**2. GPCRs regulations**

Repeated or prolonged/continues agonist stimulation of GPCRs resulted in loss of receptor response characterized as receptor desensitization. It can be described as physical uncoupling of G proteins from their associate receptors subsequent in diminish their ability to initiate intracellular signaling cascades [26]. As shown in **Figure 1**, receptor desensitization process is initiated by receptor phosphorylation. It can be mediated by G protein-coupled receptor kinases (GRKs), which phosphorylate agonist-bound active receptor inducing homologous type of receptor desensitization [27]. Receptor phosphorylation can be also mediated by second messenger kinases such as PKA and PKC, which can phosphorylate receptors regardless of whether the GPCR is occupied by agonist or not, thus producing heterologous type of receptor desensitization [27**–**29]. Receptor phosphorylation subsequently increases the affinity of the receptors for β-arrestins proteins binding, consequently prevents further receptors-G protein interactions and therefore termination of G protein-related signaling

#### **Figure 1.**

*GPCR desensitization and related signaling transductions pathways. GPCR desensitization by GRKs and β-arrestins process initiated with ligand binding, receptor activation and dissociation of G protein. Consequently, GRKs phosphorylate agonist-occupied GPCRs (activated receptors) at third intracellular loop or C-terminal. Receptor phosphorylation resulting in enhances the affinity for β-arrestin recruitment then binding to the receptors causing termination of G-protein dependent transduction signaling and receptor desensitization. After that, the receptors undergo internalization and initiation of G protein-independent transduction signaling.*

[30]. Accordingly, the phosphorylated GPCR/β-arrestin complex is subjected for clathrin-mediated endocytosis, followed by either recycling, or degradation [31**–**33]. Importantly, β-arrestins function as ligand-regulated adaptor scaffolds that enable the transduction of signaling pathways in non-canonical manner [34].
