Angiopoietins 3 and 4: diverging gene counterparts in mice and humans

The angiopoietins have recently joined the members of the vascular endothelial growth factor family as the only known growth factors largely specific for vascular endothelium. The angiopoietins include a naturally occurring agonist, angiopoietin-1, as well as a naturally occurring antagonist, angiopoietin-2, both of which act by means of the Tie2 receptor. We now report our attempts to use homology-based cloning approaches to identify new members of the angiopoietin family. These efforts have led to the identification of two new angiopoietins, angiopoietin-3 in mouse and angiopoietin-4 in human; we have also identified several more distantly related sequences that do not seem to be true angiopoietins, in that they do not bind to the Tie receptors. Although angiopoietin-3 and angiopoietin-4 are strikingly more structurally diverged from each other than are the mouse and human versions of angiopoietin-1 and angiopoietin-2, they appear to represent the mouse and human counterparts of the same gene locus, as revealed in our chromosomal localization studies of all of the angiopoietins in mouse and human. The structural divergence of angiopoietin-3 and angiopoietin-4 appears to underlie diverging functions of these counterparts. Angiopoietin-3 and angiopoietin-4 have very different distributions in their respective species, and angiopoietin-3 appears to act as an antagonist, whereas angiopoietin-4 appears to function as an agonist.

Angiopoietin-2, a natural antagonist for Tie2 that disrupts in vivo angiogenesis

Angiogenesis is thought to depend on a precise balance of positive and negative regulation. Angiopoietin-1 (Ang1) is an angiogenic factor that signals through the endothelial cell-specific Tie2 receptor tyrosine kinase. Like vascular endothelial growth factor, Ang1 is essential for normal vascular development in the mouse. An Ang1 relative, termed angiopoietin-2 (Ang2), was identified by homology screening and shown to be a naturally occurring antagonist for Ang1 and Tie2. Transgenic overexpression of Ang2 disrupts blood vessel formation in the mouse embryo. In adult mice and humans, Ang2 is expressed only at sites of vascular remodeling. Natural antagonists for vertebrate receptor tyrosine kinases are atypical; thus, the discovery of a negative regulator acting on Tie2 emphasizes the need for exquisite regulation of this angiogenic receptor system.

Isolation of angiopoietin-1, a ligand for the TIE2 receptor, by secretion-trap expression cloning

TIE2 is a receptor-like tyrosine kinase expressed almost exclusively in endothelial cells and early hemopoietic cells and required for the normal development of vascular structures during embryogenesis. We report the identification of a secreted ligand for TIE2, termed Angiopoietin-1, using a novel expression cloning technique that involves intracellular trapping and detection of the ligand in COS cells. The structure of Angiopoietin-1 differs from that of known angiogenic factors or other ligands for receptor tyrosine kinases. Although Angiopoietin-1 binds and induces the tyrosine phosphorylation of TIE2, it does not directly promote the growth of cultured endothelial cells. However, its expression in close proximity with developing blood vessels implicates Angiopoietin-1 in endothelial developmental processes.

Ligands for EPH-related receptor tyrosine kinases that require membrane attachment or clustering for activity

The EPH-related transmembrane tyrosine kinases constitute the largest known family of receptor-like tyrosine kinases, with many members displaying specific patterns of expression in the developing and adult nervous system. A family of cell surface-bound ligands exhibiting distinct, but overlapping, specificities for these EPH-related kinases was identified. These ligands were unable to act as conventional soluble factors. However, they did function when presented in membrane-bound form, suggesting that they require direct cell-to-cell contact to activate their receptors. Membrane attachment may serve to facilitate ligand dimerization or aggregation, because antibody-mediated clustering activated previously inactive soluble forms of these ligands.

LIFR beta and gp130 as heterodimerizing signal transducers of the tripartite CNTF receptor

The ciliary neurotrophic factor (CNTF) receptor complex is shown here to include the CNTF binding protein (CNTFR alpha) as well as the components of the leukemia inhibitory factor (LIF) receptor, LIFR beta (the LIF binding protein) and gp130 [the signal transducer of interleukin-6 (IL-6)]. Thus, the conversion of a bipartite LIF receptor into a tripartite CNTF receptor apparently occurs by the addition of the specificity-conferring element CNTFR alpha. Both CNTF and LIF trigger the association of initially separate receptor components, which in turn results in tyrosine phosphorylation of receptor subunits. Unlike the IL-6 receptor complex in which homodimerization of gp130 appears to be critical for signal initiation, signaling by the CNTF and LIF receptor complexes depends on the heterodimerization of gp130 with LIFR beta. Ligand-induced dimerization of signal-transducing receptor components, also seen with receptor tyrosine kinases, may provide a general mechanism for the transmission of a signal across the cell membrane.

Released form of CNTF receptor alpha component as a soluble mediator of CNTF responses

The alpha component of the receptor for ciliary neurotrophic factor (CNTF) differs from other known growth factor receptors in that it is anchored to cell membranes by a glycosylphosphatidylinositol linkage. One possible function of this type of linkage is to allow for the regulated release of this receptor component. Cell lines not normally responsive to CNTF responded to treatment with a combination of CNTF and a soluble form of the CNTF alpha receptor component. These findings not only demonstrate that the CNTF receptor alpha chain is a required component of the functional CNTF receptor complex but also reveal that it can function in soluble form as part of a heterodimeric ligand. Potential physiological roles for the soluble CNTF receptor are suggested by its presence in cerebrospinal fluid and by its release from skeletal muscle in response to peripheral nerve injury.

The alpha component of the CNTF receptor is required for signaling and defines potential CNTF targets in the adult and during development

We recently proposed that ciliary neurotrophic factor (CNTF) shares two receptor components with a generally acting cytokine, leukemia inhibitory factor (LIF), but that CNTF also requires a third receptor component (CNTFR alpha) that is mostly restricted to the nervous system in its expression. Here we demonstrate that a transfected CNTFR alpha gene is sufficient to confer CNTF responsiveness upon hemopoietic cells normally responsive only to LIF, providing evidence that CNTFR alpha is a required receptor component that uniquely characterizes CNTF-responding cells. Consistent with this notion, CNTFR alpha expression could be localized to neurons within all known peripheral targets of CNTF. CNTFR alpha was also widely expressed within neurons of the CNS, suggesting that CNTF has broader CNS actions than previously appreciated. However, in vivo localization of CNTFR alpha, as well as of CNTF itself, is consistent with a particularly important role for CNTF in motor function as well as during neuropoiesis.

The receptor for ciliary neurotrophic factor

Although neurotrophic factors were originally isolated on the basis of their ability to support the survival of neurons, these molecules are now thought to influence many aspects of the development and maintenance of the nervous system. Identifying the receptors for these neurotrophic factors should aid in identifying the cells on which these factors act and in understanding their precise mechanisms of action. A "tagged-ligand panning" procedure was used to clone a receptor for ciliary neurotrophic factor (CNTF). This receptor is expressed exclusively within the nervous system and skeletal muscle. The CNTF receptor has a structure unrelated to the receptors utilized by the nerve growth factor family of neurotrophic molecules, but instead is most homologous to the receptor for a cytokine, interleukin-6. This similarity suggestes that the CNTF receptor, like the interleukin-6 receptor, requires a second, signal-transducing component. In contrast to all known receptors, the CNTF receptor is anchored to cell membranes by a glycosyl-phosphatidylinositol linkage.

trkB encodes a functional receptor for brain-derived neurotrophic factor and neurotrophin-3 but not nerve growth factor

A variety of findings seem to functionally link brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), while distinguishing both of these factors from the third member of the neurotrophin family, nerve growth factor (NGF). Here we demonstrate that all three of these neuronal survival molecules bind similarly to the low affinity NGF receptor, but that BDNF and NT-3, unlike NGF, do not act via the high affinity NGF receptor. However, both BDNF and NT-3, but not NGF, bind to full-length and truncated forms of a receptor-like tyrosine kinase, trkB, for which no ligand had previously been identified. In addition to binding BDNF and NT-3, trkB can mediate functional responses to both of these neurotrophins when it is expressed in PC12 cells, although BDNF appears to be the more effective ligand. Thus trkB encodes an essential component of a functional receptor for BDNF and NT-3, but not for NGF. Further evidence predicts the existence of additional functional receptors for the neurotrophins.

Identification of functional receptors for ciliary neurotrophic factor on neuronal cell lines and primary neurons

The lack of reagents or molecular probes specific for the ciliary neurotrophic factor (CNTF) receptor has hindered characterization of the molecular mechanism(s) by which CNTF influences the proliferation, survival, and differentiation of cells of the vertebrate nervous system. We have developed methods for the detection and separation of cells expressing CNTF receptors by using a variety of binding assays based on a genetically engineered CNTF molecule containing an "epitope tag" at its C-terminus. These assays have allowed us to identify several neuronal cell lines, as well as embryonic and adult neurons in primary cultures, that bind CNTF and functionally respond to CNTF by rapidly activating the transcription of immediate early primary response genes.

Neurotrophic factors, their receptors, and the signal transduction pathways they activate

Our studies of the spatiotemporal availability of neurotrophic factors, coupled with tagged ligand binding assays that identify cell bearing receptors for these factors, should lead toward defining the physiological roles of these molecules in the animal. The use of the tagged ligands to identify factor-responsive cell lines has also provided new model systems for the examination of ligand-receptor interactions, as well as for the study of the subsequent induction of intracellular response pathways. To obtain insights into such intracellular pathways, we have molecularly cloned genes encoding a family of serine-threonine protein kinases, most closely related to kinases involved in the yeast response to pheromones. These kinases may be crucial regulators of early steps in the response of mammalian cells to neurotrophic factors as well as other extracellular signals.

Biological effects of a murine retrovirus carrying an activated N-ras gene of human origin

We have introduced a genomic DNA clone of a mutated human N-ras gene from a T-cell leukemia cell line into a retroviral vector equipped with a neo resistance gene and with SV40 and pBR322 origins of replication. The helper free N-ras virus, which was recovered after transfection of the construction in the psi 2 packaging cell line, contained a correctly spliced N-ras gene. Proviral DNA was amplified in cos cells and subsequently cloned in bacteria. Nucleic acid sequence analysis of the activated N-ras gene revealed a point mutation at codon 12 resulting in a glycine to aspartic acid substitution. The N-ras virus was able to transform mouse fibroblastic cell lines, but failed to fully transform mouse primary embryo fibroblasts. MoMuLV or amphotropic 4070A pseudotypes of the virus were injected intraperitoneally into newborn mice. The MoMuLV pseudotype produced only helper-virus-induced leukemias. The amphotropic pseudotype caused fibrosarcomas after a long latent period. The results of these and other in vivo experiments are discussed in relation to known pathogenic effects of other retroviruses carrying H-ras or K-ras genes.

Expression of normal and mutant ras proteins in human acute leukemia

The expression of normal and mutant ras genes in human acute leukemias was assessed by the direct analysis of p21ras polypeptides, using immunoprecipitation with monoclonal antibodies. High-resolution two-dimensional gel electrophoresis permits the identification of a wide array of activated ras alleles encoding proteins with single amino acid substitutions at any of several positions. The products of three ras genes, H-ras, N-ras, and K-ras, were detected in each of 33 specimens of fresh leukemic cells. The normal K-ras and N-ras polypeptides were substantially more abundant than H-ras p21 in all samples. In over three-fourths of the cases the total amount of p21ras exceeded that seen in control hematopoietic cell lines. The level of ras expression did not correlate simply with clinical parameters, although the two samples with the most abundant p21ras were obtained from patients with relapsed T-cell acute lymphocytic leukemia (ALL). Abnormal p21ras, consistent with oncogenic activation, was found in eight patients. Six of 11 samples from acute myelocytic leukemia (AML) patients displayed a mutant N-ras p21, while only one of 20 ALL specimens had abnormal N-ras, and one had a mutant H-ras. In every case the mutant protein comprised a minority of total p21ras. In two T-cell ALL cell lines both normal and activated N-ras gene products were expressed at equal levels. By contrast, in five fresh AML samples the abnormal N-ras protein was several-fold less abundant than the normal N-ras p21. This finding implies that only a proportion of leukemic cells in an individual patient may carry the mutant ras oncogene.

Antisera to the variable region of ras oncogene proteins, and specific detection of H-ras expression in an experimental model of chemical carcinogenesis

Antisera were prepared in mice, rats and rabbits by immunization with peptides corresponding to regions of highest variability, located near the C-termini of four ras proteins. Two of these, H-ras (171-189) and K-rasB (171-186), react uniquely with H-ras and K-rasB gene products in immunoblots and immunoprecipitation reactions. Affinity-purified rabbit H-ras (171-189) antibody detects H-ras p21 in tissue culture cells and in tissue sections. Epithelial cells in normal mouse skin and cells in papillomas and carcinomas, in a mouse model system of chemical carcinogenesis in which mutational activation of H-ras occurs with high frequency, express high levels of H-ras p21 protein. These results suggest an hypothesis to explain the mechanism and preferential activation of particular ras loci in certain neoplasia.

Expression of ras proto-oncogene proteins in normal human tissues

The expression of ras proto-oncogenes in normal human tissues was studied by immunohistochemical staining and by immunoblotting using monoclonal antibodies. We detected p21ras protein in almost every fetal and adult tissue, but the level varied significantly among cell types. In some cell lineages, immature cells capable of proliferation contain more p21ras than do mature cells. By contrast, certain fully differentiated cells, such as neurons and the epithelial cells of endocrine glands, express abundant p21ras. Among mammalian tissues the highest level of ras protein was detected in brain. Crude synaptosomal membrane preparations from rat brain contain substantially more p21ras than do plasma membranes from rat liver. The observed distribution of p21ras suggests a role for these proteins both in cellular proliferation and in certain specialized cellular functions.

Abundant expression of ras proteins in Aplysia neurons

We have cloned a DNA fragment from the marine mollusc Aplysia californica, which contains sequences homologous to mammalian ras genes, by screening a genomic library with a viral Ha-ras oncogene probe under conditions of low stringency hybridization. Nucleotide sequencing revealed a putative exon that encodes amino acids sharing 68% homology with residues 5 to 54 of mammalian p21ras polypeptides, and which therefore is likely to encode a ras-like Aplysia protein. The cloned locus, designated Apl-ras, is distinct from the Aplysia rho (ras-homologue) gene and appears to be more closely related to mammalian ras. We used a panel of monoclonal antibodies raised against v-Ha-ras p21 to precipitate an Mr 21,000 protein from extracts of Aplysia nervous tissue, ovotestis, and, to a much lesser degree, buccal muscle. Fluorescence immunocytochemistry revealed that ras-like protein is most abundant in neuronal cell bodies and axon processes, with staining most prominent at plasma membranes. Much less was present in other tissues. The prominence of ras protein in neurons, which are terminally differentiated and non-proliferating, indicates that the control of cell division is not the sole function of this proto-oncogene. The large identified neurons of Aplysia offer the opportunity to examine how ras protein might function in mature nerve cells.