Apicomplexan Parasites Co-Opt Host Calpains to Facilitate Their Escape from Infected Cells

Abstract

Apicomplexan parasites, including Plasmodium falciparum and Toxoplasma gondii (the causative agents of malaria and toxoplasmosis, respectively), are responsible for considerable morbidity and mortality worldwide. These pathogenic protozoa replicate within an intracellular vacuole inside of infected host cells, from which they must escape to initiate a new lytic cycle. By integrating cell biological, pharmacological, and genetic approaches, we provide evidence that both Plasmodium and Toxoplasma hijack host cell calpain proteases to facilitate parasite egress. Immunodepletion or inhibition of calpain-1 in hypotonically lysed and resealed erythrocytes prevented the escape of P. falciparum parasites, which was restored by adding purified calpain-1. Similarly, efficient egress of T. gondii from mammalian fibroblasts was blocked by either small interfering RNAmediated suppression or genetic deletion of calpain activity and could be restored by genetic complementation.Apicomplexan parasites are obligate intra-cellular pathogens that exhibit complex life cycles involving distinct sexual and asexual stages of growth. The asexual phase is made up of a lytic cycle in which parasites establish an intracellular niche within the host: Plasmodium species infect erythrocytes, whereas Toxoplasma gondii infects nucleated animal cells. The process of schizogony in Plasmodium (endodyogeny in Toxoplasma) involves replication within a specialized "parasitophorous vacuole" to yield multiple daughter parasites (1,2). The resulting merozoites (tachzyoites in Toxoplasma) must escape from this vacuole and the host cell to invade uninfected cells and continue the infection. Egress from the infected cell is a rapid event, requiring only seconds at the end of the ~36-to 48-hour intracellular life

Apicomplexan perforin-like proteins

“…32 Intriguingly, these proteases are activated by spikes in the local calcium concentration 33 and the Toxoplasma PVM is in tight association with the host endoplasmic reticulum, a major sites of calcium storage. 34 Secretion of TgPLP1 during egress may not only permeabilize the PVM but also act on the nearby host ER, liberating calcium locally to activate host calpains.…”

Quantitative Time-course Profiling of Parasite and Host Cell Proteins in the Human Malaria Parasite Plasmodium falciparum

“…One such remarkable example is the human protease calpain-1 whose activity is essential for egress of the P. falciparum parasites from the host erythrocyte at the end of the IDC (20). Maybe even more intriguingly, host proteins and their enzymatic activity have been reported to be present within the parasite cytoplasm during the IDC.…”

Evaluation of Five Antischizophrenic Agents Against Toxoplasma gondii in Human Cell Cultures

“…Transient rises in intracellular calcium within tachyzoites serve as a trigger for parasite egress from host cells and participate in the cell gliding motility and host-cell invasion processes critical to T. gondii survival (Hoff and Carruthers, 2002;Nagamune et al, 2008;Chandramohanadas et al, 2009;Lourido et al, 2010). Calcium binding activates calmodulin and downstream calmodulin-dependent protein kinases have been implicated in life-cycle and erythrocyte invasion of the apicomplexan, Plasmodium falciparum (Vaid et al, 2008;Wurtz et al, 2009), but these pathways are not well understood in T. gondii.…”