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From the W. M. Keck Center for Transgene Research and the Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana
| Abstract |
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-chain gene. The resulting
FG-/- animals did not possess detectable
plasma Fg. FG-/- mice were then used to
assess the roles of Fg and Fn in a bleomycin-induced acute lung injury
model. Intratracheal administration of bleomycin in wild-type and
FG-/- mice resulted in equivalent
deposition of interstitial collagen and fibrotic lesions at days 7 and
14 after administration. This indicates that Fg and/or Fn are not
essential for the development of bleomycin-induced pulmonary
fibrosis.
| Introduction |
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/Bß/
)2,1
which is altered
during blood coagulation into an insoluble polymeric network of fibers
that forms the basis of the blood clot.2
Additionally, it
is believed that Fg participates in events other than hemostasis. For
example, Fg is an acute phase reactant that responds to injury and
stress3,4
and its depletion in mice has been shown to
contribute to host susceptibility to infection.5
This
protein also seems to be necessary for early events in immune
inflammatory responses via stimulation of leukocyte adhesion and
migration across endothelial cell layers.6
Fibrin (Fn)
co-localizes with collagen-rich fibrotic lung lesions and has been
identified as a key molecular effector of the fibroproliferative
response after injury.7
In addition, Fg and Fn have been
implicated in the progression of certain types of
tumors,8-11
in severity of
atherosclerosis,12
and in the efficacy of wound
healing.13,14
The ability of Fg and Fn to interact with a
variety of cell types through integrin15
and
nonintegrin16
receptors likely accounts for the roles of
these proteins in several different cell-related pathophysiological
conditions.
Before secretion of the intact Fg, its three chains are
assembled in a stepwise manner in the rough endoplasmic
reticulum.17
If genetic defects in the A
chain,18
Bß chain,19
or
chain20,21
exist, impaired secretion can occur that
results in congenital hypofibrinogenemia or afibrinogenemia. Complexes
of A
/Bß and Bß-
chains associate with
and A
chains in
the lumenal space of the endoplasmic reticulum, forming Fg,
which is then secreted as the native dimeric protein.22
A targeted deletion of the A
chain in mice led to animals
without detectable plasma Fg levels.23
In the current
investigation, we wished to similarly generate mice with a targeted
deletion of the
-chain gene (FG-
), reasoning
that these mice would also lack plasma Fg because of the requirement
for
/
and ß/
chains to serve as intermediates for the
synthesis of the heterodimeric protein. In addition, not only would the
availability of Fg-deficient mice allow analysis of the role of Fg in
various disease models, but generation of specific
-chain-deficient
mice would allow facile and rapid development and phenotypic analyses
of transgenic animals containing mutations of the
chain, which
possibly possess loci for platelet binding,24-28
leukocyte adhesion,6,15
Fn
polymerization,21,29-31
and cleavage sites for
MMPs.32
Therefore, the
-chain gene of Fg was
deleted from mice and an analysis of the effect of a Fg deficiency on
fibrotic lesion development after acute lung injury is the subject of
the current report.
| Materials and Methods |
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Chain
FG-
chain-deficient mice were generated by targeted
inactivation of the murine
chain of Fg by homologous recombination
in R1 embryonic stem (ES) cells. The steps involved have been described
in earlier reports.33-35
Southern Blot Analysis of ES Cells
Correctly targeted ES cells were identified by Southern blot
analysis of EcoRI digests of purified ES cell genomic DNA. A
0.85-kb fragment located downstream of the 3' homologous flanking
region was used as a probe to identify an 11.5-kb WT and
6.5-kb disrupted FG-
-chain allele. A full
description of the methods used has been published.35
Polymerase Chain Reaction (PCR) Analysis of Genomic DNA
FG-
genotyping of mice was conducted by PCR analysis
of purified genomic DNA from ear-punch tissue. An aliquot of 1 µl of
genomic DNA was used in a single PCR reaction. The reaction mixture
contained the following three primers: a NEO gene-specific
primer (5'-GGTTCTAAGTACTGTGGTTTC-3'), a sequence within the
5'-noncoding flank of the FG-
chain
(5'-CACAGCGGCTTGTCATTAG-3'), and a sequence internal to the
FG-
chain (5'-CTGAAAGACCTGTCTTTGC-3'). The PCR reaction
consisted of an initial denaturation step (94°C for 3 minutes) and 30
cycles of the following (94°C for 30 seconds, 58°C for 40 seconds,
and 72°C for 50 seconds), with a final annealing step at 72°C for 5
minutes. The PCR products, 420 bp for the WT allele and 550
bp for the FG-
-chain disrupted allele, were
resolved on a 1% agarose gel.
Thrombin Clot Time of Plasma
The assay was conducted in a standard fibrometer (BBL Microbiology
Systems). Whole blood (0.8 to 0.9 ml) was isolated from the vena
cava of anesthetized WT,
FG+/-, and
FG-/- mice and placed into 0.1 ml of
a 4% sodium citrate solution. The samples were subjected to
centrifugation at 3,000 rpm for 10 minutes in a microcentrifuge and the
plasma was transferred to a fresh tube for analysis. A volume of 100
µl of 50 mmol/L Tris-HCl, pH 7.4, was incubated for 3 to 10 minutes
at 37°C in a fibrometer cup, after which 100 µl of fresh plasma was
added and allowed to incubate an additional 1 minute. After 1 minute,
100 µl of 5 NIH U/ml of human
-thrombin (Enzyme Research
Laboratories, South Bend, IN) was added and the fibrometer timer was
started. Samples were analyzed in duplicate.
Western Blot Analysis of Plasma
Western blot analysis was carried out using standard procedures using a rabbit-anti-human Fg antibody which recognizes all three chains (to differing extents) and cross-reacts with mouse Fg (DAKO, Carpinteria, CA). Plasma samples were prepared as described above for thrombin times and 0.1 µl was loaded onto a 9% sodium dodecyl sulfate gel.
Bleomycin-Induced Pulmonary Fibrosis
WT (n = 5) and FG-/- (n = 6) anesthetized mice, 4 weeks of age, were administered, intratracheally, a single dose (4.5 U/kg) of bleomycin in 25 µl saline by infusion through the vocal cords using a fiber-optic light source for illuminating the tracheal opening. Control animals of similar genotypes were treated with saline alone. At 7 and 14 days after bleomycin infusion, anesthetized mice were perfused with saline and the left lung excised for determination of the collagen content (14 days), while the right lung was processed for histological analyses (7 and 14 days).
Quantitation of Lung Collagen Content
Total lung collagen content in saline and bleomycin-treated mice was determined by a colorimetric assay measuring hydroxyproline levels of acid hydrolyzed lung tissue.36
Histology and Immunohistochemistry
Tissues were processed, embedded in paraffin, and sectioned at 3 to 4 µm. Hematoxylin and eosin (H&E) staining of tissue was performed to identify the tissue and to detect alterations in cellular architecture. Massons trichrome staining was used to identify collagen-rich fibrotic lesions in the lung.37 Fn, in the lung, was identified by immunohistochemistry using a polyclonal goat-anti-mouse fibrin(ogen) antibody (Accurate Chemical, Westbury, NY) and a secondary rabbit-anti-goat IgG (DAKO), followed by goat peroxidase/anti-peroxidase (DAKO). The substrate, 3'-aminoethyl carbazole (AEC, Biomeda, CA), was then used to detect peroxidase activity. The slides were counterstained with hematoxylin.
Statistical Analysis
Where appropriate, values were expressed as the mean ± SEM using a Microsoft Excel program.
| Results |
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Gene
A targeting vector was constructed in which the NEO
gene replaced all coding regions of the FG-
chain (Figure 1a)
. This altered gene was inserted in
place of the FG-
-chain gene in the mouse
genome by homologous recombination. Two independent correctly targeted
ES clones (Figure 1b)
were aggregated with Swiss morula stage embryos,
yielding seven chimeric animals with 80 to 95% coat color chimerism.
Chimeric mice transmitted the inactivated FG-
gene to
their offspring. Intercrossing of these heterozygous
FG-
-chain-deficient mice
(FG+/-) generated homozygous
FG-
-deficient
(FG-/-) progeny for
phenotypic analysis (Figure 1c)
. Fg antigen was not detected in
FG-
-chain null offspring as determined by
Western blot analysis of plasma (Figure 1d)
and immunostaining of liver
sections (not shown). All animals used in this study were 50%
129SvJ/50% C57Bl/6.
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A significant (
25%) percentage of
FG-/- neonates suffered from
hemorrhaging events. The bleeding was primarily abdominal but was also
observed in the cranial, neck, and nasal areas. Although these events
initially seemed dramatic, many
FG-/- mice were able to resolve the
bleeding and recover.
Thrombin Clot Time of Plasma
Thrombin-induced clot times were significantly delayed in FG-/- mice relative to WT mice (>17 minutes, n = 2, versus 35.5 ± 3.5 seconds, n = 3, respectively). When it was observed that FG-/- samples did not clot after 17 minutes, timing was stopped. Other samples left unattended never formed a clot. FG+/- mice demonstrated thrombin clot times that were similar to WT mice (35.9 ± 4.3 seconds, n = 3).
Bleomycin-Induced Pulmonary Fibrosis
WT and FG-/- mice
developed pulmonary fibrosis to a similar extent, as evidenced by the
equivalent increase, approximately two-fold, of total lung collagen
levels, reflected by the hydroxyproline content 14 days after
intratracheal administration of bleomycin relative to saline controls
(2.0 µg/mg dry lung, n = 6, versus 3.0
µg/mg dry lung, n = 4, for WT mice, and
1.3 µg/mg dry lung, n = 2, versus 2.4
µg/mg dry lung, n = 5, for
FG-/- mice). H&E staining of lung
tissue from WT and Fg-/- mice
receiving bleomycin identified focal areas of interstitial fibrosis
(Figure 2, a and d)
. Massons Trichrome
staining of lung tissue from these mice indicated that collagen
deposition was associated within the fibrotic lesions to a similar
extent in WT and FG-/-
mice (Figure 2, b and e)
. In WT mice treated with bleomycin,
Fg/Fn co-localized with the collagen-rich lesions (Figure 2c)
.
FG-/- saline-treated animals
demonstrated normal lung architecture (Figure 2g)
as well as
significantly diminished collagen levels relative to bleomycin-treated
mice (Figure 2h)
. Massons trichrome staining of lung tissue from
WT and FG-/- mice, as
early as 7 days after bleomycin treatment, indicated collagen-rich
fibrotic lesions (Figure 3, a and b)
.
Analysis of H&E-stained lung tissue from 7-day treated mice indicated
enhanced levels of neutrophils and macrophages in both WT
and FG-/- relative to their
respective saline controls (data not shown).
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| Discussion |
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In this investigation, mice deficient for the
chain of Fg, that
consequently possessed a total deficiency of plasma Fg, were generated
and the development of bleomycin-induced pulmonary fibrosis assessed.
Early inflammatory response, 7 days after bleomycin treatment, seemed
to be equivalent between WT and
FG-/- mice. It was also observed
that there was no significant difference in the development of
collagen-rich pulmonary fibrotic lesions between WT and
FG-/- mice 7 and 14 days after
administration of bleomycin. This indicates that fibrin(ogen) or
fibrinopeptides may not be required for fibroblast proliferation and
possibly other related events such as fibroblast migration after
bleomycin treatment.40
Other matrix proteins may be
involved in supporting the fibrotic process after lung injury. For
example, studies of lung biopsies from patients with pulmonary fibrosis
have indicated that fibroblasts increase their synthesis of collagen
and fibronectin, coordinately, within airspaces.41
Therefore, although Fn is not essential for the progression and
persistence of fibrosis, matrix proteins, other than collagen, may play
prominent roles in supporting the lung repair process after injury.
A recent study wherein bleomycin-induced pulmonary fibrosis was investigated in mice deficient for plasminogen (Pg), tissue-type plasminogen activator (tPA), urokinase-type plasminogen activator, and urokinase plasminogen activator receptor demonstrated enhanced fibrotic lesion formation in Pg and activator deficient mice.42 Collagen levels were especially enhanced in PG-/- and TPA-/- mice, relative to WT mice. In the PG-/- mice a persistence of Fn deposition was observed that co-localized with collagen-rich fibrotic lesions, thus implicating an involvement of Fn with the progression of the repair process. The current study with FG-/- mice presents a key investigation that expands an analysis of the involvement of the fibrinolytic process in bleomycin-induced pulmonary fibrosis and suggests another role for plasmin in this process, possibly acting as a regulatory protein of collagen turnover through activation of matrix metalloproteinases, specifically collagenases.
| Acknowledgements |
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| Footnotes |
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Supported by grants HL13423 (to F. J. C.) and HL63682 (to V. A. P.) from the National Institutes of Health, a National Scientist Development Award (9630009N) from the American Heart Association (to V. A. P.), the Kleiderer-Pezold endowed professorship (to F. J. C.), and a grant from the W. M. Keck Foundation (to F. J. C.).
Accepted for publication May 23, 2000.
| References |
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