Impaired eIF5A function causes a Mendelian disorder that is partially rescued in model systems by spermidine

The structure of proline prevents it from adopting an optimal position for rapid protein synthesis. Poly-proline-tract (PPT) associated ribosomal stalling is resolved by highly conserved eIF5A, the only protein to contain the amino acid hypusine. We show that de novo heterozygous EIF5A variants cause a disorder characterized by variable combinations of developmental delay, microcephaly, micrognathia and dysmorphism. Yeast growth assays, polysome profiling, total/hypusinated eIF5A levels and PPT-reporters studies reveal that the variants impair eIF5A function, reduce eIF5A-ribosome interactions and impair the synthesis of PPT-containing proteins. Supplementation with 1 mM spermidine partially corrects the yeast growth defects, improves the polysome profiles and restores expression of PPT reporters. In zebrafish, knockdown eif5a partly recapitulates the human phenotype that can be rescued with 1 µM spermidine supplementation. In summary, we uncover the role of eIF5A in human development and disease, demonstrate the mechanistic complexity of EIF5A-related disorder and raise possibilities for its treatment.

Klf5 down-regulation induces vascular senescence through eIF5a depletion and mitochondrial fission

Although dysregulation of mitochondrial dynamics has been linked to cellular senescence, which contributes to advanced age-related disorders, it is unclear how Krüppel-like factor 5 (Klf5), an essential transcriptional factor of cardiovascular remodeling, mediates the link between mitochondrial dynamics and vascular smooth muscle cell (VSMC) senescence. Here, we show that Klf5 down-regulation in VSMCs is correlated with rupture of abdominal aortic aneurysm (AAA), an age-related vascular disease. Mice lacking Klf5 in VSMCs exacerbate vascular senescence and progression of angiotensin II (Ang II)-induced AAA by facilitating reactive oxygen species (ROS) formation. Klf5 knockdown enhances, while Klf5 overexpression suppresses mitochondrial fission. Mechanistically, Klf5 activates eukaryotic translation initiation factor 5a (eIF5a) transcription through binding to the promoter of eIF5a, which in turn preserves mitochondrial integrity by interacting with mitofusin 1 (Mfn1). Accordingly, decreased expression of eIF5a elicited by Klf5 down-regulation leads to mitochondrial fission and excessive ROS production. Inhibition of mitochondrial fission decreases ROS production and VSMC senescence. Our studies provide a potential therapeutic target for age-related vascular disorders.

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.

Expression of eukaryotic initiation factor 5A and hypusine forming enzymes in glioblastoma patient samples: implications for new targeted therapies

Glioblastomas are highly aggressive brain tumors of adults with poor clinical outcome. Despite a broad range of new and more specific treatment strategies, therapy of glioblastomas remains challenging and tumors relapse in all cases. Recent work demonstrated that the posttranslational hypusine modification of the eukaryotic initiation factor 5A (eIF-5A) is a crucial regulator of cell proliferation, differentiation and an important factor in tumor formation, progression and maintenance. Here we report that eIF-5A as well as the hypusine-forming enzymes deoxyhypusine synthase (DHS) and deoxyhypusine hydroxylase (DOHH) are highly overexpressed in glioblastoma patient samples. Importantly, targeting eIF-5A and its hypusine modification with GC7, a specific DHS-inhibitor, showed a strong antiproliferative effect in glioblastoma cell lines in vitro, while normal human astrocytes were not affected. Furthermore, we identified p53 dependent premature senescence, a permanent cell cycle arrest, as the primary outcome in U87-MG cells after treatment with GC7. Strikingly, combined treatment with clinically relevant alkylating agents and GC7 had an additive antiproliferative effect in glioblastoma cell lines. In addition, stable knockdown of eIF-5A and DHS by short hairpin RNA (shRNA) could mimic the antiproliferative effects of GC7. These findings suggest that pharmacological inhibition of eIF-5A may represent a novel concept to treat glioblastomas and may help to substantially improve the clinical course of this tumor entity.

One Argonaute family member, Eif2c2 (Ago2), is essential for development and appears not to be involved in DNA methylation

To elucidate the epigenetic role of RNAi in mammals, we disrupted the gene for Eif2c2 (Ago2), which works as the sole slicer of RNAi in the Argonaute family. In mice, disruption of Eif2c2 leads to embryonic lethality early in development after the implantation stage. This phenotype is completely different from that in a previous report, but somewhat similar to the disruption of Dicer1, another important component of RNAi. We also show that Eif2c2 is not required for the maintenance of DNA methylation in imprinted genes, centromeric repeats, and Xist. This suggests that developmental defects in the Eif2c2-deficient mouse are caused not at the transcriptional level, but rather at the posttranscriptional level through the miRNA-protein complex.

Essential role of NAT1/p97/DAP5 in embryonic differentiation and the retinoic acid pathway

NAT1/p97/DAP5 is a newly identified protein that shares homology with the translation initiation factor eIF4G. Studies in vitro and in transfected cells indicated that NAT1 might suppress global translation, thereby repressing cellular proliferation. Here we studied the functions of NAT1 in vivo by disrupting its gene in mice. NAT1(-/-) embryos died during gastrulation, indicating a crucial role for NAT1 in embryogenesis. Undifferentiated NAT1(-/-) embryonic stem cells were normal in morphology, proliferation, global translation and gene expression profile. However, NAT1(-/-) cells exhibited an impaired ability to differentiate: they were resistant to differentiation induced by retinoic acid, and teratomas derived from them consisted of undifferentiated and poorly differentiated tissues. The expression of retinoic acid-responsive genes, such as the cell-cycle inhibitor p21(WAF1), was selectively impaired in NAT1(-/-) cells. Transcription from synthetic retinoic acid-responsive elements was also impaired. These data demonstrated that this translation initiation factor homolog controls specific gene expression pathways required for cellular differentiation.

Mutations in translation initiation factors lead to increased rates of deadenylation and decapping of mRNAs in Saccharomyces cerevisiae

The turnover of most mRNAs in Saccharomyces cerevisiae begins with deadenylation followed by decapping and 5'-->3' exonucleolytic digestion. An important question involves the mechanisms that allow particular mRNAs to exhibit different rates of both deadenylation and decapping. Since the cap structure plays a critical role in the assembly of translation initiation factors, we hypothesized that the status of the cytoplasmic cap binding complex would affect the rate of decapping. To test this hypothesis, we examined mRNA decay rates in yeast strains that were defective in several translation initiation factors that are part of the cap binding complex. These experiments yielded three significant observations. First, any mutation known to inhibit translation initiation also increased the rate of decapping. Second, decapping still occurred only after deadenylation, suggesting that the ability of the poly(A) tail to inhibit decapping does not require efficient translation of the transcript. Third, mutants with defects in translation initiation factors also showed an increase in the rate of deadenylation, suggesting that the rate of deadenylation may be controlled primarily by the translation status of the transcript. These results argue that the nature of the translation initiation complex is a critical factor in determining the mRNA half-life. This view also implies that some cis-acting sequences that modulate mRNA decay rate do so by affecting the translation status of the transcript.

Translation of ODC mRNA and polyamine transport are suppressed in ras-transformed CREF cells by depleting translation initiation factor 4E

Rapid tumor growth and metastasis require increased polyamine metabolism, which is coordinately regulated by ornithine decarboxylase (ODC) and the polyamine transporter. Both activities are stimulated by ras signalling and are dependent upon protein biosynthesis. T24ras oncogene expression in rat embryo fibroblasts (CREFT24) induces cellular transformation and malignancy, in part, by stimulating the rate-limiting translation initiation factor, eIF-4E. CREFT24 expressing antisense RNA to eIF-4E (AS4E) have markedly decreased tumor growth rates and metastatic capacity, without altered monolayer growth rates. Herein, we demonstrate that in AS4E, ODC is translationally suppressed resulting in decreased ODC activity. Additionally, exogenous polyamine uptake is suppressed in AS4E cells indicating that AS4E can neither generate nor import the polyamines necessary to support rapid tumor growth. These data provide evidence that eIF-4E is the link between ras-induced malignancy and increased polyamine metabolism and support the hypothesis that eIF-4E plays a pivotal role in mediating ras-induced malignancy.