Nicotinamide Levels in Serum, Aqueous Humor, and Tear Film in Glaucoma and Correlations With Mitochondrial Damage-Associated Molecular Patterns (mtDAMPs) and Senescence-Associated Secretory Phenotype (SASP)

Background Glaucoma is a multifactorial, chronic, and progressive disease characterized by the irreversible loss of retinal ganglion cells (RGCs). It represents the second leading cause of blindness globally, but it is the leading cause of irreversible blindness, with 76 million patients affected worldwide in 2020. Unfortunately, it is estimated that the incidence of the disease will increase significantly in the next two decades, with approximately 120 million patients affected globally, particularly in Africa and Asia. Elevated intraocular pressure (IOP) is one of the most important risk factors for the development and progression of the disease, and its reduction is the only scientifically proven approach to controlling the decline in visual function. However, despite the reduction in IOP slowing the worsening of visual fields, many glaucoma patients continue to experience progressive loss of visual function despite IOP values within the statistical normal range. This is because glaucoma is a multifactorial disease, and simply reducing IOP does not guarantee the control of other risk factors damaging RGCs.

Among the mechanisms involved in the damage and death of RGCs in glaucoma, mitochondrial dysfunction appears to be one of the most important and earliest, leading the scientific literature to consider glaucoma, at least in part, as a particular form of mitochondrial disease. The need for direct neuroprotection, in addition to indirect neuroprotection (IOP reduction), represents a significant unmet need in the management of glaucoma patients. At present, despite numerous preclinical and some clinical evidence supporting the efficacy of certain molecules (such as citicoline) in protecting RGCs, there is no scientifically supported neuroprotective therapy in humans.

Two forms of vitamin B3, nicotinamide (NAM) and nicotinamide riboside (NR), are emerging as potential adjunctive therapies for the protection and enhancement of RGCs. These compounds are precursors to nicotinamide adenine dinucleotide (NAD). NAD is essential for the proper functioning of cells and is involved in various metabolic activities, including energy production, acting as a key cofactor in redox metabolism, including glycolysis, the citric acid cycle, oxidative phosphorylation, protection against reactive oxygen species, contributing to various enzymatic activities, and maintaining mitochondrial function.

Mitochondria, traditionally recognized for their role in bioenergy production and metabolite generation, are gaining increasing recognition for their various functions, including cellular signaling, cell death, regulation of reactive oxygen species (ROS), and inflammation. Given the high susceptibility of RGCs to mitochondrial damage, mitochondrial dysfunction becomes intricately linked to a series of cellular processes (aging, excitotoxicity, oxidative stress, mitophagy deficiency, prolonged inflammatory responses) associated with the pathogenesis of glaucoma. In glaucomatous conditions, the removal of defective mitochondria becomes crucial for maintaining mitochondrial quality control, thereby protecting cellular integrity. Consequently, mitochondrial dynamics, biogenesis, and mitophagy play crucial roles in ensuring effective mitochondrial quality control. Mitochondrial damage in neurons, particularly in RGCs, leads to the release of "Mitochondrial damage-associated molecular patterns" (mtDAMPs). These molecules act as triggers for chronic inflammation through pattern recognition receptors (PRRs) on glial cells and complementary activations in glaucoma. Numerous mtDAMPs, including mtDNA, ATP, cardiolipin, mitochondrial transcription factor A (TFAM), N-formyl peptides (NFPs), and cytochrome c, have been implicated in inducing an inflammatory process. The immunogenic nature of mtDNA, similar to bacterial DNA, activates interferon (IFN)-dependent genes, TLR9, the NLRP3 inflammasome, and the cGAS-STING signaling pathway. Defects in mitochondrial synthesis and maintenance, as well as alterations in mitophagy (autophagy of mitochondria), can contribute to dysfunctional accumulation, compromising energy (ATP) production and cellular metabolism regulation. These alterations can negatively impact cellular vitality and functionality, promoting cellular senescence. Emerging evidence suggests a link between glaucomatous conditions and cellular senescence. RGC senescence may contribute to the pathogenesis of glaucoma. The accumulation of senescent cells in the eye could lead to disrupted tissue homeostasis and compromised cellular function, potentially worsening glaucoma progression. Senescent cells can secrete a variety of bioactive molecules, a phenomenon known as Senescence-Associated Secretory Phenotype (SASP). SASP components released into bodily fluids include cytokines, chemokines, growth factors, and extracellular matrix remodeling enzymes. These factors can act in a paracrine manner, influencing nearby cells and contributing to chronic inflammation and tissue malfunction. In glaucoma, the presence of SASP components in biological fluids may indicate a dynamic interaction between senescent cells and the surrounding microenvironment. Understanding the presence and impact of senescence and SASP in biological fluids is crucial for unraveling the complex molecular mechanisms underlying glaucoma progression.

Emerging evidence has demonstrated that axonal degeneration in glaucomatous cells is associated with a decline in NAD, and the supplementation of NAD or NAD precursors (NAM or NR) may be effective in preventing or delaying RGC death in vitro and in vivo. In an experimental study, Williams and colleagues showed that oral administration of NAM was able to increase NAD levels in the retina and protect RGCs from degeneration in a murine glaucoma model. Interestingly, 93% of the experimental animals supplemented orally with a high dose of NAM (2000 mg/kg/day) did not develop RGC loss despite high IOP. Given its beneficial effects and evidence of reduced NAD bioavailability with aging, numerous research directions are exploring ways to restore cellular NAD pools in humans by administering its precursors.