Phosphorylation of Bruton's tyrosine kinase by c-Abl

Progress in the development of small molecular inhibitors of the Bruton's tyrosine kinase (BTK) as a promising cancer therapy

Bruton tyrosine kinase (BTK) is a key kinase in the B cell antigen receptor signal transduction pathway, which is involved in the regulation of the proliferation, differentiation and apoptosis of B cells. BTK has become a significant target for the treatment of hematological malignancies and autoimmune diseases. Ibrutinib, the first-generation BTK inhibitor, has made a great contribution to the treatment of B cell malignant tumors, but there are still some problems such as resistance or miss target of site mutation. Therefore, there is an imperative need to develop novel BTK inhibitors to overcome these problems. Besides, proteolysis targeting chimera (PROTAC) technology has been successfully applied to the development of BTK degradation agents, which has opened a fresh way for the BTK targeted treatment. This paper reviews the biological function of BTK, the discovery and development of BTK targeted drugs as a promising cancer therapy. It mainly reviews the binding sites and structural characteristics of BTK, structure-activity relationships, activity and drug resistance of BTK inhibitors, as well as potential treatment strategies to overcome the resistance of BTK, which provides a reference for the rational design and development of new powerful BTK inhibitors.

Differential use of BTK and PLC in FcεRI- and KIT-mediated mast cell activation: A marginal role of BTK upon KIT activation

In mast cells (MCs), the TEC family kinase (TFK) BTK constitutes a central regulator of antigen (Ag)-triggered, FcεRI-mediated PLCγ phosphorylation, Ca²⁺ mobilization, degranulation, and pro-inflammatory cytokine production. Less is known about the function of BTK in the context of stem cell factor (SCF)-induced KIT signaling. In bone marrow-derived MCs (BMMCs), Ag stimulation caused intense phosphorylation of BTK at Y551 in its active center and at Y223 in its SH3-domain, whereas in response to SCF only Y223 was significantly phosphorylated. Further data using the TFK inhibitor Ibrutinib indicated that BTK Y223 is phosphorylated by a non-BTK TFK upon SCF stimulation. In line, SCF-induced PLCγ1 phosphorylation was stronger attenuated by Ibrutinib than by BTK deficiency. Subsequent pharmacological analysis of PLCγ function revealed a total block of SCF-induced Ca²⁺ mobilization by PLC inhibition, whereas only the sustained phase of Ca²⁺ flux was curtailed in Ag-stimulated BMMCs. Despite this severe stimulus-dependent difference in inducing Ca²⁺ mobilization, PLCγ inhibition suppressed Ag- and SCF-induced degranulation and pro-inflammatory cytokine production to comparable extents, suggesting involvement of additional TFK(s) or PLCγ-dependent signaling components. In addition to PLCγ, the MAPKs p38 and JNK were activated by Ag in a BTK-dependent manner; this was not observed upon SCF stimulation. Hence, FcεRI and KIT employ different mechanisms for activating PLCγ, p38, and JNK, which might strengthen their cooperation regarding pro-inflammatory MC effector functions. Importantly, our data clearly demonstrate that analyzing BTK Y223 phosphorylation is not sufficient to prove BTK activation.

Stimulus strength determines the BTK-dependence of the SHIP1-deficient phenotype in IgE/antigen-triggered mast cells

Antigen (Ag)-mediated crosslinking of IgE-loaded high-affinity receptors for IgE (FcεRI) on mast cells (MCs) triggers activation of proinflammatory effector functions relevant for IgE-associated allergic disorders. The cytosolic tyrosine kinase BTK and the SH2-containing inositol-5'-phosphatase SHIP1 are central positive and negative regulators of Ag-triggered MC activation, respectively, contrarily controlling Ca²⁺ mobilisation, degranulation, and cytokine production. Using genetic and pharmacological techniques, we examined whether BTK activation in Ship1-/- MCs is mandatory for the manifestation of the well-known hyperactive phenotype of Ship1-/- MCs. We demonstrate the prominence of BTK for the Ship1-/- phenotype in a manner strictly dependent on the strength of the initial Ag stimulus; particular importance for BTK was identified in Ship1-/- bone marrow-derived MCs in response to stimulation with suboptimal Ag concentrations. With respect to MAPK activation, BTK showed particular importance at suboptimal Ag concentrations, allowing for an analogous-to-digital switch resulting in full activation of ERK1/2 already at low Ag concentrations. Our data allow for a more precise definition of the role of BTK in FcεRI-mediated signal transduction and effector function in MCs. Moreover, they suggest that reduced activation or curtate expression of SHIP1 can be compensated by pharmacological inhibition of BTK and vice versa.

Discovery of a novel series of pyridine and pyrimidine carboxamides as potent and selective covalent inhibitors of Btk

Btk is an attractive target for the treatment of a range of Bcell malignancies as well as several autoimmune diseases such as murine lupus and rheumatoid arthritis. Several covalent irreversible inhibitors of Btk are currently in development including ibrutinib which was approved for treatment of B-cell malignancies. Herein, we describe our efforts using X-ray guided structure based design (SBD) to identify a novel chemical series of covalent Btk inhibitors. The resulting pyridine carboxamides were potent and selective inhibitors of Btk having excellent enzymatic and cellular inhibitory activity.

Discovery and biological evaluation of N5-substituted 6,7-dioxo-6,7-dihydropteridine derivatives as potent Bruton's tyrosine kinase inhibitors

Bruton's tyrosine kinase (BTK) plays a critical role in B cell receptor (BCR)-mediated signaling pathways responsible for the development and function of B cells, which makes it an attractive target for the treatment of many types of B-cell malignancies. Herein, a series of N5-substituted 6,7-dioxo-6,7-dihydropteridine-based, irreversible BTK inhibitors were reported with IC₅₀ values ranging from 1.9 to 236.6 nM in the enzymatic inhibition assay. Compounds 6 and 7 significantly inhibited the proliferation of Ramos cells which overexpress the BTK enzyme, as well as the autophosphorylation of BTK at Tyr223 and the activation of its downstream signaling molecule PLCγ2. Overall, this series of compounds could provide a promising starting point for further development of potent BTK inhibitors for B-cell malignancy treatment.

Bruton's tyrosine kinase: from X-linked agammaglobulinemia toward targeted therapy for B-cell malignancies

Discovery of Bruton's tyrosine kinase (BTK) mutations as the cause for X-linked agammaglobulinemia was a milestone in understanding the genetic basis of primary immunodeficiencies. Since then, studies have highlighted the critical role of this enzyme in B-cell development and function, and particularly in B-cell receptor signaling. Because its deletion affects mostly B cells, BTK has become an attractive therapeutic target in autoimmune disorders and B-cell malignancies. Ibrutinib (PCI-32765) is the most advanced BTK inhibitor in clinical testing, with ongoing phase III clinical trials in patients with chronic lymphocytic leukemia and mantle-cell lymphoma. In this article, we discuss key discoveries related to BTK and clinically relevant aspects of BTK inhibitors, and we provide an outlook into clinical development and open questions regarding BTK inhibitor therapy.

Ibrutinib and novel BTK inhibitors in clinical development

Small molecule inhibitors targeting dysregulated pathways (RAS/RAF/MEK, PI3K/AKT/mTOR, JAK/STAT) have significantly improved clinical outcomes in cancer patients. Recently Bruton's tyrosine kinase (BTK), a crucial terminal kinase enzyme in the B-cell antigen receptor (BCR) signaling pathway, has emerged as an attractive target for therapeutic intervention in human malignancies and autoimmune disorders. Ibrutinib, a novel first-in-human BTK-inhibitor, has demonstrated clinical effectiveness and tolerability in early clinical trials and has progressed into phase III trials. However, additional research is necessary to identify the optimal dosing schedule, as well as patients most likely to benefit from BTK inhibition. This review summarizes preclinical and clinical development of ibrutinib and other novel BTK inhibitors (GDC-0834, CGI-560, CGI-1746, HM-71224, CC-292, and ONO-4059, CNX-774, LFM-A13) in the treatment of B-cell malignancies and autoimmune disorders.

A novel isoform of the B cell tyrosine kinase BTK protects breast cancer cells from apoptosis

Tyrosine kinases orchestrate key cellular signaling pathways and their dysregulation is often associated with cellular transformation. Several recent cases in which inhibitors of tyrosine kinases have been successfully used as anticancer agents have underscored the importance of this class of proteins in the development of targeted cancer therapies. We have carried out a large-scale loss-of-function analysis of the human tyrosine kinases using RNA interference to identify novel survival factors for breast cancer cells. In addition to kinases with known roles in breast and other cancers, we identified several kinases that were previously unknown to be required for breast cancer cell survival. The most surprising of these was the cytosolic, nonreceptor tyrosine kinase, Bruton's tyrosine kinase (BTK), which has been extensively studied in B cell development. Down regulation of this protein with RNAi or inhibition with pharmacological inhibitors causes apoptosis; overexpression inhibits apoptosis induced by Doxorubicin in breast cancer cells. Our results surprisingly show that BTK is expressed in several breast cancer cell lines and tumors. The predominant form of BTK found in tumor cells is transcribed from an alternative promoter and results in a protein with an amino-terminal extension. This alternate form of BTK is expressed at significantly higher levels in tumorigenic breast cells than in normal breast cells. Since this protein is a survival factor for these cells, it represents both a potential marker and novel therapeutic target for breast cancer.

Regulation of nucleocytoplasmic shuttling of Bruton's tyrosine kinase (Btk) through a novel SH3-dependent interaction with ankyrin repeat domain 54 (ANKRD54)

Bruton's tyrosine kinase (Btk), belonging to the Tec family of tyrosine kinases (TFKs), is essential for B-lymphocyte development. Abrogation of Btk signaling causes human X-linked agammaglobulinemia (XLA) and murine X-linked immunodeficiency (Xid). We employed affinity purification of Flag-tagged Btk, combined with tandem mass spectrometry, to capture and identify novel interacting proteins. We here characterize the interaction with ankryin repeat domain 54 protein (ANKRD54), also known as Lyn-interacting ankyrin repeat protein (Liar). While Btk is a nucleocytoplasmic protein, the Liar pool was found to shuttle at a higher rate than Btk. Importantly, our results suggest that Liar mediates nuclear export of both Btk and another TFK, Txk/Rlk. Liar-mediated Btk shuttling was enriched for activation loop, nonphosphorylated Btk and entirely dependent on Btk's SH3 domain. Liar also showed reduced binding to an aspartic acid phosphomimetic SH3 mutant. Three other investigated nucleus-located proteins, Abl, estrogen receptor β (ERβ), and transcription factor T-bet, were all unaffected by Liar. We mapped the interaction site to the C terminus of the Btk SH3 domain. A biotinylated, synthetic Btk peptide, ARDKNGQEGYIPSNYVTEAEDS, was sufficient for this interaction. Liar is the first protein identified that specifically influences the nucleocytoplasmic shuttling of Btk and Txk and belongs to a rare group of known proteins carrying out this activity in a Crm1-dependent manner.

Pre-B cell receptor-mediated cell cycle arrest in Philadelphia chromosome-positive acute lymphoblastic leukemia requires IKAROS function

B cell lineage acute lymphoblastic leukemia (ALL) arises in virtually all cases from B cell precursors that are arrested at pre–B cell receptor–dependent stages. The Philadelphia chromosome–positive (Ph⁺) subtype of ALL accounts for 25–30% of cases of adult ALL, has the most unfavorable clinical outcome among all ALL subtypes and is defined by the oncogenic BCR-ABL1 kinase and deletions of the IKAROS gene in >80% of cases. Here, we demonstrate that the pre–B cell receptor functions as a tumor suppressor upstream of IKAROS through induction of cell cycle arrest in Ph⁺ ALL cells. Pre–B cell receptor–mediated cell cycle arrest in Ph⁺ ALL cells critically depends on IKAROS function, and is reversed by coexpression of the dominant-negative IKAROS splice variant IK6. IKAROS also promotes tumor suppression through cooperation with downstream molecules of the pre–B cell receptor signaling pathway, even if expression of the pre–B cell receptor itself is compromised. In this case, IKAROS redirects oncogenic BCR-ABL1 tyrosine kinase signaling from SRC kinase-activation to SLP65, which functions as a critical tumor suppressor downstream of the pre–B cell receptor. These findings provide a rationale for the surprisingly high frequency of IKAROS deletions in Ph⁺ ALL and identify IKAROS-mediated cell cycle exit as the endpoint of an emerging pathway of pre–B cell receptor–mediated tumor suppression.

Activation loop phosphorylation modulates Bruton's tyrosine kinase (Btk) kinase domain activity

Bruton's tyrosine kinase (Btk) plays a central role in signal transduction pathways regulating survival, activation, proliferation, and differentiation of B-lineage lymphoid cells. A number of cell signaling studies clearly show that Btk is activated by Lyn, a Src family kinase, through phosphorylation on activation loop tyrosine 551 (Y(551)). However, the detailed molecular mechanism regulating Btk activation remains unclear. In particular, we do not fully understand the correlation of kinase activity with Y(551) phosphorylation, and the role of the noncatalytic domains of Btk in the activation process. Insect cell expressed full-length Btk is enzymatically active, but a truncated version of Btk, composed of only the kinase catalytic domain, is largely inactive. Further characterization of both forms of Btk by mass spectrometry showed partial phosphorylation of Y(551) of the full-length enzyme and none of the truncated kinase domain. To determine whether the lack of activity of the kinase domain was due to the absence of Y(551) phosphorylation, we developed an in vitro method to generate Y(551) monophosphorylated Btk kinase domain fragment using the Src family kinase Lyn. Detailed kinetic analyses demonstrated that the in vitro phosphorylated Btk kinase domain has a similar activity as the full-length enzyme while the unphosphorylated kinase domain has a very low k(cat) and is largely inactive. A divalent magnesium metal dependence study established that Btk requires a second magnesium ion for activity. Furthermore, our analysis revealed significant differences in the second metal-binding site among the kinase domain and the full-length enzyme that likely account for the difference in their catalytic profile. Taken together, our study provides important mechanistic insights into Btk kinase activity and phosphorylation-mediated regulation.

Bruton's tyrosine kinase (Btk): function, regulation, and transformation with special emphasis on the PH domain

Bruton's agammaglobulinemia tyrosine kinase (Btk) is a cytoplasmic tyrosine kinase important in B-lymphocyte development, differentiation, and signaling. Btk is a member of the Tec family of kinases. Mutations in the Btk gene lead to X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (Xid) in mice. Activation of Btk triggers a cascade of signaling events that culminates in the generation of calcium mobilization and fluxes, cytoskeletal rearrangements, and transcriptional regulation involving nuclear factor-kappaB (NF-kappaB) and nuclear factor of activated T cells (NFAT). In B cells, NF-kappaB was shown to bind to the Btk promoter and induce transcription, whereas the B-cell receptor-dependent NF-kappaB signaling pathway requires functional Btk. Moreover, Btk activation is tightly regulated by a plethora of other signaling proteins including protein kinase C (PKC), Sab/SH3BP5, and caveolin-1. For example, the prolyl isomerase Pin1 negatively regulates Btk by decreasing tyrosine phosphorylation and steady state levels of Btk. It is intriguing that PKC and Pin1, both of which are negative regulators, bind to the pleckstrin homology domain of Btk. To this end, we describe here novel mutations in the pleckstrin homology domain investigated for their transforming capacity. In particular, we show that the mutant D43R behaves similar to E41K, already known to possess such activity.

The effects of c-Abl mutation on developing B cell differentiation and survival

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(2+) 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.

c-Abl-deficient mice exhibit reduced numbers of peritoneal B-1 cells and defects in BCR-induced B cell activation

A role for c-Abl in B cell development and signaling has been suggested by previous work showing that c-Abl-deficient mice have defects in bone marrow B cell development and that c-Abl-deficient B cells are hypoproliferative in response to antigen receptor stimulation. Here we show that in addition to defects in early B cell development, c-Abl-deficient mice have defects in peripheral B cell development, including reduced percentages of peritoneal B-1 cells as well as transitional and marginal zone B cells in the spleen. It has been shown that c-Abl kinase activity increases upon B cell receptor (BCR) stimulation and that one of the targets of tyrosine phosphorylation by c-Abl is CD19. However, the consequences of c-Abl activity on B cell activation and CD19 signaling remain unknown. Here, we show that c-Abl-deficient splenic B cells exhibit reduced calcium flux in response to CD19 cross-linking, consistent with a role for c-Abl in CD19-dependent signaling. Additionally, we show that c-Abl-deficient B cells are defective in their ability to be activated in response to antigen receptor engagement, suggesting a functional role for c-Abl in BCR-dependent activation signaling pathways.

Bruton's tyrosine kinase is not essential for Bcr-Abl-mediated transformation of lymphoid or myeloid cells

Bcr-Abl, a constitutively active tyrosine kinase, is the cause of chronic myeloid leukemia (CML) and a subset of acute lymphoblastic leukemias (ALL). Bruton's tyrosine kinase (BTK), a member of the Tec family of tyrosine kinases with a crucial role in B-cell development, is consistently tyrosine phosphorylated in Bcr-Abl expressing murine pre B cells. BTK has been implicated in Bcr-Abl-mediated B-cell transformation and resistance to imatinib, implying that inhibiting BTK may be therapeutically beneficial. We decided to test whether BTK is a critical node in Bcr-Abl transformation and potential drug target in imatinib-resistant Bcr-Abl-positive cells. We depleted BTK in Ba/F3 and 32D cells expressing native and kinase domain (KD) mutant (E255K and T315I) Bcr-Abl, using shRNA. BTK levels were reduced to <10% of controls. However, no differences in viability and cell proliferation were observed and the response to imatinib was not altered. Consistent with this, proliferation and viability were unaffected by inhibition of BTK with reversible (PC-005) and irreversible (PCI-31523) small molecule inhibitors. Lastly, BTK inhibition did not affect the ability of Bcr-Abl to transform primary murine hematopoietic cells in colony forming and B-cell transformation assays. Collectively this data argues against a critical role for BTK in Bcr-Abl-mediated leukemogenesis.

Regulation of Bruton tyrosine kinase by the peptidylprolyl isomerase Pin1

Bruton tyrosine kinase (Btk) is expressed in B-lymphocytes. Mutations in Btk cause X-linked agammaglobulinemia in humans. However, the mechanism of activation and signaling of this enzyme has not been fully investigated. We have here shown that the peptidylprolyl cis/trans isomerase (PPIase) Pin1 is a negative regulator of Btk, controlling its expression level by reducing its half-life, whereas the catalytic activity of Btk was unaffected. The negative regulatory effect of Pin1 was observed both in cell lines and in Pin(-/-) mice and was found to be dependent on a functionally intact Btk. This may constitute a feedback loop for the regulation of Btk. The target region in Btk was localized to the pleckstrin homology domain suggesting that interphase phosphorylation of serine 115 (Ser-115) in Btk is required, whereas mitosis phosphorylation of serine 21 (Ser-21) is critical. Accordingly, Pin 1 was shown to associate with Btk through binding to Ser-21 and -115, respectively, both of which lie in a classical Pin1-binding pocket. Using a phosphomitotic antibody, it was found that Btk harbors a bona fide MPM2 epitope corresponding to a phosphorylated serine or threonine residue followed by a proline. Our results indicate that the peptidylprolyl isomerase Pin1 interacts with Btk in a cell cycle-dependent manner, regulating the Btk expression level.

Involvement of SLP-65 and Btk in tumor suppression and malignant transformation of pre-B cells

Signals from the precursor-B cell receptor (pre-BCR) are essential for selection and clonal expansion of pre-B cells that have performed productive immunoglobulin heavy chain V(D)J recombination. In the mouse, the downstream signaling molecules SLP-65 and Btk cooperate to limit proliferation and induce differentiation of pre-B cells, thereby acting as tumor suppressors to prevent pre-B cell leukemia. In contrast, recent observations in human BCR-ABL1(+) pre-B lymphoblastic leukemia cells demonstrate that Btk is constitutively phosphorylated and activated by the BCR-ABL1 fusion protein. As a result, activated Btk transmits survival signals that are essential for the transforming activity of oncogenic Abl tyrosine kinase.

Mimicry of a constitutively active pre-B cell receptor in acute lymphoblastic leukemia cells

Pre-B cells undergo apoptosis unless they are rescued by pre-B cell receptor-dependent survival signals. We previously showed that the BCR-ABL1 kinase that is expressed in pre-B lymphoblastic leukemia bypasses selection for pre-B cell receptor-dependent survival signals. Investigating possible interference of BCR-ABL1 with pre-B cell receptor signaling, we found that neither SYK nor SLP65 can be phosphorylated in response to pre-B cell receptor engagement. Instead, Bruton's tyrosine kinase (BTK) is constitutively phosphorylated by BCR-ABL1. Activated BTK is essential for survival signals that otherwise would arise from the pre-B cell receptor, including activation of PLCgamma1, autonomous Ca2+ signaling, STAT5-phosphorylation, and up-regulation of BCLX(L). Inhibition of BTK activity specifically induces apoptosis in BCR-ABL1+ leukemia cells to a similar extent as inhibition of BCR-ABL1 kinase activity itself. However, BCR-ABL1 cannot directly bind to full-length BTK. Instead, BCR-ABL1 induces the expression of a truncated splice variant of BTK that acts as a linker between the two kinases. As opposed to full-length BTK, truncated BTK lacks kinase activity yet can bind to BCR-ABL1 through its SRC-homology domain 3. Acting as a linker, truncated BTK enables BCR-ABL1-dependent activation of full-length BTK, which initiates downstream survival signals and mimics a constitutively active pre-B cell receptor.

Functional replacement ofDrosophilaBtk29A with human Btk in male genital development and survival

Drosophila type 2 Btk29A reveals the highest homology to Btk among mammalian Tec kinases. In Btk29A(ficP) mutant males, the apodeme holding the penis split into two pieces. Human Btk rescued this phenotype in 39% of Btk29A(ficP) males, while the Drosophila transgenes did so in 90-100% of mutants. The Btk29A(ficP) mutation reduced adult longevity to 11% that of wild-type. This effect was counteracted by Drosophila type 2, yielding 76% of the wild-type lifespan. Human Btk extended the lifespan of Btk29A(ficP) mutants only to 20% that of wild-type. Thus human Btk can partially replace Drosophila Btk29A+ in male genital development and survival.

Crystal structures of interleukin-2 tyrosine kinase and their implications for the design of selective inhibitors

Interleukin-2 tyrosine kinase, Itk, is an important member of the Tec family of non-receptor tyrosine kinases that play a central role in signaling through antigen receptors such as the T-cell receptor, B-cell receptor, and Fcepsilon. Selective inhibition of Itk may be an important way of modulating many diseases involving heightened or inappropriate activation of the immune system. In addition to an unliganded nonphophorylated Itk catalytic kinase domain, we determined the crystal structures of the phosphorylated and nonphosphorylated kinase domain bound to staurosporine, a potent broad-spectrum kinase inhibitor. These structures are useful for the design of novel, highly potent and selective Itk inhibitors and provide insight into the influence of inhibitor binding and phosphorylation on the conformation of Itk.

Conformation of full-length Bruton tyrosine kinase (Btk) from synchrotron X-ray solution scattering

Brutons's tyrosine kinase (Btk) is a non-receptor protein tyrosine kinase (nrPTK) essential for the development of B lymphocytes in humans and mice. Like Src and Abl PTKs, Btk contains a conserved cassette formed by SH3, SH2 and protein kinase domains, but differs from them by the presence of an N-terminal PH domain and the Tec homology region. The domain structure of Btk was analysed using X-ray synchrotron radiation scattering in solution. Low resolution shapes of the full-length protein and several deletion mutants determined ab initio from the scattering data indicated a linear arrangement of domains. This arrangement was further confirmed by rigid body modelling using known high resolution structures of individual domains. The final model of Btk displays an extended conformation with no, or little, inter-domain interactions. In agreement with these results, deletion of non-catalytic domains failed to enhance the activity of Btk. Taken together, our results indicate that, contrary to Src and Abl, Btk might not require an assembled conformation for the regulation of its activity.

Therapeutic strategies for rheumatoid arthritis

Recent years have seen considerable advances in our understanding of both the clinical and basic-research aspects of rheumatoid arthritis. Clinical progress has come from a better recognition of the natural history of the disease, the development and validation of outcome measures for clinical trials and, consequently, innovative trial designs. In parallel, basic research has provided clues to the pathogenic events underlying rheumatoid arthritis, and advances in biotechnology have facilitated the development of new classes of therapeutics. Here, we summarize the fruits of these advances: innovative approaches to the use of existing, traditional disease-modifying antirheumatic drugs; novel agents approved very recently; and further avenues that are presently under investigation or which are of more distant promise.