SEARCH · PubMed Health
Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
In plants and animals, microRNAs have been shown to regulate target genes by inhibiting translation or altering target mRNA stability. In this issue of Developmental Cell, Bao et al. extend the known mechanisms of action of microRNAs to RNA-directed DNA methylation, a mechanism previously associated only with siRNA-mediated gene silencing.
microRNAs are short, endogenous non-coding RNAs that act as post-transcriptional modulators of gene expression. Important functions for microRNAs have been found in the regulation of development, cellular proliferation and differentiation, while perturbed miRNA expression patterns have been observed in many human cancers. Here we present a method for specific inhibition of miRNA function through interaction with LNA-modified antisense oligonucleotides and report the specificity of this application. We show that LNA-modified oligonucleotides can inhibit exogenously introduced miRNAs with high specificity using a heterologous reporter assay, and furthermore demonstrate their ability to inhibit an endogenous miRNA in Drosophila melanogaster cells, leading to up-regulation of the cognate target protein. The method shows stoichiometric and reliable inhibition of the targeted miRNA and can thus be applied to studies of miRNA functions and validation of putative target genes.
As knowledge of microRNAs (miRNA) grows from a compendium of sequences to annotated functional data it has become increasingly clear that a highly significant segment of regulatory biology depends on these approximately 22 nucleotide noncoding transcripts. The expression of many miRNAs in the nervous system, some with a high degree of temporal and spatial specificity, suggests that understanding miRNAs in the nervous system will yield rewarding neurobiological insights. High on the list of insights that microRNAs promise is a deeper understanding of the remarkable cellular diversity found among neurons. This review examines the interface between an emerging biology of miRNAs and their role in nervous systems.
A new species of non-coding RNA, microRNAs (miRNAs) has been identified that may regulate the expression of as many as one third to one half of all protein encoding genes. MicroRNAs are found throughout mammalian genomes, but an association between the location of these miRNAs and regions of genomic instability (or fragile sites) in humans has been suggested [1]. In this review we discuss the possible role of altered miRNA expression on human cancer and conduct an analysis correlating the physical location of murine miRNAs with sites of genetic alteration in mouse models of cancer.
Programmed cell death, or apoptosis, is ubiquitous, both during development and in the adult. Many components of the evolutionarily conserved machinery that brings about and regulates cell death have been identified, and all of these are proteins. However, in the past three years it has become clear that roughly 1% of predicted genes in animals encode small noncoding RNAs known as microRNAs, which regulate gene function. Here we review the recent identification of microRNA cell death regulators in Drosophila, hints that such regulators are also likely to exist in mammals, and more generally the approaches and tools that are now available to probe roles for noncoding RNAs in the control of cell death.
RNA interference (RNAi) provides an effective method to silence gene expression and investigate gene function. However, RNAi tools for the chicken embryo have largely been adapted from vectors designed for mammalian cells. Here we present plasmid and retroviral RNAi vectors specifically designed for optimal gene silencing in chicken cells. The vectors use a chicken U6 promoter to express RNAs modelled on microRNA30, which are embedded within chicken microRNA operon sequences to ensure optimal Drosha and Dicer processing of transcripts. The chicken U6 promoter works significantly better than promoters of mammalian origin and in combination with a microRNA operon expression cassette (MOEC), achieves up to 90% silencing of target genes. By using a MOEC, we show that it is also possible to simultaneously silence two genes with a single vector. The vectors express either RFP or GFP markers, allowing simple in vivo tracking of vector delivery. Using these plasmids, we demonstrate effective silencing of Pax3, Pax6, Nkx2.1, Nkx2.2, Notch1 and Shh in discrete regions of the chicken embryonic nervous system. The efficiency and ease of use of this RNAi system paves the way for large-scale genetic screens in the chicken embryo.
Cell proliferation, cell death, and pattern formation are coordinated in animal development. Although many proteins that control cell proliferation and apoptosis have been identified, the means by which these effectors are linked to the patterning machinery remain poorly understood. Here, we report that the bantam gene of Drosophila encodes a 21 nucleotide microRNA that promotes tissue growth. bantam expression is temporally and spatially regulated in response to patterning cues. bantam microRNA simultaneously stimulates cell proliferation and prevents apoptosis. We identify the pro-apoptotic gene hid as a target for regulation by bantam miRNA, providing an explanation for bantam's anti-apoptotic activity.
Drosophila geneticists have uncovered roles for microRNAs in the coordination of cell proliferation and cell death during development, and in stress resistance and fat metabolism. In C. elegans, a homolog of the well-known fly developmental regulator hunchback acts downstream of the microRNAs lin-4 and let-7 in a pathway controlling developmental timing.
The general basis of cancer is the loss of cell identity and inappropriate proliferation of cells. Classically, a universal paradigm in oncogenesis is the accumulation of mutations in the open reading frames of protein-encoding oncogenes and tumor suppressors. The identification of new classes of noncoding RNAs (ncRNA) important for development and cell homeostasis will likely change this current paradigm. Recent data suggests that a special class of ncRNAs called microRNAs might be involved in human disease. This review proposes a role for microRNAs in oncogenesis.
Temporal control of development is an important aspect of pattern formation that awaits complete molecular analysis. We identified lin-57 as a member of the C. elegans heterochronic gene pathway, which ensures that postembryonic developmental events are appropriately timed. Loss of lin-57 function causes the hypodermis to terminally differentiate and acquire adult character prematurely. lin-57 is hbl-1, revealing a role for the worm hunchback homolog in control of developmental time. Significantly, fly hunchback (hb) temporally specifies cell fates in the nervous system. The hbl-1/lin-57 3'UTR is required for postembryonic downregulation in the hypodermis and nervous system and contains multiple putative binding sites for temporally regulated microRNAs, including let-7. Indeed, we find that hbl-1/lin-57 is regulated by let-7, at least in the nervous system. Examination of the hb 3'UTR reveals potential binding sites for known fly miRNAs. Thus, evolutionary conservation of hunchback genes may include temporal control of cell fate specification and microRNA-mediated regulation.
MicroRNAs (miRNAs) are short RNA molecules that bind to target mRNAs, resulting in translational repression and gene silencing. Overexpression of microRNA-21 (miR-21) is associated with various human diseases, including autosomal dominant polycystic kidney disease (ADPKD) and pulmonary fibrosis. In this study, a previously described heterobifunctional molecule, TGP-21-RiboTAC, that degrades the miR-21 precursor (pre-miR-21) in triple-negative breast cancer cells was investigated in polycystic kidney cell lines and a lung fibroblast cell line. In the former, TGP-21-RiboTAC degraded pre-miR-21 and derepressed miR-21's downstream targets, programmed cell death 4 (PDCD4) and peroxisome proliferator-activated receptor alpha (PPARα), known drivers of ADPKD. The heterobifunctional molecule also inhibited cyst growth and rescued the metabolic alterations that occur in ADPKD. In the lung fibroblast cell line, MRC-5, TGP-21-RiboTAC also reduced pre- and mature miR-21 levels, rescued transforming growth factor β (TGF-β)-induced repression of SMAD family member 7 (SMAD7), and inhibited cell invasion. Collectively, these studies demonstrate the potential of targeted RNA degradation as therapeutic agents that retard the development of organ fibrosis.
Small, inhibitory RNA molecules called microRNAs cause large decreases in target protein levels through a post-transcriptional mechanism. Until recently, it was believed this mechanism operated almost exclusively at a step in translation. However, new work has revealed that microRNAs have a second, post-transcriptional mechanism that accelerates the rate of deadenylation, the initial step of mRNA decay.
How left/right functional asymmetry is layered on top of an anatomically symmetrical nervous system is poorly understood. In the nematode Caenorhabditis elegans, two morphologically bilateral taste receptor neurons, ASE left (ASEL) and ASE right (ASER), display a left/right asymmetrical expression pattern of putative chemoreceptor genes that correlates with a diversification of chemosensory specificities. Here we show that a previously undefined microRNA termed lsy-6 controls this neuronal left/right asymmetry of chemosensory receptor expression. lsy-6 mutants that we retrieved from a genetic screen for defects in neuronal left/right asymmetry display a loss of the ASEL-specific chemoreceptor expression profile with a concomitant gain of the ASER-specific profile. A lsy-6 reporter gene construct is expressed in less than ten neurons including ASEL, but not ASER. lsy-6 exerts its effects on ASEL through repression of cog-1, an Nkx-type homeobox gene, which contains a lsy-6 complementary site in its 3' untranslated region and that has been shown to control ASE-specific chemoreceptor expression profiles. lsy-6 is the first microRNA to our knowledge with a role in neuronal patterning, providing new insights into left/right axis formation.
MicroRNAs (miRNAs) are a class of small RNA molecules that regulate the stability or the translational efficiency of target messenger RNAs (mRNAs). The latency-associated transcript (LAT) of herpes simplex virus-1 (HSV-1) is the only viral gene expressed during latent infection in neurons. LAT inhibits apoptosis and maintains latency by promoting the survival of infected neurons. No protein product has been attributed to the LAT gene and the mechanism by which LAT protects cells from apoptosis is not yet known. Here we show that a miRNA encoded by the HSV-1 LAT gene confers resistance to apoptosis. Neuroblastoma cells transfected with a fragment of the LAT gene show reduced susceptibility to cell death. The anti-apoptotic function of LAT has been mapped to a region within the first exon. We have identified and characterized a microRNA (miR-LAT) generated from the exon 1 region of the HSV-1 LAT gene. The LAT miRNA was found to accumulate in cells transiently transfected with the LAT gene fragment or infected with a wild-type strain of HSV-1. A mutant virus in which a 372-nucleotide fragment encompassing the mature miRNA was deleted neither protected the infected cells from apoptosis nor generated an miRNA. miR-LAT exerts its anti-apoptotic effect by downregulation of transforming growth factor (TGF)-beta 1 and SMAD3 expression, both of which are functionally linked in the TGF-beta pathway. Our results suggest that the miRNA encoded by the HSV-1 LAT gene regulates the induction of apoptosis in infected cells by modulation of TGF-beta signalling and thus contributes to the persistence of HSV in a latent form in sensory neurons.
Deciphering the mechanisms underlying skeletal muscle-cell differentiation in mammals is an important challenge. Cell differentiation involves complex pathways regulated at both transcriptional and post-transcriptional levels. Recent observations have revealed the importance of small (20-25 base pair) non-coding RNAs (microRNAs or miRNAs) that are expressed in both lower organisms and in mammals. miRNAs modulate gene expression by affecting mRNA translation or stability. In lower organisms, miRNAs are essential for cell differentiation during development; some miRNAs are involved in maintenance of the differentiated state. Here, we show that miR-181, a microRNA that is strongly upregulated during differentiation, participates in establishing the muscle phenotype. Moreover, our results suggest that miR-181 downregulates the homeobox protein Hox-A11 (a repressor of the differentiation process), thus establishing a functional link between miR-181 and the complex process of mammalian skeletal-muscle differentiation. Therefore, miRNAs can be involved in the establishment of a differentiated phenotype - even when they are not expressed in the corresponding fully differentiated tissue.
Human adenocarcinomas commonly harbor mutations in the KRAS and MYC proto-oncogenes and the TP53 tumor suppressor gene. All three genetic lesions are potentially pro-angiogenic, as they sustain production of vascular endothelial growth factor (VEGF). Yet Kras-transformed mouse colonocytes lacking p53 formed indolent, poorly vascularized tumors, whereas additional transduction with a Myc-encoding retrovirus promoted vigorous vascularization and growth. In addition, VEGF levels were unaffected by Myc, but enhanced neovascularization correlated with downregulation of anti-angiogenic thrombospondin-1 (Tsp1) and related proteins, such as connective tissue growth factor (CTGF). Both Tsp1 and CTGF are predicted targets for repression by the miR-17-92 microRNA cluster, which was upregulated in colonocytes coexpressing K-Ras and c-Myc. Indeed, miR-17-92 knockdown with antisense 2'-O-methyl oligoribonucleotides partly restored Tsp1 and CTGF expression; in addition, transduction of Ras-only cells with a miR-17-92-encoding retrovirus reduced Tsp1 and CTGF levels. Notably, miR-17-92-transduced cells formed larger, better-perfused tumors. These findings establish a role for microRNAs in non-cell-autonomous Myc-induced tumor phenotypes.
Changes in the patterns of gene expression are widely believed to underlie many of the phenotypic differences within and between species. Although much emphasis has been placed on changes in transcriptional regulation, gene expression is regulated at many levels, all of which must ultimately be studied together to obtain a complete picture of the evolution of gene expression. Here we compare the evolution of transcriptional regulation and post-transcriptional regulation that is mediated by microRNAs, a large class of small, non-coding RNAs in plants and animals, focusing on the evolution of the individual regulators and their binding sites. As an initial step towards integrating these mechanisms into a unified framework, we propose a simple model that describes the transcriptional regulation of new microRNA genes.
Seventy-five previously known plant microRNAs (miRNAs) were classified into 14 families according to their gene sequence identity. A total of 18,694 plant expressed sequence tags (EST) were found in the GenBank EST databases by comparing all previously known Arabidopsis miRNAs to Genbank plant EST databases with BLAST algorithms. After removing the EST sequences with high numbers (more than 2) of mismatched nucleotides, a total of 812 EST contigs were identified. After predicting and scoring the RNA secondary structure of the 812 EST sequences using mFold software, 338 new potential miRNAs were identified in 60 plant species. miRNAs are widespread. Some microRNAs may highly conserve in the plant kingdom, and they may have the same ancestor in very early evolution. There is no nucleotide substitution in most miRNAs among many plant species. Some of the new identified potential miRNAs may be induced and regulated by environmental biotic and abiotic stresses. Some may be preferentially expressed in specific tissues, and are regulated by developmental switching. These findings suggest that EST analysis is a good alternative strategy for identifying new miRNA candidates, their targets, and other genes. A large number of miRNAs exist in different plant species and play important roles in plant developmental switching and plant responses to environmental abiotic and biotic stresses as well as signal transduction. Environmental stresses and developmental switching may be the signals for synthesis and regulation of miRNAs in plants. A model for miRNA induction and expression, and gene regulation by miRNA is hypothesized.