5-Hydroxytryptamine4 Receptor Activation of the Extracellular Signal-regulated Kinase Pathway Depends on Src Activation but Not on G Protein or β-Arrestin Signaling

Abstract

The 5-hydroxytryptamine4 (5-HT4) receptors have recently emerged as key modulators of learning, memory, and cognitive processes. In neurons, 5-hydroxytryptamine4 receptors (5-HT4Rs) activate cAMP production and protein kinase A (PKA); however, nothing is known about their ability to activate another key signaling pathway involved in learning and memory: the extracellular signal-regulated kinase (ERK) pathway. Here, we show that 5-HT4R stimulation, in primary neurons, produced a potent but transient activation of the ERK pathway. Surprisingly, this activation was mostly PKA independent. Similarly, using pharmacological, genetic, and molecular tools, we observed that 5-HT4Rs in human embryonic kidney 293 cells, activated the ERK pathway in a Gs/cAMP/PKA-independent manner. We also demonstrated that other classical G proteins (Gq/Gi/Go) and associated downstream messengers were not implicated in the 5-HT4R–activated ERK pathway. The 5-HT4R–mediated ERK activation seemed to be dependent on Src tyrosine kinase and yet totally independent of β-arrestin. Immunocytofluorescence revealed that ERK activation by 5-HT4R was restrained to the plasma membrane, whereas p-Src colocalized with the receptor and carried on even after endocytosis. This phenomenon may result from a tight interaction between 5-HT4R and p-Src detected by coimmunoprecipitation. Finally, we confirmed that the main route by which 5-HT4Rs activate ERKs in neurons was Src dependent. Thus, in addition to classical cAMP/PKA signaling pathways, 5-HT4Rs may use ERK pathways to control memory process.