Background:
Antigen-receptor interactions on lymphocytes result in local clustering of actin, receptors and signaling molecules into an asymmetric membrane structure termed a cap. although actin polymerization is known to be required, the mechanisms underlying cap formation are unclear. we have studied the events underlying cap formation using mice bearing a null mutation in vav (vav-/-), a gene that encodes a guanine-nucleotide exchange factor for the gtpase rac.
Results:
Lymphocytes from vav-/- mice failed to form t-cell receptor caps following activation and had a defective actin cytoskeleton. the vav-/- t cells were deficient in interleukin-2 (il-2) production and proliferation, and the peak of ca2+ mobilization was reduced although of normal duration. activation of jun n-terminal kinase or stress-activated kinase (jnk or sapk) and mitogen-activated protein kinase (mapk) and the induction of the transcription factor nf-atc1 and egr-1 genes was normal. despite the reduced ca2+ mobilization, translocation of cytoplasmic nf-atc to the nucleus was normal, reflecting that the lower levels of ca2+ in vav-/- cells were still sufficient to activate calcineurin. treatment of lymphocytes with cytochalasin d, which blocks actin polymerization, inhibited cap formation and produced defects in signaling and il-2 transcriptional induction in response to antigen-receptor signaling that were nearly identical to those seen in vav-/- cells. in transfection studies, either constitutively active vav or rac could complement constitutively active calcineurin to activate nf-at-dependent transcription.
Conclusions:
These results indicate that vav is required for cap formation in lymphocytes. furthermore, the correlation between cap formation, il-2 production and proliferation supports the hypothesis that an actin-dependent pathway is a source of specialized growth regulatory signals.