A stem cell roadmap of ribosome heterogeneity reveals a function for RPL10A in mesoderm production
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ABSTRACT: Recent findings suggest that the ribosome itself modulates gene expression. However, whether ribosomes change composition across cell types or control cell fate remains unknown. Here, employing quantitative mass spectrometry during human embryonic stem cell differentiation, we identify dozens of ribosome composition changes underlying cell fate specification. We observe upregulation of RPL10A/uL1-containing ribosomes in the primitive streak followed by progressive decreases during mesoderm differentiation. An Rpl10a loss-of-function allele in mice causes striking early mesodermal phenotypes, including posterior trunk truncations, and inhibits paraxial mesoderm production in culture. Ribosome profiling in Rpl10a loss-of-function mice reveals decreased translation of mesoderm regulators, including Wnt pathway mRNAs, which are also enriched on RPL10A/uL1-containing ribosomes. We further show that RPL10A/uL1 regulates canonical and non-canonical Wnt signaling during stem cell differentiation and in the developing embryo. These findings reveal unexpected ribosome composition modularity that controls differentiation and development through the specialized translation of key signaling networks.
Project description:Emerging studies have linked the ribosome to more selective control of gene regulation. However, an outstanding question is whether ribosome heterogeneity at the level of core ribosomal proteins (RPs) enables ribosomes to preferentially translate specific mRNAs genome-wide. Here, we measured the absolute abundance of RPs in translating ribosomes and profiled transcripts that are enriched or depleted from select subsets of ribosomes within embryonic stem cells. We find that heterogeneity in RP composition endows ribosomes with different selectivity for translating subpools of transcripts including those controlling metabolism, the cell cycle, and development. As a paradigm example, we show that mRNAs enriched in binding to RPL10A/uL1-containing ribosomes require RPL10A/uL1 for their efficient translation. Within several of these transcripts, we find this level of regulation is mediated, at least in part, by internal ribosome entry sites. Together, these results reveal a critical functional link between ribosome heterogeneity and the post-transcriptional circuitry of gene expression.
Project description:Recent findings suggest that the ribosome itself modulates gene expression. However, whether ribosomes change composition across cell types to control cell fate remains unknown. To determine the magnitude of ribosome heterogeneity and its functional contribution to cell fate specification, we measured ribosomal protein abundance in actively translating ribosomes by quantitative mass spectrometry on a day-by-day basis as human embryonic stem cells differentiate in a step-wise fashion down endoderm and mesoderm lineages. We identified numerous core ribosomal proteins (RPs) as changing significantly in abundance in actively translating ribosomes during cell fate specification, including progressive decreases in ribosome incorporation for several ribosomal proteins during mesoderm differentiation. We further traced ribosome composition changes at the cytoplasmic, whole-cell, and mRNA transcript levels and identified multiple mechanisms regulating actively translating ribosome composition. These findings reveal extensive ribosomal remodeling during differentiation, suggesting that individual ribosomal components may have cell type-specific specialized translation functions.
Project description:The transcriptome of eukaryotic cells is constantly monitored for errors to avoid the production of undesired protein variants. The evolutionarily conserved nonsense-mediated mRNA decay (NMD) pathway degrades aberrant mRNAs, but also functions in the regulation of transcript abundance in response to changed physiological states. As a result of its essential functions, only few animal models of impaired NMD activity have been developed. Here, we describe a zebrafish mutant of upf1, encoding the central component of the NMD machinery. Fish homozygous for the upf1t20450 allele (Y163X) survive until day 10 after fertilization, presenting with impaired T cell development as one of the most conspicuous features of the mutant phenotype. Analysis of differentially expressed genes identified dysregulation of the pre-mRNA splicing pathway, accompanied by perturbed auto-regulation of canonical splicing activators (SRSF) and repressors (HNRNP). In upf1-deficient mutants, NMD-susceptible transcripts of ribosomal proteins that are known for their role as non-canonical splicing regulators were greatly increased, most notably, rpl10a. When the levels of NMD-susceptible rpl10a transcripts were artifically increased in zebrafish morphants, T cell development was significantly impaired, suggesting that perturbed autoregulation of rpl10a splicing contributes to failing T cell development in upf1 deficiency. Our results identify an extra-ribosomal tissue-specific function to rpl10a in the immune system, and thus exemplify the advantages of the zebrafish model to study the effects of upf1-deficiency in the context of a vertebrate organism.
Project description:We have developed and tested the efficiency of the Tg(myo6b:GFP-2A-rpl10a-3xHA) zebrafish to specifically enrich for and evaluate the translatome of inner ear and lateral line sensory hair cells (IP) compared to the whole fish transcriptome (IN). We show through RNA-seq that HA-tagged ribosome immunoprecipitation significantly enriches for RNA transcripts of known zebrafish hair cell expressed transcripts.
Project description:Approximately 60-70% of patients with 22q11.2 deletion syndrome (22q11.2DS; velo-cardio-facial syndrome/DiGeorge syndrome) have cardiac outflow tract anomalies including persistent truncus arteriosus (PTA) as the most severe defect. Among the genes in the 22q11.2 region, TBX1, encoding a T-box transcription factor is a major candidate for cardiovascular malformations and its inactivation in mice results in a PTA. To identify novel signaling mechanisms that function downstream, we found that Tbx1 restricts canonical Wnt signaling in the pharyngeal apparatus. To test for tissue specificity within the pharyngeal apparatus, we inactivated Tbx1 in the anterior portion of the secondary heart field (AHF) mesoderm using the Mef2c-AHF-Cre allele and observed a full penetrant PTA (n = 30). Tbx1 promotes progenitor cells but restricts differentiation whereas Wnt signaling, in the AHF, promotes cardiomyocyte differentiation. To determine whether Tbx1 and canonical Wnt signaling act in opposing pathways, both alleles of Tbx1 and one β-catenin allele were inactivated in the AHF and 85% of them (n = 35) showed partial or complete rescue. The antagonistic function of the two pathways was further confirmed by gene expression profiling, indicating that these two pathways provide a key balance in the AHF to prevent premature differentiation of progenitor cells prior to reaching the cardiac outflow tract. We inactivatedTbx1 and beta-catenin allele to identify function of Tbx1 and beta-catenin in the anterior portion of the secondary heart field (AHF) mesoderm. We also inactivated both alleles of Tbx1 and one β-catenin alleles (rescue design) to determine whether Tbx1 and canonical Wnt signaling act in opposing pathways
Project description:The CCM endothelium is hypersensitive to angiogenesis and can induce a hypoxic program associated with changes in angiogenesis, inflammation, and endothelial-cell metabolism under normoxic conditions. However, the role of active drivers of angiogenesis as CCM disease modifiers in human disease remains unclear. To assess whether CCM reactive astrocytes with neuroinflammatory capacity respond to hypoxia-induced CCM exacerbation, we employed the astrocyte-specific (Aldh1l1-EGFP/Rpl10a) Translational Ribosome Affinity Purification (TRAP) system in combination with a CCM mouse model (Pdcd10BECKO;Aldh1l1-EGFP/Rpl10a). TRAP protocol was performed using Slco1c1-iCreERT2;Pdcd10fl/fl;Aldh1l1-EGFP/Rpl10a mice to isolate ribosomes from astrocytes as previously described. Astrocyte-TRAP mRNAs were from cerebral tissue of mice age P50 exposed to hypoxia or normoxia conditions.
Project description:Cells can respond to stalled ribosomes by sensing ribosome collisions and employing quality control pathways. How ribosome stalling is resolved without collisions, however, has remained elusive. Here, focusing on non-colliding stalling exhibited by decoding-defective ribosomes, we identified Fap1 as a stalling sensor triggering 18S non-functional rRNA decay via poly-ubiquitination of uS3. Ribosome profiling revealed an enrichment of Fap1 at the translation initiation site but also association with elongating individual ribosomes. Cryo-EM structures of Fap1-bound ribosomes elucidated Fap1 probing the mRNA simultaneously at both the entry and exit channels suggesting a mRNA stasis sensing activity, and Fap1 sterically hinders formation of canonical collided di-ribosomes. Our findings indicate that individual stalled ribosomes are the potential signal for ribosome dysfunction, leading to accelerated turnover of the ribosome itself.
Project description:Millions of ribosomes are packed within mammalian cells, yet we lack tools to visualize them in toto and characterize their subcellular composition. Here, we present ribosome expansion microscopy (RiboExM) to visualize individual ribosomes and an optogenetic proximity-labeling technique (ALIBi) to probe their composition. We generated a super-resolution ribosomal map, revealing unexpected subcellular translational hotspots and enrichment of 60S subunits near polysomes at the endoplasmic reticulum (ER). We found that Lsg1 tethers 60S to the ER and regulates translation of select proteins. Additionally, we discovered ribosome heterogeneity at mitochondria guiding translation of metabolism-related transcripts. Finally, we visualized ribosomes in neurons, revealing a dynamic switch between monosomes and polysomes in neuronal translation. Together, these approaches enable exploration of ribosomal localization and composition at unprecedented resolution.
Project description:Millions of ribosomes are packed within mammalian cells, yet we lack tools to visualize them in toto and characterize their subcellular composition. Here, we present ribosome expansion microscopy (RiboExM) to visualize individual ribosomes and an optogenetic proximity-labeling technique (ALIBi) to probe their composition. We generated a super-resolution ribosomal map, revealing unexpected subcellular translational hotspots and enrichment of 60S subunits near polysomes at the endoplasmic reticulum (ER). We found that Lsg1 tethers 60S to the ER and regulates translation of select proteins. Additionally, we discovered ribosome heterogeneity at mitochondria guiding translation of metabolism-related transcripts. Finally, we visualized ribosomes in neurons, revealing a dynamic switch between monosomes and polysomes in neuronal translation. Together, these approaches enable exploration of ribosomal localization and composition at unprecedented resolution.