ABSTRACT
OBJECTIVE: beta-Cell response to glucose is characterized by mitochondrial membrane potential (Delta Psi) hyperpolarization and the production of metabolites that serve as insulin secretory signals. We have previously shown that glucose-induced mitochondrial hyperpolarization accompanies the concentration-dependent increase in insulin secretion within a wide range of glucose concentrations. This observation represents the integrated response of a large number of mitochondria within each individual cell. However, it is currently unclear whether all mitochondria within a single beta-cell represent a metabolically homogenous population and whether fuel or other stimuli can recruit or silence sizable subpopulations of mitochondria. This study offers insight into the different metabolic states of beta-cell mitochondria. RESULTS: We show that mitochondria display a wide heterogeneity in Delta Psi and a millivolt range that is considerably larger than the change in millivolts induced by fuel challenge. Increasing glucose concentration recruits mitochondria into higher levels of homogeneity, while an in vitro diabetes model results in increased Delta Psi heterogeneity. Exploration of the mechanism behind heterogeneity revealed that temporary changes in Delta Psi of individual mitochondria, ATP-hydrolyzing mitochondria, and uncoupling protein 2 are not significant contributors to Delta Psi heterogeneity. We identified BAD, a proapoptotic BCL-2 family member previously implicated in mitochondrial recruitment of glucokinase, as a significant factor influencing the level of heterogeneity. CONCLUSIONS: We suggest that mitochondrial Delta Psi heterogeneity in beta-cells reflects a metabolic reservoir recruited by an increased level of fuels and therefore may serve as a therapeutic target.
Subject(s)
Glucose/pharmacology , Insulin-Secreting Cells/physiology , Insulin/metabolism , Membrane Potentials/physiology , Mitochondrial Membranes/physiology , Animals , Calcium/pharmacology , Cell Culture Techniques , Insulin Secretion , Insulin-Secreting Cells/cytology , Insulin-Secreting Cells/drug effects , Ion Channels/deficiency , Islets of Langerhans/cytology , Islets of Langerhans/drug effects , Islets of Langerhans/physiology , Magnesium/pharmacology , Male , Membrane Potentials/drug effects , Mice , Mice, Inbred C57BL , Mice, Knockout , Microscopy, Confocal , Mitochondrial Membranes/drug effects , Mitochondrial Membranes/ultrastructure , Mitochondrial Proteins/deficiency , Uncoupling Protein 2 , bcl-Associated Death Protein/deficiencyABSTRACT
An oscillatory increase in pancreatic beta cell cytoplasmic free Ca2+ concentration, [Ca2+]i, is a key feature in glucose-induced insulin release. The role of the voltage-gated Ca2+ channel beta3 subunit in the molecular regulation of these [Ca2+]i oscillations has now been clarified by using beta3 subunit-deficient beta cells. beta3 knockout mice showed a more efficient glucose homeostasis compared to wild-type mice due to increased glucose-stimulated insulin secretion. This resulted from an increased glucose-induced [Ca2+]i oscillation frequency in beta cells lacking the beta3 subunit, an effect accounted for by enhanced formation of inositol 1,4,5-trisphosphate (InsP3) and increased Ca2+ mobilization from intracellular stores. Hence, the beta3 subunit negatively modulated InsP3-induced Ca2+ release, which is not paralleled by any effect on the voltage-gated L type Ca2+ channel. Since the increase in insulin release was manifested only at high glucose concentrations, blocking the beta3 subunit in the beta cell may constitute the basis for a novel diabetes therapy.
