In a preprint published in bioRxiv, scientists have shown that long-term supplementation of nicotinamide riboside (NR), a NAD+ precursor, alleviates the progression of age-related hearing loss in a mouse model [1].
An overlooked hearing impairment
Globally, age-related hearing loss (ARHL) is the most common sensory deficit of older people. Moreover, it is associated with several problems, including cognitive decline, social isolation, and accident risk. However, due to its insidious progression, the disease is often overlooked.
The etiology of ARHL is still poorly understood. Some of its proposed hypotheses involve age-dependent changes in DNA damage accumulation (genomic instability), oxidative stress, mitochondrial dysfunction, and senescence-associated inflammation [2].
NAD+ and hearing
NAD+, a natural co-enzyme, which has been shown to ameliorate some aging hallmarks and several age-related conditions, also plays a role in the auditory system. For example, the researchers previously showed that NAD+ supplementation prevents the progression of hearing loss in a mouse model of premature aging [3]. This led them to conduct this experiment in wild-type mice, in which they expected similar results.
NAD+ in the cochlea
In this paper, the researchers compared the cellular NAD+ levels in the cochleas of young (2-month-old) and aged (12-month-old) mice and found that total NAD+ and relative NAD+/NADH levels were lower in the cochleas of the older animals. This led the researchers to hypothesize that long-term NR supplementation could reverse the age-related reduction of this critical co-enzyme.
NR prevents and alleviates the progression of ARHL in mice
To determine the effects of long-term NR administration on hearing loss in old mice, the researchers administered NR through the drinking water of a treatment group, with a control group receiving unaltered water. Both groups were then tested for hearing capability through an auditory brain response (ABR) system at the ages of 2, 8 and 12 months.
It turned out that the supplementation prevented the progression of ARHL specifically at high sound frequencies (16 and 32 kHz). Another test, the hearing threshold shift, showed that NR even improved high-frequency hearing in a subset of treated mice.
Besides the preventative benefits, the researchers wanted to investigate whether NR can also ameliorate disease progression even after hearing loss has occurred. To address this, mice with confirmed ARHL at the age of 15 months were treated with NR for 1.5 months.
Unfortunately, ABR results showed no changes to the hearing thresholds of this group at any given frequency. However, when individual threshold shifts were investigated, NR administration in the female subgroup resulted in significantly reduced threshold shifts at 24 kHz. These results suggest that NR, even when administered in later life, still benefits already affected mice at certain frequencies.
Lipids as protectors
To investigate the underlying effects of NAD+ on ARHL, the team used RNA sequencing (RNA-seq) to determine the transcriptomic profiles of the cochleas in both the treated and untreated groups. Surprisingly, mitochondria-related RNA profiles were not significantly changed, meaning that the restorative effects might not be explained by a direct NAD+ role in mitochondria homeostasis.
However, they found that NR acted along the PPARγ-CIDEC, -PLIN1, -PCK1 axis, which potentially relates to lipid droplet formation. Combined with evidence that lipid droplets could reduce excessive reactive oxygen species, this led to the hypothesis that NR administration indirectly affects mitochondrial homeostasis and function via this mechanism, thereby protecting cochlear cells from cytotoxic damage.
Conclusion
Age-related hearing loss is one of the most common and cumbersome diseases in the elderly. In a mouse model, long-term supplementation of nicotinamide riboside can increase NAD+ level in the cochlea while preventing and halting ARHL. This suggests a potential novel treatment for age-related hearing loss, and it encourages further studies that translate this mouse evidence into clinical application.
Literature
[1] Okur, M. N. et al. Long-term NAD+ supplementation prevents the progression of age-related hearing loss in mice. bioRxiv, 2022.2008.2025.505332, doi:10.1101/2022.08.25.505332 (2022).
[2] Wang, J. & Puel, J. L. Presbycusis: An Update on Cochlear Mechanisms and Therapies. J Clin Med 9, doi:10.3390/jcm9010218 (2020).
[3] Okur, M. N. et al. Short-term NAD(+) supplementation prevents hearing loss in mouse models of Cockayne syndrome. NPJ Aging Mech Dis 6, 1, doi:10.1038/s41514-019-0040-z (2020).
