microRNAs: the art of silencing in the ear

Abstract

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression through the RNA interference (RNAi) pathway and by inhibition of mRNA translation. miRNAs first made their appearance in the auditory and vestibular systems in 2005, with the discovery of a triad of hair cell-specific miRNAs later found to be involved in both human and mouse deafness. Since then, miRNAs have been implicated in other medical conditions related to these systems, such as cholesteatomas, vestibular schwannomas and otitis media. Due to the limitations in studying miRNAs and their targets derived from human inner ears, animal models are vital in this field of research. Therefore their role in inner ear development and function has been demonstrated by studies in zebrafish and mice. Transcriptomic and proteomic approaches have been undertaken to identify miRNAs and their targets. Finally, it has been suggested that miRNAs may be used in the future in regeneration of inner ear hair cells and ultimately play a role in therapeutics.

Figure 1. Timeline for miRNA expression during development and early postnatal stages of the inner ear for a subset of miRNAs

The most well-studied miRNAs, mir-96, -182- and -183, were first detected in the otic vesicle at E9.5, progressed to the cochlea-vestibule ganglion and the neural tube at E11.5, and by E17.5 had strong expression in the cochlear hair cells. The expression continued to at least p30, by some reports. The expression of other miRNAs detected by in situ hybridization are shown as well. Those marked with # were only examined at the stage indicated.

Figure 2. miRNA target identification

Computational and experimental approaches have been taken to identify target of miRNA in the inner ear. An example of such a miR-target pair is miR-135 and Psip, PC4- and SF-2 interacting protein/Ledgf (Elkan-Miller et al, 2011), which has been implicated in transcriptional regulation of stress-related genes, having an anti-apoptotic effect, involved in mRNA splicing, cell survival and is part of a fusion gene in leukaemia.

1. miR-135 is reduced in the cochlear hair cells, while its expression is high in vestibular hair cells.

2. Psip1, one of its targets, is expressed in the nucleus of the hair cells.

3. The pathways shown demonstrate potential inner ear functional pathways implicated in the miR135b-Psip regulatory network. Psip1 is a transcriptional regulator that plays a role in retinoic acid production (Fatma et al, 2004). Retinoic acid is crucial for hair cell development (Raz & Kelley, 1999). This model suggests that miR-135b regulation of Psip1 plays a role in hair cell development and survival.

Figure 3. miRNA involvement in three human ear disease phenotypes

1. The human ear is composed of the external, middle and inner ear.

2. Otitis media (OM) is a middle ear inflammation. Inflammation was induced in HMEEC cells and their miRNA levels were examined by microarray analysis, revealing up- and down-regulation of a number of miRNAs involved in inflammation and growth (Song et al, 2011).

3. A study examining cholesteatomas revealed upregulation of miR-21, leading to a model explaining the potential growth of this benign yet potentially harmful growth in the middle ear or the mastoid bone (Friedland et al, 2009).

4. Vestibular schwannomas have also been found to contain increased levels of miR-21 (Cioffi et al, 2010).