Gli proteins encode context-dependent positive and negative functions: implications for development and disease.
Several lines of evidence implicate zinc finger proteins of the Gli family in the final steps of Hedgehog signaling in normal development and disease. C-terminally truncated mutant GLI3 proteins are also associated with human syndromes, but it is not clear whether these C-terminally truncated Gli proteins fulfil the same function as full-length ones. Here, structure-function analyses of Gli proteins have been performed using floor plate and neuronal induction assays in frog embryos, as well as induction of alkaline phosphatase (AP) in SHH-responsive mouse C3H10T1/2 (10T1/2) cells. These assays show that C-terminal sequences are required for positive inducing activity and cytoplasmic localization, whereas N-terminal sequences determine dominant negative function and nuclear localization. Analyses of nuclear targeted Gli1 and Gli2 proteins suggest that both activator and dominant negative proteins are modified forms. In embryos and COS cells, tagged Gli cDNAs yield C-terminally deleted forms similar to that of Ci. These results thus provide a molecular basis for the human Polydactyly type A and Pallister-Hall Syndrome phenotypes, derived from the deregulated production of C-terminally truncated GLI3 proteins. Analyses of full-length Gli function in 10T1/2 cells suggest that nuclear localization of activating forms is a regulated event and show that only Gli1 mimics SHH in inducing AP activity. Moreover, full-length Gli3 and all C-terminally truncated forms act antagonistically whereas Gli2 is inactive in this assay. In 10T1/2 cells, protein kinase A (PKA), a known inhibitor of Hh signaling, promotes Gli3 repressor formation and inhibits Gli1 function. Together, these findings suggest a context-dependent functional divergence of Gli protein function, in which a cell represses Gli3 and activates Gli1/2 prevents the formation of repressor Gli forms to respond to Shh. Interpretation of Hh signals by Gli proteins therefore appears to involve a fine balance of divergent functions within each and among different Gli proteins, the misregulation of which has profound biological consequences.
Ruiz i Altaba
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The Skirball Institute, Developmental Genetics Program and Department of Cell Biology, NYU School of Medicine, New York, NY 10016, USA. ria@saturn.med.nyu.edu
D000469:Alkaline Phosphatase / Q000378:metabolism; D000818:Animals; D019556:COS Cells / Q000378:metabolism; D002454:Cell Differentiation / Q000235:genetics; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D002490:Central Nervous System / Q000196:embryology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism*; D004625:Embryo, Nonmammalian / Q000378:metabolism; D004627:Embryonic Induction / Q000235:genetics*; D053823:Hedgehog Proteins; D006801:Humans; D051741:Kruppel-Like Transcription Factors; D051379:Mice; D009419:Nerve Tissue Proteins; D009474:Neurons / Q000378:metabolism; D009687:Nuclear Proteins; D015513:Oncogene Proteins / Q000235:genetics / Q000378:metabolism; D011506:Proteins / Q000235:genetics / Q000378:metabolism; D011898:Ranidae / Q000196:embryology; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins; D015534:Trans-Activators; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism*; D029867:Xenopus Proteins; D000071676:Zinc Finger Protein GLI1; D000075746:Zinc Finger Protein Gli2; D000074290:Zinc Finger Protein Gli3
D004268:DNA-Binding Proteins; C082593:GLI2 protein, human; C108003:GLI3 protein, Xenopus; C065683:GLI3 protein, human; C000618551:Gli2 protein, mouse; C465571:Gli3 protein, mouse; D053823:Hedgehog Proteins; D051741:Kruppel-Like Transcription Factors; D009419:Nerve Tissue Proteins; D009687:Nuclear Proteins; D015513:Oncogene Proteins; D011506:Proteins; D011994:Recombinant Proteins; D012097:Repressor Proteins; C509239:SHH protein, human; D015534:Trans-Activators; D014157:Transcription Factors; D029867:Xenopus Proteins; D000071676:Zinc Finger Protein GLI1; D000075746:Zinc Finger Protein Gli2; D000074290:Zinc Finger Protein Gli3; D017868:Cyclic AMP-Dependent Protein Kinases; D000469:Alkaline Phosphatase