The Role of Decorin in the Pathogenesis of Primary Open Angle Glaucoma

Primary open angle glaucoma (POAG), one of the leading causes for blindness in the western world, is defined as a progressive optic neuropathy which is characterized by the loss of retinal ganglion cells. Various randomized multicenter studies could show that an elevated intraocular pressure (IOP), which is too high for the health of the eye, is the main risk factor to develop POAG (Collaborative-Normal-Tension-Glaucoma-Study-Group, 1998; Gordon et al., 2002; Johnson et al., 2002; Leske et al., 2003) Today it is widely acknowledged that the increase in IOP is due to a decreased outflow facility in the trabecular meshwork (TM). The drainage of aqueous humor in the TM is reduced by an increased contractility of TM cells and by an accumulation of extracellular matrix (ECM) in the juxtacanalicular TM (Fuchshofer et al., 2007; Lutjen-Drecoll et al., 2001; Lutjen-Drecoll et al., 1986a; Okka et al., 2004). The increase in IOP in combination with structural changes of the ECM in the region of the lamina cribrosa (LC) lead to a reduced axonal transport of neurotrophic factors to the retina ganglion cells (RGC). The lack of neurotropic factors leads to RGC death which causes the neuropathy of the optic nerve and the optical field defects characteristic for POAG (Guo et al., 2005; Fuchshofer 2011; Zode et al. 2011). Many studies could show that the growth factors TGFβ and CTGF may play an important part in the pathogenesis of POAG (Fuchshofer et al., 2005; Fuchshofer, 2011; Junglas et al., 2012; Junglas et al., 2009; Tamm & Fuchshofer, 2007). Therefore proteins able to inhibit these factors are of high interest as possible therapeutic agents. This work focuses on the small leucine-rich proteoglycan Decorin (DCN). DCN inhibits TGFβ via direct binding or via the up regulation of Fibrillin 1 synthesis (Neill et al., 2012; Yamaguchi, 1990). Possible interactions between DCN, TGFβ and CTGF should be analyzed in vitro and in vivo. Further it was aim of this study to analyze if DCN is able to inhibit the expression of ECM proteins in cell types involved in the pathogenesis of POAG. Additionally it was investigated if the AKT signaling pathway is involved in the regulation of TGFβ and CTGF via DCN in optic nerve astrocytes. The final aspect of this work was the observation of the effects of a DCN knockout in vivo in the eye in mice of mixed background and in mice of a pure CD1 background. The experiments showed that a treatment with TGFβ and CTGF leads to a decreased synthesis and expression of DCN in various cell types of the eye which play an important part in the development of POAG. On the other hand, a treatment with DCN led to a decreased synthesis of TGFβ-1, TGFβ-2 and CTGF in these cell types. Additionally, the synthesis of ECM proteins Fibronectin and Collagen IV was down regulated after treatments with DCN. We could further show that DCN regulates TGFβ-1, TGFβ-2 and CTGF via the AKT signaling pathway. Interestingly we were not able to cultivate optic nerve astrocytes of DCN knockout mice, while optic nerves astrocytes of wildtype littermates grow well in culture. Immunohistochemical staining showed reduced levels of DCN in the TM of glaucomatous eyes in comparison to the TM of healthy donors. In vivo a lens specific overexpression of CTGF, which leads to the development of POAG (Junglas et al., 2012) resulted in a reduced DCN Expression in the region of the TM, but not in the optic nerve. The eyes of DCN knockout mice showed a significant increase in IOP and a significantly reduced number of axons, which are typical characteristics of POAG. These might be due to the enhanced expression of TGFβ, CTGF and ECM proteins in the chamber angle, which could be observed on RNA and protein level. Furthermore levels of pAKT were reduced in the TM, the retina and in the glial lamina of DCN knockout mice. The results show a clear interaction between DCN and TGFβ/CTGF as between DCN and ECM-Proteins in vitro and in vivo. DCN seems to play an important role in inhibiting processes which can cause POAG. It possibly causes the balance between the necessary and pathogenic expression of TGFβ, CTGF and ECM proteins in the eye. The results lead to the conclusion that DCN is a highly promising candidate for a therapeutic against POAG. It will be of high interest to observe the effects of DCN treatments or an adenoviral gene transfer of DCN to the eyes of βB1-CTGF and DCN knockout mice. We hypothesize that such treatments will lead to a decrease in IOP and to a reduction of axonal loss.