Cofactors of LIM Domains Associate with Estrogen Receptor α to Regulate the Expression of Noncoding RNA H19 and Corneal Epithelial Progenitor Cell Function.

Cofactors of LIM domain proteins, CLIM1 and CLIM2, are widely expressed transcriptional cofactors that are recruited to gene regulatory regions by DNA-binding proteins, including LIM domain transcription factors. In the cornea, epithelium-specific expression of a dominant negative (DN) CLIM under the keratin 14 (K14) promoter causes blistering, wounding, inflammation, epithelial hyperplasia, and neovascularization followed by epithelial thinning and subsequent epidermal-like differentiation of the corneal epithelium. The defects in corneal epithelial differentiation and cell fate determination suggest that CLIM may regulate corneal progenitor cells and the transition to differentiation. Consistent with this notion, the K14-DN-Clim corneal epithelium first exhibits increased proliferation followed by fewer progenitor cells with decreased proliferative potential. In vivo ChIP-sequencing experiments with corneal epithelium show that CLIM binds to and regulates numerous genes involved in cell adhesion and proliferation, including limbally enriched genes. Intriguingly, CLIM associates primarily with non-LIM homeodomain motifs in corneal epithelial cells, including that of estrogen receptor α. Among CLIM targets is the noncoding RNA H19 whose deregulation is associated with Silver-Russell and Beckwith-Wiedemann syndromes. We demonstrate here that H19 negatively regulates corneal epithelial proliferation. In addition to cell cycle regulators, H19 affects the expression of multiple cell adhesion genes. CLIM interacts with estrogen receptor α at the H19 locus, potentially explaining the higher expression of H19 in female than male corneas. Together, our results demonstrate an important role for CLIM in regulating the proliferative potential of corneal epithelial progenitors and identify CLIM downstream target H19 as a regulator of corneal epithelial proliferation and adhesion.

Altered Mucin and Glycoprotein Expression in Dry Eye Disease.

PURPOSE: Mucins are among the many important constituents of a healthy tear film. Mucins secreted and/or associated with conjunctival goblet cells, ocular mucosal epithelial cells, and the lacrimal gland must work together to create a stable tear film. Although many studies have explored the mechanism(s) whereby mucins maintain and protect the ocular surface, the effects of dry eye on the structure and function of ocular mucins are unclear. Here, we summarize current findings regarding ocular mucins and how they are altered in dry eye. METHODS: We performed a literature review of studies exploring the expression of mucins produced and/or associated with tissues that comprise the lacrimal functional unit and how they are altered in dry eye. We also summarize new insights on the immune-mediated effects of aqueous tear deficiency on ocular surface mucins that we discovered using a mouse model of dry eye. RESULTS: Although consistent decreases in MUC5AC and altered expression of membrane-bound mucins have been noted in both Sjögren and non-Sjögren dry eye, many reports of altered mucins in dry eye are contradictory. Mechanistic studies, including our own, suggest that changes in the glycosylation of mucins rather than the proteins themselves may occur as the direct result of local inflammation induced by proinflammatory mediators, such as interleukin-1. CONCLUSIONS: Altered expression of ocular mucins in dry eye varies considerably from study to study, likely attributed to inherent difficulties in analyzing small-volume tear samples, as well as differences in tear collection methods and disease severity in dry eye cohorts. To better define the functional role of ocular mucin glycosylation in the pathogenesis of dry eye disease, we propose genomic and proteomic studies along with biological pathway analysis to reveal novel avenues for exploration.

The Ets transcription factor EHF as a regulator of cornea epithelial cell identity.

The cornea is the clear, outermost portion of the eye composed of three layers: an epithelium that provides a protective barrier while allowing transmission of light into the eye, a collagen-rich stroma, and an endothelium monolayer. How cornea development and aging is controlled is poorly understood. Here we characterize the mouse cornea transcriptome from early embryogenesis through aging and compare it with transcriptomes of other epithelial tissues, identifying cornea-enriched genes, pathways, and transcriptional regulators. Additionally, we profiled cornea epithelium and stroma, defining genes enriched in these layers. Over 10,000 genes are differentially regulated in the mouse cornea across the time course, showing dynamic expression during development and modest expression changes in fewer genes during aging. A striking transition time point for gene expression between postnatal days 14 and 28 corresponds with completion of cornea development at the transcriptional level. Clustering classifies co-expressed, and potentially co-regulated, genes into biologically informative categories, including groups that exhibit epithelial or stromal enriched expression. Based on these findings, and through loss of function studies and ChIP-seq, we show that the Ets transcription factor EHF promotes cornea epithelial fate through complementary gene activating and repressing activities. Furthermore, we identify potential interactions between EHF, KLF4, and KLF5 in promoting cornea epithelial differentiation. These data provide insights into the mechanisms underlying epithelial development and aging, identifying EHF as a regulator of cornea epithelial identity and pointing to interactions between Ets and KLF factors in promoting epithelial fate. Furthermore, this comprehensive gene expression data set for the cornea is a powerful tool for discovery of novel cornea regulators and pathways.

Pax6 downregulation mediates abnormal lineage commitment of the ocular surface epithelium in aqueous-deficient dry eye disease.

Keratinizing squamous metaplasia (SQM) of the ocular surface is a blinding consequence of systemic autoimmune disease and there is no cure. Ocular SQM is traditionally viewed as an adaptive tissue response during chronic keratoconjunctivitis sicca (KCS) that provokes pathological keratinization of the corneal epithelium and fibrosis of the corneal stroma. Recently, we established the autoimmune regulator-knockout (Aire KO) mouse as a model of autoimmune KCS and identified an essential role for autoreactive CD4+ T cells in SQM pathogenesis. In subsequent studies, we noted the down-regulation of paired box gene 6 (Pax6) in both human patients with chronic KCS associated with Sjögren's syndrome and Aire KO mice. Pax6 encodes a pleiotropic transcription factor guiding eye morphogenesis during development. While the postnatal function of Pax6 is largely unknown, we hypothesized that its role in maintaining ocular surface homeostasis was disrupted in the inflamed eye and that loss of Pax6 played a functional role in the initiation and progression of SQM. Adoptive transfer of autoreactive T cells from Aire KO mice to immunodeficient recipients confirmed CD4+ T cells as the principal downstream effectors promoting Pax6 downregulation in Aire KO mice. CD4+ T cells required local signaling via Interleukin-1 receptor (IL-1R1) to provoke Pax6 loss, which prompted a switch from corneal-specific cytokeratin, CK12, to epidermal-specific CK10. The functional role of Pax6 loss in SQM pathogenesis was indicated by the reversal of SQM and restoration of ocular surface homeostasis following forced expression of Pax6 in corneal epithelial cells using adenovirus. Thus, tissue-restricted restoration of Pax6 prevented aberrant epidermal-lineage commitment suggesting adjuvant Pax6 gene therapy may represent a novel therapeutic approach to prevent SQM in patients with chronic inflammatory diseases of the ocular surface.