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Originally published online as doi:10.1189/jlb.0108016 on March 11, 2008

Published online before print March 11, 2008
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(Journal of Leukocyte Biology. 2008;83:1404-1412.)
© 2008 by Society for Leukocyte Biology

Matrix metalloproteinase-7 (matrilysin) controls neutrophil egress by generating chemokine gradients

Mei Swee*,{dagger}, Carole L. Wilson{ddagger}, Ying Wang*,{dagger}, John K. McGuire*,{dagger},§ and William C. Parks*,{dagger},1

* Center for Lung Biology and Institute for Stem Cell and Regenerative Medicine and Departments of
{dagger} Medicine (Pulmonary and Critical Care Medicine),
{ddagger} Pathology, and
§ Pediatrics (Critical Care Medicine), University of Washington, Seattle, Washington, USA

1Correspondence: Center for Lung Biology, University of Washington, 815 Mercer Street, Seattle, WA 98109, USA. E-mail: parksw{at}u.washington.edu


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ABSTRACT
 
Matrilysin [matrix metalloproteinase 7 (MMP7)] is induced by mucosal injury of many tissues. To assess function of this proteinase, we subjected wild-type and Mmp7–/– mice to acute colon injury. When matrilysin expression was increasing, 73% of wild-type mice died, whereas only 32% of Mmp7–/– mice succumbed. Although re-epithelialization was delayed in Mmp7–/– mice, overall injury did not differ markedly between genotypes. We hypothesized that differences in acute inflammation caused increased mortality in wild-type mice. Indeed, whereas overall neutrophil influx into tissue was similar in wild-type and Mmp7–/– mice, their location and extent of migration differed between genotypes. Neutrophils were dispersed throughout the mucosa and within the lumen of wild-type mice, but these leukocytes were largely confined to the submucosa in Mmp7–/– mice. The levels of neutrophil chemokines, keratinocyte-derived chemokine and MIP-2, increased in the colon tissue of both genotypes, but these factors were detected only in lumenal lavages of wild-type mice. Our findings indicate that matrilysin mediates beneficial and deleterious effects in response to injury. On one hand, it promotes re-epithelialization, but it also controls the transepithelial influx of neutrophils, which if excessive, can lead to tissue damage.

Key Words: inflammation • epithelium • protease


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INTRODUCTION
 
A hallmark of acute inflammation in response to injury is the rapid influx of neutrophils [1 2 3 4 5 6 ]. Although an essential arm of innate immunity, an excess of neutrophils can contribute greatly to the tissue damage associated with a variety of conditions, such as inflammatory bowel disease [4 , 7 , 8 ], septic shock [9 ], acute severe asthma [10 , 11 ], toxic epidermal necrolysis [12 ], transfusion-related acute lung injury [13 ], mortality associated with lung transplantation [14 , 15 ], and much more. However, tissue damage likely does not result from just the presence of neutrophils but rather, from the indiscriminate damage caused by the massive oxidative burst of the fully activated cell [16 , 17 ].

Neutrophil influx into wounded or infected tissue is stimulated and guided, to a large part, by an activated epithelium, and despite their specialization to serve distinct functions in different tissues, epithelia respond similarly to injury and regulate local inflammation by equivalent mechanisms. Following injury, epithelial cells initiate a programmed series of coordinated responses, such as proliferation and migration, to restore tissue integrity. By their production of chemoattractants, adhesion molecules, and other proteins, epithelial cells recruit and confine the influx of inflammatory cells to sites of injury. Although seemingly divergent events, the epithelial programs regulating repair and inflammation, as well as other innate defense mechanisms, may have coevolved, particularly the proteins that serve essential functions in these processes. Hence, many of the epithelial products associated with any one of these events are likely common to all. Although long thought to function primarily in the turnover and degradation of the extracellular matrix, matrix metalloproteinases (MMPs) have emerged as a family of extracellular processing enzymes that control the activity of several diverse proteins with predominant roles in repair and inflammation [18 ].

Studies by us and others [19 20 21 ] indicate that matrilysin (MMP7), which is expressed almost exclusively by mucosal epithelia, functions in various processes of innate immunity. For example, matrilysin is highly expressed in infected tissue [22 ], and its expression is markedly up-regulated by exposure to gram-negative bacteria [23 , 24 ]. In mice, matrilysin activates intestinal pro-{alpha}-defensins (cryptdins), a family of antimicrobial peptides, and as a result of the lack of mature {alpha}-defensins, Mmp7–/– mice have an impaired ability to battle enteric pathogens [25 ]. Matrilysin also sheds membrane-bound Fas ligand [26 ], and its ability to promote apoptosis may be another point of action for this proteinase in innate immunity [18 ]. Matrilysin is also prominently expressed by migrating epithelium in injured airway [27 ], suggesting an additional function in epithelial repair. Indeed, whereas wounds made in wild-type trachea repair rapidly, Mmp7–/– tissue shows no evidence of epithelial migration, even several days post-injury [27 , 28 ]. Furthermore, by its ability to shed cell surface proteoglycans, matrilysin affects chemokine and growth factor activity [29 , 30 ].

To test further the role of matrilysin in response to tissue injury, we studied the response of Mmp7–/– mice subjected to acute colon injury. We focused on matrilysin in this model, as it is prominently expressed by epithelial cells adjacent to ulcers in patients with inflammatory bowel disease [31 ] and by the inflamed mucosa of IL-10 null mice [32 ]. Our findings in mice indicate that matrilysin mediates beneficial and deleterious effects in response to tissue injury. On one hand, matrilysin is required for repair of damaged epithelium; on the other, matrilysin controls the transepithelial influx of neutrophils via its ability to affect chemokine gradients, which if excessive, can lead to severe tissue damage. We propose that matrilysin functions as a checkpoint barring inappropriate neutrophil activation at the mucosal surface.


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MATERIALS AND METHODS
 
Animals and injury model
Colon injury was induced in litter-matched wild-type and Mmp7–/– mice (8–10 weeks old; C57BL/6) by adding 2.5% (wt/vol) dextran sodium sulfate (DSS; MW 30–40,000, MP Biomedicals, Solon, OH, USA) in drinking water for 7 days [33 ]. Mice were killed by carbon dioxide asphyxiation at various times, up to 38 days, after the start of DSS exposure or if moribund. The Institutional Animal Care and Use Committee at the University of Washington (Seattle, WA, USA; protocol 4065-01) has approved this protocol. At sacrifice, the large intestines were dissected, measured, and weighed. The segment from the cecum–colon junction to the anal verge was removed. Colons were lavaged by flushing with 3 ml cold PBS using a 20 g x 1.1/2-feeding needle, vortexed, and centrifuged at 8000 rpm to separate lavage fluid from fecal material.

Quantitative PCR
Total RNA was isolated with Trizol (Invitrogen, Carlsbad, CA, USA). Primers and TaqMan probes (FAM dye-labeled) for MMP7, {alpha}1(I) collagen, and GAPDH were added to cDNA and synthesized from 5 µg total RNA with a High-Capacity cDNA Archive kit (Applied Biosystems, Foster City, CA, USA), and product amplification was measured with an ABI HT7900 Fast Real-Time PCR system. The threshold cycle (Ct) was obtained from duplicate samples and averaged, and the Ct range for each is shown in the figure legends. The {Delta}Ct was the difference between the average Ct for the specific cDNAs and the average Ct for GAPDH, which ranged from 16.97 to 14.16. The {Delta}{Delta}Ct was the average {Delta}Ct at a given time-point minus the average {Delta}Ct of Day 0 samples. The data are expressed as relative quantification (RQ), which is the fold change, and calculated as 2–{Delta}{Delta}Ct.

Histology
Washed colons were fixed in 10% formalin overnight and then dehydrated in 70% ethanol. Colons were opened longitudinally and rolled with the mucosa facing outwards using a modified Swiss-roll technique [34 ]. Intestinal rolls were dipped into a cryomold containing 5% formalin and 2% agarose and processed for routine paraffin embedding. Sections (4 µm) were stained with H&E or processed for immunohistochemistry or in situ hybridization. To quantify injury, we determined the crypt scores [33 ]. This morphometric analysis presents injury as the percent of the total colon length with a specific grade of lesion, from 0 (unaffected) to 4 (most severe). Specifically, the grading was as follows: Grade 0, intact crypts; Grade 1, loss of basal one-third of the crypt; Grade 2, loss of basal two-thirds of the crypt; Grade 3, loss of entire crypt but surface epithelium intact; and Grade 4, loss of entire crypt including surface epithelium. Samples were assessed in a blinded manner by two authors. Images were captured using an Olympus BX-51 fluorescence/differential interference contrast (DIC) microscope with U plan Apo 1.25x/0.04, 2x/0.05, 10x/0.3, and 20x/0.70 objectives and an Olympus DP25 5.5 megapixel digital camera.

Immunohistochemistry/in situ hybridization
Serial sections were deparaffinized, immersed in 10 mM citrate buffer (pH 6.0) at 98°C for 40 min for antigen retrieval, and processed for immunostaining using Vectastain ABC Elite kits (Vector Laboratories, Burlingame, CA, USA) as described [27 ]. Matrilysin was detected with a rat anti-human mAb [35 ]. Neutrophils were identified with MCA771GA (1:1,000, Serotec, Oxford, UK), and macrophages were stained for F4/80 (1:50; MCAP497, Serotec). Controls were processed with matching preimmune sera or nonimmune ascites. In vitro-transcribed antisense and sense RNA probes for matrilysin were labeled with {alpha}[35S]-UTP, and sections were hybridized as described [36 ].

Protein assays
Myeloperoxidase (MPO) levels were determined by ELISA (HyCult Biotech, The Netherlands). Colon tissue was homogenized in 2.5 g hexadecyltri-methylammonium bromide buffer in 500 ml Milli-Q water. The homogenate was centrifuged at 8000 g, and the supernatant was collected. For chemokines and cytokines, colon samples were homogenized in 2 ml 150 mM NaCl, 15 mM Tris (pH 7.2), 1 mM MgCl2, 1 mM CaCl2, 1% Triton X-100, and Complete Miniproteinase inhibitor cocktail (Roche Molecular Biochemicals, Indianapolis, IN, USA). Homogenates were centrifuged at 10,000 g for 15 min at 4ºC, and the supernatants were collected. Reagents used for the Luminex assay were purchased from R&D Systems (Minneapolis, MN, USA) and include the mouse Fluorokine MAP kit for keratinocyte-derived chemokine (KC), MIP-2, MCP-1, TNF-{alpha}, IL-6, IL-12, IFN-{gamma}, IL-1β, IL-4, and IL-10. KC and MIP-2 levels in colon lavages were measured using Quantikine ELISA kits from R&D Systems.


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RESULTS
 
Decreased lethality in injured Mmp7–/– mice
Mmp7–/– mice are healthy and do not present a phenotype until challenged [18 , 37 ]. To assess if matrilysin functions in intestinal repair, we exposed wild-type and Mmp7–/– mice to 2.5% DSS in their drinking water for 7 days. In this model of acute colon injury, weight loss, ulceration, and inflammation occur within 4–7 days after the onset of DSS, persist for up to 14 days, and are largely resolved by 21 days (i.e., 14 days after the cessation of DSS) [33 , 38 ]. After the first 5 days of DSS treatment, progressive weight loss was seen up to Day 10 in both genotypes (Fig. 1A ); however, from Days 6 to 10, wild-type mice lost weight more rapidly than did Mmp7–/– mice (P<0.001–0.034). Changes in colon length (Fig. 1B) and weight (Fig. 1C) did not differ significantly between genotypes during the first 10 days post-DSS.


Figure 1
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  		Figure 1. Sustained injury in Mmp7–/– mice. (A) Weight (Wt) change was plotted as percent of weight at Day 0 for wild-type (n=37) and Mmp7–/– (n=32) mice. The arrow indicates the cessation of DSS at Day 7. Data are the mean ± SEM of three separate experiments. (B, C) Colon length and weight were measured after the start of DSS exposure. Data are the mean ± SEM of six samples/genotype/day. Length: P <0.05 at Days 15 and 21; weight: P = 0.0033 at Day 21. (D) Survival of wild-type (n=132; 53 females and 79 males) and Mmp7–/– (n=110; 43 females and 67 males) mice was followed for 21 days in eight experiments; P < 0.0001.

In contrast, differences in these parameters were apparent during the recovery phase. The rate of weight recovery, as determined by the slope of the percent weight-change line between Days 10 and 22, was about twice as fast in wild-type mice (slope=2.53% increase per day per mouse; r2=0.997; n=11–15 per datum point) than in Mmp7–/– mice (slope=1.27% per day; r2=0.917; n=20–24 per datum point). The more rapid recovery in wild-type compared with Mmp7–/– mice was also reflected in the colon length and to a lesser degree, weight (Fig. 1B and 1C) . Expression of {alpha}1(I) collagen mRNA, used as a marker of interstitial scarring, and total tissue protein did not differ significantly between genotypes at any time-point (data not shown).

Although we saw only a small, yet significant, difference in weight loss between genotypes during the onset of colitis (Days 0–10), susceptibility to the lethal effects of DSS-induced injury was much more pronounced in wild-type than in Mmp7–/– mice (Fig. 1D) . Whereas 73% of wild-type mice (n=132 at Day 0) died between 8 and 12 days after the onset of 2.5% DSS, which agrees with the lethality noted for C57BL/6 mice by others [39 , 40 ], only 32% of Mmp7–/– mice (n=110) succumbed over this period (Fig. 1D) . No additional mice of either genotype died after Day 15. Resistance to lethality in Mmp7–/– mice was not a result of reduced consumption of DSS–water. Over the 7 days of treatment, Mmp7–/– mice drank slightly more DSS–water (2.03±0.26 l/g body weight) than did wild-type mice (1.76±0.23 l/g). Although this small difference reached statistical significance (P=0.044; n=12), it did not correlate with mortality.

Persistent injury in Mmp7–/– mice
To dissect further the role of matrilysin, we examined sections of colons for degree of injury and inflammation (Fig. 2 ). In both genotypes, tissue injury was most severe in the middle colon, less in the distal colon, and least in the proximal colon. Using an established injury-scoring method [33 ], based largely on epithelial crypt morphology, less-severe Grades 1 and 2 lesions dominated the early stages (Days 4–7) after onset of DSS and were more prevalent and appeared earlier in wild-type mice than in Mmp7–/– mice. At Day 4, 24.7% ± 5 of the mucosal surface of wild-type colons had type 1 focal injuries and 5.5% ± 1 as type 2 lesions. In contrast, only 7% ± 2 of Mmp7–/– colons showed damage, and essentially, all of these focal injuries were classified as type 1 lesions. At Day 7, more Grade 1 injuries were still evident in wild-type colons, but the extent of Grades 2, 3, and 4 injuries did not differ between genotypes.


Figure 2
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  		Figure 2. Delayed yet persistent ulceration in Mmp7–/– colons, which from wild-type (WT) and Mmp7–/– mice were harvested at Days (D) 4, 7, 11, and 21 post-DSS and examined by H&E staining. Several areas are numbered indicating the crypt score assigned to that site. Original bar = 2 mm.

Although colon injury appeared earlier in wild-type mice, repair of these lesions, as evidenced by re-epithelialization, hyperplastic epithelium, and re-establishment of normal crypt structures, proceeded more quickly than in Mmp7–/– mice. By Day 11, re-epithelialization was evident in wild-type mice resulting in fewer Grade 4 lesions, interrupted by short segments of Grade 3 lesions. However, at this time, 39 ± 1% of the colon in Mmp7–/– mice had epithelial erosion (Grade 4 lesion) compared with 25 ± 6% in wild-type mice. Persistent ulcerations in Mmp7–/– mice were also apparent at Days 15 and 18 but were not seen in wild-type colons (see Go Go Go Fig. 6 ). By Day 21, normal crypt architecture was mostly restored in the wild-type mice, but large patches of epithelial erosion, crypt loss, and abnormal crypt structures (Grades 2–4 lesions) remained in the knockout mice.


Figure 3
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  		Figure 3. Matrilysin expression. (A) At the indicated days post-onset of 2.5% DSS, colons from wild-type mice were harvested and divided into proximal (P) and distal (D) halves. Total RNA was isolated, and matrilysin and GAPDH mRNA levels were assessed by Northern hybridization. Autoradiograms from three experiments were scanned (representative blots are shown), and the densitometric values for matrilysin mRNA were normalized to those for GAPDH mRNA and averaged. (B) Total RNA was isolated from colons of wild-type mice (n=6) at various days after the onset of exposure to 2.5% DSS, and mRNA levels were detected by quantitative RT-PCR. The data are expressed as RQ, as described in Materials and Methods. The Ct range for the MMP7 probe set among experiments was 27.91–23.95.


Figure 4
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  		Figure 4. Matrilysin expression in DSS-induced colon injury. By immunohistochemistry, matrilysin protein was detected in the mucosal folds of the proximal colon (A), within regenerating epithelial (B–E). Using in situ hybridization of serial sections, matrilysin mRNA was also detected in regenerating areas and near ulcerations (D', arrowheads), but the location of the mRNA-positive cells did not always overlap with the protein-positive cells (*). However, several examples of a coincident signal for matrilysin protein and mRNA were seen, often in the mid-to-basal crypt areas (D, D', E, and E', thick arrows). Original bars = 100 µm.


Figure 5
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  		Figure 5. Delayed and restrained neutrophil influx at Days 4 and 7 in Mmp7–/– mice. Colons from wild-type and Mmp7–/– mice at Days 4 and 7 were harvested and embedded, and neutrophils were identified by staining with antibody MCA771GA (brown). The boxed areas in the large micrographs (D4 and D7) were captured under higher magnification with DIC optics and are displayed below. Arrows mark the mucosal–submucosal interface. Original bars = 2 mm (D4, D7); 100 µm (D7'); 50 µm (insets).


Figure 6
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  		Figure 6. Impaired re-epithelialization in Mmp7–/– colons. Sections of colons from wild-type and Mmp7–/– (knockout) mice at Days 15 and 18 were stained for neutrophils. Ongoing (D15) and completed (D18) re-epithelialization (arrows) was seen over ulcers in wild-type colons, restrained epithelial migration at the edge of wounds, and persistent ulcerations were common in Mmp7–/– colons. Original bars = 100 µm.

Matrilysin expression
In control mice, matrilysin mRNA was expressed at low levels in the proximal colon and was not detected in the distal colon (Fig. 3A ). In the whole colon, matrilysin expression increased after the onset of injury, peaked between Days 11 and 14, and dropped thereafter (Fig. 3A and 3B) . Extensive staining for matrilysin protein was seen in epithelial cells near the base of crypts in the proximal colon and in regenerating epithelium of the middle colon (Fig. 4A 4B 4C 4D 4E ). Similar to that reported for human specimens [31 ], mouse matrilysin was limited to epithelial cells, mostly by the regenerating epithelium close to ulcerations, and no signal was seen in interstitial cells or leukocytes. Throughout the course of injury, immunoreactivity for matrilysin was not detected in the distal third of the colon nor in colon sections from Mmp7–/– mice (data not shown).

The presence of staining in epithelial cells at the base of and along the length of the crypts and on the mucosal surface between crypts (Fig. 4A 4B 4C 4D) suggested that matrilysin was produced by basal epithelial cells that had migrated up the length of the crypts to the mucosal surface and that the proteinase was released apically. Indeed, by in situ hybridization, we detected matrilysin mRNA in epithelial cells at the base of crypts in the regenerative epithelium in the middle and proximal colon (Fig. 4 , D' and E'). Matrilysin mRNA was often detected in epithelial cells at the base of the crypts, whereas its protein was seen in epithelial cells further up the crypts (Fig. 4D and 4E) . This pattern suggests that matrilysin was expressed in cells at the base and was turned off in daughter cells that moved up along the length of the crypts onto the mucosal surface.

Altered neutrophil influx in Mmp7–/– mice
Matrilysin was expressed during periods of injury, lethality, and repair, and we hypothesized that increased mortality and more rapid onset of injury in wild-type mice resulted from exacerbation of acute inflammation. Few yet equal numbers of neutrophils were present in lamina propria of control (Day 0) wild-type and Mmp7–/– mice (data not shown). At Day 4 post-DSS, neutrophil influx was evident in wild-type mice, with pronounced accumulation in the middle colon near the base of crypts in the mucosa and submucosa of Grades 1 and 2 lesions (Fig. 5 ). At this stage, fewer neutrophils had infiltrated into Mmp7–/– colons.

At Day 7, prominent neutrophil influx was evident in the middle colon of all mice. Although total MPO levels indicated that the number of infiltrated neutrophils was similar in tissue extracts between wild-type and Mmp7–/– mice at all times post-DSS (see Fig. 7 E), we saw a clear distinction between genotypes, where these leukocytes accumulated. In wild-type mice, most neutrophils had migrated into the mucosal layer, but in Mmp7–/– mice, neutrophils did not advance beyond the submucosa (Fig. 5 , D7'). In several areas of the middle colon of Mmp7–/– mice, neutrophils were densely packed in the submucosa just below the muscularis mucosa. At later times, the distribution and accumulation of neutrophils did not differ between wild-type and Mmp7–/– mice, and their numbers began to wane by Day 18 (Fig. 6 ). Consistent with the pattern of MMP12 mRNA, macrophage infiltration increased from Days 1 to 14 and decreased thereafter (data not shown). However, unlike neutrophils, we did not observe any difference in the location of macrophages between genotypes.


Figure 7
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  		Figure 7. Altered neutrophil migration and chemokine compartmentalization in Mmp7–/– mice. Whole colon homogenates (Tissue) and colon lavage were collected at various times after the onset of DSS exposure. Chemokine levels were quantified using a Luminex assay (A, B) or by ELISA (C, D). (E, F) MPO levels were determined by ELISA (n=4–6 per point). MPO levels differed significantly between genotypes at Day 11 (P=0.0420) but not at the other times. KC and MIP levels differed significantly between genotypes at Day 7 (P=0.0387 and 0.0188, respectively) but not at the other times.

To quantify neutrophil influx and advancement across the mucosal layer, we measured total MPO levels in tissue homogenates and lavage samples of colon lumens. In human studies, measurement of neutrophil markers, including MPO and other granule proteins and chemokines, in fecal contents and intestine lavage, is used for diagnosis and to monitor therapy and correlate well with other parameters, such as endoscopic grade of inflammation [41 42 43 44 45 46 ]. In mice, macrophages do not express MPO; only neutrophils do [47 , 48 ]. Thus, total MPO levels are a reliable marker of neutrophil influx into mouse tissue.

Consistent with the immunostaining observations, MPO levels in colon tissue increased progressively over the first 2 weeks after onset of DSS, dropped thereafter, and did not differ between genotypes (Fig. 7E ). The temporal increase and drop in MPO levels correlated with the changes seen in matrilysin expression (Fig. 4C) . In contrast, MPO levels detected in colon lavage differed markedly between genotypes (Fig. 7F) . In wild-type mice, MPO levels in colon lavage peaked at Day 11, at which time, the influx of neutrophils into the tissue and matrilysin expression were increasing, and dropped thereafter. However, in Mmp7–/– mice, little MPO was detected in colon lavage at any time. Although the levels of total MPO measured in the wild-type colon lumens were much less than the levels measured in tissue, we cannot use these numbers to accurately determine what percent of neutrophils advanced into the lumen. In colon homogenates, MPO levels reflect the number of neutrophils that had accumulated in the tissue; however, in the colon lavage samples, MPO levels indicate the number of neutrophils present in the lumen at the time of sacrifice and that have not yet been eliminated. Regardless of these considerations, the MPO data (Fig. 7E and 7F) combined with the immunostaining observations (Fig. 5) indicate that the transmucosal migration of neutrophils was retarded in Mmp7–/– mice.

Chemokine expression and compartmentalization
To explore the cause for the retarded advancement of neutrophils in Mmp7–/– mice, we assessed the levels of various chemokines and inflammatory cytokines in tissue homogenates and colon lavage samples. Overall, there was no significant difference in the chemokine/cytokine levels in colon homogenates between wild-type and Mmp7–/– mice at any time post-DSS. For example, KC and MIP-2, acute-phase CXC chemokines and potent neutrophil chemotactic factors, were induced in response to DSS, although KC levels peaked predictably sooner than did MIP-2 and returned to basal levels during the reparative phase (Fig. 7A and 7C) . Similarly, the levels of IFN-{gamma}, IL-1β, IL-6, IL-10, IL-12/p70, CCL2/MCP-1, and TNF-{alpha} increased rapidly, reaching a peak by Day 7, and decreased thereafter toward baseline in both genotypes (data not shown). There was little change in IL-4 levels.

To assess if a defect in chemokine gradient formation contributed to the retarded transepithelial advancement of neutrophils that we saw in Mmp7–/– colons, we assessed the levels of KC and MIP-2 in intestinal lavage fluid and feces. Baseline levels of both CXC chemokines were detected in Day 4 colon lavage of both genotypes (Fig. 7B and 7D) . Coincident with the peak levels detected in tissue homogenates (Fig. 7A and 7C) , relatively high levels of KC and MIP-2 protein were measured in lumenal lavages collected at Day 7 from wild-type mice, mirroring the levels detected in tissue, and dropped thereafter. In contrast, the levels of KC and MIP-2 in colon lavage from Mmp7–/– mice remained at baseline values at all times examined (Fig. 7B and 7D) . These data suggest that matrilysin controls neutrophil movement by generating transmucosal gradients of CXC chemokines. As the peak in chemokine levels measured in colon lavage preceded the peak in MPO levels, the amount of KC and MIP-2 detected did not reflect a contribution from neutrophils.


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DISCUSSION
 
The presence of MMPs in inflammatory diseases has largely been interpreted to indicate a causative role in disease pathogenesis and tissue destruction [49 ]. Indeed, administration of broad-spectrum metalloproteinase inhibitors reduced inflammation and disease indices in animal models of colon injury [50 , 51 ], as well as others. However, it is unlikely that the MMP family evolved to contribute to disease progression, and blocking the activity of several distinct proteinases will not address the specific and important functions of selective members. Our findings indicate that matrilysin mediates beneficial and deleterious effects in response to injury. On one hand, matrilysin is required for efficient repair of damaged epithelium, but it also controls the transepithelial influx of neutrophils across the colonic mucosa, similar to functions we reported in lung models [27 28 29 ]. Re-epithelialization is inarguably a desired outcome of any form of injury in any tissue. In contrast, although neutrophils are an essential arm of innate immunity, an overabundance of these granulocytes can cause indiscriminate, severe, and potentially mortal damage.

In addition to impaired re-epithelialization, we observed impaired neutrophil infiltration in an inflamed Mmp7–/– colon. Although essentially equivalent numbers (based on total MPO levels) of neutrophils had effluxed from the vasculature into injured colons, their further advancement was retarded in Mmp7–/– mice. Whereas neutrophils were dispersed throughout the mucosa/submucosa in wild-type mice, they were largely confined to the submucosa (in which resides the vasculature bed from which they emigrate) in Mmp7–/– mice. As neutrophils do not express matrilysin [27 , 29 ], the defect in neutrophil migration into and across the mucosal layer was a consequence of a lack of expression by epithelial cells. We saw no overt difference in the influx of macrophages between the genotypes, indicating that matrilysin functions to specifically control neutrophil influx. Similarly, in acute lung injury, the influx of neutrophils, but not of macrophages, is impaired in Mmp7–/– mice [29 ].

The difference in DSS-mediated lethality was quite striking between genotypes. Recently, another group, using mice from our colony, reported that Mmp7–/– mice are more susceptible to DSS-induced mortality than wild-type mice [52 ]. Although seemingly opposed to our findings, we attribute this discrepancy to the relatively small number of mice (n=5 per genotype) of unstated gender used in their study. Our lethality findings are based on data from eight experiments incorporating over 100 mice per genotype, of which data from six of these reflect the overall population trend shown in Figure 1D . In two experiments, incorporating nine and 15 wild-type males and eight and 16 Mmp7–/– males, respectively, we observed no significant difference between genotypes (males only) in percent survival, which was overall ~30%. In these two experiments, there was a large difference in survival between wild-type (5%) and Mmp7–/– (74%) female mice. In another four experiments, essentially all wild-type male mice (94–100%) were dead within 12 days. Furthermore, we noted a significant gender bias in susceptibly to DSS-induced injury, which was consistently seen in all but one experiment, in which 90% of the wild-type females died (n=10 at Day 0) versus 60% of the wild-type males (n=15). Thus, in addition to sample size, gender differences may have skewed the findings reported by Shi et al. [52 ].

We conclude that the resistance of the Mmp7–/– mice was not related to re-epithelialization, which was impaired in the knockout mice. Overall, the extent of osmotic injury caused by DSS consumption did not differ between phenotypes. Indeed, our observations demonstrate that the onset and initial degree of colon injury were similar between genotypes. However, once injury had begun, the wild-type tissue was apparently more effective at recruiting neutrophils and guiding these cells into and beyond the mucosal layer than the Mmp7–/– colon. Thus, matrilysin functions as part of an epithelial mechanism that permits neutrophil advancement across a wounded mucosal surface and their subsequent activation.

The impaired transepithelial advancement of neutrophils suggests that matrilysin also controls the activity or compartmentalization of chemokines. In mice, acute neutrophil influx is guided from the vasculature and into and through tissue by the CXC chemokines KC and MIP-2 [53 ]. Although we saw no difference in the levels of these chemokines in colon tissue homogenates between genotypes, we did see markedly reduced levels of both factors in colon lavages of Mmp7–/– mice. Similarly, in a model of lung injury, matrilysin generated a transepithelial gradient of KC, but in the lung, we saw no difference in the distribution of MIP-2 [29 ], suggesting that the mechanism of matrilysin activity may not be the same among tissues. In the lung injury model, we determined that KC accumulates on the glycosaminoglycan side-chains of syndecan-1, a transmembrane heparan sulfate proteoglycan, and that this complex is shed from the surface of epithelial cells by matrilysin, thereby establishing a chemokine gradient. Furthermore, in the lung injury, we reported that shedding of E-cadherin is dependent on matrilysin and speculated that proteolysis of this junctional protein is required for re-epithelialization [28 ]. However, in the colon model, we found no difference in shedding of syndecan-1 or E-cadherin in vivo or in tissue explants (data not shown), indicating that other proteinases function in release of membrane molecules in the intestine. Thus, although matrilysin affects similar processes among injured tissues (i.e., re-epithelialization, chemokine gradient formation, and transepithelial influx and activation of neutrophils), the mechanisms seem distinct.

Determining precisely how matrilysin controls neutrophil influx in the colon may point to a mechanism that governs acute inflammation, and our ongoing studies are designed to identifying the important substrate(s). We do not propose, however, that matrilysin controls the only mechanism involved in the transepithelial migration of neutrophils. For example, hepoxilin A3, an epithelial-derived eicosanoid, is required for neutrophils to cross an epithelial barrier in response to bacterial infection [54 , 55 ]. We do not yet know, however, if the production or activity of this lipid mediator is influenced by matrilysin. We predict that in the absence of matrilysin, neutrophils are stopped at the epithelial/interstitial interface, possibly by ligation to a cell-bound substrate of the proteinase. Thus, the matrilysin-controlled mechanism may function as a checkpoint to prevent premature neutrophil activation.

The observation that Mmp7–/– mice are protected against acute injury and lethality does not mean that this MMP is a "bad" proteinase. Indeed, matrilysin serves beneficial functions in immunity, epithelial migration, and neutrophil influx. Neutrophils comprise an essential cellular component of innate immunity, particularly in defense against bacteria. However, with extensive injury, as produced in many experimental models, a massive neutrophil influx would cause indiscriminate, severe, and potentially mortal damage. We have described what we believe to be a normal and required repair process, which if unregulated and exuberant, can be harmful. A lack of neutrophil activation and in turn, the oxidative damage it can cause, would be consistent with the protective effect we observed in the Mmp7–/– mice.

In summary, our studies show that matrilysin functions in re-epithelialization and more importantly, the transepithelial movement of neutrophils in injured colon and apparently, by mechanisms distinct from those in other tissues. These findings demonstrate that MMPs can serve beneficial functions, but they also indicate that MMPs can contribute to disease pathogenesis at the same time and place. Clearly, this family of proteinases did not expand during evolution to cause disease but rather to serve needed functions, which typically involve processes related to immunity and wound repair—in other words, re-establishment of tissue homeostasis. However, when overexpressed, MMPs can cause exuberant responses leading to tissue damage and disease.


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ACKNOWLEDGEMENTS
 
This work was supported by NIH grants HL082658, HL077555, HL029594, HL068780, HL073725, and DK52574 and Washington University Digestive Diseases Research Core Center Pilot and Feasibility program (St. Louis, MO, USA). The authors have no conflicts or commercial affiliations to disclose. We thank Hui-Qing Yin, Terese Tolley, Darlene Stewart, and Amy Schmidt for excellent help with all histology procedures, Dr. Shawn Skerrett and John Ruzinski for help with the Luminex assays, Dr. Daniel Schuster (Washington University) for help with the glucose uptake assay, and Dr. Thaddeus Stappenbeck (Department of Pathology, Washington University) for advice about evaluating injury by the crypt score.

Received January 9, 2008; revised February 13, 2008; accepted February 14, 2008.


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