Grace X. Y. Zheng

MicroRNAs can generate thresholds in target gene expression

MicroRNAs (miRNAs) are short, highly conserved noncoding RNA molecules that repress gene expression in a sequence-dependent manner. We performed single-cell measurements using quantitative fluorescence microscopy and flow cytometry to monitor a target genes protein expression in the presence and absence of regulation by miRNA. We find that although the average level of repression is modest, in agreement with previous population-based measurements, the repression among individual cells varies dramatically. In particular, we show that regulation by miRNAs establishes a threshold level of target mRNA below which protein production is highly repressed. Near this threshold, protein expression responds sensitively to target mRNA input, consistent with a mathematical model of molecular titration. These results show that miRNAs can act both as a switch and as a fine-tuner of gene expression.

Control of somatic tissue differentiation by the long non-coding RNA TINCR

Several of the thousands of human long non-coding RNAs (lncRNAs) have been functionally characterized; however, potential roles for lncRNAs in somatic tissue differentiation remain poorly understood. Here we show that a 3.7-kilobase lncRNA, terminal differentiation-induced ncRNA (TINCR), controls human epidermal differentiation by a post-transcriptional mechanism. TINCR is required for high messenger RNA abundance of key differentiation genes, many of which are mutated in human skin diseases, including FLG, LOR, ALOXE3, ALOX12B, ABCA12, CASP14 and ELOVL3. TINCR-deficient epidermis lacked terminal differentiation ultrastructure, including keratohyalin granules and intact lamellar bodies. Genome-scale RNA interactome analysis revealed that TINCR interacts with a range of differentiation mRNAs. TINCR–mRNA interaction occurs through a 25-nucleotide ‘TINCR box’ motif that is strongly enriched in interacting mRNAs and required for TINCR binding. A high-throughput screen to analyse TINCR binding capacity to approximately 9,400 human recombinant proteins revealed direct binding of TINCR RNA to the staufen1 (STAU1) protein. STAU1-deficient tissue recapitulated the impaired differentiation seen with TINCR depletion. Loss of UPF1 and UPF2, both of which are required for STAU1-mediated RNA decay, however, did not have differentiation effects. Instead, the TINCR–STAU1 complex seems to mediate stabilization of differentiation mRNAs, such as KRT80. These data identify TINCR as a key lncRNA required for somatic tissue differentiation, which occurs through lncRNA binding to differentiation mRNAs to ensure their expression.

Massively parallel digital transcriptional profiling of single cells

Characterizing the transcriptome of individual cells is fundamental to understanding complex biological systems. We describe a droplet-based system that enables 3′ mRNA counting of tens of thousands of single cells per sample. Cell encapsulation, of up to 8 samples at a time, takes place in ∼6 min, with ∼50% cell capture efficiency. To demonstrate the systems technical performance, we collected transcriptome data from ∼250k single cells across 29 samples. We validated the sensitivity of the system and its ability to detect rare populations using cell lines and synthetic RNAs. We profiled 68k peripheral blood mononuclear cells to demonstrate the systems ability to characterize large immune populations. Finally, we used sequence variation in the transcriptome data to determine host and donor chimerism at single-cell resolution from bone marrow mononuclear cells isolated from transplant patients.

Suppression of progenitor differentiation requires the long noncoding RNA ANCR

Long noncoding RNAs (lncRNAs) regulate diverse processes, yet a potential role for lncRNAs in maintaining the undifferentiated state in somatic tissue progenitor cells remains uncharacterized. We used transcriptome sequencing and tiling arrays to compare lncRNA expression in epidermal progenitor populations versus differentiating cells. We identified ANCR (anti-differentiation ncRNA) as an 855-base-pair lncRNA down-regulated during differentiation. Depleting ANCR in progenitor-containing populations, without any other stimuli, led to rapid differentiation gene induction. In epidermis, ANCR loss abolished the normal exclusion of differentiation from the progenitor-containing compartment. The ANCR lncRNA is thus required to enforce the undifferentiated cell state within epidermis.

A Latent Pro-Survival Function for the Mir-290-295 Cluster in Mouse Embryonic Stem Cells

MicroRNAs (miRNAs) post-transcriptionally regulate the expression of thousands of distinct mRNAs. While some regulatory interactions help to maintain basal cellular functions, others are likely relevant in more specific settings, such as response to stress. Here we describe such a role for the mir-290-295 cluster, the dominant miRNA cluster in mouse embryonic stem cells (mESCs). Examination of a target list generated from bioinformatic prediction, as well as expression data following miRNA loss, revealed strong enrichment for apoptotic regulators, two of which we validated directly: Caspase 2, the most highly conserved mammalian caspase, and Ei24, a p53 transcriptional target. Consistent with these predictions, mESCs lacking miRNAs were more likely to initiate apoptosis following genotoxic exposure to gamma irradiation or doxorubicin. Knockdown of either candidate partially rescued this pro-apoptotic phenotype, as did transfection of members of the mir-290-295 cluster. These findings were recapitulated in a specific mir-290-295 deletion line, confirming that they reflect miRNA functions at physiological levels. In contrast to the basal regulatory roles previously identified, the pro-survival phenotype shown here may be most relevant to stressful gestations, where pro-oxidant metabolic states induce DNA damage. Similarly, this cluster may mediate chemotherapeutic resistance in a neoplastic context, making it a useful clinical target.

Non-equivalence of Wnt and R-spondin ligands during Lgr5 + intestinal stem-cell self-renewal

The canonical Wnt/β-catenin signalling pathway governs diverse developmental, homeostatic and pathological processes. Palmitoylated Wnt ligands engage cell-surface frizzled (FZD) receptors and LRP5 and LRP6 co-receptors, enabling β-catenin nuclear translocation and TCF/LEF-dependent gene transactivation. Mutations in Wnt downstream signalling components have revealed diverse functions thought to be carried out by Wnt ligands themselves. However, redundancy between the 19 mammalian Wnt proteins and 10 FZD receptors and Wnt hydrophobicity have made it difficult to attribute these functions directly to Wnt ligands. For example, individual mutations in Wnt ligands have not revealed homeostatic phenotypes in the intestinal epithelium—an archetypal canonical, Wnt pathway-dependent, rapidly self-renewing tissue, the regeneration of which is fueled by proliferative crypt Lgr5+ intestinal stem cells (ISCs). R-spondin ligands (RSPO1–RSPO4) engage distinct LGR4–LGR6, RNF43 and ZNRF3 receptor classes, markedly potentiate canonical Wnt/β-catenin signalling, and induce intestinal organoid growth in vitro and Lgr5+ ISCs in vivo. However, the interchangeability, functional cooperation and relative contributions of Wnt versus RSPO ligands to in vivo canonical Wnt signalling and ISC biology remain unknown. Here we identify the functional roles of Wnt and RSPO ligands in the intestinal crypt stem-cell niche. We show that the default fate of Lgr5+ ISCs is to differentiate, unless both RSPO and Wnt ligands are present. However, gain-of-function studies using RSPO ligands and a new non-lipidated Wnt analogue reveal that these ligands have qualitatively distinct, non-interchangeable roles in ISCs. Wnt proteins are unable to induce Lgr5+ ISC self-renewal, but instead confer a basal competency by maintaining RSPO receptor expression that enables RSPO ligands to actively drive and specify the extent of stem-cell expansion. This functionally non-equivalent yet cooperative interaction between Wnt and RSPO ligands establishes a molecular precedent for regulation of mammalian stem cells by distinct priming and self-renewal factors, with broad implications for precise control of tissue regeneration.

Dicer-microRNA-Myc circuit promotes transcription of hundreds of long noncoding RNAs

Long noncoding RNAs (lncRNAs) are important regulators of cell fate, yet little is known about mechanisms controlling lncRNA expression. Here we show that transcription is quantitatively different for lncRNAs and mRNAs—as revealed by deficiency of Dicer (Dcr), a key RNase that generates microRNAs (miRNAs). Dcr loss in mouse embryonic stem cells led unexpectedly to decreased levels of hundreds of lncRNAs. The canonical Dgcr8-Dcr-miRNA pathway is required for robust lncRNA transcriptional initiation and elongation. Computational and genetic epistasis analyses demonstrated that Dcr activation of the oncogenic transcription factor cMyc is partly responsible for lncRNA expression. A quantitative metric of mRNA-lncRNA decoupling revealed that Dcr and cMyc differentially regulate lncRNAs versus mRNAs in diverse cell types and in vivo. Thus, numerous lncRNAs may be modulated as a class in development and disease, notably where Dcr and cMyc act.

Genome-wide impact of a recently expanded microRNA cluster in mouse

Variations in microRNA (miRNA) gene and/or target repertoire are likely to be key drivers of phenotypic differences between species. To better understand these changes, we developed a computational method that identifies signatures of species-specific target site gain and loss associated with miRNA acquisition. Interestingly, several of the miRNAs implicated in mouse 3′ UTR evolution derive from a single rapidly expanded rodent-specific miRNA cluster. Located in the intron of Sfmbt2, a maternally imprinted polycomb gene, these miRNAs (referred to as the Sfmbt2 cluster) are expressed in both embryonic stem cells and the placenta. One abundant miRNA from the cluster, miR-467a, functionally overlaps with the mir-290-295 cluster in promoting growth and survival of mouse embryonic stem cells. Predicted novel targets of the remaining cluster members are enriched in pathways regulating cell survival. Two relevant species-specific target candidates, Lats2 and Dedd2, were validated in cultured cells. We suggest that the rapid evolution of the Sfmbt2 cluster may be a result of intersex conflict for growth regulation in early mammalian development and could provide a general model for the genomic response to acquisition of miRNAs and similar regulatory factors.

Noncoding RNA Landmarks of Pluripotency and Reprogramming

Noncoding RNAs have emerged as important determinants of pluripotency and reprogramming. In this issue of Cell Stem Cell, Kosik and colleagues now provide a detailed map of microRNA expression patterns to infer the biological states of embryonic and induced pluripotent stem cells.

Erratum: Genome-wide identification of Ago2 binding sites from mouse embryonic stem cells with and without mature microRNAs (Nature Structural and Molecular Biology (2011) 18 (237-244))

MicroRNAs (miRNAs) are 19-22nt non-coding RNAs that post-transcriptionally regulate mRNA targets. To identify endogenous miRNA binding sites, we performed photo-crosslinking immunoprecipitation using Ago2 antibodies, followed by deep-sequencing of RNAs (CLIP-seq) in mouse embryonic stem cells (mESCs). We also performed CLIP-seq in Dicer−/− mESCs that lack mature miRNAs, allowing us to define whether the association of Ago2 with the identified sites was miRNA-dependent. A significantly enriched motif, GCACUU, was identified only in wildtype mESCs in 3′ untranslated and coding regions. This motif matches the seed of a miRNA family that constitutes ∼68% of the mESC miRNA population. Unexpectedly, a G-rich motif was enriched in sequences crosslinked to Ago2 in the presence and absence of miRNAs. Expression analysis and reporter assays confirmed that the seed-related motif confers miRNA-directed regulation on host mRNAs and that the G-rich motif can modulate this regulation. miRNAs are key regulators of gene expression in fundamental processes including cell proliferation, cell death, cell differentiation and cellular responses to the environment1-3. These short non-coding RNAs guide a ribonucleoprotein complex, containing a member of the conserved Argonaute (Ago) protein family, to sites predominantly in the 3′UTRs of their target mRNAs, resulting in the destabilization of the message and/or inhibition of translation4,5. Biochemical and computational studies have shown that base-pairing between the “seed” (2nd -7th nucleotide) at the 5′ end of the miRNA and mRNA target is Users may view, print, copy, download and text and datamine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature.com/authors/editorial_policies/license.html#terms Correspondence should be addressed to P.A.S. sharppa@mit.edu. 4Present address: Salk Institute for Biological Studies, La Jolla, California, USA (A.G.Y.); Howard Hughes Medical Institute and Program in Epithelial Biology, Stanford University School of Medicine, Stanford, California, USA (G.X.Z.); Michael Smith Genome Sciences Centre, Vancouver, British Columbia, Canada (C.B.N.). 5These authors contributed equally to this work. Accession codes. Microarray and short RNA sequence files have been deposited at the Gene Expression Omnibus database repository under accession number GSE25310. BED files for clusters in all libraries are available for download from http://rowley.mit.edu/pubs/ Ago2_CLIP/Ago2_CLIP.html Note: Supplementary information is available on the Nature Structural & Molecular Biology website. Author Contributions: A.K.L. and A.G.Y. designed and performed the experiments; A.K.L., A.G.Y. and P.A.S. wrote the paper; A.D.B. performed experiments; A.B., C.B.N, and G.X.Z. performed the bioinformatics analyses. All authors reviewed and approved the manuscript. Competing Interest Statement: The authors declare no competing financial interests. NIH Public Access Author Manuscript Nat Struct Mol Biol. Author manuscript; available in PMC 2011 August 1. Published in final edited form as: Nat Struct Mol Biol. 2011 February ; 18(2): 237–244. doi:10.1038/nsmb.1991. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript important for this regulation in animals6-10. Comparative genomic analysis for miRNA seed sites in 3′UTRs suggests that miRNAs regulate ∼60% of all mammalian mRNAs11. Moreover, both comparative genomic analysis and emerging data from a handful of genes suggest that miRNAs also target coding sequences8,12,13, but the prevalence of this interaction is unclear. Therefore, recent efforts14-16, including the study presented here, have aimed at identifying bona fide miRNA binding sites on a genome-wide scale in samples from whole mouse brain and whole-animal nematodes. However, one challenge of these studies is to deconvolute the miRNA-target relationships in the mixed cell types from these samples15,16.