Decorin Modulates Wound Healing in Experimental Glaucoma Filtration Surgery: A Pilot Study

Abstract

purpose. To analyze the effect of perioperative decorin in an experimental setting of glaucoma filtration surgery.

methods. Glaucoma filtration surgery, similar to that performed in clinical practice, was performed on 35 chinchilla rabbits (ChBB:CH). The animals received a unilateral subconjunctival injection of decorin (40–100 μg) or the vehicle alone before surgery and at different time intervals thereafter. Antifibrotic efficacy was established by clinical response and histologic examination. The animals were killed on day 14, and the eyes processed for histology.

results. Both the vehicle and the decorin solution were well tolerated. No adverse effects such as inflammation or blurring of the optical media were observed. Conjunctival scarification occurred within 1 week in the control groups but was suppressed in the experimental groups. The intraocular pressure correlated with the fibrotic process and reached normal levels within 7 days after surgery in control animals, but remained significantly (P < 0.001) reduced in the experimental groups. Histologic examination of the surgical area 14 days after surgery disclosed massive fibrosis in the control animals, but little deposition of extracellular matrix in the experimental groups.

conclusions. The data of this pilot study suggest that perioperative subconjunctival decorin applications significantly affect conjunctival scarring and surgical outcome of glaucoma filtration treatments in rabbits.

Glaucoma filtration surgery is the most frequent technique applied to reduce the intraocular pressure (IOP) in cases of uncontrolled glaucoma. Though this method has an immediate effect on IOP, the long-term success is often impaired by the postoperative wound-healing process. ^{1 2 3} Characteristically, fibroblasts from Tenon’s capsule and the episclera are involved in the fibroproliferative process that finally leads to the obstruction of the created fistula and subconjunctival filtration area. To increase the success of this type of surgery, several substances such as corticosteroids ⁴ and antifibrotics ^{5 6 7} have been used as antifibrotic therapy. Antifibrotics, such as fluorouracil and mitomycin C, however, work by causing widespread cell death, ^{8 9 10} and despite the positive effect on the success rate, diffusion into adjacent ocular tissues may result in impairment of other than the targeted cells and in sight-threatening complications. ^{11 12 13} New strategies are being developed, to find a more selective and intelligent way to suppress the wound-healing process. ^{14 15 16 17 18} A promising approach to impair the wound-healing cascade seems to be the inhibition of the major players of this complex process. A pivotal role in scarring has long been attributed to transforming growth factor (TGF)-β. ^{19 20 21 22} Neutralizing TGF-β antibodies have been shown to reduce experimental dermal scarring ²³ and have also been introduced recently into glaucoma filtration surgery. ^{14 24}  

Inspired by these studies we wanted to analyze whether a naturally occurring TGF-β inhibitor, decorin, ²⁵ could be used instead of the potentially immunogenic polyclonal antibodies ²³ or the more demanding recombinant human anti-TGF-β antibodies. ²⁴ The objectives of this study were to investigate the use of subconjunctival decorin in a rabbit model of glaucoma filtration surgery and to assess the antiscarring potential, the safety, and the tolerance of this TGF-β inhibitor. 

Materials and Methods

An established rabbit model of glaucoma filtration surgery was used to investigate the effect of decorin on the wound-healing process after glaucoma filtration surgery. All animal procedures and methods used for securing the animal tissue were humane and complied with the ARVO Statement for the Use of Animals in Ophthalmic Research and our institutional guidelines. 

Animals

All experiments were performed with female chinchilla bastard rabbits (ChBB:CH), 3 to 6 months old and weighing 1.5 to 2.5 kg. Animals were obtained from Charles River Laboratories (Sulzfeld, Germany) and acclimatized for 1 week before the experiments started. 

Surgical Procedure

Surgery was performed on the right eye only under general anesthesia with intramuscular injections of ketamine (Ketanest; Parke Davis, Berlin, Germany) and xylazine (Rompun; Bayer, Leverkusen, Germany) and local anesthesia with oxybuprocaine drops (Novesine 0.4%; Novartis, Nürnberg, Germany). Briefly, after a lid speculum was placed, a partial-thickness 8-0 silk corneal traction suture (Ethikon, Edinburgh, UK) was placed superiorly, and the eye was pulled down. A peritomy at 5 mm limbus distance was used to form a limbus-based conjunctival flap. A limbus-based rectangular (2.5 × 2.5-mm) scleral flap was outlined with a steel blade and carefully dissected. A trephine (diameter, 1.5 mm) was then used to create the entry into the anterior chamber at the surgical limbus. Consequently, the trephine block of tissue was excised and a peripheral iridectomy performed. Because of the known aggressive wound healing in this model, no suturing of the scleral and conjunctival flap was necessary. On three consecutive days after surgery, each animal received once-a-day drops containing a steroid and an antibiotic (Dexamytrex; Dr. Mann Pharma, Berlin, Germany). 

Preparation and Administration of Decorin

Decorin (D-8428; Sigma-Aldrich, Steinheim, Germany) was dissolved in physiologic saline (Balanced Salt Solution [BSS]; Pharmacia, Groningen, The Netherlands) at 0.4 or 1 mg/mL. The saline solution alone served as the vehicle control. Both were administered with animals under general and topical anesthesia. A 30-gauge needle was used to inject 100 μL of either the decorin solution or the vehicle alone. The needle was placed at the nasal margin of the superior rectus muscle, so that a visible bleb was formed on the supranasal quadrant (Fig. 1) . 

Treatment Regimens

Thirty rabbits received filtration surgery to the right eye only with allocation to one of the two control groups or the experimental groups. An additional control group A of animals that received decorin but did not have surgery was installed, to assure that the substance alone does not decrease the intraocular pressure. The sample size calculation was based on the standard deviation of the IOP ratio in previous experiments (0.09.) With power ε = 0.9 and relevant difference δ = 0.2, the sample size was calculated for a two-tailed test (n = 5). The groups were subdivided as shown in Table 1and described in the following (in all groups, n = 5): Group A was a control group of animals that received subconjunctival injection of decorin (100 μg) only. Group B was a control group that underwent filtration surgery only. Group C, the third control group, received surgery and the most frequent application of the vehicle (0.1 mL physiologic saline solution)—15 minutes before surgery and on the four consecutive postoperative days. The experimental groups received subconjunctival injections of either 40 μg (group D) or 100 μg (group E) decorin dissolved in 0.1 mL vehicle solution 15 minutes before surgery and on the four postoperative days. Group F received only a preoperative injection of 100 μg decorin. Animals in group G received 100 μg decorin before surgery and on postoperative days 1, 3, and 7. 

Clinical Evaluation

Clinical examination was performed to evaluate the general appearance of the treated eyes, to assess local toxicity and ocular intolerance, and to measure the IOP. Tonometry was performed both with an applanation tonometer (Tono-Pen XL; Mentor, Norwell, MA) and an impression tonometer (Schiötz; Winter/Medton, Jüngingen, Germany) with animals under general and topical anesthesia. IOP was recorded in both eyes before surgery as the baseline and after surgery on the designated days. To exclude interindividual, cyclic, and anesthesia-related variations, IOP was compared between the experimental right eye and the left control eye. The measurements were performed in triplicate. The difference in measured IOP was expressed as the right-to-left (R/L) eye ratio. Preoperative R/L ratios ranged from 0.92 to 1.1. Success was defined by ≥20% difference in IOP, reflected as an IOP ratio of ≤0.8. 

Histologic Evaluation

On postoperative day 14, the eyes were enucleated together with the conjunctiva to preserve the bleb. The globes were immediately fixed in 10% formaldehyde for at least 24 hours. Consequently, the eyes were examined, and a ring in the sagittal axis comprising the relevant area excised. Tissue samples were then dehydrated and embedded in paraffin, and 5-μm serial sections were cut, rehydrated, stained with hematoxylin and eosin and the Masson technique, and coverslipped. 

Statistical Evaluation

Statistical analysis was performed to determine the differences in IOP between the control and experimental groups. A two-tailed Student’s t-test was performed, and significance was assumed if P < 0.05. 

Photographs

Clinical photographs were produced with a digital camera (model DCR-TRV50E; Sony, Tokyo, Japan). Histologic specimens were examined and documented with a microscope (Axioskop; Carl Zeiss Meditec, Oberkochen, Germany) connected to a digital camera (model HC-300Z; Fujix, Tokyo, Japan) and the appropriate hard- and software (Image Access, ver. 3.2; Imaging Bildverarbeitung AG, Glattbrogge, Switzerland). 

Results

Clinical Appearance

The fate of the experimental and control eyes was evaluated clinically daily within the first week and on the designated days thereafter. In group A, the subconjunctival decorin depot (100 μg in 0.1 m) was well tolerated, and no signs of an inflammatory reaction or infection was detected at all. Inflammatory response in surgical eyes such as conjunctival hyperemia was mild in all groups and lasted for 1 week. There was no remarkable difference between the control and experimental groups. Not one case of endophthalmitis was observed. No corneal edema or other type of corneal damage was detected in any group. Both the control and experimental groups showed no sign of cataract development during the experimental period. 

Wound healing was clinically observed as the loss of conjunctival transparency and thickening due to the deposition of fibrotic tissue (Fig. 2A) . In both control groups B and C, scarring of the conjunctiva was prominent. The subconjunctival fibrotic tissue covered and impaired the view of the scleral flap. No bleb formation was detectable on day 14 after surgery. In contrast, suppression of scarring maintained a translucent conjunctiva (Fig. 2B) . This situation was consistent in groups E and G, but less pronounced in groups D and F. No cystic bleb formation was observed in groups E and G. The filtering zone was diffusely elevated. 

Intraocular Pressure

Intraocular pressure was measured as the mean result of triplicate experiments on the designated days (Fig. 3) . Preoperative intraocular pressure ranged between 15 and 25 mm Hg. Preoperative IOP ratios ranged from 0.92 to 1.1. This IOP ratio level was maintained in group A, but was drastically reduced in all surgical groups within the third postoperative day, with a mean ratio of 0.53 ± 0.12. On postoperative day 7, control groups B and C showed elevated IOP ratios, with mean ratios of 0.92 ± 0.07 and 0.9 ± 0.18, respectively (Fig. 3A) . 

Animals in group D, injected with 40 μg decorin maintained decreased IOP ratios, with a mean of 0.63 ± 0.05 on day 7 (P < 0.05), 0.71 ± 0.08 on day 10 (P < 0.01), and 0.75 ± 0.09 on day 14 (P < 0.01; Fig. 3A ). 

The higher concentration (100 μg) of decorin seemed to yield better results without increasing the risk of side effects. Group E, which received the same regimen but a higher dose of decorin than in group D, showed mean ratios of 0.60 ± 0.02 on day 7 (P < 0.05), 0.59 ± 0.02 on day 10 (P < 0.001), and 0.58 ± 0.11 on day 14 (P < 0.001; Fig. 3A ). However, there was no significant difference between groups E and D. 

To examine whether a single preoperative injection of 100 μg decorin would have an effect, a single-dose regimen was applied in group F (Fig. 3B) . On day 7, the mean value was 0.70 ± 0.15 (P < 0.05) with two eyes outside of the defined success limit (IOP ratio ≤0.8). On days 10 and 14, there was no statistically significant difference from control animals. 

To cover the critical wound-healing phase, but reduce the number of injections, in group G, 100 μg of decorin was applied before surgery and on postoperative days 1, 3, and 7 (Fig. 3B) . This dose regimen could retain significantly low IOP-ratios over the time of evaluation. The mean ratios were 0.60 ± 0.1 on day 7 (P < 0.05), 0.58 ± 0.04 on day 10 (P < 0.001), and 0.66 ± 0.03 on day 14 (P < 0.001). 

Histologic Appearance

All rabbits in groups B to G were killed on postoperative day 14, and the treated eyes were enucleated for histologic examination. The tissues were stained with hematoxylin and eosin to give an overall impression and with the Masson technique to determine the collagenous extracellular matrix (ECM) deposition. 

Histologic analysis of the specimens was performed at the center of the sclerotomy site as indicated by the location of the iridectomy. Histologic profiles revealed massive subconjunctival scarring in the control groups (Fig. 4A) . The subepithelial connective tissue consisted of a dense collagenous connective tissue. The sclerotomy site was infiltrated by hypercellular fibrotic tissue (Fig. 4C) . Experimental groups, in contrast, showed only a mild fibrotic response (Fig. 4B) . The biomicroscopic impression of a translucent conjunctiva with no or mild ECM deposition was supported by the histologic examination. The conjunctival epithelium looked healthy, and there was only a mild deposition of collagen in the subconjunctival space (Fig. 4D) . The subepithelial connective tissue was loosely arranged and contained histologically clear spaces. Furthermore, the eyes did not reveal an intra- or extraocular inflammatory reaction. The corneal epithelium and endothelium, as well as the lens and the ciliary body did not show light-microscopically detectable signs of toxic effects. 

Discussion

Decorin is a small proteoglycan that consists of a single glycosaminoglycan side chain linked to a core protein containing leucine-rich repeats of approximately 24 amino acids. ²⁶ Decorin is found in the ECM of a variety of tissues and interacts with a variety of proteins that are involved in matrix assembly ²⁷ and in the regulation of fundamental biological functions as cell adhesion, ²⁸ migration, ²⁹ and proliferation. ³⁰ Decorin exerts an antiproliferative effect, either directly by upregulating cyclin-dependent kinase inhibitors such as p21 and p27 or through its ability to interact with growth factors. ^{31 32} It is well established that decorin binds to the potent wound-healing factor TGF-β and inhibits its biological activity in a number of cell types. ^{33 34}  

An anti-fibrotic effect of decorin has been demonstrated in experimental kidney, ^{24 35} lung, ³⁶ cerebral, ³⁷ and muscular ³⁸ fibrosis. Ours is the first report of an antifibrotic effect of decorin in a rabbit model of filtration surgery. 

Because of the aggressive wound-healing response in rabbits, this animal model is believed to be equivalent to high-risk eyes in humans, and surgical failure results within 1 week. The use of antifibrotic drugs, such as 5-fluouracil and mitomycin C, prolongs bleb survival. However, the effect is not permanent and, depending on drug concentration, it will subside within 2 weeks. ¹⁸  

Our experiments show that filtration surgery success can be prolonged when decorin is applied locally. As demonstrated in control group B, wound closure occurred and IOP increased within 1 week. This time until surgical failure was similar to that reported in previous studies. Group C demonstrates that the injection of the vehicle alone and the procedure of punctation and injection does not significantly affect the time of surgical failure. 

Decorin was administered on day 0, approximately 15 minutes before surgery (groups D–G). Further injections were administered on days 1, 2, 3, and 4 (groups D and E) or on days 1, 3, and 7 (group G). The rationale for this dose regimen was to achieve local availability of decorin at the critical time and to mimic clinical application that would be practicable. 

In this study, multiple applications of 40-μg decorin (group D) showed a delayed increase of IOP and a decreased deposition of fibrotic tissue within the surgical area. In group E, 100 μg decorin was highly effective in retaining significantly reduced IOP R/L ratios. This was reflected in reduced scar tissue formation. The reduced fibrous response was observed biomicroscopically and supported by the histologic sections obtained at postsurgical day 14. 

In a previous study, Cordeiro et al. ¹⁴ demonstrated the effectiveness of a novel recombinant monoclonal neutralizing antibody to human TGF-β2 on cultured conjunctival fibroblasts and in a rabbit model of glaucoma filtration surgery. These British colleagues used a modified filtration surgery technique that includes the insertion of a 22-gauge, 25-mm Venflon 2 (Ohmeda AB, Helsingborg, Sweden) intravenous cannula through a scleral tunnel into the anterior chamber. Using this technique, they maintained a patent channel through the sclera to maximize aqueous outflow. This seems, on the one hand, to allow a better bleb evaluation to define clinical success; on the other hand no significant changes in IOP were detected. ¹⁴ In our study, we chose to use a surgical technique that is close to that used in our clinical practice. This procedure included the preparation of a scleral flap, partial sclerectomy with a trephine, and basal iridectomy with forceps. We described the appearance of the superior bulbar conjunctiva, but we did not quantify bleb survival. We defined clinical success as dependent on changes in IOP. In our experience, ¹⁵ IOP measurements are less subjective and better quantifiable than bleb appearance and survival. Furthermore, we excluded interindividual, cyclic, and anesthesia-related variations by comparing the IOP between the experimental right eye and the left control eye. The difference in measured IOP was expressed as the R/L IOP ratio. Using this technique, a reduced IOP ratio was found in all animals at the third day after surgery (Fig. 3) . The aggressive scarring in this animal model, however, consistently leads to preoperative levels within 1 week. 

The successful clinical and histologic anti-scarring effect of decorin supports the data found with the anti-TGF-β2 antibody. We assumed in addition that decorin may be even more potent, because its inhibitory effect is not restricted to the TGF-β2 isoform. Furthermore, it has been demonstrated recently that decorin has an inhibitory effect on platelet-derived growth factor (PDGF), a further major player in wound healing. ³⁹  

This pilot study demonstrated that locally applied decorin improved glaucoma filtration surgery in an animal model of aggressive conjunctival scarring. Furthermore, subconjunctival decorin application appeared clinically safe and well tolerated within the experimental time. No signs of an inflammatory or toxic response were seen either biomicroscopically or histologically. Further studies are needed to examine the long-term effect and safety of perioperative subconjunctival decorin applications. 

 Supported by a Tübingen Fortune grant.

 Submitted for publication July 29, 2004; revised September 17, 2004; accepted October 7, 2004.

 Disclosure: S. Grisanti, None; P. Szurman, None; M. Warga, None; R. Kaczmarek, None; F. Ziemssen, None; O. Tatar, None; K.U. Bartz-Schmidt, None

 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.

 

Figure 1.

 

Image of the anterior segment of a rabbit eye after a subconjunctival injection of decorin (100 μg) dissolved in physiologic saline (0.1 mL). The subconjunctival bleb was situated at the planned area of surgery.

Table 1.

 

Characteristics of Controls and Experimental Groups

Group	Application of Decorin (μg)	Application Time
A	100	Once
B	None	None
C	Vehicle	15 Minutes before surgery and on postoperative days 1, 2, 3, and 4
D	40	15 Minutes before surgery and on postoperative days 1, 2, 3, and 4
E	100	15 Minutes before surgery and on postoperative days 1, 2, 3, and 4
F	100	15 Minutes before surgery
G	100	15 Minutes before surgery and on postoperative days 1, 3, and 7

  n = 5 in all groups.

Figure 2.

 

Images of the anterior segment of rabbits from control group C (A) and experimental group E (B) 14 days after surgery. (A) The conjunctiva in the surgical area (superior nasal quadrant) appeared thickened due to the subconjunctival fibrotic tissue. (B) The conjunctiva was transparent, and the rectangular scleral flap was identifiable in the superior nasal quadrant.

Figure 3.

 

Intraocular pressure (IOP) measured as IOP ratios between experimental right (R) and control left (L) eyes before surgery, on surgery day (day 1), and after surgery at distinct time points. (A) Group A (×) received a subconjunctival injection of decorin (100 μg in 0.1 mL physiologic saline vehicle) only. Group B ( ) received only surgery. Group C (♦) received surgery and subconjunctival injections of the vehicle (0.1 mL) 15 minutes before surgery and on the four consecutive postoperative days. Groups D (▴) and E (○) underwent surgery and received subconjunctival injections of 40 μg decorin dissolved in 0.1 mL vehicle 15 minutes before surgery and on the four consecutive postoperative days. (B) Group F ( ) underwent surgery and received a single subconjunctival injection of 100 μg decorin dissolved in 0.1 mL vehicle 15 minutes before surgery. Group G (•) underwent surgery and received subconjunctival injections of 100 μg decorin dissolved in 0.1 mL vehicle 15 minutes before surgery and on postoperative days 1, 3, and 7. The data are expressed as the mean ± SD of results in five rabbits per group.

Figure 4.

 

Histologic micrographs of the surgical area in rabbits from control group C (A, C) and experimental group E (B, D) 14 days after surgery. (A, C) The subconjunctival area was infiltrated by densely packed collagenous fibrotic tissue. (B, D) There was almost no deposition of fibrotic ECM and the subepithelial connective tissue was loosely arranged. Specimens were stained with hematoxylin and eosin (A, B) and the Masson technique (C, D).