BS3054_GProteinIndependentSignalling_LECTURE1_2024_Bb

At the end of these two lectures, students should be able to:
- Provide detailed accounts of receptor desensitization mechanisms, internalization, re-sensitization, and down-regulation.
- Identify the roles of different families of signalling proteins, such as G Protein-Coupled Receptor Kinases (GRKs) and arrestins, in receptor regulation.
- Explain how receptor phosphorylation and arrestin binding can lead to G protein-independent cell signalling.
- Understand the significance of GRK/arrestin interactions in the drug discovery process.

GPCRs (G Protein-Coupled Receptors) respond to ligands by altering their conformation leading to the activation of G proteins.
Activated GPCR facilitates the exchange of GTP for GDP on the Gα subunit, initiating various signalling pathways, including PLC and ERK pathways.

A critical question in pharmacology is how GPCR signalling is regulated, specifically through phenomena like tachyphylaxis, drug tolerance, and desensitization.
Tachyphylaxis refers to the rapid decrease in response to a drug after its administration.

Molecular Mechanisms of Desensitization:
- Desensitization correlates with receptor phosphorylation; increases in phosphorylation result in desensitization.
- Two types of desensitization:
  - Homologous Desensitization: Specific to the receptor being stimulated, leading to decreased responsiveness.
  - Heterologous Desensitization: Affects collateral receptor signalling, inhibiting responses in receptors not directly exposed to the agonist.

Phosphorylation is a universal regulatory mechanism across GPCRs:
- Nearly all GPCRs undergo rapid phosphorylation after agonist stimulation.
- Sites of phosphorylation primarily occur in the C-terminal tail and the third intracellular loop (i3).
- Multiple phosphorylation sites are common, predominantly serine and to a lesser extent threonine and rarely tyrosine residues.

Identification of the proteins regulating homologous desensitization focuses on:
- βARK (β-adrenergic receptor kinase): Purified enzyme responsible for phosphorylating the β-adrenoceptor, thus facilitating desensitization.
- The correlation between receptor phosphorylation and desensitization is crucial; however, further investigation is required to ascertain if desensitization is solely due to phosphorylation or if additional mechanisms exist.

Arrestins function as important scaffolding proteins in receptor endocytosis; they aid in clathrin-mediated endocytosis of the receptor after phosphorylation.
Non-visual arrestins specifically interact with G protein-coupled receptors to promote internalization when activated by agonists.
These interactions significantly influence the regulation of GPCR signalling and the subsequent trafficking of receptors within the cell.

Understanding the complex interplay between GPCRs, GRKs, arrestins, and their signalling pathways is essential for developing therapies targeting these receptors and for advancing drug discovery methodologies.