Understanding the retinoid-x-receptor biology and manipulating it as a novel strategy to protect the retina

Glaucoma is a degenerative optic neuropathy affecting nearly 80 million individuals by 2020 worldwide. Glaucoma is mainly manifested as alterations of the optic disc with progressive degeneration of retinal ganglion cells (RGCs) contributing to visual field loss. High intraocular pressure (IOP) is considered as the main risk factor of glaucoma. Unfortunately, a significant number of patients show disease progression despite treating with IOP lowering drugs. So, RGC degeneration cannot be prevented only by reducing eye pressure. There is need for development of more novel strategies targeting retinal neuroprotection. Within this context, this PhD project aimed to assess the potential neuroprotective effect of RXR activation by its agonist bexarotene both in vitro studies as well as in vivo acute and chronic glaucoma models. Retinoid X receptors (RXRs) are ligand-dependent transcription factors that belong to the nuclear receptor (NR) superfamily. RXRs have three different isoforms (α, β, and γ) and can form both homo- and heterodimers with other nuclear receptors. Numerous studies have linked RXR modulation with neuroprotection. This thesis was mainly focused to elucidate the role of bexarotene as RXR modulator in glaucoma pathology. In first part of this study I demonstrated the expression and regulation of RXR receptors in SH-SY5Y cells using different concentrations of bexarotene. I also studied the role of TrkB signalling with RXR pathway in regulating endoplasmic reticulum stress response and apoptotic pathway activation. The results obtained from in vitro studies demonstrated that optimum concentrations of bexarotene upregulated the expression of all the three isoforms of RXRs. Also my studies revealed that higher concentrations of bexarotene upregulates ER stress proteins and BAD which can be prevented by pharmacological targeting of the TrkB receptor. I further extended my studies to assess the neuroprotective effects of RXR activation in mice in preventing loss of retinal ganglion cells (RGCs) under experimental glaucoma conditions. Two models i.e increased excitotoxicity mediated glutamate model (acute model) and microbead induced increased intraocular pressure model (chronic 2 months model) of RGC degeneration were used for in vivo studies. Bexarotene treatment showed enhanced expression of RXRs as compared to control and glaucoma mice retinal sections. Furthermore it was seen that bexarotene maintained inner retinal functional and structural integrity confirmed by electroretinography, H and E staining and Bieschowlsky silver staining of optic nerve sections. Moreover, docking studies also validated binding of bexarotene to RXR receptors. This thesis represents an integration of three different methodologies i.e in vitro, in vivo, in silico. The results from this thesis provide evidence to the hypothesis that RXR activation can be neuroprotective to RGCs in preventing apoptosis and cell death. Bexarotene or other RXR agonists may have potential for future therapeutic management of glaucoma.