ISL1 is an essential determinant of structural and functional tonotopic representation of sound

A cardinal feature of the auditory pathway is frequency selectivity represented in the form of a tonotopic map from the cochlea to the cortex. The molecular determinants of the auditory frequency map are unknown. Here, we discovered that the transcription factor ISL1 regulates features of auditory neurons, including the formation of the spiral ganglion neuron (SGN) and peripheral and central processes that shape the tonotopic representation of the auditory map. We selectively knocked out Isl1 in auditory neurons using Neurod1Cre strategies. In the absence of Isl1, SGNs migrate into the central cochlea and beyond. The cochlear wiring is profoundly reduced and disrupted. The central axons of Isl1 mutants lose their topographic projections and segregation at the cochlear nucleus. Transcriptome analysis of SGNs shows that Isl1 regulates neurogenesis, axonogenesis, migration, and the functional properties of neurons. Surprisingly, notable auditory processing features are preserved despite the significant hearing impairment, revealing central auditory pathway resilience and plasticity in mutant mice. Mutant mice demonstrate altered acoustic startle reflex, prepulse inhibition, characteristics of compensatory neural hyperactivity centrally. Our findings show that the Isl1 is one of the obligatory factors required to sculpt auditory structural and functional tonotopic maps. Still, upon Isl1 deletion, the ensuing compensatory plasticity of the auditory pathway does not suffice to overcome developmental changes at the peripheral sensory organ. 12 samples are analyzed (biological replicates of 6 control and 6 mutant samples) each containing 100 FACS-sorted tdTomato+ neurons.听觉通路的核心特征之一为频率选择性，其以从耳蜗延伸至皮层的音调拓扑图谱（tonotopic map）形式呈现。目前，听觉频率图谱的分子决定因素仍未明确。本研究发现，转录因子ISL1（transcription factor ISL1）可调控听觉神经元的多项特征，包括螺旋神经节神经元（spiral ganglion neuron, SGN）的形成，以及塑造听觉图谱音调拓扑表征的外周与中枢突起。我们利用Neurod1Cre策略在听觉神经元中特异性敲除Isl1基因。在缺失Isl1的情况下，螺旋神经节神经元会迁移至中枢耳蜗及以外区域，耳蜗的神经投射大幅减少且形态紊乱。Isl1突变体的中枢轴突丧失了在耳蜗核内的拓扑投射模式与分区特性。对螺旋神经节神经元的转录组分析显示，Isl1可调控神经元的神经发生、轴突发生、迁移以及功能特性。令人意外的是，尽管存在明显的听力缺陷，小鼠仍保留了关键的听觉处理功能，这揭示了突变小鼠中枢听觉通路的代偿可塑性。突变小鼠表现出听觉惊反射（acoustic startle reflex）异常、前脉冲抑制（prepulse inhibition）发生改变，以及中枢存在代偿性神经活动亢进的特征。我们的研究结果表明，Isl1是塑造听觉结构与功能音调拓扑图谱的必需调控因子之一。然而，在敲除Isl1后，听觉通路随之产生的代偿性可塑性不足以弥补外周感觉器官的发育缺陷。本研究共分析12个样本（6个对照样本与6个突变样本的生物学重复），每个样本包含100个经荧光激活细胞分选（Fluorescence-Activated Cell Sorting, FACS）获得的tdTomato阳性神经元。