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CONCISE REVIEW
Stem Cell Therapy for Erectile Dysfunction:
A Critical Review
Ching-Shwun Lin,
1
Zhong-Cheng Xin,
2
Zhong Wang,
3
Chunhua Deng,
4
Yun-Ching Huang,
5
Guiting Lin,
1
and Tom F. Lue
1
Erectile dysfunction (ED) is a prevailing health problem that seriously impacts quality of life. Current treatment
options are less effective for patients having cavernous nerve (CN) injury or diabetes mellitus-related ED. These
2 types of ED are thus the main focus of past and current stem cell (SC) therapy studies. In a total of 16 studies so
far, rats were exclusively used as disease models and SCs were mostly derived from bone marrow, adipose
tissue, or skeletal muscle. For tracking, SCs were labeled with LacZ, green fluorescent protein, 4’,6-diamidino-2-
phenylindole, DiI, bromodeoxyuridine, or 5-ethynyl-2-deoxyuridine, some of which might have led to data
misinterpretation. SC transplantation was done exclusively by intracavernous (IC) injection, which has been
recently shown to have systemic effects. Functional assessment was done exclusively by measuring increases of
IC pressure during electrostimulation of CN. Histological assessment usually focused on endothelial, smooth
muscle, and CN contents in the penis. In general, favorable outcomes have been obtained in all trials so far,
although whether SCs had differentiated into specific cell lineages remains controversial. Recent studies have
shown that intracavernously injected SCs rapidly escaped the penis and homed into bone marrow. This could
perhaps explain why intracavernously injected SCs had systemic antidiabetic effects and prolonged anti-ED
effects. These hypotheses and the differentiation-versus-paracrine debate require further investigation.
Introduction
Erectile dysfunction (ED) is a prevailing health problem
that seriously impacts the quality of life of men and their
spouses or partners [1]. In the United States alone, an esti-
mated 30 million men suffer from different degrees of ED [2].
The majority of ED patients can now be treated satisfactorily
with phosphodiesterase type-5 (PDE5) inhibitors, such as
sildenafil (Viagra; Pfizer Inc., New York, NY), vardenafil
(Levitra; Bayer AG, Leverkusen, Germany), and tadalafil
(Cialis; Lily-ICOS, Indianapolis, IN) [3]. However, PDE5 in-
hibitors are strictly contraindicated with concomitant nitrates
because of the danger of their synergistic hypotensive effects
[3]. PDE5 inhibitors are known to cause a variety of adverse
side effects that may reduce their suitability for some patients
[3]. More importantly, PDE5 inhibitors are only partially ef-
fective in treating certain types of ED including those associ-
ated with diabetes mellitus (DM) and surgery-induced CN
injuries (mainly due to radical prostatectomy) [3]. As such,
alternative treatments, particularly those that can treat the
underlying disease process of ED, are highly desirable. In this
regard, one of the strategies currently being evaluated is stem
cell (SC) therapy.
Rationale for Using SC Therapy
Despite being conventionally classified in the urinary
system, the penis is in fact a vascular organ. The penile
corpora cavernosa are composed of sinusoids that are lined
with a single layer of endothelial cells (ECs) and are sur-
rounded by multiple layers of circular and longitudinal
cavernous smooth muscle cells (CSMCs) (Fig. 1). In the
flaccid penis, CSMCs are in a contracted state and maintain a
small amount of blood flow in and out of the sinusoids.
When a man is sexually aroused, nitric oxide (NO) is re-
leased from terminal fibers of cavernous nerves (CNs) and
enters the neighboring CSMCs, resulting in CSMC relaxa-
tion. Blood rushes in as a consequence and engorges the
sinusoids, leading to the initial phase of penile erection.
Maintenance of erection, that is, continued CSMC relaxation,
is believed to derive from additional NO release from the
sinusoidal ECs. The further sinusoidal engorgement causes
1
Knuppe Molecular Urology Laboratory, Department of Urology, School of Medicine, University of California, San Francisco, California.
2
Andrology Center, Peking University First Hospital, Beijing, China.
3
Department of Urology, Ninth People’s Hospital Affiliated to Medical College of Shanghai Jiao-Tong University, Shanghai, China.
4
Department of Urology, First Affiliated Hospital of Sun Yet-Sen University, Guangzhou, China.
5
Division of Urology, Department of Surgery, Chang Gung Memorial Hospital, Chia-Yi, Taiwan, China.
STEM CELLS AND DEVELOPMENT
Volume 21, Number 3, 2012
Mary Ann Liebert, Inc.
DOI: 10.1089/scd.2011.0303
343
the compression of venules located between the trabeculae
and the tunica albuginea, resulting in nearly total occlusion
of venous outflow. This combination of sinusoidal engorge-
ment and venous occlusion leads to the full erection of the
penis [4]. Thus, structurally, the key components of erection
are the ECs, CSMCs, and CN [more specifically, neuronal
nitric oxide synthase (nNOS)-positive nerves], and func-
tionally, accurate interactions among these 3 constituents are
critical. In different types of ED, these structures and/or in-
teractions are often altered, as described briefly in the fol-
lowing text.
The purpose of a radical prostatectomy is to remove the
cancerous prostate; however, the procedure can damage the
CN, which run alongside the prostate [5]. The short-term
consequence of CN injury is neurogenic ED, which is re-
versible, whereas the long-term consequence is atrophy of
CSMCs, which can lead to irreversible ED [6]. In men with
DM, the high blood glucose also causes reduction of CN, EC,
and CSMC contents [7]. In men with hyperlipidemia, im-
paired EC function is well documented, but whether it is
accompanied with structural changes is not as certain [7]. In
our recent studies we have quantified structural changes in
the penises of rats with CN injury, type 2 DM (T2DM), and
hyperlipidemia [8–10]; these changes are summarized in Fig.
2. Note that hyperlipidemia was induced by high-fat feeding,
and T2DM by high-fat feeding plus streptozotocin injection.
High-fat feeding causes increased CSM content in the rat
[10], and this may have compensated the CSM loss that is
usually associated with DM, resulting in the preservation of
CSM in the T2DM rats [8].
SCs are believed to be able to differentiate into various cell
types including ECs, smooth muscle cells (SMCs), Schwann
cells, and neurons [11]. Therefore, SC therapy for ED was
originally based on the hypothesis that transplantation of
SCs into penis through intracavernous (IC) injection might
replenish the depleted EC and/or CSMC pools. However, in
published studies whether cellular differentiation occurred
in the treated animals is controversial and will be discussed
in more details in a later section under Histological assessment.
On the other hand, there were also considerations that IC
transplanted SCs might encourage the regeneration of the
host’s own ECs and CSMCs or might restore proper inter-
actions between ECs and SMCs. In other words, paracrine
actions as opposed to cellular differentiation are responsible
for SCs’ therapeutic efficacy, and based on several of our
studies in both ED and non-ED fields, this seems to be the
main mechanism [12].
With regard to restoring damaged CN, it should be
pointed out that the nerve cell bodies are located in the major
pelvic ganglia (MPG) some distance away from the penis. As
such, in the first SC-for-ED study, SCs were injected into the
MPG of one group of CN injury rats, and the results showed
improved erectile function [13]. Importantly, in the same
study, SCs were also injected into the penis of another group
of CN injury rats, and similar results were obtained. This
finding is cited as basis for using IC injection in the next SCs
for CN injury study [14], and since then this injection method
has been apparently accepted as legitimate, although no one
knew how it worked until now (see Stem cell transplantation
section).
Current Status
There are currently 16 publications in the field of SC
therapy for ED (Table 1). Five additional publications tested
SC transplantation without assessing erectile function and
are thus not SC therapy per se.
Clinical Trial
A clinical trial of SC therapy for ED has been carried out in
Korea [25]. In this study, 7 T2DM men ranging from 57 to 87
years of age were each treated with IC injection of 15 million
allogeneic umbilical cord blood SCs. Morning erection was
regained in 3 patients within 1 month and in 6 patients
within 3 months. However, despite having increased penile
rigidity, none was able to achieve vaginal penetration unless
aided by taking sildenafil before coitus. During 11-month
follow-up, only one treated subject maintained erection suf-
ficient for coitus. Interestingly, SC therapy appears to have
antidiabetes effects as all treated subjects except the oldest
had reduced blood glucose and glycosylated hemoglobin
levels. These results provide further evidence not only for the
frequently observed antidiabetes effects of SCs but also for
IC injection being a systemic application as we recently re-
ported (see Stem cell transplantation section). Also noteworthy
is that, despite being allogeneic in the absence of immuno-
suppressant, SC transplantation did not cause any adverse
effects, thus providing further evidence for the immuno-
suppressive effects of SCs.
Preclinical Trials
A typical preclinical trial of SC therapy for ED is sche-
matically depicted in Fig. 3. It involves the isolation,
FIG. 1. Penile histology of a
normal rat. The cross-section
of rat penis was stained with
Alexa-488-conjugated phal-
loidin, which detects smooth
muscle (green stain), with
Alexa-594-conjugated anti-
RECA antibody, which de-
tects endothelium (red stain),
and with DAPI, which detects
cell nuclei (blue stain). DAPI,
4’,6-diamidino-2-phenylindole;
CC, corpus cavernosum; Pha,
phalloidin; RECA, rat en-
dothelial cell antigen.
344 LIN ET AL.
cultivation, sorting, and modification of SCs, followed by
labeling them with a cell-tracking agent. The labeled SCs
were then injected into the corpus cavernosum (IC injection)
of an ED animal model. Weeks or months later, the animals
are tested for erectile function, usually by measurement of
increases in intracavernous pressure (ICP) during electro-
stimulation of CN. The animals are then sacrificed for his-
tological assessment of corpus cavernosum and tracking of
injected SCs.
Animal models
Rat is the most commonly used animal in ED research and
was used in all preclinical SC-for-ED studies thus far. Un-
derscoring clinical needs, CN injury and DM are the most
commonly tested disease models. CN injury, tested in 7
studies, was induced by either crush or resection of CN bi-
laterally. T2DM patients outnumber T1DM patients 9 to 1,
yet most experimental studies chose T1DM over T2DM. This
FIG. 2. Histological changes
in the penises of ED rats. Re-
presentative images from
normal rats and indicated ED
models illustrate ED-associated
changes in smooth muscle (left
column), endothelium (middle
column), and nNOS-positive
nerves (right column). Quantifi-
cation of these changes in 9 rats
in each group of rats was done
as described in our previous
studies, and the results are
shown at the bottom.For
smooth muscle and endothe-
lium, the unit on the Y-axis is
number of pixels/high-power
field (HPF). For nNOS, the unit
is number/HPF, where ‘‘num-
ber’’ refers to the red dots seen in
the images. Note that all tissue
sections were costained with
DAPI for the visualization of
cell nuclei (blue). ED, erectile
dysfunction; nNOS, neuronal
nitric oxide synthase.
STEM CELL THERAPY FOR ERECTILE DYSFUNCTION 345
Table 1. Erectile Dysfunction-Related Stem Cell Transplantation Studies
Publication year/first
author
Animal
model/patients Cell type
Transplantation
method/cell
number
Cell tracking
method
Assessment
time point
ICP
assessment
Penile histological
assessment
2003/Deng [15] 25-month-old rats Rat eNOS-BMSC IC/500,000 None 1 week No IHC for eNOS
2004/Bochinski [13] CN injury rats Rat BDNF-ESC IC or Intra-MPG/
500,000
GFP label 3 months Yes NADPH, IHC for TH,
neurofilament
2006/Kim [14] CN resection rats Rat SkMDC IC/1,000,000 LacZ label 2 and 4 weeks Yes IHC for PGP9.5
2007/Bivalacqua [16] 25-month-old rats Rat eNOS-BMSC IC/500,000 LacZ label 1 and 2 weeks Yes IF for SMA, vWF,
eNOS, PECAM,
SM-MHC, CD45
2007/Song [17] 10-week-old rats Human v-myc-BMSC IC/1,000,000 IF for human
nucleus
2 weeks No IF for desmin, calponin,
SMA, vWF, CD31
2008/Song [18] 10-week-old rats Human v-myc-NCSC IC/1,000,000 IF for human
nucleus
2 weeks No IF for desmin, calponin,
SMA, vWF, CD31
2008/Nolazco [19] 20-month-old rats Mouse SkMSC IC/500,000–
1,000,000
DAPI label 2 and 4 weeks Yes IHC for CD34, Sca1
2009/Fall [20] CN resection rats Rat BMMN cells IC/10
7
PKH-26 label 3 and 5 weeks Yes IHC for SMA, CD31,
vimentin
2009/Song [21] 6-month-old rats Rat fetal brain stem cells IC/1,000,000 GFP label 2, 4, and 6 weeks No IF for SMA, calponin,
VEGF
2010/Garcia [22] Zucker T2DM rats Autologous ADSC IC/1,000,000 BrdU label 3 weeks Yes IHC for RECA, nNOS
2010/Huang [23] Hyper-lipidemic rats Autologous ADSC IC/2,000,000 EdU label 3, 14, 28 days for
histology.
28 days for ICP
Yes IF for RECA, nNOS,
SMA
2010/Abdel Aziz [24] 2 to 2.5-year-old rats Rat BMSC IC/1,000,000 GFP label 3 and 4 weeks; 3
and 4 months
Yes HE stain
2010/Bahk [25] T2DM patients Umbilical cord blood
stem cells
IC/15,000,000 None Up to 11 months No None
2010/Albersen [26] CN injury rats Autologous ADSC and
lysate
IC/1,000,000 EdU label 4 weeks Yes IF for nNOS, SMA,
b-III-tubulin
2010/Kendirci [27] CN injury rats P75-selected rat BMSC IC/500,000 GFP transgene 4 weeks Yes IF for GFP
2011/Qiu [28] STZ T1DM rats Rat BMSC IC/400,000 DiI label 4 weeks Yes IF for calponin, SMA,
vWF, CD31
2011/Qiu [29] STZ T1DM rats Rat VEGF-BMSC IC/500,000 DiI label 4 weeks Yes IF for SMA, CD31
2011/Gou [30] STZ T1DM rats VEGF-transfected EPC IC/2,000,000 DAPI 3 weeks Yes IHC for CD34
2011/Lin [31] CN resection rats ADSC-seeded adipose
matrix
Nerve grafts EdU label 3 months Yes None
2011/Lin [32] CN resection rats Rat ADSC IC/1,000,000 EdU label 2 and 7 days No None
2011/Fandel [33] CN resection rats Autologous ADSC IC/2,000,000 EdU label 1, 3, 7, and 28
days
Yes IF for nNOS, S100,
SDF-1. Trichrome
ICP, intracavernous pressure; eNOS, endothelial nitric oxide synthase; CN, cavernous nerve; BMSC, bone marrow stem cells; IC, intracavernous; IHC, immunohistochemistry; BDNF, brain-derived
neurotropic factor; ESC, embryonic stem cell; MPG, major pelvic ganglia; TH, tyrosine hydroxylase; SkMDC, skeletal muscle-derived cells; PGP, protein gene product; IF, immunofluorescence; vWF, von
Willebrand factor; PECAM, platelet endothelial cell adhesion molecule; SM-MHC, smooth muscle myosin heavy chain; DAPI, 4’,6-diamidino-2-phenylindole; BMMN, bone marrow mononuclear; T2DM,
type 2 diabetes mellitus; ADSC, adipose-derived stem cell; EdU, 5-ethynyl-2-deoxyuridine; BrdU, 5-bromo-2-deoxyuridine; VEGF, vascular endothelial growth factor; RECA, rat endothelial cell antigen;
nNOS, neuronal nitric oxide synthase; HE, hematoxylin and eosin; SMA, smooth muscle actin; STZ, streptozotocin; DiI, one of several dialkylcarbocyanines; EPC, endothelial progenitor cells.
346
is due to the fact that T1DM can be easily induced by in-
traperitoneal injection of streptozotocin, whereas T2DM is
more difficult to induce or requires the purchase of costly
genetically modified animals. So, in SC-for ED field, T1DM
model was used in 3 studies and T2DM in 1. Aging-related
ED is largely manageable with PDE5 inhibitors, so SC
studies in this category (a total of 3) were most likely moti-
vated by not requiring any special treatment on the animal.
Finally, hyperlipidemia-associated ED, which requires feed-
ing animals with costly high-fat diet, was used in one study.
Stem cells
Types of SCs that have been used in experimental ED
treatment (number of studies in brackets) are bone marrow
(6), adipose (5), skeletal muscle (2), embryonic (1), endothe-
lial progenitor (1), and umbilical cord blood (1, a clinical
trial). Some studies have provided reason for choosing 1
particular SC type over the other; however, personal pref-
erences and/or preexisting circumstances (eg, prior experi-
ence with a particular type of SC) probably played bigger
roles than sound scientific rationales did. In any case, the
pros and cons of various types of SCs can be found abun-
dantly elsewhere and will not be discussed here.
Nearly all transplantations were done either autologously
or allogeneically, the single exception being from mouse to
rat. The transplanted cell number averages around 1 million
per recipient rat. Most studies (total of 10) employed un-
modified SCs, whereas others used SCs that were transfected
or fractionated or in combination with other agents. Trans-
fection or fractionation inevitably introduces risk factors (eg,
virus) into the system and/or substantially reduces the
number of treatment cells. Thus, knowing that the majority
of studies employed unmodified SCs with satisfactory
outcomes, the need to use modified or sorted SCs requires
additional evidence.
Cell labeling
A wide variety of methods have been used to monitor the
distribution and survival of transplanted SCs. In SC-ED field,
2 studies by the same group of researchers transplanted
human SCs to rats; therefore, cell tracking was done by the
identification of human-specific nuclear protein. However,
FIG. 3. A schematic repre-
sentation of the experimental
procedures of a typical pre-
clinical stem cell therapy for
ED. The donor rat and recip-
ient rat can be the same
(autologous) or different (al-
logeneic). The isolation and
cultivation of SCs vary from
one type to another. Mod-
ification and sorting of SCs are
desired by some researchers,
although a higher benefit ver-
sus risk ratio has not been
demonstrated. Labeling of
SCs, which is unnecessary if
from a GFP donor rat, usually
incorporates a chemical agent
that can be later detected by
color or fluorescence. Trans-
plantation of the labeled SCs
has so far been conducted
universally by IC injection, as
indicated with the cross-
sectional view of the penis of
an animal whose erectile func-
tion has been compromised by
various means, for example,
CN injury and streptozotocin
injection. Weeks or months
after SC transplantation, the
animals are tested for erectile
function, usually by measure-
ment of increases in intra-
cavernous pressure (ICP)
during electrostimulation of
CN. The animals are then sacrificed for histological assessment of corpus cavernosum and identification of the injected SCs. SCs,
stem cells; GFP, green fluorescence protein; IC, intracavernous; CN, cavernous nerve.
STEM CELL THERAPY FOR ERECTILE DYSFUNCTION 347
these studies did not assess erectile function and thus will
not be discussed further. All other SC-ED studies used cells
labeled with LacZ, 4’,6-diamidino-2-phenylindole (DAPI),
green fluorescent protein (GFP), DiI (also known as PKH-26),
bromodeoxyuridine (BrdU), or 5-ethynyl-2-deoxyuridine
(EdU). Because the accuracy of data interpretation depends
on the reliability of these labeling methods, potential prob-
lems are summarized below.
LacZ is a bacterial gene that encodes b-galactosidase (b-
gal); however, many mammalian cells and tissues contain
endogenous b-gal, making the detection of LacZ-transfected
cells after their transplantation technically challenging [34].
GFP is a protein from jellyfish. However, because of auto-
fluorescence in mammalian tissues, GFP detection can seem
like ‘‘seeing the wood through the trees’’ [35]. DAPI binds to
DNA noncovalently; therefore, it can leak from labeled cells
after transplantation and be adsorbed by host cells, resulting
in false-positive detection [36]. DiI binds to cell membrane
noncovalently and can leak from transplanted cells to host
cells. In addition, because of DiI’s cytotoxicity, transplanted
cell preparations may contain debris of dead cells. Adsorp-
tion of this DiI-labeled debris by host cells can lead to false-
positive identification [37–41]. BrdU is incorporated into
newly synthesized DNA and such labeled cells are detected
with anti-BrdU antibody. However, the immnodetection of
BrdU requires harsh treatment of tissue samples, resulting in
distorted histological images [42]. In addition, the denaturing
treatment can cause loss of antigenicity of cellular proteins,
making it impossible to detect cellular differentiation
through immunohistochemical colocalization of cell type-
specific protein. Even if the protein of interest survives the
denaturing treatment, it is still difficult to identify the BrdU
label with confidence, because its brown color cannot be
easily distinguished from the purplish nuclear stain [43].
EdU is a newer thymidine analog and is detected by a simple
chemical reaction that requires no special tissue treatment
[42]. However, similar to BrdU, if a labeled cell is replicative
after transplantation, its EdU label gets diluted with each
round of cell division. So, long-term detection of trans-
planted cells is possible only if the cells are relatively qui-
escent. In our experience with EdU-labeled adipose-derived
SCs (ADSCs), their detection in transplanted tissue is possi-
ble for at least 5 months after transplantation.
Stem cell transplantation
Before the introduction of PDE5 inhibitors, which are ta-
ken orally, IC injection of erectogenic agents was the most
effective treatment for ED [4]. Indeed, even nowadays pa-
tients who do not respond to or cannot take PDE5 inhibitors
are still prescribed with IC injection of vasodilators. As this
route of drug administration targets the organ of failure di-
rectly, it is commonly believed that IC injection is a locally
applied intervention. However, we have observed that IC
injected growth factors were able to restore erectile function
through repair of damaged CN, whose cell bodies reside in
the MPG [44–46]. Further, in our first SC-for-ED study, we
observed that intracavernously injected SCs could treat CN
injury-related ED [13], and all subsequent studies using dif-
ferent types of SCs confirmed the validity of this injection
method for treating CN injury-related ED. Together, these
data strongly suggest that IC injection is systemic in nature.
In all of our published SC-for-ED studies we reported
difficulties in finding the transplanted SCs in penile tissues
even though the animals clearly demonstrated functional
and structural improvements [13,22,23,26]. In studies pub-
lished by others, intracavernously injected SCs were simi-
larly difficult to find. In one of our studies we examined the
presence of SCs in the penis at 2, 14, and 28 days after IC
injection; the results clearly showed a time-dependent de-
cline of the number of SCs [23]. In our recent studies we
conducted more definitive quantitative analyses and the re-
sults showed that the majority of intracavernously injected
SCs exited the penis within 1 day [32,33]. Further, in 1 of
these 2 studies we showed that intracavernously injected SCs
preferentially traveled to the bone marrow [32]; in the other
we found that intracavernously injected SCs also traveled to
the MPG of CN injury rats, and this appears to be mediated
by upregulated SDF-1 in the MPG [33]. Together, these data
suggest that (i) IC injection is essentially like intravenous (IV)
injection—because of the fact that the cavernous sinusoids
are essentially bundled venules (Fig. 1), (ii) the therapeutic
efficacy of SCs for CN injury is due to SC trafficking to the
MPG, (iii) systemically (IC or IV) applied SCs home-in to
bone marrow, in support of the concept that mesenchymal
SCs originate from bone marrow, and (iv) home-in of SCs to
bone marrow may permit establishment of SC reservoirs for
sustained regenerative and/or repair activities.
Functional assessment
In animal experimentation, the most commonly used
method for functional assessment of erection is measurement
of intracavernous pressure (ICP) during electrostimulation of
CN. This procedure requires laparotomy followed by sacri-
ficing the animals; therefore, it is done near the end (most
commonly at 1 month post-treatment) of a preclinical trial.
As mentioned in the Rationale for Using SC Therapy section
earlier, sexual stimulation triggers CN to release NO, which
then causes CSMC relaxation and sinusoidal engorgement.
In animal experiments, stimulation of CN with electric cur-
rent mimics sexual stimulation and causes an increase of ICP
that can approach systemic blood pressure, depending on
the amperage of the applied electric current. Typically, at
settings of 1.5 mA, 20 Hz, and pulse width 0.2 ms, the elec-
trostimulation causes an increase of ICP (in cmH
2
O) from a
baseline of 20 to around 100 in normal rats. In ED rats, the
rise is usually to around 30, and a successful SC treatment
usually restores the value to about 70.
Histological assessment
At the end of functional assessment, penile tissues are
commonly prepared for examination by immunohisto-
chemistry or immunofluorescence. The purposes of these
examinations are to (i) locate transplanted SCs, (ii) correlate
structural with functional changes, and (iii) identify possible
SC differentiation. Localization of transplanted cells was
discussed earlier under Cell labeling section. Assessment of
structural changes invariably focuses on the 3 key compo-
nents that regulate penile erection, ECs, CSMCs, and CN.
ECs are commonly identified with antibodies against rat
endothelial cell antigen, CD31, endothelial nitric oxide syn-
thase, and/or von Willebrand factor (vWF). CSMCs are most
348 LIN ET AL.
commonly detected with anti-smooth muscle actin (anti-
SMA) antibody. The most functionally relevant marker for
CN is nNOS, as it identifies NO-releasing nerve fibers.
The concept of SC therapy was originally based on the
premise that SCs have the ability to differentiate into var-
ious cell lineages. Thus, most SC therapy studies have
strived to identify such events by checking whether the
labeled SCs express cell type-specific proteins such as CD31
for ECs and SMA for CSMCs. So, it is obvious that the
accurate identification of cell differentiation depends on the
reliability of the SC trait/label, the differentiated cell mar-
ker, and the histological image. As discussed earlier in the
Cell labeling section, with the exception of EdU, cell labels
that have been employed in SC-ED studies cannot be de-
tected with confidence. Moreover, histological images pre-
sented in most SC-ED studies are of low resolution and
thus difficult to judge whether the so-called protein ex-
pression is indeed cellularly localized. In our experience,
seemingly colocalized stains at low magnification often
turned out not to be cellularly associated when viewed at
higher mag nifications (eg, 1,00 0 ·). Thus, it is crucial that
claims of cell differentiation be backed by clearly discern-
able histological images.
Future Directions
As CN injury- and DM-related ED patients are less re-
sponsive to PDE5 inhibitors, these 2 types of ED will con-
tinue to be the main targets for future research. Current CN
injury rat models have been well characterized and are
clinically relevant; they can thus continue to be used for fu-
ture SCs for ED research. On the other hand, current DM
models either are too costly or do not adequately represent
clinical situations. To mitigate these problems, we recently
established an inexpensive T2DM rat model that exhibits ED
symptoms and penile structural changes that resemble those
of T2DM patients [8]. This model can thus generate more
clinically relevant data in future SC-ED studies.
With regard to choosing a particular SC type, it is im-
portant to consider what is most practically applicable in
clinical situations. At present, ADSCs is the only cell type
that can be isolated and autologously transplanted on a
same-day basis. Further, several devices for automated iso-
lation of ADSCs are now commercially available. Thus, in
terms of cost, risks, ethics, expediency, and effectiveness,
ADSCs should compete very favorably. The most significant
risk—promotion of tumor growth—is shared by different
types of SCs and requires further research.
As we have now demonstrated that IC injection is similar
to IV injection [32], it is advisable that future SC-ED studies
examine the systemic distribution of the transplanted SCs.
This of course requires that the cells be labeled with a reliable
tracking dye. In our experience with many commonly em-
ployed dyes such as BrdU, DiI, DAPI, GFP, and EdU, we
have found that labeling with EdU is the easiest and most
reliable. With regard to SC distribution, we advise the ex-
amination of bone marrow as SCs seem to have a tendency to
travel there. What do these SCs do in bone marrow is the
next question that needs to be addressed. Specifically, do
they play any role in terms of short-term and long-term
therapeutic efficacy? Finally, the issue of cellular differenti-
ation versus paracrine action needs to be further investi-
gated. To do so, first, it is important to know that, although
cellular differentiation is a presumed SC property, it does not
have to happen in order for SC to exert therapeutic effects.
With this concept in mind, then, a reliable tracking dye is
used to label the SCs, followed by generating high-quality
high-resolution histological images, and it should be possible
to make accurate and unbiased interpretations.
Acknowledgments
This work was supported by grants from the Arthur
Rock Foundation and the National Institutes of Health
(DK045370).
Author Disclosure Statement
No competing financial interests exist.
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Address correspondence to:
Dr. Ching-Shwun Lin
Knuppe Molecular Urology Laboratory
Department of Urology
School of Medicine
University of California
San Francisco, CA 94143-0738
E-mail: clin@urology.ucsf.edu
Received for publication June 13, 2011
Accepted after revision July 26, 2011
Prepublished on Liebert Instant Online July 27, 2011
STEM CELL THERAPY FOR ERECTILE DYSFUNCTION 351