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 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.
Holsinger
L J
LJ
Department of Pathology, Howard Hughes Medical Institute, Beckman Center for Molecular and Genetic Medicine, Stanford University School of Medicine, California 94305, USA.
Graef
I A
IA
Swat
W
W
Chi
T
T
Bautista
D M
DM
Davidson
L
L
Lewis
R S
RS
Alt
F W
FW
Crabtree
G R
GR
D000199:Actins / Q000378:metabolism*; D000818:Animals; D018797:Cell Cycle Proteins; D003599:Cytoskeleton; D004268:DNA-Binding Proteins / Q000378:metabolism; D006801:Humans; D019169:Jurkat Cells; D051379:Mice; D050778:NFATC Transcription Factors; D009687:Nuclear Proteins; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000502:physiology*; D051567:Proto-Oncogene Proteins c-vav; D017260:Receptor-CD3 Complex, Antigen, T-Cell / Q000378:metabolism; D011948:Receptors, Antigen, T-Cell / Q000378:metabolism; D015398:Signal Transduction; D013601:T-Lymphocytes / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism; D014158:Transcription, Genetic
D000199:Actins; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D050778:NFATC Transcription Factors; C494003:NFATC1 protein, human; D009687:Nuclear Proteins; D011518:Proto-Oncogene Proteins; D051567:Proto-Oncogene Proteins c-vav; D017260:Receptor-CD3 Complex, Antigen, T-Cell; D011948:Receptors, Antigen, T-Cell; D014157:Transcription Factors; C496618:VAV1 protein, human; C496619:Vav1 protein, mouse