Primary open angle glaucoma (POAG) is the most common form of glaucoma and the 2nd most common cause of irreversible vision loss in the United States. Nearly 67 million people have the disease worldwide including >3 million in the United States. A major risk factor for POAG is an elevation in intraocular pressure (IOP). The increase in IOP is believed to be caused by an increase in the deposition of extracellular matrix proteins, in particular fibronectin, in a region of the eye known as the trabecular meshwork (TM). How fibronectin contributes to the increase in IOP is not well understood. The increased density of fibronectin fibrils is thought to increase IOP by altering the compliance of the trabecular meshwork. Recent studies, however, also suggest that the composition and organization of fibronectin fibrils would affect IOP by changing the cell-matrix signaling events that control the functional properties of the cells in the trabecular meshwork. In this article, we will discuss how changes in the properties of fibronectin and fibronectin fibrils could contribute to the regulation of IOP.

Studies from our laboratory have suggested that activation of αvβ3 integrin-mediated signaling could contribute to the fibrotic-like changes observed in primary open angle glaucoma (POAG) and glucocorticoid-induced glaucoma. To determine how αvβ3 integrin signaling could be involved in this process, RNA-Seq analysis was used to analyze the transcriptomes of immortalized trabecular meshwork (TM) cell lines overexpressing either a control vector or a wild type (WT) or a constitutively active (CA) αvβ3 integrin. Compared to control cells, hierarchical clustering, PANTHER pathway and protein-protein interaction (PPI) analysis of cells overexpressing WT-αvβ3 integrin or CA-αvβ3 integrin resulted in a significant differential expression of genes encoding for transcription factors, adhesion and cytoskeleton proteins, extracellular matrix (ECM) proteins, cytokines and GTPases. Cells overexpressing a CA-αvβ3 integrin also demonstrated an enrichment for genes encoding proteins found in TGFβ2, Wnt and cadherin signaling pathways all of which have been implicated in POAG pathogenesis. These changes were not observed in cells overexpressing WT-αvβ3 integrin. Our results suggest that activation of αvβ3 integrin signaling in TM cells could have significant impacts on TM function and POAG pathogenesis.

Although elevated TGFβ2 levels appear to be a causative factor in glaucoma pathogenesis, little is known about how TGFβ2 expression is regulated in the trabecular meshwork (TM). Here, we investigated if activation of the cytokine regulator NFATc1 controlled transcription of TGFβ2 in human TM cells by using dexamethasone (DEX) to induce NFATc1 activity. The study used both proliferating and cell cycle arrested quiescent cells. Cell cycle arrest was achieved by either cell–cell contact inhibition or serum starvation. β-catenin staining and p21 and Ki-67 nuclear labeling were used to verify the formation of cell–cell contacts and activity of the cell cycle. NFATc1 inhibitors cyclosporine A (CsA) or 11R-VIVIT were used to determine the role of NFATc1. mRNA levels were determined by RT-qPCR. DEX increased TGFβ2 mRNA expression by 3.5-fold in proliferating cells but not in quiescent cells or serum-starved cells, and both CsA and 11R-VIVIT inhibited this increase. In contrast, the expression of other DEX/NFATc1-induced mRNAs (myocilin and β3 integrin) occurred regardless of the proliferative state of the cells. These studies show that NAFTc1 regulates TGFβ2 transcription in TM cells and reveals a previously unknown connection between the TM cell cycle and modulation of gene expression by NFATc1 and/or DEX in TM cells.

Age is a major risk factor for primary open-angle glaucoma (POAG), an ocular disease associated with elevated intraocular pressure (IOP) due to the restricted outflow of aqueous humor fluid from the anterior chamber. This restriction is caused by an induction of fibrogenic pathways that increase deposition of extracellular matrix (ECM) and alter the contractile properties of trabecular meshwork (TM) cells. Integrins, a family of heterodimeric cell surface receptors that bind ECM, play a key role in regulating fibrogenic pathways. In this study, we investigated whether age-related changes in integrin subunit expression and activity signifies an early event in initiating fibrotic-like changes in the TM. TM cells were isolated from young and old human donor eyes. Flow cytometry, RT-PCR, and immunofluorescent microscopy revealed a significant decrease in α5 integrin expression in TM cells from older individuals. This loss was accompanied by an increase in activated, but not total αvβ3 integrin in focal adhesions. TM cells from older donors expressed higher levels of αSMA mRNA, assembled αSMA-containing stress fibers, and contracted collagen gels significantly more than young TM cells. TM cells from old donors also assembled higher levels of insoluble fibronectin fibrils and contained higher levels of the EDB+ isoform of fibronectin in their ECM. shRNA knockdown of α5 integrin subunits showed that the increase in αvβ3 integrin activity was due to lower levels of α5 integrin expression. These studies suggest that age-related dysregulation of α5β1 and αvβ3 integrin signaling may represent an important early molecular event in inducing fibrogenic pathways associated with POAG.