BCR mediated signal transduction in immature and mature B cells.
Ligation of B cell receptors (BCR) on immature B cells may induce apoptosis, while in mature B cells it stimulates cell activation and growth. The signaling pathway regulating the differential functional response, death or survival of the B cell is not fully characterized. We have tested the intracellular signaling requirement of these processes using B cells isolated from the spleen of irradiated auto-reconstituted (transitional immature B cells) and untreated mice (mature B cells), respectively. We compared the BCR induced intracellular [Ca2+] transient, protein tyrosine phosphorylation and ERK phosphorylation, furthermore, the activation of Elk-1 and CREB transcription factors. The BCR induced rise of intracellular [Ca2+] did not significantly differ in the two populations, only a slight difference in the late phase of the response was observed. Immature B cells responded with a maximum tyrosine phosphorylation to a five times lower dose of anti-IgM compared to the mature population. Most importantly, we have found a significant difference in the tyrosine phosphorylation of the Gab family adaptor proteins, Gab1/2. In contrast to mature B cells, crosslinking of BCR on immature B cells did not induce tyrosine phosphorylation of Gab2, thus the Gab2-organized signal amplification complex could not be produced. Furthermore, we detected a significant difference in the kinetics of BCR induced ERK, Elk-1 and CREB phosphorylation. In immature B cells, ERK was transiently phosphorylated, ceasing after 120 min, while in mature cells, ERK phosphorylation was sustained. Elk-1 and CREB activation was also transient in immature B cells, followed the kinetics of ERK phosphorylation. The lack of sustained Erk1/2 activation suppresses the transcription factors necessary for the proliferation signal. Since ERK is regulated by the phosphorylated Gab1/2, these data demonstrate that BCR triggered phosphorylation and signal amplification of Gab1/2 is a critical step in a life or death decision of B cells.
Koncz
Gábor
G
Research Group of the Hungarian Academy of Sciences at the Department of Immunology, L. Eötvös University, Budapest, Hungary.
Bodor
Csaba
C
Kövesdi
Dorottya
D
Gáti
Róbert
R
Sármay
Gabriella
G
D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D000944:Antigens, Differentiation, B-Lymphocyte / Q000032:analysis; D017209:Apoptosis; D016175:B-Lymphocyte Subsets / Q000145:classification; D001402:B-Lymphocytes / Q000276:immunology*; D002118:Calcium / Q000378:metabolism; D002454:Cell Differentiation; D002478:Cells, Cultured; D007700:Kinetics; D008213:Lymphocyte Activation; D051379:Mice; D008807:Mice, Inbred BALB C; D020928:Mitogen-Activated Protein Kinases / Q000502:physiology; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D011947:Receptors, Antigen, B-Cell / Q000276:immunology*; D015398:Signal Transduction; D013154:Spleen / Q000166:cytology / Q000276:immunology; D013234:Stem Cells / Q000276:immunology; D014157:Transcription Factors / Q000378:metabolism
D048868:Adaptor Proteins, Signal Transducing; D000944:Antigens, Differentiation, B-Lymphocyte; C490894:Gab2 protein, mouse; D010750:Phosphoproteins; D011947:Receptors, Antigen, B-Cell; D014157:Transcription Factors; D020928:Mitogen-Activated Protein Kinases; D002118:Calcium