INSIGHTS | PERSPECTIVES
antimalarial regime (7), the spread of a
multidrug-resistant P. falciparum lineage
(PfPailin) in the Greater Mekong area of
Southeast Asia was recently reported (8).
This strain contains a dominant mutation
in Pfkelch13 (mutations in this gene are associated with artemisinin resistance) and is
also resistant to piperaquine. The spread of
this evolutionarily “fit” multidrug-resistant
malaria parasite between endemic countries is extremely concerning.
Cowell et al. used experimental evolution
to identify new P. falciparum drug targets
while also anticipating their possible resistance mechanisms. Experimental evolution
is the maintenance of populations of organisms in controlled environments where
changes in genotype and phenotype can be
evaluated over thousands of generations; it
is most often applied to microbes because
of their rapid generation times and small
sizes (9). In these studies, environmental
conditions, such as changes in nutrient
availability or antimicrobial drugs, can be
manipulated to explore adaptation of the
organisms being studied. Probably the most
widely known example of experimental
evolution is the “Escherichia coli long-term
evolution experiment” (LTEE) that has been
tracking genetic changes in 12 originally
identical populations of E. coli maintained
in continuous culture since 1988 (10).
Experimental evolution is not new to
malaria research; the first evolution experiments on malaria were carried out in
the 1940s, using a species of bird malaria
parasite that was propagated in laboratory
chickens (11). In later decades, researchers
generated drug-resistant strains of rodent
malaria parasites in laboratory mice that, in
the absence of high-throughput sequencing
methods, were subsequently investigated
by genetic crossing experiments and linkage mapping (12). Such studies provided
important data regarding the single-gene or
multigenic nature of the induced drug resistance, as well as identifying resistance loci
and their alleles. More recently, the technique has been used to identify resistance
loci in the rodent malaria species
Plasmodium chabaudi (13) and P. falciparum
(14) that were subjected to artemisinin.
Although these evolution experiments can
take years, they may provide immense scientific reward, as evidenced by the eventual
identification of Pfkelch13 as an artemisinin
resistance–associated locus after a 5-year
evolution experiment (14).
Whereas these previous studies evalu-
ated the response to known antimalari-
als, or focused on mutations in one target
gene, Cowell et al. examined the parasite’s
genetic response to a large number of
new inhibitors. Three well-studied P. fal-
ciparum laboratory clonal isolates were
individually subjected to experimental
evolution over 3 to 6 months in vitro us-
ing 37 publicly available compounds with
proven antimalarial activity to various
stages of the P. falciparum life cycle. Com-
parison of the genomes of > 200 genetically
stable, compound-resistant clones revealed
a remarkable enrichment of mutations,
including single-nucleotide variations, in
just 57 genes, and 150 copy number varia-
tions. These loci are informative as either
genes involved in resistance (because a
particular gene was mutated repeatedly
in response to multiple compounds), or as
potential drug targets, although in many
cases the authors were unable to discern
between these two possibilities. One in-
teresting example of a resistance gene is
a putative ABC (ATP-binding cassette)
transporter, Pfabc13, that harbored point
mutations and was involved in 12 different
amplification events with four separate
compounds. Four target-inhibitor pairs
were also singled out as possible new an-
timalarial drug targets, identified because
of their enzymatic function that enabled
docking and homology modeling. Ulti-
mately, one gene (either a new drug target
or a new drug resistance locus) was identi-
fied for each compound examined.
This rich data set increases our understanding of the biology and evolution of
P. falciparum, providing a powerful contribution toward basic research for malaria
elimination. In the end, designing “
resis-tance-proof” drugs may be the best strategy
for controlling malaria. There have been
several developments toward this goal (7),
including targeting host factors required
for parasite growth (15), although the field
has some way to mature. j
1. World Health Organization, “World Malaria Report 2016”
2. A. N. Cowell etal.,Science 359, 191 (2018).
3. S. L. Perkins et al ., J. Parasitol .100, 11 (2014).
4. M. Ghorbal et al., Nat. Biotechnol. 32, 819 (2014).
5. J. C. Wagner etal ., Nat.Methods 11, 915 (2014).
6. K. L. Olszewski et al. , Cell Host Microbe 5, 191 (2009).
7. B. Blasco et al ., Nat. Med. 23, 917 (2017).
8. M. Im wong et al ., Lancet Infect. Dis. 17, 1022 (2017).
9. J. E. Barrick, R. E. Lenski, Nat. Rev. Genet. 14, 827 (2013).
10. R. Maddamsetti et al. , Genome Biol. Evol. 9, 1072 (2017).
11. J. Williamson, E. M. Lourie, Ann. Trop. Med. Parasitol.41,
12. J. M. Carlton et al ., Trends Parasitol. 17, 236 (2001).
13. P. Hunt et al. , BMC Genomics 11, 499 (2010).
14. F. Ariey et al., Nature 505, 50 (2014).
15. A. Zumla et al ., Lancet Infect. Dis.16, e47 (2016).
By Chanhyung Bae, Andres Jara-Oseguera,
Kenton J. Swartz
Transient receptor potential (TRP) channels were first identified in pho- toreceptors of the fruit fly (1, 2). In mammals, six major families of TRP channels play key roles in sensing stimuli such as light, temperature,
membrane lipids, and intracellular Ca2+. In
2013, two landmark publications revealed
the cryo–electron microscopy (cryo-EM)
structure of the heat- and capsaicin-acti-vated TRPV1 channel (3, 4). Two articles in
this issue report cryo-EM structures of cat-ion-selective TRPM channels. On page 228,
Autzen et al. (5) describe TRPM4, which
is activated by intracellular Ca2+ and involved in controlling arterial tone, cardiac
rhythm, and the immune response (6). On
page 237, Yin et al. (7) report on TRPM8,
which senses cold and menthol and may
serve as a cancer biomarker (8).
Autzen et al. solved structures of TRPM4
in the absence and presence of the activating ion Ca2+. The transmembrane domains of
TRPM4 resemble those of other TRP channels and of voltage-activated K+ (Kv) channels (3, 4, 9). Each subunit in the tetrameric
complex contains six transmembrane helices,
with S5-S6 forming the ion permeation pathway at the central axis and S1-S4 forming a
peripheral domain (see the figure). In the
Ca2+-bound TRPM4 structure, the ion binds
in a cavity within the S1-S4 domain that faces
the cytoplasm (see the figure).
Both the Ca2+-bound and -unbound TRPM4
structures have been captured in states in
which the intracellular entrance of the pore
is sealed off by hydrophobic residues in the
S6 helix, similar to the closed structures of
other TRP and Kv channels (3, 4, 9, 10). The
Molecular Physiology and Biophysics Section, Porter
Neuroscience Research Center, National Institute of
Neurological Disorders and Stroke, National
Institutes of Health, Bethesda, MD 20892, USA.
The structures of TRPM
channels help to explain
how they can sense
160 12 JANUARY 2018 • VOL 359 ISSUE 6372
Center for Genomics and Systems Biology, Department
of Biology, Ne w York University, New York, NY 10003, USA.