38-B 3 JANUARY 2020 • VOL 367 ISSUE 6473 sciencemag.org SCIENCE
RESEARCH
NEUROSCIENCE
The neural substrate
of memory
The ability to form memory is
an essential trait that allows
learning and the accumula-
tion of knowledge. But what is
a memory? There has been a
long history of searching for the
neuronal substrate that forms
memory in the brain, and the
emerging view is that ensembles
of engram cells explain how
memories are formed and
retrieved. In a Review, Josselyn
and Tonegawa discuss the
evidence for engram cells as a
substrate of memory, particu-
larly in rodents; what we have
learned so far about the features
of memory, including memory
formation, retrieval over time,
and loss; and future directions
to understand how memory
becomes knowledge. —GKA
Science, this issue p. 39
GLOBAL HEALTH
Cancer in diverse
populations
Much of what is known about
the signature genetic altera-
tions and clinical features of
cancer comes from studying
Caucasian patients in high-
income countries. However, this
does not reflect the diversity of
heritage in most societies and
therefore does not allow patients
to be diagnosed and treated
effectively. In a Perspective,
Rebbeck discusses the emerging
evidence that cancers in patients
in sub-Saharan Africa are clini-
cally and molecularly distinct.
Understanding these differences
should improve treatment of
patients in sub-Saharan Africa as
well as the diaspora. Assessment
of the diversity of genetic
alterations can also improve how
cancer is diagnosed and classi-
fied in all populations. —GKA
Science, this issue p. 27
MALARIA
An artemisinin resistance
mechanism
Species of the malaria parasite
Plasmodium live in red blood
cells and possess a highly
conserved gene called kelch13.
Single point mutations in this
gene are associated with resis-
tance to the frontline artemisinin
drugs. Birnbaum et al. found that
Kelch13 and associated proteins
comprise an endocytic compart-
ment associated with feeding
on host erythrocytes (see the
Perspective by Marapana and
Cowman). Hot targets for arte-
misinin research also occur in
this compartment, including the
proteins UBP1, AP-2m, and the
parasite homolog of the endocy-
tosis protein Eps15. Inactivation
of Kelch13 compartment
proteins revealed that these
are required for endocytosis of
host hemoglobin. Artemisinins
are activated by hemoglobin
degradation products, so these
mutations render the parasite
resistant to these drugs to differ-
ent extents. —CA
Science, this issue p. 51;
see also p. 22TOPOLOGICAL OPTICS
Optically contorting into
new dimensions
Creating synthetic dimensions
has generated interest in many
branches of science, ranging
from ultracold atomic physics
to photonics. The ability to do
so provides a versatile platform
for realizing effective gauge
potentials and novel topological
physics that might be difficult
or impossible to realize in real
systems. Dutt et al. show that a
structured optical ring cavity can
sustain more than one synthetic
dimension. Under modulation,
coupling the different degrees of
freedom within the resonator is
used synthesize two additional
dimensions. The authors are
then able to emulate many com-
plex physical phenomena usuallyassociated with condensed mat-
ter systems. —ISO
Science, this issue p. 59TOPOLOGICAL MATTER
Looking for chiral
Majoranas
Chiral Majorana modes have been
predicted to exist in heterostruc-
tures consisting of a quantum
anomalous Hall insulator and a
superconductor. Kayyalha et al.
fabricated more than 30 such
samples and used transport
measurements to look for signa-
tures of the Majorana modes. The
data indicated that the transport
signatures previously thought
to be associated with Majorana
physics could, in their samples, be
explained using a more mundane
mechanism. —JS
Science, this issue p. 64TOPOLOGICAL MATTER
A possible propagating
Majorana
Majorana states in solid-state
systems may one day form a
basis for topological quantum
computing. Most of the candi-
dates identified so far have been
Majorana bound states, but
theorists have predicted that
propagating Majorana states may
exist as well. Wang et al. looked
for such a state on the surface
of the material FeSe0.45Te0.55
(see the Perspective by Tewari
and Stanescu). Using scanning
tunneling spectroscopy, the
researchers measured a flat,
bias-independent density of
states along a particular type of
domain wall, which was consis-
tent with a theoretical prediction
for a propagating Majorana
state in this material. Although
topologically trivial origins of this
finding are difficult to completely
rule out, the work is likely to
stimulate interest in iron-based
superconductors as hosts of
Majorana states. —JS
Science, this issue p. 104;
see also p. 23PLANT SCIENCE
Subtle origin for
complex shapes
The cup-shaped leaves of
carnivorous plants have evolved
multiple times from ancestors
with flat leaves. Studying devel-
opment of the carnivorous trap
in the humped bladderwort,
Utricularia gibba, Whitewoods
et al. identified genes similar to
those expressed in surfaces of
flat leaves (see the Perspective
by Moulton and Goriely).
Ectopic expression and compu-
tational modeling reveals how
slight shifts in gene expression
domains make the difference
between a flat leaf and a convo-
luted trap structure. Flexibility
in growth rates in orthogonal
polarity fields allows for diver-
sity in shapes formed through
development. —PJH
Science, this issue p. 91;
see also p. 24TUBULIN
Mechanism of tubulin
autoregulation
Cells tightly control the abun-
dance of key housekeeping
factors, such as ribosomes and
chaperones, to maintain them
at optimal levels needed for
homeostasis. Most abundance
control mechanisms involve
feedback regulation of mRNA
transcription, but others, such as
tubulins, are regulated by highly
specific mRNA degradation. Lin
et al. found that tetratricopep-
tide protein 5 (TTC5) binds to
nascent alpha and beta tubulins
on translating ribosomes to
trigger degradation of their
associated mRNAs when excess
tubulin is present (see the
Perspective by Shoshani and
Cleveland). In the absence of
TTC5-mediated tubulin autoreg-
ulation, cells display error-prone
chromosome segregation, a
process critically dependent on
tubulin concentration. —SMH
Science, this issue p. 100;
see also p. 29Edited by Michael FunkALSO IN SCIENCE JOURNALS
Published by AAAS