A major effect QTL on chromosome 18 for noise injury to the mouse cochlear lateral wall

MicroRNAs in acquired sensorineural hearing loss

OBJECTIVE

This review summarises the current literature on the role of microRNAs in presbyacusis (age-related hearing loss) and sudden sensorineural hearing loss.

METHODS

Medline, PubMed, Web of Science and Embase databases were searched for primary English-language studies, published between 2000 and 2017, which investigated the role of microRNAs in the pathogenesis of presbyacusis or sudden sensorineural hearing loss. Quality of evidence was assessed using the National Institutes of Health quality assessment tool.

RESULTS

Nine of 207 identified articles, 6 of good quality, satisfied the review's inclusion criteria. In presbyacusis, microRNAs in pro-apoptotic and autophagy pathways are upregulated, while microRNAs in proliferative and differentiation pathways are downregulated. Evidence for microRNAs having an aetiological role in sudden hearing loss is limited.

CONCLUSION

A shift in microRNA expression, leading to reduced cellular activity and impaired inner-ear homeostasis, may contribute to the pathogenesis of presbyacusis.

The endocochlear potential as an indicator of reticular lamina integrity after noise exposure in mice

The endocochlear potential (EP) provides part of the electrochemical drive for sound-driven currents through cochlear hair cells. Intense noise exposure (110 dB SPL, 2 h) differentially affects the EP in three inbred mouse strains (C57BL/6 [B6], CBA/J [CBA], BALB/cJ [BALB]) (Ohlemiller and Gagnon, 2007, Hearing Research 224:34-50; Ohlemiller et al., 2011, JARO 12:45-58). At least for mice older than 3 mos, B6 mice are unaffected, CBA mice show temporary EP reduction, and BALB mice may show temporary or permanent EP reduction. EP reduction was well correlated with histological metrics for injury to stria vascularis and spiral ligament, and little evidence was found for holes or tears in the reticular lamina that might 'short out' the EP. Thus we suggested that the genes and processes that underlie the strain EP differences primarily impact cochlear lateral wall, not the organ of Corti. Our previous work did not test the range of noise exposure conditions over which strain differences apply. It therefore remained possible that the relation between exposure severity and acute EP reduction simply has a higher exposure threshold in B6 mice compared to CBA and BALB. We also did not test for age dependence. It is well established that young adult animals are especially vulnerable to noise-induced permanent threshold shifts (NIPTS). It is unknown, however, whether heightened vulnerability of the lateral wall contributes to this condition. The present study extends our previous work to multiple noise exposure levels and durations, and explicitly compares young adult (6-7 wks) and older mice (>4 mos). We find that the exposure level-versus-acute EP relation is dramatically strain-dependent, such that B6 mice widely diverge from both CBA and BALB. For all three strains, however, acute EP reduction is greater in young mice. Above 110 dB SPL, all mice exhibited rapid and severe EP reduction that is likely related to tearing of the reticular lamina. By contrast, EP-versus-noise duration examined at 104 dB suggested that different processes contribute to EP reduction in young and older mice. The average EP falls to a constant level after ∼7.5 min in older mice, but progressively decreases with further exposure in young mice. Confocal microscopy of organ of Corti surface preparations stained for phalloidin and zonula occludens-1 (ZO-1) indicated this corresponds to rapid loss of outer hair cells (OHCs) and formation of both holes and tears in the reticular lamina of young mice. In addition, when animals exposed at 119 dB were allowed to recover for 1 mo, only young B6 mice showed collapse of the EP to ≤5 mV. Confocal analysis suggested novel persistent loss of tight junctions in the lateral organ of Corti. This may allow paracellular leakage that permanently reduces the EP. From our other findings, we propose that noise-related lateral wall pathology in young CBA and BALB mice promotes hair cell loss and opening of the reticular lamina. The heightened vulnerability of young adult animals to noise exposure may in part reflect special sensitivity of the organ of Corti to acute lateral wall dysfunction at younger ages. This feature appears genetically modifiable.

RNA sequencing uncovers the key microRNAs potentially contributing to sudden sensorineural hearing loss

This study aimed to identify miRNAs that may contribute to the pathogenesis of sudden sensorineural hearing loss (SSNHL) by RNA-seq (RNA-sequencing).RNA was extracted from SSNHL patients and healthy volunteers, respectively. Sequencing was performed on HiSeq4000 platform. After filtering, clean reads were mapped to the human reference genome hg19. Differential expression analysis of miRNAs between the SSNHL samples and the normal samples was performed using DEseq to identify differentially expressed microRNAs (DEMs). The target genes of the DEMs were predicted using the online tool miRWalk, which were then mapped to DAVID (https://david.ncifcrf.gov/) for functional annotation based on GO database and for pathway enrichment analysis based on KEGG. Finally, a miRNA-target-protein-protein interaction (PPIs) network was constructed using the DEMs and their target genes with interaction.Differential expression analysis reveals 24 DEMs between the SSNHL group and control group. A total of 1083 target genes were predicted. GO functional annotation analysis reveals that the target genes in the top 10 terms are mainly related to the development of salivary glands, neurotransmission, dendritic development, and other processes. KEGG pathway enrichment analysis reveals that the target genes were functionally enriched in pathways arachidonic acid metabolism, complement and coagulation cascades, linoleic acid metabolism, and MAPK signaling pathway. In the miRNA-target-PPI network, hsa-miR-34a/548n/15a/143/23a/210/1255a/18b/ /1180/99b had the most target genes; genes YWHAG, GSK3B, CDC42, NR3C1, LCK, UNC119, SIN3A, and NFKB2, interact with most other genes among all the predicted target genes.Hsa-miR-34a/15a/23a/210/18b/548n/143 is likely to have a role in the pathogenesis of SSNHL.

QTL Mapping of Endocochlear Potential Differences between C57BL/6J and BALB/cJ mice

We reported earlier that the endocochlear potential (EP) differs between C57BL/6J (B6) and BALB/cJ (BALB) mice, being lower in BALBs by about 10 mV (Ohlemiller et al. Hear Res 220: 10-26, 2006). This difference corresponds to strain differences with respect to the density of marginal cells in cochlear stria vascularis. After about 1 year of age, BALB mice also tend toward EP reduction that correlates with further marginal cell loss. We therefore suggested that early sub-clinical features of the BALB stria vascularis may predispose these mice to a condition modeling Schuknecht's strial presbycusis. We further reported (Ohlemiller et al. J Assoc Res Otolaryngol 12: 45-58, 2011) that the acute effects of a 2-h 110 dB SPL noise exposure differ between B6 and BALB mice, such that the EP remains unchanged in B6 mice, but is reduced by 40-50 mV in BALBs. In about 25 % of BALBs, the EP does not completely recover, so that permanent EP reduction may contribute to noise-induced permanent threshold shifts in BALBs. To identify genes and alleles that may promote natural EP variation as well as noise-related EP reduction in BALB mice, we have mapped related quantitative trait loci (QTLs) using 12 recombinant inbred (RI) strains formed from B6 and BALB (CxB1-CxB12). EP and strial marginal cell density were measured in B6 mice, BALB mice, their F1 hybrids, and RI mice without noise exposure, and 1-3 h after broadband noise (4-45 kHz, 110 dB SPL, 2 h). For unexposed mice, the strain distribution patterns for EP and marginal cell density were used to generate preliminary QTL maps for both EP and marginal cell density. Six QTL regions were at least statistically suggestive, including a significant QTL for marginal cell density on chromosome 12 that overlapped a weak QTL for EP variation. This region, termed Maced (Marginal cell density QTL) supports the notion of marginal cell density as a genetically influenced contributor to natural EP variation. Candidate genes for Maced notably include Foxg1, Foxa1, Akap6, Nkx2-1, and Pax9. Noise exposure produced significant EP reductions in two RI strains as well as significant EP increases in two RI strains. QTL mapping of the EP in noise-exposed RI mice yielded four suggestive regions. Two of these overlapped with QTL regions we previously identified for noise-related EP reduction in CBA/J mice (Ohlemiller et al. Hear Res 260: 47-53, 2010) on chromosomes 5 and 18 (Nirep). The present map may narrow the Nirep interval to a ~10-Mb region of proximal Chr. 18 that includes Zeb1, Arhgap12, Mpp7, and Gjd4. This study marks the first exploration of natural gene variants that modulate the EP. Their orthologs may underlie some human hearing loss that originates in the lateral wall.

Genetic background of Prop1(df) mutants provides remarkable protection against hypothyroidism-induced hearing impairment

Hypothyroidism is a cause of genetic and environmentally induced deafness. The sensitivity of cochlear development and function to thyroid hormone (TH) mandates understanding TH action in this sensory organ. Prop1(df) and Pou1f1(dw) mutant mice carry mutations in different pituitary transcription factors, each resulting in pituitary thyrotropin deficiency. Despite the same lack of detectable serum TH, these mutants have very different hearing abilities: Prop1(df) mutants are mildly affected, while Pou1f1(dw) mutants are completely deaf. Genetic studies show that this difference is attributable to the genetic backgrounds. Using embryo transfer, we discovered that factors intrinsic to the fetus are the major contributor to this difference, not maternal effects. We analyzed Prop1(df) mutants to identify processes in cochlear development that are disrupted in other hypothyroid animal models but protected in Prop1(df) mutants by the genetic background. The development of outer hair cell (OHC) function is delayed, but Prestin and KCNQ4 immunostaining appear normal in mature Prop1(df) mutants. The endocochlear potential and KCNJ10 immunostaining in the stria vascularis are indistinguishable from wild type, and no differences in neurofilament or synaptophysin staining are evident in Prop1(df) mutants. The synaptic vesicle protein otoferlin normally shifts expression from OHC to IHC as temporary afferent fibers beneath the OHC regress postnatally. Prop1(df) mutants exhibit persistent, abnormal expression of otoferlin in apical OHC, suggesting delayed maturation of synaptic function. Thus, the genetic background of Prop1(df) mutants is remarkably protective for most functions affected in other hypothyroid mice. The Prop1(df) mutant is an attractive model for identifying the genes that protect against deafness.

Different cellular and genetic basis of noise-related endocochlear potential reduction in CBA/J and BALB/cJ mice

The acute and permanent effects of noise exposure on the endocochlear potential (EP) and cochlear lateral wall were evaluated in BALB/cJ (BALB) inbred mice, and compared with CBA/J (CBA) and C57BL/6 (B6) mice. Two-hour exposure to broadband noise (4-45 kHz) at 110 dB SPL leads to a approximately 50 mV reduction in the EP in BALB and CBA, but not B6. EP reduction in BALB and CBA is reliably associated with characteristic acute cellular pathology in stria vascularis and spiral ligament. By 8 weeks after exposure, the EP in CBA mice has returned to normal. In BALBs, however, the EP remains depressed by an average approximately 10 mV, so that permanent EP reduction contributes to permanent threshold shifts in these mice. We recently showed that the CBA noise phenotype in part reflects the influence of a large effect quantitative trait locus on Chr. 18, termed Nirep (Ohlemiller et al., Hear Res 260:47-53, 2010b). While CBA "EP susceptibility" alleles are dominant to those in B6, examination of (B6 × BALB) F1 hybrid mice and (F1 × BALB) N2 backcross mice revealed that noise-related EP reduction and associated cell pathology in BALBs are inherited in an autosomal recessive manner, and are dependent on multiple genes. Moreover, while N2 mice formed from B6 and CBA retain strong correspondence between acute EP reduction, ligament pathology, and strial pathology, N2s formed from B6 and BALB include subsets that dissociate pathology of ligament and stria. We conclude that the genes and cascades that govern the very similar EP susceptibility phenotypes in BALB and CBA mice need not be the same. BALBs appear to carry alleles that promote more pronounced long term effects of noise on the lateral wall. Separate loci in BALBs may preferentially impact stria versus ligament.

Divergence of noise vulnerability in cochleae of young CBA/J and CBA/CaJ mice

CBA/CaJ and CBA/J inbred mouse strains appear relatively resistant to age- and noise-related cochlear pathology, and constitute the predominant 'good hearing' control strains in mouse studies of hearing and deafness. These strains have often been treated as nearly equivalent in their hearing characteristics, and have even been mixed in some studies. Nevertheless, we recently showed that their trajectories with regard to age-associated cochlear pathology diverge after one year of age (Ohlemiller et al., 2010a). We also recently reported that they show quite different susceptibility to cochlear noise injury during the 'sensitive period' of heightened vulnerability to noise common to many models, CBA/J being far more vulnerable than CBA/CaJ (Fernandez et al., 2010 J. Assoc. Res. Otolaryngol. 11:235-244). Here we explore this relation in a side-by-side comparison of the effect of varying noise exposure duration in young (6 week) and older (6 month) CBA/J and CBA/CaJ mice, and in F1 hybrids formed from these. Both the extent of permanent noise-induced threshold shifts (NIPTS) and the probability of a defined NIPTS were determined as exposure to intense broadband noise (4-45 kHz, 110 dB SPL) increased by factors of two from 7 s to 4 h. At 6 months of age the two strains appeared very similar by both measures. At 6 weeks of age, however, both the extent and probability of NIPTS grew much more rapidly with noise duration in CBA/J than in CBA/CaJ. The 'threshold' exposure duration for NIPTS was <1.0 min in CBA/J versus >4.0 min in CBA/CaJ. F1 hybrid mice showed both NIPTS and hair cell loss similar to that in CBA/J. This suggests that dominant-acting alleles at unknown loci distinguish CBA/J from CBA/CaJ. These loci have novel effects on hearing phenotype, as they come into play only during the sensitive period, and may encode factors that demarcate this period. Since the cochlear cells whose fragility defines the early window appear to be hair cells, these loci may principally impact the mechanical or metabolic resiliency of hair cells or the organ of Corti.