Science 14Feb2020

sciencemag.org SCIENCE

RESEARCH

MEDICINE

From animal models

to humans

A major challenge in medi-
cal research is the translation
of results in animal models to
humans. Animal models are
essential to be able to under-
take rigorous experimentation
to understand mechanisms of
disease and therapeutic oppor-
tunities. However, disappointing
outcomes of clinical trials in
patients emphasize the difficulty
in ensuring that the data from
animal models are relevant to
human disease. In a Perspective,
Brubaker and Lauffenburger
discuss how machine-learning
strategies can aid cross-species
translation to improve outcomes
in humans, but they emphasize
that such approaches also come
with their own challenges. —GKA
Science, this issue p. 742

DEVELOPMENTAL BIOLOGY

Mapping cell fate during

hematopoiesis

Biologists have long attempted
to understand how stem and
progenitor cells in regenerating
and embryonic tissues differ-
entiate into mature cell types.
Through the use of recent techni-
cal advances to sequence the
genes expressed in thousands
of individual cells, differentiation
mechanisms are being revealed.
Weinreb et al. extended these
methods to track clones of cells
(cell families) across time. Their
approach reveals differences in
cellular gene expression as cells
progress through hematopoiesis,
which is the process of blood pro-
duction. Using machine learning,
they tested how well gene expres-
sion measurements account for
the choices that cells make. This
work reveals that a considerable
gap still exists in understanding
differentiation mechanisms, and
future methods are needed to fully
understand—and ultimately con-
trol—cell differentiation. —BAP
Science, this issue p. 755

Edited by Michael Funk

ALSO IN SCIENCE JOURNALS

754-B 14 FEBRUARY 2020 • VOL 367 ISSUE 6479

NEUROSCIENCE

Memory suppression

can help after trauma

Therapists have discussed for a

long time whether attempts to

voluntarily suppress the intru-

sion of trauma memories are

helpful to combat the distress-

ing impacts of trauma. Mary

et al. studied survivors of the

2015 Paris terrorist attacks who

developed posttraumatic stress

disorder and those who did not

(see the Perspective by Ersche).

Using functional magnetic reso-

nance imaging, they investigated

the neural networks underlying

the control and suppression of

memory retrieval. The results

suggest that the characteristic

symptoms of the disorder are

not related to the memory itself

but to its maladaptive control.

These results offer new insights

into the development of post-

traumatic stress disorder and

potential avenues for treatment.

—PRS

Science, this issue p. 756;

see also p. 734

SOLID-STATE PHYSICS

An unusual conductance

sequence

Effects of correlations between

electrons are enhanced in

systems of reduced dimensions.

The two-dimensional interface

between two oxide materials,

lanthanum aluminate (LaAlO 3 )

and strontium titanate (SrTiO 3 ),

exhibits magnetism and

superconductivity. In even lower-

dimensional systems fabricated

in similar heterostructures,

electrons can pair without going

superconducting. Briggeman et

al. have now observed another

exotic effect in LaAlO 3 /SrTiO 3

waveguides: At certain magnetic

fields, the conductance in these

one-dimensional systems exhib-

its steps of an unconventional

sequence. To understand the

experimental data, the research-

ers used a model that accounted

for interactions between

electrons and found that the

phenomenology was consistent

with the formation of a series of

correlated phases characterized

by bound states of three or more

electrons. —JS

Science, this issue p. 769

MATERIALS SCIENCE

Ionic elastomeric

material electronics

Wearable devices often need to

be soft or flexible, and ideally,

these properties would extend

beyond packaging material to

also include the electronics.

Some soft ionic conductors have

been made in the form of flex-

ible, stretchable, and transparent

devices, but leaks from these

materials is a concern. Kim et al.

demonstrate ionic elastomeric

diodes and transistors that

harness ionic double layers to

rectify and switch ionic currents

(see the Perspective by Gao and

Lee). This is achieved without

trapped liquids by fixing the

anions or cations to an elasto-

mer network while the other

species of ions remain mobile.

—MSL

Science, this issue p. 773;

see also p. 735

TOPOLOGICAL MATTER

Controlling

the interactions

Near charge neutrality and sub-

ject to perpendicular magnetic

fields, graphene is expected

to become a ferromagnet with

edge states not unlike those in

two-dimensional topological

insulators. Observing this effect

experimentally has proven tricky

because very large magnetic

fields are needed to overcome

the effect of electron-electron

interactions, which drive the

system to competing states.

Instead of amping up the field,

Ve y r a t et al. placed their gra-

phene samples on a substrate

made out of strontium titanate,

which effectively screened the

interactions. Transport measure-

ments confirmed the formation

of the characteristic edge states.

—JS

Science, this issue p. 781

DRYLAND ECOLOGY

Thresholds of aridity

Increasing aridity due to climate

change is expected to affect

multiple ecosystem structural

and functional attributes in

global drylands, which cover

~45% of the terrestrial globe.

Berdugo et al. show that increas-

ing aridity promotes thresholds

on the structure and functioning

of drylands (see the Perspective

by Hirota and Oliveira). Their

database includes 20 variables

summarizing multiple aspects

and levels of ecological orga-

nization. They found evidence

for a series of abrupt ecological

events occurring sequentially in

three phases, culminating with

a shift to low-cover ecosystems

that are nutrient- and species-

poor at high aridity values. They

estimate that more than 20% of

land surface will cross at least

one of the thresholds by 2100,

which can potentially lead to

widespread land degradation

and desertification worldwide.

—AMS

Science, this issue p. 787;

see also p. 739

CARBON CYCLE

Breaking up is easy to do

Sinking particles transport

carbon to the seafloor, where

they are buried in sediments

and either provide food for

benthic organisms or seques-

ter the carbon they contain.

However, only ~30% of the

maximum flux reaches depths

of a kilometer. This loss cannot

be fully accounted for by current

measurements. Briggs et al.

used data collected by robotic

Biogeochemical-Argo floats to

quantify total mesopelagic frag-

mentation and found that this

process accounts for roughly

half of the observed flux loss

Published by AAAS