cDNA cloning and characterization of eck, an epithelial cell receptor protein-tyrosine kinase in the eph/elk family of protein kinases

The CLAVATA1 gene encodes a putative receptor kinase that controls shoot and floral meristem size in Arabidopsis

The shoot apical meristem is responsible for above-ground organ initiation in higher plants, accomplishing continuous organogenesis by maintaining a pool of undifferentiated cells and directing descendant cells toward organ formation. Normally, proliferation and differentiation are balanced, so that the structure and size of the shoot meristem is maintained. However, Arabidopsis plants homozygous for mutations at the CLAVATA1 (CLV1) locus accumulate excess undifferentiated cells. We describe the molecular cloning and expression pattern of the CLV1 gene. It encodes a putative receptor kinase, suggesting a role in signal transduction. The extracellular domain is composed of 21 tandem leucine-rich repeats that resemble leucine-rich repeats found in animal hormone receptors. We provide evidence that CLV1 expression in the inflorescence is specifically associated with meristematic activity.

ECK, a human EPH-related gene, maps to 1p36.1, a common region of alteration in human cancers

Mouse eck, a member of the EPH gene family, has been mapped to mouse chromosome 4. The syntenic relationship between this chromosome and human chromosome 1 suggests that the human ECK gene maps to the distal short arm of human chromosome 1 (1p). Since this region is frequently deleted or altered in certain tumors of neuroectodermal origin, it is important to define the specific chromosomal localization of the human ECK gene. PCR screening of a rodent-human somatic cell hybrid panel by ECK-specific primers showed that ECK is indeed localized to human chromosome 1. Additional PCR screening of a regional screening panel for chromosome 1p indicated that ECK is localized to 1p36, distal to FUCA1. Furthermore, fluorescence in situ hybridization analysis with an ECK-specific P1 clone showed that ECK maps proximal to genetic marker D1S228. Taken together, the data suggest that ECK maps to 1p36.1, a region that is frequently deleted in neuroblastoma, melanoma, and other neuroectodermal tumors.

Cell-cell adhesion mediated by binding of membrane-anchored ligand LERK-2 to the EPH-related receptor human embryonal kinase 2 promotes tyrosine kinase activity

Human embryonal kinase 2 (HEK2) is a protein-tyrosine kinase that is a member of the EPH family of receptors. Transcripts for HEK2 have a wide tissue distribution. Recently, a still growing family of ligands, which we have named LERKs, for ligands of the eph-related kinases, has been isolated. In order to analyze functional effects between the LERKs and the HEK2 receptor, we expressed HEK2 cDNA in an interleukin-3-dependent progenitor cell line 32D that grows as single cells in culture. Within the group of LERKs, LERK-2 and -5 were shown to bind to HEK2. Membrane-bound and soluble forms of LERK-2 were demonstrated to signal through HEK2 as judged by receptor phosphorylation. Coincubation of HEK2 and LERK-2 expressing cells induced cell-cell adhesion and formation of cell aggregates. This interaction could be inhibited by preincubation of HEK2 expressing cells with soluble LERK-2. Coexpression of HEK2 and LERK-2 in 32D cells showed reduced kinase activity and autophosphorylation of HEK2 compared with the juxtacrine stimulation, which seems to be due to a reduced sensitivity of the receptor.

Embryonic expression of eph signalling factors in Xenopus

Cellular communication in the developing embryo is mediated by receptor-ligand interactions at the cell surface. Receptor protein tyrosine kinases (RTKs) have been shown to play a critical role in the development of the vertebrate embryo. The eph receptors are a large subclass of RTKs for which a corresponding ligand family has only recently been described. The restricted expression patterns of several eph receptors imply roles for these molecules in early vertebrate development. We have isolated both a ligand of the eph ligand family (ELF), that we have named XELF-a, and an eph-related receptor, XE10, the likely homolog of the murine eck/Sek-2 receptor. At least two forms of the XELF-a transcript are present in the developing embryo. A truncated form of the XELF-a ligand, XELF-á, is the first ELF ligand isolated that lacks both the membrane-spanning and membrane-anchoring motifs conserved among this family, suggesting that ELF ligands can function as fully soluble molecules in vivo. XELF-a and XE10 are expressed maternally and throughout early embryogenesis, while XELF-á is only expressed zygotically. The dynamic expression patterns of these signalling molecules, in both mesoderm and neurectoderm, suggest that they may play a role in the patterning of the early vertebrate embryo.

Molecular cloning of a novel diacylglycerol kinase isozyme with a pleckstrin homology domain and a C-terminal tail similar to those of the EPH family of protein-tyrosine kinases

A fourth member of the diacylglycerol kinase (DGK) gene family termed DGK delta was cloned from the human testis cDNA library. The cDNA sequence contains an open reading frame of 3,507 nucleotides encoding a putative DGK protein of 130,006 Da. Interestingly, the new DGK isozyme contains a pleckstrin homology domain found in a number of proteins involved in signal transduction. Furthermore, the C-terminal tail of this isozyme is very similar to those of the EPH family of receptor tyrosine kinases. The primary structure of the delta-isozyme also has two cysteine-rich zinc finger-like structures (C3 region) and the C-terminal C4 region, both of which have been commonly found in the three isozymes previously cloned (DGKs alpha, beta and gamma). However, DGK delta lacks the EF-hand motifs (C2) and contains a long Glu- and Ser-rich insertion (317 residues), which divides the C4 region into two portions. Taken together, these structural features of DGK delta indicate that this isozyme belongs to a DGK subfamily distinct from that consisting of DGKs alpha, beta, and gamma. Increased DGK activity without marked preference to arachidonoyl type of diacylglycerol was detected in the particulate fraction of COS-7 cells expressing the transfected DGKdelta cDNA. The enzyme activity was independent of phosphatidylserine, which is a common activator for the previously sequenced DGKs. Northern blot analysis showed that the DGK delta mRNA (approximately 6.3 kilobases) is most abundant in human skeletal muscle but undetectable in the brain, thymus, and retina. This expression pattern is different from those of the previously cloned DGKs. Our results show that the DGK gene family consists of at least two subfamilies consisting of enzymes with distinct structural characteristics and that each cell type probably expresses its own characteristic repertoire of DGKs whose functions may be regulated through different signal transduction pathways.

B61, a ligand for the Eck receptor protein-tyrosine kinase, exhibits neurotrophic activity in cultures of rat spinal cord neurons

Although the Eph subfamily represents the largest group of receptor protein-tyrosine kinases, the biological roles of the Eph-related receptors and their ligands are not well understood. B61 has been identified recently by receptor affinity chromatography as a ligand for the Eph-related receptor Eck (Bartley et al.: Nature 368:558-560, 1994). Here we show that Eck immunoreactivity is localized in areas of the embryonic rat spinal cord that are rich in axons, suggesting that Eck plays a role in this region of the developing nervous system. To examine the biological function of Eck, monolayer cultures of dissociated cells from embryonic rat spinal cord were treated with soluble B61. With an ED50 of approximately 10 ng/ml, B61 treatment improved the survival of the overall neuronal population. Furthermore, in the presence of B61 neurites were longer and more elaborated. B61 similarly affected survival and neurite length in cultures enriched in motor neurons. These neurotrophic effects of B61 were not observed in the presence of anti-Eck antibodies, indicating that these effects are likely to be mediated by the Eck receptor.

Eph receptor tyrosine kinases and their ligands in neural development

Receptor tyrosine kinases play important roles in many developmental phenomena, including the regulation of cell proliferation, differentiation, and survival. The largest subfamily of receptor tyrosine kinases are the Eph receptors, which are widely expressed in the nervous system. With the recent identification of several Eph ligands, it has become evident that Eph receptors and their ligands are involved in the guidance of retinal axons and in the process of hindbrain segmentation.

Ligands for EPH-related tyrosine kinase receptors are developmentally regulated in the CNS

Elk is a member of the eph family of receptor-like tyrosine kinases. Although its function is unknown, elk is postulated to play a role in nervous system development. Using Northern analysis, we examined the developmental regulation of RNAs encoding elk, and several ligands for the eph family of RTKs, the LERKs. Expression of elk, LERK-1, and LERK-2 RNAs is high in all regions examined in the embryonic and postnatal rat brain and decreases to low levels with age. One exception is the adult olfactory bulb which continues to express a moderate level of LERK-2. In contrast, moderate LERK-4 expression was limited to the developing hippocampus and cerebral cortex. These data indicate that elk and some of the LERKs may play a role in nervous system development, maintenance, and/or regeneration.

Characterization of a novel Src-like adapter protein that associates with the Eck receptor tyrosine kinase

The Eph family of receptor protein tyrosine kinases (RPTKs) is the largest family of RPTKs. The signal transduction pathways initiated by this family have only recently begun to be explored. Using a yeast two-hybrid screen to identify molecules that interact with the cytoplasmic domain of Eck, it was previously shown that activated Eck RPTK bound to and stimulated phosphatidylinositol 3-kinase (Pandey, A., Lazar, D.F., Saltiel, A. R., and Dixit, V.M. (1994) J. Biol. Chem. 269, 30154-30157). Also isolated from this same screen was a novel protein containing SH3 and SH2 adapter modules that had striking homology to those found in the Src family of non-receptor tyrosine kinases. However, unlike other Src family members, it lacked a catalytic tyrosine kinase domain. Hence, this protein was designated SLAP for Src-like adapter protein. Using glutathione S-transferase fusion Proteins, it was demonstrated that SLAP bound to activated Eck receptor tyrosine kinase. Therefore, SLAP is a novel candidate downstream signaling intermediate and the first member of the Src family that resembles an adapter molecule.

In vitro guidance of retinal ganglion cell axons by RAGS, a 25 kDa tectal protein related to ligands for Eph receptor tyrosine kinases

The results of previous in vitro experiments indicate that a glycosylphosphatidylinositol (GPI)-anchored protein may play an important role in the guidance of temporal retinal axons during the formation of the topographically ordered retinotectal projection. We have purified and cloned a GPI-anchored, 25 kDa glycoprotein that is a good candidate for a molecule involved in this process. During the time of innervation by retinal ganglion cells, this protein is gradedly expressed in the posterior part of the developing tectum. In two different in vitro assay systems, the recombinant protein induces growth cone collapse and repulsion of retinal ganglion cell axons. These phenomena are observed for axons of temporal as well as nasal origin, indicating that an additional activity may be necessary to confer the nasotemporal specificity observed in previous assays. We named the protein RAGS (for repulsive axon guidance signal). The sequence of RAGS shows significant homology to recently identified ligands for receptor tyrosine kinases of the Eph subfamily.

Embryo brain kinase: a novel gene of the eph/elk receptor tyrosine kinase family

A new gene belonging to the Eph/Eck/Elk receptor tyrosine kinase family has been cloned from mouse brain. The gene maps to mouse chromosome 4. In the adult brain it is expressed exclusively and abundantly in the hippocampus. We propose to name it Ebk (embryo brain kinase), as in situ hybridisation shows expression in many parts of the developing mouse brain. The most abundant expression is in the subcommissural organ, and the earliest expression is in the forebrain neural folds, in rhombomeres 2-6, and in somites and heart. Other regions positive at various stages include the cochlear duct, trigeminal ganglion, lung, first branchial arch, and tooth primordia. Also positive are areas of mesenchyme underlying various epithelia during morphogenesis, especially in the mouth and nose, as well as in the eyelids and toes. We compare these patterns with the available data on the 12 other known members of this gene family. Most of them, like Ebk, are expressed in brain (especially adult hippocampus and embryonic rhombomeres) and in organs rich in epithelia (especially lung), although the spatial and temporal patterns differ. We suggest that combinatorial patterns of these receptors act as labels for the regional identity of neurons and epithelia, and could mediate fine control of neurite pathfinding and epithelial morphogenesis.

Tyrosine kinases in megakaryocytopoiesis

Protein-tyrosine kinases (PTKs) are of vital importance in a variety of cell functions. Recent studies have provided considerable insight into the binding of growth factors to tyrosine kinase receptors and the consequent induction of signal pathways that lead to a biologic response. Future studies will further delineate the signals that result in a proliferative response and those that induce a differentiation response. Current studies, reviewed here, indicate an important biologic role for PTKs in the regulation of megakaryocyte development and maturation. Whether PTKs function in megakaryocytes in signaling pathways that are similar to pathways in other cells will need to be examined in future studies.

Molecular cloning of tyrosine kinases in the early Xenopus embryo: identification of Eck-related genes expressed in cranial neural crest cells of the second (hyoid) arch

Growth factors and their receptors play an important role in controlling cellular proliferation, migration, and differentiation during vertebrate embryogenesis. We have used the reverse transcription-polymerase chain reaction to survey the repertoire of receptor tyrosine kinases (TK) expressed during early embryogenesis of Xenopus laevis. Twelve distinct Xenopus TK cDNA classes were identified among a total of 352 cDNAs screened. A single TK cDNA class has been described previously and encodes the fibroblast growth factor receptor FGFR-A1. The remaining 11 TK cDNA classes appear to encode novel genes of the FGFR, platelet-derived growth factor receptor (PDGFR), Eph, Csk, Tyk2, and Klg subfamilies. By RNase protection assays, Xenopus TK mRNAs are rare transcripts (< 10(7) mRNA molecules/embryo), and are usually found to be expressed also maternally in the embryo. Most Xenopus TK genes examined by whole-mount in situ hybridization were expressed widely in tissues derived from multiple germ layers. Two Eck-related genes, however, were found to be restricted in their expression to neural crest of the second (hyoid) arch. Our findings are consistent with the proposed function of TKs in the regulation of specification and differentiation of embryonic tissues.

Differential expressions of the eph family of receptor tyrosine kinase genes (sek, elk, eck) in the developing nervous system of the mouse

To examine the roles of the eph subfamily of receptor tyrosine kinase (RTK), we isolated mouse cDNAs for sek, elk, and eck and localized their mRNAs in the developing mouse, with particular reference to the CNS development, by in situ hybridization. sek mRNA is most abundantly expressed throughout development; sek was detected in the germinal layer of the embryonic CNS during mid- to late-gestation and was widely expressed in the early postnatal brain. elk was expressed in the mantle layer of the embryonic CNS and showed a distribution complementary to that of sek. Differential expression of sek and elk was also observed in the early postnatal cerebellum; sek was expressed in the Purkinje cells, while elk was detected in the granule cells. eck was moderately expressed in the germinal layer of the embryonic CNS at mid-gestation, but its expression decreased as development proceeded. These spatio-temporally different patterns of gene expression suggest that these RTKs have distinct roles in mouse development despite their structural homology.

Characterization of B61, the ligand for the Eck receptor protein-tyrosine kinase

B61 was originally described as a novel secreted tumor necrosis factor-alpha-inducible gene product in endothelial cells (Holzman, L. B., Marks, R. M., and Dixit, V. M. (1990) Mol. Cell. Biol. 10, 5830-5838). It was recently discovered that soluble recombinant B61 could serve as a ligand for the Eck receptor protein-tyrosine kinase, a member of the Eph/Eck subfamily of receptor protein-tyrosine kinases (Bartley, T.D., Hunt, R. W., Welcher, A. A., Boyle, W. J., Parker, V. P., Lindberg, R. A., Lu, H. S., Colombero, A. M., Elliott, R. L., Guthrie, R. A., Holst, P. L., Skrine, J. D., Toso, R. J., Zhang, M., Fernandez, E., Trail, G., Yarnum, B., Yarden, Y., Hunter, T., and Fox, G. M. (1994) Nature 368, 558-560). We now show that B61 can also exist as a cell surface glycosylphosphatidyl-inositol-linked protein that is capable of activating the Eck receptor protein-tyrosine kinase, the first such report of a receptor protein-tyrosine kinase ligand that is glycosylphosphatidylinositol-linked. In addition, the expression patterns of B61 and Eck during mouse ontogeny were determined by in situ hybridization. Both were found to be highly expressed in the developing lung and gut, while Eck was preferentially expressed in the thymus. Finally, the gene for B61 was localized to a specific position on mouse chromosome 3 by interspecific back-cross analysis.

cDNA cloning and characterization of a Cek7 receptor protein-tyrosine kinase ligand that is identical to the ligand (ELF-1) for the Mek-4 and Sek receptor protein-tyrosine kinases

We have isolated a murine cDNA encoding a ligand for the Cek7 receptor protein-tyrosine kinase (RPTK), a member of the Eph/Eck RPTK subfamily. Sequence analysis predicts an open reading frame of 209 amino acids with a predicted molecular mass of 24 kDa. The Cek7 ligand shows a 48% sequence identity at the protein level to B61, a ligand for the related Eck RPTK, 30% to the Cek5 ligand, 59% to the recently cloned Ehk1-L, and identity to ELF-1, a recently described ligand for the Mek4 and Sek RPTKs. The expressed Cek7 ligand is functionally active as it induces autophosphorylation of the Cek7 RPTK.

Abnormal protein tyrosine kinase gene expression during melanoma progression and metastasis

Protein tyrosine kinases have been implicated in tumor initiation and progression. Here we used Northern blotting to study expression of their genes in cultured normal melanocytes and 19 melanoma cell lines from different stages of tumor progression. We detected transcripts for 2 cytoplasmic (ABL and FES) and 6 receptor (ECK, ERB-B2, FGF-R4, IGFI-R, KDR and TIE) kinases but not for receptors RET or TRK-A. Genes for ECK, FGF-R4 and TIE were expressed ectopically in melanomas (not in normal melanocytes). Similarly, ECK protein was detected by immunoblotting in metastatic melanomas but not in normal melanocytes. ECK mRNA levels tended to increase again during late melanoma progression. ECK and TIE mRNAs were also detected in highly metastatic variant cells but not in the corresponding poorly metastatic parental lines. Conversely, FES and KDR gene expression was lost in most advanced primary and metastatic melanomas. These findings suggest positive and negative roles for specific tyrosine kinases during progression.

Identification of novel protein kinases expressed in the myocardium of the developing mouse heart

In Drosophila and Caenorhabditis, signal transduction pathways initiated by the activation of receptor-protein tyrosine kinases can mediate developmental fate decisions. In order to examine whether similar mechanisms are employed during mammalian embryogenesis, we undertook a search for novel protein kinases expressed during heart development in the mouse. The primitive mouse heart is formed between 7.75 and 8.5 days post coitum (dpc) and consists of myocardial and endocardial cells. A reverse transcriptase polymerase chain reaction-based approach was used to amplify protein kinase specific products from cDNAs obtained from 8.5 dpc heart tissue. Twenty independent PCR products corresponding to either protein serine/threonine or tyrosine kinases were identified. In this report, we describe the characterization of two of the genes corresponding to the novel PCR products (designated Hek2 and msk). Hek2 encodes the mouse ortholog of human HEK2, a recently identified member of the eph receptor-protein tyrosine kinase gene family. Prior to and at the time of heart formation (7.5-8.0 dpc), Hek2 is expressed in the cranial (rostral) region of the embryo from which a subpopulation of cells will give rise to the rudimentary heart. Between 8.0 and 9.5 dpc, Hek2 mRNA expression is observed in myocardial cells, head mesenchyme and paraxial mesoderm. Hek2 transcripts are not detected in endocardial cells. After 9.5 dpc, Hek2 expression is downregulated. msk (for myocardial SNF1-like kinase) encodes a putative protein serine/threonine kinase most similar to the yeast gene SNF1. msk mRNA expression is restricted to myocardial cells and their progenitors in the 7.75-8.5 dpc developing heart. Subsequently, msk mRNA expression is rapidly downregulated. The patterns of Hek2 and msk expression suggest that these protein kinases may function during development of the primitive heart.

Expression and developmental regulation of Ehk-1, a neuronal Elk-like receptor tyrosine kinase in brain

Protein tyrosine kinases are pivotal in central nervous tissue development and maintenance. Here we focus on the expression of Ehk-1, a novel Elk-related receptor tyrosine kinase. Ehk-1 gene expression is observed in the developing and adult central nervous system and is highly regulated throughout development at both the messenger RNA and protein levels. Three messenger RNA transcripts of 8.5, 5.9 and 5.1 kb are detectable in the rat brain and a variety of splice possibilities have been identified. However, a major protein species of around M(r) 120,000 predominates throughout development. Ehk-1 messenger RNA and protein levels are highest in the first postnatal week. By in situ messenger RNA hybridization the gene is expressed by all neurons of the adult brain, but mostly in the hippocampus, cerebral cortex and large neurons of the deep cerebellar nuclei, as well as the Purkinje and granular cells of the cerebellum. At earlier stages of development, transcripts are most prominent in the periventricular germinal layers of the brain. Immunohistochemistry reveals a pronounced membrane associated protein expression in immature neurons. In the adult animal, peak reactivity was found in the neuropil with sparing of most perikarya. The spatial and temporal pattern of ehk-1 gene expression suggests a role in both the development and maintenance of differentiated neurons of the central nervous system.

Several receptor tyrosine kinase genes of the Eph family are segmentally expressed in the developing hindbrain

Pattern formation in the hindbrain involves a segmentation process leading to the formation of metameric units, manifested as successive swellings known as rhombomeres (r). In search for genes involved in cell-cell interactions during hindbrain segmentation, we have screened for protein kinase genes with restricted expression patterns in this region of the CNS. We present the cloning of three novel mouse genes, Sek-2, Sek-3 and Sek-4 (members of the Eph subfamily of putative transmembrane receptor protein tyrosine kinases (RTKs)), the identification of their chromosomal locations, and the analysis of their expression between 7.5 and 10.5 days of development. Before morphological segmentation, Sek-2 is transcribed in a transverse stripe corresponding to prospective r4 and the adjacent mesoderm, suggesting possible roles both in hindbrain segmentation and signalling between neuroepithelium and mesoderm. Sek-3 and Sek-4 have common domains of expression, including r3, r5 and part of the midbrain, as well as specific domains in the diencephalon, telencephalon, spinal cord and in mesodermal and neural crest derivatives. Together with our previous finding that Sek (Sek-1) is expressed in r3 and r5 (Gilardi-Hebenstreit et al., 1992; Nieto et al., 1992), these data indicate that members of the Eph family of RTKs may co-operate in the segmental patterning of the hindbrain.

Pagliaccio, a member of the Eph family of receptor tyrosine kinase genes, has localized expression in a subset of neural crest and neural tissues in Xenopus laevis embryos

Cranial neural crest cells arise from neural folds in the embryonic head and differentiate to produce most of the cartilages and bones of the skull and the somatosensory ganglia of several cranial nerves, among other tissues. Since the molecular basis of the determination of these cells is poorly understood, we have begun a search for molecules involved in signal transduction in cranial neural crest. From a Xenopus laevis cranial neural crest cDNA bank, we have cloned a cDNA encoding a putative receptor tyrosine kinase, which we call Pagliaccio (Pag). Pag RNA is present transiently in visceral arch 3, probably representing neural crest cells in this tissue. Pag is also expressed in the forebrain, rhombomeres r3 and r5 of the hindbrain and in the pronephros. Based on this localized expression, we propose that Pag may play a role in the differentiation of cranial neural crest and other tissues.

The expression of the receptor-protein tyrosine kinase gene, eck, is highly restricted during early mouse development

Cell-cell interactions during embryogenesis have been shown to be important for establishing developmental fates in a number of organisms such as Drosophila and Caenorhabditis. One class of genes shown to mediate this process are receptor-protein tyrosine kinases (R-PTKs). To examine whether R-PTKs might participate in similar mechanisms operating in the mammalian embryo, an important prerequisite is to show that these genes are expressed in spatial and temporal patterns consistent with such a role. Here, we analyze the expression of eck, a member of the eph family of R-PTKs, during gastrulation and early organogenesis by in situ hybridization. eck transcripts are first detected in gastrulation stage embryos (6.5-7.5 days post coitum (dpc)) in ectodermal cells adjacent to the distal region of the primitive streak. By the neural plate stage (approximately 7.5 dpc), eck expression becomes restricted to the extreme distal end or node of the primitive streak. After the beginning of somitogenesis (approximately 8.0 dpc), eck expression persists in the node as this structure regresses toward the caudal end of the embryo. In addition, beginning at the mid head fold stage (approximately 7.75 dpc), we observe that eck exhibits a dynamic and spatially restricted expression pattern in the prospective hindbrain region. eck transcripts are initially detected in a 5-cell wide strip of mesodermal cells underlying prospective rhombomere 4 (r4). Subsequently at the beginning of somitogenesis, eck mRNA expression is observed in prospective r4. At the 4--8-somite stage, eck transcripts are observed in r4, mesenchymal cells underlying r4, and surface ectoderm in the vicinity of the developing second branchial arch. By the 10-somite stage, eck mRNA expression in these cells is downregulated. Additionally, at the 5--8-somite stage, eck transcripts are detected initially in the lateral mesenchyme immediately underlying the surface ectoderm adjacent to r5 and r6, and subsequently in surface ectoderm overlying the developing third branchial arch. These data suggest that eck may be involved in cell-cell interactions guiding early hindbrain development.

A newly identified tyrosine kinase is preferentially expressed in the gastrointestinal tract

We have identified a novel protein tyrosine kinase, cloned from rat intestinal mRNA, which is selectively expressed at high levels in intestinal cells, but is expressed at low or insignificant levels in a variety of other epithelial and non-epithelial organs. Gastrointestinal associated kinase (GASK) mRNA is present at high levels in stomach, small intestine and colon. GASK may play an important role in the growth, differentiation and/or development of the gastrointestinal tract.

B61 is a ligand for the ECK receptor protein-tyrosine kinase

A protein ligand for the ECK receptor protein-tyrosine kinase has been isolated by using the extracellular domain (ECK-X) of the receptor as an affinity reagent. Initially, concentrated cell culture supernatants were screened for receptor binding activity using immobilized ECK-X in a surface plasmon resonance detection system. Subsequently, supernatants from selected cell lines were fractionated directly by receptor affinity chromatography, resulting in the single-step purification of B61, a protein previously identified as the product of an early response gene induced by tumour necrosis factor-alpha. We report here that recombinant B61 induces autophosphorylation of ECK in intact cells, consistent with B61 being an authentic ligand for ECK. ECK is a member of a large orphan receptor protein-tyrosine kinase family headed by EPH, and we suggest that ligands for other members of this family will be related to B61, and can be isolated in the same way.

Spatially regulated expression of three receptor tyrosine kinase genes during gastrulation in the zebrafish

We describe the isolation and early developmental expression of three novel zebrafish genes (rtk1-3) that encode members of the eph family of receptor tyrosine kinases. At the onset of gastrulation, rtk1 is expressed in the shield region corresponding to the future dorsal side of the embryo. As gastrulation proceeds, both rtk1 and rtk2 are expressed within the axial hypoblast along the entire axis of the embryo. After the gastrula stage is complete, expression of both genes is maintained in precursor cells of the notochord in the tail bud but is downregulated in other regions of the axial hypoblast, rtk3 is expressed in anterior axial hypoblast including the ‘pillow’ at the anterior tip of the hypoblast and in paraxial tissue in posterior regions of the embryo. We show that the precise spatial regulation of expression of rtk genes, ntl and goosecoid along the anteroposterior axis is maintained in embryos that have no dorsoventral axis. This indicates that the mechanisms that regulate gene expression along the anteroposterior and dorsoventral axes of the hypoblast may be independent.

Isolation and characterization of Bsk, a growth factor receptor-like tyrosine kinase associated with the limbic system

Neuronal degeneration has been shown to be involved in various neurological disorders. Growth/trophic factors and their receptors are known to be important for the regeneration and survival of neurons. We report here the molecular cloning of a receptor-like protein tyrosine kinase, bsk, (for brain specific kinase). Bsk is highly related to the eph/elk receptor-like kinase family members. Northern blot analysis shows that it is expressed specifically in the brain, with no expression detected in adult heart, spleen, lung, liver, skeletal muscle, and kidney. In situ hybridization analysis of adult mouse brain sections indicates that bsk is expressed at high levels in the hippocampus, tenia tecta, indusium griseum, and the piriform cortex, major components of the limbic system that are important for learning and memory. In addition, elevated levels of expression are found in other areas of the limbic system such as the amygdala, medial septum, and nucleus of the diagonal band, and in the olfactory bulb, which has close connections to the limbic system. The highest level of expression is found in the CA3 region of the hippocampus and the pyramidal cell layer of the piriform cortex. In 16.5 day mouse embryos, bsk is expressed predominantly in the primordial cortex of the telencephalon. An antibody against a C-terminal peptide of bsk recognized a 105 kD protein in the 16.5 day embryonic head extract. Our analysis shows that bsk is a growth factor receptor-like protein tyrosine kinase and that its greatest expression in the adult brain is associated with components of the limbic system.

Novel and known protein tyrosine kinases and their abnormal expression in human melanoma

We have used the polymerase chain reaction and Northern blotting to identify protein tyrosine kinases that may play an important role in the process of melanoma initiation and progression. Degenerate primers from the conserved catalytic domain of tyrosine kinase genes were used to amplify and clone partial cDNA sequences from a human melanoma cell line (DX3-LT5.1) and normal human melanocytes. When the melanoma reaction products were sequenced, 13 distinct clones were found, of which one is novel to date and has provisionally been named MEK (for melanocytic kinase). Of the remaining 12 known kinases, only two, ERB-B2 and IGF1-R, have previously been reported in pigment cells. Reaction products from melanocytes included only eight of these 13 sequences. To test for quantitative differences in tyrosine kinase expression between normal and malignant cells, a panel of eight melanoma lines and normal melanocytes was analyzed by Northern blotting. Two tyrosine kinases (JTK-14/TIE and TYRO-9) were detected in some melanomas but were not found in normal melanocytes, whereas others, including MEK, appeared to be overexpressed in some malignant lines. A minority of kinases showed either no change or a reduction in the level of mRNA. Expression of tyrosine kinases varied independently, and individual lines contained various combinations of these enzymes. Our findings are consistent with an increased overall expression of these putative growth factor receptors during melanoma development.

Molecular cloning of a ligand for the inducible T cell gene 4-1BB: a member of an emerging family of cytokines with homology to tumor necrosis factor

4-1BB is an inducible T cell antigen that shows sequence homology to members of an emerging family of cytokine receptors, including those for tumor necrosis factor and nerve growth factor. To aid in the analysis of the function of 4-1BB we have utilized a soluble form of the molecule as a probe to identify and clone the gene which encodes its ligand. The ligand for 4-1BB is a type II membrane glycoprotein that has homology to tumor necrosis factor, lymphotoxin, and the ligands for CD40 and CD27, all of which are themselves ligands to receptors in this superfamily. The gene for 4-1BB is on mouse chromosome 4 and maps close to the p80 form of the tumor necrosis factor receptor as well as the gene for CD30. The gene for 4-1BB ligand maps to mouse chromosome 17, but considerably distal to the tumor necrosis factor and lymphotoxin genes. Interaction of 4-1BB with its ligand induces the proliferation of activated thymocytes and splenic T cells, a response which is mimicked on similar cell populations stimulated with an antibody to 4-1BB.

A novel tyrosine kinase, hyk, expressed in murine embryonic stem cells

To identify tyrosine kinases which play roles in mammalian early development, the 3' rapid amplification of cDNA ends (RACE) was performed on mouse embryonic stem (ES) cells. Among eight tyrosine kinases thus identified, we report here a novel tyrosine kinase, hyk (adhesion structures linked tyrosine kinase). The sequences of the 4.7 kb cDNA indicated the presence of RGD motif and three epidermal growth factor-like domains put between two immunoglobulin-like domains and three fibronectin type III domains in its extracellular region. It is strongly expressed in ES cells and later stages of embryos, but at low levels in midgestation embryos. It is also expressed at a low level in neural precursor cells from 10-day embryos, but at high levels in embryonic day 15 and neonatal brains. In adult tissues it is expressed ubiquitously.

A gene deleted in Kallmann's syndrome shares homology with neural cell adhesion and axonal path-finding molecules

Kallmann's syndrome (clinically characterized by hypogonadotropic hypogonadism and inability to smell) is caused by a defect in the migration of olfactory neurons, and neurons producing hypothalamic gonadotropin-releasing hormone. A gene has now been isolated from the critical region on Xp22.3 to which the syndrome locus has been assigned: this gene escapes X inactivation, has a homologue on the Y chromosome, and shows an unusual pattern of conservation across species. The predicted protein has significant similarities with proteins involved in neural cell adhesion and axonal pathfinding, as well as with protein kinases and phosphatases, which suggests that this gene could have a specific role in neuronal migration.

The receptor encoded by the human c-MET oncogene is expressed in hepatocytes, epithelial cells and solid tumors

The human c-MET oncogene encodes a transmembrane tyrosine kinase (p190c-met) with structural and functional features of a growth-factor receptor. Monoclonal antibodies (MAbs) have been used to investigate the distribution of the c-Met protein in human normal and neoplastic tissues. By immunofluorescence microscopy homogeneous expression was detected in normal hepatocytes as well as in epithelial cells lining the stomach, the small and the large intestine. Positive staining was also found in epithelial cells of the endometrium and ovary, and in basal keratinocytes of esophagus and skin. By Northern blot analysis, high levels of c-met messenger RNA were detected in specimens of liver, gastro-intestinal tract and kidney. c-met-specific mRNA was also found in thyroid, pancreas and placenta, in which organs c-Met protein was barely detectable by immunofluorescence. The antibodies revealed expression of c-MET protein in hepatomas (11/14), carcinomas of colon and rectum (19/21), stomach (11/22), kidney (16/19), ovary (9/17) and skin (7/17). Carcinomas of the lung (13/20), thyroid (11/13) and pancreas (5/7) were also positive. In these last cases (lung, thyroid and pancreas) tumor cells were homogeneously stained by the antibodies, whereas in their normal counterparts staining was barely detectable. These data suggest that the receptor encoded by c-MET plays a physiological role in epithelial cell growth and that its expression is altered in human carcinomas.

Oncogenes: a review of their clinical application

One objective of this review is to sort through and collate the recent data that suggest that human cellular oncogenes, which have been implicated as the etiologic agents in both animal and human malignancies, have also the potential to be employed as clinical tools in the struggle against cancer. For nearly 10 years, reports have been suggesting that advantage can be taken of cellular oncogenes as to their use as diagnostic and prognostic indicators of cancer and eventually as therapeutic cancer agents. It is also the purpose of this review to give an objective evaluation of these predictions. Moreover, this review will try to highlight some of the significant advances in this most rapidly evolving field of biology. Although the enormity of what has been learned about cellular oncogenes is nothing less than impressive, it is the view here that the routine implementation of oncogenes into the clinical setting will not become evident as early as the many predictions had purported.

The trkB tyrosine protein kinase is a receptor for brain-derived neurotrophic factor and neurotrophin-3

trkB is a tyrosine protein kinase gene highly related to trk, a proto-oncogene that encodes a receptor for nerve growth factor (NGF) and neurotrophin-3 (NT-3). trkB expression is confined to structures of the central and peripheral nervous systems, suggesting it also encodes a receptor for neurotrophic factors. Here we show that brain-derived neurotrophic factor (BDNF) and NT-3, but not NGF, can induce rapid phosphorylation on tyrosine of gp145trkB, one of the receptors encoded by trkB. BDNF and NT-3 can induce DNA synthesis in quiescent NIH 3T3 cells that express gp145trkB. Cotransfection of plasmids encoding gp145trkB and BDNF or NT-3 leads to transformation of recipient NIH 3T3 cells. In these assays, BDNF elicits a response at least two orders of magnitude higher than NT-3. Finally, 125I-NT-3 binds to NIH 3T3 cells expressing gp145trkB; binding can be competed by NT-3 and BDNF but not by NGF. These findings indicate that gp145trkB may function as a neurotrophic receptor for BDNF and NT-3.

An extended family of protein-tyrosine kinase genes differentially expressed in the vertebrate nervous system

We have used PCR to identify 13 novel protein-tyrosine kinase genes (tyro-1 to -13), six of which (tyro-1 to -6) are preferentially expressed in the developing vertebrate nervous system. The tyro-2 and tyro-9 genes encode kinase domains that exhibit strong amino acid sequence similarity to the equivalent regions of the receptors for EGF and FGF, respectively, and may encode novel receptors for these or related polypeptide ligands. The tyro-1 to -6 genes are all expressed during central nervous system neurogenesis and exhibit distinct and highly regionalized patterns of expression in the adult brain. Together with recent studies in invertebrates, these data are consistent with the hypothesis that protein-tyrosine kinases play a central role in neural development.