Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic apicomplexa (Plasmodium, Toxoplasma)

Abstract

The importance of Ca2+ -signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives. the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca²⁺ signaling in the two closely related groups. Acidic Ca²⁺ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca²⁺-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca²⁺-ATPases (PMCA, SERCA). respectively. Only recently was it possible to identifY in Paramecium a numberofhomologs ofryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP₃R)and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP₃Rs are constitutively active in the contractile vacuole complex; RYR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca²⁺ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.