Multiple Wnts act synergistically to induce Chk1/Grapes expression and mediate G2 arrest in Drosophila tracheoblasts

Abstract

30Larval tracheae of Drosophila harbor progenitors of the adult tracheal system 31 (tracheoblasts). Thoracic tracheoblasts are arrested in the G2 phase of the cell 32 cycle in an ATR (mei-41)-Checkpoint Kinase1 (grapes, Chk1) dependent manner 33 prior to mitotic re-entry. Here we investigate developmental regulation of Chk1 34 activation. We report that Wnt signaling is high in tracheoblasts and is necessary 35 for high levels of activated (phosphorylated) Chk1. We find that canonical Wnt 36 signaling facilitates this by transcriptional upregulation of Chk1 expression in 37 cells that have ATR kinase activity. Wnt signalling is dependent on four Wnts 38 (Wg, Wnt5, 6,10) that are expressed at high levels in arrested tracheoblasts and 39 downregulated at mitotic re-entry. Interestingly, none of the Wnts are dispensable 40 and act synergistically to induce Chk1. Finally, we show that downregulation of 41 Wnt signalling and Chk1 expression leads to mitotic re-entry and the concomitant 42 upregulation of Dpp signalling, driving tracheoblast proliferation. 43 44 45 3