Table 1.
Studies assessing the association between otologic diseases and ER stress.
| Associated Diseases | Study Design | Species and/or Tissue Type | Detection Method | Target Gene(s) or Pathway(s) Associated with ER Stress | Results/Conclusion | |
|---|---|---|---|---|---|---|
| Dong SH, et al. [32] | COM | Cross-sectional study | Human: inflammatory mucosal tissue in the middle ear cavity | qRT-PCR | IRE1α, sXBP1, PERK, CHOP, ATF6, BiP | sXBP1 appears to be involved in COM-associated inflammation, including otorrhea. ATF6 is associated with the destruction of ossicles. |
| Kang DW, et al. [35] | OME | Cross-sectional study | Human: effusion in the middle ear cavity | qRT-PCR | IRE1α, sXBP1, PERK, CHOP, ATF6, BiP | Expression of CHOP was higher in the otitis-prone group than in the non-otitis-prone group. The most common type of fluid was mucoid, and IRE1α expression was higher in other fluid types. |
| Herranen A, et al. [36] | HL | Animal model study | Mice: OHCs in the cochlea | Histochemistry, immunostaining, cytocochleogram | MANF | MANF inactivation resulted in the death of only OHCs. This robust OHC loss was accompanied by strongly elevated hearing thresholds. A MANF deficiency became detrimental when accompanied by gene mutations that predispose to HL through intensification of ER dyshomeostasis. |
| Fujinami Y, et al. [37] | HL (3-NP–induced) | Animal model study | Rat | qRT-PCR, immunohistochemistry | CHOP, ATF4 | In a 3-NP animal model of acute HL, expression of the ER stress marker genes CHOP and ATF4 were upregulated in the cochlear lateral wall in cases where primary injuries were detected, suggesting that ER stress plays a role during the onset or exacerbation of HL in some types of auditory disorders. |
| Fujinami Y, et al. [38] | HL (tunicamycin-induced) | Animal model study | Rat | Light microscopy, fluorescence microscopy, qRT-PCR, TEM | CHOP, ATF4, BiP, GRP94, GAPDH | In the tunicamycin-induced HL model, subacute and progressive HL was observed at all sound frequencies studied, and induction of ER stress marker genes was noted in the cochlea. Among cells in the cochlea, OHCs were the most sensitive to ER stress. This study also showed degeneration of subcellular organelles of inner hair cells and nerve endings of spiral ganglion cells. |
| Kishino, A, et al. [39] | HL (tunicamycin-induced) | In vitro cell line study | HEI-OC1 cell line | Fluorescence microscopy, flow cytometry, Western blotting, co-immunoprecipitation, TEM, RT-PCR | IRE1α, XBP1, FoxO1, LC3-II | Tunicamycin-induced ER stress resulted in IRE1α-mediated XBP1 mRNA splicing and autophagy. XBP1 mRNA splicing and FoxO1 were found to be involved in ER stress-induced autophagy. This inference was based on the observation that expression of LC3-II was suppressed by knockdown of IRE1α, XBP1, or FoxO1. The relationship between XBP1 and FoxO1 revealed by siRNA-mediated knockdown showed a paradoxical negative regulation of FoxO1 expression by XBP1. |
| Tu Y, et al. [40] | HL (kanamycin-induced) | Animal model study | Rats | TEM, immunohistochemistry, Western blotting | BiP, IRE1α, ATF6α, p-PERK, p-eIF2α, CHOP, caspase-12 | ER stress contributed to kanamycin-induced apoptosis of spiral ganglion neurons. Kanamycin treatment-induced apoptosis of spiral ganglion neurons was mediated, at least in part, by ER stress-induced upregulation of caspase-12 and CHOP. |
| Oishi N, et al. [41] | HL (aminoglycoside-induced) | Animal model study | XBP1-haploinsufficient (XBP1+/−) mice | Microarray analysis, proteome analysis, histochemistry, immunofluorescence assay, RT-PCR, Western blotting | XBP1 | Intra-tympanic aminoglycoside treatment caused high-frequency HL in XBP1+/− mice, but not in wild-type littermates. Densities of spiral ganglion cells and synaptic ribbons were decreased in gentamicin-treated XBP1+/− mice, whereas sensory cells were preserved. These results suggest that aminoglycoside-induced ER stress and cell death in spiral ganglion neurons are mitigated by XBP1, masking aminoglycoside neurotoxicity at the organismal level. |
| Kishino a, et al. [42] | HL (tunicamycin-induced) |
In vitro cell line study | HEI-OC1 cell line | Western blotting, co-immunoprecipitation, TEM, flow cytometry | Caspase-3, caspase-8, caspase-9 | ER stress not only initiated caspase-9/caspase-3–dependent intrinsic apoptosis, it also induced RIPK1-dependent necroptosis in auditory cells. ER stress-induced necroptosis was dependent on the induction of RIPK1 in auditory cells, which was negatively regulated by caspase-8. Thus, caspase-8 regulates ER stress-induced necroptosis independent of the apoptosis pathway in auditory cells. |
| Park C, et al. [43] | HL (pyridoxine-induced) | Ex vivo animal and in vitro cell line study | Rat (organ of Corti explants), VOT-33 cell line | Fluorescence microscopy, immunostaining, flow cytometry, Western blotting | PERK, ATF4, BiP, CHOP, caspase-12 | Pyridoxine induced VOT-33 apoptosis, as indicated by accumulation of a sub-G0/G1 fraction, caspase-3 activation, and PARP cleavage. In addition, pyridoxine induced ROS generation and alteration of MPT, including expression of Bcl-2 family protein and consequent accumulation of Ca2+ and changes in expression of the ER stress-related proteins, PERK, caspase-12, BiP, and CHOP. |
| Xia K, et al. [44] | HL | Animal model study | Mouse (Cx31wt vs. Cx31 mutant) | Immunofluorescence, immunostaining, immunoblotting | BiP | Exogenously expressed Cx31wt forms functional gap junction at cell-cell contacts. In contrast, HL-associated Cx31 mutants resided primarily in the ER and intracellular Golgi-like punctate structures and failed to mediate Lucifer yellow transfer. Expression of Cx31 mutants, but not Cx31wt, led to upregulation of the ER chaperone BiP and increased Cx31–BiP association, indicating induction of ER stress. |
| Li J, et al. [45] | HL | Animal model study | Mice | qRT-PCR, immunofluorescence, flow cytometry, Western blotting | CHOP, s-XBP1, BiP, caspase-3, Tmtc4 | Inactivation of the gene Tmtc4, which is broadly expressed in the mouse cochlea, caused acquired HL in mice. TMTC4 was enriched in the ER and functioned by regulating Ca2+ dynamics and the UPR. ISRIB, a small-molecule modulator of the UPR and stress response that activates eIF2, prevented HL in a mouse model. Moreover, mice with homozygous inactivation of both Tmtc4 and Ddit3 (encoding CHOP) caused less HL than knockout of Tmtc4 alone. |
| Hu J, et al. [46] | HL | Animal model study | Mice (Cdh23erl/erl mutant mice vs. control B6 mice) | RT-PCR, Western blotting, immunofluorescence staining, SEM | BiP, CHOP, p-eIF2α, caspase-3 | Cdh23 was unable to fully reach the top of hair bundles and became co-localized with BiP in subapical regions of OHCs in Cdh23erl/erl mutant mice. The PERK arm of the UPR was activated in the cochleae of Cdh23erl/erl mutant mice. Disruption of the gene encoding CHOP (Ddit3) protected OHCs and preserved hearing in Cdh23erl/erl mutant mice. Sal prevented HL and protected against OHC death in Cdh23erl/erl mutant mice. |
| Xue Q, et al. [47] | HL (noise-induced) | Animal model study (case-control) | Guinea pig | Immunohistochemistry, Western blotting, fluorescence microscopy | BiP, CHOP | BiP levels were significantly higher in all three experimental groups compared with the control group. CHOP levels were increased 1 day after exposure, reaching a peak that was maintained until 4 days before returning to baseline levels by 14 days post-exposure. |
| Wang W, et al. [48] | HL (age-related) | Animal model study | Mice (C57BL/6 mouse model of presbycusis) | Immunofluorescence staining, Western blotting | BiP, CHOP, caspase-12, caspase-3, caspase-8, PARP-1 | BiP expression was reduced and the number of ubiquitinated proteins was increased in cochleae of aged mice. In aged mice, expression of the ER-related pro-apoptotic factor CHOP was markedly increased. Caspase-9, caspase-3 and PARP1 cleavage were significantly increased in aged cochleae. |
| Kalinec GM, et al. [49] | HL (APAP-induced) | In vitro cell line study (case-control) | HEI-OC1 cell line | Western blotting, RT-PCR, immunofluorescence, TEM, genomic and proteomic studies | CHOP, BiP, eIF2α, p-eIF2α, ATF4, XBP1 | APAP induced expression of the ER stress markers, CHOP and BiP, in the HEI-OC1 cell line. The cytotoxic effect of APAP on HEI-OC1 cells was dependent on eIF2a and CHOP, but independent of IRE1-, ATF6-, and ATF4-mediated signaling. |
| Zong S, et al. [50] | HL (cisplatin-induced) | Animal model study | Rat | Immunofluorescence, Western blotting | CHOP, BiP, caspase-3, caspase-9, caspase-12 | Expression of active caspase-12 was upregulated in cochlear cells of cisplatin-treated rats, indicative of cisplatin-induced activation of ER-specific apoptosis. Increased expression of CHOP and cleaved caspase-9 suggested a close relationship between severe ER stress and mitochondria-dependent apoptosis in cochlear cells of cisplatin-treated rats. |
| Schiavon E, et al. [51] | HL(bilirubin-induiced) | Animal model study | Mice | Immunohistochemistry, immuno blotting, whole genome gene expression analysis | BiP, PERK, IRE1 | Bilirubin increased the hearing threshold through mechanisms such as increased ER stress, induction of inflammatory responses, and activation of the NF-kB pathway. |
ER, endoplasmic reticulum; COM, chronic otitis media; OME, otitis media with effusion; RT-PCR, reverse transcription polymerase chain reaction; IRE1α, inositol-requiring enzyme 1α; XBP1, X-box-binding protein 1; PERK, endoplasmic reticulum kinase; CHOP, C/EBP-homologous; ATF, activating transcription factor; BiP, immunoglobulin heavy chain-binding protein; HL, hearing loss; OHC, outer hair cell; MANF, mesencephalic astrocyte-derived neurotrophic factor; 3-NP, mitochondrial toxin 3-nitropropionic acid; qRT-PCR, quantitative RT-PCR; TEM, transmission electron microscopy; GRP, glucose-regulated protein; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; HEI-OC1, House Ear Institute-Organ of Corti 1; FoxO1, forkhead box O1; LC3-II, microtubule associated protein 1 light chain 3-II; siRNA, small interfering RNA; eIF, eukaryotic initiation factor; RIPK1, receptor-interacting serine/threonine kinase 1; VOT-N33, ventral otocyst-neuroblast cell line number 33; MPT, mitochondrial membrane potential transition; Bcl-2, B-cell lymphoma 2; Cx31, connexin-31; WT, wild type; Tmtc4, transmembrane and tetratricopeptide repeat 4; UPR, unfolded protein response; ISIRB, integrated stress response inhibitor; erl, erlong; SEM, scanning electron microscope; Sal, salubrinal; PARP, polyp(ADP-ribose) polymerase; APAP, N-acetyl-para-aminophenol (aka acetaminophen).