Introduction

Presbycusis, or age-related hearing loss, is the gradual loss of hearing during the aging process. Most people are familiar with this phenomenon due to the extreme prevalence of the condition among older individuals. Over 50% of individuals over the age of 75 experience this type of hearing loss, and there is currently no cure or measure of prevention besides recommendations to avoid loud noises earlier in life.

Hearing cells, called hair cells, are non-regenerative mechanosensory cells located in the inner ear that are solely responsible for the transduction of sound waves into electrical signals that can be understood and interpreted by the brain. Due to the non-regenerative nature of these cells, hair cells are particularly sensitive to the cellular damage associated with the aging of tissues.  Hair cells are also sensitive to damage caused by exposure to loud sounds (Seymour and Pereira, 2015). Over the course of a lifetime, it is difficult if not impossible to avoid loud noise exposure, even more so for individuals who work in industrial settings or live in urban environments (Menardo et al. 2012). This type of damage compounds with increasing exposure to loud noises, thus contributing to hearing loss with increasing age.

The initial cause of presbycusis is still unknown, although certain cellular processes associated with aging have been implicated as key players. One of these processes is chronic inflammation in aging tissues, known as “inflammaging” (Morrisette-Thomas et al., 2014). During normal cellular aging, metabolic processes can damage tissues, leading to the recruitment of immune cells to repair the damage. Cytokines and chemokines are recruited for this purpose, which then leads to low level chronic inflammation.  This inflammation can also increase concentrations of reactive oxygen species (ROS) which are harmful to cells and can also lead to further inflammation, representing a positive feedback loop (Morrisette-Thomas et al., 2014). Both inflammation and increased ROS load (called oxidative stress) are hallmarks of the cellular aging process, and thus are hypothesized to have a role in the onset of presbycusis (Benkafadar et al. 2019). Due to the feedback loop of inflammation and oxidative stress in aging cells, it is still unclear which process comes first in the initial stages of presbycusis and exactly how the two processes interact.

Zebrafish are a unique model that can be used to study presbycusis and other auditory disorders due to the zebrafish’s exterior hair cells that are homologous to the human inner ear hair cell. The lines of hair cells on the zebrafish’s body, called the lateral line, represent an easily manipulated and optically accessible hair cell model that can infer physiological properties about hair cells more generally. Additionally, lateral line hair cells are regenerative and in a constant state of turnover, providing unique access to hair cells of different ages which is crucial in studies of cellular aging processes (Coffin et al., 2010). Animal model studies are incredibly valuable in the study of presbycusis, as they may eventually lead to preventative or restorative treatments for one of the most prevalent auditory disorders globally.

Inflammatory cascades are commonly seen in aging cochleae

As tissue ages, cells become more susceptible to buildup of harmful cellular waste and chronic immune responses. Inflammation is a commonly observed hallmark in aging cells from many types of tissue (Morrisette-Thomas et al., 2014), and has become a hypothesized mechanism of the initial cascades of presbycusis (Shi et al., 2017). Investigations of cochlear tissue in aging rodents have reported activation of inflammosomes (Shi et al., 2017) and recruitment of pro-inflammatory cytokines, such as interleukin-1 beta (IL-1B), IL-18, and Tumor Necrosis Factor alpha (TNFa) (Menardo et al., 2012)(Morrisette-Thomas et al., 2014)(Shi et al., 2017). Researchers, however, have not yet determined the initial cause of this inflammatory recruitment and many disagree about the precise role of inflammation in the progression of presbycusis.

Studies have found that the chronic inflammation seen in aging cochleae is accompanied by activated apoptotic pathways (Menardo et al., 2012). It is still unclear whether the increased inflammation is the sole trigger for apoptosis in cochlear hair cells, but this finding has been replicated in studies using various models of presbycusis, particularly often in rodents (Menardo et al., 2012)(Shi et al., 2017).

While the chronic inflammation seen in aging cochleae could simply be due to the prolonged exposure to physical and chemical irritants, some hypothesize that the inflammation is a result of an immune response to oxidative stress in the cochlear tissues (Shi et al., 2017)(Menardo et al., 2012). While some may suggest that this role of oxidative stress is causative in the triggering of inflammatory processes, there are very few studies investigating the feedback loop between the two processes and thus it is still a worthwhile endeavor to continue investigating the role and precise timing of inflammation in the initial stages of presbycusis. Future studies should examine the timeline of inflammatory activation alongside ROS accumulation to further clarify the temporal order of the processes.

Oxidative stress as a potential precursor to inflammation in presbycusis

Mitochondrial dysfunction is another hallmark of cellular aging that has been implicated in presbycusis. Several mitochondrial changes take place in the cellular aging process, including the leakage of mitochondrial DNA (mtDNA) into the cytosol and the cytotoxic accumulation of ROS.

In murine models, aged cochleae have shown significantly increased concentrations of cytosolic mtDNA. Cytosolic mtDNA is accompanied by increased cytosolic cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING), which act together in the cGAS-STING pathway to induce type 1 interferon and inflammatory responses (Liu et al. 2022). Considering the abundance of evidence showing a chronic inflammatory response in the aging cochlea, findings of cGAS-STING pathway activation in cochlear hair cells are intriguing and may pose a hypothesized mechanism as to the initial cause of the inflammation.

ROS accumulation and reductions of endogenous antioxidants are also observed in aging hair cells and other cochlear tissues. It is still unclear what leads to the generation of these changes, whether it be the result of increased ROS production, reduced antioxidant generation, or in response to other mitochondrial dysfunction, and it is unknown whether inflammatory responses precede or follow ROS accumulation. While the temporal dynamics of these processes are still unclear, it is certain that both ROS and inflammation are apparent components of the cellular aging process. Many studies in rodent models have found ROS accumulation in aging cochlear tissues, and accompaniment of reductions in endogenous antioxidant activity (Menardo et al., 2012)(Jiang et al., 2007). In longitudinal studies, these increases in ROS and decreases in antioxidants worsen with advanced age (Jiang et al., 2007)(Benkafadar et al., 2019). A common target for analysis of antioxidant activity in presbycusis studies is superoxide dismutase (SOD), an endogenous antioxidant produced in response to increased ROS generation. Several murine models of presbycusis show reduced SOD concentration in aging cells (McFadden et al., 1999)(Menardo et al., 2012), and complete knockout of this antioxidant results in accelerated hair cell loss with age (McFadden et al., 1999), implicating an important role of antioxidants in the maintenance of hair cell longevity.

MPV17-/- zebrafish as a model for oxidative stress investigations

Genetic manipulation has proven to be an invaluable resource in producing animal models that reliably mimic human disorders. While there are several murine models of presbycusis, studies using these models are limited by the time necessary for cochlear tissues to age (between 12 and 23 months)(Jiang et al., 2007) and thus the throughput rate of such models.

 Taking experimental timeline into consideration, zebrafish pose themselves as a unique model in the study of presbycusis. With regenerative hair cells on the body’s surface, it is possible to watch the aging process of hair cells over a period of a few days, as compared to the months or years required in murine models. This timeline of hair cell turnover is an excellent way to observe aging hair cells and manipulate conditions in vivo.

One particular genetic line of zebrafish has recently proven useful in the studies of presbycusis. The MPV17 or “Roy Orbison” zebrafish line have a mutation in the mpv17 gene, which encodes for a mitochondrial transporter protein. This protein malformation leads to faulty ion transport across the mitochondrial membrane, and results in chronically expressed increased mitochondrial oxidative stress (Antonenkov et al., 2015). In murine models, mutation of this gene leads to induction of cell-death pathways in hair cells and early onset sensorineural hearing loss (Meyer zum Gottesberge et al., 2012) as well as accelerated aging of mitochondria (Antonenkov et al., 2015). In zebrafish, however, this mutation has only just begun to attract attention. One study using this line came to similar conclusions as those from murine investigations, finding chronically increased mitochondrial oxidative stress conditions in lateral line hair cells and increased vulnerability to mechanical damage (Holmgren & Sheets, 2021).

The MPV17 line of zebrafish may thus become a useful model for further investigation of oxidation states in the progression of presbycusis. With chronically high oxidative stress states and optically and physically accessible hair cells, it may be possible to observe the influence of antioxidants on hair cell longevity and perhaps even to better understand the relationship between oxidative stress and inflammation.

Conclusion

Despite the overwhelming prevalence of presbycusis in the population and the great efforts of auditory scientists, there is still no accepted theory of the initial causes of presbycusis. In studies of aging cochleae, many researchers have found evidence of chronic inflammatory responses and oxidative stress, but even the precise order in which the two processes take place is still under debate. Inflammation and oxidative stress are tied together in a positive feedback loop, which creates a problem in the search for an initial trigger of the presbycusis cascade. Therefore, both processes must be further investigated individually and in conjunction to better understand the complex relationship between the two, and more models of presbycusis should be developed to allow for a fuller understanding of hair cell aging across species. The MPV17 zebrafish line may be a promising candidate for furthering of oxidative stress studies, representing a high mitochondrial oxidation model. This model can allow for manipulation of inflammation in a high-oxidation state, as well as observational understanding of the effects of oxidative stress on hair cells as they age. Animal models are an imperative tool in the search for the initial causes of presbycusis, and studies using these models will eventually bring us closer to preventative and/or rescue therapeutics for one of the most common auditory disorders in the world.

[+] References

[+] Other Work By Emily Dale