cAMP-induced cytoskeleton rearrangement increases calcium transients through the enhancement of capacitative calcium entry

In this report we investigated the correlation between cell morphology and regulation of cytosolic calcium homeostasis. Type I astrocytes were differe ntiated to stellate process-bearing cells by a 100-min exposure to cAMP, Di fferentiation of cortical astrocytes increased the magnitude and duration o f calcium transients elicited by phospholipase C-activating agents as measu red by single cell Fura-a-based imaging. Calcium imaging showed differences in the spatial pattern of the response. In both differentiated and the con trol cells, the response originated in the periphery and gradually extended into the center of the cell. However, the elevation of cytosolic calcium c oncentration ([Ca2+](i)) was particularly evident within the processes and adjacent to the inner cell membrane of the differentiated astrocytes, In ad dition, differentiation significantly prolonged the duration of the [Ca2+]( i) elevation. Potentiation of the calcium transients was mimicked by forsko lin-induced differentiation and abolished by a specific protein kinase-A bl ocker. Conversely, the enhancement of the calcium transients was not mimick ed by brief exposure to cAMP not causing morphological differentiation, and in PC12 cells that did not undergo morphological changes after 100 min of cAMP treatment. Impairing cAMP-induced cytoskeleton re-organization, by mea ns of cytochalasin D and nocodazole, prevented the potentiation of the calc ium transients in cAMP-treated astrocytes. Phospholipase C activity and sen sitivity to inositol (1,4,5)-trisphosphate were not involved in the enhance ment of the calcium responses. Also, potentiation of the calcium transients was dependent on extracellular calcium. Calcium storage and thapsigargin-d epletable intracellular calcium reservoirs were analogously not increased i n differentiated astrocytes, Rearrangement of the cell shape also caused a condensation of the endoplasmic reticulum and altered the spatial relations hip between the endoplasmic reticulum and the cell membrane. In conclusion, morphological rearrangements of type I astrocytes increase the magnitude a nd the duration of agonist-induced calcium transients via enhancement of ca pacitative calcium entry and is associated with a spatial reorganization of the relationship between cell membrane and the endoplasmic reticulum struc tures.