Eukaryotic initiation factor eIF6 modulates the expression of Kermit 2/XGIPC in IGF- regulated eye development.

The eukaryotic initiation translation factor eIF6 is a highly conserved, essential protein implicated in translation. eIF6 is regulated in vivo by extracellular signals, such as IGF signaling (for a review see Miluzio et al., 2009). In Xenopus, eif6 over-expression causes a delay in eye development (De Marco et al., 2011). In this study we showed that eif6 co-immunoprecipitates with the insulin-like growth factor receptor (igfr) and may function downstream of igf in eye formation. The relationship between eif6 and gipc2, a protein partner of a variety of molecules including membrane proteins, was investigated. gipc2 is required for maintaining igf-induced akt activation on eye development (Wu et al., 2006). Significantly eif6 and gipc2 have opposite effects in eye development. While eif6 is required for eye formation below threshold levels, gipc2 knockdown impairs eye development (De Marco et al., 2011; Wu et al., 2006). In this study, it was shown that in eif6 over-expressors, the delay in eye morphogenesis is reversed by gipc2 injection, while the injection of eif6 down-regulates gipc2 expression. Real-time-PCR indicates that eif6 regulates gipc2 expression in a dose-dependent manner. In contrast, gipc2 knockdown has no significant effect on eif6 mRNA levels. These results suggest that eif6 regulation of gipc2 enables correct morphogenesis of Xenopus eye and stimulate questions on the molecular network implicated in this process.

Eukaryotic translation initiation factor 6 is a novel regulator of reactive oxygen species-dependent megakaryocyte maturation.

BACKGROUND: Ribosomopathies constitute a class of inherited disorders characterized by defects in ribosome biogenesis and function. Classically, bone marrow (BM) failure is a clinical symptom shared between these syndromes, including Shwachman-Bodian-Diamond syndrome (SBDS). Eukaryotic translation initiation factor 6 (eIF6) is a critical translation factor that rescues the quasilethal effect of the loss of the SBDS protein. OBJECTIVES: To determine whether eIF6 activity is necessary for BM development. METHODS: We used eIF6(+/-) mice and primary BM megakaryocytes to investigate the involvement of eIF6 in the regulation of hematopoiesis. RESULTS: We provide evidence that reduced eIF6 expression negatively impacts on megakaryopoiesis. We show that inhibition of eIF6 leads to a reduction in cell size and mean ploidy level of megakaryocytes and a delay in megakaryocyte maturation by blocking the G1 /S transition. Consistent with this phenotype, only few megakaryocyte-forming proplatelets were found in eIF6(+/-) cells. We also discovered that, in eIF6(+/-) cells, the steady-state abundance of mitochondrial respiratory chain complex I-encoding mRNAs is decreased, resulting in decreased reactive oxygen species (ROS) production. Intriguingly, connectivity map analysis showed that eIF6-mediated changes overlap with specific translational inhibitors. eIF6 is a translation factor acting downstream of insulin/phorbol 12-myristate 13-acetate (PMA) stimulation. PMA treatment significantly restored eIF6(+/-) megakaryocyte maturation, indicating that activation of eIF6 is essential for the rescue of the phenotype. CONCLUSIONS: Taken together, our results show a role for eIF6-driven translation in megakaryocyte development, and unveil the novel connection between translational control and ROS production in this cell subset.

Translation factors and ribosomal proteins control tumor onset and progression: how?

Gene expression is shaped by translational control. The modalities and the extent by which translation factors modify gene expression have revealed therapeutic scenarios. For instance, eukaryotic initiation factor (eIF)4E activity is controlled by the signaling cascade of growth factors, and drives tumorigenesis by favoring the translation of specific mRNAs. Highly specific drugs target the activity of eIF4E. Indeed, the antitumor action of mTOR complex 1 (mTORc1) blockers like rapamycin relies on their capability to inhibit eIF4E assembly into functional eIF4F complexes. eIF4E biology, from its inception to recent pharmacological targeting, is proof-of-principle that translational control is druggable. The case for eIF4E is not isolated. The translational machinery is involved in the biology of cancer through many other mechanisms. First, untranslated sequences on mRNAs as well as noncoding RNAs regulate the translational efficiency of mRNAs that are central for tumor progression. Second, other initiation factors like eIF6 show a tumorigenic potential by acting downstream of oncogenic pathways. Third, genetic alterations in components of the translational apparatus underlie an entire class of inherited syndromes known as 'ribosomopathies' that are associated with increased cancer risk. Taken together, data suggest that in spite of their evolutionary conservation and ubiquitous nature, variations in the activity and levels of ribosomal proteins and translation factors generate highly specific effects. Beside, as the structures and biochemical activities of several noncoding RNAs and initiation factors are known, these factors may be amenable to rational pharmacological targeting. The future is to design highly specific drugs targeting the translational apparatus.

p27(BBP)/eIF6 acts as an anti-apoptotic factor upstream of Bcl-2 during Xenopus laevis development.

p27(BBP)/eIF6 (beta4-binding protein/eukaryotic initiation factor 6) regulates the joining of 40S and 60S ribosomal subunits, on receptor for activated C kinase 1 binding and protein kinase C phosphorylation in serine 235. In Xenopus, p27(BBP)/eIF6 is abundantly expressed in the majority of the embryonic anlagen. Although p27(BBP)/eIF6 abundance may be required for a general regulation of protein synthesis, our data suggest that p27(BBP)/eIF6 may target the translation of specific mRNAs. We injected Xp27(BBP)/eIF6 mRNA in one blastomere of two-cell-stage embryos and obtained a bent phenotype, the curvature being lateral with respect to the embryo antero-posterior axis. The injected side had fewer apoptotic cells than the uninjected side, whereas cell proliferation appeared unaffected. Accordingly, in Xp27(BBP)/eIF6 morphants, endogenous apoptosis increased. Injection of Xp27(BBP)/eIF6 point mutants indicated that the anti-apoptotic action of Xp27(BBP)/eIF6 requires the conserved S235. The bent phenotype was also obtained with B-cell lymphoma gene-2 (Bcl-2) overexpression and was rescued by Bcl-2-associated X protein (Bax)/Xp27(BBP)/eIF6 co-injection. In addition, embryos overexpressing Xp27(BBP)/eIF6 had a higher amount of Bcl-2 and an unchanged amount of Bax with respect to controls. In Xp27(BBP)/eIF6 morphants, Bcl-2 levels were unaffected and Bax levels were higher than in the controls. Thus, we propose that Xp27(BBP)/eIF6 is part of a mechanism acting on the specific translation of messengers regulating cell survival. In particular, we suggest that Xp27(BBP)/eIF6 may regulate the translation of factors upstream of Bcl-2/Bax.

Eukaryotic initiation factor 6 selectively regulates Wnt signaling and beta-catenin protein synthesis.

Eukaryotic initiation factor 6 (eIF6), an essential protein important in ribosome biosynthesis and assembly, was identified as an interacting partner of the beta-catenin C terminus in the yeast two-hybrid assay. Independent studies identified Drosophila eIF6 (DeIF6) in a genetic screen designed to detect new genes involved in the regulation of the Wnt/Wg (wingless) pathway. Ectopic expression of DeIF6 in wing discs results in a Wg phenotype. Expression of eIF6 in adenomatous polyposis coli (APC)-mutant colon cancer cells, which express high levels of active beta-catenin, showed that eIF6 selectively inhibits the Wnt pathway at the level of beta-catenin protein independently of proteasomal degradation. Incorporation of radiolabeled amino acids into beta-catenin was selectively decreased in cells that overexpressed eIF6. A similar inverse relationship of the two proteins was observed in the APC(min/+) mouse intestine, in which beta-catenin levels are very high. Taken together these data reveal a link between eIF6 and Wnt signaling, perhaps at the level of ribosome recycling on beta-catenin mRNA.

Phosphorylation of p27(BBP)/eIF6 and its association with the cytoskeleton are developmentally regulated in Xenopus oogenesis.

p27BBP/eIF6 is an evolutionarily conserved regulator of ribosomal function. It is necessary for 60S biogenesis and impedes improper joining of 40S and 60S subunits, regulated by protein kinase C or Efl1p. No data on p27BBP/eIF6 during early development of Metazoa are available. We studied the distribution, post-translational changes and association with the cytoskeleton of p27BBP/ eIF6 during Xenopus oogenesis and early development. Results indicate that p27BBP/eIF6 is present throughout oogenesis, partly associated with 60S subunits, partly free and with little cytoskeleton bound. During prophase I, p27BBP/eIF6 is detected as a single band of 27-kDa. Upon maturation induced by progesterone or protein kinase C, a serine-phosphorylated 29 kDa isoform appears and is kept throughout development to the neurula stage. Confocal microscopy showed that the distribution of p27BBP/eIF6 and its association with the cytoskeleton varies according to oogenesis stages. Briefly, in stage 6 oocytes, p27BBP/eIF6 has a limited dot-like distribution, and does not co-localize with cytokeratin, whereas upon maturation it spreads throughout the cytoplasm. After fertilization, a large fraction coalesces around cytomembranes and a cytochalasin B-sensitive co-localization with cytokeratin occurs. RNAse removes p27BBP/eIF6 from the cytokeratin fibres. Developmental data suggest a role of p27BBP/eIF6 in controlling ribosomal availability or regulating cross-talk between ribosomes and the cytoskeleton.

Immunocytochemical localization of protein p27BBP in human skin and invertebrate (Sepia officinalis) integument.

The protein p27BBP (alias eIF6) occurs in yeast and mammalian epithelial cells. It is essential for ribosome genesis and has also been implicated in the functionality of integrins and intermediate filaments. By immunoblot, we show that homogenized integument from Sepia officinalis (Cephalopoda, Mollusca) contains a protein with immunological properties that closely resemble those of p27BBP. We also demonstrate, by immunogold electron microscopy with an indirect immunoreaction technique on ultrathin sections of human skin and Sepia integument, that p27BBP is constantly present in both species in epithelial cells, fibroblasts, and muscle fibers. It is found in the vicinity of intermediate filaments, in nucleoli, along the internal wall of the nuclear membrane, and in association with desmosomes and hemidesmosomes and occasionally occurs extracellularly. Thus, the structure and function of p27BBP seem to have been highly conserved throughout evolution; the protein appears to be essential in eukaryotic cells in which it interacts with several ultrastructural components of diverse function.

The human ITGB4BP gene is constitutively expressed in vitro, but highly modulated in vivo.

The ITGB4BP gene encodes for a highly conserved protein, named p27BBP (also known as eIF6), originally identified in mammals as a cytoplasmic interactor of beta4 integrin. In vitro and in vivo studies demonstrated that p27BBP is essential for cell viability and has a primary function in the biogenesis of the 60S ribosomal subunit. Here we report the genomic organization of the human ITGB4BP gene and show that its gene product is expressed with features of a housekeeping element in vitro, but is regulated in a cell specific fashion in vivo. The human gene spans 10 kb and comprises seven exons and six introns. The 5' flanking region shows a TATA-less promoter, canonical CpG islands, and binding sites for serum responsive elements. In cultured cells, p27BBP mRNA and protein are constitutively expressed and stable. A gradual loss of p27BBP mRNA can be observed only after prolonged serum starvation, and heat shock treatment. In contrast, p27BBP mRNA and protein levels in vivo are variable among different organs. More strikingly, immunohistochemical analysis shows that the p27BBP protein is present in a cell specific fashion, even within the same tissue. Taken together, these data show that ITGB4BP gene expression is highly regulated in vivo, possibly by the combination of tissue specific factors and protein synthesis pathways.

Expression of a highly conserved protein, p27BBP, during the progression of human colorectal cancer.

The highly conserved protein p27BBP is a cytoplasmic interactor of integrin beta4 expressed in epithelia. p27BBP is found in two pools: one nuclear pool enriched in the perinucleolar region, and one cytoplasmic pool. Deletion of p27BBP in yeast is lethal as a result of loss of the ribosomal 60S subunit. The aim of this study was to investigate the distribution of p27BBP in gut epithelium and its behavior during progression of human colorectal carcinomas. Results indicated that p27BBP is high in rapidly cycling cells and decreased in villous cells committed to apoptotic cell death. In dysplastic adenomas and carcinomas, p27BBP displayed a large increase of its nucleolar component that was superimposable to argyrophylic nucleolar organizing region-associated proteins and was associated with the nuclear matrix. Western blotting confirmed increased p27BBP in dysplastic adenomas and in carcinomas. In particular, p27BBP increased progressively from adenomas to carcinomas and, in the latter, was related to the tumor stage. The overexpression of p27BBP corresponded to mRNA up-regulation in carcinomas, supporting the idea of transcriptional or post-transcriptional regulation of its expression. Results suggested that p27BBP alterations are an early event in the transition from benign to malignant colorectal phenotypes and provide a novel tool in surgical pathology.

The beta4 integrin interactor p27(BBP/eIF6) is an essential nuclear matrix protein involved in 60S ribosomal subunit assembly.

p27(BBP/eIF6) is an evolutionarily conserved protein that was originally identified as p27(BBP), an interactor of the cytoplasmic domain of integrin beta4 and, independently, as the putative translation initiation factor eIF6. To establish the in vivo function of p27(BBP/eIF6), its topographical distribution was investigated in mammalian cells and the effects of disrupting the corresponding gene was studied in the budding yeast, Saccharomyces cerevisiae. In epithelial cells containing beta4 integrin, p27(BBP/eIF6) is present in the cytoplasm and enriched at hemidesmosomes with a pattern similar to that of beta4 integrin. Surprisingly, in the absence and in the presence of the beta4 integrin subunit, p27(BBP/eIF6) is in the nucleolus and associated with the nuclear matrix. Deletion of the IIH S. cerevisiae gene, encoding the yeast p27(BBP/eIF6) homologue, is lethal, and depletion of the corresponding gene product is associated with a dramatic decrease of the level of free ribosomal 60S subunit. Furthermore, human p27(BBP/eIF6) can rescue the lethal effect of the iihDelta yeast mutation. The data obtained in vivo suggest an evolutionarily conserved function of p27(BBP/eIF6) in ribosome biogenesis or assembly rather than in translation. A further function related to the beta4 integrin subunit may have evolved specifically in higher eukaryotic cells.

Isolation of a novel beta4 integrin-binding protein (p27(BBP)) highly expressed in epithelial cells.

The integrin beta4 has a long cytodomain necessary for hemidesmosome formation. A yeast two-hybrid screen using beta4 cytodomain uncovered a protein called p27(BBP) that represents a beta4 interactor. Both in yeast and in vitro, p27(BBP) binds the two NH2-terminal fibronectin type III modules of beta4, a region required for signaling and hemidesmosome formation. Sequence analysis of p27(BBP) revealed that p27(BBP) was not previously known and has no homology with any isolated mammalian protein, but 85% identical to a yeast gene product of unknown function. Expression studies by Northern analysis and in situ hybridization showed that, in vivo, p27(BBP) mRNA is highly expressed in epithelia and proliferating embryonic epithelial cells. An antibody raised against p27(BBP) COOH-terminal domain showed that all beta4-containing epithelial cell lines expressed p27(BBP). The p27(BBP) protein is insoluble and present in the intermediate filament pool. Furthermore, subcellular fractionation indicated the presence of p27(BBP) both in the cytoplasm and in the nucleus. Confocal analysis of cultured cells showed that part of p27(BBP) immunoreactivity was both nuclear and in the membrane closely apposed to beta4. These results suggest that the p27(BBP) is an in vivo interactor of beta4, possibly linking beta4 to the intermediate filament cytoskeleton.

Development and migration of olfactory neurones in the nervous system of the neonatal opossum.

The neonatal opossum (Monodelphis domestica) was used to assess how different populations of cells are generated in the olfactory region, and how they migrate along pathways to the central nervous system. Developing nerve cells were immunocytochemically labelled using antisera directed against two specific markers of olfactory receptor neurones: olfactory marker protein (OMP) and the dipeptide carnosine. In new-born opossums both carnosine and OMP are already co-expressed in primary olfactory neurones and in those axons that extend towards the olfactory bulb. Expression of these markers in olfactory receptor neurones during the first postnatal days reflects the advanced developmental state of this system compared to other regions of the central nervous system (such as the cortex and cerebellum), which are highly immature and less developed in comparison with those of new-born rats or mice. A second, distinct population of carnosine/OMP expressing cells was also identified during the first postnatal week. These neurones were present as clusters along the olfactory nerve bundles, on the ventral-medial aspect of the olfactory bulb and in the basal prosencephalon. The distribution of this cell population was compared to another group of well characterized migratory neurones derived from the olfactory placode, which express the decapeptide GnRH (Gonadotropin-releasing hormone, also known as LHRH). GnRH was never co-localized with carnosine/OMP in the same migratory cells. These observations show that distinct cell populations arise from the olfactory placode in the neonatal opossum and that they migrate to colonize the central nervous system by following common pathways.

Selective binding and internalisation by truncated receptors restrict the availability of BDNF during development.

The tyrosine kinase receptor trkB is thought to mediate the biological actions of brain-derived neurotrophic factor. This receptor is expressed by a large variety of neurons during development. Truncated trkB molecules lacking the tyrosine kinase domain have also been described, but their functions remain elusive. In order to gain insight into their role, we studied the pattern of expression and properties of these truncated receptors in the chick embryo. mRNA coding for truncated trkB was detected already early during neurogenesis and in situ hybridisation experiments indicated that the expression was in non-neuronal cells, as previously observed in the brain of adult rodents. Ependymal and leptomeningeal cells expressing high levels of truncated trkB were found to completely surround the developing brain and the spinal cord throughout development. In the otic vesicle, mesenchymal cells expressing truncated trkB surround cells producing brain-derived neurotrophic factor, as well as neurons expressing trkB with its tyrosine kinase domain. Non-neuronal cells were found not to express trkB mRNA coding for the tyrosine kinase domain. Studies with radioiodinated brain-derived neurotrophic factor performed on frozen sections of the chick embryo revealed that non-neuronal cells expressing truncated trkB bind brain-derived neurotrophic factor with high affinity and selectivity. In addition, experiments with dissociated leptomeningeal cells revealed that binding is rapidly followed by selective internalisation of the ligand. These results suggest that truncated trkB molecules form an efficient and selective barrier preventing the diffusion of brain-derived neurotrophic factor and eliminating it by internalisation.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular control of neuronal survival in the chick embryo.

Neurotrophins are structurally related proteins which promote the survival and differentiation of specific neuronal populations during the development of vertebrate embryos. Like many growth factors, the neurotrophins mediate their actions by binding to membrane proteins that have a ligand-activated tyrosine kinase activity. The interactions of the neurophins with their neuronal receptors have been mostly studied using chick embryonic neurons. These neurons are also extensively used to characterise biological responses to neurotrophins in physiologically relevant systems. We have recently cloned and expressed the chick homologue of trkB (ctrkB), thought to be a receptor for BDNF, and examined by in situ hybridisation the pattern of expression of the ctrkB gene during development of the chick embryo. We found that whereas the sequence of ctrkB shows a high degree of conservation with the mammalian homologues in the intracellular tyrosine kinase domain, the extracellular binding domain is less well conserved. As in mammals, ctrkB mRNAs appear to exist in differentially spliced forms that result in a full length and a truncated receptor lacking the tyrosine kinase domain. These two forms are differentially expressed in neurons and non-neuronal cells respectively. The binding characteristics of ctrkB expressed in a transfected cell line are similar, but not identical to those of the BDNF binding sites on primary chick neurons, specially with regard to the affinity of BDNF.

In vitro study of olfactory receptor neurones expressing the dipeptide carnosine.

Olfactory neuroepithelial (OE) cells were dissociated from late stage embryonic mice and analysed for carnosine expression. The yield of carnosine neurones was twice as high when the OE cells were seeded along with the olfactory bulb cells. Carnosine neurones resulted from both in vitro survival and neurogenesis, and were associated with clusters of underlying flat cells immunopositive for keratin. Our results demonstrate that olfactory neurones expressing their neurotransmitter carnosine can be studied in culture, and the close association with keratin-immunopositive basal cells suggests that they are dependent on these cells for survival and/or differentiation.

Expression and binding characteristics of the BDNF receptor chick trkB.

Previous studies using transfected cells have indicated that the mammalian receptor tyrosine kinase trkB binds the neurotrophins brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4. However, most studies demonstrating that these neurotrophins prevent the death of embryonic neurons and have specific neuronal receptors have been performed with chick neurons. In order to explore the possibility that trkB is the molecular entity representing the high-affinity receptor for brain-derived neurotrophic factor on embryonic chick neurons, we cloned and expressed a chick trkB cDNA. In situ hybridisation results indicate that the distribution of trkB mRNA in the peripheral nervous system of the developing chick embryo correlates well with the structures known to respond to brain-derived neurotrophic factor. Binding studies performed with a cell line stably transfected with the ctrkB cDNA indicate a dissociation constant for brain-derived neurotrophic factor of 9.9 x 10(-10) M, which is distinctly higher than that found on primary chick sensory neurons (1.5 x 10(-11) M). When binding of brain-derived neurotrophic factor was determined in the presence of other neurotrophins, neurotrophin-3 was found efficiently to prevent the binding of brain-derived neurotrophic factor to both the ctrkB cell line and embryonic sensory neurons. In vitro, neurotrophin-3 at high concentrations completely blocked the survival normally seen with brain-derived neurotrophic factor. Thus, unlike previous cases of receptor occupancy by heterologous neurotrophins (which resulted in agonistic effects), the interaction between the brain-derived neurotrophic factor receptor and neurotrophin-3 on sensory neurons is antagonistic.

The use of radioactively labelled riboprobes for in situ hybridization: background and examples of application.

An overview is provided concerning the preparation and use of radiolabelled single-stranded RNA probes (riboprobes) for in situ hybridization. All theoretical aspects are covered as well as some practical examples from the authors' experience. The use of riboprobes for in situ hybridization represents a valid and highly sensitive alternative to oligonucleotide probes. A sample protocol is finally described in detail: this should enable any investigators to design the ISH assay that best fits their particular needs.