Involvement of Fyn tyrosine kinase in progression of cytokinesis of B lymphocyte progenitor

Masitinib for the treatment of Alzheimer's disease

The actual standard treatment for mild-to-moderately severe Alzheimer's disease only attacks its symptoms. Masitinib is a potent and selective phenylaminothiazole-type tyrosine kinase inhibitor which is currently in Phase III studies for the treatment of Alzheimer's disease (AD) with the aim of modifying its evolution and with multiple pharmacological targets such as inhibition of mast cells activity, inhibition of microglia activation, modulation of Aβ and Tau protein signaling pathway and prevention of synaptic damage. Here, we review the preclinical and clinical studies that investigated the administration of masitinib treatment in monotherapy in AD. All research studies revealed positive effects concerning the cognitive functions in AD and generally with good safety and tolerability.

Src Family Protein Kinase Controls the Fate of B Cells in Autoimmune Diseases

There are more than 80 kinds of autoimmune diseases known at present, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), systemic sclerosis (SSc), inflammatory bowel disease (IBD), as well as other disorders. Autoimmune diseases have a characteristic of immune responses directly attacking own tissues, leading to systematic inflammation and subsequent tissue damage. B cells play a vital role in the development of autoimmune diseases and differentiate into plasma cells or memory B cells to secrete high-affinity antibody or provide long-lasting function. Drugs targeting B cells show good therapeutic effects for the treatment of autoimmune diseases, such as rituximab (anti-CD20 antibody). Src family protein kinases (SFKs) are believed to play important roles in a variety of cellular functions such as growth, proliferation, and differentiation of B cell via B cell antigen receptor (BCR). Lck/Yes-related novel protein tyrosine kinase (LYN), BLK (B lymphocyte kinase), and Fyn are three different kinds of SFKs mainly expressed in B cells. LYN has a dual role in the BCR signal. On the one hand, positive signals are beneficial to the development and maturation of B cells. On the other hand, LYN can also inhibit excessively activated B cells. BLK is involved in the proliferation, differentiation, and immune tolerance of B lymphocytes, and further affects the function of B cells, which may lead to autoreactive or regulatory cellular responses, increasing the risk of autoimmune diseases. Fyn may affect the development of autoimmune disorders via the differentiation of B cells in the early stage of B cell development. This article reviews the recent advances of SFKs in B lymphocytes in autoimmune diseases.

Antibody therapeutics and immunoregulation in cancer and autoimmune disease

Cancer and autoimmune disease are closely related, and many therapeutic antibodies are widely used in clinics for the treatment of both diseases. Among them, the anti-CD20 antibody has proven to be effective against both lymphoid malignancy and autoimmune disease. Moreover, immune checkpoint blockade using the anti-PD1/PD-L1/CTLA4 antibody has improved the prognosis of patients with refractory solid tumors. At the same time, however, over-enhancement of immunoreaction can induce autoimmune reaction. Although anti-TNF antibody therapies represent a breakthrough in the treatment of autoimmune diseases, optimal management is required to control the serious associated issues, including development and progression of cancer, and it is becoming more and more important to control the immunoreaction. In addition, next-generation antibody therapeutics such as antibody-drug conjugates and bispecific antibodies, are anticipated to treat uncontrolled cancer and autoimmune disease. IL-7R signaling plays an important role in the development and progression of both lymphoid malignancy and autoimmune disease. In addition, abnormal homing activity and steroid resistance caused by IL-7R signaling may worsen prognosis. Therefore, anti-IL-7R targeting antibody therapies that enable suppression of such pathophysiological status have the potential to be beneficial for the treatment of both diseases. In this review, we discuss current antibody therapeutics in cancer and autoimmune disease, and describe a new therapeutic strategy for immunoregulation including IL-7R targeting antibodies.

Immunoregulation by IL-7R-targeting antibody-drug conjugates: overcoming steroid-resistance in cancer and autoimmune disease

Steroid-resistance is a common complication in the treatment of malignancies and autoimmune diseases. IL-7/IL-7R signaling, which regulates lymphocyte growth and survival, has been implicated in the development of malignancies and autoimmune diseases. However, the biological significance of IL-7/IL-7R signaling in steroid treatment is poorly understood. Here, we identified a novel relationship between IL-7R signaling and steroid-resistance, and showed that an anti-IL-7R antibody conjugated with SN-38 (A7R-ADC-SN-38) has strong anti-tumor effects against both parental and steroid-resistant malignant cells. Furthermore, inflammation in the mouse autoimmune arthritis model was suppressed to greater extent by A7R-ADC conjugated to MMAE than by A7R-ADC-SN-38. Given that an increased proportion of IL-7R-positive cells is a common mechanism underlying the pathogenesis of autoimmunity, we found that specific depletion of this cell population abrogated the progression of disease. This suggests that the cytotoxicity and immunosuppressive capacity of A7R-ADC could be modulated to treat specific malignancies or autoimmune diseases through the introduction of different payloads, and represents a novel alternative to steroid therapy.

Masitinib for the treatment of mild to moderate Alzheimer's disease

Alzheimer's disease (AD) is a degenerative neurological disorder that is the most common cause of dementia and disability in older patients. Available treatments are symptomatic in nature and are only sufficient to improve the quality of life of AD patients temporarily. A potential strategy, currently under investigation, is to target cell-signaling pathways associated with neurodegeneration, in order to decrease neuroinflammation, excitotoxicity, and to improve cognitive functions. Current review centers on the role of neuroinflammation and the specific contribution of mast cells to AD pathophysiology. The authors look at masitinib therapy and the evidence presented through preclinical and clinical trials. Dual actions of masitinib as an inhibitor of mast cell-glia axis and a Fyn kinase blocker are discussed in the context of AD pathology. Masitinib is in Phase III clinical trials for the treatment of malignant melanoma, mastocytosis, multiple myeloma, gastrointestinal cancer and pancreatic cancer. It is also in Phase II/III clinical trials for the treatment of multiple sclerosis, rheumatoid arthritis and AD. Additional research is warranted to better investigate the potential effects of masitinib in combination with other drugs employed in AD treatment.

SRC-family tyrosine kinases in oogenesis, oocyte maturation and fertilization: an evolutionary perspective

The oocyte is a highly specialized cell poised to respond to fertilization with a unique set of actions needed to recognize and incorporate a single sperm, complete meiosis, reprogram maternal and paternal genomes and assemble them into a unique zygotic genome, and finally initiate the mitotic cell cycle. Oocytes accomplish this diverse series of events through an array of signal transduction pathway components that include a characteristic collection of protein tyrosine kinases. The src-family protein kinases (SFKs) figure importantly in this signaling array and oocytes characteristically express certain SFKs at high levels to provide for the unique actions that the oocyte must perform. The SFKs typically exhibit a distinct pattern of subcellular localization in oocytes and perform critical functions in different subcellular compartments at different steps during oocyte maturation and fertilization. While many aspects of SFK signaling are conserved among oocytes from different species, significant differences exist in the extent to which src-family-mediated pathways are used by oocytes from species that fertilize externally vs those which are fertilized internally. The observation that several oocyte functions which require SFK signaling appear to represent common points of failure during assisted reproductive techniques in humans, highlights the importance of these signaling pathways for human reproductive health.

UNC119a bridges the transmission of Fyn signals to Rab11, leading to the completion of cytokinesis

Src family kinases (SFKs) regulate the completion of cytokinesis through signal transduction pathways that lead to the Rab11-dependent phosphorylation of ERK and its localization to the midbody of cytokinetic cells. We find that UNC119a, a known activator of SFKs, plays essential roles in this signaling pathway. UNC119a localizes to the centrosome in interphase cells and begins to translocate from the spindle pole to the spindle midzone after the onset of mitosis; it then localizes to the intercellular bridge in telophase cells and to the midbody in cytokinetic cells. We show that the midbody localization of UNC119a is dependent on Rab11, and that knocking down UNC119a inhibits the Rab11-dependent phosphorylation and midbody localization of ERK and cytokinesis. Moreover, we demonstrate that UNC119a interacts with a Src family kinase, Fyn and is required for the activation of this kinase. These results suggest that UNC119a plays a key role in the Fyn signal transduction pathway, which regulates the completion of cytokinesis via Rab11.

Src protein kinases in mouse and rat oocytes and embryos

Meiosis of the mammalian oocytes is a specialized cell division, initiated during the female's embryonic life. It arrests at the germinal vesicle (GV) stage and resumes with GV breakdown, followed by segregation of the chromosomes and extrusion of the first polar body in an asymmetric cell division that concludes the first meiotic division, before arresting at metaphase of the second meiotic division (MII). Once fertilized, the oocyte exits from MII, extrudes the second polar body, and the developing zygote will continue dividing to create a blastocyst. Although the two processes of meiosis and mitosis have different developmental functions, it is believed that they share similar mechanisms. Src family kinases (SFKs) are nine non-receptor protein tyrosine kinases that regulate many key cellular functions including meiotic and mitotic cell cycles. In this review we discuss the involvement of SFKs in meiotic and mitotic cell cycle key processes as nuclear envelope breakdown, spindle stabilization, karyokinetic exit from metaphase, regulation of cortical actin, and cytokinetic cleavage furrow ingression.

The conformation and activation of Fyn kinase in the oocyte determine its localisation to the spindle poles and cleavage furrow

Several lines of evidence imply the involvement of Fyn, a Src family kinase, in cell-cycle control and cytoskeleton organisation in somatic cells. By live cell confocal imaging of immunostained or cRNA-microinjected mouse oocytes at metaphase of the second meiotic division, membrane localisation of active and non-active Fyn was demonstrated. However, Fyn with a disrupted membrane-binding domain at its N-terminus was targeted to the cytoplasm and spindle in its non-active form and concentrated at the spindle poles when active. During metaphase exit, the amount of phosphorylated Fyn and of spindle-poles Fyn decreased and it started appearing at the membrane area of the cleavage furrow surrounding the spindle midzone, either asymmetrically during polar body II extrusion or symmetrically during mitosis. These results demonstrate that post-translational modifications of Fyn, probably palmitoylation, determine its localisation and function; localisation of de-palmitoylated active Fyn to the spindle poles is involved in spindle pole integrity during metaphase, whereas the localisation of N-terminus palmitoylated Fyn at the membrane near the cleavage furrow indicates its participation in furrow ingression during cytokinesis.

Fyn kinase is involved in cleavage furrow ingression during meiosis and mitosis

Fertilization of mammalian oocytes triggers their exit from the second meiotic division metaphase arrest. The extrusion of the second polar body (PBII) that marks the completion of meiosis is followed by the first mitotic cleavage of the zygote. Several lines of evidence in somatic cells imply the involvement of Fyn, an Src family kinase (SFK), in cell cycle control and actin functions. In this study, we demonstrate, using live cell confocal imaging and microinjection of Fyn cRNAs, the recruitment of Fyn to the oocyte's cortical area overlying the chromosomes and its colocalization with filamentous actin (F-actin) during exit from the meiotic metaphase. Fyn concentrated asymmetrically at the cortical site designated for ingression of the PBII cleavage furrow, where F-actin had already been accumulated, and then redispersed throughout the entire cortex only to be recruited again to the cleavage furrow during the first mitotic division. Although microinjection of dominant negative Fyn did not affect initiation of the cleavage furrow, it prolonged the average duration of ingression, decreased the rates of PB extrusion and of the first cleavage, and led to the formation of bigger PBs and longer spindles. Extrusion of the PBII was blocked in oocytes exposed to SU6656, an SFK inhibitor. Our results demonstrate, for the first time, a continuous colocalization of Fyn and F-actin during meiosis and imply a role for the SFKs, in general, and for Fyn, in particular, in regulating pathways that involve actin cytoskeleton, during ingression of the meiotic and mitotic cleavage furrows.

Stress-evoked tyrosine phosphorylation of signal regulatory protein α regulates behavioral immobility in the forced swim test

Severe stress induces changes in neuronal function that are implicated in stress-related disorders such as depression. The molecular mechanisms underlying the response of the brain to stress remain primarily unknown, however. Signal regulatory protein alpha (SIRPalpha) is an Ig-superfamily protein that undergoes tyrosine phosphorylation and binds the protein tyrosine phosphatase Shp2. Here we show that mice expressing a form of SIRPalpha that lacks most of the cytoplasmic region manifest prolonged immobility (depression-like behavior) in the forced swim (FS) test. FS stress induced marked tyrosine phosphorylation of SIRPalpha in the brain of wild-type mice through activation of Src family kinases. The SIRPalpha ligand CD47 was important for such SIRPalpha phosphorylation, and CD47-deficient mice also manifested prolonged immobility in the FS test. Moreover, FS stress-induced tyrosine phosphorylation of both the NR2B subunit of the NMDA subtype of glutamate receptor and the K+-channel subunit Kvbeta2 was regulated by SIRPalpha. Thus, tyrosine phosphorylation of SIRPalpha is important for regulation of depression-like behavior in the response of the brain to stress.

Decreased expression of Fyn protein and disbalanced alternative splicing patterns in platelets from patients with schizophrenia

Fyn, a Src-family kinase, is highly expressed in brain tissue and blood cells. In the mouse brain, Fyn participates in brain development, synaptic transmission through the phosphorylation of N-methyl-d-aspartate (NMDA) receptor subunits, and the regulation of emotional behavior. Recently, we found that Fyn is required for the signal transduction in striatal neurons that is initiated by haloperidol, an antipsychotic drug. To determine whether Fyn abnormalities are present in patients with schizophrenia, we analyzed Fyn expression in platelet samples from 110 patients with schizophrenia, 75 of the patients' first-degree relatives, and 130 control subjects. A Western blot analysis revealed significantly lower levels of Fyn protein among the patients with schizophrenia and their relatives, compared with the level in the control group. At the mRNA level, the splicing patterns of fyn were altered in the patients and their relatives; specifically, the ratio of fynDelta7, in which exon 7 is absent, was elevated. An expression study in HEK293T cells revealed that FynDelta7 had a dominant-negative effect on the phosphorylation of Fyn's substrate. These results suggest novel deficits in Fyn function, manifested as the downregulation of Fyn protein or the altered transcription of the fyn gene, in patients with schizophrenia.

Activation of Fyn tyrosine kinase in the mouse dorsal hippocampus is essential for contextual fear conditioning

Fyn-tyrosine-kinase-deficient mice exhibit defects in the Morris water maze test and long-term potentiation (LTP) induction in the hippocampus, and given that LTP has been postulated as the neural basis for memory formation, Fyn may be required for hippocampus-dependent memory formation. However, how Fyn is involved in the process of memory formation is unclear. To investigate the role of Fyn in hippocampal memory formation, we first tested the behavior of Fyn-deficient mice by contextual fear conditioning. A mouse was placed in a context and a foot shock was delivered, so that the mouse associated the context with the shock. We found that the freezing response of Fyn-deficient mice to the context was impaired at 24 h after conditioning. We then measured freezing at 1 h after conditioning, and found that their short-term contextual fear memory was also impaired. We used Western blotting to examine the mode of Fyn activation in dorsal hippocampal tissue following contextual fear conditioning. Fyn activation peaked as early as 5-10 min after contextual fear conditioning and persisted for at least 40 min. Concomitant increases in tyrosine phosphorylation of several proteins, including NR2B, were also observed, but no increases in tyrosine phosphorylation were observed in Fyn-deficient mice. Thus, both short-term and long-term (24-h) contextual fear memory were impaired in Fyn-deficient mice, and Fyn activation in the dorsal hippocampus transiently increased after contextual fear conditioning. These findings strongly suggest that activation of the Fyn signaling pathway is involved in hippocampus-dependent formation of contextual fear memory.

Fyn Is Required for Haloperidol-induced Catalepsy in Mice

Fyn-mediated tyrosine phosphorylation of N-methyl-D-aspartate (NMDA) receptor subunits has been implicated in various brain functions, including ethanol tolerance, learning, and seizure susceptibility. In this study, we explored the role of Fyn in haloperidol-induced catalepsy, an animal model of the extrapyramidal side effects of antipsychotics. Haloperidol induced catalepsy and muscle rigidity in the control mice, but these responses were significantly reduced in Fyn-deficient mice. Expression of the striatal dopamine D(2) receptor, the main site of haloperidol action, did not differ between the two genotypes. Fyn activation and enhanced tyrosine phosphorylation of the NMDA receptor NR2B subunit, as measured by Western blotting, were induced after haloperidol injection of the control mice, but both responses were significantly reduced in Fyn-deficient mice. Dopamine D(2) receptor blockade was shown to increase both NR2B phosphorylation and the NMDA-induced calcium responses in control cultured striatal neurons but not in Fyn-deficient neurons. Based on these findings, we proposed a new molecular mechanism underlying haloperidol-induced catalepsy, in which the dopamine D(2) receptor antagonist induces striatal Fyn activation and the subsequent tyrosine phosphorylation of NR2B alters striatal neuronal activity, thereby inducing the behavioral changes that are manifested as a cataleptic response.

NMDA-receptor proteins are upregulated in the hippocampus of postnatal heterozygous reeler mice

Reelin is a large glycoprotein that is secreted into the extracellular matrix. In the embryonic brain, the binding of Reelin to its receptors ApoER2 and VLDLR induces subcellular events that include the activation Fyn tyrosine kinase, and plays a crucial role in cortical formation. Reelin signaling is also involved in postnatal brain functions such as dendrite development and synaptic plasticity. However, the molecular events involved in Reelin signaling in the postnatal brain remain to be elucidated. Here, we evaluated the proteins downstream of Reelin signaling by comparing the tyrosine-phosphorylated proteins in the postnatal hippocampus of heterozygous and homozygous reeler and wild-type mice, by Western blot analyses. We found that the levels of several phosphoproteins were highest in the hippocampus of the heterozygous reeler mice. The most prominent increase was of two 180-kDa phosphoproteins, which were identified as the NR2A and NR2B subunits of NMDA-R. The amounts of these proteins also increased in the hippocampus of heterozygous reeler mice. However, the mRNA levels of the NMDA-R subunits, determined by quantitative RT-PCR, were the same as in wild-type mice. We also found that the increase in NR2A and NR2B proteins in heterozygous reeler was dependent on Fyn, because this change was absent in heterozygous reeler/homozygous Fyn-deficient double-mutant mice. Thus, the NMDA-R protein level is regulated by the Reelin protein level in a Fyn-dependent manner in the mouse brain.

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK)--endogenous negative regulators of Src-family protein kinases

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK) are endogenous inhibitors of the Src-family protein tyrosine kinases (SFKs). Since constitutive activation of SFKs contributes to cancer formation and progression, to prevent excessive activation of SFKs, their activity in normal cells is kept at the basal level by CSK and CHK. CSK and CHK inactivate SFKs by specifically phosphorylating a consensus tyrosine (called Y(T)) near their C-termini. Upon phosphorylation, the phospho-Y(T) engages in intramolecular interactions that lock the SFK molecule in an inactive conformation. SFKs are anchored to the plasma membrane, while CSK and CHK are localized predominantly in the cytosol. To inhibit SFKs, CSK and CHK need to translocate to the plasma membrane. Recruitment of CSK and CHK to the plasma membrane is mediated by the binding of their SH2, SH3 and/or kinase domains to specific transmembrane proteins, G-proteins and adaptor proteins located near the plasma membrane. For CSK, membrane recruitment often accompanies activation. CSK and CHK employ two types of direct interactions with SFKs to achieve efficient Y(T) phosphorylation: (i) short-range interactions involving binding of the active sites of CSK and CHK to specific residues near Y(T), (ii) long-range non-catalytic interactions involving binding of SFKs to motifs located distally from the active sites of CSK and CHK. The interactions between CSK and SFKs are transient in nature. Unlike CSK, CHK binds tightly to SFKs to form stable protein complexes. The binding is non-catalytic as it is independent of Y(T). More importantly, the tight binding alone is sufficient to completely inhibit SFKs. This non-catalytic inhibitory binding represents a novel mechanism employed by CHK to inhibit SFKs. Given that SFKs are implicated in cancer development, compounds mimicking the non-catalytic inhibitory mechanism of CHK are potential anti-cancer therapeutics.

Tau and src family tyrosine kinases

The interaction between tau and src family non-receptor tyrosine kinases represents a new function for tau. Mediated by the proline-rich region of tau and the SH3 domain of fyn or src, this interaction has the potential to confer novel cellular activities for tau in the growth cone and in the membrane. The subsequent finding that tau is tyrosine phosphorylated has led to the observation that tau in neurofibrillary tangles is tyrosine phosphorylated. Therefore, a role for tyrosine kinases such as fyn in neuropathogenesis is predicted.

Fyn kinase–tubulin interaction during meiosis of rat eggs

Prior to fertilization, the spindle of vertebrate eggs must remain stable and well organized during the second meiotic meta-phase arrest (MII). In a previous study we have determined that the completion of meiosis is a Src family kinase (SFK)-dependent event. In the current study we have used the SFK inhibitors, SU6656 and PP2, and demonstrated that inhibition of SFKs caused the formation of a disorganized spindle. The observation that proper organization of an MII spindle is an SFK-dependent process, combined with our previous finding that Fyn kinase is localized at the microtubules (MTs), prompted us to examine the potential role of Fyn in MT signaling. Our results show an association between Fyn and tubulin, the ability of Fyn to phosphorylate tubulinin vitroand stimulation of meiosis completion by injection of a constitutively active form of Fyn (CAF).

We suggested that SFKs mediate significant functions during the organization of the MII spindle. In view of CAF injection experiments, and of the pronounced concentration of Fyn kinase at the spindle, we propose that Fyn may play an important role in some aspects of the spindle functions, possibly those involving the MTs.

Inhibition of JNK2 disrupts anaphase and produces aneuploidy in mammalian cells

The JNK family members JNK1 and JNK2 regulate tumor growth and are essential for transformation by oncogenes such as constitutively activated Ras. The mechanisms downstream of JNK that regulate cell cycle progression and transformation are unclear. Here we show that inhibition of JNK2, but not JNK1, with either a dominant-negative mutant, a pharmacological inhibitor, or RNA interference caused an accumulation of mammalian cells with 4N DNA content. When observed by immunofluorescence, these cells progressed to metaphase without apparent defects in spindle formation or chromosome alignment to the metaphase plate, suggesting that the 4N accumulation is a result of postmetaphase defects. Consistent with this prediction, when JNK activity was suppressed, we observed defects in central spindle formation and chromosome segregation during anaphase. In contrast, cyclin-dependent kinase 1 activity, cyclin B1 protein, and Polo-like kinase 1 protein turnover remained intact when JNK was inhibited. In addition, continued inhibition of JNK activity did not block reentry into subsequent cell cycles but instead resulted in polyploidy. This evidence suggests that JNK2 functions in maintaining the genomic stability of mammalian cells by signaling that is independent of cyclin-dependent kinase 1/cyclin B1 down-regulation.

Phosphorylation of tau by fyn: implications for Alzheimer's disease

The abnormal phosphorylation of tau protein on serines and threonines is a hallmark characteristic of the neurofibrillary tangles of Alzheimer's disease (AD). The discovery that tau could be phosphorylated on tyrosine and evidence that Abeta signal transduction involved tyrosine phosphorylation led us to question whether tyrosine phosphorylation of tau occurred during the neurodegenerative process. In this study we determined that human tau tyr18 was phosphorylated by the src family tyrosine kinase fyn. By developing both polyclonal and monoclonal probes specific for phospho-tyr18, we found that the phosphorylation of tau at tyr18 occurred at early developmental stages in mouse but was absent in the adult. Our phosphospecific probes also revealed that paired helical filament preparations exhibited phospho-tyr18 reactivity that was sensitive to phosphotyrosine-specific protein phosphatase treatment. Moreover, immunocytochemical studies indicated that tyrosine phosphorylated tau was present in the neurofibrillary tangles in AD brain. However, the staining pattern excluded neuropil threads and dystrophic neurites indicating that tyrosine phosphorylated tau was distributed in AD brain in a manner dissimilar from other abnormally phosphorylated tau. We also found evidence suggesting that differentially phosphorylated tau existed within degenerating neurons. Our data add new support for a role for fyn in the neurodegenerative process.

Defective neocortical development in Fyn-tyrosine-kinase-deficient mice

Fyn tyrosine kinase is involved in the tyrosine-phosphorylation of Disabled-1 in the Reelin signaling pathway, and absence of Fyn is expected to result in a reeler-like phenotype. Thus, this study investigated neocortical development in Fyn-deficient mice. Bromodeoxyuridine labeling revealed the under-migration of later-generated neurons despite the normal placement of earlier-generated neurons. Calbindin- and alpha-calcium/calmodulin-dependent protein kinase II-immunohistochemistry showed that layer II-III neurons were aberrantly stratified, but the neurons in the deeper layers showed little evidence of abnormality. Fyn was intensely expressed in the leading process of migratory cortical neurons generated in the later stage. These findings strongly suggest that Fyn is required for the migration of later-generated neurons, but that it is dispensable for the Reelin-dependent inside-out layer formation.

Characterization of the in vivo sites of serine phosphorylation on Lck identifying serine 59 as a site of mitotic phosphorylation

The lymphocyte-specific protein-tyrosine kinase Lck plays a critical role in T cell activation. In response to T cell antigen receptor binding Lck undergoes phosphorylation on serine residues that include serines 59 and 194. Serine 59 is phosphorylated by ERK mitogen-activated protein kinase. Recently, we showed that in mitotic T cells Lck becomes hyper-phosphorylated on serine residues. In this report, using one-dimensional phosphopeptide mapping analysis, we identify serine 59 as a site of in vivo mitotic phosphorylation in Lck. The mitotic phosphorylation of serine 59 did not require either the catalytic activity or functional SH2 or SH3 domains of Lck. In addition, the presence of ZAP-70 also was dispensable for the phosphorylation of serine 59. Although previous studies demonstrated that serine 59 is a substrate for the ERK MAPK pathway, inhibitors of this pathway did not block the mitotic phosphorylation of serine 59. These results identify serine 59 as a site of mitotic phosphorylation in Lck and suggest that a pathway distinct from that induced by antigen receptor signaling is responsible for its phosphorylation. Thus, the phosphorylation of serine 59 is the result of two distinct signaling pathways, differentially activated in response to the physiological state of the T cell.

Fyn tyrosine kinase in Sertoli cells is involved in mouse spermatogenesis

Fyn is a member of the Src family of non-receptor-type tyrosine kinases and plays an important role in signal transductions regulating cell proliferation and differentiation. Fyn immunoreactivity was localized in the Sertoli cells of mouse testes. Although fyn-deficient adult male mice were fertile, a significant reduction in testis weight and degenerated germ cells were observed at 3 and 4 wk of age. Electron microscopic examination revealed that fyn -/- testis has ultrastructural abnormalities in the specialized junctional structures of the Sertoli cells, the ectoplasmic specializations. Unusual vesicular structures were found in the actin filament layers of the ectoplasmic specializations of mutant mice. Immunohistochemical studies demonstrated that both Fyn and actin filaments were concentrated in the areas of ectoplasmic specializations. At these sites, a high level of phosphotyrosine was also immunostained in wild-type testes, whereas phosphotyrosine immunoreactivity was reduced in fyn -/- testes. Immunoblot analyses revealed that Fyn was mainly distributed within the Triton X-100-insoluble cytoskeletal fraction prepared from wild-type testes, suggesting that Fyn might be associated with cytoskeletal proteins such as actin filaments. These findings suggest that Fyn kinase functions at the ectoplasmic specializations of the Sertoli cells in the testes, regulating the dynamics of cytoskeletal proteins. Fyn-mediated signal transduction in the Sertoli cells may affect the survival and differentiation of germ cells at a specific stage during spermatogenesis.

Intracellular adapter molecules

Lymphocyte antigen receptor engagement leads to the initiation of numerous signal transduction pathways that direct ultimate cellular responses. In recent years, it has become apparent that adapter molecules regulate the coupling of receptor-proximal events, such as protein tyrosine kinase activation, with end results such as inducible gene expression and cytoskeletal rearrangements. While adapter molecules possess no intrinsic enzymatic activity, their ability to mediate protein-protein interactions is vital for the integration and propagation of signal transduction cascades in lymphocytes. Recent studies demonstrate that intracellular adapter molecules function as both positive and negative regulators of lymphocyte activation.

Tau interacts with src-family non-receptor tyrosine kinases

Tau and other microtubule-associated proteins promote the assembly and stabilization of neuronal microtubules. While each microtubule-associated protein has distinct properties, their in vivo roles remain largely unknown. Tau is important in neurite outgrowth and axonal development. Recently, we showed that the amino-terminal region of tau, which is not involved in microtubule interactions, is important in NGF induced neurite outgrowth in PC12 cells. Here we report that a proline rich sequence in the amino terminus of tau interacts with the SH3 domains of fyn and src non-receptor tyrosine kinases. Tau and fyn were co-immunoprecipitated from human neuroblastoma cells and co-localization of tau and fyn was visualized in co-transfected NIH3T3 cells. Co-transfection of tau and fyn also resulted in an alteration in NIH3T3 cell morphology, consistent with an in vivo interaction. Fyn-dependent tyrosine phosphorylation of tau occurred in transfected cells and tyrosine phosphorylated tau was identified in human neuroblastoma cells as well. Our data suggest that tau is involved in signal transduction pathways. An interaction between tau and fyn may serve as a mechanism by which extracellular signals influence the spatial distribution of microtubules. The tyrosine phosphorylation of tau by fyn may also have a role in neuropathogenesis, as fyn is upregulated in Alzheimer's disease.

Interaction of p59fyn Kinase with the Dynein Light Chain, Tctex-1, and Colocalization During Cytokinesis

The protein tyrosine kinase p59fyn (Fyn) plays important roles in both lymphocyte Ag receptor signaling and cytokinesis of proB cells. We utilized yeast two-hybrid cloning to identify the product of the tctex-1 gene as a protein that specifically interacts with Fyn, but not with other Src family kinases. Tctex-1 was recently identified as a component of the dynein cytoskeletal motor complex. The capacity of a Tctex-1-glutathione S-transferase fusion protein to effectively bind Fyn from cell lysates confirmed the authenticity of this interaction. Tctex-1 binding required the first 19 amino acids of Fyn and integrity of two lysine residues within this sequence that were previously shown to be important for Fyn interactions with the immunoreceptor tyrosine-based activation motifs (ITAMs) of lymphocyte Ag receptors. Expression of tctex-1 mRNA and protein was observed in all lymphoma lines analyzed, and immunofluorescence confocal microscopy localized the protein to the perinuclear region. Analysis of a T cell hybridoma revealed prominent colocalization of Tctex-1 and Fyn at the cleavage furrow and mitotic spindles in cells undergoing cytokinesis. Our results provide a unique insight into a mechanism by which Tctex-1 might mediate specific recruitment of Fyn to the dynein complex in lymphocytes, which may be a critical event in mediating the previously defined role of Fyn in cytokinesis.

Diversity revealed by a novel family of cadherins expressed in neurons at a synaptic complex

In mammals, neurons are highly differentiated and play distinctive functions even in the same brain region. We found a novel cadherin-related neuronal receptor (Cnr) gene family by studying Fyn-binding activity in mouse brain. CNR1 protein is located in the synaptic junction and forms a complex with Fyn. Sequence analysis of eight Cnr products of approximately 20 genes indicates that these comprise a novel cadherin family of the cadherin superfamily. The expression patterns of each member of this novel family were grossly similar to each other but restricted to subpopulations of neurons of the same type. The diversity of the Cnr family genes suggests that there are molecular mechanisms that govern highly differentiated neural networks in the mammalian CNS.

Microtubule-entrained kinase activities associated with the cortical cytoskeleton during cytokinesis

Research over the past few years has demonstrated the central role of protein phosphorylation in regulating mitosis and the cell cycle. However, little is known about how the mechanisms regulating the entry into mitosis contribute to the positional and temporal regulation of the actomyosin-based contractile ring formed during cytokinesis. Recent studies implicate p34cdc2 as a negative regulator of myosin II activity, suggesting a link between the mitotic cycle and cytokinesis. In an effort to study the relationship between protein phosphorylation and cytokinesis, we examined the in vivo and in vitro phosphorylation of actin-associated cortical cytoskeletal (CSK) proteins in an isolated model of the sea urchin egg cortex. Examination of cortices derived from eggs or zygotes labeled with 32P-orthophosphate reveals a number of cortex-associated phosphorylated proteins, including polypeptides of 20, 43 and 66 kDa. These three major phosphoproteins are also detected when isolated cortices are incubated with [32P]ATP in vitro, suggesting that the kinases that phosphorylate these substrates are also specifically associated with the cortex. The kinase activities in vivo and in vitro are stimulated by fertilization and display cell cycle-dependent activities. Gel autophosphorylation assays, kinase assays and immunoblot analysis reveal the presence of p34cdc2 as well as members of the mitogen-activated protein kinase family, whose activities in the CSK peak at cell division. Nocodazole, which inhibits microtubule formation and thus blocks cytokinesis, significantly delays the time of peak cortical protein phosphorylation as well as the peak in whole-cell histone H1 kinase activity. These results suggest that a key element regulating cortical contraction during cytokinesis is the timing of protein kinase activities associated with the cortical cytoskeleton that is in turn regulated by the mitotic apparatus.

Oncoprotein signalling and mitosis

Studies of the roles of oncoproteins in cell cycle progression have concentrated on G1 because transformation is frequently associated with loss of G1 checkpoint control. However, it has become evident that G2 and mitotic checkpoints are often compromised in transformed cells and that many tumour suppressor proteins and oncoprotein kinases regulate and/or are activated in G2 and M. Disruption of p53 and ATM tumour suppressor protein functions can eliminate G2 and M checkpoints. The Src family kinases are activated in mitosis and collectively play an indispensable role in progression through G2/M. In addition, evidence suggests that Mos and elements of the Ras/Raf/MAPK cascade are also active in mitosis and appear likely to regulate G2 and/or M. Potential targets of these kinases include likely regulators of gene expression and microtubule dynamics such as Sam68 and Oncoprotein 18/stathmin. The ability of some oncoproteins to perturb orderly progression through both G1 and/or S and G2 and/or M is probably important for transformation.