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Send Orders for Reprints to [email protected] 1018 Current Neuropharmacology, 2018, 16, 1018-1035 REVIEW ARTICLE Glaucoma Pathogenesis and Neurotrophins: Focus on the Molecular and Genetic Basis for Therapeutic Prospects Nitin Chitranshi1,*, Yogita Dheer1, Mojdeh Abbasi1, Yuyi You2, Stuart L Graham1,2 and Vivek Gupta1 1Faculty of Medicine and Health Sciences, Macquarie University, F10A, 2 Technology Place, North Ryde, NSW 2109, Australia; 2Save Sight Institute, Sydney University, Sydney NSW 2000, Australia Abstract: Background: Retinal ganglion cell (RGC) degeneration is a major feature of glaucoma pathology. Neuroprotective approaches that delay or halt the progression of RGC loss are needed to prevent vision loss which can occur even after conventional medical or surgical treatments to lower intraocular pressure. Objective: The aim of this review was to examine the progress in genetics and cellular mechanisms associated with endoplasmic reticulum (ER) stress, RGC dysfunction and cell death pathways in glaucoma. Materials and Methods: Here, we review the involvement of neurotrophins like brain derived neu- A R T I C L E H I S T O R Y rotrophic factor (BDNF) and its high affinity receptor tropomyosin receptor kinase (TrkB) in glaucoma. The role of ER stress markers in human and animal retinas in health and disease conditions is Received: July 02, 2017 Revised: April 10, 2018 also discussed. Further, we analysed the literature highlighting genetic linkage in the context of Accepted: April 18, 2018 primary open angle glaucoma and suggested mechanistic insights into potential therapeutic options DOI: relevant to glaucoma management. 10.2174/1570159X16666180419121247 Results: The literature review of the neurobiology underlying neurotrophin pathways, ER stress and gene associations provide critical insights into association of RGCs death in glaucoma. Alteration in signalling pathway is associated with increased risk of misfolded protein aggregation in ER promot- ing RGC apoptosis. Several genes that are linked with neurotrophin signalling pathways have been reported to be associated with glaucoma pathology. Conclusion: Understanding genetic heterogeneity and involvement of neurotrophin biology in glaucoma could help to understand the complex pathophysiology of glaucoma. Identification of novel molecular targets will be critical for drug development and provide neuroprotection to the RGCs and optic nerve. Keywords: Neurotrophins, retinal ganglion cells, gene, glaucoma, apoptosis, endoplasmic reticulum stress. 1. INTRODUCTION of the optic nerve head (ONH) [2]. Genome-wide association studies (GWAS) and other genomic analyses have success- Glaucoma is a neurodegenerative disease of the eye and fully identified over 70 single nucleotide polymorphism is the major cause of irreversible vision loss. It has been pro- (SNPs) associated with primary open angle glaucoma jected to affect approximately 60 million people by 2020, (POAG) [3]. The molecular basis of cell signalling pathways translating to more than 8.4 million cases of irreversible and genetic risk factors can be studied together to understand blindness worldwide [1]. The underlying pathological their contribution to this complex disease and could provide mechanism associated with disease progression is still under cell signalling and gene regulatory based screening tests. In investigation, but evidence from both human and experimen- this review, we summarize the recent advances in the neu- tal models suggest that elevated intraocular pressure (IOP) rotrophin signalling pathways and genomics associations that obstructs retrograde and anterograde axonal transport of neu- are identified in glaucoma (Fig. 1). Deprivation of nerve rotrophins in retinal ganglion cells (RGCs) axons at the level growth factor (NGF) and other neurotrophic factors contributes to disease progression. A better understanding of the *Address correspondence to this author at the Faculty of Medicine and gene linked molecular events is necessary to produce novel Health Sciences, 75, Talavera Road, Macquarie University, Sydney, NSW therapeutic targeted treatment, which may give hope to pre- 2109, Australia; Tel: +61-298502760; E-mail: [email protected] serve vision. 1570-159X/18 $58.00+.00 ©2018 Bentham Science Publishers Glaucoma Pathogenesis and Neurotrophins Current Neuropharmacology, 2018, Vol. 16, No. 7 1019 Fig. (1). Schematic representation from brain to eye highlighting association of neurotrophins and genes in glaucoma (NGF, nerve growth factor; TrkB, tropomyosin receptor kinase B; Shp2, SH2 domain containing protein tyrosine phosphatase; p75NTR, p75 neurotrophin receptor; ER, endoplasmic reticulum; IOP, intraocular pressure; NTF4, neurotrophin factor 4; BDNF, brain derived neurotrophic factor; MYOC, myocilin; OPTN, optineurin; WDR36, WD repeat domain 36; TMCO1, transmembrane and coiled-coil domains 1). 2. NEUROTROPHIN ROLE IN RGCs grade transport of brain-derived neurotrophic factor (BDNF) from SC to the RGCs soma. Deprivation of BDNF to RGCs Neurotrophins play a key role in neuronal cell survival. contributes to the loss of visual signal [11]. NGF also dif- In healthy or normal conditions, RGCs can receive neurotro- fuses to presynaptic axon terminals of neurons such as the phin support from Muller cells [4] or retrograde axonal cholinergic neurons where it binds its receptor TrkA, bring- transport from the brain directly. This has been visualized ing its dimerization, phosphorylation and the formation of an using live-cell imaging in vitro [5]. Human and mouse adult extended complex of proteins involved in signalling path- RGCs not only express Trk family receptors but also abun- ways. These complexes are then internalized in part by dantly express p75NTR receptor during retinal development clathrin-coated membranes and moved to early endosomes [6]. The p75NTR receptor is predominantly expressed in that are then retrogradely transported from axons to the cell Müller glial cells [6, 7]. The immature form of neurotrophin, body through a dynein-microtubule based mechanism. When such as pro-NGF binding to p75NTR leads to its activation the signalling endosome reaches the cell body, the signal is and programmed cell death via tumor necrosis factor-alpha communicated to the cytoplasm and the nucleus, triggering (TNFα) production in glial cells [8]. NGF binding to the gene expression. Lysosomes promote the degradation of this p75NTR receptor in the absence of TrkA has been suggested signalling complex [13]. This model proposed for NGF may to induce apoptosis in developing RGCs [9]. Lack of neu- well apply for other neurotrophins. Many studies have led to rotrophin support has been suggested as a reason for RGCs the conclusion that BDNF is also locally synthesized in the degeneration and death in glaucoma [10]. A significant ques- retina. The RGCs and neighbouring amacrine cells produce tion is where do neurotrophins originate, and how do neu- BDNF that may act in a paracrine fashion. The RGCs them- rotrophins function upon engaging receptors on RGCs? Dif- selves act in autocrine fashion and these neurons have been ferential activation of neurotrophin receptors can occur when shown to survive in absence of additional BDNF from the neurotrophins act at the axonal terminal or at the neuronal brain and are able to express it in culture [14, 15]. Other neu- cell body [11, 12]. Neurotrophins are also shown to transport rotrophic factors such as ciliary neurotrophic factor (CNTF) retrogradely from the superior colliculus (SC) in the central and glial cell-line derived neurotrophic factor (GDNF) have nervous system (CNS). Retrograde transportation of neu- also been reported to be involved in RGCs maintenance and rotrophin take place from SC to RGCs cell bodies. Studies in survival [16, 17]. rats demonstrated that acute IOP elevation inhibits the retro- 1020 Current Neuropharmacology, 2018, Vol. 16, No. 7 Chitranshi et al. 3. NEUROTROPHINS DEPRIVATION THEORY IN signalling by its phosphorylation at Ser9, thereby inactivating GLAUCOMA it [38]. The active form of GSK3β (GSK3β phosphorylation) inhibits collapsin response mediator protein-2 (CRMP-2) Neurotrophins are transported to the retina by retrograde binding to tubulin and promotes microtubule dynamics along manner. They are responsible for regulation of neuronal with regulating axonal transport and outgrowth in neurons. growth, function and survival. The long-range retrograde In order to understand the mechanism of BDNF/ TrkB sig- neurotrophin transportation is probably facilitated via en- nalling in RGCs, we have demonstrated that activation of the dosome signalling [18]. Neurotrophins bind to the Trk recep- BDNF-TrkB signalling in PC-12 cell lines regulate GSK3β tors at axon terminals which is then taken up by cell body activity which is involved in cellular survival, cell fate de- retrogradely [19]. In glaucomatous conditions, due to high termination and proliferation [39]. BDNF knockdown in PC- IOP the retrograde transport is thought to be blocked at the 12 cell lines resulted in marked downregulation of GSK3β ONH region and RGCs fail to receive BDNF and TrkB sup- phosphorylation [39]. In addition, ONH of BDNF+/− mice port for survival [11, 20]. This deprivation causes an altera- illustrated reduced GSK3β phosphorylation with age [39]. tion of axoplasmic transport of neurotrophins from target Further studies to stimulate BDNF-TrkB signalling in RGCs neurons in the SC and lateral geniculate nucleus in the brain neuroprotection may

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