Widening Spectrum of Cellular and Subcellular Expression of Human GLUD1 and GLUD2 Glutamate Dehydrogenases Suggests Novel Functions

Mammalian glutamate dehydrogenase1 (GDH1) (E.C. 1.4.1.3) is a mitochondrial enzyme that catalyzes the reversible oxidative deamination of glutamate to α-ketoglutarate and ammonia while reducing NAD+ and/or NADP+ to NADH and/or NADPH. It links amino acid with carbohydrate metabolism, contributing to Krebs cycle anaplerosis, energy production, ammonia handling and redox homeostasis. Although GDH1 was one of the first major metabolic enzymes to be studied decades ago, its role in cell biology is still incompletely understood. There is however growing interest in a novel GDH2 isoenzyme that emerged via duplication in primates and underwent rapid evolutionary selection concomitant with prefrontal human cortex expansion. Also, the anaplerotic function of GDH1 and GDH2 is currently under sharp focus as this relates to the biology of glial tumors and other neoplasias. Here we used antibodies specific for human GDH1 (hGDH1) and human GDH2 (hGDH2) to study the expression of these isoenzymes in human tissues. Results revealed that both hGDH1 and hGDH2 are expressed in human brain, kidney, testis and steroidogenic organs. However, distinct hGDH1 and hGDH2 expression patterns emerged. Thus, while the Sertoli cells of human testis were strongly positive for hGDH2, they were negative for hGDH1. Conversely, hGDH1 showed very high levels of expression in human liver, but hepatocytes were virtually devoid of hGDH2. In human adrenals, both hGDHs were densely expressed in steroid-producing cells, with hGDH2 expression pattern matching that of the cholesterol side chain cleavage system involved in steroid synthesis. Similarly in human ovaries and placenta, both hGDH1 and hGDH2 were densely expressed in estrogen producing cells. In addition, hGDH1, being a housekeeping enzyme, was also expressed in cells that lack endocrine function. Regarding human brain, study of cortical sections using immunofluorescence (IF) with confocal microscopy revealed that hGDH1 and hGDH2 were both expressed in the cytoplasm of gray and white matter astrocytes within coarse structures resembling mitochondria. Additionally, hGDH1 localized to the nuclear membrane of a subpopulation of astrocytes and of the vast majority of oligodendrocytes and their precursors. Remarkably, hGDH2-specific staining was detected in human cortical neurons, with different expression patterns having emerged. One pattern, observed in large cortical neurons (some with pyramidal morphology), was a hGDH2-specific labeling of cytoplasmic structures resembling mitochondria. These were distributed either in the cell body-axon or on the cell surface in close proximity to astrocytic end-feet that encircle glutamatergic synapses. Another pattern was observed in small cortical neurons with round dense nuclei in which the hGDH2-specific staining was found in the nuclear membrane. A detailed description of these observations and their functional implications, suggesting that the GDH flux is used by different cells to serve some of their unique functions, is presented below.