Ligand-Independent Spontaneous Activation of Purinergic P2Y₆ Receptor Under Cell Culture Soft Substrate

G protein-coupled receptors (GPCRs) exist in the conformational equilibrium between inactive state and active state, where the proportion of active state in the absence of a ligand determines the basal activity of GPCRs. Although many GPCRs have different basal activity, it is still unclear whether physiological stresses such as substrate stiffness affect the basal activity of GPCRs. In this study, we identified that purinergic P2Y₆ receptor (P2Y₆R) induced spontaneous Ca²⁺ oscillation without a nucleotide ligand when cells were cultured in a silicon chamber. This P2Y₆R-dependent Ca²⁺ oscillation was absent in cells cultured in glass dishes. Coating substrates, including collagen, laminin, and fibronectin, did not affect the P2Y₆R spontaneous activity. Mutation of the extracellular Arg-Gly-Asp (RGD) motif of P2Y₆R inhibited spontaneous activity. Additionally, extracellular Ca²⁺ was required for P2Y₆R-dependent spontaneous Ca²⁺ oscillation. The GPCR screening assay identified cells expressing 10 GPCRs, including purinergic P2Y₁R, P2Y₂R, and P2Y₆R, that exhibited spontaneous Ca²⁺ oscillation under cell culture soft substrate. Our results suggest that stiffness of the cell adhesion surface modulates spontaneous activities of several GPCRs, including P2Y₆R, through a ligand-independent mechanism.