SummaryIntracellular targeting of mRNAs has long been recognized as a means to produce proteins locally, but has only recently emerged as a prevalent mechanism used by a wide variety of polarized cell types. Localization of mRNA molecules within the cytoplasm provides a basis for cell polarization, thus underlying developmental processes such as asymmetric cell division, cell migration, neuronal maturation and embryonic patterning. In this review, we describe and discuss recent advances in our understanding of both the regulation and functions of RNA localization during animal development.Key words: RNA localization, RNA transport, Local translation, Cell polarity, Post-transcriptional gene regulation Introduction Establishment of cell polarity is crucial for the execution of developmental programmes governing key processes, including specification of cell fates, individual or collective cell movements and specialization of somatic cell types. Cell polarization depends on the asymmetric segregation of organelles and various molecules within the cell. Polarized accumulation of RNA molecules was first visualized nearly 30 years ago, when -actin mRNA was found to be asymmetrically localized within ascidian eggs and embryos (Jeffery et al., 1983). Following this, the discovery of the first localized maternal mRNAs in Xenopus (Rebagliati et al., 1985) and Drosophila oocytes (Frigerio et al., 1986;Berleth et al., 1988) provided evidence for the earlier proposal that localized RNA determinants could be responsible for early embryonic patterning (Kandler-Singer and Kalthoff, 1976). mRNAs were soon found to be asymmetrically distributed within differentiated somatic cells, such as fibroblasts (Lawrence and Singer, 1986), oligodendrocytes (Trapp et al., 1987) and neurons (Garner et al., 1988), and to colocalize with their encoded proteins, establishing intracellular transport of mRNAs as a potential mechanism used to target the production of selected proteins to discrete sites.Significant improvements in RNA detection methods led to the identification of a growing number of localized mRNAs. Still, in the early 2000s, the set of described targeted mRNAs was limited to ~100 (reviewed by Bashirullah et al., 1998;Palacios and St Johnston, 2001) and the process of mRNA localization was thought to be restricted to specific cell types. However, recent genome-wide analyses (see Table 1) have changed this view dramatically, and strongly suggest that subcellular targeting of mRNAs is a prevalent mechanism used by polarized cells to establish functionally distinct compartments (Fig. 1). Particularly striking was the discovery that >70% of the 2314 expressed transcripts analysed in a highresolution in situ hybridization screen were subcellularly localized in Drosophila embryos (Lécuyer et al., 2007). Moreover, hundreds to thousands of mRNAs have been detected in cellular compartments as diverse as the mitotic apparatus (Blower et al., 2007;Sharp et al., 2011), pseudopodia (Mili et al., 2008), dendrites (Moccia et al., 2003;Poon et a...
In mammals, the JAK/STAT (Janus Kinase/Signal Transducer and Activator of Transcription) signaling pathway is activated in response to cytokines and growth factors to control blood cell development, proliferation and cell determination. In Drosophila, a conserved JAK/STAT signaling pathway controls segmentation in embryos, as well as blood cell development and other processes in larvae and adults. During embryogenesis, transduction of the Unpaired [Upd; also known as Outstretched (Os)] ligand through the JAK/STAT pathway requires Domeless, a putative membrane protein with distant homology to vertebrate type I cytokine receptors. We have isolated domeless(dome) in a screen to identify genes essential in epithelial morphogenesis during oogenesis. The level of dome activity is critical for proper border cell migration and is controlled in part through a negative feedback loop. In addition to its essential role in border cells, we show that dome is required in the germarium for the polarization of follicle cells during encapsulation of germline cells. In this process,dome controls the expression of the apical determinant Crumbs. In contrast to the ligand Upd, whose expression is limited to a pair of polar cells at both ends of the egg chamber, dome is expressed in all germline and follicle cells. However, the Dome protein is specifically localized at apicolateral membranes and undergoes ligand-dependent internalization in the follicle cells. dome mutations interact genetically with JAK/STAT pathway genes in border cell migration and abolish the nuclear translocation of Stat92E in vivo. We also show that domefunctions downstream of upd and that both the extracellular and intracellular domains of Dome are required for JAK/STAT signaling. Altogether,our data indicate that Dome is an essential receptor molecule for Upd and JAK/STAT signaling during oogenesis.
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