Overexpression of X-Linked Inhibitor of Apoptotic Protein (XIAP) reduces age-related neuronal degeneration in the mouse cochlea

Overexpression of XIAP inhibits cisplatin-induced hair cell loss

Cisplatin is a platinum-containing drug with ototoxicity commonly used clinically and has significant efficacy against a variety of solid tumors. One of the most important mechanisms of ototoxicity is that cisplatin induces apoptosis of hair cells. According to relevant literature, X-linked inhibitor of apoptosis protein (XIAP, anti-apoptotic protein) could inhibit the apoptotic pathway. We hypothesized that this protein might protect cochlear hair cells from cisplatin-induced injury. To figure it out, we treated cochlea of normal mice with various concentrations of cisplatin to observe the response and morphology of hair cells and determine a reasonable concentration. Next, Western Blot and quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) experiments were conducted to make an investigation about the expression of XIAP protein and mRNA. In addition, we constructed and identified XIAP overexpressing mice. Finally, we treated cochlear tissues of normal and overexpressing mice with cisplatin to investigate the cyto-protection of XIAP on hair cells, respectively. It was found that 50 μmol/L cisplatin resulted in significant loss and disorganization of hair cells, while simultaneously downregulating the protein and mRNA of XIAP. In XIAP overexpressing mice, the loss and disorganization of hair cells were significantly lessened. These results showed that XIAP can lessen cisplatin-induced hair cell loss and play a role in otoprotection.

Mild Therapeutic Hypothermia and Putative Mechanisms of Hair Cell Survival in the Cochlea

Significance: Sensorineural hearing loss has significant implications for quality of life and risk for comorbidities such as cognitive decline. Noise and ototoxic drugs represent two common risk factors for acquired hearing loss that are potentially preventable. Recent Advances: Numerous otoprotection strategies have been postulated over the past four decades with primary targets of upstream redox pathways. More recently, the application of mild therapeutic hypothermia (TH) has shown promise for otoprotection for multiple forms of acquired hearing loss. Critical Issues: Systemic antioxidant therapy may have limited application for certain ototoxic drugs with a therapeutic effect on redox pathways and diminished efficacy of the primary drug's therapeutic function (e.g., cisplatin for tumors). Future Directions: Mild TH likely targets multiple mechanisms, contributing to otoprotection, including slowed metabolics, reduced oxidative stress, and involvement of cold shock proteins. Further work is needed to identify the mechanisms of mild TH at play for various forms of acquired hearing loss.

XIAP inhibits gentamicin-induced hair cell damage and ototoxicity through the caspase-3/9 pathway

Gentamicin (GM), an aminoglycoside antibiotic, is one commonly used clinical drugs with ototoxic side effects. One of the most principal mechanisms of its ototoxicity is that GM can activate caspase-mediated cell death pathways in the cochlea. Since the anti-apoptotic protein known as X-linked Inhibitor of Apoptosis Protein (XIAP) has been reported to directly bind to activated caspase protein and inhibit their activities, we hypothesized that it might protect cochlea hair cells from GM ototoxicity. To evaluate this hypothesis, postnatal day 2-3 (P2-3) transgenic (TG) mice, in which XIAP gene is over-expressed under a pure C57BL/6J genetic background was constructed. We first extracted the cochlea tissue of normal mice and treated them with different concentrations of GM, and the number of hair cells were observed to determine the concentration of GM used in subsequent experiments. Next, we used Western Blot experiment to examine the effect of GM on XIAP protein expression in normal mouse cochlea, and then Western Blot and RT-PCR experiments were used to identify the transgenic mice. Finally, immunofluorescence assays were used to detect the effect of GM on the expression of caspase protein and verify the protective effect of XIAP. We found that GM at a concentration of 0.5 mM significantly affected the function of cochlea hair cells, up-regulating the expression of cleaved-caspase-3 and cleaved-caspase-9 protein but down-regulating XIAP protein. In the cochlea tissues of TG mice, this effect of GM was suppressed, and the destruction of hair cells was significantly reduced, and the cleaved-caspase-3 and cleaved-caspase-9 proteins were significantly suppressed. These results suggested that XIAP reduces GM-induced ototoxicity and caspase-3/9 pathway is associated with this process.

Chronic Oral Selegiline Treatment Mitigates Age-Related Hearing Loss in BALB/c Mice

Age-related hearing loss (ARHL), a sensorineural hearing loss of multifactorial origin, increases its prevalence in aging societies. Besides hearing aids and cochlear implants, there is no FDA approved efficient pharmacotherapy to either cure or prevent ARHL. We hypothesized that selegiline, an antiparkinsonian drug, could be a promising candidate for the treatment due to its complex neuroprotective, antioxidant, antiapoptotic, and dopaminergic neurotransmission enhancing effects. We monitored by repeated Auditory Brainstem Response (ABR) measurements the effect of chronic per os selegiline administration on the hearing function in BALB/c and DBA/2J mice, which strains exhibit moderate and rapid progressive high frequency hearing loss, respectively. The treatments were started at 1 month of age and lasted until almost a year and 5 months of age, respectively. In BALB/c mice, 4 mg/kg selegiline significantly mitigated the progression of ARHL at higher frequencies. Used in a wide dose range (0.15-45 mg/kg), selegiline had no effect in DBA/2J mice. Our results suggest that selegiline can partially preserve the hearing in certain forms of ARHL by alleviating its development. It might also be otoprotective in other mammals or humans.

Toward Cochlear Therapies

Sensorineural hearing impairment is the most common sensory disorder and a major health and socio-economic issue in industrialized countries. It is primarily due to the degeneration of mechanosensory hair cells and spiral ganglion neurons in the cochlea via complex pathophysiological mechanisms. These occur following acute and/or chronic exposure to harmful extrinsic (e.g., ototoxic drugs, noise...) and intrinsic (e.g., aging, genetic) causative factors. No clinical therapies currently exist to rescue the dying sensorineural cells or regenerate these cells once lost. Recent studies have, however, provided renewed hope, with insights into the therapeutic targets allowing the prevention and treatment of ototoxic drug- and noise-induced, age-related hearing loss as well as cochlear cell degeneration. Moreover, genetic routes involving the replacement or corrective editing of mutant sequences or defected genes are showing promise, as are cell-replacement therapies to repair damaged cells for the future restoration of hearing in deaf people. This review begins by recapitulating our current understanding of the molecular pathways that underlie cochlear sensorineural damage, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. It then guides the reader through to the recent discoveries in pharmacological, gene and cell therapy research towards hearing protection and restoration as well as their potential clinical application.

Cholinergic Hypofunction in Presbycusis-Related Tinnitus With Cognitive Function Impairment: Emerging Hypotheses

Presbycusis (age-related hearing loss) is a potential risk factor for tinnitus and cognitive deterioration, which result in poor life quality. Presbycusis-related tinnitus with cognitive impairment is a common phenotype in the elderly population. In these individuals, the central auditory system shows similar pathophysiological alterations as those observed in Alzheimer's disease (AD), including cholinergic hypofunction, epileptiform-like network synchronization, chronic inflammation, and reduced GABAergic inhibition and neural plasticity. Observations from experimental rodent models indicate that recovery of cholinergic function can improve memory and other cognitive functions via acetylcholine-mediated GABAergic inhibition enhancement, nicotinic acetylcholine receptor (nAChR)-mediated anti-inflammation, glial activation inhibition and neurovascular protection. The loss of cholinergic innervation of various brain structures may provide a common link between tinnitus seen in presbycusis-related tinnitus and age-related cognitive impairment. We hypothesize a key component of the condition is the withdrawal of cholinergic input to a subtype of GABAergic inhibitory interneuron, neuropeptide Y (NPY) neurogliaform cells. Cholinergic denervation might not only cause the degeneration of NPY neurogliaform cells, but may also result in decreased AChR activation in GABAergic inhibitory interneurons. This, in turn, would lead to reduced GABA release and inhibitory regulation of neural networks. Reduced nAChR-mediated anti-inflammation due to the loss of nicotinic innervation might lead to the transformation of glial cells and release of inflammatory mediators, lowering the buffering of extracellular potassium and glutamate metabolism. Further research will provide evidence for the recovery of cholinergic function with the use of cholinergic input enhancement alone or in combination with other rehabilitative interventions to reestablish inhibitory regulation mechanisms of involved neural networks for presbycusis-related tinnitus with cognitive impairment.

Gene transfection mediated by polyethyleneimine-polyethylene glycol nanocarrier prevents cisplatin-induced spiral ganglion cell damage

Polyethyleneimine-polyethylene glycol (PEI-PEG), a novel nanocarrier, has been used for transfection and gene therapy in a variety of cells. In our previous study, we successfully carried out PEI-PEG-mediated gene transfer in spiral ganglion cells. It remains unclear whether PEI-PEG could be used for gene therapy with X-linked inhibitor of apoptosis protein (XIAP) in the inner ear. In the present study, we performed PEI-PEG-mediated XIAP gene transfection in the cochlea of Sprague-Dawley rats, via scala tympani fenestration, before daily cisplatin injections. Auditory brainstem reflex tests demonstrated the protective effects of XIAP gene therapy on auditory function. Immunohistochemical staining revealed XIAP protein expression in the cytoplasm of cells in the spiral ganglion, the organ of Corti and the stria vascularis. Reverse transcription-PCR detected high levels of XIAP mRNA expression in the cochlea. The present findings suggest that PEI-PEG nanocarrier-mediated XIAP gene transfection results in XIAP expression in the cochlea, prevents damage to cochlear spiral ganglion cells, and protects hearing.