Neuron 51, 187–199, July 20, 2006 ª2006 Elsevier Inc.DOI 10.1016/j.neuron.2006.06.012
PDGFRa-Positive B Cells Are Neural Stem Cells
in the Adult SVZ that Form Glioma-like Growths
in Response to Increased PDGF Signaling
Erica L. Jackson,1Jose Manuel Garcia-Verdugo,3
Sara Gil-Perotin,3Monica Roy,1
Alfredo Quinones-Hinojosa,1,4Scott VandenBerg,2
and Arturo Alvarez-Buylla1,*
1Department of Neurological Surgery and
Program in Developmental and Stem Cell Biology
2Departments of Neurological Surgery and Pathology
University of California, San Francisco
San Francisco, California 94143
3University of Valencia
Neurons and oligodendrocytes are produced in the
cells (B cells), which express GFAP and have morpho-
logical properties of astrocytes. We report here on the
identification B cells expressing the PDGFRa in the
adult SVZ. Specifically labeled PDGFRa expressing B
cells in vivo generate neurons and oligodendrocytes.
Conditional ablation of PDGFRa in a subpopulation
of postnatal stem cells showed that this receptor is re-
quired for oligodendrogenesis, but not neurogenesis.
Infusion of PDGF alone was sufficient to arrest neuro-
blast production and induce SVZ B cell proliferation
contributing to the generation of large hyperplasias
that PDGFRa signaling occurs early in the adult stem
cell lineage and may help regulate the balance be-
sive PDGF activation in the SVZ in stem cells is suffi-
cient to induce hallmarks associated with early
stages of tumor formation.
The subventricular zone (SVZ) in the walls of the lateral
ventricles is the largest germinal center and source of
stem cells in the adult brain. The stem cells in the SVZ
of the adult rodent brain continue to generate neurons
destined for the olfactory bulb (reviewed in Lim and Al-
varez-Buylla ). A population of GFAP-expressing
cells in the SVZ (B cells) are the primary precursor cells.
They first produce a transit-amplifying cell population (C
cells) that then give rise to the neuroblasts (A cells) that
migrate to the olfactory bulb where they mature into
neurons (Doetsch et al., 1999a; Garcia et al., 2004; Par-
ras et al., 2004). Although the SVZ stem cells have char-
acteristics of astrocytes and express GFAP, additional
markers that enable further characterization of the
stem cells are lacking. New oligodendrocytes are also
generated in the SVZ of adult mice (Nait-Oumesmar
et al., 1999), and B cells located in this region also serve
as their primary progenitors (Menn et al., 2006). Again,
additional markers and factors regulating SVZ oligoden-
drogenesis have not been well characterized.
During mouse development, the platelet-derived
growth factor receptor-a (PDGFRa) is expressed by
neuroepithelial cells as early as E8.5 (Andrae et al.,
2001). However, the role of PDGF in neuroepithelial
stem cells is not understood. Instead, PDGF is consid-
ered to function later in development as a potent mito-
gen of oligodendrocyte precursor cells (OPCs). OPCs
were first identified and characterized in cultures from
rat perinatal optic nerve (Raff et al., 1983), where they
give rise to either oligodendrocytes or type-2 astro-
cytes. In the rat optic nerve and spinal cord, OPCs ex-
press PDGFRa (Hall et al., 1996), and PDGF is important
for regulating OPC number and oligodendrocyte pro-
duction in vivo (Calver et al., 1998; Fruttiger et al.,
1999). While most OPCs differentiate into mature oligo-
dendrocytes early in postnatal life, it is believed that
tributed within the adult brain (Ffrench-Constant et al.,
1986; Wolswijk and Noble, 1989). The adult CNS retains
the capacity to generate new oligodendrocytes (McCar-
thy and Leblond, 1988), and here PDGF is also thought
to regulate OPC number (Woodruff et al., 2004).
In addition to these developmental roles, PDGF sig-
naling has been widely implicated in the formation of
brain tumors. Activation of the pathway has been ob-
served in >80% of oligodendrogliomas and in 50%–
100% of astrocytomas and astrocytoma cell lines
(Guha et al., 1995; Varela et al., 2004) occurring mainly
through overexpression of the ligand and cognate re-
ceptor. Overexpression of PDGF/PDGFR occurs with
equal frequency in low- and high-grade tumors, sug-
gesting that activation of the pathway may be important
for tumor initiation. Thus, understanding which cells in
the adult brain normally respond to PDGF may provide
insights into the glioma cell of origin and events regulat-
ing brain tumor initiation.
Some tumors in the adult human brain have anatomi-
cal connections with the ventricular wall (Globus and
Kuhlenbeck, 1944), and it has been suggested that brain
tumors may arise from neural stem or progenitor cells
(Dai et al., 2001; Recht et al., 2003). Support for this hy-
pothesis comes from oncogenic transformation of pro-
genitor cells (Holland et al., 1998) and from a murine as-
trocytoma model in which tumors appear to initiate in
the SVZ (Zhu et al., 2005). Further support comes from
work demonstrating the presence of neural stem-like
cells within brain tumors (Ignatova et al., 2002; Hemmati
et al., 2003; Galli et al., 2004; Singh et al., 2004). How-
ever, it has not been shown in vivo that tumor stem cells
are derived from normal stem cells or that a specific
population of cells with demonstrated stem cell proper-
ties is capable of initiating tumor formation.
We report here the identification of PDGFRa-express-
ing astrocytes (B cells) in the adult SVZ that are neural
stem cells. We show that PDGF stimulates growth and
self-renewal of SVZ stem cells in neurosphere cultures
4Present address: Department of Neurosurgery, Johns Hopkins
University, Baltimore, Maryland 21218.
in vitro and that PDGFRa+B cells generate new neurons
and oligodendrocytes in vivo. In addition, we find that
these cells are sensitive to alterations in the PDGF path-
way and form atypical hyperplasias in vivo in response
to excess PDGF signaling.
PDGFRa Is Expressed by a Subpopulation of SVZ B
The expression of PDGFRa was analyzed by immunos-
taining in the adult mouse SVZ using a rabbit polyclonal
and a mouse monoclonal antibody, which showed very
similar staining patterns. Large numbers of PDGFRa-
expressing cells were observed in the dorsal region of
present in the central and ventral portions (Figure 1B).
PDGFRa-expressing cells were also observed in the me-
dial wall of the lateral ventricle. PDGFRb was detected in
cortical neurons (Smits et al., 1991), but not in the SVZ.
We analyzed sections coimmunostained for the
PDGFRa and SVZ cell type-specific markers at the con-
focal microscope to identify which cells in the adult
mouse SVZ expressed the PDGFRa. A subset of
GFAP+SVZ astrocytes (B cells) expressed the PDGFRa.
The PDGFRa signal was highest in the astrocytic pro-
cesses, with only a subset of cells exhibiting immunos-
taining in the cell body (Figure 1A). To determine what
Figure 1. PDGFRa Expression in the Adult
Confocal optical sections of coronal sections
of the mouse SVZ coimmunostained for: (A)
PDGFRa (red) and GFAP (green). Numerous
cells express both PDGFRa and GFAP in
the dorsal wedge region of the SVZ (top
panels). The bottom panels show double-
positive cells located more ventrally, about
midway between the dorsal and ventral
SVZ. Note that the staining is mainly localized
in the processes and that many of the
PDGFRa+cells have a short radial process.
(B) Nestin (red) and PDGFRa (green). Some
PDGFRa-expressing cells also express Nes-
(C) Confocal optical sections of adult human
SVZ coimmunostained for PDGFRa (red)
and GFAP (green). Note that a subpopulation
of the B cells express the PDGFRa (arrows).
(D) Confocal optical section of mouse SVZ
showing costaining for PDGFRa (red, arrow)
and EGFR (green). Note the absence of dou-
Scale bars = 20 mm.
proportion of SVZ B cells express the PDGFRa, we ana-
lyzed only those cells for which we could detect GFAP
immunostaining in the cell body encompassing the nu-
cleus. Using the mouse anti-PDGFRa 82.5% 6 4.9%
(mean 6 SEM, n = 3 mice) of SVZ B cells were PDGFRa+.
This was very similar to results obtained using rabbit
anti-PDGFRa in which case 78.1% 6 0.95% (n = 3) of
the B cells were PDGFRa+.
In the SVZ, PDGFRa was not detected on the Dlx2+C
cells, nor did the staining colocalize with that of PSA-
NCAM or doublecortin, which mark A cells (Bonfanti
and Theodosis, 1994; Doetsch et al., 1999a; see Figures
S1C and S1D in the Supplemental Data available with
this article online). Intriguingly, many of the PDGFRa+
cells also expressed Nestin (Figure 1B), a marker of neu-
ral stem and progenitor cells (Lendahl et al., 1990). Fur-
thermore, the PDGFRa was not expressed by EGFR+
cells in the SVZ (Figure 1D), most of which are transit-
amplifying C cells (Doetsch et al., 2002). To further verify
the identity of the PDGFRa+cells in the murine SVZ, we
performed pre-embedding immunostaining in conjunc-
tion with electron microscopy (EM). Analysis of semithin
sections revealed thatSVZ Bcells with variousmorphol-
ogies expressed the PDGFRa (Figure S1A). Under the
electron microscope, cells expressing the PDGFRa
had ultrastructural characteristics of astrocytes includ-
ing a light cytoplasm and thick bundles of intermediate
filaments (Figure S1B).
We next determined whether PDGF-expressing cells
existed in the adult human SVZ, where a subpopulation
of astrocytes can function as stem cells in vitro (Sanai
et al., 2004). Costaining for PDGFRa and GFAP was per-
formed on sections of postmortem adult human brain.
Interestingly, a subset of the GFAP+SVZ astrocytes ex-
pressed the PDGFRa (Figure 1C).
SVZ B Cells Activate the PDGFRa in Response to
We next investigated whether PDGF signaling occurs in
adult SVZ B cells. The presence of the activated recep-
tor was examined by staining with antibodies specific to
the phosphorylated PDGFRa (phospho-Tyr720). This
analysis revealed a small subpopulation of SVZ B cells
in which the PDGFRa was activated (Figure 2A). Inter-
estingly, Western blots of SVZ lysates revealed abun-
dant levels of PDGF-A ligand relative to levels in the
cortex (Figure 2B, left). The converse was true of the
PDGF-B ligand (Figure 2B, right), and PDGF-C ligand
was not detected in Western blots.
To explore whether SVZ cells respond to increased
levels of PDGF ligand, we injected PDGF-AA, which
binds specifically to the PDGFRa, or vehicle into the lat-
eral ventricle. The number of phospho-PDGFRa+B cells
was dramatically increased in the PDGF-injected mice
(Figure 2D) as compared to vehicle infused or untreated
brain (Figure 2A). Together, these data indicate that
Figure 2. PDGFRa Signaling in Astrocytes of
the Adult SVZ
(A) Confocal optical sections showing cos-
taining for GFAP (red) and PDGFRa-phos-
phoTyr720 (green) on coronal sections of
brain from untreated mice.Note the presence
of GFAP+/phospho-PDGFRa+astrotcytes in
the untreated brain ([A], inset).
(B) Western blots of lysates from SVZ and
cortex probed for PDGF-A ligand (left) or
PDGF-B ligand (right). Note the abundant ex-
pression of PDGF-A (w30 kDa band) in the
SVZ as compared to the cortex, while the op-
posite is true for PDGF-B.
(C) Levels of PDGFRa (left) or phospho-
PDGFRa-Tyr720(right) inlysates fromcortex,
SVZ, and SVZ after injection of PDGF-A li-
gand into the lateral ventricle. A single band
of w170 kDa is observed.
(D) Confocal optical sections showing cos-
taining for GFAP (red) and phospho-PDGFRa
(green) on coronal sections of brain from
mice given two injections of PDGF-A ligand
into the lateral ventricle separated by 20
min. Many more phospho-PDGFRa+B cells
are present after injection of ligand. Scale
bar = 50 mm. Inset scale bar = 20 mm.
(E) Costaining for GFAP (red) and phospho-
PDGFRa (green) on sections of adult human
SVZ demonstrating the occurrence of endog-
enous PDGF signaling. Scale bar = 20 mm.
PDGFRa+B Cells in the SVZ Are Adult Neural Stem Cells
PDGF signaling occurs in the normal SVZ as a result of
endogenous signals and that signaling through this
pathway can be stimulated by the administration of ex-
We also examined the human SVZ for the presence of
astrocytes in which the PDGFRa was actively signaling.
Again, costaining for phospho-PDGFRa revealed the
presence of a subset of GFAP+astrocytes in which the
PDGFRa was phosphorylated (Figure 2E), suggesting
that endogenous signaling also occurs in adult human
PDGF Works in Conjunction with FGF to Stimulate
Neurosphere Formation In Vitro
Based on our findings that some B cells expressed the
PDGFRa, we hypothesized that PDGF signaling regu-
lates stem cell properties such as survival, proliferation,
or self-renewal. Cells from the SVZ grown in vitro in the
presence of EGF, bFGF, or both, form neurospheres
(NS), which contain multipotent neural stem cells (Mors-
head et al., 2003). We studied the effects of PDGF on
SVZ stem cells in NS assays. Equal numbers of cells
dissociated from the SVZ of adult mice were divided
into culture media containing PDGF-AA, bFGF, or bFGF
and PDGF-AA. Although PDGF-AA alone did not
support NS growth, there was a significant increase
(45% 6 9%, p < 0.001) in the number of primary NS gen-
erated in media containing bFGF and PDGF-AA as com-
pared to bFGF alone (Figure 3A), suggesting that PDGF
promotes the survival and/or proliferation of SVZ stem
cells. In addition, there was a 76% increase (p = 0.001)
in the number of secondary NS generated from a single
primary NS in the presence of bFGF and PDGF-AA com-
pared to bFGF alone (Figure 3B), indicating that PDGF-
AA also stimulates self-renewal. Upon differentiation,
NS derived in bFGF and PDGF-AA were capable of gen-
erating neurons, oligodendrocytes, and astrocytes,
tained their multipotency (Figure 3D). The cooperative
effects of PDGF-AA and bFGF observed in vitro promp-
ted us to compare the expression patterns of the FGFR
and PDGFR in the SVZ. Many of the PDGFRa-express-
ing cells were also FGFR+(Figure 3E, arrows). In con-
trast to the cooperative effects of bFGF and PDGF-AA,
there was no difference in the number of NS derived
from cells cultured in EGF alone versus EGF and PDGF
(Figure 3C). These data, together with the nonoverlap-
ping staining pattern of PDGFRa and EGFR (Figure 1D),
suggest that these two growth factors affect different
subsets of SVZ cells.
PDGFRa+B Cells in the Adult SVZ Give Rise
to Neurons and Oligodendrocytes
(Ara-C) kills the rapidly dividing cells, depleting the
SVZ of C and A cells but sparing a subpopulation of
slowly dividing B cells (Doetsch et al., 1999b). Following
Ara-C treatment, B cells divide to repopulate the SVZ,
producing C cells which in turn generate A cells. We
used this method to determine if the repopulating stem
cells express the PDGFRa. Mice were infused with
Ara-C for 6 days and sacrificed 12 hr later after receiving
a single injection of BrdU 1 hr prior to sacrifice. Costain-
ing for PDGFRa and GFAP revealed PDGFRa+B cells
Figure 3. PDGF Cooperates with bFGF toEn-
hance Neurosphere Formation and Stimulate
Stem Cell Self-Renewal
(A) Freshly dissociated SVZ cells were cul-
tured under neurosphere conditions in the
presence of PDGF, bFGF, or bFGF + PDGF.
No NS grew in PDGF alone, but the addition
of PDGF to bFGF containing growth media
resulted in an increase in the number of pri-
mary neuroshperes generated over bFGF
alone (data from three independent experi-
ments; mean 6 SEM).
the dissociation of a single primary NS in-
creased 76% when cultured in bFGF +
PDGF compared to bFGF alone.
(C) Freshly dissociated SVZ cells were cul-
tured under neurosphere conditions in the
presence of PDGF, EGF, or EGF + PDGF.
PDGF did not enhance NS formation in con-
junction with EGF.
(D) Triple labeling of a differentiated NS de-
rived in bFGF + PDGF showing immunoreac-
tivity for astrocyte (GFAP in blue), oligoden-
drocyte (O4 in green), and neuron-specific
(Tuj1 in red) markers.
(E) Confocal optical section of a coronal sec-
tion of the SVZ showing costaining for FGFR
(red) and PDGFRa (green).
remaining in the SVZ (Figure 4A), consistent with the hy-
pothesis that they function as primary progenitors not
transit-amplifying precursors. We also performed cos-
taining for PDGFRa and BrdU to determine whether
the PDGFRa+cells contribute to regeneration of the
SVZ. Because the PDGFR signal is localized to the pro-
cesses, it was difficult to colocalize this marker with the
nuclear BrdU staining. However, we could be certain
that a subset of the BrdU+B cells were also PDGFRa+
(Figure 4B). This is consistent with the idea that the
PDGFRa+B cells are induced to re-enter the cell cycle
after removal of Ara-C.
In several organs the slowly dividing, label-retaining
cells (LRCs) correspond to the resident stem cell popu-
lation (Bickenbach, 1981; Morris and Potten, 1994). We
examined LRCs in the SVZ to see if they were PDGFRa+.
BrdU was administered in the drinking water for one
week and then removed for 3 weeks, sufficient time for
the rapidly dividing C and A cells that were in the SVZ
at the time of BrdU administration to complete division
and/or migrate to the olfactory bulb (Lois and Alvarez-
Buylla, 1994; Luskin and Boone, 1994). Thus, any LRCs
remaining in the SVZ at this time are good candidates
for stem cells that divided and remained in the germinal
zone. Again, we found that a subset of the BrdU-labeled
B cells were also PDGFRa+(Figure 4C).
While the above experiments indicate that PDGFRa+
B cells divide and possibly function as neural stem cells
in vivo, they do not directly demonstrate that the
PDGFRa+B cells generate neurons in vivo. To do this,
we needed a method to specifically label these cells
and follow the fates of their progeny. Recent work iden-
tified the PDGFR as a receptor for adeno-associated vi-
rus serotype 5 (AAV5; Di Pasquale et al., 2003; Lotery
et al., 2003). AAV5 integrates into the host genome and
is stably inherited by the daughter cells. We first verified
that AAV5 infection in the SVZ is limited to PDGFRa+
cells. Freshly isolated SVZ cells were infected in vitro
with an AAV5-encoding GFP (AAV5-GFP), and coex-
pression of PDGFRa and GFP was evaluated 24 hr later
by costaining. We analyzed all of the cells in 15 random
fields. Of 155 cells analyzed, we found 73 GFP+(AAV5-
infected) cells, all of which expressed the PDGFRa
(Figure 5A). Furthermore, we used conditional ablation
of the PDGFRa to confirm that it is required for AAV5 in-
fection of SVZ cells in vivo. Adenovirus encoding Cre re-
combinase (AdCre, n = 4) or LacZ (AdLacZ, n = 4) as
a control was injected stereotaxically in the SVZ of
mice homozygous for a floxed allele of PDGFRa
(PDGFRaFl/Fl; Klinghoffer et al., 2002). Nine days later,
AAV5-GFP was injected into the SVZ. PDGFRaFl/Fl
mice that received AdCre had 5-fold fewer GFP+cells
than those that received AdLacZ (p = 0.002; Figure S3A).
Of the GFP+cells present in the four mice injected with
AdCre, only seven cells also expressed Cre; all these
cells still exhibited PDGFRa protein expression (Fig-
ure S3B). Thus, AAV5-GFP virus provides a method to
specifically label B cells expressing PDGFRa.
To confirm that PDGFRa+B cells are neurogenic stem
the SVZ then labeled remaining PDGFRa+B cells by in-
fection with AAV5-GFP. Twelve days after infection, nu-
merous GFP+neurons were present in the OB, including
both granule and periglomerular cells, which were iden-
tified by their characteristic morphology (Figure 5B) and
by costaining for GFP and doublecortin (Figure 5D).
GFP+/doublecortin+neuroblasts were also present in
the migratory stream between the SVZ and OB (Fig-
ure 5C), suggesting the continuing production of neu-
rons by PDGFRa+B cells. Together, these data demon-
strate that the PDGFRa+B cells function as precursors
of new neurons. Since AAV5-GFP infection was done
Figure 4. PDGFRa-Expressing B Cells Have
Characteristics of Stem Cells
(A and B) PDGFRa+B cells remain in the SVZ
after elimination of C cells and neuroblasts by
Ara-C treatment, and some of these cells
divide after removal of the drug.
(A) Costaining for GFAP (green) and PDGFRa
(red) after 6 day Ara-C infusion.
(B) Costaining for BrdU (green) and PDGFRa
(red) 12 hr after Ara-C treatment.
(C) Some label-retaining cells express the
PDGFRa. Mice received BrdU continuously
in the drinking water for 1 week followed by
a 3 weeksurvival. Costaining for BrdU (green)
and PDGFRa (red).
PDGFRa+B Cells in the SVZ Are Adult Neural Stem Cells
after elimination of transit-amplifying C cells in the SVZ,
this experiment further suggests that PDGFRa+B cells
correspond to the primary neural precursors.
We also noted the presence of GFP+oligodendro-
cytes in the CC, suggesting that the PDGFRa+B cells
in the SVZ also generated oligodendrocytes. However,
we could not rule out infection of local PDGFRa+OPCs
in the CC by the AAV5-GFP because the injection needle
passed through the CC. Therefore, to specifically label
PDGFRa+B cells, we performed a coinfection in
GFAP-Tva mice, which express the receptor for RCAS
virus only in GFAP-expressing cells. Infection of Gtv-
a mice with an RCAS virus encoding alkaline phospha-
tase (RCAS-AP) results in specific infection of dividing
GFAP-positive cells and labeling of their progeny (Hol-
land and Varmus, 1998; Doetsch et al., 1999a). Costain-
ing showed that expression of tvaR only occurs in
GFAP-positive cells in Gtva-a mice (Figure S3C). Injec-
tion of RCAS-AP virus into nontransgenic mice did not
result in AP labeling. Thus, after confirming the specific-
ity of RCAS infection, we injected a mixture RCAS-AP
and AAV5-GFP into the SVZ. The mice were sacrificed
21 days later, and the CC was analyzed for the presence
of AP/GFP-double-positive cells. Indeed, AP+/GFP+
cells with obvious oligodendrocyte morphology were
present in the CC (Figure 5E), corresponding to the
progeny of PDGFRa+B cells.
These experiments indicate that PDGFRa+B cells in
the SVZ produce both neurons and oligodendrocytes.
In addition, numerous GFP+/GFAP+cells were present
in the SVZ after AAV5-GFP infection (Figure S4). How-
ever, we could not distinguish whether these were the
original B cells that were infected by the AAV5 virus or
if they were newly generated SVZ astrocytes.
Infusion of PDGF-A into the Lateral Ventricle Blocks
Neuroblast Differentiation and Induces the Growth
of Atypical Hyperplasias in the SVZ
primary SVZ progenitors, we examined their sensitivity
to alterations in PDGF signaling. PDGF-AA ligand, which
specifically binds to the PDGFRa, was infused into the
Figure 5. Specific Labeling of PDGFRa-Ex-
pressing B Cells Demonstrates that These
Cells Generate Neurons and Oligodendro-
cytes In Vivo
(A) Immunostaining of dissociated SVZ cells
after 24 hr AAV5-GFP infection for PDGFRa
(red) and GFP (green). The merged image
shows that all AAV5-infected GFP+cells ex-
press the PDGFRa. See Figure S4 for addi-
(B–E) Mice were treated with Ara-C for 6 days
to eliminate C and A cells. Twenty-four hours
after Ara-C treatment, the animals were in-
to survive for 12 days.
(B) Section of OB immunostained for GFP
showing numerous GFP+cells.
(C)Confocal optical section of the RMS coim-
munostained for doublecortin (red) and GFP
(green). The arrow indicates a GFP/double-
(D–E) Confocal optical section of OB coim-
munostained for doublecortin (red) and GFP
(green). The arrow indicates a GFP/double-
cortin+neuron in (D) granule cell layer or (E)
a periglomerular cell. Scale bar = 20 mm.
(F) Section of CC of a mouse 21 days after co-
infection with AAV5-GFP and RCAS-AP
showing immunostaining for GFP in brown
and NBT/BCIP histochemistry for AP in pur-
ple. Note the presence of a GFP+/AP+cell
with characteristic oligodendrocyte morphol-
lateral ventricle for 6 days. One hundred percent of the
PDGF-AA infused mice(n=5) developed atypical hyper-
ventricle (Figure 6A). The hyperplastic nodules showed
infiltrating cells invading surrounding tissues including
the striatum, CC, and/or septum (Figure 6A), and some
also grew inside the lateral ventricles. Furthermore,
they contained a large proportion of proliferating cells
as determined byBrdUincorporation (Figure 6B).Exam-
ination of hematoxylin and eosin-stained sections re-
vealed a heterogeneous population of cells that varied
in cytoarchitecture from polygonal cells with short stout
processes to more fusiform cells with delicate pro-
cesses. The cells within the hyperplastic nodules had
a high nuclear to cytoplasmic ratio and displayed nu-
clear pleiomorphism and hyperchromasia.
To ascertain whether the atypical hyperplasias were
derived from B cells or their immediate progeny, we la-
beled a subset of SVZ B cells by infection with RCAS-
GFP virus in GFAP-Tva mice, immediately prior to the
onset of PDGF-AA infusion. Many cells in the hyperpla-
sias were GFP labeled, indicating that they were derived
from SVZ B cells (Figure 6C). Since all the mice studied
(n = 6) had hyperplasias that included both positive
and negative cells, we conclude that these growths
have a polyclonal origin, and it is possible that other
PDGFRa+cells contribute to their growth.
Immunohistochemistry was performed for a variety of
cell type-specific markers to further characterize the
cells comprisingtheatypical hyperplasias.Asexpected,
the cells showed robust expression of the PDGFRa
(Figure S5A). Interestingly, the majority of cells within
the hyperplastic nodule did not express the normal
markers of cells in the SVZ lineage, such as Dlx2, PSA-
NCAM, or doublecortin, suggesting that the cells were
no longer undergoing their normal differentiation pro-
gram. Only a few cells expressed the B cell marker
GFAP, but based on morphology they appeared to be
a subpopulation of the hyperproliferating cells rather
than reactive astrocytes (Figure S5B). Cells in the hyper-
plastic nodule did express Nestin, and the majority of
cells stained positively for Olig2 (Figure 6D), which is ex-
pressed by OPCs and isalso a marker ofhuman gliomas
of all subtypes (Lu et al., 2001).
While examining marker expression in the hyperpla-
sias, we noted a striking absence of A cells close to
the nodule. This suggested that activation of the PDGF
pathway interferes with the ability of B cells to generate
neurons. To address this, we infused PDGF-AA into the
lateral ventricle for 6 days at a higher concentration and
faster flow rate (10 ng/ml; 1 ml/hr) to provide enough
PDGF-AA to stimulate a larger population of B cells in
the SVZ and not just adjacent to the infusion cannula.
Mice were given a single injection of BrdU 1 hr prior to
a 7.7-fold reduction in the number of proliferating neuro-
hibited neuroblast production and resulted in the
formation of large atypical hyperplasias.
Removal of PDGF-AA Results in Regression
of Hyperplasias and Increased Oligodendrocyte
Next, we determined whether the hyperproliferative
cells continued to require exogenous PDGF-AA for their
proliferation and survival. We infused PDGF-AA into the
lateral ventricle for 14 days and waited 21 days before
sacrificing the animals. Strikingly, the SVZ architecture
of the PDGF-infused animals looked comparable to
that of saline infused controls, indicating a complete re-
gression of the nodule upon growth factor withdrawal
Figure 6. PDGF Induces the Hyperprolifera-
tion of B cells Resulting in the Formation of
(A–C) Six day infusion of vehicle or PDGF-A
(40 ng/day) into the lateral ventricle.
(A) Hematoxylin and eosin-stained sections
of vehicle-infused (left panel) or PDGF-in-
fused brains (two right panels).
(B) BrdU immunostaining of vehicle (left
panel) or PDGF-infused brain (right panel).
Mice received a single injection of BrdU 1 hr
prior to sacrifice.
(C) Photomicrograph of SVZ of mice infected
with RCAS-GFP virus at the time of initiation
of PDGF infusion showing that a subpopula-
tion of the cells in the tumor-like growths
are derived from SVZ B cells.
(D) Section of hyperplasia coimmunostained
for Olig2 (green) and nestin (red). The right
panel shows the merge of the green and red
panels showing that most, but not all, cells
are double labeled.
PDGFRa+B Cells in the SVZ Are Adult Neural Stem Cells
AA, we infected the SVZ with RCAS-GFP virus immedi-
ately prior to a 6 day infusion of high concentration
PDGF-AA into the lateral ventricle and analyzed brains
21 days latter. GFP-positive neurons were detected in
the OBs of both saline-infused and PDGF-infused
mice, however the PDGF-infused mice had significantly
fewer (99.6 6 13.4) OB cells than vehicle-infused mice
(191.3 6 26.5, p = 0.005; Figures 8B and 8E). In contrast,
there were significantly more GFP-positive cells in the
CC of the PDGF-infused animals (178 6 71.3) than in
the vehicle infused mice (92.7 6 38, p = 0.004; Figures
8C–8E). These cells had the typical branched morphol-
ogy of nonmyelinating and myelinating oligodendro-
cytes. This provides further evidence that the PDGFRa+
B cells also generate oligodendrocytes and suggests
that PDGF stimulates oligodendrocyte production by
PDGFRa Signaling Is Required for SVZ
Oligodendrogenesis but Not for Neurogenesis
To further elucidate the role of PDGFRa signaling in the
SVZ, we performed a loss-of-function analysis using
Cre-mediated gene ablation in PDGFRaFl/Flmice. These
mice were crossed to the Z/EG reporter strain (Novak
et al., 2000) that expresses GFP upon Cre-mediated re-
combination to enable permanent GFP labeling of cells
that lose PDGFRa. Adeno-Cre infection was targeted
to a small subpopulation of radial glia in neonatal (P0)
mice (Merkle et al., 2004). These cells function as the
progenitors of adult neural stem cells. PDGFRaFlox/+;
Z/EG and PDGFRa+/+; Z/EG littermates were infected
as controls. The animals were sacrificed at 8 weeks of
age. While GFP+neurons were detected at similar num-
bers in the olfactory bulbs of PDGFRaFlox/Flox(155.8 6
34.8) and control mice (181.4 6 36.3; Figure 8F), there
was a 19-fold reduction (p = 0.044) in the number of
GFP+oligodendrocytes (Figure 8G) present in the CC
of PDGFRaFlox/Floxmice (n = 4) compared to control
mice (n = 4).
We have identified a subpopulation of PDGFRa-ex-
pressing B cells in the adult murine SVZ and have dem-
onstrated that these cells function as progenitors of
neurons and oligodendrocytes in vivo. We describe
the effects of PDGF stimulation on neurosphere forma-
tion in vitro and show that PDGFRa+B cells act as pri-
mary progenitors during SVZ regeneration in vivo after
elimination of transit-amplifying precursors and migrat-
ing neuroblasts. Furthermore, we show that increased
PDGF signaling in these cells stimulates their prolifera-
tion and blocks their ability to give rise to differentiated
progeny, causing them to form tumor-like growths re-
sembling astrocytomas. Interestingly, we also found
that similar PDGFRa-expressing astrocytes exist in the
adult human SVZ.
Identification of PDGFRa+SVZ Type B Cells
in the Adult Brain
Several lines of evidence indicate that oligodendrocytes
are produced from theadult SVZ.Studies ofdemyelinat-
ing lesions ofthe CC suggest thatprogenitors in theSVZ
Figure 7. PDGF Arrests Neuroblast Forma-
(A and B) Coronal sections of SVZ from ani-
mals labeled with RCAS-GFP before 6 day in-
fusion with (A) PDGF-A or (B) vehicle showing
immunostaining for GFP (green) and double-
cortin (red). Note the increased number of
GFP+cells and absence of doublecortin+
cells in the SVZ of PDGF-infused mice.
(C) Histogram showing a 7.7-fold decrease in
the number (mean 6 SEM) of doublecortin+
neuroblasts in the SVZ of PDGF infused
mice (n = 3; p < 0.001).
produce oligodendrocytes after injury (Nait-Oumesmar
et al., 1999) and that PDGF promotes this process (La-
chapelle et al., 2002). We have recently determined
that B cells are the primary progenitors of SVZ-derived
oligodendrocytes (Menn et al., 2006). PDGF has long
been recognized as a mitogen for OPCs that gives rise
to oligodendrocytes and type 2 astrocytes in vitro
(Raff et al., 1983; Ffrench-Constant et al., 1986).
Although OPCs have been viewed as glial-restricted
progenitors, it was recently demonstrated that they
can be reprogrammed to a neural stem cell-like cell
(NSLC) in vitro after first being induced to form type 2
astrocytes (Kondo and Raff, 2000). However, the rele-
vance of this finding to in vivo neurogenesis was not
Our analysis of PDGF receptor expression in the adult
SVZ revealed for the first time the presence of
a PDGFRa-expressing GFAP+B cell population present
in both the murine and human brain. Furthermore, we
demonstrate that these PDGFRa+B cells are not only
precursors of oligodendrocytes, but also of neurons.
Depleting SVZ C and A cells and labeling the remaining
PDGFRa+B cells with AAV5-GFP resulted in many la-
beled granule and periglomerular neurons. In addition,
by coinfecting the SVZ with RCAS-AP virus and AAV5-
GFP, we showed that PDGFRa+B cells in the SVZ also
generate oligodendrocytes. The PDGFRa is commonly
used as a marker of OPCs or glial-restricted progenitors
(GRPs) in the CNS (Grinspan and Franceschini, 1995;
Redwine et al., 1997). Our work demonstrates that these
PDGFRa+B cells are not GRPs but instead function as
the primary progenitors of both oligodendrocytes and
new neurons produced throughout adult life. A common
progenitor population of neurons and oligodendrocytes
also exists during development. Oligodendrocytes and
neurons are derived from the same population of pro-
genitors in the ventral region of the embryonic spinal
cord (Lu et al., 2002; Takebayashi et al., 2002; Zhou
and Anderson, 2002) and in the developing cerebral cor-
tex (He et al., 2001). Furthermore, it has recently been
shown that PDGF-responsive progenitors capable of
generating multipotent neurospheres can be isolated
from the embryonic medial ganglionic eminence (Choj-
nacki and Weiss, 2004).
Figure 8. PDGF Infusion Results in Increased
Production of Oligodendrocytes, and De-
creased Production of Neurons from the
SVZ and PDGFRa Is Required for SVZ-De-
(A) H&E-stained section of brain immediately
after (left) and 21 days after (right) 14 day
PDGF infusion. No atypical hyperplasia is ob-
served in the SVZ 21 days after infusion.
(B–E) Mice were infected with RCAS-GFP vi-
rus immediately prior to 6 day infusion of
high concentration PDGF followed by a 21
day recovery period.
(B) IF for GFP (green) on the CC of vehicle-in-
fused (left) and PDGF-infused (right) mice.
(C) Example of characteristic morphology of
GFP+cells in the CC.
(D) GFP immunostaining (green) of olfactory
bulb of vehicle-infused (left) and PDGF-in-
(E) Histogram showing the number (mean 6
SEM) of GFP+cells in the CC (total cells
counted) and OB (cells per section) of vehi-
cle-infused (n = 4) or PDGF-infused (n = 4)
mice. Note the decreased number of neurons
(p = 0.005) and increased number of oligo-
dendrocytes (p = 0.004) in the PDGF-infused
oligodendrogenesis but not neurogenesis.
PDGFRaFlox/Flox; Z/EG mice and their litter-
mates were infected with Adeno-Cre virus at
population of stem cells.
(F) Histogram showing the number (mean 6
SEM) of GFP+OB neurons in PDGFRaFlox/Flox
(n = 4) and control mice (n = 4).
(G) Histogram showing the number (mean 6
SEM) of GFP+oligodendrocytes in the CC
mice and their litter-
mate controls. Note the reduction in the
number of GFP+oligodendrocytes in the
PDGFRaFlox/Floxmice (p = 0.044).
PDGFRa+B Cells in the SVZ Are Adult Neural Stem Cells
Our data suggest that PDGF signaling may be in-
volved in the normal regulation of oligodendrogenesis
and neurogenesis in the SVZ. We confirmed the pres-
ence of PDGF-A and to a lesser extent PDGF-B ligand
in the SVZ by Western blot, and cells actively signaling
from the PDGFRa were revealed by immunostaining
for phospho-PDGFRa. Analysis of PDGFRaFl/Fl; Z/EG
animals demonstrated that the PDGFRa is required for
SVZ-derived oligodendrogenesis, and prelabeling B
cells with RCAS-GFP demonstrated that PDGF infusion
increased oligodendrocyte production from the SVZ. In
contrast, loss of the PDGFRa did not interfere with the
production of neurons, and PDGF infusion blocked neu-
roblast production during infusion and resulted in de-
creased numbers of GFP+neurons present in the OB
21 days after treatment. Together, these data are con-
sistent with a role for PDGF signaling in balancing neu-
ron and oligodendrocyte production from PDGFRa+
SVZ B cells. The robust expression of the bHLH tran-
scription factor Olig2 by the cells responding to PDGF
stimulation suggests a mechanism for this regulation.
The Olig family of transcription factors is required for ol-
igodendrocyte development (Lu et al., 2002; Zhou and
Anderson, 2002). Olig2 repressor function is sufficient
to repress neuronal differentiation in the postnatal (Mar-
shall et al., 2005) and adult SVZ (Hack et al., 2005). Thus,
PDGF may function upstream of Olig2 in regulating fate
choice in the SVZ. It will be important to determine the
source of PDGF in vivo.
In contrast to our findings, previous in vitro studies
addressing the effects of PDGF stimulation on embry-
onic CNS stem/progenitor cells indicate that PDGF di-
rects neuronal differentiation from these cells (Johe
that PDGF increased proliferation of immature neurons
and delayed their differentiation (Erlandsson et al.,
2001). This discrepancy could be explained by differ-
ences in growth factor responsiveness between embry-
onic and adult cells. It is also likely that cells in vivo
encounter a complex network of secreted, ECM-associ-
ated, and cell surface-associated factors that in combi-
nation elicit responses different to that of any single
factor in vitro. Our findings that PDGF alone does not
support neurosphere growth but robustly stimulates B
cell proliferation in vivo are consistent with this notion.
PDGF Blocks the Differentiation of SVZ Stem Cell
Progeny and Induces the Formation of Tumor-like
The ‘‘blocked differentiation’’ model of cancer proposes
ity to differentiate, holding the cell in a proliferative state
(Buick et al., 1979). Human gliomas often express the
progenitor marker Nestin (Tohyama et al., 1992), sup-
porting the notion that the tumor cells do not fully differ-
entiate. Moreover, differentiation state is one criteria
used to distinguish among low-grade astrocytoma, ana-
plastic astrocytoma, and GBM. Previous studies also
suggested that blocking the differentiation of postnatal
glial progenitor cells by increased PDGF signaling con-
tributed to the formation of oligodendrogliomas in
gest that an uncoupling of proliferation and differentia-
tion by increased PDGF signaling may be important for
Activation of the PDGF pathway is a common event in
gliomagenesis and has been implicated in tumor initia-
tion, since PDGF/PDGFR overexpression occurs with
equal frequency in both low- and high-grade gliomas
(Varela et al., 2004). Our work provides compelling evi-
dence that activation of the PDGF pathway in adult mu-
rine SVZ stem cells (B cells) is sufficient to induce the
formation of atypical hyperplasias. Importantly, we
show that PDGF stimulation blocks the ability of B cells
to give rise to differentiated progeny, resulting in a dra-
matic reduction in the number of doublecortin+neuro-
blasts in the SVZ. This leads to the accumulation of
a large number of rapidly proliferating cells in the SVZ
that invade the adjacent parenchyma. Interestingly, we
found that most SVZ C and A cells do not normally ex-
press the PDGFRa. Together these data suggest that
PDGF signaling may be involved in the regulation of pri-
mary precursors, being downregulated during neuronal
differentiation but maintained in the early stages oligo-
dendrocyte formation (Hart et al., 1989; Barres et al.,
1992; Ellison and De Vellis, 1994).
We do not propose that the PDGFRa+B cells undergo
neoplastic or malignant transformation upon PDGF infu-
sion. However, increasing the levels of PDGF ligand
available to the B cells is sufficient to induce atypical hy-
perplasias with a similar degree of cellular atypia and
pleomorphism as that found in low- to intermediate-
grade human gliomas. This suggests that these cells
may be particularly susceptible to neoplastic transfor-
mation. Activation of PDGFR signaling may provide
a first step in this process. There is a long-held belief
that cells must accumulate numerous genetic alter-
ations to be transformed. For example, cultured human
astrocytes require overexpression of hTERT, Ras, and
human papillomavirus E6/E7 to form xenograft tumors
(Sonoda et al., 2001). However, the initial growth events
that allow for the accumulations of these mutations are
poorly understood, and new evidence suggests that
physiological levels of oncogene expression may be
sufficient to initiate transformation (Tuveson et al.,
2004). Targeting mutations to the appropriate cell of or-
igin may be crucial for understanding requirements for
transformation. This study identifies a subpopulation
of cells that under epigenetic stimulation can give rise
to large hyperplasias that may serve as a substrate for
accumulation of transforming mutations.
The robust expression of the bHLH transcription fac-
tor Olig2 by a large proportion of the cells responding
to PDGF stimulation is also intriguing. In addition to its
physiological roles in specifying oligodendrocyte devel-
opment, overexpression of Olig2 is also a common
event in gliomas of all grades and classes (Lu et al.,
2001;Ligon etal.,2004),and Olig2hasalso beendirectly
implicated intumorigenesis inother tissues(Wang etal.,
Olig2 is a critical mediator of PDGF-induced hyperprolif-
eration may yield new insights into the putative growth-
promoting properties of this transcription factor.
We have identified a population of PDGFRa+B cells in
the adult SVZ and shown that they give rise to both neu-
rons and oligodendrocytes in vivo. These findings are
significant due to our limited knowledge of surface
markers for neural stem cells. Our data also provide ev-
idence of a link between these PDGFRa+B cells and the
early changes associated with tumor initiation, suggest-
ing they may be targets of neoplastic transformation.
The regression of atypical hyperplasia after PDGF re-
moval described here suggests that inhibition of PDGF
signaling could provide a useful therapy for those glio-
mas in which the pathway is upregulated, especially
given the recovery of the normal architecture after re-
gression of the hyperplasia.
For all adult experiments, we used 2- to 4-month-old CD-1 or GFAP-
Tva mice. For PDGFR loss-of-function experiments, we used P0
PDGFRaFlox/Flox; Z/EG mice. All animal procedures were approved
by the Institutional Animal Care and Use Committee.
Human Postmortem Specimens
Three brains (ages 18, 50, and 80 years) were collected at autopsy.
All specimens were obtained within 12 hr of death from patients
without clinical or postmortem evidence of brain pathology and pro-
pathology specimens was in accordance with the guidelines of the
UCSF Committee on Human Research (Approval # H11170-19113-
Mice wereperfusedwith4% paraformaldehyde, andthe brains were
Vibratome, blocked in 5% goat serum in PBS (except where noted),
and incubated overnight at 4?C in primary antibody. Polyclonal
PDGFRa (required antigen retrieval by boiling in 10 mMcitrate buffer
for 10 min, 1:50 dilution; Spring Biosciences), polyclonal PDGFRa-
phosphoTyr720 (Blocked in 5% donkey serum, 1:50 dilution in
PBS containing 1.5% donkey serum; Santa Cruz Biotechnology),
monoclonal FGF-R1 and 2 (1:500; Chemicon), monoclonal GFAP
(1:1000; Chemicon), polyclonal GFAP (1:500; DAKO), PSA-NCAM
(1:400;AbCys),Doublecortin (1:500+ 0.5% Triton-X 100; Chemicon),
gift of Charles Stiles), O4 (1:100; Chemicon), Tuj1 (1:500; Covance),
and GFP (1:500; Aves Labs). Controls in which primary antibody
was omitted resulted in no immunostaining. Preincubation of the
rabbit polyclonal PDGFRa antibody with blocking peptide resulted
in no immunostaining for this antibody.
For receptor activation analysis, two injections of PDGF-AA (R&D
Systems; 6.6 ng) in vehicle (2 ml) or vehicle (1 mg/ml BSA/0.9% sa-
fused immediately after the last injection. For infusions, PDGF-AA
(40 ng/day for low-concentration and 240 ng/day for high-concen-
tration experiments) in vehicle or vehicle alone was infused into
the lateral ventricle for 6 or 14 days with a miniosmotic pump (Alzet).
At least three mice per group were infused for all experiments.
Two percent Ara-C (Sigma) was infused as previously described
(Doetsch et al., 1999b).
Neurospheres were prepared from the SVZ of adult mice (five mice
per experiment; Doetsch et al.,1999a). Cells were plated at a density
of 25 cells/ml in the presence of bFGF (10 ng/ml), PDGF-AA (20 ng/
ml), or EGF (20 ng/ml) as indicated. Primary neurospheres were
counted 9 days after plating, and secondary neurospheres were
counted 7 days after passaging. Neurosphere differentiation was
performed as described Doetsch et al. (1999a).
0.1ml of AAV5-GFP (1012PFU/ml) was injected into the SVZ of CD-1
mice 24 hr after Ara-C treatment at three coordinates: anterior, lat-
eral, depth (mm) relative to bregma, the midline, and the surface of
the brain, respectively (0, 1.4, 1.6; 0.5, 1.1, 1.7; and 1, 1, 2.3). (See
Supplemental Experimental Procedures for in vivo controls for the
specificity of AAV5-GFP infection in PDGFRa-expressing cells).
RCAS Infection with PDGF Infusion and Analysis
0.2 ml of RCAS-GFP virus (w2 3 107PFU/ml) was injected into the
SVZ of GFAP-Tva mice at three coordinates: (0, 1.4, 1.6; 0.5, 1.1,
1.7; and 1, 1, 2.3). PDGF or vehicle was infused at the time of retro-
virus injection,and mice weresacrificed 6 days (n = 3)or27days (n=
4)after infection. For quantification of GFP+cells 21 days after pump
removal, 50 mm coronal vibratome sections from four saline-infused
and four PDGF-infused mice, were immunostained for GFP. The to-
tal number of greencells present inthe CC wascountedin threesets
of six sections spanning the SVZ from anterior to posterior. For
quantification of GFP+cells present in the olfactory bulb, all green
cells present in three equivalent OB sections were counted per
Coinfection with AAV5-GFP and RCAS-AP
0.2 ml of a 1:1 mixture of AAV5-GFP (1012PFU/ml) and RCAS-AP vi-
rus (w107PFU/ml) was injected into the SVZ of GFAP-Tva mice at
three coordinates: (0, 1.4, 1.6; 0.5, 1.1, 1.7; and 1, 1, 2.3). Mice
were sacrificed 21 days after infection. Mice were perfused with
4%paraformaldehydeandpostfixed overnight. The brainswerecry-
sections) on a cryostat. Endogenous AP was inactivated by heating
at 65?C in PBS for 30 min. The sections were then washed in AP de-
tectionbuffer(100 mMTris-HCl[pH9.5], 100mMNaCl,5 mMMgCl2)
then incubated in AP buffer containing NBT/BCIP (200 ml/10ml;
Roche) overnight at room temperature. The reaction was halted by
washing in PBS and IHC for GFP was performed.
P0 Infection of PDGFRaFl/Fl; Z/EG Mice with AdCre
P0 infections were performed as previously described (Merkle et al.,
2004). Brains were analyzed at 8 weeks of age. For quantification of
GFP+cells, 50 mm coronal vibratome sections from four control and
four PDGFRaFl/Flmice were immunostained for GFP. The total num-
ber of green cells present in the CC was counted in six sections
spanning the SVZ from anterior to posterior. For quantitation of
GFP+neurons, all green cells present in three equivalent OB sec-
tions were counted per mouse.
tase inhibitors. SDS-PAGE was performed using 20 mg total protein
per lane. Western blotting was performed using the SuperSignal
West Pico kit (Pierce Biotechnology) as per manufacturer’s instruc-
tions. The primary antibodies used were as follows: PDGF-A (1:200;
Santa Cruz), PDGF-B (1:200; Santa Cruz); rabbit anti-PDGFRa
(1:200; Spring Bioscience); phospho-PDGFRa (1:100; Santa Cruz).
HD-32116. E.L.J. was supported by American Brain Tumor Associa-
tion and American Cancer Society Fellowships. A.A.-B. holds the
Heather and Melanie Muss Endowed Chair. The authors would
also like to acknowledge John and Frances Bowes for their support
of this research. We are grateful to C. Stiles and D. Rowitch for pro-
viding us with the Olig2 antibody and B. Davidson for providing
AAV5-GFP. The PDGFRaFl/Flmice were a generous gift from P. Sor-
iano, and GFAP-Tva mice were a kind gift from E. Holland. We thank
eral discussion and support of this work. We also thank three anon-
ymous reviewers for their comments, which helped improve this
PDGFRa+B Cells in the SVZ Are Adult Neural Stem Cells
Received: January 4, 2006
Revised: April 19, 2006
Accepted: June 9, 2006
Published: July 19, 2006
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PDGFRa+B Cells in the SVZ Are Adult Neural Stem Cells