATP demand in many stimulated mammalian cells. The effects of calcium ions on FAD-isocitrate dehydrogenase involve binding to an EF-hand binding motif within this enzyme but the binding sites involved in the effects of calcium ions on the three intramitochondrial dehydrogenases remain to be fully established. It is also emphasised in this article that these three dehydrogenases appear only to be regulated by calcium ions in vertebrates and that this raises some interesting and potentially important developmental issues.

A mutation in the inner mitochondrial membrane peptidase 2-like gene (Immp2l) affects mitochondrial function and impairs fertility in mice

The mitochondrion is involved in energy generation, apoptosis regulation, and calcium homeostasis. Mutations in genes involved in mitochondrial processes often result in a severe phenotype or embryonic lethality, making the study of mitochondrial involvement in aging, neurodegeneration, or reproduction challenging. Using a transgenic insertional mutagenesis strategy, we generated a mouse mutant, Immp2lTg(Tyr)979Ove, with a mutation in the inner mitochondrial membrane peptidase 2-like (Immp2l) gene. The mutation affected the signal peptide sequence processing of mitochondrial proteins cytochrome c1 and glycerol phosphate dehydrogenase 2. The inefficient processing of mitochondrial membrane proteins perturbed mitochondrial function so that mitochondria from mutant mice manifested hyperpolarization, higher than normal superoxide ion generation, and higher levels of ATP. Homozygous Immp2lTg(Tyr)979Ove females were infertile due to defects in folliculogenesis and ovulation, whereas mutant males were severely subfertile due to erectile dysfunction. The data suggest that the high superoxide ion levels lead to a decrease in the bioavailability of nitric oxide and an increase in reactive oxygen species stress, which underlies these reproductive defects. The results provide a novel link between mitochondrial dysfunction and infertility and suggest that superoxide ion targeting agents may prove useful for treating infertility in a subpopulation of infertile patients.

Isolation of a FAD-GPDH gene encoding a mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase from Dunaliella salina

The mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase (FAD-GPDH), recently reported in plants, has been detailed in yeast and animal systems. It oxidizes glycerol-3-phosphate (G-3-P) to dihydroxyacetone phosphate (DHAP) on the outer surface of mitochondrial inner membrane. A cDNA encoding the Dunaliella salina mitochondrial glycerol-3-phosphate dehydrogenase (DsFAD-GPDH) has been cloned and sequenced. The full length cDNA is 2791 bp, with an open reading frame (ORF) encoding 650 predicted amino acids, which show strong homology to reported FAD-GPDHs and have an apparent mitochondrial targeting sequence in the N-terminal. The sequence has been submitted to the GenBank database under Accession No. DQ916107. Results of Real-Time Quantitative PCR and enzymatic assays show that expression of DsFAD-GPDH is enhanced at first by salt treatment, and repressed by oxygen deficiency and cold stress.

Mitochondrial Transporters as Novel Targets for Intracellular Calcium Signaling

Ca2+signaling in mitochondria is important to tune mitochondrial function to a variety of extracellular stimuli. The main mechanism is Ca2+entry in mitochondria via the Ca2+uniporter followed by Ca2+activation of three dehydrogenases in the mitochondrial matrix. This results in increases in mitochondrial NADH/NAD ratios and ATP levels and increased substrate uptake by mitochondria. We review evidence gathered more than 20 years ago and recent work indicating that substrate uptake, mitochondrial NADH/NAD ratios, and ATP levels may be also activated in response to cytosolic Ca2+signals via a mechanism that does not require the entry of Ca2+in mitochondria, a mechanism depending on the activity of Ca2+-dependent mitochondrial carriers (CaMC). CaMCs fall into two groups, the aspartate-glutamate carriers (AGC) and the ATP-Mg/Picarriers, also named SCaMC (for short CaMC). The two mammalian AGCs, aralar and citrin, are members of the malate-aspartate NADH shuttle, and citrin, the liver AGC, is also a member of the urea cycle. Both types of CaMCs are activated by Ca2+in the intermembrane space and function together with the Ca2+uniporter in decoding the Ca2+signal into a mitochondrial response.

The mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase of Trypanosomatidae and the glycosomal redox balance of insect stages of Trypanosoma brucei and Leishmania spp

The genes for the mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase were identified in Trypanosoma brucei and Leishmania major genomes. We have expressed the L. major gene in Saccharomyces cerevisiae and confirmed the subcellular localization and activity of the produced enzyme. Using cultured T. brucei procyclic and Leishmania mexicana promastigote cells with a permeabilized plasma membrane and containing intact glycosomes, it was shown that dihydroxyacetone phosphate is converted into pyruvate, and stimulates oxygen consumption, indicating that all components of the glycerol 3-phosphate/dihydoxyacetone phosphate shuttle between glycosomes and mitochondrion are present in these insect stages of both organisms. A computer model has been prepared for the energy and carbohydrate metabolism of these cells. It was used in an elementary mode analysis to get insight into the metabolic role of the shuttle in these insect-stage parasites. Our analysis suggests that the shuttle fulfils important roles for these organisms, albeit different from its well-known function in the T. brucei bloodstream form. It allows (1) a high yield of further metabolizable glycolytic products by decreasing the need to produce a secreted end product of glycosomal metabolism, succinate; (2) the consumption of glycerol and glycerol 3-phosphate derived from lipids; and (3) to keep the redox balance of the glycosome finely tuned due to a highly flexible and redundant system.

Substrate specificity of inner membrane peptidase in yeast mitochondria

The inner membrane protease (IMP) cleaves intra-organelle sorting peptides from precursor proteins in mitochondria of the yeast Saccharomyces cerevisiae. An unusual feature of the IMP is the presence of two catalytic subunits, Imp1p and Imp2p, which recognize distinct substrate sets even though both enzymes belong to the same protease family. This nonoverlapping substrate specificity was hypothesized to result from the recognition of distinct residues at the P'1 position (also termed +1 position) in the protease substrates. Here, we constructed an extensive series of mutations to obtain a profile of the critical cleavage site residues in IMP substrates and conclude that Imp1p, and not Imp2p, recognizes specific P'1 residues. In addition to its specificity for P'1 residues, Imp1p also shows substrate specificity for the P3 (-3) position. In contrast, Imp2p recognizes the P1 (-1) position and the P3 position. Based on this new understanding of IMP substrate specificity, we conducted a survey for candidate IMP substrates in mammalian mitochondria and found consensus Imp2p cleavage sites in mammalian precursors to cytochrome c1 and glycerol-3-phosphate (G-3-P) dehydrogenase. Presence of a putative Imp2p cleavage site in G-3-P dehydrogenase was surprising, as its yeast ortholog contains an Imp1p cleavage site. To address this issue experimentally, we performed the first co-expression of mammalian IMP with proposed mammalian IMP precursors in yeast and show that murine precursors to cytochrome c1 and G-3-P dehydrogenase are cleaved by murine Imp2p. These results suggest, surprisingly, G-3-P dehydrogenase has switched from Imp1p in yeast to Imp2p in mammals.

Differential requirements of calcium for oxoglutarate dehydrogenase and mitochondrial nitric-oxide synthase under hypoxia: Impact on the regulation of mitochondrial oxygen consumption

Studies with isolated mitochondria are performed at artificially high pO(2) (220 to 250 microM oxygen), although this condition is hyperoxic for these organelles. It was the aim of this study to evaluate the effect of hypoxia (20-30 microM) on the calcium-dependent activation of 2-oxoglutarate dehydrogenase (or 2-ketoglutarate dehydrogenase; OGDH) and mitochondrial nitric-oxide synthase (mtNOS). Mitochondria had a P/O value 15% higher in hypoxia than that in normoxia, indicating that oxidative phosphorylation and electron transfer were more efficiently coupled, whereas the intramitochondrial free calcium concentrations were higher (2-3-fold) at lower pO(2). These increases were abrogated by ruthenium red indicating that the higher uptake via the calcium uniporter was involved in this process. Mitochondria at high calcium concentration microdomains may produce nitric oxide, given the K(0.5) of calcium for OGDH (0.16 microM) and mtNOS (approximately 1 microM). Nitric oxide, by binding to cytochrome oxidase in competition with oxygen, decreases the rate of oxygen consumption. This condition is highly beneficial for the following reasons: i, these mitochondria are still able to produce ATP and support calcium clearance; ii, it prevents the accumulation of ROS by slowing the rate of oxygen consumption (hence ROS production); iii, the onset of anoxia is delayed, allowing oxygen to diffuse back to these sites, thereby ameliorating the oxygen gradient between regions of high and low calcium concentration. In this way, oxygen depletion at the latter sites is prevented. This, in turn, assures continued aerobic metabolism which may involve the activated dehydrogenases.

pH difference across the outer mitochondrial membrane measured with a green fluorescent protein mutant

In this study we have generated a EYFP targeted to the mitochondrial intermembrane space (MIMS-EYFP) to determine for the first time the pH within this compartment. The fragment encoding HAI-tagged EYFP was fused with the C-terminus of glycerol-phosphate dehydrogenase, an integral protein of the inner mitochondrial membrane. Human ECV304 cells transiently transfected with MIMS-EYFP showed the typical mitochondrial network, co-localized with MitoTracker Red. Following the calibration procedure, an estimation of the pH value in the intermembrane space was obtained. This value (6.88+/-0.09) was significantly lower than that determined in the cytosol after transfection with a cytosolic EYFP (7.59+/-0.01). Further, the pH of the mitochondrial matrix, determined with a EYFP targeted to this subcompartment, was 0.9 pH units higher than that in the intermembrane space. In conclusion, MIMS-EYFP represents a novel powerful tool to monitor pH changes in the mitochondrial intermembrane space of live cells.

Recombinant expression of the Ca(2+)-sensitive aspartate/glutamate carrier increases mitochondrial ATP production in agonist-stimulated Chinese hamster ovary cells

The Ca(2+)-sensitive dehydrogenases of the mitochondrial matrix are, so far, the only known effectors to allow Ca2+ signals to couple the activation of plasma membrane receptors to the stimulation of aerobic metabolism. In this study, we demonstrate a novel mechanism, based on Ca(2+)-sensitive metabolite carriers of the inner membrane. We expressed in Chinese hamster ovary cells aralar1 and citrin, aspartate/glutamate exchangers that have Ca(2+)-binding sites in their sequence, and measured mitochondrial Ca2+ and ATP levels as well as cytosolic Ca2+ concentration with targeted recombinant probes. The increase in mitochondrial ATP levels caused by cell stimulation with Ca(2+)-mobilizing agonists was markedly larger in cells expressing aralar and citrin (but not truncated mutants lacking the Ca(2+)-binding site) than in control cells. Conversely, the cytosolic and the mitochondrial Ca2+ signals were the same in control cells and cells expressing the different aralar1 and citrin variants, thus ruling out an indirect effect through the Ca(2+)-sensitive dehydrogenases. Together, these data show that the decoding of Ca2+ signals in mitochondria depends on the coordinate activity of mitochondrial enzymes and carriers, which may thus represent useful pharmacological targets in this process of major pathophysiological interest.

Identification of a mitochondrial glycerol-3-phosphate dehydrogenase from Arabidopsis thaliana: evidence for a mitochondrial glycerol-3-phosphate shuttle in plants

We report molecular characterization of an Arabidopsis gene encoding a mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase (FAD-GPDH) that oxidizes glycerol-3-phosphate (G-3-P) to dihydroxyacetone phosphate. We demonstrate through in vitro targeting assays that the encoded gene product can be imported into mitochondrial membrane systems. Enzyme activity of the protein was confirmed through heterologous expression in Escherichia coli. The Arabidopsis gene is expressed throughout plant development, but at the highest level during seed germination. We also show that expression of the Arabidopsis FAD-GPDH gene is coupled to oxygen consumption and affected by ABA and stress conditions. Together with an NAD(+)-dependent GPDH, this enzyme could form a G-3-P shuttle, as previously established in other eukaryotic organisms, and links cytosolic G-3-P metabolism to carbon source utilization and energy metabolism in plants.

Normal thyroid thermogenesis but reduced viability and adiposity in mice lacking the mitochondrial glycerol phosphate dehydrogenase

The mitochondrial glycerol phosphate dehydrogenase (mGPD) is important for metabolism of glycerol phosphate for gluconeogenesis or energy production and has been implicated in thermogenesis induced by cold and thyroid hormone treatment. mGPD in combination with the cytosolic glycerol phosphate dehydrogenase (cGPD) is proposed to form the glycerol phosphate shuttle, catalyzing the interconversion of dihydroxyacetone phosphate and glycerol phosphate with net oxidation of cytosolic NADH. We made a targeted deletion in Gdm1 and produced mice lacking mGPD. On a C57BL/6J background these mice showed a 50% reduction in viability compared with wild-type littermates. Uncoupling protein-1 mRNA levels in brown adipose tissue did not differ between mGPD knockout and control pups, suggesting normal thermogenesis. Pups lacking mGPD had decreased liver ATP and slightly increased liver glycerol phosphate. In contrast, liver and muscle metabolites were normal in adult animals. Adult mGPD knockout animals had a normal cold tolerance, normal circadian rhythm in body temperature, and demonstrated a normal temperature increase in response to thyroid hormone. However, they were found to have a lower body mass index, a 40% reduction in the weight of white adipose tissue, and a slightly lower fasting blood glucose than controls. The phenotype may be secondary to consequences of the obligatory production of cytosolic NADH from glycerol metabolism in the mGPD knockout animal. We conclude that, although mGPD is not essential for thyroid thermogenesis, variations in its function affect viability and adiposity in mice.

Polyhydroxybenzoates inhibit ascorbic acid activation of mitochondrial glycerol-3-phosphate dehydrogenase: implications for glucose metabolism and insulin secretion

Glycerol-3-phosphate dehydrogenase from pig brain mitochondria was stimulated 2.2-fold by the addition of 50 microm l-ascorbic acid. Enzyme activity, dependent upon the presence of l-ascorbic acid, was inhibited by lauryl gallate, propyl gallate, protocatechuic acid ethyl ester, and salicylhydroxamic acid. Homogeneous pig brain mitochondrial glycerol-3-phosphate dehydrogenase was activated by either 150 microm L-ascorbic acid (56%) or 300 microm iron (Fe(2+) or Fe(3+) (62%)) and 2.6-fold by the addition of both L-ascorbic acid and iron. The addition of L-ascorbic acid and iron resulted in a significant increase of k(cat) from 21.1 to 64.1 s(-1), without significantly increasing the K(m) of L-glycerol-3-phosphate (10.0-14.5 mm). The activation of pure glycerol-3-phosphate dehydrogenase by either L-ascorbic acid or iron or its combination could be totally inhibited by 200 microm propyl gallate. The metabolism of [5-(3)H]glucose and the glucose-stimulated insulin secretion from rat insulinoma cells, INS-1, were effectively inhibited by 500 microm or 1 mm propyl gallate and to a lesser extent by 5 mm aminooxyacetate, a potent malate-aspartate shuttle inhibitor. The combined data support the conclusion that l-ascorbic acid is a physiological activator of mitochondrial glycerol-3-phosphate dehydrogenase, that the enzyme is potently inhibited by agents that specifically inhibit certain classes of di-iron metalloenzymes, and that the enzyme is chiefly responsible for the proximal signal events in INS-1 cell glucose-stimulated insulin release.

Functional analysis of two promoters for the human mitochondrial glycerol phosphate dehydrogenase gene

Mitochondrial glycerol phosphate dehydrogenase (mGPD) is abundant in the normal pancreatic insulin cell, but its level is lowered 50% by diabetes. To evaluate mGPD expression, we cloned and characterized the 5'-flanking region of the human mGPD gene. The gene has two alternative first exons and two promoters. The downstream promoter (B) is 10 times more active than the upstream promoter (A) in insulin-secreting cells (INS-1) and HeLa cells. Promoter B has higher activity in INS-1 than in non-beta cells. Deletion and mutation analysis suggested that a NRF-2 binding site at -94 to -101 and an E2F binding site at -208 to -215 are important regulatory cis elements in promoter B. Gel mobility shift assays indicated that the -94 to -101 region binds the NRF-2 protein. When INS-1 cells were maintained in the presence of high glucose (25 mm) for 7 days, mGPD was the only 1 of 6 enzyme activities lowered (53%). mGPD promoter B activity was reduced by 60% in INS-1 cells by the high glucose, but in HepG2 cells and HeLa cells, promoter B activity was unchanged or slightly increased. Deletion analysis indicated the glucose responsiveness was distributed across the region from -340 to -260 in promoter B. The results indicate that mGPD gene transcription in the beta cell is regulated differently from other cells and that decreased mGPD promoter B transcription is at least in part the cause of the decreased beta cell mGPD levels in diabetes.

Metabolic adaptation of the hypertrophied heart: role of the malate/aspartate and alpha-glycerophosphate shuttles

Activation of the malate/aspartate and alpha -glycerophosphate shuttles (the NADH shuttles) has been identified in glycolytically active newborn myocardium. The goal of this study was to determine if the NADH shuttles and their regulatory genes are activated in hypertrophied myocardium as substrate utilization shifts away from fatty acids and toward glucose and lactate. Capacity of the shuttles was determined in cardiac mitochondria isolated one week, one month, and three months following aortic banding or sham operation. Myocardial steady-state mRNA and protein levels of regulatory enzymes were also measured. Despite a significant increase in left ventricular mass and activation of the atrial natriuretic peptide gene, no change in malate/aspartate nor alpha -glycerophosphate shuttle capacity was found at any of the three time points studied. Reactivation of the genes encoding the regulatory inner mitochondrial membrane proteins was not found in the hypertrophied myocardium, though these genes were down regulated one week following aortic-banding. These results suggest that sufficient malate/aspartate and alpha -glycerophosphate shuttle capacity exists in cardiac mitochondria to accommodate increased shuttle flux as hypertrophied myocardium becomes more glycolytically active.

Multiple promoters direct the tissue-specific expression of rat mitochondrial glycerol-3-phosphate dehydrogenase

The mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase (mGPDH) is an essential component of the glycerol phosphate shuttle which transfers reduction equivalents from the cytoplasm into the mitochondria. We analyzed the distribution of different exon 1-containing transcripts by RT-PCR in various tissues in vivo. Exon 1 a was predominantly expressed in brain, brown adipose tissue and pancreas, exon 1b was ubiquitously expressed, and exon 1c was exclusively expressed in testis. In transient transfection assays the ubiquitous promoter B showed a detectable activity, whereas promoters A and C were completely silent. A deletion mutational analysis located the basal promoter B activity to a 316 bp core sequence upstream of the transcription start site.

Thyroid hormone regulation of the NADH shuttles in liver and cardiac mitochondria

Thyroid hormone can potentially regulate the malate/aspartate and alpha-glycerophosphate shuttle pathways in cardiac mitochondria either directly, by altering gene expression, or indirectly, by increasing myocardial workload. The goal of the current study was to determine the influence of thyroid hormone on the NADH shuttles in cardiac and liver mitochondria. Malate/aspartate and alpha-glycerophosphate shuttle capacities were significantly increased in cardiac mitochondria from adult rats treated for 9 days with T3 compared to saline-treated controls. Liver mitochondria demonstrated a significant increase in alpha-glycerophosphate and no change in malate/aspartate shuttle capacity. T3 increased steady-state mRNA levels and activity of mitochondrial alpha-glycerophosphate dehydrogenase in both myocardium and liver. Quantitative immunoblot studies demonstrated a significant increase in aspartate-glutamate carrier levels in T3-treated myocardium suggesting a regulatory role of the aspartate/glutamate carrier in T3-treated hearts. Thyroid hormone effects on the NADH shuttles are tissue-specific. Changes in the NADH shuttles in the presence of thyroid hormone excess occur both directly at the gene level and indirectly as an adaptive response.

Mitochondria as biosensors of calcium microdomains

The notion that the agonist-dependent increases in intracellular Ca2+ concentration, on ubiquitous signalling mechanism, occur with a tightly regulated spatio-temporal pattern has become an established concept in modern cell biology. As a consequence, the concept is emerging that the recruitment of specific intracellular targets and effector system mechanisms depends on exposure to local [Ca2+] that differs substantially from the mean [Ca2+]. A striking example is provided by mitochondria, intracellular organelles that have been overlooked for a long time in the field of calcium signalling due to the low affinity of their Ca(2+)-uptake pathways. We will summarize here some of the evidence indicating that these organelles actively participate in Ca2+ homeostasis in physiological conditions (with consequences not only for the control of their function, but also for the modulation of the complexity of calcium signals) because they have the capability to respond to microdomains of high [Ca2+] transiently generated in their proximity by the opening of Ca2+ channels.

Hexachlorobenzene, a dioxin-type compound, increases malic enzyme gene transcription through a mechanism involving the thyroid hormone response element

Hexachlorobenzene (HCB) is a dioxin-type chemical that acts mainly through the aryl hydrocarbon receptor. Chronic exposure of rats to HCB increases the activity of malic enzyme (ME). In this report, we show that this increase is correlated with an induction of ME messenger RNA (mRNA) levels, with the maximal HCB effect achieved after 9 days of intoxication. This effect is specific for ME, as other liver enzymes, such as glyceraldehyde-3-phosphate dehydrogenase, phosphoenol pyruvate carboxykinase, and mitochondrial alpha-glycerol-3-phosphate dehydrogenase, are not affected by HCB. The induction of ME mRNA levels is accompanied by an increase in ME promoter activity, as demonstrated by transient transfection experiments performed in rat hepatoma H35 cells. In an attempt to identify the cis-regulatory elements responsible for the HCB effect, different promoter deletions and mutations were used. The results obtained localize the responsive region between positions -315 and -177. This region does not contain either consensus xenobiotic response or activating protein-1 elements, the two main mediators of dioxin compounds described to date. In contrast, a thyroid hormone response element (TRE) is located between -281 to -261. Deletions and mutations of the TRE element do not respond to HCB, demonstrating that this element mediates the response of this dioxin-type compound. As ME gene expression is regulated mainly by thyroid hormones, we next investigated the role of T3 receptor (T3R) in the ME gene transcriptional induction mediated by HCB. Using Scatchard analysis, we show that neither T3R binding features for its ligand nor alpha1 or beta1T3R mRNA levels are changed with the toxic. In gel shift assays, however, we observed that protein/DNA complexes formed on TRE from the ME promoter were induced by HCB. Using an oligonucleotide with a mutation that eliminates the TRE function, we demonstrate a loss of the induced protein/DNA complexes. Together, these data suggest that the dioxin-type compound HCB increases ME gene transcription by modulating the levels of still unidentified nuclear proteins that bind to the TRE element of the ME promoter.

The soluble alpha-glycerophosphate oxidase from Enterococcus casseliflavus. Sequence homology with the membrane-associated dehydrogenase and kinetic analysis of the recombinant enzyme

The soluble flavoprotein alpha-glycerophosphate oxidase from Enterococcus casseliflavus catalyzes the oxidation of a "non-activated" secondary alcohol, in contrast to the flavin-dependent alpha-hydroxy- and alpha-amino acid oxidases. Surprisingly, the alpha-glycerophosphate oxidase sequence is 43% identical to that of the membrane-associated alpha-glycerophosphate dehydrogenase from Bacillus subtilis; only low levels of identity (17-22%) result from comparisons with other FAD-dependent oxidases. The recombinant alpha-glycerophosphate oxidase is fully active and stabilizes a flavin N(5)-sulfite adduct, but only small amounts of intermediate flavin semiquinone are observed during reductive titrations. Direct determination of the redox potential for the FAD/FADH2 couple yields a value of -118 mV; the protein environment raises the flavin potential by 100 mV in order to provide for a productive interaction with the reducing substrate. Steady-state kinetic analysis, using the enzyme-monitored turnover method, indicates that a ping-pong mechanism applies and also allows the determination of the corresponding kinetic constants. In addition, stopped-flow studies of the reductive half-reaction provide for the measurement of the dissociation constant for the enzyme. alpha-glycerophosphate complex and the rate constant for reduction of the enzyme flavin. These and other results demonstrate that this enzyme offers a very promising paradigm for examining the protein determinants for flavin reactivity and mechanism in the energy-yielding metabolism of alpha-glycerophosphate.

Rat mitochondrial glycerol-3-phosphate dehydrogenase gene: multiple promoters, high levels in brown adipose tissue, and tissue-specific regulation by thyroid hormone

Mitochondrial FAD-linked glycerol-3-phosphate dehydrogenase (mtGPDH) is one of the two enzymes of the glycerol phosphate shuttle. This shuttle transfers reducing equivalents from the cytoplasm to the mitochondria in a unidirectional, exothermic manner. Here, the isolation and characterization of the rat nuclear gene (Gpd2) encoding mtGPDH is reported. The mtGPDH gene spans 100 kb and consists of 17 exons. The use of alternate promoters was suggested by the presence of three different first exons and confirmed by transient expression for two of them. The first exons are expressed in a tissue-restricted manner. Exon 1a was found primarily in brain, exon 1b was used in all tissues examined, and exon 1c was detected predominantly in testis. Depending on the tissue, different transcript lengths were also observed: 5.9 kb (all tissues), 3.6 kb (skeletal muscle), and 2.5 kb (testis). The length isoforms are attributable to alternate splicing and polyadenylation site use. Very high mtGPDH mRNA levels were found in brown adipose tissue, 75 fold greater than in white adipose tissue. Thyroid hormone increased mtGPDH mRNA levels in liver and heart but not in brown adipose tissue, brain, or testis. This pattern corresponds to that of thyroid hormone-induced oxygen consumption and is consistent with a role for mtGPDH in thyroid hormone-induced thermogenesis. Both thyroid-responsive and nonresponsive tissues used promoter 1b, suggesting that tissue-specific factor(s) contribute to the tissue-restricted responsiveness to thyroid hormone.

Role of mitochondria in metabolism-secretion coupling of insulin release in the pancreatic beta-cell

The control of insulin secretion from the pancreatic beta-cell involves a complex cascade of events in which the mitochondria play a central role. In the consensus model this role is essentially restricted to the production of ATP promoting membrane depolarisation and a rise in cytosolic [Ca2+]. Evidence for the generation of an additional mitochondrial factor implicated in metabolism-secretion coupling is provided in this review. Although not yet identified, the formation of this putative factor requires an increase of [Ca2+] in the mitochondrial matrix together with a supply of carbons to the tricarboxylic acid (TCA) cycle. In this model, calcium activates matrix dehydrogenases, in particular those of the TCA cycle. This enables the synthesis of the mitochondrial factor from the TCA cycle intermediates. Experimental evidence gathered in permeabilised cells largely supports this model.

New light on mitochondrial calcium

The possibility of specifically addressing recombinant probes to mitochondria is a novel, powerful way to study these organelles within living cells. We first showed that the Ca(2+)-sensitive photoprotein aequorin, modified by the addition of a mitochondrial targeting sequence, allows to monitor specifically the Ca2+ concentration in the mitochondrial matrix ([Ca2+]m) of living cells. With this tool, we could show that, upon physiological stimulation, mitochondria undergo a major rise in [Ca2+]m, well in the range of the Ca2+ sensitivity of the matrix dehydrogenases, in a wide variety of cell types, ranging from non excitable, e.g., HeLa and CHO, and excitable, e.g., cell lines to primary cultures of various embryological origin, such as myocytes and neurons. This phenomenon, while providing an obvious mechanism for tuning mitochondrial activity to cell needs, appeared at first in striking contrast with the low affinity of mitochondrial Ca2+ uptake mechanisms. Based on indirect evidence, we proposed that the mitochondria might be close to the source of the Ca2+ signal and thus exposed to microdomains of high [Ca2+], hence allowing the rapid accumulation of Ca2+ into the organelle. In order to verify this intriguing possibility, we followed two approaches. In the first, we constructed a novel aequorin chimera, targeted to the mitochondrial intermembrane space (MIMS), i.e., the region sensed by the low-affinity Ca2+ uptake systems of the inner mitochondrial membrane. With this probe, we observed that, upon agonist stimulation, a portion of the MIMS is exposed to saturating Ca2+ concentrations, thus confirming the occurrence of microdomains of high [Ca2+] next to mitochondria. In the second approach, we directly investigated the spatial relationship of the mitochondria and the ER, the source of agonist-releasable Ca2+ in non-excitable cells. For this purpose, we constructed GFP-based probes of organelle structure; namely, by targeting to these organelles GFP mutants with different spectral properties, we could label them simultaneously in living cells. By using an imaging system endowed with high speed and sensitivity, which allows to obtain high-resolution 3D images, we could demonstrate that close contacts (< 80 nm) occur in vivo between mitochondria and the ER.

Mutation in the calcium-binding domain of the mitochondrial glycerophosphate dehydrogenase gene in a family of diabetic subjects

The Ca(2+)-sensitive and mitochondrial enzyme FAD-linked glycerophosphate dehydrogenase (m-GDH) represents an essential component of the pancreatic B-cell glucose-sensing device. This report deals with the first identified case of mutation in the calcium-binding domain of the m-GDH gene in a patient with type-2 diabetes and his glucose-intolerant half sister. Single strand conformation polymorphism analysis indeed revealed an abnormal mobility of the 32P-labelled polymerase chain reaction product in these two subjects. The corresponding base pair mutations and amino acid changes were documented. In the diabetic proband, the relative extent of the Ca(2+)-induced activation of m-GDH in CD3+ T-lymphocytes was lower than in his brother with a normal m-GDH gene sequence.

An antibiotic, ascofuranone, specifically inhibits respiration and in vitro growth of long slender bloodstream forms of Trypanosoma brucei brucei

Ascofuranone, a prenylphenol antibiotic isolated from a phytopathogenic fungus, Ascochyta visiae, strongly inhibited both glucose-dependent cellular respiration and glycerol-3-phosphate-dependent mitochondrial O2 consumption of long slender bloodstream forms of Trypanosoma brucei brucei. This inhibition was suggested to be due to inhibition of the mitochondrial electron-transport system, composed of glycerol-3-phosphate dehydrogenase (EC 1.1.99.5) and plant-like alternative oxidase. Ascofuranone noncompetitively inhibited the reduced coenzyme Q1-dependent O2 uptake of the mitochondria with respect to ubiquinol (Ki = 2.38 nM). Therefore, the susceptible site is deduced to be the ubiquinone redox machinery which links the two enzyme activities. Further, ascofuranone in combination with glycerol completely blocked energy production, and potently inhibited the in vitro growth of the parasite. Our findings suggest that ascofuranone might be a promising candidate for the chemotherapeutic agents of African trypanosomiasis.

An antibiotic, ascofuranone, specifically inhibits respiration and in vitro growth of long slender bloodstream forms of Trypanosoma brucei brucei

Ascofuranone, a prenylphenol antibiotic isolated from a phytopathogenic fungus, Ascochyta visiae, strongly inhibited both glucose-dependent cellular respiration and glycerol-3-phosphate-dependent mitochondrial O2 consumption of long slender bloodstream forms of Trypanosoma brucei brucei. This inhibition was suggested to be due to inhibition of the mitochondriai electron-transport system, composed of glycerol-3-phosphate dehydrogenase (EC 1.1.99.5) and plant-like alternative oxidase. Ascofuranone noncompetitively inhibited the reduced coenzyme Q1-dependent O2 uptake of the mitochondria with respect to ubiquinol (Ki = 2.38 nM). Therefore, the susceptible site is deduced to be the ubiquinone redox machinery which links the two enzyme activities. Further, ascofuranone in combination with glycerol completely blocked energy production, and potently inhibited the in vitro growth of the parasite. Our findings suggest that ascofuranone might be a promising candidate for the chemotherapeutic agents of African trypanosomiasis.

Structural organization and mapping of the human mitochondrial glycerol phosphate dehydrogenase-encoding gene and pseudogene

Mitochondrial glycerol phosphate dehydrogenase (mtGPD) is the rate-limiting enzyme in the glycerol phosphate shuttle, which is thought to play an important role in cells that require an active glycolytic pathway. Abnormalities in mtGPD have been proposed as a potential cause for non-insulin-dependent diabetes mellitus. To facilitate genetic studies, we have isolated genomic clones containing the coding regions of the human mtGPD-encoding gene (GPDM). The gene contains 17 exons and is estimated to span more than 80 kb. All splice junctions contain GT/AG consensus sequences. Introns interrupt the sequences encoding the leader peptide, the FAD-binding site, the calcium-binding regions, and a conserved central element postulated to play a role in glycerol phosphate binding. Fluorescence in situ hybridization was used to map this gene to chromosome 2, band q24.1. A retropseudogene was identified and mapped to chromosome 17.

Defective glycolysis and calcium signaling underlie impaired insulin secretion in a transgenic mouse

Pancreatic beta cells from mice that overexpress the Ca(2+)-binding protein calmodulin have a unique secretory defect that leads to chronic hyperglycemia. To further understand the molecular basis underlying this defect, we have studied signaling pathways in these beta cells. Measurements of cytosolic free Ca2+ concentration ([Ca2+]i) using fura-2 or indo-1 revealed a markedly reduced response when glucose was the stimulant. However, eliciting membrane depolarization with 50 mM K+ or the addition of the ATP-sensitive K+ (K+ ATP) channel antagonist tolbutamide restored [Ca2+]i transients to near normal levels. Electrophysiological analysis of the beta cell ion channels revealed that Ca2+ currents, delayed rectifier K+ currents, and K+ATP channel currents were similar in transgenic and nontransgenic cells, suggesting that these ion channels were able to function normally. However, whereas K+ATP channel currents in control cells were reduced by 50% by the presence of high glucose, those in transgenic cells were unaltered. Addition of tolbutamide inhibited this channel and enhanced the secretion of insulin in response to glucose for both control and transgenic cells. As these observations implicated a metabolic defect, glucose utilization, which is an indicator of glucose metabolism and ATP production in beta cells, was measured and found to be reduced by 40% in the transgenic cells. These data support the contention that excessive levels of calmodulin may compromise the ability of the beta cell to metabolize glucose and to modulate the state of the K+ATP channel, resulting in an inadequate control of the membrane potential, which collectively impair [Ca2+]i and thus insulin secretion in response to glucose.

Towards the molecular basis for the regulation of mitochondrial dehydrogenases by calcium ions

In mammalian cells, increases in calcium concentration cause increases in oxidative phosphorylation. This effect is mediated by the activation of four mitochondrial dehydrogenases by calcium ions; FAD-glycerol 3-phosphate dehydrogenase, pyruvate dehydrogenase, NAD-isocitrate dehydrogenase and oxoglutarate dehydrogenase. FAD-glycerol 3-phosphate dehydrogenase, being located on the outer surface of the inner mitochondrial membrane, is exposed to fluctuations in cytoplasmic calcium concentration. The other three enzymes are located within the mitochondrial matrix. While the kinetic properties of all of these enzymes are well characterised, the molecular basis for their regulation by calcium is not. This review uses information derived from calcium binding studies, analysis of conserved calcium binding motifs and comparison of amino acid sequences from calcium sensitive and non-sensitive enzymes to discuss how the recent cloning of several subunits from the four dehydrogenases enhances our understanding of the ways in which these enzymes bind calcium. FAD-glycerol 3-phosphate dehydrogenase binds calcium ions through a domain which is part of the polypeptide chain of the enzyme. In contrast, it is possible that the calcium sensitivity of the other three dehydrogenases may involve separate calcium binding subunits.