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Egress of Plasmodium falciparum merozoites from host erythrocytes is a critical step in multiplication of blood-stage parasites. A cascade of proteolytic events plays a major role in degradation of membranes leading to egress of merozoites. However, the signals that regulate the temporal activation and/or secretion of proteases upon maturation of merozoites in intra-erythrocytic schizonts remain unclear. Here, we have tested the role of intracellular Ca2+ in regulation of egress of P. falciparum merozoites from schizonts. A sharp rise in intracellular Ca2+ just before egress, observed by time-lapse video microscopy, suggested a role for intracellular Ca2+ in this process. Chelation of intracellular Ca2+ with chelators such as BAPTA- AM or inhibition of Ca2+ release from intracellular stores with a phospholipase C ( PLC) inhibitor blocks merozoite egress. Interestingly, chelation of intracellular Ca2+ in schizonts was also found to block the discharge of a key protease PfSUB1 (subtilisin-like protease 1) from exonemes of P. falciparum merozoites to parasitophorous vacuole ( PV). This leads to inhibition of processing of PfSERA5 (serine repeat antigen 5) and a block in parasitophorous vacuolar membrane ( PVM) rupture and merozoite egress. A complete understanding of the steps regulating egress of P. falciparum merozoites may provide novel targets for development of drugs that block egress and limit parasite growth. [ABSTRACT FROM AUTHOR]

Motility and phagocytosis are the two important processes that are intricately linked to survival and virulence potential of the protist parasite Entamoeba histolytica. These processes primarily rely on actin‐dependent pathways, and regulation of these pathways is critical for understanding the pathology of E. histolytica. Generally, phosphoinositides dynamics have not been explored in amoebic actin dynamics and particularly during phagocytosis in E. histolytica. We have explored the roles of PtdIns(4,5)P2 as well as the enzyme that produces this metabolite, EhPIPKI during phagocytosis. Immunofluorescence and live cell images showed enrichment of EhPIPKI in different stages of phagocytosis from initiation till the cups progressed towards closure. However, the enzyme was absent after phagosomes are pinched off from the membrane. Overexpression of a dominant negative mutant revealed a reduction in the formation of phagocytic cups and inhibition in the rate of engulfment of erythrocytes. Moreover, EhPIPKI binds directly to F and G‐actin unlike PIPKs from other organisms. PtdIns(4,5)P2, the product of the enzyme, also followed a similar distribution pattern during phagocytosis as determined by a GFP‐tagged PH‐domain from PLCδ, which specifically binds PtdIns(4,5)P2 in trophozoites. In summary, EhPIPKI regulates initiation of phagocytosis by regulating actin dynamics. [ABSTRACT FROM AUTHOR]

Motility and phagocytosis are the two important processes that are intricately linked to survival and virulence potential of the protist parasite Entamoeba histolytica. These processes primarily rely on actin-dependent pathways, and regulation of these pathways is critical for understanding the pathology of E. histolytica. Generally, phosphoinositides dynamics have not been explored in amoebic actin dynamics and particularly during phagocytosis in E. histolytica. We have explored the roles of PtdIns(4,5)P 2 as well as the enzyme that produces this metabolite, EhPIPKI during phagocytosis. Immunofluorescence and live cell images showed enrichment of EhPIPKI in different stages of phagocytosis from initiation till the cups progressed towards closure. However, the enzyme was absent after phagosomes are pinched off from the membrane. Overexpression of a dominant negative mutant revealed a reduction in the formation of phagocytic cups and inhibition in the rate of engulfment of erythrocytes. Moreover, EhPIPKI binds directly to F and G-actin unlike PIPKs from other organisms. PtdIns(4,5)P 2 , the product of the enzyme, also followed a similar distribution pattern during phagocytosis as determined by a GFP-tagged PH-domain from PLCδ, which specifically binds PtdIns(4,5)P 2 in trophozoites. In summary, EhPIPKI regulates initiation of phagocytosis by regulating actin dynamics., (© 2019 John Wiley & Sons Ltd.)

Egress of Plasmodium falciparum merozoites from host erythrocytes is a critical step in multiplication of blood-stage parasites. A cascade of proteolytic events plays a major role in degradation of membranes leading to egress of merozoites. However, the signals that regulate the temporal activation and/or secretion of proteases upon maturation of merozoites in intra-erythrocytic schizonts remain unclear. Here, we have tested the role of intracellular Ca(2+) in regulation of egress of P. falciparum merozoites from schizonts. A sharp rise in intracellular Ca(2+) just before egress, observed by time-lapse video microscopy, suggested a role for intracellular Ca(2+) in this process. Chelation of intracellular Ca(2+) with chelators such as BAPTA-AM or inhibition of Ca(2+) release from intracellular stores with a phospholipase C (PLC) inhibitor blocks merozoite egress. Interestingly, chelation of intracellular Ca(2+) in schizonts was also found to block the discharge of a key protease PfSUB1 (subtilisin-like protease 1) from exonemes of P. falciparum merozoites to parasitophorous vacuole (PV). This leads to inhibition of processing of PfSERA5 (serine repeat antigen 5) and a block in parasitophorous vacuolar membrane (PVM) rupture and merozoite egress. A complete understanding of the steps regulating egress of P. falciparum merozoites may provide novel targets for development of drugs that block egress and limit parasite growth., (© 2012 John Wiley & Sons Ltd.)