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E.C. Johnson, L. Jia, W.O. Cepurna, J.C. Morrison; Microarray Analysis Implicates a Mechanical Stretch Response Contribution to Pressure–Induced Optic Nerve Head (ONH) Injury . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1827.
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Ischemic, mechanical and immune mechanisms have been proposed to play important roles in glaucomatous ONH injury. Here we used our rat glaucoma model and microarray analysis to identify significant changes in gene expression in the ONH injured by elevated pressure. We then examined 10 published array studies that used the injury paradigms of stretch, compression, ischemia, and inflammatory injury, to identify genes that were altered similarly to those identified in ONH injured by elevated intraocular pressure (IOP).
ONH RNA was obtained from eyes with extensive nerve injury following 5 weeks of IOP elevation and controls (N=6 per group). Relative gene expression for each sample was determined using 12 separate cDNA arrays and a reference standard study design. Significant changes in gene expression (q<0.02, >1.3 fold change, median FDR=2) were identified by SAM and affected processes found using DAVID and EASE websites. Finally, these genes were compared to those identified in studies of non–neural cells exposed to mechanical stretch (4), compressed neural cells (2), ischemic brain tissue (2), and multiple sclerosis/EAE inflammation of white matter (2).
The majority of significantly altered genes in pressure–damaged ONH were upregulated (1269 of 1892), primarily affecting lysosomal, immune response, extracellular matrix, cell cycle, adhesion and ribosomal gene classes (p<0.005). For gene expression changes identified in both the ONH and three of the four injury conditions, the majority were in the same direction (stretch 76±10%, compression 83±3% and ischemia 89±3%, for inflammation only 56±6%). The specific genes differed by functional class. ONH injury and stretch responses were dominated by cell adhesion and extracellular matrix (ECM) genes, including collagens, while ONH injury and ischemia upregulated cation binding, stress response, and some ECM genes, but not collagens. ONH injury and compression increased the expression of proliferation genes. Finally, membrane attack complex components were upregulated by both ONH injury and EAE.
This analysis suggests that, in addition to ischemic, compressive and immune components, mechanical stretch may be significant contributor to injury from elevated IOP. We hypothesize that pressure–induced increases in scleral wall stress are primarily sensed by ONH astrocytes as stretch and result in altered gene expression, leading to compromised axonal support and/or direct axonal injury.
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