Multiple essential functions of Plasmodium falciparum actin-1 during malaria blood-stage development

Sujaan Das,Leandro Lemgruber,Markus Meissner,Jake Baum,Chwen L Tay

doi:10.1186/s12915-017-0406-2

Abstract

BackgroundThe phylum Apicomplexa includes intracellular parasites causing immense global disease burden, the deadliest of them being the human malaria parasite Plasmodium falciparum, which invades and replicates within erythrocytes. The cytoskeletal protein actin is well conserved within apicomplexans but divergent from mammalian actins, and was primarily reported to function during host cell invasion. However, novel invasion mechanisms have been described for several apicomplexans, and specific functions of the acto-myosin system are being reinvestigated. Of the two actin genes in P. falciparum, actin-1 (pfact1) is ubiquitously expressed in all life-cycle stages and is thought to be required for erythrocyte invasion, although its functions during parasite development are unknown, and definitive in vivo characterisation during invasion is lacking.ResultsHere we have used a conditional Cre-lox system to investigate the functions of PfACT1 during P. falciparum blood-stage development and host cell invasion. We demonstrate that PfACT1 is crucially required for segregation of the plastid-like organelle, the apicoplast, and for efficient daughter cell separation during the final stages of cytokinesis. Surprisingly, we observe that egress from the host cell is not an actin-dependent process. Finally, we show that parasites lacking PfACT1 are capable of microneme secretion, attachment and formation of a junction with the erythrocyte, but are incapable of host cell invasion.ConclusionsThis study provides important mechanistic insights into the definitive essential functions of PfACT1 in P. falciparum, which are not only of biological interest, but owing to functional divergence from mammalian actins, could also form the basis for the development of novel therapeutics against apicomplexans.

Highlights

The phylum Apicomplexa includes intracellular parasites causing immense global disease burden, the deadliest of them being the human malaria parasite Plasmodium falciparum, which invades and replicates within erythrocytes
In order to not disrupt native actin function, we avoided the use of any epitope tag and employed the strategy of introducing a loxP site within a heterologous intron in the middle of the pfact1 gene
Similar filaments were never observed in P. falciparum actin-1 (PfACT1) KO parasites, demonstrating specificity of this antibody, as described previously [42]. These results suggest a conserved function of Plasmodium and Toxoplasma actin-1 [8] in apicoplast segregation

Summary

Introduction

The phylum Apicomplexa includes intracellular parasites causing immense global disease burden, the deadliest of them being the human malaria parasite Plasmodium falciparum, which invades and replicates within erythrocytes. Novel invasion mechanisms have been described for several apicomplexans, and specific functions of the acto-myosin system are being reinvestigated. Of the two actin genes in P. falciparum, actin-1 (pfact1) is ubiquitously expressed in all life-cycle stages and is thought to be required for erythrocyte invasion, its functions during parasite development are unknown, and definitive in vivo characterisation during invasion is lacking. Malaria causes almost half a million deaths and immeasurable morbidity every year, with most deaths attributable to Plasmodium falciparum, the deadliest of the five parasite species capable of infecting. The role of the actomyosin system during the intracellular development of apicomplexan parasites is largely unknown. In Plasmodium spp. intracellular functions of actin have been suggested, such as during endocytosis [11], secretion [12] and antigenic variation [13, 14]

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