oa Calcium Waves in the Xenopus Oocytes Trigger Store Operated Calcium Entry

The oscillation of cytoplasmic calcium concentration exists in a large number of non-excitable cells such as astrocytes in the brain, intestinal cells and oocytes (Fig.1). This rhythmic activity carries information into the cell, regulating various processes such as gene expression or transmitter release. In some cells, such as Xenopus oocytes, the oscillations can rely only on calcium being released in the cytoplasm from intracellular stores after stimulation of inositol triphosphate (IP3) receptors located on the endoplasmic reticulum (ER). To refill the intracellular stores in the ER, the cell has to import extracellular calcium. This is achieved through the STIM/Orai pathway where an ER calcium sensor (STIM) is activated by store depletion and in turn interacts and opens a calcium channel in the plasma membrane (Orai) leading to calcium influx, a process known as Store Operated Calcium Entry (SOCE). In the present work we evaluated the interactions between SOC and intracellular calcium oscillations in Xenopus oocytes. Intracellular calcium levels were monitored using the amplitude of the endogenous calcium-activated chloride currents. Following injection into the cells of a non-hydrolysable analog of IP3 (IP3-DF), long lasting calcium oscillations could be triggered as well as SOCE ( Fig. 1 ). The calcium oscillations were more efficient in promoting SOC than store depletion by ionomycin (2.34 ± 0.55 nA, n=13 vs 0.05 ± 0.02 nA, n=8), the mechanisms involved are currently under study. We also observed that activation of SOC could trigger a calcium wave in a dose-dependent manner following a “Calcium Induced Calcium Release” pattern. This suggests that SOC might not only be involved in replenishing the calcium stores but also in the regulation of the amplitude and timing of calcium oscillations. The well known down-regulation of SOC during oocyte maturation should therefore have important functional consequences on intracellular calcium oscillations.