Individual neurons or muscle cells express many G-protein-coupled receptors (GPCRs) for neurotransmitters and neuropeptides, yet it remains unclear how cells integrate multiple GPCR signals that all must activate the same few G-proteins. We analyzed this issue in the Caenorhabditis elegans egg-laying system, where multiple GPCRs on muscle cells promote contraction and egg laying. We genetically manipulated individual GPCRs and G-proteins specifically in these muscle cells within intact animals and then measured egg laying and muscle calcium activity. Two serotonin GPCRs on the muscle cells, Gα_q-coupled SER-1 and Gα_s-coupled SER-7, together promote egg laying in response to serotonin. We found that signals produced by either SER-1/Gα_q or SER-7/Gα_s alone have little effect, but these two subthreshold signals combine to activate egg laying. We then transgenically expressed natural or designer GPCRs in the muscle cells and found that their subthreshold signals can also combine to induce muscle activity. However, artificially inducing strong signaling through just one of these GPCRs can be sufficient to induce egg laying. Knocking down Gα_q and Gα_s in the egg-laying muscle cells induced egg-laying defects that were stronger than those of a SER-1/SER-7 double knockout, indicating that additional endogenous GPCRs also activate the muscle cells. These results show that in the egg-laying muscles multiple GPCRs for serotonin and other signals each produce weak effects that individually do not result in strong behavioral outcomes. However, they combine to produce sufficient levels of Gα_q and Gα_s signaling to promote muscle activity and egg laying.

SIGNIFICANCE STATEMENT How can neurons and other cells gather multiple independent pieces of information from the soup of chemical signals in their environment and compute an appropriate response? Most cells express >20 GPCRs that each receive one signal and transmit that information through three main types of G-proteins. We analyzed how this machinery generates responses by studying the egg-laying system of C. elegans, where serotonin and multiple other signals act through GPCRs on the egg-laying muscles to promote muscle activity and egg laying. We found that individual GPCRs within an intact animal each generate effects too weak to activate egg laying. However, combined signaling from multiple GPCR types reaches a threshold capable of activating the muscle cells.

单个神经元或肌肉细胞表达许多神经递质和神经肽的 G 蛋白偶联受体 (GPCR)，但目前尚不清楚细胞如何整合多个 GPCR 信号，而这些信号都必须激活相同的少数 G 蛋白。我们在秀丽隐杆线虫产卵系统中分析了这个问题，其中肌肉细胞上的多个 GPCR 促进收缩和产卵。我们对完整动物的这些肌肉细胞中的个体 GPCR 和 G 蛋白进行了基因操作，然后测量了产卵和肌肉钙活性。肌肉细胞上的两个血清素 GPCR，Gα _q偶联的 SER-1 和 Gα _s偶联的 SER-7，共同响应血清素促进产卵。我们发现，单独由 SER-1/Gα _q或 SER-7/Gα _s产生的信号几乎没有影响，但这两个阈下信号结合起来激活产卵。然后，我们在肌肉细胞中转基因表达天然或设计的 GPCR，发现它们的阈下信号也可以结合起来诱导肌肉活动。然而，仅通过这些 GPCR 之一人为诱导强信号传导就足以诱导产卵。敲除产卵肌细胞中的 Gα _q和 Gα _s诱导的产卵缺陷比 SER-1/SER-7 双敲除的结果更强，表明额外的内源性 GPCR 也会激活肌细胞。这些结果表明，在产卵肌肉中，血清素和其他信号的多个 GPCR 各自产生微弱的影响，单独不会导致强烈的行为结果。 然而，它们结合产生足够水平的 Gα _q和 Gα _s信号传导，以促进肌肉活动和产卵。

意义陈述神经元和其他细胞如何从其环境中的化学信号汤中收集多个独立的信息并计算适当的反应？大多数细胞表达超过 20 个 GPCR，每个 GPCR 接收一个信号并通过三种主要类型的 G 蛋白传输该信息。我们通过研究线虫的产卵系统来分析这种机制如何产生反应，其中血清素和多种其他信号通过 GPCR 作用于产卵肌肉，以促进肌肉活动和产卵。我们发现，完整动物体内的各个 GPCR 产生的效应都太弱，无法激活产卵。然而，来自多种 GPCR 类型的组合信号达到能够激活肌肉细胞的阈值。