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Critical role of ATP-induced ATP release for Ca2+ signaling in nonsensory cell networks of the developing cochlea.

ABSTRACT: Spatially and temporally coordinated variations of the cytosolic free calcium concentration ([Ca²⁺]_c) play a crucial role in a variety of tissues. In the developing sensory epithelium of the mammalian cochlea, elevation of extracellular adenosine trisphosphate concentration ([ATP]_e) triggers [Ca²⁺]_c oscillations and propagation of intercellular inositol 1,4,5-trisphosphate (IP₃)-dependent Ca²⁺ waves. What remains uncertain is the relative contribution of gap junction channels and connexin hemichannels to these fundamental mechanisms, defects in which impair hearing acquisition. Another related open question is whether [Ca²⁺]_c oscillations require oscillations of the cytosolic IP₃ concentration ([IP₃]_c) in this system. To address these issues, we performed Ca²⁺ imaging experiments in the lesser epithelial ridge of the mouse cochlea around postnatal day 5 and constructed a computational model in quantitative adherence to experimental data. Our results indicate that [Ca²⁺]_c oscillations are governed by Hopf-type bifurcations within the experimental range of [ATP]_e and do not require [IP₃]_c oscillations. The model replicates accurately the spatial extent and propagation speed of intercellular Ca²⁺ waves and predicts that ATP-induced ATP release is the primary mechanism underlying intercellular propagation of Ca²⁺ signals. The model also uncovers a discontinuous transition from propagating regimes (intercellular Ca²⁺ wave speed > 11 ?m?s^-1) to propagation failure (speed = 0), which occurs upon lowering the maximal ATP release rate below a minimal threshold value. The approach presented here overcomes major limitations due to lack of specific connexin channel inhibitors and can be extended to other coupled cellular systems.

SUBMITTER: Ceriani F

PROVIDER: S-EPMC5135323 | biostudies-literature | 2016 Nov

REPOSITORIES: biostudies-literature

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Critical role of ATP-induced ATP release for Ca2+ signaling in nonsensory cell networks of the developing cochlea.

Ceriani Federico F Pozzan Tullio T Mammano Fabio F

Proceedings of the National Academy of Sciences of the United States of America 20161102 46

Spatially and temporally coordinated variations of the cytosolic free calcium concentration ([Ca2+]c) play a crucial role in a variety of tissues. In the developing sensory epithelium of the mammalian cochlea, elevation of extracellular adenosine trisphosphate concentration ([ATP]e) triggers [Ca2+]c oscillations and propagation of intercellular inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ waves. What remains uncert ...[more]

PMID: 27807138

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Project description:Mutations in the genes encoding for gap junction proteins connexin 26 (Cx26) and connexin 30 (Cx30) have been linked to syndromic and nonsyndromic hearing loss in mice and humans. The release of ATP from connexin hemichannels in cochlear nonsensory cells has been proposed to be the main trigger for action potential activity in immature sensory inner hair cells (IHCs), which is crucial for the refinement of the developing auditory circuitry. Using connexin knock-out mice, we show that IHCs fire spontaneous action potentials even in the absence of ATP-dependent intercellular Ca2+ signaling in the nonsensory cells. However, this signaling from nonsensory cells was able to increase the intrinsic IHC firing frequency. We also found that connexin expression is key to IHC functional maturation. In Cx26 conditional knock-out mice (Cx26Sox10-Cre), the maturation of IHCs, which normally occurs at approximately postnatal day 12, was partially prevented. Although Cx30 has been shown not to be required for hearing in young adult mice, IHCs from Cx30 knock-out mice exhibited a comprehensive brake in their development, such that their basolateral membrane currents and synaptic machinery retain a prehearing phenotype. We propose that IHC functional differentiation into mature sensory receptors is initiated in the prehearing cochlea provided that the expression of either connexin reaches a threshold level. As such, connexins regulate one of the most crucial functional refinements in the mammalian cochlea, the disruption of which contributes to the deafness phenotype observed in mice and DFNB1 patients. SIGNIFICANCE STATEMENT:The correct development and function of the mammalian cochlea relies not only on the sensory hair cells, but also on the surrounding nonsensory cells. Although the nonsensory cells have been largely implicated in the general homeostasis in the mature cochlea, their involvement in the initial functional differentiation of the sensory inner hair cells is less clear. Using mutant mouse models for the most common form of congenital deafness in humans, which are knock-outs for the gap-junction channels connexin 26 and connexin 30 genes, we show that defects in nonsensory cells prevented the functional maturation of inner hair cells. In connexin knock-outs, inner hair cells remained stuck at a prehearing stage of development and, as such, are unable to process sound information.

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Critical role of ATP-induced ATP release for Ca2+ signaling in nonsensory cell networks of the developing cochlea.

Publications

Critical role of ATP-induced ATP release for Ca2+ signaling in nonsensory cell networks of the developing cochlea.

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