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Terete Borras, Matthew H Smith, LaKisha Buie, MdZahid Karim; Matrix GLA (MGP) as an anti-stiffness candidate gene in the peripapillary sclera (ppSC) of the mouse. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):4823.
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© ARVO (1962-2015); The Authors (2016-present)
Soft tissue calcification is a pathological condition. In arterial calcification, the elastic lamina is the first site of mineral deposition leading to increased arterial stiffness and higher systolic blood pressure. MGP is a potent mineralization inhibitor secreted by chondrocytes in cartilage and VSMC in arteries where colocalizes with elastin. Mgp-deficient mice die at 6 wks due to massive artery calcification. An Mgp Knock-In (KI) mouse showed high Mgp expression in the eye uniquely targeted to trabecular meshwork (TM) and sclera. Since scleral stiffness affects optic nerve damage, we investigated scleral’s Mgp spatial/temporal distribution in the KI and ppSC’s calcification in the KO.
The KI DNA in our Mgp-Cre mouse contains the Mgp gene, 2 kb promoter, 3 kb 3’UTR and an IRES-Cre cassette inserted in between. This mouse was crossed with a Cre-mediated reporter line R26R-lacZ. Their offspring expresses lacZ (β-gal) where Mgp is transcribed. Eyes from MgpCre/+;R26RlacZ/+ and controls, 1-8 months were assayed for β-gal, embedded and analyzed by histology. An Mgp-deficient mouse Mgp+/- was intercrossed to produce KO Mgp-/- mice. Sclera of 5 wks homos were assessed for calcification staining.
In the KI, we examined 43 experimental and 8 control mice (2 eyes each). Mice were selected from different breeding pairs and different founders. In all cases, eyes from Mgp-lacZ exhibited intense blue staining, all with similar distribution pattern (TM&sclera). In the sclera, staining was negative in the periphery, increased posteriorly and was located immediately underneath the choroid, in the chondrocyte but not in the fibroblast layer. There was a “burst of blue” expression at the ppSC. Expression levels were similar at 2, 4 & 6 mo, but diminished at the 8 mo last point. In the Mgp KO mouse, VonKossa staining was positive at mid-posterior and ppSC with dark calcification deposits similar to those observed in the sclera of aging rats (AmJAnat,189,61,1990).
MGP’s anti-calcification/anti-stiffness properties in the vascular tissue, plus its high eye’s ppSC expression, place this gene as a strong candidate for sclera stiffness regulation in glaucoma. KI vector specific sequences plus a potential suprachoroidal space delivery (in progress) could be used to target therapeutic genes to the sclera. MGP’s TM&ppSC expression offers an opportunity for dual targeting in glaucoma.
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