title: Phosphofructo-2-kinase/fructose-2,6-bisphosphatase modulates oscillations of pancreatic islet metabolism.
abstract: Pulses of insulin from pancreatic beta-cells help maintain blood glucose in a narrow range, although the source of these pulses is unclear. It has been proposed that a positive feedback circuit exists within the glycolytic pathway, the autocatalytic activation of phosphofructokinase-1 (PFK1), which endows pancreatic beta-cells with the ability to generate oscillations in metabolism. Flux through PFK1 is controlled by the bifunctional enzyme PFK2/FBPase2 (6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase) in two ways: via (1) production/degradation of fructose-2,6-bisphosphate (Fru2,6-BP), a potent allosteric activator of PFK1, as well as (2) direct activation of glucokinase due to a protein-protein interaction. In this study, we used a combination of live-cell imaging and mathematical modeling to examine the effects of inducibly-expressed PFK2/FBPase2 mutants on glucose-induced Ca(2+) pulsatility in mouse islets. Irrespective of the ability to bind glucokinase, mutants of PFK2/FBPase2 that increased the kinase:phosphatase ratio reduced the period and amplitude of Ca(2+) oscillations. Mutants which reduced the kinase:phosphatase ratio had the opposite effect. These results indicate that the main effect of the bifunctional enzyme on islet pulsatility is due to Fru2,6-BP alteration of the threshold for autocatalytic activation of PFK1 by Fru1,6-BP. Using computational models based on PFK1-generated islet oscillations, we then illustrated how moderate elevation of Fru-2,6-BP can increase the frequency of glycolytic oscillations while reducing their amplitude, with sufficiently high activation resulting in termination of slow oscillations. The concordance we observed between PFK2/FBPase2-induced modulation of islet oscillations and the models of PFK1-driven oscillations furthermore suggests that metabolic oscillations, like those found in yeast and skeletal muscle, are shaped early in glycolysis.
authors.affiliation: Department of Pharmacology and Brehm Diabetes Center, University of Michigan, Ann Arbor, Michigan, United States of America. mmerrins@umich.edu
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Merrins
authors.forename: Richard
authors.initials: R
authors.lastname: Bertram
authors.forename: Arthur
authors.initials: A
authors.lastname: Sherman
authors.forename: Leslie S
authors.initials: LS
authors.lastname: Satin
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D050260:Carbohydrate Metabolism; D005941:Glucokinase / Q000378:metabolism; D005947:Glucose / Q000378:metabolism*; D007328:Insulin / Q000378:metabolism*; D050417:Insulin-Secreting Cells / Q000378:metabolism; D007515:Islets of Langerhans / Q000378:metabolism*; D051379:Mice; D008954:Models, Biological; D025481:Phosphofructokinase-2 / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation
chemical_list: D007328:Insulin; D025481:Phosphofructokinase-2; D005941:Glucokinase; D005947:Glucose; D002118:Calcium