Generation and Characterization of a Novel Mouse Line, Keratocan-rtTA (KeraRT), for Corneal Stroma and Tendon Research

SMAD4-Dependent Signaling Pathway Involves in the Pathogenesis of TGFBR2-Related CE-like Phenotype

(1) Background: Our previous data indicated that disturbance of the Transforming Growth Factor beta (TGFB) signaling pathway via its Type-2 Receptor (TGFBR2) can cause a Corneal Ectasia (CE)-like phenotype. The purpose of this study is to elucidate whether the SMAD4-dependent signaling pathway is involved in the TGFBR2-related CE-like pathogenesis. (2) Methods: Smad4 was designed to be conditionally knocked out from keratocytes. Novel triple transgenic mice, KerartTA; Tet-O-Cre; Smad4flox/flox (Smad4kera-cko), were administered with doxycycline (Dox). Optical Coherence Tomography (OCT) was performed to examine Central Corneal Thickness (CCT), Corneal Radius, Anterior Chamber and CE-like phenotype and compared to the littermate Control group (Smad4Ctrl). (3) Results: The OCT revealed normal cornea in the Smad4Ctrl and a CE-like phenotype in the Smad4kera-cko cornea, in which the overall CCT in Smad4kera-cko was thinner than that of Smad4Ctrl at P42 (n = 6, p < 0.0001) and showed no significant difference when compared to that in Tgfbr2kera-cko. Furthermore, the measurements of the Anterior Chamber and Corneal Radius indicated a substantial ectatic cornea in the Smad4kera-cko compared to Smad4Ctrl. The H&E staining of Smad4kera-cko mimics the finding in the Tgfbr2kera-cko. The positive immunostaining of cornea-specific marker K12 indicating the cell fate of cornea epithelium remained unchanged in Smad4kera-cko and the Proliferating Cell Nuclear Antigen (PCNA) immunostaining further indicated an enhanced proliferation in the Smad4kera-cko. Both immunostainings recapitulated the finding in Tgfbr2kera-cko. The Masson's Trichrome staining revealed decreased collagen formation in the corneal stroma from both Smad4kera-cko and Tgfbr2kera-cko. The collagen type 1 (Col1a1) immunostaining further confirmed the reduction in collagen type 1 formation in Smad4kera-cko. (4) Conclusions: The aforementioned phenotypes in the Smad4kera-cko strain indicated that the SMAD4-dependent signaling pathway is involved in the pathogenesis of the CE-like phenotype observed in Tgfbr2kera-cko.

The RBPMSCreERT2-tdTomato mouse line for studying retinal and vascular relevant diseases

RNA-binding protein with multiple splicing (RBPMS) plays a crucial role in cardiac mesoderm specification and cardiovascular development, as well as being a typical marker for whole retinal ganglion cells (RGCs). However, there is a lack of animal models to spatiotemporally trace the location and function of RBPMS-expressing cells in vivo. In this study, we develop a tamoxifen-inducible RBPMS-tdTomato reporter mouse line to track RBPMS-expressing cells during embryogenesis and adulthood. This mouse line allows us to identify and locate RBPMS-tdTomato-positive cells among various tissues, especially in RGCs and smooth muscle cells, which assist to simulate related retinal degenerative diseases, model and examine choroidal neovascularization non-invasively in vivo. Our results show that the RBPMSCreERT2-tdTomato mouse line is a valuable tool for lineage tracing, disease modeling, drug screening, as well as isolating specific target cells.

Keratocyte-Derived Myofibroblasts: Functional Differences With Their Fibroblast Precursors

Purpose

In this study, we aim to elucidate functional differences between fibroblasts and myofibroblasts derived from a keratocyte lineage to better understand corneal scarring.

Methods

Corneal fibroblasts, derived from a novel triple transgenic conditional KeraRT/tetO-Cre/mTmG mouse strain that allows isolation and tracking of keratocyte lineage, were expanded, and transformed by exposure to transforming growth factor (TGF)-β1 to myofibroblasts. The composition and organization of a fibroblast-built matrix, deposited by fibroblasts in vitro, was analyzed and compared to the composition of an in vitro matrix built by myofibroblasts. Second harmonic generation microscopy (SHG) was used to study collagen organization in deposited matrix. Different extracellular matrix proteins, expressed by fibroblasts or myofibroblasts, were analyzed and quantified. Functional assays compared latent (TGF-β) activation, in vitro wound healing, chemotaxis, and proliferation between fibroblasts and myofibroblasts.

Results

We found significant differences in cell morphology between fibroblasts and myofibroblasts. Fibroblasts expressed and deposited significantly higher quantities of fibril forming corneal collagens I and V. In contrast, myofibroblasts expressed and deposited higher quantities of fibronectin and other non-collagenous matrix components. A significant difference in the activation of latent TGF-β activation exists between fibroblasts and myofibroblasts when measured with a functional luciferase assay. Fibroblasts and myofibroblasts differ in their morphology, extracellular matrix synthesis, and deposition, activation of latent TGF-β, and chemotaxis.

Conclusions

The differences in the expression and deposition of extracellular matrix components by fibroblasts and myofibroblasts are likely related to critical roles they play during different stages of corneal wound healing.

Role of Fibroblast Growth Factor Receptor 2 (FGFR2) in Corneal Stromal Thinning

Purpose

To determine the role of fibroblast growth factor receptor 2 (FGFR2)-mediated signaling in keratocytes during corneal development, a keratocyte-specific FGFR2-knockout (named FGFR2cKO) mouse model was generated, and its phenotypic characteristics were determined.

Methods

A FGFR2cKO mouse model was generated by the following method: FGFR2 flox mice were crossed with the inducible keratocyte specific-Cre mice (Kera-rtTA/tet-O-Cre). Both male and female FGFR2cKO- and control mice (1 to 3-months-old) were analyzed for changes in corneal topography and pachymetry maps using the optical coherence tomography (OCT) method. The comparative TUNEL assay and immunohistochemical analyses were performed using corneas of FGFR2cKO and control mice to determine apoptotic cells, and expression of collagen-1 and fibronectin. Transmission electron microscopic analysis was conducted to determine collagen structures and their diameters in corneas of FGFR2cKO and control mice.

Results

OCT-analyses of corneas of FGFR2cKO mice (n = 24) showed localized central thinning and an increased corneal steepness compared to control mice (n = 23). FGFR2cKO mice further showed a decreased expression in collagen-1, decreased collagen diameters, acute corneal hydrops, an increased fibronectin expression, and an increased number of TUNEL-positive cells suggesting altered collagen structures and keratocytes' apoptosis in the corneas of FGFR2cKO mice compared to control mice.

Conclusions

The FGFR2cKO mice showed several corneal phenotypes (as described above in the results) that are also exhibited by the human keratoconus corneas. The results suggested that the FGFR2cKO mouse model serves to elucidate not only the yet unknown role of FGFR2-mediated signaling in corneal physiology but also serves as a model to determine molecular mechanism of human keratoconus development.

Stromal matrix directs corneal fibroblasts to re-express keratocan after injury and transplantation

Every tissue has an extracellular matrix (ECM) with certain properties unique to it - the tissue 'niche' - that are necessary for normal function. A distinct specific population of quiescent keratocan-expressing keratocytes populate the corneal stroma during homeostasis to maintain corneal function. However, during wound healing, when there is alteration of the niche conditions, keratocytes undergo apoptosis, and activated corneal fibroblasts and myofibroblasts attempt to restore tissue integrity and function. It is unknown what the fate of activated and temporary fibroblasts and myofibroblasts is after the wound healing process has resolved. In this study, we used several strategies to elucidate the cellular dynamics of corneal wound healing and the fate of corneal fibroblasts. We injured the cornea of a novel mouse model that allows cell-lineage tracing, and we transplanted a cell suspension of in vitro-expanded corneal fibroblasts that could be tracked after being relocated into normal stroma. These transplanted fibroblasts regained expression of keratocan in vivo when relocated to a normal stromal niche. These findings suggest that transformed fibroblasts maintain plasticity and can be induced to a keratocyte phenotype once relocated to an ECM with normal signaling ECM.

Ablation of collagen XII disturbs joint extracellular matrix organization and causes patellar subluxation

Collagen XII, belonging to the fibril-associated collagens, is a homotrimeric secreted extracellular matrix (ECM) protein encoded by the COL12A1 gene. Mutations in the human COL12A1 gene cause an Ehlers-Danlos/myopathy overlap syndrome leading to skeletal abnormalities and muscle weakness. Here, we studied the role of collagen XII in joint pathophysiology by analyzing collagen XII deficient mice and human patients. We found that collagen XII is widely expressed across multiple connective tissue of the developing joint. Lack of collagen XII in mice destabilizes tendons and the femoral trochlear groove to induce patellar subluxation in the patellofemoral joint. These changes are associated with an ECM damage response in tendon and secondary quadriceps muscle degeneration. Moreover, patellar subluxation was also identified as a clinical feature of human patients with collagen XII deficiency. The results provide an explanation for joint hyperlaxity in mice and human patients with collagen XII deficiency.

Small leucine rich proteoglycans: Biology, function and their therapeutic potential in the ocular surface

Small leucine rich proteoglycans (SLRPs) are the largest family of proteoglycans, with 18 members that are subdivided into five classes. SLRPs are small in size and can be present in tissues as glycosylated and non-glycosylated proteins, and the most studied SLRPs include decorin, biglycan, lumican, keratocan and fibromodulin. SLRPs specifically bind to collagen fibrils, regulating collagen fibrillogenesis and the biomechanical properties of tissues, and are expressed at particularly high levels in fibrous tissues, such as the cornea. However, SLRPs are also very active components of the ECM, interacting with numerous growth factors, cytokines and cell surface receptors. Therefore, SLRPs regulate major cellular processes and have a central role in major fundamental biological processes, such as maintaining corneal homeostasis and transparency and regulating corneal wound healing. Over the years, mutations and/or altered expression of SLRPs have been associated with various corneal diseases, such as congenital stromal corneal dystrophy and cornea plana. Recently, there has been great interest in harnessing the various functions of SLRPs for therapeutic purposes. In this comprehensive review, we describe the structural features and the related functions of SLRPs, and how these affect the therapeutic potential of SLRPs, with special emphasis on the use of SLRPs for treating ocular surface pathologies.

Genipin increases extracellular matrix synthesis preventing corneal perforation

PURPOSE

Corneal melting and perforation are feared sight-threatening complications of infections, autoimmune disease, and severe burns. Assess the use of genipin in treating stromal melt.

METHODS

A model for corneal wound healing was created through epithelial debridement and mechanical burring to injure the corneal stromal matrix in adult mice. Murine corneas were then treated with varying concentrations of genipin, a natural occurring crosslinking agent, to investigate the effects that matrix crosslinking using genipin has in wound healing and scar formation. Genipin was used in patients with active corneal melting.

RESULTS

Corneas treated with higher concentrations of genipin were found to develop denser stromal scarring in a mouse model. In human corneas, genipin promoted stromal synthesis and prevention of continuous melt. Genipin mechanisms of action create a favorable environment for upregulation of matrix synthesis and corneal scarring.

CONCLUSION

Our data suggest that genipin increases matrix synthesis and inhibits the activation of latent transforming growth factor-β. These findings are translated to patients with severe corneal melting.

Inducible Slc4a11 Knockout Triggers Corneal Edema Through Perturbation of Corneal Endothelial Pump

Purpose

The conventional Slc4a11 knockout (KO) shows significant corneal edema at eye opening, a fact that complicates the study of the initial events leading to edema. An inducible KO would provide opportunities to examine early events following loss of Slc4a11 activity.

Methods

Slc4a11 Flox (SF) mice were crossed with mice expressing the estrogen receptor Cre Recombinase fusion protein and fed tamoxifen (Tm) for two weeks. Corneal thickness (CT) was measured by OCT. At eight weeks endpoint, oxidative damage, tight junction integrity, stromal lactate concentration, endothelial permeability, differentially expressed transporters, and junction proteins were determined. Separately, a keratocyte only inducible Slc4a11 KO was also examined.

Results

At four weeks post-Tm induction Slc4a11 transcript levels were 2% of control. Corneal thickness increased gradually and was 50% greater than Wild Type (WT) after eight weeks with significantly altered endothelial morphology, increased nitrotyrosine staining, significantly higher stromal lactate, decreased expression of lactate transporters and Na-K ATPase activity, higher ATP, altered expression of tight and adherens junctions, and increased fluorescein permeability. No significant differences in CT were found between WT and keratocyte only Slc4a11 KO.

Conclusions

The Slc4a11 inducible KO shows development of a similar phenotype as the conventional KO, thereby validating the model and providing a tool for further use in examining the sequence of cellular events by use of noninvasive in vivo physiological probes.

Corneal fibroblasts: Function and markers

Corneal stromal keratocytes contribute to the maintenance of corneal transparency and shape by synthesizing and degrading extracellular matrix. They are quiescent in the healthy cornea, but they become activated in response to insults from the external environment that breach the corneal epithelium, with such activation being associated with phenotypic transformation into fibroblasts. Corneal fibroblasts (activated keratocytes) act as sentinel cells to sense various external stimuli-including damage-associated molecular patterns derived from injured cells, pathogen-associated molecular patterns of infectious microorganisms, and inflammatory mediators such as cytokines-under pathological conditions such as trauma, infection, and allergy. The expression of various chemokines and adhesion molecules by corneal fibroblasts determines the selective recruitment and activation of inflammatory cells in a manner dependent on the type of insult. In infectious keratitis, the interaction of corneal fibroblasts with various components of microbes and with cytokines derived from infiltrated inflammatory cells results in excessive degradation of stromal collagen and consequent corneal ulceration. Corneal fibroblasts distinguish between type 1 and type 2 inflammation through recognition of corresponding cytokines, with their activation by type 2 cytokines contributing to the pathogenesis of corneal lesions in severe ocular allergic diseases. Pharmacological targeting of corneal fibroblast function is thus a potential novel therapeutic approach to prevention of excessive corneal stromal inflammation, damage, and scarring.

Composition, structure and function of the corneal stroma

No other tissue in the body depends more on the composition and organization of the extracellular matrix (ECM) for normal structure and function than the corneal stroma. The precise arrangement and orientation of collagen fibrils, lamellae and keratocytes that occurs during development and is needed in adults to maintain stromal function is dependent on the regulated interaction of multiple ECM components that contribute to attain the unique properties of the cornea: transparency, shape, mechanical strength, and avascularity. This review summarizes the contribution of different ECM components, their structure, regulation and function in modulating the properties of the corneal stroma. Fibril forming collagens (I, III, V), fibril associated collagens with interrupted triple helices (XII and XIV), network forming collagens (IV, VI and VIII) as well as small leucine-rich proteoglycans (SLRP) expressed in the stroma: decorin, biglycan, lumican, keratocan, and fibromodulin are some of the ECM components reviewed in this manuscript. There are spatial and temporal differences in the expression of these ECM components, as well as interactions among them that contribute to stromal function. Unique regions within the stroma like Bowman's layer and Descemet's layer are discussed. To define the complexity of corneal stroma composition and structure as well as the relationship to function is a daunting task. Our knowledge is expanding, and we expect that this review provides a comprehensive overview of current knowledge, definition of gaps and suggests future research directions.

Distribution and Function of Glycosaminoglycans and Proteoglycans in the Development, Homeostasis and Pathology of the Ocular Surface

The ocular surface, which forms the interface between the eye and the external environment, includes the cornea, corneoscleral limbus, the conjunctiva and the accessory glands that produce the tear film. Glycosaminoglycans (GAGs) and proteoglycans (PGs) have been shown to play important roles in the development, hemostasis and pathology of the ocular surface. Herein we review the current literature related to the distribution and function of GAGs and PGs within the ocular surface, with focus on the cornea. The unique organization of ECM components within the cornea is essential for the maintenance of corneal transparency and function. Many studies have described the importance of GAGs within the epithelial and stromal compartment, while very few studies have analyzed the ECM of the endothelial layer. Importantly, GAGs have been shown to be essential for maintaining corneal homeostasis, epithelial cell differentiation and wound healing, and, more recently, a role has been suggested for the ECM in regulating limbal stem cells, corneal innervation, corneal inflammation, corneal angiogenesis and lymphangiogenesis. Reports have also associated genetic defects of the ECM to corneal pathologies. Thus, we also highlight the role of different GAGs and PGs in ocular surface homeostasis, as well as in pathology.

Excess Transforming Growth Factor-α Changed the Cell Properties of Corneal Epithelium and Stroma

Purpose

This study is to investigate the corneal anomaly caused by excess transforming growth factor-α (TGF-α) during mouse development.

Methods

Bitransgenic Kera^RT/TGF-α mice, generated via cross-mating tetO-TGF-α and Kera^RT mice, were induced to overexpress TGF-α by doxycycline commencing at embryonic day 0 or postnatal day 0 to different developmental stages. Bitransgenic mice with doxycycline induction were defined as TGF-α^ECK mice (TGF-α excess expression by corneal keratocytes). Mouse eyes were examined by hematoxylin and eosin staining, immunofluorescent staining and transmission electron microscopy. Protein and RNA from mouse cornea were subjected to western blotting and real-time quantitative polymerase chain reaction.

Results

In TGF-α^ECK mice, TGF-α overexpression resulted in corneal opacity. Excess TGF-α initially caused corneal epithelial hyperplasia and subsequent epithelium degeneration as the mouse developed, which was accompanied by gradually diminished K12 expression from the periphery of corneal epithelium and increased K13 expression toward the corneal center. Interestingly, K14 was detected in all layers of corneal epithelium of TGF-α^ECK mice, whereas it was limited at basal layer of controls. Transmission electron microscopy showed desmosome loss between corneal epithelial cells of TGF-α^ECK mice. In TGF-α^ECK mice, keratocan expression was abolished; α-SMA expression was increased while expression of Col1a1, Col1a2, and Col5a1 was diminished. Cell proliferation increased in the corneal epithelium and stroma, but not in the endothelium of TGF-α^ECK mice.

Conclusions

Excess TGF-α had detrimental effects on corneal morphogenesis during mouse development in that it changed the cell fate of corneal epithelial cells to assume conjunctival phenotypic expression of K13, and keratocytes to myofibroblast phenotype.

Primary cilia deficiency in neural crest cells models anterior segment dysgenesis in mouse

Defects affecting tissues of the anterior segment (AS) of the eye lead to a group of highly debilitating disorders called Anterior Segment Dysgenesis (ASD). Despite the identification of some causative genes, the pathogenesis of ASD remains unclear. Interestingly, several ciliopathies display conditions of the AS. Using conditional targeting of Ift88 with Wnt1-Cre, we show that primary cilia of neural crest cells (NCC), precursors of most AS structures, are indispensable for normal AS development and their ablation leads to ASD conditions including abnormal corneal dimensions, defective iridocorneal angle, reduced anterior chamber volume and corneal neovascularization. Mechanistically, NCC cilia ablation abolishes hedgehog (Hh) signaling in the periocular mesenchyme (POM) canonically activated by choroid-secreted Indian Hh, reduces proliferation of POM cells surrounding the retinal pigment epithelium and decreases the expression of Foxc1 and Pitx2, two transcription factors identified as major ASD causative genes. Thus, we uncovered a signaling axis linking cilia and ASD.

Repressed Wnt Signaling Accelerates the Aging Process in Mouse Eyes

PURPOSE

Ocular aging is a natural process of functional decline in vision. When the process reaches a point that compromised vision affects normal daily activity, it manifests as age-related ocular diseases, such as age-related macular degeneration, cataracts, glaucoma, and pseudoexfoliation syndrome. We previously reported that repressed Wnt signaling accelerated the maturation of corneal epithelium during tissue development. Here, we explore the hypothesis that repressed Wnt signaling is associated with accelerated aging in mouse eyes.

METHODS

Wnt ligand antagonist secreted frizzled-related protein 1 (sFRP1) was expressed in the corneal stroma by a tissue-specific, inducible, bitransgenic system. Tissue structure was analyzed for signs of aging. Signal transduction analysis was performed to determine the cellular response to sFRP1.

RESULTS

Mouse eyes with sFRP1 expression showed signs of accelerated aging, resembling those found in pseudoexfoliation (PEX) syndrome, a known age-related disease. Specific findings include granular deposition on the surface of the anterior lens capsule, pigment loss from the anterior surface of the iris, the presence of fibrillary material in the anterior chamber, and changes in cell size (polymegethism) and shape (pleomorphism) of the corneal endothelial cells. In vitro studies demonstrated that sFRP1 did not inhibit Wnt5a function and that cells responded to sFRP1 and Wnt5a in a very similar manner.

CONCLUSION

The expression of sFRP1 accelerates the aging process in mouse eyes and future studies are warranted to elucidate the underlying mechanisms.

Aberrant expression of a stabilized β-catenin mutant in keratocytes inhibits mouse corneal epithelial stratification

We previously reported that genetic deletion of β-catenin in mouse corneal keratocytes resulted in precocious corneal epithelial stratification. In this study, to strengthen the notion that corneal keratocyte-derived Wnt/β-catenin signaling regulates corneal epithelial stratification during mouse development, we examined the consequence of conditional overexpression of a stabilized β-catenin mutant (Ctnnb1ΔE3) in corneal keratocytes via a doxycycline (Dox)-inducible compound transgenic mouse strain. Histological analysis showed that conditional overexpression of Ctnnb1ΔE3 in keratocytes inhibited corneal epithelial stratification during postnatal development. Unlike the corneal epithelium of the littermate controls, which consisted of 5-6 cell layers at postnatal day 21 (P21), the mutant corneal epithelium contained 1-2 or 2-3 cell layers after Dox induction from embryonic day 0 (E0) to P21 and from E9 to P21, respectively. X-gal staining revealed that Wnt/β-catenin signaling activity was significantly elevated in the corneal keratocytes of the Dox-induced mutant mice, compared to the littermate controls. Furthermore, RT-qPCR and immunostaining data indicated that the expression of Bmp4 and ΔNp63 was downregulated in the mutant corneas, which was associated with reduced corneal epithelial proliferation in mutant epithelium, as revealed by immunofluorescent staining. However, the expression of Krt12, Krt14 and Pax6 in the mutant corneas was not altered after overexpression of Ctnnb1ΔE3 mutant protein in corneal keratocytes. Overall, mutant β-catenin accumulation in the corneal keratocytes inhibited corneal epithelial stratification probably through downregulation of Bmp4 and ΔNp63 in the corneal epithelium.