Alterations in scleral TGF beta isoforms and their role in extracellular matrix remodeling during myopia induction

Abstract

Abstract: : Purpose: The mammalian TGF–ß isoforms, TGF–ß1, –ß2, and –ß3 are known to be involved in extracellular matrix (ECM) remodeling, such as occurs in the sclera during myopic eye growth. The purpose of this study was to assess TGF–ß isoform expression within the tree shrew sclera during myopia induction and to determine whether changes in expression profile could result in the alterations in collagen metabolism reported in vivo. Methods: Myopia was induced monocularly in tree shrews and TGF–ß isoform expression was determined at 1 and 5 days post induction, using quantitative real–time PCR. Scleral fibroblast cultures were exposed to individual, or combinations of, TGF–ß isoforms and collagen synthesis was investigated using ³H proline incorporation. Isoform combinations reflected the in vivo TGF–ß profiles estimated from quantitative expression data. Collagen synthesis was also assayed after the individual isoforms were selectively altered to approximate the changes observed during myopia induction. Results: All three TGF–ß mammalian isoforms were expressed in the tree shrew sclera, with TGF–ß2 the predominant isoform (TGF–ß2, 90%; TGF–ß1, 7%, TGF–ß3, 3%). After 1 day of myopia induction, TGF–ß2 levels decreased by 27%, while TGF–ß3 levels reduced by 42%. All three isoforms exhibited reduced expression at 5 days, with TGF–ß1 and –ß3 having decreases of 37% and 36% respectively. TGF–ß2 levels decreased a further 23% compared to 1 day levels (50% reduction). All TGF–ß isoforms increased collagen synthesis, however, there was isoform–specificity. Modeling the in vivo decrease in TGF–ß levels observed during myopia induction, resulted in a significant reduction in collagen synthesis (approximately 15%). Conclusion: All mammalian TGF–ß isoforms are expressed in the sclera, with TGF–ß2 predominant. The isoforms are regulated both before and after extensive structural change associated with myopia has occurred. These changes are capable of altering scleral collagen synthesis in a quantitatively similar manner to that observed in vivo. Thus, TGF–ß isoforms appear to be major factors in the initiation and maintenance of ECM remodeling in the sclera of myopic eyes.

Keywords: myopia • sclera • growth factors/growth factor receptors