Experimental dry eye stimulates expression and production of IL-1beta, TNF-alpha, and MMP-9 and activates MAPK signaling pathways on the ocular surface. MAPKs are known to stimulate the production of inflammatory cytokines and MMPs, and they could play an important role in the induction of these factors that have been implicated in the pathogenesis of dry eye disease.

This study evaluated proposed molecular markers related to stem cell (SC) properties with the intention of characterizing a putative SC phenotype in human limbal epithelia. Human corneal and limbal tissues were cut in the vertical and horizontal meridians for histology, transmission electron microscopy (TEM), and immunostaining. Semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization were used to evaluate gene expression. TEM showed that the limbal basal cells were small primitive cells. Immunostaining disclosed that p63, ABCG2 and integrin α α9 were primarily expressed by the basal epithelial cells of limbus. Antibodies against integrin β β1, epidermal growth factor receptor (EGFR), K19, enolase-α α, and CD71 stained the basal cells of the limbus more brightly than the suprabasal epithelia. Integrin α α6, nestin, E-cadherin and connexin 43 did not stain the limbal basal cells, but the suprabasal epithelia of the cornea and limbus showed strong immunoreactivity. K3 and involucrin stained only corneal and limbal superficial cells. RT-PCR showed higher levels of p63, ABCG2 and integrin α α9 mRNA, but lower levels of K3, K12 and connexin 43 expressed in the limbal epithelia than the corneal epithelia. In situ hybridization showed that p63 transcripts were located in basal layer of the limbal epithelium. This work suggests that the basal epithelial cells of the limbus are p63, ABCG2 and integrin α α9 positive, and nestin, E-cadherin, connexin 43, involucrin, K3, and K12 negative, with relatively higher expression of integrin β β1, EGFR, K19, and enolase-α α. This putative SC phenotype may facilitate the identification and isolation of limbal epithelial SCs.

Altered corneal epithelial barrier function is the cause for ocular irritation and visual morbidity in dry eye disease. Increased matrix metalloproteinase (MMP)-9 activity has been observed in the tear fluid of dry eye patients. To determine the pathogenic role of MMP-9 in the corneal epithelial disease of dry eye, the effects of experimentally induced dry eye on corneal epithelial morphology and barrier function were compared in MMP-9 knockout mice and their wild-type littermates. Dry eye was created through cholinergic blockade and exposure to a desiccating environment. The tear fluid MMP-9 concentration increased in response to dryness in wild-type mice. Corneal epithelial permeability to three different-sized molecules increased in dry eye wild-type mice, but not in MMP-9 knockout mice. Topical administration of active MMP-9 to dry eye MMP-9 knockout mice significantly increased corneal epithelial permeability. Compared to MMP-9 knockout mice, wild-type mice showed greater desquamation of differentiated apical corneal epithelial cells that expressed the tight junction protein occludin in response to dryness. This was accompanied by an increase in lower sized (50 kd) occludin in the corneal epithelia of wild-type mice. These findings could be replicated in cultured human corneal epithelial cells that were treated with active MMP-9. These studies indicate that increased MMP-9 activity on the ocular surface in response to dryness disrupts corneal epithelial barrier function. This appears to be because Corneal epithelial disease, termed keratitis sicca, is a severe and sight-threatening complication of dry eye. 1 A key clinical feature of keratitis sicca is disruption of corneal epithelial barrier function. [2][3][4] This results in eye irritation, corneal surface irregularity, blurred and fluctuating vision, and increased risk for corneal ulceration. 4 -8 Ocular surface inflammation has been implicated in the pathogenesis of keratitis sicca. Elevated levels of proinflammatory cytokines, such as interleukin (IL)-1, have been detected in the tear fluid of patients with this condition. 9 -11 Furthermore, the concentration and activity of matrix metalloproteinase (MMP)-9 in the tear fluid was found to be significantly increased in these eyes, with the highest concentrations observed in eyes with the severe corneal epithelial disease or sterile corneal ulcers. 10 -12 We hypothesize that MMP-9 plays an important role in the disruption of corneal epithelial barrier function in dry eye. We previously reported an experimental murine model of dry eye that disrupts corneal epithelial barrier function similar to human dry eye disease. 13 The purpose of this study was to compare the effects of experimentally induced dry eye (EIDE) on corneal epithelial morphology and barrier function in MMP-9 knockout mice and their wild-type (WT) littermates. Materials and Methods MiceThis research protocol was approved by the Baylor College of Medicine Center for Comparative Medicine and it conformed to the standards in the Association...