1013 Fairchild Center, M.C. 2445
New York, N.Y. 10027
Our earlier genetic investigations of developmental roles of Protein Kinase A (PKA) in Drosophila led to three current projects described below.
1. Mechanism of Hedgehog Signal Transduction.
Hedgehog (Hh) signaling controls a multitude of developmental processes in Drosophila and other organisms, including vertebrates. This requires spatially regulated Hh expression and induction of cell-specific transcriptional responses in nearby cells. PKA activity is required to prevent ectopic Hh signal transduction in the absence of ligand. The major role of PKA is to phosphorylate the transcription factor Cubitus interruptus (Ci). This phosphorylation is required for full-length Ci-155 to be partially proteolyzed to a shorter form, Ci-75, that acts as a transcriptional repressor. In the absence of PKA or in the presence of Hh, Ci-155 is not converted to Ci-75 and activation of Hh-target genes is favored. Hh signaling also increases the specific activity of Ci-155 as a transcriptional activator. PKA phosphorylation of Ci primes further phosphorylation at neighboring sites by Glycogen Synthase Kinase 3 (GSK3) and Casein Kinase I (CK1) activities. GSK3 sites, CK1 sites and the activity of the GSK3 named Shaggy (Sgg) are all required for conversion of Ci-155 to Ci-75. We are currently trying to determine whether Hh signaling regulates phosphorylation of Ci-155 by PKA, GSK3 and CK1 and how this leads to partial proteolysis.
2. Regulating somatic stem cells in the Drosophila ovary
Hedgehog acts as a stem cell factor in Drosophila ovaries. Somatic ovarian stem cells that lose the ability to transduce a Hh signal cannot self-renew. Conversely, excessive Hh signal transduction pathway activity allows a somatic stem cell to produce two (instead of the usual one) daughter stem cells. It is generally believed that the fundamental capacity of stem cells for self-renewal depends on numerous extracellular factors, the presence of which defines a “niche” that can harbor stem cells. We are testing whether other known signal transduction pathways affect ovarian somatic stem cell behavior and we are screening for mutations that modify the over-proliferation of somatic ovarian cells caused by excessive Hh signaling in the hope that this might reveal effector genes that are induced by Hh signaling.
3. PKA and Anterior-Posterior Polarity during oogenesis.
PKA is essential for correct localization of mRNAs (bicoid and oskar) at the anterior and posterior poles of developing Drosophila oocytes. These localizations are themselves essential for directing embryonic patterning. In the absence of PKA a population of microtubules nucleated at the extreme posterior of the oocyte fails to disassemble during a microtubule re-organization that takes place during mid-oogenesis. This re-organization normally provides a substrate of appropriate polarity for motor proteins to transport specific mRNAs (such as bicoid and oskar) to their appropriate destinations along the anterior-posterior (AP) axis. Similar phenotypes are produced by mutations that affect the specification of the follicle cells that surround the oocyte, leading to the idea that these follicle cells signal to the oocyte to induce de-polymerization of posterior microtubules. We are trying to identify components of this pathway by identifying dominant enhancers of a weak PKA mutant phenotype. We are mapping and identifying the affected genes for those enhancer mutations that disrupt AP polarity by themselves.
Price MA, Kalderon D.
(2002). Proteolysis of the Hedgehog signaling effector
Cubitus interruptus requires phosphorylation by Glycogen Synthase Kinase 3 and
Casein Kinase 1. Cell 108: 823-35.
Zhang Y, Kalderon D. (2001). Hedgehog acts as a somatic stem cell factor in the Drosophila ovary. Nature 410: 599-604.
PMID: 11279500 [PubMed - indexed for MEDLINE]
Zhang Y, Kalderon D. (2000). Regulation of cell proliferation and patterning in Drosophila oogenesis by Hedgehog signaling.
Development. 127: 2165-76.
Price MA, Kalderon D. (1999). Proteolysis of cubitus interruptus in Drosophila requires phosphorylation by protein kinase A. Development.126: 4331-9.
Ohlmeyer JT, Kalderon D. (1998). Hedgehog stimulates maturation of Cubitus interruptus into a labile transcriptional activator. Nature. 396: 749-53.
Ohlmeyer JT, Kalderon D. (1997). Dual pathways for induction of wingless expression by protein kinase A and Hedgehog in Drosophila embryos.
Genes Dev. 11: 2250-8.
Li W, Ohlmeyer JT, Lane ME, Kalderon D. (1995). Function of protein kinase A in hedgehog signal transduction and Drosophila imaginal disc development.
Cell. 80: 553-62.
Lane ME, Kalderon D. (1994). RNA localization along the anteroposterior axis of the Drosophila oocyte requires PKA-mediated signal transduction to direct normal microtubule organization. Genes Dev. 8: 2986-95.