Effects of DNA damage and vesicular exchange in P. falciparum

Abstract

Plasmodium falciparum causes human malaria and is a global leading cause of mortality from parasitic infections. While decades of concerted effort has yielded significant result in reducing the endemicity, there is evidence of a recent resurgence in transmission intensity.

Notably, Plasmodium falciparum parasites are master adapter to various environmental pressures, including stresses mounted by the host immune system as well as assaults from an arsenal of anti-malarial drugs. Intuitively, this adept adaptability constitutes a major impedance to a continued success in the control program. The mechanisms that confer such a high adaptability to parasite is currently unclear, but the plastic AT-rich genome, a versatile but enigmatic transcription network as well as the recently characterized cell-cell communication module via extracellular vesicles are likely contributors to such cause. Through experimental designs that sought to investigate the dynamics of these aforementioned molecular and cellular programs on both population and single-cell level, the works presented in this thesis aim to further the current understandings on how the parasites respond and adapt to various environmental pressures.

In paper I, we investigated the short-term effect of anti-malarial drugs on the genomic plasticity of parasite. We revealed that all the tested anti-malarial drugs can cause acute DNA damages and trigger a robust response to the damages by the upregulation of specific DNA repair pathways. We further demonstrated that the DNA damages elicited by the drug action can be erroneously repaired to incorporate random mutations. Therefore, due to the random nature of DNA damages and the subsequent error-prone repair, this can serve as a mechanism to rapidly diversify the genetic pools of the exposed parasite population and pre-deposit it to a rapid selection of resistant genotypes.

In paper II, we described the role of extra-cellular vesicles (EVs) during malaria infection. Infected RBCs are previously characterized for their enhanced capacity to generate EVs. In this study, we discovered that EVs originated from infected cell contain a subset of host miRNAs and the Ago2 proteins. These EVs are readily internalized by endothelial cells and that the miRNAs trafficked within these EVs can accumulate in the endothelial cells and exert a global effect on the post-transcriptional gene regulatory network. We show that this unilateral cellular communication can contribute to vascular dysfunction, local and systemic immunological modulation as well as endothelial activation. In particular, we note that endothelial activation can promote sequestration of infected RBCs and, thereby, serve to avoid splenic clearance.

In paper III, we developed and detailed a technical platform that enables single-cell transcriptomic analysis of individual Plasmodium falciparum parasites. We then utilized the method to decipher the transcriptional cascade underlying the process of gametogenesis, which is triggered by yet undetermined environmental cues. Interestingly, we revealed huge heterogeneity even within a highly synchronous parasite population, supporting the presence of a versatile transcriptional network. Moreover, we identified a distinct gene signature that is associated with sexually committed and differentiating parasites. This work has generated important knowledge that can be exploited for the design of transmission blocking drugs.