of an electropositive patch, with P4 contacting a
conserved charged region on Xrn1’s winged helix
This putative P4-Xrn1 interaction could serve
to stabilize the xrRNA’s pseudoknot interaction
and thus enhance resistance to the enzyme, or P4
may form sequence-specific interactions with Xrn1
or with Xrn1-bound proteins. Also, because the
winged helix domain is important for processive
Xrn1 function (29), the bound ZIKV xrRNA may
prevent conformational changes in the enzyme
that are important for processivity. The new structure and derived hypotheses point the way to
future studies that may clarify the formation and
function of ZIKV sfRNAs, with implications for
the development of interventions or vaccines.
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We thank the members of the Kieft lab for discussions; C. Musselman
and M. Stone for critical reading of this manuscript; and R. Soto from
the M. Garcia-Blanco and S. Bradrick laboratories for sharing their
protocol for sfRNA blots. Supported by NIH fellowship F32GM117730
(B.M.A.); an HHMI Early Career Scientist award and NIH grants
R35GM118070 and R01GM081346 (J.S.K.); a HHMI–Burroughs
Wellcome Fund medical research fellowship (H.M.L.); University of
Colorado School of Medicine and Department of Medicine institutional
pilot funds (A.R.M. and J.D.B.); a postdoctoral fellowship from
Novartis Institutes for BioMedical Research (X. X.); and a University of
Texas Medical Branch startup award, a University of Texas STARs
award, and NIH grant R01AI087856 (P.-Y.S.). The UC Denver X-ray
Facility is supported by UC Cancer Center support grant
P30CA046934 and NIH grant S10OD012033. The Advanced Light
Source is supported by the Office of Science, Office of Basic Energy
Sciences, of the U.S. Department of Energy under contract DE-AC02-
05CH11231. The data presented in this manuscript are tabulated in
the main paper and in the supplementary materials. Coordinates and
structure factors have been deposited with PDB accession code 5TPY.
Materials and Methods
Figs. S1 to S8
20 June 2016; accepted 26 October 2016
Published online 10 November 2016
Depleting dietary valine permits
hematopoietic stem cell transplantation
Yuki Taya,1 Yasunori Ota,2 Adam C. Wilkinson,1,3 Ayano Kanazawa,1
Hiroshi Watarai,4,5 Masataka Kasai,1 Hiromitsu Nakauchi,1,3 Satoshi Yamazaki1*
A specialized bone marrow microenvironment (niche) regulates hematopoietic stem cell
(HSC) self-renewal and commitment. For successful donor-HSC engraftment, the niche
must be emptied via myeloablative irradiation or chemotherapy. However, myeloablation
can cause severe complications and even mortality. Here we report that the essential
amino acid valine is indispensable for the proliferation and maintenance of HSCs. Both
mouse and human HSCs failed to proliferate when cultured in valine-depleted conditions.
In mice fed a valine-restricted diet, HSC frequency fell dramatically within 1 week. Furthermore,
dietary valine restriction emptied the mouse bone marrow niche and afforded donor-HSC
engraftment without chemoirradiative myeloablation. These findings indicate a critical
role for valine in HSC maintenance and suggest that dietary valine restriction may reduce
iatrogenic complications in HSC transplantation.
Although much is known about the molecules and signaling pathways regulating hema- topoietic stem cells (HSCs), our understand- ing of the HSC bone marrow (BM) niche is less clear. The availability of niche “space”
appears to be a limiting factor for engraftment in
HSC transplantation (HSCT) (1). Ionizing radiation
and/or high-dose chemotherapy are most commonly used to empty the BM niche. However,
severe side effects limit the success of these treatments. Young patients are particularly at risk of
experiencing late effects, including secondary
malignancy, endocrinopathy, and reproductive
Numerous cellular and molecular components
of the HSC niche have previously been proposed
(3). These include several stromal and neuronal
cell types, as well as various signaling molecules
including the cytokines stem cell factor and
thrombopoietin. In 1946, Kornberg et al. reported
that rats fed a low-protein diet developed severe
granulocytopenia or anemia that was corrected
by administration of purified amino acids (AAs)
(4, 5). On the basis of these findings, we hypoth-
esized that specific AAs may be indispensable for
the BM niche and could influence HSC fate.
We initially quantified AA concentrations in BM
and peripheral blood (PB) by high-performance
liquid chromatography (HPLC). The BM contained
>100-fold higher concentrations of all 20 AAs and
also displayed a distinct AA profile (or AA ba-
lance), as compared with the PB (fig. S1, A and B).
To identify the specific AAs essential for the
maintenance of hematopoiesis, CD34–Kit+Sca1+
Lin– cells (HSCs) (6) were cultured for 1 week in
various media lacking single AAs (Fig. 1A and
fig. S1C). Proliferation was significantly retarded
when cysteine or valine was absent (–Cys, –Val)
(Fig. 1B and fig. S2A). We also carried out similar
analyses using CD34+Kit+Sca1+Lin– hematopoietic
progenitor cells (HPCs). HPCs displayed significant growth retardation in medium lacking cysteine or lysine (fig. S2, B and C). Growth of HPCs
1152 2 DECEMBER 2016 • VOL 354 ISSUE 6316
1Division of Stem Cell Therapy, Center for Stem Cell Biology
and Regenerative Medicine, The Institute of Medical Science,
University of Tokyo, Tokyo 108-8639, Japan. 2Department of
Pathology, Research Hospital, The Institute of Medical
Science, University of Tokyo, Tokyo 108-8639, Japan.
3Institute for Stem Cell Biology and Regenerative Medicine,
Stanford University School of Medicine, Lorry I. Lokey Stem
Cell Research Building, 265 Campus Drive, Stanford, CA
94305-5461, USA. 4Division of Stem Cell Cellomics, Center
for Stem Cell Biology and Regenerative Medicine, The
Institute of Medical Science, University of Tokyo, Tokyo 108-
8639, Japan. 5Impulsing Paradigm Change through
Disruptive Technologies Program (ImPACT), Japan Science
and Technology Agency (JST), Tokyo 102-0076, Japan.
*Corresponding author. Email: email@example.com (S. Y.);