Molecular basis of human glutamate dehydrogenase regulation under changing energy demands

Human GLUD2 glutamate dehydrogenase is expressed in neural and testicular supporting cells

Mammalian glutamate dehydrogenase (GDH) is an allosterically regulated enzyme that is expressed widely. Its activity is potently inhibited by GTP and thought to be controlled by the need of the cell for ATP. In addition to this housekeeping human (h) GDH1, humans have acquired (via a duplication event) a highly homologous isoenzyme (hGDH2) that is resistant to GTP. Although transcripts of GLUD2, the gene encoding hGDH2, have been detected in human neural and testicular tissues, data on the endogenous protein are lacking. Here, we developed an antibody specific for hGDH2 and used it to study human tissues. Western blot analyses revealed, to our surprise, that endogenous hGDH2 is more densely expressed in testis than in brain. At the subcellular level, hGDH2 localized to mitochondria. Study of testicular tissue using immunocytochemical and immunofluorescence methods revealed that the Sertoli cells were strongly labeled by our anti-hGDH2 antibody. In human cerebral cortex, a robust labeling of astrocytes was detected, with neurons showing faint hGDH2 immunoreactivity. Astrocytes and Sertoli cells are known to support neurons and germ cells, respectively, providing them with lactate that largely derives from the tricarboxylic acid cycle via conversion of glutamate to alpha-ketoglutarate (GDH reaction). As hGDH2 is not subject to GTP control, the enzyme is able to metabolize glutamate even when the tricarboxylic acid cycle generates GTP amounts sufficient to inactivate the housekeeping hGDH1 protein. Hence, the selective expression of hGDH2 by astrocytes and Sertoli cells may provide a significant biological advantage by facilitating metabolic recycling processes essential to the supportive role of these cells.

Human GLUD1 and GLUD2 glutamate dehydrogenase localize to mitochondria and endoplasmic reticulum

Mammalian glutamate dehydrogenase (GDH), an enzyme central to glutamate metabolism, is thought to localize to the mitochondrial matrix, although there are also suggestions for the extramitochondrial presence of this protein. Whereas GDH in mammals is encoded by the GLUD1 gene, humans and the great apes have, in addition, a GLUD2 gene showing a distinct expression pattern. The encoded hGDH1 and hGDH2 isoenzymes are highly homologous, but their leader sequences are more divergent. To explore their subcellular targeting, we constructed expression vectors in which hGDH1 or hGDH2 was fused with the enhanced green fluorescent protein (EGFP) and used these to transfect COS 7, HeLa, CHO, HEK293, or neuroblastoma SHSY-5Y cells. Confocal microscopy revealed GDH-EGFP fluorescence in the cytoplasm within coarse structures. Cotransfection experiments using organelle-specific markers revealed that hGDH1 or hGDH2 colocalized with the mitochondrial marker DsRed2-Mito and to a lesser extent with the endoplasmic reticulum marker DsRed2-ER. Western blots detected two GDH-EGFP specific bands: a ~90 kDa band and a ~95 kDa band associated with the mitochondria and the endoplasmic reticulum containing cytosol, respectively. Deletion of the signal sequence, while altering drastically the fluoresce distribution within the cell, prevented GDH from entering the mitochondria, with the ~90 kDa band being retained in the cytosol. In addition, the deletion eliminated the ~95 kDa band from cell lysates, thus confirming that it represents the full-length GDH. Hence, while most of the hGDHs translocate into the mitochondria (a process associated with cleavage of the signal sequence), part of the protein localizes to the endoplasmic reticulum, probably serving additional functions.

Glutamate dehydrogenase activity in lymphocytes of B-cell chronic lymphocytic leukaemia patients

OBJECTIVES

To investigate the pattern of glutamate dehydrogenase (GLDH) activity, GLUD1 and GLUD2 expressions in peripheral blood mononuclear cells (PBMC) of untreated B-chronic lymphocytic leukemia (B-CLL) in healthy individuals (HI) and patients with infectious mononucleosis (IM).

DESIGN AND METHODS

GLDH activity was determined in a supernatant obtained from pelleted PBMC. GLUD1 and GLUD2 mRNA expression was determined using a quantitative real-time polymerase chain reaction. CD19(+) B cells from PBMC were purified by using positive selection.

RESULTS

The highest GLDH activity was found in PBMC of the B-CLL group followed by the HI group and IM group. The PBMC GLDH activity was higher in 60% of the B-CLL patients according to the established reference interval for our HI (2.17-5.70 microkat/g protein). The greater GLDH activity was also found in the CD19(+) cell preparation of the B-CLL patients (two of the three) but not in HI (n=3). The median value of GLUD1 expression was highest in the IM group (n=11), followed by the HI (n=14) and B-CLL groups (n=59) (median 4.69/3.78, P<0.005 and 4.69/2.91, P<0.0005, respectively). GLUD2 expression was not significantly different between groups.

CONCLUSIONS

The increased GLDH activity is specific for the PBMC of B-CLL patients. The GLUD1 but not the GLUD2 gene expression pattern is different between the PBMC of IM and B-CLL patients.

The human GLUD2 glutamate dehydrogenase: localization and functional aspects

In all mammals, glutamate dehydrogenase (GDH), an enzyme central to the metabolism of glutamate, is encoded by a single gene (GLUD1 in humans) which is expressed widely (housekeeping). Humans and other primates also possess a second gene, GLUD2, which encodes a highly homologous GDH isoenzyme (hGDH2) expressed predominantly in retina, brain and testis. There is evidence that GLUD1 was retro-posed <23 million years ago to the X chromosome, where it gave rise to GLUD2 through random mutations and natural selection. These mutations provided the novel enzyme with unique properties thought to facilitate its function in the particular milieu of the nervous system. hGDH2, having been dissociated from GTP control (through the Gly456Ala change), is mainly regulated by rising levels of ADP/l-leucine. To achieve full-range regulation by these activators, hGDH2 needs to set its basal activity at low levels (<10% of full capacity), a property largely conferred by the evolutionary Arg443Ser change. Studies of structure/function relationships have identified residues in the regulatory domain of hGDH2 that modify basal catalytic activity and regulation. In addition, enzyme concentration and buffer ionic strength can influence basal enzyme activity. While mature hGDH1 and hGDH2 isoproteins are highly homologous, their predicted leader peptide sequences show a greater degree of divergence. Study of the subcellular sites targeted by hGDH2 in three different cultured cell lines using a GLUD2/EGFP construct revealed that hGDH2 localizes mainly to mitochondria and to a lesser extent to the endoplasmic reticulum of these cells. The implications of these findings for the potential role of this enzyme in the biology of the nervous system in health and disease are discussed.

RNA-based gene duplication: mechanistic and evolutionary insights

Gene copies that stem from the mRNAs of parental source genes have long been viewed as evolutionary dead-ends with little biological relevance. Here we review a range of recent studies that have unveiled a significant number of functional retroposed gene copies in both mammalian and some non-mammalian genomes. These studies have not only revealed previously unknown mechanisms for the emergence of new genes and their functions but have also provided fascinating general insights into molecular and evolutionary processes that have shaped genomes. For example, analyses of chromosomal gene movement patterns via RNA-based gene duplication have shed fresh light on the evolutionary origin and biology of our sex chromosomes.

Mitochondrial targeting adaptation of the hominoid-specific glutamate dehydrogenase driven by positive Darwinian selection

Many new gene copies emerged by gene duplication in hominoids, but little is known with respect to their functional evolution. Glutamate dehydrogenase (GLUD) is an enzyme central to the glutamate and energy metabolism of the cell. In addition to the single, GLUD-encoding gene present in all mammals (GLUD1), humans and apes acquired a second GLUD gene (GLUD2) through retroduplication of GLUD1, which codes for an enzyme with unique, potentially brain-adapted properties. Here we show that whereas the GLUD1 parental protein localizes to mitochondria and the cytoplasm, GLUD2 is specifically targeted to mitochondria. Using evolutionary analysis and resurrected ancestral protein variants, we demonstrate that the enhanced mitochondrial targeting specificity of GLUD2 is due to a single positively selected glutamic acid-to-lysine substitution, which was fixed in the N-terminal mitochondrial targeting sequence (MTS) of GLUD2 soon after the duplication event in the hominoid ancestor ∼18–25 million years ago. This MTS substitution arose in parallel with two crucial adaptive amino acid changes in the enzyme and likely contributed to the functional adaptation of GLUD2 to the glutamate metabolism of the hominoid brain and other tissues. We suggest that rapid, selectively driven subcellular adaptation, as exemplified by GLUD2, represents a common route underlying the emergence of new gene functions.

Little is known about the functional evolution of new hominoid genes. In this study, we utilized a combination of evolutionary analyses and cell biology experiments to unveil a novel mode by which the human- and ape-specific glutamate dehydrogenase enzyme (GLUD2) functionally adapted. We find that whereas the GLUD1 parental protein (present in all mammals) localizes to mitochondria and also to the cytoplasm, GLUD2 is specifically targeted to mitochondria. Using resurrected ancestral proteins and site-directed mutagenesis, we show that the optimized mitochondrial targeting capacity of GLUD2 is due to a single positively selected amino acid substitution in its N-terminal targeting sequence, which occurred soon after the duplication event in the ape ancestor 18–25 million years ago. The specialization in mitochondrial localization is probably linked to the function of GLUD2 in the glutamate metabolism of the brain (recycling of glutamate in astrocytes), but is likely also of functional relevance in other tissues in which GLUD2 is expressed. We suggest that in addition to the traditionally considered modes of functional adaptation (changes in gene expression and/or the biochemical function of the protein), rapid and selectively driven subcellular adaptation to specific ancestral compartments may represent a common yet previously little-considered mechanism for the origin of new gene functions.

Tissue specificity of mitochondrial glutamate pathways and the control of metabolic homeostasis

Glutamate is implicated in numerous metabolic and signalling functions that vary according to specific tissues. Glutamate metabolism is tightly controlled by activities of mitochondrial enzymes and transmembrane carriers, in particular glutamate dehydrogenase and mitochondrial glutamate carriers that have been identified in recent years. It is remarkable that, although glutamate-specific enzymes and transporters share similar properties in most tissues, their regulation varies greatly according to particular organs in order to achieve tissue specific functions. This is illustrated in this review when comparing glutamate handling in liver, brain, and pancreatic beta-cells. We describe the main cellular glutamate pathways and their specific functions in different tissues, ultimately contributing to the control of metabolic homeostasis at the organism level.

Identification of amino acid residues responsible for different GTP preferences of human glutamate dehydrogenase isozymes

Human glutamate dehydrogenase isozymes (hGDH1 and hGDH2) differ markedly in their inhibition by GTP. These regulatory preferences must arise from amino acid residues that are not common between hGDH isozymes. We have constructed chimeric enzymes by reciprocally switching the corresponding amino acid segments 390-465 in hGDH isozymes that are located within or near the C-terminal 48-residue antenna helix, which is thought to be part of the regulatory domain of mammalian GDHs. These resulted in triple mutations in amino acid sequences at 415, 443, and 456 sites that are not common between hGDH1 and hGDH2. The chimeric enzymes did not change their enzyme efficiency (k(cat)/K(m)) and expression level. Functional analyses, however, revealed that the chimeric mutants almost completely acquired the different GTP regulatory preference between hGDH isozymes. These results suggest that the 415, 443, and 456 residues acting in concert are responsible for the GTP inhibitory properties of hGDH isozymes.

Metabolic and regulatory roles of leucine in neural cells

Dietary leucine transported into the brain parenchyma serves several functions. Most prominent is the role of leucine as a metabolic precursor of fuel molecules, alpha-ketoisocaproate and ketone bodies. As alternatives to glucose, these compounds are forwarded by the producing astrocytes to the adjacent neural cells. Leucine furthermore participates in the maintenance of the nitrogen balance in the glutamate/glutamine cycle pertinent to the neurotransmitter glutamate. Leucine also serves as a regulator of the activity of some enzymes important for brain energy metabolism. Another role of leucine as an informational molecule is in mTOR signaling that participates in the regulation of food ingestion. The importance of leucine for brain function is stressed by the fact that inborn errors in its metabolism cause metabolic diseases often associated with neuropathological symptoms. In this overview, the current knowledge on the metabolic and regulatory roles of this essential amino acid in neural cells are briefly summarized.

Amino Acid Changes within Antenna Helix Are Responsible for Different Regulatory Preferences of Human Glutamate Dehydrogenase Isozymes

Human glutamate dehydrogenase (hGDH) exists in hGDH1 (housekeeping isozyme) and in hGDH2 (nerve-specific isozyme), which differ markedly in their allosteric regulation. Because they differ in only 16 of their 505 amino acids, the regulatory preferences must arise from amino acid residues that are not common between hGDH1 and hGDH2. To our knowledge none of the mutagenesis studies on the hGDH isozymes to date have identified the amino acid residues fully responsible for the different regulatory preferences between hGDH1 and hGDH2. In this study we constructed hGDH1(hGDH2(390-448))hGDH1 (amino acid segment 390-448 of hGDH1 replaced by the corresponding hGDH2 segment) and hGDH2(hGDH1(390-448))hGDH2 (amino acid segment 390-448 of hGDH2 replaced by the corresponding hGDH1 segment) by swapping the corresponding amino acid segments in hGDH1 and hGDH2. The chimeric enzymes by reciprocal swapping resulted in double mutations in amino acid sequences at 415 and 443 residues that are not common between hGDH1 and hGDH2 and are located in the C-terminal 48-residue "antenna" helix, which is thought to be part of the regulatory domain of mammalian GDHs. Functional analyses revealed that the doubly mutated chimeric enzymes almost completely acquired most of the different regulatory preferences between hGDH1 and hGDH2 for electrophoretic mobility, heat-stability, ADP activation, palmitoyl-CoA inhibition, and l-leucine activation, except for GTP inhibition. Our results indicate that substitutions of the residues in the antenna region may be important evolutionary changes that led to the adaptation of hGDH2 to the unique metabolic needs of the nerve tissue.

N-Acetylaspartate in the CNS: from neurodiagnostics to neurobiology

The brain is unique among organs in many respects, including its mechanisms of lipid synthesis and energy production. The nervous system-specific metabolite N-acetylaspartate (NAA), which is synthesized from aspartate and acetyl-coenzyme A in neurons, appears to be a key link in these distinct biochemical features of CNS metabolism. During early postnatal central nervous system (CNS) development, the expression of lipogenic enzymes in oligodendrocytes, including the NAA-degrading enzyme aspartoacylase (ASPA), is increased along with increased NAA production in neurons. NAA is transported from neurons to the cytoplasm of oligodendrocytes, where ASPA cleaves the acetate moiety for use in fatty acid and steroid synthesis. The fatty acids and steroids produced then go on to be used as building blocks for myelin lipid synthesis. Mutations in the gene for ASPA result in the fatal leukodystrophy Canavan disease, for which there is currently no effective treatment. Once postnatal myelination is completed, NAA may continue to be involved in myelin lipid turnover in adults, but it also appears to adopt other roles, including a bioenergetic role in neuronal mitochondria. NAA and ATP metabolism appear to be linked indirectly, whereby acetylation of aspartate may facilitate its removal from neuronal mitochondria, thus favoring conversion of glutamate to alpha ketoglutarate which can enter the tricarboxylic acid cycle for energy production. In its role as a mechanism for enhancing mitochondrial energy production from glutamate, NAA is in a key position to act as a magnetic resonance spectroscopy marker for neuronal health, viability and number. Evidence suggests that NAA is a direct precursor for the enzymatic synthesis of the neuron specific dipeptide N-acetylaspartylglutamate, the most concentrated neuropeptide in the human brain. Other proposed roles for NAA include neuronal osmoregulation and axon-glial signaling. We propose that NAA may also be involved in brain nitrogen balance. Further research will be required to more fully understand the biochemical functions served by NAA in CNS development and activity, and additional functions are likely to be discovered.

Properties and molecular evolution of humanGLUD2 (neural and testicular tissue-specific) glutamate dehydrogenase

Glutamate dehydrogenase (GDH) is an enzyme central to the metabolism of glutamate that also plays a role in cellular energetics. In the human, GDH exists in a housekeeping isoenzyme (hGDH1) encoded by the GLUD1 gene and a neural and testicular tissue-specific isoform (hGDH2) encoded by the GLUD2 gene. There is evolutionary evidence that the GLUD1 was retroposed to the X chromosome in the ape ancestor (<23 million years ago), where it gave rise to GLUD2 through random mutations and directional selection. In the human, the two mature GDH isoproteins are highly homologous, differing in only 16 of their 505 amino acid residues. Functional analyses of highly purified recombinant wild-type hGDH2 revealed that this adaptive evolution dissociated the enzyme from GTP control, permitted regulation almost entirely by ADP and/or L-leucine, and fine-tuned its activity to the relatively low cellular pH that occurs in synaptic astrocytes during excitatory transmission. Study of structure-function relationships, using site-directed mutagenesis of GLUD1 at single sites differing from GLUD2, showed that the Arg443Ser and the Gly456Ala change reproduced some, but not all, of the properties of hGDH2. In addition, we created a double hGDH1 mutant that had both Arg443Ser and Gly456Ala in the same polypeptide chain. Functional analyses revealed that the doubly mutated enzyme did not acquire all the characteristics of the wild-type hGDH2. Hence, additional amino acid changes, acting in concert with Arg443Ser and Gly456Ala, ought to be responsible the unique properties of the brain-specific human isoenzyme.

A critical role for peroxisomal proliferator-activated receptor-alpha nuclear receptors in the development of cardiomyocyte degeneration and necrosis

Peroxisomal proliferator-activated receptor (PPAR)-alpha is a ligand-activated transcriptional factor that regulates genes involved in lipid metabolism and energy homeostasis. PPAR-alpha activators, including fibrates, have been used to treat dyslipidemia for several decades. In contrast to their known effects on lipids, the pharmacological consequences of PPAR-alpha activation on cardiac metabolism and function are not well understood. Therefore, we evaluated the role that PPAR-alpha receptors play in the heart. Our studies demonstrate that activation of PPAR-alpha receptors using a selective PPAR-alpha ligand results in cardiomyocyte necrosis in mice. Studies in PPAR-alpha-deficient mice demonstrated that cardiomyocyte necrosis is a consequence of the activation of PPAR-alpha receptors. Cardiac fatty acyl-CoA oxidase mRNA levels increased at doses in which cardiac damage was observed and temporally preceded cardiomyocyte degeneration, suggesting that peroxisomal beta-oxidation correlates with the appearance of microscopic injury and cardiac injury biomarkers. Increased myocardial oxidative stress was evident in mice treated with the PPAR-alpha agonists coinciding with increased peroxisomal biomarkers of fatty acid oxidation. These findings suggest that activation of PPAR-alpha leads to increased cardiac fatty acid oxidation and subsequent accumulation of oxidative stress intermediates resulting in cardiomyocyte necrosis.