A new mode of cAMP signalling: the adenylyl cyclase-IP3 receptor junction

所有的细胞都把自己包裹在一层不可渗透的屏障——质膜中，将自己与周围环境隔离开来。没有质膜，就不可能保持细胞内与细胞外的不同组成。但质膜也是与周围环境基本交流的障碍。跨越质膜的受体蛋白通过跨越屏障传递信息来克服这个问题。这些受体选择性地结合细胞外环境中的化学物质，改变它们的形状，然后改变形状的蛋白质的细胞内部分可以与细胞内蛋白质相互作用，启动信号级联反应，导致细胞活性的变化。通常，这些蛋白-蛋白在质膜上的相互作用会产生可溶性的细胞内化学物质，即第二信使，它将信息传递给许多不同的细胞内目标。这些信号级联最显著的特征是它们的经济性：数千种不同的细胞外刺激，由相似数量的受体检测到，聚集在一起调节细胞内少量第二信使的浓度。然后，它们选择性地调节数百种不同的细胞过程，包括神经递质的释放、收缩、基因表达和细胞生长和死亡。这些信号通路中的缺陷是常见疾病的原因，所涉及的蛋白质是最成功的药物靶点之一。这么少的第二信使是如何在如此多的细胞外刺激和如此多的细胞反应之间进行特定的通信的？有三个重要的特性促成了这种多功能性。首先，第二信使，如Ca2+和cAMP，是在细胞内空间组织的：细胞一部分Ca2+浓度的增加可以引起与其他地方相同的增加非常不同的影响。其次，信号通路之间的相互作用允许复杂的信息处理，因此只有当两个通路同时活跃时才可能发生响应。第三，信号蛋白，如制造cAMP的腺苷酸环化酶，有不同的色调（亚型），它们的调节和与其他蛋白质的相互作用不同。我们对这些特征之间的相互作用及其对可靠信息处理的重要性的理解仍处于起步阶段，但治疗干预的新机会确实有希望。我们的工作涉及两种普遍存在的信号蛋白：腺苷酸环化酶（AC）和IP3受体（IP3R）。后者介导了动物细胞外刺激引起的大多数Ca2+信号。我们最近的工作表明，这两种蛋白之间存在一种完全出乎意料的通信模式，cAMP直接从AC的特定亚型（AC6）传递到IP3R的特定亚型（IP3R2），以增加其对IP3的敏感性。我们认为这些信号连接可能是第二信使途径的普遍特征，允许质膜和细胞内蛋白质之间的强大信号传递以及每个连接内的信息自主处理。我们的工作旨在确定IP3R上新的cAMP结合位点，定义AC6-IP3R连接的组成和结构，其调控及其对cAMP信号的差异解码的贡献。我们的工作解决了细胞生物学中的一个基本问题，即信息如何从质膜可靠地传递到细胞内部，并为确定新的未来药物靶点铺平了道路。

All cells separate themselves from their surroundings by wrapping themselves in an impermeable barrier, the plasma membrane, without which it would be impossible to maintain an intracellular composition distinct from that outside. But the plasma membrane is also a barrier to essential communication with the surroundings. Receptor proteins that span the plasma membrane overcome this problem by transmitting information across the barrier. These receptors selectively bind chemicals in the extracellular environment, change their shape, and the re-shaped intracellular part of the protein can then interact with intracellular proteins to initiate signalling cascades that lead to changes in cellular activity. Very commonly these protein-protein interactions at the plasma membrane lead to production of soluble intracellular chemicals, second messengers, that convey information to many different intracellular targets. The most remarkable feature of these signalling cascades is their economy: thousands of different extracellular stimuli, detected by a similar number of receptors converge to regulate the intracellular concentration of just a handful of second messengers. These then selectively regulate many hundreds of different cellular processes, including release of neurotransmitters, contraction, gene expression and cell growth and death. Defects within these signalling pathways are causes of common diseases and the proteins involved are among the most successful of drug targets. How can so few second messengers function to allow specific communication between so many extracellular stimuli and so many cellular responses? Three important features contribute to this versatility. First the second messengers, like Ca2+ and cAMP, are spatially organized within cells: an increase in Ca2+ concentration in one part of a cell can evoke very different effects to the same increase elsewhere. Second, interactions between signalling pathways allow complex information processing, such that a response may occur only when 2 pathways are simultaneously active. Third, signalling proteins, like the adenylyl cyclase that makes cAMP, come in different hues (subtypes) that differ in their regulation and interactions with additional proteins. Our understanding of the interplay between these features and their significance for reliable information processing is still in its infancy but there is real promise of new opportunities for therapeutic intervention. Our work is concerned with 2 ubiquitous signalling proteins: adenylyl cyclase (AC) and IP3 receptors (IP3R). The latter mediate most Ca2+ signals evoked by extracellular stimuli in animal cells. Our recent work suggests an entirely unexpected mode of communication between these 2 proteins, with cAMP passing directly from a specific subtype of AC (AC6) to a specific subtype of IP3R (IP3R2) to increase its sensitivity to IP3. We suggest that these signalling junctions, which may be a universal feature of second messenger pathways, allow robust signalling between the plasma membrane and intracellular proteins and autonomous processing of information within each junction. Our work seeks to identify the novel cAMP-binding site on the IP3R, to define the composition and structure of the AC6-IP3R junction, its regulation and its contribution to differential decoding of cAMP signals by the proteins that respond to it. Our work addresses a fundamental issue in cell biology, how information passes reliably from the plasma membrane to the cell interior, and it paves the way to identifying novel future drug targets.