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.     

Molecular interactions and signaling mechanisms during erythrocyte invasion by malaria parasites

Invasion of erythrocytes by Plasmodium merozoites is a complex process that is mediated by specific molecular interactions. Here, we review recent studies on interactions between erythrocyte binding antigens (EBA) and PfRH proteins from the parasite and erythrocyte receptors involved in invasion. The timely release of these parasite ligands from internal organelles such as micronemes and rhoptries to the merozoite surface is critical for receptor-engagement leading to successful invasion. We review information on signaling mechanisms that control the regulated secretion of parasite proteins during invasion. Erythrocyte invasion involves the formation and movement of a junction between the invading merozoite and host erythrocyte. We review recent studies on the molecular composition of the junction and the molecular motor that drives movement of the junction.     

Molecular mechanisms exploited by Listeria monocytogenes during host cell invasion

The facultative intracellular bacterial pathogen Listeria monocytogenes has evolved multiple strategies to invade a large panel of mammalian cells. Host cell invasion is critical for several stages of listeriosis pathology such as the initial crossing of the host intestinal barrier and the successive colonization of diverse target organs including the placenta. In this review, we address the main molecular mechanisms known to be used by L. monocytogenes during invasion of nonphagocytic cells and host tissues.     

Molecular mechanisms of host cell invasion by Trypanosoma cruzi

The protozoan parasite Trypanosoma cruzi, the etiologic agent of Chagas disease, is an obligate intracellular protozoan pathogen. Overlapping mechanisms ensure successful infection, yet the relationship between these cellular events and clinical disease remains obscure. This review explores the process of cell invasion from the perspective of cell surface interactions, intracellular signaling, modulation of the host cytoskeleton and endosomal compartment, and the intracellular innate immune response to infection.     

Gliding motility and cell invasion by Apicomplexa: insights from the Plasmodium sporozoite

Apicomplexa constitute one of the largest phyla of protozoa. Most Apicomplexa, including those pathogenic to humans, are obligate intracellular parasites. Their extracellular forms, which are highly polarized and elongated cells, share two unique abilities: they glide on solid substrates without changing their shape and reach an intracellular compartment without active participation from the host cell. There is now ample ultrastructural evidence that these processes result from the backward movement of extracellular interactions along the anteroposterior axis of the parasite. Recent work in several Apicomplexa, including genetic studies in the Plasmodium sporozoite, has provided molecular support for this 'capping' model. It appears that the same machinery drives both gliding motility and host cell invasion. The cytoplasmic motor, a transmembrane bridge and surface ligands essential for cell invasion are conserved among the main apicomplexan pathogens.     

Molecular mechanisms of invasion by cancer cells, leukocytes and microorganisms

Invasion is a phenotype common to cancer cells, leukocytes, parasites, bacteria and viruses, involving cell-cell adhesion, cell-matrix adhesion, proteolysis and motility. These activities are regulated by the cross talk between invaders and host. We discuss the invasion-related molecular interactions of E-cadherin, integrins, matrix metalloproteinases and the chemokine receptor RANTES.     

A mosquito-specific protein family includes candidate receptors for malaria sporozoite invasion of salivary glands

We describe a previously unrecognized protein family from Aedes and Anopheles mosquitoes, here named SGS proteins. There are no SGS homologues in Drosophila or other eukaryotes, but SGS presence in two mosquito genera suggests that the protein family is widespread among mosquitoes. Ae. aegypti aaSGS1 mRNA and protein are salivary gland specific, and protein is localized in the basal lamina covering the anatomical regions that are preferentially invaded by malaria sporozoites. Anti-aaSGS1 antibodies inhibited sporozoite invasion into the salivary glands in vivo, confirming aaSGS1 as a candidate sporozoite receptor. By homology to aaSGS1 we identified the complete complement of four SGS genes in An. gambiae, which were not recognized in the genome annotation. Two An. gambiae SGS genes display salivary gland specific expression like aaSGS1. Bioinformatic analysis predicts that SGS proteins possess heparin-binding domains, and have among the highest density of tyrosine sulphation sites of all An. gambiae proteins. The major sporozoite surface proteins (CS and TRAP) also bind heparin, and interact with sulphoconjugates during liver cell invasion. Thus, we speculate that sporozoite invasion of mosquito salivary glands and subsequently the vertebrate liver may share similar mechanisms based on sulphation. Phylogenomic analysis suggests that an SGS ancestor was involved in a lateral gene transfer.     

Molecular mechanisms of cytoadherence in malaria

Microbialpathogens subvert host adhesion molecules to disseminate or to enterhost cells to promote their own survival. One such subversion is thecytoadherence of Plasmodiumfalciparum-infected erythrocytes (IRBC) to vascularendothelium, which protects the parasite from being removed by thespleen. The process results in microcirculatory obstruction andsubsequent hypoxia, metabolic disturbances, and multiorgan failure,which are detrimental to the host. Understanding the molecular eventsinvolved in these adhesive interactions is therefore critical both interms of pathogenesis and implications for therapeutic intervention.Under physiological flow conditions, cytoadherence occurs in a stepwisefashion through parasite ligands expressed on the surface of IRBCand the endothelial receptors CD36, intercellular adhesionmolecule-1 (ICAM-1), P-selectin, and vascular adhesion molecule-1.Moreover, rolling on ICAM-1 and P-selectin increases subsequentadhesion to CD36, indicating that receptors can act synergistically.Cytoadherence may activate intracellular signaling pathways in bothendothelial cells and IRBC, leading to gene expression of mediatorssuch as cytokines, which could modify the outcome of the infection.

     

Proteases in host cell invasion by the malaria parasite

The life cycle of the malaria parasite contains three distinct invasive forms, or zoites. For at least two of these--the sporozoite and the blood-stage merozoite--invasion into their respective host cell requires the activity of parasite proteases. This review summarizes the evidence for this, discusses selected well-described proteolytic modifications linked to invasion, and describes recent progress towards identifying the proteases involved.     

Rhoptry neck protein 11 has crucial roles during malaria parasite sporozoite invasion of salivary glands and hepatocytes

The malaria parasite sporozoite sequentially invades mosquito salivary glands and mammalian hepatocytes; and is the Plasmodium lifecycle infective form mediating parasite transmission by the mosquito vector. The identification of several sporozoite-specific secretory proteins involved in invasion has revealed that sporozoite motility and specific recognition of target cells are crucial for transmission. It has also been demonstrated that some components of the invasion machinery are conserved between erythrocytic asexual and transmission stage parasites. The application of a sporozoite stage-specific gene knockdown system in the rodent malaria parasite, Plasmodium berghei, enables us to investigate the roles of such proteins previously intractable to study due to their essentiality for asexual intraerythrocytic stage development, the stage at which transgenic parasites are derived. Here, we focused on the rhoptry neck protein 11 (RON11) that contains multiple transmembrane domains and putative calcium-binding EF-hand domains. PbRON11 is localised to rhoptry organelles in both merozoites and sporozoites. To repress PbRON11 expression exclusively in sporozoites, we produced transgenic parasites using a promoter-swapping strategy. PbRON11-repressed sporozoites showed significant reduction in attachment and motility in vitro, and consequently failed to efficiently invade salivary glands. PbRON11 was also determined to be essential for sporozoite infection of the liver, the first step during transmission to the vertebrate host. RON11 is demonstrated to be crucial for sporozoite invasion of both target host cells – mosquito salivary glands and mammalian hepatocytes – via involvement in sporozoite motility.     

Molecular mechanisms of invasion by Entamoeba histolytica

Amebiasis is the third leading cause of death due to parasitic disease. Adherence to and contact-dependent killing of host cells requires the galactose-inhibitable lectin, a heterodimeric glycoprotein composed of heavy and light subunits. The cysteine-rich extracellular domain of the heavy subunits has identity with β₁ integrins, complement receptor CD59, and complement components C8 and C9; the light subunit sequence is unlike any other sequenced protein. Monoclonal antibodies to the cysteine-rich domain identify pathogenic-specific domains, have adherence-inhibitory and -enhancing properties, block contact-dependent cytotoxicity, and abrogate complement C5b-9 resistance. The purified lectin has galactose-binding activity and confers C5b-9 resistance to susceptible amebae. The accumulated evidence points to the same cell surface galactose-inhibitable lectin as a mediator of two activities required for invasion: adherence and complement resistance.     

Analysis of the mechanisms of Salmonella-induced actin assembly during invasion of host cells and intracellular replication

Salmonella enterica serovar Typhimurium (S. typhimurium) induces actin assembly both during invasion of host cells and during the course of intracellular bacterial replication. In this study, we investigated the involvement in these processes of host cell signalling pathways that are frequently utilized by bacterial pathogens to manipulate the eukaryotic actin cytoskeleton. We confirmed that Cdc42, Rac, and Arp3 are involved in S. typhimurium invasion of HeLa cells, and found that N-WASP and Scar/WAVE also play a role in this process. However, we found no evidence for the involvement of these proteins in actin assembly during intracellular replication. Cortactin was recruited by Salmonella during both invasion and intracellular replication. However, RNA interference directed against cortactin did not inhibit either invasion or intracellular actin assembly, although it resulted in increased cell spreading and a greater number of lamellipodia. We also found no role for either the GTPase dynamin or the formin family member mDia1 in actin assembly by intracellular bacteria. Collectively, these data provide evidence that signalling pathways leading to Arp2/3-dependent actin nucleation play an important role in S. typhimurium invasion, but are not involved in intracellular Salmonella-induced actin assembly, and suggest that actin assembly by intracellular S. typhimurium may proceed by a novel mechanism.     

Molecular mechanisms of gossypol action on sperm motility

1. Gossypol acetic acid inhibits collective motility of ejaculated ram spermatozoa. 2. Oxygen consumption was stimulated at low gossypol concentrations and inhibited as the concentrations are increased. 3. Gossypol inhibits respiration of permeabilized spermatozoa supported by durohydroquinome, which indicates a direct inhibition of mitochondrial electron transport chain. 4. The rapid reduction of mitochondrial dependent motility, high uncoupling effect and almost complete inhibition of mitochondrial calcium accumulation, indicate that gossypol inhibits motility in a mechanism by which mitochondrial uncoupling is involved.     

How do sperm swim? Molecular mechanisms underlying sperm motility.

Motility is a characteristic function of the male gamete, which allows spermatozoa to actively reach and penetrate the female gamete in organisms with internal and external fertilization. Sperm motility is acquired under the control of many extrinsic and intrinsic factors and is based on the specialized structure of the sperm flagellum. After a brief overview of how the sperm flagellum is organized and works to support cell motility, the present review focuses on the molecular mechanisms and factors involved in the development and maintenance of sperm motility. Data obtained both in organisms with external fertilization, such as fishes and sea urchin, and with internal fertilization, such as Mammals, are critically analyzed. In particular, a great attention has been put on the ionic mechanisms and on the involvement of protein kinases and phosphatases in regulation of sperm motility. A brief overview of the pharmacological and physiological molecules which have been studied for their possible application as therapeutic molecules for in vitro treatment of defects of sperm motility in asthenozoospermic human subjects, is presented. Moreover, we show some preliminary data obtained in our laboratory on the involvement of the phosphatydilinositol 3-kinase and the A kinase anchoring protein (AKAP3) in regulation of motility in human spermatozoa. The last section is dedicated to hyperactivation, a peculiar pattern of motility which is developed in association with capacitation occurring during sperm transit through the female genital tract and which can also be obtained in vitro by incubation in defined media.     

Participation of myosin in gliding motility and host cell invasion by Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite that actively invades mammalian cells using a unique form of gliding motility that critically depends on actin filaments in the parasite. To determine if parasite motility is driven by a myosin motor, we examined the distribution of myosin and tested the effects of specific inhibitors on gliding and host cell invasion. A single 90 kDa isoform of myosin was detected in parasite lysates using an antisera that recognizes a highly conserved myosin peptide. Myosin was localized in T. gondii beneath the plasma membrane in a circumferential pattern that overlapped with the distribution of actin. The myosin ATPase inhibitor, butanedione monoxime (BDM), reversibly inhibited gliding motility across serum-coated slides. The myosin light-chain kinase inhibitor, KT5926, also blocked parasite motility and greatly reduced host cell attachment; however, these effects were primarily caused by its ability to block the secretion of microneme proteins, which are involved in cell attachment. In contrast, while BDM partially reduced cell attachment, it prevented invasion even under conditions in which microneme secretion was not affected, indicating a potential role for myosin in cell entry. Collectively, these results indicate that myosin(s) probably participate(s) in powering gliding motility, a process that is essential for cell invasion by T. gondii .