MicroRNAs (miRNAs) have been implicated in cell-cycle regulation and in some cases shown to have a role in tissue growth control. Depletion of miRNAs was found to have an effect on tissue growth rates in the wing primordium of Drosophila, a highly proliferative epithelium. Dicer-1 (Dcr-1) is a double-stranded RNAseIII essential for miRNA biogenesis. Adult cells lacking dcr-1, or with reduced dcr-1 activity, were smaller than normal cells and gave rise to smaller wings. dcr-1 mutant cells showed evidence of being susceptible to competition by faster growing cells in vivo and the miRNA machinery was shown to promote G 1 -S transition. We present evidence that Dcr-1 acts by regulating the TRIM-NHL protein Mei-P26, which in turn regulates dMyc protein levels. Mei-P26 is a direct target of miRNAs, including the growth-promoting bantam miRNA. Thus, regulation of tissue growth by the miRNA pathway involves a double repression mechanism to control dMyc protein levels in a highly proliferative and growing epithelium. The EMBO Journal (2010) IntroductionRegulation of gene expression at the transcriptional level has a central role in development and physiology; however, the relevance of post-transcriptional gene regulation is increasingly recognized. MicroRNAs (miRNAs), endogenous small non-coding RNAs, 22 nucleotides long, that repress target transcripts (Flynt and Lai, 2008), confer a novel layer of posttranscriptional regulation. Dicer-1 (Dcr-1) is a crucial element for miRNA biogenesis (Lee et al, 2003). Therefore, impairing Dcr-1 activity provides a means to assess the role of the miRNA pathway in a given biological process. Loss of dcr-1 produces defects in Drosophila and vertebrate stem cell maintenance and causes a delay in G 1 -S transition in these cells (Hatfield et al, 2005;Jin and Xie, 2007;Wang et al, 2007). In the developing mouse limb, loss of dcr-1 leads to growth defects (Harfe et al, 2005). The main effectors mediating the activity of Dcr-1 in these processes have not been identified. The wing imaginal disc of Drosophila is a very suitable model system to analyse at a cellular level the role of miRNAs in a highly proliferative epithelium and to identify such effectors.The fly wing primordium arises as a group of 30-40 cells in the embryonic ectoderm that proliferates during 5 days to reach a final size of around 50 000 cells and gives rise after metamorphosis to the adult wing (García-Bellido and Merriam, 1971;Madhavan and Schneiderman, 1977). Here we have analysed the role of Dcr-1 in growth control in the developing wing. Dcr-1 is required for cell and tissue growth, promotes G 1 -S transition and dcr-1 mutant cells are eliminated by a process of cell competition. We present evidence that the dMyc proto-oncogene (Johnston et al, 1999) contributes to the role of the miRNA pathway in these processes. TRIM32, the mouse orthologue of Drosophila Mei-P26 (Page et al, 2000), has been shown to show ubiquitin ligase activity, bind to c-Myc and target it for degradation (Schwamborn et al, 2009). We present ev...

A stable pool of morphogen-producing cells is critical for the development of any organ or tissue. Here we present evidence that JAK/STAT signalling in the Drosophila wing promotes the cycling and survival of Hedgehog-producing cells, thereby allowing the stable localization of the nearby BMP/Dpp-organizing centre in the developing wing appendage. We identify the inhibitor of apoptosis dIAP1 and Cyclin A as two critical genes regulated by JAK/STAT and contributing to the growth of the Hedgehog-expressing cell population. We also unravel an early role of JAK/STAT in guaranteeing Wingless-mediated appendage specification, and a later one in restricting the Dpp-organizing activity to the appendage itself. These results unveil a fundamental role of the conserved JAK/STAT pathway in limb specification and growth by regulating morphogen production and signalling, and a function of pro-survival cues and mitogenic signals in the regulation of the pool of morphogen-producing cells in a developing organ.

TRIM-NHL proteins are a family of translational regulators that control cell growth, proliferation, and differentiation during development. Drosophila Brat and Mei-P26 TRIM-NHL proteins serve as tumor suppressors in stem cell lineages and have been proposed to exert this action, in part, via the repression of the protooncogene dMyc. Here we analyze the role of Brat, Mei-P26, and dMyc in regulating growth in Drosophila imaginal discs. As in stem cell lineages, Brat and Mei-P26 repress dMyc in epithelial cells by acting at the post-transcriptional and protein level, respectively. Analysis of cell and organ size unravel that Mei-P26 mediates tissuespecific responses to Brat and dMyc activities. Loss-of-function of brat and overexpression of dMyc induce overgrowth in stem cell lineages and eventually can participate in tumor formation. In contrast, an increase in Mei-P26 levels inhibits growth of epithelial cells in these two conditions. Upon depletion of Brat, Mei-P26 up-regulation prevents an increase in dMyc protein levels and leads to tissue undergrowth. This mechanism appears to be tissue-specific since Mei-P26 is not upregulated in brain tumors resulting from brat lossof-function. Driving Mei-P26 expression in these tumors -mimicking the situation in epithelial cells-is sufficient to prevent dMyc accumulation, thus rescuing the overgrowth. Finally, we show that Mei-P26 upregulation mediates dMyc-induced apoptosis and limits dMyc growth potential in epithelial cells. These findings shed light on the tumor suppressor roles of TRIM-NHL proteins and underscore a new mechanism that maintains tissue homeostasis upon dMyc deregulation.

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How cells acquiring mutations in tumor suppressor genes outcompete neighboring wild-type cells is poorly understood. The phosphatidylinositol 3-kinase (PI3K)–phosphatase with tensin homology (PTEN) and tuberous sclerosis complex (TSC)-target of rapamycin (TOR) pathways are frequently activated in human cancer, and this activation is often causative of tumorigenesis. We utilized the Gal4-UAS system in Drosophila imaginal primordia, highly proliferative and growing tissues, to analyze the impact of restricted activation of these pathways on neighboring wild-type cell populations. Activation of these pathways leads to an autonomous induction of tissue overgrowth and to a remarkable nonautonomous reduction in growth and proliferation rates of adjacent cell populations. This nonautonomous response occurs independently of where these pathways are activated, is functional all throughout development, takes place across compartments, and is distinct from cell competition. The observed autonomous and nonautonomous effects on tissue growth rely on the up-regulation of the proteoglycan Dally, a major element involved in modulating the spreading, stability, and activity of the growth promoting Decapentaplegic (Dpp)/transforming growth factor β(TGF-β) signaling molecule. Our findings indicate that a reduction in the amount of available growth factors contributes to the outcompetition of wild-type cells by overgrowing cell populations. During normal development, the PI3K/PTEN and TSC/TOR pathways play a major role in sensing nutrient availability and modulating the final size of any developing organ. We present evidence that Dally also contributes to integrating nutrient sensing and organ scaling, the fitting of pattern to size.