Distinct roles of LARP1 and 4EBP1/2 in regulating translation and stability of 5'TOP mRNAs.

A central mechanism of mTOR complex 1 (mTORC1) signaling is the coordinated translation of ribosomal protein and translation factor mRNAs mediated by the 5'-terminal oligopyrimidine motif (5'TOP). Recently, La-related protein 1 (LARP1) was proposed to be the specific regulator of 5'TOP mRNA translation downstream of mTORC1, while eIF4E-binding proteins (4EBP1/2) were suggested to have a general role in translational repression of all transcripts. Here, we use single-molecule translation site imaging of 5'TOP and canonical mRNAs to study the translation of single mRNAs in living cells. Our data reveal that 4EBP1/2 has a dominant role in repression of translation of both 5'TOP and canonical mRNAs during pharmacological inhibition of mTOR. In contrast, we find that LARP1 selectively protects 5'TOP mRNAs from degradation in a transcriptome-wide analysis of mRNA half-lives. Our results clarify the roles of 4EBP1/2 and LARP1 in regulating 5'TOP mRNAs and provide a framework to further study how these factors control cell growth during development and disease.

The transcription factor Zic4 promotes tentacle formation and prevents epithelial transdifferentiation in Hydra.

The molecular mechanisms that maintain cellular identities and prevent dedifferentiation or transdifferentiation remain mysterious. However, both processes are transiently used during animal regeneration. Therefore, organisms that regenerate their organs, appendages, or even their whole body offer a fruitful paradigm to investigate the regulation of cell fate stability. Here, we used Hydra as a model system and show that Zic4, whose expression is controlled by Wnt3/ß-catenin signaling and the Sp5 transcription factor, plays a key role in tentacle formation and tentacle maintenance. Reducing Zic4 expression suffices to induce transdifferentiation of tentacle epithelial cells into foot epithelial cells. This switch requires the reentry of tentacle battery cells into the cell cycle without cell division and is accompanied by degeneration of nematocytes embedded in these cells. These results indicate that maintenance of cell fate by a Wnt-controlled mechanism is a key process both during homeostasis and during regeneration.

Mechanical oscillations orchestrate axial patterning through Wnt activation in Hydra.

Mechanical input shapes cell fate decisions during development and regeneration in many systems, yet the mechanisms of this cross-talk are often unclear. In regenerating Hydra tissue spheroids, periodic osmotically driven inflation and deflation cycles generate mechanical stimuli in the form of tissue stretching. Here, we demonstrate that tissue stretching during inflation is important for the appearance of the head organizer­a group of cells that secrete the Wnt3 ligand. Exploiting time series RNA expression profiles, we identify the up-regulation of Wnt signaling as a key readout of the mechanical input. In this system, the levels of Wnt3 expression correspond to the levels of stretching, and Wnt3 overexpression alone enables successful regeneration in the absence of mechanical stimulation. Our findings enable the incorporation of mechanical signals in the framework of Hydra patterning and highlight the broad significance of mechanochemical feedback loops for patterning epithelial lumens.

Dynamics of alternative splicing during somatic cell reprogramming reveals functions for RNA-binding proteins CPSF3, hnRNP UL1, and TIA1.

BACKGROUND: Somatic cell reprogramming is the process that allows differentiated cells to revert to a pluripotent state. In contrast to the extensively studied rewiring of epigenetic and transcriptional programs required for reprogramming, the dynamics of post-transcriptional changes and their associated regulatory mechanisms remain poorly understood. Here we study the dynamics of alternative splicing changes occurring during efficient reprogramming of mouse B cells into induced pluripotent stem (iPS) cells and compare them to those occurring during reprogramming of mouse embryonic fibroblasts. RESULTS: We observe a significant overlap between alternative splicing changes detected in the two reprogramming systems, which are generally uncoupled from changes in transcriptional levels. Correlation between gene expression of potential regulators and specific clusters of alternative splicing changes enables the identification and subsequent validation of CPSF3 and hnRNP UL1 as facilitators, and TIA1 as repressor of mouse embryonic fibroblasts reprogramming. We further find that these RNA-binding proteins control partially overlapping programs of splicing regulation, involving genes relevant for developmental and morphogenetic processes. CONCLUSIONS: Our results reveal common programs of splicing regulation during reprogramming of different cell types and identify three novel regulators of this process and their targets.

Cooperation between HDAC3 and DAX1 mediates lineage restriction of embryonic stem cells.

Mouse embryonic stem cells (mESCs) are biased toward producing embryonic rather than extraembryonic endoderm fates. Here, we identify the mechanism of this barrier and report that the histone deacetylase Hdac3 and the transcriptional corepressor Dax1 cooperatively limit the lineage repertoire of mESCs by silencing an enhancer of the extraembryonic endoderm-specifying transcription factor Gata6. This restriction is opposed by the pluripotency transcription factors Nr5a2 and Esrrb, which promote cell type conversion. Perturbation of the barrier extends mESC potency and allows formation of 3D spheroids that mimic the spatial segregation of embryonic epiblast and extraembryonic endoderm in early embryos. Overall, this study shows that transcriptional repressors stabilize pluripotency by biasing the equilibrium between embryonic and extraembryonic lineages that is hardwired into the mESC transcriptional network.

A unique bipartite Polycomb signature regulates stimulus-response transcription during development.

Rapid cellular responses to environmental stimuli are fundamental for development and maturation. Immediate early genes can be transcriptionally induced within minutes in response to a variety of signals. How their induction levels are regulated and their untimely activation by spurious signals prevented during development is poorly understood. We found that in developing sensory neurons, before perinatal sensory-activity-dependent induction, immediate early genes are embedded into a unique bipartite Polycomb chromatin signature, carrying active H3K27ac on promoters but repressive Ezh2-dependent H3K27me3 on gene bodies. This bipartite signature is widely present in developing cell types, including embryonic stem cells. Polycomb marking of gene bodies inhibits mRNA elongation, dampening productive transcription, while still allowing for fast stimulus-dependent mark removal and bipartite gene induction. We reveal a developmental epigenetic mechanism regulating the rapidity and amplitude of the transcriptional response to relevant stimuli, while preventing inappropriate activation of stimulus-response genes.

Cell Types of the Human Retina and Its Organoids at Single-Cell Resolution.

Human organoids recapitulating the cell-type diversity and function of their target organ are valuable for basic and translational research. We developed light-sensitive human retinal organoids with multiple nuclear and synaptic layers and functional synapses. We sequenced the RNA of 285,441 single cells from these organoids at seven developmental time points and from the periphery, fovea, pigment epithelium and choroid of light-responsive adult human retinas, and performed histochemistry. Cell types in organoids matured in vitro to a stable "developed" state at a rate similar to human retina development in vivo. Transcriptomes of organoid cell types converged toward the transcriptomes of adult peripheral retinal cell types. Expression of disease-associated genes was cell-type-specific in adult retina, and cell-type specificity was retained in organoids. We implicate unexpected cell types in diseases such as macular degeneration. This resource identifies cellular targets for studying disease mechanisms in organoids and for targeted repair in human retinas.

The RNA hairpin binder TRIM71 modulates alternative splicing by repressing MBNL1.

TRIM71/LIN-41, a phylogenetically conserved regulator of development, controls stem cell fates. Mammalian TRIM71 exhibits both RNA-binding and protein ubiquitylation activities, but the functional contribution of either activity and relevant primary targets remain poorly understood. Here, we demonstrate that TRIM71 shapes the transcriptome of mouse embryonic stem cells (mESCs) predominantly through its RNA-binding activity. We reveal that TRIM71 binds targets through 3' untranslated region (UTR) hairpin motifs and that it acts predominantly by target degradation. TRIM71 mutations implicated in etiogenesis of human congenital hydrocephalus impair target silencing. We identify a set of primary targets consistently regulated in various human and mouse cell lines, including MBNL1 (Muscleblind-like protein 1). MBNL1 promotes cell differentiation through regulation of alternative splicing, and we demonstrate that TRIM71 promotes embryonic splicing patterns through MBNL1 repression. Hence, repression of MBNL1-dependent alternative splicing may contribute to TRIM71's function in regulating stem cell fates.

Smu1 and RED are required for activation of spliceosomal B complexes assembled on short introns.

Human pre-catalytic spliceosomes contain several proteins that associate transiently just prior to spliceosome activation and are absent in yeast, suggesting that this critical step is more complex in higher eukaryotes. We demonstrate via RNAi coupled with RNA-Seq that two of these human-specific proteins, Smu1 and RED, function both as alternative splicing regulators and as general splicing factors and are required predominantly for efficient splicing of short introns. In vitro splicing assays reveal that Smu1 and RED promote spliceosome activation, and are essential for this step when the distance between the pre-mRNA's 5' splice site (SS) and branch site (BS) is sufficiently short. This Smu1-RED requirement can be bypassed when the 5' and 3' regions of short introns are physically separated. Our observations suggest that Smu1 and RED relieve physical constraints arising from a short 5'SS-BS distance, thereby enabling spliceosomes to overcome structural challenges associated with the splicing of short introns.

Functional Network Analysis Reveals the Relevance of SKIIP in the Regulation of Alternative Splicing by p38 SAPK.

Alternative splicing is a prevalent mechanism of gene regulation that is modulated in response to a wide range of extracellular stimuli. Stress-activated protein kinases (SAPKs) play a key role in controlling several steps of mRNA biogenesis. Here, we show that osmostress has an impact on the regulation of alternative splicing (AS), which is partly mediated through the action of p38 SAPK. Splicing network analysis revealed a functional connection between p38 and the spliceosome component SKIIP, whose depletion abolished a significant fraction of p38-mediated AS changes. Importantly, p38 interacted with and directly phosphorylated SKIIP, thereby altering its activity. SKIIP phosphorylation regulated AS of GADD45α, the upstream activator of the p38 pathway, uncovering a negative feedback loop involving AS regulation. Our data reveal mechanisms and targets of SAPK function in stress adaptation through the regulation of AS.

Self-organization and symmetry breaking in intestinal organoid development.

Intestinal organoids are complex three-dimensional structures that mimic the cell-type composition and tissue organization of the intestine by recapitulating the self-organizing ability of cell populations derived from a single intestinal stem cell. Crucial in this process is a first symmetry-breaking event, in which only a fraction of identical cells in a symmetrical sphere differentiate into Paneth cells, which generate the stem-cell niche and lead to asymmetric structures such as the crypts and villi. Here we combine single-cell quantitative genomic and imaging approaches to characterize the development of intestinal organoids from single cells. We show that their development follows a regeneration process that is driven by transient activation of the transcriptional regulator YAP1. Cell-to-cell variability in YAP1, emerging in symmetrical spheres, initiates Notch and DLL1 activation, and drives the symmetry-breaking event and formation of the first Paneth cell. Our findings reveal how single cells exposed to a uniform growth-promoting environment have the intrinsic ability to generate emergent, self-organized behaviour that results in the formation of complex multicellular asymmetric structures.

Impaired Spermatogenesis, Muscle, and Erythrocyte Function in U12 Intron Splicing-Defective Zrsr1 Mutant Mice.

The U2AF35-like ZRSR1 has been implicated in the recognition of 3' splice site during spliceosome assembly, but ZRSR1 knockout mice do not show abnormal phenotypes. To analyze ZRSR1 function and its precise role in RNA splicing, we generated ZRSR1 mutant mice containing truncating mutations within its RNA-recognition motif. Homozygous mutant mice exhibited severe defects in erythrocytes, muscle stretch, and spermatogenesis, along with germ cell sloughing and apoptosis, ultimately leading to azoospermia and male sterility. Testis RNA sequencing (RNA-seq) analyses revealed increased intron retention of both U2- and U12-type introns, including U12-type intron events in genes with key functions in spermatogenesis and spermatid development. Affected U2 introns were commonly found flanking U12 introns, suggesting functional cross-talk between the two spliceosomes. The splicing and tissue defects observed in mutant mice attributed to ZRSR1 loss of function suggest a physiological role for this factor in U12 intron splicing.

UNR/CSDE1 Drives a Post-transcriptional Program to Promote Melanoma Invasion and Metastasis.

RNA binding proteins (RBPs) modulate cancer progression through poorly understood mechanisms. Here we show that the RBP UNR/CSDE1 is overexpressed in melanoma tumors and promotes invasion and metastasis. iCLIP sequencing, RNA sequencing, and ribosome profiling combined with in silico studies unveiled sets of pro-metastatic factors coordinately regulated by UNR as part of RNA regulons. In addition to RNA steady-state levels, UNR was found to control many of its targets at the level of translation elongation/termination. Key pro-oncogenic targets of UNR included VIM and RAC1, as validated by loss- and gain-of-function studies. Our results identify UNR as an oncogenic modulator of melanoma progression, unravel the underlying molecular mechanisms, and identify potential targets for this therapeutically challenging malignancy.

The Spliceosome: The Ultimate RNA Chaperone and Sculptor.

The spliceosome, one of the most complex machineries of eukaryotic cells, removes intronic sequences from primary transcripts to generate functional messenger and long noncoding RNAs (lncRNA). Genetic, biochemical, and structural data reveal that the spliceosome is an RNA-based enzyme. Striking mechanistic and structural similarities strongly argue that pre-mRNA introns originated from self-catalytic group II ribozymes. However, in the spliceosome, protein components organize and activate the catalytic-site RNAs, and recognize and pair together splice sites at intron boundaries. The spliceosome is a dynamic, reversible, and flexible machine that chaperones small nuclear (sn) RNAs and a variety of pre-mRNA sequences into conformations that enable intron removal. This malleability likely contributes to the regulation of alternative splicing, a prevalent process contributing to cell differentiation, homeostasis, and disease.

Reconstruction of composite regulator-target splicing networks from high-throughput transcriptome data.

We present a computational framework tailored for the modeling of the complex, dynamic relationships that are encountered in splicing regulation. The starting point is whole-genome transcriptomic data from high-throughput array or sequencing methods that are used to quantify gene expression and alternative splicing across multiple contexts. This information is used as input for state of the art methods for Graphical Model Selection in order to recover the structure of a composite network that simultaneously models exon co-regulation and their cognate regulators. Community structure detection and social network analysis methods are used to identify distinct modules and key actors within the network. As a proof of concept for our framework we studied the splicing regulatory network for Drosophila development using the publicly available modENCODE data. The final model offers a comprehensive view of the splicing circuitry that underlies fly development. Identified modules are associated with major developmental hallmarks including maternally loaded RNAs, onset of zygotic gene expression, transitions between life stages and sex differentiation. Within-module key actors include well-known developmental-specific splicing regulators from the literature while additional factors previously unassociated with developmental-specific splicing are also highlighted. Finally we analyze an extensive battery of Splicing Factor knock-down transcriptome data and demonstrate that our approach captures true regulatory relationships.

Relationship between nucleosome positioning and progesterone-induced alternative splicing in breast cancer cells.

Splicing of mRNA precursors can occur cotranscriptionally and it has been proposed that chromatin structure influences splice site recognition and regulation. Here we have systematically explored potential links between nucleosome positioning and alternative splicing regulation upon progesterone stimulation of breast cancer cells. We confirm preferential nucleosome positioning in exons and report four distinct profiles of nucleosome density around alternatively spliced exons, with RNA polymerase II accumulation closely following nucleosome positioning. Hormone stimulation induces switches between profile classes, correlating with a subset of alternative splicing changes. Hormone-induced exon inclusion often correlates with higher nucleosome occupancy at the exon or the preceding intronic region and with higher RNA polymerase II accumulation. In contrast, exons skipped upon hormone stimulation display low nucleosome densities even before hormone treatment, suggesting that chromatin structure primes alternative splicing regulation. Skipped exons frequently harbor binding sites for hnRNP AB, a hormone-induced splicing regulator whose knock down prevents some hormone-induced skipping events. Collectively, our results argue that a variety of chromatin architecture mechanisms can influence alternative splicing decisions.

Genome-wide identification of Fas/CD95 alternative splicing regulators reveals links with iron homeostasis.

Alternative splicing of Fas/CD95 exon 6 generates either a membrane-bound receptor that promotes, or a soluble isoform that inhibits, apoptosis. Using an automatized genome-wide siRNA screening for alternative splicing regulators of endogenous transcripts in mammalian cells, we identified 200 genes whose knockdown modulates the ratio between Fas/CD95 isoforms. These include classical splicing regulators; core spliceosome components; and factors implicated in transcription and chromatin remodeling, RNA transport, intracellular signaling, and metabolic control. Coherent effects of genes involved in iron homeostasis and pharmacological modulation of iron levels revealed a link between intracellular iron and Fas/CD95 exon 6 inclusion. A splicing regulatory network linked iron levels with reduced activity of the Zinc-finger-containing splicing regulator SRSF7, and in vivo and in vitro assays revealed that iron inhibits SRSF7 RNA binding. Our results uncover numerous links between cellular pathways and RNA processing and a mechanism by which iron homeostasis can influence alternative splicing.