Ci: a complex transducer of the Hedgehog signal

doi:10.1016/S0168-9525(99)01869-7

Trends in Genetics

Volume 15, Issue 11, 1 November 1999, Pages 458-462

https://doi.org/10.1016/S0168-9525(99)01869-7 Get rights and content

Abstract

Recent progress has unveiled Cubitus interruptus (Ci) as a complex transcription factor whose diverse activities as an activator and repressor are regulated by its proteolysis, localization and concentration. The principal role of Ci is to elaborate the developmental program directed by the morphogen Hedgehog (Hh), and it uses its various activities to target the expression of key downstream genes to different spatial domains. Here, we highlight recent advances in the Ci story, and discuss remaining questions whose resolution promise to help explain how morphogens like Hh signal their distant targets.

Section snippets

The ci gene

The Drosophila ci gene was identified in 1933 with a viable allele (ci¹) that arose spontaneously and caused disruptions in the cubital vein in the P compartment of the wing blade³⁴. Today there are, in addition to a number of deletions, a small number of other alleles – three that are recessive and viable (ci¹, ci^56l and ci^57g), two that are recessive lethal (l(4)17, 94), and three recessive lethals that have dominant wing phenotypes (ci^D, ci^W and ci^Ce). Complementation among these alleles is

The Ci protein and its signal-transduction partners

Ci has a variety of functions in its role as a transcription regulator in the A compartment of the wing disc. It represses hh and dpp. It also responds to Hh, both by upregulating ptc, thereby helping to restrict the reach of the Hh signal, and by inducing dpp (18, 19, 42, 43). The key to understanding these different roles was the finding that Ci protein can exist in different forms and that the relative amounts of these forms is regulated by Hh. Ci exists as a cytoplasmic protein18, 25, 26, 44

Nuclear/cytoplasmic concentration

Ci is the type of transcription factor whose activity is regulated in part by its relative nuclear/cytoplasmic concentration. Inhibition of nuclear export leads to the accumulation of nuclear Ci (Ref. 51), and a domain that is C-terminal to the zinc fingers is responsible for localizing the full-length protein in the cytoplasm¹⁸. Proteolysis separates the N-terminal, zinc-finger-containing portion that lacks the cytoplasmic localization domain, and redirects the processed protein (Ci^R) to the

Concluding comments

The many ways that cells respond to Hh appear to be related, in part, to their distance from the source of this morphogen, and the ability of Ci to induce targets differentially is key. We now know that the active form of Ci changes in response to Hh, but we still lack a full understanding of the different forms that it takes and the ways by which its activities are regulated. Future progress in answering the many remaining questions promises to provide insights into the mechanisms that define

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Non-syndromic mitral valve prolapse (MVP) is the most common heart valve disease affecting 2.4% of the population. Recent studies have identified genetic defects in primary cilia as causative to MVP, although the mechanism of their action is currently unknown. Using a series of gene inactivation approaches, we define a paracrine mechanism by which endocardially-expressed Desert Hedgehog (DHH) activates primary cilia signaling on neighboring valve interstitial cells. High-resolution imaging and functional assays show that DHH de-represses smoothened at the primary cilia, resulting in kinase activation of RAC1 through the RAC1-GEF, TIAM1. Activation of this non-canonical hedgehog pathway stimulates α-smooth actin organization and ECM remodeling. Genetic or pharmacological perturbation of this pathway results in enlarged valves that progress to a myxomatous phenotype, similar to valves seen in MVP patients. These data identify a potential molecular origin for MVP as well as establish a paracrine DHH-primary cilium cross-talk mechanism that is likely applicable across developmental tissue types.
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Canonical Hedgehog (HH) signaling leads to the regulation of the GLI code: the sum of all positive and negative functions of all GLI proteins. In humans, the three GLI factors encode context-dependent activities with GLI1 being mostly an activator and GLI3 often a repressor. Modulation of GLI activity occurs at multiple levels, including by co-factors and by direct modification of GLI structure. Surprisingly, the GLI proteins, and thus the GLI code, is also regulated by multiple inputs beyond HH signaling. In normal development and homeostasis these include a multitude of signaling pathways that regulate proto-oncogenes, which boost positive GLI function, as well as tumor suppressors, which restrict positive GLI activity. In cancer, the acquisition of oncogenic mutations and the loss of tumor suppressors – the oncogenic load – regulates the GLI code toward progressively more activating states. The fine and reversible balance of GLI activating GLI^A and GLI repressing GLI^R states is lost in cancer. Here, the acquisition of GLI^A levels above a given threshold is predicted to lead to advanced malignant stages. In this review we highlight the concepts of the GLI code, the oncogenic load, the context-dependency of GLI action, and different modes of signaling integration such as that of HH and EGF. Targeting the GLI code directly or indirectly promises therapeutic benefits beyond the direct blockade of individual pathways.
Wing tips: The wing disc as a platform for studying Hedgehog signaling
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Citation Excerpt :
An exciting new model for how Hh moves through tissues came from the recent discovery that Hh navigates wing disc epithelia along the exterior of cellular extensions of the basolateral membrane called cytonemes that can span up to 70 μm in length [29,30,43,44]. In most cases, Hh signaling controls tissue development by regulating post-translational modifications of the Cubitus Interruptus (Ci) transcription factor and, by extension, the genes Ci regulates [45,46] (Fig. 2). In the absence of Hh ligands, the kinases Protein Kinase A (PKA), Casein Kinase 1 (CK1), and Glycogen Synthase Kinase 3 (GSK3) phosphorylate the C-terminus of full-length (155 kDa) Ci, which is termed CiF.
Hedgehog (Hh) signal transduction is necessary for the development of most mammalian tissues and can go awry and cause birth defects or cancer. Hh signaling was initially described in Drosophila, and much of what we know today about mammalian Hh signaling was directly guided by discoveries in the fly. Indeed, Hh signaling is a wonderful example of the use of non-vertebrate model organisms to make basic discoveries that lead to new disease treatment. The first pharmaceutical to treat hyperactive Hh signaling in Basal Cell Carcinoma was released in 2012, approximately 30 years after the isolation of Hh mutants in Drosophila. The study of Hh signaling has been greatly facilitated by the imaginal wing disc, a tissue with terrific experimental advantages. Studies using the wing disc have led to an understanding of Hh ligand processing, packaging into particles for transmission, secretion, reception, signal transduction, target gene activation, and tissue patterning. Here we describe the imaginal wing disc, how Hh patterns this tissue, and provide methods to use wing discs to study Hh signaling in Drosophila. The tools and approaches we highlight form the cornerstone of research efforts in many laboratories that use Drosophila to study Hh signaling, and are essential for ongoing discoveries.
Deciphering the onychophoran 'segmentation gene cascade': Gene expression reveals limited involvement of pair rule gene orthologs in segmentation, but a highly conserved segment polarity gene network
2013, Developmental Biology
Citation Excerpt :
Hh signalling towards posterior is impossible since posterior cells do not express ptc. As a result of Hh binding to Ptc, in Drosophila, the transcription factor cubitus-interruptus (ci), which is expressed in non-engrailed expressing cells, is activated and transferred into the nucleus where it positively regulates the transcription of wg (reviewed in Aza-Blanc and Kornberg, 1999). In Euperipatoides this interaction is also likely conserved since ci is expressed in virtually all cells that do not express en (e.g. Eaton and Kornberg, 1990; Orenic et al., 1990; Damen, 2002; Janssen et al., 2004, 2008; Farzana and Brown, 2008).
The hallmark of the arthropods is their segmented body, although origin of segmentation, however, is unresolved. In order to shed light on the origin of segmentation we investigated orthologs of pair rule genes (PRGs) and segment polarity genes (SPGs) in a member of the closest related sister-group to the arthropods, the onychophorans. Our gene expression data analysis suggests that most of the onychophoran PRGs do not play a role in segmentation. One possible exception is the even-skipped (eve) gene that is expressed in the posterior end of the onychophoran where new segments are likely patterned, and is also expressed in segmentation-gene typical transverse stripes in at least a number of newly formed segments. Other onychophoran PRGs such as runt (run), hairy/Hes (h/Hes) and odd-skipped (odd) do not appear to have a function in segmentation at all. Onychophoran PRGs that act low in the segmentation gene cascade in insects, however, are potentially involved in segment-patterning. Most obvious is that from the expression of the pairberry (pby) gene ortholog that is expressed in a typical SPG-pattern. Since this result suggested possible conservation of the SPG-network we further investigated SPGs (and associated factors) such as Notum in the onychophoran. We find that the expression patterns of SPGs in arthropods and the onychophoran are highly conserved, suggesting a conserved SPG-network in these two clades, and indeed also in an annelid. This may suggest that the common ancestor of lophotrochozoans and ecdysozoans was already segmented utilising the same SPG-network, or that the SPG-network was recruited independently in annelids and onychophorans/arthropods.
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In Drosophila, a single Gli protein, named Cubitus interruptus (Ci), is present. Upon ligand binding, the cleavage of Ci into a repressor isoform is inhibited, and the activator isoform transcriptionally upregulates target genes [58]. Sixty-four unique mutations in the SHH gene have been linked to holoprosencephaly [59].
In this review, we highlight recent literature concerning the signaling mechanisms underlying the development of two neural birth defects, holoprosencephaly and coloboma. Holoprosencephaly, the most common forebrain defect, occurs when the cerebral hemispheres fail to separate and is typically associated with mispatterning of embryonic midline tissue. Coloboma results when the choroid fissure in the eye fails to close. It is clear that Sonic hedgehog (Shh) signaling regulates both forebrain and eye development, with defects in Shh, or components of the Shh signaling cascade leading to the generation of both birth defects. In addition, other intercellular signaling pathways are known factors in the incidence of holoprosencephaly and coloboma. This review will outline recent advances in our understanding of forebrain and eye embryonic pattern formation, with a focus on zebrafish studies of Shh and retinoic acid pathways. Given the clear overlap in the mechanisms that generate both diseases, we propose that holoprosencephaly and coloboma can represent mild and severe aspects of single phenotypic spectrum resulting from aberrant forebrain development. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

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Trends in Genetics

ReviewCi: a complex transducer of the Hedgehog signal

Abstract

Section snippets

The ci gene

The Ci protein and its signal-transduction partners

Nuclear/cytoplasmic concentration

Concluding comments

Cell

J. Biol. Chem.

Cell

Cell

Cell

Dev. Biol.

Cell

Cell

Mech. Dev.

Cell

Cell

Cell

Cell

Cell

Mech. Dev.

Dev. Biol.

Mech. Dev.

Cell

Cell

J. Biol. Chem.

Mech. Dev.

Autoproteolysis in hedgehog protein biogenesis

Science

Cholesterol modification of hedgehog signaling proteins in animal development

Science

Relationship between dose, distance and time in Sonic Hedgehog-mediated regulation of anteroposterior polarity in the chick limb

Development

A Hedgehog activity gradient contributes to AP axial patterning of the Drosophila wing

Development

Hedgehog activity, independent of decapentaplegic, participates in wing disc patterning

Development

Tout-velu is a Drosophila homologue of the putative tumour suppressor EXT-1 and is needed for Hh diffusion

Nature

A protein with several possible membrane-spanning domains encoded by the Drosophila segment polarity gene patched

Nature

smoothened encodes a receptor-like serpentine protein required for hedgehog signalling

Nature

Review
Ci: a complex transducer of the Hedgehog signal