Localization of Human Glutamate Dehydrogenases Provides Insights into Their Metabolic Role and Their Involvement in Disease Processes

Abstract

Glutamate dehydrogenase (GDH) catalyzes the reversible deamination of L-glutamate to α-ketoglutarate and ammonia. In mammals, GDH contributes to important processes such as amino acid and carbohydrate metabolism, energy production, ammonia management, neurotransmitter recycling and insulin secretion. In humans, two isoforms of GDH are found, namely hGDH1 and hGDH2, with the former being ubiquitously expressed and the latter found mainly in brain, testis and kidney. These two iso-enzymes display highly divergent allosteric properties, especially concerning their basal activity, ADP activation and GTP inhibition. On the other hand, both enzymes are thought to predominantly localize in the mitochondrial matrix, even though alternative localizations have been proposed. To further study the subcellular localization of the two human iso-enzymes, we created HEK293 cell lines stably over-expressing hGDH1 and hGDH2. In these cell lines, immunofluorescence and enzymatic analyses verified the overexpression of both hGDH1 and hGDH2 iso-enzymes, whereas subcellular fractionation followed by immunoblotting showed their predominantly mitochondrial localization. Given that previous studies have only indirectly compared the subcellular localization of the two iso-enzymes, we co-expressed them tagged with different fluorescent dyes (green and red fluorescent protein for hGDH1 and hGDH2, respectively) and found them to co-localize. Despite the wealth of information related to the functional properties of hGDH1 and hGDH2 and the availability of the hGDH1 structure, there is still an ongoing debate concerning their metabolic role and their involvement in disease processes. Data on the localization of hGDHs, as the ones presented here, could contribute to better understanding of the function of these important human enzymes.

Glutamate-glutamine homeostasis is perturbed in neurons and astrocytes derived from patient iPSC models of frontotemporal dementia

“…Together, these parallel approaches have allowed us to implicate the involvement of a crucial metabolic enzyme, GDH in FTD3 pathogenesis. Strikingly, recent evidence shows that GDH may be a potential therapeutic target in neurodegeneration: increasing its activity might enable it to play a neuroprotective role in energy-depleted conditions [22,48].…”

Glutamate-Glutamine Homeostasis is Perturbed in Neurons and Astrocytes derived from Patient iPSC Models of Frontotemporal Dementia

“…Together, these parallel approaches have allowed us to implicate the involvement of a crucial metabolic enzyme, GDH in FTD3 pathogenesis. Strikingly, recent evidence shows that GDH may be a potential therapeutic target in neurodegeneration: increasing its activity might enable it to play a neuroprotective role in energydepleted conditions 22,48 .…”

The Effects of Oxytocin on Appetite Regulation, Food Intake and Metabolism in Humans

“…More recent human studies employed methods of monoclonal antibodies [ 52 ] as well as extensive gene mapping of the OXTR in human brains, showing a widespread distribution of the OXTR throughout the brain which was reproducible regardless of ethnicity and sex when tested with the latter method [ 53 ]. Interestingly, an exploratory analysis of the anatomical proximity between the OXTR and more than 20,000 other protein coding genes in the brain showed that genes regulating feeding ( NTSR2 ) [ 54 ] and metabolism ( Glud1 and Glud2 ) [ 55 ] had robust overlap in brain expression patterns. Providing further confirmation of the link between the anatomical location of the OXTR and human eating behavior, a recent study found that OXTR brain expression patterns were strongly correlated with fMRI brain activation of areas categorized as anticipatory and appetitive [ 53 ].…”