Academic journal article Genetics

Wg Signaling Via Zw3 and Mad Restricts Self-Renewal of Sensory Organ Precursor Cells in Drosophila

Academic journal article Genetics

Wg Signaling Via Zw3 and Mad Restricts Self-Renewal of Sensory Organ Precursor Cells in Drosophila

Article excerpt

ABSTRACT It is well known that the Dpp signal transducer Mad is activated by phosphorylation at its carboxy-terminus. The role of phosphorylation on other regions of Mad is not as well understood. Here we report that the phosphorylation of Mad in the linker region by the Wg antagonist Zw3 (homolog of vertebrate Gsk3-β) regulates the development of sensory organs in the anterior-dorsal quadrant of the wing. Proneural expression of Mad-RNA interference (RNAi) or a Mad transgene with its Zw3/Gsk3-β phosphorylation sites mutated (MGM) generated wings with ectopic sensilla and chemosensory bristle duplications. Studies with pMad-Gsk (an antibody specific to Zw3/Gsk3-β-phosphorylated Mad) in larval wing disks revealed that this phosphorylation event is Wg dependent (via an unconventional mechanism), is restricted to anterior-dorsal sensory organ precursors (SOP) expressing Senseless (Sens), and is always co-expressed with the mitotic marker phospho-histone3. Quantitative analysis in both Mad-RNAi and MGM larval wing disks revealed a significant increase in the number of Sens SOP. We conclude that the phosphorylation of Mad by Zw3 functions to prevent the self-renewal of Sens SOP, perhaps facilitating their differentiation via asymmetric division. The conservation of Zw3/Gsk3-β phosphorylation sites in vertebrate homologs of Mad (Smads) suggests that this pathway, the first transforming growth factor β-independent role for any Smad protein, may be widely utilized for regulating mitosis during development.

INTERCELLULAR signaling is essential for proper development of multicellular organisms. In all animals, highly conserved proteins belonging to the transforming growth factor b (TGFb) family perform a multitude of tasks. TGFb proteins can be parsed into the TGFb/Activin or Dpp/BMP subfamilies. In Drosophila, Dpp signals utilize the type I receptor Thickveins (Tkv), and signal transduction proceeds via Tkv phosphorylation of carboxy-terminal serines in the signal transducer Mothers against dpp (Mad). Once Receptor phosphorylated, Mad nuclear import occurs, and Mad then forms a complex with Medea. Mad/Medea complexes regulate gene expression together with tissue-specific transcription factors (Derynck and Miyazono 2008).

Mad and Medea are members of a highly conserved Smad family of TGFb signal transducers. Mad and Smads1/5/8 in vertebrates signal for Dpp/BMP subfamily proteins while Medea and Smad4 in vertebrates form complexes with Smads that signal for all TGFb proteins (Newfeld and Wisotzkey 2006). There are many instances during development when interactions between the TGFb pathway and the equally ancient Wnt-signaling pathway are required. In brief, canonical Wg signal transduction begins with the Frizzled2 Receptor and proceeds via activation of Dishevelled (Dsh). Dsh then relays the signal to a ubiquitous cytoplasmic complex that includes Zw3 (Gsk3-b in vertebrates), dAPC, dAxin, and Armadillo (Arm; b-catenin in vertebrates). Under nonsignaling conditions, Zw3 phosphorylation continuously shunts the ubiquitously expressed Arm into the proteasome pathway for degradation. Upon receiving a Dsh signal, Zw3 is prevented from phosphorylating Arm. This leads to Arm nuclear accumulation and activation of gene expression in cooperation with transcription factors such as dTCF (Logan and Nusse 2004).

Frequently, the molecular mechanism underlying TGFb- Wnt interactions is binding of Smad proteins to b-catenin and/or TCF. These complexes synergystically activate target genes via bipartite enhancer sequences (e.g., Nishita et al. 2000). However, a phylogenetic analysis suggested the existence of another mechanism (Newfeld and Wisotzkey 2006). Conserved Zw3/Gsk3-b (serine-threonine kinase) sites were identified in all Mad/Smad1/5/8 subfamily members. Thus, it was predicted that Mad/Smad1 phosphorylation by Zw3/Gsk3-b represented a cytoplasmic mechanism of Smad-Wnt interaction. This prediction was subsequently confirmed. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.