The effect of nicotinamide on retinal ganglion cell metabolism and mitochondria in glaucoma

<p dir="ltr">Glaucoma is the leading cause of irreversible blindness, affecting over 111 million people worldwide by 2040 (Tham et al., 2014a) and accounting for over 8% of all blindness cases, which equates to approximately 3.6 million individuals. The main risk factors are age, genetic, and elevated intraocular pressure (IOP). To date, the only available therapeutic strategy is the IOP reduction (eye drops, laser or surgical form) which role is to limit the retinal ganglion cells (RGCs) degeneration and the vision loss.</p><p dir="ltr">In this work I compile three articles which collectively treats about biologic challenge faced by the RGCs during the neurodegenerative disease affecting the visual system. The RGCs are neuronal cells present in the retina which project axons to form the optic nerve (ON). This thesis focuses primarily on glaucoma, but another form of optic neuropathy, Leber's Hereditary Optic Neuropathy (LHON) is also studied. The critical point in both pathologies is the mitochondrial dysfunction leading to the metabolic failure. This translates by the RGCs vulnerability, a progressive dysfunction and eventually a cell death. In this context, a promising strategy have been identified: the nicotinamide (NAM) supplementation. NAM is an early precursor to nicotinamide adenine dinucleotide (NAD) NAD in the salvage pathway, previous research has shown neuroprotective capacities across different model.</p><p dir="ltr">With an aging population and a longer life expectancy, we are likely to see a drastic increased number of patients presenting glaucoma, this stresses even more the necessity to improve the treatment.</p><p dir="ltr"><b>Paper I</b>: Retinal Ganglion Cell Degeneration in a Rat Magnetic Bead Model of Ocular Hypertensive Glaucoma.</p><p dir="ltr">In this study we use a previously published glaucomatous model: the rat beads model. This inducible ocular hypertensive model by magnetic microbeads injection in the anterior segment of the brown Norwegian rat constitutes a relevant glaucomatous pre-clinical tool to study the consequence of the IOP component. With highly relevant clinical feature met in patients such as the compartmentalized RGCs degeneration, the optic nerve head (ONH) excavation, the axoplasmic and vascular impairment, this model offers a great platform for the characterization and further understanding of the mechanical, molecular and metabolic processes lead by the elevation of the IOP in the glaucomatous pathology.</p><p dir="ltr"><b>Paper II</b>: Nicotinamide provides neuroprotection in glaucoma by protecting against mitochondrial and metabolic dysfunction.</p><p dir="ltr">This second article constitutes the continuity of paper I as we use the same model and further explore the impact of the ocular hypertension (OHT) on the RGCs specific mitochondria and the cellular metabolic changes taking place after the OHT induction. Using similar methodology, we study the impact of the metabolic disruption following the intravitreal injection of rotenone, a specific Complex I inhibitor, which leads to a severe metabolic deprivation. We detailed the impacts of these glaucomatous injuries throughout the visual track (RGCs soma, dendrites and axons) before to target the NAD, a co-factor necessary for the neuronal survival. NAD is known to decrease in retina in both animal models and glaucoma patients, and we demonstrate the neuroprotective effect of NAM under IOP and metabolic stressor when pre-treated. These finding support a neuroprotective action of NAM against different glaucomatous injuries, and more generally against neurodegenerative diseases.</p><p dir="ltr"><b>Paper III</b>: Prophylactic nicotinamide treatment protects from rotenone-induced neurodegeneration by increasing mitochondrial content and volume.</p><p dir="ltr">The LHON is driven by mutation affecting the gene which code for the mitochondrial Complex I, resulting into a progressive RGC dysfunction and eventually cell death. The RGCs are highly metabolically vulnerable and do not tolerate the mitochondrial dysfunction, unfortunately there are no treatment or neuroprotection, which leads to long-term vision loss. NAM is a NAD precursor, and it has been shown to have neuroprotective effect against mitochondrial and metabolic dysfunction in the scope of glaucoma. In this study, we model LHON with intravitreal injection of rotenone (a complex I inhibitor) in mice reporting model (MitoV), causing a drastic loss of RGCs. We then demonstrate the prevention of RGCs loss when mice are treated in a prophylactic manner with NAM and detail the mechanism of protection under complex I inhibition. NAM protects the mitochondria by increasing the total number of mitochondria and their cristae density. This well-established component with a safe profile is being tested for glaucomatous patients, and could hold some promises for other mitochondrial optic neuropathies as LHON to slower or prevent the progression of the neurodegenerative process.</p><h3>List of scientific papers</h3><p dir="ltr">I. Tribble JR, <b>Otmani A,</b> Kokkali E, Lardner E, Morgan JE, Williams PA; Retinal Ganglion Cell Degeneration in a Rat Magnetic Bead Model of Ocular Hypertensive Glaucoma. Translational vision science & technology. 2021;10(1):21. <a href="https://doi.org/10.1167/tvst.10.1.21">https://doi.org/10.1167/tvst.10.1.21</a></p><p dir="ltr">II. Tribble JR, <b>Otmani A,</b> Sun S, Ellis SA, Cimaglia G, Vohra R, Jöe M, Lardner E, Venkataraman AP, Domínguez-Vicent A, Kokkali E, Rho S, Jóhannesson G, Burgess RW, Fuerst PG, Brautaset R, Kolko M, Morgan JE, Crowston JG, Votruba M, Williams PA., Nicotinamide provides neuroprotection in glaucoma by protecting against mitochondrial and metabolic dysfunction, Redox Biology, Volume 43, 2021, 101988, ISSN 2213-2317, <a href="https://doi.org/10.1016/j.redox.2021.101988">https://doi.org/10.1016/j.redox.2021.101988</a></p><p dir="ltr">III. <b>Otmani A,</b> Johannesson G, Brautaset R, Tribble JR, Williams PA. Prophylactic nicotinamide treatment protects from rotenone- induced neurodegeneration by increasing mitochondrial content and volume. Acta Neuropathologica Communications 12, 37 (2024).<br><a href="https://doi.org/10.1186/s40478-024-01724-z">https://doi.org/10.1186/s40478-024-01724-z</a><br></p>