Disease relevance of ci

• Haploinsufficiency for the ci-related transcription factor Gli3 causes phenotypic changes in mice (known as 'extra-toes) and humans (Greig's cephalopolysyndactyly syndrome) that have similarities to Rubinstein-Taybi syndrome [1].
• In vertebrates, three Ci-related transcription factors (glioblastoma gene products (GLIs) 1, 2, and 3) were identified, but their functional difference in Hh signal transduction is unknown [2].
• Second, overexpression of Cubitus interruptus causes the suppression of denticles (as Wingless does) in absence of Wingless activity in the anterior trunk [3].

High impact information on ci

• The secreted signaling molecule Hedgehog regulates gene expression in target cells in part by preventing proteolysis of the full-length Cubitus interruptus (Ci-155) transcriptional activator to the Ci-75 repressor form [4].
• Moreover, we show that Engrailed (En), in the absence of Ci, is sufficient to specify "P-type" sorting [5].
• The Gli homolog Cubitus interruptus (Ci) is involved in controlling the transcription of Hh target genes [6].
• Transcriptional activation of Hedgehog (Hh) target genes requires Cubitus interruptus, a 155 kDa cytoplasmic zinc finger protein (Ci155), which in the absence of Hh signaling is processed to form a nuclear repressor (Ci75) [7].
• Nuclear trafficking of Cubitus interruptus in the transcriptional regulation of Hedgehog target gene expression [7].

Biological context of ci

• These mutant proteins are unable to transactivate a reporter gene regulated by ci binding sites and have a lower dCBP-stimulated activity than wild-type CI [8].
• This model is consistent with genetic data demonstrating that Su(fu) is not required for Hh signal transduction proper and with the elaborate genetic interactions observed among Su(fu), fu, cos2, and ci [9].
• Coincidentally, pan and ci are adjacent genes on the fourth chromosome in a head-to-head orientation [10].
• Other loss-of-function en alleles also enhance the ci phenotype, with the en lethal alleles (and deletions) showing the strongest effect, while the homozygous viable en alleles show weaker enhancement [11].
• As a double heterozygote, it also enhances the ci wing vein phenotype of the dominant alleles ciW and ciCe2, but not ciD [11].

Anatomical context of ci

• We propose that the association of the Cos2 protein complex with Smo at the plasma membrane controls the stability of the complex and allows Ci activation, eliciting its nuclear translocation [12].
• Specifically, we show that the mechanism by which Hh signaling controls initiation of photoreceptor differentiation is to alleviate repression of eya and decapentaplegic (dpp) expression by the zinc-finger transcription factor Cubitus interruptus (Ci(rep)) [13].
• Fu, Cos-2 and Ci are co-associated in vivo in large complexes that are bound to microtubules in a Hh-dependent manner [6,7] [14].
• We show that the Drosophila Schneider 2 (S2) cell line has the potential to transduce the Hh-triggered intracellular signals, leading to the activation of target gene expression, when a transcription factor, Cubitus interruptus (Ci), is provided exogenously [15].
• Consequently, the transcription of CI-dependent genes leads to stem cell proliferation [16].

Associations of ci with chemical compounds

• In addition, we demonstrate that, in cell culture, the mutation of any one of the three serine-containing PKA sites abolishes the proteolytic processing of CI [17].
• This effect requires the cholesterol but not palmitoyl modification on Hh, and expands the zone of full-length Ci expression [18].
• The complex includes the products of at least three genes: fused (a protein-serine/threonine kinase), cubitus interruptus (a transcription factor), and costal2 (a kinesin-like protein) [19].
• The nematode serotonin receptor, designated 5HT-Ce, was permanently expressed in murine Ltk-cells, where it mediates adenylate cyclase attenuation [20].

Physical interactions of ci

• Here we show that the latter response is mediated principally by regulatory elements other than Ci binding sites and not by altered Smo phosphorylation [21].
• Genetic evidence for a protein kinase A/cubitus interruptus complex that facilitates processing of cubitus interruptus in Drosophila [22].
• In these cells, Fu may be involved in the stabilization of a large protein complex which is probably responsible for the regulation of Ci cleavage and/or targeting to nucleus [23].
• We show that Cos2 binds Ci, prevents its nuclear import, and inhibits its activity via this domain [24].
• The Drosophila cubitus interruptus (ci) gene encodes a sequence-specific DNA-binding protein that regulates transcription of Hedgehog (Hh) target genes [25].

Enzymatic interactions of ci

• Here we demonstrate that the PKA pathway antagonizes the hh pathway by phosphorylating CI [26].
• We propose that Cos2 recruits multiple kinases to efficiently phosphorylate Ci and that Hh inhibits Ci phosphorylation by specifically interfering with kinase recruitment [27].

Regulatory relationships of ci

• It is now shown that anterior cells lacking ci express hh and adopt posterior properties without expressing en [28].
• The default activity of Ptc is to inhibit Ci function; when Ptc binds Hh, this inhibition is released and Ci can control wg transcription [29].
• Interactions with Costal2 and suppressor of fused regulate nuclear translocation and activity of cubitus interruptus [24].
• These effects were all enhanced in a fu mutant context and were suppressed by cubitus interruptus (Ci) overexpression [30].
• Mitotic clones induced at the beginning of the second larval instar do not cross the boundary between the engrailed-expressing and cubitus interruptus-expressing domains, indicating that these domains are true genetic compartments [31].

Other interactions of ci

• Increased levels of Ci can induce the expression of the Hh target gene decapentaplegic (dpp) in a Hh-independent manner [28].
• When expressed exogenously in embryos, the CI point mutants cannot activate endogenous wg expression [8].
• Activity of this element depends on ptc, but it contains no consensus Ci-binding sites [29].
• Engrailed and polyhomeotic maintain posterior cell identity through cubitus-interruptus regulation [32].
• We have identified a 125 amino acid domain in the C-terminal part of Ci that mediates response to Cos2 inhibition [24].

Analytical, diagnostic and therapeutic context of ci

• Using chromatin immunoprecipitation (ChIP), we identified a 1 kb genomic fragment located 4 kb upstream of the ci coding region that is responsible for the regulation of ci [32].
• Our genetic and in situ hybridization data indicate that ciD is a mutation affecting both ci and pan [10].
• We provide evidence that Hh signaling controls A/P cell segregation not by directly modifying structural components but by a Cubitus interruptus (Ci)-mediated transcriptional response [5].
• Using the yeast two-hybrid method and an in vitro binding assay, we show that Su(fu), Ci and Fu can interact directly to form a trimolecular complex, with Su(fu) binding to both its partners simultaneously [14].
• Genetic dissection of the Drosophila Cubitus interruptus signaling complex [33].

References