Effect of manganese and manganese plus noise on auditory function and cochlear structures

Sox2 overexpression alleviates noise-induced hearing loss by inhibiting inflammation-related hair cell apoptosis

Background

The transcription factor Sox2 plays important roles in the developmental processes of multiple organs and tissues. However, whether Sox2 can protect mature or terminally differentiated cells against injury is still unknown.

Methods

We investigated the roles of Sox2 in cochlear hair cells, which are terminally differentiated cells, using conditional transgenic mice and several hearing loss models.

Results

Sox2 overexpression dramatically mitigated the degree of cochlear hair cell loss when exposed to ototoxic drugs. Noise-induced apoptosis of cochlear hair cells and hearing loss were also significantly alleviated by Sox2 overexpression. Notably, noise-induced upregulation of pro-inflammatory factors such as TNF-α and IL6 was inhibited by Sox2 overexpression. Then we used lipopolysaccharide to clarify the effect of Sox2 on cochlear inflammation, and Sox2 overexpression significantly inhibited lipopolysaccharide-induced upregulation of pro-inflammatory factors and alleviated inflammation-related cochlear hair cell death.

Conclusions

These results demonstrate a novel protective role of Sox2 in mature and terminally differentiated cochlear hair cells by inhibiting inflammation.

Experimental animal models of drug-induced sensorineural hearing loss: a narrative review

Objective

This narrative review describes experimental animal models of sensorineural hearing loss (SNHL) caused by ototoxic agents.

Background

SNHL primarily results from damage to the sensory organ within the inner ear or the vestibulocochlear nerve (cranial nerve VIII). The main etiology of SNHL includes genetic diseases, presbycusis, ototoxic agents, infection, and noise exposure. Animal models with functional and anatomic damage to the sensory organ within the inner ear or the vestibulocochlear nerve mimicking the damage seen in humans are employed to explore the mechanism and potential treatment of SNHL. These animal models of SNHL are commonly established using ototoxic agents.

Methods

A literature search of PubMed, Embase, and Web of Science was performed for research articles on hearing loss and ototoxic agents in animal models of hearing loss.

Conclusions

Common ototoxic medications such as aminoglycoside antibiotics (AABs) and platinum antitumor drugs are extensively used to induce SNHL in experimental animals. The effect of ototoxic agents in vivo is influenced by the chemical mechanisms of the ototoxic agents, the species of animal, routes of administration of the ototoxic agents, and the dosage of ototoxic agents. Animal models of drug-induced SNHL contribute to understanding the hearing mechanism and reveal the function of different parts of the auditory system in humans.

The combined effects of occupational exposure to noise and other risk factors - a systematic review

PURPOSE

Noise-induced health effects exacerbate by many other risk factors. This systematic review aims at shedding light on the combined effects of co-exposure to occupational noise and other factors.

MATERIAL AND METHODS

A literature search in Web of Science, Scopus, PubMed, Science Direct, and Google Scholar, with appropriate keywords on combined effects of occupational noise, and co-exposure to noise and other factors, revealed 7928 articles which were screened by two researchers. A total of 775 articles were reviewed in full text. We found 149 articles that were relevant and had sufficient quality for analysis.

RESULTS

We identified 16 risk factors that exacerbate occupational noise-induced health effects. These factors were classified into four groups: chemical (carbon monoxide (CO), solvents, heavy metals, and other chemicals), physical (lighting, heat, vibration, and cold), personal (age, gender, genetics, smoking, medication, contextual diseases) and occupational (workload and shift work). Hearing loss, hypertension, reduced performance, and cardiovascular strains, are the most important risk factors combined effects due to concurrent exposure to noise and other risk factors.

CONCLUSION

Evidences of combined effects of solvents, vibration, heavy metals, CO, smoking, chemicals, aging, heat, and shiftwork were respectively stronger than for other factors. Most of the studies have investigated only the combined effects of risk factors on hearing, and the evidence for non-auditory effects is still limited, and more studies are warranted. Therefore, in the Hearing Conservation Programs, besides noise, aggravating factors of noise effects should also be taken into account.

Salidroside protects inner ear hair cells and spiral ganglion neurons from manganese exposure by regulating ROS levels and inhibiting apoptosis

Manganese (Mn) is an essential cofactor for many enzymes and thus plays an important role in normal growth and development. However, persistent exposure to high Mn concentrations can result in deleterious effects on not only the central nervous system but also peripheral nerves, including nerves associated with the auditory system. Our initial research on cochlear organotypic cultures in vitro showed that N-acetylcysteine (NAC) clearly decreases Mn-induced losses in hair cells (HCs), auditory nerve fibers (ANFs) and spiral ganglion neurons (SGNs) in a concentration-dependent manner. Salidroside (SAL) (p-hydroxyphenethyl-b-d-glucoside; C14H20O7), which is extracted from Rhodiola rosea L, has many pharmacological actions and antioxidative, antiaging, neuroprotective and anticancer effects. We hypothesized that SAL could also protect HCs, ANFs and SGNs from Mn injury. Cochlear organotypic cultures were treated with 1 mM Mn alone or combined with SAL (1-1000 μM). The neurofilament staining results showed that HCs, ANFs and SGNs were seriously damaged at high concentrations (100-1000 μM) but less damaged at low concentrations (1-10 μM). SAL may protect against 1 mM Mn-induced HC loss and axonal degeneration, suggesting that SAL could be a promising drug for clinical applications.

N-Acetylcysteine protects inner ear hair cells and spiral ganglion neurons from manganese exposure by regulating ROS levels

Manganese (Mn) is an indispensable cofactor for many enzymes and a basic factor for many reproductive and metabolic pathways. However, exposure to high concentrations of Mn can result in deleterious effects on the central nervous system and peripheral nerves, including nerves associated with the auditory system. Based on our studies of cochlear organotypic cultures, Mn exposure induces a significant loss of hair cells (HCs), auditory nerve fibers (ANFs) and spiral ganglion neurons (SGNs) in a concentration-dependent manner. Additionally, N-acetylcysteine (NAC), a glutathione (GSH) provider and a direct scavenger of reactive oxygen species (ROS), clearly decreases Mn-induced ROS accumulation, caspase-3 activation and TUNEL staining, which indicate increased cell survival. Based on these results, Mn exposure exerts ototoxic and neurotoxic effects on the auditory system. Furthermore, 20mM NAC may prevent 1mM Mn-induced hair cell loss and axonal degeneration, indicating that NAC could be a promising drug for clinical applications.