The use of transgenic and knock-out mice in the investigation of ocular surface cell biology

Keratin expression by corneal and limbal stem cells during development

Keratins are the forming units of intermediate filaments (IF) that provide mechanical support, and formation of desmosomes between cells and hemi desmosomes with basement membranes for epithelium integrity. Keratin IF are polymers of obligate heterodimer consisting one type I keratin and one type II keratin molecules. There are 54 functional keratin genes in human genome, which are classified into three major groups, i.e., epithelial keratins, hair follicle cell-specific epithelial keratins and hair keratins. Their expression is cell type-specific and developmentally regulated. Corneal epithelium expresses a subgroup of keratins similar to those of epidermal epithelium. Limbal basal stem cells express K5/K14, and K8/K18 and K8/K19 IF suggesting that there probably are two populations of limbal stem cells (LSCs). In human, LSCs at limbal basal layer can directly stratify and differentiate to limbal suprabasal cells that express K3/K12 IF, or centripetally migrate then differentiate to corneal basal transient amplifying cells (TAC) that co-express both K3/K12 and K5/K14 prior to moving upward and assuming suprabasal cells phenotype of only K3/K12 expression that signifies corneal type epithelium differentiation. In rodent, the differentiated cornea epithelial cells express K5/K12 in lieu of K3/K12, because K3 allele exists as a pseudogene and does not encode a functional K3 protein. The basal corneal cells of new-born mice originate from surface ectoderm during embryonic development slowly commit to differentiation of becoming TAC co-expressing K5/K12 and K5/K14 IF. However, the centripetal migration may still occur at a slower rate in young mice, which is accelerated during wound healing. In this review, we will discuss and compare the cornea-specific keratins expression patterns between corneal and epidermal epithelial cells during mouse development, and between human and mouse during development and homeostasis in adult, and pathology caused by a mutation of keratins.

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

Purpose

We created a novel inducible mouse line Keratocan-rtTA (KeraRT) that allows specific genetic modification in corneal keratocytes and tenocytes during development and in adults.

Methods

A gene-targeting vector (Kera- IRES2-rtTA3) was constructed and inserted right after the termination codon of the mouse Kera allele via gene targeting techniques. The resulting KeraRT mouse was crossed to tet-O-Hist1H2B-EGFP (TH2B-EGFP) to obtain KeraRT/TH2B-EGFP compound transgenic mice, in which cells expressing Kera are labeled with green fluorescence protein (GFP) by doxycycline (Dox) induction. The expression patterns of GFP and endogenous Kera were examined in KeraRT/TH2B-EGFP. Moreover, KeraRT was bred with tet-O-TGF-α to generate a double transgenic mouse, KeraRT/tet-O-TGF-α, to overexpress TGF-α in corneal keratocytes upon Dox induction.

Results

Strong GFP-labeled cells were detected in corneal stroma, limbs, and tail when KeraRT/TH2B-EGFP mice were fed Dox chow. There was no GFP in any single transgenic KeraRT or TH2B-EGFP mouse. Histological analysis showed that GFP in the cornea was limited to stromal keratocytes of KeraRT/TH2B-EGFP, which is consistent with Kera expression. Induction of GFP occurred in 24 hours and reached a plateau by 7 days after Dox induction. GFP could be detected 3-months after induction of KeraRT/TH2B-EGFP. Ectopic expression of TGF-α in corneal keratocytes caused hyperplasia in the corneal epithelium and stroma.

Conclusions

The novel Dox inducible KeraRT driver mouse line is a useful genetic tool for gene manipulation and elucidating gene functions in corneal stroma and tendons of limbs and tail during embryonic development, homeostasis and pathogenesis.

Keratocan is expressed by osteoblasts and can modulate osteogenic differentiation

Keratocan is an extracellular matrix protein that belongs to the small leucine-rich proteoglycan family that also includes lumican, biglycan, decorin, mimecan, and fibromodulin. Members of this family are known to play a role in regulating cellular processes such as proliferation and modulation of osteoprogenitor lineage differentiation. The aims of this study were to evaluate the expression pattern of the keratocan within the osteoprogenitor lineage and to assess its role in regulating osteoblast maturation and function. Results from gene expression analyses of cells at different maturation stages within the osteoblast lineage indicate that keratocan is differentially expressed by osteoblasts and shows little or no expression by osteocytes. During primary osteoblast cultures, high keratocan mRNA expression was observed on day 14, whereas lower expression was detected at days 7 and 21. To assess the effects of keratocan on osteoprogenitor cell differentiation, we evaluated primary calvarial cell cultures from keratocan-deficient mice. The mineralization of calvarial osteoblast cultures derived from keratocan null (Kera-/-) mice was lower than in wild-type osteoblast cultures. Furthermore, analysis of RNA derived from Kera-/- calvarial cell cultures showed a reduction in the mature osteoblast differentiation markers, that is, bone sialoprotein and osteocalcin. In addition, we have evaluated the bone formation in keratocan-deficient mice. Histomorphometric analysis indicated that homozygous knockout mice have significantly decreased rates of bone formation and mineral apposition. Taken together, our results demonstrate the expression of keratocan by osteoblast lineage cells and its ability to modulate osteoblast function.

Corneal morphogenesis during development and diseases

OBJECTIVE

To review the use of genetically modified mouse lines for elucidating corneal morphogenesis during embryonic development and diseases.

METHODS

Transgenesis and gene-targeting techniques were used to create doxycycline-inducible mouse models (tet-On) to express transgenes or ablation of LoxP-modified genes or both in corneal cells, e.g., epithelial cells, and keratocytes and periocular mesenchymal cells of neural crest origin.

RESULTS

Two driver mouse lines, i.e., Krt12-rtTA and Kera-rtTA, were created, which express reverse tetracycline transcription activator (rtTA) in corneal epithelial cells and keratocytes, respectively. Bitransgenic (Krt12-rtTA/tet-o-FGF7) and triple transgenic mice (Krt12rtTA/tet-o-Cre/Ctnnb1 and Kera-rtTA/tet-o-Cre/Ctnnb1) were obtained through cross-breeding tet-o-FGF7, tet-o-Cre, and Ctnnb1 mice. On doxycycline induction, overexpression of FGF7 by corneal epithelial cells of bitransgenic Krt12-rtTA/tet-o-FGF7 mice caused nuclear translocation of beta-catenin and epithelium hyperplasia resembling human ocular surface squamous neoplasia; in triple transgenic mice (Krt12rtTA/tet-o-Cre/Ctnnb1), constitutive nuclear translocation of mutant beta-catenin (loss of exon 3) leads to hyper proliferation of corneal epithelial cells; in comparison of expression of beta-catenin mutant protein by migrating, periocular mesenchymal cells of Kera-rtTA/tet-o-Cre/Ctnnb1 caused eyelid malformation.

CONCLUSIONS

Use of genetically modified mice is of great value to study the pathophysiology of ocular surface defects resulting from genetic mutations.

Potential localization of putative stem/progenitor cells in human bulbar conjunctival epithelium

Although the conjunctival fornix appears to contain the greatest proportion of stem cells, it is likely that pockets of conjunctival epithelial stem cells may also exist throughout the conjunctival epithelium. This study was to investigate the potential localization of putative stem/progenitor cells in the human bulbar conjunctival epithelium by evaluating 6 keratins and 13 molecules that have been previously proposed stem cell associated or differentiation markers. We found that cornea specific cytokeratin (CK) 3 was not expressed by the bulbar conjunctival epithelial cells. In contrast, CK4 and CK7 were expressed by the superficial cells of bulbar conjunctival epithelium. CK14 and CK15 were confined to the basal cell layer. CK19 was strongly expressed by all layers of the bulbar conjunctival epithelium. The expression patterns of molecular markers in the basal cells of human bulbar conjunctival epithelium were found to be similar to the corneal epithelium. Basal conjunctival epithelial cells strongly expressed stem cell associated markers, including ABCG2, p63, nerve growth factor (NGF) with its receptors tyrosine kinase receptor A (TrkA) and neurotrophin low-affinity receptor p75NTR, glial cell-derived neurotrophic factor (GDNF) with its receptor GDNF family receptor alpha 1 (GFRalpha-1), integrin beta1, alpha-enolase, and epidermal growth factor receptor (EGFR). The differentiation associated markers nestin, E-cadherin and involucrin were not expressed by these cells. These findings indicate that the basal cells of bulbar conjunctival epithelium shares a similar expression pattern of stem cell associated markers to the corneal epithelium, but has a unique pattern of differentiation associated cytokeratin expression.

Four-dimensional multiphoton confocal microscopy: the new frontier in cellular imaging

This paper reviews new developments in microscopy that combine gene transfer technology, multiphoton confocal fluorescence microscopy, live cell imaging and digital imaging techniques that provide unique insights into the complex physiological processes involved in tissue function at the cellular and subcellular level. The evolution of this novel, new technology is discussed with particular attention to earlier achievements in noninvasive ocular surface imaging. The practical basis of confocal microscopy, multiphoton confocal fluorescence microscopy, and the vital fluorescent labeling of cells in living tissues are also discussed. Additionally, one application using retroviral gene transfer to express enhanced green fluorescent protein in living wound healing fibroblasts is presented as an example of how living biology can be studied in situ in four dimensions (x, y, z, time).

Keratopathy in congenital aniridia

Although the most apparent clinical finding in aniridia is the absence of iris tissue, additional ocular structures are often affected. Mutations of the Pax 6 gene, which is important for eye development, have been identified in families with members affected by aniridia. Poor vision in aniridic eyes may be the result of macular hypoplasia, nystagmus, amblyopia, cataracts, glaucoma, and corneal disease, termed aniridic keratopathy. Advances in surgical techniques have improved management of some of the visually disabling manifestations of aniridia, but aniridic keratopathy remains a significant source of visual loss. We have conducted a large, retrospective study of patients with aniridia to gain information about the natural course of aniridic keratopathy. In this paper, we report the results of our study, as well as findings reported in the literature. Penetrating keratoplasty alone has not been a successful treatment for severe stromal scarring, as it does not treat the underlying epithelial causes of corneal disease. However, it has been successful in corneas that have achieved stable epithelium following limbal stem cell transplantation.

Ocular Surface Tissue Morphogenesis in Normal and Disease States Revealed by Genetically Modified Mice

Many transgenic and knockout mice exhibit pathogenic processes resembling human ocular surface diseases. Thus, the clinical manifestations of mouse lines can provide clues for identifying heritable human diseases of unknown etiology. However, mouse lines using conventional techniques of transgenesis and gene targeting often exhibit embryonic lethality and congenital defects, which preclude the use of such mouse models to study acquired ocular surface tissue diseases. These difficulties can be in part overcome by preparing mouse lines of inducible transgene expression, tissue-specific gene ablation, and inducible tissue-specific gene ablation. Conditional transgenic mouse lines live normally until administration of doxycycline and hormones that induce expression of the transgene and ablation of gene of interest. Toward this goal, we prepared 2 groups of genetically modified mouse lines: (1) transgenesis using keratocan promoter was used to create Kera-rtTA mice (doxycycline-inducible mice) and Cre-LoxP system (ie, Kera-Cre mice; conditional gene ablation in neural crest cell lineage and adult stromal keratocyte) and Kera-CrePR mice (RU-486 inducible); and (2) knock-in strategies were used to create Krt12-rtTA mice (doxycycline inducible), Krt12-Cre mice (conditional ablation in corneal epithelium), and Krt12rtTA-tet-O-Cre mice (doxycycline-inducible corneal epithelium-specific gene ablation). Using these mouse lines, we showed that transforming growth factor (TGF)-beta2 is essential for eye morphogenesis, TGF-alpha is a morphogen for eyelid formation, and lumican is a matrikine that has multiple regulatory functions on cell activities (eg, migration proliferation and gene expression) besides serving as a regulatory molecule of collagen fibrillogenesis. These mouse lines can also be used as models for development of therapeutic treatment regimens of ocular surface diseases using gene therapy and stem cell strategies.

Excess biglycan causes eyelid malformation by perturbing muscle development and TGF-alpha signaling

Tissue morphogenesis during development is regulated by growth factors and cytokines, and is characterized by constant remodeling of extracellular matrix (ECM) in response to signaling molecules, for example, growth factors, cytokines, and so forth. Proteoglycans that bind growth factors are potential regulators of tissue morphogenesis during embryonic development. In this study, we showed that transgenic mice overexpressing biglycan under the keratocan promoter exhibited exposure keratitis and premature eye opening from noninfectious eyelid ulceration due to perturbation of eyelid muscle formation and the failure of meibomian gland formation. In addition, in vitro analysis revealed that biglycan binds to TGF-alpha, thus interrupting EGFR signaling pathways essential for mesenchymal cell migration induced by eyelid epithelium. The defects of TGF-alpha signaling by excess biglycan were further augmented by the interruption of the autocrine or paracrine loop of the EGFR signaling pathway of HB-EGF expression elicited by TGF-alpha. These results are consistent with the notion that under physiological conditions, biglycan secreted by mesenchymal cells serves as a regulatory molecule for the formation of a TGF-alpha gradient serving as a morphogen of eyelid morphogenesis.