Age-regulated function of autophagy in the mouse inner ear

Abstract

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.

Autophagy in the Vertebrate Inner Ear

“…A temporal analysis of autophagic proteolysis in the cochlea and vestibule confirms the induction of autophagy in adults rather than E18.5 embryos. Moreover, there is significantly less SQSTM1/p62 at P270 than at E.18.5, whilst the relative LC3-II levels increase in the cochlea and vestibule (de Iriarte Rodríguez et al, 2015; Figure 2B). Indeed, autophagosomes are clearly visible in adult neurons of the spiral ganglion but not at earlier stages.…”

“…Significance: * P < 0.05, ** P < 0.01, and *** P < 0.001 vs. E18.5; # P < 0.05, ## P < 0.01, and ### P < 0.001 vs. P0; and & P < 0.05 and && P < 0.01 vs. P30–60. E, embryonic day and P, postnatal day (adapted from de Iriarte Rodríguez et al, 2015). …”

“…The transcriptome of the E18.5 mouse cochlea shows that a wide variety of Atg genes are expressed, underlining the relevance of autophagy at perinatal stages (de Iriarte Rodríguez et al, 2015). Furthermore, several key genes of the autophagic molecular machinery ( Becn1, Atg4b, Atg5 , and Atg9 ) are expressed in the mouse vestibule and cochlea throughout development and adulthood (de Iriarte Rodríguez et al, 2015; summarized in Figure 2B). The expression of these genes is significantly enhanced from the perinatal stages (E18.5 and P0) to adulthood (P30) as the inner ear acquires its complete functionality (Rueda et al, 1996).…”

“…LC3 is converted into the cytosolic form, LC3-I, by cleavage of ATG4B, and into the membrane associated form, LC3-II, by conjugation with phosphoethanolamine via ATG5 (and the remaining components of the ATG12-Ubl system). SQSTM1/p62 (p62) binds to ubiquitinated proteins and carries them to the autophagosome (adapted from de Iriarte Rodríguez et al, 2015). (B) Anatomy of the adult mouse inner ear.…”

Cell apoptosis, autophagy and necroptosis in osteosarcoma treatment

“…There are several main steps in autophagy: induction, vesicle nucleation and elongation, autophagosome formation and autolysosome formation [85]. To date, more than 30 autophagy- related genes (Atg) have been found to be involved, and several molecular complexes are essential in the process of autophagy [86, 87]. These are mainly divided into five groups according to its function in the process of autophagy: Atg1 protein kinase complex, class III phosphatidylinositol 3 kinase (PI3K)-Beclin 1 complex, Atg12 conjugation system, Atg8 conjugation system, and Atg9 [88].…”