Defective Glycolysis and Calcium Signaling Underlie Impaired Insulin Secretion in a Transgenic Mouse

Abstract

] i transients to near normal levels. Electrophysiological analysis of the beta cell ion channels revealed that Ca 2؉ currents, delayed rectifier K ؉ currents, and K ؉ ATP channel currents were similar in transgenic and nontransgenic cells, suggesting that these ion channels were able to function normally. However, whereas K ؉ ATP channel currents in control cells were reduced by 50% by the presence of high glucose, those in transgenic cells were unaltered. Addition of tolbutamide inhibited this channel and enhanced the secretion of insulin in response to glucose for both control and transgenic cells. As these observations implicated a metabolic defect, glucose utilization, which is an indicator of glucose metabolism and ATP production in beta cells, was measured and found to be reduced by 40% in the transgenic cells. These data support the contention that excessive levels of calmodulin may compromise the ability of the beta cell to metabolize glucose and to modulate the state of the K ؉ ATP channel, resulting in an inadequate control of the membrane potential, which collectively impair [Ca 2؉ ] i and thus insulin secretion in response to glucose.

Alterations in Calcium Channel Currents Underlie Defective Insulin Secretion in a Transgenic Mouse

“…We find that the primary defect in beta-cells from CaM-8 mice is a remarkable reduction in the amount of Ca 2ϩ flowing through voltage-gated Ca 2ϩ channels, which results in abnormally small rises in [Ca 2ϩ ] i during glucose stimulation and presumably leads to or directly accounts for the diabetic phenotype of these animals. These results confirm that the nature and origin of the insulin secretory defect exhibited in CaM-8 mice is quite different from that of CaM mice (13) and provide a useful experimental system for the study of intracellular Ca 2ϩ signaling and Ca 2ϩ channel regulation.…”

“…These mice have defective insulin secretion at an early age, due to an impairment in the metabolism of glucose and the subsequent generation of ATP (12,13). Another mouse line, referred to as CaM-8, expresses in its beta-cells a mutant form of CaM that lacks 8 amino acids in its central helix and does not activate calmodulin-dependent proteins in vitro even though this protein binds Ca 2ϩ with normal affinity (14).…”

Calbindin-D28k Controls [Ca2+] and Insulin Release

“…Studies using transgenic mice with the calcium-binding protein calmodulin overexpressed in the pancreatic ␤ cells show that these mice (referred to as CaM mice) have decreased plasma insulin levels, leading to increased serum glucose levels and early onset of diabetes (47). Impairment in the metabolism of glucose and the subsequent generation of ATP was reported to be the underlying mechanism involved in the defective insulin secretion in the calmodulin transgenic mouse (48). Further studies were done on another mouse transgenic line, referred to as CaM-8.…”

Abnormalities of pancreatic islets by targeted expression of a dominant-negative K ATP channel

“…Pancreatic beta cells isolated from transgenic (lines M4 and M10) and control mice were cultured in DMEM supplemented with 10% fetal bovine serum, plated into 3.5-cm dishes containing CELLocate Coverslips (Eppendorf, Hamburg, Germany), and incubated at 37°C for 24-72 hours before the experiments. Whole-cell current recordings were made with a patch-clamp amplifier EPC-7 (List Electronics, Darmstadt, Germany), according to the method described by Ribar et al (30). The extracellular solution contained 135 mM NaCl, 5 mM KCl, 5 mM CaCl 2 , 2 mM MgSO 4 , and 5 mM Hepes (pH 7.4).…”