Graded changes in dose of a Xenopus activin A homologue elicit stepwise transitions in embryonic cell fate.

The protein XTC-MIF, a Xenopus homologue of activin A and a potent mesoderm-inducing factor, can induce responding animal pole explants to form several different cell types in a dose-dependent manner, higher doses eliciting more dorso-anterior tissues. This graded response, characteristic of classically postulated morphogens, may underlie pattern formation, but the response of intact animal caps to XTC-MIF provides only a crude indication of trends. Here we report the effects of XTC-MIF on dispersed blastomeres rather than intact animal caps. Under these conditions, responding cells distinguish sharply between doses of pure XTC-MIF differing by less than 1.5-fold. Two different response thresholds have been found, defining three cell states. This suggests that XTC-MIF has an instructive effect. Notochord and muscle are both induced in the same narrow dose-range. Mixing treated with untreated cells does not seem to shift the dose thresholds, showing that at least some cells can stably record the received dose of inducing factor.     

A truncated activin receptor inhibits mesoderm induction and formation of axial structures in Xenopus embryos.

Activins can induce mesoderm in embryonic explants and have been proposed as the natural inducer in Xenopus. A mutant activin receptor that inhibits activin signalling is used to show that activin is required for the induction of mesoderm in vivo and the patterning of the embryonic body plan. Blocking the activin signal transduction pathway also reveals autonomous induction of a neural marker and unmasks a relationship between activin and fibroblast growth factor.     

A transglutaminase homologue as a condensation catalyst in antibiotic assembly lines

The unrelenting emergence of antibiotic-resistant bacterial pathogens demands the investigation of antibiotics with new modes of action. The pseudopeptide antibiotic andrimid is a nanomolar inhibitor of the bacterial acetyl-CoA carboxylase that catalyses the first committed step in prokaryotic fatty acid biosynthesis. Recently, the andrimid (adm) biosynthetic gene cluster was isolated and heterologously expressed in Escherichia coli. This establishes a heterologous biological host in which to rapidly probe features of andrimid formation and to use biosynthetic engineering to make unnatural variants of this important and promising new class of antibiotics. Bioinformatic analysis of the adm cluster revealed a dissociated biosynthetic assembly system lacking canonical amide synthases between the first three carrier protein domains. Here we report that AdmF, a transglutaminase (TGase) homologue, catalyses the formation of the first amide bond, an N-acyl-beta-peptide link, in andrimid biosynthesis. Hence, AdmF is a newly discovered biosynthetic enzyme that acts as a stand-alone amide synthase between protein-bound, thiotemplated substrates in an antibiotic enzymatic assembly line. TGases (enzyme class (EC) 2.3.2.13) normally catalyse the cross-linking of (poly)peptides by creating isopeptidic bonds between the gamma-carboxamide group of a glutamine side chain of one protein and various amine donors, including lysine side chains. To the best of our knowledge, the present study constitutes the first report of a TGase-like enzyme recruited for the assembly of an antibiotic. Moreover, genome mining using the AdmF sequence yielded additional TGases in unassigned natural product biosynthetic pathways. With many more microbial genomes being sequenced, such a strategy could potentially unearth biosynthetic pathways producing new classes of antibiotics.     

Identification of an actin-binding protein from Dictyostelium as elongation factor 1a

Indirect evidence has implicated an interaction between the cytoskeleton and the protein synthetic machinery. Two recent reports have linked the elongation factor 1a (EF-1a) which is involved in protein synthesis, with the microtubular cytoskeleton. In situ hybridization has, however, revealed that the messages for certain cytoskeletal proteins are preferentially associated with actin filaments. ABP-50 is an abundant actin filament bundling protein of native relative molecular mass 50,000 (50K) isolated from Dictyostelium discoideum. Immunofluorescence studies show that ABP-50 is present in filopodia and other cortical regions that contain actin filament bundles. In addition, ABP-50 binds to monomeric actin in the cytosol of unstimulated cells and the association of ABP-50 with the actin cytoskeleton is regulated during chemotaxis. Through complementary DNA sequencing and subsequent functional analysis, we have identified ABP-50 as D. discoideum EF-1a. The ability of EF-1a to bind reversibly to the actin cytoskeleton upon stimulation could provide a mechanism for spatially and temporally regulated protein synthesis in eukaryotic cells.     

Activin-like factor from a Xenopus laevis cell line responsible for mesoderm induction

Induction of mesoderm during early amphibian embryogenesis can be mimicked in vitro by adding growth factors, including heparin-binding and type-beta transforming growth factors (TGF-beta), to isolated ectoderm explants from Xenopus laevis embryos. Although the mesoderm-inducing factor (MIF) from X. laevis XTC cells (XTC-MIF) has properties similar to TGF-beta, this factor is still unidentified. Recently, we obtained a number of homogeneous cell lines from the heterogeneous XTC population, which differ in their MIF production. Only one, XTC-GTX-11, produced MIF, although it was similar to the rest of the clones in its production of known growth factors, including TGF-beta activity. This observation, together with the identification of activin A as a potent MIF led us to study the parallel activities of MIF and activin. Here we report an analysis of activin-like activity from XTC cells and some of the XTC clones, including XTC-GTX-11. There is a clear consistent correlation between MIF activity and presence of activin activity, indicating that XTC-MIF is the Xenopus homologue of mammalian activin.     

Corticotropin-releasing factor is a potent inhibitor of sexual receptivity in the female rat

Corticotropin-releasing factor (CRF), the recently characterized and synthesized 41-amino acid polypeptide isolated from ovine hypothalami, has been shown to be a potent stimulator of adenohypophyseal beta-endorphin and corticotropin (ACTH) secretion both in vitro and in vivo. In common with other regulatory peptides, CRF has also been demonstrated to possess extra-hypophysiotropic roles. Indeed, intracerebroventricularly (i.c.v.) administered CRF elicits several endocrine and behavioural responses compatible with the concept that this peptide could be a key signal in coordinating the organism's endocrine and behavioural responses to stressful and other adaptive stimuli. We now provide the first evidence for neurally placed CRF in the control of a specific hormone-dependent behavioural response and unequivocally demonstrate an extremely potent suppressive effect of CRF on sexual behaviour in the female rat when microinfused into the arcuate-ventromedial area of the hypothalamus (ARC-VMH) and the mesencephalic central grey (MCG).     

Interleukin-1 beta as a potent hyperalgesic agent antagonized by a tripeptide analogue

Interleukin-1 (IL-1) describes two inflammatory proteins, IL-1 alpha and IL-1 beta, produced by activated macrophages and other cell types and encoded by two genes. Their amino acid sequences have only 26% similarity, but their biological activities are comparable, with a few exceptions; indeed, both molecules appear to act at the same receptor. As IL-1 release prostaglandins which sensitize nociceptors in man and in experimental animals, we tested IL-1 alpha and IL-1 beta in rats for hyperalgesic (nociceptive) activity. Our results show that IL-1 beta given systemically is an extremely potent hyperalgesic agent with a probable peripheral site of action; IL-1 alpha is approximately 3,000 times less active than IL-1 beta. We have delineated the region of IL-1 beta mediating the hyperalgesic effect and developed an analgesic tripeptide analogue of IL-1 beta which antagonizes hyperalgesia evoked by IL-1 beta and by the inflammatory agent carrageenan.     

Identification of the BAL-labile factor

One of us has previously reported that treatment of the Keilin and Hartree heart-muscle preparation with 2,3-dimercaptopropanol (BAL), in the presence of air, leads to the complete inactivation of the succinate oxidase system with little if any effect on the activities of succinate dehydrogenase (until more than half the BAL was oxidized) or cytochrome c oxidase. The inactivation of the complete succinate oxidase system requires the oxidation of BAL by air in the presence of the enzyme. It is not caused by H2O2 or BAL disulphides produced during the oxidation of BAL. Spectroscopic studies identified the block as lying between cytochromes b and c. It was suggested that a BAL-labile factor is present which transfers electrons from cytochrome b to cytochrome c and which is destroyed by coupled oxidation with BAL. The factor is also required for NADH oxidation. Subsequent work showed it is not identical with cytochrome c1 (ref. 4), myoglobin present in the preparation or the antimycin-binding site. We report here that this factor is identical to the iron-sulphur protein in the central portion of the respiratory chain first identified by Rieske.     

Identification of a factor that links apoptotic cells to phagocytes

Apoptotic cells are rapidly engulfed by phagocytes to prevent the release of potentially noxious or immunogenic intracellular materials from the dying cells, thereby preserving the integrity and function of the surrounding tissue. Phagocytes engulf apoptotic but not healthy cells, indicating that the apoptotic cells present a signal to the phagocytes, and the phagocytes recognize the signal using a specific receptor. Here, we report a factor that links apoptotic cells to phagocytes. We found that milk fat globule-EGF-factor 8 (MFG-E8), a secreted glycoprotein, was produced by thioglycollate-elicited macrophages. MFG-E8 specifically bound to apoptotic cells by recognizing aminophospholipids such as phosphatidylserine. MFG-E8, when engaged by phospholipids, bound to cells via its RGD (arginine-glycine-aspartate) motif--it bound particularly strongly to cells expressing alpha(v)beta(3) integrin. The NIH3T3 cell transformants that expressed a high level of alpha(v)beta(3) integrin were found to engulf apoptotic cells when MFG-E8 was added. MFG-E8 carrying a point mutation in the RGD motif behaved as a dominant-negative form, and inhibited the phagocytosis of apoptotic cells by peritoneal macrophages in vitro and in vivo. These results indicate that MFG-E8 secreted from activated macrophages binds to apoptotic cells, and brings them to phagocytes for engulfment.     

Identification of a secretory granule-binding protein as caldesmon

Stimulation of adrenal chromaffin cells results in a rise in the concentration of intracellular free calcium which initiates catecholamine secretion by exocytosis. An understanding of the molecular basis of exocytosis will require knowledge of the sites of action of calcium. A role for calmodulin has been implicated in secretion from chromaffin cells, and isolated granule membranes bind both calmodulin and a series of cytosolic proteins in a calcium-dependent fashion. Here, we demonstrate that one of the cytosolic granule-binding proteins with a relative molecular mass (Mr) of 70,000 (70K) is a form of the calmodulin-regulated actin-binding protein caldesmon, first isolated from smooth muscle. Cytoplasmic gels assembled from an adrenal medullary extract in the absence of Ca2+ contained actin and the 70K protein. The association of both of these proteins with the cytoplasmic gel was inhibited by a micromolar concentration of Ca2+. In addition, we have demonstrated that the 70K protein is localized at the periphery of chromaffin cells. These results are consistent with the notion that 70K protein (caldesmon) has a role in regulating the organization of actin filaments of the cell periphery during the secretory process.     

Identification of Tim4 as a phosphatidylserine receptor

In programmed cell death, a large number of cells undergo apoptosis, and are engulfed by macrophages to avoid the release of noxious materials from the dying cells. In definitive erythropoiesis, nuclei are expelled from erythroid precursor cells and are engulfed by macrophages. Phosphatidylserine is exposed on the surface of apoptotic cells and on the nuclei expelled from erythroid precursor cells; it works as an 'eat me' signal for phagocytes. Phosphatidylserine is also expressed on the surface of exosomes involved in intercellular signalling. Here we established a library of hamster monoclonal antibodies against mouse peritoneal macrophages, and found an antibody that strongly inhibited the phosphatidylserine-dependent engulfment of apoptotic cells. The antigen recognized by the antibody was identified by expression cloning as a type I transmembrane protein called Tim4 (T-cell immunoglobulin- and mucin-domain-containing molecule; also known as Timd4). Tim4 was expressed in Mac1+ cells in various mouse tissues, including spleen, lymph nodes and fetal liver. Tim4 bound apoptotic cells by recognizing phosphatidylserine via its immunoglobulin domain. The expression of Tim4 in fibroblasts enhanced their ability to engulf apoptotic cells. When the anti-Tim4 monoclonal antibody was administered into mice, the engulfment of apoptotic cells by thymic macrophages was significantly blocked, and the mice developed autoantibodies. Among the other Tim family members, Tim1, but neither Tim2 nor Tim3, specifically bound phosphatidylserine. Tim1- or Tim4-expressing Ba/F3 B cells were bound by exosomes via phosphatidylserine, and exosomes stimulated the interaction between Tim1 and Tim4. These results indicate that Tim4 and Tim1 are phosphatidylserine receptors for the engulfment of apoptotic cells, and may also be involved in intercellular signalling in which exosomes are involved.     

A human homologue of the yeast HDEL receptor

Retention of resident proteins in the lumen of the endoplasmic reticulum is achieved in both yeast and animal cells by their continual retrieval from the cis-Golgi, or a pre-Golgi compartment. Sorting of these proteins is dependent on a C-terminal tetrapeptide signal, usually Lys-Asp-Glu-Leu (KDEL in the single letter code) in animal cells, His-Asp-Glu-Leu (HDEL) in Saccharomyces cerevisiae. There is evidence that the ERD2 gene encodes the sorting receptor that recognizes HDEL in yeast; its product is an integral membrane protein of relative molecular mass 26,000 (26K) that is not glycosylated. In contrast, Vaux et al. suggest that the mammalian KDEL receptor is a 72K glycoprotein that they detected using an anti-idiotypic antibody approach. If this were so, it would indicate a surprising divergence of the retrieval machinery between yeast and animal cells. We report here that human cells express a protein similar in sequence, size and properties to the ERD2 product, and propose that this protein is the human KDEL receptor.     

Neuropeptide Y functions as a neuroproliferative factor

Neuropeptide Y (NPY) has a number of functions in mammalian physiology. Here we identify a role for NPY in promoting proliferation of postnatal neuronal precursor cells. NPY is synthesized in the postnatal olfactory epithelium by sustentacular cells, previously proposed to function only in structural support. Mice with a targeted deletion of NPY contain half as many dividing olfactory neuronal precursor cells as do controls. Furthermore, NPY-deficient mice develop significantly fewer olfactory neurons by adulthood. NPY acts on multipotent neuronal precursor or basal cells to activate rapidly and transiently the extracellular signal-regulated kinase (ERK)1/2 subgroup of mitogen-activated protein kinases. The NPY Y1 receptor subtype appears to mediate this effect. The ability of NPY to induce neuronal precursor proliferation is mediated by protein kinase C (PKC), indicating an upstream PKC-dependent activation of ERK1/2. These results indicate that NPY may regulate neuronal precursor proliferation in the adult mammal.     

Identification and characterization of a novel member of the nerve growth factor/brain-derived neurotrophic factor family

The survival and functional maintenance of vertebrate neurons critically depends on the availability of specific neurotrophic factors. So far, only two such factors, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) have been characterized and shown to have the typical features of secretory proteins. This characterization has been possible because of the extraordinarily large quantities of NGF in some adult tissues, and the virtually unlimited availability of brain tissue from which BDNF was isolated. Both NGF and BDNF promote the survival of distinct neuronal populations in vivo and are related in their primary structure, suggesting that they are members of a gene family. Although there is little doubt about the existence of other such proteins, their low abundance has rendered their identification and characterization difficult. Taking advantage of sequence identities between NGF and BDNF, we have now identified a third member of this family, which we name neurotrophin-3. Both the tissue distribution of the messenger RNA and the neuronal specificity of this secretory protein differ from those of NGF and BDNF. Alignment of the sequences of the three proteins reveals a remarkable number of amino acid identities, including all cysteine residues. This alignment also delineates four variable domains, each of 7-11 amino acids, indicating structural elements presumably involved in the neuronal specificity of these proteins.     

Identification of the V factor needed for synthesis of the iron-molybdenum cofactor of nitrogenase as homocitrate

Nitrogenase catalyses the ATP-dependent reduction of N2 to NH3, and is composed of two proteins, dinitrogenase (MoFe protein or component I) and dinitrogenase reductase (Fe protein or component II). Dinitrogenase contains a unique prosthetic group (iron-molybdenum cofactor, FeMoco) comprised of Fe, Mo and S, which has been proposed as the site of N2 reduction. Biochemical and genetic studies of Nif- (nitrogen fixation) mutants of Klebsiella pneumoniae which are defective in nitrogen fixation, have shown that the nifB, nifQ, nifN, nifE and nifV genes are required for the biosynthesis of FeMo-co. Recently, a system for in vitro synthesis of FeMoco was described. The assay requires at least the nifB, nifN and nifE gene products, and a low-molecular-weight factor (V factor) produced in the presence of the nifV gene product. We have used this system to study FeMoco biosynthesis. We report here the isolation of V factor and identify it as homocitric acid ([R]2-hydroxy-1,2,4-butanetricarboxylic acid).     

A zebrafish homologue of the chemokine receptor Cxcr4 is a germ-cell guidance receptor

Germ cells preserve an individual's genetic information and transmit it to the next generation. Early in development germ cells are set aside and undergo a specialized developmental programme, a hallmark of which is the migration from their site of origin to the future gonad. In Drosophila, several factors have been identified that control germ-cell migration to their target tissues; however, the germ-cell chemoattractant or its receptor have remained unknown. Here we apply genetics and in vivo imaging to show that odysseus, a zebrafish homologue of the G-protein-coupled chemokine receptor Cxcr4, is required specifically in germ cells for their chemotaxis. odysseus mutant germ cells are able to activate the migratory programme, but fail to undergo directed migration towards their target tissue, resulting in randomly dispersed germ cells. SDF-1, the presumptive cognate ligand for Cxcr4, shows a similar loss-of-function phenotype and can recruit germ cells to ectopic sites in the embryo, thus identifying a vertebrate ligand-receptor pair guiding migratory germ cells at all stages of migration towards their target.