Abstract
Precise control of the range of signalling molecule action is crucial for correct cell fate patterning during development. For example, Drosophila ovarian germline stem cells (GSCs) are maintained by exquisitely short-range BMP signalling from the niche. In the absence of BMP signalling, one GSC daughter differentiates into a cystoblast (CB) and this fate is stabilised by Brain tumour (Brat) and Pumilio (Pum)-mediated post-transcriptional repression of mRNAs, including that encoding the Dpp transducer, Mad. However, the identity of other repressed mRNAs and the mechanism of post-transcriptional repression are currently unknown. Here, we identify the Medea and schnurri mRNAs, which encode transcriptional regulators required for activation and/or repression of Dpp target genes, as additional Pum-Brat targets, suggesting that tripartite repression of the transducers is deployed to desensitise the CB to Dpp. In addition, we show that repression by Pum-Brat requires recruitment of the CCR4 and Pop2 deadenylases, with knockdown of deadenylases in vivo giving rise to ectopic GSCs. Consistent with this, Pum-Brat repression leads to poly(A) tail shortening and mRNA degradation in tissue culture cells, and we detect a reduced number of Mad and shn transcripts in the CB relative to the GSC based on single molecule mRNA quantitation. Finally, we show generality of the mechanism by demonstrating that Brat also attenuates pMad and Dpp signalling range in the early embryo. Together our data serve as a platform for understanding how post-transcriptional repression restricts interpretation of BMPs and other cell signals in order to allow robust cell fate patterning during development.
Highlights
The bone morphogenetic proteins (BMPs) constitute a major conserved family of signalling proteins that regulate a range of cellular processes during development and homeostasis, including cell fate determination (Ramel and Hill, 2012)
As we have previously provided evidence that Pum-Brain tumour (Brat) post-transcriptionally represses Mothers against dpp (Mad) mRNA via its 3′ UTR (Harris et al, 2011) we wished to determine whether similar regulation exists for Med and shn mRNAs
As Mad mRNA regulation by Pum-Brat occurs in the absence of Nos in the CB, we mimicked this situation by reducing Nos levels in S2 cells using nos RNAi, as described previously (Harris et al, 2011)
Summary
The bone morphogenetic proteins (BMPs) constitute a major conserved family of signalling proteins that regulate a range of cellular processes during development and homeostasis, including cell fate determination (Ramel and Hill, 2012). *Present address: Department of Molecular and Cell Biology, Life Sciences Addition Building, University of California, Berkeley, CA 94720, USA. When a GSC divides, one daughter remains within the niche while the other, the cystoblast (CB) is displaced posteriorly and receives a lower level of Dpp signal This results in de-repression of Bam and subsequent differentiation (Kirilly and Xie, 2007; Harris and Ashe, 2011)
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