Artemisinin Action and Resistance in Plasmodium falciparum

Abstract

The worldwide use of artemisinin-based combination therapies (ACTs) has contributed in recent years to a substantial reduction in deaths resulting from Plasmodium falciparum malaria. Resistance to artemisinins, however, has emerged in Southeast Asia. Clinically, resistance is defined as a slower rate of parasite clearance in patients treated with an artemisinin derivative or an ACT. These slow clearance rates associate with enhanced survival rates of ring-stage parasites briefly exposed in vitro to dihydroartemisinin. We describe recent progress made in defining the molecular basis of artemisinin resistance, which has identified a primary role for the P. falciparum K13 protein. Using K13 mutations as molecular markers, epidemiological studies are now tracking the emergence and spread of artemisinin resistance. Mechanistic studies suggest potential ways to overcome resistance.

Keywords: Kelch13; artemisinin resistance; malaria.; proteasome; ubiquitination; unfolded protein response.

Figure 1. Structures of endoperoxide antimalarials

These are shown for the lactone artemisinin, its lactol derivatives dihydroartemisinin, artemether and artesunate, and the fully synthetic ozonides OZ277 and OZ439.

Figure 2. Diagram of putative cell death- and survival-promoting events following treatment with artemisinin (ART)

ART and its derivatives are activated by a reduced iron source (probably mainly heme released from hemoglobin digestion) to produce activated ART (ART*). Activated ART* reacts promiscuously with nucleophile-harboring cellular components, leading to damage and ultimately parasite death (black arrows). Parasites are thought to mount a stress response that engages the unfolded protein response (UPR), including the ubiquitin-proteasome system (green arrows). Recent evidence suggests that the stress response in K13 mutants is enhanced. Proteasome inhibitors are proposed to decrease the stress response (red), thus promoting parasite death.

Figure 3. K13 and KEAP1 domains

Diagram of K13 domain structure illustrating the conserved Plasmodium-specific N-terminal domain, a BTB/POZ domain and a C-terminal domain containing six Kelch motifs, compared with human KEAP1, which has an E3 ligase-binding BACK domain adjacent to the BTB/POZ domain.

Figure 4. Proposed role of Kelch proteins as an E3 ligase substrate adaptor

The BTB/POZ domain of Kelch proteins contains a binding site for an E3 ligase, while the Kelch domain acts as an adaptor for a putative transcription factor substrate (S) that can be polyubiquitinated by an E2 enzyme. This would result in proteasomal degradation of the transcription factor. In stressed conditions the transcription factor is not degraded and can promote transcription of cellular defense proteins.