Organization of lamellae in the corneal stroma: Mouse models for better understanding of the pathomechanism of keratoconus

Abstract

Purpose : In keratoconus (KC), structural and compositional changes lead to disruptions of the lamellar organization with thinning and scarring of the cornea. Both genetic and environmental factors have been associated with KC and recent studies suggest that inflammation (e.g. high TNFα levels) could trigger the onset of KC. Therefore, we analyzed human corneas as well as corneas of hTNFtg mice and syndecan-1 and -4 deficient mice.

Methods : Human and mouse corneas were analyzed by TEM and stromal collagen degradation was visualized by the collagen hybridizing peptide B-CHP. Moreover, 3D-cell cultures of human keratocytes were analyzed by TEM and for activity of the cross-linking enzyme tissue transglutaminase (TG). Furthermore, collagenous structures of healthy and KC corneas were analyzed by P-SHG microscopy and OCT analysis was used for 3D imaging of hTNFtg and wt corneas.

Results : Sheets of orthogonally arranged collagen fibrils were found in the stroma of wt mice and human controls. In contrast, lamellae were disrupted and fibril diameter increased in hTNFtg and in both syndecan-deficient mice. Interestingly, stromal morphology of hTNFtg mice was similar to that of KC patients. We found an invasion of macrophages and apoptotic keratocytes. 3D-cell cultures of KC keratocytes generated a structurally altered ECM with reduced TG-activity. Moreover, binding of B-CHP was significantly stronger in KC and hTNFtg mice. In addition, OCT analysis revealed a slightly altered shape of the cornea of hTNFtg mice. Alterations in the Bowman`s membrane together with disrupted cell-cell-contacts were found in all mouse models. P-SHG analysis revealed changes in lamellae orientation in KC. The entropy was higher and the orientation index was lower demonstrating a disorganization of stromal lamellae compared to controls.

Conclusions : Disruptions of the lamellar organization of collagen fibrils in hTNFtg and syndecan-1 and -4 deficient mice are similar to KC patients. Thus, inflammation and altered cross-links could be crucial factors for the onset of KC. The strong binding of B-CHP supports our hypothesis of enhanced collagen degradation in the stroma. Thus, hTNFtg mice as model for KC will help to better understand the pathomechanism of KC and P-SHG analysis provides new parameters for characterization of lamellar changes in KC.

This is a 2021 ARVO Annual Meeting abstract.