Glaucoma neurodegeneration ameliorated by mitochondrial support

Glaucoma is fundamentally a neurodegenerative disease, that becomes apparent with progressive loss of function of retinal ganglion cells (RGC). Elevated intraocular pressure (IOP) and increasing age are the major risk factors. A fascinating study, recently published in Science and reviewed in NEJM (New England Journal of Medicine), demonstrates that, like other neurodegenerative diseases, it may be ameliorated by support for aging neuronal mitochondrial function that renders the RGCs more tolerant of elevated IOP and other stressors.

Glaucoma susceptibility increases with age-related neurodegeneration

The authors state:

"Glaucoma is one of the most common neurodegenerative diseases worldwide, affecting over 70 million people. High intraocular pressure (IOP) and increasing age are important risk factors for glaucoma. However, specific mechanisms rendering retinal ganglion cells more vulnerable to damage with age are unknown. Here, we address how increasing age and high IOP interact to drive neurodegeneration using DBA/2J (D2) mice, a widely used model of chronic, age-related, inherited glaucoma."

They determined age and IOP-dependent molecular changes within RGCs that precede the glaucomatous neurodegeneration in 9 month old (mo) D2 mice (termed early glaucoma - high IOP and molecular changes but lacking neurodegeneration; 4 month old D2 mice (precedes high IOP) and age, sex and strain matched D2-Gpnmb⁺ controls that do not develop high IOP or ocular disease. This yielded a high correlation of transcriptome changes with increasing glaucomatous disease:

"As disease progressed, there was an increase in transcript abundance that was most pronounced for mitochondrial reads. Emerging evidence suggests that imbalances in the relative proportions of mitochondrial proteins encoded by nuclear and mitochondrial genomes negatively impact mitochondrial function. In D2 Groups 2 to 4, differential expression of genes encoding mitochondrial proteins, and significant enrichment of differentially expressed (DE) genes in the mitochondrial dysfunction and oxidative phosphorylation pathways further point to mitochondrial abnormalities...Extending previous studies, our data demonstrate that mitochondrial perturbations are among the very first changes occurring within RGCs during glaucoma"

Retinal metabolites

The authors demonstrated that degradation of mitochondrial health reflecting reflected in deficiencies of retinal metabolites is a key factor in the progression to vision loss.

"Guided by the above data, we assessed metabolites in retinas with increasing age and disease. We detected early decreases in metabolites that are central to healthy mitochondrial metabolism and protection from oxidative stress (NAD⁺ and NADH [total NAD; NAD(t)], GSH and GSSG [total glutathione; glutathione(t)])."

Importantly, this mitochondrial metabolic degradation occurs without increasing IOP, but renders the the RGCs more vulnerable to the stress of IOP when it occurs.

"These age-dependent decreases were not a response to IOP-insult(s) as they also occurred in control D2-Gpnmb⁺ retinas. These decreases are expected to sensitize retinal neurons to disease related stresses and mitochondrial dysfunction. Suggesting greater metabolic stress in RGCs than other retinal neurons, HIF-1α (a key metabolic regulator during perturbed redox states) is induced in the ganglion cell layer early in glaucoma."

Progression to vision loss is multifactorial

Vision loss occurs as a result of causes that combine ocular stress, including increased IOP, acting on RGCs that have become sensitized to stress due to mitochondrial dysfunction.

"Our data suggest that RGCs go through a period of mitochondrial stress and metabolite depletion, potentially moving towards fatty acid metabolism. Fatty acid β-oxidation can increase generation of free radicals/reactive oxygen species (ROS). Both RNA-seq and γ-H2AX immunostaining support increased ROS and DNA damage within RGCs early in glaucoma. Providing a link between DNA damage, and increased metabolic stress, PARP activity (NAD consuming) is induced in RGCs with age."

Supplementation protects against glaucoma

It stands to reason that supporting mitochondrial health may render the RGCs more resistant to damage that leads to vision loss; the author's data show this to be correct.

"Our data support a model where age-dependent declines of NAD⁺ and glutathione in the retina render RGCs vulnerable to damage from elevated IOP. Thus, increasing NAD levels would be predicted to protect IOP-insulted eyes from glaucomatous changes, by decreasing the probability of metabolic/energetic failure and rendering the RGCs more resilient to IOP-induced stress. Oral supplementation of vitamin B₃/nicotinamide (NAM; a precursor of NAD) has been successfully used to correct disturbances in NAD⁺metabolism in two mouse models of pre-eclampsia. Accordingly, we administered NAM to D2 mice, initially at the same dose (550 mg/kg/d, NAM^Lo). NAM administration in drinking water prevented the decline of NAD levels through to 12 mo (a standard end stage for assessing neurodegeneration in this glaucoma model)."

It's of major clinical significance that vitamin B₃ protected without altering IOP.

"Supporting our neuronal vulnerability hypothesis, NAM^Lo did not alter IOP, but protected from glaucoma. NAM was protective both prophylactically (starting at 6 mo, prior to IOP elevation in most eyes in our colony) and interventionally (starting at 9 mo, when the majority of eyes have had continuing IOP elevation). NAM significantly reduced the incidence of optic nerve degeneration, prevented RGC soma loss and retinal nerve fiber layer thinning, and protected visual function as assessed by PERG. NAM prevented RGC axonal loss, and these axons continued to support anterograde axonal. NAM administration was sufficient to inhibit the formation of dysfunctional mitochondria with abnormal cristae and also limited synapse loss that occurs in this model. Lipid droplet formation was also prevented in aged D2 retinas. NAM also decreased PARP activation, limited levels of DNA damage, and transcriptional induction of HIF-1α reflecting less perturbed cellular metabolism. NAM prevented even the earliest molecular signs of glaucoma in most treated eyes as assessed by RNA-seq and prevented the majority of age-related gene expression changes within RGCs. This highlights the unexpected potency of NAM in decreasing metabolic disruption and prevention of glaucoma."

Even more was better; even demonstrating some reduction in IOP.

"Attempting to further decrease the probability of glaucoma, we administered a higher dose of NAM (2000mg/kg/d; NAM^Hi). NAM^Hi was extremely protective with 93% of treated eyes having no optic nerve damage. The degree of protection afforded by administering this single molecule is unprecedented and unanticipated. Although NAM^Lo demonstrates a clear neuroprotective effect (no effect on IOP), NAM^Hi lessens the degree of IOP elevation. This indicates that NAM can protect against age-related pathogenic processes in additional cell types to RGCs. Therefore vitamin B₃/NAM, a single molecule that protects against both IOP elevation and neural vulnerability, has great potential for glaucoma treatment..."

Enzyme activity using vitamin B₃ to produce NAD diminishes with stress and age

Alzheimer's disease, not surprisingly, is also associated with degraded enzyme activity.

"NMNAT2 is emerging as an important NAD producing enzyme in axons, protecting from axon degeneration (20). Ongoing stress negatively impacts Nmnat2 expression in RGCs. This decline of NMNAT2 may induce vulnerability to axon degeneration in glaucoma. NMNAT2 expression is decreased in brains with Alzheimer’s disease and is highly variable in aged postmortem human brains. Such variation in expression may contribute to individual differences in vulnerability to various neurodegenerations."

Perspective

Commentators writing in the New England Journal of Medicine further place the stunning value of these findings in context.

"Glaucomatous optic neuropathy is the most common form of neurodegeneration involving the central nervous system and the leading cause of irreversible vision loss worldwide. Although age is an important risk factor, early onset is not uncommon and may result in severe vision loss in younger persons, even when the rate of disease progression is slow.The critical neurons that are damaged in glaucoma are the retinal ganglion cells, which reside in the inner retina and serve as neuronal intermediaries between the photosensitive outer retina and the brain. They transmit visual information to the visual cortex through synapses in the lateral geniculate nucleus. The site of injury in the glaucomatous human optic nerve is thought to be the lamina cribrosa, where the unmyelinated axons of the retinal ganglion cells exit the eye through collagenous pores in the sclera and are susceptible to perturbations in their microenvironment and microarchitecture. The progressive loss of neuronal elements leads to irreversible structural damage and functional loss.For more than 150 years, the only proven treatment for glaucoma has been the reduction of intraocular pressure with either drugs or surgical approaches. As with the study of other multifactorial neurodegenerative conditions, the ultimate goal in glaucoma research is the identification of treatment interventions that directly target neuronal health and enhance neuronal survival. Unfortunately, several drugs designed to protect against glaucomatous neurodegeneration have failed in clinical trials.

The investigators proposed that the differences in gene expression and total NAD levels affect the function of the retinal ganglion cells partly by limiting the energy produced by — and thus available within — neurons. Although decreased NAD levels alone did not result in cell death, Williams et al. hypothesized that reduced levels do destabilize metabolism during periods of stress and that the age-dependent decline in NAD levels, when combined with stress from elevated intraocular pressure, has a negative effect on mitochondrial function. This compromise in function leads to increases in the metabolism of fatty acids and the generation of free radicals, and thus an impaired response to metabolic stress, which in turn leads to loss of retinal ganglion cells.To test the “NAD-deficit” hypothesis, Williams et al. supplemented the mouse diet with nicotinamide (the amide of vitamin B₃ and a precursor to NAD⁺) to enhance cellular energy production. At the lowest dose studied (equivalent to about 2.5 g per day for a person weighing 60 kg), the authors found that nicotinamide prevented the structural and functional loss of retinal ganglion cells despite the continued elevation of intraocular pressure. The dose-dependent protective effect was evident at different points in disease progression, and the authors did not observe adverse effects."

Clinical Note

This wonderful study illustrates a vital clinical point: supporting mitochondrial function can protect from neuronal damage that occurs with stress and age, and rehabilitate to a very meaningful degrees damages neuronal function. And is resulted from supporting with only one cofactor, vitamin B₃/NAM. Therefore, consider how much more effective we can be by targeting other mitochondrial cofactors found objectively to be suboptimal by organic acid testing, along with adding glutathione with shown to be deficient. Again, referring to one cofactor among the numerous ones that ensure mitochondrial integrity...

"Given these protections against severe acute insults, NAM could have broad implications for treating glaucoma and potentially other age-related neurodegenerative diseases."

The authors conclude:

"In conclusion, we show that dietary supplementation with a single molecule (vitamin B₃/nicotinamide), or Nmnat1 gene therapy, significantly reduces vulnerability to glaucoma by supporting mitochondrial health and metabolism. Combined with established medications that lower IOP, NAM treatment (and/or Nmnat1gene therapy) may be profoundly protective. By providing a new molecular and metabolic link between increased neuronal vulnerability with age and neurodegeneration these findings are of critical importance for glaucoma and possibly other age-related diseases."

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