Autophagy in the Vertebrate Inner Ear.

Autophagy is a conserved catabolic process that results in the lysosomal degradation of cell components. During development, autophagy is associated with tissue and organ remodeling, and under physiological conditions it is tightly regulated as it plays a housekeeping role in removing misfolded proteins and damaged organelles. The vertebrate inner ear is a complex sensory organ responsible for the perception of sound and for balance. Cell survival, death and proliferation, as well as cell fate specification and differentiation, are processes that are strictly coordinated during the development of the inner ear in order to generate the more than a dozen specialized cell types that constitute this structure. Here, we review the existing evidence that implicates autophagy in the generation of the vertebrate inner ear. At early stages of chicken otic development, inhibiting autophagy impairs neurogenesis and causes aberrant otocyst morphogenesis. Autophagy provides energy for the clearing of dying cells and it favors neuronal differentiation. Moreover, autophagy is required for proper vestibular development in the mouse inner ear. The autophagy-related genes Becn1, Atg4g, Atg5, and Atg9, are expressed in the inner ear from late developmental stages to adulthood, and Atg4b mutants show impaired vestibular behavior associated to defects in otoconial biogenesis that are also common to Atg5 mutants. Autophagic flux appears to be age-regulated, augmenting from perinatal stages to young adulthood in mice. This up-regulation is concomitant with the functional maturation of the hearing receptor. Hence, autophagy can be considered an intracellular pathway fundamental for in vertebrate inner ear development and maturation.

Age-regulated function of autophagy in the mouse inner ear.

Autophagy is a highly conserved catabolic process essential for embryonic development and adult homeostasis. The autophagic machinery supplies energy by recycling intracellular components and facilitates the removal of apoptotic cells. In the inner ear, autophagy has been reported to play roles during early development in the chicken embryo and in the response to otic injury in the adult mouse. However, there are no studies on the expression of the autophagy machinery in the postnatal and adult inner ear. Insulin-like growth factor 1 (IGF-1) is one of the factors that regulate both otic development and cochlear postnatal maturation and function. Here, we hypothesised that autophagy could be one of the processes involved in the cochlear development and functional maturation. We report that autophagy-related genes (ATG) Becn1, Atg4g and Atg5 are expressed in the mouse cochlea, vestibular system and brainstem cochlear nuclei from late developmental stages to adulthood. Atg9 was studied in the mouse cochlea and showed a similar pattern. The presence of autophagic flux was confirmed by decreased sequestosome 1 (SQSTM1/p62) and increased relative levels of microtubule-associated protein light chain 3-II (LC3-II). Inner ear autophagy flux is developmentally regulated and is lower at perinatal stages than in the adult mouse, where an expression plateau is reached at the age of two-months, coinciding with the age at which full functional activity is reached. Expression is maintained in adult mice and declines after the age of twelve months. LC3B labelling showed that autophagy was primarily associated with spiral ganglion neurons. Over time, Igf1 wild type mice showed lower expression of genes coding for IGF-1 high affinity receptor and the family factor IGF-2 than null mice. Parallel analysis of autophagy machinery gene expression showed no significant differences between the genotypes over the lifespan of the null mice. Taken together, these results show that the autophagy machinery expression in the inner ear is regulated with age but is not compromised by the chronic absence of IGF-1. Our data also strongly support that the up-regulation of autophagy machinery genes is concomitant with the functional maturation of the inner ear.

C-Raf deficiency leads to hearing loss and increased noise susceptibility.

The family of RAF kinases transduces extracellular information to the nucleus, and their activation is crucial for cellular regulation on many levels, ranging from embryonic development to carcinogenesis. B-RAF and C-RAF modulate neurogenesis and neuritogenesis during chicken inner ear development. C-RAF deficiency in humans is associated with deafness in the rare genetic insulin-like growth factor 1 (IGF-1), Noonan and Leopard syndromes. In this study, we show that RAF kinases are expressed in the developing inner ear and in adult mouse cochlea. A homozygous C-Raf deletion in mice caused profound deafness with no evident cellular aberrations except for a remarkable reduction of the K(+) channel Kir4.1 expression, a trait that suffices as a cause of deafness. To explore the role of C-Raf in cellular protection and repair, heterozygous C-Raf (+/-) mice were exposed to noise. A reduced C-RAF level negatively affected hearing preservation in response to noise through mechanisms involving the activation of JNK and an exacerbated apoptotic response. Taken together, these results strongly support a role for C-RAF in hearing protection.