Distinct p53/56lyn and p59fyn domains associate with nonphosphorylated and phosphorylated Ig-alpha

Spatial Stochastic Model of the Pre-B Cell Receptor

Survival and proliferation of immature B lymphocytes requires expression and tonic signaling of the pre-B cell receptor (pre-BCR). This low level, ligand-independent signaling is likely achieved through frequent, but short-lived, homo interactions. Tonic signaling is also central in the pathology of precursor B acute lymphoblastic leukemia (B-ALL). In order to understand how repeated, transient events can lead to sustained signaling and to assess the impact of receptor accumulation induced by the membrane landscape, we developed a spatial stochastic model of receptor aggregation and downstream signaling events. Our rule- and agent-based model builds on previous mature BCR signaling models and incorporates novel parameters derived from single particle tracking of pre-BCR on surfaces of two different B-ALL cell lines, 697 and Nalm6. Live cell tracking of receptors on the two cell lines revealed characteristic differences in their dimer dissociation rates and diffusion coefficients. We report here that these differences affect pre-BCR aggregation and consequent signal initiation events. Receptors on Nalm6 cells, which have a lower off-rate and lower diffusion coefficient, more frequently form higher order oligomers than pre-BCR on 697 cells, resulting in higher levels of downstream phosphorylation in the Nalm6 cell line.

Src-related thrombocytopenia: a fine line between a megakaryocyte dysfunction and an immune-mediated disease

Src-related thrombocytopenia (SRC-RT) is a rare autosomal dominant inherited platelet disorder due to the p.E527K heterozygous germline gain-of-function variant of Src. To date, genetic diagnosis of the disease has only been reported in seven patients from three unrelated families. The clinical features ranged from isolated thrombocytopenia to complex syndromic manifestations characterized by thrombocytopenia, bleeding, myelofibrosis, splenomegaly, and bone disease. We report a new three-generation kindred with the Src p.E527K variant. Patients presented with rather variable platelet counts (38 - 139 x 109/L), mildly impaired platelet function, >15% immature platelet fraction, and with a significant proportion of large-giant platelets. Four adults from the family were diagnosed with immune thrombocytopenia (ITP) and underwent splenectomy, achieving sustained platelet counts above 75 x 109/L for several years; increases in platelet counts were also observed after corticosteroid therapy. Four of seven Src p.E527K variant carriers showed immune defects and recurrent infections. In addition, a range of neurological symptoms, from specific language impairment to epilepsy was seen in some family members. Patient platelets exhibited constitutive Src, BTK, and PLC2 activation, and after stimulating CD19 cells by crosslinking surface IgM, phosphorylated ERK was significantly increased in B cells from individuals carrying the Src p.E527K substitution. In summary, in addition to causing impaired platelet production, SRC-RT may associate immune dysregulation, and increased platelet consumption. In families in whom several members are responsive to ITP directed therapies, an underlying Src p.E527K variant should be excluded.

Role of inhibitory signaling in peripheral B cell tolerance*

At least 20% of B cells in the periphery expresses an antigen receptor with a degree of self-reactivity. If activated, these autoreactive B cells pose a risk as they can contribute to the development of autoimmune diseases. To prevent their activation, both B cell-intrinsic and extrinsic tolerance mechanisms are in place in healthy individuals. In this review article, I will focus on B cell-intrinsic mechanisms that prevent the activation of autoreactive B cells in the periphery. I will discuss how inhibitory signaling circuits are established in autoreactive B cells, focusing on the Lyn-SHIP-1-SHP-1 axis, how they contribute to peripheral immune tolerance, and how disruptions of these circuits can contribute to the development of autoimmunity.

Fyn Kinase Activity and Its Role in Neurodegenerative Disease Pathology: a Potential Universal Target?

Fyn is a non-receptor tyrosine kinase belonging to the Src family of kinases (SFKs) which has been implicated in several integral functions throughout the central nervous system (CNS), including myelination and synaptic transmission. More recently, Fyn dysfunction has been associated with pathological processes observed in neurodegenerative diseases, such as multiple sclerosis (MS), Alzheimer's disease (AD) and Parkinson's disease (PD). Neurodegenerative diseases are amongst the leading cause of death and disability worldwide and, due to the ageing population, prevalence is predicted to rise in the coming years. Symptoms across neurodegenerative diseases are both debilitating and degenerative in nature and, concerningly, there are currently no disease-modifying therapies to prevent their progression. As such, it is important to identify potential new therapeutic targets. This review will outline the role of Fyn in normal/homeostatic processes, as well as degenerative/pathological mechanisms associated with neurodegenerative diseases, such as demyelination, pathological protein aggregation, neuroinflammation and cognitive dysfunction.

B cell receptor accessory molecule CD79 gets involved in response against Streptococcus agalactiae infection and BCR signaling in Nile tilapia (Oreochromis niloticus)

CD79, composed of two distinct chains called CD79a and CD79b, is a transmembrane protein that forms a B cell antigen receptor with membrane immunoglobulin, and generates a signal following antigen recognition by the B cell receptor. In this study, the CD79a (OnCD79a) and CD79b (OnCD79b) were cloned and identified from Nile tilapia (Oreochromis niloticus). The cDNA of ORF for OnCD79a and OnCD79b are 669 and 627 bp, coding 222 and 208 amino acids, respectively. The deduced protein analysis showed that both CD79a andCD79b contain an immunoreceptor tyrosine-based activation motif in their intracellular tails that used to propagate a signal in a B cell. Expression analysis revealed that both CD79a and CD79b expressed at high levels in immune tissues, such as anterior kidney and spleen, and in IgM⁺ B cells. Upon Streptococcus agalactiae (S. agalactiae) infection, the expressions of OnCD79a and OnCD79b were significantly up-regulated in anterior kidney and spleen. The significant up-regulations of OnCD79a and OnCD79b were also detected in leukocytes after in vitro challenge with S. agalactiae. Further, stimulations of LPS and anti-OnIgM monoclonal antibody induced significant up-regulations of OnCD79a and OnCD79b in leukocytes. Taken together, the results of this study indicated that CD79 molecule, playing roles in BCR signaling, was likely to get involved in host defense against bacterial infection in Nile tilapia.

A novel SRC-2-dependent regulation of epithelial-mesenchymal transition in breast cancer cells

Steroid receptor coactivator 2 (SRC-2) is a nuclear receptor coactivator, important for the regulation of estrogen receptor alpha (ERα)-mediated transcriptional activity in breast cancer cells. However, the transcriptional role of SRC-2 in breast cancer is still ambiguous. Here we aimed to unravel a more precise transcriptional role of SRC-2 and uncover unique target genes in MCF-7 breast cancer cells, as opposed to the known oncogene SRC-3. Gene expression analyses of cells depleted of either SRC-2 or SRC-3 showed that they transcriptionally regulate mostly separate gene sets. However, individual unique gene sets were implicated in some of the same major gene ontology biological processes, such as cellular structure and development. This finding was supported by three-dimensional cell cultures, demonstrating that depletion of SRC-2 and SRC-3 changed the morphology of the cells into epithelial-like hollow acinar structures, indicating that both SRC proteins are involved in maintaining the hybrid E/M phenotype. In clinical ER-positive, HER2-negative breast cancer samples the expression of SRC-2 was negatively correlated with the expression of MCF-7-related luminal, cell cycle and cellular morphogenesis genes. Finally, elucidating SRC-2 unique transcriptional effects, we identified Lyn kinase (an EMT biomarker) to be upregulated exclusively after SRC-2 depletion. In conclusion, we show that both SRC-2 and SRC-3 are essential for the EMT in breast cancer cells, controlling different transcriptional niches.

Activation of the B cell receptor leads to increased membrane proximity of the Igα cytoplasmic domain

Binding of antigen to the B cell receptor (BCR) induces conformational changes in BCR's cytoplasmic domains that are concomitant with phosphorylation of the immunoreceptor tyrosine-based activation motifs (ITAMs). Recently, reversible folding of the CD3ε and ξ chain ITAMs into the plasma membrane has been suggested to regulate T cell receptor signaling. Here we show that the Igα and Igβ cytoplasmic domains of the BCR do not associate with plasma membrane in resting B cells. However, antigen binding and ITAM phosphorylation specifically increased membrane proximity of Igα, but not Igβ. Thus, BCR activation is accompanied by asymmetric conformational changes, possibly promoting the binding of Igα and Igβ to differently localized signaling complexes.

HIV-1 infection of T cells and macrophages are differentially modulated by virion-associated Hck: a Nef-dependent phenomenon

The proline repeat motif (PxxP) of Nef is required for interaction with the SH3 domains of macrophage-specific Src kinase Hck. However, the implication of this interaction for viral replication and infectivity in macrophages and T lymphocytes remains unclear. Experiments in HIV-1 infected macrophages confirmed the presence of a Nef:Hck complex which was dependent on the Nef proline repeat motif. The proline repeat motif of Nef also enhanced both HIV-1 infection and replication in macrophages, and was required for incorporation of Hck into viral particles. Unexpectedly, wild-type Hck inhibited infection of macrophages, but Hck was shown to enhance infection of primary T lymphocytes. These results indicate that the interaction between Nef and Hck is important for Nef-dependent modulation of viral infectivity. Hck-dependent enhancement of HIV-1 infection of T cells suggests that Nef-Hck interaction may contribute to the spread of HIV-1 infection from macrophages to T cells by modulating events in the producer cell, virion and target cell.

Role of Src homology domain binding in signaling complexes assembled by the murid γ-herpesvirus M2 protein

γ-Herpesviruses express proteins that modulate B lymphocyte signaling to achieve persistent latent infections. One such protein is the M2 latency-associated protein encoded by the murid herpesvirus-4. M2 has two closely spaced tyrosine residues, Tyr(120) and Tyr(129), which are phosphorylated by Src family tyrosine kinases. Here we used mass spectrometry to identify the binding partners of tyrosine-phosphorylated M2. Each M2 phosphomotif is shown to bind directly and selectively to SH2-containing signaling molecules. Specifically, Src family kinases, NCK1 and Vav1, bound to the Tyr(P)(120) site, PLCγ2 and the SHP2 phosphatase bound to the Tyr(P)(129) motif, and the p85α subunit of PI3K associated with either motif. Consistent with these data, we show that M2 coordinates the formation of multiprotein complexes with these proteins. The effect of those interactions is functionally bivalent, because it can result in either the phosphorylation of a subset of binding proteins (Vav1 and PLCγ2) or in the inactivation of downstream targets (AKT). Finally, we show that translocation to the plasma membrane and subsequent M2 tyrosine phosphorylation relies on the integrity of a C-terminal proline-rich SH3 binding region of M2 and its interaction with Src family kinases. Unlike other γ-herpesviruses, that encode transmembrane proteins that mimic the activation of ITAMs, murid herpesvirus-4 perturbs B cell signaling using a cytoplasmic/membrane shuttling factor that nucleates the assembly of signaling complexes using a bilayered mechanism of phosphotyrosine and proline-rich anchoring motifs.

A computational model for early events in B cell antigen receptor signaling: analysis of the roles of Lyn and Fyn

BCR signaling regulates the activities and fates of B cells. BCR signaling encompasses two feedback loops emanating from Lyn and Fyn, which are Src family protein tyrosine kinases (SFKs). Positive feedback arises from SFK-mediated trans phosphorylation of BCR and receptor-bound Lyn and Fyn, which increases the kinase activities of Lyn and Fyn. Negative feedback arises from SFK-mediated cis phosphorylation of the transmembrane adapter protein PAG1, which recruits the cytosolic protein tyrosine kinase Csk to the plasma membrane, where it acts to decrease the kinase activities of Lyn and Fyn. To study the effects of the positive and negative feedback loops on the dynamical stability of BCR signaling and the relative contributions of Lyn and Fyn to BCR signaling, we consider in this study a rule-based model for early events in BCR signaling that encompasses membrane-proximal interactions of six proteins, as follows: BCR, Lyn, Fyn, Csk, PAG1, and Syk, a cytosolic protein tyrosine kinase that is activated as a result of SFK-mediated phosphorylation of BCR. The model is consistent with known effects of Lyn and Fyn deletions. We find that BCR signaling can generate a single pulse or oscillations of Syk activation depending on the strength of Ag signal and the relative levels of Lyn and Fyn. We also show that bistability can arise in Lyn- or Csk-deficient cells.

B cell activation triggered by the formation of the small receptor cluster: a computational study

We proposed a spatially extended model of early events of B cell receptors (BCR) activation, which is based on mutual kinase-receptor interactions that are characteristic for the immune receptors and the Src family kinases. These interactions lead to the positive feedback which, together with two nonlinearities resulting from the double phosphorylation of receptors and Michaelis-Menten dephosphorylation kinetics, are responsible for the system bistability. We demonstrated that B cell can be activated by a formation of a tiny cluster of receptors or displacement of the nucleus. The receptors and Src kinases are activated, first locally, in the locus of the receptor cluster or the region where the cytoplasm is the thinnest. Then the traveling wave of activation propagates until activity spreads over the whole cell membrane. In the models in which we assume that the kinases are free to diffuse in the cytoplasm, we found that the fraction of aggregated receptors, capable to initiate B cell activation decreases with the decreasing thickness of cytoplasm and decreasing kinase diffusion. When kinases are restricted to the cell membrane - which is the case for most of the Src family kinases - even a cluster consisting of a tiny fraction of total receptors becomes activatory. Interestingly, the system remains insensitive to the modest changes of total receptor level. The model provides a plausible mechanism of B cells activation due to the formation of small receptors clusters collocalized by binding of polyvalent antigens or arising during the immune synapse formation.

B cells are activated in response to binding of appropriate ligands, which induces the aggregation of B cell receptors. The formation of even small clusters containing less than 1% of all the receptors is sufficient for activation. This observation led us to a model in which the receptor cluster serves only as a switch that turns on the activation process involving also the remaining receptors. The idea of the model exploits the fact the Src kinase - BCR system is bistable, and thus its local activation may start the propagation of a traveling wave, which spreads activation over the entire membrane. We found that the minimal size of the activatory cluster decreases with the thickness of the cytoplasm and kinase diffusion coefficient. It is particularly small when kinases are restricted to the membrane. These findings are consistent with the properties of B cells, which prior to activation have extremely thin cytoplasmic layer and in which Src family kinases (interacting with the receptors) are tethered to the membrane.

Exact solutions to a spatially extended model of kinase-receptor interaction

B and Mast cells are activated by the aggregation of the immune receptors. Motivated by this phenomena we consider a simple spatially extended model of mutual interaction of kinases and membrane receptors. It is assumed that kinase activates membrane receptors and in turn the kinase molecules bound to the active receptors are activated by transphosphorylation. Such a type of interaction implies positive feedback and may lead to bistability. In this study we apply the Steklov eigenproblem theory to analyze the linearized model and find exact solutions in the case of non-uniformly distributed membrane receptors. This approach allows us to determine the critical value of receptor dephosphorylation rate at which cell activation (by arbitrary small perturbation of the inactive state) is possible. We found that cell sensitivity grows with decreasing kinase diffusion and increasing anisotropy of the receptor distribution. Moreover, these two effects are cooperating. We showed that the cell activity can be abruptly triggered by the formation of the receptor aggregate. Since the considered activation mechanism is not based on receptor crosslinking by polyvalent antigens, the proposed model can also explain B cell activation due to receptor aggregation following binding of monovalent antigens presented on the antigen presenting cell.

Fyn binds to and phosphorylates T cell immunoglobulin and mucin domain-1 (Tim-1)

The gene encoding T cell immunoglobulin and mucin domain-1 (Tim-1) is linked to atopy and asthma susceptibility in mice and humans. Tim-1 is a transmembrane protein expressed on activated lymphocytes and appears to have a role as a co-stimulatory receptor in T cells. The protein has not been shown to have enzymatic activity but contains a site within its cytoplasmic tail predicted to be a target for tyrosine kinases. Here, we show that Tim-1 can associate with the kinase Fyn, a member of the Src family of tyrosine kinases. This association does not require Fyn's kinase activity and is independent of the phosphorylation of a conserved tyrosine present within the cytoplasmic tail of Tim-1. Fyn is necessary for phosphorylation of this tyrosine in Tim-1 and the phosphorylation of Tim-1 varies with the levels of Fyn present in cells. These data suggest a role for Fyn in the signaling downstream of Tim-1.

Structural characterization of the natively unfolded N-terminal domain of human c-Src kinase: insights into the role of phosphorylation of the unique domain

The N-terminal regions of the members of Src family of non-receptor protein tyrosine kinases are intrinsically unfolded and contain the maximum sequence divergence among them. In this study, we have addressed the structural characterization by nuclear magnetic resonance of this region of 84 residues that encompasses the SH4 and the unique domains (USrc) of the human c-Src. With this aim, the backbone assignment was performed using (13)C-detected experiments that overcome the spectral resolution problems and the large number of prolines that are typical for intrinsically unfolded proteins. The analysis of the residual dipolar couplings measured for the USrc indicates the presence of a low populated helical structure in the 60-75 region. No long-range contacts between remote fragments of the chain were detected with paramagnetic relaxation enhancement experiments. The structural characterization was extended to two different phosphorylation states of USrc that encompassed three different phosphorylated sites, Ser17, Thr37, and Ser75. The structural and conformational changes upon phosphorylation were monitored through chemical shift perturbations and residual dipolar couplings, indicating that modifications occur at local level and no global rearrangements were apparent. These results suggest a scenario where phosphorylation induces a global electrostatic perturbation that could be involved in the membrane unbinding of c-Src and that could be related with the localization of the enzyme. These observations suggest the unique domain of Src kinases as a source of selectivity and reinforce the relevant role of intrinsically disordered proteins in biological processes.

Epstein-Barr virus latent membrane protein 2A preferentially signals through the Src family kinase Lyn

Latent membrane protein 2A (LMP2A) is a viral protein expressed during Epstein-Barr virus (EBV) latency in EBV-infected B cells both in cell culture and in vivo. LMP2A has important roles in modulating B-cell receptor signal transduction and provides survival and developmental signals to B cells in vivo. Although Lyn has been shown to be important in mediating LMP2A signaling, it is still unclear if Lyn is used preferentially or if LMP2A associates promiscuously with other Src family kinase (SFK) members. To investigate the role of various SFKs in LMP2A signaling, we crossed LMP2A transgenic mice (TgE) with Lyn(-/-), Fyn(-/-), or Blk(-/-) mice. TgE Lyn(-/-) mice had a larger immunoglobulin M (IgM)-positive B-cell population than TgE mice, suggesting that the absence of Lyn prevents LMP2A from delivering survival and developmental signals to the B cells. Both TgE Fyn(-/-) and TgE Blk(-/-) mice have an IgM-negative population of splenic B cells, similar to the TgE mice. LMP2A was also transiently transfected into the human EBV-negative B-cell line BJAB to determine which SFK members associate with LMP2A. Lyn was detected in LMP2A immunoprecipitates, whereas Fyn was not. Both Lyn and Fyn were able to bind to an LMP2A mutant which contained a sequence shown previously to bind tightly to the SH2 domain of multiple SFK members. From these results, we conclude that LMP2A preferentially associates with and signals through Lyn compared to its association with other SFKs. This preferential association is due in part to the SH2 domain of Lyn associating with LMP2A.

Conformational plasticity and navigation of signaling proteins in antigen-activated B lymphocytes

Over the past two decades our view of the B cell antigen receptor (BCR) has fundamentally changed. Being initially regarded as a mute antibody orphan of the B cell surface, the BCR turned out to be a complex multimolecular machine monitoring almost all stages of B cell development, selection, and activation through a plethora of ubiquitously and cell-type-specific effector proteins. A comprehensive understanding of the many BCR signaling facets is still out but a few common biochemical principles outlined in this review operate at the level of receptor activation and orchestrate specific wiring of intracellular transducer cascades. First, initiation and processing of antigen-induced signal transduction relies on transient conformational changes in the signaling proteins to trigger their physical interaction with downstream elements. Second, this dynamic assembly of signalosomes occurs at distinct subcellular locations, most prominently the plasma membrane, which requires dynamic relocalization of one or more of the engaged molecules. For both, precise complex formation and efficient subcellular targeting, B cell signaling components are equipped with a variety of protein interaction domains. Here we provide an overview on how these simple rules are applied by a limited number of transmembrane and cytosolic proteins to convert BCR ligation into Ca(2+) mobilization and Ras activation in an adjustable manner.

Tonic B-cell and viral ITAM signaling: context is everything

The presence of an immunoreceptor tyrosine-based activation motif (ITAM) makes immunoreceptors different from other signaling receptors, like integrins, G-coupled protein receptors, chemokine receptors, and growth factor receptors. This unique motif has the canonical sequence D/Ex(0-2)YxxL/Ix(6-8)YxxL/I, where x represents any amino acid and is present at least once in all immunoreceptor complexes. Immunoreceptors can promote survival, activation, and differentiation by transducing signals through these highly conserved motifs. Traditionally, ITAM signaling is thought to occur in response to ligand-induced aggregation, although evidence indicates that ligand-independent tonic signaling also provides functionally relevant signals. The majority of proteins containing ITAMs are transmembrane proteins that exist as part of immunoreceptor complexes. However, oncogenic viruses also have ITAM-containing proteins. In this review, we discuss what is known about tonic signaling by both cellular and viral ITAM-containing proteins and speculate what we might learn from each context.

Basal B cell receptor-directed phosphatidylinositol 3-kinase signaling turns off RAGs and promotes B cell-positive selection

PI3K plays key roles in cell growth, differentiation, and survival by generating the second messenger phosphatidylinositol-(3,4,5)-trisphosphate (PIP3). PIP3 activates numerous enzymes, in part by recruiting them from the cytosol to the plasma membrane. We find that in immature B lymphocytes carrying a nonautoreactive Ag receptor, PI3K signaling suppresses RAG expression and promotes developmental progression. Inhibitors of PI3K signaling abrogate this positive selection. Furthermore, immature primary B cells from mice lacking the p85alpha regulatory subunit of PI3K suppress poorly RAG expression, undergo an exaggerated receptor editing response, and, as in BCR-ligated cells, fail to progress into the G1 phase of cell cycle. Moreover, immature B cells carrying an innocuous receptor have sustained elevation of PIP3 levels and activation of the downstream effectors phospholipase C (PLC)gamma2, Akt, and Bruton's tyrosine kinase. Of these, PLCgamma2 appears to play the most significant role in down-regulating RAG expression. It therefore appears that when the BCR of an immature B cell is ligated, PIP3 levels are reduced, PLCgamma2 activation is diminished, and receptor editing is promoted by sustained RAG expression. Taken together, our results provide evidence that PI3K signaling is an important cue required for fostering development of B cells carrying a useful BCR.

Epstein-barr virus latent membrane protein 2B (LMP2B) modulates LMP2A activity

Latent membrane protein 2A (LMP2A) and LMP2B are viral proteins expressed during Epstein-Barr virus (EBV) latency in EBV-infected B cells both in cell culture and in vivo. LMP2A has important roles in modulating B-cell receptor (BCR) signal transduction by associating with the cellular tyrosine kinases Lyn and Syk via specific phosphotyrosine motifs found within the LMP2A N-terminal tail domain. LMP2A has been shown to alter normal BCR signal transduction in B cells by reducing levels of Lyn and by blocking tyrosine phosphorylation and calcium mobilization following BCR cross-linking. Although little is currently known about the function of LMP2B in B cells, the similarity in structure between LMP2A and LMP2B suggests that they may localize to the same cellular compartments. To investigate the function of LMP2B, B-cell lines expressing LMP2A, LMP2B, LMP2A/LMP2B, and the relevant vector controls were analyzed. As was previously shown, cells expressing LMP2A had a dramatic block in normal BCR signal transduction as measured by calcium mobilization and tyrosine phosphorylation. There was no effect on BCR signal transduction in cells expressing LMP2B. Interestingly, when LMP2B was expressed in conjunction with LMP2A, there was a restoration of normal BCR signal transduction upon BCR cross-linking. The expression of LMP2B did not alter the cellular localization of LMP2A but did bind to and prevent the phosphorylation of LMP2A. A restoration of Lyn levels, but not a change in LMP2A levels, was also observed in cells coexpressing LMP2B with LMP2A. From these results, we conclude that LMP2B modulates LMP2A activity.

Membrane-targeted peptides derived from Igalpha attenuate B-cell antigen receptor function

Within the B-cell antigen receptor (BCR), heterodimers of Igalpha/Igbeta couple the receptor to intracellular signaling pathways. In the resting state, Igalpha associates with Src-family tyrosine kinases (SFTKs) which contain some basal activity. Upon engagement of the receptor, the SFTKs phosphorylate tyrosine residues in the BCR that recruit and activate the tyrosine kinase Syk, initiating signaling pathways. To test the hypothesis that disrupting the association between the resting receptor and the SFTKs would attenuate both basal and induced receptor activities, we expressed non-phosphorylatable membrane-targeted analogs of Igalpha (Igalpha/M) or Igbeta (Igbeta/M) in B lymphocytes. Both Igalpha/M and Igbeta/M inhibited BCR-induced calcium mobilization, but only Igalpha/M was able to diminish tyrosine phosphorylation. In an immature B-cell line, Igalpha/M attenuated both receptor-induced and basal apoptosis. Taken together, these data demonstrate the importance of the resting receptor complex and suggest therapeutic strategies for regulating receptor-mediated functions.

Dual Requirement for the Igα Immunoreceptor Tyrosine-Based Activation Motif (ITAM) and a Conserved Non-Igα ITAM Tyrosine in Supporting Igαβ-Mediated B Cell Development

Surface Ig (sIg) expression is a critical checkpoint during avian B cell development. Only cells that express sIg colonize bursal follicles, clonally expand, and undergo Ig diversification by gene conversion. Expression of a heterodimer, in which the extracellular and transmembrane domains of murine CD8alpha or CD8beta are fused to the cytoplasmic domains of chicken Igalpha (chIgalpha) or Igbeta, respectively (murine CD8alpha (mCD8alpha):chIgalpha + mCD8beta:chIgbeta), or an mCD8alpha:chIgalpha homodimer supported bursal B cell development as efficiently as endogenous sIg. In this study we demonstrate that B cell development, in the absence of chIgbeta, requires both the Igalpha ITAM and a conserved non-ITAM Igalpha tyrosine (Y3) that has been associated with binding to B cell linker protein (BLNK). When associated with the cytoplasmic domain of Igbeta, the Igalpha ITAM is not required for the induction of strong calcium mobilization or BLNK phosphorylation, but is still necessary to support B cell development. In contrast, mutation of the Igalpha Y3 severely compromised calcium mobilization when expressed as either a homodimer or a heterodimer with the cytoplasmic domain of Igbeta. However, coexpression of the cytoplasmic domain of Igbeta partially complemented the Igalpha Y3 mutation, rescuing higher levels of BLNK phosphorylation and, more strikingly, supporting B cell development.

Src-family kinases in B-cell development and signaling

The Src-family protein tyrosine kinases (SFKs) are known to play key roles in initiating signal transduction by the B-cell antigen receptor (BCR). In addition, numerous studies have shown that this family of molecules also contributes to signaling by BCR surrogates during B-lymphocyte lineage development and maturation. Paradoxically, ablation of SFKs not only results in obvious defects in B-cell development but also in the onset of autoimmunity. Thus SFKs, most notably Lyn, play both activating and inhibitory roles in B-cell function. Confounding analyses of SFK function in B cells is the varied coexpression of family members that mediate redundant as well as unique functions. In this review, we will focus mainly on the role of Lyn in mediating positive and negative roles in B-cell activation and how these affect immune signaling and disease progression.

Positive and negative selection during B lymphocyte development

Our laboratory is interested in a variety of issues related to lymphocyte development. More specifically, we have focused on the processes that regulate the decision to commit to the B lymphocyte (B cell) lineage, then the subsequent signals that are involved in maintaining this commitment to the B cell lineage. These signals result in the positive selection of those B cells that properly execute the complex genetic changes associated with B cell development, then trigger the elimination of B cells that are responsive to self-antigens and, therefore, possess the potential to mediate autoimmune disease. Our general experimental approach has been to address these issues from the perspective of signal transduction. Our goal is to define the biochemical and genetic processes that are integrated in order to accomplish these selection processes. To do so, we employ in vivo animal models as well as more defined in vitro studies, using both primary and transformed cell lines. For the past several years, we have been primarily interested in the precise mechanisms by which the B cell antigen receptor (BCR), and intermediate forms of this receptor, regulate these complex developmental processes. We have used the ongoing studies described below as two representative examples of how we are approaching these issues and some of the insights that we have made. To place both of these studies in context, we will begin with a brief introduction into B cell development.

SYK and LYN mediate B-cell receptor-independent calcium-induced apoptosis in DT-40 lymphoma B-cells

Here, we report that the calcium ionophore ionomycin induces a massive Ca2+-dependent apoptosis in wildtype DT-40 chicken B lymphoma cells, as well as in BTK-deficient, PLCgamma2-deficient and IP3 receptor-deficient DT-40 cells, but not in LYN- or SYK-deficient DT-40 cells. Notably, the deficiency of CSK, a negative regulator of Src-family PTK, promoted ionomycin-induced apoptosis of DT-40 cells. Reconstitution of SYK-deficient cells with wild-type SYK restored the apoptotic response of the cells to ionomycin, but the expression of FYN or LCK in LYN-deficient cells did not restore the apoptotic response of LYN-deficient cells. Taken together, our data suggests that both LYN and SYK, but not BTK, FYN or LCK, are crucial mediators of BCR-independent Ca2+-induced apoptosis in DT-40 lymphoma B cells.

Lyn tyrosine kinase is important for IL-5-stimulated eosinophil differentiation

IL-5 plays a pivotal role in growth and differentiation of eosinophils. The signal transduction mechanism of IL-5Ralpha is largely unknown. We have demonstrated that IL-5 induces tyrosine phosphorylation of IL-5Ralpha in eosinophils. To identify IL-5Ralpha-associated tyrosine kinases, we have examined the expression of Src family tyrosine kinases in eosinophils. Among the Src family members, Lyn, Hck, Fgr, and Lck are present in eosinophils, and, among these four kinases, only Lyn is associated with the IL-5Ralpha under basal conditions. We also confirm the association of Janus kinase (Jak)2 with IL-5Ralpha. Lyn kinase phosphorylates both IL-5Ralpha and betacR in vitro. The importance of Lyn kinase for eosinophil differentiation was studied using antisense oligodeoxynucleotides. Lyn antisense oligodeoxynucleotide blocks eosinophil differentiation from stem cells in a dose-dependent manner. The Jak2 inhibitor tyrphostin AG490 also inhibits eosinophil differentiation. The importance of Lyn for eosinophil differentiation was further studied using Lyn knockout mice. The IL-5-stimulated eosinophil differentiation from bone marrow cells is significantly inhibited in Lyn(-/-) mice as compared with that in control mice. We conclude that both Lyn and Jak2 play an essential role in IL-5Ralpha signaling, leading to eosinophil differentiation. The effect of Lyn appears to be relatively specific for the eosinophilic lineage.

Ligand-independent signaling functions for the B lymphocyte antigen receptor and their role in positive selection during B lymphopoiesis

Signal transduction through the B cell antigen receptor (BCR) is determined by a balance of positive and negative regulators. This balance is shifted by aggregation that results from binding to extracellular ligand. Aggregation of the BCR is necessary for eliciting negative selection or activation by BCR-expressing B cells. However, ligand-independent signaling through intermediate and mature forms of the BCR has been postulated to regulate B cell development and peripheral homeostasis. To address the importance of ligand-independent BCR signaling functions and their regulation during B cell development, we have designed a model that allows us to isolate the basal signaling functions of immunoglobulin (Ig)alpha/Igbeta-containing BCR complexes from those that are dependent upon ligand-mediated aggregation. In vivo, we find that basal signaling is sufficient to facilitate pro-B --> pre-B cell transition and to generate immature/mature peripheral B cells. The ability to generate basal signals and to drive developmental progression were both dependent on plasma membrane association of Igalpha/Igbeta complexes and intact immunoregulatory tyrosine activation motifs (ITAM), thereby establishing a correlation between these processes. We believe that these studies are the first to directly demonstrate biologically relevant basal signaling through the BCR where the ability to interact with both conventional as well as nonconventional extracellular ligands is eliminated.

Interference with immunoglobulin (Ig)alpha immunoreceptor tyrosine-based activation motif (ITAM) phosphorylation modulates or blocks B cell development, depending on the availability of an Igbeta cytoplasmic tail

To determine the function of immunoglobulin (Ig)alpha immunoreceptor tyrosine-based activation motif (ITAM) phosphorylation, we generated mice in which Igalpha ITAM tyrosines were replaced by phenylalanines (Igalpha(FF/FF)). Igalpha(FF/FF) mice had a specific reduction of B1 and marginal zone B cells, whereas B2 cell development appeared to be normal, except that lambda1 light chain usage was increased. The mutants responded less efficiently to T cell-dependent antigens, whereas T cell-independent responses were unaffected. Upon B cell receptor ligation, the cells exhibited heightened calcium flux, weaker Lyn and Syk tyrosine phosphorylation, and phosphorylation of Igalpha non-ITAM tyrosines. Strikingly, when the Igalpha ITAM mutation was combined with a truncation of Igbeta, B cell development was completely blocked at the pro-B cell stage, indicating a crucial role of ITAM phosphorylation in B cell development.

CD79: a review

CD79 is composed of CD79a and CD79b components expressed almost exclusively on B cells and B-cell neoplasms. CD79a and CD79b expression precedes immunoglobulin (Ig) heavy-chain gene rearrangement and CD20 expression during B-cell ontogeny and disappears later than CD20 in the late (plasma cell) stage of B-cell differentiation. Therefore, antibodies to CD79a and CD79b are useful in the differential diagnosis of B-cell neoplasms from T-cell neoplasms or myeloid neoplasms, or L and H lymphocyte predominance Hodgkin's lymphoma from classic Hodgkin's lymphoma. In addition, CD79a and CD79b antibodies are useful markers in the diagnosis of precursor B-acute lymphoblastic leukemia (pre-B-ALL) because many of these tumors are negative for other B-cell markers, such as CD20 and CD45RA. Furthermore, for B-cell neoplasms, wherein CD20 expression is aberrantly lost, such as in diffuse large B-cell lymphoma, or for B-cell neoplasms after CD20-antibody therapy, CD79a may be used as a first-line B-cell marker for the diagnosis. In this review, the authors discuss the molecular biology of CD79 and the frequency and usefulness of CD79 expression in these neoplasms.

Genetic ablation of the src kinase p59fynT exacerbates pulmonary inflammation in an allergic mouse model

p59fynT is a protein tyrosine kinase in the src family that has been associated with and believed to function in the signaling of many receptors, including the T-cell receptor. A role for the kinase in antigen-driven pulmonary inflammation was examined using mice whose p59fynT gene had been genetically ablated. FynKO mice that were sensitized to ovalbumin exhibited a marked increase in bronchoalveolar lavage eosinophils and cytokines, including interleukin (IL)-4 and IL-5, relative to wild-type mice in response to antigen aerosol exposure. Ovalbumin-stimulated IL-5 production was also increased in cultured splenocytes derived from fynKO mice relative to wild-type mice, whereas interferon-gamma levels were unchanged. Diminished concanavalin A--stimulated IL-4 levels from fynKO splenocytes were consistent with reduced serum immunoglobulin (Ig)E levels observed in sensitized/saline aerosol-challenged animals and may reflect defective natural killer 1.1(+) T cell development. Normalization of IgE levels in sensitized fynKO mice relative to wild-type mice occurred after repeat antigen challenge, which suggests a secondary source of IL-4. Overall, these data demonstrate fyn is a negative regulator of allergic airway inflammation in mice because its absence promotes a shift to a T helper-2 phenotype that may reflect the kinase's role in T-cell receptor signaling.

WW- and SH3-domain interactions with Epstein-Barr virus LMP2A

Epstein-Barr virus (EBV) is a human herpesvirus which establishes a lifelong latent infection in B lymphocytes. Latent membrane protein 2A (LMP2A) is expressed in both humans with EBV latent infection and EBV immortalized cell lines grown in culture. Previous studies have shown that the amino terminal domain of LMP2A, which contains eight tyrosines, associates with a variety of cellular proteins via SH2-phosphotyrosine interactions. Also contained within the LMP2A amino terminal domain are five proline-rich regions, three of which possess the PxxP core consensus sequence required for interacting with SH3 domains and two of which possess the PPxY core consensus sequence (PY motif) required for interacting with class I type WW domains. In the current study, the ability of LMP2A to interact with either modular SH3 or WW domains was investigated. The results of these studies indicate that the two LMP2A PY motifs interact strongly with representative class I WW domains, but not with representative class II WW domains. In contrast, no interactions were detected between LMP2A and any of the five different SH3 domains tested. These data demonstrate that a subset of the conserved proline-rich motifs within the amino terminus of LMP2A can potentially mediate interactions with cellular proteins and may play a role in EBV-mediated latency and/or transformation.

Characteristic expression of Hck in human B-cell precursors

OBJECTIVE

To identify molecules involved in signaling for early B-cell development, we investigated the expression of signal transduction-related proteins in B-cell progenitors.

MATERIALS AND METHODS

[corrected] Normal as well as leukemic B-cell progenitors were examined by immunoblotting and immunofluorescence study.

RESULTS

[corrected] In a survey of the expression of a broad range of signal transduction molecules, the Src-family protein tyrosine kinases were found to be differentially expressed in early B-cell differentiation. [corrected] Analysis of freshly prepared precursor-B acute lymphoblastic leukemia cells and B-lineage cell lines showed Hck and Lyn are major Src-family protein tyrosine kinases expressed in this type of leukemic blasts. [corrected] However, heterogeneity of Hck and Lyn expression was found in these cells, and precursor-B acute lymphoblastic leukemia cells subsequently were classified according to the expression pattern of Hck and Lyn as Hck/Lyn dual-negative, Hck-predominant, Hck/Lyn dual-positive, and Lyn-predominant. Further studies on normal B-lineage cells indicated that the Src-family protein tyrosine kinases are expressed sequentially in a differentiation-dependent fashion during B-cell ontogeny and that the predominant expression of Hck is a common feature in B-cell progenitors, whereas Lyn expression is more significant in mature B cells.

CONCLUSIONS

Although the biologic significance remains unknown, sequential expression of Src-family protein tyrosine kinases should play a role in regulation of early B-cell differentiation.

Distinctive roles of Fyn and Lyn in IgD- and IgM-mediated signaling

Src family kinases Fyn and Lyn associate with the B cell antigen receptor (BCR). Accumulating data show that Lyn plays important roles in BCR-mediated signaling, while the role of Fyn remains obscure. Here we dissected the role of Fyn and Lyn in BCR signaling using B cells from fyn(-/-), lyn(-/-) and fyn/lyn double-deficient (fyn(-/-)lyn(-/-)) mice. In contrast to previous reports, fyn(-/-) B cells were slightly hyporeactive to both anti-IgM and anti-IgD-dextran. Although lyn(-/-) B cells were hyper-reactive to anti-IgM, anti-IgD-induced proliferation was impaired in lyn(-/-) B cells. Most of the other phenotypes of fyn(-/-)lyn(-/-) mice were similar to that of lyn(-/-) mice, except that proliferative responses of B cells to various stimuli, such as BCR cross-linking and lipopolysaccharide, were significantly lower in fyn(-/-)lyn(-/-) mice than in lyn(-/-) mice. Finally, immune responses to thymus-independent type 2 antigen were affected in these mutant mice. These observations suggest that Fyn and Lyn are involved in B cell functions, and play similar, but partly distinct, roles in BCR signaling.

A Novel Lyn-Binding Peptide Inhibitor Blocks Eosinophil Differentiation, Survival, and Airway Eosinophilic Inflammation1, 2

Receptor antagonists block all receptor-coupled signaling pathways indiscriminately. We introduce a novel class of peptide inhibitors that is designed to block a specific signal from a receptor while keeping other signals intact. This concept was tested in the model of IL-5 signaling via Lyn kinase. We have previously mapped the Lyn-binding site of the IL-5/GM-CSF receptor common β (βc) subunit. In the present study, we designed a peptide inhibitor using the Lyn-binding sequence. The peptide was N-stearated to enable cellular internalization. The stearated peptide blocked the binding of Lyn to the βc receptor and the activation of Lyn. The lipopeptide did not affect the activation of Janus kinase 2 or its association with βc. The inhibitor blocked the Lyn-dependent functions of IL-5 in vitro (e.g., eosinophil differentiation from stem cells and eosinophil survival). It did not affect eosinophil degranulation. When applied in vivo, the Lyn-binding peptide significantly inhibited airway eosinophil influx in a mouse model of asthma. The lipopeptide had no effect on basophil histamine release or on the proliferation of B cells and T cells. To our knowledge, this is the first report on an inhibitor of IL-5 that blocks eosinophil differentiation, survival, and airway eosinophilic inflammation. This novel strategy to develop peptide inhibitors can be applied to other receptors.

Modifications of Igalpha and Igbeta expression as a function of B lineage differentiation

Transcription of the mb1 and B29 genes is initiated when lymphoid progenitors enter the B cell differentiation pathway, and their transmembrane Igalpha and Igbeta products constitute essential signaling components of pre-B and B cell antigen receptors. We analyzed Igalpha/Igbeta biosynthesis, heterogeneity, and molecular interactions as a function of human B lineage differentiation in cell lines representative of the pro-B, pre-B, and B cell stages. All B lineage representatives produced a 36-kDa Igbeta form and three principal Igalpha forms, transient 33/40-kDa species and a mature 44-kDa glycoprotein. Deglycosylation revealed a major Igalpha core protein of 25 kDa and a minor 21-kDa Igalpha protein, apparently the product of an alternatively spliced mRNA. In pro-B cells, the Igalpha and Igbeta molecules existed primarily in separate unassembled pools, exhibited an immature glycosylation pattern, did not associate with surrogate light chain proteins, and were retained intracellularly. Their unanticipated association with the Lyn protein-tyrosine kinase nevertheless suggests functional potential for the Igalpha/Igbeta molecules in pro-B cells. Greater heterogeneity of the Igalpha and Igbeta molecules in pre-B and B cell lines was attributable to increased glycosylation complexity. Finally, the Igalpha/Igbeta heterodimers associated with fully assembled IgM molecules as a terminal event in B cell receptor assembly.

Genetic analysis of B cell antigen receptor signaling

In B lymphocytes, a signaling complex that contributes to cell fate decisions is the B cell antigen receptor (BCR). Data from knockout experiments in cell lines and mice have revealed distinct functions for the intracellular protein tyrosine kinases (Lyn, Syk, Btk) in BCR signaling and B cell development. Combinations of intracellular signaling pathways downstream of these PTKs determine the quality and quantity of BCR signaling. For example, concerted actions of the PLC-gamma 2 and PI3-K pathways are required for proper calcium responses. Similarly, the regulation of ERK and JNK responses involves both PLC-gamma 2 and GTPases pathways. Since the immune response in vivo is regulated by alteration of these signaling outcomes, achieving a precise understanding of intracellular molecular events leading to B lymphocyte proliferation, deletion, anergy, receptor editing, and survival still remains a challenge for the future.

Signaling complex formation of CD44 with src-related kinases

The complex formation of murine CD44 with the src-like protein tyrosine kinases, lck and lyn, was investigated. In accordance with previous observations, stable CD44-lck and CD44-lyn complexes were detected in nonstimulated lymphoid T- and B-cells, respectively. In addition, a direct modulation of lck and lyn by CD44 was observed as revealed by the CD44-dependent translocation of these enzymes to the Triton X-100 resistant cell fraction. To clarify which receptor domain is responsible for the association, peptide binding assays were performed. Interestingly, the synthetic peptide pCD44 (ILAVCIAVNSRRR), which corresponds to the plasma membrane-cytoplasmic interface region of murine CD44, exhibited a high capacity to bind lck and lyn. A single amino acid modification in the position of the cysteine residue completely abolished this interaction, while the truncation of the three tandem arginines significantly decreased it. Remarkably, similar sequences were found in a number of other molecules including subunits of receptors recognizing antigens, immunoglobulins, extracellular matrix components, accessory molecules, cytokines and also in certain viral gene products. Synthetic peptides corresponding to the homologous regions found in CD28 and FcepsilonRIbeta were also studied and comparable lck-lyn-binding potentials were detected. These data suggest a novel interaction between src-family kinases and CD44, CD28, FcepsilonRIbeta, and provide a simple model for the association of src-like kinases with transmembrane proteins.

Dual fatty acylation of p59(Fyn) is required for association with the T cell receptor zeta chain through phosphotyrosine-Src homology domain-2 interactions

The first 10 residues within the Src homology domain (SH)-4 domain of the Src family kinase Fyn are required for binding to the immune receptor tyrosine-based activation motif (ITAM) of T cell receptor (TCR) subunits. Recently, mutation of glycine 2, cysteine 3, and lysines 7 and 9 was shown to block binding of Fyn to TCR zeta chain ITAMs, prompting the designation of these residues as an ITAM recognition motif (Gauen, L.K.T., M.E. Linder, and A.S. Shaw. 1996. J. Cell Biol. 133:1007-1015). Here we show that these residues do not mediate direct interactions with TCR ITAMs, but rather are required for efficient myristoylation and palmitoylation of Fyn. Specifically, coexpression of a K7,9A-Fyn mutant with N-myristoyltransferase restored myristoylation, membrane binding, and association with the cytoplasmic tail of TCR zeta fused to CD8. Conversely, treatment of cells with 2-hydroxymyristate, a myristoylation inhibitor, blocked association of wild-type Fyn with zeta. The Fyn NH2 terminus was necessary but not sufficient for interaction with zeta and both Fyn kinase and SH2 domains were required, directing phosphorylation of zeta ITAM tyrosines and binding to zeta ITAM phosphotyrosines. Fyn/zeta interaction was sensitive to octylglucoside and filipin, agents that disrupt membrane rafts. Moreover, a plasma membrane bound, farnesylated Fyn construct, G2A,C3S-FynKRas, was not enriched in the detergent insoluble fraction and did not associate with zeta. We conclude that the Fyn SH4 domain provides the signals for fatty acylation and specific plasma membrane localization, stabilizing the interactions between the Fyn SH2 domain and phosphotyrosines in TCR zeta chain ITAMs.

Requirement of PEST domain tyrosine phosphatase PEP in B cell antigen receptor-induced growth arrest and apoptosis

Signaling events leading to B cell growth or apoptosis are beginning to be unravelled, but detailed information is still lacking. To identify signaling molecules involved in B cell antigen receptor (BCR)-initiated pathways, we used the immature B cell line, WEHI-231, to investigate protein tyrosine phosphatases (PTP) whose expression was modulated by BCR ligation. Among the PTP cloned by reverse transcription-PCR, mRNA expression of the proline-, glutamic acid-, serine- and threonine-rich (PEST) domain phosphatase (PEP) was selectively elevated 3.1-fold within 3 h after anti-IgM antibody stimulation. In contrast, expression of another PEST domain phosphatase, PTP-PEST, was unaffected. Western blot analysis revealed that 71% of PEP was located in the cytosolic fraction, while 29% was in the membrane fraction. To examine the direct contribution made by PEP to BCR-initiated signal transduction, we transfected an antisense PEP cDNA into WEHI-231 cells. Two stable clones were established in which PEP expression was reduced by 34% and 47%, respectively. Strikingly, BCR-mediated inhibition of DNA synthesis was significantly rescued in the clones, and G1 phase cell cycle arrest and apoptosis were almost completely ablated. Considered collectively, these results indicate that PEP is a positive, crucial regulator in determining B cell fate triggered by BCR engagement.

The Mapping of the Lyn Kinase Binding Site of the Common β Subunit of IL-3/Granulocyte-Macrophage Colony- Stimulating Factor/IL-5 Receptor

It has been shown that a membrane-proximal region within common β (βc) receptor of IL-3/granulocyte-macrophage CSF/IL-5 (amino acids 450–517) is important for Lyn binding. We have shown previously that Lyn kinase is physically associated with the IL-5R βc subunit in unstimulated cells. The result suggests that this association involves binding modules that are not activation or phosphorylation dependent. The objective of this study was to map the exact Lyn binding site on βc. Using overlapping and/or sequential peptides derived from βc 450–517, we narrowed down the Lyn binding site to nine amino acid residues, βc 457–465. The P→A mutation in this region abrogated the binding to Lyn, indicating a critical role of proline residues. We created a cell-permeable Lyn-binding peptide by N-stearation. This cell-permeable peptide blocked the association of Lyn, but not Jak2 with βc in situ. We also investigated the βc binding site of Lyn kinase. Our results suggest that the N-terminal unique domain of Lyn kinase is important for binding to βc receptor. To our knowledge, this is the first molecular identification of the Lyn binding site of βc receptor. This finding may help develop specific inhibitors of Lyn-coupled signaling pathways.