Daniel Kalderon
Columbia University
Biological Sciences
1013 Fairchild Center, M.C. 2445
New York, N.Y. 10027
212-854-6469
E-mail
ddk1@columbia.edu
Current Lab Members ... Past Lab Members ... Selected References
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.