How malaria parasites choose between us and our evolutionary cousins

3rd December 2013
by Madushi Wanaguru

The binding of P. falciparum RH5 to primate Basigin proteins is species-specific. HsBSG= human BSG, PtBSG= chimpanzee (Pan troglodytes) BSG, GgBSG= Gorilla (Gorilla gorilla) BSG

The binding of P. falciparum RH5 to primate Basigin proteins is species-specific. HsBSG= human BSG, PtBSG= chimpanzee (Pan troglodytes) BSG, GgBSG= Gorilla (Gorilla gorilla) BSG

Plasmodium falciparum, the most lethal of the human malaria parasites, is evolutionarily more closely related to a family of gorilla and chimpanzee parasites called Lavernia, than to other malaria parasites known to infect humans.

Interestingly, even though humans, chimpanzees and gorillas are physiologically very alike and share more than 90 per cent similarity in most protein sequences, Lavernia parasites (including P. falciparum) appear to discriminate between these primates, showing strict host preferences. Understanding the molecular basis of this host-specificity is important as it would reveal how ape parasites could develop the ability to infect humans and vice versa, which would have serious implications for malaria eradication.

However, identifying the factors that determine host-specificity is a rather difficult area for experimental exploration as none of the ape Lavernia parasites have so far been adapted for growth in lab cultures and the use of endangered African great apes for invasive infection studies breaches ethical boundaries.

To overcome these problems, we were able to use genetic material recovered from ape faeces samples to provide parasite sequence data and express the parasite and ape proteins artificially in the lab so that interactions between them could be studied.

Host restriction may be imposed at one or more of the stages that make up the complex life cycle of Lavernia parasites. However, we know that the highest increase in parasite number (103 to 109 fold) occurs during its blood-stage, which is also responsible for all the symptoms of malaria. The entry of the parasite into the host’s red blood cells is therefore likely to be an important point of selection.

To explore this, we needed to look at the interactions between cell surface proteins on the host and parasite cells that mediate red blood cell entry. We focused on two interactions known to be important, parasite EBA175 with host Glycophorin A (GYPA) and parasite RH5 with host Basigin (BSG), and compared their relative contributions to determining Lavernia host-specificity.

The genomes of only two Laverania parasites, P. falciparum and the chimpanzee-restricted P. reichenowi, have so far been sequenced, so we determined the EBA175 sequences of the other Lavernia parasites using genetic material recovered from the faeces of wild-living chimpanzees and gorillas, a non-invasive method that does not interfere with ape conservation efforts.

We went on to express the soluble extracellular regions of the selected parasite and ape proteins and to analyse the interactions between them using biochemical and biophysical assays. By using proteins expressed in the lab for our experiments, we were able to avoid the need for live ape and parasite material.

In the past it’s been difficult to study interactions between cell surface proteins as they tend to be very weak with half-lives of less than 0.5 seconds. We used AVEXIS, a platform developed in our lab as a primary tool to study the interactions between our ape and parasite proteins. In AVEXIS, both the ‘bait’ and the ‘prey’ proteins are multimerised so that the duration (and therefore the chance of detection) of potential interactions between them is increased.

In our study we found that EBA175 proteins from P. falciparum and the chimpanzee-restricted parasites P. reichenowi and P. billcollinsi, bound human GYPA with fairly similar affinity, suggesting that this interaction is unlikely to be the sole determinant of host-specificity at the stage of red blood cell entry. P. falciparum RH5 on the other hand, showed no binding to gorilla BSG and very weak binding to chimpanzee BSG, relative to human BSG, thereby mimicking the known host preferences of P. falciparum.

Madushi was a PhD student at the Wellcome Trust Sanger Institute. She worked under the supervision of Gavin Wright and Julian Rayner on the red blood cell invasion pathways of Plasmodium falciparum.