Partial ligand-receptor engagement yields functional bias at the human complement receptor, C5aR1

The human complement component, C5a, binds two different seven transmembrane receptors termed C5aR1 and C5aR2. C5aR1 is a prototypical G protein-coupled receptor (GPCR) that couples to the Gαi sub-family of heterotrimeric G-proteins and β-arrestins (βarr) following C5a stimulation. Peptide fragments derived from the carboxyl-terminus of C5a can still interact with the receptor, albeit with lower affinity, and can act as agonists or antagonists. However, whether such fragments might display ligand bias at C5aR1 remains unexplored. Here, we compare C5a and a modified C-terminal fragment of C5a, C5apep, in terms of G protein coupling, βarr recruitment, endocytosis and ERK1/2 MAP kinase activation at the human C5aR1. We discover that C5apep acts as a full-agonist for Gαi-coupling as measured by cAMP response and ERK1/2 phosphorylation, but it displays partial agonism for βarr recruitment and receptor endocytosis. Interestingly, C5apep exhibits full-agonist efficacy with respect to inhibiting LPS induced IL-6 secretion in human macrophages, but its ability to induce human neutrophil migration is substantially lower compared to C5a although both these responses are sensitive to Pertussis toxin (PTX) treatment. Taken together, our data reveal that compared to C5a, C5apep exerts partial efficacy for βarr recruitment, receptor trafficking and neutrophil migration. Our findings therefore uncover functional bias at C5aR1, and also provide a framework that can potentially be extended to chemokine receptors which also typically interact with chemokines through a biphasic mechanism.