N-homocysteinylation of β-arrestins biases GPCR signaling and promotes platelet activation.

Hyperhomocysteinemia (HHcy) is strongly associated with cardiovascular diseases (CVDs), and it has been identified as a risk factor for thrombotic diseases. Most patients with HHcy die from various complications closely related to thrombotic diseases. However, the underlying mechanisms have not been fully elucidated. G protein-coupled receptors (GPCRs), the central regulators of the cardiovascular system, primarily control platelet activation. By examining the effects of HHcy on a panel of GPCRs involved in platelet aggregation, we found that HHcy systematically modulated biased GPCR signaling through the inhibition of desensitization by β-arrestins and the amplification of G protein signals. We further revealed that the N-homocysteinylation of β-arrestin1/2 at lysine (K) residues (K294/K296) disrupted the interaction between β-arrestins and GPCRs. The aforementioned phenomenon may be universal because HHcy was found to modulate the signaling bias of 9 other randomly selected GPCRs. Moreover, we found that the proinflammatory effects of homocysteine and homocysteine thiolactone were weakened in Arrb2-/- mice and that the reintroduction of wild-type but not K296R β-arrestin2 mutants (in mice) into primary peritoneal macrophages reversed these effects. Notably, in Arrb2K296R mice, HHcy-induced thrombus formation and platelet aggregation were reversed. Our results suggest that a G-biased agonist could be a better choice for disease therapy under HHcy conditions. Collectively, our findings demonstrate that the N-homocysteinylation of β-arrestin1/β-arrestin2 actively modulates the biased property of GPCR signaling, which contributes to the pathophysiology of HHcy-related CVDs and provides insight into the selection of agonists for the treatment of diseases under HHcy conditions.