The role of TMTC4, endoplasmic reticulum Ca2+ flux, and the unfolded protein response in noise-induced hearing loss

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Noise-induced hearing loss (NIHL) is a significant public health problem, affecting nearly 40 million Americans. We have made the exciting discovery that NIHL may be linked to the unfolded protein response (UPR), a critical early response mechanism to cellular stress that has downstream effectors that can promote both cell survival and apoptosis. In support of this, we have additionally identified and characterized a novel deafness gene in mice, Tmtc4, which has also been recently identified as a potential deafness gene in a human family. Mice in which Tmtc4 is genetically absent (Tmtc4 knockout (KO) mice) hear normally at the onset of hearing but rapidly become deaf within 2 weeks and have markedly increased susceptibility to NIHL. We have found that Tmtc4 is broadly expressed in cochlear hair cells and supporting cells, both of which degenerate over time in Tmtc4 KO mice. We have shown that Tmtc4 is part of a macromolecular complex involved in clearing calcium (Ca2+) from the cytoplasm into the endoplasmic reticulum (ER), and that cochlear cells from Tmtc4 KO mice have impairments in intracellular Ca2+ homeostasis and dynamics. This impairment in Ca2+ management leads to upregulation of the UPR and cell death in the Tmtc4 KO cochlea. In parallel with this genetic deafness model of UPR dysregulation, we have found that NIHL in wild-type (WT) mice results in UPR upregulation within 2 hours of noise exposure; this hearing loss could be prevented in part by treatment with one drug, ISRIB, that specifically targets the UPR, or a second drug, CDN1163, that facilitates Ca2+ reuptake into the ER. These preliminary findings strongly implicate the UPR as an early mediator of cellular stress in the cochlea, upstream of other previously studied apoptotic mechanisms, and thus is a potential therapeutic target for a wide range of acquired and genetic forms of hearing loss. In this proposal, our specific aims are to investigate 1) how, in cell lines, TMTC4 dysfunction, including human variants associated with hearing loss, affect ER Ca2+ flux and, subsequently, UPR activation; 2) how, in the cochlea, noise-induced trauma in the form of hair-cell tip-link disruption and ER Ca2+ depletion activate the UPR to induce hair-cell loss; and 3) how, in in vivo models of hearing loss, the UPR is modulated to give rise to different patterns of hearing loss and hair-cell death. These Aims will be achieved using a multidisciplinary set of physiologic, biochemical, pharmacologic, and genetic techniques including ER Ca2+ imaging, mRNA transcriptional analysis, and genetic TMTC4 conditional knockout mice. Through these experiments, we will gain valuable insight into the mechanisms by which ER Ca2+ flux and the UPR are involved in genetic and noise-induced hearing loss, laying the foundation for development of targeted therapies for NIHL, a critical unmet clinical need.