Mammalian Piezo2 channels are essential for transduction of innocuous mechanical forces by proprioceptors and cutaneous touch receptors. In contrast, mechanical responses of somatosensory nociceptor neurons evoking pain, remain intact or are only partially reduced in Piezo2-deficient mice. In the eye cornea, comparatively low mechanical forces are detected by polymodal and pure mechanosensory trigeminal ganglion neurons. Their activation always evokes ocular discomfort or pain and protective reflexes, thus being a unique model to study mechanotransduction mechanisms in this particular class of nociceptive neurons. Cultured male and female mouse mechano- and polymodal nociceptor corneal neurons display rapidly, intermediately and slowly adapting mechanically activated currents. Immunostaining of the somas and peripheral axons of corneal neurons responding only to mechanical force (pure mechano-nociceptor) or also exhibiting TRPV1 (transient receptor potential cation channel subfamily V member 1) immunoreactivity (polymodal nociceptor) revealed that they express Piezo2. In sensory-specific Piezo2-deficient mice, the distribution of corneal neurons displaying the three types of mechanically evoked currents is similar to the wild type; however, the proportions of rapidly adapting neurons, and of intermediately and slowly adapting neurons were significantly reduced. Recordings of mechano- and polymodal-nociceptor nerve terminals in the corneal surface of Piezo2 conditional knock-out mice revealed a reduced number of mechano-sensitive terminals and lower frequency of nerve terminal impulse discharges under mechanical stimulation. Eye blinks evoked by von Frey filaments applied on the cornea were lower in Piezo2-deficient mice compared with wild type. Together, our results provide direct evidence that Piezo2 channels support mechanically activated currents of different kinetics in corneal trigeminal neurons and contributes to transduction of mechanical forces by corneal nociceptors.

SIGNIFICANCE STATEMENT The cornea is a richly innervated and highly sensitive tissue. Low-threshold mechanical forces activate corneal receptors evoking discomfort or pain. To examine the contribution of Piezo2, a low-threshold mechanically activated channel, to acute ocular pain, we characterized the mechanosensitivity of corneal sensory neurons. By using Piezo2 conditional knock-out mice, we show that Piezo2 channels, present in the cell body and terminals of corneal neurons, are directly involved in acute corneal mechano-nociception. Inhibition of Piezo2 for systemic pain treatment is hindered because of its essential role for mechano-transduction processes in multiple body organs. Still, topical modulation of Piezo2 in the cornea may be useful to selectively relief unpleasant sensations and pain associated with mechanical irritation accompanying many ocular surface disorders.

哺乳动物Piezo2通道对于本体感受器和皮肤接触受体的无害机械力传导至关重要。相反，在缺乏Piezo2的小鼠中，引起疼痛的体感伤害感受神经元的机械反应保持完整或仅部分降低。在眼角膜中，多峰和纯机械感觉的三叉神经节神经元检测到较低的机械力。它们的激活总是会引起眼部不适或疼痛以及保护性反射，因此是研究此类伤害性神经元中机械转导机制的独特模型。培养的雄性和雌性小鼠机械和多峰伤害感受器角膜神经元显示快速，中度和缓慢适应机械激活电流。角膜神经元的躯体和周围轴突的免疫染色仅对机械力作出反应（纯机械感受器）或还表现出TRPV1（瞬时受体电位阳离子通道亚家族V成员1）免疫反应性（多峰伤害感受器）表明它们表达了Piezo2。在感官上缺乏Piezo2的小鼠，显示三种机械诱发电流的角膜神经元分布与野生型相似。但是，快速适应的神经元以及中度和缓慢适应神经元的比例显着降低。Piezo2条件性基因敲除小鼠角膜表面的机械和多模态伤害感受器神经末梢的记录显示，在机械刺激下，机械敏感末梢的数目减少，神经末梢冲动放电的频率降低。在Piezo2中，涂在角膜上的冯·弗雷丝引起的眨眼次数更低缺陷小鼠与野生型相比。在一起，我们的结果提供直接的证据表明Piezo2通道支持角膜三叉神经元中不同动力学的机械激活电流，并有助于角膜伤害感受器的机械力传导。

意义声明角膜是一种高度支配和高度敏感的组织。低阈值机械力激活角膜受体，引起不适或疼痛。为了检查Piezo2（低阈值机械激活通道）对急性眼痛的作用，我们表征了角膜感觉神经元的机械敏感性。通过使用Piezo2条件性基因敲除小鼠，我们显示存在于细胞体和角膜神经元末端的Piezo2通道直接参与急性角膜机械伤害感受。Piezo2对于全身性疼痛治疗的抑制作用受到阻碍，因为它对于多种身体器官的机械传导过程至关重要。仍然，局部调节角膜中的Piezo2对于选择性缓解与伴随许多眼表疾病的机械刺激有关的不愉快的感觉和疼痛可能是有用的。