Abstract
To ensure precision and specificity of ligand-receptor-induced signaling, co-receptors and modulatory factors play important roles. The membrane-bound ligand Nogo-A (an isoform encoded by RTN4) induces inhibition of neurite outgrowth, cell spreading, adhesion and migration through multi-subunit receptor complexes. Here, we identified the four-transmembrane-spanning protein tetraspanin-3 (TSPAN3) as a new modulatory co-receptor for the Nogo-A inhibitory domain Nogo-A-Δ20. Single-molecule tracking showed that TSPAN3 molecules in the cell membrane reacted to binding of Nogo-A with elevated mobility, which was followed by association with the signal-transducing Nogo-A receptor sphingosine-1-phosphate receptor 2 (S1PR2). Subsequently, TSPAN3 was co-internalized as part of the Nogo-A-ligand-receptor complex into early endosomes, where it subsequently separated from Nogo-A and S1PR2 to be recycled to the cell surface. The functional importance of the Nogo-A-TSPAN3 interaction is shown by the fact that knockdown of TSPAN3 strongly reduced the Nogo-A-induced S1PR2 clustering, RhoA activation, cell spreading and neurite outgrowth inhibition. In addition to the modulatory functions of TSPAN3 on Nogo-A-S1PR2 signaling, these results illustrate the very dynamic spatiotemporal reorganizations of membrane proteins during ligand-induced receptor complex organization.