Matrix metalloproteinases and their inhibitors in ocular neovascularization

Abstract

Growth of pathological new vessels (angiogenesis) in the eye is a leading cause of severe vision-loss. Ocular angiogenesis is seen in the anterior part of the eye as in corneal neovascularization, or in the posterior part as in diabetic retinopathy, retinopathy of prematurity or age-related macular degeneration. Angiogenesis is a multi-step event that is dependent of extracellular proteolysis. Matrix metalloproteinases (MMPs) are a family of zinc dependent enzymes that includes the type IV collagenases MMP-2 and -9 both being implicated in neovascular responses.

In the present studies the expression and role of different angiogenic factors in ocular angiogenesis of the choroid and the cornea is studied with special emphasis on the MMPs and tissue inhibitors of matrix metalloproteinases (TIMPs).

The expression of MMP-2 and -9 mRNA was studied on surgically removed choroidal neovascular membranes from patients with AMD. The localization of MMP-2 and -9 mRNA to areas of new vessel formation and to the enveloping Bruch-like membrane, respectively, suggests that MMP-2 and -9 may be cooperatively involved in the progressive growth of CNV membranes. Experimental CNV was induced in rats and mice by krypton laser and MMP-2 and MMP-9 mRNA expression was detected by in situ hybridization and immunohistochemistry. MMP-2 mRNA and protein was up-regulated in the CNV lesions. The CNV membrane thickness was significantly reduced in MMP-2-deficient as compared to wild-type mice. These results show that MMP-2 is involved in experimental CNV formation and suggest that pharmacological targeting of MMP-2 may reduce CNV formation in conditions such as age-related macular degeneration.

Histologic material from a patient with Sorsby fundus dystrophy, a hereditary retinal disorder with a mutation in the TIMP-3 gene resulting in an accumulation of TIMP-3 protein, was analysed for MMP-2, -9 and TIMP-1, -2 and -3 expression. There is a reduction in of MMP-2 and TIMP-3 expression in the retinal pigment epithelium of Sorsby eyes. It is concluded that the increased TIMP-3 protein accumulation is not due to an increased expression of TIMP-3 mRNA.

Experimentally neovascularized rat and mouse corneas were analyzed for MMP-2 mRNA expression. There was a temporal and spatial up-regulation of MMP-2 during new vessel formation in the cornea. Moreover, analysis of vascularized corneal flatmounts showed a delayed neovascularization in MMP-2-deficient as compared to wild-type mice. These results support a role for MMP-2 in inflammation-associated corneal neovascularization.

Collectively, our present studies suggest that MMP-2 is directly involved in neovascularization of ocular tissues, whereas MMP-9 rather plays a role in epithelial and basement membrane remodelling.