GPCR oligomer

Receptor homodimers – which consist of two identical GPCRs – are the simplest homomeric GPCR oligomers. Receptor heterodimers – which consist of two different GPCRs – are the simplest heteromeric GPCR oligomers.

The existence of receptor oligomers is a general phenomenon, whose discovery has superseded the prevailing paradigmatic concept of the function of receptors as plain monomers, and has far-reaching implications for the understanding of neurobiological diseases as well as for the development of drugs.

Discovery

For a long time it was assumed that receptors transmitted their effects exclusively from their basic functional forms – as monomers. The first clue to the existence of GPCR oligomers goes back to 1975 when Robert Lefkowitz observed that β-adrenoceptors display negative binding cooperativity. At the beginning of the 1980s, it was hypothesized, receptors could form larger complexes, the so-called mosaic form, where two receptors may interact directly with each other. Mass determination of β-adrenoceptors (1982) and muscarinic receptors (1983), supported the existence of homodimer or tetrameric complexes. In 1991, the phenomenon of receptor crosstalk was observed between adenosine A_2A (A2A) and dopamine D₂ receptor (DRD2) thus suggesting the formation of heteromers. While initially thought to be a receptor heterodimer, a review from 2015 determined that the A2A-DRD2 heteromer is a heterotetramer composed of A2A and DRD2 homodimers (i.e., two adenosine A2A receptors and two dopamine D2 receptors). Maggio and co-workers showed in 1993 the ability of the muscarinic M₃ receptor and α2C-adrenoceptor to heterodimerize. The first direct evidence that GPCRs functioned as oligomers in vivo came from Overton and Blumer in 2000 by fluorescence resonance energy transfer (FRET) analysis of the α-factor receptor in the yeast Saccharomyces cerevisiae. In 2005, further evidence was provided that receptor oligomerization plays a functional role in a living organism with regulatory implication. The crystal structure of the CXCR4 dimer was published in 2010.