CD28-CAR-T cell activation through FYN kinase signaling rather than LCK enhances therapeutic performance

Signal transduction induced by chimeric antigen receptors (CARs) is generally believed to rely on the activity of the SRC family kinase (SFK) LCK, as is the case with T cell receptor (TCR) signaling. Here, we show that CAR signaling occurs in the absence of LCK. This LCK-independent signaling requires the related SFK FYN and a CD28 intracellular domain within the CAR. LCK-deficient CAR-T cells are strongly signaled through CAR and have better in vivo efficacy with reduced exhaustion phenotype and enhanced induction of memory and proliferation. These distinctions can be attributed to the fact that FYN signaling tends to promote proliferation and survival, whereas LCK signaling promotes strong signaling that tends to lead to exhaustion. This non-canonical signaling of CAR-T cells provides insight into the initiation of both TCR and CAR signaling and has important clinical implications for improvement of CAR function.

Sulforaphane prevents chromium-induced lung injury in rats via activation of the Akt/GSK-3β/Fyn pathway

Chromium (Cr) is an internationally recognized carcinogenic hazard that causes serious pulmonary toxicity. However, Cr-induced pulmonary toxicity lacks effective treatment to date. Sulforaphane (SFN), a well-known organosulfur compound, has gained increasing attention because of its unique biological function. This study investigates if SFN could decrease K₂Cr₂O₇-induced pulmonary toxicity and a potential mechanism involved using a rat 35-day Cr-induced pulmonary toxicity model and the mouse alveolar type II epithelial cell line (MLE-12). The results showed that SFN prevented Cr-induced oxidative stress, histopathological lesions, inflammation, apoptosis, and changes in protein kinase B (Akt) and glycogen synthase kinase 3 beta (GSK-3β) levels in vivo and in vitro. However, SFN can not play the protective effect against K₂Cr₂O₇-induced cell injury after treating by an Akt-specific inhibitor (MK-2206 2HCl) in MLE-12 cells. Furthermore, SFN increased the expression of nuclear factor-E2-related factor-2 (Nrf2) phase II detoxification enzymes. Collectively, this study demonstrates that SFN prevents K₂Cr₂O₇-induced lung toxicity in rats through enhancing Nrf2-mediated exogenous antioxidant defenses via activation of the Akt/GSK-3β/Fyn signaling pathway. SFN may be a novel natural substance to cure Cr-induced lung toxicity.

Regulation of AMPK activation by CD36 links fatty acid uptake to β-oxidation

Increases in muscle energy needs activate AMPK and induce sarcolemmal recruitment of the fatty acid (FA) translocase CD36. The resulting rises in FA uptake and FA oxidation are tightly correlated, suggesting coordinated regulation. We explored the possibility that membrane CD36 signaling might influence AMPK activation. We show, using several cell types, including myocytes, that CD36 expression suppresses AMPK, keeping it quiescent, while it mediates AMPK activation by FA. These dual effects reflect the presence of CD36 in a protein complex with the AMPK kinase LKB1 (liver kinase B1) and the src kinase Fyn. This complex promotes Fyn phosphorylation of LKB1 and its nuclear sequestration, hindering LKB1 activation of AMPK. FA interaction with CD36 dissociates Fyn from the protein complex, allowing LKB1 to remain cytosolic and activate AMPK. Consistent with this, CD36(-/-) mice have constitutively active muscle and heart AMPK and enhanced FA oxidation of endogenous triglyceride stores. The molecular mechanism described, whereby CD36 suppresses AMPK, with FA binding to CD36 releasing this suppression, couples AMPK activation to FA availability and would be important for the maintenance of cellular FA homeostasis. Its dysfunction might contribute to the reported association of CD36 variants with metabolic complications of obesity in humans.

Distinct and overlapping functional roles of Src family kinases in mouse platelets

BACKGROUND AND OBJECTIVES

Src family kinases (SFKs) play a critical role in initiating and propagating signals in platelets. The aims of this study were to quantitate SFK members present in platelets and to analyze their contribution to platelet regulation using glycoprotein VI (GPVI) and intregrin αIIbβ3, and in vivo.

METHODS AND RESULTS

Mouse platelets express four SFKs, Fgr, Fyn, Lyn and Src, with Lyn expressed at a considerably higher level than the others. Using mutant mouse models, we demonstrate that platelet activation by collagen-related peptide (CRP) is delayed and then potentiated in the absence of Lyn, but only marginally reduced in the absence of Fyn or Fgr, and unaltered in the absence of Src. Compound deletions of Lyn/Src or Fyn/Lyn, but not of Fyn/Src or Fgr/Lyn, exhibit a greater delay in activation relative to Lyn-deficient platelets. Fibrinogen-adherent platelets show reduced spreading in the absence of Src, potentiation in the absence of Lyn, but no change in the absence of Fyn or Fgr. In mice double-deficient in Lyn/Src or Fgr/Lyn, the inhibitory role of Lyn on spreading on fibrinogen is lost. Lyn is the major SFK-mediating platelet aggregation on collagen at arterial shear and its absence leads to a reduction in thrombus size in a laser injury model.

CONCLUSION

These results demonstrate that SFKs share individual and overlapping roles in regulating platelet activation, with Lyn having a dual role in regulating GPVI signaling and an inhibitory role downstream of αIIbβ3, which requires prior signaling through Src.

Fc{epsilon}RI-mediated mast cell degranulation requires calcium-independent microtubule-dependent translocation of granules to the plasma membrane

The aggregation of high affinity IgE receptors (Fcɛ receptor I [FcɛRI]) on mast cells is potent stimulus for the release of inflammatory and allergic mediators from cytoplasmic granules. However, the molecular mechanism of degranulation has not yet been established. It is still unclear how FcɛRI-mediated signal transduction ultimately regulates the reorganization of the cytoskeleton and how these events lead to degranulation. Here, we show that FcɛRI stimulation triggers the formation of microtubules in a manner independent of calcium. Drugs affecting microtubule dynamics effectively suppressed the FcɛRI-mediated translocation of granules to the plasma membrane and degranulation. Furthermore, the translocation of granules to the plasma membrane occurred in a calcium-independent manner, but the release of mediators and granule–plasma membrane fusion were completely dependent on calcium. Thus, the degranulation process can be dissected into two events: the calcium-independent microtubule-dependent translocation of granules to the plasma membrane and calcium-dependent membrane fusion and exocytosis. Finally, we show that the Fyn/Gab2/RhoA (but not Lyn/SLP-76) signaling pathway plays a critical role in the calcium-independent microtubule-dependent pathway.

Regulation of natural cytotoxicity by the adaptor SAP and the Src-related kinase Fyn

SAP is an adaptor protein that is expressed in NK and T cells. It is mutated in humans who have X-linked lymphoproliferative (XLP) disease. By interacting with SLAM family receptors, SAP enables tyrosine phosphorylation signaling of these receptors by its ability to recruit the Src-related kinase, Fyn. Here, we analyzed the role of SAP in NK cell functions using the SAP-deficient mouse model. Our results showed that SAP was required for the ability of NK cells to eliminate tumor cells in vitro and in vivo. This effect strongly correlated with expression of CD48 on tumor cells, the ligand of 2B4, a SLAM-related receptor expressed in NK cells. In keeping with earlier reports that studied human NK cells, we showed that SAP was necessary for the ability of 2B4 to trigger cytotoxicity and IFN-γ secretion. In the absence of SAP, 2B4 function was shifted toward inhibition of NK cell–mediated cytotoxicity. By analyzing mice lacking Fyn, we showed that similarly to SAP, Fyn was strictly required for 2B4 function. Taken together, these results provide evidence that the 2B4-SAP-Fyn cascade defines a potent activating pathway of natural cytotoxicity. They also could help to explain the high propensity of patients who have XLP disease to develop lymphoproliferative disorders.

Lyn kinase activity is the predominant cellular SRC kinase activity in glioblastoma tumor cells

Cellular Src activity modulates cell migration, proliferation, and differentiation, and recent reports suggest that individual members of the Src family may play specific roles in these processes. As we have found that Lyn, but not Fyn, activity promotes migration of glioblastoma cells in response to the cooperative signal generated by platelet-derived growth factor receptor beta and integrin alpha(v)beta3, we compared the activity and expression of Lyn and Fyn in glioblastoma (grade IV) tumor biopsy samples with that in anaplastic astrocytoma (grade III) tumors, nonneoplastic brain, and normal autopsy brain samples. Lyn kinase activity was significantly elevated in glioblastoma tumor samples. Notably, the Lyn kinase activity accounted for >90% of pan-Src kinase activity in glioblastoma samples but only approximately 30% of pan-Src kinase activity in the other groups. The levels of phosphorylation of the autophosphorylation site were consistent with significantly higher Lyn activity in glioblastoma tumor tissue than nonneoplastic brain. Although the normalized levels of Lyn protein and the relative levels of Lyn message were significantly higher in glioblastoma samples than nonneoplastic brain, the normalized levels of Lyn protein did not correlate with Lyn activity in the glioblastoma samples. There was no significant difference in the normalized levels of c-Src and Fyn protein and message in the glioblastoma and nonneoplastic brain. Immunostaining revealed that Lyn is located primarily in the glioblastoma cells in the tumor biopsies. These data indicate that Lyn kinase activity is significantly elevated in glioblastoma tumors and suggest that it is the Lyn activity that promotes the malignant phenotype in these tumors.

Fyn/Yes and non-canonical Wnt signalling converge on RhoA in vertebrate gastrulation cell movements

Convergent extension (CE) cell movements during gastrulation mediate extension of the anterior-posterior body axis of vertebrate embryos. Non-canonical Wnt5 and Wnt11 signalling is essential for normal CE movements in vertebrate gastrulation. Here, we show that morpholino (MO)-mediated double knock-down of the Fyn and Yes tyrosine kinases in zebrafish embryos impaired normal CE cell movements, resembling the silberblick and pipetail mutants, caused by mutations in wnt11 and wnt5, respectively. Co-injection of Fyn/Yes- and Wnt11- or Wnt5-MO was synergistic, but wnt11 or wnt5 RNA did not rescue the Fyn/Yes knockdown or vice versa. Remarkably, active RhoA rescued the Fyn/Yes knockdown as well as the Wnt11 knockdown, indicating that Fyn/Yes and Wnt11 signalling converged on RhoA. Our results show that Fyn and Yes act together with non-canonical Wnt signalling via RhoA in CE cell movements during gastrulation.

Engineering a signal transduction mechanism for protein-based biosensors

Hybridization-induced conformational changes have been successfully used in biosensors for the transduction of DNA-binding events into readily observable optical or electronic signals. Similar signal transduction has not, however, proven of equal utility in protein-based biosensors. The discrepancy arises because, unlike ssDNA, most proteins do not undergo significant conformational changes upon ligand binding. Here, we describe a solution to this problem. We show that an arbitrarily selected, normally well folded protein can be rationally engineered such that it undergoes ligand-induced folding. The engineered protein responds rapidly (milliseconds) and selectively to its target, and it couples recognition with the largest possible conformational change: folding. These traits suggest that ligand-induced folding could serve as an ideal signal-transduction mechanism. Consistent with this claim, we demonstrate a label-free optical biosensor based on the effect that is sufficiently selective to detect its target even in complex, contaminant-ridden samples such as blood serum.

Evidence for Involvement of p59fyn in Fasting-Induced Thymic Involution

p59fyn, a member of the src-family protein tyrosine kinase, is expressed abundantly in thymus. We examined the possible involvement of p59fyn in thymic involution induced by a fasting stress in Fyn-/- mice. An acute 48 h fast resulted in severe atrophy of the thymus and a marked decrease of the total thymocyte number with depletion of the CD4+CD8+[double positive (DP)] population in Fyn+/+ (control) mice. A remarkable increase in terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling-positive signals was detected in the fasted group of control mice. However, these findings were not observed in Fyn-/- mice. Interestingly, MRL/MPJ-lpr/lpr, a Fas-deficient model animal, also showed no significant decrease of DP cell numbers in the fasted group. p59fyn is known to interact with Fas signalling, and these findings suggest that p59fyn is involved in fasting-induced thymic involution, raising the possibility that Fas/p59fyn-mediated signalling may, at least partially, be associated with the phenomenon.

Inferential structure determination

Macromolecular structures calculated from nuclear magnetic resonance data are not fully determined by experimental data but depend on subjective choices in data treatment and parameter settings. This makes it difficult to objectively judge the precision of the structures. We used Bayesian inference to derive a probability distribution that represents the unknown structure and its precision. This probability distribution also determines additional unknowns, such as theory parameters, that previously had to be chosen empirically. We implemented this approach by using Markov chain Monte Carlo techniques. Our method provides an objective figure of merit and improves structural quality.

Fluorescence-Based Cloning of a Protein Tyrosine Kinase with a Yeast Tribrid System

Post-translational modifications of proteins control myriad biological functions. However, relatively few methods exist for the identification of the enzymes that catalyze these modifications. To expand this repertoire, we report a yeast genetic approach that enables the identification of protein tyrosine kinases (PTKs) from cDNA libraries. Yeasts were transformed with four vectors encoding: 1) a potentially universal PTK substrate fused to the LexA DNA binding domain, 2) the Grb2-SH2 domain fused to the B42 activation domain, 3) a fluorescent reporter gene controlled by LexA DNA sites, and 4) a Jurkat cDNA library. Transient expression of PTKs, such as the lymphocyte-specific kinase Fyn, resulted in phosphorylation of the DNA-bound substrate, recruitment of the Grb2-SH2 domain, and activation of the fluorescent reporter gene. This brief induction of protein expression circumvented the potential toxicity of PTKs to the yeast. Fluorescence activated cell sorting (FACS) enabled isolation of PTKs, and these enzymes were further characterized by flow cytometry and immunoblotting. This approach provides a potentially general method for the identification and evaluation of enzymes involved in the post-translational modification of proteins.

Fyn is a downstream target of the pleiotrophin/receptor protein tyrosine phosphatase β/ζ-signaling pathway: Regulation of tyrosine phosphorylation of Fyn by pleiotrophin

Pleiotrophin (PTN the protein, Ptn the gene) signals downstream targets through inactivation of its receptor, the transmembrane receptor protein tyrosine phosphatase (RPTP)beta/zeta, disrupting the balanced activity of RPTPbeta/zeta and the activity of a constitutively active tyrosine kinase. As a consequence of the inactivation of RPTPbeta/zeta, PTN stimulates a sharp increase in the levels of tyrosine phosphorylation of the substrates of RPTPbeta/zeta in PTN-stimulated cells. We now report that the Src family member Fyn interacts with the intracellular domain of RPTPbeta/zeta in a yeast two-hybrid system. We further demonstrate that Fyn is a substrate of RPTPbeta/zeta, and that tyrosine phosphorylation of Fyn is sharply increased in PTN-stimulated cells. In previous studies, we demonstrated that beta-catenin and beta-adducin are targets of the PTN/RPTPbeta/zeta-signaling pathway and defined the mechanisms through which tyrosine phosphorylation of beta-catenin and beta-adducin disrupts cytoskeletal protein complexes. We conclude that Fyn is a downstream target of the PTN/RPTPbeta/zeta-signaling pathway and suggest that PTN coordinately regulates tyrosine phosphorylation of beta-catenin, beta-adducin, and Fyn through the PTN/RPTPbeta/zeta-signaling pathway and that together Fyn, beta-adducin, and beta-catenin may be effectors of the previously described PTN-stimulated disruption of cytoskeletal stability, increased cell plasticity, and loss of cell-cell adhesion that are characteristic of PTN-stimulated cells and a feature of many human malignant cells in which mutations have established constitutive expression of the Ptn gene.

Development and function of CD1d-restricted NKT cells: influence of sphingolipids, SAP and sex

Natural killer T (NKT) cells are innate-like lymphocytes with immunoregulatory properties. They express an invariant and conserved T-cell receptor that recognizes glycolipids presented by CD1d. Manipulation, activation or loss of NKT cells has profound effects on diverse immune functions and can influence desirable responses to infectious organisms and tumors, as well as undesirable responses to autoantigens. Here, we discuss recent progress in the identification of natural endogenous and exogenous CD1d-presented antigens, the understanding of factors that regulate development of NKT cells in the thymus and of factors that influence their function in the periphery. Clinical implications of these findings are briefly touched on.

Inhibition of the antigen-induced activation of rodent mast cells by putative Janus kinase 3 inhibitors WHI-P131 and WHI-P154 in a Janus kinase 3-independent manner

We analyzed the effects of the Janus kinase 3 (Jak3)-specific inhibitor WHI-P131 (4-(4'-hydroxyphenyl)-amino-6,7-dimethoxyquinazoline) and the Jak3/Syk inhibitor WHI-P154 (4-(3'-bromo-4'-hydroxyphenyl)-amino-6,7-dimethoxyquinazoline) on the antigen-induced activation of mast cells. In the rat mast cell line RBL-2H3, both WHI-P131 and WHI-P154 inhibited the antigen-induced degranulation and phosphorylation of p44/42 mitogen-activated protein kinase (MAPK), p38 MAPK and c-Jun N-terminal kinase (JNK). The phosphorylation of Gab2, Akt and Vav was also inhibited by WHI-P131 and WHI-P154, indicating that these inhibitors suppress the activation of phosphatidylinositol 3-kinase (PI3K). In bone marrow-derived mast cells (BMMCs) from Jak3-deficient (Jak3-/-) mice, degranulation and activation of MAPKs were induced by the antigen in almost the same extent as in BMMCs from wild-type mice. In addition, the antigen-induced degranulation and activation of MAPKs were inhibited by WHI-P131 and WHI-P154 in both groups of BMMCs, indicating that these compounds inhibit a certain step except for Jak3. The antigen-induced increase in the activity of Fyn, a probable tyrosine kinase of Gab2, was also inhibited by WHI-P131 and WHI-P154 in RBL-2H3 cells. In BMMCs from Jak3-/- mice, the antigen stimulation induced tyrosine phosphorylation of Fyn, which was inhibited by WHI-P131, as well as in BMMCs from wild-type mice and in RBL-2H3 cells. These findings suggest that Jak3 does not play a significant role in the antigen-induced degranulation and phosphorylation of MAPKs, and that WHI-P131 and WHI-P154 inhibit the PI3K pathway by preventing the antigen-induced activation of Fyn, thus inhibiting the antigen-induced degranulation and phosphorylation of MAPKs in mast cells.

Prion protein recruits its neuronal receptor NCAM to lipid rafts to activate p59fyn and to enhance neurite outgrowth

In spite of advances in understanding the role of the cellular prion protein (PrP) in neural cell interactions, the mechanisms of PrP function remain poorly characterized. We show that PrP interacts directly with the neural cell adhesion molecule (NCAM) and associates with NCAM at the neuronal cell surface. Both cis and trans interactions between NCAM at the neuronal surface and PrP promote recruitment of NCAM to lipid rafts and thereby regulate activation of fyn kinase, an enzyme involved in NCAM-mediated signaling. Cis and trans interactions between NCAM and PrP promote neurite outgrowth. When these interactions are disrupted in NCAM-deficient and PrP-deficient neurons or by PrP antibodies, NCAM/PrP-dependent neurite outgrowth is arrested, indicating that PrP is involved in nervous system development cooperating with NCAM as a signaling receptor.

Biphasic functions of the kinase-defective Ephb6 receptor in cell adhesion and migration

EphB6 is a unique member in the Eph family of receptor tyrosine kinases in that its kinase domain contains several alterations in conserved amino acids and is catalytically inactive. Although EphB6 is expressed both in a variety of embryonic and adult tissues, biological functions of this receptor are largely unknown. In the present study, we examined the function of EphB6 in cell adhesion and migration. We demonstrated that EphB6 exerted biphasic effects in response to different concentrations of the ephrin-B2 ligand; EphB6 promoted cell adhesion and migration when stimulated with low concentrations of ephrin-B2, whereas it induced repulsion and inhibited migration upon stimulation with high concentrations of ephrin-B2. A truncated EphB6 receptor lacking the cytoplasmic domain showed monophasic-positive effects on cell adhesion and migration, indicating that the cytoplasmic domain is essential for the negative effects. EphB6 is constitutively associated with the Src family kinase Fyn. High concentrations of ephrin-B2 induced tyrosine phosphorylation of EphB6 through an Src family kinase activity. These results indicate that EphB6 can both positively and negatively regulate cell adhesion and migration, and suggest that tyrosine phosphorylation of the receptor by an Src family kinase acts as the molecular switch for the functional transition.

Mice lacking the IFN-gamma receptor or fyn develop severe experimental autoimmune uveoretinitis characterized by different immune responses

Endogenous interferon (IFN)-gamma negatively regulates experimental autoimmune uveoretinitis (EAU), a Th1-mediated disease. Although it is well known that IFN-gamma exerts its effects by binding to the IFN-gamma receptor (IFN-gammaR), the role that IFN-gammaR plays in the development of EAU has not been investigated. Fyn has been reported to inhibit Th2 differentiation. We aimed to investigate how endogenous IFN-gammaR and fyn, which influence Th1/Th2 differentiation, participate in the development of EAU. Sex-matched 6- to 10-week-old C57BL/6 wild-type (WT), IFN-gammaR knockout (GRKO) and fyn knockout (fyn KO) mice were compared. Mice were immunized subcutaneously with human interphotoreceptor retinoid-binding protein peptide 1-20 emulsified in Freund's complete adjuvant together with an intraperitoneal injection of Bordetella pertussis toxin. Three weeks later, mice were sacrificed, and their eyes and spleens were harvested for histopathologic analyses and examination of cellular immune responses, respectively. Cellular immune responses were evaluated by measuring the proliferative responses and cytokine production [interleukin (IL)-4, IL-5, IL-6, IL-13, IFN-gamma and tumor necrosis factor (TNF)-alpha] of splenocytes. The incidence of EAU was 40.0% in WT mice, 59.3% in GRKO mice and 78.6% in fyn KO mice. The average EAU score was 0.294 in WT mice, 0.917 in GRKO mice and 1.063 in fyn KO mice. Upon EAU induction, significant infiltration of eosinophils into the eyes was observed in GRKO and fyn KO mice compared to WT mice. Splenocytes from GRKO mice proliferated against the antigen and a mitogen more vigorously than those from WT and fyn KO mice. Stimulation of splenocytes with the antigen induced a higher production of IL-4, IL-6, IL-13 and IFN-gamma in GRKO mice compared to WT and fyn KO mice. In contrast, IL-5 and TNF-alpha were most abundantly produced by splenocytes from fyn KO mice compared to WT and GRKO mice. The incidence and mean severity of EAU were significantly higher in GRKO and fyn KO mice than in WT mice, suggesting that endogenous IFN-gammaR and fyn negatively regulate the development of EAU. The different cytokine production patterns by the GRKO and fyn KO mice indicate that the negative regulatory mechanism mediated by IFN-gammaR and fyn may differ.

Impairment of conditioned freezing to tone, but not to context, in Fyn-transgenic mice: relationship to NMDA receptor subunit 2B function

We previously demonstrated that transgenic mice overexpressing Fyn tyrosine kinase exhibit higher seizure susceptibility and enhanced tyrosine phosphorylation of several proteins, including the N-methyl-D-aspartate (NMDA) receptor subunit 2B (NR2B). In the present study, we analysed behavioural phenotypes, especially conditioned fear responses, of Fyn-transgenic (TG) mice to better understand the role of Fyn in learned emotional behaviour. Tone-dependent conditioned freezing was significantly attenuated in Fyn-TG mice, whereas context-dependent freezing was unaffected. Neither massed nor spaced conditioning ameliorated the attenuation of tone-dependent freezing. However, the selective NR2B antagonist ifenprodil, when administered before conditioning, restored tone-dependent freezing in Fyn-TG mice at a dose that did not affect freezing in wild-type (WT) mice. These results suggest that impairment of tone-dependent conditioned freezing in Fyn-TG mice is caused by disruption of the NR2B-containing NMDA receptor function. Tyrosine phosphorylation of brain proteins, including NR2B, was enhanced in Fyn-TG mice compared with that in WT mice. We also found that ifenprodil significantly suppressed the enhanced tyrosine phosphorylation. Thus, our data support the notion that NMDA receptor activity is tightly correlated with protein tyrosine phosphorylation, and Fyn might be one key molecule that controls tone-dependent conditioned freezing through the regulation of NMDA receptor function.

Rapid immunosuppressive effects of glucocorticoids mediated through Lck and Fyn

Glucocorticoids (GCs) are effective immunosuppressive agents and mediate well-defined transcriptional effects via GC receptors. There is increasing evidence that GCs also initiate rapid nongenomic signaling events. Using activated human CD4(+) lymphocytes and a peptide array containing 1176 different kinase consensus substrates, we generated a comprehensive profile of GC-induced rapid effects on signal transduction. The results show marked early differences in phosphorylation between GC-pretreated cells and control cells, including impaired phosphorylation of p56lck/p59fyn (Lck/Fyn) consensus substrates. Immunoprecipitation and in vitro kinase assays reveal rapid GC-induced down-modulation of Lck and Fyn kinases using SAM68 (Src [pp60c-src]-associated in mitosis 68 kDa) as a substrate. Additionally, immunoprecipitation experiments revealed reduced Lck-CD4 and Fyn-CD3 associations, suggesting GC inhibited recruitment of these kinases to the T-cell receptor complex. Western blot analysis revealed reduced phosphorylation of a series of downstream signaling intermediates following GC treatment, including protein kinase B (PKB), protein kinase C (PKC), and mitogen-activated protein kinases (MAPKs). Experiments with GC receptor-negative Jurkat cells and a pharmacologic GC receptor ligand (RU486) indicated that rapid inhibition of Lck and Fyn kinases is GC receptor dependent. Parallel experiments conducted following the application of GCs in healthy individuals confirmed suppression of Lck/Fyn in T cells within 1 hour in vivo. These results identify the inhibition of Lck and Fyn kinases as rapid targets of GCs, mediated via a GC receptor-dependent pathway.

The tyrosine kinases Fyn and Hck favor the recruitment of tyrosine-phosphorylated APOBEC3G into vif-defective HIV-1 particles

The main function of Vif is to limit the antiviral activity of APOBEC3G by counteracting its packaging into HIV-1 virions. In this work, we examine the possible functional interactions between Vif, APOBEC3G, and two Src family tyrosine kinases, Fyn and Hck, present in T lymphocytes and in monocyte-macrophages, respectively. By GST pull-down, we show that the SH3 domains of Fyn and Hck, and the corresponding full-length proteins bind Vif of HIV-1. One consequence of this interaction is a reduction in their catalytic activity. Interestingly, we also observed that APOBEC3G can be phosphorylated on tyrosine in the presence of Fyn or Hck, suggesting that both kinases may regulate APOBEC3G function. Accordingly, we demonstrate that in the presence of Fyn or Hck and in the absence of Vif, the overall level of APOBEC3G incorporated into HIV-1 particles is decreased, whereas the level of encapsidation of its phosphorylated form is significantly enhanced.

LINGO-1 negatively regulates myelination by oligodendrocytes

The control of myelination by oligodendrocytes in the CNS is poorly understood. Here we show that LINGO-1 is an important negative regulator of this critical process. LINGO-1 is expressed in oligodendrocytes. Attenuation of its function by dominant-negative LINGO-1, LINGO-1 RNA-mediated interference (RNAi) or soluble human LINGO-1 (LINGO-1-Fc) leads to differentiation and increased myelination competence. Attenuation of LINGO-1 results in downregulation of RhoA activity, which has been implicated in oligodendrocyte differentiation. Conversely, overexpression of LINGO-1 leads to activation of RhoA and inhibition of oligodendrocyte differentiation and myelination. Treatment of oligodendrocyte and neuron cocultures with LINGO-1-Fc resulted in highly developed myelinated axons that have internodes and well-defined nodes of Ranvier. The contribution of LINGO-1 to myelination was verified in vivo through the analysis of LINGO-1 knockout mice. The ability to recapitulate CNS myelination in vitro using LINGO-1 antagonists and the in vivo effects seen in the LINGO-1 knockout indicate that LINGO-1 signaling may be critical for CNS myelination.

Modulation of the Cardiac Sodium Channel Na V 1.5 by Fyn, a Src Family Tyrosine Kinase

Dynamic modulation of ion channels can produce dramatic alterations of electrical excitability in cardiac myocytes. This study addresses the effects of the Src family tyrosine kinase Fyn on Na V 1.5 cardiac sodium channels. Sodium currents were acquired by whole cell recording on HEK-293 cells transiently expressing Na V 1.5. Acute treatment of cells with insulin caused a depolarizing shift in steady-state inactivation, an effect eliminated by the Src-specific tyrosine kinase inhibitor PP2. Sodium channels were coexpressed with either constitutively active (Fyn CA ) or catalytically inactive (Fyn KD ) variants of Fyn. Fyn CA caused a 10-mV depolarizing shift of steady-state inactivation compared with Fyn KD without altering the activation conductance-voltage relationship. Comparable effects of these Fyn variants were obtained with whole-cell and perforated-patch recording. Tyrosine phosphorylation of immunoprecipitated Na V 1.5 was increased in cells expressing Fyn CA compared with Fyn KD . We show that Fyn is present in rat cardiac myocytes, and that Na V 1.5 channels from these myocytes are tyrosine-phosphorylated. In HEK-293 cells the effect of Fyn CA on Na V 1.5 inactivation is abolished by the single point mutation Y1495F, a residue located within the cytoplasmic linker between the third and fourth homologous domains of the sodium channel. We provide evidence that this linker is a substrate for Fyn in vitro, and that Y1495 is a preferred phosphorylation site. These results suggest that cardiac sodium channels are physiologically relevant targets of Src family tyrosine kinases.

Nicotinic acetylcholine receptor subunits and associated proteins in human sperm

We demonstrated previously the involvement of a nicotinic acetylcholine receptor containing an alpha7 subunit in the human sperm acrosome reaction (a modified exocytotic event essential to fertilization). Here we report the presence in human sperm of alpha7, alpha9, alpha3, alpha5, and beta4 nicotinic acetylcholine receptor subunits and the following proteins known to be associated with the receptor in the somatic cell: rapsyn and the tyrosine kinases c-SRC and FYN. The alpha7 subunit appears to exist as a homomer in the posterior post-acrosomal and neck regions of sperm and is probably linked to the cytoskeleton via rapsyn. The alpha3, alpha5, and beta4 subunits are present in the sperm flagellar mid-piece of sperm and possibly exist as alpha3alpha5beta4 and/or alpha3beta4 channels. The alpha9 subunit is present in the sperm mid-piece. We detected the FYN and c-SRC tyrosine kinases in the flagellar mid-piece region. Both co-precipitated only with the nicotinic acetylcholine receptor beta4 subunit. Immunolocalization with a C-terminal SRC kinase antibody, which recognizes several members of SRC kinase family, detected a SRC kinase co-localized with the alpha7 subunit in the neck region of sperm. Immunoprecipitation studies with that antibody demonstrated that the alpha7 subunit is associated with a SRC kinase. Antagonists of tyrosine phosphorylation inhibited the acetylcholine-initiated acrosome reaction, suggesting the involvement of a SRC kinase in the acrosome reaction.

Regulated expression of galectin-1 during T-cell activation involves Lck and Fyn kinases and signaling through MEK1/ERK, p38 MAP kinase and p70S6 kinase

Recent evidence has implicated galectins and their carbohydrate ligands as novel regulators of T-cell homeostasis. Galectin-1 (Gal-1), a member of this family, inhibits clonal expansion, induces apoptosis of antigen-primed T lymphocytes and suppresses the development of T-cell-mediated autoimmune diseases in vivo. Because the beta-galactoside-binding protein is expressed in activated but not resting T cells, it has been hypothesized that Gal-1-induced apoptosis may constitute an autocrine suicide mechanism to eliminate activated T cells contributing to the termination of an effector immune response. We undertook this study to investigate the signals and intracellular pathways leading to Gal-1 expression during T-cell activation. When T cells were stimulated either with anti-CD3 or anti-CD28 monoclonal antibody plus PMA in the presence of accessory cells, a sustained up-regulation of Gal-1 was observed, reaching a plateau between days 3 and 5 following CD3 engagement or costimulation through CD28. Investigation of the signal transduction events involved in this process revealed a role for Lck and Fyn kinases, since the Src kinase inhibitor PP1 inhibited the up-regulated expression of Gal-1 following T-cell activation. Downstream signaling routes involve mitogen-activated protein kinase (MAPK) kinase (MEK)1/extracellular signal-regulated kinase (ERK) and p38 MAPK, as Gal-1 expression was prevented by U0126 and SB202190. In addition, expression of Gal-1 involves interleukin (IL)-2-dependent signaling routes triggered by p70S6 kinase, as it could be inhibited by rapamycin. This is the first demonstration of the intracellular pathways that control activation-induced expression of Gal-1, which may reveal potential targets for immune intervention to modulate expression of this beta-galactoside-binding protein in pathological disorders.

2-Arachidonoylglycerol stimulates activator protein-1-dependent transcriptional activity and enhances epidermal growth factor-induced cell transformation in JB6 P+ cells

2-Arachidonoylglycerol (2-AG) is the most abundant endocannabinoid, and it plays a critical role in cannabinoid receptor-mediated cell signaling. Although 2-AG was shown to induce ERK activation via the cannabinoid receptor 1 (CB1), only a nonspecific CB receptor agonist and antagonist was used in those studies. Whether cannabinoid receptor 2 (CB2) is involved in 2-AG-induced ERK activation is still unclear. Moreover, whether 2-AG is involved in mediation of AP-1 activity and cell transformation is also not known. In the present study, we show that 2-AG stimulates AP-1-dependent transcriptional activity and enhances epidermal growth factor-induced cell transformation in mouse epidermal JB6 P+ Cl41 cells. Using JB6 P+ C141 cells, stably transfected with an AP-1 luciferase reporter, we found that 10 microm 2-AG induced up to a 3-fold stimulation of AP-1 transcriptional activity. The AP-1 stimulation appeared to be mediated by ERK but not JNK or p38 kinase. PD98059, a specific inhibitor of MEK1, almost completely blocked 2-AG-induced ERK phosphorylation and AP-1 activation. Using CB1/2-/- murine embryonic fibroblasts, we present the first direct evidence that both cannabinoid receptors 1 and 2 (CB1/2) are involved in 2-AG-induced ERK activation. 2-AG could not stimulate ERK phosphorylation or Fyn kinase activity in dominant negative Fyn. In addition, the Fyn inhibitor PP2 blocked 2-AG-induced Fyn kinase activity and ERK phosphorylation and activity. Small interfering RNA Fyn also suppressed 2-AG-induced ERK phosphorylation. Interestingly, 2-AG enhanced epidermal growth factor-induced AP-1 DNA binding and cell transformation. Taken together, our data provide direct evidence suggesting that 2-AG may have a novel role in cell transformation and carcinogenesis in a signaling pathway involving CB1/2 and activation of Fyn, ERKs, and AP-1.

Further studies on hepatitis C virus NS5A-SH3 domain interactions: identification of residues critical for binding and implications for viral RNA replication and modulation of cell signalling

The NS5A protein of hepatitis C virus has been shown to interact with a subset of Src homology 3 (SH3) domain-containing proteins. The molecular mechanisms underlying these observations have not been fully characterized, therefore a previous analysis of NS5A-SH3 domain interactions was extended. By using a semi-quantitative ELISA assay, a hierarchy of binding between various SH3 domains for NS5A was demonstrated. Molecular modelling of a polyproline motif within NS5A (termed PP2.2) bound to the FynSH3 domain predicted that the specificity-determining RT-loop region within the SH3 domain did not interact directly with the PP2.2 motif. However, it was demonstrated that the RT loop did contribute to the specificity of binding, implicating the involvement of other intermolecular contacts between NS5A and SH3 domains. The modelling analysis also predicted a critical role for a conserved arginine located at the C terminus of the PP2.2 motif; this was confirmed experimentally. Finally, it was demonstrated that, in comparison with wild-type replicon cells, inhibition of the transcription factor AP-1, a function previously assigned to NS5A, was not observed in cells harbouring a subgenomic replicon containing a mutation within the PP2.2 motif. However, the ability of the mutated replicon to establish itself within Huh-7 cells was unaffected. The highly conserved nature of the PP2.2 motif within NS5A suggests that functions involving this motif are of importance, but are unlikely to play a role in replication of the viral RNA genome. It is more likely that they play a role in altering the cellular environment to favour viral persistence.

Altered p59Fyn kinase expression accompanies disease progression in Alzheimer's disease: implications for its functional role

Alzheimer's disease (AD) is characterized by progressive decline in memory and other cognitive domains, accompanied by early loss of presynaptic terminals, amyloid-bearing neuritic plaques and neurofibrillary tangles containing hyperphosphorylated tau. The mechanisms leading to neurodegeneration are not completely understood, however, recent evidence suggests that alterations in p59Fyn kinase, an Src family tyrosine kinase, might contribute to AD pathogenesis. In this context, the main objective of the present study was to investigate the relationship between Fyn protein levels and the neurological and neuropathological alterations in AD. We found, by quantitative immunoblotting, that in AD, Fyn levels were increased in the insoluble fraction and decreased in the soluble fraction. Soluble Fyn levels were directly correlated with the cognitive scores and levels of synaptophysin immunoreactivity, and inversely correlated with neurofibrillary tangle counts in the frontal cortex. Consistent with these findings, the immunocytochemical analysis showed that in AD cases, Fyn levels were decreased in the synapses and increased in the neuronal cell bodies where it was colocalized with neurofibrillary tangles. Taken together, these findings suggest that alterations in Fyn localization might be associated with neurofibrillary pathology and synapse loss in AD.

Involvement of CD45 in central nervous system myelination

Myelin is a multi-layered membranous lipid insulator surrounding axons that allows the rapid conduction of neuronal impulses. In the central nervous system (CNS), myelin is produced by oligodendrocytes. During development, morphologically immature oligodendrocyte precursor cells (OPCs) arise from neural stem cells before differentiating into myelinating oligodendrocytes shortly after birth. Fyn tyrosine kinase (Fyn) has been shown to play a central role during OPC differentiation, including inducing morphological changes in the cells and initiating the expression of myelin basic protein (MBP), a major structural protein required for the compaction of myelin sheaths. Recently, we have shown that signaling via the gamma chain of immunoglobulin Fc receptors (FcRgamma) induces the Fyn-MBP cascade and promotes the morphological differentiation of OPCs. The protein tyrosine phosphatases that are responsible for the positive regulation of Fyn tyrosine kinase activity during this cascade, however, remained unknown. Here we report that a protein tyrosine phosphatase, CD45, is involved in this process. Fyn co-immunoprecipitated with CD45 from differentiating wild-type OPCs in vitro, while CD45-deficient OPCs failed to differentiate. Additionally, dysmyelination was observed in CD45-deficient mice in vivo. Our findings suggest that CD45 is a key phosphatase involved in OPC differentiation and provide a preliminary explanation for the previously reported CD45 mutations observed in some multiple sclerosis (MS) patients.

SH2 domain-mediated activation of an SRC family kinase is not required to initiate Ca2+ release at fertilization in mouse eggs

SRC family kinases (SFKs) function in initiating Ca2+release at fertilization in several species in the vertebrate evolutionary line, but whether they play a similar role in mammalian fertilization has been uncertain. We investigated this question by first determining which SFK proteins are expressed in mouse eggs, and then measuring Ca2+release at fertilization in the presence of dominant negative inhibitors. FYN and YES proteins were found in mouse eggs, but other SFKs were not detected; based on this, we injected mouse eggs with a mixture of FYN and YES Src homology 2 (SH2) domains. These SH2 domains were effective inhibitors of Ca2+release at fertilization in starfish eggs, but did not inhibit Ca2+release at fertilization in mouse eggs. Thus the mechanism by which sperm initiate Ca2+release in mouse eggs does not depend on SH2 domain-mediated activation of an SFK. We also tested the small molecule SFK inhibitor SU6656, and found that it became compartmentalized in the egg cytoplasm, thus suggesting caution in the use of this inhibitor. Our findings indicate that although the initiation of Ca2+release at fertilization of mammalian eggs occurs by a pathway that has many similarities to that in evolutionarily earlier animal groups, the requirement for SH2 domain-mediated activation of an SFK is not conserved.

Process elongation of oligodendrocytes is promoted by the Kelch-related actin-binding protein Mayven

Rearrangement of the cytoskeleton leading to the extension of cellular processes is essential for the myelination of axons by oligodendrocytes. We observed that the actin-binding protein, Mayven, is expressed during all stages of the oligodendrocyte lineage, and that its expression is up-regulated during oligodendrocyte differentiation. Mayven is localized in the cytoplasm and along the cell processes. Mayven also binds actin, and is involved in the cytoskeletal reorganization in oligodendrocyte precursor cells (O-2A cells) that leads to process elongation. Mayven overexpression resulted in an increase in the process outgrowth of O-2A cells and in the lengths of the processes, while microinjection of Mayven-specific antibodies inhibited process extension in these cells. Furthermore, O-2A cells transduced with recombinant retroviral sense Mayven (pMIG-W-Mayven) showed an increase in the number of oligodendrocyte processes with outgrowth, while recombinant retroviral antisense Mayven (pMIG-W-Mayven-AS) blocked O-2A process extension. Interestingly, co-localization and association of Mayven with Fyn kinase were found in O-2A cells, and these interactions were increased during the outgrowth of oligodendrocyte processes. This association was mediated via the SH3 domain ligand (a.a. 1-45) of Mayven and the SH3 domain of Fyn, suggesting that Mayven may act as a linker to bind Fyn, via its N-terminus. Thus, Mayven plays a role in the dynamics of cytoskeletal rearrangement leading to the process extension of oligodendrocytes.

Defective Vav expression and impaired F-actin reorganization in a subset of patients with common variable immunodeficiency characterized by T-cell defects

Common variable immunodeficiency (CVID) is a primary immune disorder characterized by impaired antibody production, which is in many instances secondary to defective T-cell function (T-CVID). We have previously identified a subset of patients with T-CVID characterized by defective T-cell receptor (TCR)-dependent protein tyrosine phosphorylation. In these patients, ZAP-70 fails to be recruited to the TCR as the result of impaired CD3zeta phosphorylation, which is, however, not dependent on defective Lck expression or activity. Here we show that neither Fyn nor CD45 is affected in these patients. On the other hand, T-CVID T cells show dramatic defects in the Vav/Rac pathway controlling F-actin dynamics. A significant deficiency in Vav protein was indeed observed; in 3 of 4 patients with T-CVID, it was associated with reduced VAV1 mRNA levels. The impairment in Vav expression correlated with defective F-actin reorganization in response to TCR/CD28 co-engagement. Furthermore, TCR/CD28-dependent up-regulation of lipid rafts at the cell surface, which requires F-actin dynamics, was impaired in these patients. The actin cytoskeleton defect could be reversed by reconstitution of Vav1 expression in the patients' T cells. Results demonstrate an essential role of Vav in human T cells and strongly suggest Vav insufficiency in T-CVID.

Cross-correlated spin relaxation effects in methyl 1H CPMG-based relaxation dispersion experiments: complications and a simple solution

Artifacts associated with the measurement of methyl (1)H single quantum CPMG-based relaxation dispersion profiles are described. These artifacts arise due to the combination of cross-correlated spin relaxation effects involving intra-methyl (1)H-(1)H dipolar interactions and imperfections in (1)H refocusing pulses that are applied during CPMG intervals that quantify the effects of chemical exchange on measured transverse relaxation rates. As a result substantial errors in extracted exchange parameters can be obtained. A simple 'work-around' is presented where the (1)H chemical shift difference between the exchanging states is extracted from a combination of (13)C single quantum and (13)C-(1)H multiple quantum dispersion profiles. The approach is demonstrated with an application to a folding/unfolding reaction involving a G48M mutant Fyn SH3 domain.

The Stomatin/Prohibitin/Flotillin/HflK/C Domain of Flotillin-1 Contains Distinct Sequences That Direct Plasma Membrane Localization and Protein Interactions in 3T3-L1 Adipocytes

Flotillin-1 is a lipid raft-associated protein that has been implicated in various cellular processes. We examined the subcellular distribution of flotillin-1 in different cell types and found that localization is cell type-specific. Flotillin-1 relocates from a cytoplasmic compartment to the plasma membrane upon the differentiation of 3T3-L1 adipocytes. To delineate the structural determinants necessary for its localization, we generated a series of truncation mutants of flotillin-1. Wild type flotillin-1 has two putative hydrophobic domains and is localized to lipid raft microdomains at the plasma membrane. Flotillin-1 fragments lacking the N-terminal hydrophobic stretch are excluded from the lipid raft compartments but remain at the plasma membrane. On the other hand, mutants with the second hydrophobic region deleted fail to traffic to the plasma membrane but are instead found in intracellular granule-like structures. Flotillin-1 specifically interacts with the adaptor protein CAP, the Src family kinase Fyn, and cortical F-actin in lipid raft microdomains in adipocytes. Furthermore, CAP and Fyn associate with different regions in the N-terminal sequences of flotillin-1. These results furthered our understanding for how flotillin-1 can function as a molecular link between lipid rafts of the plasma membrane and a multimeric signaling complex at the actin cytoskeleton.

Differential effects of Hsp90 inhibition on protein kinases regulating signal transduction pathways required for myoblast differentiation

As derivatives of the Hsp90-inhibitor and tumoricidal agent geldanamycin move into phase II clinical trials, its potential for triggering adverse effects in non-tumor cell populations requires closer examination. In this report, the effect of geldanamycin on the differentiation and survival of C2C12 myoblasts was investigated. Treatment of differentiating C2C12 myoblasts with geldanamycin blocked myogenin expression, inhibited myotubule formation, and led to the depletion of three Hsp90-dependent protein kinases, ErbB2, Fyn, and Akt, and induction of apoptosis. ErbB2 levels declined rapidly, while Fyn and Akt levels decreased at a slower rate. Geldanamycin blocked the interaction of Hsp90 and its "kinase-specific" co-chaperone Cdc37 with Fyn, indicating that Fyn is an Hsp90-dependent kinase. Pulse-chase experiments indicated that geldanamycin caused newly synthesized Akt and Fyn to be degraded rapidly, but geldanamycin had little effect on the turnover rate of mature Fyn and Akt. Curiously, total cellular Src (c-Src) protein levels and the turnover rate of newly synthesized c-Src were unaffected by geldanamycin. While, geldanamycin had no effect on the levels of the putative Hsp90 client protein MyoD expressed in C2C12 cells, geldanamycin disrupted the interaction of Cdc37 with MyoD. Thus, inhibition of Hsp90 caused C2C12 cells to become depleted of multiple signal transduction proteins whose functions are essential for myoblast differentiation, and muscle cell survival, suggesting that geldanamycin derivatives may have the prospective of adversely affecting the physiology of certain sensitive muscle cell populations in vivo.

Fyn binding protein, Fyb, interacts with mammalian actin binding protein, mAbp1

The immune cell specific protein Fyn-T binding protein (Fyb) has been identified as a target of the Yersinia antiphagocytic effector Yersinia outer protein H (YopH), but its role in macrophages is unknown. By using Fyb domains as bait to screen a mouse lymphoma cDNA library, we identified a novel interaction partner, mammalian actin binding protein 1 (mAbp1). We show that mAbp1 binds the Fyb N-terminal via its C-terminally located src homology 3 domain. The interaction between Fyb and mAbp1 is detected in macrophage lysates and the proteins co-localize with F-actin in the leading edge. Hence, mAbp1 is likely to constitute a downstream effector of Fyb involved in F-actin dynamics.

Signaling for NKT cell development: the SAP-FynT connection

New studies demonstrate a critical role for the adaptor protein SAP (SLAM-associated protein) during NKT cell development. By connecting homotypic SLAM family receptor interactions with the FynT Src kinase, SAP may integrate a set of long-standing yet seemingly disparate observations characterizing NKT cell development. In fact, SAP-dependent signaling may underlie the development of multiple unconventional T cell lineages whose thymic selection relies on homotypic interactions between hematopoietic cells.

Off-resonance R(1rho) NMR studies of exchange dynamics in proteins with low spin-lock fields: an application to a Fyn SH3 domain

An (15)N NMR R(1rho) relaxation experiment is presented for the measurement of millisecond time scale exchange processes in proteins. On- and off-resonance R(1rho) relaxation profiles are recorded one residue at a time using a series of one-dimensional experiments in concert with selective Hartmann-Hahn polarization transfers. The experiment can be performed using low spin-lock field strengths (values as low as 25 Hz have been tested), with excellent alignment of magnetization along the effective field achieved. Additionally, suppression of the effects of cross-correlated relaxation between dipolar and chemical shift anisotropy interactions and (1)H-(15)N scalar coupled evolution is straightforward to implement, independent of the strength of the (15)N spin-locking field. The methodology is applied to study the folding of a G48M mutant of the Fyn SH3 domain that has been characterized previously by CPMG dispersion experiments. It is demonstrated through experiment that off-resonance R(1rho) data measured at a single magnetic field and one or more spin-lock field strengths, with amplitudes on the order of the rate of exchange, allow a complete characterization of a two-site exchange process. This is possible even in the case of slow exchange on the NMR time scale, where complementary approaches involving CPMG-based experiments fail. Advantages of this methodology in relation to other approaches are described.

Molecular basis for positive and negative signaling by the natural killer cell receptor 2B4 (CD244)

Triggering of 2B4 (CD244) can induce natural killer (NK)-cell activation, costimulation, or even inhibition of NK-cell activity. Here, we investigate the molecular basis for the different signals generated by 2B4. We show that the first immunoreceptor tyrosine-based switch motif (ITSM) within the cytoplasmic tail of 2B4 is sufficient for 2B4-mediated NK-cell activation, whereas the third ITSM can negatively influence 2B4 signaling. We further identify signaling molecules that associate with 2B4. Signaling lymphocyte activation molecule-associated protein (SAP) can bind to all 4 ITSMs of 2B4 in a phosphorylation-dependent manner. The phosphorylated third ITSM can additionally recruit the phosphatases SHP-1, SHP-2, SHIP, and the inhibitory kinase Csk. SAP acts as an inhibitor of interactions between 2B4 and these negative regulatory molecules, explaining how 2B4 inhibits NK-cell activation in the absence of functional SAP, as occurs in cells from patients with X-linked lymphoproliferative syndrome (XLP). Recently, another function for SAP was proposed: SAP can recruit the kinase Fyn to the SLAM (CD150) immune receptor. We now show that Fyn can also associate with phosphorylated 2B4. Finally, we demonstrate that Fyn and Csk can both phosphorylate 2B4, suggesting a possible mechanism of 2B4 phosphorylation.

Pigment epithelium-derived factor inhibits fibroblast-growth-factor-2-induced capillary morphogenesis of endothelial cells through Fyn

Pigment epithelium-derived factor (PEDF) exerts anti-angiogenic actions. However, the signal-transduction pathways regulated by PEDF remain to be elucidated. We show here that PEDF inhibited fibroblast growth factor 2 (FGF-2) induced capillary morphogenesis of a murine brain capillary endothelial cell line (IBE cells) and of human umbilical-vein endothelial cells (HUVECs) cultured on growth-factor-reduced Matrigel. We previously showed that FGF-2-mediated capillary morphogenesis was blocked by the Src-kinase inhibitor PP2 and that expression of dominant negative Fyn in IBE cells inhibited capillary morphogenesis. We examined the effect of PEDF on kinase activity of Fyn and found that PEDF downregulated FGF-2-promoted Fyn activity by tyrosine phosphorylation at the C-terminus in a Fes-dependent manner. In a stable IBE cell line expressing kinase-inactive Fes (KE5-15 Fes cells), PEDF failed to inhibit FGF-2-induced capillary morphogenesis or Fyn activity. PEDF induced the colocalization of Fyn and Fes in IBE cells expressing wild-type Fes, but not in KE5-15 Fes cells. In addition, wild-type Fes increased the tyrosine phosphorylation of Fyn in vitro, suggesting that Fes might directly phosphorylate Fyn. Expression of constitutively active Fyn (Y531F) in IBE cells exhibited capillary morphogenesis in the absence of FGF-2 and was resistant for PEDF treatment. Our results suggest that PEDF downregulates Fyn through Fes, resulting in inhibition of FGF-2-induced capillary morphogenesis of endothelial cells.

Correlation between semaphorin3A-induced facilitation of axonal transport and local activation of a translation initiation factor eukaryotic translation initiation factor 4E

An impressive body of evidence has been accumulated indicating that local protein synthesis is implicated in navigation of neurite extension induced by guidance cues, such as semaphorin3A (Sema3A). We found previously that a Src type tyrosine kinase Fyn and cyclin-dependent kinase 5 (Cdk5) mediate Sema3A-signaling. We also showed that Sema3A elicits axonal transport through neuropilin-1, a receptor for Sema3A, located at the growth cones. Here, we investigate the relationship between Sema3A-induced local signaling, protein synthesis, and axonal transport. Lavendustin A, a tyrosine kinase inhibitor, and olomoucine, a cyclin-dependent kinase inhibitor, suppressed Sema3A-induced facilitation of anterograde and retrograde axonal transport in dorsal root ganglion (DRG) neuron with and without the cell body. Sema3A-induced facilitation of axonal transport was attenuated in DRG neurons of fyn- (fyn-/-) and a Cdk5 activator, p35 (p35-/-)-deficient mice when compared with those of wild-type or heterozygous mice. Inhibition of protein synthesis suppressed Sema3A-induced facilitation of axonal transport in the DRG neuron with and without the cell body. Sema3A enhanced the level of immunoreactivity of phosphorylated eukaryotic translation initiation factor 4E (eIF-4E) within 5 min in growth cones in a time course similar to that of the facilitated axonal transport. This enhanced signal for phospho-eIF4E was blocked by lavendustin A or olomoucine and was not detected in the fyn-/- and p35-/- neurons. These results provide evidence for a mutual regulatory mechanism between local protein synthesis and axonal transport.

Development of CD1d-restricted NKT cells in the mouse thymus

Using genetic and phenotypic analyses, we have analyzed the developmental pathway of mouse CD1d-restricted invariant NKT cells. We provide strong evidence that similar to conventional T cells, positive selection of NKT cells occurs during a CD4(+)CD8(+) stage. Later stages of NKT cell development involved the down-regulation of both TCR and CD4 levels and therefore diverge from conventional T cell development pathways. A unique and complete dependency for development on Fyn, a Src family kinase member, also distinguishes the NKT cell and conventional T cell populations.

p59(fyn)-mediated phosphorylation regulates the activity of the tissue-specific splicing factor rSLM-1

The Sam68-like mammalian protein SLM-1 is a member of the STAR protein family and is related to SAM68 and SLM-2. Here, we demonstrate that rSLM-1 interacts with itself, scaffold-attachment factor B, YT521-B, SAM68, rSLM-2, SRp30c, and hnRNP G. rSLM-1 regulates splice site selection in vivo via a purine-rich enhancer. In contrast to the widely expressed SAM68 and rSLM-2 proteins, rSLM-1 is found primarily in brain and, to a much smaller degree, in testis. In the brain, rSLM-1 and rSLM-2 are predominantly expressed in different neurons. In the hippocampal formation, rSLM-1 is present only in the dentate gyrus, whereas rSLM-2 is found in the pyramidal cells of the CA1, CA3, and CA4 regions. rSLM-1, but not rSLM-2, is phosphorylated by p59(fyn). p59(fyn)-mediated phosphorylation abolishes the ability of rSLM-1 to regulate splice site selection, but has no effect on rSLM-2 activity. This suggests that rSLM-1-positive cells could respond with a change of their splicing pattern to p59(fyn) activation, whereas rSLM-2-positive cells would not be affected. Together, our data indicate that rSLM-1 is a tissue-specific splicing factor whose activity is regulated by tyrosine phosphorylation signals emanating from p59(fyn).

Signaling from integrins to Fyn to Rho family GTPases regulates morphologic differentiation of oligodendrocytes

Differentiation of oligodendrocyte progenitor cells requires activation of the Src family kinase Fyn. The signals that are upstream and downstream of Fyn in oligodendrocytes remain essentially unknown. Here we show that extracellular matrix engagement regulates the morphology of oligodendrocytes and activates Fyn. Infection of primary oligodendrocyte cultures with recombinant adenovirus revealed that expression of Fyn or its downstream target p190RhoGAP induced process extension. This phenotypic change was not observed when kinase-inactive Fyn or GAP-defective p190 mutants were expressed. Because Rho family proteins are regulated by p190, we monitored the effects of introducing dominant-negative (DN) or constitutively activated (CA) versions of Rho, Rac1, or Cdc42 into primary oligodendrocyte cultures. Expression of DN Rho, CA Rac1, or CA Cdc42 induced outgrowth of oligodendrocyte processes, whereas introduction of CA Rho, DN Rac1, or DN cdc42 inhibited oligodendrocyte differentiation, indicating that Rho and Cdc42-Rac1 exert opposing effects on oligodendrocyte differentiation. Direct measurement of Rho family activity revealed that RhoA was downregulated, and Cdc42 and Rac1 were upregulated during differentiation of primary oligodendrocytes. Moreover, inhibition of integrin engagement or of Fyn activation blocked activation of Rac1 and cdc42 as well as myelin basic protein expression. Taken together, these results suggest a linear signal transduction pathway of integrin-Fyn-Rho family GTPases that controls morphologic differentiation of oligodendrocytes.

Phosphorylation of DCC by Fyn mediates Netrin-1 signaling in growth cone guidance

Netrin-1 acts as a chemoattractant molecule to guide commissural neurons (CN) toward the floor plate by interacting with the receptor deleted in colorectal cancer (DCC). The molecular mechanisms underlying Netrin-1–DCC signaling are still poorly characterized. Here, we show that DCC is phosphorylated in vivo on tyrosine residues in response to Netrin-1 stimulation of CN and that the Src family kinase inhibitors PP2 and SU6656 block both Netrin-1–dependent phosphorylation of DCC and axon outgrowth. PP2 also blocks the reorientation of Xenopus laevis retinal ganglion cells that occurs in response to Netrin-1, which suggests an essential role of the Src kinases in Netrin-1–dependent orientation. Fyn, but not Src, is able to phosphorylate the intracellular domain of DCC in vitro, and we demonstrate that Y1418 is crucial for DCC axon outgrowth function. Both DCC phosphorylation and Netrin-1–induced axon outgrowth are impaired in Fyn^−/− CN and spinal cord explants. We propose that DCC is regulated by tyrosine phosphorylation and that Fyn is essential for the response of axons to Netrin-1.

Apparent Debye−Huckel Electrostatic Effects in the Folding of a Simple, Single Domain Protein

We have monitored the effects of salts and denaturants on the folding of the simple, two-state protein FynSH3. As predicted by Debye-Huckel limiting law, both the stability and (log) folding rate of FynSH3 increase nearly perfectly linearly (r(2)> 0.99) with the square root of ionic strength upon increasing concentrations of the relatively nonchaotropic salt sodium chloride. The stability of FynSH3 is also linear in square root ionic strength when the relatively nonchaotropic salts sodium bromide, potassium bromide, and potassium chloride are employed. Comparison of the kinetic and equilibrium effects of sodium chloride suggests that the electrostatic interactions formed in the folding transition state are approximately 50% as destabilizing as those formed in the native state, presumably reflecting the more compact nature of the latter. In contrast, the relationship between concentration and folding kinetics is more complex when the highly chaotropic salt guanidine hydrochloride (GuHCl) is employed. At moderate to high GuHCl concentrations the net effect of the linear, presumably chaotrope-induced deceleration and the presumed, square root-dependent ionic strength-induced acceleration is well approximated as linear, thus accounting for the observation of "chevron behavior" (log folding rate linear in denaturant concentration) typically reported for the folding of single domain proteins. At very low GuHCl concentrations, however, significant kinetic rollover is observed. This rollover is reasonably well fitted as a sum of a linear, presumably chaotropic effect and a square root-dependent, presumably electrostatic effect. These results thus not only provide insight into the nature of the folding transition state but also suggest that caution is in order when extrapolating GuHCl-based chevrons to estimate folding rates in the absence of denaturant and in interpreting deviations from chevron linearity as evidence for non-two-state kinetics.

T-cell factor-4-dependent up-regulation of fibronectin is involved in fibroblast growth factor-2-induced tube formation by endothelial cells

Deletion of fibronectin or its receptor, alpha(5) integrin, interferes with the formation of a functional circulation in mice. We hypothesized that alpha(5)beta(1) integrin/fibronectin interaction may be involved in differentiation of endothelial cells during angiogenesis. We examined the effect of blocking antibody against alpha(5)beta(1) integrin in fibroblast growth factor-2 (FGF-2)-induced angiogenesis by Matrigel plug assay. Although the antibody did not inhibit the recruitment of endothelial cells into plugs, it inhibited organization of lumen-containing capillaries. The antibody also inhibited FGF-2-induced tube formation by murine brain capillary endothelial cells (IBE cells) cultured in type I collagen gels. We previously showed that FGF-2 failed to induce tube formation by IBE cells expressing kinase-dead c-Fyn (KDFyn cells). Association with beta-catenin enhances the transcriptional activity of T-cell factor-4 (TCF-4), which up-regulates the expression of fibronectin. FGF-2 induced association of beta-catenin with TCF-4 and up-regulation of fibronectin in IBE cells, but not in KDFyn cells. Expression of mutant TCF-4, which does not associate with beta-catenin, inhibited FGF-2-induced tube formation and expression of fibronectin in IBE cells. FGF-2-induced tyrosine phosphorylation of beta-catenin, and association with TCF-4 was increased in IBE cells, but not in KDFyn cells. Taken together, interaction of alpha(5)beta(1) integrin and fibronectin is involved in FGF-2-induced tube formation by endothelial cells and up-regulation of fibronectin through TCF-4 seemed to be involved in this process.

Regulation of Ultraviolet B-induced Phosphorylation of Histone H3 at Serine 10 by Fyn Kinase

Ultraviolet B (UVB) induces phosphorylation of histone H3 at serine 10, and mitogen-activated protein kinases are involved in this signal transduction pathway. Here we provide evidence that Fyn kinase, a member of the Src kinase family, is involved in the UVB-induced phosphorylation of histone H3 at serine 10. UVB distinctly increased Fyn kinase activity and phosphorylation. Fyn kinase inhibitors 4-amino-5-(4-chlorophenyl)-7(t-butyl)pyrazol(3,4-d)pyramide and leflunomide, an Src kinase inhibitor, suppressed both UVB-induced phosphorylation of histone H3 at serine 10 and Fyn kinase activity and phosphorylation. UVB-induced phosphorylation of histone H3 at serine 10 was blocked by either a dominant-negative mutant of Fyn (DNM-Fyn) kinase or small interfering RNA of Fyn kinase. UVB-induced phosphorylation and activities of ERKs and protein kinase B/Akt were markedly inhibited by DNM-Fyn kinase. However, DNM-Fyn kinase did not inhibit UVB-induced phosphorylation of p38 MAPK or c-Jun N-terminal kinases. Active Fyn kinase phosphorylated histone H3 at serine 10 in vitro, and the phosphorylated Fyn kinase could translocate into the nucleus of HaCaT cells. These results indicate that Fyn kinase plays a key role in the UVB-induced phosphorylation of histone H3 at serine 10.

RPTPalpha is essential for NCAM-mediated p59fyn activation and neurite elongation

The neural cell adhesion molecule (NCAM) forms a complex with p59^fyn kinase and activates it via a mechanism that has remained unknown. We show that the NCAM140 isoform directly interacts with the intracellular domain of the receptor-like protein tyrosine phosphatase RPTPα, a known activator of p59^fyn. Whereas this direct interaction is Ca²⁺ independent, formation of the complex is enhanced by Ca²⁺-dependent spectrin cytoskeleton–mediated cross-linking of NCAM and RPTPα in response to NCAM activation and is accompanied by redistribution of the complex to lipid rafts. Association between NCAM and p59^fyn is lost in RPTPα-deficient brains and is disrupted by dominant-negative RPTPα mutants, demonstrating that RPTPα is a link between NCAM and p59^fyn. NCAM-mediated p59^fyn activation is abolished in RPTPα-deficient neurons, and disruption of the NCAM–p59^fyn complex in RPTPα-deficient neurons or with dominant-negative RPTPα mutants blocks NCAM-dependent neurite outgrowth, implicating RPTPα as a major phosphatase involved in NCAM-mediated signaling.