The mitochondrial glycerol phosphate dehydrogenase (mGPD) is important for metabolism of glycerol phosphate for gluconeogenesis or energy production and has been implicated in thermogenesis induced by cold and thyroid hormone treatment. mGPD in combination with the cytosolic glycerol phosphate dehydrogenase (cGPD) is proposed to form the glycerol phosphate shuttle, catalyzing the interconversion of dihydroxyacetone phosphate and glycerol phosphate with net oxidation of cytosolic NADH. We made a targeted deletion in Gdm1 and produced mice lacking mGPD. On a C57BL/6J background these mice showed a 50% reduction in viability compared with wild-type littermates. Uncoupling protein-1 mRNA levels in brown adipose tissue did not differ between mGPD knockout and control pups, suggesting normal thermogenesis. Pups lacking mGPD had decreased liver ATP and slightly increased liver glycerol phosphate. In contrast, liver and muscle metabolites were normal in adult animals. Adult mGPD knockout animals had a normal cold tolerance, normal circadian rhythm in body temperature, and demonstrated a normal temperature increase in response to thyroid hormone. However, they were found to have a lower body mass index, a 40% reduction in the weight of white adipose tissue, and a slightly lower fasting blood glucose than controls. The phenotype may be secondary to consequences of the obligatory production of cytosolic NADH from glycerol metabolism in the mGPD knockout animal. We conclude that, although mGPD is not essential for thyroid thermogenesis, variations in its function affect viability and adiposity in mice.The glycerol phosphate shuttle, composed of the FAD-dependent mitochondrial glycerol phosphate dehydrogenase (mGPD, 1 EC 1.1.99.5) and the NAD(H)-dependent cytosolic glycerol phosphate dehydrogenase (cGPD, EC 1.1.1.8), is generally considered to play a role in the oxidation of cytosolic NADH formed during glycolysis. In mice the mitochondrial enzyme is encoded by a single gene, Gdm1, on chromosome 2 (1). The cytosolic enzyme has both an adult form, encoded by Gdc1 on chromosome 15 (2) and an embryonic form, encoded by Gdc2 on chromosome 9 (3). The embryonic form has not been found in liver or kidney during gestation but persists in brain for several weeks following birth (4) and in the epididymal white adipose tissue until at least 5 days of age (5). It has recently been reported that mice lacking the adult form of cGPD have elevated dihydroxyacetone phosphate and decreased glycerol phosphate and ATP levels in muscle following exercise (6), although these mice otherwise appear normal, having normal weights and litter sizes (7). Surprisingly, these cGPD-deficient mice grow normally even on a diet essentially free of glycerol (6), presumably by using the dihydroxyacetone phosphate acyl transferase pathway for lipid synthesis. The findings suggest that a lack of cGPD alters the cellular redox status in muscle, whereas pancreatic islet function is relatively normal and liver is only mildly affected, confirming the ability of alter...
The activities of either the mitochondrial or cytosolic glycerol phosphate dehydrogenase (mGPD, cGPD) plus that of glycerol kinase are required for the use of glycerol in aerobic metabolism and gluconeogenesis. A knockout mouse lacking mGPD has reduced body weight and fertility but shows remarkably normal liver and muscle metabolite levels. The BALB/cHeA mouse strain, which lacks cGPD, breeds well and is phenotypically normal, although it demonstrates metabolite abnormalities in certain tissues. Crosses were made between these two strains, and mice were generated that lacked both dehydrogenases. These mice, although active and nursing well for several days, failed to grow, and usually died within the first week. Liver glycerol phosphate levels were elevated 30-fold, whereas liver ATP, ADP, and AMP levels were reduced by 30 -40%. Plasma glycerol was elevated 30-to 50-fold to 30 -50 mM, and urine glycerol exceeded 0.45 M (4% w/v). GPD-deficient mice were hypoglycemic, had a 50% increase in plasma free fatty acids, and developed ketonuria within the first day of life. Uncoupling protein-1 mRNA in brown adipose tissue was reduced 60%. These mice share some features of both glycerol kinase deficiency and hereditary fructose intolerance, suggesting the phenotype may be due to the combined effects of the loss of a gluconeogenic substrate, the osmotic effects of glycerol, and the metabolic effects of the accumulation of a phosphorylated metabolite.Mammalian cells contain two glycerol phosphate dehydrogenase enzymes. The NADH-dependent cytosolic enzyme (EC 1.1.1.8) catalyzes the conversion of dihydroxyacetone phosphate (DHAP) 1 to glycerol phosphate. This reaction is reversible, with a strong preference under physiologic conditions for the production of glycerol phosphate. In the mouse, this enzyme is encoded by Gdc1. An embryonic form of the enzyme is encoded by Gdc2 but has not been found in liver or kidney during gestation, although it persists in brain of the neonate for several weeks (1) and in epididymal white fat for at least 5 days after birth (2). The FAD-dependent mitochondrial GPD (EC 1.1.99.5) is encoded by a single gene, Gdm1, on chromosome 2 (3) and is located on the outer surface of the mitochondrial inner membrane. mGPD catalyzes the irreversible conversion of glycerol phosphate to DHAP, with transfer of electrons from bound FAD through ubiquinone to complex III of the electron transport chain. These two enzymes form the glycerol phosphate shuttle, which cycles glycerol phosphate and DHAP to oxidize NADH formed in the cytosol. A spontaneous mutation in Gdc1 in the inbred strain BALB/cHeA results in an altered mRNA size and the loss of cGPD enzyme activity (4). cGPD-deficient BALB/cHeA animals are viable and fertile, although they demonstrate some evidence of an inhibition of glycolysis at glyceraldehyde phosphate dehydrogenase in skeletal muscle (5). We (40) and others (6) have produced knockout mice-deficient in mGPD. The mGPD knockout mice have decreased adiposity and reduced fertility and viability (40...
We studied a mouse doubly homozygous for mutations in the genes encoding malic enzyme (EC 1.1.1.40) and cytosolic glycerol phosphate dehydrogenase (EC 1.1.1.8) (cGPD). This mouse, which we call the mmgg mouse and which is the product of intercrosses between the Mod-1 mouse and the BALB/cHeA mouse, lacks activity of both enzymes. Like both parental strains the mmgg mouse is completely normal in appearance. cGPD is one of the two enzymes that catalyze the reactions of the glycerol phosphate shuttle. The activity of the other enzyme of the glycerol phosphate shuttle, mitochondrial glycerol phosphate dehydrogenase (EC 1.1.99.5) (mGPD), is abundant in tissues, such as brain, skeletal muscle and the pancreatic islet, suggesting that the glycerol phosphate shuttle is important in these tissues which rapidly metabolize glucose. Cytosolic malic enzyme activity is important for shuttles which transport NADPH equivalents from mitochondria to the cytosol. The major finding of the study was a highly abnormal metabolite pattern in tissues of the mmgg mouse suggesting a block in the glycerol phosphate shuttle due to cGPD deficiency. The metabolite pattern did not suggest that malic enzyme deficiency caused an abnormality. Tissue levels of glycerol phosphate (low) and dihydroxyacetone phosphate (high) were only abnormal in skeletal muscle. Glycolytic intermediates, situated at or before the triose phosphates in the pathway, such as fructose bisphosphate and glyceraldehyde phosphate were increased depending on the tissue. Taken together with previous extensive data on the mouse deficient only in cGPD, this suggests a block in glycolysis at the step catalyzed by glyceraldehyde phosphate dehydrogenase caused by an abnormally low NAD/NADH ratio resulting from a nonfunctional glycerol phosphate shuttle. Consistent with this idea the lactate/pyruvate ratio was high in skeletal muscle signifying a low cytosolic NAD/NADH ratio. The mmgg mouse was normal in all other factors studied including blood glucose and serum insulin levels, pancreatic islet mass, insulin release from isolated pancreatic islets, as well as the activities of five metabolic enzymes, including mGPD, in liver, kidney, skeletal muscle and pancreatic islets. cGPD enzyme activity was undetectable in pancreatic islets, 0.5% of normal in liver, and 2.1% of normal in kidney and skeletal muscle. Malic enzyme activity was undetectable in these same tissues.
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