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Please cite this article in press as: Smith-Berdan et al., ROBO4-Mediated Vascular Integrity Regulates the Directionality of Hematopoietic
Stem Cell Trafficking, Stem Cell Reports (2015), http://dx.doi.org/10.1016/j.stemcr.2014.12.013
Stem Cell Reports
Ar ticle
ROBO4-Mediated Vascular Integrity Regulates the Directionality of
Hematopoietic Stem Cell Trafficking
Stephanie Smith-Berdan,1 Andrew Nguyen,1 Matthew A. Hong,1 and E. Camilla Forsberg1,*
1Institute
for the Biology of Stem Cells, Department of Biomolecular Engineering, University of California, Santa Cruz, Santa Cruz, CA 95064, USA
*Correspondence: cforsber@soe.ucsc.edu
http://dx.doi.org/10.1016/j.stemcr.2014.12.013
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).
SUMMARY
Despite the use of hematopoietic stem cells (HSCs) in clinical therapy for over half a century, the mechanisms that regulate HSC trafficking, engraftment, and life-long persistence after transplantation are unclear. Here, we show that the vascular endothelium regulates
HSC trafficking into and out of bone marrow (BM) niches. Surprisingly, we found that instead of acting as barriers to cellular entry,
vascular endothelial cells, via the guidance molecule ROBO4, actively promote HSC translocation across vessel walls into the BM space.
In contrast, we found that the vasculature inhibits the reverse process, as induced vascular permeability led to a rapid increase in HSCs in
the blood stream. Thus, the vascular endothelium reinforces HSC localization to BM niches both by promoting HSC extravasation from
blood-to-BM and by forming vascular barriers that prevent BM-to-blood escape. Our results uncouple the mechanisms that regulate the
directionality of HSC trafficking and show that the vasculature can be targeted to improve hematopoietic transplantation therapies.
INTRODUCTION
Hematopoietic stem cells (HSCs) reside primarily in the
bone marrow (BM). This selective location results in part
from the unique ability of BM niches to support HSC selfrenewal and long-term maintenance. Intense interest in
the complex regulation of HSC self-renewal has led to significant progress in understanding the cellular and molecular
composition of BM niches (reviewed in Ugarte and Forsberg,
2013). Because osteoblasts are only present in bone, they
may provide an environment that helps to regulate the
selective location of HSCs to BM. Several lines of evidence
support this notion (reviewed in Krause et al., 2013). Recent
evidence also points to the vascular endothelium and associated cells as important regulators of HSC maintenance
and location (Ding and Morrison, 2013; Ding et al., 2012;
Greenbaum et al., 2013; Kunisaki et al., 2013; Me´ndez-Ferrer
et al., 2010; Sacchetti et al., 2007; Sugiyama et al., 2006;
Ugarte and Forsberg, 2013), and most HSCs localize near sinusoidal endothelial cells (SECs) (Kiel et al., 2005). Thus,
accumulating evidence indicates that vascular structures
within the BM are necessary for optimal HSC function.
Another mechanism that is likely involved in specifying
HSC location to the BM is regulated trafficking between the
BM and vasculature. HSC residence in BM niches is far
from static, with circulation in the blood stream occurring
under steady-state physiological conditions (Massberg
et al., 2007; Wright et al., 2001), between different hematopoietic organs during development, and as an essential
requirement for successful hematopoietic transplantation
therapies. During trafficking to and from the BM, HSCs
have to traverse the vascular endothelium. Differential
vascular structures of different organs that either prevent
or allow HSC entry likely play important roles in guiding
HSCs specifically to the BM. Here, we show that the integrity of the vascular endothelium and its ability to regulate
directional HSC trafficking to the BM depend on the single
transmembrane cell-surface receptor ROBO4.
We recently reported that ROBO4, expressed by HSCs,
promotes HSC localization to BM niches at steady state
and upon transplantation (Forsberg et al., 2005, 2010;
Smith-Berdan et al., 2011). ROBO4 is a member of the
ROBO family of guidance receptors that respond to Slits,
secreted proteins that are essential for neuronal development (Brose et al., 1999; Long et al., 2004). ROBO4 was previously identified as an EC-selective protein (Huminiecki
et al., 2002; Park et al., 2003) and its support of vascular
integrity seems to be particularly important in dynamic situations such as vascular stress, inflammation, and pregnancy
(Jones et al., 2008; London et al., 2010; Marlow et al., 2010).
ROBO4 was found by our group and others to also be expressed by HSCs, but not hematopoietic progenitor or
mature cells (Forsberg et al., 2005, 2010; Ivanova et al.,
2002; Shibata et al., 2009; Smith-Berdan et al., 2011). We previously reported that hematopoietic ROBO4 acts as an HSCselective adhesion molecule that promotes HSC location to
BM niches (Smith-Berdan et al., 2011). ROBO4 deletion led
to increased numbers of HSCs in the peripheral blood (PB)
at steady state and reduced engraftment upon competitive
transplantation into wild-type (WT) mice. We also found
that CXCR4, a G protein-coupled receptor and well-established regulator of HSC location (Nagasawa et al., 1998; Peled
et al., 1999; Zou et al., 1998), was upregulated on ROBO4deficient HSCs, mitigating the effects of ROBO4 loss.
Consequently, ROBO4-deficient HSCs displayed heightened
responsiveness to mobilization with the CXCR4 inhibitor
Stem Cell Reports j Vol. 4 j 1–14 j February 10, 2015 j ª2015 The Authors 1
Please cite this article in press as: Smith-Berdan et al., ROBO4-Mediated Vascular Integrity Regulates the Directionality of Hematopoietic
Stem Cell Trafficking, Stem Cell Reports (2015), http://dx.doi.org/10.1016/j.stemcr.2014.12.013
A
% Donor
//
R4-/Wt WBM
7.5x106 cells
PB mature cells
80
Wt
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60
*** *** ***
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40
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PB analysis BM analysis
4,8,12,16 wks
>16 wks
*
35
30
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Wt R4-/-
0
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8 12 16 20
Weeks post transplant
Wt
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R4-/PB analysis
4,8,12,16 wks
% Donor
Wt HSC
(400 GFP+
KLS/Flk2-/CD150+
cells)
6
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AMD
Wt
R4-/-
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BM analysis
>16 wks
40
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8 12 16 20
Weeks post transplant
Transplantation into hematopoietic chimeras
Wt HSC
(400 Tomato+
KLS/Flk2-/CD150+
cells)
BM HSC
**
Wt R4-/-
PB chimerism week 16
NS
PB
analysis
16 wks
Wt:R4-/Chimeras
% Donor
80
60
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20
Chimeras
Wt:Wt
***
20
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Primary Donor: Wt
!"#
$%&%'()(#
R4-/-
Tomato HSC engraftment
100
% Donor Engraftment in
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***
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***
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Wt BM
CD45.2
PB mature cells
0
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8 12 16 20
Weeks post transplant
0
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10
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PB analysis
4,8,12,16 wks
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transplant
% Donor
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*** *** ***
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PB mature cells
10
//
80
NS
60
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Resident Wt HSC
Resident R4-/- HSC
20
0
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12 16 20 24
Weeks post transplant
28
Figure 1. Recipient ROBO4 Is Necessary for Efficient HSC Engraftment
(A and B) Engraftment of WT BM cells (A) and purified HSCs (B) is significantly lower in Robo4/ compared with WT recipient mice. Middle
panels: percent donor contribution to mature cells in the PB over 16 weeks. Right: HSC chimerism in the BM (A) and mature cell chimerism
in the PB (B, with donor HSCs from UBC-GFP transgenic mice) at 16 weeks posttransplantation into sublethally irradiated recipients. (A)
(legend continued on next page)
2 Stem Cell Reports j Vol. 4 j 1–14 j February 10, 2015 j ª2015 The Authors
Please cite this article in press as: Smith-Berdan et al., ROBO4-Mediated Vascular Integrity Regulates the Directionality of Hematopoietic
Stem Cell Trafficking, Stem Cell Reports (2015), http://dx.doi.org/10.1016/j.stemcr.2014.12.013
AMD3100. Functional differences in the hematopoietic system upon ROBO4 deletion were highly selective for HSCs
and did not involve alterations in the number or function
of hematopoietic progenitors or mature cells. We also did
not detect a defect in cell-cycle status or proliferation of
either HSCs or their progeny upon ROBO4 loss or in
response to Slits. Similar results were reported independently by others (Goto-Koshino et al., 2012; Shibata et al.,
2009). Collectively, these data demonstrated that ROBO4
on HSCs promotes HSC localization to the BM. Here, we
report that in addition to ROBO4 expressed by HSCs, endothelial ROBO4 is essential for efficient HSC engraftment. Using a combination of in vitro and in vivo assays, we identify
the cellular and molecular mechanisms by which endothelial ROBO4 promotes HSC location to the BM, and reveal
strategies for manipulating HSC location.
RESULTS
Recipient ROBO4 Promotes HSC Engraftment
Previous competitive transplantation assays (Smith-Berdan
et al., 2011) showed that WT HSCs outcompeted Robo4/
HSCs upon transplantation into WT mice. Here, we tested
the hypothesis that WT HSCs would also outcompete
Robo4/ HSCs upon transplantation into Robo4/ recipients and therefore engraft with high efficiency in
ROBO4-deficient hosts. Surprisingly, we found the opposite: engraftment of WT HSCs was significantly poorer in
Robo4/ compared with WT recipient mice (Figures 1A
and 1B; Figure S1 available online). Similar results were obtained whether HSCs were delivered as unfractionated BM
(Figure 1A) or purified HSCs (Figure 1B), and the reconstitution of all mature cell types tested, as well as BM HSCs, was
affected (Figures 1A and 1B). Because we previously found
that CXCR4 upregulation on Robo4/ HSCs attenuates
HSC mobilization in response to cytoxan/G-CSF, and
mobilization efficiency can be restored with the CXCR4 inhibitor AMD3100 (Smith-Berdan et al., 2011), we tested
whether AMD3100 pretreatment of recipient mice could
rescue the engraftment deficiency in Robo4/ hosts by
clearing the host BM niches of resident HSCs. However,
similar to what was observed for irradiated recipients,
HSC engraftment was more efficient in WT versus
Robo4/ hosts preconditioned with a combination of irradiation and AMD3100 (Figures 1C and S1). Thus, just as
Robo4/ HSCs engraft poorly in WT hosts (Smith-Berdan
et al., 2011), WT HSCs engraft poorly in Robo4/ hosts.
We therefore hypothesized that an ROBO4-mediated
HSC-HSC interaction between resident and transplanted
HSCs facilitates the engraftment of incoming HSCs. We
tested this hypothesis in chimeric mice generated by transplantation of WT mice with either WT or Robo4/ BM (Figure 1D). Mice with high (>90%) and stable hematopoietic
chimerism (Figure 1D) were then transplanted with Tomato-expressing WT HSCs. The engraftment efficiency
was indistinguishable between recipient mice that were
first repopulated with WT or Robo4/ hematopoietic cells
(Figure 1E), indicating that the defect in engraftment in
Robo4/ mice is not due to the absence of ROBO4 on resident HSCs. Thus, we turned our attention to the function
of ROBO4 in nonhematopoietic cells.
Endothelial ROBO4 Promotes Vascular Integrity and
HSC Trafficking to the BM
In addition to HSCs, ECs also express ROBO4 (Huminiecki
et al., 2002; Park et al., 2003). We recently showed that
BM ECs, but not other BM stromal cells, express ROBO4
mRNA and cell-surface protein (Smith-Berdan et al.,
2012). Therefore, we tested whether endothelial ROBO4
is responsible for the poor HSC engraftment observed in
Robo4/ mice (Figures 1A–1C). Using intravenously (i.v.)
injected Evans Blue in a modified Miles assay (Miles and
Miles, 1952), we found that Robo4/ mice displayed significantly higher vascular leak in several organs at steady state
(Figure 2A). Interestingly, we detected increased vascular
leak in the BM of Robo4/ mice upon irradiation, but not
at steady state (Figure 2B). These results are consistent
with previous studies implicating ROBO4 in supporting
vascular integrity, in particular under stress conditions
(Jones et al., 2008, 2009; Koch et al., 2011), and with
n = 22 (WT recipients) and 21 (Robo4/ recipients) in 4 independent experiments. (B) n = 16 (WT recipients) and 15 (Robo4/ recipients) in 3 independent experiments.
(C) Engraftment of WT BM cells is significantly lower in Robo4/ compared with WT recipient mice preconditioned with both sublethal
irradiation and AMD3100. Middle: percent donor contribution in the PB over 16 weeks. Right: HSC chimerism in the BM at 16 weeks
posttransplantation. n = 10 recipient mice of each phenotype in 2 independent experiments.
(D) Transplantation of WT or Robo4/ BM cells (CD45.2) into lethally irradiated WT (CD45.1) recipients to generate mice with either a WT
or Robo4/ hematopoietic system on a WT background.
(E) Mice from (D) with >90% chimerism at 16 weeks posttransplantation were then transplanted with Tomato-expressing WT HSCs into
sublethally irradiated recipients and assessed for Tomato-expressing donor cells in PB over time. WT HSC contribution to PB cells was equal
in WT recipient mice repopulated by either WT or Robo4/ hematopoietic cells. n = 10 mice in 3 independent experiments.
R4/, ROBO4-deficient mice; PB, peripheral blood; BM, bone marrow. HSCs in (B) and (E) were isolated as c-KIT+LinSCA1+ (‘‘KLS’’)
FLK2CD150+ BM cells. Error bars represent SEM. *p < 0.05, **p < 0.005, ***p < 0.0005. See also Figure S1.
Stem Cell Reports j Vol. 4 j 1–14 j February 10, 2015 j ª2015 The Authors 3
Please cite this article in press as: Smith-Berdan et al., ROBO4-Mediated Vascular Integrity Regulates the Directionality of Hematopoietic
Stem Cell Trafficking, Stem Cell Reports (2015), http://dx.doi.org/10.1016/j.stemcr.2014.12.013
Figure 2. Transplanted HSCs Are Trapped
in the Vasculature of ROBO4-Deficient
Lung
Small Intestine
Foot
BM
BM
Mice despite Increased Vascular PermeNS
2.5
0.020
**
***
**
*
0.004
0.005
0.0020
ability
2
0.016
(A and B) Loss of ROBO4 leads to vascular
0.004
0.0016
0.003
leak in lung, small intestine, and foot,
1.5
0.012
0.003
0.0012
0.002
but not the BM, at steady state (A) and
1
0.008
0.002
0.0008
in the BM upon lethal irradiation (B) as
0.001
0.5
0.004
0.0004
0.001
measured by leakage of the albumin0
0.000
0.0000
binding dye Evans Blue into the indicated
0
0
Wt R4-/Wt R4-/Wt R4-/Wt R4-/Wt R4-/tissue 5 min after i.v. injection. n = 9–15
per cohort in 3–5 independent experiC Vascular Leak
D Short-term Homing
ments.
Control
Irradiated
(C) Sublethal irradiation induces vascular
WT
Robo4-/1.2
*
permeability as evidenced by the leakage
of Evans Blue dye at the site of tail injection
0.8
in white (CD1) WT mice.
0.4
(D) WT HSCs accumulate in the PB of
Robo4/ compared with WT mice,
0
2
accompanied by poor HSC recovery from
*
1.6
BM, but not spleen, at 3 hr post1.2
transplantation. GFP-expressing KLS (cKI0.8
E CFU-S
T+LinSCA1+) cells were injected into WT
6
0.4
NS
or Robo4/ recipient mice that had
5
0
been lethally irradiated 24 hr before.
NS
2
4
BM, blood, and spleen were analyzed for
1.5
3
GFP+ cells 3 hr posttransplantation by
1
2
flow cytometry and quantified as percent
0.5
1
of injected cells recovered from each tis0
0
sue. n = 3–5 mice per cohort in 5 indeSSC-A
-/-/Wt R4
Wt R4
pendent experiments.
(E) The ability of transplanted WT HSCs (c-KIT+LinSCA1+FLK2CD34 cells) to form spleen colonies (CFU-S12) is equal in lethally
irradiated WT and Robo4/ recipients. n = 16–20 per cohort in 4 independent experiments.
Error bars represent SEM. *p < 0.05, **p < 0.005, ***p < 0.0005.
A In Vivo Permeability
% Recovery
compensatory mechanisms attenuating the effects of
ROBO4 loss in the BM (see below). We hypothesized
that the increased vascular permeability associated with
ROBO4 deletion caused transplanted cells to leak out of
Robo4/ vessels, similar to the leakage of Evans Blue at
the tail injection site of irradiated animals (Figure 2C), leading to reduced delivery of HSCs to hematopoietic organs
and subsequently lower engraftment levels. However,
short-term homing assays revealed that rather than leaking
out of vessels, cells transplanted into Robo4/ mice were
trapped within the vasculature (Figure 2D), with significantly reduced relocation to the BM extravascular space
(Figure 2D). Short-term trafficking to the spleen (Figure 2D)
and the capability of transplanted HSCs to form spleen colony-forming units (CFU-S) (Figure 2E) did not differ significantly between WT and Robo4/ recipients. Thus, ROBO4
deletion selectively affected the ability of WT HSCs to
reconstitute the BM, but not the spleen (Figures 1A–1C,
2D, and 2E). Collectively, these results indicate that the
poor engraftment in Robo4/ recipients is not due to premature HSC leakage out of the vasculature prior to reaching
4 Stem Cell Reports j Vol. 4 j 1–14 j February 10, 2015 j ª2015 The Authors
% Recovery
Spleen
GFP
CFU-S per spleen
Blood
BM
% Recovery
Fold Difference
OD/Mass Tissue
B
the BM, and that recipient ROBO4 on endothelial, but not
hematopoietic, cells affects the engraftment efficiency of
transplanted HSCs.
Endothelial ROBO4 Promotes HSC Translocation
across Vascular Barriers
The observation that transplanted cells accumulated in
the vasculature (Figure 2D) raised the possibility that endothelial ROBO4 promotes HSC extravasation. To test this
directly, we established an in vitro assay system that
enabled quantification of both passive and active translocation across endothelial barriers. We isolated primary ECs
from the BM, lungs, and kidney of WT and Robo4/ mice
and grew confluent monolayers on porous transwell inserts. We first tested the integrity of these EC layers by
measuring passive diffusion of FITC-labeled dextran, an
40 kD macromolecule, to the bottom well (Figure 3A).
Although cell phenotype, confluency, and proliferation
rates were indistinguishable between WT and Robo4/
ECs (Figures S2 and S3B), significantly more FITC-dextran
diffused across Robo4/ EC layers regardless of the tissue
Please cite this article in press as: Smith-Berdan et al., ROBO4-Mediated Vascular Integrity Regulates the Directionality of Hematopoietic
Stem Cell Trafficking, Stem Cell Reports (2015), http://dx.doi.org/10.1016/j.stemcr.2014.12.013
A
C
Passive Permeability
E
Active Migration
*
NS
% HSC Migration
0.7
Hematopoietic
Cells
FITC-Dextran
Endothelial cell
monolayer
Pores
Lung
25
*
20
15
Kidney
BM
1.6
14
1.4
12
1.2
8
0.8
10
5
0
EC layer Wt R4-/-
6
0.6
0.4
4
0.2
2
0
**
10
1
Wt R4-/-
0
D
Wt R4-/-
**
*
0.2
0.1
+
**
**
*** ***
3.0
+
-
+
R4-/-
Wt
BM
*** ***
3.5
*
3.0
2.5
2.5
0.8
2.0
0.6
1.5
0.4
1.0
1.0
0.2
0.5
0.5
EC layer
+
-
Kidney
1
0
SDF1
NS
0.3
Wt HSC
R4-/- HSC
EC layer
Lung
1.2
% HSC Migration
% of Total Fluorescence
B
0.5
0.4
0
SDF1
**
0.6
2.0
-
+
Wt
-
+
R4-/-
0.0
***
-
+
Wt
-
**
1.5
+
R4-/-
0.0
-
+
Wt
-
+
R4-/-
Figure 3. Endothelial ROBO4 Promotes Endothelial Barrier Formation but Is Necessary for Efficient HSC Transendothelial Migration
(A and C) Schematic of the strategy to measure passive permeability (A) and active cell migration (C) across WT and Robo4/ endothelial
cell (EC) layers.
(B) Monolayers of Robo4/ ECs from lung, kidney, and BM are more permeable to passive diffusion of FITC-dextran compared with WT EC
layers. n = 2–5 independent experiments, with 2–3 wells per cohort and experiment.
(D) WT HSCs (c-KIT+LinSCA1+FLK2CD34 cells) migrate with significantly reduced efficiency across lung, kidney, and BM Robo4/ EC
layers. n = 2–7 independent experiments, with 2–3 wells per cohort and experiment.
(E) HSCs (cKIT+LinSCA1+FLK2CD34 cells) from Robo4/ mice migrate with the same efficiency as WT HSCs across WT EC layers, and
their migration is equally impaired across Robo4/ ECs. n = 3–7 independent experiments.
Error bars represent SEM. *p < 0.05, **p < 0.01, ***p < 0.005. See also Figure S2.
origin of the ECs (Figures 3B and S3C). These results are
consistent with the vascular leak in vivo (Figures 2A and
2B) and with previous findings of increased vascular permeability of Robo4/ EC layers (Jones et al., 2008). We then
tested the ability of HSCs to migrate across lung-, BM-,
and kidney-derived EC barriers in response to SDF1 (aka
CXCL12), a strong HSC chemoattractant and CXCR4
ligand (Smith-Berdan et al., 2011; Wright et al., 2002; Figure 3C). Remarkably, despite the increased passive permeability of Robo4/ relative to WT EC layers, HSC migration
across Robo4/ EC layers was significantly reduced regardless of the tissue origin of the ECs (Figure 3D). These results
show that endothelial ROBO4 facilitates HSC translocation
across vascular barriers and that increased passive permeability is not sufficient to overcome the requirement for
ROBO4 in promoting active cell translocation. Our findings
also identify impaired HSC extravasation as a cause of poor
engraftment in Robo4/ recipients (Figures 1A–1C).
Hematopoietic ROBO4 Is Not Necessary for Efficient
Transendothelial Migration of HSCs
We previously showed that Robo4/ HSCs engraft poorly
in WT hosts and attributed this to impaired HSC retention
in BM niches, mediated by ROBO4 on HSCs (Smith-Berdan
et al., 2011). Because it is possible that ROBO4 on HSCs is
also required for efficient extravasation, we tested whether
HSC-expressed ROBO4 was important for efficient translocation across EC layers. However, HSCs from Robo4/
mice migrated as efficiently as WT HSCs across WT EC
barriers and were equally poor at crossing Robo4/ EC
layers (Figure 3E). These data are consistent with our previous finding that Robo4/ HSCs do not display impaired
Stem Cell Reports j Vol. 4 j 1–14 j February 10, 2015 j ª2015 The Authors 5
Please cite this article in press as: Smith-Berdan et al., ROBO4-Mediated Vascular Integrity Regulates the Directionality of Hematopoietic
Stem Cell Trafficking, Stem Cell Reports (2015), http://dx.doi.org/10.1016/j.stemcr.2014.12.013
migration toward SDF1 across artificial membranes (SmithBerdan et al., 2011). Furthermore, the reduced migration
across Robo4/ EC layers was not specific for HSCs, but
also applied to hematopoietic progenitor and mature cells
(Figure S3D) that do not express ROBO4 (Smith-Berdan
et al., 2011). Because CXCR4 plays a role in vascular development (Tachibana et al., 1998) and Robo4/ HSCs upregulate CXCR4 to compensate for the loss of ROBO4 (SmithBerdan et al., 2011), we tested whether the differential
migration across Robo4/ EC layers was a result of a differential response to SDF1, the CXCR4 ligand used as the
attractant in the transwell assays, on Robo4/ versus WT
ECs. We did not detect robust levels of CXCR4 on freshly
isolated primary BM ECs or cultured EC monolayers by
either quantitative RT-PCR or cell-surface stains (Figures
S3E and S3F). Neither SDF1 nor other Cxcl genes, SCF, laminins, or PDGFb were differentially expressed by WT versus
Robo4/ ECs (Figure S3G). Moreover, SDF1 did not induce
detectable differences in EC permeability in vitro (Figure S3H), and IL6-induced migration of B and T cells was
less efficient across Robo4/ compared with WT ECs (Figure S3I). Thus, the ability of endothelial ROBO4 to promote
transendothelial migration is independent of the SDF1/
CXCR4 axis.
ROBO4 Deletion Leads to Alterations in BM Sinusoidal
Cell Numbers and Organization
To further understand the mechanisms behind endothelial ROBO4-mediated HSC engraftment, we tested
the consequences of ROBO4 deletion on the BM vasculature. Quantification of endothelial subpopulations by
flow cytometry revealed that the total number of ECs
(CD45TER119CD31+SCA1+ BM cells) was increased in
the BM (Figures 4A and 4B), but not in the spleen (Figure S4A), of Robo4/ mice. The elevated numbers of BM
ECs may prevent detection of increased permeability by
Evans Blue leak in Robo4/ BM at steady state (Figure 2A).
Splenic ECs also expressed lower levels of ROBO4
compared with BM ECs (Figure S3B), indicating that
ROBO4 may play less important roles in the splenic vasculature. These data are consistent with the poor trafficking of
HSCs selectively to the BM, but not the spleen, in Robo4/
mice (Figures 2D and 2E).
We also found that a distinct subpopulation of BM ECs,
defined by low levels of SCA1, expressed robust levels of
VCAM1 in WT mice, whereas the equivalent VCAM1high
population was drastically decreased in Robo4/ mice
(Figures 4C and 4D). Similarly, VCAM1 mRNA, but not
VEGFR2 or VE-Cadherin, was downregulated in ROBO4deficient ECs (Figure S4C). The SCA1low EC subpopulation
expressed high levels of ROBO4, as well as VEGFR2
and VEGFR3 (Figure S4D), and was specifically labeled by
i.v. injections of Dil-Ac-LDL, a sinusoidal-cell-specific dye
6 Stem Cell Reports j Vol. 4 j 1–14 j February 10, 2015 j ª2015 The Authors
(Kunisaki et al., 2013; Li et al., 2009; Figure 4E). These characteristics are all consistent with an SEC identity (Hooper
et al., 2009; Kunisaki et al., 2013; Li et al., 2009). Whether
defined by high VCAM1 expression or Dil-Ac-LDL labeling,
the number of SECs was significantly reduced in Robo4/
BM (Figures 4D and 4F). Consistent with a direct role for
VCAM1 in HSC extravasation, blocking antibodies to
its binding partner, integrin a4, impaired HSC transendothelial migration in transwell assays (Figure 4G). Equivalent experiments using an antibody reported to block
CD31(PECAM1)-mediated leukocyte extravasation (Bogen
et al., 1994) had no effect on HSC migration efficiency (Figure S4E). Sinusoidal cells expressing VCAM1 and interacting with integrin a4 thus appear to be important for HSC
extravasation into the BM space. Indeed, an examination
of BM sections revealed poorly formed sinusoids in
Robo4/ mice, which was associated with a significant
decrease in the total sinusoidal area and average size of individual sinusoids compared with WT BM (Figures 4H–4J
and S5). Collectively, these results indicate that ROBO4
loss leads to a poorly developed BM vasculature, characterized by reduced numbers of VCAM1-expressing sinusoidal
cells and small, narrow sinusoids. Moreover, the impaired
trafficking across ROBO4-deficient endothelium points to
BM sinusoids as important conduits for HSC extravasation
and subsequent long-term engraftment.
Vascular Permeability Regulates HSC Mobilization
from BM to Blood, but Not HSC Extravasation from
Blood to BM
The finding that loss of endothelial ROBO4 impairs transendothelial migration despite causing increased vascular
permeability was surprising because we had postulated
that increased permeability of vascular barriers would
facilitate cellular trafficking. Therefore, we tested whether
other means of inducing permeability affected the efficiency of HSC transendothelial migration. We treated EC
monolayers with VEGF, the prototype vascular permeability factor (Senger et al., 1993), and confirmed the
increased permeability of both WT and Robo4/ EC layers
by measuring the passive diffusion of FITC-dextran (Figure 5A). In contrast to the impaired translocation of
HSCs across the hyperpermeable ROBO4-deficient monolayers (Figures 3B and 3D), VEGF-induced permeability
led to significantly improved HSC migration (Figure 5B).
Although Robo4/ EC monolayers also exhibited increased
passive permeability in response to VEGF (Figure 5A), the
resulting improvement in cellular translocation was not
sufficient to restore transendothelial migration efficiency
across Robo4/ layers to that observed across WT ECs (Figures 5B, S6A, and S6B). Similar results for both passive
permeability and active HSC migration were obtained using
histamine (Figures 5A, 5B, S6C, and S6D), a compound that,
Please cite this article in press as: Smith-Berdan et al., ROBO4-Mediated Vascular Integrity Regulates the Directionality of Hematopoietic
Stem Cell Trafficking, Stem Cell Reports (2015), http://dx.doi.org/10.1016/j.stemcr.2014.12.013
Stroma
-
EC
(CD45 Ter119- )
Wt
R4-/-
3
Sca1low
Wt
8
CD31
SSC-A
R4-/-
41
90
D
VCAM1
1.6
1.2
0.8
0.4
Wt
ITG 4 Blocking
**
3
1
0.8
0.6
0.4
0.2
1
0.8
0.6
0.4
0.2
0
R4-/-
Wt
Total Sinusoid
Area
Wt
25
**
20
15
10
R4-/-
5
0
Wt R4-/-
Average
Sinusoid Size
0.12
***
0.1
0.08
0.06
0.04
0.02
0
Wt R4-/-
J
Stroma
-
EC
(CD45 Ter119- )
3
I
**
1.2
E
*
Wt BM
Laminin VCAM1
VCAM1high
2
R4-/- BM
Laminin VCAM1
1
0
SDF1
-ITGa4
-
+
-
+
+
SSC-A
1
Sca1
VCAM1low
% Max
2
CD31
% HSC Migration
R4-/-
*
1.2
0
F Dil-Ac-LDL+ SEC
Fold Difference
Dil-Ac-LDL+ cells
**
Fold Difference Vcam1+
SECs
Fold Difference Total ECs
G
200µm
Vcam1high SEC
EC
0
200µm
15
Sca1
B
2
Sca1high
Relative Sinusoid Size
Sca1
7
Laminin
Laminin
89
% Max
8
R4-/- BM
H Wt BM
C
Relative Sinusoid Area
A
50µm
VCAM1
50µm
Dil-Ac-LDL
Figure 4. BM Sinusoidal Cell Number and Organization Are Altered in Robo4/ Mice
(A–D) Flow-cytometry analysis of BM EC populations from WT and Robo4/ mice reveals that total EC (CD45TER119CD31+SCA1+)
numbers are increased (B) and SEC (CD45TER119CD31+SCA1lowVCAM1high) numbers are decreased (D) in Robo4/ compared with WT
BM. Broad gray arrows point from representative data to quantification of multiple experiments: (B) represents quantification of ECs as
shown in (A), and (D) represents quantification of VCAM1+ SECs as shown in (C). n = 3–5 independent experiments.
(E) VCAM1high BM ECs (CD45TER119CD31+SCA1+) are selectively labeled by i.v. injection of the sinusoidal-cell-selective dye Dil-Ac-LDL,
whereas nonsinusoidal (CD45TER119CD31+ SCA1+VCAM1low) ECs are negative for Dil-Ac-LDL. Mice were injected i.v. with Dil-Ac-LDL 4 hr
prior to EC harvesting from the BM and flow-cytometry analysis.
(F) Robo4/ mice have significantly fewer Dil-AC-LDL+ BM ECs (CD45TER119CD31+SCA1+Dil-Ac-LDL+ cells) compared with WT mice.
Quantification of ECs as shown in (E). n = 7–8 mice in 3 independent experiments.
(G) Transendothelial migration of HSCs (c-KIT+LinSCA1+FLK2CD34 cells) across WT EC layers in vitro is blocked by preincubation of ECs
with an antibody to the VCAM1-binding protein integrin a4. n = 3 independent experiments, with duplicate wells for each experiment and
cohort.
(H and J) Sinusoids are poorly formed in Robo4/ compared with WT mice. BM sections from WT and Robo4/ were stained with
a-laminin (H) or with a-laminin and a-VCAM1 antibodies (J) and examined for sinusoidal structures by fluorescence microscopy. Note the
abundance of sinusoids with a clearly defined lumen in WT BM sections (white arrows in H; laminin+/VCAM1+ circular structures in J),
whereas the sinusoids in Robo4/ mice are smaller, with a narrower lumen.
(I) The total area and size of the sinusoids from BM sections in (H) were significantly decreased in Robo4/ compared with WT BM. n = 3–4
mice for each cohort.
Error bars represent SEM. *p < 0.05, **p < 0.005, ***p < 0.0005. See also Figures S3 and S4.
Stem Cell Reports j Vol. 4 j 1–14 j February 10, 2015 j ª2015 The Authors 7
Please cite this article in press as: Smith-Berdan et al., ROBO4-Mediated Vascular Integrity Regulates the Directionality of Hematopoietic
Stem Cell Trafficking, Stem Cell Reports (2015), http://dx.doi.org/10.1016/j.stemcr.2014.12.013
B
40
**
30
**
*
**
25
15
20
10
10
0
VEGF
EC layer
5
-
+
Wt
-
+
R4-/-
0
Hist
-
+
Wt
*
2
20
C
Active Migration
*
**
*
*
8
1.5
*
0
VEGF
SDF1
EC layer
NS
-
- +
+ +
Wt
2
0
- - + SDF1
- + + Hist
R4-/-
-
+
-
+
+
1.0
0.001
0.004
4
0.8
0.0008
3
0.003
0.6
0.0006
2
0.002
0.4
0.0004
0.2
0.0002
1
0.001
00
VEGF
*
0.0014
1.4
0.0012
1.2
*
0.005
5
4
0.5
*
***
0.007
7
0.006
6
6
1
In Vivo Permeability: BM
OD/Mass Tissue (10^-3)
Passive Permeability
% HSC Migration
% Total Fluorescence
A
-
+
Wt
-
+
R4-/-
00
Hist
-
+
Wt
Wt
Figure 5. Induced Vascular Permeability Facilitates HSC Transendothelial Migration
(A) VEGF and histamine treatment increased the diffusion of FITC-dextran across WT (VEGF and histamine) and Robo4/ (VEGF) EC
monolayers in vitro. Transwell assays were performed as in Figure 3A except that certain EC layers were treated with VEGF or histamine to
induce permeability prior to addition of FITC-dextran to the top wells. n = 3 independent experiments.
(B) VEGF- and histamine-induced vascular permeability significantly improved SDF1-induced HSC (c-KIT+LinlowSCA1+FLK2CD34)
migration across EC layers in vitro, but VEGF-induced permeability did not rescue the transmigration defect across Robo4/ ECs. Transwell
assays were performed as outlined in Figure 3C except that specific EC layers were treated with VEGF or histamine to induce permeability
prior to addition of hematopoietic cells to the top wells. n = 3 independent experiments.
(C) VEGF and histamine treatment induced vascular permeability in the BM of WT (VEGF and histamine) and Robo4/ (VEGF) mice. Mice
were injected i.v. with Evans Blue and VEGF or histamine, and the amount of Evans Blue leakage into the BM was measured by spectrophotometry. n = 10–19 mice in 4 independent experiments.
Error bars represent SEM. *p < 0.05, **p < 0.005, ***p < 0.0005. See also Figure S6.
like VEGF, is capable of rapidly inducing vascular permeability (Ehringer et al., 1996; Woodward and Ledgard,
1986). Thus, the barrier posed by ECs can be reduced
by either histamine- or VEGF-mediated permeabilization.
Strikingly, the requirement for ROBO4 to promote cell
translocation overrides the benefit of the increased vascular
permeability caused by either loss of ROBO4 or by VEGF or
histamine treatment, resulting in a net decrease in HSC
transendothelial migration across Robo4/ ECs.
Our demonstration here that ROBO4 is necessary for efficient translocation across endothelial barriers seemingly
contradicts our previous finding that AMD3100-mediated
HSC mobilization from BM to blood is more efficient
in Robo4/ mice (Smith-Berdan et al., 2011). Therefore,
we tested whether VEGF-induced vascular permeability
or ROBO4 affected the directionality of HSC movement
into and out of the BM space in vivo. As expected, VEGF injections induced robust vascular leak in vivo (Figure 5C).
However, VEGF-induced permeability of recipient mice
as a pretransplantation conditioning, either by itself or
in combination with low-dose irradiation or AMD3100,
did not improve short-term or long-term engraftment of
transplanted HSCs (Figures 6A–6D). Thus, VEGF-induced
permeability did not facilitate HSC translocation from
blood to BM. In contrast, VEGF significantly improved
HSC mobilization from BM to blood in WT mice (Figure 7B). Remarkably, VEGF significantly enhanced
AMD3100-mediated mobilization, leading to extremely
8 Stem Cell Reports j Vol. 4 j 1–14 j February 10, 2015 j ª2015 The Authors
rapid and robust increases in phenotypic (Figure 7B) and
functional, engraftable HSCs in the PB (Figure 7C), without
affecting the relative lineage output of transplanted cells
(Figure 7C). Mobilization to the PB was transient, as the
numbers of HSCs in the PB, as well as in the BM and spleen,
returned to normal levels within 24 hr after drug treatment
(Figure S7A). As in our previous report (Smith-Berdan et al.,
2011), AMD3100-induced mobilization was more efficient
in Robo4/ mice (Figure 7B), indicating that endothelial
ROBO4 is not necessary for efficient HSC exit from the
BM space. Consistent with our in vitro experiments (Figure 5A) and previously published results (Jones et al.,
2008), Robo4/ mice also responded to VEGF permeabilization (Figure 5C). However, VEGF did not further improve
AMD3100-mediated HSC mobilization in Robo4/ mice
(Figures 7B and S7B), likely due to the already compromised vasculature of Robo4/ mice. Like VEGF treatment,
histamine injections also led to increased Evans Blue leak
(Figure 5C) and to an increase in HSCs in the PB of WT,
but not Robo4/, mice (Figure 7A), substantiating our
conclusion that induced vascular permeability results in
HSC mobilization from BM to PB. In contrast to VEGF
and histamine, AMD3100 did not induce vascular permeability in vivo (Figure S7B), supporting the notion that
AMD3100 acts by directly inhibiting CXCR4-mediated adhesive interactions between hematopoietic cells and the
BM environment (Broxmeyer et al., 2005; Smith-Berdan
et al., 2011). Hematopoietic progenitors, including MPPs
Please cite this article in press as: Smith-Berdan et al., ROBO4-Mediated Vascular Integrity Regulates the Directionality of Hematopoietic
Stem Cell Trafficking, Stem Cell Reports (2015), http://dx.doi.org/10.1016/j.stemcr.2014.12.013
Figure 6. VEGF-Induced Vascular Permeability Does Not Result in Detectable Increases in HSC Engraftment
AMD VEGF
AMD
(A) Schematic of recipient mouse preconditioning with AMD3100, VEGF, and/or
//
irradiation prior to BM transplantation.
(B) VEGF-induced permeability does not
improve engraftment of transplanted HSCs.
PB analysis
Donor contribution after transplantation of
4,8,12,16 wks
7.5 3 106 BM cells from WT (CD45.1) mice
was similar whether recipient mice were
Rad only
AMD + Rad
PB Wk 16
B
C
D
pretreated with radiation alone (518 rad) or
AMD + VEGF + Rad
VEGF + Rad
radiation plus VEGF (2 mg/mouse, i.v.). No
80
90
100
differences were detectable between pre80
70
treatment conditions for up to 16 weeks;
70
60
GM
80
data shown were obtained at 8 weeks
60
T
50
posttransplantation. n = 3 mice per cohort
60
50
B
40
in each of 3 independent experiments.
40
30
40
(C) VEGF-induced permeability does not
30
20
improve engraftment of transplanted HSCs
20
20
10
in recipients preconditioned with radiation
10
0
and AMD3100. Experiments were performed
0
0
AMD - - + +
B
T
GM
B
T
GM
as in (B) except that recipient mice were
VEGF - + - +
RAD + + + +
also pretreated with AMD3100 (5 mg/kg
subcutaneously), as indicated in (A). Two
alternative time courses of preconditioning were tested without detectable differences in engraftment. Recipient pretreatment with VEGF
alone or in combination with AMD3100 was not sufficient to detect engraftment. n = 3–4 mice per cohort in each of 3 independent
experiments.
(D) VEGF pretreatment of recipient mice did not alter the lineage distribution of donor cells in recipient mice from (B) and (C). One
representative experiment out of three independent experiments is shown. n = 3–4 mice per cohort and experiment.
Error bars represent SEM. No comparisons were significantly different. See also Figure S7.
1
% Donor
% Donor
0
Total
B220 B
CD3 T
1:45
2 Hr
BM
transplant
% Donor
A
GM
and myeloid progenitors, also mobilized in response to
VEGF (Figures S7D–S7F), consistent with VEGF acting on
the vasculature as opposed to specific hematopoietic subpopulations. Similar to what was observed for HSCs,
VEGF also had a greater effect on progenitor mobilization
in WT compared with Robo4/ mice (Figures S7D–
S7F). Because hematopoietic progenitors do not express
ROBO4, the attenuated response to VEGF in Robo4/
mice suggests that endothelial ROBO4 helps maintain cells
in the BM by preventing vascular leak. Collectively, these
results provide compelling evidence that induced vascular
permeability enhances HSC mobilization from BM to
blood, and that HSC translocation into and out of the
BM is regulated by different mechanisms.
DISCUSSION
Endothelial ROBO4 Promotes Unidirectional HSC
Trafficking across Vessel Walls of the BM
The results presented here show that endothelial ROBO4 is
necessary for efficient HSC trafficking from the blood to the
BM space. By using a combination of in vitro and in vivo
assays, we were able to pinpoint the poor engraftment of
WT HSCs in Robo4/ mice to defective transendothelial
migration. Our finding that HSCs migrate poorly across
ROBO4-deficient EC layers despite their increased permeability suggests that the vascular endothelium actively promotes the HSC extravasation process. We found that the
number and structure of VCAM1+ SECs are compromised
in Robo4/ BM, and that the VCAM1/ITGA4 interaction
is essential for transendothelial migration of HSCs.
Together, these findings point to VCAM1+ SECs as likely
sites for HSC extravasation from blood to BM. As most
HSCs reside in close proximity to sinusoidal cells (Kiel
et al., 2005), HSCs that exit the blood via sinusoidal structures may take up residence on the BM side of sinusoids.
Thus, ROBO4 and VCAM1 on SECs may promote HSC
extravasation in the optimal location in the BM.
Intriguingly, the reverse process of HSC escape from the
BM to the blood does not require ROBO4. AMD3100-mediated mobilization of progenitor cells was as efficient in
the absence of Robo4/. Furthermore, AMD3100-mediated
HSC mobilization was significantly more efficient in
ROBO4/ mice, consistent with our previous conclusion
that ROBO4 on HSCs mediates adhesive interactions with
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Stem Cell Trafficking, Stem Cell Reports (2015), http://dx.doi.org/10.1016/j.stemcr.2014.12.013
0
5
10
Histamine
15 Min
perfuse
B
0
AMD
40
VEGF
800
500
400
300
***
***
***
***
100
Wt
+
-
+
R4-/-
2500
2000
1500
***
*** NS
***
***
1000
NS
**
500
0
AMD
VEGF
Recipient
-
+
Wt
+ +
- +
-
+
+ +
- +
PBS
AMD
AMD + VEGF
80
70
60
50
40
30
20
10
0
**
200
***
PB analysis
4,8,12,16 wks
***
No. of HSC/2ml PB
600
0
Hist
*
3000
700
Mobilized
Blood
**
No. of HSC /2ml PB
NS
**
//
PB analysis;
transplant
PB
analysis
900
C
60 Min
perfuse
% Donor
A
0
4
8
12
16
Weeks post transplant
R4-/-
20
90
80
70
60
50
40
30
20
10
0
***
***
*
***
*
*
*
*
*
GM
PBS
B
AMD
T
AMD + VEGF
Figure 7. Induced Vascular Permeability Facilitates HSC Mobilization from BM to the Blood Stream
(A) Histamine injections induce HSC (c-KIT+LinlowSCA1+CD27+FLK2) mobilization to the blood stream in WT, but not Robo4/, mice.
Three consecutive injections of histamine were followed by perfusion and quantification of HSC numbers in the blood by flow cytometry.
n = 3–4 mice per experiment in 3 independent experiments.
(B) VEGF, alone or in combination with AMD3100, enhances HSC (c-KIT+LinlowSCA1+CD27+FLK2) mobilization to the blood stream in WT,
but not Robo4/, mice. n = 10–18 mice in 3 independent experiments.
(C) Long-term, multilineage engraftment of mobilized blood cells from WT mice treated with AMD3100 alone or in combination with VEGF.
Blood collected from mice in (E) was transplanted into sublethally irradiated recipient mice, followed by assessment of long-term multilineage reconstitution by flow-cytometry analysis of donor-derived mature cells in the PB of recipient mice. n = 8–9 mice for each cohort
in 3 independent experiments.
Error bars represent SEM. *p < 0.05, **p < 0.005, ***p < 0.0005. See also Figure S7.
BM niches (Smith-Berdan et al., 2011). Thus, neither
ROBO4 nor VCAM1+ SECs seem to be essential for efficient
HSC mobilization. The alterations in vascular permeability
in Robo4/ mice led us to also test the role of vascular
integrity in HSC retention in the BM. We found that
induced vascular permeability, via VEGF and histamine
injections, very rapidly mobilized HSCs to the blood
stream. This action of VEGF appears to be different from
the previously reported VEGF-mediated vascular remodeling and associated HSC relocation that occurs over a period
of several days (Hattori et al., 2001). HSC mobilization
upon induced vascular permeability indicates that the
vasculature acts as a barrier to prevent HSC escape into
the blood. Addition of AMD3100, acting on hematopoietic
CXCR4, led to increased HSC mobilization compared with
ROBO4 loss alone, or VEGF or histamine injection alone.
Thus, targeting both the vascular barriers and hematopoietic adhesive forces had additive effects on HSC mobilization, identifying a promising strategy for very rapid and
efficient harvesting of HSCs for cell therapies.
Just as ROBO4 and VCAM1 are necessary for efficient
HSC extravasation but appear to be dispensable for efficient
HSC mobilization, induced vascular permeability improves
mobilization but does not appear to enhance HSC extravasation. VEGF preconditioning of recipient mice under conditions that induce vascular permeability did not lead to
measureable improvements in HSC engraftment. These observations very clearly separate the mechanisms that regulate HSC trafficking into the BM from those that regulate
reentry into the blood. Uncoupling of these mechanisms
has been suggested previously (Adams et al., 2009; Me´ndez-Ferrer and Frenette, 2009). Our results indicate that
SECs expressing ROBO4 and VCAM1 promote HSC extravasation, that vascular barriers prevent leakage back to the
blood, and that adhesion molecules on HSCs, including
ROBO4 and CXCR4, anchor the HSCs to BM niches. In
combination, these directional mechanisms result in a
net accumulation of HSCs in the BM. Tipping this balance
in either direction by modulating these mechanisms, one
by one or in combination, would favor HSC location to
either the blood for HSC harvesting or to the BM to promote HSC engraftment upon transplantation.
Vascular Regulation of HSC Location
In addition to affecting directional specificity across vessels, ROBO4 selectively regulates HSC location to the BM.
10 Stem Cell Reports j Vol. 4 j 1–14 j February 10, 2015 j ª2015 The Authors
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Loss of either endothelial or hematopoietic ROBO4 failed
to affect HSC localization to the spleen. Differences in
vascular structures throughout the body are likely an
important factor in regulating HSC location. Although
HSCs can infiltrate different peripheral tissues (Massberg
et al., 2007), the vessel walls of most organs likely limit
HSC extravasation. In contrast, the splenic vasculature
is discontinuous, resulting in unrestricted access of
circulating HSCs. HSCs fail to remain in the spleen
long term, however, due to the inability of the spleen
vasculature to prevent escape to the blood and to support
self-renewal. The specialized vascular architecture of the
BM, which includes fenestrae and sinusoids, allows
HSC entry, long-term engraftment, and regulated reentry
to the blood.
Our findings define two distinct mechanisms by which
the vascular endothelium promotes HSC location to the
BM. By expressing ROBO4, ECs actively promote HSC
extravasation from the blood stream to the BM space. In
addition, ECs prevent HSCs from traveling in the reverse direction, from the BM to the blood. This vascular barrier formation is supported by ROBO4 and can be manipulated by
acute exposure of the endothelium to VEGF or histamine.
Our findings reinforce the previously established importance of the vascular endothelium, in particular SECs, in
long-term HSC maintenance (Ding and Morrison, 2013;
Ding et al., 2012; Greenbaum et al., 2013; Ugarte and Forsberg, 2013) and raise the possibility that secreted niche factors influence HSC location by modulating vascular permeability. Our uncoupling of the mechanisms that regulate
the directionality of HSC trafficking and identification of
separate molecular targets for each process point to manipulation of vascular integrity as a strategy for improving
both HSC mobilization and engraftment in clinical
therapy.
EXPERIMENTAL PROCEDURES
EC Isolation and Culturing
ECs were isolated from lungs, kidney, or BM from WT or Robo4/
mice and treated with collagenase as previously described (Dong
et al., 1997; Jones et al., 2008; Sobczak et al., 2010). Freshly isolated
cells were used for gene and protein expression analyses. For transwell assays, magnetic-bead-enriched cells were cultured for up to
four passages under conditions that promote EC growth (Sobczak
et al., 2010).
Transendothelial Migration Assays
Primary ECs were seeded onto 0.5% gelatin-treated transwell inserts and grown to confluency. BM cells (lineage depleted by magnetic selection when appropriate) from WT or Robo4/ mice were
preincubated at 37 C for 1 hr and then placed in the upper chamber of a transwell insert (5 mm pore size). The bottom wells contained SDF1 (100 ng/ml) or IL-6 (100 ng/ml) (Weissenbach et al.,
2004) as indicated. Cells were allowed to migrate for 2 hr at 37 C
before harvesting and analysis by flow cytometry as described previously (Smith-Berdan et al., 2011). In some cases, permeability
was induced by exposing starved cells to 2.4 nM rhVEGF-165 for
3.5 hr or to 32 mM histamine for 45 min prior to migration. For
blocking assays, ECs were pretreated with anti-ITGa4 (clone PS/
2) (Bowden et al., 2002) or anti-CD31 (clone 2h8) (Bogen et al.,
1994) antibodies at 10 mg/ml for 30 min prior to cell migration toward SDF1.
Vascular Permeability Assays
A modified Miles assay was utilized to assess in vivo vascular
permeability (Miles and Miles, 1952). Mice were injected i.v. with
Evans Blue (50 mg/kg) and then euthanized by isoflurane inhalation. Vascular leak was determined by isolating tissues at 5–
10 min postinjection and measuring Evans Blue absorbance, expressed as OD650/tissue mass. For induced permeability assays,
VEGF (2 mg/mouse) was injected once i.v., followed by Evans
Blue dye 5 min later. The dye was allowed to leak into the tissues
for an additional 15 min prior to tissue harvest, whereas histamine
(100 mg/mouse) was injected i.v. three times 5 min apart, followed
by Evans Blue dye i.v. 5 min later and 5 min prior to tissue harvest.
For radiation permeability assays, mice were treated with a lethal
dose of radiation 3 days prior to the Miles assay.
Immunohistochemistry
Mice
Mice were maintained by the University of California, Santa Cruz,
animal facility according to approved protocols. Robo4/ mice on
the C57Bl6 background were described previously (Jones et al.,
2008; London et al., 2010; Marlow et al., 2010). UBC-GFP and
mTmG transgenic mice (both from JAX) were described previously
(Boyer et al., 2011, 2012; Muzumdar et al., 2007; Schaefer et al.,
2001).
Bones were embedded into OCT cryopreservation media on an
ethanol/dry ice slurry immediately after dissection from WT or
Robo4/ mice and stored at 80 C. BM sections (7 mm thick)
were cut with a tungsten blade and fixed with acetone at 20 C
for 10 min or with 4% paraformaldehyde at 4 C for 20 min.
Sections were blocked with 10% goat serum prior to overnight
antibody staining at 4 C, followed by incubation for 1 hr with fluorescently conjugated secondary antibodies. Samples were imaged
with a Keyance microscope.
Transplantation Assays
Long-term reconstitution, short-term homing, and CFU-S assays were performed similarly to previous protocols (Boyer
et al., 2011; Forsberg et al., 2006; Smith-Berdan et al., 2011)
and are described in detail in Supplemental Experimental
Procedures.
Mobilization
Mice were injected with either histamine (5 mg/kg i.v.)
or AMD3100 (5 mg/kg subcutaneously) and/or rhVEGF-165
(2 mg/mouse, i.v.) as indicated. Total blood was isolated by perfusion with PBS/20 mM EDTA and processed for cell counts and
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flow-cytometry analysis to determine the number and frequency
of each cell population as described previously (Smith-Berdan
et al., 2011). Reconstitution assays on mobilized blood were performed by transplanting one-half of the blood mouse equivalent
into a lethally irradiated host (1,024 rads). Recipient mice were
bled at the indicated intervals after transplantation via the tail
vein, and PB was analyzed for donor chimerism as described above
and previously (Beaudin et al., 2014; Boyer et al., 2011, 2012; Forsberg et al., 2006; Ooi et al., 2009; Smith-Berdan et al., 2011).
Statistics
Statistically significant differences for all comparisons were calculated using two-tailed t tests unless stated otherwise. One-sided
t tests were used for comparison with zero values (Figure 5B).
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental
Procedures and seven figures and can be found with this article online at http://dx.doi.org/10.1016/j.stemcr.2014.12.013.
ACKNOWLEDGMENTS
We thank Dr. Michael Halbisen for advice on statistics; Drs. David
Alexander, Amy Ralston, Anna Beaudin, and Fernando Ugarte for
comments on the manuscript; and Susan Calhoun, Herman Tsang,
Gaby Sanchez, Felicia Kemp, and Alan Ly for assistance with
experiments. This work was supported by an American Cancer
Society Research Scholar Award (RSG-13-193-01-DDC), the
Cancer Research Coordinated Committee, an NIH/NIAID award
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