Gliding motility: the molecules behind the motion

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.

IMC1b is a putative membrane skeleton protein involved in cell shape, mechanical strength, motility, and infectivity of malaria ookinetes

Membrane skeletons are cytoskeletal elements that have important roles in cell development, shape, and structural integrity. Malaria parasites encode a conserved family of putative membrane skeleton proteins related to articulins. One member, IMC1a, is expressed in sporozoites and localizes to the pellicle, a unique membrane complex believed to form a scaffold onto which the ligands and glideosome are arranged to mediate parasite motility and invasion. IMC1b is a closely related structural paralogue of IMC1a, fostering speculation that it could be functionally homologous but in a different invasive life stage. Here we have generated genetically modified parasites that express IMC1b tagged with green fluorescent protein, and we show that it is targeted exclusively to the pellicle of ookinetes. We also show that IMC1b-deficient ookinetes display abnormal cell shape, reduced gliding motility, decreased mechanical strength, and reduced infectivity. These findings are consistent with a membrane skeletal role of IMC1b and provide strong experimental support for the view that membrane skeletons form an integral part of the pellicle of apicomplexan zoites and function to provide rigidity to the pellicular membrane complex. The similarities observed between the loss-of-function phenotypes of IMC1a and IMC1b show that membrane skeletons of ookinetes and sporozoites function in an overall similar way. However, the fact that ookinetes and sporozoites do not use the same IMC1 protein implies that different mechanical properties are required of their respective membrane skeletons, likely reflecting the distinct environments in which these life stages must operate.

Developmental biology of sporozoite-host interactions in Plasmodium falciparum malaria: implications for vaccine design

The Plasmodium falciparum sporozoite infects different types of cells in a mosquito's salivary glands and human epithelial and Kuppfer cells and hepatocytes. These become differentiated later on, transforming themselves into the invasive red blood cell form, the merozoite. The ability of sporozoites to interact with different types of cells requires a wide variety of mechanisms allowing them to survive in both hosts: mobility, receptor-ligand interactions with different cellular receptors, and transformation and development into other invasive parasite forms, which are vitally important for parasite survival. Sporozoite complexity is reflected in the large quantity of proteins that can be expressed. Some of them have been extensively studied, such as CSP, TRAP, STARP, LSA-1, LSA-3, SALSA, SPECT1, SPECT2, MAEBL, and SPATR, due to their importance in infection and their potential use as vaccines. Our work has been focused on the search for the molecular mechanisms of parasite-host cellular receptor-ligand interactions by identifying amino acid sequences and the critical binding residues from these proteins relevant to parasite invasion. Once such sequences have been identified, it will be possible to modify them to induce a strong immune response against P. falciparum in the experimental Aotus monkey model. This all leads towards developing multistage, multicomponent, subunit-based vaccines that will be effective in eradicating or controlling malaria caused by P. falciparum.

Structure of the MTIP-MyoA complex, a key component of the malaria parasite invasion motor

The causative agents of malaria have developed a sophisticated machinery for entering multiple cell types in the human and insect hosts. In this machinery, a critical interaction occurs between the unusual myosin motor MyoA and the MyoA-tail Interacting Protein (MTIP). Here we present one crystal structure that shows three different conformations of Plasmodium MTIP, one of these in complex with the MyoA-tail, which reveal major conformational changes in the C-terminal domain of MTIP upon binding the MyoA-tail helix, thereby creating several hydrophobic pockets in MTIP that are the recipients of key hydrophobic side chains of MyoA. Because we also show that the MyoA helix is able to block parasite growth, this provides avenues for designing antimalarials.

An introduction to cell motility for the physical scientist

Directed, purposeful movement is one of the qualities that we most closely associate with living organisms, and essentially all known forms of life on this planet exhibit some type of self-generated movement or motility. Even organisms that remain sessile most of the time, like flowering plants and trees, are quite busy at the cellular level, with large organelles, including chloroplasts, constantly racing around within cellular boundaries. Directed biological movement requires that the cell be able to convert its abundant stores of chemical energy into mechanical energy. Understanding how this mechanochemical energy transduction takes place and understanding how small biological forces generated at the molecular level are marshaled and organized for large-scale cellular or organismal movements are the focus of the field of cell motility. This tutorial, aimed at readers with a background in physical sciences, surveys the state of current knowledge and recent advances in modeling cell motility.

Plasmodium falciparum myosins: transcription and translation during asexual parasite development

Six myosins genes are now annotated in the Plasmodium falciparum Genome Project. Malaria myosins have been named alphabetically; accordingly, we refer to the two latest additions as Pfmyo-E and Pfmyo-F. Both new myosins contain regions characteristic of the functional motor domain of "true" myosins and, unusually for P. falciparum myosins, Pfmyo-F encodes two consensus IQ light chain-binding motifs. Phylogenetic analysis of the 17 currently known apicomplexan myosins together with one representative of each myosin class clusters all but one of the apicomplexan sequences together in Class XIV. This refines the earlier definition of the Class XIV Subclasses XIVa and XIVb. RT-PCR on blood stage parasite mRNA amplifies a specific product for all six myosins and each shows developmentally regulated transcription. Thus: Pfmyo-A and Pfmyo-B genes are transcribed throughout development; Pfmyo-C is predominant in trophozoites; Pfmyo-D occurs in trophozoites and schizonts; Pfmyo-E though barely present in earlier stages is abundant in schizonts; Pfmyo-F increases steadily throughout development and maturation. It is known that Pfmyo-A and Pfmyo-B are synthesised during late schizogony and we now show that Pfmyo-D expression is also temporally regulated to late trophozoites and schizonts where it distributes close to segregating nuclei. Thus, in asexual stages myosin synthesis does not always parallel transcript accumulation, showing that translation is also regulated. The implication is that the mRNAs are either subjected to turnover, synthesised and degraded, or that they are sequestered in an inactivate form until required for protein synthesis.

The elusive malaria sporozoite in the mammalian host

Malaria infection is initiated when Plasmodium sporozoites are injected into a host during the bite of an infected mosquito. In the mammal, the sporozoite must rapidly reach an intravacuolar niche within a hepatocyte, where it will generate the parasite stage that invades red blood cells and causes the symptoms of the disease. Herein, we describe our understanding of the way in which sporozoites travel from the site of the mosquito bite to the liver, arrest in the liver, cross the sinusoidal barrier and eventually gain access to hepatocytes. We also highlight some of the recent advances in our understanding of these processes at the molecular level.

Actin and myosin in Gregarina polymorpha

Actin and two class XIV unconventional myosins have been cloned from Gregarina polymorpha, a large protozoan parasite inhabiting the gut of the mealworm Tenebrio molitor. These proteins were most similar to their homologues expressed in the coccidian and haemosporidian Apicomplexa such as Toxoplasma and Plasmodium despite the significant morphological differences among these parasites. Both actin and G. polymorpha myosin A (GpMyoA), a 92.6-kDa protein characterized by a canonical myosin head domain and short, highly basic tail, localized to both the longitudinally-disposed surface membrane folds (epicytic folds) of the parasite as well as to the subjacent rib-like myonemes that gird the parasite cortex. G. polymorpha myosin B (GpMyoB), a 96.3-kDa myosin, localized exclusively to the epicytic folds of the parasite. Both myosins were tightly associated with the cortical cytoskeleton and were solubilized only with a combination of high salt and detergent. Both GpMyoA and GpMyoB could bind to actin in an ATP-sensitive fashion. The distribution of actin and the unconventional myosins in G. polymorpha was consistent with their proposed participation in both the rapid (1-10 microm/sec) gliding motility exhibited by the gregarines as well as the myoneme-mediated bending motions that have been observed in these parasites.

Differential transcriptome profiling identifies Plasmodium genes encoding pre-erythrocytic stage-specific proteins

Invasive sporozoite and merozoite stages of malaria parasites that infect mammals enter and subsequently reside in hepatocytes and red blood cells respectively. Each invasive stage may exhibit unique adaptations that allow it to interact with and survive in its distinct host cell environment, and these adaptations are likely to be controlled by differential gene expression. We used suppression subtractive hybridization (SSH) of Plasmodium yoelii salivary gland sporozoites versus merozoites to identify stage-specific pre-erythrocytic transcripts. Sequencing of the SSH library and matching the cDNA sequences to the P. yoelii genome yielded 25 redundantly tagged genes including the only two previously characterized sporozoite-specific genes encoding the circumsporozoite protein (CSP) and thrombospondin-related anonymous protein (TRAP). Twelve novel genes encode predicted proteins with signal peptides, indicating that they enter the secretory pathway of the sporozoite. We show that one novel protein bearing a thrombospondin type 1 repeat (TSR) exhibits an expression pattern that suggests localization in the sporozoite secretory rhoptry organelles. In addition, we identified a group of four genes encoding putative low-molecular-mass proteins. Two proteins in this group exhibit an expression pattern similar to TRAP, and thus possibly localize in the sporozoite secretory micronemes. Proteins encoded by the differentially expressed genes identified here probably mediate specific interactions of the sporozoite with the mosquito vector salivary glands or the mammalian host hepatocyte and are not used during merozoite-red blood cell interactions.

A small-molecule approach to studying invasive mechanisms of Toxoplasma gondii

Toxoplasma gondii is the most common protozoan parasite of humans. Infection with T. gondii can lead to life-threatening disease as a result of repeated cycles of host cell invasion, parasite replication, and host cell lysis. Relatively little is known about the invasive mechanisms of T. gondii and related parasites within the Phylum Apicomplexa (including Plasmodium spp., the causative agents of malaria), due to difficulties associated with studying genes essential to invasion in haploid obligate intracellular organisms. To circumvent this problem, we have developed a high-throughput microscope-based assay, which we have used to screen a collection of 12,160 structurally diverse small molecules for inhibitors of T. gondii invasion. A total of 24 noncytotoxic invasion inhibitors were identified. Secondary assays demonstrated that different inhibitors perturb different aspects of invasion, including gliding motility, secretion of host cell adhesins from apical organelles (the micronemes), and extension of a unique tubulin-based structure at the anterior of the parasite (the conoid). Unexpectedly, the screen also identified six small molecules that dramatically enhance invasion, gliding motility, and microneme secretion. The small molecules identified here reveal a previously unrecognized complexity in the control of parasite motility and microneme secretion, and they constitute a set of useful probes for dissecting the invasive mechanisms of T. gondii and related parasites. Small-molecule-based approaches provide a powerful means to address experimentally challenging problems in host-pathogen interaction, while simultaneously identifying new potential targets for drug development.