6-Phosphofructo-2-Kinase/Fructose-2,6-Bisphosphatase (PFKFB) Modulates Slow Oscillations in Pancreatic Islets

Abstract

Glucose-dependent insulin secretion from beta-cells of pancreatic islets is pulsatile, but the source of the underlying oscillations remains unclear. We have developed a computational model of the beta-cell, termed the ‘Dual Oscillator Model’ (DOM), based on the hypothesis that slow oscillations in insulin secretion reflect slow oscillations in glycolysis, which then interact with fast oscillations arising from membrane electrical activity and Ca2+. One version of the DOM predicts that glycolytic oscillations are generated by phosphofructokinase-1 (PFK1) via allosteric activation by its product fructose-1,6-bisphosphate (FBP), and terminated by depletion of the substrate fructose-6-phosphate (F6P). To directly determine whether PFK1 is a control point for slow metabolic oscillations, we experimentally manipulated the flux through PFK1 by altering the activity of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFKFB). This bifunctional enzyme, which contains an N-terminal kinase domain (PFK2) and C-terminal phosphatase domain (FBPase2), strongly activates PFK1 activity via conversion of F6P to fructose-2,6-bisphosphate, a potent allosteric activator of PFK1. PFKFB has also been proposed to bind and directly activate glucokinase, further accelerating glycolytic flux. Using optical measurements of NAD(P)H and Ca2+, we found that increasing the level of PFK2, but not FBPase2, in islets strongly decreased both the period and amplitude of slow oscillations by ∼40%. In many of the PFK2-expressing islets slow oscillations (period >120s) were either converted to fast oscillations (period <120s) or were abolished. These data are consistent with the predictions of the DOM after modification of the model to include regulation of PFK1 by PFKFB, and furthermore support the hypothesis that slow oscillations are driven by oscillations in glycolysis. Supported by F32DK085960 (M.J.M.), NSF-DMS0917664 (R.B.), NIH/NIDDK Intramural Research Program (A.S.), and R01DK46409 (L.S.).