G protein-coupled receptors (GPCRs) are a vast family of membrane-bound proteins crucial for transmitting external stimuli into intracellular signals, thereby influencing numerous physiological processes. These receptors typically engage specific G protein subtypes, such as Gs, Gi/o, Gq/11, and G12/13, at the initial GPCR-G protein association step, ensuring precise downstream signalling activation. Traditionally, it was believed that GPCRs selectively activate only their cognate G protein subtypes, avoiding interactions with non-cognate G proteins [1]. However, recent studies have unveiled a complex layer of GPCR functioning, introducing the concept of 'unproductive coupling'[1] and a fascinating phenomenon known as GPCR priming [2].
The concept of unproductive coupling was highlighted by Okashah et al. in 2020 [1]. Using BRET based approaches, the Gs-coupled vasopressin V2 receptor (V2R) was shown to form a stable complex with the non-cognate G12 protein in response to arginine vasopressin. Surprisingly, this interaction did not result in G12 activation, even in the presence of GTP, which is typically necessary for G protein activation. Instead, G12 overexpression inhibited V2R downstream signalling, including β-arrestin recruitment and receptor internalization. This unproductive coupling revealed that non-cognate G protein interactions could modulate GPCR signalling pathways without initiating traditional activation sequences.
In 2017, Gupte et al. introduced GPCR priming, which posits that non-cognate G proteins can enhance the coupling efficiency of cognate G proteins to GPCRs, thereby amplifying canonical downstream signalling [2]. This priming effect occurs through non-functional interactions between non-cognate G proteins and GPCRs, facilitating subsequent coupling of cognate G proteins and promoting enhanced signalling. Gupte et al. utilized a systematic protein affinity strength modulation (SPASM) technique to show that Gq proteins could bolster Gs dependent-β2-adrenergic receptor-mediated cAMP signalling, while Gs proteins similarly enhanced Gq dependent-vasopressin receptor-mediated IP1 accumulation.
The enhanced signalling observed in GPCR priming is attributed to the formation of temporal non-cognate-GPCR conformational states, which enable more efficient interactions between the GPCR and the cognate Gα C terminus [2, 3]. This suggests that the non-cognate G proteins, although not activating downstream pathways directly, prepare the GPCR in a manner that optimizes subsequent cognate G protein activation. Using CRISPR-Cas9 knockout HEK cells lacking Gs proteins, a study by Stallaert et al. in 2017 showed that Gs played a pivotal role in calcium signalling at the β2-adrenergic receptors (β2AR) [4]. Overall, the increased calcium signalling was attributed to the release of ATP via β2AR-Gs mediated activation, which subsequently transactivates purinergic receptors through Gq in intracellular stores [4]. Thus, this finding raises the possibility that GPCR priming might result not only from intricate receptor-G protein interactions but also from the influence of downstream effector mechanisms. However, IP1 signalling through the non-cognate Gq was not important for the increased β2AR-Gs-mediated cAMP levels in response to isoproterenol. [2].
GPCR priming opens new avenues for understanding the nuanced regulatory mechanisms governing GPCR signalling. It challenges the traditional view of strict cognate G protein coupling and suggests a more dynamic interaction landscape where non-cognate G proteins play a critical preparatory role. This has significant implications for drug development, as targeting these non-cognate interactions could lead to novel therapeutic strategies for modulating GPCR activity.
In conclusion, GPCR priming represents a compelling area of research that promises to deepen our understanding of cellular signalling networks. Continued exploration into the molecular mechanisms underlying this phenomenon could unveil new dimensions of GPCR functionality and offer innovative approaches to treating a wide range of diseases.
References
1. Okashah, N., et al., Agonist-induced formation of unproductive receptor-G(12) complexes. Proc Natl Acad Sci U S A, 2020. 117(35): p. 21723-21730.
2. Gupte, T.M., et al., Priming GPCR signaling through the synergistic effect of two G proteins. Proceedings of the National Academy of Sciences, 2017. 114(14): p. 3756-3761.
3. Hilger, D., et al., Structural insights into differences in G protein activation by family A and family B GPCRs. Science, 2020. 369(6503).
4. Stallaert, W., et al., Purinergic Receptor Transactivation by the β(2)-Adrenergic Receptor Increases Intracellular Ca(2+) in Nonexcitable Cells. Mol Pharmacol, 2017. 91(5): p. 533-544.
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