MANF supports the inner hair cell synapse and the outer hair cell stereocilia bundle in the cochlea.

Failure in the structural maintenance of the hair cell stereocilia bundle and ribbon synapse causes hearing loss. Here, we have studied how ER stress elicits hair cell pathology, using mouse models with inactivation of Manf (mesencephalic astrocyte-derived neurotrophic factor), encoding an ER-homeostasis-promoting protein. From hearing onset, Manf deficiency caused disarray of the outer hair cell stereocilia bundle and reduced cochlear sound amplification capability throughout the tonotopic axis. In high-frequency outer hair cells, the pathology ended in molecular changes in the stereocilia taper region and in strong stereocilia fusion. In high-frequency inner hair cells, Manf deficiency degraded ribbon synapses. The altered phenotype strongly depended on the mouse genetic background. Altogether, the failure in the ER homeostasis maintenance induced early-onset stereociliopathy and synaptopathy and accelerated the effect of genetic causes driving age-related hearing loss. Correspondingly, MANF mutation in a human patient induced severe sensorineural hearing loss from a young age onward. Thus, we present MANF as a novel protein and ER stress as a mechanism that regulate auditory hair cell maintenance in both mice and humans.

Deficiency of the ER-stress-regulator MANF triggers progressive outer hair cell death and hearing loss.

The non-conventional neurotrophic factor mesencephalic astrocyte-derived neurotrophic factor (MANF) is an endoplasmic reticulum (ER)-resident protein that promotes ER homeostasis. MANF has a cytoprotective function, shown in the central nervous system neurons and pancreatic beta cells. Here, we report that MANF is expressed in the hair cells and neurons and in selected non-sensory cells of the cochlea and that Manf inactivation triggers upregulation of the ER chaperones in these cells. However, Manf inactivation resulted in the death of only outer hair cells (OHCs), the cells responsible for sound amplification in the cochlea. All OHCs were formed in Manf-inactivated mice, but progressive OHC death started soon after the onset of hearing function. The robust OHC loss was accompanied by strongly elevated hearing thresholds. Conditional Manf inactivation demonstrated that MANF has a local function in the cochlea. Immunostainings revealed the upregulation of CHOP, the pro-apoptotic component of the unfolded protein response (UPR), in Manf-inactivated OHCs, linking the UPR to the loss of these cells. The phenotype of Manf-inactivated OHCs was distinctly dependent on the mouse strain, such that the strains characterized by early-onset age-related hearing loss (C57BL/6J and CD-1) were affected. These results suggest that Manf deficiency becomes detrimental when accompanied by gene mutations that predispose to hearing loss, by intensifying ER dyshomeostasis. Together, MANF is the first growth factor shown to antagonize ER stress-mediated OHC death. MANF might serve as a therapeutic candidate for protection against hearing loss induced by the ER-machinery-targeting stressors.

The Stress Response in the Non-sensory Cells of the Cochlea Under Pathological Conditions-Possible Role in Mediating Noise Vulnerability.

Various stressors, such as loud sounds and the effects of aging, impair the function and viability of the cochlear sensory cells, the hair cells. Stressors trigger pathophysiological changes in the cochlear non-sensory cells as well. We have here studied the stress response mounted in the lateral wall of the cochlea during acute noise stress and during age-related chronic stress. We have used the activation of JNK/c-Jun, ERK, and NF-κB pathways as a readout of the stress response, and the expression of the FoxO3 transcription factor as a possible additional player in cellular stress. In the aging cochlea, NF-κB transcriptional activity was strongly induced in the stria vascularis of the lateral wall. This induction was linked with the atrophy of the stria vascularis, suggesting a role for NF-κB signaling in mediating age-related strial degeneration. Acutely following noise exposure, the JNK/c-Jun, ERK, and NF-κB pathways were activated in the spiral ligament of the lateral wall of CBA/Ca mice. This activation was concomitant with the morphological transformation of macrophages, suggesting that the upregulation of stress signaling leads to macrophage activation. In contrast, C57BL/6J mice lacked these responses. Only the combination of noise exposure and a systemic stressor, lipopolysaccharide, exceeded the threshold for the activation of stress signaling in the lateral wall of C57BL/6J mice. In addition, we found that, at the young adult age, outer hair cells of CBA/Ca mice are much more vulnerable to loud sounds compared to these cells of C57BL/6J mice. These results suggest that the differential stress response in the lateral wall of the two mouse strains underlies, in part, the differential noise vulnerability of their outer hair cells. Together, we propose that the molecular stress response in the lateral wall modulates the outcome of the stressed cochlea.