Johnson Stuart L, Ceriani Federico, Houston Oliver, Polishchuk Roman, Polishchuk Elena, Crispino Giulia, Zorzi Veronica, Mammano Fabio, Marcotti Walter
Department of Biomedical Science, University of Sheffield, S10 2TN Sheffield, United Kingdom.
Department of Physics and Astronomy "G. Galilei," University of Padua, 35131 Padua, Italy.
J Neurosci. 2017 Jan 11;37(2):258-268. doi: 10.1523/JNEUROSCI.2251-16.2016.
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 Ca 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 (Cx26), 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.
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.
编码间隙连接蛋白连接蛋白26（Cx26）和连接蛋白30（Cx30）的基因突变已与小鼠和人类的综合征性及非综合征性听力损失相关联。有人提出，耳蜗非感觉细胞中连接蛋白半通道释放ATP是未成熟感觉性内毛细胞（IHC）动作电位活动的主要触发因素，这对发育中的听觉回路的完善至关重要。利用连接蛋白基因敲除小鼠，我们发现即使在非感觉细胞中不存在ATP依赖的细胞间钙信号传导的情况下，IHC仍能产生自发性动作电位。然而，来自非感觉细胞的这种信号传导能够增加IHC的固有放电频率。我们还发现连接蛋白的表达是IHC功能成熟的关键。在Cx26条件性基因敲除小鼠（Cx26）中，通常在出生后第12天左右发生的IHC成熟过程受到部分阻碍。尽管已表明成年幼鼠的听力不需要Cx30，但Cx30基因敲除小鼠的IHC在发育过程中表现出全面的阻滞，以至于它们的基底外侧膜电流和突触机制保留了听力前的表型。我们提出，只要连接蛋白中的任何一种表达达到阈值水平，IHC向成熟感觉受体的功能分化就在听力前的耳蜗中开始。因此，连接蛋白调节哺乳动物耳蜗中最关键的功能完善之一，其破坏导致在小鼠和DFNB1患者中观察到的耳聋表型。
哺乳动物耳蜗的正确发育和功能不仅依赖于感觉毛细胞，还依赖于周围的非感觉细胞。尽管非感觉细胞在很大程度上与成熟耳蜗的一般内环境稳定有关，但其在感觉性内毛细胞初始功能分化中的作用尚不清楚。利用针对人类最常见的先天性耳聋形式的突变小鼠模型，即间隙连接通道连接蛋白26和连接蛋白30基因的敲除模型，我们表明非感觉细胞中的缺陷阻止了内毛细胞的功能成熟。在连接蛋白基因敲除小鼠中，内毛细胞停留在听力前的发育阶段，因此无法处理声音信息。
