|
|
||||||||
1 From the Department of Immunology and Microbiology and the 2 Cytology Research Core Facility, Wayne State University School of Medicine, Detroit, Michigan.
| Abstract |
|---|
|
|
|---|
METHODS. C57BL/6J mice were intracorneally infected with P. aeruginosa. The expression of MT4-, MT5-, and MT6-MMP was detected at both the mRNA and protein levels by RT-PCR and immunoblot analysis. Immunohistochemical staining was performed to localize the expression of MT4- and MT5-MMP in the mouse corneas.
RESULTS. Expression of MT4- and MT5-MMP was detected in the normal (uninfected) cornea by RT-PCR and immunoblot analysis. When infected with P. aeruginosa, the corneas showed significant induction of each MT-MMP. Localization of MT4- and MT5-MMP revealed that the expression of MT5-MMP was restricted to the epithelial tissue in the normal cornea, whereas the induced expression of MT4- and MT5-MMP was predominantly in the substantia propria, which contained most of the infiltrating cells. MT6-MMP expression was not detected in the uninfected cornea but was upregulated in the infected corneas.
CONCLUSIONS. Expression of MT4-, MT5-, and MT6-MMP was induced in corneas infected with P. aeruginosa. Immunohistochemistry showed predominant immunoreactivity of MT4- and MT5-MMP in the substantia propria. Previous histologic studies have revealed different patterns of inflammatory cell infiltration with an increased number of polymorphonuclear neutrophils (PMNs) during the early stage of inflammation and increased macrophages during the late stage. These results indicate a good correlation between the overexpression of the MT-MMPs in the infected corneas and the inflammatory responsethat is, leukocyte infiltrationindicating that inflammatory cells such as macrophages and PMNs may play a role in the upregulation of MT-MMPs during corneal infection, which in turn can cause the destruction of corneal tissue.
| Introduction |
|---|
|
|
|---|
MMPs are a group of structurally related and zinc-dependent enzymes that play a crucial role in physiologic and pathologic processes, such as embryonic development, endometrial cycling, wound healing, rheumatoid arthritis, tumor invasion, and metastasis.2 MMPs have been classified into five broad categories based on their substrate specificity and domain organization. They include collagenases (MMP-1, -8, -13, and -18), stromelysins (MMP-3, -10, and -11), gelatinases (MMP-2 and -9), MT-MMPs (MMP-14, -15, -16, -17, -24, and -25), and other MMPs (MMP-7, -12, -19, -20, -21/22, and -26).3 4 5 6 7 Since the first member was identified by Sato et al. in 1994,8 the MT-MMP subgroup has grown rapidly and now has six members named after the transmembrane domains that anchor the molecules on the plasma membrane.3 5 6 MT-MMPs are important in turnover of the extracellular matrix (ECM), because of their dual functions. Similar to other MMPs, they directly cleave ECM proteins, including type I and III collagens, fibronectin, and laminin. In addition, they activate other MMPs such as pro-MMP-2 (gelatinase A) and pro-MMP-13 (collagenase 3) and form an enzymatic cascade for regulation of ECM degradation.9 10
Although classified in the same subgroup, MT4-, MT5-, and MT6-MMP are
different from the other three MT-MMPs in some respects. For example,
MT4-MMP has the least sequence identity with other MT-MMPs, except for
MT6-MMP.11
12
Both MT4-MMP and MT6-MMP are predominantly
expressed in leukocytes, and both are
glycosylphosphatidylinositol-anchored MMPs.12
13
14
15
In
addition, MT4-MMP also shows TNF-
convertase activity and can
potentially process pro-TNF-
to the 17-kDa form.11
These characteristics of MT4-MMP and MT6-MMP suggest that they play a
role in inflammation. However, the normal functions of MT4-, MT5-, and
MT6-MMP remain unknown. Their expression and regulation in the cornea
have not been reported.
In the present study, we explored expression of newly identified MT4-, MT5-, and MT6-MMP in the corneas of naïve C57BL/6J mice infected with P. aeruginosa. RT-PCR, immunoblot analysis, and immunohistochemical staining revealed the significant induction of MT4- and MT5-MMP expression at both the mRNA and protein levels in naïve mice over an 8-day period. Similar results were obtained for MT6-MMP gene expression using only RT-PCR due to the absence of available antibody to mouse MT6-MMP.
| Methods |
|---|
|
|
|---|
Infection of Animals and Corneal Sample Collection
All animals were treated in accordance with the ARVO Statement
for the Use of Animals in Ophthalmic and Vision Research. Age-matched
C57BL/6J mice (Jackson Laboratories, Bar Harbor, ME), each weighing 18
to 22 g, were infected at 14 weeks of age. Before infection, they
were lightly anesthetized with ether and placed beneath a stereoscopic
microscope. The corneal surface was then gently incised with three 1-mm
incisions using a sterile 26-gauge needle, taking care not to penetrate
the anterior chamber or to damage the sclera. A bacterial suspension (5
µl) containing 108 colony-forming units was
topically delivered onto the wounded cornea using a micropipette with a
sterile disposable tip. Mice were examined 24 hours later to verify
infection. Controls consisted of scratched and unscratched mice that
were uninfected.
At selected time points after infection, mice were killed and corneas were excised. Individual samples consisted of 12 pooled corneas per time point for RT-PCR and immunoblot analysis. Single corneas from different time points were used for immunohistochemical staining and histologic study. Immediately after isolation, corneas were rinsed in sterile saline and then processed for the purpose of the different assays. Control mice were treated similarly.
Semiquantitative RT-PCR
Total RNA was extracted from the harvested corneas with
extraction reagent (TRIzol; Gibco, Grand Island, NY) according to the
manufacturers instruction. The total RNA was dissolved in water
treated with diethylpyrocarbonate (DEPC), and the concentration was
measured by a spectrophotometer (UV-1601; Shimadzu, Kyoto, Japan). The
samples were treated with DNase I (0.2 U/µl; Ambion, Austin, TX) to
remove possible DNA contamination. All the reagents needed for RT-PCR
were purchased from Perkin Elmer (Norwalk, CT) except Taq
DNA polymerase and the dNTP mixture, which were purchased from Gibco.
RT-PCR was performed as previously described.1
Briefly, RT
was performed in 0.65-ml RNase-free tubes under optimized conditions in
a DNA thermal cycler (model 480; Perkin Elmer). Equal amounts of total
RNA (500 ng) from each sample were used for this reaction. The whole
product of RT in each tube was amplified by PCR. Cycle parameters were
generally a 1-minute melting step at 95°C, a 1-minute annealing step
at 55°C, and a 2-minute extension step at 72°C. Thirty cycles were
selected for amplification of all target genes, based on results of the
experiments that tested the linear range of amplification with
different cycles. The following specific primers for MT4-, MT5-, and
MT6-MMP were designed and prepared according to the available
information for these specific genes (GenBank, provided in the public
domain by the National Center for Biotechnology information, Bethesda,
MD, and available at http://www.ncbi.nlm.nih.gov/genbank). Because the
mRNA sequence for mouse MT6-MMP has not been reported, the primers for
human MT6-MMP were prepared and used, assuming there is high homology
between human and mouse MT6-MMP. The primers of mouse MT4-MMP (5'-TGG
TCT GAT GGT GCA TCC TA-3' and 5'-TGC AGG AAC ATA CTG CCA GA-3')
amplified a 205-bp product (gene sequence 1550-1754); the primers of
mouse MT5-MMP (5'-GTG ACA GCT CCC CAT TTG AT-3' and 5'-TAG AGT GCT CCA
AGC CCA GT-3') amplified a 192-bp product (682-873); and the primers of
human MT6-MMP (5'-ATG GCC TGC AGC AAC TCT AT-3' and 5'-AGG GGC CTT TGA
AGA AGA AA-3') amplified a
200-bp product. There were two negative
controls: one without reverse transcriptase and the other one without
specific primers. The housekeeping gene 18S RNA (Ambion) was also
amplified and used as an internal control for the comparison of all
time point target genes. Finally, the amplified genes were resolved by
1% agarose gels and revealed by ethidium bromide staining. RT-PCR for
each MT-MMP was repeated at least twice.
To confirm the specificity of the primers, the amplified DNA sequences
were subjected to restriction enzyme digestion. The 205-bp PCR product
was treated with SmaI (Gibco), which recognizes the sequence
CCC
GGG. The 192-bp product was treated with MboI (Gibco),
which recognizes the sequence
GATC. Each of them was incubated with
respective enzyme (5 U) for 90 minutes, with different reaction buffer
and at different temperature, according to the manufacturers
instruction. The results were revealed by 3% agarose gel
electrophoresis.
The
200-bp PCR product was purified with a kit (MinElute; Qiagen,
Valencia, CA) according to the manufacturers instruction. The
concentration of the purified DNA was measured by the spectrophotometer
at 260 nm. The purified DNA at a concentration of 10 ng/µl and human
MT6-MMP primers at a concentration of 1 pM/µl were then prepared. The
DNA samples were analyzed in the DNA Sequencing Facility at the Center
for Molecular Medicine and Genetics (Wayne State University, Detroit,
MI).
Immunoblot Analysis
Cornea samples were immediately frozen with liquid nitrogen
after isolation from mice. To enrich the extracts with membrane-bound
proteins, TBS-CM buffer (50 mM Tris [pH 7.5], 150 mM NaCl, 1 mM
CaCl2, and 1.5% Triton X-114) was used. Samples
were homogenized with TBS-CM buffer on ice and then centrifuged at
12,000 rpm at 4°C for 2 minutes. The supernatants were collected and
incubated at 37°C for 3 minutes, followed by centrifugation at 12,000
rpm for 2 minutes. The detergent phase, which contained enriched
membrane-bound proteins, was saved for immunoblot analysis. The
concentrations of the total protein were measured with a bicinchoninic
acid (BCA) protein assay. Equal amounts of individual samples (5 µg)
were mixed with 5 µl of 4x sample loading buffer (0.125 M Tris-HCl
[pH 6.8], 4% SDS, 40% glycerol, and 0.02% bromphenol blue)
containing ß-mercaptoethanol and boiled for 5 minutes. The samples
and a prestained molecular weight marker (Bio-Rad, Cambridge, MA) were
electrophoresed on 12% SDS gels and subsequently transferred to
nitrocellulose membranes. The membranes were blocked for 30 minutes in
blocking reagent (Blotto; Santa Cruz Biotechnology, Santa Cruz, CA;
TBS, containing 0.5% Tween 20, 3% nonfat milk, and 2% bovine serum
albumin) and then incubated with a polyclonal anti-human MT4-MMP
(AB854,1 µg/ml; Chemicon, Temecula, CA) and a polyclonal anti-human
MT5-MMP antibody (AB924, 0.2 µg/ml; R&D, Minneapolis, MN),
respectively, on a rocker at room temperature for 2 hours. The species
reactivity for MT5-MMP is positive to both human and mouse according to
the information from R&D. In addition, alignment of mouse MT4- and
MT5-MMP protein sequences with that of human revealed 82.0% and 92.6%
of homology, respectively. Samples treated with nonimmune (normal)
rabbit IgG or without primary antibody treatment were processed as the
negative control. Afterward, the blot was incubated with anti-rabbit
secondary antibody conjugated with horseradish peroxidase (0.5 µg/ml;
Roche Molecular Biochemicals, Indianapolis, IN) at room temperature for
1 hour. Finally, the blot was developed by a chemiluminescence kit
(Amersham, Arlington Heights, IL), and MT4- and MT5-MMP were visualized
as dark bands with corresponding molecular weights. Immunoblot analysis
of each MT-MMP was repeated at least twice.
Immunohistochemical Staining
The corneas, harvested on days 0 (uninfected) and 6 after
infection (peak inflammatory response), were fixed with 10% buffered
formalin overnight, dehydrated with increased concentrations of ethanol
and 100% xylene, infiltrated with paraffin overnight, and embedded in
fresh paraffin. The tissues were sectioned (4 µm) to prepare slides,
and the slides were deparaffinized, rehydrated, and treated with
proteinase K (Sigma, St. Louis, MO). Normal horse serum and 3%
hydrogen peroxide were applied separately to reduce nonspecific
staining and to remove endogenous peroxidase. The slides were treated
with specific primary antibodies recognizing mouse MT4- and MT5-MMP
(the same as used for immunoblot analysis) in a humidified chamber at
room temperature for 2 hours, followed by subsequent treatment with a
biotinylated anti-rabbit secondary antibody and the ABC reagents
(Vectastain; Vector Laboratories, Burlingame, CA). Positive staining
was exhibited by diaminobenzidine (DAB; Vector) treatment appearing as
brown granules.
| Results |
|---|
|
|
|---|
200-bp mRNA segment that was amplified by
human MT6-MMP primers and peaked at day 4. DNA sequencing and alignment
with human MT6-MMP revealed 46.6% of gene homology of this DNA segment
when compared with human MT6-MMP (data not shown). In addition, there
was no significant similarity between the amplified DNA and other known
genes.
|
There was no difference in any MT-MMP mRNA expression between the scratched corneas and the unscratched corneas on day 0, indicating that corneal abrasion did not affect the mRNA expression (data not shown). In addition, negative control samples (those not treated with reverse transcriptase or amplified without specific primers) did not exhibit any MT-MMP expression (data not shown).
To confirm MT-MMP expression at the protein level and to compare MT-MMP
expression in the corneas at different time points during the
infection, immunoblot analysis of corneal extracts was performed. As
shown in Figure 2
, MT4-MMP with a molecular weight of
68 kDa was detected in the
uninfected mouse corneas (day 0 sample), which had a molecular weight
similar to human MT4-MMP identified in brain
extracts.13
In corneas infected with P.
aeruginosa, the expression of MT4-MMP was gradually induced and
remained at a high level on day 8. MT5-MMP expression in corneas was
different from that of MT4-MMP. MT5-MMP with a molecular weight of
39 kDa, probably the soluble species, was found in normal
corneas.5
The expression was reduced from day 1 through
day 5 after infection. There was a significant induction of MT5-MMP
expression on day 7 after infection. These results were consistent with
the gene expression of MT5-MMP during the infection.
|
Localization of MT4- and MT5-MMPs in Mouse Corneas
To localize MT-MMP expression in the mouse corneas,
immunohistochemical staining was performed using the corneal samples of
normal and infected mice. As shown in Figure 3
, no positive staining of MT4-MMP was found in the normal cornea,
although immunoblot analysis showed a weak band of MT4-MMP on day 0
(normal cornea), perhaps because there is much less sensitivity in
immunohistochemical staining than in immunoblot analysis. On day 6,
MT4-MMP expression was localized in both epithelial tissue (weak
positive staining) and substantia propria (strong positive staining).
MT5-MMP expression was observed in corneas of normal mice, mainly
localized in the epithelium. On the sixth day after infection, the
distribution of MT5-MMP was not restricted to the epithelium but was
also detected in the substantia propria.
|
| Discussion |
|---|
|
|
|---|
Although a number of studies have begun to reveal the effects of some MMPs on corneal tissues under physiological and pathologic conditions, little is known about the expression and regulation of MT-MMPs in the cornea. Our recent work on MT1-, MT2-, and MT3-MMP showed that these three MT-MMPs are all present in the normal mouse cornea and that there is a significant induction of each of them in the corneas of naïve (unimmunized) mice intracorneally infected with P. aeruginosa, whereas the induction in the infected corneas of immunized mice reaches a much lower level and has a shorter duration.1
In the present study, we further investigated the expression and regulation of another three recently identified MT-MMPsMT4-, MT5-, and MT6-MMP, in the mouse corneas infected with P. aeruginosa. MT4- and MT5-MMP were identified in the normal (uninfected) mouse cornea at both the mRNA and protein levels, whereas MT6-MMP was not detectable by RT-PCR. Similar to MT1-, MT2-, and MT3-MMP, expression of MT4-, MT5-, and MT6-MMP also showed a significant induction in corneas when mice were intracorneally infected with P. aeruginosa. However, the induction pattern was different from that previously observed (MT1-, MT2-, and MT3-MMP). MT4-MMP expression in the infected corneas did not change dramatically until day 5 after infection. It was reported that MT4-MMP was mainly expressed by leukocytes, especially by monocytes and macrophages.11 Indeed, our data showed strong positive staining of MT4-MMP in the cornea on day 6 after infection. Moreover, our previous histologic study also showed increased macrophage infiltration of the corneas during days 6 and 9 after infection.1 Therefore, the infiltrating macrophages seemed responsible for the induction of MT4-MMP expression in the cornea during the late stage of the inflammation. Although MT5-MMP was primarily identified in human brain tumor,24 basal expression of MT5-MMP was also found in our corneal studies, primarily localized in the epithelial tissue.
Downregulation of MT5-MMP during the early stage of the infection (days 24) possibly resulted from injury of epithelial cells, because they seem to be the only source of MT5-MMP in the normal cornea, according to our studies of immunohistochemical staining. Later induction of MT5-MMP was probably also due to the infiltration of macrophages into the infected cornea. MT6-MMP, also called leukolysin,12 was not detected in the normal cornea by RT-PCR, but with infection, there was a significant induction of MT6-MMP expression in corneas. It was reported that MT6-MMP was predominantly expressed in leukocytes.6 12 Our data regarding MT6-MMP induction correlate well with our previous findings on corneal inflammation as quantified by bacterial numbers, PMNs measured by myeloperoxidase, arachidonic acid metabolites, and cytokines.25 26 27 In addition, previous histologic studies showed that PMNs peak at day 6 after infection and then diminish gradually, which has good correlation with the MT6-MMP induction pattern.1 Thus, induced expression of MT6-MMP in the cornea may be produced by PMNs.
As members of the MMP family, MT4- and MT5-MMP hydrolyze a variety of ECM components, such as fibronectin, gelatin, fibrinogen, fibrin, and proteoglycans.11 28 The induced enzymatic activity during the inflammation may cause degradation of the Bowman membranes and ECM components, leading to severe corneal damage. Similar to other MT-MMPs, MT4-, MT5-, and MT6-MMP are also capable of activating pro-MMP-2.5 6 29 Therefore, the induction of these MT-MMPs in the mouse corneas infected with P. aeruginosa may also be responsible for corneal damage caused by other proteases that are activated by these MT-MMPs.
| Acknowledgements |
|---|
| Footnotes |
|---|
Submitted for publication January 26, 2001; revised August 10, 2001; accepted August 13, 2001.
Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked
"advertisement" in accordance with 18 U.S.C.
1734
solely to indicate this fact.
Corresponding author: Richard S. Berk, Department of Immunology and Microbiology, Wayne State University School of Medicine, 540 E. Canfield, Detroit, MI 48201. rberk{at}med.wayne.edu
| References |
|---|
|
|
|---|
This article has been cited by other articles:
![]() |
P. Karicherla and J. A. Hobden Nona-D-Arginine Therapy for Pseudomonas aeruginosa Keratitis Invest. Ophthalmol. Vis. Sci., January 1, 2009; 50(1): 256 - 262. [Abstract] [Full Text] [PDF] |
||||
![]() |
S. A. McClellan, X. Huang, R. P. Barrett, S. Lighvani, Y. Zhang, D. Richiert, and L. D. Hazlett Matrix Metalloproteinase-9 Amplifies the Immune Response to Pseudomonas aeruginosa Corneal Infection Invest. Ophthalmol. Vis. Sci., January 1, 2006; 47(1): 256 - 264. [Abstract] [Full Text] [PDF] |
||||
![]() |
N. Kumagai, K. Fukuda, Y. Fujitsu, Y. Lu, N. Chikamoto, and T. Nishida Lipopolysaccharide-Induced Expression of Intercellular Adhesion Molecule-1 and Chemokines in Cultured Human Corneal Fibroblasts Invest. Ophthalmol. Vis. Sci., January 1, 2005; 46(1): 114 - 120. [Abstract] [Full Text] [PDF] |
||||
![]() |
R. S. Berk, Z. Dong, S. Alousi, M. A. Kosir, Y. Wang, and I. Vlodavsky Murine Ocular Heparanase Expression before and during Infection with Pseudomonas aeruginosa Invest. Ophthalmol. Vis. Sci., April 1, 2004; 45(4): 1182 - 1187. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |