Dysregulation of neurotrophic factors and mechanisms of neurodegeneration in the visual pathway
Creators: name: Joseph, Chitra
Publisher: name: Macquarie University
Year: 2022
Subjects: Other education not elsewhere classified

description: Neurons are terminally differentiated cells incapable of further cell division processes, and thus neuronal damage and loss associated with neurodegenerative disorders are difficult to treat. Neurotrophic factors play an important role in the proper functioning of neurons in the central nervous system and have been shown to augment the survival of neuronal cells under stress conditions. Neurotrophins regulate a spectrum of key signalling networks implicating their significance in determining cell fate in neuronal pathophysiology. Brain-derived neurotrophic factor (BDNF) is the most widely studied neurotrophin crucial for neuronal survival and maintains the growth and differentiation of neurons. In the brain, BDNF is found vital for neuronal synapses which assists learning and other cognitive functions. Thus, deleterious alterations in BDNF signalling can predispose neuronal cells to apoptosis contributing to the pathogenesis of major disorders like Alzheimer's disease (AD), Parkinson’s disease (PD), Amyotrophic Lateral Sclerosis (ALS), and ophthalmic disease, principally glaucoma. The positive or negative effects of BDNF on neuronal cells is mediated by the high affinity Tropomysin receptor kinase B (TrkB) receptor or low affinity p75 neurotrophin receptor (p75NTR). BDNF induced TrkB phosphorylation activates three downstream signalling pathways; Mitogen- activated protein kinase (MAPK) pathway, the phosphatidylinositol 3-kinase (PI3K)-Akt pathway and the Phospholipase C γ (PLCγ) pathway all of which subsequently increase the transcription of cAMP-response element binding protein (CREB) in the nucleus. Among the multiple processes that regulate BDNF expression, this thesis discusses the functional single nucleotide polymorphism (SNP) that leads to valine to methionine substitution at position 66 in the BDNF pro-domain and alters the intracellular processing and activity-dependent secretion of BDNF. Reliable models are required to better understand the pathophysiologic changes inherent in neurodegenerative disorders. Owing to its unique hierarchical architecture and the ease of assessing physiologic and functional changes, the visual pathway represents an ideal model to measure and examine degenerative changes in neuronal cells. Faulty axonal transport of trophic factors along the visual pathway is one reason that causes retinal ganglion cell loss during optic neuropathies. This thesis also discusses the molecular events associated with the spread of neurodegeneration along the visual pathway. Firstly, in this thesis, I have investigated the differential regulation of BDNF on the downstream signalling pathway in different neuronal cells using phosphoproteomic analysis. The analysis showed activation of distinct downstream molecules in response to BDNF in a cell type specific manner. I have further looked at the potential of BDNF and BDNFV66M variant to activate the immediate downstream targets of BDNF in three different neuronal cells, and the results illustrated the reduced functional compliance of the variant in mediating neuronal survival. To understand the course of degenerative events in neurodegenerative disorders, I studied the transsynaptic degeneration process happening in the visual pathway in a mice model of optic nerve transection. The data showed evidence of myelin pathology before axonal injury in the transneuronal spread of damage. Finally, using BDNF transgenic mice, the retinal phenotype of wildtype and Met carriers were compared and further role of this polymorphism under induced glaucoma condition was studied. Although under healthy conditions, the mice did not show any retinal phenotype, under high intraocular pressure (IOP) induced glaucoma condition, the polymorphic mice suffered more severe inner retinal damage compared to control mice. These findings indicate that BDNFVal66Met has impaired neuronal cell survival signalling and has augmented susceptibility to glaucomatous damage.