Selective regulation of MMP and TIMP mRNA levels in tree shrew sclera during minus lens compensation and recovery

Previous studies have provided evidence that a visually guided emmetropization mechanism adjusts the growth of the eye to reduce any mismatch between the eye's axial length and optical power.1-4 Experimental manipulation of refractive state in animal models provides particularly strong evidence for the existence of this mechanism. For example, the eyes of avians5-8 and mammals9-13 can be induced to increase their axial elongation rate within days or weeks by placing a minus-power lens or diffuser in front of the eye. In response to a minus lens, the eye elongates until the increase in vitreous chamber depth “compensates” for the decrease in optical power and then adjusts its elongation rate to maintain the match. When the minus lens is removed, the eye is myopic and responds by slowing its axial elongation rate producing refractive recovery. Manipulations of the visual environment that trigger a response from the emmetropization mechanism also induce tissue remodeling in the sclera.14-19 In tree shrews, which, like other eutherian mammals, have an all-fibrous sclera,20 the tissue remodeling does not appear to modulate growth per se, but rather alters the mechanical properties of the sclera and, as a consequence, alters the ability of the sclera to resist the relatively low but constant expansive force of intraocular pressure.21,22 In a previous study,21 it was found that changes in creep rate (a quantitative measure of viscoelasticity) were closely associated with changes in axial elongation rate during minus-lens treatment (creep rate higher) and recovery (creep rate lower), suggesting that modulation of the mechanical properties of the sclera may control axial elongation. Precisely controlled scleral tissue remodeling is probably responsible for the changes in the mechanical properties of the sclera. Tissue remodeling is a complex process that involves both synthesis and degradation of the extracellular matrix (ECM). Numerous proteins are involved, including structural components such as collagen and proteoglycans,23 enzymes such as the matrix metalloproteinases (MMPs) that degrade ECM proteins,24 and tissue inhibitors of metalloproteinases (TIMPs) which bind to and inhibit the activity of the MMPs. In tree shrews, scleral remodeling in eyes developing induced myopia is characterized by decreased levels of collagen,16,25 decreased levels of sulfated and unsulfated glycosaminoglycans (GAGs)16,19 (Norton TT et al. IOVS 1998;39:ARVO Abstract 2312; German A et al. IOVS 1999;40:ARVO Abstract 2387), and increased levels of gelatinase-A (MMP-2).26 In a previous study in tree shrews that involved monocular form deprivation (MD) to induce myopia,27 we found that mRNA levels for MMP-2, MMP-3, collagen 1(I), and TIMP-1 were altered during form deprivation and recovery, suggesting that modulation of scleral gene expression may control scleral tissue remodeling and scleral extensibility. When MD is used to induce myopia in one eye, the axial elongation rate and scleral creep rate increase and remain elevated. When minus-power lenses are used, axial elongation and scleral extensibility rise at first. Then, as the axial length reaches its new target, they return to normal.21 If the pattern of changes in mRNA levels is causally related to the tissue remodeling and scleral extensibility, then inducing myopia with a minus-power lens should cause changes in mRNA levels for the same proteins as occurs when MD is used to induce myopia. However, the time course of the changes should differ, in keeping with the different time course of the changes in scleral extensibility and axial elongation rate. In the present study, we focused on the mRNA levels of MMPs and TIMPs that data suggest are involved in the remodeling of fibrous tissue such as sclera. For example, MMP-2 appears to be involved in scleral remodeling,26-28 and an important mechanism for the activation of pro-MMP-2 is thought to be cell surface activation by MT1-MMP,29,30 a mechanism that also involves TIMP-2 and possibly TIMP-3. Activation of MMP-2 through this mechanism is dependent on the relative levels of these molecules, and appropriate changes in their mRNA levels can suggest that this mechanism is active. MT1-MMP may also directly degrade scleral ECM components. Fibroblasts from MT1-MMP deficient mice have virtually no degradative effect on type I collagen,31 suggesting that MT1-MMP may be critical in producing the loss of collagen that occurs during the development of experimental myopia in tree shrews.16,25