New roles for perforins and proteases in apicomplexan egress

Abstract

Egress is a pivotal step in the life cycle of intracellular pathogens initiating the transition from an expiring host cell to a fresh target cell. While much attention has been focused on understanding cell invasion by intracellular pathogens, recent work is providing a new appreciation of mechanisms and therapeutic potential of microbial egress. This review highlights recent insight into cell egress by apicomplexan parasites and emerging contributions of membranolytic and proteolytic secretory products, along with host proteases. New findings suggest that Toxoplasma gondii secretes a pore-forming protein, TgPLP1, during egress that facilitates parasite escape from the cell by perforating the parasitophorous membrane. Also, in a cascade of proteolytic events, Plasmodium falciparum late-stage schizonts activate and secrete a subtilisin, PfSUB1, which processes enigmatic putative proteases called serine-repeat antigens that contribute to merozoite egress. A new report also suggests that calcium-activated host proteases called calpains aid parasite exit, possibly by acting upon the host cytoskeleton. Together these discoveries reveal important new molecular players involved in the principal steps of egress by apicomplexans.

Figure 1

Common steps in Toxoplasma tachyzoite and Plasmodium merozoite egress. Each step is indicated in the nucleus of Toxoplasma (A) or Plasmodium (B). (1) Transmission of an egress signal. (2) Protein secretion from micronemes (Toxoplasma) or exonemes (Plasmodium) and activation of the motility system (blue line underneath the zoite plasma membrane). (3) Disruption of the PVM. (4) Disruption of the host cytoskeleton and HPM. (5) Active exit from the host cell.