Glaucoma as a Metabolic Optic Neuropathy: Making the Case for Nicotinamide Treatment in Glaucoma

Abstract

Mitochondrial dysfunction may be an important, if not essential, component of human glaucoma. Using transcriptomics followed by molecular and neurobiological techniques, we have recently demonstrated that mitochondrial dysfunction within retinal ganglion cells is an early feature in the DBA/2J mouse model of inherited glaucoma. Guided by these findings, we discovered that the retinal level of nicotinamide adenine dinucleotide (NAD, a key molecule for mitochondrial health) declines in an age-dependent manner. We hypothesized that this decline in NAD renders retinal ganglion cells susceptible to damage during periods of elevated intraocular pressure. To replete NAD levels in this glaucoma, we administered nicotinamide (the amide of vitamin B3). At the lowest dose tested, nicotinamide robustly protected from glaucoma (~70% of eyes had no detectable glaucomatous neurodegeneration). At this dose, nicotinamide had no influence on intraocular pressure and so its effect was neuroprotective. At the highest dose tested, 93% of eyes had no detectable glaucoma. This represents a ~10-fold decrease in the risk of developing glaucoma. At this dose, intraocular pressure still became elevated but there was a reduction in the degree of elevation showing an additional benefit. Thus, nicotinamide is unexpectedly potent at preventing this glaucoma and is an attractive option for glaucoma therapeutics. Our findings demonstrate the promise for both preventing and treating glaucoma by interventions that bolster metabolism during increasing age and during periods of elevated intraocular pressure. Nicotinamide prevents age-related declines in NAD (a decline that occurs in different genetic contexts and species). NAD precursors are reported to protect from a variety of neurodegenerative conditions. Thus, nicotinamide may provide a much needed neuroprotective treatment against human glaucoma. This manuscript summarizes human data implicating mitochondria in glaucoma, and argues for studies to further assess the safety and efficacy of nicotinamide in human glaucoma care.

Figures

Figure 1. Retinal ganglion cell protection following nicotinamide treatment in the DBA/2J mouse model of inherited glaucoma

Nicotinamide profoundly protects retinal ganglion cells and prevents optic nerve degeneration in a dose dependent manner. Top row shows flat-mounted retinas stained with an anti-RBPMS antibody that specifically labels retinal ganglion cells (red) and counterstained with DAPI that stains nuclei (blue). There is a significant loss of retinal ganglion cells following periods of elevated IOP (top row, middle panel), which is prevented by nicotinamide treatment (top row, right panel). Bottom row shows cross sections of the optic nerve stained with PPD. Following periods of elevated IOP retinal ganglion cell axons in the optic nerve degenerate and glial scars are formed (bottom row, middle panel). Nicotinamide treatment (NAMLo) robustly protected the axons, and the number of optic nerves with glaucoma was significantly decreased. At a higher dose (NAMHi) 93% of optic nerves did not develop glaucoma (bottom row, chart). Scale bars = 20μm (top row), 50μm (bottom row). ** = P < 0.01, *** = P < 0.001, Student’s t-test (top), Fisher’s exact test (bottom). All images are for mice that were 12 months of age. See references , for more details.

Figure 2. NAD protects from retinal ganglion cell degeneration and glaucoma

NAD levels decrease with age rendering retinal ganglion cells vulnerable to IOP-induced damage. Preventing this age-dependent change by increasing NAD availability robustly protects from all assessed signs of glaucoma. Increasing NAD using nicotinamide , Nmnat1 gene therapy , the WldS allele , , or combinations of these robustly protect retinal ganglion cells from degeneration. This protection is neuroprotective, as neither the low dose that we used (NAMLo) or the other treatments changed the IOP. Higher doses of nicotinamide may have an additional benefit by also limiting the degree of IOP elevation . The inhibitory affect of nicotinamide on various enzymes may enhance its potency against glaucoma through different mechanisms (see text). Left shows a retinal ganglion cell undergoing glaucomatous changes due to high IOP. Right shows a retinal ganglion cell that is protected from the detrimental effects of IOP due to increased NAD levels presumably within the RGC. The affects of NAD on other cell types may also be important. See references , , for more details.

Figure 3. NAD synthesis

The chemical structure of nicotinamide adenine dinucleotide (NAD) (top left) and four major NAD precursors; nicotinic acid (NA), nicotinamide (NAM), nicotinamide mononucleotide (NMN), and nicotinamide riboside (NR) (top right). NAD can be produced de novo from dietary tryptophan. Alternatively NAD can be produced through two other core pathways; the Preiss-Handler pathway from NA, or through the salvage pathway from NAM (bottom panel). NAM is available in diet and readily absorbed as a major NAD precursor through the salvage pathway in vivo. Enzymes: NAPRT, nicotinic acid phosphoribosyltransferase; NADSYN, NAD synthetase 1; NAMPT, nicotinamide phosphoribosyltransferase; NMNAT1, -2, and -3 nicotinamide nucleotide adenylytransferases 1, 2 and 3; NRK1, -2, nicotinamide riboside kinases (mouse gene names Nmrk1, -2); NP, purine nucleoside phosphorylase.