Erythropoiesis and molecular mechanisms for sexual determination in malaria parasites

Malaria parasites proliferate asexually within the vertebrate host but must undergo sexual reproduction for transmission to mosquitoes and hence infection of new hosts. The developmental pathways controlling gametocytogenesis are not known, but several protein kinases and other putative signal transduction elements possibly involved in this phenomenon have been found in Plasmodium. Recently, another developmental pathway, that of Plasmodium sex determination (male or female), has been shown to be triggered by erythropoiesis in the host. Rapid progress is being made in our understanding of the molecular basis of mammalian erythropoiesis, revealing kinase pathways that are essential to cellular responses triggered by the hormone erythropoietin. Although the molecular mechanisms whereby this hormone modulates the sex ratio of malaria parasites remain to be elucidated, it probably activates, within the parasite, transduction pathways similar to those found in other eukaryotes. Indeed, enzymes belonging to protein kinase families known to be involved in the response of mammalian cells to erythropoietin (such as the mitogen-activated protein kinases) have been identified in P. falciparum gametocytes. Some of these enzymes differ markedly from their mammalian homologs; therefore, identification of the transduction pathways of the parasite that are responsible for its developmental response to erythropoietin opens the way to the development of transmission-blocking drugs based on kinase inhibitors.     

Revealing the molecular determinants of neurotoxin specificity for calcium-activated versus voltage-dependent potassium channels

Potassium channel dysfunction underlies diseases such as epilepsy, hypertension, cardiac arrhythmias, and multiple sclerosis. Neurotoxins that selectively inhibit potassium channels, alpha-KTx, have provided invaluable information for dissecting the contribution of different potassium channels to neurotransmission, vasoconstriction, and lymphocyte proliferation. Thus, alpha-KTx specificity comprises an important first step in potassium channel-directed drug discovery for these diseases. Despite extensive functional and structural studies of alpha-KTx-potassium channel complexes, none have predicted the molecular basis of alpha-KTx specificity. Here we show that by minimizing the differences in binding free energy between selective and nonselective alpha-KTx we are able to identify all of the determinants of alpha-KTx specificity for calcium-activated versus voltage-dependent potassium channels. Because these determinants correspond to unique features of the two types of channels, they provide a way to develop more accurate models of alpha-KTx-potassium channel complexes that can be used to design novel selective alpha-KTx inhibitors.     

Chromosomes of malaria parasites

The advent of pulsed field gradient electrophoresis has proved remarkably useful for studying chromosomes of the genetically intractable malaria parasite Plasmodium falciparum. Advances include determination of the karyotype, a linkage map and restriction maps of individual chromosomes that enable the ordering of genes. The structural basis underlying a frequently occurring form of chromosome size polymorphism is now understood and other polymorphisms are providing tantalizing clues to the mechanisms underlying drug resistance.     

Calcium signalling in malaria parasites

Ca²⁺ is a ubiquitous intracellular messenger in malaria parasites with important functions in asexual blood stages responsible for malaria symptoms, the preceding liver‐stage infection and transmission through the mosquito. Intracellular messengers amplify signals by binding to effector molecules that trigger physiological changes. The characterisation of some Ca²⁺ effector proteins has begun to provide insights into the vast range of biological processes controlled by Ca²⁺ signalling in malaria parasites, including host cell egress and invasion, protein secretion, motility and cell cycle regulation. Despite the importance of Ca²⁺ signalling during the life cycle of malaria parasites, little is known about Ca²⁺ homeostasis. Recent findings highlighted that upstream of stage‐specific Ca²⁺ effectors is a conserved interplay between second messengers to control critical intracellular Ca²⁺ signals throughout the life cycle. The identification of the molecular mechanisms integrating stage‐transcending mechanisms of Ca²⁺ homeostasis in a network of stage‐specific regulator and effector pathways now represents a major challenge for a meaningful understanding of Ca²⁺ signalling in malaria parasites.     

Signal transduction in malaria parasites

Over the past few years, several reports have been published about the characterization of Plasmodium genes that are thought, on the basis of sequence homology with eukaryotic genes of known function, to be involved in the regulation of growth and differentiation of the parasite. Taken together with phenomenological observations on the regulation of developmental stages in the malaria life cycle, these data form the basis of an informative, albeit incomplete, picture of signal transtruction in Plasmodium. Christian Doerig here reviews Plasmodium elements that are presumably part of major regulatory pathways conserved in eukaryotes, and addresses the problem of how to pursue such studies beyond the stage of gene identification.     

Avian malaria parasites (

There is concern that avian malaria may be partly responsible for fluctuations in yellow-eyed penguin (Megadyptes antipodes) populations in New Zealand. Recent findings, however, have provided no evidence of avian malaria parasites infecting yellow-eyed penguins on the Otago Peninsula, raising questions as to whether this area is currently free of such parasites. To test this possibility we collected blood samples from 109 individuals of five non-native bird species known to carry malarial parasites elsewhere in New Zealand. Molecular screening by polymerase chain reaction revealed 6% of the sampled birds were positive for malarial parasites, indicating that a local reservoir of infection is present. Sequence data revealed a generalist strain of Plasmodium is present, one that infects a number of native and non-native bird species elsewhere in the country. The absence of this generalist strain in yellow-eyed penguins, some of which were sampled during the same period as the current study, may be due to low levels of mosquito vectors of disease during the study period, low densities of non-native birds around yellow-eyed penguin colonies, or infected penguins dying before they could be sampled. Continued monitoring of mosquito populations and the factors that affect their densities should be included in the future management of native birds in this area.     

Measurably recombining malaria parasites

Genomic epidemiology has guided research and policy for various viral pathogens and there has been a parallel effort towards using genomic epidemiology to combat diseases that are caused by eukaryotic pathogens, such as the malaria parasite. However, the central concept of viral genomic epidemiology, namely that of measurably mutating pathogens, does not apply easily to sexually recombining parasites. Here we introduce the related but different concept of measurably recombining malaria parasites to promote convergence around a unifying theoretical framework for malaria genomic epidemiology. Akin to viral phylodynamics, we anticipate that an inferential framework developed around recombination will help guide practical research and thus realize the full public health potential of genomic epidemiology for malaria parasites and other sexually recombining pathogens.     

Cysteine proteases of malaria parasites

A number of cysteine proteases of malaria parasites have been described, and many more putative cysteine proteases are suggested by analysis of the Plasmodium falciparum genome sequence. Studies with protease inhibitors have suggested roles for cysteine proteases in hemoglobin hydrolysis, erythrocyte rupture, and erythrocyte invasion by erythrocytic malaria parasites. The best characterised Plasmodium cysteine proteases are the falcipains, a family of papain-family (clan CA) enzymes. Falcipain-2 and falcipain-3 are hemoglobinases that appear to hydrolyse host erythrocyte hemoglobin in the parasite food vacuole. This function was recently confirmed for falcipain-2, with the demonstration that disruption of the falcipain-2 gene led to a transient block in hemoglobin hydrolysis. A role for falcipain-1 in erythrocyte invasion was recently suggested, but disruption of the falcipain-1 gene did not alter parasite development. Other papain-family proteases predicted by the genome sequence include dipeptidyl peptidases, a calpain homolog, and serine-repeat antigens. The serine-repeat antigens have cysteine protease motifs, but in some the active site Cys is replaced by a Ser. One of these proteins, SERA-5, was recently shown to have serine protease activity. As SERA-5 and some other serine-repeat antigens localise to the parasitophorous vacuole in mature parasites, they may play a role in erythrocyte rupture. The P. falciparum genome sequence also predicts more distantly related (clan CD and CE) cysteine proteases, but biochemical characterisation of these proteins has not been done. New drugs for malaria are greatly needed, and cysteine proteases may provide useful new drug targets. Cysteine protease inhibitors have demonstrated potent antimalarial effects, and the optimisation and testing of falcipain inhibitor antimalarials is underway.     

Cyclic nucleotide signalling in malaria parasites

Cyclic nucleotides are so-called intracellular second messenger molecules used by all cells to transform environmental signals into an appropriate response. Interest in the cyclic nucleotides cAMP and cGMP in malaria parasites followed early observations that both molecules might be involved in distinct differentiation events within the sexual phase of the life cycle that is required for transmission of parasites to the mosquito vector. Completed genome sequences combined with biochemical and genetic studies have confirmed the presence of the main enzymatic components of cyclic nucleotide signalling in the parasite. Dissection of their functions is underway and is giving initial insights into some of the cellular processes, which are regulated by these signalling pathways. Malaria parasites occupy terminally differentiated red blood cells for a significant proportion of their life cycle, but although there is some evidence of potential roles for the residual host cell signalling machinery in parasite development, details are few. A major gap in our knowledge is the nature of the cell surface receptors, which might trigger cyclic nucleotide signalling in the parasite.