Redundancy in B Cell Developmental Pathways: c-Cbl Inactivation Rescues Early B Cell Development through a B Cell Linker Protein-Independent Pathway (original) (raw)
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The Journal of Immunology, 2003
The Tec family member Bruton's tyrosine kinase (Btk) is a cytoplasmic protein tyrosine kinase that transduces signals from the pre-B and B cell receptor (BCR). Btk is involved in pre-B cell maturation by regulating IL-7 responsiveness, cell surface phenotype changes, and the activation of L chain gene rearrangements. In mature B cells, Btk is essential for BCR-mediated proliferation and survival. Upon BCR stimulation, Btk is transphosphorylated at position Y551, which promotes its catalytic activity and subsequently results in autophosphorylation at position Y223 in the Src homology 3 domain. To address the significance of Y223 autophosphorylation and the requirement of enzymatic activity for Btk function in vivo, we generated transgenic mice that express the autophosphorylation site mutant Y223F and the kinase-inactive mutant K430R, respectively. We found that Y223 autophosphorylation was not required for the regulation of IL-7 responsiveness and cell surface phenotype changes in differentiating pre-B cells, or for peripheral B cell differentiation. However, expression of the Y223F-Btk transgene could not fully rescue the reduction of L chain usage in Btk-deficient mice. In contrast, transgenic expression of kinase-inactive K430R-Btk completely reconstituted usage in Btk-deficient mice, but the defective modulation of pre-B cell surface markers, peripheral B cell survival, and BCRmediated NF-B induction were partially corrected. From these findings, we conclude that: 1) autophosphorylation at position Y223 is not essential for Btk function in vivo, except for regulation of L chain usage, and 2) during B cell development, Btk partially acts as an adapter molecule, independent of its catalytic activity.
Role of the BCR Complex in B Cell Development, Activation, and Leukemic Transformation
Immunologic Research, 2003
A primary focus of signal transduction in B cells, from the pre-B cell to the mature B cell, is the B cell receptor complex. Here we describe work demonstrating the importance of signaling via the pre-B cell receptor complex (pre-BCR) to the pre-B cell transition, the central checkpoint in B-cell development. We have shown tht pre-BCR complex components Igα and Igβ are critical to allowing the pre-B cell to move through this transition, but may not be required for allelic exclusion. Pre-BCR expression also directly affects the response of leukemic cells to steroid treatment, suggesting that signals initiated by the pre-BCR complex may present therapeutic targets in acute leukemia. Additionally, interleukin-7 may also modulate the response of leukemic cells arising from early B-cell stages to treatment. This observation has lead directly to proposals to test drugs which may antagonize early B-cell growth signals, such as rapamycin, in acute lymphoid leukemia.
Regulation of B Lymphocyte Development and Activation by Bruton's Tyrosine Kinase
Immunologic Research, 2001
The generation and maintenance of B lymphocytes is controlled by biochemical signals transmitted by the B cell antigen receptor (BCR) complex. These signals are transduced by multiple cytoplasmic protein tyrosine kinases (PTKs) including Lyn, Syk, and Bruton's tyrosine kinase (BTK). Upon BCR engagement, these PTKs activate downstream effectors, including transcription factors that modulate gene expression. In turn, activation of downstream effectors is critical for B cell survival, cell cycle progression, and antibody production. Our studies focus on the role of BTK in these biological responses. We have discovered that BTK is required for activation of the BCR-responsive transcription factor, NF-κB. Furthermore, BTK-dependent activation of NF-κB is essential for reprogramming the expression of genes that control B cell survival and proliferation. The biochemical mechanisms by which BTK regulates signaling components that activate NF-κB, and the identification of BTK-responsive genes are under investigation. Elucidation of these regulatory mechanisms is expected to reveal new therapeutic targets for B cell pathologies involving defects in BTK, including X-linked agammaglobulinemia (XLA).
The effects of c-Abl mutation on developing B cell differentiation and survival
International Immunology, 2009
c-Abl is a widely expressed Src family protein tyrosine kinase that is activated by chromosomal translocation in certain human leukemias. While shown in various experimental systems to regulate cell division and stress responses, its biological functions remain poorly understood. Although expressed at similar levels throughout B cell development, we found that the fraction of phosphorylated, active c-Abl peaks at the pro-B stage. We went on to perform a detailed analysis of B cell development in c-Abl-deficient mice. We confirmed a striking but variable decrease in pro-and pre-B cell numbers, a decrease in pre-B cell growth and an increase in pre-B cell apoptosis. This phenotype was not rescued by transgenic expression of a functional IgHC transgene and only partially rescued by the anti-apoptosis gene Bcl-x. Unlike their wild-type counterparts, c-Abl-deficient pre-B cells show a defect in Ca 21 flux upon cross-linking of CD19, a co-receptor known to be involved in pre-B cell receptor signaling and failed to express CD25 on the cell surface. Despite these pre-B cell-signaling defects, selection for in-frame heavy-chain rearrangements was intact in the mutant mice. Remarkably, we were able to rescue the proliferative defect by culturing cells in vitro with large amounts of rIL-7. We conclude that c-Abl is required for normal B cell differentiation and survival.
Mediation of transitional B cell maturation in the absence of functional Bruton’s tyrosine kinase
Scientific Reports, 2017
X-linked immune-deficient (Xid) mice, carrying a mutation in Bruton’s tyrosine kinase (Btk), have multiple B cell lineage differentiation defects. We now show that, while Xid mice showed only mild reduction in the frequency of the late transitional (T2) stage of peripheral B cells, the defect became severe when the Xid genotype was combined with either a CD40-null, a TCRbeta-null or an MHC class II (MHCII)-null genotype. Purified Xid T1 and T2 B cells survived poorlyin vitrocompared to wild-type (WT) cells. BAFF rescued WT but not Xid T1 and T2 B cells from death in culture, while CD40 ligation equivalently rescued both. Xid transitional B cellsex vivoshowed low levels of the p100 protein substrate for non-canonical NF-kappaB signalling.In vitro, CD40 ligation induced equivalent activation of the canonical but not of the non-canonical NF-kappaB pathway in Xid and WT T1 and T2 B cells. CD40 ligation efficiently rescued p100-null T1 B cells from neglect-induced deathin vitro. These da...
Bruton's tyrosine kinase is required for signaling the CD79b-mediated pro-B to pre-B cell transition
International Immunology, 2001
However, its molecular mechanisms in early B cell development are not fully understood. To examine whether BTK functions in CD79b-mediated signaling for the pro-B/pre-B transition, we utilized RAG2/BTK double-knockout (DKO) mice. Pro-B cells from RAG2/BTK-DKO mice did not differentiate into pre-B cells following CD79b cross-linking, although tyrosine phosphorylation of cellular proteins including Erk1/2 and phospholipase C-γ2 was induced in the same manner as RAG2-KO mice. BTK is phosphorylated after cross-linking of CD79b on RAG2-deficient pro-B cells. These findings suggest that BTK-dependent pathways downstream of CD79b are critical for the pro-B/pre-B transition and BTK-independent signaling pathways are also activated via the pre-BCR complex.
Insight into B cell development and differentiation
Acta Paediatrica, 2007
The main topic of this article is B cell development and differentiation, with a special focus on the mechanisms and molecules that regulate the expression of humoral immunity. Molecular epidemiological analysis was performed on the genes responsible for the X-linked agammaglobulinemia (XLA) phenotype of the majority of Italian patients and their distinct mutations were characterized. Mutations in Bruton's tyrosine kinase (BTK), a member of Tec Family of protein tyrosine kinases, have been found to be mainly responsible for XLA disease. The exact function of BTK in signal transduction is not yet known; thus, the specific role of BTK in receptor-dependent calcium signaling and the pro-antiapoptotic regulatory activity was addressed by transfecting RAMOS-1, a BTK-deficient human Burkitt's/B cell leukemia line with wild-type and mutant constructs. This work may provide clues about critical sites in the molecule and give support for gene therapy as a potential successful approach to XLA. Another aspect of this research is the identification and dissection of the molecular events that are likely to be directly related to the ability to express various isotypes of immunoglobulin with differing function and certain B cell immunodeficiency, mainly common variable disease and non-X-linked hyperIgM. B cell development and maturation steps in different compartments of the immune system are tracked by the analysis of cell-surface molecules and components of the signal transduction pathways, i.e. CD40, CD30, CD27, CD38, CD22 and CD24. A few components involved in B cell development, maturation and differentiation and their specific functional role are at least partially known, but these are far from fitting into an understandable pathway at present.