S01-03 miRNAs couple cell cycle, cell fate and developmental timing in retinal histogenesis

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available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/modo

Symposium – Non-coding RNA in development S01-01

rapid progress has been made in identifying new miRNAs and

The diverse roles of small, non-coding RNA in plants

in determining factors important for their expression and func-

David Baulcombe

tion. However, the mechanism by which these regulatory RNAs

University of Cambridge, Cambridge, United Kingdom Eukaryotes contain small regulatory RNAs that have been referred to as the dark matter of genetics. They are typically 21– 24 nucleotides long, associated with Argonaut or Piwi proteins. Some of these small RNAs guide the Argonaut/Piwi protein to a complementary RNA and they are negative regulators of gene expression acting at the level of messenger RNA turnover or translation. Others participate in more complex regulatory systems affecting transcription or they act as part of an RNA signal that moves between cells. In this talk I will focus the small RNA systems affecting chromatin modification. In order to understand the RNA silencing pathways we have been characterizing siRNAs and miRNAs using high throughput sequencing technology in wild type and mutant plants and by tracking their association with Argonaute proteins. The results indicate that at 24nt size class of siRNA are derived from thou-

repress specific target mRNAs is still unclear. My lab primarily focuses on the founding miRNA genes, lin-4 and let-7, and their genetically defined targets in Caenorhabitis elegans to understand how these miRNAs control development of the animal. The majority of animal miRNAs, including lin-4 and let-7, recognize sites of partial complementarity in the 3 untranslated regions (UTRs) of target genes, and this typically results in translational repression and mRNA degradation. Currently, we are exploring the generality of mRNA degradation as an outcome of regulation by miRNAs. We are also attempting to identify the mRNA degradation intermediates and the protein factors that participate in this mechanism of regulation by miRNAs. Studying miRNAs and their targets in C. elegans allows us to analyze the effect of authentic mutants in miRNA genes on endogenous gene expression. By elucidating the function of miRNAs in this organism, we hope to contribute to the general understanding of this newly discovered and widely conserved mode of gene regulation.

sands of loci in the Arabidopsis genome that are associated chromatin modification. There are at least four different AGO proteins

doi:10.1016/j.mod.2009.06.1069

that bind these RNAs in a locus-dependent manner. Surprisingly, given that there are so many of them, the absence of these polIV-dependent RNAs has only minor effects on growth and

S01-03

development of plants. Based on our recent discovery that these

miRNAs couple cell cycle, cell fate and developmental timing in

24ntsiRNAs are most abundant in the endosperm of developing

retinal histogenesis

seeds where they are uniparentally expressed (maternal) we pro-

Federico Cremisi1, Sarah Decembrini2, Dario Bressan1,

pose they mediate interactions between maternal and paternal

Robert Vignali2, Letizia Pitto3, Giuseppe Rainaldi3, Xiumei Wang4,

genomes. I will discuss the implications of these findings for

Monica Evangelista3, Giuseppina Barsacchi2

the properties of hybrid plants. doi:10.1016/j.mod.2009.06.1068

S01-02 Regulation of gene expression by MicroRNAs in Caenorhabitis elegans Amy Pasquinelli UCSD, La Jolla, United States

1

Scuola Normale Superiore, Pisa, Pisa, Italy

2

Department of Biology, Pisa, Pisa, Italy

3

Consiglio Nazionale delle Ricerche, Pisa, Pisa, Italy

4

Center for Brain and Cognitive Science, Beijin, China Early retinal progenitor cells can generate different types of

neurons depending on the time when they exit the cell cycle. In Xenopus, the late translational de-repression of Xvsx1 and Xotx2 homeobox genes commits the latest progenitor cells towards bipolar cell fate. Xvsx1 and Xotx2 mRNAs contain 3UTR cis-acting signals that inhibit their translation in early, rapidly dividing pro-

MicroRNAs (miRNAs) comprise a new class of small regulatory

genitors. However, the nature of the trans-acting molecules that

RNAs that inhibit the expression of protein-coding target genes.

control this inhibition and/or the timing of its release, is

There are hundreds, perhaps thousands of miRNAs encoded by

unknown. Here we show that four miRNAs, which we selected

the human genome and already mis-expression of specific miR-

by a high throughput screening and functionally assayed invivo,

NA genes has been linked to abnormal development and disease.

play the inhibition. These miRNAs, mir-129, miR-155, miR-214

Since the recent emergence of this family of regulatory RNAs,

and miR-222, are highly expressed in early retinal progenitors

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and their expression decreases over time during retinogenesis as

expressed in gonads, and function in genome surveillance

the cell cycle lengthens. The four miRNAs act on the 3UTR of

through association with the PIWI proteins by silencing mobile

Xvsx1 and Xotx2 by repressing their translation and inhibit the

repetitive elements. Deep-sequencing analyses of piRNAs in

generation of bipolar cells during the early stages of retina differ-

Drosophila gonads suggested that piRNAs are produced by two

entiation. We anticipate our findings to be a starting point to

systems, the primary processing pathway and the amplification

investigate the role of miRNAs in the co-ordination of cell prolifer-

loop, from repetitive genes and piRNA clusters in the genome.

ation, fate and timing of differentiation of neural embryonic cells.

The amplification loop occurs in a Dicer-independent, PIWI-Slicer-dependent manner. However, the primary processing path-

doi:10.1016/j.mod.2009.06.1070

way remains largely conceptual. Recently we show that in ovarian somatic cells, which lack Aub and AGO3 but express Piwi, the primary processing pathway for piRNAs indeed exists. In

S01-04 piRNA biogenesis pathways in Drosophila germline cells Mikiko Siomi Keio University School of Medicine, Tokyo, Japan

addition to flamenco, a Piwi-specific piRNA cluster, traffic jam (tj), a large Maf-coding gene, was determined as a new piRNA cluster. piRNAs arising from tj correspond to the untranslated regions of tj mRNA and are all in sense. In tj and piwi mutant larval ovaries, somatic cells fail to intermingle with germ cells. Loss of tj abolishes Piwi expression in gonadal somas. Thus in gonadal

Gene silencing pathways triggered by small RNAs are generi-

somatic cells, tj gives rise simultaneously to two different mole-

cally called RNA silencing. Members of the Argonaute family play

cules, the TJ protein, which activates Piwi expression, and piR-

important roles in the pathways. In Drosophila, the Argoanute

NAs, which define the Piwi targets for silencing.

family consists of five distinct members (AGO1, AGO2, AGO3, Piwi, and Aubergine). Piwi, Aubergine, and AGO3 (the PIWI proteins) are specifically expressed in germlines and associated with Piwi-interacting RNAs (piRNAs). piRNAs are 24–30 nt long, only

doi:10.1016/j.mod.2009.06.1071