Marine Biological Laboratory, Woods Hole, Massachussets, United States of America.
PLoS One. 2012;7(1):e29813. doi: 10.1371/journal.pone.0029813. Epub 2012 Jan 3.
Melanopsin, the receptor molecule that underlies light sensitivity in mammalian 'circadian' receptors, is homologous to invertebrate rhodopsins and has been proposed to operate via a similar signaling pathway. Its downstream effectors, however, remain elusive. Melanopsin also expresses in two distinct light-sensitive cell types in the neural tube of amphioxus. This organism is the most basal extant chordate and can help outline the evolutionary history of different photoreceptor lineages and their transduction mechanisms; moreover, isolated amphioxus photoreceptors offer unique advantages, because they are unambiguously identifiable and amenable to single-cell physiological assays. In the present study whole-cell patch clamp recording, pharmacological manipulations, and immunodetection were utilized to investigate light transduction in amphioxus photoreceptors. A G(q) was identified and selectively localized to the photosensitive microvillar membrane, while the pivotal role of phospholipase C was established pharmacologically. The photocurrent was profoundly depressed by IP₃ receptor antagonists, highlighting the importance of IP₃ receptors in light signaling. By contrast, surrogates of diacylglycerol (DAG), as well as poly-unsaturated fatty acids failed to activate a membrane conductance or to alter the light response. The results strengthen the notion that calcium released from the ER via IP₃-sensitive channels may fulfill a key role in conveying--directly or indirectly--the melanopsin-initiated light signal to the photoconductance; moreover, they challenge the dogma that microvillar photoreceptors and phoshoinositide-based light transduction are a prerogative of invertebrate eyes.
黑视蛋白是哺乳动物“生物钟”受体中光敏感性的受体分子，与无脊椎动物视蛋白同源，并且据推测通过类似的信号通路发挥作用。然而，其下游效应物仍不明确。黑视蛋白也在文昌鱼神经管中的两种不同的光敏感细胞类型中表达。文昌鱼是现存最基础的脊索动物，可以帮助描绘不同光感受器谱系及其转导机制的进化历史；此外，分离的文昌鱼光感受器具有独特的优势，因为它们可以明确识别并且易于进行单细胞生理测定。在本研究中，利用全细胞膜片钳记录、药理学操作和免疫检测来研究文昌鱼光感受器中的光转导。鉴定出一种 G(q)，并选择性地定位于感光微绒毛膜，同时药理学建立了磷脂酶 C 的关键作用。IP₃ 受体拮抗剂显著抑制了光电流，突出了 IP₃ 受体在光信号中的重要性。相比之下，二酰基甘油 (DAG) 的替代物以及多不饱和脂肪酸既不能激活膜电导，也不能改变光反应。结果强化了这样的观点，即通过 IP₃ 敏感通道从内质网释放的钙可能直接或间接在将黑视蛋白引发的光信号传递到光电导中发挥关键作用；此外，它们挑战了微绒毛光感受器和基于磷酸肌醇的光转导是无脊椎动物眼睛的特权的教条。
