Piccolino is required for ribbon architecture at cochlear inner hair cell synapses and for hearing

Abstract

Cochlear inner hair cells (IHCs) form specialized ribbon synapses with spiral ganglion neurons that tirelessly transmit sound information at high rates over long time periods with extreme temporal precision. This functional specialization is essential for sound encoding and is attributed to a distinct molecular machinery with unique players or splice variants compared to conventional neuronal synapses. Among these is the active zone (AZ) scaffold protein piccolo/aczonin, which is represented by its short splice variant piccolino at cochlear and retinal ribbon synapses. While the function of piccolo at synapses of the central nervous system has been intensively investigated, the role of piccolino at IHC synapses remains unclear. In this study, we characterize the structure and function of IHC synapses in piccolo gene‐trap mutant rats (Pclogt/gt). We find a mild hearing deficit with elevated thresholds and reduced amplitudes of auditory brainstem responses. Ca2+ channel distribution and ribbon morphology are altered in apical IHCs, while their presynaptic function seems to be unchanged. We conclude that piccolino contributes to the AZ organization in IHCs and is essential for normal hearing.

Establishing synthetic ribbon-type active zones in a heterologous expression system

“…Changes in Ca 2+ channel physiology upon deletion of RIBEYE were also found for CaV1.4 at ribbon-less rod photoreceptor AZs (Grabner & Moser, 2021). Finally, IHCs lacking piccolino, a ribbonsynapse specific isoform of the AZ protein piccolo, show AZs with smaller synaptic ribbons and altered clustering yet normal complement of Ca 2+ channels (Michanski et al, 2023).…”

“…We note that other ribbon-resident proteins such as piccolino likely contribute to this fine-tuning of the size and shape of ribbon-type AZs. Indeed, disruption of piccolino in IHCs reduced the average ribbon size (Müller et al, 2019;Michanski et al, 2023). One exciting example of ribbon synapse heterogeneity manifests itself in IHCs where synapses show a spatial size gradient that likely relates to the diverse molecular and functional properties of the postsynaptic SGNs (reviewed in Moser et al, 2023).…”

Microtubule networks in zebrafish hair cells facilitate presynapse transport and fusion during development

“…In mice, work has shown that in sensory cells lacking Bassoon ribbons are not properly anchored at the presynaptic AZ, and synapse function is disrupted (Dick et al, 2003; Frank et al, 2010; Khimich et al, 2005). In rats, loss of Piccolino also impacts ribbon morphology (Michanski et al, 2023; Regus-Leidig et al, 2014). Currently how these key molecular players fit within the dynamics underlying ribbon formation is not completely understood.…”

Slow kinesin-dependent microtubular transport facilitates ribbon synapse assembly in developing cochlear inner hair cells

“…Such ‘free-floating’ ribbon precursors can occur at significant distances to the AZ and have previously been observed not only in auditory IHCs, but also pinealocytes and retinal photoreceptors (Hermes et al, 1992; Regus-Leidig et al, 2009; Spiwoks-Becker et al, 2008, 2004). Across all systems, floating precursors were shown to tether a cohort of SVs and consist not only of the main scaffold RIBEYE, but also contain other AZ proteins such as Piccolino, a short splice variant of Piccolo (Michanski et al, 2023, 2019; Regus-Leidig et al, 2013). Hence, these precursors can be considered as presynaptic ‘building blocks’ for rapid AZ establishment or supplementation at nascent afferent contacts.…”