Coordinated calcium signalling in cochlear sensory and non‐sensory cells refines afferent innervation of outer hair cells

Abstract

Outer hair cells (OHCs) are highly specialized sensory cells conferring the fine‐tuning and high sensitivity of the mammalian cochlea to acoustic stimuli. Here, by genetically manipulating spontaneous Ca2+ signalling in mice in vivo, through a period of early postnatal development, we find that the refinement of OHC afferent innervation is regulated by complementary spontaneous Ca2+ signals originating in OHCs and non‐sensory cells. OHCs fire spontaneous Ca2+ action potentials during a narrow period of neonatal development. Simultaneously, waves of Ca2+ activity in the non‐sensory cells of the greater epithelial ridge cause, via ATP‐induced activation of P2X3 receptors, the increase and synchronization of the Ca2+ activity in nearby OHCs. This synchronization is required for the refinement of their immature afferent innervation. In the absence of connexin channels, Ca2+ waves are impaired, leading to a reduction in the number of ribbon synapses and afferent fibres on OHCs. We propose that the correct maturation of the afferent connectivity of OHCs requires experience‐independent Ca2+ signals from sensory and non‐sensory cells.

Organ-on-chip model shows that ATP release through connexin hemichannels drives spontaneous Ca2+ signaling in non-sensory cells of the greater epithelial ridge in the developing cochlea

“…20 Therefore, it remains to be established if and how these two types of spontaneous activities correlate with each other. This stands out a key open question for cochlear physiopathology, as connexin expression and spontaneous Ca 2+ signaling in the GER are essential for normal development of the sensory epithelium, hair cell functional maturation, hearing acquisition, [20][21][22][23] redox homeostasis and age-related hearing loss. 24 We are confident that the technological advances presented here have the potential to shed light on this as well as a plethora of other unrelated open issues that concern the role of paracrine signaling in physiology and pathology 93 and cannot be addressed with standard methods.…”

The Purinergic Receptor P2rx3 is Required for Spiral Ganglion Neuron Branch Refinement during Development

“…Recently, outer hair cells were also shown to exhibit spontaneous calcium transients, which were stimulated by neighboring Deiters' cells. Pharmacological blockade of P2rx3 receptors inhibited the firing dynamics of outer hair cells, and this altered normal patterns of outer hair cell ribbon synapse distribution and innervation (Ceriani et al, 2019). Extracellular ATP is also known to excite Type II SGNs (Weisz et al, 2009), and recent studies have shown that purinergic signaling occurs in Type II SGNs in response to hair cell damage (Liu et al, 2015).…”

Postnatal Development of the Subcellular Structures and Purinergic Signaling of Deiters’ Cells along the Tonotopic Axis of the Cochlea

“…This sensory input-independent firing of IHCs is fundamental for the maturation, pruning, and maintenance of the hair cell—auditory neuron synapses [7,8,25,27]. A recent study demonstrated that this autocrine and paracrine purinergic signaling does not only drive the development of IHCs in the greater epithelial ridge (GER, Kölliker’s organ), but it is also crucial for OHC maturation in the lesser epithelial ridge (LER) [28]. Deiters’ cells, the supporting cells in the LER, are paramount players in this process.…”