Rek, a gene expressed in retina and brain, encodes a receptor tyrosine kinase of the Axl/Tyro3 family

Therapeutic targeting of the functionally elusive TAM receptor family

The TAM receptor family of TYRO3, AXL and MERTK regulates tissue and immune homeostasis. Aberrant TAM receptor signalling has been linked to a range of diseases, including cancer, fibrosis and viral infections. Specifically, the dysregulation of TAM receptors can enhance tumour growth and metastasis due to their involvement in multiple oncogenic pathways. For example, TAM receptors have been implicated in the epithelial-mesenchymal transition, maintaining the stem cell phenotype, immune modulation, proliferation, angiogenesis and resistance to conventional and targeted therapies. Therapeutically, multiple TAM receptor inhibitors are in preclinical and clinical development for cancers and other indications, with those targeting AXL being the most clinically advanced. Although there has been notable clinical advancement in recent years, challenges persist. This Review aims to provide both biological and clinical insights into the current therapeutic landscape of TAM receptor inhibitors, and evaluates their potential for the treatment of cancer and non-malignant diseases.

Chicken-derived MERTK protein inhibits Newcastle disease virus replication by increasing STAT1 phosphorylation in DF-1 cells

The receptor tyrosine kinases TYRO3, AXL, and MERTK (TAM) are transmembrane proteins associated with the regulation of the innate immune response. In this study, the role of the chicken-derived MERTK protein (chMertk) in the regulation of the type I interferon (IFN) signaling pathway and its antiviral effect were investigated in vitro. Newcastle disease (ND) caused by the Newcastle disease virus (NDV) is able to widely spread in chickens and give rise to massive losses in the poultry industry around the world. We found that the overexpression of the exogenous chMertk upregulated the STAT1 phosphorylation and the expression of IFN-stimulated gene IFITM3 and significantly reduced the NDV titer (p < 0.05). A mutation assay showed that three tyrosine residues (Y739, Y743, and Y744) in chMertk promoted STAT1 phosphorylation and inhibited NDV replication. However, the chicken-derived E3 ubiquitin ligase CBL significantly negatively regulated chMertk expression, thus attenuating STAT1 phosphorylation. chMertk function was restored by the ubiquitin-proteasome inhibitor MG132, demonstrating that chMertk was controlled by Casitas B-lineage proto-oncogene (CBL) ubiquitination and degradation. Together, these results suggested that chMertk participated in regulating the immune responses to NDV infection, and that CBL significantly downregulated the expression of chMertk through its ubiquitination and degradation, to maintain cellular homeostasis. Overall, our study provided new insights into the role of chMertk in regulating the innate immune response and its anti-NDV activity.

Exploring the Expression and Function of cTyro3, a Candidate Zika Virus Receptor, in the Embryonic Chicken Brain and Inner Ear

The transmembrane protein Axl was proposed as an entry receptor for Zika virus (ZIKV) infection in vitro, but conflicting results from in vivo studies have made it difficult to establish Axl as a physiologically relevant ZIKV receptor. Both the functional redundancy of receptors and the experimental model used can lead to variable results. Therefore, it can be informative to explore alternative animal models to analyze ZIKV receptor candidates as an aid in discovering antivirals. This study used chicken embryos to examine the role of chicken Tyro3 (cTyro3), the equivalent of human Axl. Results show that endogenous cTyro3 mRNA expression overlaps with previously described hot spots of ZIKV infectivity in the brain and inner ear. We asked if ectopic expression or knockdown of cTyro3 influenced ZIKV infection in embryos. Tol2 vectors or replication-competent avian retroviruses were used in ovo to introduce full-length or truncated (presumed dominant-negative) cTyro3, respectively, into the neural tube on embryonic day two (E2). ZIKV was delivered to the brain 24 h later. cTyro3 manipulations did not alter ZIKV infection or cell death in the E5/E6 brain. Moreover, delivery of truncated cTyro3 variants to the E3 otocyst had no effect on inner ear formation on E6 or E10.

Suspected Neratinib Macular Toxicity Presenting As Macular Telangiectasia Type II

The purpose of this case report is to present the first case of neratinib maculopathy. We describe the initial presentation, baseline characteristics, imaging findings, and outcomes. The case report is accompanied by a thorough literature review including possible mechanisms of tyrosine kinase inhibitor (TKI) maculopathy. Neratinib is a novel TKI that is commonly used in the treatment of breast-associated malignancies. Neratinib toxicity presents similarly to macular telangiectasia type II but differs with the fine granular hypofluorescent areas spanning the limit of the posterior pole and vascular arcades as well as the nasal aspect of the optic nerve.  We report a case of suspected macular toxicity secondary to neratinib. Concomitant use of neratinib in conjunction with docetaxel and other chemotherapeutics with known retinal side effects should alert clinicians of an increase in the risk of macular toxicity. Albeit commonly reported ocular side effects of TKIs, maculopathy is a rare and potentially overlooked side effect. Patients that have planned chemotherapy should undergo a baseline retinal examination.

Recent advancements in role of TAM receptors on efferocytosis, viral infection, autoimmunity, and tissue repair

Evolutionarily conserved highly regulated process of apoptosis has been a major physiological process throughout the entire evolutionary history of living beings that has impacted the process of evolution itself. One of the key features of this highly researched field of science is the process of phosphatidylserine (PS) externalization by the different membrane bound enzymes. The process is a result of series of biological events and is associated with various biological outcomes depending on the proper recognition of this ligand. In this review, we will briefly summarize the recent advancement in the field pertaining to the set of receptors, known as TAM (Tyro3, Axl and Mertk) receptors, for their influence in the recognition of various PS externalization events and mediation of pathological outcomes such as autoimmunity, cancer, and tissue repair.

Macrophage-tumor crosstalk: role of TAMR tyrosine kinase receptors and of their ligands

Ample clinical and preclinical evidence indicates that macrophages interact with tumor cells as well as with virtually all populations of host cells present in the tumor microenvironment. This crosstalk can strongly promote malignancy, but also has in principle the potential to inhibit tumor growth. Thus, it is of the utmost importance to improve our understanding of the mechanisms driving the pro- and antimalignant behavior of tumor-associated macrophages (TAMs) in order to develop better anticancer therapies. In this review, we discuss the biological consequences of reciprocal interactions between TAMs, cancer cells, endothelial cells, fibroblasts and other leukocyte subfractions within tumors. It was recently elucidated that tumors specifically educate macrophages to secrete growth arrest-specific gene 6 (Gas6), the common ligand of the Tyro3, Axl, Mer receptor (TAMR) family. In turn, Gas6 fosters tumor growth by promoting cancer cell proliferation. Therefore, the Gas6-TAMR axis might represent a novel target for disrupting tumor-macrophage crosstalk. We summarize here what is known about TAMR and their ligands in (human) cancer biology. In order to shed more light on the role of macrophages in human cancer, we additionally provide an overview of what is currently known about the prognostic impact of TAMs in human cancer.

TAM receptor tyrosine kinases: biologic functions, signaling, and potential therapeutic targeting in human cancer

Tyro-3, Axl, and Mer constitute the TAM family of receptor tyrosine kinases (RTKs) characterized by a conserved sequence within the kinase domain and adhesion molecule-like extracellular domains. This small family of RTKs regulates an intriguing mix of processes, including cell proliferation/survival, cell adhesion and migration, blood clot stabilization, and regulation of inflammatory cytokine release. Genetic or experimental alteration of TAM receptor function can contribute to a number of disease states, including coagulopathy, autoimmune disease, retinitis pigmentosa, and cancer. In this chapter, we first provide a comprehensive review of the structure, regulation, biologic functions, and downstream signaling pathways of these receptors. In addition, we discuss recent evidence which suggests a role for TAM receptors in oncogenic mechanisms as family members are overexpressed in a spectrum of human cancers and have prognostic significance in some. Possible strategies for targeted inhibition of the TAM family in the treatment of human cancer are described. Further research will be necessary to evaluate the full clinical implications of TAM family expression and activation in cancer.

Soluble Axl is generated by ADAM10-dependent cleavage and associates with Gas6 in mouse serum

Axl receptor tyrosine kinase exists as a transmembrane protein and as a soluble molecule. We show that constitutive and phorbol 12-myristate 13-acetate-induced generation of soluble Axl (sAxl) involves the activity of disintegrin-like metalloproteinase 10 (ADAM10). Spontaneous and inducible Axl cleavage was inhibited by the broad-spectrum metalloproteinase inhibitor GM6001 and by hydroxamate GW280264X, which is capable of blocking ADAM10 and ADAM17. Furthermore, murine fibroblasts deficient in ADAM10 expression exhibited a significant reduction in constitutive and inducible Axl shedding, whereas reconstitution of ADAM10 restored sAxl production, suggesting that ADAM10-mediated proteolysis constitutes a major mechanism for sAxl generation in mice. Partially overlapping 14-amino-acid stretch deletions in the membrane-proximal region of Axl dramatically affected sAxl generation, indicating that these regions are involved in regulating the access of the protease to the cleavage site. Importantly, relatively high circulating levels of sAxl are present in mouse sera in a heterocomplex with Axl ligand Gas6. Conversely, two other family members, Tyro3 and Mer, were not detected in mouse sera and conditioned medium. sAxl is constitutively released by murine primary cells such as dendritic and transformed cell lines. Upon immobilization, sAxl promoted cell migration and induced the phosphorylation of Axl and phosphatidylinositol 3-kinase. Thus, ADAM10-mediated generation of sAxl might play an important role in diverse biological processes.

Identification of genes regulating sensory neuron genesis and differentiation in the avian dorsal root ganglia

The dorsal root ganglia (DRG) derive from a population of migrating neural crest cells that coalesce laterally to the neural tube. As the DRG matures, discrete cell types emerge from a pool of differentiating progenitor cells. To identify genes that regulate sensory genesis and differentiation, we have designed screens to identify members from families of known regulatory molecules such as receptor tyrosine kinases, and generated full-length and subtractive cDNA libraries between immature and mature DRG for identifying novel genes not previously implicated in DRG development. Several genes were identified in these analyses that belong to important regulatory gene families. Quantitative PCR confirmed differential expression of candidate cDNAs identified from the subtraction/differential screening. In situ hybridization further validated dynamic expression of several cDNAs identified in our screens. Our results demonstrate the utility of combining specific and general screening approaches for isolating key regulatory genes involved in the genesis and differentiation of discrete cell types and tissues within the classic embryonic chick model system.

The interface between ErbB and non-ErbB receptors in tumor invasion: clinical implications and opportunities for target discovery

The molecular switches by which malignant cancer cells evolve from a confined to an invasive state are poorly understood, but seem to involve a progressive activation of a signaling network shared by several growth factor receptors and non-receptor molecules. Abnormal expression of ErbB tyrosine kinase receptors, commonly seen in cancer, is an early event in the invasive process, which makes these receptors exciting targets for drug discovery. The past few years have been full of promise for ErbB targeting in the context of receptor overexpression, but also fraught with disappointment as clinical efficacy has often been hampered by potential problems such as the heterogeneity of receptor expression within the same tumor, and the extensive cooperative signaling among ErbB and non-ErbB receptors. Cooperative signaling is a common characteristic of invasive cancer cells, and is believed to dictate the genetic program that controls invasion switches. Molecular studies on the combinatorial signaling involved in tumor invasion are becoming a fertile area for target discovery in cancer. This review discusses how cooperative signaling between ErbB and non-ErbB receptors regulates tumor invasion and hence provides multiple opportunities for drug discovery, and how current therapies and investigational drugs could pave the way to even more potent alternative combinatorial therapeutic approaches for invasive cancers.

Mer receptor tyrosine kinase signaling: prevention of apoptosis and alteration of cytoskeletal architecture without stimulation or proliferation

Mer is a member of the Axl/Mer/Tyro3 receptor tyrosine kinase family, a family whose physiological function is not well defined. We constructed a Mer chimera using the epidermal growth factor receptor (EGFR) extracellular and transmembrane domains and the Mer cytoplasmic domain. Stable transfection of the Mer chimera into interleukin 3 (IL-3)-dependent murine 32D cells resulted in ligand-activable surface receptor that tyrosine autophosphorylated, stimulated intracellular signaling, and dramatically reduced apoptosis initiated by IL-3 withdrawal. However, unlike multiple other ectopically expressed receptor tyrosine kinases including full-length EGFR or an EGFR/Axl chimera, the Mer chimera did not stimulate proliferation. Moreover, and in contrast to EGFR, Mer chimera activation induced adherence and cell flattening in the normally suspension-growing 32D cells. The Mer chimera signal also blocked IL-3-dependent proliferation leading to G(1)/S arrest, dephosphorylation of retinoblastoma protein, and elongation of cellular processes. Unlike other agonists that lead to a slow (4-8 days) ligand-dependent differentiation of 32D cells, the combined Mer and IL-3 signal resulted in differentiated morphology and growth cessation in the first 24 h. Thus the Mer chimera blocks apoptosis without stimulating growth and produces cytoskeletal alterations; this outcome is clearly separable from the proliferative signal produced by most receptor tyrosine kinases.

Molecular cloning, expression and partial characterization of Xksy, Xenopus member of the Sky family of receptor tyrosine kinases

We isolated a cDNA encoding the Xenopus member of Sky/Axl/Mer receptor tyrosine kinase family (referred as Sky family), termed Xksy. The predicted Xksy protein has conserved structural characteristics of the Sky family: an unique extracellular domain of two immunoglobulin (Ig)-like repeats, two fibronectin type III (FNIII)-like repeats and an intracellular tyrosine kinase. Homology analysis of Xksy showed the highest identity to mammalian Sky protein. In contrast to the predominant expression of sky mRNA in the adult mammalian nervous system, Northern blot analysis showed ubiquitous expression of a single 5.2-kb Xksy mRNA in tissues of the adult Xenopus. RNase protection assays revealed that, during development, Xksy mRNA is expressed from mid neurulation stage. Levels increase through the tadpole stage and become restricted to the head region in embryos by stage 40. Whole-mount in situ hybridization analyses revealed that expression of Xksy is localized to the nervous system of the tadpole stage, including origins of sensory organs and branchial arches. When a chimeric receptor (EGFR-Xksy), composed of the extracellular region of epidermal growth factor (EGF) receptor and the transmembrane/intracellular regions of Xksy, was expressed in a doxycycline repressive manner in HEK 293 cells, EGF-stimulus without doxycycline induced tyrosine phosphorylation of the chimeric receptor and evoke morphological changes. EGF treatment also induced growth modifications of EGFR-Xksy cells. And doxycycline pre-treatment eliminated these activities. These findings suggest that Xksy may play an important role in growth, differentiation and the accurate migration of cells during embryogenesis and early neural development.

Myocyte enhancer factors-2B and -2C are required for adhesion related kinase repression of neuronal gonadotropin releasing hormone gene expression

Synthesis of the hypothalamic peptide, gonadotropin releasing hormone (GnRH), is paramount for reproductive function. GnRH neurons originate in the olfactory region and migrate into the forebrain during development. We recently implicated adhesion related kinase (Ark) in GnRH neuron development based on its differential expression in two GnRH producing cell lines, GT1-7 and Gn10. The Ark membrane receptor encodes an extracellular domain resembling cell adhesion molecules and an intracellular tyrosine kinase. Ark is expressed in Gn10 cells derived from migrating GnRH neurons but not GT1-7 cells of the post-migratory phenotype. Here, we show that Ark and GnRH transcripts are colocalized in the cribriform plate at midgestation, suggesting that Ark is expressed in migrating GnRH neurons in vivo. Furthermore, we have identified the GnRH gene as a downstream target of Ark signaling. Ark inhibits GnRH gene expression in GnRH neuronal cells via the coordinated binding of myocyte enhancer factor-2B and -2C (MEF-2B and -2C) and a putative homeoprotein within the proximal rat GnRH promoter. Given that MEF-2 proteins are widely expressed in the brain, these studies provide further evidence for MEF-2 action during neuronal development. Moreover, our studies elucidate a potential role for Ark in regulating GnRH gene expression during GnRH neuronal migration.

Cellular signaling by neural cell adhesion molecules of the immunoglobulin superfamily

Neural cell adhesion molecules (CAMs) of the immunoglobulin superfamily nucleate and maintain groups of cells at key sites during early development and in the adult. In addition to their adhesive properties, binding of CAMs can affect intracellular signaling. Their ability to influence developmental events, including cell migration, proliferation, and differentiation can therefore result both from their adhesive as well as their signaling properties. This review focuses on the two CAMs for which the most information is known, the neural CAM, N-CAM, and L1. N-CAM was the first CAM to be characterized and, therefore, has been studied extensively. The binding of N-CAM to cells leads to a number of signaling events, some of which result in changes in gene expression. Interest in L1 derives from the fact that mutations in its gene lead to human genetic diseases including mental retardation. Much is known about modifications of the L1 cytoplasmic domain and its interaction with cytoskeletal molecules. The study of CAM signaling mechanisms has been assay-dependent rather than molecule-dependent, with particular emphasis on assays of neurite outgrowth and gene expression, an emphasis that is maintained throughout the review. The signals generated following CAM binding that lead to alterations in cell morphology and gene expression have been linked directly in only a few cases. We also review information on other CAMs, giving special consideration to those that are anchored in the membrane by a phospholipid anchor. These proteins, including a form of N-CAM, are presumed to be localized in lipid rafts, membrane substructures that include distinctive subsets of cytoplasmic signaling molecules such as members of the src-family of nonreceptor protein tyrosine kinases. In the end, these studies may reveal that what CAMs do after they bind cells together may have as profound consequences for the cells as the adhesive interactions themselves. This area will therefore remain a rich ground for future studies.

ETK2 receptor tyrosine kinase promotes survival of factor-dependent FDC-P1 progenitor cells

By virtue of its high expression in both developing hematopoietic tissues and many myeloid leukemia cells lines, the embryonic tyrosine kinase receptor ETK2 (also known as Tyro3, Sky, and Rse) has been postulated to play a role in early hematopoiesis. To investigate this role, we expressed murine ETK2 in the interleukin 3 (IL-3) dependent myeloid progenitor cell line FDC-P1 and examined its effect on growth factor dependence.ETK2 cDNAs encoding full-length or kinase domain-deleted receptor were retrovirally transduced into murine FDC-P1 cells. Survival, cell cycle status, and proliferative responses of ETK2 expressing clones were studied at normal and reduced growth factor concentrations. ETK2 was expressed as a functional tyrosine kinase of 110 and 150 kDa. This proto-oncogene altered the growth of FDC-P1 cells, allowing survival at reduced growth factor concentrations and delaying apoptosis after IL-3 withdrawal. ETK2-expressing clones contained a higher fraction of cells in the S/G2/M phases of the cell cycle, both after cytokine withdrawal and in the presence of IL-3. Furthermore, these cells had a modestly enhanced proliferative response to IL-3 and granulocyte-macrophage colony-stimulating factor, suggesting that ETK2 intracellular signaling may converge with that of hematopoietic growth factors. The effects of ETK2 expression on viability and proliferation were largely dependent on a functional intracellular tyrosine kinase domain. These results support a role for ETK2 in the survival and/or expansion of primitive hematopoietic cells and suggest that this tyrosine kinase may be implicated in myeloid leukemogenesis as well.

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.

The first laminin G-type domain in the SHBG-like region of protein S contains residues essential for activation of the receptor tyrosine kinase sky

Vitamin K-dependent protein S and the product of growth-arrest-specific gene 6 (Gas6) both possess the ability to phosphorylate members of the Axl/Sky subfamily of receptor tyrosine kinases. However, Gas6 appears to be the bona fide ligand for these receptors in man, as human protein S has been demonstrated to activate murine Sky but not the human orthologue. In contrast, bovine protein S is able to stimulate human Sky despite its high degree of sequence identity with human protein S. The domain organisations of protein S and Gas6 are virtually identical and the C-terminal SHBG-like region, containing two globular (G) domains, has been shown to play a crucial role in the receptor stimulation. In order to further localise the area responsible for the interaction, a number of protein chimeras were used to stimulate human Sky. Each chimera had one part of the human protein S SHBG-like region replaced by the corresponding part of bovine protein S or human Gas6. We found that human protein S may indeed activate human Sky but only above physiological plasma concentrations. The human-bovine protein S chimeras provided new information implying that the first G domain contains critical residues for the interaction with the Sky receptor. Moreover, these residues do not seem to be clustered but rather to be distributed at various positions in the first G domain.

Transforming activity of receptor tyrosine kinase Tyro3 is mediated, at least in part, by the PI3 kinase-signaling pathway

Protein tyrosine phosphorylation is an integral part of cytokine-induced proliferation and differentiation of hematopoietic cells. The authors previously reported cloning and characterization of the receptor tyrosine kinase Tif, also termed Tyro3. Using the yeast 2-hybrid technology, they recently identified that the p85 subunit of phosphatidylinositol 3-kinase (PI3 kinase) interacted with the cytoplasmic domain of Tyro3. On treatment with epidermal growth factor (EGF), NIH3T3 cells expressed EGFR/Tyro3 (a fusion receptor with the extracellular domain from epidermal growth factor receptor and the transmembrane and cytoplasmic domains from Tyro3), and EGFR/Tyro3 was rapidly phosphorylated on tyrosine residues. The interaction between Tyro3 and p85 was also confirmed by glutathione S-transferase (GST) pull-down experiments. Co-immunoprecipitation followed by Western blot analysis revealed that PI3 kinase was associated with and phosphorylated by the activated Tyro3. Tyro3-associated PI3 kinase exhibited an enhanced kinase activity. In addition, EGF treatment of EGFR/Tyro3-expressing cells led to enhanced phosphorylation of Akt, a downstream component of PI3 kinase. Treatment of NIH3T3 cells expressing a full length of rat Tyro-3, but not NIH3T3 cells, with protein S also resulted in phosphorylation of Akt. Soft agar colony assays showed that the addition of EGF to EGFR/Tyro3-transfected cells, but not to the parental NIH3T3 cells, resulted in a concentration-dependent increase in the formation of anchorage-independent colonies. Tyro3-mediated transformation of NIH3T3 cells was significantly blocked by wortmannin, a PI3 kinase-specific inhibitor. Results of these combined studies strongly suggested that the oncogenic transforming ability of Tyro3 was mediated at least in part by the PI3 kinase pathway.

The effect of nitric oxide on the contractile tone of Müller cells

The effect of nitric oxide (NO) on the contractile tone of Müller cells was investigated. Müller cells, isolated from the retina of chick embryos, were cultured on thin sheets of silicone. S-Nitro-N-acetyl-DL-penicillamine (SNAP), an NO donor, was applied in the presence or the absence of carboxyl phenyltetramethylimidazole oxide (C-PTIO), an NO scavenger. The contractile tone of Müller cells was assessed by the extent of wrinkles created on the silicone sheets. The change of contractile tone was evaluated quantitatively by digitizing the photograph before and after the application of SNAP. Relaxation of wrinkles was induced by SNAP. C-PTIO inhibited the SNAP-induced relaxation of wrinkles. These results suggest that NO affects the contractility of embryonic Müller cells and could thus modulate ocular development.

Gas6-mediated survival in NIH3T3 cells activates stress signalling cascade and is independent of Ras

Gas6 is a growth factor membrane of the vitamin K-dependent family of proteins which is preferentially expressed in quiescent cells. Gas6 was identified as the ligand for Axl tyrosine kinase receptor family. Consistent with this, Gas6 was previously reported to induce cell cycle re-entry of serum-starved NIH3T3 cells and to prevent cell death after complete growth factor withdrawal, the survival effect being uncoupled from Gas6-induced mitogenesis. We have previously demonstrated that both Gas6 mitogenic and survival effects are mediated by Src and the phosphatidylinositol3-OH kinase (PI3K). Here we report that Ras is required for Gas6 mitogenesis but is dispensable for its survival effect. Gas6-induced survival requires the activity of the small GTPases of the Rho family, Rac and Rho, together with the downstream kinase Pak. Overexpression of the respective dominant negative constructs abrogates Gas6-mediated survival functions. Addition of Gas6 to serum starved cells results in the activation of AKT/PKB and in the phosphorylation of the Bcl-2 family member, Bad. By ectopic expression of a catalytically inactive form of AKT/PKB, we demonstrate that AKT/PKB is necessary for Gas6-mediated survival functions. We further show evidence that Gas6 stimulation of serum starved NIH3T3 cells results in a transient ERK, JNK/SAPK and p38 MAPK activation. Blocking ERK activation did not influence Gas6-induced survival, suggesting that such pathway is not involved in Gas6 protection from cell death. On the contrary we found that the late constitutive increase of p38 MAPK activity associated with cell death was downregulated in Gas6-treated NIH3T3 cells thus suggesting that Gas6 might promote survival by interfering with this pathway. Taken together the evidence here provided identity elements involved in Gas6 signalling more specifically elucidating the pathway responsible for Gas6-induced cell survival under conditions that do not allow cell proliferation.

A Novel Receptor Tyrosine Kinase, Mer, Inhibits TNF-α Production and Lipopolysaccharide-Induced Endotoxic Shock

The regulation of monocyte function and the inhibition of TNF-α production during bacterial sepsis are critical in attenuating adverse host responses to endotoxemia. To study the function of a novel receptor tyrosine kinase, mer, that is expressed in monocytes, we generated mice (merkd) that lack the signaling tyrosine kinase domain. Upon LPS challenge, merkd animals died of endotoxic shock (15/17, 88.2%), whereas control wild-type mice survived (1/15, 6.7% died). Susceptible merkd mice exhibited edema, leukocyte infiltration, and signs of endotoxic shock that correlated with higher levels of TNF-α found in the serum of merkd mice as compared with wild-type control animals. Death due to LPS-induced endotoxic shock in merkd mice was blocked by administration of anti-TNF-α Ab, suggesting that overproduction of this cytokine was principally responsible for the heightened suseptibility. The increase in TNF-α production appeared to be the result of a substantial increase in the LPS-dependent activation of NF-κB nuclear translocation resulting in greater TNF-α production by macrophages from merkd mice. Thus, Mer receptor tyrosine kinase signaling participates in a novel inhibitory pathway in macrophages important for regulating TNF-α secretion and attenuating endotoxic shock.

Biological effects of c-Mer receptor tyrosine kinase in hematopoietic cells depend on the Grb2 binding site in the receptor and activation of NF-kappaB

The c-Mer receptor tyrosine kinase (RTK) is most closely related to chicken c-Eyk and belongs to the Axl RTK subfamily. Although not detected in normal lymphocytes, c-Mer is expressed in B- and T-cell leukemia cell lines, suggesting an association with lymphoid malignancies. To gain an understanding of the role of this receptor in lymphoid cells, we expressed in murine interleukin-3 (IL-3)-dependent Ba/F3 pro-B-lymphocyte cells a constitutively active receptor, CDMer, formed from the CD8 extracellular domain and the c-Mer intracellular domain. Cells transfected with a plasmid encoding the CDMer receptor became IL-3 independent. When tyrosine (Y)-to-phenylalanine (F) mutations were introduced into c-Mer, only the Y867 change significantly reduced the IL-3-independent cell proliferation. The Y867 residue in the CDMer receptor mediated the binding of Grb2, which recruited the p85 phosphatidylinositol 3-kinase (PI 3-kinase). Despite the difference in promotion of proliferation, both the CDMer and mutant F867 receptors activated Erk in transfected cells. On the other hand, we found that both transcriptional activation of NF-kappaB and activation of PI 3-kinase were significantly suppressed with the F867 mutant receptor, suggesting that the activation of antiapoptotic pathways is the major mechanism for the observed phenotypic difference. Consistent with this notion, apoptosis induced by IL-3 withdrawal was strongly prevented by CDMer but not by the F867 mutant receptor.

A single amino acid substitution in the v-Eyk intracellular domain results in activation of Stat3 and enhances cellular transformation

The receptor tyrosine kinase Eyk, a member of the Axl/Tyro3 subfamily, activates the STAT pathway and transforms cells when constitutively activated. Here, we compared the potentials of the intracellular domains of Eyk molecules derived from c-Eyk and v-Eyk to transform rat 3Y1 fibroblasts. The v-Eyk molecule induced higher numbers of transformants in soft agar and stronger activation of Stat3; levels of Stat1 activation by the two Eyk molecules were similar. A mutation in the sequence Y933VPL, present in c-Eyk, to the v-Eyk sequence Y933VPQ led to increased activation of Stat3 and increased transformation efficiency. However, altering another sequence, Y862VNT, present in both Eyk molecules to F862VNT markedly decreased transformation without impairing Stat3 activation. These results indicate that activation of Stat3 enhances transformation efficiency and cooperates with another pathway to induce transformation.

Gas6, a ligand for the receptor protein-tyrosine kinase Tyro-3, is widely expressed in the central nervous system

Gas6 (growth arrest specific gene-6) is a ligand for members of the Axl subfamily of receptor protein-tyrosine kinases. One of these receptors, Tyro-3, is widely expressed in the central nervous system. We have used biochemical and histological techniques, including in situ hybridization, to determine the expression patterns of Gas6 mRNA and protein during development. Gas6 is widely expressed in the rat central nervous system (CNS) beginning at late embryonic stages and its levels remain high in the adult. Gas6 is detected as a single 85 kDa protein, which is encoded by a single 2.5 kb mRNA species. At embryonic day 14 it is detected in the heart, blood vessels, testes, choroid plexus, and in the ventral spinal cord. In the adult, Gas6 is expressed in the cerebral cortex, (predominantly in layer V), the piriform cortex, and the hippocampus (areas CA1, CA3 and the dentate gyrus). It is also expressed in thalamic and hypothalamic structures, the midbrain, and in a subset of motor and trigeminal nuclei. In the cerebellum, it is expressed in Purkinje neurons and deep cerebellar nuclei. Protein S, a protein related to Gas6, is only detected at low levels in the CNS. The spatial and temporal profiles of Gas6 expression suggest that it could potentially serve as the physiologically relevant ligand for Tyro-3 in the postnatal rat nervous system.