Entamoeba histolytica and pathogenesis: A calcium connection

Abstract

Calcium signaling plays a key role in many essential processes in almost all eukaryotic systems. It is believed that it may also be an important signaling system of the protist parasite Entamoeba histolytica. Motility, adhesion, cytolysis, and phagocytosis/trogocytosis are important steps in invasion and pathogenesis of E. histolytica, and Ca2+ signaling is thought to be associated with these processes leading to tissue invasion. There are a large number of Ca2+-binding proteins (CaBPs) in E. histolytica, and a number of these proteins appear to be associated with different steps in pathogenesis. The genome encodes 27 EF-hand-containing CaBPs in addition to a number of other Ca2+-binding domain/motif-containing proteins, which suggest intricate calcium signaling network in this parasite. Unlike other eukaryotes, a typical calmodulin-like protein has not been seen in E. histolytica. Though none of the CaBPs display sequence similarity with a typical calmodulin, extensive structural similarity has been seen in spite of lack of significant functional overlap with that of typical calmodulins. One of the unique features observed in E. histolytica is the identification of CaBPs (EhCaBP1, EhCaBP3) that have the ability to directly bind actin and modulate actin dynamics. Direct interaction of CaBPs with actin has not been seen in any other system. Pseudopod formation and phagocytosis are some of the processes that require actin dynamics, and some of the amoebic CaBPs (EhC2Pk, EhCaBP1, EhCaBP3, EhCaBP5) participate in this process. None of these E. histolytica CaBPs have any homolog in organisms other than different species of Entamoeba, suggesting a novel Ca2+ signaling pathway that has evolved in this genus.

Fig 1. Model depicting the intracellular calcium dynamics in E. histolytica.

Model summarizes the possible pathways of intracellular calcium dynamics after stimulus. The activation of receptor/channel leads to release of calcium from the ER-like structures into the cytosol. Second messengers such as Ins(1,4,5)P3 helps in the release of the internal Ca²⁺ pool in the cytosol. CaBPs, calpain-like protein, Ca²⁺-nucleotidase, CaM kinases, and other proteins modulates these Ca²⁺ signals into cellular response. Later on, Ca²⁺ is sequestered back into the internal sources by Ca²⁺-ATPase pumps such as SERCA and PMCA, which are located on the membrane of ER-like structures and in vacuoles, respectively. CCXs and SPCAs are other pumps localized on the vesicles of many organisms. Dotted arrows (in red) show the speculated Ca²⁺ ion flow. Because of the lack of evidence, it is difficult to speculate on the localization of different pumps on the vesicles, and the schematic diagram has been simplified for clarity. CaBP, Ca²⁺-binding protein; CaM kinases, Ca²⁺ modulated protein kinases; CCX, Ca²⁺/cation exchanger; ER, endoplasmic reticulum; Ins(1,4,5)P3, inositol 1,4,5-triphosphate; PMCA, plasma membrane Ca²⁺-ATPase; SER, smooth endoplasmic reticulum; SERCA, sarco/endoplasmic reticulum ATPase; SPCA, secretory pathway calcium ATPase.

Fig 2. Domain organization of CaBPs.

(A) Schematic representation of the domain organization of different CaBPs. (B) The linker regions of EhCaBP1 and EhCaBP2 between EF II and EF III are shown. Though EF sequences are highly conserved, the linker region sequences have extensive variation. aa, amino acid; CaBP, Ca²⁺-binding protein; EhCaBP, E. histolytica Ca²⁺-binding protein.