DEFECTS IN ACTIN-CAP FORMATION IN VAV-DEFICIENT MICE IMPLICATE AN ACTIN REQUIREMENT FOR LYMPHOCYTE SIGNAL-TRANSDUCTION

Background: Antigen-receptor interactions on lymphocytes result in loc al clustering of actin, receptors and signaling molecules into an asym metric membrane structure termed a cap. Although actin polymerization is known to be required, the mechanisms underlying cap formation are u nclear. We have studied the events underlying cap formation using mice bearing a null mutation in vav (vav(-/-)), a gene that encodes a guan ine-nucleotide exchange factor for the GTPase Pac. Results: Lymphocyte s from vav(-/-) mice failed to form T-cell receptor caps following act ivation and had a defective actin cytoskeleton. The vav(-/-) cells wer e 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 o r SAPK) and mitogen-activated protein kinase (MAPK) and the induction of the transcription factor NF-ATc1 and egr-1 genes was normal. Despit e the reduced Ca2+ mobilization, translocation of cytoplasmic NF-ATc t o the nucleus was normal, reflecting that the lower levels of Ca2+ in vav(-/-) cells were still sufficient to activate calcineurin. Treatmen t of lymphocytes with cytochalasin D, which blocks actin polymerizatio n, inhibited cap formation and produced defects in signaling and IL-2 transcriptional induction in response to antigen-receptor signaling th at were nearly identical to those seen in vav(-/-) cells. In transfect ion studies, either constitutively active Vav or Rac could complement constitutively active calcineurin to activate NF-AT-dependent transcri ption. Conclusions: These results indicate that Vav is required for ca p formation in lymphocytes, Furthermore, the correlation between cap f ormation, IL-2 production and proliferation supports the hypothesis th at an actin-dependent pathway is a source of specialized growth regula tory signals. (C) Current Biology Ltd ISSN 0960-9822.