Department of Pharmacology, Faculty of Medicine, University of Montreal, and Centre de Recherche de l'Hôpital Ste Justine, Montreal, Quebec, Canada.
Cell Signal. 2010 Mar;22(3):501-9. doi: 10.1016/j.cellsig.2009.11.004.
Serotonin 4 receptors (5-HT4Rs) are particularly abundant within the limbic system, where they constitute potential targets for the development of novel, rapid acting antidepressants. However, the population of limbic 5-HT4Rs is not homogenous, comprising various isoforms of which 5-HT4(a) and 5-HT4(b) are among the most abundant variants. Sequence divergence at their C-termini is predictive of specificity in isoform signalling and regulation, but the differences, if any, remain ill-defined. The present study compared isoforms 5-HT4(a) and 5-HT4(b) in their ability to undergo endocytic regulation following exposure to 5-HT and to the putatively fast acting antidepressant RS67333. Both ligands differed in their ability to induce internalization of either isoform, 5-HT being more effective than RS67333 in HEK293 cells and in neurons. In contrast, trafficking induced by 5-HT was isoform-specific. In particular, while PKC, GRK2 and betaarrestin were necessary for 5-HT4(a)R internalization, sequestration of 5-HT4(b)Rs required PKC but not GRK2 and relied significantly less on betaarrestin. After endocytosis, isoform (b) appeared scattered throughout the intracellular compartment and efficiently recycled to the membrane upon agonist removal. Isoform (a) accumulated in the perinuclear compartment and displayed little recycling. Isoform-specific subcellular distribution was present in HEK293 cells and in neurons. In neurons, where internalization by RS67333 was more pronounced than in HEK293 cells, receptors internalized by this ligand followed the same distribution pattern as observed with 5-HT. These results point to isoform-related differences in the way that 5-HTRs respond to different ligands. Such diversity should be taken into account when developing therapeutic agents that target 5-HT4Rs.
5-羟色胺 4 受体（5-HT4Rs）在边缘系统中特别丰富，它们是开发新型快速抗抑郁药的潜在靶点。然而，边缘系统中的 5-HT4R 群体并非同质的，由各种亚型组成，其中 5-HT4(a)和 5-HT4(b)是最丰富的变体之一。它们 C 端的序列差异预示着亚型信号转导和调节的特异性，但差异如果存在的话，仍然定义不明确。本研究比较了 5-HT4(a)和 5-HT4(b)亚型在暴露于 5-HT 和假定的快速抗抑郁药 RS67333 后发生内吞调节的能力。两种配体在诱导任一亚型内化的能力上有所不同，5-HT 在 HEK293 细胞和神经元中的作用比 RS67333 更有效。相比之下，5-HT 诱导的转运是亚型特异性的。特别是，虽然 PKC、GRK2 和β-arrestin 是 5-HT4(a)R 内化所必需的，但 5-HT4(b)R 的隔离需要 PKC，但不需要 GRK2，并且对β-arrestin的依赖性显著降低。内吞作用后，亚型(b)似乎散布在整个细胞内区室中，并在激动剂去除后有效地再循环到膜上。亚型(a)在核周区室中积累，很少再循环。亚型特异性亚细胞分布存在于 HEK293 细胞和神经元中。在神经元中，RS67333 的内化作用比在 HEK293 细胞中更为明显，用该配体内化的受体遵循与 5-HT 观察到的相同的分布模式。这些结果表明 5-HT4R 对不同配体的反应方式存在亚型相关差异。在开发针对 5-HT4R 的治疗药物时，应考虑这种多样性。
