New insights into the regulation and functions of Tec family tyrosine kinases in the immune system

Interaction of T-cell-specific adapter protein with Src- and Tec-family kinases

Adapter proteins are flexible and dynamic modulators of cellular signalling that are important for immune cell function. One of these, the T-cell-specific adapter protein (TSAd), interacts with the non-receptor tyrosine kinases Src and Lck of the Src family kinases (SFKs) and Itk of the Tec family kinases (TFKs). Three tyrosine residues in the TSAd C-terminus are phosphorylated by Lck and serve as docking sites for the Src homology 2 (SH2) domains of Src and Lck. The TSAd proline-rich region (PRR) binds to the Src homology 3 (SH3) domains found in Lck, Src and Itk. Despite known interactors, the role TSAd plays in cellular signalling remains largely unknown. TSAd's ability to bind both SFKs and TFKs may point to its function as a general scaffold for both kinase families. Using GST-pulldown as well as peptide array experiments, we found that both the SH2 and SH3 domains of the SFKs Fyn and Hck, as well as the TFKs Tec and Txk, interact with TSAd. This contrasts with Itk, which interacts with TSAd only through its SH3 domain. Although our analysis showed that TSAd is both co-expressed and may interact with Fyn, we were unable to co-precipitate Fyn with TSAd from Jurkat cells, as detected by Western blotting and affinity purification mass spectrometry. This may suggest that TSAd-Fyn interaction in intact cells may be limited by other factors, such as the subcellular localization of the two molecules or the co-expression of competing binding partners.

Whole genome bisulfite sequencing reveals DNA methylation roles in the adaptive response of wildness training giant pandas to wild environment

DNA methylation modification can regulate gene expression without changing the genome sequence, which helps organisms to rapidly adapt to new environments. However, few studies have been reported in non-model mammals. Giant panda (Ailuropoda melanoleuca) is a flagship species for global biodiversity conservation. Wildness and reintroduction of giant pandas are the important content of giant pandas’ protection. However, it is unclear how wildness training affects the epigenetics of giant pandas, and we lack the means to assess the adaptive capacity of wildness training giant pandas. We comparatively analyzed genome-level methylation differences in captive giant pandas with and without wildness training to determine whether methylation modification played a role in the adaptive response of wildness training pandas. The whole genome DNA methylation sequencing results showed that genomic cytosine methylation ratio of all samples was 5.35%–5.49%, and the methylation ratio of the CpG site was the highest. Differential methylation analysis identified 544 differentially methylated genes (DMGs). The results of KEGG pathway enrichment of DMGs showed that VAV3, PLCG2, TEC and PTPRC participated in multiple immune-related pathways, and may participate in the immune response of wildness training giant pandas by regulating adaptive immune cells. A large number of DMGs enriched in GO terms may also be related to the regulation of immune activation during wildness training of giant pandas. Promoter differentially methylation analysis identified 1,199 genes with differential methylation at promoter regions. Genes with low methylation level at promoter regions and high expression such as, CCL5, P2Y13, GZMA, ANP32A, VWF, MYOZ1, NME7, MRPS31 and TPM1 were important in environmental adaptation for wildness training giant pandas. The methylation and expression patterns of these genes indicated that wildness training giant pandas have strong immunity, blood coagulation, athletic abilities and disease resistance. The adaptive response of giant pandas undergoing wildness training may be regulated by their negatively related promoter methylation. We are the first to describe the DNA methylation profile of giant panda blood tissue and our results indicated methylation modification is involved in the adaptation of captive giant pandas when undergoing wildness training. Our study also provided potential monitoring indicators for the successful reintroduction of valuable and threatened animals to the wild.

Recognition of Immune Cell Markers of COVID-19 Severity with Machine Learning Methods

COVID-19 is hypothesized to be linked to the host's excessive inflammatory immunological response to SARS-CoV-2 infection, which is regarded to be a major factor in disease severity and mortality. Numerous immune cells play a key role in immune response regulation, and gene expression analysis in these cells could be a useful method for studying disease states, assessing immunological responses, and detecting biomarkers. Here, we developed a machine learning procedure to find biomarkers that discriminate disease severity in individual immune cells (B cell, CD4+ cell, CD8+ cell, monocyte, and NK cell) using single-cell gene expression profiles of COVID-19. The gene features of each profile were first filtered and ranked using the Boruta feature selection method and mRMR, and the resulting ranked feature lists were then fed into the incremental feature selection method to determine the optimal number of features with decision tree and random forest algorithms. Meanwhile, we extracted the classification rules in each cell type from the optimal decision tree classifiers. The best gene sets discovered in this study were analyzed by GO and KEGG pathway enrichment, and some important biomarkers like TLR2, ITK, CX3CR1, IL1B, and PRDM1 were validated by recent literature. The findings reveal that the optimal gene sets for each cell type can accurately classify COVID-19 disease severity and provide insight into the molecular mechanisms involved in disease progression.

The Importance of Being PI3K in the RAS Signaling Network

Ras proteins are essential mediators of a multitude of cellular processes, and its deregulation is frequently associated with cancer appearance, progression, and metastasis. Ras-driven cancers are usually aggressive and difficult to treat. Although the recent Food and Drug Administration (FDA) approval of the first Ras G12C inhibitor is an important milestone, only a small percentage of patients will benefit from it. A better understanding of the context in which Ras operates in different tumor types and the outcomes mediated by each effector pathway may help to identify additional strategies and targets to treat Ras-driven tumors. Evidence emerging in recent years suggests that both oncogenic Ras signaling in tumor cells and non-oncogenic Ras signaling in stromal cells play an essential role in cancer. PI3K is one of the main Ras effectors, regulating important cellular processes such as cell viability or resistance to therapy or angiogenesis upon oncogenic Ras activation. In this review, we will summarize recent advances in the understanding of Ras-dependent activation of PI3K both in physiological conditions and cancer, with a focus on how this signaling pathway contributes to the formation of a tumor stroma that promotes tumor cell proliferation, migration, and spread.

Formula Diet Alters the Ileal Metagenome and Transcriptome at Weaning and during the Postweaning Period in a Porcine Model

Exclusive breastfeeding impacts the intestinal microbiome and is associated with a better immune function than is seen with milk formula (MF) feeding in infants and yet with mechanisms poorly defined. The porcine model was used to evaluate the impact of MF on ileum microbial communities and gene expression relative to human milk (HM)-fed piglets. Fifty-two Dutch Landrace male piglets were fed an isocaloric diet of either HM (n = 26) or MF (n = 26) from day 2 through day 21 of age and weaned to a solid diet until day 51. Eleven piglets from each group were euthanized at day 21, while the remaining piglets (HM, n = 15; MF, n = 15) were euthanized at day 51 to collect ileal epithelium (EP) scrapings and ileal (IL) tissues. The epithelial mucosa was subjected to shotgun metagenome sequencing, and EP and IL tissues were used for transcriptome analysis. On day 21, transcriptome data revealed that the levels of pathways involved in inflammation and apoptosis were significantly higher in MF piglets than in HM piglets, whereas the levels of tight junctions and pathogen detection systems were lower in MF piglets than in HM piglets. The MF impacts on the small intestine were maintained over the postweaning period (day 51) as indicated by higher levels of Dialister invisus bacteria and higher levels of expression of genes associated with inflammation and apoptosis pathways relative to HM group. The current study demonstrated that MF might impact local intestinal inflammation, apoptosis, and tight junctions and might suppress pathogen recognition in the small intestine compared with HM.IMPORTANCE Exclusive human milk (HM) breastfeeding for the first 6 months of age in infants is recommended to improve health outcomes during early life and beyond. When women are unable to provide sufficient HM, milk formula (MF) is often recommended as a complementary or alternative source of nutrition. Previous studies in piglets demonstrated that MF alters the gut microbiome and induces inflammatory cytokine production. The links between MF feeding, gut microbiome, and inflammation status are unclear due to challenges associated with the collection of intestinal samples from human infants. The current report provides the first insight into MF-microbiome-inflammation connections in the small intestine compared with HM feeding using a porcine model. The present results showed that, compared with HM, MF might impact immune function through the induction of ileal inflammation, apoptosis, and tight junction disruptions and likely compromised immune defense against pathogen detection in the small intestine relative to piglets that were fed HM.

T-cell expression of Bruton's tyrosine kinase promotes autoreactive T-cell activation and exacerbates aplastic anemia

The role of Bruton’s tyrosine kinase (BTK) in BCR signaling is well defined, and BTK is involved in B-cell development, differentiation, and malignancies. However, the expression of Btk in T cells and its role in T-cell function remain largely unknown. Here, we unexpectedly found high expression and activation of BTK in T cells. Deficiencies in BTK resulted in the impaired activation and proliferation of autoreactive T cells and ameliorated bone marrow failure (BMF) in aplastic anemia. Mechanistically, BTK is activated after TCR engagement and then phosphorylates PLCγ1, thus promoting T-cell activation. Treatment with acalabrutinib, a selective BTK inhibitor, decreased T-cell proliferation and ameliorated BMF in mice with aplastic anemia. Our results demonstrate an unexpected role of BTK in optimal T-cell activation and in the pathogenesis of autoimmune aplastic anemia, providing insights into the molecular regulation of T-cell activation and the pathogenesis of T-cell-mediated autoimmune disease.

Effects of Tec Tyrosine Kinase Inhibition on the Inflammatory Response of Severe Acute Pancreatitis-Associated Acute Lung Injury in Mice

BACKGROUND

The Tec kinase family is involved in acute and chronic inflammatory diseases, but its relationship with severe acute pancreatitis (SAP) remains unclear.

AIMS

To investigate whether Tec tyrosine kinase can be used as a target for severe acute pancreatitis-associated acute lung injury (PALI).

METHODS

A total of 90 mice were randomly assigned into four groups: SAP (n = 15), control (n = 15), SAP + α-cyano-β-hydroxy-β-methyl-N-(2,5-dibromophenyl)propenamide (LFM-A13) (pretreated with Tec kinase inhibitor LFM-A13, n = 15), and SAP + Tec siRNA (pretreated with PBS/negative control siRNA/Tec siRNA, n = 45). SAP was induced by caerulein and lipopolysaccharide. Animals were sacrificed at 0, 3, 24, 48, and 72 h, respectively. Pathological changes and scores of the lung and pancreas were determined using hematoxylin-eosin staining. Expression of Tec and phosphorylated Tec (p-Tec) were examined by real-time polymerase chain reaction, Western blot, and immunoprecipitation. Serum levels of amylase, myeloperoxidase, and pro-inflammatory cytokines were measured by ELISA.

RESULTS

The expression of Tec in lung tissue was significantly higher in the SAP group than in the control group (p < 0.05), and p-Tec expression gradually increased with time. Furthermore, p-Tec expression was significantly lower in the SAP + LFM-A13 group than in the SAP group (p < 0.05); however, Tec expression did not vary. Tec inhibitors, LFM-A13 and Tec siRNA, alleviated pathological damage and release of inflammatory cytokines (p < 0.05).

CONCLUSIONS

Tec tyrosine kinase plays a key role in PALI, and is therefore a potential target for clinical treatment.

Transcriptomic and epigenetic analyses reveal a gender difference in aging-associated inflammation: the Vitality 90+ study

Aging is associated with a pro-inflammatory state, often referred to as inflammaging. The origin of the pro-inflammatory mediators and their role in the pathogenesis of the aging-associated diseases remain poorly understood. As aging is also associated with profound changes in the transcriptomic and epigenetic (e.g., DNA methylation) profiles of cells in the peripheral blood, we analyzed the correlation of these profiles with inflammaging using the "classical" marker interleukin-6 as an indicator. The analysis of the whole-genome peripheral blood mononuclear cell (PBMC) gene expression revealed 62 transcripts with expression levels that significantly correlated with the plasma interleukin-6 (IL-6) levels in men, whereas no correlations were observed in women. The Gene Ontology analysis of plasma IL-6-associated transcripts in men revealed processes that were linked to the inflammatory response. Additionally, an Ingenuity Pathway Analysis (IPA) pathway analysis identified Tec kinase signaling as an affected pathway and upstream regulator analysis predicted the activation of IL-10 transcript. DNA methylation was assessed using a HumanMethylation450 array. Seven genes with expression profiles that were associated with the plasma IL-6 levels in men were found to harbor CpG sites with methylation levels that were also associated with the IL-6 levels. Among these genes were IL1RN, CREB5, and FAIM3, which mapped to a network of inflammatory response genes. According to our results, inflammaging is manifested differently at the genomic level in nonagenarian men and women. Part of this difference seems to be of epigenetic origin. These differences point to the genomic regulation of inflammatory response and suggest that the gender-specific immune system dimorphism in older individuals could be accounted for, in part, by DNA methylation.

Inhibition of the Inositol Kinase Itpkb Augments Calcium Signaling in Lymphocytes and Reveals a Novel Strategy to Treat Autoimmune Disease

Emerging approaches to treat immune disorders target positive regulatory kinases downstream of antigen receptors with small molecule inhibitors. Here we provide evidence for an alternative approach in which inhibition of the negative regulatory inositol kinase Itpkb in mature T lymphocytes results in enhanced intracellular calcium levels following antigen receptor activation leading to T cell death. Using Itpkb conditional knockout mice and LMW Itpkb inhibitors these studies reveal that Itpkb through its product IP4 inhibits the Orai1/Stim1 calcium channel on lymphocytes. Pharmacological inhibition or genetic deletion of Itpkb results in elevated intracellular Ca²⁺ and induction of FasL and Bim resulting in T cell apoptosis. Deletion of Itpkb or treatment with Itpkb inhibitors blocks T-cell dependent antibody responses in vivo and prevents T cell driven arthritis in rats. These data identify Itpkb as an essential mediator of T cell activation and suggest Itpkb inhibition as a novel approach to treat autoimmune disease.

Experimental and computational tools for analysis of signaling networks in primary cells

Cellular information processing in signaling networks forms the basis of responses to environmental stimuli. At any given time, cells receive multiple simultaneous input cues, which are processed and integrated to determine cellular responses such as migration, proliferation, apoptosis, or differentiation. Protein phosphorylation events play a major role in this process and are often involved in fundamental biological and cellular processes such as protein-protein interactions, enzyme activity, and immune responses. Determining which kinases phosphorylate specific phospho sites poses a challenge; this information is critical when trying to elucidate key proteins involved in specific cellular responses. Here, methods to generate high-quality quantitative phosphorylation data from cell lysates originating from primary cells, and how to analyze the generated data to construct quantitative signaling network models, are presented. These models can subsequently be used to guide follow-up in vitro/in vivo validation studies.

PI3K regulates MEK/ERK signaling in breast cancer via the Rac-GEF, P-Rex1

The PI3K pathway is genetically altered in excess of 70% of breast cancers, largely through PIK3CA mutation and HER2 amplification. Preclinical studies have suggested that these subsets of breast cancers are particularly sensitive to PI3K inhibitors; however, the reasons for this heightened sensitivity are mainly unknown. We investigated the signaling effects of PI3K inhibition in PIK3CA mutant and HER2 amplified breast cancers using PI3K inhibitors currently in clinical trials. Unexpectedly, we found that in PIK3CA mutant and HER2 amplified breast cancers sensitive to PI3K inhibitors, PI3K inhibition led to a rapid suppression of Rac1/p21-activated kinase (PAK)/protein kinase C-RAF (C-RAF)/ protein kinase MEK (MEK)/ERK signaling that did not involve RAS. Furthermore, PI3K inhibition led to an ERK-dependent up-regulation of the proapoptotic protein, BIM, followed by induction of apoptosis. Expression of a constitutively active form of Rac1 in these breast cancer models blocked PI3Ki-induced down-regulation of ERK phosphorylation, apoptosis, and mitigated PI3K inhibitor sensitivity in vivo. In contrast, protein kinase AKT inhibitors failed to block MEK/ERK signaling, did not up-regulate BIM, and failed to induce apoptosis. Finally, we identified phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchanger 1 (P-Rex1) as the PI(3,4,5)P3-dependent guanine exchange factor for Rac1 responsible for regulation of the Rac1/C-RAF/MEK/ERK pathway in these cells. The expression level of P-Rex1 correlates with sensitivity to PI3K inhibitors in these breast cancer cell lines. Thus, PI3K inhibitors have enhanced activity in PIK3CA mutant and HER2 amplified breast cancers in which PI3K inhibition down-regulates both the AKT and Rac1/ERK pathways. In addition, P-Rex1 may serve as a biomarker to predict response to single-agent PI3K inhibitors within this subset of breast cancers.

A case of Fabry's disease with congenital agammaglobulinemia

Fabry's disease is an X-linked lysosomal storage disorder caused by abnormalities in the α-galactosidase A (GLA) gene, which leads to a GLA deficiency and to the intracellular deposition of globotriaosylceramide (Gb3) within vascular endothelium and other tissues. It manifests as progressive multiple organ dysfunctions caused by the deposition of Gb3. On the other hand, congenital agammaglobulinemia is usually caused by mutations in Bruton's tyrosine kinase (Btk) gene with X-linked dominence, suppresses B cell maturation, and causes recurrent pyogenic infections. In former reports, the distance between the loci in the Xq22 region of the human X chromosome was found to be about 69 kilobases. A 23-yr-old man diagnosed with congenital agammaglobulinemia at age 5, showed typical clinical and laboratory and histopathological findings of Fabry's disease. The genetic basis of this combination of the two syndromes was studied in this patient. Here, we report a case of Fabry's disease with congenital agammaglobulinemia.

Disrupting the intermolecular self-association of Itk enhances T cell signaling

The Tec family tyrosine kinase (Itk), is a key component of the TCR signaling pathway. Biochemical studies have shown that Itk activation requires recruitment of Itk to the membrane via its pleckstrin homology domain, phosphorylation of Itk by the Src kinase, Lck, and binding of Itk to the SLP-76/LAT adapter complex. However, the regulation of Itk enzymatic activity by Itk domain interactions is not yet well understood. In this study, we show that full-length Itk self-associates in an intermolecular fashion. Using this information, we have designed an Itk variant that exhibits reduced self-association but maintains normal binding to exogenous ligands via each of its regulatory domains. When expressed in insect cells, the Itk substrate phospholipase Cgamma1 is phosphorylated more efficiently by the Itk variant than by wild-type Itk. Furthermore, expression of the Itk variant in primary murine T cells induced higher ERK activation and increased calcium flux following TCR stimulation compared with that of wild-type Itk. Our results indicate that the Tec kinase Itk is negatively regulated by intermolecular clustering and that disruption of this clustering leads to increased Itk kinase activity following TCR stimulation.

Tec protein tyrosine kinase inhibits CD25 expression in human T-lymphocyte

The Tec protein tyrosine kinase (PTK) belongs to a group of structurally related nonreceptor PTKs that also includes Btk, Itk, Rlk, and Bmx. Previous studies have suggested that these kinases play important roles in hematopoiesis and in the lymphocyte signaling pathway. Despite evidence suggesting the involvement of Tec in the T-lymphocyte activation pathway via T-cell receptor (TCR) and CD28, Tec's role in T-lymphocytes remains unclear because of the lack of apparent defects in T-lymphocyte function in Tec-deficient mice. In this study, we investigated the role of Tec in human T-lymphocyte using the Jurkat T-lymphoid cell line stably transfected with a cDNA encoding Tec. We found that the expression of wild-type Tec inhibited the expression of CD25 induced by TCR cross-linking. Second, we observed that LFM-A13, a selective inhibitor of Tec family PTK, rescued the suppression of TCR-induced CD25 expression observed in wild-type Tec-expressing Jurkat cells. In addition, expression of kinase-deleted Tec did not alter the expression level of CD25 after TCR ligation. We conclude that Tec PTK mediates signals that negatively regulate CD25 expression induced by TCR cross-linking. This, in turn, implies that this PTK plays a role in the attenuation of IL-2 activity in human T-lymphocytes.

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.

Mechanism and functional significance of Itk autophosphorylation

Tec family non-receptor tyrosine kinases (Itk, Btk, Tec, Rlk and Bmx) are characterized by the presence of an autophosphorylation site within the non-catalytic Src homology 3 (SH3) domain. The full-length Itk mutant containing phenylalanine in place of the autophosphorylated tyrosine has been studied in Itk-deficient primary T cells. These studies revealed that the non-phosphorylated enzyme restores Itk mediated signaling only partially. In spite of these insights, the precise role of the Tec kinase autophosphorylation site is unclear and the mechanism of the autophosphorylation reaction within the Tec kinases is not known. Here, we show both in vitro and in vivo that Itk autophosphorylation on Y180 within the SH3 domain occurs exclusively via an intramolecular, in cis mechanism. Using an in vitro kinase assay, we show that mutation of the Itk autophosphorylation site Y180 to Phe decreases kinase activity of the full-length enzyme by increasing Km for a peptide substrate. Moreover, mutation of Y180 to Glu, a residue chosen to mimic the phosphorylated tyrosine, alters the ligand-binding capability of the Itk SH3 domain in a ligand-dependent fashion. NMR chemical shift mapping gives residue-specific structural insight into the effect of the Y180E mutation on ligand binding. These data provide a molecular level context with which to interpret in vivo functional data and allow development of a structural model for Itk autophosphorylation.

Analysis on the protein structure and function of retinoic acid induced 16

AIM: To analyze the protein structure and function of Homo sapiens retinoic acid induced 16 (RAI16) cDNA nucleotides sequence.

METHODS: Using some related computer software and database, RAI16 cDNA sequence and the deduced amino acid (AA) sequence were analyzed and compared with those of other species, and the secondary structure and hydrophobicity of the protein were determined and predicted.

RESULTS: RAI16 cDNA sequence consisted of 2863 nucleotides encoding a 759-AA protein. RAI16 protein included a 31-AA signal peptide. The protein had no transmembrane domain, and its subcellular localization was in endoplasmic reticulum. The secondary structure contained several kinds of conformations. RAI16 protein appeared as a compact and globular domain, and was a secretory and hydrophilicity protein (grand average of hydropathicity: -0.136). The protein was also unstable (the unstable coefficient: 53.11), with many phosphorylation and proteinase sites.

CONCLUSION: As a new signal protein, RAI16 may be involved in the signal transduction and transcription of hepatoma cell differentiation by interacting with Tec protein.

Use of Tag single nucleotide polymorphisms (SNPs) to screen PTPN21: no association with Graves' disease

OBJECTIVE

The protein-tyrosine-phosphate nonreceptor 22 gene (PTPN22) has recently been identified as a susceptibility locus for a number of autoimmune diseases including Graves' disease (GD). PTPN21 is another member of the PTPN family and its gene PTPN21 maps to the first reported region of genetic linkage to GD, GD-1, on chromosome 14q31. The aim of this study was to determine whether PTPN21 is acting as a GD susceptibility locus in UK Caucasian subjects.

DESIGN

A case control association study of seven Tag single nucleotide polymorphisms (SNPs) (rs1469602, rs8007288, rs1998670, rs11622270, rs2274736, rs2295136 and rs366476) selected to predict 51 un-genotyped polymorphisms present within PTPN21.

PATIENTS

Unrelated Caucasian patients of UK origin with GD and ethnically and gender matched control subjects with no family history of autoimmune disease were recruited. In total, DNA was obtained from 768 GD patients and 768 control subjects.

RESULTS

No association of any of the seven Tag SNPs was detected with GD. Preliminary evidence of association of rs2274736 was found with younger age of GD onset (0-30 years) (OR = 1. 48 [95% CI = 1.11-1.97]). No other correlations with clinical phenotype or previously established susceptibility loci were detected.

CONCLUSIONS

Using a Tag SNP approach we screened PTPN21 as a susceptibility locus for GD and found no evidence for association with disease. Preliminary evidence for association of rs2274736 with younger age of GD onset requires replication in similar sized data sets to exclude a false positive result. Methods such as the Tag SNP approach significantly reduce the amount of genotyping required when screening candidate loci, including those within regions of chromosomal linkage.

Distinct gene expression signature in Btk-defective T1 B-cells

Bruton's tyrosine kinase (Btk) is a cytoplasmic tyrosine kinase important for B-lymphocyte maturation. Mutations in Btk give rise to the primary immunodeficiency disease X-linked agammaglobulinemia (XLA) in man and X-linked immunodeficiency (Xid) in mice. Recent studies have subdivided the mouse immature, or transitional, B-cells into two distinct subsets according to their respective surface markers. Transitional type 1 (T1) and transitional type 2 (T2) cells are also located in distinct anatomic locations. Based on a limited number of markers it has previously been reported that the earliest phenotypic sign of Btk deficiency is manifested at the T2 stage in mice. Here, we report on distinct genome-wide transcriptomic signature differences found in T1 B-lymphocytes from Btk-defective compared to normal mice and demonstrate that Btk deficiency is visible already at this stage.

CD28 and ICOS: Similar or separate costimulators of T cells?

Numerous studies have revealed that the B7.1/B7.2-CD28 and B7RP-1-ICOS (Inducible COStimulator) pathways provide crucial costimulatory signals to T cells. We have compared the contribution of these pathways during primary and effector responses, in vitro and in vivo, molecularly as well as functionally. This comparison between CD28 an ICOS after initiation of T cell activation demonstrates that both CD28 and ICOS function similarly during expansion, survival and differentiation of T cells and that both CD28 and ICOS are necessary for proper IgG responses. The major differences between CD28 and ICOS are differences in expression of both receptors and ligands, and the fact that CD28 induces IL-2 production, whereas ICOS does not. In addition, ICOS is more potent in the induction of IL-10 production, a cytokine important for suppressive function of T regulatory cells. All data available at present indicate that both molecules are very suitable candidates for immunotherapy, each in their own unique way.

Identification of committed placental stem cell lines for studies of differentiation

Trophoblasts provide a model to investigate fundamental mechanisms of stem cell differentiation, but the availability of trophoblast stem cell lines is limited. Here we report the development of an RT-PCR-based lineage-specific profile as a method to identify the lineages of placental trophoblast cells routinely and specifically. This profiling method was used to analyze the mouse SM10 and rat HRP-1 cell lines, isolated from a region of the placental labyrinth, but of previously unidentified lineage. Using this profile, the expression of trophoblast stem cell markers was detected in the SM10 and HRP-1 cells. In contrast, no expression of a marker of differentiated labyrinthine trophoblast was detected. Additionally, both cell lines expressed labyrinthine trophoblast-specific genes and did not express lineage-specific markers of spongiotrophoblasts or trophoblast giant cells. Our results suggest that SM10 and HRP-1 cell lines are trophoblast stem cell-like cell lines that can be maintained in undifferentiated but committed state in cell culture. These cell lines express labyrinthine-specific genes and are committed to differentiate solely into functional labyrinthine trophoblasts. Our profiling method provides a new technique to identify stem cells and their lineage-specific differentiation. This method additionally indicates that SM10 and HRP-1 cell lines provide new systems for future studies of stem cell differentiation, allowing investigation of basic mechanisms of differentiation, which may provide insights into the biophysics of development of a specialized system. This method should also prove to be useful for identification of other stem cell lines and examination of lineage-specific commitment.

LIME acts as a transmembrane adapter mediating BCR-dependent B-cell activation

Assembly of a signaling complex around the transmembrane adapter LAT is essential for the transmission of T-cell receptor (TCR)-mediated signaling. However, a LAT-like molecule responsible for the initial activation events in B-cell receptor (BCR) signaling has not yet been identified. Here, we show that LIME is a transmembrane adaptor required for BCR-mediated B-cell activation. LIME was found to be expressed in mouse splenic B cells. Upon BCR cross-linking, LIME was tyrosine phosphorylated by Lyn and associated with Lyn, Grb2, PLC-gamma2, and PI3K. Reduction of LIME expression by the introduction of siRNA resulted in the disruption of BCR-mediated activation of MAPK, calcium flux, NF-AT, PI3K, and NF-kappaB. Taken together, these results establish that LIME is an essential transmembrane adaptor linking BCR ligation to the downstream signaling events that lead to B-cell activation.

Distinct roles for multiple Src family kinases at fertilization

Egg activation at fertilization requires the release of Ca2+ from the endoplasmic reticulum of the egg. Recent evidence indicates that Src family kinases (SFKs) function in the signaling pathway that initiates this Ca2+ release in the eggs of many deuterostomes. We have identified three SFKs expressed in starfish (Asterina miniata) eggs, designated AmSFK1, AmSFK2 and AmSFK3. Antibodies made against the unique domains of each AmSFK protein revealed that all three are expressed in eggs and localized primarily to the membrane fraction. Both AmSFK1 and AmSFK3 (but not AmSFK2) are necessary for egg activation, as determined by injection of starfish oocytes with dominant-interfering Src homology 2 (SH2) domains, which specifically delay and reduce the initial release of Ca2+ at fertilization. AmSFK3 exhibits a very rapid and transient kinase activity in response to fertilization, peaking at 30 seconds post sperm addition. AmSFK1 kinase activity also increases transiently at fertilization, but peaks later, at 2 minutes. These results indicate that there are multiple SFKs present in starfish eggs with distinct, perhaps sequential, signaling roles.

A role for the Tec family kinase ITK in regulating SEB-induced interleukin-2 production in vivo via c-jun phosphorylation

Background

Exposure to Staphylococcal Enterotoxin B (SEB), a bacterial superantigen secreted by the Gram-positive bacteria Staphyloccocus aureus, results in the expansion and eventual clonal deletion and anergy of Vβ8⁺ T cells, as well as massive cytokine release, including Interleukin-2 (IL-2). This IL-2 is rapidly secreted following exposure to SEB and may contribute to the symptoms seen following exposure to this bacterial toxin. The Tec family kinase ITK has been shown to be important for the production of IL-2 by T cells stimulated in vitro and may represent a good target for blocking the production of this cytokine in vivo. In order to determine if ITK represents such a target, mice lacking ITK were analyzed for their response to SEB exposure.

Results

It was found that T cells from mice lacking ITK exhibited significantly reduced proliferative responses to SEB exposure in vitro, as well as in vivo. Examination of IL-2 production revealed that ITK null mice produced reduced levels of this cytokine in vitro, and more dramatically, in vivo. In vivo analysis of c-jun phosphorylation, previously shown to be critical for regulating IL-2 production, revealed that this pathway was specifically activated in SEB reactive Vβ8⁺ (but not non-reactive Vβ6⁺) T cells from WT mice, but not in Vβ8⁺ T cells from ITK null mice. However, toxicity analysis indicated that both WT and ITK null animals were similarly affected by SEB exposure.

Conclusion

These data show that ITK is required for IL-2 production induced by SEB in vivo, and may regulate signals leading IL-2 production, in part by regulating phosphorylation of c-jun. The data also suggest that perturbing T cell activation pathways leading to IL-2 does not necessarily lead to improved responses to SEB toxicity.

B cell signaling and tumorigenesis

The proliferation and differentiation of lymphocytes are regulated by receptors localized on the cell surface. Engagement of these receptors induces the activation of intracellular signaling proteins that transmit the receptor signals to distinct targets and control the cellular responses. The first signaling proteins to be discovered in higher organisms were the products of oncogenes. For example, the kinases Src and Abelson (Abl) were originally identified as oncogenes and were later characterized as important proteins for signal transduction in various cell types, including lymphocytes. Now, as many cellular signaling molecules have been discovered and ordered into certain pathways, we can better understand why particular signaling proteins are associated with tumorigenesis. In this review, we discuss recent progress in unraveling the molecular mechanisms of signaling pathways that control the proliferation and differentiation of early B cells. We point out the concepts of auto-inhibition and subcellular localization as crucial aspects in the regulation of B cell signaling.

Dynamic Regulation of Tec Kinase Localization in Membrane-proximal Vesicles of a T Cell Clone Revealed by Total Internal Reflection Fluorescence and Confocal Microscopy

Tec family tyrosine kinases are key regulators of lymphocyte activation and effector function. Several Tec family kinases (Tec, Itk, Rlk/Txk) are expressed in T cells, but it is still not clear to what degree these are redundant or have unique functions. We recently demonstrated that Tec alone, among the Tec kinase family members examined, can induce nuclear factor of activated T cell-dependent transcription. This unique functional characteristic correlated with a unique pattern of subcellular localization, as Tec (but not other family members) was found in small vesicles, the appearance of which requires signaling through the T cell receptor for antigen. Here we report on our studies of these Tec-containing structures in live T cells, using total internal reflection fluorescence microscopy. With this technique, we showed that, in live T cells, the Tec vesicles are located at the plasma membrane, the vesicles are unique to Tec (and not the related kinase Itk), and their formation and maintenance require T cell receptor signaling through Src family kinases and PI 3-kinase. Finally, we have imaged isolated T cell membranes by confocal microscopy, confirming the membrane-proximal location of Tec vesicles, as well as demonstrating overlap of these vesicles with the tyrosine kinase Lck, the Tec substrate PLC-gamma1, and the early endosomal antigen 1 marker EEA1.

Itk is not essential for CD28 signaling in naive T cells

Itk, a member of the Tec family of tyrosine kinases, is critical for TCR signaling, leading to the activation of phospholipase C gamma1. Early biochemical studies performed in tumor cell lines also implicated Itk in CD28 signaling. These data were complemented by functional studies on primary Itk-/- T cells that suggested a negative role for Itk in CD28 signaling. In this report, we describe a thorough analysis of CD28-mediated responses in T cells lacking Itk. Using purified naive CD4+ T cells from Itk-/- mice, we examine a range of responses dependent on CD28 costimulation. We also analyze Akt and glycogen synthase kinase-3beta phosphorylation in response to stimulation of CD28 alone. Overall, these experiments demonstrate that CD28 signaling, as well as CD28-mediated costimulation of TCR signaling, function efficiently in the absence of Itk. These findings indicate that Itk is not essential for CD28 signaling in primary naive CD4+ T cells.

Signaling through CD16b in human neutrophils involves the Tec family of tyrosine kinases

Tec kinases belong to the second largest family of nonreceptor tyrosine kinases. Although these kinases are expressed in myeloid cells, little is known about their implication in neutrophil function. We recently reported the participation of Tec kinases in the responses of human neutrophils to the bacterial peptide N-formyl-l-methionyl-l-leucyl-l-phenylalanine via G-coupled protein receptors. In this study, we extended our investigations of Tec kinases to the signaling of the glycosylphosphatidylinositol-linked receptor CD16b, which is highly and specifically expressed in neutrophils. The results obtained indicate that Tec is translocated to the plasma membrane, phosphorylated, and activated upon CD16b cross-linking and that the activation of Tec is inhibited by Src-specific inhibitors as well as by the phosphatidylinositol-3 kinase inhibitor, wortmannin. As no specific inhibitor of Tec exists, the role of Tec kinases was further investigated using a-Cyano-b-hydroxy-b-methyl-N-(2,5-dibromophenyl)propenamide (LFM-A13), a compound known to inhibit Bruton's tyrosine kinase. We show that this compound also inhibits the kinase activity of Tec and provide evidence that the mobilization of intracellular calcium and the tyrosine phosphorylation of phospholipase Cgamma2 (PLCgamma2) induced upon CD16b engagement are inhibited by LFM-A13. We also show that Tec kinases are important for CD16b-dependent degranulation of neutrophils. In summary, we provide direct evidence for the implication of Tec in CD16b signaling and suggest that Tec kinases are involved in the phosphorylation and activation of PLCgamma2 and subsequently, in the mobilization of calcium in human neutrophils.

Signaling pathways in Th2 development

In order for an immune response to be successful, it must be of the appropriate type and magnitude. Intracellular residing pathogens require a cell-mediated immune response, whereas extracellular pathogens evoke a humoral immune response. T-helper (Th) cells orchestrate the immune response and are divided into two subsets, Th1 and Th2 cells. Here, we discuss the mechanisms of Th2 development with a focus on signal transduction pathways that influence Th2 differentiation.

SHIP Family Inositol Phosphatases Interact with and Negatively Regulate the Tec Tyrosine Kinase

The Tec family of protein-tyrosine kinases (PTKs), that includes Tec, Itk, Btk, Bmx, and Txk, plays an essential role in phospholipase Cgamma (PLCgamma) activation following antigen receptor stimulation. This function requires activation of phosphatidylinositol 3-kinase (PI 3-kinase), which promotes Tec membrane localization through phosphatidylinositol 3,4,5-trisphosphate (PtdIns 3,4,5-P(3)) generation. The mechanism of negative regulation of Tec family PTKs is poorly understood. In this study, we show that the inositol 5' phosphatases SHIP1 and SHIP2 interact preferentially with Tec, compared with other Tec family members. Four lines of evidence suggest that SHIP phosphatases are negative regulators of Tec. First, SHIP1 and SHIP2 are potent inhibitors of Tec activity. Second, inactivation of the Tec SH3 domain, which is necessary and sufficient for SHIP binding, generates a hyperactive form of Tec. Third, SHIP1 inhibits Tec membrane localization. Finally, constitutively targeting Tec to the membrane relieves SHIP1-mediated inhibition. These data suggest that SHIP phosphatases can interact with and functionally inactivate Tec by de-phosphorylation of local PtdIns 3,4,5-P(3) and inhibition of Tec membrane localization.

Identification and Specificity Studies of Small-Molecule Ligands for SH3 Protein Domains

The Src Homology 3 (SH3) domains are small protein-protein interaction domains that bind proline-rich sequences and mediate a wide range of cell-signaling and other important biological processes. Since deregulated signaling pathways form the basis of many human diseases, the SH3 domains have been attractive targets for novel therapeutics. High-affinity ligands for SH3 domains have been designed; however, these have all been peptide-based and no examples of entirely nonpeptide SH3 ligands have previously been reported. Using the mouse Tec Kinase SH3 domain as a model system for structure-based ligand design, we have identified several simple heterocyclic compounds that selectively bind to the Tec SH3 domain. Using a combination of nuclear magnetic resonance chemical shift perturbation, structure-activity relationships, and site-directed mutagenesis, the binding of these compounds at the proline-rich peptide-binding site has been characterized. The most potent of these, 2-aminoquinoline, bound with Kd = 125 microM and was able to compete for binding with a proline-rich peptide. Synthesis of 6-substituted-2-aminoquinolines resulted in ligands with up to 6-fold improved affinity over 2-aminoquinoline and enhanced specificity for the Tec SH3 domain. Therefore, 2-aminoquinolines may potentially be useful for the development of high affinity small molecule ligands for SH3 domains.

Cyclophilin A regulates TCR signal strength in CD4+ T cells via a proline-directed conformational switch in Itk

Cyclophilin A (CypA/Ppia) is a peptidyl-prolyl isomerase (PPIase) that binds the immunosuppressive drug cyclosporine. The resulting complex blocks T cell function by inhibiting the calcium-dependent phosphatase calcineurin. To identify the native function of CypA, long suspected of regulating signal transduction, we generated mice lacking the Ppia gene. These animals develop allergic disease, with elevated IgE and tissue infiltration by mast cells and eosinophils, that is driven by CD4+ T helper type II (Th2) cytokines. Ppia(-/-) Th2 cells were hypersensitive to TCR stimulation, a phenotype consistent with increased activity of Itk, a Tec family tyrosine kinase crucial for Th2 responses. CypA bound Itk via the PPIase active site. Mutation of a conformationally heterogeneous proline in the SH2 domain of Itk disrupted interaction with CypA and specifically increased Th2 cytokine production from wild-type CD4+ T cells. Thus, CypA inhibits CD4+ T cell signal transduction in the absence of cyclosporine via a regulatory proline residue in Itk.

A phosphorylation site in Bruton's tyrosine kinase selectively regulates B cell calcium signaling efficiency by altering phospholipase C-gamma activation

Loss of function of Bruton's tyrosine kinase (Btk) causes X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency in mice (xid). By using MS analysis and phosphopeptide-specific antibodies, we identified a tyrosine phosphorylation site (Y617) near the carboxyl terminus of the Btk domain from Btk expressed in 293T as well as DT-40 cells. Y617 is conserved in all Tec family kinases except murine Tec. Replacement of Y617 with a negatively charged glutamic acid (E) suppressed Btk-mediated phospholipase Cgamma2 activation and calcium response in DT-40 cells, whereas Akt activation was not affected. The Btk Y617E mutant could partially restore conventional B cell development and proliferation in Btk(-)/Tec(-) mice but failed to rescue CD5(+) B-1 cell development and the TI-II immune response to 2,4,6,-trinitrophenyl-Ficoll. These data suggest that Y617 phosphorylation or a negative charge at this site may down-regulate the function of Btk by selectively suppressing the B cell calcium signaling pathway.

Signaling through Itk promotes T helper 2 differentiation via negative regulation of T-bet

The Tec family tyrosine kinase, Itk, is critical for PLC-gamma1 activation downstream of the TCR. Studies of Itk-/- mice have demonstrated a requirement for Itk in Th2 cytokine production and protective immunity to parasitic infections. Here we address the mechanism by which Itk regulates Th2 differentiation. We find that naive Itk-/- CD4+ T cells respond normally to cytokine skewing signals and can differentiate efficiently into either Th1 or Th2 lineage cells. In the absence of skewing cytokines, wild-type CD4+ T cells stimulated with low-avidity ligands preferentially express GATA-3 mRNA and differentiate into Th2 cells. Under these same stimulation conditions, Itk-/- T cells produce large amounts of T-bet mRNA and differentiate into IFN-gamma-producing cells. Furthermore, Itk is upregulated during Th2 differentiation, while Rlk, a related Tec kinase, disappears rapidly from differentiating Th2 cells. Together, these findings provide a molecular explanation for the essential role of Itk in Th2 differentiation.

Tec Kinase Migrates to the T Cell-APC Interface Independently of Its Pleckstrin Homology Domain

Tec is the prototypical member of the Tec tyrosine kinases family, which plays an important role in T cell signaling. We show in this study that Tec translocates to the immunological synapse when a T cell contacts a dendritic cell. Surprisingly, the presence of the pleckstrin homology (PH) domain of Tec is not required for this accumulation, and despite a strong activation of 3'-phosphorylated phosphoinositide lipids synthesis during the synapse formation, the Tec PH domain is not redistributed to the T cell plasma membrane. In contrast, we demonstrate that an active Src homology 3 domain is absolutely required, underlining the essential role played by this part of the molecule in the recruitment and/or stabilization of Tec at the immunological synapse. Our results nevertheless suggest that the PH domain controls the kinase activity of the molecule in vivo. We finally demonstrate that the two domains are necessary to trigger transcriptional events following Ag presentation. These data support a model in which the plasma membrane recruitment of the PH-containing protein Tec is not dependent on the production of 3'-phosphorylated phosphoinositide lipids by the PI3K, but rather on an intact Src homology 3 domain.

The role of tyrosine kinase Etk/Bmx in EGF-induced apoptosis of MDA-MB-468 breast cancer cells

Etk/Bmx, a member of the Tec family of tyrosine kinases, mediates various signaling pathways and confers several cellular functions. In the present study, we have explored the functional role of Etk in mediating EGF-induced apoptosis, using MDA-MB-468 cell line as a model. We first demonstrated that EGF treatment induces Etk tyrosine phosphorylation in both HeLa and MDA-MB-468 cells. Overexpression of Etk by recombinant adenovirus in MDA-MB-468 cells potentiates the extent of EGF-induced cell apoptosis. The observed Etk-enhanced MDA-MB-468 cell apoptosis is associated with the Stat1 activation, as demonstrated by electrophoresis mobility shift assays and reporter gene assays. By contrast, a kinase domain deletion mutant EtkDeltaK, functioning as a dominant-negative mutant, ameliorates EGF-induced Stat1 activation and apoptosis in MDA-MB-468 cells. To explore whether the activated Etk alone is sufficient for inducing apoptosis, a conditionally activated Etk (DeltaEtk-ER), a chimeric fusion protein of PH domain-truncated Etk and ligand-binding domain of estrogen receptor, was introduced into MDA-MB-468 cells. Upon beta-estradiol ligand activation, the DeltaEtk-ER could stimulate Stat1 activity and confer cell apoptosis independent of EGF treatment. Taken together, our findings indicate that Etk is a downstream signaling molecule of EGF receptor and suggest that Etk activation is essential for transducing the EGF-induced apoptotic signaling.

Tyrosine kinase inhibitors: a new approach for asthma

The pathogenesis of allergic asthma involves the interplay of inflammatory cells and airway-resident cells, and of their secreted mediators including cytokines, chemokines, growth factors and inflammatory mediators. Receptor tyrosine kinases are important for the pathogenesis of airway remodeling. Activation of epidermal growth factor (EGF) receptor kinase and platelet-derived growth factor (PDGF) receptor kinase leads to hyperplasia of airway smooth muscle cells, epithelial cells and goblet cells. Stimulation of non-receptor tyrosine kinases (e.g. Lyn, Lck, Syk, ZAP-70, Fyn, Btk, Itk) is the earliest detectable signaling response upon antigen-induced immunoreceptor activation in inflammatory cells. Cytokine receptor dimerization upon ligand stimulation induces activation of Janus tyrosine kinases (JAKs), leading to recruitment and phosphorylation of signal transducer and activator of transcription (STAT) for selective gene expression regulation. Activation of chemokine receptors can trigger JAK-STAT pathway, Lck, Fyn, Lyn, Fgr, and Syk/Zap-70 to induce chemotaxis of inflammatory cells. Inhibitors of tyrosine kinases have been shown in vitro to block growth factor-induced hyperplasia of airway-resident cells; antigen-induced inflammatory cell activation and cytokine synthesis; cytokine-mediated pro-inflammatory gene expression in inflammatory and airway cells; and chemokine-induced chemotaxis of inflammatory cells. Recently, anti-inflammatory effects of tyrosine kinase inhibitors (e.g. genistein, tyrphostin AG213, piceatannol, tyrphostin AG490, WHI-P97, WHI-P131, Syk antisense) in animal models of allergic asthma have been reported. Therefore, development of inhibitors of tyrosine kinases can be a very attractive strategy for the treatment of asthma.

The role of Tec family kinases in myeloid cells

Members of the Tec kinase family (Bmx, Btk, Itk, Rlk and Tec) are primarily expressed in the hematopoietic system and form, after the Src kinase family, the second largest class of non-receptor protein tyrosine kinases. During lymphocyte development and activation Tec kinases have important functions in signaling pathways downstream of the antigen receptors. Tec family kinases are also expressed in cells of the myeloid lineage. However, with the exception of mast cells and platelets, their biological role in the myeloid system is only poorly understood. This review summarizes the current knowledge about the function of Tec family kinases in hematopoietic cells of the myeloid lineage.

Control of TCR-Mediated Activation of β1 Integrins by the ZAP-70 Tyrosine Kinase Interdomain B Region and the Linker for Activation of T Cells Adapter Protein

One of the earliest functional responses of T lymphocytes to extracellular signals that activate the Ag-specific CD3/TCR complex is a rapid, but reversible, increase in the functional activity of integrin adhesion receptors. Previous studies have implicated the tyrosine kinase zeta-associated protein of 70 kDa (ZAP-70) and the lipid kinase phosphatidylinositol 3-kinase, in the activation of beta(1) integrins by the CD3/TCR complex. In this report, we use human ZAP-70-deficient Jurkat T cells to demonstrate that the kinase activity of ZAP-70 is required for CD3/TCR-mediated increases in beta(1) integrin-mediated adhesion and activation of phosphatidylinositol 3-kinase. A tyrosine to phenylalanine substitution at position 315 in the interdomain B of ZAP-70 inhibits these responses, whereas a similar substitution at position 292 enhances these downstream signals. These mutations in the ZAP-70 interdomain B region also specifically affect CD3/TCR-mediated tyrosine phosphorylation of residues 171 and 191 in the cytoplasmic domain of the linker for activation of T cells (LAT) adapter protein. CD3/TCR signaling to beta(1) integrins is defective in LAT-deficient Jurkat T cells, and can be restored with expression of wild-type LAT. Mutant LAT constructs with tyrosine to phenylalanine substitutions at position 171 and/or position 191 do not restore CD3/TCR-mediated activation of beta(1) integrins in LAT-deficient T cells. Thus, these studies demonstrate that the interdomain B region of ZAP-70 regulates beta(1) integrin activation by the CD3/TCR via control of tyrosine phosphorylation of tyrosine residues 171 and 191 in the LAT cytoplasmic domain.

Expression and function of Tec, Itk, and Btk in lymphocytes: evidence for a unique role for Tec

The Tec protein tyrosine kinase is the founding member of a family that includes Btk, Itk, Bmx, and Txk. Btk is essential for B-cell receptor signaling, because mutations in Btk are responsible for X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice, whereas Itk is involved in T-cell receptor signaling. Tec is expressed in both T and B cells, but its role in antigen receptor signaling is not clear. In this study, we show that Tec protein is expressed at substantially lower levels in primary T and B cells relative to Itk and Btk, respectively. However, Tec is up-regulated upon T-cell activation and in Th1 and Th2 cells. In functional experiments that mimic Tec up-regulation, we find that Tec overexpression in lymphocyte cell lines is sufficient to induce phospholipase Cgamma (PLC-gamma) phosphorylation and NFAT (nuclear factor of activated T cells) activation. In contrast, overexpression of Btk, Itk, or Bmx does not induce NFAT activation. Tec-induced NFAT activation requires PLC-gamma, but not the adapters LAT, SLP-76, and BLNK, which are required for Btk and Itk to couple to PLC-gamma. Finally, we show that the unique effector function for Tec correlates with a unique subcellular localization. We hypothesize that Tec functions in activated and effector T lymphocytes to induce the expression of genes regulated by NFAT transcription factors.

Function of Bruton's tyrosine kinase during B cell development is partially independent of its catalytic activity

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 lambda 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 lambda L chain usage in Btk-deficient mice. In contrast, transgenic expression of kinase-inactive K430R-Btk completely reconstituted lambda usage in Btk-deficient mice, but the defective modulation of pre-B cell surface markers, peripheral B cell survival, and BCR-mediated NF-kappaB 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 lambda L chain usage, and 2) during B cell development, Btk partially acts as an adapter molecule, independent of its catalytic activity.