DAP5 promotes cap-independent translation of Bcl-2 and CDK1 to facilitate cell survival during mitosis

O-GlcNAcylation of TRIM29 and OGT translation forms a feedback loop to promote adaptive response of PDAC cells to glucose deficiency

PURPOSE

Glucose not only provides energy for tumor cells, but also provides various biomolecules that are essential for their survival, proliferation and invasion. Therefore, it is of great clinical significance to understand the mechanism of how tumor cells adapt to metabolic stress and maintain their survival. The aim of this research was to study the critical role of OGT and TRIM29 O-GlcNAc modification driven adaptability of PDAC cells to low glucose stress, which might have important medical implications for PDAC therapy.

METHODS

Western blotting, mass spectrometry and WGA-immunoprecipitation were used to examined the levels of OGT and O-GlcNAc glycosylated proteins in BxPC3 and SW1990 cells in normal culture and under glucose deprivation conditions. Crystal violet assay, flow cytometry, RIP, RT-qPCR, protein stability assay, biotin pull down were used to investigate the mechanism of OGT and TRIM29-mediated adaptive response to glucose deficiency in PDAC cells.

RESULTS

The current study found that under the condition of low glucose culture, the levels of OGT and O-GlcNAc glycosylation in PDAC cells were significantly higher than those in normal culture. Moreover, the high expression of OGT has a protective effect on PDAC cells under low glucose stress. This study confirmed that there was no significant change in mRNA level and protein degradation of OGT under low glucose stress, which was mainly reflected in the increase of protein synthesis. In addition, O-GlcNAc modification at T120 site plays a critical role in the metabolic adaptive responses mediated by TRIM29.

CONCLUSIONS

Taken together, our study indicated that O-GlcNAcylation of TRIM29 at T120 site and OGT translation forms a loop feedback to facilitate survival of PDAC under glucose deficiency.

Non-canonical mRNA translation initiation in cell stress and cancer

The now well described canonical mRNA translation initiation mechanism of m7G 'cap' recognition by cap-binding protein eIF4E and assembly of the canonical pre-initiation complex consisting of scaffolding protein eIF4G and RNA helicase eIF4A has historically been thought to describe all cellular mRNA translation. However, the past decade has seen the discovery of alternative mechanisms to canonical eIF4E mediated mRNA translation initiation. Studies have shown that non-canonical alternate mechanisms of cellular mRNA translation initiation, whether cap-dependent or independent, serve to provide selective translation of mRNAs under cell physiological and pathological stress conditions. These conditions typically involve the global downregulation of canonical eIF4E1/cap-mediated mRNA translation, and selective translational reprogramming of the cell proteome, as occurs in tumor development and malignant progression. Cancer cells must be able to maintain physiological plasticity to acquire a migratory phenotype, invade tissues, metastasize, survive and adapt to severe microenvironmental stress conditions that involve inhibition of canonical mRNA translation initiation. In this review we describe the emerging, important role of non-canonical, alternate mechanisms of mRNA translation initiation in cancer, particularly in adaptation to stresses and the phenotypic cell fate changes involved in malignant progression and metastasis. These alternate translation initiation mechanisms provide new targets for oncology therapeutics development.

Upstream open reading frames: new players in the landscape of cancer gene regulation

The translation of RNA by ribosomes represents a central biological process and one of the most dysregulated processes in cancer. While translation is traditionally thought to occur exclusively in the protein-coding regions of messenger RNAs (mRNAs), recent transcriptome-wide approaches have shown abundant ribosome activity across diverse stretches of RNA transcripts. The most common type of this kind of ribosome activity occurs in gene leader sequences, also known as 5' untranslated regions (UTRs) of the mRNA, that precede the main coding sequence. Translation of these upstream open reading frames (uORFs) is now known to occur in upwards of 25% of all protein-coding genes. With diverse functions from RNA regulation to microprotein generation, uORFs are rapidly igniting a new arena of cancer biology, where they are linked to cancer genetics, cancer signaling, and tumor-immune interactions. This review focuses on the contributions of uORFs and their associated 5'UTR sequences to cancer biology.

eIF4A controls translation of estrogen receptor alpha and is a therapeutic target in advanced breast cancer

The majority of human breast cancers are dependent on hormone-stimulated estrogen receptor alpha (ER) and are sensitive to its inhibition. Treatment resistance arises in most advanced cancers due to genetic alterations that promote ligand independent activation of ER itself or ER target genes. Whereas re-targeting of the ER ligand binding domain (LBD) with newer ER antagonists can work in some cases, these drugs are largely ineffective in many genetic backgrounds including ER fusions that lose the LBD or in cancers that hyperactivate ER targets. By identifying the mechanism of ER translation, we herein present an alternative strategy to target ER and difficult to treat ER variants. We find that ER translation is cap-independent and mTOR inhibitor insensitive, but dependent on 5' UTR elements and sensitive to pharmacologic inhibition of the translation initiation factor eIF4A, an mRNA helicase. EIF4A inhibition rapidly reduces expression of ER and short-lived targets of ER such as cyclin D1 and other components of the cyclin D-CDK complex in breast cancer cells. These effects translate into suppression of growth of a variety of ligand-independent breast cancer models including those driven by ER fusion proteins that lack the ligand binding site. The efficacy of eIF4A inhibition is enhanced when it is combined with fulvestrant-an ER degrader. Concomitant inhibition of ER synthesis and induction of its degradation causes synergistic and durable inhibition of ER expression and tumor growth. The clinical importance of these findings is confirmed by results of an early clinical trial (NCT04092673) of the selective eIF4A inhibitor zotatifin in patients with estrogen receptor positive metastatic breast cancer. Multiple clinical responses have been observed on combination therapy including durable regressions. These data suggest that eIF4A inhibition could be a useful new strategy for treating advanced ER+ breast cancer.

Multimodal stimulation screens reveal unique and shared genes limiting T cell fitness

Genes limiting T cell antitumor activity may serve as therapeutic targets. It has not been systematically studied whether there are regulators that uniquely or broadly contribute to T cell fitness. We perform genome-scale CRISPR-Cas9 knockout screens in primary CD8 T cells to uncover genes negatively impacting fitness upon three modes of stimulation: (1) intense, triggering activation-induced cell death (AICD); (2) acute, triggering expansion; (3) chronic, causing dysfunction. Besides established regulators, we uncover genes controlling T cell fitness either specifically or commonly upon differential stimulation. Dap5 ablation, ranking highly in all three screens, increases translation while enhancing tumor killing. Loss of Icam1-mediated homotypic T cell clustering amplifies cell expansion and effector functions after both acute and intense stimulation. Lastly, Ctbp1 inactivation induces functional T cell persistence exclusively upon chronic stimulation. Our results functionally annotate fitness regulators based on their unique or shared contribution to traits limiting T cell antitumor activity.

Loss of EIF4G2 mediates aggressiveness in distinct human endometrial cancer subpopulations with poor survival outcome in patients

The non-canonical translation initiation factor EIF4G2 plays essential roles in cellular stress responses via translation of selective mRNA cohorts. Currently there is limited and conflicting information regarding its involvement in cancer development and progression. Here we assessed its role in endometrial cancer (EC), in a cohort of 280 EC patients across different types, grades, and stages, and found that low EIF4G2 expression highly correlated with poor overall- and recurrence-free survival in Grade 2 EC patients, monitored over a period of up to 12 years. To establish a causative connection between low EIF4G2 expression and cancer progression, we stably knocked-down EIF4G2 in two human EC cell lines in parallel. EIF4G2 depletion resulted in increased resistance to conventional therapies and increased the prevalence of molecular markers for aggressive cell subsets, altering their transcriptional and proteomic landscapes. Prominent among the proteins with decreased abundance were Kinesin-1 motor proteins, KIF5B and KLC1, 2, 3. Multiplexed imaging of the EC patient tumor cohort showed a correlation between decreased expression of the kinesin proteins, and poor survival in patients with tumors of certain grades and stages. These findings reveal potential novel biomarkers for Grade 2 EC with ramifications for patient stratification and therapeutic interventions.

Loss-of-function cancer-linked mutations in the EIF4G2 non-canonical translation initiation factor

Tumor cells often exploit the protein translation machinery, resulting in enhanced protein expression essential for tumor growth. Since canonical translation initiation is often suppressed because of cell stress in the tumor microenvironment, non-canonical translation initiation mechanisms become particularly important for shaping the tumor proteome. EIF4G2 is a non-canonical translation initiation factor that mediates internal ribosome entry site (IRES)- and uORF-dependent initiation mechanisms, which can be used to modulate protein expression in cancer. Here, we explored the contribution of EIF4G2 to cancer by screening the COSMIC database for EIF4G2 somatic mutations in cancer patients. Functional examination of missense mutations revealed deleterious effects on EIF4G2 protein-protein interactions and, importantly, on its ability to mediate non-canonical translation initiation. Specifically, one mutation, R178Q, led to reductions in protein expression and near-complete loss of function. Two other mutations within the MIF4G domain specifically affected EIF4G2's ability to mediate IRES-dependent translation initiation but not that of target mRNAs with uORFs. These results shed light on both the structure-function of EIF4G2 and its potential tumor suppressor effects.

To initiate or not to initiate: A critical assessment of eIF2A, eIF2D, and MCT-1·DENR to deliver initiator tRNA to ribosomes

Selection of the correct start codon is critical for high-fidelity protein synthesis. In eukaryotes, this is typically governed by a multitude of initiation factors (eIFs), including eIF2·GTP that directly delivers the initiator tRNA (Met-tRNAi Met ) to the P site of the ribosome. However, numerous reports, some dating back to the early 1970s, have described other initiation factors having high affinity for the initiator tRNA and the ability of delivering it to the ribosome, which has provided a foundation for further work demonstrating non-canonical initiation mechanisms using alternative initiation factors. Here we provide a critical analysis of current understanding of eIF2A, eIF2D, and the MCT-1·DENR dimer, the evidence surrounding their ability to initiate translation, their implications in human disease, and lay out important key questions for the field. This article is categorized under: RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes Translation > Mechanisms Translation > Regulation.

Dissecting the roles of EIF4G homologs reveals DAP5 as a modifier of CGG repeat-associated toxicity in a Drosophila model of FXTAS

Neurodegeneration in Fragile X-associated tremor/ataxia syndrome (FXTAS) is caused by a CGG trinucleotide repeat expansion in the 5' UTR of FMR1. Expanded CGG repeat RNAs form stable secondary structures, which in turn support repeat-associated non-AUG (RAN) translation to produce toxic peptides. The parameters that impact RAN translation initiation efficiency are not well understood. Here we used a Drosophila melanogaster model of FXTAS to evaluate the role of the eIF4G family of eukaryotic translation initiation factors (EIF4G1, EIF4GII and EIF4G2/DAP5) in modulating RAN translation and CGG repeat-associated toxicity. DAP5 knockdown robustly suppressed CGG repeat-associated toxicity and inhibited RAN translation. Furthermore, knockdown of initiation factors that preferentially associate with DAP5 (such as EIF2β, EIF3F and EIF3G) also selectively suppressed CGG repeat-induced eye degeneration. In mammalian cellular reporter assays, DAP5 knockdown exhibited modest and cell-type specific effects on RAN translation. Taken together, these data support a role for DAP5 in CGG repeat associated toxicity possibly through modulation of RAN translation.

Breast cancer cell mesenchymal transition and metastasis directed by DAP5/eIF3d-mediated selective mRNA translation

Cancer cell plasticity enables cell survival in harsh physiological environments and fate transitions such as the epithelial-to-mesenchymal transition (EMT) that underlies invasion and metastasis. Using genome-wide transcriptomic and translatomic studies, an alternate mechanism of cap-dependent mRNA translation by the DAP5/eIF3d complex is shown to be essential for metastasis, EMT, and tumor directed angiogenesis. DAP5/eIF3d carries out selective translation of mRNAs encoding EMT transcription factors and regulators, cell migration integrins, metalloproteinases, and cell survival and angiogenesis factors. DAP5 is overexpressed in metastatic human breast cancers associated with poor metastasis-free survival. In human and murine breast cancer animal models, DAP5 is not required for primary tumor growth but is essential for EMT, cell migration, invasion, metastasis, angiogenesis, and resistance to anoikis. Thus, cancer cell mRNA translation involves two cap-dependent mRNA translation mechanisms, eIF4E/mTORC1 and DAP5/eIF3d. These findings highlight a surprising level of plasticity in mRNA translation during cancer progression and metastasis.

Specific mechanisms of translation initiation in higher eukaryotes: the eIF4G2 story

The eukaryotic initiation factor 4G2 (eIF4G2, DAP5, Nat1, p97) was discovered in 1997. Over the past two decades, dozens of papers have presented contradictory data on eIF4G2 function. Since its identification, eIF4G2 has been assumed to participate in noncanonical translation initiation mechanisms, but recent results indicate that it can be involved in scanning as well. In particular, eIF4G2 provides leaky scanning through some upstream open reading frames (uORFs), which are typical for long 5' UTRs of mRNAs from higher eukaryotes. It is likely the protein can also help the ribosome overcome other impediments during scanning of the 5' UTRs of animal mRNAs. This may explain the need for eIF4G2 in higher eukaryotes, as many mRNAs that encode regulatory proteins have rather long and highly structured 5' UTRs. Additionally, they often bind to various proteins, which also hamper the movement of scanning ribosomes. This review discusses the suggested mechanisms of eIF4G2 action, denotes obscure or inconsistent results, and proposes ways to uncover other fundamental mechanisms in which this important protein factor may be involved in higher eukaryotes.

Eukaryotic translation initiation factor eIF4G2 opens novel paths for protein synthesis in development, apoptosis and cell differentiation

Protein translation is a critical regulatory event involved in nearly all physiological and pathological processes. Eukaryotic translation initiation factors are dedicated to translation initiation, the most highly regulated stage of protein synthesis. Eukaryotic translation initiation factor 4G2 (eIF4G2, also called p97, NAT1 and DAP5), an eIF4G family member that lacks the binding sites for 5' cap binding protein eIF4E, is widely considered to be a key factor for internal ribosome entry sites (IRESs)-mediated cap-independent translation. However, recent findings demonstrate that eIF4G2 also supports many other translation initiation pathways. In this review, we summarize the role of eIF4G2 in a variety of cap-independent and -dependent translation initiation events. Additionally, we also update recent findings regarding the role of eIF4G2 in apoptosis, cell survival, cell differentiation and embryonic development. These studies reveal an emerging new picture of how eIF4G2 utilizes diverse translational mechanisms to regulate gene expression.

The HIPK2/CDC14B-MeCP2 axis enhances the spindle assembly checkpoint block by promoting cyclin B translation

Mitotic perturbations activate the spindle assembly checkpoint (SAC) that keeps cells in prometaphase with high CDK1 activity. Prolonged mitotic arrest is eventually bypassed by gradual cyclin B decline followed by slippage of cells into G1 without chromosome segregation, a process that promotes cell transformation and drug resistance. Hitherto, the cyclin B1 decay is exclusively defined by mechanisms that involve its proteasomal degradation. Here, we report that hyperphosphorylated HIPK2 kinase accumulates in mitotic cells and phosphorylates the Rett syndrome protein MeCP2 at Ser92, a regulation that is counteracted by CDC14B phosphatase. MeCP2S92 phosphorylation leads to the enhanced translation of cyclin B1, which is important for cells with persistent SAC activation to counteract the proteolytic decline of cyclin B1 and therefore to suspend mitotic slippage. Hence, the HIPK2/CDC14B-MeCP2 axis functions as an enhancer of the SAC-induced mitotic block. Collectively, our study revises the prevailing view of how cells confer a sustainable SAC.

LINC02870 facilitates SNAIL translation to promote hepatocellular carcinoma progression

Exploring the roles of long noncoding RNAs (lncRNAs) in tumorigenesis and metastasis could contribute to the recognition of novel diagnostic and therapeutic targets. LINC02870 is a novel lncRNA, whose role in tumors has not been reported. Herein, we focused on the function and mechanism of LINC02870 in human hepatocellular carcinoma (HCC). We first carried out a pan-cancer study of LINC02870 expression and its relationship to prognosis, and LINC02870 was determined to be a possible oncogene in HCC. Upregulated expressions of LINC02870 were also found in our HCC samples compared to the para-tumor samples. Moreover, overexpression of LINC02870 promoted the growth, migration, and invasion of HCC cells. Subsequently, binding proteins of LINC02870 were identified by a number of in silico analyses, including correlation analysis, signaling network analysis, and survival analysis. Intriguingly, the most promising binding protein of LINC02870 was predicted and confirmed to be eukaryotic translation initiation factor 4 gamma 1 (EIF4G1), an important component of the eukaryotic translation initiation factor 4F complex that initiates cap-dependent translation. Further investigation showed that LINC02870 increased the translation of SNAIL to induce malignant phenotypes in HCC cells. Additionally, HCC patients with higher expression levels of LINC02870 and EIF4G1 had shorter survival times than those with lower expression levels. Thus, our findings suggested that LINC02870 induced SNAIL translation and correlated with poor prognosis and tumor progression in HCC.

DAP5 enables main ORF translation on mRNAs with structured and uORF-containing 5' leaders

Half of mammalian transcripts contain short upstream open reading frames (uORFs) that potentially regulate translation of the downstream coding sequence (CDS). The molecular mechanisms governing these events remain poorly understood. Here, we find that the non-canonical initiation factor Death-associated protein 5 (DAP5 or eIF4G2) is required for translation initiation on select transcripts. Using ribosome profiling and luciferase-based reporters coupled with mutational analysis we show that DAP5-mediated translation occurs on messenger RNAs (mRNAs) with long, structure-prone 5' leader sequences and persistent uORF translation. These mRNAs preferentially code for signalling factors such as kinases and phosphatases. We also report that cap/eIF4F- and eIF4A-dependent recruitment of DAP5 to the mRNA facilitates main CDS, but not uORF, translation suggesting a role for DAP5 in translation re-initiation. Our study reveals important mechanistic insights into how a non-canonical translation initiation factor involved in stem cell fate shapes the synthesis of specific signalling factors.

DAP5 drives translation of specific mRNA targets with upstream ORFs in human embryonic stem cells

Death associated protein 5 (DAP5/eIF4G2/NAT1) is a member of the eIF4G translation initiation factors that has been shown to mediate noncanonical and/or cap-independent translation. It is essential for embryonic development and for differentiation of embryonic stem cells (ESCs), specifically its ability to drive translation of specific target mRNAs. In order to expand the repertoire of DAP5 target mRNAs, we compared ribosome profiles in control and DAP5 knockdown (KD) human ESCs (hESCs) to identify mRNAs with decreased ribosomal occupancy upon DAP5 silencing. A cohort of 68 genes showed decreased translation efficiency in DAP5 KD cells. Mass spectrometry confirmed decreased protein abundance of a significant portion of these targets. Among these was KMT2D, a histone methylase previously shown to be essential for ESC differentiation and embryonic development. We found that nearly half of the cohort of DAP5 target mRNAs displaying reduced translation efficiency of their main coding sequences upon DAP5 KD contained upstream open reading frames (uORFs) that are actively translated independently of DAP5. This is consistent with previously suggested mechanisms by which DAP5 mediates leaky scanning through uORFs and/or reinitiation at the main coding sequence. Crosslinking protein-RNA immunoprecipitation experiments indicated that a significant subset of DAP5 mRNA targets bound DAP5, indicating that direct binding between DAP5 protein and its target mRNAs is a frequent but not absolute requirement for DAP5-dependent translation of the main coding sequence. Thus, we have extended DAP5's function in translation of specific mRNAs in hESCs by a mechanism allowing translation of the main coding sequence following upstream translation of short ORFs.

The metaphorical swiss army knife: The multitude and diverse roles of HEAT domains in eukaryotic translation initiation

Biomolecular associations forged by specific interaction among structural scaffolds are fundamental to the control and regulation of cell processes. One such structural architecture, characterized by HEAT repeats, is involved in a multitude of cellular processes, including intracellular transport, signaling, and protein synthesis. Here, we review the multitude and versatility of HEAT domains in the regulation of mRNA translation initiation. Structural and cellular biology approaches, as well as several biophysical studies, have revealed that a number of HEAT domain-mediated interactions with a host of protein factors and RNAs coordinate translation initiation. We describe the basic structural architecture of HEAT domains and briefly introduce examples of the cellular processes they dictate, including nuclear transport by importin and RNA degradation. We then focus on proteins in the translation initiation system featuring HEAT domains, specifically the HEAT domains of eIF4G, DAP5, eIF5, and eIF2Bϵ. Comparative analysis of their remarkably versatile interactions, including protein–protein and protein–RNA recognition, reveal the functional importance of flexible regions within these HEAT domains. Here we outline how HEAT domains orchestrate fundamental aspects of translation initiation and highlight open mechanistic questions in the area.

Protein synthesis control in cancer: selectivity and therapeutic targeting

Translational control of mRNAs is a point of convergence for many oncogenic signals through which cancer cells tune protein expression in tumorigenesis. Cancer cells rely on translational control to appropriately adapt to limited resources while maintaining cell growth and survival, which creates a selective therapeutic window compared to non-transformed cells. In this review, we first discuss how cancer cells modulate the translational machinery to rapidly and selectively synthesize proteins in response to internal oncogenic demands and external factors in the tumor microenvironment. We highlight the clinical potential of compounds that target different translation factors as anti-cancer therapies. Next, we detail how RNA sequence and structural elements interface with the translational machinery and RNA-binding proteins to coordinate the translation of specific pro-survival and pro-growth programs. Finally, we provide an overview of the current and emerging technologies that can be used to illuminate the mechanisms of selective translational control in cancer cells as well as within the microenvironment.

Circular RNA-UBE2D2 accelerates the proliferation and metastasis of non-small cell lung cancer cells via modulating microRNA-376a-3p/Eukaryotic Translation Initiation Factor 4γ2 axis

Non-small cell lung cancer (NSCLC) ranks first in the morbidity and mortality of malignant tumors in China. As reported, circular RNAs (circRNAs) are emerged in the progress of NSCLC. The study was to figure out the potential mechanism of circ-UBE2D2 in the progression of NSCLC. First, plasmid vectors intervening circ-UBE2D2, microRNA (miR)-376a-3p or Eukaryotic Translation Initiation Factor 4γ2 (EIF4G2) expression were transfected into NSCLC cells, and the expression of circ-UBE2D2, miR-376a-3p and EIF4G2 was detected by reverse transcription quantitative polymerase chain reaction or Western blot. Then, cell proliferation was detected by Cell counting kit-8 assay and plate cloning. Cell apoptosis was tested by flow cytometry. Plate scratches and Transwell were used to detect cell migration and invasion. Finally, the binding sites of circRNA UBE2D2, EIF4G2 and miR-376a-3p were verified by bioinformatics website starBase analysis and dual luciferase reporter gene assay. The results manifested the up-regulation of circ-UBE2D2 expression in NSCLC tissues and cells. Circ-UBE2D2 promoted the proliferation, migration and invasion, but repressed apoptosis of NSCLC cells. Interestingly, circ-UBE2D2 directly targeted miR-376a-3p and up-regulated miR-376a-3p restrained proliferation, migration and invasion, but accelerated apoptosis of NSCLC cells. More importantly, EIF4G2 was the target of miR-376a-3p, and overexpression of EIF4G2 reversed the effects of circ-UBE2D2 downregulation on proliferation, migration, invasion and apoptosis of NSCLC cells. These results suggest that circ-UBE2D2 promotes the proliferation, migration and invasion but restrains apoptosis of lung cancer cells by regulating miR-376a-3p/EIF4G2 axis.

Ribosome profiling reveals novel regulation of C9ORF72 GGGGCC repeat-containing RNA translation

GGGGCC (G₄C₂) repeat expansion in the first intron of C9ORF72 causes amyotrophic lateral sclerosis and frontotemporal dementia. Repeat-containing RNA is translated into dipeptide repeat (DPR) proteins, some of which are neurotoxic. Using dynamic ribosome profiling, we identified three translation initiation sites in the intron upstream of (G₄C₂) repeats; these sites are detected irrespective of the presence or absence of the repeats. During translocation, ribosomes appear to be stalled on the repeats. An AUG in the preceding C9ORF72 exon initiates a uORF that inhibits downstream translation. Polysome isolation indicates that unspliced (G₄C₂) repeat-containing RNA is a substrate for DPR protein synthesis. (G₄C₂) repeat-containing RNA translation is 5' cap-independent but inhibited by the initiation factor DAP5, suggesting an interplay with uORF function. These results define novel translational mechanisms of expanded (G₄C₂) repeat-containing RNA in disease.

Ribosomal leaky scanning through a translated uORF requires eIF4G2

eIF4G2 (DAP5 or Nat1) is a homologue of the canonical translation initiation factor eIF4G1 in higher eukaryotes but its function remains poorly understood. Unlike eIF4G1, eIF4G2 does not interact with the cap-binding protein eIF4E and is believed to drive translation under stress when eIF4E activity is impaired. Here, we show that eIF4G2 operates under normal conditions as well and promotes scanning downstream of the eIF4G1-mediated 40S recruitment and cap-proximal scanning. Specifically, eIF4G2 facilitates leaky scanning for a subset of mRNAs. Apparently, eIF4G2 replaces eIF4G1 during scanning of 5′ UTR and the necessity for eIF4G2 only arises when eIF4G1 dissociates from the scanning complex. In particular, this event can occur when the leaky scanning complexes interfere with initiating or elongating 80S ribosomes within a translated uORF. This mechanism is therefore crucial for higher eukaryotes which are known to have long 5′ UTRs with highly frequent uORFs. We suggest that uORFs are not the only obstacle on the way of scanning complexes towards the main start codon, because certain eIF4G2 mRNA targets lack uORF(s). Thus, higher eukaryotes possess two distinct scanning complexes: the principal one that binds mRNA and initiates scanning, and the accessory one that rescues scanning when the former fails.

Structured elements drive extensive circular RNA translation

The human genome encodes tens of thousands circular RNAs (circRNAs) with mostly unknown functions. Circular RNAs require internal ribosome entry sites (IRES) if they are to undergo translation without a 5' cap. Here, we develop a high-throughput screen to systematically discover RNA sequences that can direct circRNA translation in human cells. We identify more than 17,000 endogenous and synthetic sequences as candidate circRNA IRES. 18S rRNA complementarity and a structured RNA element positioned on the IRES are important for driving circRNA translation. Ribosome profiling and peptidomic analyses show extensive IRES-ribosome association, hundreds of circRNA-encoded proteins with tissue-specific distribution, and antigen presentation. We find that circFGFR1p, a protein encoded by circFGFR1 that is downregulated in cancer, functions as a negative regulator of FGFR1 oncoprotein to suppress cell growth during stress. Systematic identification of circRNA IRES elements may provide important links among circRNA regulation, biological function, and disease.

ERα is an RNA-binding protein sustaining tumor cell survival and drug resistance

Estrogen receptor α (ERα) is a hormone receptor and key driver for over 70% of breast cancers that has been studied for decades as a transcription factor. Unexpectedly, we discover that ERα is a potent non-canonical RNA-binding protein. We show that ERα RNA binding function is uncoupled from its activity to bind DNA and critical for breast cancer progression. Employing genome-wide cross-linking immunoprecipitation (CLIP) sequencing and a functional CRISPRi screen, we find that ERα-associated mRNAs sustain cancer cell fitness and elicit cellular responses to stress. Mechanistically, ERα controls different steps of RNA metabolism. In particular, we demonstrate that ERα RNA binding mediates alternative splicing of XBP1 and translation of the eIF4G2 and MCL1 mRNAs, which facilitates survival upon stress conditions and sustains tamoxifen resistance of cancer cells. ERα is therefore a multifaceted RNA-binding protein, and this activity transforms our knowledge of post-transcriptional regulation underlying cancer development and drug response.

Eukaryotic initiation factor 4 gamma 2 contributes to neuropathic pain through down-regulation of Kv1.2 and the mu opioid receptor in mouse primary sensory neurones

BACKGROUND

Nerve injury-induced changes in gene expression in the dorsal root ganglion (DRG) contribute to neuropathic pain genesis. Eukaryotic initiation factor 4 gamma 2 (eIF4G2) is a general repressor of cap-dependent mRNA translation. Whether DRG eIF4G2 participates in nerve injury-induced alternations in gene expression and nociceptive hypersensitivity is unknown.

METHODS

The expression and distribution of eIF4G2 mRNA and protein in mouse DRG after spinal nerve ligation (SNL) were assessed. Effects of eIF4G2 siRNA microinjected through a glass micropipette into the injured DRG on the SNL-induced DRG mu opioid receptor (MOR) and Kv1.2 downregulation and nociceptive hypersensitivity were examined. In addition, effects of DRG microinjection of adeno-associated virus 5-expressing eIF4G2 (AAV5-eIF4G2) on basal DRG MOR and Kv1.2 expression and nociceptive thresholds were analysed.

RESULTS

eIF4G2 protein co-expressed with Kv1.2 and MOR in DRG neurones. Levels of eIF4G2 mRNA (1.7 [0.24] to 2.3 [0.14]-fold of sham, P<0.01) and protein (1.6 [0.14] to 2.5 [0.22]-fold of sham, P<0.01) in injured DRG were time-dependently increased on days 3-14 after SNL. Blocking increased eIF4G2 through microinjection of eIF4G2 siRNA into the injured DRG attenuated SNL-induced downregulation of DRG MOR and Kv1.2 and development and maintenance of nociceptive hypersensitivities. DRG microinjection of AAV5-eIF4G2 reduced DRG MOR and Kv1.2 expression and elicited hypersensitivities to mechanical, heat and cold stimuli in naïve mice.

CONCLUSIONS

eIF4G2 contributes to neuropathic pain through participation in downregulation of Kv1.2 and MOR in injured DRG and is a potential target for treatment of this disorder.

MiR-144-3p-mediated dysregulation of EIF4G2 contributes to the development of hepatocellular carcinoma through the ERK pathway

Background

Hepatocellular carcinoma (HCC) is one of the most common cancers with high incidence and mortality. However, the underlying mechanisms of HCC still remain unclear. Eukaryotic translation initiation factors (eIFs) have a substantial effect on tumor development. In this study, we were aimed to investigate the role of eukaryotic translation initiation factor 4 gamma 2 (EIF4G2) in HCC.

Methods

Western blot (WB) of 30 paired HCC tissues and tissue microarrays (TMAs) conducted by immunohistochemistry (IHC) in 89 paired HCC samples were performed to assess EIF4G2 expression. Clone formation, real-time cell analysis (RTCA), wound healing and transwell assays were adopted to evaluate the role of EIF4G2 on HCC cell proliferation, migration and invasion abilities. The function of EIF4G2 in HCC tumor growth was assessed in a xenograft nude mouse model in vivo. The regulation of EIF4G2 by miR-144-3p was performed by luciferase reporter assay and WB.

Results

The EIF4G2 protein was clearly upregulated in HCC tissues, and high EIF4G2 expression was closely related to HCC prognosis. EIF4G2 silencing could inhibit HCC cell growth and metastasis in vitro, and suppress tumorigenesis in vivo by repressing the ERK signaling pathway. The results of luciferase reporter assays, WB and IHC staining verified that EIF4G2 was negatively regulated by miR-144. And re-expression of EIF4G2 could partially reverse the inhibiting effect of miR-144 in HCC.

Conclusion

In summary, our study revealed the role of EIF4G2 in HCC development via the activation of the ERK pathway. We also found that EIF4G2 could be negatively regulated by the tumor suppressor miR-144. Our investigations indicated that EIF4G2 might be a promising therapeutic target in HCC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-021-01853-6.

Maternal immune activation in mice disrupts proteostasis in the fetal brain

Maternal infection and inflammation during pregnancy are associated with neurodevelopmental disorders in offspring, but little is understood about the molecular mechanisms underlying this epidemiologic phenomenon. Here, we leveraged single-cell RNA sequencing to profile transcriptional changes in the mouse fetal brain in response to maternal immune activation (MIA) and identified perturbations in cellular pathways associated with mRNA translation, ribosome biogenesis and stress signaling. We found that MIA activates the integrated stress response (ISR) in male, but not female, MIA offspring in an interleukin-17a-dependent manner, which reduced global mRNA translation and altered nascent proteome synthesis. Moreover, blockade of ISR activation prevented the behavioral abnormalities as well as increased cortical neural activity in MIA male offspring. Our data suggest that sex-specific activation of the ISR leads to maternal inflammation-associated neurodevelopmental disorders.

A cyclin-dependent kinase, CDK11/p58, represses cap-dependent translation during mitosis

During mitosis, translation of most mRNAs is strongly repressed; none of the several explanatory hypotheses suggested can fully explain the molecular basis of this phenomenon. Here we report that cyclin-dependent CDK11/p58-a serine/threonine kinase abundantly expressed during M phase-represses overall translation by phosphorylating a subunit (eIF3F) of the translation factor eIF3 complex that is essential for translation initiation of most mRNAs. Ectopic expression of CDK11/p58 strongly repressed cap-dependent translation, and knockdown of CDK11/p58 nullified the translational repression during M phase. We identified the phosphorylation sites in eIF3F responsible for M phase-specific translational repression by CDK11/p58. Alanine substitutions of CDK11/p58 target sites in eIF3F nullified its effects on cell cycle-dependent translational regulation. The mechanism of translational regulation by the M phase-specific kinase, CDK11/p58, has deep evolutionary roots considering the conservation of CDK11 and its target sites on eIF3F from C. elegans to humans.

Deciphering the organelle genomes and transcriptomes of a common ornamental plant Ligustrum quihoui reveals multiple fragments of transposable elements in the mitogenome

Ligustrum quihoui (L. quihoui) is an important hedge material for landscaping and also possesses medicinal value. To generate genomic resources for better understanding the evolutionary history of this important plant, the organelle genomes of L. quihoui are de novo assembled and functionally annotated. Compared with other Oleaceae species, the 163,069 bp chloroplast genome of L. quihoui exhibits a typical quadripartite structure with highly conserved gene content and gene order, while the 848,451 bp mitochondrial genome of L. quihoui exhibits highly divergent genome size and gene content. Codon usage analyses show that genes related with photosynthesis and mitochondrial respiratory chain show inconsistent codon biases. A total of 48,760 bp transposable elements (TEs) fragments and 41,887 bp chloroplast-like sequences are found in the L. quihoui mitochondrial genome. A striking discrepancy of RNA editing between the two organelle genomes is found in L. quihoui, in which 146 mitochondrial editing sites coexist with only 43 such sites in chloroplast. Based on DNA and RNA-Seq data, we propose that GTG may act as the start codon of mitochondrial rpl16 in Oleaceae species. Phylogenetic analysis based on chloroplast genome shows that L. quihoui and L. japonicum form a sister clade within the genus Ligustrum.

Internal Ribosome Entry Sites Mediate Cap-Independent Translation of Bmi1 in Nasopharyngeal Carcinoma

Bmi1 is overexpressed in multiple human cancers. We previously reported the oncogenic function and the transcription regulation mechanisms of Bmi1 in nasopharyngeal carcinoma (NPC). In this study, we observed that the mRNA and the protein levels of Bmi1 were strictly inconsistent in NPC cell lines and cancer tissues. The inhibitors of proteasome and lysosome could not enhance the protein level of Bmi1, indicating that Bmi1 may be post-transcriptionally regulated. The IRESite analysis showed that there were two potential internal ribosome entry sites (IRESs) in the 5'-untranslated region (5'-UTR) of Bmi1. The luciferase assay demonstrated that the 5'-UTR of Bmi1 has IRES activity, which may mediate cap-independent translation. The IRES activity of the Bmi1 5'-UTR was significantly reduced after the mutation of the two IRES elements. Taken together, these results suggested that the IRES elements mediating translation is a novel post-transcriptional regulation mechanism of Bmi1.

Translation Regulation by eIF2α Phosphorylation and mTORC1 Signaling Pathways in Non-Communicable Diseases (NCDs)

Non-communicable diseases (NCDs) are medical conditions that, by definition, are non-infectious and non-transmissible among people. Much of current NCDs are generally due to genetic, behavioral, and metabolic risk factors that often include excessive alcohol consumption, smoking, obesity, and untreated elevated blood pressure, and share many common signal transduction pathways. Alterations in cell and physiological signaling and transcriptional control pathways have been well studied in several human NCDs, but these same pathways also regulate expression and function of the protein synthetic machinery and mRNA translation which have been less well investigated. Alterations in expression of specific translation factors, and disruption of canonical mRNA translational regulation, both contribute to the pathology of many NCDs. The two most common pathological alterations that contribute to NCDs discussed in this review will be the regulation of eukaryotic initiation factor 2 (eIF2) by the integrated stress response (ISR) and the mammalian target of rapamycin complex 1 (mTORC1) pathways. Both pathways integrally connect mRNA translation activity to external and internal physiological stimuli. Here, we review the role of ISR control of eIF2 activity and mTORC1 control of cap-mediated mRNA translation in some common NCDs, including Alzheimer’s disease, Parkinson’s disease, stroke, diabetes mellitus, liver cirrhosis, chronic obstructive pulmonary disease (COPD), and cardiac diseases. Our goal is to provide insights that further the understanding as to the important role of translational regulation in the pathogenesis of these diseases.

The diversity, plasticity, and adaptability of cap-dependent translation initiation and the associated machinery

Translation initiation is a critical facet of gene expression with important impacts that underlie cellular responses to stresses and environmental cues. Its dysregulation in many diseases position this process as an important area for the development of new therapeutics. The gateway translation factor eIF4E is typically considered responsible for ‘global’ or ‘canonical’ m⁷G cap-dependent translation. However, eIF4E impacts translation of specific transcripts rather than the entire translatome. There are many alternative cap-dependent translation mechanisms that also contribute to the translation capacity of the cell. We review the diversity of these, juxtaposing more recently identified mechanisms with eIF4E-dependent modalities. We also explore the multiplicity of functions played by translation factors, both within and outside protein synthesis, and discuss how these differentially contribute to their ultimate physiological impacts. For comparison, we discuss some modalities for cap-independent translation. In all, this review highlights the diverse mechanisms that engage and control translation in eukaryotes.

Critical Roles of Translation Initiation and RNA Uridylation in Endogenous Retroviral Expression and Neural Differentiation in Pluripotent Stem Cells

Previous studies have suggested that the loss of the translation initiation factor eIF4G1 homolog NAT1 induces excessive self-renewability of naive pluripotent stem cells (PSCs); yet the role of NAT1 in the self-renewal and differentiation of primed PSCs is still unclear. Here, we generate a conditional knockout of NAT1 in primed PSCs and use the cells for the functional analyses of NAT1. Our results show that NAT1 is required for the self-renewal and neural differentiation of primed PSCs. In contrast, NAT1 deficiency in naive pluripotency attenuates the differentiation to all cell types. We also find that NAT1 is involved in efficient protein expression of an RNA uridyltransferase, TUT7. TUT7 is involved in the neural differentiation of primed PSCs via the regulation of human endogenous retrovirus accumulation. These data demonstrate the essential roles of NAT1 and TUT7 in the precise transition of stem cell fate.

Ribosome and Translational Control in Stem Cells

Embryonic stem cells (ESCs) and adult stem cells (ASCs) possess the remarkable capacity to self-renew while remaining poised to differentiate into multiple progenies in the context of a rapidly developing embryo or in steady-state tissues, respectively. This ability is controlled by complex genetic programs, which are dynamically orchestrated at different steps of gene expression, including chromatin remodeling, mRNA transcription, processing, and stability. In addition to maintaining stem cell homeostasis, these molecular processes need to be rapidly rewired to coordinate complex physiological modifications required to redirect cell fate in response to environmental clues, such as differentiation signals or tissue injuries. Although chromatin remodeling and mRNA expression have been extensively studied in stem cells, accumulating evidence suggests that stem cell transcriptomes and proteomes are poorly correlated and that stem cell properties require finely tuned protein synthesis. In addition, many studies have shown that the biogenesis of the translation machinery, the ribosome, is decisive for sustaining ESC and ASC properties. Therefore, these observations emphasize the importance of translational control in stem cell homeostasis and fate decisions. In this review, we will provide the most recent literature describing how ribosome biogenesis and translational control regulate stem cell functions and are crucial for accommodating proteome remodeling in response to changes in stem cell fate.

Hallmarks and Determinants of Oncogenic Translation Revealed by Ribosome Profiling in Models of Breast Cancer

Gene expression is extensively and dynamically modulated at the level of translation. How cancer cells prioritize the translation of certain mRNAs over others from a pool of competing mRNAs remains an open question. Here, we analyze translation in cell line models of breast cancer and normal mammary tissue by ribosome profiling. We identify key recurrent themes of oncogenic translation: higher ribosome occupancy, greater variance of translational efficiencies, and preferential translation of transcriptional regulators and signaling proteins in malignant cells as compared with their nonmalignant counterpart. We survey for candidate RNA interacting proteins that could associate with the 5′untranslated regions of the transcripts preferentially translated in breast tumour cells. We identify SRSF1, a prototypic splicing factor, to have a pervasive direct and indirect impact on translation. In a representative estrogen receptor–positive and estrogen receptor–negative cell line, we find that protein synthesis relies heavily on SRSF1. SRSF1 is predominantly intranuclear. Under certain conditions, SRSF1 translocates from the nucleus to the cytoplasm where it associates with MYC and CDK1 mRNAs and upregulates their internal ribosome entry site–mediated translation. Our results point to a synergy between splicing and translation and unveil how certain RNA-binding proteins modulate the translational landscape in breast cancer.

PTBP1 mRNA isoforms and regulation of their translation

Polypyrimidine tract-binding proteins (PTBPs) are RNA binding proteins that regulate a number of posttranscriptional events. Human PTBP1 transits between the nucleus and cytoplasm and is thought to regulate RNA processes in both. However, information about PTBP1 mRNA isoforms and regulation of PTPB1 expression remains incomplete. Here we mapped the major PTBP1 mRNA isoforms in HEK293T cells and identified alternative 5' and 3' untranslated regions (5'-UTRs, 3'-UTRs), as well as alternative splicing patterns in the protein coding region. We also assessed how the observed PTBP1 mRNA isoforms contribute to PTBP1 expression in different phases of the cell cycle. Previously, PTBP1 mRNAs were shown to crosslink to eukaryotic translation initiation factor 3 (eIF3). We find that eIF3 binds differently to each PTBP1 mRNA isoform in a cell cycle dependent manner. We also observe a strong correlation between eIF3 binding to PTBP1 mRNAs and repression of PTBP1 levels during the S phase of the cell cycle. Our results provide evidence of translational regulation of PTBP1 protein levels during the cell cycle, which may affect downstream regulation of alternative splicing and translation mediated by PTBP1 protein isoforms.

The CUL5 ubiquitin ligase complex mediates resistance to CDK9 and MCL1 inhibitors in lung cancer cells

Overexpression of anti-apoptotic proteins MCL1 and Bcl-xL are frequently observed in many cancers. Inhibitors targeting MCL1 are in clinical development, however numerous cancer models are intrinsically resistant to this approach. To discover mechanisms underlying resistance to MCL1 inhibition, we performed multiple flow-cytometry based genome-wide CRISPR screens interrogating two drugs that directly (MCL1i) or indirectly (CDK9i) target MCL1. Remarkably, both screens identified three components (CUL5, RNF7 and UBE2F) of a cullin-RING ubiquitin ligase complex (CRL5) that resensitized cells to MCL1 inhibition. We find that levels of the BH3-only pro-apoptotic proteins Bim and Noxa are proteasomally regulated by the CRL5 complex. Accumulation of Noxa caused by depletion of CRL5 components was responsible for re-sensitization to CDK9 inhibitor, but not MCL1 inhibitor. Discovery of a novel role of CRL5 in apoptosis and resistance to multiple types of anticancer agents suggests the potential to improve combination treatments.

eIF4G2 balances its own mRNA translation via a PCBP2-based feedback loop

Poly(rC)-binding protein 2 (PCBP2, hnRNP E2) is one of the most abundant RNA-binding proteins in mammalian cells. In humans, it exists in seven isoforms, which are assumed to play similar roles in cells. The protein is shown to bind 3'-untranslated regions (3'-UTRs) of many mRNAs and regulate their translation and/or stability, but nothing is known about the functional consequences of PCBP2 binding to 5'-UTRs. Here we show that the PCBP2 isoform f interacts with the 5'-UTRs of mRNAs encoding eIF4G2 (a translation initiation factor with a yet unknown mechanism of action, also known as DAP5) and Cyclin I, and inhibits their translation in vitro and in cultured cells, while the PCBP2 isoform e only affects Cyclin I translation. Furthermore, eIF4G2 participates in a cap-dependent translation of the PCBP2 mRNA. Thus, PCBP2 and eIF4G2 seem to regulate one another's expression via a novel type of feedback loop formed by the translation initiation factor and the RNA-binding protein.

Mammalian cell growth dynamics in mitosis

The extent and dynamics of animal cell biomass accumulation during mitosis are unknown, primarily because growth has not been quantified with sufficient precision and temporal resolution. Using the suspended microchannel resonator and protein synthesis assays, we quantify mass accumulation and translation rates between mitotic stages on a single-cell level. For various animal cell types, growth rates in prophase are commensurate with or higher than interphase growth rates. Growth is only stopped as cells approach metaphase-to-anaphase transition and growth resumes in late cytokinesis. Mitotic arrests stop growth independently of arresting mechanism. For mouse lymphoblast cells, growth in prophase is promoted by CDK1 through increased phosphorylation of 4E-BP1 and cap-dependent protein synthesis. Inhibition of CDK1-driven mitotic translation reduces daughter cell growth. Overall, our measurements counter the traditional dogma that growth during mitosis is negligible and provide insight into antimitotic cancer chemotherapies.

Noncanonical Translation Initiation in Eukaryotes

The vast majority of eukaryotic messenger RNAs (mRNAs) initiate translation through a canonical, cap-dependent mechanism requiring a free 5' end and 5' cap and several initiation factors to form a translationally active ribosome. Stresses such as hypoxia, apoptosis, starvation, and viral infection down-regulate cap-dependent translation during which alternative mechanisms of translation initiation prevail to express proteins required to cope with the stress, or to produce viral proteins. The diversity of noncanonical initiation mechanisms encompasses a broad range of strategies and cellular cofactors. Herein, we provide an overview and, whenever possible, a mechanistic understanding of the various noncanonical mechanisms of initiation used by cells and viruses. Despite many unanswered questions, recent advances have propelled our understanding of the scope, diversity, and mechanisms of alternative initiation.

IRES Trans-Acting Factors, Key Actors of the Stress Response

The cellular stress response corresponds to the molecular changes that a cell undergoes in response to various environmental stimuli. It induces drastic changes in the regulation of gene expression at transcriptional and posttranscriptional levels. Actually, translation is strongly affected with a blockade of the classical cap-dependent mechanism, whereas alternative mechanisms are activated to support the translation of specific mRNAs. A major mechanism involved in stress-activated translation is the internal ribosome entry site (IRES)-driven initiation. IRESs, first discovered in viral mRNAs, are present in cellular mRNAs coding for master regulators of cell responses, whose expression must be tightly controlled. IRESs allow the translation of these mRNAs in response to different stresses, including DNA damage, amino-acid starvation, hypoxia or endoplasmic reticulum stress, as well as to physiological stimuli such as cell differentiation or synapse network formation. Most IRESs are regulated by IRES trans-acting factor (ITAFs), exerting their action by at least nine different mechanisms. This review presents the history of viral and cellular IRES discovery as well as an update of the reported ITAFs regulating cellular mRNA translation and of their different mechanisms of action. The impact of ITAFs on the coordinated expression of mRNA families and consequences in cell physiology and diseases are also highlighted.

The ribosomal RPL10 R98S mutation drives IRES-dependent BCL-2 translation in T-ALL

The R98S mutation in ribosomal protein L10 (RPL10 R98S) affects 8% of pediatric T-cell acute lymphoblastic leukemia (T-ALL) cases, and was previously described to impair cellular proliferation. The current study reveals that RPL10 R98S cells accumulate reactive oxygen species which promotes mitochondrial dysfunction and reduced ATP levels, causing the proliferation defect. RPL10 R98S mutant leukemia cells can survive high oxidative stress levels via a specific increase of IRES-mediated translation of the anti-apoptotic factor B-cell lymphoma 2 (BCL-2), mediating BCL-2 protein overexpression. RPL10 R98S selective sensitivity to the clinically available Bcl-2 inhibitor Venetoclax (ABT-199) was supported by suppression of splenomegaly and the absence of human leukemia cells in the blood of T-ALL xenografted mice. These results shed new light on the oncogenic function of ribosomal mutations in cancer, provide a novel mechanism for BCL-2 upregulation in leukemia, and highlight BCL-2 inhibition as a novel therapeutic opportunity in RPL10 R98S defective T-ALL.

Translational Control of Canonical and Non-Canonical Translation Initiation Factors at the Sea Urchin Egg to Embryo Transition

Sea urchin early development is a powerful model to study translational regulation under physiological conditions. Fertilization triggers an activation of the translation machinery responsible for the increase of protein synthesis necessary for the completion of the first embryonic cell cycles. The cap-binding protein eIF4E, the helicase eIF4A and the large scaffolding protein eIF4G are assembled upon fertilization to form an initiation complex on mRNAs involved in cap-dependent translation initiation. The presence of these proteins in unfertilized and fertilized eggs has already been demonstrated, however data concerning the translational status of translation factors are still scarce. Using polysome fractionation, we analyzed the impact of fertilization on the recruitment of mRNAs encoding initiation factors. Strikingly, whereas the mRNAs coding eIF4E, eIF4A, and eIF4G were not recruited into polysomes at 1 h post-fertilization, mRNAs for eIF4B and for non-canonical initiation factors such as DAP5, eIF4E2, eIF4E3, or hnRNP Q, are recruited and are differentially sensitive to the activation state of the mechanistic target of rapamycin (mTOR) pathway. We discuss our results suggesting alternative translation initiation in the context of the early development of sea urchins.

A widespread alternate form of cap-dependent mRNA translation initiation

Translation initiation of most mammalian mRNAs is mediated by a 5′ cap structure that binds eukaryotic initiation factor 4E (eIF4E). However, inactivation of eIF4E does not impair translation of many capped mRNAs, suggesting an unknown alternate mechanism may exist for cap-dependent but eIF4E-independent translation. We show that DAP5, an eIF4GI homolog that lacks eIF4E binding, utilizes eIF3d to facilitate cap-dependent translation of approximately 20% of mRNAs. Genome-wide transcriptomic and translatomic analyses indicate that DAP5 is required for translation of many transcription factors and receptor capped mRNAs and their mRNA targets involved in cell survival, motility, DNA repair and translation initiation, among other mRNAs. Mass spectrometry and crosslinking studies demonstrate that eIF3d is a direct binding partner of DAP5. In vitro translation and ribosome complex studies demonstrate that DAP5 and eIF3d are both essential for eIF4E-independent capped-mRNA translation. These studies disclose a widespread and previously unknown mechanism for cap-dependent mRNA translation by DAP5-eIF3d complexes.

Binding of eIF4E to the 5′ cap of mRNAs is a key early step in canonical translation initiation, but the requirement for eIF4E is not universal. Here the authors show that the eIF4G homolog DAP5 interacts with eIF3 to promote cap-dependent translation of a significant number of mRNA in an eIF4E-independent manner.

Two distinct nodes of translational inhibition in the Integrated Stress Response

The Integrated Stress Response (ISR) refers to a signaling pathway initiated by stress-activated eIF2α kinases. Once activated, the pathway causes attenuation of global mRNA translation while also paradoxically inducing stress response gene expression. A detailed analysis of this pathway has helped us better understand how stressed cells coordinate gene expression at translational and transcriptional levels. The translational attenuation associated with this pathway has been largely attributed to the phosphorylation of the translational initiation factor eIF2α. However, independent studies are now pointing to a second translational regulation step involving a downstream ISR target, 4E-BP, in the inhibition of eIF4E and specifically cap-dependent translation. The activation of 4E-BP is consistent with previous reports implicating the roles of 4E-BP resistant, Internal Ribosome Entry Site (IRES) dependent translation in ISR active cells. In this review, we provide an overview of the translation inhibition mechanisms engaged by the ISR and how they impact the translation of stress response genes.

Cap-Independent Translation: What's in a Name?

Eukaryotic translation initiation relies on the m⁷G cap present at the 5' end of all mRNAs. Some viral mRNAs employ alternative mechanisms of initiation based on internal ribosome entry. The 'IRES ideology' was adopted by researchers to explain the differential translation of cellular mRNAs when the cap recognition is suppressed. However, some cellular IRESs have already been challenged and others are awaiting their validation. As an alternative cap-independent mechanism, we propose adopting the concept of cap-independent translation enhancers (CITEs) for mammalian mRNAs. Unlike IRESs, CITEs can be located both within 5' and 3' UTRs and bind mRNA-recruiting translational components. The respective 5' UTRs are then inspected by the scanning machinery essentially in the same way as under cap-dependent translation.

Translatome analysis at the egg-to-embryo transition in sea urchin

Early embryogenesis relies on the translational regulation of maternally stored mRNAs. In sea urchin, fertilization triggers a dramatic rise in translation activity, necessary for the onset of cell division. Here, the full spectrum of the mRNAs translated upon fertilization was investigated by polysome profiling and sequencing. The translatome of the early sea urchin embryo gave a complete picture of the polysomal recruitment dynamics following fertilization. Our results indicate that only a subset of maternal mRNAs were selectively recruited onto polysomes, with over-represented functional categories in the translated set. The increase in translation upon fertilization depends on the formation of translation initiation complexes following mTOR pathway activation. Surprisingly, mTOR pathway inhibition differentially affected polysomal recruitment of the newly translated mRNAs, which thus appeared either mTOR-dependent or mTOR-independent. Therefore, our data argue for an alternative to the classical cap-dependent model of translation in early development. The identification of the mRNAs translated following fertilization helped assign translational activation events to specific mRNAs. This translatome is the first step to a comprehensive analysis of the molecular mechanisms governing translation upon fertilization and the translational regulatory networks that control the egg-to-embryo transition as well as the early steps of embryogenesis.

Regulation of Hypoxia-Inducible Factor 1α during Hypoxia by DAP5-Induced Translation of PHD2

Death-associated protein 5 (DAP5) is an atypical isoform of the translation initiation scaffolds eukaryotic initiation factor 4GI (eIF4GI) and eIF4GII (eIF4GI/II), which recruit mRNAs to ribosomes in mammals. Unlike eIF4GI/II, DAP5 binds eIF2β, a subunit of the eIF2 complex that delivers methionyl-tRNA to ribosomes. We discovered that DAP5:eIF2β binding depends on specific stimuli, e.g., protein kinase C (PKC)-Raf-extracellular signal-regulated kinase 1/2 (ERK1/2) signals, and determines DAP5's influence on global and template-specific translation. DAP5 depletion caused an unanticipated surge of hypoxia-inducible factor 1α (HIF-1α), the transcription factor and master switch of the hypoxia response. Physiologically, the hypoxia response is tempered through HIF-1α hydroxylation by the oxygen-sensing prolyl hydroxylase-domain protein 2 (PHD2) and subsequent ubiquitination and degradation. We found that DAP5 regulates HIF-1α abundance through DAP5:eIF2β-dependent translation of PHD2. DAP5:eIF2-induced PHD2 translation occurred during hypoxia-associated protein synthesis repression, indicating a role as a safeguard to reverse HIF-1α accumulation and curb the hypoxic response.

Co-translational protein targeting facilitates centrosomal recruitment of PCNT during centrosome maturation in vertebrates

As microtubule-organizing centers of animal cells, centrosomes guide the formation of the bipolar spindle that segregates chromosomes during mitosis. At mitosis onset, centrosomes maximize microtubule-organizing activity by rapidly expanding the pericentriolar material (PCM). This process is in part driven by the large PCM protein pericentrin (PCNT), as its level increases at the PCM and helps recruit additional PCM components. However, the mechanism underlying the timely centrosomal enrichment of PCNT remains unclear. Here, we show that PCNT is delivered co-translationally to centrosomes during early mitosis by cytoplasmic dynein, as evidenced by centrosomal enrichment of PCNT mRNA, its translation near centrosomes, and requirement of intact polysomes for PCNT mRNA localization. Additionally, the microtubule minus-end regulator, ASPM, is also targeted co-translationally to mitotic spindle poles. Together, these findings suggest that co-translational targeting of cytoplasmic proteins to specific subcellular destinations may be a generalized protein targeting mechanism.

Before a cell divides, it creates a copy of its genetic material (DNA) and evenly distributes it between the new ‘daughter’ cells with the help of a complex called the mitotic spindle. This complex is made of long cable-like protein chains called microtubules.

To ensure that each daughter cell receives an equal amount of DNA, structures known as centrosomes organize the microtubules during the division process. Centrosomes have two rigid cores, called centrioles, which are surrounded by a matrix of proteins called the pericentriolar material. It is from this material that the microtubules are organized.

The pericentriolar material is a dynamic structure and changes its size by assembling and disassembling its protein components. The larger the pericentriolar material, the more microtubules can form. Before a cell divides, it rapidly expands in a process called centrosome maturation. A protein called pericentrin initiates the maturation by helping to recruit other proteins to the centrosome. Pericentrin molecules are large, and it takes the cell between 10 and 20 minutes to make each one. Nevertheless, the cell can produce and deliver large quantities of pericentrin to the centrosome in a matter of minutes. We do not yet know how this happens.

To investigate this further, Sepulveda, Antkowiak, Brust-Mascher et al. used advanced microscopy to study zebrafish embryos and human cells grown in the laboratory. The results showed that cells build and transport pericentrin at the same time. Cells use messenger RNA molecules as templates to build proteins. These feed into protein factories called ribosomes, which assemble the building blocks in the correct order. Rather than waiting for the pericentrin production to finish, the cell moves the active factories to the centrosome with the help of a molecular motor called dynein. By the time the pericentrin molecules are completely made by ribosomes, they are already at the centrosome, ready to help with the recruitment of other proteins during centrosome maturation.

These findings improve our understanding of centrosome maturation. The next step is to find out how the cell coordinates this process with the recruitment of other proteins to the centrosome. It is also possible that the cell uses similar processes to deliver other proteins to different parts of the cell.

Global Effects of DDX3 Inhibition on Cell Cycle Regulation Identified by a Combined Phosphoproteomics and Single Cell Tracking Approach

DDX3 is an RNA helicase with oncogenic properties. The small molecule inhibitor RK-33 is designed to fit into the ATP binding cleft of DDX3 and hereby block its activity. RK-33 has shown potent activity in preclinical cancer models. However, the mechanism behind the antineoplastic activity of RK-33 remains largely unknown. In this study we used a dual phosphoproteomic and single cell tracking approach to evaluate the effect of RK-33 on cancer cells. MDA-MB-435 cells were treated for 24 hours with RK-33 or vehicle control. Changes in phosphopeptide abundance were analyzed with quantitative mass spectrometry using isobaric mass tags (Tandem Mass Tags). At the proteome level we mainly observed changes in mitochondrial translation, cell division pathways and proteins related to cell cycle progression. Analysis of the phosphoproteome indicated decreased CDK1 activity after RK-33 treatment. To further evaluate the effect of DDX3 inhibition on cell cycle progression over time, we performed timelapse microscopy of Fluorescent Ubiquitin Cell Cycle Indicators labeled cells after RK-33 or siDDX3 exposure. Single cell tracking indicated that DDX3 inhibition resulted in a global delay in cell cycle progression in interphase and mitosis. In addition, we observed an increase in endoreduplication. Overall, we conclude that DDX3 inhibition affects cells in all phases and causes a global cell cycle progression delay.