Nicotinamide riboside protects noise-induced hearing loss by recovering the hair cell ribbon synapses

The Role of Molecular and Cellular Aging Pathways on Age-Related Hearing Loss

Aging, a complex process marked by molecular and cellular changes, inevitably influences tissue and organ homeostasis and leads to an increased onset or progression of many chronic diseases and conditions, one of which is age-related hearing loss (ARHL). ARHL, known as presbycusis, is characterized by the gradual and irreversible decline in auditory sensitivity, accompanied by the loss of auditory sensory cells and neurons, and the decline in auditory processing abilities associated with aging. The extended human lifespan achieved by modern medicine simultaneously exposes a rising prevalence of age-related conditions, with ARHL being one of the most significant. While our understanding of the molecular basis for aging has increased over the past three decades, a further understanding of the interrelationship between the key pathways controlling the aging process and the development of ARHL is needed to identify novel targets for the treatment of AHRL. The dysregulation of molecular pathways (AMPK, mTOR, insulin/IGF-1, and sirtuins) and cellular pathways (senescence, autophagy, and oxidative stress) have been shown to contribute to ARHL. However, the mechanistic basis for these pathways in the initiation and progression of ARHL needs to be clarified. Therefore, understanding how longevity pathways are associated with ARHL will directly influence the development of therapeutic strategies to treat or prevent ARHL. This review explores our current understanding of the molecular and cellular mechanisms of aging and hearing loss and their potential to provide new approaches for early diagnosis, prevention, and treatment of ARHL.

Regulation of the p53/SLC7A11/GPX4 Pathway by Gentamicin Induces Ferroptosis in HEI-OC1 Cells

BACKGROUND

Gentamicin is a commonly used aminoglycoside antibiotic, with ototoxicity as a significant side effect. Ferroptosis, an iron-dependent form of cell death, has been implicated in a variety of disorders. Whether ferroptosis impacts gentamicin ototoxicity is not yet known. The current work used an in-vitro model to examine the influence of gentamicin-induced ferroptosis on cochlear hair cell damage and probable molecular biological pathways.

METHODS

House Ear Institute-Organ of Corti 1 (HEI-OC1) cells were treated with different concentrations of gentamicin for 24 hours, with or without ferrostatin-1 pretreatment, to observe gentamicin-induced ferroptosis. The role of p53/solute carrier family 7 member 11 (SLC7A11)/glutathione peroxidase 4 (GPX4) signaling in gentamicin-induced ferroptosis was explored by pretreating cells with the p53 inhibitor pifithrin-α (PFT-α). We investigated the effect of gentamicin on cells by assessing cell viability. Cellular proteins were isolated and Western blots were performed to detect changes in the expression of p53, SLC7A11, and GPX4. Fluorescence staining was used to assess levels of reactive oxygen species. An enzymatic detection kit was used to detect glutathione, Fe, and malondialdehyde markers.

RESULTS

Gentamicin reduced cell viability, glutathione content, and SLC7A11 and GPX4 protein levels, and increased levels of p53 protein, reactive oxygen species, malondialdehyde, and Fe. These effects were largely blocked by pretreatment with ferrostatin-1. Pretreatment with the p53 inhibitor PFT-α prevented the gentamicin-induced reduction in SLC7A11 and GPX4, which alleviated several features of ferroptosis including glutathione depletion, iron overload, and lipid peroxidation build-up.

CONCLUSION

Gentamicin induces ferroptosis in the HEI-OC1 cell line, and the mechanism may be related to the p53/SLC7A11/GPX4 signaling pathway.

Nicotinamide Riboside Regulates Chemotaxis to Decrease Inflammation and Ameliorate Functional Recovery Following Spinal Cord Injury in Mice

Changes in intracellular nicotinamide adenine dinucleotide (NAD+) levels have been observed in various disease states. A decrease in NAD+ levels has been noted following spinal cord injury (SCI). Nicotinamide riboside (NR) serves as the precursor of NAD+. Previous research has demonstrated the anti-inflammatory and apoptosis-reducing effects of NR supplements. However, it remains unclear whether NR exerts a similar role in mice after SCI. The objective of this study was to investigate the impact of NR on these changes in a mouse model of SCI. Four groups were considered: (1) non-SCI without NR (Sham), (2) non-SCI with NR (Sham +NR), (3) SCI without NR (SCI), and (4) SCI with NR (SCI + NR). Female C57BL/6J mice aged 6-8 weeks were intraperitoneally administered with 500 mg/kg/day NR for a duration of one week. The supplementation of NR resulted in a significant elevation of NAD+ levels in the spinal cord tissue of mice after SCI. In comparison to the SCI group, NR supplementation exhibited regulatory effects on the chemotaxis/recruitment of leukocytes, leading to reduced levels of inflammatory factors such as IL-1β, TNF-α, and IL-22 in the injured area. Moreover, NR supplementation notably enhanced the survival of neurons and synapses within the injured area, ultimately resulting in improved motor functions after SCI. Therefore, our research findings demonstrated that NR supplementation had inhibitory effects on leukocyte chemotaxis, anti-inflammatory effects, and could significantly improve the immune micro-environment after SCI, thereby promoting neuronal survival and ultimately enhancing the recovery of motor functions after SCI. NR supplementation showed promise as a potential clinical treatment strategy for SCI.

The Safety and Antiaging Effects of Nicotinamide Mononucleotide in Human Clinical Trials: an Update

The importance of nicotinamide adenine dinucleotide (NAD+) in human physiology is well recognized. As the NAD+ concentration in human skin, blood, liver, muscle, and brain are thought to decrease with age, finding ways to increase NAD+ status could possibly influence the aging process and associated metabolic sequelae. Nicotinamide mononucleotide (NMN) is a precursor for NAD+ biosynthesis, and in vitro/in vivo studies have demonstrated that NMN supplementation increases NAD+ concentration and could mitigate aging-related disorders such as oxidative stress, DNA damage, neurodegeneration, and inflammatory responses. The promotion of NMN as an antiaging health supplement has gained popularity due to such findings; however, since most studies evaluating the effects of NMN have been conducted in cell or animal models, a concern remains regarding the safety and physiological effects of NMN supplementation in the human population. Nonetheless, a dozen human clinical trials with NMN supplementation are currently underway. This review summarizes the current progress of these trials and NMN/NAD+ biology to clarify the potential effects of NMN supplementation and to shed light on future study directions.

Long-term NAD+ supplementation prevents the progression of age-related hearing loss in mice

Age-related hearing loss (ARHL) is the most common sensory disability associated with human aging. Yet, there are no approved measures for preventing or treating this debilitating condition. With its slow progression, continuous and safe approaches are critical for ARHL treatment. Nicotinamide Riboside (NR), a NAD+ precursor, is well tolerated even for long-term use and is already shown effective in various disease models including Alzheimer's and Parkinson's disease. It has also been beneficial against noise-induced hearing loss and in hearing loss associated with premature aging. However, its beneficial impact on ARHL is not known. Using two different wild-type mouse strains, we show that long-term NR administration prevents the progression of ARHL. Through transcriptomic and biochemical analysis, we find that NR administration restores age-associated reduction in cochlear NAD+ levels, upregulates biological pathways associated with synaptic transmission and PPAR signaling, and reduces the number of orphan ribbon synapses between afferent auditory neurons and inner hair cells. We also find that NR targets a novel pathway of lipid droplets in the cochlea by inducing the expression of CIDEC and PLIN1 proteins that are downstream of PPAR signaling and are key for lipid droplet growth. Taken together, our results demonstrate the therapeutic potential of NR treatment for ARHL and provide novel insights into its mechanism of action.

Nicotinamide Riboside, a Promising Vitamin B3 Derivative for Healthy Aging and Longevity: Current Research and Perspectives

Many studies have suggested that the oxidized form of nicotinamide adenine dinucleotide (NAD+) is involved in an extensive spectrum of human pathologies, including neurodegenerative disorders, cardiomyopathy, obesity, and diabetes. Further, healthy aging and longevity appear to be closely related to NAD+ and its related metabolites, including nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN). As a dietary supplement, NR appears to be well tolerated, having better pharmacodynamics and greater potency. Unfortunately, NR is a reactive molecule, often unstable during its manufacturing, transport, and storage. Recently, work related to prebiotic chemistry discovered that NR borate is considerably more stable than NR itself. However, immediately upon consumption, the borate dissociates from the NR borate and is lost in the body through dilution and binding to other species, notably carbohydrates such as fructose and glucose. The NR left behind is expected to behave pharmacologically in ways identical to NR itself. This review provides a comprehensive summary (through Q1 of 2023) of the literature that makes the case for the consumption of NR as a dietary supplement. It then summarizes the challenges of delivering quality NR to consumers using standard synthesis, manufacture, shipping, and storage approaches. It concludes by outlining the advantages of NR borate in these processes.

Alpha synuclein post translational modifications: potential targets for Parkinson's disease therapy?

Parkinson's disease (PD) is the most common neurodegenerative disorder with motor symptoms. The neuropathological alterations characterizing the brain of patients with PD include the loss of dopaminergic neurons of the nigrostriatal system and the presence of Lewy bodies (LB), intraneuronal inclusions that are mainly composed of alpha-synuclein (α-Syn) fibrils. The accumulation of α-Syn in insoluble aggregates is a main neuropathological feature in PD and in other neurodegenerative diseases, including LB dementia (LBD) and multiple system atrophy (MSA), which are therefore defined as synucleinopathies. Compelling evidence supports that α-Syn post translational modifications (PTMs) such as phosphorylation, nitration, acetylation, O-GlcNAcylation, glycation, SUMOylation, ubiquitination and C-terminal cleavage, play important roles in the modulation α-Syn aggregation, solubility, turnover and membrane binding. In particular, PTMs can impact on α-Syn conformational state, thus supporting that their modulation can in turn affect α-Syn aggregation and its ability to seed further soluble α-Syn fibrillation. This review focuses on the importance of α-Syn PTMs in PD pathophysiology but also aims at highlighting their general relevance as possible biomarkers and, more importantly, as innovative therapeutic targets for synucleinopathies. In addition, we call attention to the multiple challenges that we still need to face to enable the development of novel therapeutic approaches modulating α-Syn PTMs.

Oridonin employs interleukin 1 receptor type 2 to treat noise-induced hearing loss by blocking inner ear inflammation

NOD-like receptor protein 3 (NLRP3) inflammasomes trigger the inflammatory cascades and participate in various inflammatory diseases, including noise-induced hearing loss (NIHL) caused by oxidative stress. Recently, the anti-inflammatory traditional medicine oridonin (Ori) has been reported to provide hearing protection in mice after noise exposure by blocking the NLRP3-never in mitosis gene A-related kinase 7 (NEK7)-inflammasome complex assembly. Using RNA sequencing analysis, we further elucidated that interleukin 1 receptor type 2 (IL1R2) may be another crucial factor regulated by Ori to protect NIHL. We observed that IL1R2 expression was localized in spiral ganglion neurons, inner and outer hair cells, in Ori-treated mouse cochleae. Additionally, we confirmed that ectopic overexpression of IL1R2 in the inner ears of healthy mice using an adeno-associated virus delivery system significantly reduced noise-induced ribbon synapse lesions and hearing loss by blocking the "cytokine storm" in the inner ear. This study provides a novel theoretical foundation for guiding the clinical treatment of NIHL.

Blood levels of nicotinic acid negatively correlate with hearing ability in healthy older men

BACKGROUND

Age-related hearing loss (ARHL) is a common phenomenon observed during aging. On the other hand, the decrease in Nicotinamide adenine dinucleotide (NAD +) levels is reported to be closely related to the age-related declines in physiological functions such as ARHL in animal studies. Moreover, preclinical studies confirmed NAD + replenishment effectively prevents the onset of age-related diseases. However, there is a paucity of studies on the relationship between NAD⁺ metabolism and ARHL in humans.

METHODS

This study was analyzed the baseline results of our previous clinical trial, in which nicotinamide mononucleotide or placebo was administered to 42 older men (Igarashi et al., NPJ Aging 8:5, 2022). The correlations between blood levels of NAD⁺-related metabolites at baseline and pure-tone hearing thresholds at different frequencies (125, 250, 500, 1000, 2000, 4000, and 8000 Hz) in 42 healthy Japanese men aged > 65 years were analyzed using Spearman's rank correlation. Multiple linear regression analysis was performed with hearing thresholds as the dependent variable and age and NAD⁺-related metabolite levels as independent variables.

RESULTS

Positive associations were observed between levels of nicotinic acid (NA, a NAD⁺ precursor in the Preiss-Handler pathway) and right- or left-ear hearing thresholds at frequencies of 1000 Hz (right: r = 0.480, p = 0.001; left: r = 0.422, p = 0.003), 2000 Hz (right: r = 0.507, p < 0.001, left: r = 0.629, p < 0.001), and 4000 Hz (left: r = 0.366, p = 0.029). Age-adjusted multiple linear regression analysis revealed that NA was an independent predictor of elevated hearing thresholds (1000 Hz (right): p = 0.050, regression coefficient (β) = 1610; 1000 Hz (left): p = 0.026, β = 2179; 2000 Hz (right): p = 0.022, β = 2317; 2000 Hz (left): p = 0.002, β = 3257). Weak associations of nicotinic acid riboside (NAR) and nicotinamide (NAM) with hearing ability were observed.

CONCLUSIONS

We identified negative correlations between blood concentrations of NA and hearing ability at 1000 and 2000 Hz. NAD⁺ metabolic pathway might be associated with ARHL onset or progression. Further studies are warranted.

TRIAL REGISTRATION

The study was registered at UMIN-CTR (UMIN000036321) on 1st June 2019.

Chronic nicotinamide mononucleotide supplementation elevates blood nicotinamide adenine dinucleotide levels and alters muscle function in healthy older men

Preclinical studies have revealed that the elevation of nicotinamide adenine dinucleotide (NAD + ) upon the administration of nicotinamide mononucleotide (NMN), an NAD + precursor, can mitigate aging-related disorders; however, human data on this are limited. We investigated whether the chronic oral supplementation of NMN can elevate blood NAD + levels and alter physiological dysfunctions in healthy older participants. We administered 250 mg NMN per day to aged men for 6 or 12 weeks in a placebo-controlled, randomized, double-blind, parallel-group trial. Chronic NMN supplementation was well tolerated and caused no significant deleterious effect. Metabolomic analysis of whole blood samples demonstrated that oral NMN supplementation significantly increased the NAD + and NAD + metabolite concentrations. There were nominally significant improvements in gait speed and performance in the left grip test, which should be validated in larger studies; however, NMN exerted no significant effect on body composition. Therefore, chronic oral NMN supplementation can be an efficient NAD + booster for preventing aging-related muscle dysfunctions in humans.

Balancing NAD+ deficits with nicotinamide riboside: therapeutic possibilities and limitations

Alterations in cellular nicotinamide adenine dinucleotide (NAD+) levels have been observed in multiple lifestyle and age-related medical conditions. This has led to the hypothesis that dietary supplementation with NAD+ precursors, or vitamin B3s, could exert health benefits. Among the different molecules that can act as NAD+ precursors, Nicotinamide Riboside (NR) has gained most attention due to its success in alleviating and treating disease conditions at the pre-clinical level. However, the clinical outcomes for NR supplementation strategies have not yet met the expectations generated in mouse models. In this review we aim to provide a comprehensive view on NAD+ biology, what causes NAD+ deficits and the journey of NR from its discovery to its clinical development. We also discuss what are the current limitations in NR-based therapies and potential ways to overcome them. Overall, this review will not only provide tools to understand NAD+ biology and assess its changes in disease situations, but also to decide which NAD+ precursor could have the best therapeutic potential.

SIRT3-mediated deacetylation protects inner hair cell synapses in a H2O2-induced oxidative stress model in vitro

Oxidative stress is considered a driving event in the damage to inner hair cell (IHC) synapses. Mitochondrial deacetylase Sirtuin 3 (SIRT3) is an important regulator of reactive oxygen species (ROS) production. However, the effect of SIRT3 on IHC synapses remains elusive. In this study, we treated cochlear basilar membrane (CBM) with hydrogen peroxide (H₂O₂) to establish an oxidative stress model in vitro. The H₂O₂-induced CBM exhibited decreased the number of IHC synapses with low levels of ATP and mitochondrial membrane potential. Additionally, H₂O₂-incuded CBM showed markedly reduced levels of forkhead box protein O 3a (FOXO3a), superoxide dismutase 2 (SOD2), and isocitrate dehydrogenase 2 (IDH2), thereby increasing ROS generation. SIRT3 overexpression via administrating nicotinamide riboside in the H₂O₂-induced CBM protected IHC synapses against oxidative stress and inhibited hair cell apoptosis. We further demonstrated that SIRT3 overexpression led to upregulation of IDH2, and hypoacetylation of several proteins, such as FOXO3a and SOD2, which in turn reduced the levels of ROS and improved mitochondrial function. Collectively, these findings reveal that SIRT3 may be a potential therapeutic approach for damaged IHC synapses induced by oxidative stress.

A reduced form of nicotinamide riboside protects the cochlea against aminoglycoside-induced ototoxicity by SIRT1 activation

BACKGROUND

Nicotinamide adenine dinucleotide (NAD⁺), a coenzyme that plays crucial roles in many cellular processes, is a potential therapeutic target for various diseases. Dihydronicotinamide riboside (NRH), a novel reduced form of nicotinamide riboside, has emerged as a potent NAD⁺ precursor. Here, we studied the protective effects and underlying mechanism of NRH on aminoglycoside-induced ototoxicity.

METHODS

Auditory function and hair-cell (HC) morphology were examined to assess the effects of NRH on kanamycin-induced hearing loss. The pharmacokinetic parameters of NRH were measured in plasma and the cochlea using liquid chromatography tandem mass spectrometry. NAD⁺ levels in organ explant cultures were assessed to compare NRH with known NAD⁺ precursors. Immunofluorescence analysis was performed to detect reactive oxygen species (ROS) and apoptosis. We analyzed SIRT1 and 14-3-3 protein expression. EX527 and resveratrol were used to investigate the role of SIRT1 in the protective effect of NRH against kanamycin-induced ototoxicity.

RESULTS

NRH alleviated kanamycin-induced HC damage and attenuated hearing loss in mice. NRH reduced gentamicin-induced vestibular HC loss. Compared with NAD and NR, NRH produced more NAD⁺ in cochlear HCs and significantly ameliorated kanamycin-induced oxidative stress and apoptosis. NRH rescued the aminoglycoside-induced decreases in SIRT1 and 14-3-3 protein expression. Moreover, EX527 antagonized the protective effect of NRH on kanamycin-induced HC loss by inhibition of SIRT1, while resveratrol alleviated HC damage caused by EX527.

CONCLUSIONS

NRH ameliorates aminoglycoside-induced ototoxicity by inhibiting HC apoptosis by activating SIRT1 and decreasing ROS. NRH is an effective therapeutic option for aminoglycoside-induced ototoxicity.

Involvement of the SIRT1/PGC-1α Signaling Pathway in Noise-Induced Hidden Hearing Loss

Objective: To establish an animal model of noise-induced hidden hearing loss (NIHHL), evaluate the dynamic changes in cochlear ribbon synapses and cochlear hair cell morphology, and observe the involvement of the SIRT1/PGC-1α signaling pathway in NIHHL.

Methods: Male guinea pigs were randomly divided into three groups: control group, noise exposure group, and resveratrol treatment group. Each group was divided into five subgroups: the control group and 1 day, 1 week, 2 weeks, and 1 month post noise exposure groups. The experimental groups received noise stimulation at 105 dB SPL for 2 h. Hearing levels were examined by auditory brainstem response (ABR). Ribbon synapses were evaluated by inner ear basilar membrane preparation and immunofluorescence. The cochlear morphology was observed using scanning electron microscopy. Western blotting analysis and immunofluorescence was performed to assess the change of SIRT1/PGC-1α signaling. Levels of superoxide dismutase (SOD), malondialdehyde (MDA), catalase (CAT), ATP and SIRT1 activity were measured using commercial testing kits.

Results: In the noise exposure group, hearing threshold exhibited a temporary threshold shift (TTS), and amplitude of ABR wave I decreased irreversibly. Ribbon synapse density decreased after noise exposure, and the stereocilia were chaotic and then returned to normal. The expression and activity of SIRT1 and PGC-1α protein was lower than that in the control group. SOD, CAT and ATP were also influenced by noise exposure and were lower than those in the control group, but MDA showed no statistical differences compared with the control group. After resveratrol treatment, SIRT1 expression and activity showed a significant increase after noise exposure, compared with the noise exposure group. In parallel, the PGC-1α and antioxidant proteins were also significantly altered after noise exposure, compared with the noise exposure group. The damage to the ribbon synapses and the stereocilia were attenuated by resveratrol as well. More importantly, the auditory function, especially ABR wave I amplitudes, was also promoted in the resveratrol treatment group.

Conclusion: The SIRT1/PGC-1α signaling pathway and oxidative stress are involved in the pathogenesis of NIHHL and could be potential therapeutical targets in the future.

Heme Oxygenase-1 Protects Hair Cells From Gentamicin-Induced Death

Gentamicin ototoxicity can generate free radicals within the inner ear, leading to permanent damage to sensory hair cells (HCs) and eventually hearing loss. The following study examined the alterations of oxidative damage-related genes in the cochlea and important molecules responsible for oxidation following gentamicin injury in vitro. The RT² Profiler polymerase chain reaction (PCR) array was used to screen candidate targets for treatment to prevent hearing loss caused by gentamicin. We found that during gentamicin-induced death in HCs, Heme oxygenase-1 (HO-1) had a high fold change in the HCs of the cochlea. Moreover, the use of CoPPIX to induce HO-1 inhibited gentamicin-induced HC death, while HO-1 inhibitors ZnPPIX after CoPPIX reversed this process. Furthermore, the inhibitors of NF-E2-related factor-2 (Nrf2) reduced the expression of HO-1 and inhibited the protective effect of HO-1 after gentamicin, thus suggesting that the Nrf2/HO-1 axis might regulate gentamicin-associated ototoxicity. We further demonstrated that induction of HO-1 up-regulated the expression of Nrf2 in both cochlear and HEI-OC1 cells. In summary, these findings indicated that HO-1 protects HCs from gentamicin by up-regulating its expression in HCs and interacting with Nrf2 to inhibit reactive oxygen species (ROS).

The NADPARK study: A randomized phase I trial of nicotinamide riboside supplementation in Parkinson's disease

We conducted a double-blinded phase I clinical trial to establish whether nicotinamide adenine dinucleotide (NAD) replenishment therapy, via oral intake of nicotinamide riboside (NR), is safe, augments cerebral NAD levels, and impacts cerebral metabolism in Parkinson's disease (PD). Thirty newly diagnosed, treatment-naive patients received 1,000 mg NR or placebo for 30 days. NR treatment was well tolerated and led to a significant, but variable, increase in cerebral NAD levels-measured by ³¹phosphorous magnetic resonance spectroscopy-and related metabolites in the cerebrospinal fluid. NR recipients showing increased brain NAD levels exhibited altered cerebral metabolism, measured by ¹⁸fluoro-deoxyglucose positron emission tomography, and this was associated with mild clinical improvement. NR augmented the NAD metabolome and induced transcriptional upregulation of processes related to mitochondrial, lysosomal, and proteasomal function in blood cells and/or skeletal muscle. Furthermore, NR decreased the levels of inflammatory cytokines in serum and cerebrospinal fluid. Our findings nominate NR as a potential neuroprotective therapy for PD, warranting further investigation in larger trials.

mito-TEMPO Attenuates Oxidative Stress and Mitochondrial Dysfunction in Noise-Induced Hearing Loss via Maintaining TFAM-mtDNA Interaction and Mitochondrial Biogenesis

The excessive generation of reactive oxygen species (ROS) and mitochondrial damage have been widely reported in noise-induced hearing loss (NIHL). However, the specific mechanism of noise-induced mitochondrial damage remains largely unclear. In this study, we showed that acoustic trauma caused oxidative damage to mitochondrial DNA (mtDNA), leading to the reduction of mtDNA content, mitochondrial gene expression and ATP level in rat cochleae. The expression level and mtDNA-binding function of mitochondrial transcription factor A (TFAM) were impaired following acoustic trauma without affecting the upstream PGC-1α and NRF-1. The mitochondria-target antioxidant mito-TEMPO (MT) was demonstrated to enter the inner ear after the systemic administration. MT treatment significantly alleviated noise-induced auditory threshold shifts 3d and 14d after noise exposure. Furthermore, MT significantly reduced outer hair cell (OHC) loss, cochlear ribbon synapse loss, and auditory nerve fiber (ANF) degeneration after the noise exposure. In addition, we found that MT treatment effectively attenuated noise-induced cochlear oxidative stress and mtDNA damage, as indicated by DHE, 4-HNE, and 8-OHdG. MT treatment also improved mitochondrial biogenesis, ATP generation, and TFAM-mtDNA interaction in the cochlea. These findings suggest that MT has protective effects against NIHL via maintaining TFAM-mtDNA interaction and mitochondrial biogenesis based on its ROS scavenging capacity.

Strategies to protect against age-related mitochondrial decay: Do natural products and their derivatives help?

Mitochondria serve vital roles critical for overall cellular function outside of energy transduction. Thus, mitochondrial decay is postulated to be a key factor in aging and in age-related diseases. Mitochondria may be targets of their own decay through oxidative damage. However, treating animals with antioxidants has been met with only limited success in rejuvenating mitochondrial function or in increasing lifespan. A host of nutritional strategies outside of using traditional antioxidants have been devised to promote mitochondrial function. Dietary compounds are under study that induce gene expression, enhance mitochondrial biogenesis, mitophagy, or replenish key metabolites that decline with age. Moreover, redox-active compounds may now be targeted to mitochondria which improve their effectiveness. Herein we review the evidence that representative dietary effectors modulate mitochondrial function by stimulating their renewal or reversing the age-related loss of key metabolites. While in vitro evidence continues to accumulate that many of these compounds benefit mitochondrial function and/or prevent their decay, the results using animal models and, in some instances human clinical trials, are more mixed and sometimes even contraindicated. Thus, further research on optimal dosage and age of intervention are warranted before recommending potential mitochondrial rejuvenating compounds for human use.

Pharmacology and Potential Implications of Nicotinamide Adenine Dinucleotide Precursors

Coenzyme I (nicotinamide adenine dinucleotide, NAD⁺/NADH) and coenzyme II (nicotinamide adenine dinucleotide phosphate, NADP^+/NADPH) are involved in various biological processes in mammalian cells. NAD⁺ is synthesised through the de novo and salvage pathways, whereas coenzyme II cannot be synthesised de novo. NAD⁺ is a precursor of coenzyme II. Although NAD⁺ is synthesised in sufficient amounts under normal conditions, shortage in its supply due to over consumption and its decreased synthesis has been observed with increasing age and under certain disease conditions. Several studies have proved that in a wide range of tissues, such as liver, skin, muscle, pancreas, and fat, the level of NAD⁺ decreases with age. However, in the brain tissue, the level of NADH gradually increases and that of NAD⁺ decreases in aged people. The ratio of NAD⁺/NADH indicates the cellular redox state. A decrease in this ratio affects the cellular anaerobic glycolysis and oxidative phosphorylation functions, which reduces the ability of cells to produce ATP. Therefore, increasing the exogenous NAD⁺ supply under certain disease conditions or in elderly people may be beneficial. Precursors of NAD⁺ have been extensively explored and have been reported to effectively increase NAD⁺ levels and possess a broad range of functions. In this review article, we discuss the pharmacokinetics and pharmacodynamics of NAD⁺ precursors.

Sirtuin-3 Protects Cochlear Hair Cells Against Noise-Induced Damage via the Superoxide Dismutase 2/Reactive Oxygen Species Signaling Pathway

Mitochondrial oxidative stress is involved in hair cell damage caused by noise-induced hearing loss (NIHL). Sirtuin-3 (SIRT3) plays an important role in hair cell survival by regulating mitochondrial function; however, the role of SIRT3 in NIHL is unknown. In this study, we used 3-TYP to inhibit SIRT3 and found that this inhibition aggravated oxidative damage in the hair cells of mice with NIHL. Moreover, 3-TYP reduced the enzymatic activity and deacetylation levels of superoxide dismutase 2 (SOD2). Subsequently, we administered adeno-associated virus-SIRT3 to the posterior semicircular canals and found that SIRT3 overexpression significantly attenuated hair cell injury and that this protective effect of SIRT3 could be blocked by 2-methoxyestradiol, a SOD2 inhibitor. These findings suggest that insufficient SIRT3/SOD2 signaling leads to mitochondrial oxidative damage resulting in hair cell injury in NIHL. Thus, ameliorating noise-induced mitochondrial redox imbalance by intervening in the SIRT3/SOD2 signaling pathway may be a new therapeutic target for hair cell injury.

NAD+ and NAFLD - caution, causality and careful optimism

The prevalence of non-alcoholic fatty liver disease (NAFLD) is increasing worldwide, and new treatments are sorely needed. Nicotinamide adenine dinucleotide (NAD⁺ ) has been proposed as a potential target to prevent and reverse NAFLD. NAD⁺ is an important redox factor for energy metabolism and is used as a substrate by a range of enzymes, including sirtuins (SIRT), which regulates histone acetylation, transcription factor activity and mitochondrial function. NAD⁺ is also a precursor for reduced nicotinamide adenine dinucleotide phosphate (NADPH), which is an important component of the antioxidant defense system. NAD⁺ precursors such as nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) are available as over-the-counter dietary supplements, and oral supplementation with these precursors increases hepatic NAD⁺ levels and prevents hepatic lipid accumulation in pre-clinical models of NAFLD. NAD⁺ precursors have also been found to improve hepatic mitochondrial function and decrease oxidative stress in pre-clinical NAFLD models. NAD⁺ repletion also prevents NAFLD progression to non-alcoholic steatohepatitis (NASH), as NAD⁺ precursor supplementation is associated with decreased hepatic stellate cell activation, and decreased fibrosis. However, initial clinical trials have only shown modest effects when NAD⁺ precursors were administrated to people with obesity. We review the available pre-clinical investigations of NAD⁺ supplementation for targeting NAFLD, and discuss how data from the first clinical trials can be reconciled with observations from preclinical research.

Protection of Cochlear Ribbon Synapses and Prevention of Hidden Hearing Loss

In the auditory system, ribbon synapses are vesicle-associated structures located between inner hair cells (IHCs) and spiral ganglion neurons that are implicated in the modulation of trafficking and fusion of synaptic vesicles at the presynaptic terminals. Synapse loss may result in hearing loss and difficulties with understanding speech in a noisy environment. This phenomenon happens without permanent hearing loss; that is, the cochlear synaptopathy is "hidden." Recent studies have reported that synapse loss might be critical in the pathogenesis of hidden hearing loss. A better understanding of the molecular mechanisms of the formation, structure, regeneration, and protection of ribbon synapses will assist in the design of potential therapeutic strategies. In this review, we describe and summarize the following aspects of ribbon synapses: (1) functional and structural features, (2) potential mechanisms of damage, (3) therapeutic research on protecting the synapses, and (4) the role of synaptic regeneration in auditory neuropathy and the current options for synapse rehabilitation.