Developmental refinement of hair cell synapses tightens the coupling of Ca²⁺influx to exocytosis

Abstract

Cochlear inner hair cells (IHCs) develop from pre-sensory pacemaker to sound transducer. Here, we report that this involves changes in structure and function of the ribbon synapses between IHCs and spiral ganglion neurons (SGNs) around hearing onset in mice. As synapses matured they changed from holding several small presynaptic active zones (AZs) and apposed postsynaptic densities (PSDs) to one large AZ/PSD complex per SGN bouton. After the onset of hearing (i) IHCs had fewer and larger ribbons; (ii) Ca V 1.3 channels formed stripe-like clusters rather than the smaller and round clusters at immature AZs; (iii) extrasynaptic Ca V 1.3-channels were selectively reduced, (iv) the intrinsic Ca 2+ dependence of fast exocytosis probed by Ca 2+ uncaging remained unchanged but (v) the apparent Ca 2+ dependence of exocytosis linearized, when assessed by progressive dihydropyridine block of Ca 2+ influx. Biophysical modeling of exocytosis at mature and immature AZ topographies suggests that Ca 2+ influx through an individual channel dominates the [Ca 2+ ] driving exocytosis at each mature release site. We conclude that IHC synapses undergo major developmental refinements, resulting in tighter spatial coupling between Ca 2+ influx and exocytosis.

Rab3-interacting molecules 2α and 2β promote the abundance of voltage-gated Ca V 1.3 Ca 2+ channels at hair cell active zones

“…5C) and Ca 2+ efficiency (Fig. 5D) suggests that coupling of Ca 2+ influx to exocytosis (1,4,54) was largely unaltered in RIM2α-deficient IHCs, which contrasts findings at the in RIM1/2-deficient IHCs calyx of Held (18), which might reflect compensation by other molecular links between channels and release sites at the IHC AZ. Alternatively, a subtle alteration of Ca 2+ channel position relative to the vesicular Ca 2+ sensor on the scale of few nanometers might have gone undetected in our measurements of exocytic membrane capacitance changes.…”

“…Rab3-interacting molecule | active zone | ribbon synapse | Ca 2+ channel | vesicle T ens of Ca V 1.3 Ca 2+ channels are thought to cluster within the active zone (AZ) membrane underneath the presynaptic density of inner hair cells (IHCs) (1)(2)(3)(4). They make up the key signaling element, coupling the sound-driven receptor potential to vesicular glutamate release (5)(6)(7).…”

Innervation regulates synaptic ribbons in lateral line mechanosensory hair cells

“…They are thought to store a ready-releasable pool of synaptic vesicles and coordinate synchronous vesiclerelease at the synapse (Khimich et al, 2005;Buran et al, 2010;Matthews and Fuchs, 2010;Maxeiner et al, 2016). Voltage gated Ltype Ca 2+ channels are found to drive exocytosis at ribbon synapses and are localized at the base of ribbons (Platzer et al, 2000;Brandt et al, 2003;Dou et al, 2004;Sidi et al, 2004;Sheets et al, 2012;Wong et al, 2014). In zebrafish, these channels are also found to play a role in regulating ribbon size, number and maintenance of ribbons at the hair cell synapse (Sheets et al, 2012).…”

“…During early development of auditory hair cells and retinal photoreceptor cells, electron-dense ribbon precursors are found apically in the cytoplasm (Blanks et al, 1974;Sobkowicz et al, 1982Sobkowicz et al, , 1986Regus-Leidig et al, 2009;Sheets et al, 2011;Wong et al, 2014). These cytoplasmic ribbon precursors in retinal photoreceptor cells colocalize with the cytomatrix proteins bassoon and piccolo and are thought to be shuttled together to the active zone (Regus-Leidig et al, 2009).…”

Innervation regulates synaptic ribbons in lateral line mechanosensory hair cells

“…As synaptogenesis proceeds, the total number of ribbons decreases, ribbons become localized at the active zone and fewer ribbon precursors are found in the cytoplasm. Whereas multiple ribbons are found at the site of immature hair cell synapses, mature synapses have single ribbons (Pujol et al, 1979;Sobkowicz et al, 1986;Pujol et al, 1997;Wong et al, 2014).…”