Gremlin Induces Ocular Hypertension in Mice Through Smad3-Dependent Signaling

Abstract

Purpose: Transforming growth factor–β2 induces extracellular matrix (ECM) remodeling, which likely contributes to the defective function of the trabecular meshwork (TM) leading to glaucomatous ocular hypertension. Bone morphogenetic proteins (BMPs) inhibit these profibrotic effects of TGFβ2. The BMP antagonist gremlin is elevated in glaucomatous TM cells and increases IOP in an ex vivo perfusion culture model. The purpose of this study was to determine whether gremlin regulates ECM proteins in the TM, signals through the Smad3-dependent pathway, and induces ocular hypertension in mice.

Methods: Ad5.Gremlin or Ad5.TGFβ2 was injected intravitreally into one eye of each mouse. Intraocular pressure measurements were taken using a TonoLab tonometer. Gremlin, TGFβ2, fibronectin (FN), and collagen-1 (Col-1) expression in the TM was determined by immunofluorescence, Western immunoblot, and quantitative (q)PCR analyses.

Results: Ad5.Gremlin or Ad5.TGFβ2 each caused significant IOP elevation in mice. Immunofluorescence and Western blot analysis demonstrated that gremlin and TGFβ2 reciprocally increased the expression of each other, and both increased FN expression in the TM and surrounding tissues. Ad5.Gremlin elevated IOP and increased Fn and Col-1 gene expression in the TM of Smad3 wild-type (WT) mice, but had no effect in Smad3 HET or Smad3 KO mice.

Conclusions: Our results demonstrate that intravitreal injections of either Ad5.Gremlin or Ad5.TGFβ2 elevate IOP and upregulate the ECM protein FN in the TM of mice. These data show that gremlin signals through the Smad3-dependent pathway in the TM to elevate IOP. We determined for the first time gremlin's role in inducing ocular hypertension in an in vivo model system.

Elevated IOP is a well-known causative risk factor for both the development and progression of glaucoma. Intraocular pressure is regulated by aqueous humor (AH) production in the ciliary body and drainage from the eye via the trabecular meshwork (TM) and uveoscleral pathway. The role of the TM and the surrounding extracellular matrix (ECM) in IOP regulation has been extensively studied. Primary open-angle glaucoma (POAG) is associated with changes in the ECM composition within the TM, increased AH outflow resistance, and elevated IOP.^1,2 

Transforming growth factor–β2 signaling pathway has been shown to be an important regulator of ECM proteins in the TM,^3–9 and TGFβ2 has been found to be elevated in the AH and TM of POAG patients.^7,10–13 Transforming growth factor–β2 has also been shown to cause ocular hypertension in both ex vivo anterior segment perfusion organ culture models,^14,15 and by overexpression of a bioactivated form of TGFβ2 in mouse eyes.^4,16 Transforming growth factor–β2 is known to regulate the expression of ECM proteins through the canonical Smad pathway as well as noncanonical signaling pathways.^17–20 We have previously demonstrated that TGFβ2 signals through the canonical Smad and non-Smad pathways and alters the ECM in human TM cells.^5,7 We have also determined that TGFβ2 signaling through the canonical Smad pathway is essential for TGFβ2-induced ocular hypertension in mice.²¹ 

Transforming growth factor–β2 signaling is regulated by bone morphogenetic proteins (BMPs) that control a variety of functions in different cell types, and are present in the TM.^9,22 Bone morphogenetic proteins are regulated by BMP antagonists, such as gremlin, which bind BMP ligands and block BMP binding to their receptors.^23,24 We have previously shown higher expression of gremlin in glaucomatous TM cells and tissues.⁹ In addition, we have shown that gremlin antagonizes BMP4 inhibition of TGFβ2-induced ECM proteins in TM cells. Gremlin and TGFβ2 are involved in a “feed-forward” pathogenic fibrotic pathway in the TM, where gremlin increases TGFβ2 expression and TGFβ2 increases gremlin expression. Gremlin also elevates IOP in perfusion-cultured human anterior segments.^5,9 Here, we demonstrate that gremlin induces ocular hypertension and alters the ECM composition in the TM of mice. We also show that gremlin signaling through the canonical Smad pathway is essential for this gremlin-induced ocular hypertension. 

Methods

Animals

Both A/J and 129-Smad3^tm1Par/J mice were obtained from The Jackson Laboratory (Bar Harbor, ME, USA) and subsequently bred and aged at University of North Texas Health Science Center (UNTHSC; Fort Worth, TX, USA). We used A/J mice to test the effect of TGFβ2 and gremlin on ocular hypertension. We used homozygous wild-type (WT), heterozygous (HET), and homozygous knockout (KO) 129-Smad3^tm1Par/J mice to test the influence of Smad3 on gremlin signaling and ocular hypertension. Both male and female mice were used, and all mice were 5 to 6 months old at the start of the experiments. All experiments were conducted in compliance with the ARVO Statement of the Use of Animals in Ophthalmic and Vision Research and the UNTHSC Animal Care and Use Committee regulations. 

Adenovirus Injections

We used adenovirus 5 (Ad5) viral vectors to overexpress gremlin and TGFβ2 in the TM of mouse eyes. We have previously reported the preparation and use of Ad5.hTGFβ2^226/228 (hereafter referred to as Ad5.TGFβ2 throughout the manuscript) and Ad5.null vectors.¹⁶ The human gremlin gene was PCR-amplified from an expression vector (Origene, Rockville, MD, USA) with XhoI and BamHI restriction sites introduced into 5′ and 3′ ends, respectively. The PCR product was ligated with the pGEMT vector (Promega, Madison, WI, USA), transformed into Escherichia coli, screened, and amplified. The gremlin fragment was digested with XhoI and BamHI (Promega) and inserted into the pacAd5CMV shuttle vector (University of Iowa, Iowa City, IA, USA). After sequence verification (Genewiz, South Plainfield, NJ, USA), the pacAd5CMV-Gremlin vector was sent to University of Iowa Gene Transfer Core for adenovirus production. Two microliters of Ad5.TGFβ2, Ad5.Gremlin, or Ad5.null (2.5 × 10⁷ plaque-forming units [pfu]) were intravitreally injected in one eye of each animal, and the contralateral uninjected eyes were used as negative controls. Previously we have shown that intravitreal injections of Ad5 transgenes produce a more pronounced and consistent IOP response compared with intracameral injections in mouse eyes.^16,21,25,26 We have also demonstrated that Ad5.null, Ad5.GFP, and transgenes not associated with glaucomatous phenotypes have no effect on ocular hypertension.^16,21,25,26 

IOP Measurement

Intraocular pressure was measured as previously described.^21,27 In all experiments involving A/J mice, IOP was measured using a noninvasive method in conscious mice with the TonoLab tonometer (Colonial Medical Supply, Franconia, NH, USA). In all experiments involving 129-Smad3^tm1Par/J mice, IOP was measured after anesthesia with 2.5% isoflurane + 100% oxygen with the TonoLab tonometer. All measurements were made during the same 3-hour period of the lights-on phase. This study included IOP measurements in A/J mice: Ad5.TGFβ2-injected animals (n = 9), Ad5.Gremlin-injected animals (n = 9), and Ad5.null-injected animals (n = 5); as well as Ad5.Gremlin-injected 129-Smad3^tm1Par/J mice: WT (n = 5), HET (n = 8), and KO mice (n = 8). Area under the curve (AUC) was calculated for each individual mouse and then averaged for each mouse strain. The IOP exposure was calculated by subtracting the AUC of uninjected control eyes from the AUC of the Ad5.TGFβ^2226/228- and Ad5.Gremlin-injected eyes. Statistical significance was calculated by one-way ANOVA and Tukey post hoc analysis. All data are reported as mean ± SEM. 

Immunohistochemistry

At the end of the 3-week time course, whole eyes were removed and processed for immunohistochemistry to detect TGFβ2, gremlin, and fibronectin (FN) expression in Ad5.TGFβ2- (n = 4) and Ad5.Gremlin- (n = 4) injected eyes. Eyes were fixed in 4% paraformaldehyde for 24 hours, processed, and embedded in paraffin. Five-micrometer sections were cut, and sections were transferred to glass slides. Paraffin sections were dewaxed two times in xylene, 100% ethanol, and 95% ethanol for 2 minutes each. Slides were then soaked in PBS for 5 minutes. Rabbit anti-TGFβ2 (Santa Cruz Biotechnology, Santa Cruz, CA, USA), rabbit anti-gremlin (Abcam, Cambridge, MA, USA), and rabbit anti-FN (EMD Millipore; Billerica, MA, USA) antibodies were used at a 1:1000 dilution, followed by Alexa-Fluor–labeled anti-rabbit antibody. Slides were mounted with ProLong Gold mounting medium with DAPI (Life Technologies, Carlsbad, CA, USA) and imaged using a fluorescent microscope. Hematoxylin and eosin (H&E) staining was performed on sections from Ad5.Gremlin-injected eyes and is visible as pink and purple stain in the images. All images were taken at ×400 magnification; scale bars represent 50 μm. 

Quantitative (q)PCR and Western Blot Analysis

In order to detect RNA and protein changes at the peak of the IOP increase, eyes were harvested at day 14 post injection. Whole eyes from each group of mice were collected for qPCR analysis (n = 3–5 mice/genotype). The TM rings were carefully dissected from the whole eye. The TM rings contained mainly TM tissue and small amounts of sclera and cornea. Great effort was made to dissect away as much of the sclera and cornea as possible. Samples were homogenized and RNA extracted in Isol-RNA Lysis Reagent (5PRIME, Gaithersburg, MD, USA) and reverse-transcribed to cDNA (Bio-Rad iScript cDNA synthesis Kit; Bio-Rad, Hercules, CA, USA). Each PCR reaction contained: 10 μL 2X iQ SYBR Green Supermix (Bio-Rad), 0.25 μL forward primer (100 μM), 0.25 μL reverse primer (100 μM), 8.5 μL dH₂O, and 1.0 μL cDNA template (25 ng/ul). Primer pairs used in PCR reactions include FN (5′-GGTGACACTTATGAGCGCCCTA-3′, 5′-AACATGTAGCCACCAGTCTCAT-3′), COL1 (5′-GGAATGAAAGGGACACAGAGG-3′, 5′-TAGCACCATCATTTCCACGA-3′), and GAPDH (5′-ACTCCACTCACGGCAAATTC-3′, 5′-TCTCCATGGTGGTGAAGAACA-3′). Polymerase chain reaction conditions were: 95°C for 1 minute and 40× (95°C for 1 minute, 65°C for 45 seconds, 72°C for 45 seconds). Fold change was calculated using the ΔΔC_t method comparing expression to glutaraldehyde-3-phosphate dehydrogenase (GAPDH) and the uninjected control eye. Statistical significance was calculated by one-way ANOVA and Tukey post hoc analysis. All data are reported as mean ± SEM. 

Similarly, tissue was collected for Western blot analysis by dissecting the TM rings as described above (n = 4 mice/treatment). The tissue was placed in lysis buffer and manually homogenized using a small plastic tissue homogenizer in a 1.5-mL microcentrifuge tube. Protein concentration was determined using DC protein assay system (Bio-Rad). Proteins were separated on denaturing polyacrylamide gels and then transferred to polyvinylidene difluoride (PVDF) membranes by electrophoresis. Membranes were blocked with SuperBlock blocking buffer (Pierce Biotechnology, Rockford, IL, USA) for 1 hour and then incubated overnight with primary antibodies: rabbit anti-TGFβ2 (Santa Cruz Biotechnology) rabbit anti-gremlin (Abnova, Walnut, CA, USA), and rabbit anti-FN (EMD Millipore). The membranes were washed with TBST and processed with goat anti-rabbit horseradish peroxidase–conjugated secondary antibody (Santa Cruz Biotechnology). The proteins were then visualized (Fluor Chem 8900 imager; Alpha Innotech, San Leandro, CA, USA) using ECL detection reagent (SuperSignal West Femto Maximum Sensitivity Substrate; Pierce Biotechnology, Waltham, MA, USA). Beta-actin expression (mouse anti-β-actin; EMD Millipore) was used as a loading control for each blot. 

Results

Gremlin is a known regulator of the TGFβ2/BMP signaling pathway. Gremlin antagonizes BMP4, which in turn allows uninhibited TGFβ2-induction of ECM proteins such as FN in human TM cells.⁹ We have also shown that gremlin alone can induce these ECM proteins in TM cells.⁵ The TM is a key tissue in IOP regulation. Therefore, we determined the effect of gremlin overexpression on IOP in mice (Fig. 1). Ad5.Gremlin was injected intravitreally into one eye of each animal, with the contralateral eye as an uninjected control. Intraocular pressure was significantly increased at day 11 (P < 0.05), day 14 (P < 0.01), and day 18 (P < 0.05) post injection (Fig. 1A). Previously, we reported that TGFβ2 induces ocular hypertension in mice.^4,16 Here, we confirm that Ad5.TGFβ2 does indeed increase IOP with significant IOP differences at all time points measured post injection (days 3–18, P < 0.001; day 21, P < 0.05; Fig. 1A). Significance was calculated by one-way ANOVA with Tukey post hoc analysis at each time point and P values reported are comparison of injected eye with the corresponding uninjected control eye for each treatment (n = 9 mice/group). Ad5.null was tested as a negative control, showing no change in IOP at any time point post injection (n = 5; Fig. 1B). 

Figure 1

 

Ad5.Gremlin and Ad5.TGFβ2 induce ocular hypertension in mice. All mice were injected in one eye with 2.5 × 10⁷ pfu Ad5.Gremlin, Ad5.hTGFβ2, or Ad5.null, and IOP was measured for 21 days post injection. The contralateral uninjected eye served as a control. (A) Ad5.Gremlin significantly increased IOP at day 11 (*P < 0.05), day 14 (**P < 0.01), and day 18 (*P < 0.05) post injection (n = 9 mice). Ad5.TGFβ2 significantly elevated IOP at days 3 to 18 (***P < 0.001) and day 21 (*P < 0.05; n = 9 mice). (B) Ad5.null had no significant effect on IOP at any time point (n = 5 mice). Significance was calculated by one-way ANOVA with Tukey post hoc analysis at each time point and P values reported are comparison of injected eye with the corresponding uninjected control eye for each treatment.

In order to determine the expression levels of gremlin and TGFβ2 in the TM post injection, Western blot analysis was performed. Eyes were harvested 14 days post injection of Ad5.Gremlin or Ad5.TGFβ2. The TM ring was carefully dissected and protein extracted. As expected, in the Ad5.Gremlin-injected eyes there was a significant increase in gremlin protein expression (Figs. 2A, 2B). There was also a trend for increased TGFβ2 expression in the Ad5.Gremlin-injected eyes compared with the uninjected control eye (Figs. 2A, 2D). In the Ad5.TGFβ2-injected eyes, there was a significant increase in TGFβ2 protein expression and gremlin protein expression (Figs. 2A, 2C, 2E). We also analyzed the Ad5.Gremlin- and Ad5.TGFβ2-injected eyes by immunohistochemistry and showed a similar effect with overexpression of TGFβ2 inducing gremlin expression and overexpression of gremlin-inducing TGFβ2 expression in the TM and surrounding tissue (Fig. 3). These data suggest a feed-forward pathogenic pathway. This feed-forward pathway was previously demonstrated in primary TM cell cultures.⁵ Both gremlin and TGFβ2 have been associated with increased production of ECM proteins, and these data suggest that a feed-forward mechanism could further effect the ECM deposition within the TM, possibly leading to increased aqueous humor outflow resistance and IOP elevation. 

Figure 2

 

Gremlin and TGFβ2 reciprocally induce expression of each other in the mouse TM. Mice were injected in one eye with 2.5 × 10⁷ pfu Ad5.Gremlin or Ad5.hTGFβ2 and the contralateral uninjected eye served as controls. At 14 days post injection, the eyes were harvested, and the TM ring was carefully dissected, protein isolated, and Western blot analysis performed. (A) Representative Western blots of gremlin and TGFβ2 expression in the TM of Ad5.Gremlin and Ad5.TGFβ2-injected eyes. (B) Ad5.Gremlin significantly increased expression in gremlin protein (1110.36 ± 186.49%) compared with uninjected control (100.00 ± 13.48%; P < 0.05, n = 4). (C) Ad5.TGFβ2 significantly increased gremlin expression (192.68% ± 13.92%) compared with uninjected controls (100.00 ± 22.09%; P < 0.05, n = 4). (D) Ad5.Gremlin appeared to increase expression of TGFβ2 (348.79 ± 130.11%) compared with uninjected controls (100.00 ± 92.84; statistically not significant, n = 4). (E) Ad5TGFβ2 significantly increased TGFβ2 expression (180.35 ± 69.38%) compared with uninjected control (100.00 ± 56.79%; P < 0.05, n = 4). All values are mean ± SEM, statistical significance determined by Student's t-test.

Figure 3

 

Immunohistochemical analysis of gremlin and TGFβ2 expression in the mouse TM. Mice were injected in one eye with 2.5 × 10⁷ pfu Ad5.Gremlin or Ad5.hTGFβ2 and the contralateral uninjected eye served as a control. (A–C) Ad5.Gremlin and Ad5TGFβ2 increased gremlin expression in the TM and closely surrounding tissues compared with the uninjected control eyes. (D–F) Ad5.Gremlin and Ad.TGFβ2 increased expression of TGFβ2 in the TM and closely surrounding tissues compared with the uninjected control eyes. Scale bar: 50 μm (n = 4).

In order to test whether there was increased ECM deposition in the TM and surrounding tissue in the Ad5.Gremlin- and Ad5.TGFβ2-injected eyes, we performed Western blot and immunohistochemical analysis on FN expression (Fig. 4). In both Ad5.Gremlin- (Figs. 4A, 4C) and Ad5.TGFβ2- (Figs. 4B, 4C) injected eyes, there was a significant increase in FN expression in the TM analyzed by Western blot compared with the uninjected control eyes. These results were recapitulated in immunohistochemical analysis with an increase in FN expression in the TM and surrounding tissues of Ad5.Gremlin- and Ad5.TGFβ2-injected eyes compared with uninjected control eyes (Figs. 4D–F). These data show that both gremlin and TGFβ2 induce expression of the ECM protein FN, and overexpression of gremlin and TGFβ2 increase IOP in mice. 

Figure 4

 

Fibronectin expression is increased in the TM of Ad5.Gremlin- and Ad5.TGFβ2-injected eyes. Mice were injected in one eye with 2.5 × 10⁷ pfu Ad5.Gremlin or Ad5.hTGFβ2 and the contralateral uninjected eyes served as a control. (A) Ad5.Gremlin significantly increased expression in FN protein (175 ± 17.41%) compared with uninjected control (100.00 ± 10.33%; P < 0.05, n = 4). (B) Ad5.TGFβ2 significantly increased FN expression (142.38% ± 6.51%) compared with uninjected control (100.00 ± 9.35%; P < 0.05, n = 4). (C) Representative blot of FN expression in the TM and surrounding tissue of Ad5.Gremlin- and Ad5.TGFβ2-injected eyes (n = 4, quantified in [A, B]). All values are mean ± SEM, statistical significance determined by Student's t-test. (D–F) Fibronectin expression is increased in the TM and surrounding tissues of Ad5.Gremlin- and Ad5.TGFβ2-injected eyes analyzed by immunohistochemistry. Scale bar: 50 μm (n = 4).

Previously, we reported that TGFβ2 alters the ECM composition^5,7 and induces ocular hypertension through a Smad3-dependent pathway.⁴ Here, we used our in vivo mouse model system to test whether gremlin overexpression alters the ECM composition in the TM and elevates IOP via the Smad3-dependent signaling pathway. Homozygous WT, HET, and homozygous KO 129-Smad3^tm1Par/J mice were used in each of our experiments. We have previously reported that there is no difference in the gross morphology of the TM, gross morphology of the anterior chamber, or FN protein expression in the TM between Smad3 genotypes.⁴ We have also previously reported that adenoviral injection can cause mild inflammation in the anterior chamber, but this inflammation does not affect ocular hypertension phenotypes based on the lack of ocular hypertension in animals injected with Ad5.null, Ad5.GFP, Ad5.MYOC^WT, or Ad5hTGFβ2^WT.^4,16,25,28 We recapitulate these data here by injecting Ad5.Gremlin into one eye of each Smad3 genotype. Eyes were harvested 14 days post injection and analyzed by immunohistochemistry. Hematoxylin and eosin staining revealed similar TM morphology and organization in WT, HET, and KO mice injected with Ad5.Gremlin (Fig. 5). The iridiocorneal angles were open in all eyes examined. We did observe mild inflammation in eyes of each genotype post injection of Ad5.Gremlin; however, this did not affect ocular hypertension as IOP remained normal in the HET and KO mice throughout an entire 28-day time course (Figs. 6A, 6B). 

Figure 5

 

Histologic analysis of anterior chambers after Ad5.Gremlin injection. All mice were injected in one eye with 2.5 × 10⁷ pfu Ad5.Gremlin, and tissue was harvested 14 days post injection. (A–F) Hematoxylin and eosin staining revealed similar TM morphology and organization in WT, HET, and KO eyes post injection with Ad5.Gremlin (n = 4, WT; n = 4, HET, n = 3 KO). The iridiocorneal angle was open in all eyes examined. Scale bars: 50 μm.

Figure 6

 

Smad3 is necessary for Ad5.Gremlin-induced ocular hypertension. Mice were injected in one eye with 2.5 × 10⁷ pfu Ad5.Gremlin and the contralateral uninjected eye served as a control. (A) Intraocular pressure was significantly elevated at day 13 (P < 0.01), day 17 (P < 0.001), and day 20 (P < 0.001) in WT mice. No significant IOP elevation was observed at any time point in the Smad3 HET and KO mice (n = 5 mice/genotype). (B) Intraocular pressure exposure was calculated as millimeters of mercury multiplied by the days and also showed a significant difference between WT and HET mice (P < 0.05) and WT and KO mice (P < 0.01). (C) Ad5.Gremlin transduction overexpressed gremlin in the TM of each mouse genotype. (D) Fibronectin and collagen-1 gene expression was elevated in Ad5.Gremlin WT animals (P < 0.05) compared with HET and KO animals 14 days post injection (n = 3–5 mice/genotype). Significance was calculated by one-way ANOVA with Tukey post hoc analysis at each time point and P values reported are comparison of injected eye with the corresponding uninjected control eye for each treatment.

Ocular hypertension experiments were conducted by injecting Ad5.Gremlin into one eye of each animal, the contralateral uninjected eye serving as a control. Intraocular pressure was significantly elevated at day 13 (P < 0.01), day 17 (P < 0.001), and day 20 (P < 0.001) in WT mice, while no significant IOP elevation was observed at any time-point in the HET and KO mice, n = 5 to 8 mice/genotype (Fig. 6A). Significance was calculated by one-way ANOVA with Tukey post hoc analysis at each time point and P values reported are comparison of injected eye compared with the corresponding uninjected control eye for each treatment. Intraocular pressure exposure was calculated as millimeters of mercury multiplied by the days and also showed a significant difference between WT and HET mice (P < 0.05) and WT and KO mice (P < 0.01), but no statistical difference between HET and KO mice, analyzed by one-way ANOVA with Tukey post hoc analysis (Fig. 6B). These data are similar to the IOP response shown in SMAD3 WT, HET, and KO mice injected with Ad5.TGFβ2⁴ and demonstrate that two copies of Smad3 are necessary for the induction of ocular hypertension. In order to demonstrate that the Ad5.Gremlin virus was overexpressing gremlin in the TM for each mouse genotype, Western blot analysis was performed (Fig. 6C). A trace amount of endogenous gremlin is evident in the uninjected control eyes. Because the peak IOP elevation occurred at approximately 14 days post injection, this time point was used to determine a change in ECM gene expression. Both fibronectin and collagen-1 gene expression was elevated in the WT animals compared with HET and KO animals (Fig. 6D). These data indicate that gremlin, like TGFβ2, signals through a Smad3-dependent pathway in the TM, and overexpression of gremlin causes increased ECM gene expression. 

Discussion

Previous studies have shown that TGFβ2/BMP signaling regulates the ECM architecture in the TM. Recently, we identified that Smad3 is necessary for TGFβ2-indcued ocular hypertension in mice.^4,16 Gremlin is an antagonist of BMP4, and has also been shown to be an important regulator of ECM proteins in TM cells and glaucomatous tissues.^5,9,29 Combined, these data suggest that gremlin may play a role in ocular hypertension and be an important regulator of ECM proteins in the glaucomatous TM. 

To directly test the hypothesis that overexpression of gremlin will increase IOP, we used our mouse model system of chronically elevating IOP with an Ad5 expression vector for gremlin or a bioactivated form of TGFβ2. Both gremlin and TGFβ2 induced ocular hypertension in mice. Similar to the data reported in TM cells, each of these molecules increased expression of the other protein, creating a feed-forward loop, where TGFβ2 increased expression of gremlin and gremlin increased expression of TGFβ2 in the TM. Both gremlin and TGFβ2 induced expression of FN in the TM. In addition to increasing expression of TGFβ2, gremlin, and FN in the TM, we also saw increased FN expression in the surrounding tissues. Given that both TGFβ2 and gremlin are secreted proteins, they could be affecting the expression levels in tissues surrounding the TM as well. Our results are similar to those previously reported using the Ad5.TGFβ2 virus.⁴ These data provide further support to our previously published data and demonstrate that gremlin is an important regulator of the ECM in the TM and IOP in vivo. 

The signaling pathways involved in the TGFβ2 induced changes to the ECM in the TM and ocular hypertension are complex.^4,5,30 Both Smad and non-Smad signaling pathways are activated by TGFβ2 in TM cells and increase expression of ECM proteins as well as ECM cross-linking enzymes.^5,6,30 We recently found that Smad3 was necessary for TGFβ2-induced ocular hypertension in mice.⁴ In cell culture studies, we showed that gremlin activates only the canonical Smad2/3 pathway, and inhibition of Smad signaling blocked gremlin's effect on TM ECM expression.⁵ However, we also reported that gremlin's induction of ECM cross-linking lysyl oxidase genes uses both the canonical and noncanonical TGFβ2-signaling pathways.²⁹ Here, we tested the Smad3-signaling pathway involved in gremlin-induced ocular hypertension using Smad3 KO mice. Although the WT animals developed increased IOP following Ad5.Gremlin injection, the Smad3 KO and Smad3 HET mice had no significant IOP elevation. These data show that Smad3 also is necessary for gremlin-induced ocular hypertension in vivo. 

Gremlin and TGFβ2 are important regulators of the ECM architecture, and overexpression of these proteins increased IOP in vivo. Transforming growth factor–β2 has been shown to be elevated in the aqueous humor and TM of POAG,^7,10–13 and gremlin has been shown to be increased in both glaucomatous TM cells and tissues.⁹ However, the mechanistic pathway regulating increased expression of each of these proteins has not been elucidated. Given the feed-forward pathogenic pathway between TGFβ2 and gremlin in both TM cells and in our mouse model system, it is conceivable that this relationship is also occurring in human glaucoma. We tested the potential role of matrix metalloproteases (MMP), MMP2, MMP3, MMP9, and MMP14, in the TM of Ad5.Gremlin-injected eyes and found no statistical difference in expression compared with uninjected control eyes (data not shown). Additional studies are necessary to determine the precise insult that is producing the initial increased expression of these molecules. Our data demonstrate that these molecules are important regulators of the ECM in the TM and cause ocular hypertension in mice. 

Acknowledgments

The authors thank Robert Wordinger for many helpful discussions. 

Supported by Grants R21EY019977 and R01EY017374 from the National Institutes of Health (Bethesda, MD, USA). 

Disclosure: C.M. McDowell, None; H. Hernandez, None; W. Mao, None; A.F. Clark, None