Lethal Hypoglycemic Ketosis and Glyceroluria in Mice Lacking Both the Mitochondrial and the Cytosolic Glycerol Phosphate Dehydrogenases

Abstract

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...

ChREBP•Mlx Is the Principal Mediator of Glucose-induced Gene Expression in the Liver

“…Several lines of evidence have connected GPDH with obesity. For instance, mice lacking both GPDH1 (cytosolic) and GPDH2 (mitochondrial) display a decreased adiposity and reduced body weight (30). In addition, enhanced GPDH activity was observed in adipose tissue of obese humans (31).…”

“…Sequences that matched one or more of the test sequences at a minimum of 9 of 12 positions and found in all three genomes within a region of sequence homology were selected. An electrophoretic mobility shift assay was conducted as described previously (30). FLAG-tagged ChREBP (S196A/ T666A) and HA-tagged Mlx were expressed in 293 cells, and cell lysates were prepared using the whole cell extract protocol (Active Motif, Carlsbad, CA).…”

PPARα governs glycerol metabolism

“…The contribution of glycerol to hepatic glucose production greatly depends on the nutritional state and may vary from 5% postprandially in humans (28) to being the main gluconeogenic precursor in rodents after prolonged fasting (29). The significance of glycerol as a gluconeogenic precursor is supported by the episodic hypoglycemia observed in patients with isolated glycerol kinase deficiency (30) and by the phenotype of mice lacking both cGPDH and mGPDH (31), which suffer from elevated plasma glycerol concentrations and hypoglycemia before dying within the first week of life. Inasmuch as glycerol is an important gluconeogenic precursor during fasting, and its conversion to glucose in liver is impaired in the absence of PPARα, defective synthesis of glucose from glycerol may explain the fastinginduced hypoglycemia in PPARα-null mice (12)(13)(14).…”

High Expression of Thyroid Hormone Receptors and Mitochondrial Glycerol-3-phosphate Dehydrogenase in the Liver Is Linked to Enhanced Fatty Acid Oxidation in Lou/C, a Rat Strain Resistant to Obesity

“…Hence, due to its role in the regulation of the respective flux through these two shuttles, G3P metabolism and its modulation by mGPdH activity play an important role in cellular energetic homeostasis. Indeed, knock-out mice for both mitochondrial and cytosolic GPdH died a few days after birth (16). mGPdH is present in most tissues of various animal species, but its activity varies from very high (brown adipose tissue and muscle) to very low (the heart and liver) (17).…”