Science - USA (2020-01-03)

INSIGHTS

sciencemag.org SCIENCE

GRAPHIC: KELLIE HOLOSKI/

SCIENCE

By Danushka Marapana and Alan F. Cowman

T

he identification of artemisinin (ART)

in 1971 allowed treatment of malaria

resistant to chloroquine, the prevail-

ing drug at the time, and provided

hope for a malaria-free world ( 1 ). To-

day, malaria control efforts have been

very successful, with 32% fewer deaths over

the past 8 years ( 2 ). However, the emergence

of resistance to ART and other antimalari-

als threatens to become a major problem in

the continuing program to eliminate and

eventually eradicate malaria ( 3 ). ART com-

bination therapies (ACTs) are the

current gold standard for the treat-

ment and control of malaria, and

how parasites that cause malaria

in humans mediate this resistance

is of intense interest for preventing

the spread of drug resistance. On

page 51 of this issue, Birnbaum et

al. ( 4 ) answer this critical question

by identifying the molecular mecha-

nism of ART resistance in the most

lethal human malaria parasite, Plas-

modium falciparum.

As malaria parasites infect and

grow within human erythrocytes

(red blood cells), they actively en-

gulf and digest host hemoglobin

in a dedicated parasite-derived

food vacuole (see the figure).

Hemoglobin is broken down into

amino acids, which are used for the

synthesis of parasite proteins, and

iron-bound heme, which is gradu-

ally detoxified within hemozoin

crystals. Heme binding results in

cleavage of the endoperoxide bond

of ART, enabling drug activation. Active

ART produces free radicals and reactive

oxygen species that attack protein and

lipid molecules of the developing para-

site. ART treatment of intraerythrocytic

Plasmodium parasites results in rapid kill-

ing of all life cycle stages, including the

young “ring” stage of infection, which is

resistant to most antimalarial drugs ( 5 ).

However, the initial identification of ART

resistance in western Cambodia in 2008

followed by rapid spread throughout the

Greater Mekong Subregion of Southeast

Asia forewarned of a major issue for the

malaria elimination agenda in that re-

gion ( 3 ). Additionally, the potential spread

of ART resistance to sub-Saharan Africa

would make elimination programs even

more challenging.

ART resistance manifests as enhanced

survival and delayed clearance of young

ring-stage parasites after a concen-

trated exposure to ART or its derivatives.

Molecular genotyping of ART-resistant par-

asites uncovered single nonsynonymous

mutations in a P. falciparum–specific gene

called Pfkelch13 ( 6 ). The PfKelch13 protein

contains three main functional regions: a

parasite-specific localization sequence, a

BTB/POZ domain that typically facilitates

ubiquitin-mediated degradation, and a

carboxyl-terminal Kelch propeller repeat

region that is predicted to function as a

scaffold for protein-protein interactions

( 6 ). Nearly all clinically relevant ART-

resistance mutations are localized within

this carboxyl-terminal Kelch-repeat region.

Directed mutagenesis studies of both ART-

naïve strains and clinical isolates have

confirmed the causal role of PfKelch13 in

mediating ART resistance ( 7 , 8 ). Multiple

hypotheses implicate PfKelch13 as a re-

sponder to downstream effects of ART

activation, especially in up-regulation of

pathways involved in the cellular stress

response and reduced protein translation

in the presence of ART-induced stress ( 9 –

11 ). However, the possibility of PfKelch13

acting upstream as a conduit for ART ac-

tivation was not investigated prior to the

Birnbaum et al. s tudy.

Birnbaum et al. found that

PfKelch13 and its interacting pro-

teins are localized in vesicles close

to cytostomes, which are erythro-

cyte-cytosol containing structures

produced by the parasite. They

discovered that proteins in this

PfKelch13 compartment are re-

quired for the endocytic uptake of

hemoglobin; however, PfKelch13 it-

self is essential for this process only

in the young ring-stage parasites.

By investigating the link between

PfKelch13, hemoglobin uptake, and

ART activation, Birnbaum et al.

found that PfKelch13-inactivated

parasites and those carrying ART

resistance–conferring Pfkelch13

mutations display depleted con-

centrations of the protein. These

PfKelch13-depleted parasites ex-

hibit decreased hemoglobin uptake

as well as enhanced ring-stage sur-

vival during high-dose ART treat-

ment. Thus, PfKelch13 is a facilita-

tor of ART activation, and Birnbaum et al.

propose an elegant model to merge a large

body of prior research into a comprehen-

sive pathway of ART resistance.

In this model, mutations in the Kelch

propeller domains of PfKelch13 impair the

ability of parasites to endocytose hemoglo-

bin during the young ring stage of infection.

Consequently, hemoglobin degradation is

reduced, and less heme becomes available

for ART activation. Hemoglobin catabolism

also provides the parasite with a source of

amino acids for protein synthesis. Therefore,

INFECTIOUS DISEASE

Uncovering the ART of antimalarial resistance

A key mechanism of resistance to the antimalarial drug artemisinin is identified

Hemoglobin

PfKelch13

vesicles

Cytostome

Plasmodium

falciparum

parasite

Endoplasmic

reticulum

Nucleus

Heme

Food

vacuole

Red blood

cell

ART

Department of Medical Biology, Faculty of Medicine,

Dentistry and Health Sciences, University of Melbourne,

Parkville, VIC 3052, Australia. Email: [email protected]

PERSPECTIVES

Gateway for activation

Malaria parasites ingest hemoglobin to produce heme from the

host red blood cell using cytostomes. The interaction of heme in the

parasite food vacuole with artemisinin (ART) causes activation of

the drug and parasite killing. PfKelch13-containing vesicles regulate

the uptake of hemoglobin by Plasmodium falciparum and thereby

affect the amount of ART activation through reduced heme availability.

22 3 JANUARY 2020 • VOL 367 ISSUE 6473

Published by AAAS