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Activation of T cells leads to the formation of the immunological synapse (IS) with antigen presenting cells. This requires T cell polarization and coordination between the actomyosin and microtubule cytoskeletons. The interactions between these two cytoskeletal components during T cell activation are not well understood. Here, we elucidate the interactions between microtubules and actin at the IS with high-resolution fluorescence microscopy. We show that microtubule growth dynamics in the peripheral actin-rich region are distinct from those in the central actin-free region. We further demonstrate that these differences arise from differential involvement of Arp2/3- and formin-nucleated actin structures. Formin inhibition results in a moderate decrease in microtubule growth rates, which is amplified in the presence of integrin engagement. In contrast, Arp2/3 inhibition leads to an increase in microtubule growth rates. We find that microtubule filaments are more deformed and exhibit greater shape fluctuations in the periphery of the IS compared to the center. Using small molecule inhibitors, we show that actin dynamics and actomyosin contractility play key roles in defining microtubule deformations and shape fluctuations. Our results indicate a mechanical coupling between the actomyosin and microtubule systems during T cell activation, whereby different actin structures influence microtubule dynamics in distinct ways. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] 

Regulation of membrane receptor mobility tunes cellular response to external signals, such as in binding of B cell receptors (BCR) to antigen, which initiates signaling. However, whether BCR signaling is regulated by BCR mobility, and what factors mediate this regulation, are not well understood. Here we use single molecule imaging to examine BCR movement during signaling activation and a novel machine learning method to classify BCR trajectories into distinct diffusive states. Inhibition of actin dynamics downstream of the actin nucleating factors, Arp2/3 and formin, decreases BCR mobility. Constitutive loss or acute inhibition of the Arp2/3 regulator, N-WASP, which is associated with enhanced signaling, increases the proportion of BCR trajectories with lower diffusivity. Furthermore, loss of N-WASP reduces the diffusivity of CD19, a stimulatory co-receptor, but not that of FcγRIIB, an inhibitory co-receptor. Our results implicate a dynamic actin network in fine-tuning receptor mobility and receptor-ligand interactions for modulating B cell signaling.

 

When B cells are exposed to antigens, they use their B‐cell receptors (BCRs) to transduce this external signal into internal signaling cascades and uptake antigen, which activate transcriptional programs. Signaling activation requires complex cytoskeletal remodeling initiated by BCR signaling. The actin cytoskeletal remodeling drives B‐cell morphological changes, such as spreading, protrusion, contraction, and endocytosis of antigen by mechanical forces, which in turn affect BCR signaling. Therefore, the relationship between the actin cytoskeleton and BCR signaling is a two‐way feedback loop. These morphological changes represent the indirect ways by which the actin cytoskeleton regulates BCR signaling. Recent studies using high spatiotemporal resolution microscopy techniques have revealed that actin also can directly influence BCR signaling. Cortical actin networks directly affect BCR mobility, not only during the resting stage by serving as diffusion barriers, but also at the activation stage by altering BCR diffusivity through enhanced actin flow velocities. Furthermore, the actin cytoskeleton, along with myosin, enables B cells to sense the physical properties of its environment and generate and transmit forces through the BCR. Consequently, the actin cytoskeleton modulates the signaling threshold of BCR to antigenic stimulation. This review discusses the latest research on the relationship between BCR signaling and actin remodeling, and the research techniques. Exploration of the role of actin in BCR signaling will expand fundamental understanding of the relationship between cell signaling and the cytoskeleton and the mechanisms underlying cytoskeleton‐related immune disorders and cancer.