The role of epithelial membrane-associated mucin 4 in ocular surface health and corneal wound healing

The purpose of this study was to determine the role of MUC4, a corneal membrane-associated mucin, on ocular surface health and corneal wounding healing using a <i>Muc4</i> knockout (KO) mouse model. Complete ophthalmic examinations were performed on wildtype (WT), <i>Muc4</i> heterozygous (Het) and <i>Muc4</i> knockout (KO) mice, including slit lamp biomicroscopy, phenol red thread test (PRTT), intraocular pressure (IOP), and fluorescein staining. The mice were also assessed using optical coherence tomography (OCT), an advanced imaging technique. Dynamic contact angle goniometry was performed on <i>ex vivo</i> globes of WT, <i>Muc4</i> Het and <i>Muc4</i> KO mice to calculate contact angle hysteresis as a novel measure of the adherence properties of the corneal epithelium. To determine the effect of <i>Muc4</i> in corneal wound healing, a phototherapeutic keratectomy (PTK) was performed on the right eye. After PTK wounding, the corneas were fluorescein stained, imaged, and the wound size was quantified using ImageJ at 0-, 24-, 36-, and 48-h post-wounding. There were no phenotypic differences identified between WT, <i>Muc4</i> Het and <i>Muc4</i> KO mice on clinical examination, diagnostic testing, advanced imaging, and histology. While there was no difference in mucin 1 (<i>Muc1</i>) mRNA expression between WT and KO mice, there was a compensatory upregulation of a previously unreported murine corneal mucin, <i>Muc20</i> mRNA expression, in corneal epithelium of <i>Muc4</i> KO mice. No differences were detected between WT and <i>Muc4</i> KO mice using dynamic contact angle goniometry. The <i>Muc4</i> KO mice had significantly slower healing rates at 24 and 36 h post-wounding when compared with WT mice <i>(P</i> < 0.05) and all healed by 48 h post-wounding. These results support further investigation into compensatory roles of glycoproteins on the ocular surface, namely <i>Muc4</i> and <i>Muc20</i>, and the role of <i>Muc4</i> in epithelial cell migration in corneal wound healing.