The Glutamate Dehydrogenase Pathway and Its Roles in Cell and Tissue Biology in Health and Disease

Role of upstream stimulatory factor 2 in glutamate dehydrogenase gene transcription

Glutamate dehydrogenase (Gdh) plays a central role in ammonia detoxification by catalysing reversible oxidative deamination of l-glutamate into α-ketoglutarate using NAD⁺ or NADP⁺ as cofactor. To gain insight into transcriptional regulation of glud, the gene that codes for Gdh, we isolated and characterised the 5' flanking region of glud from gilthead sea bream (Sparus aurata). In addition, tissue distribution, the effect of starvation as well as short- and long-term refeeding on Gdh mRNA levels in the liver of S. aurata were also addressed. 5'-Deletion analysis of glud promoter in transiently transfected HepG2 cells, electrophoretic mobility shift assays, chromatin immunoprecipitation (ChIP) and site-directed mutagenesis allowed us to identify upstream stimulatory factor 2 (Usf2) as a novel factor involved in the transcriptional regulation of glud Analysis of tissue distribution of Gdh and Usf2 mRNA levels by reverse transcriptase-coupled quantitative real-time PCR (RT-qPCR) showed that Gdh is mainly expressed in the liver of S. aurata, while Usf2 displayed ubiquitous distribution. RT-qPCR and ChIP assays revealed that long-term starvation down-regulated the hepatic expression of Gdh and Usf2 to similar levels and reduced Usf2 binding to glud promoter, while refeeding resulted in a slow but gradual restoration of both Gdh and Usf2 mRNA abundance. Herein, we demonstrate that Usf2 transactivates S. aurata glud by binding to an E-box located in the proximal region of glud promoter. In addition, our findings provide evidence for a new regulatory mechanism involving Usf2 as a key factor in the nutritional regulation of glud transcription in the fish liver.

Glutamate Dehydrogenase as a Neuroprotective Target Against Neurodegeneration

Regulation of glutamate metabolism via glutamate dehydrogenase (GDH) might be the promising therapeutic approach for treating neurodegenerative disorders. In the central nervous system, glutamate functions both as a major excitatory neurotransmitter and as a key intermediate metabolite for neurons. GDH converts glutamate to α-ketoglutarate, which serves as a TCA cycle intermediate. Dysregulated GDH activity in the central nervous system is highly correlated with neurological disorders. Indeed, studies conducted with mutant mice and allosteric drugs have shown that deficient or overexpressed GDH activity in the brain can regulate whole body energy metabolism and affect early onset of Parkinson's disease, Alzheimer's disease, temporal lobe epilepsy, and spinocerebellar atrophy. Moreover, in strokes with excitotoxicity as the main pathophysiology, mice that overexpressed GDH exhibited smaller ischemic lesion than mice with normal GDH expression. In additions, GDH activators improve lesions in vivo by increasing α-ketoglutarate levels. In neurons exposed to an insult in vitro, enhanced GDH activity increases ATP levels. Thus, in an energy crisis, neuronal mitochondrial activity is improved and excitotoxic risk is reduced. Consequently, modulating GDH activity in energy-depleted conditions could be a sound strategy for maintaining the mitochondrial factory in neurons, and thus, protect against metabolic failure.

Adaptive Evolution of the GDH2 Allosteric Domain Promotes Gliomagenesis by Resolving IDH1R132H-Induced Metabolic Liabilities

Hotspot mutations in the isocitrate dehydrogenase 1 (IDH1) gene occur in a number of human cancers and confer a neomorphic enzyme activity that catalyzes the conversion of α-ketoglutarate (αKG) to the oncometabolite D-(2)-hydroxyglutarate (D2HG). In malignant gliomas, IDH1R132H expression induces widespread metabolic reprogramming, possibly requiring compensatory mechanisms to sustain the normal biosynthetic requirements of actively proliferating tumor cells. We used genetically engineered mouse models of glioma and quantitative metabolomics to investigate IDH1R132H-dependent metabolic reprogramming and its potential to induce biosynthetic liabilities that can be exploited for glioma therapy. In gliomagenic neural progenitor cells, IDH1R132H expression increased the abundance of dipeptide metabolites, depleted key tricarboxylic acid cycle metabolites, and slowed progression of murine gliomas. Notably, expression of glutamate dehydrogenase GDH2, a hominoid-specific enzyme with relatively restricted expression to the brain, was critically involved in compensating for IDH1R132H-induced metabolic alterations and promoting IDH1R132H glioma growth. Indeed, we found that recently evolved amino acid substitutions in the GDH2 allosteric domain conferred its nonredundant, glioma-promoting properties in the presence of IDH1 mutation. Our results indicate that among the unique roles for GDH2 in the human forebrain is its ability to limit IDH1R132H-mediated metabolic liabilities, thus promoting glioma growth in this context. Results from this study raise the possibility that GDH2-specific inhibition may be a viable therapeutic strategy for gliomas with IDH mutations.Significance: These findings show that the homonid-specific brain enzyme GDH2 may be essential to mitigate metabolic liabilities created by IDH1 mutations in glioma, with possible implications to leverage its therapeutic management by IDH1 inhibitors. Cancer Res; 78(1); 36-50. ©2017 AACR.

[Comparative study of glutamate dehydrogenase in the brain of patients with schizophrenia and mentally healthy people]

AIM

To compare the glutamate dehydrogenase (GDH) activity and amounts of GDHI, GDHII, and GDHIII immunoreactive forms in prefrontal, anterior and posterior cingulate cortex and cerebellar cortex of patients with schizophrenia and control subjects.

MATERIAL AND METHODS

GDH enzymatic activity was measured and levels of GDH immunoreactive forms were determined in extracts of autopsied samples of prefrontal, anterior and posterior cingulate cortex (areas 10, 24, and 23 by Brodmann), and cerebellar cortex of patients with schizophrenia (n=8) and controls (n=9).

RESULTS AND CONCLUSION

GDH enzymatic activity was significantly increased in the prefrontal cortex (area 10) (p<0.004), the posterior cingulate cortex (area 23) (p<0.05) and the cerebellar cortex (p<0.002) and was unchanged in the anterior cingulate cortex (area 24) in patients with schizophrenia compared to controls. The levels of immunoreactive GDH I, GDH II and GDH III were significantly higher in the prefrontal cortex of patients with schizophrenia than in controls (p<0.008, p<0.003, and p<0.0001, respectively). Levels of all three immunoreactive GDH forms were unchanged in the anterior cingulate cortex (area 24), but they were increased in the posterior cingulate cortex (area 23) (p<0.004, p<0.001 and p<0.02, respectively). The levels of immunoreactive GDH II and GDH III, but not GDH I, were significantly increased in the cerebellar cortex of patients with schizophrenia compared with the control group (p<0.02 and p<0.001, respectively). The alteration in the levels of GDH immunoreactive forms in the brain of patients with schizophrenia is one of the causes of impaired brain glutamate metabolism and an important aspect of schizophrenia pathogenesis.

Modeling of autophagy-related gene expression dynamics during long term fasting in European eel (Anguilla anguilla)

Autophagy is an evolutionary conserved cellular self-degradation process considered as a major energy mobilizing system in eukaryotes. It has long been considered as a post-translationally regulated event, and the importance of transcriptional regulation of autophagy-related genes (atg) for somatic maintenance and homeostasis during long period of stress emerged only recently. In this regard, large changes in atg transcription have been documented in several species under diverse types of prolonged catabolic situations. However, the available data primarily concern atg mRNA levels at specific times and fail to capture the dynamic relationship between transcript production over time and integrated phenotypes. Here, we present the development of a statistical model describing the dynamics of expression of several atg and lysosomal genes in European glass eel (Anguilla anguilla) during long-term fasting at two temperatures (9 °C and 12 °C) and make use of this model to infer the effect of transcripts dynamics on an integrated phenotype – here weight loss. Our analysis shows long-term non-random fluctuating atg expression dynamics and reveals for the first time a significant contribution of atg transcripts production over time to weight loss. The proposed approach thus offers a new perspective on the long-term transcriptional control of autophagy and its physiological role.