: prevention and treatment

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The fourth lecture (March 20, 2014) focused on the auditory system's means of defense against its aggressors, preventive and curative treatments.

Initially, additional information was provided on the biogenesis and functioning of peroxisomes.

The passage of proteins from peroxisomes to other cellular compartments was highlighted and documented: (i) ATM kinase, defective in ataxia telangiectasia, hitherto described as present in the nucleus, has recently been found to be associated with the peroxisome; (ii) nicotinamidase, localized in the peroxisome, is also present in the cytoplasm and nucleus ; (iii) NAD-dependent glycerol-3-phosphate dehydrogenase, which carries the PTS2 tag at its N-terminus, and is therefore addressed to the peroxisome matrix, is redirected to the nucleus in the event of osmotic stress (the dynamics of protein localization in the various cellular compartments is often an integral part of the stress response).

An excellent recent review summarizes the pathways of peroxisome biogenesis (Smith & Aitchison, 2013). Note that both 11 α and ß peroxynes are involved in peroxisome formation from a pre-existing peroxisome. Peroxyn 11ß is required for constitutive peroxisome proliferation (Schrader et al., 1998). Its overexpression induces elongation of pre-existing peroxisomes, followed by fission. Mice with the corresponding gene inactivated do not survive. Peroxisome 11α has been implicated in induced peroxisome proliferation. Recently, the Pex11αgene has been inactivated (Weng et al., 2013): the mice are viable, but peroxisome proliferation in the liver normally induced by a high-calorie diet is defective.

The peroxisome, which produces and degrades ROS, has sensors for the cell's redox status. They are far from all identified. They include: peroxyn 11ß, which is active only in reduced, monomeric form (Marshall et al., 1996); yeast GPx1, an atypical peroxyredoxin (Ohdate & Inoue, 2012); and a Prx homologous to mammalian Prx5 (Bener Aksam et al., 2008); peroxyn 5 behaves like a redox sensor, both for the charge of its cargo and for its release within the peroxisome (Bindoli et al., 2008): it can only release its cargo if the environment inside the peroxisome is reducing. Peroxisome 5 possesses three cysteine residues, whose substitution by serine residues has made it possible to establish their respective roles in these two activities (Ma et al., 2013). Note that the ubiquitination of peroxisome proteins, which determines their fate (degradation or recycling), could also depend on redox status; indeed that of peroxyne 5 has the particularity of occurring on a cysteine residue. Finally, peroxisome ageing appears to be accompanied by a decline in the reducing power of their internal contents. This is similar to the observation that, during the establishment of fibroblasts in culture, these cells contain little _H2O2 during the first passages, but accumulate_H2O2_afterseveral passages (Legakis et al., 2002).

Peroxisome-related diseases have been briefly discussed (Trompier et al., 2014). Diseases related to impaired peroxisome biogenesis - Zellweger syndrome, neonatal adrenoleukodystrophy, infantile Refsum disease, and rhizomelic chondrodysplasia punctata type 1 - affect neuronal migration during embryonic development, myelination of nerve fibers (Kassmann, 2014) and lead to inflammation associated with the overexpression of proinflammatory cytokines (interleukin 1, TNFα..). Auditory neurosensory damage is observed; it is attributed to defects in myelination, essential for the temporal precision of sensory signal propagation in the auditory pathways.

The dialogue between the peroxisome and mitochondria has also attracted our attention (Dirkx et al., 2005; Islinger et al., 2012).

Anti-oxidant therapies were discussed. N-acetylcysteine can reduce protein disulfide bridges, but competes in this function with thioredoxins and glutaredoxins, which are particularly effective. Its role would therefore be limited to cysteines located deep inside molecules, which these enzymes would be unable to reach due to their size. In contrast, N-acetylcysteine is able to block glutathionylation of cysteines and the creation of disulfide bridges between cysteines in sulfenic form.

To protect against oxidative stress, the most powerful antioxidant is glutathione. Other antioxidants are copper, zinc, manganese, selenium and iron, which are necessary for the activity of antioxidant enzymes. However, they can be spontaneously oxidized by the Fenton and Haber-Weiss reactions, creating hydroxyl radicals. Antioxidants include ubiquinones and cytochrome c. Ubiquinones play a role in ROS production, and protect lipids against peroxidation. Cytochrome c is believed to capture free electrons from the ^O2- anion in the mitochondrial respiratory chain. Vitamin E and vitamin C act as antioxidants. Vitamin E is fat-soluble; it binds to membranes and sequesters free radicals. Vitamin C is water-soluble; it can scavenge _H2O2 and OH radicals. D-methionine and ebselene (C13H9NOSe) have the same effect on oxidative stress.

As we have seen, SODs prevent the formation of the ^O2- anion that gives rise to _H2O2. In a relatively early study, the joint injection of an SOD and allopurinol (a xanthine oxidase inhibitor) in rats produced a significant protective effect against the effects of sound overexposure (Seidman et al., 1993). This injection protected these animals from the effects produced by exposure to broad-spectrum sound (100 Hz to 20 kHz), at an intensity of 90 dB for 60 hours; the rise in hearing threshold was less than in the absence of this treatment.

Hearing loss can be prevented by sound preconditioning. This is an interesting avenue to explore from a therapeutic perspective, as the protection obtained could be applied to a variety of auditory system stress situations. Sound preconditioning produces an increase in "chaperone" proteins. These proteins, discovered in response to heat shock, are also known as heat shock proteins (HSP) (May et al., 2013). The HSP70 protein protects the sensory cells of the utricle against the deleterious effects of aminoglycosides. It is secreted by supporting cells, and is thought to act on sensory cells. In a recent article (Roy et al., 2013), the sound preconditioning protocol used led to a significant rise in HSP32 and very high levels of HSP70, which protected the hearing of animals subjected to cisplatin or aminoglycosides...

Caloric restriction has a major effect on redox status. It slows the progression of age-related deafness, or presbycusis, in C57BL/6J mice (Someya & Prolla, 2010). It protects cochlear ganglion neurons and sensory cells in the cochlea of C57BL/6J mice. Its effect on auditory threshold involves sirtuin 3, located in mitochondria, whose expression increases in response to caloric diet. Sirtuins are ^{NAD+-dependent} deacetylases. They control transcription, acting on histones and numerous transcription factors; they also act on enzymes. When the gene encoding sirtuin-3 is inactivated, the effect of caloric diet on the progression of presbycusis remains limited. The calorie diet leads to a drop in oxidized glutathione, without any change in reduced glutathione. In sirtuin-3-deficient mice on the calorie diet, oxidized glutathione increases. During the caloric diet, sirtuin 3 appears to lead to a more reductive environment within the mitochondria, enhancing the glutathione-linked antioxidant defense system. Sirtuin 3 deacetylates mitochondrial isocitrate dehydrogenase 2, stimulating its enzymatic activity (conversion of ^NADP+ to NADPH). The two mitochondrial antioxidant pathways that convert _H2O2 into_H2O, the Grx pathway and the Prx-Trx pathway, use NADPH reductases. This is the first time that a molecular explanation has been put forward for the beneficial effect of caloric restriction on the progression of age-related hearing loss.