Insights into vertebrate electroreceptor evolution from the lamprey lateral line

我们的听觉，以及我们用来保持平衡的信息，来自于内耳感觉“毛细胞”上的微小“毛发”的运动，这是对内耳内液体运动的反应。在鱼类和两栖动物身上，体表上也发现了毛细胞，在头部和身体上排列成行的感觉器官（“神经鞘”）中。这些“侧线”毛细胞检测局部水流（“远距离触摸”）。许多鱼类和两栖动物（如鲨鱼、鲟鱼、蝾螈）也有电感觉侧线器官，可以探测水中动物周围的弱电场（主要用于狩猎活的猎物）。在电感受器的鱼类和两栖动物中，这两种类型的感觉器官，加上从毛细胞和电感受器向大脑传递信息的神经元（神经细胞），都是从胚胎的“侧线基板”（头部增厚的皮肤斑块）发育而来的。电感受是一种古老的感觉，和脊椎动物一样古老。我们之所以知道这一点，是因为七鳃鳗是唯一存活下来的无颌脊椎动物，它们具有电感受力。七鳃鳗的电感受器似乎与有颌脊椎动物的电感受器的工作方式相同，并且与有颌电感受器相同的侧线神经（前神经）将它们的信息传递到大脑的同一个专门部分，这表明这个系统存在于所有现存脊椎动物的共同祖先中。电感受器是如何进化的？我们以前的研究表明硬骨鱼的电感受器与毛细胞密切相关。例如，我们发现毛细胞形成所需的“转录因子”基因（编码控制其他基因活性的蛋白质）在壶腹器官的发育中也很活跃。这表明电感受器可能是从侧线毛细胞进化而来的。独特的是，七鳃鳗的侧线神经也需要在远离光线的地方进行反射游泳反应。因此，在七鳃鳗中，侧线系统包括对光有反应的“光感受器”细胞，对局部水运动有反应的“机械感受器”（毛细胞），以及对电场有反应的电感受器。在有颌脊椎动物的侧线中没有光敏感性的报道，但最近的一些研究报道了斑马鱼神经鞘（包括毛细胞）中编码视蛋白的基因的表达，视蛋白可以探测光。因此，部分或全部七鳃鳗神经肥大毛细胞可能对光敏感，也可能对水的运动有反应。在这项拨款申请中，我们的目标是通过研究海七鳃鳗胚胎和幼虫阶段毛细胞、电感受器和光感受器（可能是毛细胞）的发育，来阐明电感受器的进化和避光游泳反射。如果电感受器在所有现存脊椎动物的共同祖先中进化，我们预计七鳃鳗的电感受器将从侧线基板发育而来，并表达与有颌脊椎动物相似的基因。为了验证这一点，我们将注射重要的荧光染料，以跟踪侧线基板的发育和侧线神经的投影（只有前侧线神经支配电感受器，包括树干上的电感受器）；研究已知在颌脊椎动物电感受器中活跃的基因的表达，以及斑马鱼神经鞘中报道的视蛋白基因；并采用无偏倚的“单细胞”测序方法来鉴定每种受体细胞类型表达的新基因。最后，我们将使用CRISPR对其中一些基因进行突变，以确定它们对电感受器的形成或光感受器的功能是否重要。总的来说，这项工作将提供关于七鳃鳗侧线受体细胞发育的第一个信息，确定神秘的侧线介导的避光反射的细胞和分子基础，并揭示脊椎动物感觉受体细胞的进化。

Our sense of hearing, and the information we use for balance, results from the movement of tiny 'hairs' on sensory 'hair cells' in our inner ears, in response to fluid motion within the inner ear. In fish and amphibians, hair cells are also found on the body surface, in sense organs ('neuromasts') arranged in lines over the head and body. These 'lateral line' hair cells detect local water flow ('touch at a distance'). Many fish and amphibians (e.g., sharks, sturgeons, salamanders) also have electrosensory lateral line organs that detect the weak electric fields around animals in water (primarily used for hunting live prey). In electroreceptive fish and amphibians, both types of sense organ, plus the neurons (nerve cells) that transmit information from hair cells and electroreceptors to the brain, develop from embryonic 'lateral line placodes' (patches of thickened skin on the head). Electroreception is an ancient sense, as old as the vertebrates. We know this because lampreys, from the only surviving lineage of jawless vertebrates, are electroreceptive. Lamprey electroreceptors seem to work in the same way as those in jawed vertebrates, and the same lateral line nerve (the anterior nerve) transmits their information to the same specialised part of the brain as in electroreceptive jawed vertebrates, suggesting this system was present in the common ancestor of all living vertebrates. How did electroreceptors evolve? We previously showed that bony fish electroreceptors are closely related to hair cells. For example, we found that 'transcription factor' genes (encoding proteins that control the activity of other genes) required for hair-cell formation are also active in developing ampullary organs. This suggests that electroreceptors likely evolved from lateral line hair cells. Uniquely, lateral line nerves in the lamprey are also needed for a reflex swimming response away from light. In lampreys, therefore, the lateral line system includes 'photoreceptor' cells that respond to light, as well as 'mechanoreceptors' (hair cells) that respond to local water movement, and electroreceptors that respond to electric fields. Light sensitivity has not been reported in the lateral line of jawed vertebrates, but several recent studies reported the expression of genes encoding opsins, which can detect light, in zebrafish neuromasts, including in hair cells. So, it is possible that some or all lamprey neuromast hair cells may be light-sensitive, as well as responding to water movement.In this grant proposal, we aim to shed light both on electroreceptor evolution and on the light-avoidance swimming reflex, by studying the development of hair cells, electroreceptors and photoreceptors (which may be hair cells) in embryos and larval stages of the sea lamprey. If electroreceptors evolved in the common ancestor of all living vertebrates, we would expect lamprey electroreceptors to develop from lateral line placodes and to express similar genes as in jawed-vertebrate electroreceptors. To test this, we will inject vital fluorescent dyes to follow the development of the lateral line placodes and the projections of the lateral line nerves (only the anterior lateral line nerve innervates electroreceptors, including those on the trunk); study the expression of genes known to be active in jawed-vertebrate electroreceptors, and the opsin genes reported in zebrafish neuromasts; and take an unbiased 'single-cell' sequencing approach to identify novel genes expressed by each receptor cell-type. Finally, we will use CRISPR to mutate some of these genes, to determine whether they are important for the formation of electroreceptors or the function of photoreceptors.Overall, this work will yield the first information on lamprey lateral line receptor-cell development, identify the cellular and molecular basis of the enigmatic lateral line-mediated light-avoidance reflex, and shed light on the evolution of vertebrate sensory receptor cells.