Calcium storage and function in apicomplexan parasites

Calcium is relevant for several vital functions in apicomplexan parasites, including host cell invasion, parasite motility and differentiation. The ER (endoplasmic reticulum) and calcium-rich acidocalcisomes have been identified as major calcium stores. Other potential calcium-storage organelles include the Golgi, the mitochondrion, the apicoplast and the recently described plant-like vacuole in Toxoplasma gondii. Compared with most eukaryotic systems, apicomplexan parasites contain a reduced number of calcium-related genes, a vast majority of which remain uncharacterized. Several Ca²⁺-ATPases have been described in apicomplexans, several of which are annotated in the different genomes. There is experimental evidence for an IP3 (inositol 1,4,5-trisphosphate)-dependent calcium response in Plasmodium spp. and T. gondii, although no IP3 or ryanodine receptors have been identified. Genes encoding potential calcium channels are present in T. gondi, but not in Plasmodium spp. and Cryptosporidium spp. Effector calcium-binding proteins including calmodulins and CDPK (calcium-dependent protein kinase) genes mainly found in plants have also been described. The characterized CDPKs were found to play important roles in protein secretion, host cell invasion and parasite differentiation. Taken together, the available information on calcium storage and function in apicomplexans, although fragmented, suggest the existence of unique calcium-mediated pathways in these parasites. An in-depth functional characterization of the apicomplexan calcium-related genes could lead to the identification of novel therapeutic targets, and will improve our understanding of the role of calcium in parasite development and virulence.