Interactions of endoplasmic reticulum and mitochondria Ca²⁺ stores with capacitative calcium entry

Thiamine dependent enzymes are diminished in Alzheimer’s disease (AD). Thiamine deficiency in vitro and in rodents is a useful model of this reduction. Thiamine interacts with cellular calcium stores. To directly test the relevance of the thiamine dependent changes to dynamic processes in AD, the interactions must be studied in cells from patients with AD. These studies employed fibroblasts. Mitochondrial dysfunction including reductions in thiamine dependent enzymes and abnormalities in calcium homeostasis and oxidative processes occur in fibroblasts from Alzheimer’s Disease (AD) patients. Bombesin-releasable calcium stores (BRCS) from the endoplasmic reticulum (ER) are exaggerated in fibroblasts from patients with AD bearing a presenilin-1 (PS-1) mutation and in control fibroblasts treated with oxidants. ER calcium regulates calcium entry into the cell through capacitative calcium entry (CCE), which is reduced in fibroblasts and neurons from mice bearing PS-1 mutations. Under physiological conditions, mitochondria and ER play important and interactive roles in the regulation of Ca²⁺ homeostasis. Thus, the interactions of mitochondria and oxidants with CCE were tested. Inhibition of ER Ca²⁺-ATPase by cyclopiazonic acid (CPA) stimulates CCE. CPA-induced CCE was diminished by inhibition of mitochondrial Ca²⁺ export (−60 %) or import (−40 %). Different aspects of mitochondrial Ca²⁺ coupled to CPA-induced-CCE were sensitive to select oxidants. The effects were very different when CCE was examined in the presence of InsP₃, a physiological regulator of ER calcium release, and subsequent CCE. CCE under these conditions was only mildly reduced (20–25 %) by inhibition of mitochondrial Ca²⁺ export, and inhibition of mitochondrial Ca²⁺ uptake exaggerated CCE (+53 %). However, t-BHP reversed both abnormalities. The results suggest that in the presence of InsP₃, mitochondria buffer the local Ca²⁺ released from ER following rapid activation of InsP₃R and serve as a negative feedback to the CCE. The results suggest that mitochondrial Ca²⁺ modifies the depletion and refilling mechanism of ER Ca²⁺ stores.