Isolation and functional characterization of a dynamin-like gene from Plasmodium falciparum

The dynamin-related protein Dyn2 is essential for both apicoplast and mitochondrial fission in Plasmodium falciparum

Dynamins, or dynamin-related proteins (DRPs), are large mechano-sensitive GTPases mediating membrane dynamics or organellar fission/fusion events. Plasmodium falciparum encodes three dynamin-like proteins whose functions are poorly understood. Here, we demonstrate that PfDyn2 mediates both apicoplast and mitochondrial fission. Using super-resolution and ultrastructure expansion microscopy, we show that PfDyn2 is expressed in the schizont stage and localizes to both the apicoplast and mitochondria. Super-resolution long-term live cell microscopy shows that PfDyn2-deficient parasites cannot complete cytokinesis because the apicoplast and mitochondria do not undergo fission. Further, the basal complex or cytokinetic ring in Plasmodium cannot fully contract upon PfDyn2 depletion, a phenotype secondary to physical blockage of undivided organelles in the middle of the ring. Our data suggest that organellar fission defects result in aberrant schizogony, generating unsuccessful merozoites. The unique biology of PfDyn2, mediating both apicoplast and mitochondrial fission, has not been observed in other organisms possessing two endosymbiotic organelles.

Highlights

PfDyn2 is essential for schizont-stage development.PfDyn2 mediates both apicoplast and mitochondrial fission.Deficiency of PfDyn2 leads to organellar fission failures and blockage of basal complex contraction.Addition of apicoplast-derived metabolite IPP does not rescue the growth defects.

Apicoplast Dynamics During Plasmodium Cell Cycle

The deadly malaria parasite, Plasmodium falciparum, contains a unique subcellular organelle termed the apicoplast, which is a clinically-proven antimalarial drug target. The apicoplast is a plastid with essential metabolic functions that evolved via secondary endosymbiosis. As an ancient endosymbiont, the apicoplast retained its own genome and it must be inherited by daughter cells during cell division. During the asexual replication of P. falciparum inside human red blood cells, both the parasite, and the apicoplast inside it, undergo massive morphological changes, including DNA replication and division. The apicoplast is an integral part of the cell and thus its development is tightly synchronized with the cell cycle. At the same time, certain aspects of its dynamics are independent of nuclear division, representing a degree of autonomy in organelle biogenesis. Here, we review the different aspects of organelle dynamics during P. falciparum intraerythrocytic replication, summarize our current understanding of these processes, and describe the many open questions in this area of parasite basic cell biology.

Organelle Dynamics in Apicomplexan Parasites

Apicomplexan parasites, such as Toxoplasma gondii and Plasmodium falciparum, are the cause of many important human and animal diseases. While T. gondii tachyzoites replicate through endodyogeny, during which two daughter cells are formed within the parental cell, P. falciparum replicates through schizogony, where up to 32 parasites are formed in a single infected red blood cell and even thousands of daughter cells during mosquito- or liver-stage development. These processes require a tightly orchestrated division and distribution over the daughter parasites of one-per-cell organelles such as the mitochondrion and apicoplast. Although proper organelle segregation is highly essential, the molecular mechanism and the key proteins involved remain largely unknown. In this review, we describe organelle dynamics during cell division in T. gondii and P. falciparum, summarize the current understanding of the molecular mechanisms underlying organelle fission in these parasites, and introduce candidate fission proteins.

The Ringleaders: Understanding the Apicomplexan Basal Complex Through Comparison to Established Contractile Ring Systems

The actomyosin contractile ring is a key feature of eukaryotic cytokinesis, conserved across many eukaryotic kingdoms. Recent research into the cell biology of the divergent eukaryotic clade Apicomplexa has revealed a contractile ring structure required for asexual division in the medically relevant genera Toxoplasma and Plasmodium; however, the structure of the contractile ring, known as the basal complex in these parasites, remains poorly characterized and in the absence of a myosin II homolog, it is unclear how the force required of a cytokinetic contractile ring is generated. Here, we review the literature on the basal complex in Apicomplexans, summarizing what is known about its formation and function, and attempt to provide possible answers to this question and suggest new avenues of study by comparing the Apicomplexan basal complex to well-studied, established cytokinetic contractile rings and their mechanisms in organisms such as S. cerevisiae and D. melanogaster. We also compare the basal complex to structures formed during mitochondrial and plastid division and cytokinetic mechanisms of organisms beyond the Opisthokonts, considering Apicomplexan diversity and divergence.

Biogenesis and maintenance of the apicoplast in model apicomplexan parasites

The apicoplast is a non-photosynthetic relict plastid of Apicomplexa that evolved from a secondary symbiotic system. During its evolution, most of the genes derived from its alga ancestor were lost. Only genes involved in several valuable metabolic pathways, such as the synthesis of isoprenoid precursors, heme, and fatty acids, have been transferred to the host genome and retained to help these parasites adapt to a complex life cycle and various living environments. The biological function of an apicoplast is essential for most apicomplexan parasites. Considering their potential as drug targets, the metabolic functions of this symbiotic organelle have been intensively investigated through computational and biological means. Moreover, we know that not only organellar metabolic functions are linked with other organelles, but also their biogenesis processes have developed and evolved to tailor their biological functions and proper inheritance. Several distinct features have been found in the biogenesis process of apicoplasts. For example, the apicoplast borrows a dynamin-related protein (DrpA) from its host to implement organelle division. The autophagy system has also been repurposed for linking the apicoplast and centrosome during replication and the division process. However, many vital questions remain to be answered about how these parasites maintain and properly inherit this symbiotic organelle. Here we review our current knowledge about its biogenesis process and discuss several critical questions remaining to be answered in this field.

Mitochondrial dynamics in parasitic protists

The shape and number of mitochondria respond to the metabolic needs during the cell cycle of the eukaryotic cell. In the best-studied model systems of animals and fungi, the cells contain many mitochondria, each carrying its own nucleoid. The organelles, however, mostly exist as a dynamic network, which undergoes constant cycles of division and fusion. These mitochondrial dynamics are driven by intricate protein machineries centered around dynamin-related proteins (DRPs). Here, we review recent advances on the dynamics of mitochondria and mitochondrion-related organelles (MROs) of parasitic protists. In contrast to animals and fungi, many parasitic protists from groups of Apicomplexa or Kinetoplastida carry only a single mitochondrion with a single nucleoid. In these groups, mitochondrial division is strictly coupled to the cell cycle, and the morphology of the organelle responds to the cell differentiation during the parasite life cycle. On the other hand, anaerobic parasitic protists such as Giardia, Entamoeba, and Trichomonas contain multiple MROs that have lost their organellar genomes. We discuss the function of DRPs, the occurrence of mitochondrial fusion, and mitophagy in the parasitic protists from the perspective of eukaryote evolution.

Host cell remodeling by pathogens: the exomembrane system in Plasmodium-infected erythrocytes

Malaria is caused by infection of erythrocytes by parasites of the genus Plasmodium. To survive inside erythrocytes, these parasites induce sweeping changes within the host cell, one of the most dramatic of which is the formation of multiple membranous compartments, collectively referred to as the exomembrane system. As an uninfected mammalian erythrocyte is devoid of internal membranes, the parasite must be the force and the source behind the formation of these compartments. Even though the first evidence of the presence these of internal compartments was obtained over a century ago, their functions remain mostly unclear, and in some cases completely unknown, and the mechanisms underlying their formation are still mysterious. In this review, we provide an overview of the different parts of the exomembrane system, describing the parasitophorous vacuole, the tubovesicular network, Maurer's clefts, the caveola-vesicle complex, J dots and other mobile compartments, and the small vesicles that have been observed in Plasmodium-infected cells. Finally, we combine the data into a simplified view of the exomembrane system and its relation to the alterations of the host erythrocyte.

Plasmodium parasites remodel the host erythrocyte in various ways, including the formation of several membranous compartments, together referred to as the exomembrane system, within the erythrocyte cytosol that together are key to the sweeping changes in the host cell.

Defining the morphology and mechanism of the hemoglobin transport pathway in Plasmodium falciparum-infected erythrocytes

Hemoglobin degradation during the asexual cycle of Plasmodium falciparum is an obligate process for parasite development and survival. It is established that hemoglobin is transported from the host erythrocyte to the parasite digestive vacuole (DV), but this biological process is not well characterized. Three-dimensional reconstructions made from serial thin-section electron micrographs of untreated, trophozoite-stage P. falciparum-infected erythrocytes (IRBC) or IRBC treated with different pharmacological agents provide new insight into the organization and regulation of the hemoglobin transport pathway. Hemoglobin internalization commences with the formation of cytostomes from localized, electron-dense collars at the interface of the parasite plasma and parasitophorous vacuolar membranes. The cytostomal collar does not function as a site of vesicle fission but rather serves to stabilize the maturing cytostome. We provide the first evidence that hemoglobin transport to the DV uses an actin-myosin motor system. Short-lived, hemoglobin-filled vesicles form from the distal end of the cytostomes through actin and dynamin-mediated processes. Results obtained with IRBC treated with N-ethylmaleimide (NEM) suggest that fusion of hemoglobin-containing vesicles with the DV may involve a soluble NEM-sensitive factor attachment protein receptor-dependent mechanism. In this report, we identify new key components of the hemoglobin transport pathway and provide a detailed characterization of its morphological organization and regulation.

Investigation of the Plasmodium falciparum food vacuole through inducible expression of the chloroquine resistance transporter (PfCRT)

Haemoglobin degradation during the erythrocytic life stages is the major function of the food vacuole (FV) of Plasmodium falciparum and the target of several anti-malarial drugs that interfere with this metabolic pathway, killing the parasite. Two multi-spanning food vacuole membrane proteins are known, the multidrug resistance protein 1 (PfMDR1) and Chloroquine Resistance Transporter (PfCRT). Both modulate resistance to drugs that act in the food vacuole. To investigate the formation and behaviour of the food vacuole membrane we have generated inducible GFP fusions of chloroquine sensitive and resistant forms of the PfCRT protein. The inducible expression system allowed us to follow newly-induced fusion proteins, and corroborated a previous report of a direct trafficking route from the ER/Golgi to the food vacuole membrane. These parasites also allowed the definition of a food vacuole compartment in ring stage parasites well before haemozoin crystals were apparent, as well as the elucidation of secondary PfCRT-labelled compartments adjacent to the food vacuole in late stage parasites. We demonstrated that in addition to previously demonstrated Brefeldin A sensitivity, the trafficking of PfCRT is disrupted by Dynasore, a non competitive inhibitor of dynamin-mediated vesicle formation. Chloroquine sensitivity was not altered in parasites over-expressing chloroquine resistant or sensitive forms of the PfCRT fused to GFP, suggesting that the PfCRT does not mediate chloroquine transport as a GFP fusion protein.

Identification of a vaccine candidate antigen, PfMAg-1, from Plasmodium falciparum with monoclonal antibody M26-32

Monoclonal antibody M26-32 has been shown to strongly inhibit the growth of Plasmodium falciparum in vitro. To identify the target antigen of M26-32, a P. falciparum Dd2 asexual stage cDNA expression library was screened with this antibody, and a full open reading frame cDNA was obtained. This gene, named pfmag-1, encodes a polypeptide of 589 amino acids. The protein PfMAg-1 was characterized as a membrane-associated protein that expressed on the surface of merozoite during erythrocytic stage. Remarkably, at the C terminus of PfMAg-1, there are 14 copies of a deca-peptide sequence of QTDEIKND (H/N) I. This tandem repeat domain was identified to harbor the epitope of the protective M26-32 monoclonal antibody, and was also recognized by sera of patients infected with P. falciparum. Rabbit antibody elicited against this deca-peptide repeat domain effectively inhibited P. falciparum invasion in vitro. Our work suggests that PfMAg-1 is a promising malaria vaccine candidate.

Plasmodium falciparum GPI mannosyltransferase-III has novel signature sequence and is functional

The glycosylphosphatidylinositol (GPI) anchors of Plasmodium falciparum are indispensable for parasite survival since merozoite surface proteins-1, -2, -4, -5, and -10, crucial for erythrocyte invasion, are GPI-anchored. Therefore, the GPI biosynthetic pathway can offer potential targets for novel anti-malarial drugs. Here, we characterized the putative P. falciparum PIG-B gene (PfPIGB) that encodes mannosyltransferase-III of GPI biosynthesis. PfPIGB mRNA is transcribed in a developmental stage specific manner. A protein corresponding to the expected size of PfPIG-B is expressed by the parasite and is localized in the endoplasmic reticulum. Treatment of parasites with PfPIG-B specific siRNA caused reduction in GPI synthesis, affecting the PIG-B specific GPI intermediate. These data demonstrate that PfPIG-B is functional and encodes mannosyltransferase-III of the parasite GPI biosynthesis. The parasite PfPIG-B is novel in that its signature sequence HKEHKI is unique and is only partially conserved as compared to HKEXRF signature motif of mammalian PIG-B enzymes.

Characterization of PfDYN2, a dynamin-like protein of Plasmodium falciparum expressed in schizonts

Dynamin superfamily members are large GTPases conserved through evolution mainly described as mechanochemical enzymes involved in membrane scission events. The Plasmodium falciparum dynamin-2 (Pfdyn2) gene was cloned from the FcB1 strain. PfDYN2 belongs to the dynamin-like protein subgroup of the dynamin superfamily since it possesses a large GTPase domain together with the conserved dynamin_M and GED domains. Recombinant PfDYN2 was able to bind GTP, to hydrolyze GTP into GDP and to self-associate in low-salt conditions. PfDYN2 expression was restricted to schizonts where it localized in punctuate structures within the parasite cytoplasm. PfDYN2 partly co-localized with markers of the parasite endoplasmic reticulum, Golgi apparatus and apicoplast, suggesting it could be implicated in vesicular trafficking and/or organelle fission events known to occur during the last hours of the parasite development in erythrocytes. PfDYN2 and the previously described PfDYN1 are the only two dynamin superfamily members identified in the P. falciparum genome and the available data suggest that this situation is conserved in the Apicomplexa phylum.

Toward silencing the burden of malaria: progress and prospects for RNAi-based approaches

The discovery of RNA interference (RNAi) is one of the most significant of recent years, with potential for application beyond the laboratory to the clinic. As a tool for functional genomics, RNAi has permitted the characterization of genes in organisms that had previously remained recalcitrant to targeted gene manipulation. Efforts to understand its mode of action have revealed a central role in gene regulation and host defense. Finally, as a therapeutic tool, it has shown enormous promise in the control of a large array of diseases. Here we examine how RNAi is revolutionizing malaria research in an organism, the Anopheles mosquito, that until recently was essentially resistant to genetic study, and show how its application in both the mosquito vector and the Plasmodium parasite might ultimately lead to new ways of controlling and perhaps even eradicating this devastating disease.

Structure, function and evolution of the mitochondrial division apparatus

Mitochondria are derived from free-living alpha-proteobacteria that were engulfed by eukaryotic host cells through the process of endosymbiosis, and therefore have their own DNA which is organized using basic proteins to form organelle nuclei (nucleoids). Mitochondria divide and are split amongst the daughter cells during cell proliferation. Their division can be separated into two main events: division of the mitochondrial nuclei and division of the matrix (the so-called mitochondrial division, or mitochondriokinesis). In this review, we first focus on the cytogenetical relationships between mitochondrial nuclear division and mitochondriokinesis. Mitochondriokinesis occurs after mitochondrial nuclear division, similar to bacterial cytokinesis. We then describe the fine structure and dynamics of the mitochondrial division ring (MD ring) as a basic morphological background for mitochondriokinesis. Electron microscopy studies first identified a small electron-dense MD ring in the cytoplasm at the constriction sites of dividing mitochondria in the slime mold Physarum polycephalum, and then two large MD rings (with outer cytoplasmic and inner matrix sides) in the red alga Cyanidioschyzon merolae. Now MD rings have been found in all eukaryotes. In the third section, we describe the relationships between the MD ring and the FtsZ ring descended from ancestral bacteria. Other than the GTPase, FtsZ, mitochondria have lost most of the proteins required for bacterial cytokinesis as a consequence of endosymbiosis. The FtsZ protein forms an electron transparent ring (FtsZ or Z ring) in the matrix inside the inner MD ring. For the fourth section, we describe the dynamic association between the outer MD ring with a ring composed of the eukaryote-specific GTPase dynamin. Recent studies have revealed that eukaryote-specific GTPase dynamins form an electron transparent ring between the outer membrane and the MD ring. Thus, mitochondriokinesis is thought to be controlled by a mitochondrial division (MD) apparatus including a dynamic trio, namely the FtsZ, MD and dynamin rings, which consist of a chimera of rings from bacteria and eukaryotes in primitive organisms. Since the genes for the MD ring and dynamin rings are not found in the prokaryotic genome, the host genomes may make these rings to actively control mitochondrial division. In the fifth part, we focus on the dynamic changes in the formation and disassembly of the FtsZ, MD and dynamin rings. FtsZ rings are digested during a later period of mitochondrial division and then finally the MD and dynamin ring apparatuses pinched off the daughter mitochondria, supporting the idea that the host genomes are responsible for the ultimate control of mitochondrial division. We discuss the evolution, from the original vesicle division (VD) apparatuses to VD apparatuses including classical dynamin rings and MD apparatuses. It is likely that the MD apparatuses involving the dynamic trio evolved into the plastid division (PD) apparatus in Bikonta, while in Opisthokonta, the MD apparatus was simplified during evolution and may have branched into the mitochondrial fusion apparatus. Finally, we describe the possibility of intact isolation of large MD/PD apparatuses, the identification of all their proteins and their related genes using C. merolae genome information and TOF-MS analyses. These results will assist in elucidating the universal mechanism and evolution of MD, PD and VD apparatuses.