The Basal Complex Protein PfMORN1 Is Not Required for Asexual Replication of Plasmodium falciparum

Elucidating the spatio-temporal dynamics of the Plasmodium falciparum basal complex

Asexual replication of Plasmodium falciparum occurs via schizogony, wherein 16-36 daughter cells are produced within the parasite during one semi-synchronized cytokinetic event. Schizogony requires a divergent contractile ring structure known as the basal complex. Our lab has previously identified PfMyoJ (PF3D7_1229800) and PfSLACR (PF3D7_0214700) as basal complex proteins recruited midway through segmentation. Using ultrastructure expansion microscopy, we localized both proteins to a novel basal complex subcompartment. While both colocalize with the basal complex protein PfCINCH upon recruitment, they form a separate, more basal subcompartment termed the posterior cup during contraction. We also show that PfSLACR is recruited to the basal complex prior to PfMyoJ, and that both proteins are removed unevenly as segmentation concludes. Using live-cell microscopy, we show that actin dynamics are dispensable for basal complex formation, expansion, and contraction. We then show that EF-hand containing P. falciparum Centrin 2 partially localizes to this posterior cup of the basal complex and that it is essential for growth and replication, with variable defects in basal complex contraction and synchrony. Finally, we demonstrate that free intracellular calcium is necessary but not sufficient for basal complex contraction in P. falciparum. Thus, we demonstrate dynamic spatial compartmentalization of the Plasmodium falciparum basal complex, identify an additional basal complex protein, and begin to elucidate the unique mechanism of contraction utilized by P. falciparum, opening the door for further exploration of Apicomplexan cellular division.

Essential function of alveolin PfIMC1g in the Plasmodium falciparum asexual blood stage

IMPORTANCE

Infection by the Plasmodium falciparum parasite is responsible for the most severe form of human malaria. The asexual blood stage of the parasite, which occurs inside human red blood cells, is responsible for the symptoms of malaria and is the target of most antimalarial drugs. Plasmodium spp. rely on their highly divergent cytoskeletal structures to scaffold their cell division, sustain the mechanical stress of invasion, and survive in both the human bloodstream and the mosquito. We investigate the function of a class of divergent intermediate filament-like proteins called alveolins in the clinically important blood stage. The functional role of individual alveolins in Plasmodium remains poorly understood due to pleiotropic effects of gene knockouts and redundancy among alveolins. We evaluate the localization and essentiality of the four asexual-stage alveolins and find that PfIMC1g and PfIMC1c are essential. Furthermore, we demonstrate that PfIMC1g is critical for survival of the parasite post-invasion.

A PPP-type pseudophosphatase is required for the maintenance of basal complex integrity in Plasmodium falciparum

During its asexual blood stage, P. falciparum replicates via schizogony, wherein dozens of daughter cells are formed within a single parent. The basal complex, a contractile ring that separates daughter cells, is critical for schizogony. In this study, we identify a Plasmodium basal complex protein essential for basal complex maintenance. Using multiple microscopy techniques, we demonstrate that PfPPP8 is required for uniform basal complex expansion and maintenance of its integrity. We characterize PfPPP8 as the founding member of a novel family of pseudophosphatases with homologs in other Apicomplexan parasites. By co-immunoprecipitation, we identify two additional new basal complex proteins. We characterize the unique temporal localizations of these new basal complex proteins (late-arriving) and of PfPPP8 (early-departing). In this work, we identify a novel basal complex protein, determine its specific role in segmentation, identify a new pseudophosphatase family, and establish that the P. falciparum basal complex is a dynamic structure.

Plasmodium schizogony, a chronology of the parasite's cell cycle in the blood stage

Malaria remains a significant threat to global health, and despite concerted efforts to curb the disease, malaria-related morbidity and mortality increased in recent years. Malaria is caused by unicellular eukaryotes of the genus Plasmodium, and all clinical manifestations occur during asexual proliferation of the parasite inside host erythrocytes. In the blood stage, Plasmodium proliferates through an unusual cell cycle mode called schizogony. Contrary to most studied eukaryotes, which divide by binary fission, the parasite undergoes several rounds of DNA replication and nuclear division that are not directly followed by cytokinesis, resulting in multinucleated cells. Moreover, despite sharing a common cytoplasm, these nuclei multiply asynchronously. Schizogony challenges our current models of cell cycle regulation and, at the same time, offers targets for therapeutic interventions. Over the recent years, the adaptation of advanced molecular and cell biological techniques have given us deeper insight how DNA replication, nuclear division, and cytokinesis are coordinated. Here, we review our current understanding of the chronological events that characterize the unusual cell division cycle of P. falciparum in the clinically relevant blood stage of infection.

Identification of basal complex protein that is essential for maturation of transmission-stage malaria parasites

Malaria remains a global driver of morbidity and mortality. To generate new antimalarials, one must elucidate the fundamental cell biology of Plasmodium falciparum, the parasite responsible for the deadliest cases of malaria. A membranous and proteinaceous scaffold called the inner membrane complex (IMC) supports the parasite during morphological changes, including segmentation of daughter cells during asexual replication and formation of transmission-stage gametocytes. The basal complex lines the edge of the IMC during segmentation and likely facilitates IMC expansion. It is unknown, however, what drives IMC expansion during gametocytogenesis. We describe the discovery of a basal complex protein, PfBLEB, which we find to be essential for gametocytogenesis. Parasites lacking PfBLEB harbor defects in IMC expansion and are unable to form mature gametocytes. This article demonstrates a role for a basal complex protein outside of asexual division, and, importantly, highlights a potential molecular target for the ablation of malaria transmission.

Toxoplasma gondii’s Basal Complex: The Other Apicomplexan Business End Is Multifunctional

The Apicomplexa are famously named for their apical complex, a constellation of organelles at their apical end dedicated to invasion of their host cells. In contrast, at the other end of the cell, the basal complex (BC) has been overshadowed since it is much less prominent and specific functions were not immediately obvious. However, in the past decade a staggering array of functions have been associated with the BC and strides have been made in understanding its structure. Here, these collective insights are supplemented with new data to provide an overview of the understanding of the BC in Toxoplasma gondii. The emerging picture is that the BC is a dynamic and multifunctional complex, with a series of (putative) functions. The BC has multiple roles in cell division: it is the site where building blocks are added to the cytoskeleton scaffold; it exerts a two-step stretch and constriction mechanism as contractile ring; and it is key in organelle division. Furthermore, the BC has numerous putative roles in ‘import’, such as the recycling of mother cell remnants, the acquisition of host-derived vesicles, possibly the uptake of lipids derived from the extracellular medium, and the endocytosis of micronemal proteins. The latter process ties the BC to motility, whereas an additional role in motility is conferred by Myosin C. Furthermore, the BC acts on the assembly and/or function of the intravacuolar network, which may directly or indirectly contribute to the establishment of chronic tissue cysts. Here we provide experimental support for molecules acting in several of these processes and identify several new BC proteins critical to maintaining the cytoplasmic bridge between divided parasites. However, the dispensable nature of many BC components leaves many questions unanswered regarding its function. In conclusion, the BC in T. gondii is a dynamic and multifunctional structure at the posterior end of the parasite.