N-WASP regulates the mobility of the B cell receptor and co-receptors during signaling activation

Regulation of membrane receptor mobility is important in tuning the cells response to external signals. This is particularly relevant in the context of immune receptor signaling. The binding of B cell receptors(BCR) to antigen induces B cell receptoractivation. While actin dynamics and BCR signaling are known to be linked, the role of actin dynamics in modulating receptor mobility is not well understood. Here, we use single molecule imaging to examine BCR movement during signaling activation and examine the role of actin dynamics on BCR mobility. We use a novel machine learning based method to classify BCR trajectories into distinct diffusive states and show that the actin regulatory protein N-WASP regulates receptor mobility. Constitutive loss or acute inhibition of N-WASP, which is associated with enhanced signaling, leads to a predominance of BCR trajectories with lower diffusivity and is correlated with a decrease in actin dynamics. Furthermore, loss of N-WASP reduces diffusivity of CD19, a stimulatory co-receptorof the BCR but not that of unstimulated FcYRIIB, an inhibitory co-receptor. The effect of N-WASP is mirrored by inhibition of the Arp2/3 complexand formins. Our results implicate the dynamic actin network in fine-tuning receptor mobility and receptor-ligand interactions, thereby modulating B cell signaling.