The gene involved in X-linked agammaglobulinaemia is a member of the src family of protein-tyrosine kinases

Immunoglobulin recombinase gene activity is modulated reciprocally by interleukin 7 and CD19 in B cell progenitors

Bone marrow stromal cells promote B cell development involving recombinase gene-directed rearrangement of the immunoglobulin genes. We observed that the stromal cell-derived cytokine interleukin 7 (IL-7) enhances the expression of CD19 molecules on progenitor B-lineage cells in human bone marrow samples and downregulates the expression of terminal deoxynucleotidyl transferase (TdT) and the recombinase-activating genes RAG-1 and RAG-2. Initiation of the TdT downregulation on the first day of treatment, CD19 upregulation during the second day, and RAG-1 and RAG-2 downmodulation during the third day implied a cascade of IL-7 effects. While CD19 ligation by divalent antibodies had no direct effect on TdT or RAG gene expression, CD19 cross-linkage complete blocked the IL-7 downregulation of RAG expression without affecting the earlier TdT response. These results suggest that signals generated through CD19 and the IL-7 receptor could modulate immunoglobulin gene rearrangement and repertoire diversification during the early stages of B cell differentiation.

Src family protein tyrosine kinases induce autoactivation of Bruton's tyrosine kinase

Bruton's tyrosine kinase (Btk) is tyrosine phosphorylated and enzymatically activated following ligation of the B-cell antigen receptor. These events are temporally regulated, and Btk activation follows that of various members of the Src family of protein tyrosine kinases, thus raising the possibility that Src kinases participate in the Btk activation process. We have evaluated the mechanism underlying Btk enzyme activation and have explored the potential regulatory relationship between Btk and Src protein kinases. We demonstrate in COS transient-expression assays that Btk can be activated through intramolecular autophosphorylation at tyrosine 551 and that Btk autophosphorylation is required for Btk catalytic functions. Coexpression of Btk with members of the Src family of protein tyrosine kinases, but not Syk, led to Btk tyrosine phosphorylation and activation. Using a series of point mutations in Blk (a representative Src protein kinase) and Btk, we show that Src kinases activate Btk through an indirect mechanism that requires membrane association of the Src enzymes as well as functional Btk SH3 and SH2 domains. Our results are compatible with the idea that Src protein tyrosine kinases contribute to Btk activation by indirectly stimulating Btk intramolecular autophosphorylation.

Activation of Tsk and Btk tyrosine kinases by G protein beta gamma subunits

Tsk/Itk and Btk are members of the pleckstrin-homology (PH) domain-containing tyrosine kinase family. The PH domain has been demonstrated to be able to interact with beta gamma subunits of heterotrimeric guanine nucleotide-binding proteins (G proteins) (G beta gamma) and phospholipids. Using cotransfection assays, we show here that the kinase activities of Tsk and Btk are stimulated by certain G beta gamma subunits. Furthermore, using an in vitro reconstitution assay with purified bovine brain G beta gamma subunits and the immunoprecipitated Tsk, we find that Tsk kinase activity is increased by G beta gamma subunits and another membrane factor(s). These results indicate that this family of tyrosine kinases could be an effector of heterotrimeric G proteins.

Defective B cell development and function in Btk-deficient mice

Mutations in the Bruton's tyrosine kinase (Btk) gene have been linked to severe early B cell developmental blocks in human X-linked agammaglobulinemia (XLA), and to milder B cell activation deficiencies in murine X-linked immune deficiency (Xid). To elucidate unequivocally potential Btk functions in mice, we generated mutations in embryonic stem cells, which eliminated the ability to encode Btk pleckstrin homology or kinase domains, and assayed their effects by RAG2-deficient blastocyst complementation or introduction into the germline. Both mutations block expression of Btk protein and lead to reduced numbers of mature conventional B cells, severe B1 cell deficiency, serum IgM and IgG3 deficiency, and defective responses in vitro to various B cell activators and in vivo to immunization with thymus-independent type II antigens. These results prove that lack of Btk function results in an Xid phenotype and further suggest a differential requirement for Btk during the early stages of murine versus human B lymphocyte development.

Role of tyrosine kinases in lymphocyte activation: targets for drug intervention

Recent developments in our understanding of lymphocyte receptor-associated signalling events have offered many new potential targets for modifying antigen and cytokine receptor signalling events in immune-related diseases such as allergy, autoimmunity and transplant rejection. As discussed below, these targets are largely tissue-restricted and are functionally confined to a limited set of receptors. Therefore, it is anticipated that selective inhibitors of these signalling events would offer safe and effective therapies for immunologically-based diseases. First, we review T and B cell antigen receptor signalling as targets for inhibiting lymphocyte responses. Second, targets in lymphocyte cytokine receptor signalling pathways are discussed. Finally, we review strategies for inhibition of receptor signalling.

An SH3-binding site conserved in Bruton's tyrosine kinase and related tyrosine kinases mediates specific protein interactions in vitro and in vivo

Mutations in Bruton's tyrosine kinase (Btk) have been associated with immunodeficiencies in man and in the mouse. Btk and two related proteins, Itk and Tec, are members of a distinct family of tyrosine kinases. These kinases are believed to function in various receptor-mediated signaling pathways, but their specific functions are as yet undefined. Btk and its homologues share extensive sequence similarity, including a conserved region, the Tec-homology (TH) domain, that has been proposed to mediate specific intermolecular or intramolecular interactions. The TH region of Btk contains a functional SH3-binding site at residues 189-192. SH3 binding is selective: Btk is retained by the SH3 domain of Fyn but not by that of Blk, another Src-type kinase. TH-SH3 binding in vitro is abolished by specific, single amino acid substitutions within the Btk TH domain or the Fyn SH3 domain. We provide two lines of evidence that the SH3-binding site in the Btk TH domain mediates protein interactions in intact cells. First, treatment of cells with pervanadate induces an increase in the phosphotyrosine content of kinase-inactive Btk; this response is substantially reduced by a mutation that inactivates the SH3-binding site in the Btk TH domain. Second, in cell lysates Btk is found in association with an as yet unidentified 72-kDa phosphotyrosine-containing protein; this interaction requires a functional SH3-binding site in the TH domain. The TH domain may therefore interact in vivo with other proteins that regulate the phosphorylation state of Btk.

Impaired expansion of mouse B cell progenitors lacking Btk

Mutations in the gene encoding the protein tyrosine kinase Btk are associated with the human B cell immunodeficiency X-linked agammaglobulinemia (XLA). In the mouse, a point mutation in the Btk pleckstrin homology domain segregates with a milder X-linked immunodeficiency (xid). To assess the importance of Btk function in murine lymphopoiesis, we generated multiple embryonic stem cell clones bearing a targeted disruption of the btk gene and examined their potential to produce lymphocytes in both C57BL/6 and RAG2-/- host chimeric animals. These mice provide a complementary set of in vivo competition assays that formally establish the genetic basis for the xid phenotype. Although the null mutation yields a phenotype quite similar to that of xid, it also compromises expansion of B cell precursors. Our results suggest that the murine and human consequences of Btk deficiency differ only quantitatively, and represent the same disease process.

Involvement of p59fynT in interleukin-5 receptor signaling

Previous studies implicate the nonreceptor protein tyrosine kinase (PTK) p59fyn in the propagation of signals from the B cell antigen receptor. To elucidate the functions of this kinase, we examined B cell responsiveness in mice engineered to lack the hematopoietic isoform of p59fyn. Remarkably, antigen receptor signaling was only modestly defective in fynTnull B cells. In contrast, signaling from the interleukin (IL)-5 receptor which ordinarily provides a comitogenic stimulus with antiimmunoglobulin, was completely blocked. Our results document the importance of p59fynT in IL-5 responses in B cells, and they support a general model for cytokine receptor signal transduction involving the simultaneous recruitment of at least three families of PTK.

Structural basis for X-linked agammaglobulinemia (XLA): mutations at interacting Btk residues R562, W563, and A582

It has been suggested that tryptophan 563 is sandwiched between residues R562 and A582 in Bruton's agammaglobulinemia tyrosine kinase (Btk). Mutations of the surrounding residues have been shown to cause X-linked agammaglobulinemia. Substitutions R562P and A582V were noticed to have impaired kinase activity. However, based on Western blot analysis, the mutant proteins were expressed at normal levels. Molecular modeling of the kinase domain has previously indicated that these residues presumably govern the position of the W563 side chain, which is thought to interact with the catalytic loop. W563 is inside the molecule and too far away from the catalytic center to interact directly with the substrate or cofactors. To prove these model-based conclusions, a conservative substitution with phenylalanine for W563 was made, and the resultant mutant lacked kinase activity. These results confirm our previous assumption that the side chain of W563, invariant in protein tyrosine kinases, is crucial for Btk kinase activity. Mutations in the surrounding residues seem to inactivate Btk by affecting the location of W563.

Nonreceptor protein tyrosine kinase involvement in signal transduction and immunodeficiency disease

The nonreceptor protein tyrosine kinases (PTKs) have been grouped into 10 different enzyme families based on predicted amino acid sequences. As the number of enzymes belonging to the nonreceptor class of PTK is increasing, one challenge is to determine how these various classes of PTKs interact within the cell to promote signal transduction. Herein, the activation of four classes of nonreceptor PTKs is discussed in relation to their interactions with each other as well as with other signaling molecules during the process of lymphocyte surface antigen receptor-mediated activation. Recent findings of nonreceptor PTK loss-of-function mutations in different immunodeficiency diseases has revealed the important contribution of this group of enzymes to lymphocyte development.

GapIII, a new brain-enriched member of the GTPase-activating protein family

Ras GTPase-activating proteins (GAPs) are negative regulators of ras, which controls proliferation and differentiation in many cells. Ras GAPs have been found in a variety of species from yeast to mammals. We describe here a newly identified mammalian GAP, GapIII, which was obtained by differential screening of a rat oligodendrocyte cDNA library. GapIII putatively encodes a 834 amino acid protein with a predicted molecular weight of 96 kDa, which contains a consensus GAP-related domain (GRD). The protein encoded by this cDNA has high homology with Gap1m, which was recently identified as a putative mammalian homolog of Drosophila Gap1. These proteins contain three structural domains, an N-terminal calcium-dependent phospholipid binding domain, GRD, and a C-terminal PH/Btk domain. Because of the sequence homology and the structural similarities of this protein with Gap1m, we hypothesize that GapIII and Gap1m may be members of a mammalian GAP gene family, separate from p120GAP, neurofibromin (NF1), and IQGAP. To confirm the GapIII protein activity, constructs containing different GapIII-GRD domains were transformed into iral mutant yeast to determine their relative ability to replace IRA1 functionally. Constructs that contained essentially the full-length protein (all three domains), the GRD alone, or the GRD plus PH/Btk domain suppressed heat shock sensitivity of ira1, whereas constructs that contained the GRD with part of the PH/Btk domain had only a weak ability to suppress heat shock sensitivity. These results suggest that the GapIII GRD itself is sufficient to down-regulate ras proteins in yeast. Expression of GapIII mRNA (4.2 kb) was examined by Northern analysis and in situ hybridization. This mRNA was expressed at highest levels in the brain, where its expression increased with development. Lower levels of the mRNA were expressed in the spleen and lung. Among neural cells, GapIII mRNA was expressed in neurons and oligodendrocytes, but not in astrocytes. Interestingly, the expression pattern in brain is reminiscent of type 1 NF1 expression reported by Gutmann et al. (Cell Growth Differ in press, 1995). We propose that in addition to p120GAP and neurofibromin, the GapIII/Gap1m family may be important for modulating ras activity in neurons and oligodendrocytes during normal brain development and in particular in the adult brain.

Protein tyrosine phosphorylation as a mechanism of signalling in mast cells and basophils

Tyrosine phosphorylation of proteins is a mechanism of signalling for different receptors and is important for cell growth and differentiation. Mast cells and basophils are secretory cells that play a role in inflammatory and immediate allergic reactions. The activation/aggregation of different surface receptors on these cells induces tyrosine phosphorylation of proteins. Because these signals are essential for the function of basophils and mast cells, characterizing these pathways could provide methods to specifically regulate the function of these cells. Here we discuss the signals generated by three receptors: the high affinity IgE receptor (Fc epsilon RI) the growth factor receptor, Kit, and integrins.

Common variable immunodeficiency: clinical aspects and recent progress in identifying the immunological defect(s)

Common variable immunodeficiency (CVID) comprises a heterogeneous group of patients with as yet undefined genetic defects. Patients with CVID have in common a decrease in the levels of one or more serum immunoglobulin isotypes and a severe defect in the production of specific antibodies. Typically, the patients suffer from recurrent infections of the upper and lower respiratory tract or the gastrointestinal tract. In consequence of these infections patients may develop severe organ damage, such as chronic pulmonary disease with bronchiectases, leading to pulmonary failure. Early diagnosis of CVID is important, as antibody deficiency can efficiently be treated by regular intravenous IgG (IVIG) substitution therapy. IVIG therapy prevents the occurrence of further acute infectious episodes and the development of long-term complications. The basic immunological defect(s) in patients with CVID are still unknown. There is currently no convincing evidence for an intrinsic B-cell defect in patients with CVID. A defect in T-cell activation due to impaired signal transduction upon T-cell receptor triggering has been described in a large subgroup of patients with CVID. Defective T-cell activation may lead to an impairment in cognate T-B-cell interaction due to impaired expression of CD40 ligand and/or abnormalities in the production T-cell-derived cytokines required for fully functional B-cell activation, proliferation and/or differentiation which could indeed explain the impairment in antibody production present in CVID patients.

The protein product of the c-cbl protooncogene is phosphorylated after B cell receptor stimulation and binds the SH3 domain of Bruton's tyrosine kinase

X-linked agammaglobulinemia, a B cell immunodeficiency, is caused by mutations in the Bruton's tyrosine kinase (Btk) gene. The absence of a functional Btk protein leads to a failure of B cell differentiation and antibody production. B cell receptor stimulation leads to the phosphorylation of the Btk protein and it is, therefore, likely that Btk is involved in B cell receptor signaling. As a nonreceptor tyrosine kinase, Btk is likely to interact with several proteins within the context of a signal transduction pathway. To understand such interactions, we have generated glutathione S-transferase fusion proteins corresponding to different domains of the human Btk protein. We have identified a 120-kD protein present in human B cells as being bound by the SH3 domain of Btk and which, after B cell receptor stimulation, is one of the major substrates of tyrosine phosphorylation. We have shown that this 120-kD protein is the protein product of c-cbl, a protooncogene, which is known to be phosphorylated in response to T cell receptor stimulation and to interact with several other tyrosine kinases. Association of the SH3 domain of Btk with p120cbl provides evidence for an analogous role for p120cbl in B cell signaling pathways. The p120cbl protein is the first identified ligand of the Btk SH3 domain.

Protein kinases--structure and function

The solution of crystal structures from half a dozen protein kinases during the last four years in different laboratories has deepened our understanding of the catalysis and regulation of this enzyme class, and given a vigorous impetus to the whole field. Due to the great degree of sequence conservation among protein kinases the informational yield with every new structure is high, as each is a representative of the enzyme family in general and most often of a subclass in particular. This review will focus on the active site structure of cAMP-dependent protein kinase (cAPK) with special regard to two new crystal structures; one of an active protein kinase CK1*, which may represent an as yet unsolved step in the kinetic pathway, and the other of the insulin receptor kinase domain, the first structure of a tyrosine kinase.

Genetic immunodeficiencies: cutaneous manifestations and recent progress

In recent years, remarkable progress has been made in elucidating the pathophysiology of genetic immunodeficiency disorders. Dermatologic manifestations are prominent in these conditions; because of advances in diagnosis and therapy, patients are living longer, increasing the likelihood that dermatologists will encounter patients with these diseases. The genes of many of these disorders have been cloned, including chronic granulomatous disease, X-linked immunodeficiencies, and myeloperoxidase deficiency. Understanding the regulation and function of these genes will not only affect patients with these rare disorders, but may provide an insight into common dermatologic conditions, such as eczema and cutaneous infection. Diagnosis, dermatologic manifestations, and therapy are discussed.

The SH2 domains of Src family kinases associate with Syk

Src family kinases (Lyn, Fyn, Lck, and Blk) and Syk, a tandem SH2 domain containing tyrosine kinase, have been demonstrated to be associated with the antigen receptor in B cells. Both of these categories of tyrosine kinases are presumed to be critical players in the process of antigen-mediated signal transduction. Cross-linking of membrane immunoglobulin on the surface of B cells leads to the activation of Lyn, Fyn, and Blk, which presumably associate with the cytoplasmic tails of the membrane immunoglobulin-associated Ig alpha/beta heterodimer. Receptor ligation also leads to the tyrosine phosphorylation and catalytic activation of Syk, but the mechanism of association of this kinase with the antigen receptor remains to be established. A number of phosphoproteins that can associate with the SH2 domains of Blk, Lyn, and Fyn have been described in activated B cells. We demonstrate here that Syk is one of the proteins in the lysates of activated B cells which bind to the SH2 domains of Src family kinases. Syk binds directly to the SH2 domain of Blk and complexes in vivo with Lyn and Blk in activated B cells.

B-cell-specific demethylation of BTK, the defective gene in X-linked agammaglobulinemia

BTK, the gene that is defective in X-linked agammaglobulinemia, encodes a cytoplasmic tyrosine kinase that is critical for B-cell proliferation, or survival. To identify regulatory elements that control the expression of BTK we evaluated the methylation pattern of this gene in cell lines and in freshly isolated cells. An Hpa II site that was specifically demethylated in mature B cells but not in pre-B cells, T cells, neutrophils, or nonhematopoietic cells was identified in the tenth intron of BTK. In a 40 kilobase (kb) segment of DNA spanning the entire coding region of BTK plus 3 kb upstream of the first exon there were no other sites that demonstrated lineage-specific demethylation. The B-cell-specific demethylation site in intron 10, which falls within the SH2 domain, 26 kb distal to the first exon, occurs in a region rich in regulatory elements including two E2 boxes, two AP-2 sites, and a cAMP response element. It is likely that this site plays a role in maintaining BTK transcription in mature B cells.

Production of monoclonal antibodies to Bruton's tyrosine kinase

Bruton's X-linked agammaglobulinemia is caused by mutations in a cytoplasmic protein tyrosine kinase termed Bruton's tyrosine kinase (BTK). The protein is expressed in all members of the B cell lineage and is critical for B cell development. The protein consists of several modules, including a pleckstrin homology domain and the Src homology domains SH1, SH2, and SH3. We report here the production of monoclonal antibodies against the pleckstrin homology domain of human BTK. The antibody was produced by immunizing mice with a FLAG-BTK fusion protein. Hybridoma supernatants were screened by ELISA using a GST-BTK fusion protein as the antigen. Selected monoclonal antibodies recognize denatured BTK on Western blots of peripheral blood mononuclear cell lysates. Mouse BTK protein is also detected. These antibodies should be useful in assessing patients with immune deficiency, as well as in studying normal B cell development.

In-vitro analyses of mechanisms of B-cell development

B-cell lymphopoiesis in vivo is very complex due to the influences of cooperating cells, cytokines and other receptor-ligand interactions which appear to occur developmentally at different cellular stages. Therefore in-vitro models will help to unravel this complex situation. Here, we review our and others' work on in-vitro models of B-cell development. The role of stromal cells, cytokines, surrogate light chain and products of rearranged Ig-loci in the developmentally different cellular stages will be discussed.

Syk protein-tyrosine kinase is regulated by tyrosine-phosphorylated Ig alpha/Ig beta immunoreceptor tyrosine activation motif binding and autophosphorylation

Syk is a cytoplasmic protein-tyrosine kinase containing two amino-terminal Src homology 2 domains that is activated following ligation of the B cell antigen receptor. Syk activation in B cells correlates with Syk tyrosine phosphorylation as well as with Syk SH2-mediated association with the tyrosine-phosphorylated Ig alpha and Ig beta B cell antigen receptor subunits. Tyrosine-phosphorylated peptide 20-mers representing Ig alpha and Ig beta immunoreceptor tyrosine activation motifs were synthesized and found to stimulate the specific activity of Syk by as much as 10-fold in vitro. Maximal phosphopeptide-induced Syk activation required both Syk SH2 domains and phosphorylation of both tyrosine residues present in the immunoreceptor tyrosine activation motif. The biochemical mechanism responsible for the phosphopeptide-induced Syk enzyme activation appears to be a function of Syk autophosphorylation. Our observations suggest the association of Syk tandem SH2 domains with the tyrosine-phosphorylated Ig alpha and/or Ig beta immunoreceptor tyrosine activation motifs in B cells stimulates Syk autophosphorylation leading to Syk enzyme activation.

Activation of Fes tyrosine kinase by gp130, an interleukin-6 family cytokine signal transducer, and their association

gp130 is a signal-transducing subunit of receptors for the interleukin-6 (IL-6)-related cytokine subfamily including IL-6, leukemia inhibitory factor, oncostatin M, IL-11, and ciliary neurotrophic factor, indicating that gp130-mediated signals are involved in the immune response, hematopoiesis, inflammation, and endocrine and nervous system activity. We previously showed that gp130 stimulation rapidly activates Jak, Btk, and Tec tyrosine kinases, all of which constitutively associate with gp130. To further elucidate intracellular signal transduction through gp130, we examined the possible involvement of another nonreceptor tyrosine kinase, p92c-fes (Fes). We showed that gp130 stimulation rapidly induced tyrosine phosphorylation of Fes and actually activated its kinase activity in hematopoietic lineage cells. Furthermore, Fes associated with gp130 independently of ligand stimulation like Jak, Btk, and Tec tyrosine kinases. These results indicate that multiple nonreceptor tyrosine kinases are involved in the gp130-mediated signal transduction pathway. Because both gp130 and Fes are expressed not only in hematopoietic lineage cells but also in heart and nerve cells, Fes may play a role in signal transduction through gp130 in these tissues.

Activation of Bruton's tyrosine kinase (BTK) by a point mutation in its pleckstrin homology (PH) domain

Bruton's tyrosine kinase (BTK) is a nonreceptor tyrosine kinase critical for B cell development and function. Mutations in BTK result in X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice. Using a random mutagenesis scheme, we isolated a gain-of-function mutant called BTK* whose expression drives growth of NIH 3T3 cells in soft agar. BTK* results from a single point mutation in the pleckstrin homology (PH) domain, where a Glu is replaced by Lys at residue 41. BTK* shows an increase in phosphorylation on tyrosine residues and an increase in membrane targeting. Transforming activity requires kinase activity, a putative autophosphorylation site, and a functional PH domain. Mutation of the SH2 or SH3 domains did not affect the activity of BTK*. Expression of BTK* could also relieve IL-5 dependence of a B lineage cell line. These results show that transformation activation and regulation of BTK are critically dependent on the PH domain.

Sixty-nine kilobases of contiguous human genomic sequence containing the alpha-galactosidase A and Bruton's tyrosine kinase loci

Several disease loci have been mapped to the Xq21.3-Xq22 region of the human X Chromosome (Chr) including X-linked agammaglobulinemia (XLA), Fabry disease, Alport syndrome, and Pelizaeus Merzbacher disease. Upon cloning of the XLA gene, Bruton's tyrosine kinase (btk), both Fabry disease and XLA were mapped within the same 50- to 70-kb interval. In order to investigate the genomic organization of the region surrounding btk and the Fabry disease gene, alpha-galactosidase A (gla), we constructed a 6-cosmid contig spanning the region from 5' of gla to 3' of btk. Two of these cosmids spanning most of the coding sequence and the upstream region of btk and gla, U237D10 and U230D1, were sequenced by a random shotgun strategy combined with automated sequencing, resulting in 69 kb of contiguous genomic sequence. Sequencing of U237D10 showed btk to be comprised of 19 exons spanning over 35 kb. Sequencing of U230D1 showed that the 3' end of gla is 9 kb from the 5' end of btk and also demonstrated the presence of two additional genes in the region immediately 5' to btk. The surprisingly high gene density is similar to that seen previously only in the human major histocompatibility locus.

YAC contigs mapping the human COL4A5 and COL4A6 genes and DXS118 within Xq21.3-q22

Sequence-tagged sites (STSs) were developed for three loci of uncertain X chromosomal localization (DXS122, DXS137, and DXS174) and were used to seed YAC contigs. Two contigs now total about 3.3 Mb formatted with 34 STSs. One contains DXS122 and DXS174 within 250 kb on single YACs; it is placed in Xq21.3-q22.1 by FISH analysis, which is consistent with somatic cell hybrid panel analyses and with the inclusion of a probe that detects polymorphism at the DXS118 locus already assigned to that general region. The other contig, which contains DXS137, is in Xq22.2 by FISH, consistent with cell hybrid analyses and with the finding that it covers the human COL4A5 and COL4A6 genes known to be in that vicinity. In addition to extending the cloned coverage of this portion of the X chromosome, these materials should aid, for example, in the further analysis of Alport syndrome.

AH/PH domain-mediated interaction between Akt molecules and its potential role in Akt regulation

The cytoplasmic serine-threonine protein kinase coded for by the c-akt proto-oncogene features a protein kinase C-like catalytic domain and a unique NH2-terminal domain (AH domain). The AH domain is a member of a domain superfamily whose prototype was observed in pleckstrin (pleckstrin homology, or PH, domain). In this communication, we present evidence that the AH/PH domain is a domain of protein-protein interaction which mediates the formation of Akt protein complexes. The interaction between c-akt AH/PH domains is highly specific, as determined by the failure of this domain to bind AKT2. The AH/PH domain-mediated interactions depend on the integrity of the entire domain. Akt molecules with deletions of the NH2-terminal portion (amino acids 11 to 60) and AH/PH constructs with deletions of the C-terminal portion of this domain (amino acids 107 to 147) fail to interact with c-akt. To determine the significance of these findings, we carried out in vitro kinase assays using Akt immunoprecipitates from serum-starved and serum-starved, platelet-derived growth factor-stimulated NIH 3T3 cells. Addition of maltose-binding protein-AH/PH fusion recombinant protein, which is expected to bind Akt, to the immunoprecipitates from serum-starved cells induced the activation of the Akt kinase.

Transmembrane signaling by antigen receptors of B and T lymphocytes

The specificity of immune responses depends upon the activation of only those lymphocytes that recognize the introduced antigen. In recent years, a great deal has been learned about the structure of lymphocyte receptors for antigens and about their signal transduction mechanism. These receptors activate intracellular protein tyrosine kinases of at least two families, the Src family and the Syk/ZAP-70 family. Recent studies have given us considerable insight into the interactions of these two types of kinases and how they mediate antigen receptor signaling.

Structure-function relationships in Src family and related protein tyrosine kinases

There is increasing evidence to suggest that cytoplasmic tyrosine kinases of the Src family have a pivotal role in the regulation of a number of cellular processes. Members of this family have been implicated in cellular responses to a variety of extracellular signals, such as those arising from growth factors and cell-cell interactions, as well as in differentiative and developmental processes in both vertebrates and invertebrates. A better understanding of the regulation and of the structure-function relationships of these enzymes might aid in the development of specific ways to interfere with their action, as well as serving as a paradigm for regulation of other protein tyrosine kinases that have SH2 and SH3 domains. In this review we will first discuss the regulation of Src family protein tyrosine kinases, with particular emphasis on their SH2 and SH3 domains. We will then briefly review other non-receptor protein tyrosine kinases that have SH2 and SH3 domains.

A new X linked recessive deafness syndrome with blindness, dystonia, fractures, and mental deficiency is linked to Xq22

X linked recessive deafness accounts for only 1.7% of all childhood deafness. Only a few of the at least 28 different X linked syndromes associated with hearing impairment have been characterised at the molecular level. In 1960, a large Norwegian family was reported with early onset progressive sensorineural deafness, which was indexed in McKusick as DFN-1, McKusick 304700. No associated symptoms were described at that time. This family has been restudied clinically. Extensive neurological, neurophysiological, neuroradiological, and biochemical, as well as molecular techniques, have been applied to characterise the X linked recessive syndrome. The family history and extensive characterisation of 16 affected males in five generations confirmed the X linked recessive inheritance and the postlingual progressive nature of the sensorineural deafness. Some obligate carrier females showed signs of minor neuropathy and mild hearing impairment. Restudy of the original DFN-1 family showed that the deafness is part of a progressive X linked recessive syndrome, which includes visual disability leading to cortical blindness, dystonia, fractures, and mental deficiency. Linkage analysis indicated that the gene was linked to locus DXS101 in Xq22 with a lod score of 5.37 (zero recombination). Based on lod-1 support interval of the multipoint analysis, the gene is located in a region spanning from 5 cM proximal to 3 cM distal to this locus. As the proteolipid protein gene (PLP) is within this region and mutations have been shown to be associated with non-classical PMD (Pelizaeus-Merzbacher disease), such as complex X linked hereditary spastic paraplegia, PLP may represent a candidate gene for this disorder. This family represents a new syndrome (Mohr-Tranebjaerg syndrome, MTS) and provides significant new information about a new X linked recessive sydromic type of deafness which was previously thought to be isolated deafness.

The protein defective in X-linked agammaglobulinemia, Bruton's tyrosine kinase, shows increased autophosphorylation activity in vitro when isolated from cells in which the B cell receptor has been cross-linked

X-linked agammaglobulinemia is a primary inherited immunodeficiency resulting in a lack of or dramatic reduction in the number of mature B lymphocytes and, thus, greatly reduced levels of serum immunoglobulin. The defect results from mutations in the gene for Bruton's tyrosine kinase (Btk). Using rabbit antisera generated against Btk, we have demonstrated an increase in the level of in vitro kinase activity present in anti-Btk immunoprecipitates from B cells following stimulation with anti-immunoglobulin antibody. This increase in immune complex kinase activity is detectable 1 to 2 min following stimulation and remains elevated for over 30 min. A similar increase was not seen with two late pre-B cell lines investigated in the same way. This stimulation of activity may suggest a role for Btk in signalling through the B cell receptor or associated proteins, in mature B cells.

Purification of a major tyrosine kinase from RBL-2H3 cells phosphorylating Fc epsilon RI gamma-cytoplasmic domain and identification as the Btk tyrosine kinase

Immunoglobulin E high affinity receptor-mediated signal transduction in mast cells results in a number of protein tyrosine kinases being activated as very early events in the process leading to degranulation. Some of these, such as the src kinases and the syk kinase, are known to be involved in this receptor-associated activation. In this paper we describe the search for other activation-associated tyrosine kinases by the ability to phosphorylate a cytoplasmic domain peptide of the Fc epsilon RI gamma-subunit. In utilizing a purification step previously used to isolate the 72 kDa syk kinase, we detected another kinase of molecular weight 79 kDa which we designated cd gamma kinase. The kinase was purified to near homogeneity by Heparin-agarose, Mono Q, and CM Sepharose chromatographies. The yield of enzyme was approx. 200 micrograms/10(9) cells. We characterized this kinase by its ability to phosphorylate both the cd gamma peptide (Km = 0.2 mM) and the cytoplasmic fragment of the Band III protein. The cd gamma kinase was distinguished from syk by inability to be precipitated by anti-syk antiserum and by partial peptide mapping. Cd gamma kinase was also distinguished from syk by cd gamma peptide and Band III substrate specificity. We identified the cd gamma kinase by Western blotting and by partial phosphopeptide mapping as Btk, the B-cell tyrosine kinase found to be defective in X-linked agammaglobulinemia.

Predominance of sterile immunoglobulin transcripts in a female phenotypically resembling Bruton's agammaglobulinemia

The transcription pattern of the heavy chain immunoglobulin gene locus was analyzed in a 6-month-old female with agammaglobulinemia characterized by the absence of mature B cells in peripheral blood, arrested B cell development in the bone marrow and lack of germinal center development. DNA sequencing provided no evidence of mutations within the coding region of the Bruton's tyrosine kinase gene. Polymerase chain reaction-generated cDNA libraries from blood and bone marrow were screened initially using JH and CH oligodeoxynucleotide probes and VH family-specific probes. Only 10% of the transcripts constituted mature VDJC mu recombinations. Ninety percent of the cDNA were sterile immunoglobulin transcripts comprised of: DJC mu (DH-JHC mu), JC mu (JH-C mu), EC mu (enhancer spliced to C mu), SC mu and IC mu [corresponding to switch (S) and intron (I) regions spliced to C mu]. In the mature immunoglobulin transcripts, VH use indicated germline expression with little evidence of somatic mutation. All cDNA were of the C mu type. Different D segments, D-D joining events and unknown D-like elements were noted in the DJC mu and VDJC mu transcripts. This pattern of immunoglobulin rearrangements, along with the phenotypic cell surface antigen characteristics (CD19-), suggest that an earlier arrest in B cell development than is characteristic of Bruton's X-linked agammaglobulinemia has occurred in this patient.

Insights into lymphocyte development from X-linked immune deficiencies

The genetic immune deficiencies have drawn the attention of physicians and immunologists for more than 40 years. The selectivity of these deficiencies brings into focus the contribution of the response of each arm of the immune system to specific pathogens. Recently, the genes underlying four X-linked defects in immune development in humans have been identified by either positional cloning or candidate-gene cloning techniques. Remarkably, these genetic defects reveal a microcosm of lymphocyte developmental controls involving cell-cell interactions, combinatorial cell surface receptor specificity and lineage-specific signal transduction.

Missense mutation in exon 7 of the common gamma chain gene causes a moderate form of X-linked combined immunodeficiency

Clinical and immunologic features of a recently recognized X-linked combined immunodeficiency disease (XCID) suggested that XCID and X-linked severe combined immunodeficiency (XSCID) might arise from different genetic defects. The recent discovery of mutations in the common gamma chain (gamma c) gene, a constituent of several cytokine receptors, in XSCID provided an opportunity to test directly whether a previously unrecognized mutation in this same gene was responsible for XCID. The status of X chromosome inactivation in blood leukocytes from obligate carriers of XCID was determined from the polymorphic, short tandem repeats (CAG), in the androgen receptor gene, which also contains a methylation-sensitive HpaII site. As in XSCID, X-chromosome inactivation in obligate carriers of XCID was nonrandom in T and B lymphocytes. In addition, X chromosome inactivation in PMNs was variable. Findings from this analysis prompted sequencing of the gamma c gene in this pedigree. A missense mutation in the region coding for the cytoplasmic portion of the gamma c gene was found in three affected males but not in a normal brother. Therefore, this point mutation in the gamma c gene leads to a less severe degree of deficiency in cellular and humoral immunity than that seen in XSCID.

Structural and functional studies of the intracellular tyrosine kinase MATK gene and its translated product

We recently cloned the cDNA which encodes a novel megakaryocyte-associated tyrosine kinase termed MATK. In this study, we have cloned and characterized the human MATK gene as well as the murine homolog of human MATK cDNA and performed functional studies of its translated product. Comparison of the deduced amino acid sequences of human and murine MATK cDNAs revealed 85% homology, indicating that MATK is highly conserved in mouse and human. The human gene consists of 13 exons interrupted by 12 introns. The genetic units which encode the SH3 and SH2 domains are located on separate exons. The putative ATP binding site (GXGXXG) is localized on exon 7, and the entire catalytic domain is subdivided into seven exons (7-13). Somatic cell hybrid analysis indicated that human MATK gene is located on chromosome 19 while the murine Matk gene is located on chromosome 10. The immediate 5'-flanking region was highly rich in GC sequences, and potential cis-acting elements were identified including several SP1, GATA-1, APRE, and APRE1. Antisense oligonucleotides directed against MATK mRNA sequences significantly inhibited megakaryocyte progenitor proliferation. Functional studies indicated that MATK can phosphorylate the carboxyl-terminal conserved tyrosine of the Src protein. These results support the notion that MATK acts as a regulator of p60c-src in megakaryocytic cells and participates in the pathways regulating growth of cells of this lineage.

Identification of Rlk, a novel protein tyrosine kinase with predominant expression in the T cell lineage

The control of phosphorylation by protein tyrosine kinases represents an important regulatory mechanism in T cell growth, function, and differentiation. We have identified a 62-kDa murine protein tyrosine kinase predominantly expressed within the T cell lineage, which we have termed Rlk (for Resting lymphocyte kinase). rlk mRNA was found to be expressed in the fetal thymus as early as day 13 of embryonic development as well as in adult thymus and mature resting peripheral T cells. The sequence of rlk showed that it is most closely related to the subfamily of cytoplasmic tyrosine kinases that includes the Btk, Itk, and Tec proteins. However, Rlk differs from these kinases by virtue of its unique aminoterminal domain, which lacks a region of pleckstrin homology common to the other members of this protein subfamily. Examination of rlk abundance within different T cell subpopulations revealed preferential expression in Th1 relative to Th2 T cell clones, suggesting a possible role in signal transduction pathways that selectively regulate cytokine production in mature CD4+ T cell subsets. Rlk thus represents a novel cytoplasmic tyrosine kinase with potential functions in intrathymic T cell development and mature T cell signaling.

Cloning of BSK, a murine FRK homologue with a specific pattern of tissue distribution

A novel protein tyrosine kinase (PTK) was previously identified by us from the rat insulin-producing cell line, RINm5F, by polymerase chain reaction (PCR). By using this PCR fragment to screen a cDNA library from the mouse insulin-producing cell line beta TC-1, a cDNA clone of about 2.0 kb was obtained which encodes the entire amino acid (aa) sequence of the corresponding PTK. The deduced aa sequence reveals strong homology with the members of the SRC family of intracellular PTKs. We have designated the gene as BSK (beta-cell Src-homology tyrosine kinase). Southern blot analysis after PCR with primers specific for BSK confirmed its expression in fetal and adult islets of Langerhans, in RINm5F cells and in mouse kidney. Northern blots using poly(A)+RNA from non-beta-cell tissues showed that the BSK cDNA hybridized to three mRNA transcripts (2.9, 3.1 and 5.0 kb) present in kidney, liver and lung. Extensive homology of BSK with the recently identified human gene FRK was observed. It is concluded that Bsk is a murine Frk homologue with a specific pattern of tissue expression.

A new point mutation involving a highly conserved leucine in the Btk SH2 domain in a family with X linked agammaglobulinaemia

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