The cochlea is built to last a lifetime

Abstract

Orchestration of protein production and degradation and the regulation of protein lifetimes play a central role in many basic biological processes. Nearly all mammalian proteins are replenished by protein turnover in waves of synthesis and degradation. Protein lifetimes in vivo are typically measured in days, but a small number of extremely long-lived proteins (ELLPs) persist for months or even years. ELLPs are rare in all tissues but are enriched in tissues containing terminally differentiated post-mitotic cells and extracellular matrix. Consistently, emerging evidence suggests that the cochlea may be particularly enriched in ELLPs. Damage to ELLPs in specialized cell types, such as crystallin in the lens cells of the eye, causes organ failure such as cataracts. Similarly, damage to cochlear ELLPs is likely to occur with many insults, including acoustic overstimulation, drugs, anoxia, and antibiotics, and may play an underappreciated role in hearing loss. Furthermore, hampered protein degradation may contribute to acquired hearing loss. In this review, I highlight our knowledge of the lifetimes of cochlear proteins with an emphasis on ELLPs and the potential contribution that impaired cochlear protein degradation has on acquired hearing loss and the emerging relevance of ELLPs.

Keywords: Autophagy; Long-lived proteins; Lysosome; Noise induced hearing loss; Post-mitotic cells; Proteasome; Protein degradation; Proteostasis; Ubiquitin.

Figure 1.

Cartoon depicting canonical gene expression, protein synthesis, protein degradation, and protein lifetimes patterns. (A) Summary of gene expression and protein life cycle. (B) Schematic illustrating short- and long-lived proteins. Cartoons depicting: (Top) a short-lived single lifetime protein, note that all protein molecules in the pool are replaced; (Middle) a long-lived single lifetime protein; (Bottom) a multi-lifetime protein.

Figure 2.

Cartoon depiction of cochlear long-lived structures and regions with long-lived cells.