TUDCA Rescues γ-Cell Metabolic Oscillations from ER Stress, Revealed By NAD(P)H-FLIM and FRET

Abstract

Dysregulation of endoplasmic reticulum (ER) homeostasis contributes to β-cell dysfunction, but it remains unclear how the dynamics of metabolic oscillations are affected. Using both fitting and phasor analyses of NAD(P)H lifetimes, we observed an increase in the lifetime of bound NAD(P)H in islets harvested from ob/ob mice, an in vivo model of obesity and ER stress, relative to wild-type controls. The downstream effects of mitochondrial-driven cellular dynamics were quantified using combinations of FRET biosensors, fluorescent dyes, and genetically-encoded reporters. Analysis of the oscillations revealed that ER stress modulates the dynamics of mitochondrial NAD(P)H and membrane potential (Δψm), ATP/ADP, and cytosolic and ER Ca2+. In ob/ob islets, the β-cell's sensitivity to glucose was fundamentally increased, as evidenced by a ∼3-fold increase in oscillatory plateau fraction and augmented insulin exocytosis. In vivo administration of the chemical ER stress mitigator tauroursodeoxycholic acid (TUDCA) - currently in Phase I Clinical trials - rescued the oscillation dynamics in ob/ob islets by relaxing β-cell glucose sensitivity and reducing baseline levels of cytosolic Ca2+. Reduction of β-cell excitability may improve long term survival in obese animals.