Rapid downregulation of the Ca2+-signal after exocytosis stimulation in Paramecium cells: Essential role of a centrin-rich filamentous cortical network, the infraciliary lattice

SummaryWe analysed in Paramecium tetraurelia cells the role of the infraciliary lattice, a cytoskeletal network containing numerous centrin isoforms tightly bound to large binding proteins, in the re-establishment of Ca2+ homeostasis following exocytosis stimulation. The wild type strain d4-2 has been compared with the mutant cell line Δ-PtCenBP1 which is devoid of the infraciliary lattice (“Δ-PtCenBP1” cells). Exocytosis is known to involve the mobilization of cortical Ca2+-stores and a superimposed Ca2+-influx and was analysed using Fura Red ratio imaging. No difference in the initial signal generation was found between wild type and Δ-PtCenBP1 cells. In contrast, decay time was greatly increased in Δ-PtCenBP1 cells particularly when stimulated, e.g., in presence of 1 mM extracellular Ca2+, [Ca2+]o. Apparent halftimes of f/f0 decrease were 8.5 s in wild type and ∼125 s in Δ-PtCenBP1 cells, requiring ∼30 s and ∼180 s, respectively, to re-establish intracellular [Ca2+] homeostasis. Lowering [Ca2+]o to 0.1 and 0.01 mM caused an acceleration of intracellular [Ca2+] decay to t1/2 = 33 s and 28 s, respectively, in Δ-PtCenBP1 cells as compared to 8.1 and 5.6, respectively, for wild type cells. We conclude that, in Paramecium cells, the infraciliary lattice is the most efficient endogenous Ca2+ buffering system allowing the rapid downregulation of Ca2+ signals after exocytosis stimulation.