Forms Of Isocitrate Dehydrogenase Research Articles

Abstract A62: A cerebral organoid model of pediatric glioma

Abstract Glioblastoma is the most devastating brain cancer with dismal prognosis both in adults and children and accounts for 16% of primary brain tumors. Despite aggressive therapeutic approaches, the majority of patients die within one year after diagnosis. Current in vivo modeling of glioblastoma involves xenografts either of cultured cell lines or human biopsies transplanted into mice. These models do not faithfully recapitulate the complex biology/pathogenesis of cancer. Here we used human pluripotent embryonic stem cells (or young children fibroblast-driven induced pluripotent stem cells) to create 3D structures with histology reminiscent of human fetal brain (the so-called cerebral organoids). Cerebral organoids combine the accurate multilineage differentiation and physiology of a developing brain with the easiness of manipulation of an in vitro system. We engineered these mini-brains to develop tumors by forced expression of combinations of epigenetic and genetic hits that have previously been shown to drive pediatric glioblastoma multiform (GMB). These include the expression of a mutant histone H3 variant H3F3A (mutated at K27M or G34R) or a gain of function of a mutant form of isocitrate dehydrogenase (IDH1R132H) in combination with the loss of ATRX histone remodeling factor and/or p53 tumor suppressor. Lentiviral transfection was used to express combination of these genetic and epigenetic hits into cerebral organoids. The mutated organoids exhibited tumorigenic growth that was histologically characterized. For example, the expression of H3K27M in combination with loss of both p53 and ATRX showed a remarkable dysplasia reminiscent of primary glioblastoma. Moreover, these organoids exhibited tumor rosette structures resembling those found in ependymoma. In contrast, organoids expressing K27M alone did not show such structures. Interestingly, IDH1R132H combined with p53 and ATRX loss resulted in different tumor morphology, suggestive of a possibility to model oncogenic dependence in pediatric GBM. Cerebral organoids could also be used as a model to study tumor invasion starting with glioma stem-like cells (GSCs, which are believed to be the tumor-initiating cells in GBM). Interestingly, the GSC cell line 923 (which does not invade mouse brain in vivo) invaded and formed a focal lesion in a cocultured organoid. The organoid-based cancer model presented here carries a promising potential to uncover the initiation and pathogenesis of pediatric tumors early in the developing brain. Note: This abstract was not presented at the conference. Citation Format: Amin Ismail, Demis Balamatsias. A cerebral organoid model of pediatric glioma [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr A62.

Novel type II and monomeric NAD+ specific isocitrate dehydrogenases: phylogenetic affinity, enzymatic characterization, and evolutionary implication.

NAD+ use is an ancestral trait of isocitrate dehydrogenase (IDH), and the NADP+ phenotype arose through evolution as an ancient adaptation event. However, no NAD+-specific IDHs have been found among type II IDHs and monomeric IDHs. In this study, novel type II homodimeric NAD-IDHs from Ostreococcus lucimarinus CCE9901 IDH (OlIDH) and Micromonas sp. RCC299 (MiIDH), and novel monomeric NAD-IDHs from Campylobacter sp. FOBRC14 IDH (CaIDH) and Campylobacter curvus (CcIDH) were reported for the first time. The homodimeric OlIDH and monomeric CaIDH were determined by size exclusion chromatography and MALDI-TOF/TOF mass spectrometry. All the four IDHs were demonstrated to be NAD+-specific, since OlIDH, MiIDH, CaIDH and CcIDH displayed 99-fold, 224-fold, 61-fold and 37-fold preferences for NAD+ over NADP+, respectively. The putative coenzyme discriminating amino acids (Asp326/Met327 in OlIDH, Leu584/Asp595 in CaIDH) were evaluated, and the coenzyme specificities of the two mutants, OlIDH R326H327 and CaIDH H584R595, were completely reversed from NAD+ to NADP+. The detailed biochemical properties, including optimal reaction pH and temperature, thermostability, and metal ion effects, of OlIDH and CaIDH were further investigated. The evolutionary connections among OlIDH, CaIDH, and all the other forms of IDHs were described and discussed thoroughly.

Kinetic model of functioning and regulation of Escherichia coli isocitrate dehydrogenase

A Rapid Equilibrium Random Bi Ter mechanism of formation of two dead-end complexes was proposed to describe the experimental data on the functioning of E. coli isocitrate dehydrogenase (IDH). A kinetic model for the enzyme functioning was constructed, which assumes that it is regulated through reversible phosphorylation by its kinase/phosphatase, which in turn is regulated by IDH substrates and central metabolites such as pyruvate (Pyr), 3-phosphoglycerate (3-PG), and AMP. It was shown using the model that increasing the concentration of these effectors results in an increase of the active part of IDH, thus leading to an increase in the Krebs cycle flux. We predict that the ratio of the phosphorylated and free forms of IDH (IDHP/IDH) is more sensitive to AMP, NADPH, and isocitrate concentrations than to Pyr and 3-PG. The model allows a realistic prediction of changes in the IDHP/IDH ratio, which would occur under changes of biosynthetic and energetic loading of the E. coli cell.

Assembly and Function of a Cytosolic Form of NADH-specific Isocitrate Dehydrogenase in Yeast

Mitochondrial NAD-dependent isocitrate dehydrogenase catalyzes a rate-limiting step in the tricarboxylic acid cycle. Yeast isocitrate dehydrogenase is an octomer composed of two subunits (IDH1 and IDH2) encoded by different genes and possessing independent mitochondrial targeting presequences. Oligonucleotide-directed mutagenesis was used to remove the presequences from each gene and from both genes carried on centromere-based expression plasmids. Effects on cellular localization were examined in a yeast strain containing chromosomal disruptions of IDH1 and IDH2 loci. Each subunit was found to be dependent upon its presequence for mitochondrial localization, and the subunits are independently imported into mitochondria under most growth conditions. Furthermore, an active holoenzyme can be assembled in the cytosol and this ''cytosolic'' form of isocitrate dehydrogenase can reverse the acetate- growth phenotype characteristic of the DeltaIDH1/ DeltaIDH2 disruption strain, indicating functional replacement of the mitochondrial enzyme. However, transformants containing plasmids lacking either the IDH1 or IDH2 presequence coding regions were unexpectedly found to be capable of growth on acetate medium. Further investigation demonstrated that cellular localization of the IDH1 subunit can be biased by this stringent growth pressure.

Membrane-Associated NAD-Dependent Isocitrate Dehydrogenase in Potato Mitochondria

The oxidation isotherms for citrate and isocitrate by potato (Solanum tuberosum var. Russet Burbank) mitochondria in the presence of NAD differ markedly. Citrate oxidation shows positively cooperative kinetics with a sigmoid isotherm, whereas isocitrate oxidation shows Michaelis-Menten kinetics at concentrations up to 3 millimolar, and cooperative kinetics thereafter up to 30 millimolar. In the absence of exogenous NAD, the isocitrate isotherm is sigmoid throughout. The dual isotherm for isocitrate oxidation in the presence of exogenous NAD reflects the operation of two forms of isocitrate dehydrogenase, one in the matrix and one associated with the inner mitochondrial membrane. Whereas in intact mitochondria the activity of the membrane-bound enzyme is insensitive to rotenone, and to butylmalonate, an inhibitor of organic acid transport, isocitrate oxidation by the soluble matrix enzyme is inhibited by both. The membrane-bound isocitrate dehydrogenase does not operate through the NADH dehydrogenase on the outer face of the inner mitochondrial membrane, and is thus considered to face inward. The regulatory potential of isocitrate dehydrogenase in potato mitochondria may be realized by the apportionment of the enzyme between its soluble and bound forms.

Isolation and Characterization of Inner Membrane-Associated and Matrix NAD-Specific Isocitrate Dehydrogenase in Potato Mitochondria

The isotherm for isocitrate oxidation by potato (Solanum tuberosum L. var. Russet Burbank) mitochondria in the presence of exogenous NAD is characterized by a hyperbolic phase at isocitrate concentrations below 3 millimolar, and a sigmoid, or positively cooperative phase from approximately 3 to 30 millimolar. The two forms of isocitrate dehydrogenase were separated and characterized following the sonication of mitochondria in 15% glycerol in the absence of buffer, followed by fractionation in a density step gradient to yield inner membrane and matrix components. The membrane-associated isocitrate dehydrogenase was found to have a Hill, or cooperativity, number of 1, while the Hill number of the matrix enzyme was 2.5. Upon digitonin extraction the cooperativity number of the membrane enzyme rose to 3.5. The isocitrate K(m) for the membrane enzyme was calculated to be approximately 5.9 x 10(-4) molar, while the S(0.5) for the matrix was 6.9 x 10(-4) molar. The NAD K(m) for both enzymes was 150 micromolar. Whereas the membrane enzyme proved indifferent to adenine nucleotides, the matrix enzyme was arguably inhibited by AMP and ADP, and inhibited some 25% by 5 millimolar ATP. Both enzymes were negatively responsive to the mole fraction of NADH, the membrane enzyme being 50% inhibited at a mole fraction of 0.26, and the matrix enzyme by a mole fraction of 0.32. The suggestion is offered that the enzymes in question constitute two forms of a single enzyme, one peripherally associated with the inner membrane, and one soluble in the matrix. It is proposed that a degree of regulation may be achieved by the apportionment of the enzyme between the bound and free forms.

A catalytically inactive form of isocitrate dehydrogenase fromEscherichia coli

A protein exhibiting immunological cross-reactivity with NADP-specific isocitrate dehydrogenase, but containing no catalytic activity, has been isolated from nalidixic acid-resistantEscherichia coli. The two proteins have, within the limits of experimentation, identical molecular weight, subunit structure, and amino acid homology. The absence of catalytic activity in the protein isolated from nalidixic acid-resistant mutants may result from a mutation in the isocitrate dehydrogenase structural gene.

Genetics of NADP isocitrate dehydrogenase in Paramecium aurelia.

STARCH gel electrophoresis of extracts of Paramecium aurelia has shown that in this organism NADP-dependent isocitrate dehydrogenase occurs in two forms which are located in the mitochondrial and supernatant fractions respectively. The nuclear control of the mitochondrial enzyme β-hydroxybutyrate deydrogenase in Paramecium aurelia has been described already1, and I wish to report similar results for the mitochondrial form of isocitrate dehydrogenase. Genetic variants of both mitochondrial and supernatant (or cytoplasmic) forms of isocitrate dehydrogenase have been detected, and so it was possible to investigate whether these two forms are under independent genetic control.