guanosine-triphosphate and Huntington-Disease

Transglutaminase 2 (TG2) is the first described cellular member of an enzyme family catalyzing Ca(2+)-dependent transamidation of proteins. During the last two decades its additional enzymatic (GTP binding and hydrolysis, protein disulfide isomerase, protein kinase) and non-enzymatic (multiple interactions in protein scaffolds) activities, which do not require Ca(2+) , have been recognized. It became a prevailing view that TG2 is silent as a transamidase, except in extreme stress conditions, in the intracellular environment characterized by low Ca(2+) and high GTP concentrations. To counter this presumption a critical review of the experimental evidence supporting the role of this enzymatic activity in cellular processes is provided. It includes the structural basis of TG2 regulation through non-canonical Ca(2+) binding sites, mechanisms making it sensitive to low Ca(2+) concentrations, techniques developed for the detection of protein transamidation in cells and examples of basic cellular phenomena as well as pathological conditions influenced by this irreversible post-translational protein modification.

Topics: Aging; Alternative Splicing; Alzheimer Disease; Amino Acid Sequence; Animals; Binding Sites; Calcium; Cross-Linking Reagents; Enzyme Activation; Extracellular Space; GTP-Binding Proteins; Guanosine Triphosphate; Humans; Huntington Disease; Molecular Sequence Data; Oxidation-Reduction; Protein Glutamine gamma Glutamyltransferase 2; Protein Processing, Post-Translational; Sequence Alignment; Transglutaminases

By crossing Huntington's disease (HD) R6/1 transgenic mice with 'tissue' transglutaminase (TG2) knock-out mice, we have demonstrated that this multifunctional enzyme plays an important role in the neuronal death characterising this disorder in vivo. In fact, a large reduction in cell death is observed in R6/1, TG2(-/-) compared with R6/1 transgenic mice. In addition, we have shown that the formation of neuronal intranuclear inclusions (NII) is potentiated in absence of the 'tissue' transglutaminase. These phenomena are paralleled by a significant improvement both in motor performances and survival of R6/1, TG2(-/-) versus R6/1 mice. Taken together these findings suggest an important role for tissue transglutaminase in the regulation of neuronal cell death occurring in Huntington's disease.

Topics: Animals; Brain; Cell Death; Disease Models, Animal; Down-Regulation; Female; GTP-Binding Proteins; Guanosine Triphosphate; Huntington Disease; Immunohistochemistry; Inclusion Bodies; Longevity; Male; Mice; Mice, Knockout; Microscopy, Electron; Motor Activity; Neocortex; Neostriatum; Nerve Degeneration; Nerve Tissue Proteins; Neurons; Protein Glutamine gamma Glutamyltransferase 2; Survival Rate; Transglutaminases

Tissue transglutaminase (tTG) is a transamidating enzyme that is elevated in Huntington's disease (HD) brain and may be involved in the etiology of the disease. Further, there is evidence of impaired mitochondrial function in HD. Therefore, in this study, we examined the effects of mitochondrial dysfunction on the transamidating activity of tTG. Neuroblastoma SH-SY5Y cells stably overexpressing human tTG or mutated inactive tTG were treated with 3-nitropropionic acid (3-NP), an irreversible inhibitor of succinate dehydrogenase. 3-NP treatment of tTG-expressing cells resulted in a significant increase of TG activity in situ. In vitro measurements demonstrated that 3-NP had no direct effect on tTG activity. However, 3-NP treatment resulted in a significant decrease of the levels of GTP and ATP, two potent inhibitors of the transamidating activity of tTG. No significant changes in the intracellular levels of calcium were observed in 3-NP-treated cells. Treatment with 3-NP in combination with antioxidants significantly reduced the 3-NP-induced increase in in situ TG activity, demonstrating that oxidative stress is a contributing factor to the increase of TG activity. This study demonstrates for the first time that impairment of mitochondrial function significantly increases TG activity in situ, a finding that may have important relevance to the etiology of HD.

Topics: Adenosine Triphosphate; Antioxidants; Calcium; Dose-Response Relationship, Drug; GTP-Binding Proteins; Guanosine Triphosphate; Humans; Huntington Disease; Marine Toxins; Mitochondria; Neuroblastoma; Nitro Compounds; Oxidative Stress; Oxocins; Propionates; Protein Glutamine gamma Glutamyltransferase 2; Succinate Dehydrogenase; Transfection; Transglutaminases; Tumor Cells, Cultured

Previous studies have shown that the mammalian neostriatum contains two subtypes of D2 dopamine receptors, which can be distinguished on the basis of the ability of GTP to convert high (RH) into low (RL) affinity sites for dopamine: GTP-sensitive (GS) and GTP-insensitive (GI) D2 receptors. The GI-D2 receptors in rat and human neostriatum are confined to the corticostriatal terminals. In rats, these receptors mediate the inhibitory effect of dopamine on the release of glutamate in the striatum. Here we report that the putamen in Huntington's disease (HD) lacks GI-D2 receptors. Their absence might be responsible for an inappropriate release of glutamate, which is neurotoxic in high concentrations, and might thus contribute to striatal cell death in HD.

Topics: Aged; Binding, Competitive; Female; Globus Pallidus; Guanosine Triphosphate; Humans; Huntington Disease; Male; Middle Aged; Putamen; Receptors, Dopamine; Receptors, Dopamine D2; Spiperone