G6PC3 deficiency, characterized by neutropenia and neutrophil dysfunction, is caused by deficiencies in the endoplasmic reticulum (ER) enzyme glucose-6-phosphatase- (G6Pase- or G6PC3) that converts glucose-6-phosphate (G6P) into glucose, the primary energy source of neutrophils. Enhanced neutrophil ER stress and apoptosis underlie neutropenia in G6PC3 deficiency, but the exact functional role of G6Pase- in neutrophils remains unknown. We hypothesized that the ER recycles G6Pase--generated glucose to the cytoplasm, thus regulating the amount of available cytoplasmic glucose/ G6P in neutrophils. Accordingly, a G6Pase- deficiency would impair glycolysis and hexose monophosphate shunt activities leading to reductions in lactate production, adenosine-5-triphosphate (ATP) production, and reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity. Using annexin V-depleted neutrophils, we show that glucose transporter-1 translocation is impaired in neutrophils from G6pc3 ؊/؊ mice and G6PC3-deficient patients along with impaired glucose uptake in G6pc3 ؊/؊ neutrophils. Moreover, levels of G6P, lactate, and ATP are markedly lower in murine and human G6PC3-deficient neutrophils, compared with their respective controls. In parallel, the expression of NADPH oxidase subunits and membrane translocation of p47 phox are down-regulated in murine and human G6PC3-deficient neutrophils. The results establish that in nonapoptotic neutrophils, G6Pase- is essential for normal energy homeostasis. A G6Pase- deficiency prevents recycling of ER glucose to the cytoplasm, leading to neutrophil dysfunction. (Blood. 2010;116(15):2783-2792)
IntroductionThere are 2 enzymatically active glucose-6-phosphatases (G6Pases), the liver/kidney/intestine-restricted G6Pase-␣ (or G6PC) 1,2 and the ubiquitously expressed G6Pase- (also known as G6PC3). 3,4 Both enzymes are transmembrane endoplasmic reticulum (ER) proteins, with a similar topology, that places their active site on the luminal side of the ER membrane. 5,6 Both have similar kinetic properties 2,4 and hydrolyze glucose-6-phosphate (G6P) to glucose and phosphate when coupled with the ubiquitously expressed G6P transporter (G6PT) that translocates G6P from the cytoplasm into the lumen of the ER. 7,8 The primary role of the G6Pase/G6PT complex is to provide glucose and phosphate to the ER lumen. The G6Pase-␣/G6PT complex maintains blood glucose homeostasis between meals by hydrolyzing G6P to glucose in the terminal step of gluconeogenesis and glycogenolysis. 1,2 Deficiencies in G6Pase-␣ cause glycogen storage disease type Ia (GSD-Ia) and deficiencies in G6PT result in GSD type Ib (GSD-Ib). 1,2,9 Both GSD-Ia and GSD-Ib patients manifest a phenotype of disturbed blood glucose homeostasis with GSD-Ib patients also suffering neutropenia and neutrophil dysfunction, 1,9 reflecting a role of G6PT in tissues beyond the liver and kidney.The biologic roles of G6Pase- and the G6Pase-/G6PT complex are poorly defined. Neutrophils, which express both G6Pase- and G6PT, 10 are capable ...