Adenoviral-Mediated Glial Cell Line–Derived
Neurotrophic Factor Gene Transfer Has a Protective
Effect on Sciatic Nerve Following Constriction-Induced
Spinal Cord Injury
An-Kuo Chou1, Ming-Chang Yang2,3,4, Hung-Pei Tsai5, Chee-Yin Chai6, Ming-Hong Tai7, Aij-Li Kwan5,8*,
Yi-Ren Hong2,3,5*
1 Department of Anesthesiology, Kaohsiung Chang Gung Memorial Hospital and Chang Gung University College of Medicine, Kaohsiung, Taiwan, R.O.C., 2 Department of
Biological Sciences, National Sun Yat-Sen University, Kaohsiung, Taiwan, R.O.C., 3 Department of Biochemistry, Faculty of Medicine, College of Medicine, Kaohsiung
Medical University, Kaohsiung, Taiwan, R.O.C., 4 Laboratory of Medical Research, Kaohsiung Armed Forces General Hospital, Kaohsiung, Taiwan, R.O.C., 5 Graduate Institute
of Medicine, College of Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan, R.O.C., 6 Department of Pathology, Kaohsiung Medical University Chung-Ho Memorial
Hospital, Kaohsiung, Taiwan, R.O.C., 7 Institute of Biomedical Science, National Sun Yat-sen University, Kaohsiung, Taiwan, R.O.C., 8 Department of Neurosurgery,
Kaohsiung Medical University Chung-Ho Memorial Hospital, Kaohsiung, Taiwan, R.O.C.
Abstract
Neuropathic pain due to peripheral nerve injury may be associated with abnormal central nerve activity. Glial cell-linederived neurotrophic factor (GDNF) can help attenuate neuropathic pain in different animal models of nerve injury.
However, whether GDNF can ameliorate neuropathic pain in the spinal cord dorsal horn (SCDH) in constriction-induced
peripheral nerve injury remains unknown. We investigated the therapeutic effects of adenoviral-mediated GDNF on
neuropathic pain behaviors, microglial activation, pro-inflammatory cytokine expression and programmed cell death in a
chronic constriction injury (CCI) nerve injury animal model. In this study, neuropathic pain was produced by CCI on the
ipsilateral SCDH. Mechanical allodynia was examined with von Frey filaments and thermal sensitivity was tested using a
plantar test apparatus post-operatively. Target proteins GDNF-1, GDNFRa-1, MMP2, MMP9, p38, phospho-p38, ED1, IL6, IL1b,
AIF, caspase-9, cleaved caspase-9, caspase-3, cleaved caspase-3, PARP, cleaved PARP, SPECTRIN, cleaved SPECTRIN, Beclin-1,
PKCs, PKCc, iNOS, eNOS and nNOS were detected. Microglial activity was measured by observing changes in
immunoreactivity with OX-42. NeuN and TUNEL staining were used to reveal whether apoptosis was attenuated by
GDNF. Results showed that administrating GDNF began to attenuate both allodynia and thermal hyperalgesia at day 7. CCIrats were found to have lower GDNF and GDNFRa-1 expression compared to controls, and GDNF re-activated their
expression. Also, GDNF significantly down-regulated CCI-induced protein expression except for MMP2, eNOS and nNOS,
indicating that the protective action of GDNF might be associated with anti-inflammation and prohibition of microglia
activation. Immunocytochemistry staining showed that GDNF reduced CCI-induced neuronal apoptosis. In sum, GDNF
enhanced the neurotrophic effect by inhibiting microglia activation and cytokine production via p38 and PKC signaling.
GDNF could be a good therapeutic tool to attenuate programmed cell death, including apoptosis and autophagy,
consequent to CCI-induced peripheral nerve injury.
Citation: Chou A-K, Yang M-C, Tsai H-P, Chai C-Y, Tai M-H, et al. (2014) Adenoviral-Mediated Glial Cell Line–Derived Neurotrophic Factor Gene Transfer Has a
Protective Effect on Sciatic Nerve Following Constriction-Induced Spinal Cord Injury. PLoS ONE 9(3): e92264. doi:10.1371/journal.pone.0092264
Editor: Yael Abreu-Villaça, Universidade do Estado do Rio de Janeiro, Brazil
Received October 22, 2013; Accepted February 20, 2014; Published March 18, 2014
Copyright: ß 2014 Chou et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This study was sponsored by National Science Council-96-2314-B-182A-018-MY2, National Science Council-95-2314-B-182A-151, and Chang Gung
Memorial Hospital grant CMRP83031 to AKC. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the
manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: m835016@cc.kmu.edu.tw (YRH); aijliekwan@yahoo.com.tw (ALK)
lamina III into II in neuronal remodeling in the spinal cord might
result in the development of persistent tactile allodynia [1,2].
Recent studies have demonstrated that C-fibers appear not to
sprout outside their normal laminar distribution after injury [3].
According to current clinical experience, patients with neuropathic
pain and visceral pain commonly have poor response to ordinary
medication, and usually depend on opioid drugs for pain control
[4]. Unfortunately, long-term administration of opiates has wellknown side effects including drug addiction and tolerance,
immunosuppression, and decreased micturition reflex. New
Introduction
Neuropathic pain is caused by lesions or diseases of the
somatosensory system including peripheral nerve injury and
central nerve injury. Spontaneous pain, thermal-mediated hyperalgesia and tactile-evoked allodynia are common neuropathic pain
symptoms following peripheral nerve injury, and significantly
reduce quality of life and functional status. In clinical observation,
neuropathic pain is not confined to the innervation area of the
injured nerve, but also affects the adjacent area innervated by
other intact nerves. Previous data have shown that sprouting from
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GDNF Protects SCDH from Constriction-Induced Cell Death
forming units (pfu) in 100 ml sterile phosphate buffer saline PBS]
were administrated via the triceps brachii muscle of anesthetized
rats using a disposable insulin syringe equipped with a 27-gauge
needle. Injection was performed in a biosafety P2 laboratory, and
the care of animals receiving the adenovirus vectors conformed to
institutional guidelines.
therapeutic approaches such as gene therapy with pain-killer genes
may hold promise for treating such patients.
Glial cell line-derived neurotrophic factor (GDNF) is one of the
GDNF family of ligands (GFLs). GFLs are important for cell
survival, neurite outgrowth, cell differentiation and cell migration,
and GDNF promotes the survival of dopaminergic neurons [5].
Nerve injury downregulated GDNF and its receptor, GDNF
family receptor alpha-1 (GDNFRa-1), on dorsal root ganglia [6].
Continuous injection of GDNF by osmotic pump promotes
regeneration of sensory axons and attenuates neuropathic pain
in animal models of nerve injury [7–9]. GDNF has been used as a
therapy for neurodegenerative diseases such as Parkinson’s disease
[10,11] and amyotrophic lateral sclerosis [12,13]. However, the
underlying molecular mechanism by which GDNF ameliorates
neuropathic pain remains largely unknown. A better understanding of microglial-neuronal interactions in the SCDH will further
our understanding of neural plasticity and may also lead to novel
therapeutics for chronic pain management.
In this study, we used CCI as neuropathic pain model with
adenoviral-mediated GDNF to evaluate the therapeutic effect of
GDNF on peripheral nerve injury-induced neuropathic pain,
analyzing protein expressions and activations in different aspects
including microglia activation (MMP2, MMP9, p38, phosphop38, IL6 and IL1b), caspase-dependent apoptotic markers
(caspase-9, cleaved caspase-9, caspase-3, cleaved caspase-3, PARP,
cleaved PARP), caspase-independent apoptotic markers (AIF,
SPECTRIN and cleaved SPECTRIN), autophagy marker (Beclin1), and CCI-induced proinflammatory markers (PKCs, PKCc,
iNOS, eNOS and nNOS) to determine whether adenoviralmediated GDNF gene therapy can successfully ameliorate the
above gene expression and the different types of associated
programmed cell death.
Tissue preparation
The sciatic nerves were dissected and harvested at 7, 14, or 28
days after the CCI operation and at 28 days after viral injection
paired with the CCI operation (six animals for each time point and
group). The injured side of the sciatic nerve was cut into two
segments from the region of ligation, the adjacent proximal and
distal segments. The uninjured contralateral sciatic nerve in each
animal was used as the control.
Thermal hyperalgesia
Thermal sensitivity of the plantar hind paws was tested
according to Hargreaves’ method [17] using a Plantar Test
Apparatus (Ugo Basile, Comerio, Italy). Rats were placed
unrestrained in individual clear plastic compartments
(11 cm617 cm614 cm). When the rats were stationary and not
attending to the tester or stimulus, an infrared radiant heat source
(180 mW) was applied through a glass floor to the middle of the
plantar surface of the hind paw, between the foot pads. A photocell
automatically stopped the heat source and the timer when the rat
lifted its paw. For transected mice that were not capable of hind
paw plantar placement, the rats were held gently to assist the
plantar placement of the hind paws. Each rat was tested for five
trials on each hind paw, with at least 1 min between trials, and the
order of testing was randomized to minimize windup or avoidance
behaviors. A 20-s maximum cutoff was established to prevent
tissue damage.
Materials and Methods
Animal model
Allodynia test
Male Sprague–Dawley rats weighing (140 to 160 g at the time
of surgery (NSC Animal Center, Taiwan) were fed with standard
lab rodent chow and water ad libitum and housed individually. Rats
were anesthetized with an intraperitoneal (i.p.) injection of sodium
pentobarbital (Nembutal, 50 mg/kg), and CCI to the right sciatic
nerve (SN) was done according to the method of Bennett and Xie
(1988) [14], in which the left common sciatic nerves were exposed
in the left midthigh and loosely ligated with 4-0 silk thread in three
regions at about 1-mm intervals. Animal procedures were
performed according to a protocol approved by the Institutional
Review Board and Institutional Animal Care and Use Committee
of Chang Guang Memorial Hospital.
The hind paw withdrawal threshold to tactile stimulation was
determined using factory-calibrated Touch Test filaments (von
Frey, Semmes–Weinstein monofilaments) (Stoelting, Wood Dale,
IL, USA). Mice were placed under a small, clear compartment
(8 cm612 cm65.5 cm) on an elevated wire mesh screen to allow
the investigator free access to the plantar surface of the paws. For
transected mice that were not capable of hind paw plantar
placement, the mice were held gently to assist the plantar
placement of the hind paws. Left and right hind paws were tested
in a random order using the up-down method when the rat was
not attending to the tester or the stimulus.
Hematoxylin/eosin staining
Gene therapy
Slides were counterstained with hematoxylin and eosin (H&E)
as described elsewhere [18] for tissue examination. Briefly, 6 mm
sections were deparaffinized in Xylol (Carl-Roth, Germany) for 10
minutes, rehydrated in a descending ethanol series and rinsed in
deionized H2O for 1 minute. Sections were placed in hematoxylin
for 3 minutes, rinsed in tap water for 1 minute to allow stain to
develop and then placed in eosin for 1 minute, dehydrated and
mounted in Entellan resin (Merck, Germany). The occurrence of
clearly detectable eosinophilic spheroids indicative of dystrophic
axons [19] was quantified in approximately 90 sections from
ipsilateral SCDH so irregular results due to random deviations in
spheroid numbers could be ruled out. H&E stained axonal
spheroids were generally eosinophilic and round or oval in shape.
They varied in diameter (5–50 mm) and sometimes reached a size
larger than the nerve cells in SCDH. Morphology and density of
Recombinant adenovirus vectors encoding GDNF (Ad-GDNF)
or enhanced green fluorescent protein (Ad-GFP) were prepared as
described previously [15]. For Ad-GDNF, GDNF cDNA was
subcloned into pCA13 to yield the transfer vector, Ad5-GDNF,
which was used to transfect 293 cells with pJM17, a plasmid
containing the entire adenoviral genome, to generate recombinant
virus through homologous recombination by calcium phosphate
protocol as described previously [16]. The virus was amplified in
293 cells, purified by two rounds of cesium chloride gradient
ultracentrifugation, and dialyzed against buffer containing 10 mM
Tris, pH 7.5, 1 mM MgCl2, and 10% glycerol at 48uC. The titer
of the virus solution was determined by measuring optical density
at a wavelength of 260 nm and plaque-forming assay in 293 cells
before storage at 280 uC. Adenovirus vectors [26109 plaquePLOS ONE | www.plosone.org
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GDNF Protects SCDH from Constriction-Induced Cell Death
(OX-42, phospho-p38, NeuN) at 4uC, 3 days, followed by
repeated washing with PBS, and replaced in secondary antibodies
conjugated with Alexa 488 or Cy3 for 3 hours at room
temperature.
TUNEL test
The mode of cell death induced by CCI was determined by
morphological observations done with terminal deoxynucleotidyl
transferase (TdT)-mediated dUTP nick end labelling (TUNEL)
assay. Briefly, the tissues were fixed with 4% methanol-free
paraformaldehyde at 4 uC and washed with phosphate-buffered
saline (PBS) for 30 min. An equilibrium buffer (0.1 ml) was added
to each of the slides and covered with parafilm for 10 min at 37
uC. A mixture of 1 ml TdT enzyme, 5 ml nucleotide mix and 45 ml
equilibrium buffer was prepared in the dark and 50 ml of the
mixture was added onto each slide. Slides were incubated in the
dark for 1 or 2 h at 37 uC. SSC (2X) was added for 15 min at
room temperature to stop the TdT enzyme reaction. The
unbound fluorescent-12-dUTP was removed by washing with
PBS. The slides were then immersed in propidium iodide for
15 min in the dark to stain the cells. Slides were dried after rinsing
with de-ionized water and cover slips were later overlaid on the
cell area of the slides.
Western blot
For protein extraction, each single hemi-cord segment was
homogenized in protein lysis buffer in the presence of protease
inhibitors and incubated on ice for 10 min. Samples were
centrifuged at 13,0006rpm for 30 min at 4 uC. Total protein
content was determined in the supernatants by the Bio-Rad DC
Protein Assay Kit. For Western blot analysis, equal amounts of
total protein were separated by sodium dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE; 12%) and transferred onto
PVDF membranes. After blocking for 1 hour at room temperature
in Tris-buffered saline containing 0.05% Tween 20 (TBST) and
5% non-fat milk, the membranes were incubated overnight at 4uC
with the primary antibody including GDNF (1:100 dilutions;
Santa Cruz Biotechnology Inc.); GDNFRa-1(1:100 dilution; R&D
Systems Inc.); ERK, p-ERK, p38, phospho-p38, AIF, caspase-3,
cleaved caspase-3, caspase-9, cleaved caspase-9, Beclin-1, MMP-2,
MMP-9, iNOS, nNOS, eNOS, PARP, cleaved PARP, PKCc,
PKCd (1: 1,000 dilution; Cell Signaling Technology); ED1,
SPECTRIN and cleaved SPECTRIN (1:200 dilutions; Santa
Cruz Biotechnology Inc.) directed against the protein of interest.
After several washes, an appropriate HRP conjugated secondary
antibody (1:5000; Vector Laboratories) was applied for 1 hour at
room temperature. Peroxidase activity was visualized using the
ECL Western Blotting Detection kit and X-ray films. Quantification of western blots and TUNEL staining were the average band
intensities and/or cells with positive staining of chosen antibodies
of three independent experiments were determined using ImageJ
and plotted.
Figure 1. The effect of intramuscular delivery of Ad-GDNF on
allodynia (A) and thermal hyperalgesia (B) in the CCI model.
*P,0.05 compared with the CCI group at each time point.
doi:10.1371/journal.pone.0092264.g001
neurons within the spinal cord were assessed. To avoid examining
the same neurons twice, we left more than an 8 mm gap between
sections.
Immunohistochemistry
Paraffin embedded samples, after deparaffinization and rehydration, were treated by steam heating for antigen retrieval
(30 min) using DAKO antigen retrieval solution (DAKO,
Carpenteria, CA). Slides were washed using Tris Buffered Saline
(TBS) twice. Endogenous peroxidase was inhibited by immersing
the slides in a 3% hydrogen peroxide solution for 10 min. Slides
were then washed twice in TBS. The sections were incubated with
primary antibody against GDNF 1 hour at room temperature.
Slides were washed twice with TBS and consecutively incubated
with biotinylated secondary antibody for 30 min. Slides were
washed twice with TBS and incubated with DAB for 5 min. Slides
were washed twice again with distilled water. Immediately after
staining, slides were counterstained with hematoxylin for 1 min.
Slides were rinsed for 1 second with distilled water and dehydrated
for 1–2 seconds each with 90–100% isopropanol. Finally, samples
were immersed in xylene for 10 min each and mounted using
Permount (Fisher Scientific, Pittsburg, PA).
Data Analyses
Comparisons within groups were made by using one-way
analysis of variance (ANOVA). The comparisons across groups
were accomplished with one-way ANOVA and, if significant,
discrete comparisons were accomplished using Tukey’s method for
post-hoc testing. A p value of less than 0.05 was considered
statistically significant. Data were expressed as mean 6 SEM.
Immunofluorescent microscopy
The transversal frozen sections (10 mm) of sciatic nerves were
dried and incubated in blocking buffer containing 1.5% normal
goat serum and 0.2% Triton X-100 in PBS. The slides were
washed twice with PBS, incubated with the primary antibodies
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GDNF Protects SCDH from Constriction-Induced Cell Death
Figure 2. Immunoblotting showing protein expression level with respect to GDNF and its receptor. Western blot analysis showing the
expression levels of GDNF and its receptor, GDNFRa-1, under control, CCI, and ipsilateral SCDH with intramuscular injection of Ad-MOCK or Ad-GDNF
(A). The expression levels of GDNF and GDNFRa-1 with respect to each tested group were shown as bar charts of relative ratio (B–C).
Immunohistochemical (D–K) staining was used to confirm GDNF expression. (D–G: 200X magnification, H–K: 400X magnification) *P,0.05 **P,0.01
compared with control group.
doi:10.1371/journal.pone.0092264.g002
changes underlying the beneficial effect of Ad-GDNF at day 5 in
our study.
Results
GDNF attenuates CCI-induced allodynia and hyperalgesia
To measure the seriousness of neuropathic pain, the Von Frey
filament and hot-plantar test were used to establish the animal
model. The result showed that CCI induced allodynia and thermal
hyperagia at day 1 after surgery (Fig. 1A & 1B). Allodynia was
maintained 28 days in CCI group (Fig.1A), whereas thermal
hyperalgesia was maintained 14 days but returned to the same
level as the control group at day 28 (Fig. 1B). At day 28, rats in the
CCI group showed a significantly lower weight for the left hind
paw corresponding to the sites where the stimuli were applied (at
site of sciatic nerve constriction) compared to controls. Allodynia
and thermal hyperalgesia did not differ significantly between the
CCI and Ad-MOCK group. Ad-GDNF started to significantly
alleviate both allodynia and thermal hyperalgesia associated with
CCI at day 5 after surgery, but showed no effect at day 1 and 3
(Fig. 1A & 1B). Therefore, we further investigated the molecular
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GDNF inhibits matrix metalloproteinase expression on
SCDH
To check GDNF and GDNFRa1 protein expression on the
SCDH after adenovirus-mediated GDNF was delivered by
intramuscular injection, samples prepared from the ipsilateral
SCDH at day 5 after surgery were immunoblotted. GDNF and
GDNFRa1 expression in both the CCI and Ad-MOCK group was
significantly lower than control. After delivery of Ad-GDNF at day
1 after surgery, GDNF and GDNFRa1 expression returned to
control levels at day 5 (Fig. 2A–C). Immunohistochemical analysis
of GDNF expression was consistent with the results from
immunoblotting (Fig. 2D–K). Since Kawasaki Y et al. reported
that MMP-9 induces neuropathic pain through interleukin-1b
cleavage and microglial activation at early onset [20], we also
analyzed MMP-2 and MMP-9 expression. We found no significant
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GDNF Protects SCDH from Constriction-Induced Cell Death
Figure 3. Immunoblotting showing protein expression level with respect to MMP-2 and MMP-9. Western blot analysis showing the
expression levels of MMP-2 and MMP-9 in control, CCI, and ipsilateral SCDH with intramuscular injection of Ad-MOCK or Ad-GDNF (A). The expression
levels of MMP-2 and MMP-9 with respect to each tested group were shown as bar charts of relative ratio normalized with the expression level of bactin (B–C). *P,0.05 compared with control group.
doi:10.1371/journal.pone.0092264.g003
inter-group difference between any two groups for MMP-2
expression in the SCDH at day 5. In contrast, in the CCI group
the MMP-9 expression was significantly higher than that of
controls, but no different than the Ad-MOCK group. Administering Ad-GDNF did inhibit MMP-9 expression, which possibly
contributed to the attenuation of neuropathic pain (Fig. 3A–C).
protein expression in the Ad-GDNF group was significantly lower
than in the CCI and Ad-MOCK groups, but not the control group
(Fig. 5A–C).
GDNF prevents CCI-induced programmed cell death on
SCDH
It had been reported that inflammatory factors induced
wallerian degeneration at the lesion site following peripheral
nerve injury [22].We also observed SCDH tissue loss by H&E
staining after CCI and this hallmark was not recovered by AdMOCK administration (Fig. 6F&G). We hypothesized that the
tissue loss is possibly due to neuronal reduction caused by
programmed cell death. To address this question, we used
terminal deoxynucleotidyl transferase dUTP nick end labeling
(TUNEL) and immunofluorescent microscopy (Fig.7A–H). We
found a significant difference between the CCI, Ad-MOCK and
Ad-GDNF group by TUNEL staining (Fig.7 F–H&M), as well as
double labeling of TUNEL and NeuN (Fig. 7J–L&N). These
results may indicate that SCDH neuron cells underwent apoptosis
after CCI and that this phenomenon was reversed by Ad-GDNF.
To confirm this finding, we also detected the expressions of several
apoptotic proteins. Based on our data, the expressions of apoptosis
inducing factor (AIF), cleaved caspase-9, cleaved caspase-3,
cleaved Poly (ADP-ribose) polymerase (PARP), cleaved SPECTRIN and Beclin-1 were enhanced in the CCI group (Fig. 8A–G).
Interestingly, expression levels of these proteins were attenuated to
control group levels after administration of Ad-GDNF. These
results suggested that adenoviral-mediated delivery of GDNF
successfully inhibited CCI-induced apoptosis in the SCDH
(Fig. 8A–G).
GDNF decreases activated microglia and expression of
inflammatory factors from CCI on SCDH
A previous study reported that microglia are activated by
phosphorylation of p38 and ERK1/2 following peripheral nerve
injury including CCI [21]. To confirm this, we detected
phosphorylation of p38 and stained for ED-1 expression (microglia
marker). Western blot showed that p38 but not ERK1/2 (data not
shown) was phosphorylated. In addition, ED-1 expression was
elevated in the CCI group. These results may indicate that CCI
induced microglia activation and proliferation through the
phosphorylation of p38. In contrast, in the Ad-GDNF group the
expression of phosphorylated-p38 and ED-1 was significantly
lower than in the CCI and Ad-MOCK groups (Fig 4A–C). Double
immunofluorescent staining for phosphorylated-p38 and OX42, a
microglia marker, showed that expression levels with respect to
OX42 and phosphorylated p38 were obviously enhanced after
CCI, but phosphorylated p38 was no longer highly expressed after
administration of Ad-GDNF (Fig 4D–N). These results revealed
CCI-induced phosphorylation of p38 on microglia at SCDH,
which returned to normal after GDNF delivery. In CNS,
microglia is not only a support cell but is also involved in immune
regulation. Microglia activation releases pro-inflammatory cytokines such as TNF-a, IL-1b and IL-6. We examined expression
with respect to IL-6 and IL-1b in the different tested groups.
Immunoblotting showed that CCI up-regulated IL-6 and IL-1b in
the SCDH. After treatment with Ad-GDNF, IL-6 and IL-1b
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Figure 4. Immunoblotting showing protein expression level with respect to phosphor-p38, p38 and ED-1. Western blot analysis
showing the expression levels of phospho-p38, p38 and ED-1 in control, CCI, and ipsilateral SCDH with intramuscular injection of Ad-MOCK or AdGDNF (A). The expression levels of phospho-p38 and ED-1 with respect to each tested group were shown as bar charts of relative ratio normalized
with expression levels of p38 and b-actin, respectively (B-C). *P,0.05, **P,0.01 compared with the Ad-GDNF group. Double immunofluorescence
staining of OX42 (D–G), a microglia marker, and phosphor-p38 (H–K) in different tested groups. The expression levels with respect to OX42 and
phospho-p38 were obviously enhanced after CCI, but phospho-p38 was no longer highly expressed after administration of Ad-GDNF as shown in
merged images (L–O).
doi:10.1371/journal.pone.0092264.g004
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Figure 5. Immunoblotting showing protein expression level with respect to IL-6 and IL-1b. Western blot analysis showing the expression
levels of IL-6 and IL-1b in control, CCI, and ipsilateral SCDH with intramuscular injections of Ad-MOCK or Ad-GDNF (A). The expression levels of IL-6
and IL-1b with respect to each tested group were shown as bar charts of relative ratio normalized with the expression levels of b-actin (B–C). *P,0.05,
**P,0.01 compared with control group.
doi:10.1371/journal.pone.0092264.g005
eNOS, since NMDA/PKC signaling was associated with NOS
expression. Among these three NOS, only iNOS was increased
following CCI and this effect was reversed by Ad-GDNF (Fig. 10B).
These data suggested that GDNF may have a role in attenuating
CCI-induced PKC/iNOS signaling associated with its neuroprotective effect in the SCDH.
GDNF blocks CCI-induced cellular signaling in SCDH
Mao J et al. reported that PKCc was increased after CCI [23].
In our results, consistent with previous reports, CCI increased
both PKCd and PKCc protein expression, but in the Ad-GDNF
group expression with respect to PKCd and PKCc was significant
lower than the Ad-MOCK and CCI group, respectively (Fig. 9A–
C). These data clearly indicated that GDNF modulated both
PKCd and PKCc protein expression on the SCDH after CCI. In
addition to examining PKC signaling, we also detected the
expressions of NOS family proteins including iNOS, nNOS, and
Discussion
In the CCI-induced nerve injury animal model, microglia
activation and abnormal pro-inflammatory cytokine profiling are
Figure 6. The result of Hematoxylin-Eosin staining (H&E staining) in detecting the morphological changes after administrated with
Ad-GDNF. The morphological charges in tight junctions of ipsilateral SCDH among the different tested groups (A–D: 200X, E–G: 400X). Yellow
arrows represent possible wallerian degeneration, which was no longer observed after administration of Ad-GDNF.
doi:10.1371/journal.pone.0092264.g006
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Figure 7. Double immunofluorescent staining of TUNEL and a neuronal cell marker, NeuN, in the rat ipsilateral SCDH in different
treatment groups. Tissue samples were detected using antibodies against NeuN (A–D) and TUNEL staining for apoptosis (E–H). The merged images
show neuron apoptosis in the ipsilateral SCDH (I–L). Yellow arrows indicate TUNEL-positive neurons. The bar chart with respect to fold increase of
TUNEL staining positivity (M) and double labeling (TUNEL and NeuN, N) revealed that apoptotic events triggered by CCI were attenuated by AdGDNF.
doi:10.1371/journal.pone.0092264.g007
associated with microgliosis in the SCDH [24]. However, the
physiological role of microglia in spinal cord circuitry development
and pain transmission remains to be investigated.
Increasing evidence suggests the important role of spinal cord
microglia in the genesis of persistent pain, by releasing the
proinflammatory cytokines tumor necrosis factor-alpha (TNFa),
Interleukine-1beta (IL-1b), and brain derived neurotrophic factor
(BDNF). Nerve injury-induced microglial activation occurs by
phosphorylation of MAP kinases such as p38 MAPK kinase and
extracellular signal-related kinase (ERK) isoforms 1 and 2, and
Src-family kinases [21,25–27]. The morphological changes associated with microgliosis may also be mediated by the activation of
ERK/MAPK [28]. A number of signaling pathways such as
neuregulin-1, matrix metalloproteases (e.g. MMP-9) and multiple
chemokines enable direct communication between injured prima-
suggested to be crucial in maintaining neuropathic pain. In fact,
microglia are derived from myeloid precursor cells in the
periphery and penetrate the CNS during embryogenesis. Microglia are the resident macrophages in the CNS, and mediate
signaling crosstalk between peripheral and CNS nerves. Moreover,
microglia are also important in CNS neuroinflammation.
Mechanical or biochemical stressor insults affecting CNS homeostasis usually induce rapid responses in microglia morphology,
gene expression profile and functional behavior and these events
are collectively termed ‘microgliosis’. Interestingly, damage to the
nervous system outside the CNS, such as axotomy of a peripheral
nerve, can lead to microgliosis in the spinal cord. In addition, it is
also reported that peripheral nerve injury nociceptive inputs from
sensory neurons appear to be critical for triggering the development of spinal microgliosis. CCI-induced neuropathic pain is also
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GDNF Protects SCDH from Constriction-Induced Cell Death
Figure 8. Immunoblotting showing protein expression level with respect to apoptotic and autophagic marker. Western blot analysis of
the effect of CCI on the expression of AIF, caspase-9, cleaved caspase-9, caspase-3, cleaved caspase-3, PARP, cleaved PARP, SPECTRIN, cleaved
SPECTRIN, and Beclin-1 on ipsilateral SCDH by intramuscular injection with adenovirus plus GDNF gene (A). Ratios of AIF, cleaved caspase-9, cleaved
caspase-3, cleaved PARP, cleaved SPECTRIN, Beclin-1 with b-actin on ipsilateral SCDH were measured using western blot analysis (B–G). *P,0.05,
**P,0.01 compared with the CCI group.
doi:10.1371/journal.pone.0092264.g008
additional production of IL-1b, TNF-a, IL-6, IL-10, TGF-b,
PGE2, BDNF, and cathepsin S and promoting the deleterious
effects of microglial infiltration and phagocytosis in neuropathic
pain. Blocking the signaling pathways mediated by IL-1b or IL-6
diminishes behaviors related to neuropathic pain [39]. In this
study, GDNF inhibited the activation of microglia and IL-1band
IL-6 release. This can be one of reasons for the decline in pain
behaviors after Ad-GDNF administration.
The present study explored how CCI-induced peripheral nerve
injury can cause type I (apoptosis) and II (autophagy) programmed
cell death. These molecular events can also be attenuated by
adenoviral-mediated GDNF delivery. Consistent with other
studies, the present data revealed that pro-inflammatory cytokine
signaling, such as TNFa, p38, and MMP-9, may contribute to
apoptosis induction in the SCDH after CCI. Previous studies
suggested that either apoptosis or inflammation plays an important
role in neuropathic pain. Peripheral nerve injury produces neuron
apoptosis in the dorsal horn of the spinal cord [40–42] and DRG
[43,44]. Specifically, apoptosis in satellite glial cells (SGC) (not in
neurons) of CCI rat ipsilateral dorsal root ganglia (DRG) at day 30
after injury was revealed [45].
Cleaved caspase-3 and caspase-9 expression is the effector with
respect to apoptosis and post-mitochondrial apoptosis. Our results
showed that GDNF can reduce CCI-induced caspase expression
and activation, which may be associated with relief of pain
behavior. This data suggests that caspase signaling pathways are
involved in pain development. Apoptosis inducing factor (AIF) is
used as a marker of mitochondrial apoptosis. Both caspases and
AIF can trigger SPECTRIN cleavage by activating cleaved Poly
(ADP-ribose) polymerase (PARP). All of them are apoptosis
markers. In our results, CCI induced the expression and activation
of chosen markers involved in either mitochondrial or non-
ry afferents and microglia. According to current knowledge
regarding p38 MAPK signaling and increased the pain sensitivity,
several molecules have been reported to activate p38, such as
TNFa [29], IL-1b[30], CCL2[31], MCP-1, CX3CL1[32],
iNOS[33], MMP-9[20], P2X4 and P2X7. Some of these
microglial activators, such as ATP, CCL2, fractalkine, and
MMP-9, are suggested to be released from primary afferent
neurons [34,35]. When microglia are activated, the p38 pathway
induces expression of molecules such as NFkB, COX2, iNOS,
BDNF, TNFa, IL-1b, and IL-6 [30,36,37]. p38 activation in
microglia also results in increased release of BDNF and TNFa
[38]. Microglial production of proinflammatory cytokines can
further recruit additional microglia, activate surrounding astrocytes, and promote the sensitization of central nervous system
nociceptive circuits.
We observed CCI induced activation of microglia by phosphorylation of p38 rather than ERK. Spinal microglial activation
in both dorsal and ventral horns peaked 1 week after injury and
returned after several weeks. Our results support the idea that that
microglia affect inflammatory reactions at an early stage. Yasuhiko
Kawasaki et al. reported that after spinal nerve ligation (SNL),
MMP-9 induced neuropathic pain through interleukin-1 cleavage
and microglial activation at early times, whereas MMP-2
maintained neuropathic pain through interleukin-1 cleavage and
astrocyte activation at later times [20]. In our study, MMP-9 was
induced at day 5 after CCI, and MMP-2 showed no effect.
Microglial activation might be just the first step in a cascade of
immune responses in the CNS. Zhuang et al. showed the
activation of ERK in neurons, then microglia, and then astrocytes
in a neuropathic pain model [27]. Microglia may initiate
neuropathic pain, and astrocytes probably respond to maintain
neuropathic pain. Microglial MAP kinases can be activated by IL1b and TNF-a, inducing, via transcription factors such as NFkB,
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GDNF Protects SCDH from Constriction-Induced Cell Death
Figure 9. Immunoblotting showing protein expression level with respect to PKCd and PKCc. Western blot analysis showing the
expression levels of PKCd and PKCc in control, CCI, and ipsilateal SCDH with intramuscular injection with Ad-MOCK or Ad-GDNF (A). The expression
levels of PKCd and PKCc with respect to each tested group were shown as bar charts of relative ratio normalized with the expression levels of b-actin
(B–C). *P,0.05, **P,0.01 compared with the Ad-GDNF group.
doi:10.1371/journal.pone.0092264.g009
induction on the SCDH and GDNF prevented the elevated
expression of Beclin-1 due to CCI-induced nerve injury.
In conclusion, intramuscular injection with Ad-GDNF not only
attenuates neuropathic pain but also protects cells from neuropathic pain-associated programmed cell death (microglia inactivation through down-regulating IL-6, IL-1b, p38 and MMP-9). In
addition, administration of GDNF also enhanced expression of
inducible nitric oxide synthases by modulating the PKC pathway
in the SCDH following chronic constriction injury. Adenoviral
GDNF-based gene therapy may be an alternative therapeutic
approach for treating neuropathic pain in patients.
Although our results provide evidence that GDNF can be
applied to attenuate CCI-induced neuropathic pain, the experimental limitations should be noted. First, CCI-induced nerve
injury was recently associated with autophagy induction. Many
proteins are used as hallmarks of autophagy, such as ATG family
proteins, p62, Beclin 1 and LC3B. In this study, we only measured
Beclin 1 and LC3B to compare the results with the SNL model.
Second, for evaluating apoptotic events after CCI-induced nerve
injury on the SCDH, we only used TUNEL staining, NeuN
staining and immunostaining of apoptotic proteins. This may not
mitochondrial apoptosis. Moreover, using adenovirally delivered
GDNF successfully attenuated apoptotic protein expression.
Autophagy is an intracellular membrane trafficking pathway
controlling the delivery of cytoplasmic material to the lysosomes
for degradation. It plays an important role in cell homeostasis in
both normal settings and abnormal or stressful conditions[46].
LC3B and Beclin-1 are autophagy markers involved in different
stages of autophagy. Previous studies reported that L5 spinal nerve
ligation induced autophagy in the SCDH. Both LC3 and Beclin-1
are observed to be significantly elevated in the ipsilateral L5 spinal
dorsal horn on day 14 following spinal nerve ligation. These two
proteins are mainly located at GABAergic interneurons of the
spinal dorsal horn after SNL, indicating that autophagic disruption
in GABAergic interneurons and astrocytes following peripheral
nerve injury might be involved in the induction and maintenance
of neuropathic pain [47]. Besides, the mTOR pathway (autophagy-associated) is also reported to be activated in the SCDH in
CCI-induced neuropathic pain, and the intrathecal injection of
rapamycin can reduce mechanical allodynia [48]. In this study,
CCI-induced peripheral nerve injury did lead to autophagy
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GDNF Protects SCDH from Constriction-Induced Cell Death
Figure 10. Immunoblotting showing protein expression level with respect to different NOS isoform. Western blot analysis showing the
expression levels of iNOS, nNOS and eNOS in control, CCI, and ipsilateral SCDH with intramuscular injection with Ad-MOCK or Ad-GDNF (A). The
expression levels of iNOS, nNOS and eNOS with respect to each tested group were shown as bar charts of relative ratio normalized with the
expression levels of b-actin (B–D). **P,0.01 compared with Ad-GDNF group.
doi:10.1371/journal.pone.0092264.g010
fully characterize the apoptosis. Information regarding DNA
content and cell-cycle distribution consequent to CCI and AdGDNF treatment was not obtained. Third, the lack of another
neurotrophic factor such as BDNF, NGF and NT3 as control to
compare with results of GDNF limited our conclusions about the
therapeutic value of Ad-GDNF. Finally, to discriminate the
signaling pathways associated with the beneficial effects attributed
to GDNF in attenuating CCI-induced nerve injury, we only
directly observed the modulating role of GDNF on target protein
expressions rather than introducing any pathway inhibitors.
Our major finding was that adenovirally mediated delivery of
GDNF successfully decreased neuropathic pain behaviors and
their associated protein expressions. GDNF appears to inhibit
microglia activation, pro-inflammatory cytokine production, and
at least two types of programmed cell death (apoptosis and
autophagy). Future work on signaling pathways and cross-talk
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consequent to GDNF administration will provide further insights
into its therapeutic action in terms of CCI-induced neuropathic
pain attenuation, and provide a starting point for developing new
strategies for pain control.
Acknowledgments
We thank Gary Mawyer for English editing. Authors also grateful for
technical assistance provided from graduated students and technicians in
laboratory owned by Dr. Ming-Hong Tai in NSYSU.
Author Contributions
Conceived and designed the experiments: AKC MHT ALK YRH.
Performed the experiments: AKC MHT. Analyzed the data: MCY.
Contributed reagents/materials/analysis tools: MCY HPT CYC MHT
YRH. Wrote the paper: MCY.
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