screens, which are aided by collaborative efforts currently underway to mutate all coding genes in mouse, fly, and worm using various methods (see the databases and Web resources listed in Table 1.1). Screens in the different model systems have complemented one another and generated data sets that are overlapping but not identical. This may be in part because of the sensitivi- ty of the phenotypes scored and the types of mutation (e.g., loss-of-function versus hypomorphic alleles) introduced in each system. However, studies of the Hh signaling pathway have suggested that some species-specific mechan- isms are used to transduce cell-cell signals. Free medical books
Key components of the Hh pathway are highly conserved (reviewed in Lum and Beachy, 2004; Huangfu and Anderson, 2005b). Hh signal transduc- tion is controlled by the actions of the Patched (Ptc) receptor and the seven- pass transmembrane protein, Smoothened (Smo). In the absence of Hh, Ptc inhibits Smo from transducing signals. Hh binding to Ptc relieves this inhibi- tion and enables Smo to activate a cytoplasmic signal transduction pathway that culminates in the proteolysis and nuclear translocation of an activating transcription factor (known as Gli in vertebrates and Ci in Drosophila). In mice, ENU mutagenesis screens identified cilia and intraflagellar transport proteins as essential components of the Hh pathway that act downstream of Ptc and Smo (Huangfu et al., 2003; Huangfu and Anderson, 2005a). Func- tional studies have demonstrated that activation of the Hh pathway in verte- brates induces the localization of Smo, Gli2 and Gli3, and other relevant proteins to cilia; a cilia localization motif on Smo is essential for normal Hh responses in cultured cells and zebrafish (Corbit et al., 2005; Haycraft et al., 2005). By contrast, intraflagellar transport mutations do not cause Hh-like phenotypes in Drosophila, and, in this organism, Hh-responsive cells do not have cilia (Ray et al., 1999; Han et al., 2003; Avidor-Reiss et al., 2004). Drosophila Smo accumulates at the plasma membrane upon Hh stimulation, whereas the vertebrate ortholog gets internalized. Furthermore, mammalian and fly Smo proteins are phosphorylated on different residues in response to Hh. Phosphorylation is required for the internalization of mammalian Smo RNAi uses short, double-stranded RNAs to trigger the degradation of tar- get mRNAs species. This was developed as an experimental tool for work with C. elegans, in which it is now widely used for loss-of-function studies and phenotype-driven screens (Fire et al., 1998; Wang and Barr, 2005). Recently, genome-wide screens have been developed that use RNAi in Dro- sophila embryonic imaginal disc cell cultures (clone-8 cells) to identify novel signaling pathway components (Lum et al., 2003). In these screens, clone- 8 cells are cotransfected with a pathway-responsive luciferase reporter and a
proteoglycan (Dally-like, which was previously implicated in Wnt signaling), a homeodomain gene, three kinases (CK1 , Pitslre1, and Cdk9), and a phos- phatase (PP2A). Interestingly, the screens also indicated that the Hh pathway is affected by factors involved in more general cellular processes, including ribosome and proteosome function, RNA regulation and splicing, and vesicle
mutate all coding genes in mouse, fly, and worm using various methods (see
the databases and Web resources listed in Table 1.1). Screens in the different
model systems have complemented one another and generated data sets that
are overlapping but not identical. This may be in part because of the sensitivi-
ty of the phenotypes scored and the types of mutation (e.g., loss-of-function
versus hypomorphic alleles) introduced in each system. However, studies of
the Hh signaling pathway have suggested that some species-specific mechan-
isms are used to transduce cell-cell signals. Free medical books
Key components of the Hh pathway are highly conserved (reviewed in
Lum and Beachy, 2004; Huangfu and Anderson, 2005b). Hh signal transduc-
tion is controlled by the actions of the Patched (Ptc) receptor and the seven-
pass transmembrane protein, Smoothened (Smo). In the absence of Hh, Ptc
inhibits Smo from transducing signals. Hh binding to Ptc relieves this inhibi-
tion and enables Smo to activate a cytoplasmic signal transduction pathway
that culminates in the proteolysis and nuclear translocation of an activating
transcription factor (known as Gli in vertebrates and Ci in Drosophila). In
mice, ENU mutagenesis screens identified cilia and intraflagellar transport
proteins as essential components of the Hh pathway that act downstream of
Ptc and Smo (Huangfu et al., 2003; Huangfu and Anderson, 2005a). Func-
tional studies have demonstrated that activation of the Hh pathway in verte-
brates induces the localization of Smo, Gli2 and Gli3, and other relevant
proteins to cilia; a cilia localization motif on Smo is essential for normal Hh
responses in cultured cells and zebrafish (Corbit et al., 2005; Haycraft et al.,
2005). By contrast, intraflagellar transport mutations do not cause Hh-like
phenotypes in Drosophila, and, in this organism, Hh-responsive cells do not
have cilia (Ray et al., 1999; Han et al., 2003; Avidor-Reiss et al., 2004).
Drosophila Smo accumulates at the plasma membrane upon Hh stimulation,
whereas the vertebrate ortholog gets internalized. Furthermore, mammalian
and fly Smo proteins are phosphorylated on different residues in response to
Hh. Phosphorylation is required for the internalization of mammalian Smo
RNAi uses short, double-stranded RNAs to trigger the degradation of tar-
get mRNAs species. This was developed as an experimental tool for work
with C. elegans, in which it is now widely used for loss-of-function studies
and phenotype-driven screens (Fire et al., 1998; Wang and Barr, 2005).
Recently, genome-wide screens have been developed that use RNAi in Dro-
sophila embryonic imaginal disc cell cultures (clone-8 cells) to identify novel
signaling pathway components (Lum et al., 2003). In these screens, clone-
8 cells are cotransfected with a pathway-responsive luciferase reporter and a
proteoglycan (Dally-like, which was previously implicated in Wnt signaling),
a homeodomain gene, three kinases (CK1 , Pitslre1, and Cdk9), and a phos-
phatase (PP2A). Interestingly, the screens also indicated that the Hh pathway
is affected by factors involved in more general cellular processes, including
ribosome and proteosome function, RNA regulation and splicing, and vesicle