Partners in Mischief: Functional Networks of Heat Shock Proteins of Plasmodium falciparum and Their Influence on Parasite Virulence.

The survival of the human malaria parasite Plasmodium falciparum under the physiologically distinct environments associated with their development in the cold-blooded invertebrate mosquito vectors and the warm-blooded vertebrate human host requires a genome that caters to adaptability. To this end, a robust stress response system coupled to an efficient protein quality control system are essential features of the parasite. Heat shock proteins constitute the main molecular chaperone system of the cell, accounting for approximately two percent of the malaria genome. Some heat shock proteins of parasites constitute a large part (5%) of the 'exportome' (parasite proteins that are exported to the infected host erythrocyte) that modify the host cell, promoting its cyto-adherence. In light of their importance in protein folding and refolding, and thus the survival of the parasite, heat shock proteins of P. falciparum have been a major subject of study. Emerging evidence points to their role not only being cyto-protection of the parasite, as they are also implicated in regulating parasite virulence. In undertaking their roles, heat shock proteins operate in networks that involve not only partners of parasite origin, but also potentially functionally associate with human proteins to facilitate parasite survival and pathogenicity. This review seeks to highlight these interplays and their roles in parasite pathogenicity. We further discuss the prospects of targeting the parasite heat shock protein network towards the developments of alternative antimalarial chemotherapies.

Plasmodial Hsp40s: New Avenues for Antimalarial Drug Discovery.

Malaria, an infectious disease caused by Plasmodium spp, is one of the world's most dangerous diseases, accounting for more than half a million deaths yearly. The long years of co-habitation between the parasite and its hosts (human and mosquito), is a testimony to the parasite's ability to escape the immune system and develop drug resistance mechanisms. Currently, an important search area for improved pharmacotherapy are molecular chaperones of the heat shock protein family, abundant in Plasmodium falciparum and contributing to its continuous survival and development. Thus far, small molecule inhibitor studies on P. falciparum Hsp70s and Hsp90s have indicated that they are promising antimalarial targets. However, not much attention has been given to Hsp40s as potential antimalarial drug targets. Hsp40s are known to function as chaperones by preventing protein aggregation, and as co-chaperones, by regulating the chaperone activities of Hsp70s to ensure proper protein folding. There are only a limited number of reviews on Hsp40s as drug targets, and the few reviews on plasmodial Hsp40s tend to focus largely on the intra-erythrocytic stage of the parasite life cycle. Therefore, this review will summarize what is known about Hsp40s throughout the malaria parasite life cycle, and critically evaluate their potential to serve as new avenues for antimalarial drug discovery.

The Malarial Exported PFA0660w Is an Hsp40 Co-Chaperone of PfHsp70-x.

Plasmodium falciparum, the human pathogen responsible for the most dangerous malaria infection, survives and develops in mature erythrocytes through the export of proteins needed for remodelling of the host cell. Molecular chaperones of the heat shock protein (Hsp) family are prominent members of the exportome, including a number of Hsp40s and a Hsp70. PFA0660w, a type II Hsp40, has been shown to be exported and possibly form a complex with PfHsp70-x in the infected erythrocyte cytosol. However, the chaperone properties of PFA0660w and its interaction with human and parasite Hsp70s are yet to be investigated. Recombinant PFA0660w was found to exist as a monomer in solution, and was able to significantly stimulate the ATPase activity of PfHsp70-x but not that of a second plasmodial Hsp70 (PfHsp70-1) or a human Hsp70 (HSPA1A), indicating a potential specific functional partnership with PfHsp70-x. Protein binding studies in the presence and absence of ATP suggested that the interaction of PFA0660w with PfHsp70-x most likely represented a co-chaperone/chaperone interaction. Also, PFA0660w alone produced a concentration-dependent suppression of rhodanese aggregation, demonstrating its chaperone properties. Overall, we have provided the first biochemical evidence for the possible role of PFA0660w as a chaperone and as co-chaperone of PfHsp70-x. We propose that these chaperones boost the chaperone power of the infected erythrocyte, enabling successful protein trafficking and folding, and thereby making a fundamental contribution to the pathology of malaria.

Effects of prior administration of amodiaquine on the disposition of halofantrine in healthy volunteers.

The prevalence of multidrug-resistant malaria parasites brings about the switch from an antimalarial drug with poor therapeutic outcome to an effective alternative, resulting in overlap in the plasma drug levels. In this study, the influence of prior administration of amodiaquine on the pharmacokinetics and electrocardiographic effect of halofantrine (HF) was investigated in healthy volunteers. Ten healthy male subjects were each given single oral doses of 500 mg HF alone or with 600 mg of amodiaquine hydrochloride (AQ) administered 24 hours before the HF dose in a crossover fashion. Blood samples, collected at predetermined time intervals, were analyzed for HF and its major metabolite, desbutylhalofantrine (HFM) using a validated high-performance liquid chromatography method. Electrocardiogram for each volunteer was taken at predetermined time points. Results showed that prior administration of amodiaquine resulted in no significant changes (P > 0.05) in any of the pharmacokinetic parameters of HF. For example, the parameter values for HF alone and with AQ were: Cmax 144 +/- 53 versus 164 +/- 58 microg/L; T1/2beta 142 +/- 23 versus 139 +/- 28 hours; Cl/F 37.3 +/- 13.9 versus 32.3 +/- 11.4 L/h; and metabolic ratio 1.2 +/- 0.5 vs 1.1 +/- 0.6 Similarly, the disposition of HFM was not significantly altered (P > 0.05) after an earlier exposure to amodiaquine. In addition, the presence of AQ was linked with a further lengthening of the QT interval compared with the effect of HF alone. This study suggests that prior administration of AQ does not result in a significant alteration of the pharmacokinetics of HF but may be associated with an increased risk of QT prolongation. It may be necessary to exercise caution in the use of HF for malaria treatment in persons who have recently received AQ.