(Circulation. 2000;102:1564.)
© 2000 American Heart Association, Inc.
Basic Science Reports |
From the Departments of Cardiology (E.A.W.J.D., L.H., P.A.F.D., E.D, H.J.J.W.), Biochemistry (W.L.v.H., S.V.D.E., C.P.M.R), Surgery (M.A.R.C.D.), Pharmacology (J.F.M.S.), Electron Microscopy (P.F.), and Pathology (M.J.A.P.D.), Cardiovascular Research Institute, Maastricht, the Netherlands.
Correspondence to L. Hofstra, MD, PhD, Department of Cardiology, University Hospital Maastricht, PO Box 5800, 6202 AZ Maastricht, Netherlands. E-mail L.Hofstra{at}cardio.azm.nl
| Abstract |
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Methods and ResultsI/R was induced in mouse hearts by ligation and subsequent release of a suture around the left anterior descending coronary artery. Annexin-V (25 mg/kg) fused to a marker molecule was injected intra-arterially 30 minutes before euthanasia. After 15 minutes of ischemia followed by 30 minutes of reperfusion, 1.4±1.2% (mean±SD) of the cardiomyocytes in the area at risk were annexin-V positive (n=6). This increased to 11.4±1.9% after 15 minutes of ischemia followed by 90 minutes of reperfusion (n=7) and to 20.2±3.3% after 30 minutes of ischemia followed by 90 minutes of reperfusion (n=7). In control mice, including those injected with annexin-V at the binding site of PS, no annexin-Vpositive cells were observed. DNA gel electrophoresis showed typical laddering starting after 15 minutes of ischemia followed by 30 minutes of reperfusion, suggesting activation of the cell death program. Intervention in the cell death program by pretreatment with a novel Na+-H+ exchange inhibitor substantially decreased annexin-Vpositive cardiomyocytes from 20.2% to 2.2% in mice after 30 minutes of ischemia followed by 90 minutes of reperfusion.
ConclusionsThese data suggest that labeled annexin-V is useful for in situ detection of cell death in an in vivo model of I/R in the heart and for the evaluation of cell deathblocking strategies.
Key Words: apoptosis cells myocardial infarction reperfusion proteins
| Introduction |
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One of the earliest events after the triggering of cell death is the externalization of phosphatidylserine (PS) to the outer leaflet of the plasma membrane of the cell.5 6 7 8 In a viable cell, PS is expressed only on the inner leaflet of the cell membrane because of the active transport of PS from the outer to the inner cell membrane by an aminophospholipid translocase.9 However, after activation of the cell death program, PS is externalized rapidly to the outer leaflet of the cell membrane.10 PS externalization is considered to be in close relation to the activation of key players in the cell death program and can be induced by a variety of cell death triggers.5 6 7 11 12 13 14 15
Detection of PS exposure can be easily achieved by the phospholipid binding protein annexin-V.5 6 7 We have demonstrated in a number of in vitro and in vivo studies that annexin-V is a specific marker for the early and late stages of cells undergoing programmed cell death and that annexin-V is also suitable for the in situ detection of cell death.5 6 7 8 16 Therefore, labeled annexin-V provides a tool for in situ detection of cell death in vivo in animals and also, at least potentially, in the clinical setting. In the present study, we evaluated the potential of labeled annexin-V to detect cell death in situ induced by I/R in the murine heart. Furthermore, we determined the time frame of cell death induction in the early phase of I/R of the heart. In addition, we tested whether labeled annexin-V is useful in evaluating the effect of cell deathblocking strategies after I/R.
| Methods |
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Detection of Cardiomyocyte Death With Labeled Annexin-V
Biotinylated annexin-V (Apoptest-Biotin) and
annexin-VOregon green (Apoptest-Oregon-Green) were
provided by NeXins Research BV (Kattendijke, the Netherlands).
Biotinylated annexin-V (25 mg/kg) was injected into the carotid artery
30 minutes before excision of the heart. Annexin-V was always injected
during reperfusion. Biotinylated annexin-V was used for quantification
studies. After excision of the heart and fixation in HEPES-buffered
formalin with 2 mmol/L Ca2+ added, routine
processing, and paraffin-embedding, 4-µm-thick sections were cut
perpendicular to the long axis of the heart. This was followed by
staining with a standard avidin-biotin complex kit (ABC kit, Vector
Laboratories). Diaminobenzidine was used as the chromogen. Sections
were counterstained with hematoxylin. Control experiments were
performed with the use of biotinylated annexin-V mutated for its
binding site to PS in mice after 30 minutes of ischemia
followed by 90 minutes of reperfusion (I/R 30/90 mice, n=6). The kidney
was used as a positive control, because annexin-V is cleared by the
kidney and always shows labeling in successfully injected animals. The
total area of annexin-Vpositive cardiomyocytes was
determined by computerized morphometry (Leica Quantimet 570).
Quantitative data are expressed as the percentage (mean±SD) of the AR
staining positive for annexin-V (annexin-V/ARx100%). In a
subset of I/R 30/90 animals (n=6), Oregon greenlabeled annexin-V was
used for the detection of cardiomyocyte death by
fluorescence microscopy according to a protocol similar to
biotinylated annexin-V injection.
TUNEL Assay
The TUNEL assay (Boehringer-Mannheim) was used for
detection of DNA strand breaks in the hearts. Paraffin-fixed sections
were pretreated with 3%
H2O2, and end-labeling was
performed with TdT (0.06 U/µL) in TdT buffer containing 40 µmol
dig-11-dUTP for 1 hour. Staining with a standard ABC kit was followed
by incubation with diaminobenzidine. Nuclei were considered
apoptotic by the presence of dark brown staining. As a positive
control, we used mouse intestine.
DNA Gel Electrophoresis
The presence of nucleosomal laddering in hearts was investigated
with a commercial ligase-mediated polymerase chain reaction assay kit
(Apoalert, Clontech). Briefly, DNA was isolated from tissue samples
previously frozen at -70°C by using a commercial DNA purification
kit (Wizard, Promega) according to the manufacturers instructions.
DNA purity and concentration were determined by electrophoresis through
an 0.8% agarose gel containing ethidium bromide, followed by
visualization under UV illumination as well as by measuring absorbance
at 260/280 nm. Dephosphorylated adapters were ligated
to 5' phosphorylated blunt ends with T4 DNA ligase
(during 16 hours at 16°C) and served as primers in a ligase-mediated
polymerase chain reaction under the following conditions: hot start
(72°C for 8 minutes), 25 cycles (94°C for 60 seconds and 72°C for
180 seconds), and postcycling (72°C for 15 minutes). To confirm that
equal amounts of DNA were used for polymerase chain reaction, an
internal control using En-2 primer pairs was performed. Amplified DNA
was subjected to gel electrophoresis on a 1.2% agarose gel containing
ethidium bromide.
Immunostaining for Intracytoplasmic IgG
Immunostaining for IgG was performed to
investigate the presence of plasma cell membrane
leakage.17 18 The hearts were removed and processed as
described for annexin-Vbiotin detection. Incubation with a rabbit
anti-mouse IgG antibody (1:100) was followed by incubation with
biotinylated goat anti-rabbit as the secondary antibody. Routine
staining with a standard ABC kit was followed by incubation with
diaminobenzidine. As a positive control, mouse hearts subjected to 24
hours of ischemia were used. Quantitative data are expressed as
the percentage of AR staining positive for IgG (IgG/ARx100%).
Immunoelectron Microscopy
For analysis by electron microscopy, murine hearts
from I/R 30/90 mice were used (n=3). The hearts were fixed after
annexin-Vbiotin labeling of the heart with 2%
paraformaldehyde and 0.2%
glutaraldehyde in Apoptest binding buffer. After
sectioning, the samples were sucrose-infiltrated as follows: 1 mol/L
sucrose in 0.1 mol/L sodium phosphate buffer for several hours,
followed by 2 mol/L sucrose with 15% polyvinylpyrrolidone (molecular
weight 25 000) in 0.1 mol/L sodium phosphate buffer for at least 24
hours. The sucrose-infiltrated samples were vitrified in liquid
nitrogen. The vitrified samples were transferred to Eppendorf
cups containing methanol with 0.5% uranyl acetate at -90°C. After
freeze substitution at several temperature steps (-90°C, -70°C,
-50°C, and -30°C; each step for at least 8 hours), the samples
were embedded in Lowicryl HM20 (Electron Microscopy Sciences) and
polymerized at -30°C under indirect UV light for 24 hours, followed
by direct UV light at ambient temperature for 48 hours. The samples
were cut into ultrathin sections (
70 nm) by using a Reichert-Jung
Ultracut, followed by immunolabeling (at ambient temperature) as
follows: grids were placed on drops of PBS with 50 mmol/L glycine
for 15 minutes, transferred to blocking buffer (PBS containing 5%
[wt/vol] BSA, 10% [vol/vol] newborn calf serum, 0.1% [vol/vol]
cold water fish skin (CWFS) gelatin (Sigma) and 1% [vol/vol]
goat normal serum) for 30 minutes, washed 3 times on drops of labeling
buffer (PBS containing 0.1% [wt/vol] BSA and 0.2% [wt/vol]
BSA-c), incubated with rabbit anti-biotin IgG (Chemicon International)
for 60 minutes, washed 6 times for 5 minutes on drops of labeling
buffer, incubated with goat anti-rabbit IgG (Aurion) conjugated with 10
nm gold diluted at 1:20 for 60 minutes, washed 6 times for 5 minutes on
drops of labeling buffer, washed 5 times for 5 minutes on drops of
plain PBS, postfixed for 5 minutes on drops of PBS containing 2.5%
glutaraldehyde, washed once with PBS for 5 minutes,
washed 5 times for 2 minutes on drops of Milli Q water
(Millipore) and contrasted with 15% uranyl acetate in 50% ethanol for
10 minutes, followed by a 5-minute lead citrate staining. The labeled
sections were air-dried and examined in a Philips CM 10 microscope at
80 keV.
Data Analysis
Data are presented as mean±SD. To compare differences
between groups, the Student t test (Bonferroni correction)
was performed by using SPSS software.
| Results |
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Annexin-Vpositive cells could easily be identified as
cardiomyocytes by their characteristic morphology.
Annexin-Vpositive cardiomyocytes exhibited selective
staining at the cell membrane (Figure 1D
[cell 1] and 1E and
Figure 3A
through 3C). Some punctuated
staining could also be observed, which likely reflects the staining of
the T tubules (Figure 1D
[cell 2]). Annexin-V staining ranged
from staining limited to the cell membrane (Figure 1D
[cell
1]) to intense staining (Figure 1D
[cell 2]), probably
reflecting varying stages of cell death. The injection of colloidal ink
after reocclusion of the LAD enabled us to determine the perfusion area
of the LAD (the AR). Annexin-Vpositive cardiomyocytes
were found only in the ischemic area, devoid of ink (Figure
1F, open arrow). In areas that stained with ink (Figure
1F, closed arrow), positive cardiomyocytes were
never observed (total of n=20). In control experiments, including
sham-operated controls (n=12) and I/R 30/90 mice injected with
biotinylated annexin-V mutated for its binding site to PS (n=6), no
positive cardiomyocytes were found. In the kidney, used as
a positive control, clear annexin-Vpositive tubular epithelial cells
were observed (Figure 1H
).
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To quantify the extent of cells with plasma membrane leakage in
relation to the extent of annexin-V binding, we used IgG
immunostaining (Figure 1G
). The percentage of
IgG-positive cardiomyocytes in the AR was 0.1±0.2% for
I/R 15/30 mice, 1.0±0.7% for I/R 15/90 mice, and 3.9±3.4% for I/R
30/90 mice (Figure 2
). In mice pretreated with the
Na+-H+ exchange
inhibitor, no IgG staining could be observed for I/R 30/90
mice (Figure 2
).
Detection of Cell Death With Annexin-VOregon Green
Detection of cell death with annexin-VOregon green was tested in
I/R 30/90 mice. Evaluation with fluorescence microscopy showed
binding of the annexin-VOregon green complex to the membranes of
cardiomyocytes in the AR (Figure 3A
through 3C).
Detection of DNA Fragmentation
In cardiac specimens obtained from I/R 15/30 mice, some
laddering of DNA could be observed (Figure 4
). Clear DNA laddering could be observed
in cardiac specimens obtained from I/R 15/90 mice and I/R 30/90 mice.
DNA laddering decreased to control levels in cardiac specimens obtained
from I/R 30/90 mice that were pretreated with Eniporide. Some
background laddering was observed in cardiac tissue obtained from
sham-operated animals (controls). Detection of DNA fragmentation in
situ with TUNEL showed no staining of cardiomyocyte
nuclei.
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Annexin-VBiotin: Electron Microscopy
For electron microscopic analysis of cell death in
I/R 30/90 mouse hearts (n=3), we used small tissue sections obtained
from the AR. Sampling of these tissue specimens was guided by
biotinylated annexin-Vstained sections from serial sections of the
same heart that were analyzed by light microscopy. Immunogold
staining with an antibody against biotin demonstrated specific staining
of the cell membranes of cardiomyocytes with an oncotic
morphology, which had bound annexin-Vbiotin during the experiment
(Figure 5A
and 5B
, circles).
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| Discussion |
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A point of debate is the exclusive use of morphological criteria and internucleosomal fragmentation of DNA to study apoptosis in in vivo models of cardiac injury. Possibly, the morphology of programmed cell death in relative large cells, such as cardiomyocytes and neurons, may differ from the classic morphological appearance of smaller and rapidly dividing cells, such as Jurkat cells.21 The limitation of using morphological criteria is also suggested from in vitro studies, which showed a change from Bax-induced apoptotic cell death to membrane-permeability cell death, when caspase activation blockers are used.22 In addition, necrotic cell death and apoptotic cell death may share common biological pathways.23 Therefore, for development of cell deathblocking strategies in the heart, it may be more efficient to focus on the understanding of biochemical pathways of cell death rather than to study the morphology of dying cells.
In conclusion, our data show that labeled annexin-V is a valuable
marker for the in situ detection of cell death induced by I/R of the
mouse heart and is useful in the evaluation of cell deathblocking
strategies. Our data obtained with annexin-VOregon green support the
idea that in vivo imaging with labeled annexin-V is possible. In a
limited number of mice, detection of fluorescent annexin-V was
performed in vivo, with the use of an operation microscope with
fluorescence equipment. Annexin-Vpositive areas were observed
in the AR (Figure 3D
). The specific and massive annexin-V
staining of cardiomyocytes in the AR of the mouse hearts
subjected to I/R suggests that in vivo detection of cell death with
labeled annexin-V in patients suffering from an acute myocardial
infarction may be possible. An interesting option may be the use of
technetium-labeled annexin-V and detection with nuclear
imaging. The data in the present study provide essential
information as to the time of injection of labeled annexin-V and the
time course of binding of annexin-V to PS in the acute phase of
myocardial I/R. Preliminary data on cell death detection with
technetium-labeled annexin-V have shown promising
results.24
| Acknowledgments |
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| Footnotes |
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Received March 27, 2000; revision received May 2, 2000; accepted May 2, 2000.
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