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Matrix GLA Protein Function in Human Trabecular Meshwork Cells: Inhibition of BMP2-Induced Calcification Process

¹From the Department of Ophthalmology, University of North Carolina School of Medicine, Chapel Hill, North Carolina; the ²Department of Internal Medicine, Wake Forest University, Winston-Salem, North Carolina; and the ³Center for Cell and Gene Therapy, Baylor College of Medicine, Houston, Texas.

PURPOSE. The matrix GLA (MGP) gene has been found to be among the 10 most highly expressed genes in the human trabecular meshwork (TM), and its expression is affected by conditions associated with glaucoma. Because MGP protein has been shown to play a key role in inhibiting calcification in cartilage and arterial vessels, MGP’s function in human TM was investigated.

METHODS. Perfused TM tissue and primary human TM (HTM) cells originated from donors of nonglaucomatous eyes. MGP mRNA was assayed by relative quantitative and real-time PCR. AdhMGP recombinant adenovirus was generated by bacterial transposition. Western blot analyses were cross-reacted with MGP N-terminal- and conformational-specific antibodies. MGP/BMP2 colocalization was analyzed by confocal microscopy. -Carboxylation activity was measured by incorporation of ¹⁴CO₂ into FLEEL synthetic peptide. Alkaline phosphatase (ALP) activity was used as a marker of osteogenic differentiation and a calcification precursor. Calcification was assessed by measuring direct calcium (o-cresolphthalein). Normalization was conducted with a telomerase probe (genomic DNA).

RESULTS. HTM cells contained high levels of -carboxylase activity and were able to convert MGP to its active conformation. Overexpression of MGP in HTM cells reduced ALP activity in a model of BMP2-induced osteogenesis. MGP colocalized intracellularly with BMP2. HTM cells aged in culture exhibited increased calcium content, increased ALP, decreased normalized MGP expression and lower -carboxylase activity.

CONCLUSIONS. MGP protein is active and functions as an inhibitor of BMP2-induced ALP activity in the HTM cells. The human TM may undergo a calcification process with age. Inhibition of the calcification mechanism mediated by MGP could be used to regulate resistance and elevated IOP.

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				W. Xue, N. Comes, and T. Borras Presence of an Established Calcification Marker in Trabecular Meshwork Tissue of Glaucoma Donors Invest. Ophthalmol. Vis. Sci., July 1, 2007; 48(7): 3184 - 3194. [Abstract] [Full Text] [PDF]