Lapatinib Initially studies of GPCRs predominantly assessed
Initially, studies of GPCRs predominantly assessed the signalling pathways downstream of receptors on the cell surface. There is now an understanding that GPCRs can localize to and signal from various intracellular compartments, such as the nucleus (reviewed in ). These intracellular pools of receptors can lead to distinct signalling pathways from those activated by the same receptor at the cellular surface. In adult rat ventricular cardiomyocytes, ETBR localizes along the nuclear membrane, whereas ETAR is mainly found at the cell surface . In the α1-AR family, both subtypes found in adult cardiomyocytes localize to the nucleus and to a lesser extent the cell surface . The nuclear GPCR population activates proximal signalling pathways similar to those on the cell membrane, but also have more direct effects on nuclear activities such as transcription initiation and gene expression (reviewed in [40, 43]). Studies assessing these nuclear specific events have used both the ETR and α1-AR interchangeably as both are thought to predominantly couple to Gαq. Furthermore, the receptor subtype-specific signalling that occurs in distinct cellular compartments has not been addressed.
Here, we have used transcriptome analysis of primary neonatal rat cardiomyocytes treated with either the ETR agonist endothelin-1 or the α1-AR agonist phenylephrine to assess differences in their respective signalling networks, and further probed these differences using a panel of fluorescent resonance energy transfer (FRET)- and Lapatinib resonance energy transfer (BRET)-based biosensors. We also used genetically-encoded biosensors targeted to specific cellular compartments to compare differential signalling by distinct GPCR populations. These experiments revealed unexpected specificity in signalling function, both among the receptor subtypes tested and between subcellular compartments.
Discussion Here we have shown that two GPCR subtypes thought to trigger similar signalling events by coupling to Gαq in fact regulate different signalling networks via coupling to distinct G proteins. Thus global effects regulated by both receptors in events such as cardiac hypertrophy should be assessed independently. We first demonstrated distinct signalling pathway responses activated by the α1-AR compared to the ETR in hypertrophic cardiomyocytes. The upregulation of CREM expression after 1.5 h stimulation of the α1-AR suggested a concomitant increase in cAMP levels following receptor activation, as some CREM isoforms are upregulated in response to cAMP [56, 57]. We explored this potential signalling pathway further as upregulation of cAMP synthesis by catecholamines, the endogenous ligands for α1-AR, is usually associated with βAR receptor activation. We used a heterologous expression system with a panel of FRET- and BRET-based biosensors in HEK 293SL cells and the pertinent subtypes of the α1-AR and ETR that regulate cardiac hypertrophy. Whereas ETAR stimulation did not increase cAMP production or PKA activity, both α1A-AR and α1B-AR were able to generate cellular cAMP accumulation and PKA activation in a Gαs-dependent manner. This expands the current view of the α1-AR subfamily, which is classically associated with Gαq, to include regulation of cAMP and PKA through Gαs.