Mammalian Genetics Unit, MRC Harwell Institute, Harwell Oxford, United Kingdom.
School of Sciences, University of Sheffield, Sheffield, United Kingdom.
PLoS Genet. 2022 Jan 31;18(1):e1009937. doi: 10.1371/journal.pgen.1009937. eCollection 2022 Jan.
Mammalian hearing involves the mechanoelectrical transduction (MET) of sound-induced fluid waves in the cochlea. Essential to this process are the specialised sensory cochlear cells, the inner (IHCs) and outer hair cells (OHCs). While genetic hearing loss is highly heterogeneous, understanding the requirement of each gene will lead to a better understanding of the molecular basis of hearing and also to therapeutic opportunities for deafness. The Neuroplastin (Nptn) gene, which encodes two protein isoforms Np55 and Np65, is required for hearing, and homozygous loss-of-function mutations that affect both isoforms lead to profound deafness in mice. Here we have utilised several distinct mouse models to elaborate upon the spatial, temporal, and functional requirement of Nptn for hearing. While we demonstrate that both Np55 and Np65 are present in cochlear cells, characterisation of a Np65-specific mouse knockout shows normal hearing thresholds indicating that Np65 is functionally redundant for hearing. In contrast, we find that Nptn-knockout mice have significantly reduced maximal MET currents and MET channel open probabilities in mature OHCs, with both OHCs and IHCs also failing to develop fully mature basolateral currents. Furthermore, comparing the hearing thresholds and IHC synapse structure of Nptn-knockout mice with those of mice that lack Nptn only in IHCs and OHCs shows that the majority of the auditory deficit is explained by hair cell dysfunction, with abnormal afferent synapses contributing only a small proportion of the hearing loss. Finally, we show that continued expression of Neuroplastin in OHCs of adult mice is required for membrane localisation of Plasma Membrane Ca2+ ATPase 2 (PMCA2), which is essential for hearing function. Moreover, Nptn haploinsufficiency phenocopies Atp2b2 (encodes PMCA2) mutations, with heterozygous Nptn-knockout mice exhibiting hearing loss through genetic interaction with the Cdh23ahl allele. Together, our findings provide further insight to the functional requirement of Neuroplastin for mammalian hearing.
哺乳动物的听觉涉及耳蜗中声音诱导的流体波的机电转换 (MET)。这个过程的关键是专门的感觉耳蜗细胞，即内毛细胞 (IHC) 和外毛细胞 (OHC)。虽然遗传性听力损失高度异质，但了解每个基因的要求将有助于更好地理解听力的分子基础，也为耳聋提供治疗机会。Neuroplastin (Nptn) 基因，编码两个蛋白同工型 Np55 和 Np65，是听觉所必需的，而影响这两个同工型的纯合功能丧失突变会导致小鼠严重耳聋。在这里，我们利用几种不同的小鼠模型来详细阐述 Nptn 对听觉的空间、时间和功能要求。虽然我们证明了 Np55 和 Np65 都存在于耳蜗细胞中，但对 Np65 特异性小鼠敲除的特征分析表明，Np65 在听觉功能上是冗余的。相比之下，我们发现 Nptn 敲除小鼠的成熟 OHC 中最大 MET 电流和 MET 通道开放概率显著降低，OHC 和 IHC 也未能完全发育成熟的基底外侧电流。此外，将 Nptn 敲除小鼠的听力阈值和 IHC 突触结构与缺乏 IHC 和 OHC 中 Nptn 的小鼠进行比较表明，大部分听力缺陷是由毛细胞功能障碍引起的，异常的传入突触仅导致一小部分听力损失。最后，我们表明，成年小鼠 OHC 中持续表达 Neuroplastin 是 Plasma Membrane Ca2+ATPase 2 (PMCA2) 膜定位所必需的，PMCA2 对听力功能至关重要。此外，Nptn 杂合子缺失表型模拟 Atp2b2（编码 PMCA2）突变，杂合子 Nptn 敲除小鼠通过与 Cdh23ahl 等位基因的遗传相互作用表现出听力损失。总之，我们的研究结果为 Neuroplastin 对哺乳动物听觉的功能要求提供了进一步的见解。
