Biomedical subjects

Gábor Koncz

Publications and source records attributed to Gábor Koncz.

4 recordsLinked to original sources

PubMed2004

Functional consequences of a MAPK docking site on human FcgammaRIIb.

Type IIb Fcgamma receptors (FcgammaRIIb) have a major role in regulating B cell activation. Upon its co-aggregation with the B cell receptors (BCR) via immune complexes FcgammaRIIb become phosphorylated on tyrosine within its immunoreceptor tyrosine based inhibitory motif (ITIM) and in turn recruit protein- and inositol phosphatases, inhibiting thereby signal transduction. The intracellular domain of the human FcgammaRIIb has a membrane proximal motif that is very similar to those of MAPK docking site in MAPK-interacting molecules. Additionally, in contrast to the mouse, a serine residue is located next to this motif that is a potential phosphorylation site for Ser/Thr kinases. Our aim was to study the role of the putative MAPK docking motif on FcgammaRIIb mediated function. We report here that MAPKs bind to FcgammaRIIb affinity purified from the detergent extracts of anti-IgM activated and BCR-FcgammaRIIb co-clustered B cells. We detected extracellular signal regulated kinase (ERK) activity in FcgammaRIIb immunoprecipitates and identified the bound proteins as 85, 44 and 42kDa ERKs by Western blots. Active ERKs bound to the synthetic peptide representing the putative docking site of FcgammaRIIb on a Ser/Thr phosphatase dependent manner. The FcgammaRIIb-associated ERKs may phosphorylate the membrane proximal serine of the receptor. We examined the consequences of serine phosphorylation by comparing the proteins that interact with synthetic peptides comprising the combined sequences of the MAPK docking site and the ITIM either in phosphorylated or in non-phosphorylated forms. The results indicate that phosphorylation on serine modifies the binding of Lyn to FcgammaRIIb, thus might negatively regulate phosphorylation of ITIM.

Amino Acid Motifs↗

PubMed2004

Functional mapping of the Fc gamma RII binding site on human IgG1 by synthetic peptides.

Receptors specific for the Fc part of IgG (Fc gamma R) are expressed by several cell types and play diverse roles in immune responses. Impaired function of the activating and inhibitory Fc gamma R may result in autoimmunity. Thus, the modulation of IgG-Fc gamma R interaction can be a target for the development of treatments for some autoimmune and inflammatory diseases. This study addresses the localization and functional characterization of linear sequences in human IgG1 which bind to Fc gamma RII. Peptides with overlapping sequences derived from the CH2 domain of human IgG1 between P(234) and S(298) were synthesized and used in binding and functional experiments. Binding of the peptides to Fc gamma R was assayed in vitro and ex vivo, and peptides found to interact were functionally tested. The shortest effective peptide was T(256)-P(271), which bound to soluble recombinant Fc gamma RIIb with K(d)=6 x 10(6) M(-1). The biotinylated peptides R(255)-P(271) and T(256)-P(271) complexed by avidin exhibited functional activity; they induced Fc gamma RIIb-mediated inhibition of the BCR-triggered Ca(2+) response of human Burkitt lymphoma cells, and inflammatory cytokine production (TNF-alpha and IL-6) by the human monocyte cell line MonoMac. In conclusion, our results suggest that the selected peptides functionally represent the Fc gamma RII-binding part of IgG1.

Binding Sites↗

PubMed2002

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.

Adaptor Proteins, Signal Transducing↗

PubMed2002

Developmental differences in B cell receptor-induced signal transduction.

We have compared early signaling events at various stages of B cell differentiation using established mouse cell lines. Clustering of pre-B cell antigen receptor (BCR) or BCR induced the tyrosine phosphorylation of various proteins in all cells, although the phosphorylation pattern differed. In spite of the pre-BCR-induced tyrosine phosphorylation, we could not detect an intracellular Ca(2+) signal in pre-B cells. However, co-clustering of the pre-BCR with CD19 did induce Ca(2+) mobilization. In contrast to the immature and mature B cells, where the B cell linker protein (BLNK) went through inducible tyrosine phosphorylation upon BCR clustering, we observed a constitutive tyrosine phosphorylation of BLNK in pre-B cell lines. Both BLNK and phospholipase C (PLC)gamma were raft associated in unstimulated pre-B cells, and this could not be enhanced by pre-BCR engagement, suggesting a ligand-independent PLC gamma-mediated signaling. Further results indicate that the cell lines representing the immature stage are more sensitive to BCR-, CD19- and type II receptors binding the Fc part of IgG (Fc gamma RIIb)-mediated signals than mature B cells.

Adaptor Proteins, Signal Transducing↗