Btk/Tec kinases regulate sustained increases in intracellular Ca2+ following B-cell receptor activation

Bruton's tyrosine kinase (Btk) is essential for B-lineage development and represents an emerging family of non-receptor tyrosine kinases implicated in signal transduction events initiated by a range of cell surface receptors. Increased dosage of Btk in normal B cells resulted in a striking enhancement of extracellular calcium influx following B-cell antigen receptor (BCR) cross-linking. Ectopic expression of Btk, or related Btk/Tec family kinases, restored deficient extracellular Ca2+ influx in a series of novel Btk-deficient human B-cell lines. Btk and phospholipase Cgamma (PLCgamma) co-expression resulted in tyrosine phosphorylation of PLCgamma and required the same Btk domains as those for Btk-dependent calcium influx. Receptor-dependent Btk activation led to enhanced peak inositol trisphosphate (IP3) generation and depletion of thapsigargin (Tg)-sensitive intracellular calcium stores. These results suggest that Btk maintains increased intracellular calcium levels by controlling a Tg-sensitive, IP3-gated calcium store(s) that regulates store-operated calcium entry. Overexpression of dominant-negative Syk dramatically reduced the initial phase calcium response, demonstrating that Btk/Tec and Syk family kinases may exert distinct effects on calcium signaling. Finally, co-cross-linking of the BCR and the inhibitory receptor, FcgammaRIIb1, completely abrogated Btk-dependent IP3 production and calcium store depletion. Together, these data demonstrate that Btk functions at a critical crossroads in the events controlling calcium signaling by regulating peak IP3 levels and calcium store depletion.

BTK, the tyrosine kinase affected in X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is a heritable immunodeficiency disorder that is caused by a differentiation block leading to almost complete absence of B lymphocytes and plasma cells. The affected protein is a cytoplasmic protein tyrosine kinase, Bruton's agammaglobulinemia tyrosine kinase (Btk). Btk along with Tec, Itk and Bmx belong to a distinct family of protein kinases. These proteins contain five regions; PH, TH, SH3, SH2 and kinase domains. Mutations causing XLA may affect any of these domains. About 200 unique mutations have been identified and are collected in a mutation database, BTKbase. Here, we describle, the structure, function, and interactions of the affected signaling molecules in atomic detail.

HAEMOLYTIC ANAEMIA IN HODGKIN'S DISEASE ASSOCIATED WITH IMMUNOGLOBULIN DEFICIENCIES

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[Deficiencies in humoral immunity]

B cell immune deficiencies are characterized by inadequate production of antibodies and/or low levels of one or more classes of immune globulins (IgG, IgA, IgM) or IgG sub-classes. They include: 1) severe deficiencies involving all the immune globulins (Bruton agammaglobulinemia, variable hypogammaglobulinemia); 2) selective deficiencies in immune globulins (IgA deficiency, IgM deficiency, deficiencies in IgG sub-classes, dysgammaglobulinemias); 3) transient infantile hypogammaglobulinemia; 4) B cell immune deficiencies with normal gammaglobulin levels. Symptoms of B cell immune deficiencies are variable but respiratory manifestations are usually more prominent than the other features that include digestive disorders, fungal infections, autoimmune conditions, joint manifestations, and severe bacterial or viral infections (pneumococcus, meningococcus, enterovirus). Management of IgM and IgA deficiencies rests on antimicrobial agents and non-specific measures. The prognosis of the other B cell immune deficiencies has improved dramatically since effective replacement immune globulin therapy has become available; this treatment combined with antimicrobial therapy and chest physiotherapy can usually prevent development of bronchiectases. The main risk is chronic enteroviral neuromeningeal infection.