No abstract

In polarized epithelial cells [Ca 2؉ ] i waves are initiated in discrete regions and propagate through the cytosol. The structural basis for these compartmentalized and coordinated events are not well understood. In the present study we used a combination of [Ca 2؉ ] i imaging at high temporal resolution, recording of Ca 2؉ -activated Cl ؊ current, and immunolocalization by confocal microscopy to study the correlation between initiation and propagation of [Ca 2؉ ] i waves and localization of Ca 2؉ release channels in pancreatic acini and submandibular acinar and duct cells. In all cells Ca 2؉ waves are initiated in the luminal pole and propagate through the cell periphery to the basal pole. All three cell types express the three known inositol 1,4,5-trisphosphate receptors (IP 3 Rs). Expression of IP 3 Rs was confined to the area just underneath the luminal and lateral membranes, with no detectable receptors in the basal pole or other regions of the cells. In pancreatic acini and SMG ducts IP 3 R3 was also found in the nuclear envelope. Expression of ryanodine receptor was detected in submandibular salivary gland cells but not pancreatic acini. Accordingly, cyclic ADP ribose was very effective in mobilizing Ca 2؉ from internal stores of submandibular salivary gland but not pancreatic acinar cells. Measurement of [Ca 2؉ ] i and localization of IP 3 Rs in the same cells suggests that only a small part of IP 3 Rs participate in the initiation of the Ca 2؉ wave, whereas most receptors in the cell periphery probably facilitate the propagation of the Ca 2؉ wave. The combined results together with our previous studies on this subject lead us to conclude that the internal Ca 2؉ pool is highly compartmentalized and that compartmentalization is achieved in part by polarized expression of Ca 2؉ channels.

In contrast to what has been reported previously, endo-lysosomal dysfunction in presenilin-deficient cells does not arise from improper glycosylation of the lysosomal V-ATPase but rather from defective lysosomal calcium homeostasis.

The various splice variants of the three SERCA-and the two SPCA-pump genes in higher vertebrates encode P-type ATPases of the P 2A group found respectively in the membranes of the endoplasmic reticulum and the secretory pathway. Of these, SERCA2b and SPCA1a represent the housekeeping isoforms. The SERCA2b form is characterized by a luminal carboxy terminus imposing a higher affinity for cytosolic Ca 2þ compared to the other SERCAs. This is mediated by intramembrane and luminal interactions of this extension with the pump. Other known affinity modulators like phospholamban and sarcolipin decrease the affinity for Ca 2þ . The number of proteins reported to interact with SERCA is rapidly growing. Here, we limit the discussion to those for which the interaction site with the ATPase is specified: HAX-1, calumenin, histidine-rich Ca 2þ -binding protein, and indirectly calreticulin, calnexin, and ERp57. The role of the phylogenetically older and structurally simpler SPCAs as transporters of Ca 2þ , but also of Mn 2þ , is also addressed.