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| FACULTY BIOGRAPHY |
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| Daniel Kalderon |
| Professor |
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Our general objectives are to understand how cell behaviors are regulated by communications between cells in development by using Drosophila molecular genetics supplemented by suitable biochemical and microscopy approaches. Our earlier investigations of the functions of Protein Kinase A (PKA) led us to study how a Hedgehog signal is transduced to alter cellular behavior, and this in turn led us to investigate how somatic stem cells in the Drosophila ovary are regulated by Hedgehog and other factors.
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 silencing role of PKA is to phosphorylate the transcription factor Cubitus interruptus (Ci). PKA phosphorylation of Ci at three sites primes further phosphorylation at neighboring sites by Glycogen Synthase Kinase 3 (GSK3) and Casein Kinase I (CK1) activities. Each of these protein kinases and Ci phosphorylation sites is required for normal conversion of Ci-155 to Ci-75, which acts as a transcriptional repressor of Hh target genes. Conversion to Ci-75 depends on binding of the SCF ubiquitin ligase component, Slimb, to phospho-epitopes of Ci-155, followed by ubiquitination and incomplete proteolysis by the proteasome. Hh signaling blocks Ci-155 proteolysis. Hence, a key question is whether this is achieved by altering the phosphorylation of Ci-155, and if so, how?
In the absence of Hh, Ci-155 is only slowly converted to Ci-75 but is additionally held largely in the cytoplasm and silenced as a transcriptional activator. Phosphorylation of Ci-155 by PKA, CK1 and GSK3 may contribute to this silencing directly in addition to promoting Ci-155 proteolysis. If so, it will be important to establish the mechanistic basis for this and how Ci-155 silencing is relieved by Hh. The activity and proteolysis of GLI proteins, the veretebrate homologs, of Ci, are also regulated by the same set of protein kinases and analogous phosphorylation sites, implying that the mechanisms we are investigating are widely conserved in Hedgehog signaling. This is especially interesting in light of recent evidence that upstream aspects of Hh signal transduction may differ substantially between Drosophila and mammals.
Smoothened (Smo) is a seven transmembrane protein that is essential for transduction of all Hh signals, but both its activity and mode of regulation remain mysterious. The C-terminal tail of Drosophila Smo (but not mouse or human Smo) includes three PKA sites that prime neighboring CK1 sites, as in Ci. These PKA and CK1 sites are essential for Smo activity (defined by induction of Hh target genes) and acidic residues at these positions can confer some constitutive (Hh-independent) activity. We would like to understand how Hh influences phosphorylation at these sites and how phosphorylation contributes to Smo activity.
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 using a genetic screen to identify proteins that are required in ovarian somatic stem cells for their self-renewal. The objective is to define other external signaling molecules that influence stem cell behavior, and to understand how Hh and other factors affect the proliferation, survival, adhesion or undifferentiated state of these stem cells.
MedLine Listing of Dr. Kalderon's Publications |
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Representative Recent Publications
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- Vied, C. and Kalderon, D. (2009) Hedgehog-stimulated stem cells depend on non-canonical activity of the Notch co-activator Mastermind. Development, in press. Article
- Smelkinson, M.G., Zhou, Q. and Kalderon, D (2007) Regulation of Ci-SCFSlimb binding, Ci proteolysis, and Hedgehog pathway activity by Ci phosphorylation Dev. Cell 13: 481-495. Article
- Zhou, Q, Apionishev, S. and Kalderon, D. (2006) The contributions of protein kinase A and smoothened phosphorylation to Hedgehog signal transduction in Drosophila melanogaster Genetics 173: 2049-2062. Article
- Smelkinson MG, Kalderon D. (2006) Processing of the Drosophila hedgehog signaling effector Ci-155 to the repressor Ci-75 is mediated by direct binding to the SCF component Slimb. Curr Biol 16: 110-116. Article
- Apionishev S, Katanayeva NM, Marks SA, Kalderon D, Tomlinson A. (2005) Drosophila Smoothened phosphorylation sites essential for Hedgehog signal transduction Nat Cell Biol. 7(1): 86-92. Article
- 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. Article
- Zhang Y, Kalderon D. (2001) Hedgehog acts as a somatic stem cell factor in the Drosophila ovary. Nature 410: 599-604. Article
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