Editors' ChoiceDevelopmental Biology

Alk Regulates Gli Activity

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Science Signaling  23 Jul 2013:
Vol. 6, Issue 285, pp. ec166
DOI: 10.1126/scisignal.2004535

The activity of Gli transcription factors is regulated by Hedgehog signaling, but Popichenko et al. report that activity of the Drosophila melanogaster Gli family transcription factor Lame duck (Lmd) was inhibited by signaling from the receptor tyrosine kinase anaplastic lymphoma kinase (Alk), which is transduced through the extracellular signal–regulated protein kinase (ERK) pathway. In the visceral mesoderm of the fly embryo, founder cells (FCs) and fusion-competent myoblasts (FCMs) fuse with one another to generate myotubes. Although lmd was expressed in both FCs and FCMs, Lmd protein was restricted to the FCMs and required for FCM specification. In alk mutants, FCs were absent and Lmd was present in all visceral mesoderm cells. When Alk signaling was activated throughout the visceral mesoderm by ectopic expression of the gene encoding its ligand Jelly belly (Jeb), all cells expressed FC markers and showed ERK activation but lacked Lmd and FCM markers. The depletion of Lmd from visceral mesoderm cells in response to overexpression of jeb required the ubiquitin ligase Mib2, implying that Lmd may undergo ubiquitin-mediated degradation in FC cells. In human embryonic kidney (HEK) 293 cells, fluorescently tagged Lmd was cytoplasmically localized when Alk signaling was activated and nuclearly localized when the cells were treated with an Alk inhibitor. Live-cell imaging revealed that removing the Alk inhibitor resulted in gradual relocalization of Lmd to the cytoplasm. Treatment with an inhibitor of mitogen-activated or extracellular signal–regulated protein kinase kinase (MEK) inhibited Alk-dependent relocalization of tagged Lmd. Experiments in fly embryos and in human embryonic kidney (HEK) 293 cells indicated that the N-terminal portion of Lmd was required for Alk-dependent relocalization, and mass spectroscopy revealed that Alk signaling induced phosphorylation of several amino acids in this N-terminal region. These results imply a model in which Alk signaling through the ERK pathway promotes FC identity by stimulating Lmd phosphorylation, its exclusion from the nucleus, and its degradation in the cytoplasm, thus preventing Lmd from stimulating the expression of FCM-specific genes.

D. Popichenko, F. Hugosson, C. Sjögren, M. Dogru, Y. Yamazaki, G. Wolfstetter, C. Schönherr, M. Fallah, B. Hallberg, H. Nguyen, R. H. Palmer, Jeb/Alk signalling regulates the Lame duck GLI family transcription factor in the Drosophila visceral mesoderm. Development 140, 3156–3166 (2013). [Abstract] [Full Text]

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