Hair cell synaptic dysfunction, auditory fatigue and thermal sensitivity in otoferlin Ile515Thr mutants.

The multi-C2 domain protein otoferlin is required for hearing and mutated in human deafness. Some OTOF mutations cause a mild elevation of auditory thresholds but strong impairment of speech perception. At elevated body temperature, hearing is lost. Mice homozygous for one of these mutations, OtofI515T/I515T, exhibit a moderate hearing impairment involving enhanced adaptation to continuous or repetitive sound stimulation. In OtofI515T/I515T inner hair cells (IHCs), otoferlin levels are diminished by 65%, and synaptic vesicles are enlarged. Exocytosis during prolonged stimulation is strongly reduced. This indicates that otoferlin is critical for the reformation of properly sized and fusion-competent synaptic vesicles. Moreover, we found sustained exocytosis and sound encoding to scale with the amount of otoferlin at the plasma membrane. We identified a 20 amino acid motif including an RXR motif, presumably present in human but not in mouse otoferlin, which reduces the plasma membrane abundance of Ile515Thr-otoferlin. Together, this likely explains the auditory synaptopathy at normal temperature and the temperature-sensitive deafness in humans carrying the Ile515Thr mutation.

Disruption of adaptor protein 2µ (AP-2µ) in cochlear hair cells impairs vesicle reloading of synaptic release sites and hearing.

Active zones (AZs) of inner hair cells (IHCs) indefatigably release hundreds of vesicles per second, requiring each release site to reload vesicles at tens per second. Here, we report that the endocytic adaptor protein 2µ (AP-2µ) is required for release site replenishment and hearing. We show that hair cell-specific disruption of AP-2µ slows IHC exocytosis immediately after fusion of the readily releasable pool of vesicles, despite normal abundance of membrane-proximal vesicles and intact endocytic membrane retrieval. Sound-driven postsynaptic spiking was reduced in a use-dependent manner, and the altered interspike interval statistics suggested a slowed reloading of release sites. Sustained strong stimulation led to accumulation of endosome-like vacuoles, fewer clathrin-coated endocytic intermediates, and vesicle depletion of the membrane-distal synaptic ribbon in AP-2µ-deficient IHCs, indicating a further role of AP-2µ in clathrin-dependent vesicle reformation on a timescale of many seconds. Finally, we show that AP-2 sorts its IHC-cargo otoferlin. We propose that binding of AP-2 to otoferlin facilitates replenishment of release sites, for example, via speeding AZ clearance of exocytosed material, in addition to a role of AP-2 in synaptic vesicle reformation.

Otoferlin Is Required for Proper Synapse Maturation and for Maintenance of Inner and Outer Hair Cells in Mouse Models for DFNB9.

Deficiency of otoferlin causes profound prelingual deafness in humans and animal models. Here, we closely analyzed developmental deficits and degenerative mechanisms in Otof knock-out (Otof -/-) mice over the course of 48 weeks. We found otoferlin to be required for proper synapse development in the immature rodent cochlea: In absence of otoferlin, synaptic pruning was delayed, and postsynaptic boutons appeared enlarged at 2 weeks of age. At postnatal day 14 (P14), we found on average ∼15 synapses per inner hair cell (IHC) in Otof -/- cochleae as well as in wild-type controls. Further on, the number of synapses in Otof -/- IHCs was reduced to ∼7 at 8 weeks of age and to ∼6 at 48 weeks of age. In the same period, the number of spiral ganglion neurons (SGNs) declined in Otof -/- animals. Importantly, we found an age-progressive loss of IHCs to an overall number of 75% of wildtype IHCs. The IHC loss more prominently but not exclusively affected the basal aspects of the cochlea. For outer hair cells (OHCs), we observed slightly accelerated age-dependent degeneration from base to apex. This was associated with a progressive decay in DPOAE amplitudes for high frequency stimuli, which could first be observed at the age of 24 weeks in Otof -/- mice. Our data will help to plan and predict the outcome of a gene therapy applied at various ages of DFNB9 patients.

A dual-AAV approach restores fast exocytosis and partially rescues auditory function in deaf otoferlin knock-out mice.

Normal hearing and synaptic transmission at afferent auditory inner hair cell (IHC) synapses require otoferlin. Deafness DFNB9, caused by mutations in the OTOF gene encoding otoferlin, might be treated by transferring wild-type otoferlin cDNA into IHCs, which is difficult due to the large size of this transgene. In this study, we generated two adeno-associated viruses (AAVs), each containing half of the otoferlin cDNA Co-injecting these dual-AAV2/6 half-vectors into the cochleae of 6- to 7-day-old otoferlin knock-out (Otof-/-) mice led to the expression of full-length otoferlin in up to 50% of IHCs. In the cochlea, otoferlin was selectively expressed in auditory hair cells. Dual-AAV transduction of Otof-/- IHCs fully restored fast exocytosis, while otoferlin-dependent vesicle replenishment reached 35-50% of wild-type levels. The loss of 40% of synaptic ribbons in these IHCs could not be prevented, indicating a role of otoferlin in early synapse maturation. Acoustic clicks evoked auditory brainstem responses with thresholds of 40-60 dB. Therefore, we propose that gene delivery mediated by dual-AAV vectors might be suitable to treat deafness forms caused by mutations in large genes such as OTOF.

The fade-in--short stimulation--fade out approach to sham tDCS--reliable at 1 mA for naïve and experienced subjects, but not investigators.

OBJECTIVE: Slowly ramping down initial current intensity after a minimal interval of stimulation is the de facto standard for sham stimulation in transcranial electrical stimulation research. The aim of this study is to further investigate the effectiveness of this method of blinding. METHODS: We have investigated the time course of the cutaneous perception during 10 min of anodal, cathodal, and sham transcranial direct current stimulation, probing the perceived strength and site of the perceived sensation. We have also utilized post-stimulation assessment and measurements of sleepiness prior to and after the intervention. Previous exposure to tDCS has also been taken into account: the experiment has been repeated in naïve and experienced subject groups, and a group consisting of investigators who use tDCS as a research tool. RESULTS: Although we have observed a general reduction in the perceived strength of the stimulation with time, we have not found the complete disappearance of the cutaneous perception during either the verum or the sham conditions. Experienced subjects were more likely to be able to differentiate between trials with stimulation and non-stimulation trials and to correctly identify sham and verum stimulation conditions. CONCLUSION: When taking only naïve and experienced subjects into account, there was no significant difference between the strength of the perceived stimulation in the verum and sham conditions. The fade-in - short stimulation - fade-out sham stimulation can be indistinguishable from verum stimulation, but not because it mimics the disappearance of the cutaneous sensations associated with the verum stimulation, but because these sensations persist also in the sham stimulation. The significance of this finding with potential confounding factors and limitations are discussed.