To activate a G protein-coupled receptor permanently with cell surface photodynamic action in the gastrointestinal tract

Abstract

Different from reversible agonist-stimulated receptor activation, singlet oxygen oxidation activates permanently G protein-coupled receptor (GPCR) cholecystokinin 1 (CCK1R) in type II photodynamic action, with soluble photosensitizer dyes (sulphonated aluminum phthalocyanine, λ_max 675 nm) or genetically encoded protein photosensitizers (KillerRed λ_max 585 nm; mini singlet oxygen generator λ_max 450 nm), together with a pulse of light (37 mW/cm², 1-2 minutes). Three lines of evidence shed light on the mechanism of GPCR activated by singlet oxygen (GPCR-ABSO): (1) CCK1R is quantitatively converted from dimer to monomer; (2) Transmembrane domain 3, a pharmacophore for permanent photodynamic CCK1R activation, can be transplanted to non-susceptible M3 acetylcholine receptor; and (3) Larger size of disordered region in intracellular loop 3 correlates with higher sensitivity to photodynamic CCK1R activation. GPCR-ABSO will add to the arsenal of engineered designer GPCR such as receptors activated solely by synthetic ligands and designer receptors exclusively activated by designer drugs, but show some clear advantages: Enhanced selectivity (double selectivity of localized photosensitizer and light illumination), long-lasting activation with no need for repeated drug administration, antagonist-binding site remains intact when needed, ease to apply to multiple GPCR. This type of permanent photodynamic activation may be applied to functional proteins other than GPCR.

Keywords: Cholecystokinin 1 receptor; Singlet oxygen; G protein-coupled receptor activated by singlet oxygen; Genetically encoded protein photosensitizers; Calcium oscillations; Pancreatic acinar cells

Core Tip: G protein-coupled receptors (GPCR) are a large group of functional proteins, whose ligands account for a large proportion of clinical drugs. Important as GPCR may be, their ligand-independent pharmacology is poorly investigated. This minireview shows that the A-class GPCR cholecystokinin 1 receptor, predominantly expressed in pancreatic acini, could be activated permanently by singlet oxygen in photodynamic action. Three lines of evidence regarding mechanisms of photodynamic activation is presented. Clues suggest that permanent photodynamic activation of other GPCR and other functional proteins are possible, permanent photodynamic activation may be a general principle, both in gastrointestinal tract and elsewhere.