Fibrillin-1 and fibrillin-2 are essential for formation of thick oxytalan fibers in human nonpigmented ciliary epithelial cells in vitro

The ciliary zonule, also known as Zinn's zonule, is composed of oxytalan fibers. However, the mechanism by which epithelial cells in the ciliary body form these fibers in not fully understood. We examined human nonpigmented ciliary epithelial cells to determine the appearance and amount of oxytalan fibers in terms of positivity for their major components, fibrillin-1 and fibrillin-2. Examination of fibrillin-1 and fibrillin-2 expression by immunofluorescence revealed that thin fibers positive for fibrillin-1 on Day 2 changed to thick fibers by Day 8. The fibers positive for fibrillin-2 appeared on the thick fibrillin-1-positive fibers after Day 4. Northern blot analysis revealed that the level of fibrillin-1 did not change markedly, while induction of fibrillin-2 gene was evident on Day 5. Western blot analysis showed that fibrillin-1 deposition increased gradually, while that of fibrillin-2 increased markedly from Day 5 to Day 8. Fibrillin-1 suppression did not lead to the formation of fibrillin-2-positive thick fibers, whereas fibrillin-2 suppression led to the formation of fibrillin-1-positive thin fibers, but not thick fibers. These results suggest that both fibrillin-1 and fibrillin-2 are essential for the formation of thick oxytalan fibers in the ciliary zonule and are informative for clarifying the mechanism of homeostasis of the ocular matrix.