The Role of Transforming Growth Factor Beta 2 Signaling and MicroRNAs in Optic Nerve Head Remodeling

Primary open-angle glaucoma (POAG) is a prevalent age-related neurodegenerative disease of the visual system. There are functional and morphological changes in the retina, optic nerve head (ONH) and brain that lead to an irreversible loss of vision. POAG is characterised by degeneration of retinal ganglion cells (RGC), thinning of the neuro-retinal rim and structural deformation of the ONH. The primary site of injury is the lamina cribrosa, which is a fibro-elastic connective tissue that supports the ONH and unmyelinated RGC axons as they exit the intraocular space. Pathological changes to the lamina cribrosa include posterior displacement of the lamina cribrosa, loss of trophic support, and remodeling of the extracellular matrix (ECM). An important growth factor associated with tissue remodeling is TGFb2. TGFb2 activates the SMAD-dependent TGFb2 pathway and increases transcription of several ECM genes including collagen, fibronectin and crosslinking enzymes. In POAG, the levels of TGFb2 are increased in the lamina cribrosa and is associated with excess deposition of ECM molecules. This study proposes to investigate the intermediary mechanisms that lead to tissue remodeling. microRNAs (miRNAs) regulate gene expression by inhibiting protein translation. We hypothesized that miRNAs are dysregulated in POAG and in response to TGFb2, which leads to excess ECM synthesis and tissue remodeling. We isolated primary human ONH astrocytes and lamina cribrosa cells from POAG and normal donor eyes. We used miRNA PCR arrays to determine differentially expressed miRNAs in POAG and TGFb2 treated cells. Several anti-fibrotic miRNAs were downregulated, including downregulation of miR-29c-3p in POAG and TGFb2 treated lamina cribrosa cells, and downregulation of miR-200b-3p in TGFb2 treated ONH astrocytes. To validate mRNA targets and determine the functional role of differentially expressed miRNAs, we modulated miRNA biology using miRNA mimics and inhibitors. Overexpression of miR-29c-3p and miR-200b- decreased the expression of ECM proteins. Treatment with TGFb2 increased the expression of collagens and fibronectin and overexpression of miR-29c-3p and miR-200b-3p decreased this effect, suggesting that miR-29c-3p and miR-200b-3p regulate the TGFb2 signaling pathway. It is possible that increased TGFb2 is responsible for tissue remodeling through inhibition of anti-fibrotic miRNAs and a subsequent increase in ECM synthesis.