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Published in final edited form as:
Neuroscience. 2014 December 5; 281: 229–240. doi:10.1016/j.neuroscience.2014.09.038.
Protease Activated Receptor-1 mediates cytotoxicity during
ischemia using in vivo and in vitro models
Padmesh S Rajput1, Patrick D Lyden1, Bo Chen1,2, Jessica A Lamb1, Benedict Pereira1,
Alexander Lamb3, Lifu Zhao1, I-Farn Lei1, and Jilin Bai1
1Department
of Neurology, Cedars-Sinai Medical Center, Los Angeles, 90048, USA
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2McKinsey&Company,
Beijing, China
3Department
of Surgery, Division of Trauma and Critical Care, Cedars-Sinai Medical Center, Los
Angeles, 90048, USA
Abstract
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Protease activated receptors (PARs) populate neurons and astrocytes in the brain. The serine
protease thrombin, which activates PAR-1 during the first hours after stroke, appears to be
associated with the cytotoxicity. Thrombin antagonists and PAR-1 inhibitors have been correlated
with reduced cell death and behavioral protection after stroke, but no data yet supports a
mechanistic link between PAR-1 action and benefit. We sought to establish the essential role of
PAR-1 in mediating ischemic damage. Using a short hairpin mRNA packaged with green
fluorescent protein in a lentivirus vector, we knocked downPAR-1 in the medial caudate nucleus
prior to rat middle cerebral artery occlusion (MCAo) and in rat neurons prior to oxygen-glucose
deprivation. We also compared aged PAR-1 knockout mice with aged PAR-3, PAR-4 mice and
young wild-type mice in a standard MCAo model. Silencing PAR-1 significantly reduced
neurological deficits, reduced endothelial barrier leakage, and decreased neuronal degeneration in
vivo during MCAo. PAR-1 knock-down in the ischemic medial caudate allowed cells to survive
the ischemic injury; infected cells were negative for TUNEL and c-Fos injury markers. Primary
cultured neurons infected with PAR-1 shRNA showed increased neuroprotection during hypoxic/
aglycemic conditions with or without added thrombin. The aged PAR-1 knockout mice showed
decreased infarction and vascular disruption compared to aged controls or young wild types. We
demonstrated an essential role for PAR-1 during ischemia. Silencing or removing PAR-1
significantly protected neurons and astrocytes. Further development of agents that act at PAR-1or
its downstream pathways could yield powerful stroke therapy.
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Keywords
Protease Activated Receptor-1; Thrombin; cerebral ischemia; Lentivirus knock-down; oxygen –
glucose deprivation
Correspondence to: Patrick Lyden, MD AHSP 6417 127 S. San Vicente Blvd Los Angeles, CA 90048 310 423 5166 lydenp@cshs.org.
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1.0 Introduction
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Thrombolysis remains the mainstay of stroke therapy, while effective neuroprotection
remains under development. In addition to neuroprotection and thrombolysis, however, early
investigators pursued stroke therapies designed to limit microvascular thrombosis
downstream of an occluded cerebral artery (Ames et al., 1968, Adams et al., 2008). When
several large clinical trials of anti-thrombotic therapy for acute stroke failed, interest waned
in developing these drugs (Biller et al., 1989, Gordon et al., 1990). A new chapter in antithrombotic therapy opened with the discovery of cell-membrane receptors that could be
activated by proteases, named the protease activated receptors or PARs (Vu et al., 1991).
Thrombin, normally present in the serum as prothrombin until activated, is a serine protease
that plays a central mediating role in the coagulation cascade. Accumulating data from in
vitro studies with brain cells indicate that thrombin induces cell death in glia and neurons
(Donovan et al., 1997, Striggow et al., 2000). Furthermore, thrombin seems to be
particularly toxic to stressed neuronal cells (Weinstein et al., 1998). We showed significant
benefit after acute ischemia with direct thrombin inhibitors (Lyden et al., 2014). Thrombin
induces protection at low doses (thrombin preconditioning) but acts as a neurotoxin at high
doses, killing cells via the PARs (Xi et al., 2003).
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PARs belong to a G-protein coupled receptor family including PAR-1 to PAR-4. PAR-1 is
also known as thrombin receptor and is considered to be the main receptor subtype activated
by thrombin and plasmin (Macfarlane et al., 2001, Hollenberg and Compton, 2002). PAR-1
is activated by proteolytic cleavage of an N-terminus extra-cellular blocking segment,
unmasking an amino acid sequence that acts as an auto-ligand (Coughlin, 2000, Macfarlane
et al., 2001). Blood derived thrombin and other PAR-1 activators enter brain tissue during
traumatic brain injury, hemorrhagic stroke, ruptured aneurysms and therapeutic treatments
with rt-PA (Gingrich et al., 2000, Gingrich and Traynelis, 2000, Davalos et al., 2014).
During ischemia, thrombin enters brain via a permeable blood brain barrier (Chen et al.,
2012, Davalos et al., 2014). Thrombin injected in rat brain causes edema, neuronal cell death
and precipitates seizures (Lee et al., 1996, Lee et al., 1997). PAR-1 is up regulated during
ischemic conditions and has been associated with neuronal cell death, neurite retraction, and
glial proliferation (Gurwitz and Cunningham, 1988, Cavanaugh et al., 1990, Grabham and
Cunningham, 1995, Donovan et al., 1997). PAR-1 is expressed in most brain regions and is
associated with brain injury during hemorrhagic and ischemic stroke (Donovan et al., 1997,
Sarker et al., 1999, Rohatgi et al., 2004).
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Agents acting at the PAR-1 receptor to either block thrombin or induce cell-survival
signaling have been associated with beneficial effects after ischemia, as measured with
behavioral or histological outcomes. For example, activated protein-C (APC) and mutant
analogs that cleave the PAR-1 tail at a distinct site, causing cytoprotective signaling, have
been correlated with benefit in animal stroke models (Wang et al., 2012). Previous studies
have shown that mice lacking PAR-1 tolerate ischemia better compared to wild type mice
(Junge et al., 2003). A recent study using PAR-1 siRNA infected animals showed decrease
in lesion area and improved behavior in animals after stroke and correlated with HSP-70
expression (Zhang et al., 2012). To date, however, data confirming a required role for
presence of PAR-1 on neurons and astrocytes (cells mainly affected during ischemia) in
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mediating cytotoxicity has been lacking; there remains a possibility that previous treatment
correlations could be due to off-target effects. We sought to demonstrate obligate
involvement of the PAR-1 receptor in mediating cell death in the brain using genomic
modification. We also sought to confirm in aged animals the previous findings that PAR-1
knock out provided protection to young animals suffering ischemia. We used in vivo and in
vitro knockdown of PAR-1 in neurons and astrocytes to protect the ischemic lesion. We
subjected aged PAR-1 knockout mice to focal cerebral ischemia.
2.0 Materials and Methods
2.1 Reagents
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All analytical grade reagents were purchased from Sigma-Aldrich. Cell culture media were
purchased from Life Technologies (formerly Invitrogen). Other reagents were purchased as
noted.
2.2 Animal Surgery
Institutional Animal Care and Use Committee (IACUC) at Cedars-Sinai Medical Center
approved all animal handling and surgery protocols, as per the national guideline for the care
of experimental animals. Procedures recommended by the STAIR and RIGOR guidelines
were followed (Fisher et al., 2009, Lapchak et al., 2013).
2.3 shRNA Lentivirus
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A potential siRNA sequence of 20 nucleotide duplexes against the PAR-1 receptor
consensus coding (GenBank Accession No. NM_012950) was designed to create a small
hairpin-blocking segment (Liu et al., 2004, 2006, Villares et al., 2008). The sequence used
was specific to rat and mouse PAR-1 and was cloned in pLKO.1-puro-UbC-TurboGFPTM
vector. The sequence for shRNA of PAR-1 knockdown used was
“AATCCCAGTGAAGATACATTT”. We used a negative control siRNA that mismatches the
sequence of PAR-1mRNA (misRNA PAR-1) for comparison. Lentiviruses were packaged in
the vector by Sigma-Aldrich labs with the sequence provided. The vectors included the code
for green fluorescent protein (GFP). The vectors were titrated using flow cytometry and
PCR analysis. The titers of the vectors used in the study were in the range of 1.0 × 109 – 3.9
× 109 TU per ml.
2.4 In vivo PAR-1 knockdown
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To demonstrate cell survival after PAR-1 knockdown, male Sprague Dawley rats weighing
290–310g were divided at random into two groups and injected with GFP tagged shRNA
PAR-1 or misPAR-1 using a lentivirus delivery vector. Animals were anesthetized with 4%
isoflurane mixed in oxygen and nitrous oxide (30:70) and maintained on1.5–2% isoflurane
during the surgery. A volume of 2 μl of lentivirus was stereotaxically injected into the medial
striatum using a 10μl Hamilton syringe at a rate of 0.2 μl/min. The coordinates for striatal
injections were −0.3 mm rostral, 3.0 mm lateral and 6.5 mm ventral to bregma. Following
injections animals survived for 1 week and were then subjected to MCAo. The MCAo
procedure was performed by a surgeon blind to treatment assignment, as described
previously in our laboratory (Chen et al., 2012, Van Winkle et al., 2012). Briefly, a midline
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neck incision was made exposing the left common carotid artery. The external carotid and
occipital arteries were ligated with 4–0 silk suture and an incision was made in the wall of
the external carotid artery close to the bifurcation point of the external and internal carotid
arteries. A 4–0 heat-blunted nylon suture (Ethicon) was inserted and advanced 17.5mm from
the bifurcation point into the internal carotid arteries for 2h. After the 2h occlusion duration,
the nylon suture was removed from carotid artery to allow the reperfusion of blood flow into
the MCA. Neurological function was tested during reperfusion and 24 h after onset of
ischemia using rodent neurological grading system. Animals were killed with an overdose of
ketamine and xylazine, and then intracardially perfused with saline and 4%
paraformaldehyde.
2.5 Effect of PAR knockout
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To evaluate the role of PAR in mediating ischemic injury, PAR-1, PAR-3 and PAR-4 agematched mice from 12–14 months old were used. Young wild-type C57/bl6-J mice were
purchased from Jackson Labs (Bar Harbor, Maine). PAR-1, PAR-3 and PAR-4 knockout
animals were kindly provided by Dr. Berislav Zlokovic (University of Southern California,
Los Angeles) and Dr. Sean Coughlin (University of California, San Francisco). Animals
were anesthetized with 4% isoflurane mixed in oxygen and nitrous oxide (30:70) and
received 2MDa dextran conjugated with FITC in PBS (Sigma-Aldrich, FD2000S). The
MCAo model as described earlier was adapted for mice (Chen et al., 2012, Van Winkle et
al., 2012). Cerebral blood flow was monitored using laser Doppler flowmetry (LDF) by
fixing a probe on the skull, 4-mm lateral to the bregma (Moor Instruments, Devon, United
Kingdom). Nylon 6-0 silicon-coated suture was inserted and advanced 10 mm from the
bifurcation into the internal carotid artery. After 2 hours the suture was removed and
reperfusion confirmed with LDF. Animals survived for 24 hours after stroke and
neurological functions were examined during reperfusion and 24 hours after onset of
ischemia. Brains were removed and divided coronally into two halves: the anterior one-half
was sectioned into 2 or 3 1-mm blocks and incubated for 30min in 2,3,5triphenyltetrazolium chloride (TTC) at 37°C. The posterior half was post-fixed, frozen, and
3 evenly spaced 50μm thick sections were cut and imaged for FITC fluorescence as
described previously (Chen et al., 2009, Chen et al., 2012). The surgeon and the behavioral
examiners were blinded to the genomic status of the mice.
2.6 Cell target characterization and cell injury
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Brain sections were used for immunofluorescence studies for characterization of infected
cells using NeuN, IBA1, GFAP antibodies and Neurotrace (Nissl Staining) (details provided
in Table). Briefly, rats were anesthetized with halothane and perfused with cold saline and
4% paraformaldehyde and post-fixed overnight in 4% paraformaldehyde, followed by
cryopreservation in 30% sucrose solution. Using a sliding microtome, 50-μm-thick brain
sections were cut and free-floating sections were collected in phosphate-buffered saline
(PBS). Brain sections were incubated in 0.2% Triton X-100 in PBS for 15 min and followed
by three subsequent washes with PBS. Sections were then incubated in 5% normal goat
serum for 1 h at room temperature and followed by overnight incubation with mouse antiNeuN, GFAP and rabbit anti c-Fos and IBA1 (1:1000) antibodies. Following three
subsequent washes in PBS; brain sections were incubated with Alexa 594 (red) secondary
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antibodies for 1 h at room temperature. For Nissl staining with Neurotrace (NeuroTrace
640/660, Life Technologies) the sections were permeablised with 0.2% Triton X-100,
following three washes with PBS the sections were incubated with Neurotrace for 15 min
(1:3000). The sections were washed, mounted on slides and viewed under Leica and
Olympus (FV10i) microscopes. Images showing co-localization were generated using NIH
ImageJ software; Adobe Photoshop (San Jose, CA, USA) was used to make the composites
from smaller micrographs. Separate sections were used for Fluoro-Jade-C (Histo-Chem Inc.
Jefferson, Arkansas) and In Situ Cell Death Kit, TMR red (TUNEL kit, Roche Applied
Science, Germany catalogue number 12156792910) to determine the neuronal cell death as
per the manufacturer’s instructions as described previously (Chen et al., 2010, Chen et al.,
2012).
2.7 Image analysis
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The investigator conducting all image analysis was unaware of group assignment or genomic
status and all sections were masked and coded. Sections were decoded and assigned to
proper groups only after all image data was collected and finalized. All sections were
imaged at low power using epifluorescence microscopy with a highly sensitive air-cooled
CCD camera (Apogee, Alta U32) (Chen et al., 2010, Chen et al., 2012). For quantification of
Fluoro-Jade-C positive neurons and TUNEL positive cells NIH ImageJ software was used.
The images were converted to 8-bit grey scale format; the Image J pseudo flat field plugin
was used to control variation in the fluorescence across all sections. Using the Image J
nucleus counter particle analysis plugin, the numbers of neurons from each brain section
were automatically counted (Wang et al., 2013).
2.8 Primary Neuronal Culture
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Cortices from E14 to E16 embryos were removed and isolated, cleaned of meninges, and
finely minced. The cortical tissue was then digested in 0.25% Trypsin for 5 minutes in a
37 °C water bath with occasional gentle shaking. DNAse I was added to the cells and the
mixture was returned to the 37 °C water bath for another 5 minutes. The cells were then
removed and brought up to 50mL with warmed DMEM and centrifuged for 5 minutes at
1000g. The supernatant was aspirated and the cell pellet was resuspended in pre-warmed
complete media (DMEM containing 5% HS, 2 mM glutamine and 1% penicillin/
streptomycin). The cells were then filtered through a 70μM diameter membrane, removed
and resuspended, and counted for total cell concentration for plating. After two days
survival, cells were infected with PAR-1 shRNA lentivirus or misRNA (1μl/ml. of media) for
24 hours and medium was replaced with fresh medium. Successful infection was confirmed
using the GFP reporter; plates were not used unless more than about 80% of cells expressed
the GFP.
2.9 Oxygen-Glucose Deprivation
Neurons for oxygen-glucose deprivation (OGD) were grown on 96-well plates at a density
of 1×105 cells per well. OGD media was made with glucose-free DMEM (5% HS, 2 mM
glutamine and 1% penicillin/streptomycin) and subsequently bubbled with 95% nitrogen,
5% CO2 gas mixture for 15 minutes. The cells were then washed twice with OGD media
and placed in an anoxic incubator for 2 hours (95% N2 and 5% CO2). Wells were randomly
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divided into three groups: Control, Thrombin 50 U/ml treated during 2 hours of OGD or
Thrombin 50 U beginning and the end of OGD and continuing during 24 hours of
reoxygenation. Wells not subjected to OGD were washed twice with normal media and
randomly divided into similar groups: control, Thrombin 50U/ml for 2 hours or 24 hours and
placed in a normal incubator for an equivalent amount of time. The medium from each
treatment cohort was removed after 2 hours of OGD and replaced with normal medium.
After 24 hours medium was removed and used for lactate dehydrogenase (LDH) assay
(Cytotoxicity Detection KitPLUS (LDH) Roche Applied Science, Germany catalogue
number -04744934001). To determine a maximum LDH value for each well, after removing
the media, the remaining cells in each well were lysed using a provided lysis reagent
according to manufacturer’s instructions. Each measurement was normalized to the total of
the LDH found after 2 hours, 24 hours and lysis.
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2.10 Statistical Analysis
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3.0 Results
All continuous data were analyzed with parametric statistics and reported as mean ± SE. For
cell counting, data are presented as number of neurons that stained positive for Fluoro-JadeC or using TUNEL. Briefly, from 3–4 sections per animal, the numbers of positive neurons
were averaged over 4 high power fields per section. For c-Fos immunoreactivity the area was
measured using Image J analysis and represented as mm2. Quantitative neurological scores
were compared using an independent sample student t-test. Two-way ANOVA with
Bonferroni post-hoc test was used to compare the results of OGD experiment (LDH assay)
performed in triplicates. Areal measurements (c-Fos, TTC, and vascular disruption) were
compared using 1-way ANOVA with Dunnett’s multiple comparison post-hoc test.
GraphPad Prism 5.0 (San Diego, CA) was used to perform all the statistical analysis.
3.1 Effect of in vivo PAR-1 Knock-down
We confirmed PAR-1 knockdown and protein expression after vector infection in brain
sections (Fig 1A) and cultured cells (Fig 1B and Fig 1C) using immunohistochemistry and
western blot analysis. As shown in Figure 1A brain sections from animals injected with
misRNA PAR-1 (control) and shRNA PAR-1 were stained for PAR-1 antibody. The misRNA
PAR-1 panel shows colocalization with infected cells in green whereas; the shRNA PAR-1
panel shows no colocalization with GFP labeled lentivirus infected cells. We also confirmed
the knockdown by western blot analysis using protein lysates from cultured neuronal cells
and see significant decrease in PAR-1 protein expression in cells infected with PAR1 shRNA
in comparison to cells infected with misRNA and uninfected cells Fig (1B and 1C).
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Brain sections taken from lesioned rats one week after lentivirus injection were used for
characterization of the infected cells. Cells infected in the medial caudate—the medial MCA
perfusion bed—avidly took up the vector and expressed vector-delivered proteins (Fig. 2A
and 2B). The infected cells were further characterized using specific antibodies to label
neurons, astrocytes or microglia. Of the entire lentivirus infected cell population, about 42%
also stained for NeuN, about 28% stained for GFAP, and about 30% stained for neither (Fig.
2 C, D and E). Of considerable importance, infected cells showed no positive
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immunoreactivity when stained with IBA1—a microglial marker (Fig. 2E)—suggesting
viability, or at least the absence of phagocytosis around the infected cells. For further
confirmation of neuronal staining we used NeuroTrace Red Nissl staining to co-localize with
lentivirus infected GFP positive cells. Using CytoFx© software supplied by Dr. Kolja
Wawrowsky, we created a 3D contour image from a z-stack showing a clear co-localization
of GFP positive cells with NeuroTrace Red (Fig. 2F).
3.2 Post-ischemic cell injury
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To identify areas of cellular injury after ischemia, we labeled for c-Fos, an immediate-early
proto-oncogene whose expression is increased after stimulation (Hu et al., 1994, Sharp and
Sagar, 1994). We observed increased expression of c-Fos in ischemic areas of animals
subjected at 24 hours with 2 hours of MCAo. We noted c-Fos expression was significantly
decreased in the PAR-1 shRNA-injected animals compared to misRNA injection (Fig. 3). To
assure that this finding was not due to random variation in the sizes of the ischemic zones
induced in our model, we confirmed in a separate series that the medial caudate is involved
in every animal subject to MCAo, even though the sizes of the infarctions may vary (Fig.
2A). A signification reduction in c-Fos positive cells in PAR-1 shRNA injected animals was
regularly observed in the ischemic zone (Fig. 3D). Upon high-power magnification, there
was a clear distinction (i.e. absent co-localization) between infected cells and c-Fos positive
cells, even juxtaposed in the same ischemic bed (Fig 3E). That is, in a single ischemic zone
—in which all factors related to ischemia (temperature, cerebral blood flow, acidosis)
affected all cells equally—dead and viable cells differing only by the presence or absence of
PAR-1 could be identified next to each other, (Fig. 3E). On sections taken 24 hours after
stroke, we observed a significantly reduced number of TUNEL positive cells in animals with
PAR-1 knock-down (Fig. 4). Interestingly, the cells positive for GFP (indicating presence of
shRNA) showed no co-localization with TUNEL positivity; these viable cells were found in
the ischemic zone, and were surrounded by degenerating cells. In other words, in one
ischemic milieu, there was a clear distinction (absent co-localization) between infected cells
and degenerating cells. We used Fluoro-Jade-C, a marker for degenerating neurons, to
demonstrate a neuroprotective effect of PAR-1 shRNA knockdown. Similar to what was
observed with TUNEL staining, increased numbers of Fluoro-Jade-C positive cells were
observed in the misRNA injected animals (Fig. 5A) and this was markedly reduced in
shRNA injected animals (Fig. 5B). The quantification of Fluoro-Jade-C positive cells
demonstrated that degenerating neurons were significantly fewer in MCAo induced PAR-1
knockdown animals (Figs. 5C and 5D).
3.3 Neurological behavior outcomes
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A rodentneurological scale was used to asses the changes in the behavior of animals after
MCAo (Bederson et al., 1986). Figure 6 illustrates the effect of PAR-1 knockdown by
shRNA on neurolgical behavior 24 hours after MCAo. We observed no difference between
the groups 2 hours after stroke, suggesting that the level of acute injury was similar between
the groups. The scores were significantly lower (better) 24 hours after stroke in the animals
injected with PAR-1 shRNA lentivirus compared to misRNA injected animals (p< 0.001).
Thus, the data suggest that PAR-1 knockdown ameliorates the behavioral deficits after
ischemic stroke.
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3.4 Oxygen-Glucose Deprivation in PAR-1 knockdown primary neuronal culture
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3.5 Transient ischemia in PAR-1−/− mice
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Primary neuronal cultures were infected with misRNA or PAR-1 shRNA 1 week before
OGD. Visualizing the cells with green fluorescence microscopy and counting the GFP
positive cells we estimated the infection efficiency to be 80% (data not shown). Thrombin
concentration for the OGD experiment was first determined from a separate dose escalation
cell death assay (Fig. 7 inset). Under normoxic conditions 10U, 20U 50U and 100U per ml
thrombin treatment resulted in approximately 50% cell death during 2-hour treatment in
misRNA-infected cells while no effect was observed in shRNA infected cells (Fig. 7). After
24 hours, cell death increased in both treatment groups, but less so in the PAR-1 knockdown
cells. However, during 2 hours of OGD the PAR-1 knockdown cells showed significantly
decreased cell death with or without thrombin treatment when compared to misRNA
infected cells without thrombin p<0.05 or with thrombin p<0.01. Upon treatment with
thrombin during 24 hours reoxygenation after OGD, the PAR-1 knockdown cultured cells
showed a significant reduction in cell death as compared to misRNA-infected cells (Fig. 7,
p<0.01).
4.0 Discussion
Young PAR-1 knockout mice have been shown to resist ischemia-induced injury compared
to young wild type mice (Junge et al., 2003). Since stroke typically occurs in aged subjects,
we compared aged PAR-1−/− mice with aged PAR-3−/− and PAR-4−/− mice and compared
them to young wild type mice. Animals were excluded if they showed less than a minimum
75% LDF drop in blood flow during MCAo. Ischemic lesion volume determined using TTC
was significantly reduced in PAR-1−/− and PAR-3−/− mice compared with wild type (p<0.05)
or PAR-4−/−mice (<0.01) (Fig 8A). We also compared the area of vascular disruption
measured by high molecular weight dextran-FITC leakage in the penumbra. Vascular
disruption was reduced inPAR-1−/− and PAR-3−/− mice compared to young wild type and
PAR-4−/− mice (Fig. 8B, p<0.05).
In this study, we found that PAR-1 plays an essential role mediating damage to the
neurovascular unit during cerebral ischemia. The selective silencing of PAR-1 using RNA
interference techniques resulted in improved neurological function induced by MCAo
ischemia. PAR-1 knockdown selectively decreased the ischemia-induced expression of cFos. PAR-1 shRNA positive cells showed no co-localization with TUNEL positive cells.
PAR-1 knockdown results in 70% reduction in the number of Fluoro-Jade-C positive
degenerating neurons counted in the ischemic striatum.
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4.1 PAR-1 mediates cytotoxicity
Our data confirm an essential role for PAR-1 in mediating ischemic injury using 3 different
models. After intracerebral injection of PAR-1 silencing mRNA, we showed significant
cytoprotection in the infected cells in the ischemic area. The effect of the PAR-1 knock
down was widespread, as evidenced by resistance to injury (c-Fos expression) throughout
the ischemic bed (Fig. 3). Benefit was also expressed as improved behavioral outcome (Fig.
6) and as evidenced by several histological markers of cell injury (Figs 4, 5 and 7). Using an
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in vitro model of oxygen and glucose deprivation, cells with pre-injury PAR-1 knockdown
significantly resisted injury (Fig 7). Finally, aged PAR-1 knock out animals showed
significant protection from focal ischemia (Fig. 8).
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Previously, a role for PAR-1 mediated neuroprotection has been suggested; our data show
for the first time a direct link between PAR-1 and neurovascular injury. For example, in the
ischemic zone, we identified degenerating cells immediately juxtaposed to viable cells
lacking the PAR-1 receptor (Fig. 4c). Non-specific effects of the virus injection cannot
explain the protective effect because the cells are residing next to each other, subject to the
identical ischemic milieu. Similarly, injured cells expressing c-Fos could be identified next
to healthy cells that lacked PAR-1 (Fig. 3E). Given the identical milieu in which the viable
and dying cells are found, the only explanation for the survival is the absence of PAR-1. In
cultured neuronal and astrocyte cells, resistance to both thrombin toxicity and OGD in the
presence of thrombin was reduced by PAR-1 knockdown. In the controlled in vitro
environment, there are no other factors that could explain the protective effect.
4.2 PAR-1 knockout correlated with protection in aged animals
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Previous authors have shown that young PAR-1−/− animals resist ischemia (Junge et al.,
2003). We replicated these findings in aged animals to assure the consistency of the result
during advanced age (Fig 8A). This replication is important because knockout animals live
without the PAR-1 receptor their entire lifetime, and could develop unknown compensatory
mechanisms. Thus, compensatory effects such as upregulation of other PAR receptors could
mediate the observed resistance to ischemia. In fact, recently one group reported that young
PAR-4−/− animals showed ischemia resistance, an effect we did not see in aged animals
(Mao et al., 2010). Those authors attributed the neuroprotective effect of PAR-4 knock out to
an anti-platelet effect since the PAR-4 receptor in mice mediates platelet response to injury.
We used the identical strain of animals, so we suspect that the different results indicate either
a chance variation in experimental response, or a confounding effect of aging in our
experiment. Previous authors have confirmed significant neuroprotection from agents that
act on the PAR-1 receptor using an embolic thrombus model (Wang et al., 2012).
4.3 Astrocytes may participate in cytoprotection
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Cells infected with PAR-1 shRNA lentivirus showed co-localization with neuronal markers
as well as astrocytes (Fig. 2). In 30% of infected cells, neither neuronal nor astrocytic
markers were detected, but neither NeuN nor GFAP reliably label 100% of their respective
cell types. Infected cells showed no co-localization with microglial markers, consistent with
the viability of the cells and resistance to inflammatory attack. In previous work, we have
shown that PAR-1 mediated ischemic cell injury occurs primarily in neurons and endothelial
cells, but to a lesser extent in astrocytes. The present data suggest that there may be a
protective effect mediated via astrocytes as well. Further study will be needed to identify the
specific roles played by each component of the neurovascular unit during PAR-1 dependent
cell death. While our data clearly establish a direct link between the PAR-1 receptor and cell
death (Figs. 3, 4, 5), the widespread zone of protection seen around the injection site (Fig. 3)
raises the possibility of additional effects. Significant improvement in neurological deficit
withPAR-1 knockdown indicates a critical role of PAR-1 in mediating ischemia injury (Fig
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6). Recently, using siRNA injected into the lateral ventricle, behavioral and lesion volume
results similar to ours were obtained (Zhang et al., 2012).
4.4 Limitations
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There are some limitations to our methods that suggest a cautious interpretation. Although
we used LDF to assure ischemia, we could not perform regional cerebral blood flow studies
in each subject. Therefore, there is the remote possibility that the subjects receiving the
shRNA somehow all suffered less ischemia than the misRNA injected subjects. We view this
possibility as extremely unlikely, given the very high reproducibility of the model—as
shown in Fig. 2A—in which the medial striatum is involved in 100% of subjects, a finding
we have published consistently over the course of several years (Chen et al., 2009). To
minimize possible bias, all animals were randomly assigned to one treatment group or the
other; all surgeries and injections were done without knowledge of the treatment group
assignment; and all behavioral and imaging assessments were done by an investigator
blinded to treatment group (Lapchak et al., 2013). Stroke modeling with the nylon filament
technique, while standard in this field, is not physiologically representative; our results could
be confirmed using an embolic thrombus approach, but no data exists to suggest the findings
would be significantly different between the 2 approaches.
4.5 Clinical Translational Implications
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These data have significant implications for stroke victims. In the past, experimental
findings in pre-clinical animal models have not translated well to human stroke trials. We
and others have shown powerful benefits of drugs that block the main activator for PAR-1,
the serine protease thrombin, and we showed that a thrombin inhibitor could be effective
even if delayed to a clinically relevant 3 hours after ischemia onset (Lyden et al., 2014).
Agents that act on the PAR-1 receptor directly also show powerful neuroprotective effects in
multiple laboratories, using different models, at clinical relevant delay times (Shibata et al.,
2001). One such agent is in early phase clinical trials (Lyden et al., 2013). Our present data
suggest that the protective effect of these drugs is mediated directly by PAR-1, not by offtarget effects, thus significantly increasing the likelihood that such stroke trials could be
successful. Our data also suggest that future strategies, perhaps aimed at downstream
transduction events triggered by PAR-1 actions, could similarly be successful. Future studies
will be needed to explore these downstream actions of PAR-1 agonists, biased agonists, and
antagonists. Even though the shRNA injections occurred before the MCAo, the results
provided here indicate direct mediation of PAR-1 in several post-ischemia
pathophysiological pathways. PAR-1 is present on astrocytes and neurons along with other
cell types hence engineering various cellular targets will be critical for developing a
therapeutic for ischemic injury.
5.0 Conclusion
Our data confirms the role of thrombin-activated PAR-1 in mediating ischemic cell death.
Further, we have demonstrated for the first time that PAR-1 is both necessary and sufficient
to mediate cell death during ischemia. Treatment strategies targeting PAR-1 are expected to
protect the neurovascular unit during ischemic injury.
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Acknowledgments
This work was supported by the National Institute of Neurological Disorders and Stroke R01 NS075930 (Lyden).
PAR knock out mouse breeding pairs were kindly provided by Dr. Berislav Zlokovic and Dr. Sean Coughlin. Dr.
Kolja Wawrowsky kindly supplied the CytoFx© software.
Abbreviations
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ANOVA
analysis of variance
APC
Activated Protein-C
CCD
charge-coupled Device
DMEM
Dulbecco’s modified eagle’s medium
DNAse
deoxyribonuclease
E14
embryonic day 14
G-protein
guanosine nucleotide-binding protein
GFP
green fluorescent protein
GFAP
glial fibrillary acidic protein
IACUC
Institutional animal care and use committee
IBA1
Ionized calcium-binding adapter molecule 1
LDF
laser Doppler flowmetry
LDH
lactate dehydrogenase
MCA
middle cerebral artery
MCAo
middle cerebral artery occlusion
NeuN
neuronal nuclei
OGD
oxygen glucose deprivation
PARs
protease activated receptors
PBS
phosphate buffered saline
PCR
polymerase chain reaction
rtPA
recombinant tissue plasminogen activator
STAIR
stroke therapy academic industry roundtable
shRNA
short hairpin ribonucleic acid
siRNA
small interfering ribonucleic acid
TTC
2,3,5-triphenyl-2H-tetrazolium chloride
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TU
Titer Unit
TUNEL
terminal deoxynucleotidyltransferase mediated dUTP Nick End Labeling
U
Unit
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1.
PAR-1 knockdown neuronal and astrocytes cells are negative for TUNEL
positive cells.
2.
PAR-1 knockdown results 70% reduction of dying neurons with improved
neurological deficits.
3.
Knockdown or inhibition of PAR-1 would provide neuroprotection in brain
from ischemic injury.
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Figure 1. Effective knockdown of PAR-1 with PAR-1 shRNA lentivirus in brain sections and
transfected neuronal cells
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(A) Lentivirus was injected into rat striatum. After one week the animals were sacrificed and
brain sections were stained to determine the effective knockdown of PAR-1 mediated by
lentivirus. Sections were stained with rabbit polyclonal antibody against PAR-1 (Alexa 594
goat anti-rabbit secondary antibody.) Pictomicrograph shows cells infected with lentivirus
expressing a mis-sense RNA (misRNA) PAR-1 colocalize with PAR-1 protein indicated with
arrows, demonstrating no knock down. Cells infected with shRNA PAR-1 lentivirus show no
colocalization with PAR-1 receptor antibody, indicated with arrow heads. (B) Cell lysate
taken from infected cells was used to determine the protein levels of PAR-1 with western
blot analysis. Decreased expression of PAR-1 was observed in cells infected with PAR-1
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shRNA when compared to control cells and PAR-1 misRNA infected cells. (C)
Densitometry analysis shows a significant reduction in PAR-1 expression upon infection
with PAR-1 shRNA lentivirus *** p<0.001.
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Figure 2. Stereotaxic injection of PAR-1 shRNA lentivirus infects neurons and astrocytes in the
ischemic zone
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(A) Lentivirus was injected into rat striatum. One week later the animals were subjected to
MCAo and survived for 24 hours. A schematic representation shows TTC-negative areas in
rat coronal brain sections at the level of Bregma −0.3mm from 6 different animals. The
absent TTC staining concentrated mostly in the striatum indicated by darker color to
demonstrate the consistency of the ischemia area after 2 hr occlusion in our laboratory. The
medial striatum was lesioned in 100% of the subjects and thus the lentivirus injections
targeted this area. (B) High power photomicrograph from the medial striatum of one animal.
GFP positive cells were detected one week following injection. (C, D, F) Confocal
microscopy for several cell-type specific antigens co-localized with GFP labeled cells: NeuN
(neuronal marker), GFAP (astrocytes) and Nissl body (neurons). (E) GFP positive cells did
not co-localize with microglia marker IBA1. (G) Quantification of the proportion of GFP
positive cells co-localizing with GFAP+ or NeuN+ profiles. At least 1000–1500 cells were
counted for the quantification from 7 different animals. Scale bars (B) 100μm; (C, D, E)
50μm; (F) 20μm. In all figures, white arrows indicate GFP positive cells showing colocalization and in (E) the white arrowhead indicates GFP positive cells showing no colocalization with IBA1.
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Figure 3. Distribution of c-Fos positive neurons in the ischemic striatum
(A, B) Animals subjected to 2-hour MCAo and 24 hours reperfusion showed marked
increased expression of c-Fos, an immediate early gene associated with neuronal stress or
injury. Animals pre-injected with mismatch RNA showed greater expression of c-Fos
compared to animals pre-injected with PAR-1 shRNA. (C) Par-1 knockdown significantly
reduced the total area of c-Fos expression in ischemic animals, measured with planimetry
from stained sections, Mean ± SE (n=7). ***P<0.001, Students t-test for independent
samples. (D, E) Cells counted in 20× magnification high power fields showed essentially no
co-localization of PAR-1 shRNA positive GFP cells with c-Fos positive cells. Scale bars (A,
B) 200μm (inset: 100μm); (E) 20μm.
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Figure 4. Representative TUNEL staining of brain sections after MCAo
(A, B) Photomicrographs showing TUNEL positive cells in PAR-1 mismatch and shRNA
injected animals 24 hours after 2hr. of MCAo. (C) PAR-1 shRNA positive GFP cells show
no co-localization with TUNEL positive cells. Animals with PAR-1 knockdown show
significant reduction in dying cells. Scale bars (A, B) 200μm (inset: 100μm); (C) 20μm. (D)
Data presented as Mean ± SE (n=7). ***P<0.001, Students t-test for Independent samples.
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Figure 5. PAR-1 knockdown induces cytoprotection after MCAo
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(A, B) Representative images showing Fluoro-Jade-C positive cells indicating neuronal
damage in mismatch and shRNA PAR-1 lentivirus injected animals. (C) Fluoro-Jade-C
positive cells in a selected high power field. (D) PAR-1 shRNA injected animals show
significantly reduced number of Fluoro-Jade-C positive cells. Scale bars (A, B) 200μm; (C)
100μm. (D) Data presented as mean ± SE (n=7). ***P<0.001, Students t-test for
independent samples.
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Figure 6. Effect of PAR-1 knockdown on neurological function
Stereotaxic injection of PAR-1shRNA in animals 1 week before MCAo mediated significant
recovery of neurological functions compared to PAR-1 misRNA injection. Animals were
tested for forelimb withdrawal when suspended by tail, twisting of animal toward
contralateral side and circling behavior based on a standard rodent grading system. Each
neurological deficit accounted for 1 point for a total score of 3 points. Data presented as
mean ± SE (n=7), ***P<0.001, Students t-test for independent samples.
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Figure 7. PAR-1 knockdown mediates neuroprotection in primary cultured neurons subjected to
oxygen/glucose deprivation
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Primary cultured neurons infected with PAR-1 misRNA or shRNA for 24 hours and then
grown for 5 or 6 days until at least 80% of cells were infected. The cells were divided into
OGD (2 hours) or non-OGD groups at random. During OGD or non-OGD 50U/ml thrombin
was added to the wells. The media was removed 2 hr and 24 hr after OGD followed by LDH
assay to estimate cell death using spectrophotometric analysis at 492 nm. Data is presented
as percentage mean±SE from three different experiments, each carried out in triplicate. Cells
infected with PAR- 1 shRNA showed decreased neuronal cell death after OGD or without
OGD with thrombin treatment in comparison to PAR-1 misRNA infected cells. PAR-1
knockdown significantly rescued the cells exposed to OGD with or without thrombin
treatment compared to misRNA. Inset: Primary cultured cells were treated with increasing
concentration of thrombin (1 U/ml to 100U/ml) for 2 hours and replaced with fresh medium
after treatment. Medium withdrawn at 2 hours and 24 hours was used for LDH assay to
determine percentage LDH release. Thrombin dose above 10U/ml showed a significant
increase in cell death at 24 hours. Mean±SE. In both experiments the final results were
analyzed using two-way ANOVA and the Bonferroni test for post-hoc comparisons.
Significant difference misRNA compared to shRNA: * p<0.05, **p<0.01. Significant
difference thrombin compared to control: # p<0.05, ### p<0.01, *** p<0.001
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Figure 8. PAR-1 knockout animals show reduction in neurovascular damage after acute ischemia
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Animals were subjected to 2 hr MCAo and 48 hr of reperfusion. (A) Brains were processed
for TTC staining for cellular metabolism. Using calibrated planimetry, the area of TTC
pallor was measured and a blinded examiner compared the areas between different groups of
animals. The percent TTC corresponds to percent of the whole brain section. PAR-1−/−
animals showed a significant reduction in comparison with young wild types (* p<0.05) and
PAR-4−/− mice (##p<0.01). (B) Severe vascular disruption was quantified by measuring the
accumulation and leakage of high-molecular weight dextran-FITC fluorescence in coronal
sections. PAR-1−/− animals showed significant reduction in vascular damage compared to
wild type (* p<0.05) and age matched PAR-4 −/− (##p<0.01) mice. The PAR-3−/− animals
also showed significant reduction in vascular disruption in comparison to young wild type
and aged PAR-4−/− mice (@@@ p<.01). N=15–17 for each group; One-way ANOVA
followed by a post hoc Bonferroni adjustment for multiple comparisons.
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Table 1
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Antibody list for immunohistochemistry
Primary Antibody
Species
Vendor/Catalogue#
Secondary Antibody
Vendor/Catalogue#
NeuN
MS-monoclonal
Millipore/MAB377
A594 GtαMs
Invitrogen/A11032
GFAP
MS-monoclonal
Millipore/MAB360
A594 GtαMs
Invitrogen/A11032
C-Fos
Rb-polyclonal
Abcam/ab7963
A594 GtαRb
Invitrogen/A11037
Iba1
Rb-polyclonal
Wako/019-19741
A594 GtαRb
Invitrogen/A11037
PAR-1 receptor
Rb-polyclonal
Santa Cruz Biotechnology/H-111
Goat anti-rabbit IgG-HRP
Santa Cruz/SC-2030
NeuroTrace-Red 530/615
N/A
Invitrogen/N-21482
N/A
N/A
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