Signaling by Diffusion

Sci. Signal.  16 Mar 2010:
Vol. 3, Issue 113, pp. ec80
DOI: 10.1126/scisignal.3113ec80

The B cell receptor (BCR) consists of a membrane immunoglobulin (IgM or IgD) that binds to antigen and a transmembrane immunoglobulin heterodimer (Igα and Igβ) that mediates signaling. The BCR can generate a tonic signal in the absence of antigen stimulation, although the underlying mechanisms have been unclear (see also Sasaki and Kurosaki). Treanor et al. proposed that the mobility of the BCR in the plasma membrane was linked to its ability to induce tonic signaling. The authors performed total internal reflection fluorescence microscopy (TIRFM) and found that although some fluorescently labeled BCRs were highly mobile within the plasma membrane, most showed slow, restricted diffusion in naive, unstimulated primary B cells. A chimera consisting of the extracellular domain of IgM fused to the transmembrane and cytoplasmic domain of major histocompatibility complex (MHC) class I (IgM-H2 BCR) was more mobile than wild-type IgM, an effect that was rescued by replacing the MHC class I intracellular domain with that of Igβ. BCRs in areas of high actin density were less mobile than those in areas of low actin density. Wild-type BCRs rarely exited actin-rich areas, whereas the IgM-H2 chimera entered and exited actin-rich areas. The slow-diffusing BCRs that were detected outside actin-rich areas were located in networks rich in the cytoskeletal protein ezrin. Agents that disrupted actin polymerization (such as latrunculin A, cytochalasin D, and jasplakinolide) induced events typical of B-cell activation in naive B cells, such as intracellular increases in calcium concentration and phosphorylation of extracellular signal–regulated kinase (ERK). However, these signaling events were reduced in latrunculin A–treated cells lacking signaling molecules downstream of the BCR, including phospholipase C γ2 (PLCγ2) and the guanine nucleotide exchange factors Vav1 and Vav2. Disruption of the actin cytoskeleton increased the mobility of the BCR, and increasing concentrations of cytochalasin D correlated with increases in intracellular calcium concentration. Furthermore, BCR mobility was reduced in B cells lacking PLCγ2 or Vav1 and Vav2. Thus, the authors propose that the cytoskeleton restricts the diffusion of the BCR, and thereby its ability to initiate signal transduction events, and that tonic signaling is mediated by the mobile fraction of BCR in the plasma membrane.

B. Treanor, D. Depoil, A. Gonzalez-Granja, P. Barral, M. Weber, O. Dushek, A. Bruckbauer, F. D. Batista, The membrane skeleton controls diffusion dynamics and signaling through the B cell receptor. Immunity 32, 187–199 (2010). [PubMed]

Y. Sasaki, T. Kurosaki, Immobile BCRs: The safety on the signal trigger. Immunity 32, 143–144 (2010). [PubMed]