Programme Contents

Store operated Ca²⁺ channels and microdomains of Ca²⁺ in liver cells

Oscillatory increases in the cytoplasmic Ca²⁺ concentration ([Ca²⁺]_cyt) play essential roles in the hormonal regulation of animal cells. Increases in [Ca²⁺]_cyt require the release of Ca²⁺ from the endoplasmic reticulum (ER) and Ca²⁺ entry across the plasma membrane. Store-operated Ca²⁺ channels (SOCs), activated by a decrease in Ca²⁺ in the lumen of the ER, are responsible for maintaining adequate ER Ca²⁺. We are studying the nature and mechanism of activation of SOCs in liver cells. Patch clamp recording and fura-2 experiments indicate there is only one type of SOC in hepatocytes. These SOCs have a high selectivity for Ca²⁺ and properties essentially indistinguishable from those of Ca²⁺ release-activated Ca²⁺ (CRAC) channels. Orai1, a CRAC channel pore protein, and Stim1, a CRAC channel Ca²⁺ sensor, are components of liver cell SOCs. Recent studies in this laboratory have been directed to an investigation of the role of a sub-region of the ER in liver cell SOC activation (shown schematically in the figure). Experiments employing ectopically expressed TRPV1, localised in intracellular membranes, as an alternative method to deplete ER Ca²⁺, have provided evidence that only a small component of the ER is required for the activation of SOCs. Consistent with this conclusion are the results obtained with choleretic bile acids, which activate SOCs without detectable Ca²⁺ release from the ER. Three aspects of Ca²⁺ microdomains appear to be important in SOC action. (i) The activation process probably requires a decrease in Ca²⁺ near the SOC channels at the cytoplasmic side of the plasma membrane. (ii) There is strong feedback of Ca²⁺ entry through Ca²⁺ localised at the mouth of the channel. (iii) A number of experiments indicate that Ca²⁺ in microdomains near the SOC channel has specific regulatory functions such as regulation of adenylate cyclase. Current experiments are directed to further elucidation of these microdomains of Ca²⁺.