Synaptic Release Potentiation at Aging Auditory Ribbon Synapses

Abstract

46Cochlear inner hair cells (IHCs) harbor a peculiar presynaptic organelle, the ribbon, which is 47 essential for aggregating synaptic vesicles and organizing Ca 2+ channels at the active zone. 48 Emerging evidence suggests that damage to the ribbon synapses represents an important form 49 of cochlear synaptopathy that seems prevalent in age-related sensorineural hearing loss. The 50 functional changes occurring at these synapses during aging are not fully understood. Here, 51 we characterized the age-related changes in IHCs of C57BL/6J mice, a strain which is known 52 to carry a cadherin23 mutation and experiences early hearing loss with age. We found a 53 progressive loss of synaptic ribbons with aging, starting before postnatal day 180 (P180) and 54 reaching up to 50 % loss in middle age mice at P365. A deletion of the Otof gene, encoding 55 the Ca 2+ sensor otoferlin produced an accelerated loss of IHC ribbons with aging, with a 50 % 56 loss by P60. In both Otof +/+ and Otof -/-C57BL/6J mice, the synaptic ribbons became larger 57 with aging and IHCs displayed a drastic cell surface reduction with a large decrease in 58 extrasynaptic BK-channel expression. These changes are indicative of oxidative stress and 59 synaptic autophagy as suggested by an increased expression of the autophagosomal protein 60 LC3B. Furthermore, whole-cell patch-clamp recordings and calcium imaging in IHCs from 61 old Otof +/+ P365 C57BL/6J mice indicated an increase in Ca 2+ -channel density and a stronger 62 exocytosis at the remaining ribbon active zones, suggesting synaptic release potentiation 63 possibly explaining hyperacusis and recruitment in the elderly.64 65 3 SIGNIFICANCE STATEMENT 66 67Age-related hidden hearing loss is often described as a cochlear synaptopathy that results 68 from a progressive degeneration of the ribbon synapses contacting the inner hair cells (IHCs). 69 We show that the auditory defect of aging C57BL/6J mice is associated with a large shrinkage 70 of IHCs and a drastic enlargement of their remaining presynaptic ribbons. Synaptic Ca 2+ 71 microdomains and exocytosis were largely increased in old IHCs, while on the contrary the 72 expression of the fast repolarizing BK channels, known to negatively control transmitter 73 release, was decreased. This age-related synaptic plasticity in IHCs suggested a functional 74 potentiation of synaptic transmission at the surviving synapses, a process that could partially 75 compensate the decrease in synapse number and underlie hyperacusis. 76 77 78 4 79 80 Age-related hearing loss (ARHL or presbycusis) is the most prevalent form of sensory 81 disability in human populations and adversely affects the quality of life of many elderly 82 individuals. The development of ARHL has been generally considered to be associated with 83 primary degeneration of cochlear hair cells, with secondary deafferentation associating 84 degeneration of spiral ganglion neurons and ribbon synapses. However, recent studies have 85 resulted in a paradigm shift in the understanding of ...

Otoferlin as a multirole Ca2+ signaling protein: from inner ear synapses to cancer pathways

“…IHCs from mice lacking otoferlin, the number of synaptic ribbons is proportionally reduced with the cell size (Tertrais et al, 2019). Also in old P360 C57BL/6J mice, the reduced size of the IHCs is proportionally associated with a lower number of synaptic ribbons (Peineau et al, 2021). This correlation suggests that the number of ribbons may influence membrane exocytosis, and in turn, cell size, possibly by modifying the intracellular hydrostatic pressure.…”

“…(D) The presynaptic IHC ribbons are closely colocalized with otoferlin, as indicated by the line-scan fluorescence intensity profiles of the ribbon indicated in (C). This is an original figure built from unpublished confocal immunostaining as previously described (Peineau et al, 2021).…”

“…On the other hand, the importance of otoferlin in the maintenance or development of ribbons and the presence of corresponding synapses has been demonstrated in mice (Stalmann et al, 2021). Otoferlin depletion in zebrafish hair cells also results in abnormal synaptic ribbons and altered intracellular calcium levels (Manchanda et al, 2019), suggesting Ca 2+ as an important factor for the maintenance and survival of the ribbons as in aging auditory synapses (Peineau et al, 2021).…”