cubitus interruptus



The spatial pattern of expression of ci during development is unusual in that, unlike most other segmentation genes, it exhibits uniform expression throughout cellular blastoderm and gastrulation stages and does not resolve into a periodic pattern until the end of the fast phase of germ-band elongation [Image] when it is present in 15 broad segmentally repeating stripes along the anterior-posterior axis of the embryo. Registration of the ci stripes of expression relative to the stripes of other segment polarity genes shows that ci is expressed in the posterior three-quarters of every parasegment. This registration does not correlate with the pattern defects observed in ci mutants (Orenic, 1990).

Expression of ci is observed in the hindgut primordia and in the head region corresponding to the procephalic lobe. It accumulates in the ectoderm, nervous system and mesoderm during germ band elongation. There is very little expression by stage 15 (Orenic, 1990).

In Drosophila embryos, segment boundaries form at the posterior edge of each stripe of engrailed expression. An HRP-CD2 transgene has been used to follow by transmission electron microscopy the cell shape changes that accompany boundary formation. The first change is a loosening of cell contact at the apical side of cells on either side of the incipient boundary. Then, the engrailed-expressing cells flanking the boundary undergo apical constriction, move inwards and adopt a bottle morphology. Eventually, grooves regress, first on the ventral side, then laterally. Groove formation and regression are contemporaneous with germ band retraction and shortening, respectively, suggesting that these rearrangements could also contribute to groove morphology. The cellular changes accompanying groove formation require that Hedgehog signalling be activated, and, as a result, a target of Ci is expressed at the posterior of each boundary (obvious targets like stripe and rhomboid appear not to be involved). In addition, Engrailed must be expressed at the anterior side of each boundary, even if Hedgehog signalling is artificially maintained. Thus, there are distinct genetic requirements on either side of the boundary. In addition, Wingless signalling at the anterior of the domains of engrailed (and hedgehog) expression represses groove formation and thus ensures that segment boundaries form only at the posterior (Larsen, 2003).

Segmental boundary formation is initiated shortly after germ-band retraction has begun. They are recognizable as periodic indentations in the epidermis that separate cells expressing engrailed at the anterior from those expressing rhomboid at the posterior. To allow identification of cells in electron micrographs, a transgenic membrane marker was devised based on horseradish peroxidase (HRP), which catalyses the production of an electron-dense product from diaminobenzidine (DAB). HRP was fused to the transmembrane protein CD2 so that the marker would outline cells and thus reveal cell shapes. This inert fusion protein was expressed under the control of engrailed-Gal4, so that the membrane of engrailed-expressing cells appears dark under the electron microscope (Larsen, 2003).

Engrailed has both a cell autonomous and a non-cell autonomous function in the establishment of the compartment boundary in wing imaginal discs. Although the compartment boundary does not trace its embryonic origin to segment boundaries, there is a striking parallel between the two. For segmental grooves to form, Hedgehog signaling is required in cells at the posterior of the boundary, even if engrailed expression is artificially maintained at the anterior side. Conversely, Hedgehog signaling is not sufficient as exogenous expression of hedgehog in the absence of engrailed does not lead to groove formation (Larsen, 2003).

It is the cells that line the anterior side of segment boundaries (the most posterior engrailed-expressing cells) that undergo the most distinctive behavior during groove formation. This behavior requires Hedgehog signalling, and yet engrailed-expressing cells are not responsive to this signal. Therefore, their morphological changes must be in response to a signal originating from neighboring non-engrailed expressing cells. This could be achieved through standard paracrine signaling or by contact-dependent signal mediated by cell surface proteins. Whatever the mechanism, Hedgehog-responsive cells influence the behavior of adjoining engrailed-expressing cells across the boundary, and crosstalk between the two cells takes place. This is reminiscent of the situation found during eye morphogenesis; at rhombomere boundaries cross communication between neighboring rhombomere cells are required for rhombomere formation (Larsen, 2003).

Because boundaries form in the complete absence of Ci (in ci94), it is concluded that the activator form of Ci is not required for segment boundary formation. However, no boundary forms in ciCell mutant embryos, indicating that the presence of Ci[75] (the repressor) prevents boundary formation. It is suggest therefore that boundary formation requires the expression of a gene (x) that is repressed by Ci[75] but does not require Ci[155] to be activated. Presumably, an activator of x is constitutively present but, in the absence of Hedgehog, it is prevented from activating x expression by Ci[75]. Hedgehog signaling would remove Ci[75] and thus allow activation to occur. Two characterized target genes of Hedgehog (wingless and rhomboid) follow the same mode of regulation. For example, expression of wingless in the embryonic epidermis decays in ciCell but is still present in the complete absence of Ci, in ci94 embryos (Larsen, 2003).

Although Hedgehog signaling is activated both at the anterior and the posterior of its source, segment boundaries only form at the posterior. One reason for this asymmetry is that Wingless signaling represses boundary formation at the anterior. Indeed, in the absence of Wingless, boundaries are duplicated, as long as expression of Engrailed and Hedgehog is artificially maintained. It is concluded that expression of x is repressed by Wingless signalling. Two obvious candidates for x are Rhomboid and Stripe. Genes encoding both these proteins are activated by Hedgehog signaling and repressed by Wingless signaling and, indeed, both are expressed in cells that line the segment boundary. To determine if either gene could mediate the role of Hedgehog in boundary formation, the respective mutants were examined. No effect on grooves could be seen. It is concluded that neither rhomboid nor stripe is required for boundary formation although the possibility that these genes could contribute in a redundant fashion cannot be excluded. Overall the genetic analysis suggests that additional targets of Hedgehog must be involved in boundary formation. It will be interesting to find out whether any of these targets will turn out to be implicated in compartment boundary maintenance as well (Larsen, 2003).

Although the role of a Hedgehog target gene in boundary formation has been emphasized, it is clear from this analysis that engrailed also has a cell-autonomous role. Even though Engrailed represses ci expression, its role in boundary formation is likely to involve the transcriptional regulation of another target gene. One possibility is that Engrailed could be a repressor of x and that boundaries would form at the interface between x-expressing and non-expressing cells. However, it is thought that instead, or in addition, Engrailed has a Hedgehog-independent effect on cell affinity and that this could contribute to boundary formation. Of note is the observation that engrailed-expressing cells remain together in small groups even when boundaries are lost for lack of hedgehog. This suggests that engrailed-expressing cells have increased affinity for one another. Thus, Engrailed could specify P specific cell adhesion independently of Hedgehog. Clearly, future progress will require the identification of Engrailed target genes that control such preferential affinity and/or contribute to boundary formation (Larsen, 2003).


ci is expressed in the anterior compartment of the eye, antennal, leg, wing and haltere imaginal discs. In eye discs CI staining is elevated just anterior to the morphogenetic furrow. ci is also expressed in the larval brain lobes and along the midline of the ventral ganglia (Motzny, 1995).

Unlike the thoracic discs, the anterior and posterior compartmental organization of the genital imaginal disc is compound, consisting of three primordia ­ the female genital, male genital, and anal primordia. Each primordium is divided into anterior and posterior compartments. Genes that are known to be expressed in a compartment-specific manner in other discs (engrailed, hedgehog, patched, decapentaplegic, wingless and cubitus interruptus) are expressed in analogous patterns in each primordium of the genital disc. Specifically, engrailed and cubitus interruptus are expressed in complementary domains, while patched, decapentaplegic and wingless are expressed along the border between the two domains. en and inv are required in the posterior comparment of the genital disc to repress dpp and activate hh. 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 (Chen, 1997).

The adult clonal phenotypes of protein kinase A and engrailed-invected mutants provide a more detailed map of the adult genitalia and analia with respect to the anterior/posterior compartmental subdivision. A new model has been proposed to describe the anterior and posterior compartmental organization of the genital disc. Each of the three primordia (female, male and anal) is composed of its own anterior and posterior compartments. Each primordium has a larger anterior compartment and a smaller posterior compartment. Each genital disc is divided into anterior and posterior compartment (Chen, 1997).

Very little information is available about gene expression during the larval period, a developmental interval critical to the formation of the adult. To what extent does gene expression during this period resemble that in the embryonic stages, and how does gene expression during the larval period contribute to segment polarity in the adult? In fact, all the genes expressed during embryonic segment polarity also play a similar role in the formation of the adult. Cells destined to form the cuticle of the adult abdomen are present as clusters of small, non-dividing diploid cells (the anterior dorsal, posterior dorsal and ventral histoblast nests) located at stereotyped postions in the larval epidermis. These cells, just as do their embryonic counterparts, express engrailed, hedgehog, wingless, patched, cubitus interruptus and sloppy paired in a stereotyped manner dependent on their positions within each segment. Each segment is subdivided into an anterior (A) and posterior (P) compartment, distinguished by activity of the selector gene engrailed (en) in P but not A compartment cells. The ventral epidermis of each abdominal segment forms a flexible cuticle, the pleura, with a small plate of sclerotised cuticle, the sternite, centered on the ventral midline. The pleura is covered with a uniform lawn of hairs, all pointed posteriorly, whereas the sternite contains a stereotyped pattern of bristles. Posterior compartments are to a large degree devoid of hairs and bristles, while the sternite cuticle of the A compartment consists of an anterior-to posterior progression of six types of cuticle distinguished by ornamentation and pigmentation. Just anterior to the posterior compartment, A6 is unpigmented, with hairs and none of the larger ornaments called bristles. A5 is darkly pigmented with hairs and bristles of large size. A4 and A3 are darkly and lightly pigmented respectively with moderately sized hairs and bristles. A2 is lightly pigmented with hairs, and A1, adjacent to the next more anteriorly located "posterior" compartment is unpigmented without hairs (Struhl, 1997a).

Hedgehog (Hh), a protein secreted by engrailed expressing P compartment cells, spreads into each A compartment across the anterior and the posterior boundaries to form opposing concentration gradients that organize cell pattern and polarity. Anteriorly and posteriorly situated cells within the A compartment respond in distinct ways to Hh: they express different combinations of genes and form different cell types. patched is expressed at both boundaries. patched is expressed in a graded fashion within each stripe, just anterior to each P compartment. ci peaks at high level in those cells abutting Hh- secreting cells of the P compartment and declines progressively in cells further away. wingless is also expressed in this domain and sloppy paired is expressed in the same region as wingless. decapentaplegic is expressed only in the ventral pleura in those A compartment cells neighboring P compartment cells within the same segment. dpp is not expressed immediately behind posterior compartments (Struhl, 1997). sightless (sit) is required for the activity of Drosophila Hh in the eye and wing imaginal discs and in embryonic segmentation. sit acts in the cells that produce Hh, but does not affect hh transcription, Hh cleavage, or the accumulation of Hh protein. sit encodes a conserved transmembrane protein with homology to a family of membrane-bound acyltransferases. The Sit protein could act by acylating Hh or by promoting other modifications or trafficking events necessary for its function (Lee, 2001b).

One of the critical signals triggering photoreceptor development is Hedgehog (Hh), which is expressed at the posterior margin of the disc prior to differentiation and subsequently in the differentiating photoreceptors. Hh activates the expression of decapentaplegic (dpp) in a stripe at the front of differentiation, or morphogenetic furrow; Dpp signaling also promotes photoreceptor formation. dpp expression is lost from the morphogenetic furrow in sit mutant eye discs. Another target of Hh signaling, the proneural gene atonal, also requires sit for its expression. Despite this lack of Hh target gene expression, a hh-lacZ enhancer trap is expressed at the posterior margin of sit mutant eye discs, indicating that hh expression is established normally. This suggests that the sit phenotype could be due to a defect in Hh signaling (Lee, 2001).

Hh signaling has been extensively studied in the wing disc, where hh is expressed in the posterior compartment and signals to cells just anterior to the compartment boundary to upregulate the expression of dpp and patched (ptc). The Hh signal is mediated by the stabilization and activation of the full-length form of the transcription factor Cubitus interruptus (Ci). This stabilization can be detected with an antibody directed against the C-terminal region of Ci, which fails to recognize the cleaved form of Ci produced in the absence of Hh signaling. sit mutant wing discs show defects consistent with a lack of Hh pathway function; ptc expression is not upregulated at the compartment boundary, and dpp expression is almost completely absent. In addition, no stabilization of full-length Ci could be detected at the compartment boundary. However, hh-lacZ is expressed at wild-type levels in sit mutant discs, indicating that hh transcription is unaffected. This implicates Sit in the Hh pathway downstream of hh transcription and upstream of Ci stabilization (Lee, 2001).

>glial cells missing and gcm2 cell autonomously regulate both glial and neuronal development in the visual system of Drosophila: gcm regulates developmental processes through interaction with the Hh-signaling pathway

The transcription factors Glial cells missing (Gcm) and Gcm2 are known to play a crucial role in promoting glial-cell differentiation during Drosophila embryogenesis. A central function for gcm genes has been revealed in regulating neuronal development in the postembryonic visual system. Gcm and Gcm2 are expressed in both glial and neuronal precursors within the optic lobe. Removal of gcm and gcm2 function shows that the two genes act redundantly and are required for the formation of a subset of glial cells. They also cell-autonomously control the differentiation and proliferation of specific neurons. The transcriptional regulator Dachshund acts downstream of gcm genes and is required to make lamina precursor cells and lamina neurons competent for neuronal differentiation through regulation of epidermal growth factor receptor levels. These findings further suggest that gcm genes regulate neurogenesis through collaboration with the Hedgehog-signaling pathway (Chotard, 2005).

To explore the mechanisms by which gcm genes mediate neuronal development in the optic lobe, the role of Dac was examined because its expression depends on both the activation of the Hh pathway and on gcm and gcm2 function. Genetic analysis added two findings to an understanding as to how Hh and EGF signaling work in concert to regulate neurogenesis in the lamina. It was shown that (1) dac is not required for cell divisions of LPCs and (2) that expression of dac is necessary for the upregulation and maintenance of EGF receptor expression in lamina neurons to promote their further maturation. This is consistent with findings in the developing eye imaginal disc, demonstrating that Dac promotes early progression of the morphogenetic furrow and aspects of R-cell specification but is not required for cell proliferation. In the eye, genetic interaction assays have previously established a link between Dac and EGFR signaling because dac mutant alleles were identified as suppressors of the dominant-active EGFR allele Ellipse, although the precise mechanism underlying this interaction is unclear. The current findings present evidence for one possible mechanism by demonstrating that Dac controls EGF receptor levels in the optic lobe and, in this way, makes LPCs and their progeny competent for neuronal differentiation. In Drosophila, processing of EGF ligands by Rhomboids rather than the regulation of the receptor itself has been considered to be a limiting step in EGF receptor signaling. In the rodent retina, both ligand and receptor levels have been reported to mediate different cellular responses such as proliferation and cell-fate specification. Therefore, regulating receptor levels by Dac represents an additional mechanism to modulate activity of the EGF receptor pathway in the optic lobe of flies. gcm genes can contribute to neuronal differentiation through induction of Dac. Their role in promoting mitotic divisions of LPCs, however, must involve another mechanism. Indeed, genetic analysis suggests that gcm genes regulate both developmental processes through interaction with the Hh-signaling pathway (Chotard, 2005).

That gcm genes work in concert with the Hedgehog-signaling pathway is supported by the following findings. (1) The loss-of-function phenotypes of gcm/gcm2 and hh share three characteristics because in their absence, LPCs neither enter S phase nor express the neuronal differentiation marker Dac, and show increased levels of apoptosis. (2) gcm/gcm2 loss-of-function phenotypes can be partially rescued by overexpressing activated full-length Ci in cells homozygous mutant for gcm and gcm2. One possible explanation for the partial rescue is that levels of activated Ci need to be under tight spatial and temporal control to trigger a normal cellular response. Thus, overexpressing activated Ci at high amounts compromise the ability of gcm and gcm2 homozygous mutant LPCs to express normal levels of Dac or to divide at the correct rate (Chotard, 2005).

Epistasis analysis supports a model in which gcm genes interact with the Hedgehog pathway upstream of Ci. Because loss of gcm and gcm2 function does not interfere with the general expression of Ci in LPCs, one possible mechanism is that gcm genes may indirectly affect the production of activated Ci. In the zebrafish embryo, the Zinc-finger protein Iguana/Dzip1 has recently been implicated in regulating the balance between activator and repressor forms of the vertebrate homologs of Ci, Gli1, and Gli2, possibly by modulating their nuclear activity or import. Perhaps gcm and gcm2 act in an analogous manner and regulate the production or subcellular localization of activated Ci by promoting the expression of another member of the Hh-signaling pathway. Alternatively, gcm genes may act in parallel and cooperate with Ci at the DNA level of common target genes. The dissection of the precise mechanism underlying the genetic interaction of gcm genes and the Hh pathway will require additional genetic analysis in the future (Chotard, 2005).

Gcm genes mediate neuronal differentiation in collaboration with the Hh pathway through induction of Dac. Proliferation is likely regulated by controlling a component of the cell-cycle machinery, such as Cyclin E. Indeed, in the eye and wing imaginal discs, Ci has been shown to directly promote entry into S phase by inducing increased transcription of Cyclin E. Moreover, three consensus Ci binding sites have been found within the 5' regulatory region of cyclin E (Chotard, 2005).

Effects of Mutation or Deletion

cubitus interruptus mutation causes defects in every embryonic segment (Orenic, 1990). Three classes of existing mutations in the ci locus alter the regulation of ci expression and can be used to examine ci function during development. The first class of ci mutations causes interruptions in wing veins four and five due to inappropriate expression of the ci product in the posterior compartment of imaginal discs. The second class of mutations eliminates CI protein early in embryogenesis and causes the deletion of structures that are derived from the region including and adjacent to the engrailed expressing cells. The third class of mutations eliminates CI protein later in embryogenesis and blocks the formation of the ventral naked cuticle. The loss of ci expression at these two different stages in embryonic development correlates with the subsequent elimination of wingless expression (Slusarski, 1995)

The mutation ciD is mutant for two neighboring loci, cubitus interruptus and pangolin. In situ hybridication experiments with pan probes reveal that in ciD/+ heterozygote embryos, pan transcripts are detected in a pattern indistinguishable from that of ci transcripts, providing additional evidence that the molecular lesion of ciD disrupts both genes. It is a curious coincidence that pan and ci are adjacent genes, as ci encodes a transcription factor that is essential for transducing all examples of Hedgehog signaling, whereas the present evidence suggests an equivalent role for Pan in Wingless signal transduction. The fact that the ciD mutation abolishes both activities also calls for a reassessment of genetic epistasis experiments in which this allele is used to assay the relationship between Hedgehog and Wingless signaling (Brunner, 1997)

The Hedgehog (Hh) and Wingless (Wg) signaling pathways play important roles in animal development. The activities of the two pathways depend on each other during Drosophila embryogenesis. In the embryonic segment, Wg is required in anterior cells to sustain Hh secretion in adjacent posterior cells. In turn, Hh input is necessary for anterior cells to maintain wg expression. The Hh and Wg pathways are mediated by the transcription factors Cubitus interruptus (Ci) and Pangolin/TCF (Pan), respectively. Coincidentally, pan and ci are adjacent genes on the fourth chromosome in a head-to-head orientation. Genetic and in situ hybridization data indicate that ciD is a mutation affecting both ci and pan. Whereas pan is expressed ubiquitously during embryogenesis, in ciD mutant embryos pan is expressed in an intense, segmentally-repeated manner. This expression is intriguingly similar to that of the ci gene and it occurs both in homozygous and heterozygous mutant embryos. Moreover, it was found that in ciD/ciD homozyogous mutants, the typical striped expression of ci is absent; instead, ci transcripts are dispersed uniformly at low levels, like pan transcripts in wild type embryos. Molecular analysis reveals that the ciD allele is caused by an inversion event that swaps the promoter regions and the first exons of the two genes. The ci gene in ciD is controlled by the ubiquitous pan promoter and encodes a hybrid Ci protein that carries the N-terminal region of Pan. The predicted Ci fusion protein product consists of the first 246 amino acids of Pan fused in frame to the Ci protein, of which the first 13 N-terminal amino acids are missing. The N-terminal domain of Pan has previously been shown to bind to the beta-catenin homolog Armadillo (Arm), raising the possibility that Wg input, in addition to Hh input, modulates the activity of the hybrid CiD protein. Indeed, Wg signaling induces the expression of the Hh target gene patched (ptc) in ciD animals. Evidence is provided that this effect depends on the ability of the CiD protein to bind Arm. Genetic and molecular data indicate that wild-type Pan and CiD compete for binding to Arm, leading to a compromised transduction of the Wg signal in heterozygous ciD/+ animals and to a dramatic enhancement of the gain-of-function activity of ciD in homozygous mutants. Thus, the Hh and the Wg pathways are affected by the ciD mutation, and the CiD fusion protein integrates the activities of both (Schweizer, 1998).

It has been proposed that during embryogenesis ciD functions as a gain-of-function allele of ci. This is largely based on the striking finding that CiD can substute for Hh protein in driving expression of Hh-responsive genes. Specifically, in homozygous ciD mutant embryos, the ptc, wg and gooseberry genes are expressed in wider stripes, when compared to wild type, even in the absence of active Hh protein. However, homozygous ciD mutant embryos can be rescued to adult by introducing a duplication of the pan/ci genomic region (unpublished). Thus it is unlikely that the observed phenotypes can be ascribed merely to the dosage of CiD protein. Rather, it appears to be critical that no wild-type Pan protein accompanies the CiD protein in ciD homozygous embryos. This allows all Arm protein that accumulates in response to Wg to be unrestrained to bind to, and activate the CiD protein. It is predicted that the expression of an N-terminally complete Pan protein -- even if its DNA-binding activity has been impaired -- would abolish the extreme gain-of-function effect of CiD in homozygous mutant embryos. Thus it must be the simultaneous gain and loss of Arm binding sites by Ci and Pan, respectively, that confers these unusual properties to the ciD allele. Together with the observation that CiD interferes in a dominant-negative manner with Wg signaling, these results illustrate that the levels of free and accessible Arm protein critically determine the output of Wg signaling in wild-type and the output of CiD signaling in mutant situations (Schweizer, 1998 and references).

The Drosophila cubitus interruptus (ci) gene encodes a sequence-specific DNA-binding protein that regulates transcription of Hedgehog (Hh) target genes. Activity of the Ci protein is posttranslationally regulated by Hh signaling. In animals homozygous for the ciD mutation, however, transcription of Hh target genes is regulated by Wingless (Wg) signaling rather than by Hh signaling. ciD is shown to encode a chimeric protein composed of the regulatory domain of dTCF/Pangolin (Pan) and the DNA binding domain of Ci. Pan is a Wg-regulated transcription factor that is activated by the binding of Armadillo (Arm) to its regulatory domain. Arm is thought to activate Pan by contributing a transactivation domain. A constitutively active form of Arm potentiates activity of a CiD transgene and coimmunoprecipitates with CiD protein. The Wg-responsive activity of CiD could be explained by recruitment of the Arm transactivation function to the promoters of Hh-target genes. It is suggestrd that wild-type Ci also recruits a protein with a transactivation domain as part of its normal mechanism of activation (Von Ohlen, 1999).

In Drosophila embryos cubitus interruptus activity is both necessary and sufficient to drive expression of HH-responsive genes, including wingless, gooseberry and patched. To demonstrate that ci is required for transduction of the HH signal, expression of wg was examined in ci null embryos when HH is ubiquitously expressed under control of a heat-shock promoter (Hs-hh). In Hs-hh embryos, wg is expressed ectopically in anteriorly expanded stripes. In ci mutants Hs-hh does not induce ectopic expression of wg. Similar results were obtained for gsb. CI is a sequence-specific DNA binding protein that drives transcription from a wingless promoter in transiently transfected cells. CI binds to the same 9 bp consensus sequence -TGGGTGGTC- as mammalian Gli and Gli3. Alteration of a single nucleotide in the core sequence prevents binding. CI activates transcription from a 5-kb fragment of the wg promoter. CI binding sites in the wg promoter are necessary for this transcriptional activation. A CI element maps to a distal 1-kb region of the 5-kb fragment. The wg promoter sequence has 10 possible Gli consensus binding sites, with three pairs of sites in the distal 1.2 kb. When putatitive CI binding sites are mutagenized, mutant fragments show a greater than 90% reduction in CI-dependent transcriptional activation. Mutagenesis of these sites completely eliminates an electrophoretic mobility shift caused by binding of CI to unmutagenized sites (Van Ohlen, 1997).

Mutants of cubitus interruptus that are independent of PKA regulation are independent of hedgehog signaling

Hedgehog (HH) is an important morphogen involved in pattern formation during Drosophila embryogenesis and disc development. cubitus interruptus encodes a transcription factor responsible for transducing the hh signal in the nucleus and activating hh target gene expression. Previous studies have shown that Ci exists in two forms: a 75 kDa proteolytic repressor form and a 155 kDa activator form. The ratio of these forms, which is regulated positively by hh signaling and negatively by PKA activity, determines the on/off status of hh target gene expression. Exogenous expression of Ci that is mutant for four consensus PKA sites, CI(m1-4), causes ectopic expression of wingless in vivo and a phenotype consistent with wg overexpression. Expression of CI(m1-4), but not Ci(wt), can rescue the hh mutant phenotype and restore wg expression in hh mutant embryos. When PKA activity is suppressed by expressing a dominant negative PKA mutant, the exogenous expression of Ci(wt) results in overexpression of wg and lethality in embryogenesis, defects that are similar to those caused by the exogenous expression of CI(m1-4). In addition, in cell culture, the mutation of any one of the three serine-containing PKA sites abolishes the proteolytic processing of Ci. PKA is shown to directly phosphorylate the four consensus phosphorylation sites in vitro. Taken together, these results suggest that positive hh and negative PKA regulation of wg gene expression converge on the regulation of Ci phosphorylation (Y. Chen, 1999).

It can be determined whether PKA phosphorylates consensus PKA target sites in vitro. Ci fragments of wild type Ci and of CI(m1-4) that contain the four PKA sites (aa441-1065) were fused to GST. Two-dimensional tryptic phosphopeptide maps of the expressed fusion proteins were generated. There are at least 13 phosphopeptides that are labeled by PKA in the wild-type Ci peptide. In vitro, PKA can recognize RxS/T, the subset RRxS/T, RxxS/T and RKxxS/T. The phosphorylation of S is preferred 40:6 over T and in vivo, the RRxS site is preferred 2:1 over the others. The four consensus RRxS/T sites in Ci were chosen for mutation because they would probably be the preferred phosphorylation sites in vivo. Scanning the Ci fragment for all possible consensus PKA sites, it was found that all of the phosphopeptides can be accounted for by the number of PKA consensus sites in the fusion protein. Three of the strong spots and two weaker spots that are present in the wild-type fragment are missing in the mutant fragment, demonstrating that PKA can specifically and directly phosphorylate the four RRxS/T consensus PKA sites in vitro. The two weak spots are difficult to distinguish and may represent only one spot or incomplete digestion of a single peptide. GST alone was not phosphorylated (Y. Chen, 1999).

What of the positive regulation of Ci activity by hh? Because the genetic data suggests that hh does not regulate PKA directly, it may be that hh affects the phosphorylation state of Ci by activating a phosphatase, or through changing the accessibility of Ci to a phosphatase. In support of this idea is the observation that the phosphatase inhibitor, okadaic acid, stimulates Ci proteolysis, even in the presence of a HH signal. HH signaling stimulates fu kinase activity to transform full-length Ci to a transcriptional activator. It may also be that fu activity renders full-length Ci inaccessible to PKA phosphorylation (Y. Chen, 1999).

Cubitus interruptus-independent transduction of the Hedgehog signal in Drosophila

The manner in which Hh molecules regulate a target cell remains poorly understood. In the Drosophila embryo, Hh is produced in identical stripes of cells in the posterior compartment of each segment. From these cells a Hh signal acts in both anterior and posterior directions. In the anterior cells, the target genes wingless and patched are activated whereas posterior cells respond to Hh by expressing rhomboid and patched. This study examines the role of the transcription factor Cubitus interruptus (Ci) in this process. So far, Ci has been thought to be the most downstream component of the Hh pathway, capable of activating all Hh functions. However, the study of a null ci allele indicates that it is actually not required for all Hh functions. Whereas Hh and Ci are both required for patched expression, the target genes wingless and rhomboid have unequal requirements for Hh and Ci activity. Hh is required for the maintenance of wingless expression before embryonic stage 11 whereas Ci is necessary only later during stage 11. For rhomboid expression Hh is required positively whereas Ci exhibits negative input. These results indicate that factors other than Ci are necessary for Hh target gene regulation. Evidence is presented that the zinc-finger protein Teashirt is one candidate for this activity. It is required positively for rhomboid expression and Teashirt and Ci act in a partially redundant manner before stage 11 to maintain wingless expression in the trunk (Gallet, 2000).

Ci is required to transduce Hh signal in order to activate its target genes. In cells that do not receive Hh, Ci is cleaved and represses Hh target genes. However, compelling results point out a more complex role for Ci activity during embryonic development of Drosophila. The embryonic phenotype resulting from the complete loss of Ci function is weaker than the complete loss of Hh function. The phenotypic differences observed between hh and ci null mutations reside in the following observations: in ci94 embryos one observes (1) the presence of segmentation due to maintenance of wg expression until stage 11 and (2) the presence of denticle diversities due to an expansion of EGF signaling illustrated by an expansion of rho expression. Ci does not have a maternal contribution, since ci94 homozygotes issuing from germ-line clones homozygous for ci94, do not show a stronger phenotype than embryos lacking only zygotic Ci product, and also embryos hatching after ci RNA interference experiments show phenotypes similar to ci94 embryos. Furthermore, if rho expression present in ci mutants is due to maternal production of Ci, one has to explain how two Ci targets would behave differently in the absence of zygotic Ci contribution; wg expression disappears, whereas rho is expressed in more cells. For all these reasons it can be confidently concluded that Ci has no maternal contribution. Consequently, other factors are substituting for Ci activity in the transduction of Hh signaling (Gallet, 2000).

The ci94 phenotype could be due to the fact that the ci94 mutation disrupts both activator and repressive (Cirep) functions of Ci and that the resulting weak phenotype could thus be due in part to the loss of Cirep activity. Indeed, in the wing disc loss of ci gene alleviates the repressive function of Cirep on dpp transcription; dpp is then transcribed at a basal level. Although Cirep could repress wg transcription, Cirep absence in the hh;ci94 double mutant embryo is not sufficient to induce an upregulation of wg transcription. Hence the maintenance of wg expression until stage 11 in ci94 is not due to the loss of Cirep activity but is controlled by another Hh type of Hh input (Gallet, 2000).

Loss of ci induces an expansion of rho expression instead of a reduction, as seen in a hh loss of function, showing that Ci is not involved in the activation of rho expression. The fact that rho disappears in tsh mutant embryos strongly suggests that the Tsh zinc-finger protein regulates rho expression or is at least necessary for instructing cells to respond to Hh for rho expression. Nevertheless, one has to explain why rho expression is expanded in ci94 . Loss of Cirep activity could be responsible for this effect. Indeed, overexpression of Cirep in a ci null background or analyses of the ciCe2 mutant, which ectopically expresses Cirep, reveals a repressive effect of Cirep on rho expression. Therefore, Cirep could be used as a gatekeeper in order to repress hh target genes tightly where they should not be expressed, and thus to overcome mis-regulation of key genes such as rho or wg. Nevertheless, these observations contradict previous analyses showing that Cirep is not required for correct embryogenesis, since loss of ci function is rescued by a ci transgene lacking the Ci75 repressor form of Ci. An alternative explanation can be gleaned from the fact that ci94 cuticle phenotypes resemble those lacking Wg activity during the cell specification stage. Because it has been shown that Wg exerts a repressive role on rho expression (since absence of Wg activity promotes ectopic expression of rho), rho expansion in ci94 could be an indirect consequence of the late disappearance of wg expression during stage 10-11 (Gallet, 2000).

Before stage 11, either Tsh or Ci is sufficient for wg regulation because only the loss of both gene activities results in a downregulation of wg, a situation comparable with that observed in hh mutants. It is interesting to note that Ci seems to display differential requirements for wg maintenance and naked cuticle differentiation in the abdomen versus the thorax. While Ci is dispensable until stage 11 for wg expression and naked cuticle differentiation in the abdomen, its presence in the thorax is required. This specific Ci function in the thorax is currently being studied (Gallet, 2000).

Both Ci and Tsh transcription factors, when overexpressed can induce ectopic wg expression. The two factors do not display the same features: Tsh has three atypical, widely spaced, zinc-finger motifs, whereas Ci has conserved spacer regions between its five zinc fingers; the binding sites identified so far for these two proteins are different. It would be interesting to know whether Tsh can bind directly to the wg promoter and to identify its binding sites. It is also noteworthy that between stage 8 and 10 wg requires, in parallel to Hh, its own activity for the maintenance of its transcription. It has previously been shown that Tsh is a modulator of Wg signaling. Tsh becomes phosphorylated and accumulates at a higher level in the nucleus in Wg-receiving cells compared with cells lacking Wg signal. Hence, in the trunk Tsh could be employed both by Wg and Hh signaling in order to maintain wg transcription (Gallet, 2000).

The redundancy exhibited between Tsh and Ci for wg regulation changes after stage 10, since loss of either Ci or Tsh results in the downregulation of wg transcripts. It is not known if this observation is the result of a cooperation between Tsh and Ci. At least one other gene, gooseberry, is required for the maintenance of wg transcription at this stage, indicating that multiple inputs for the maintenance of wg expression are necessary for normal embryonic development (Gallet, 2000).

Studies on the developing wing blade show that Ci transduces all Hh-delivered information. However, this study and others on the Hh pathway support the idea that Ci is not always involved in Hh signaling, showing that branchpoints are common for distinct Hh signaling steps for the following five reasons. (1) It has been shown that neither Ci nor Fused (Fu) are involved in the Hh-dependent formation of Bolwig's organ in Drosophila. (2) A Hh-responsive wg reporter gene with no Ci-binding sites does not require Ci activity for its regulation until stage 11. (3) Studies on the talpid 3 gene in chicken suggest that Gli proteins, the vertebrate homologues of Ci, regulate only a subset of Hh target genes, the others being regulated by an unidentified transcription factor. (4) A Sonic hedgehog response element on the COUP-TFII promoter binds to a factor distinct from Gli. (5) Hh signaling does not require Ci activity to regulate rho. Although the authors favor the idea that Tsh regulates rho expression directly in response to Hh signal the hypothesis that Tsh plays a more permissive role allowing Hh to regulate rho via another factor apart from Ci cannot be excluded (Gallet, 2000 and references therein).

In conclusion, Hh requires at least two different transcription factors during Drosophila embryogenesis to regulate its multiple target genes and to instruct cells with precise behaviors. The transcription factors may act independently (e.g. Ci for ptc; Tsh for rho), cooperatively (e.g. Ci and Tsh for wg maintenance during the cell specification phase) or redundantly (e.g. Ci and Tsh for wg maintenance earlier during the stabilization phase). The possibility that other transcription factors like gooseberry might be recruited for Hh signaling cannot be excluded, especially since denticle density is weaker in tsh;ci double mutants as compared with hh single mutants. Furthermore the dorsal phenotypes of the tsh;ci double mutants are weaker than those of hh. (1) wg transcripts are still present in dorsal patches in tsh;ci mutations whereas they are not present in hh embryos. (2) Dorsal cuticle is not as severely perturbed in tsh;ci larvae as compared with hh null ones (Gallet, 2000).

Finally, pathway bifurcations are involved not only at the level of the transcription factors. The Fu kinase, which is normally required to transduce Hh signal and to convert Ci 155 into a putative Ci act form, is not necessary in all Hh-receiving cells during embryogenesis. While Fu is involved anteriorly to En/Hh-expressing cells for the maintenance of wg and ptc, it is not involved posteriorly for the maintenance of ptc. These results correlate with the Ci isoforms detected: anteriorly the putative Ci act form is present but posteriorly only the Ci 155 form is detected (Gallet, 2000 and references therein).

An absolute requirement for Cubitus interruptus in Hedgehog signaling

Is the Hh pathway distally branched? In other words, is the regulation of Ci activity the sole output of Hh signaling? Putative Ci-independent branches of Hh signaling were explored by analyzing the behavior of cells that lack Ci but that had undergone maximal activation of the Hh transduction pathway due to the removal of Patched (Ptc). The analysis of target gene expression and morphogenetic read-outs of Hh in embryonic, larval and adult stages indicates that Ci is absolutely required for all examined aspects of Hh outputs. This is interpreted as evidence against the existence of Ci-independent branches in the Hh signal transduction pathway. It is proposed that most cases of apparent Ci/Gli-independent Hh output can be attributed to the derepression of target gene expression in the absence of Ci/Gli repressor function (Methot, 2001).

The key result of this study is the observation that maximal activation of the Hh pathway (i.e. complete loss of Ptc) has no discernible effect in the absence of Ci. This is taken as evidence against a distal branching in the Hh signal transduction pathway. These results do not exclude the existence of alternative pathways between Smo and Ci, yet all these putative branches must converge at Ci. It is noted that the indispensability of Ci for Hh signaling also explains how developmental compartments are formed and maintained. The essential difference between cells on opposite sides of the anteroposterior compartment boundary is the responsiveness to Hh. Posterior compartment cells do not respond to the Hh signal, even though they are amply exposed to Hh and appear to possess all but one of the components to transduce Smo activity. The lack of Ci, however, precludes any response to Hh and is thus sufficient to create a population of cells that behaves the opposite from that of the anterior, Ci-expressing compartment (Methot, 2001).

Although it is concluded that Hh signaling has no effect in the absence of Ci, it is also concluded that the converse is not the case: Ci does have a function in the absence of Hh signaling. This can be illustrated most effectively by comparing a hh;ci double mutant embryo with a hh single mutant one. Although both animals completely lack the Hh signal, the presence of a functional ci gene considerably increases the segment polarity phenotype of hh mutants. This effect of Ci is brought about by the default state of Ci, which is the repressor function Ci possesses in the absence of Hh input. This function is critical for limb development but not essential for embryogenesis. This is because an uncleavable form of Ci, CiU, can substitute for embryonic Ci in spite of the fact that it cannot form detectable amounts of Ci[rep], the repressor form of Ci. The severe phenotype of hh mutant embryos indicates that Ci[rep] activity (although not essential in a wild-type background) can be detrimental in circumstances where Hh signaling is abolished. This situation is reminiscent of the Wg signal transduction pathway, where the nuclear mediator, dTCF/Pangolin, represses Wg target genes in the absence of Wg input. An analogous case has been described for the Notch pathway, where the DNA-binding factor Suppressor of Hairless has a repressive effect on single-minded (sim) transcription in the absence of Notch activity, yet mediates sim activation upon Notch signaling. It may be a general principle that the transcriptional targets of a signaling pathway are repressed in the absence of the signal. Signal-mediated induction, therefore, requires both the abolition of this repression and the concomitant activation of transcription (Methot, 2001).

Based on this analysis, three predictions can be made regarding the Hh pathway in other systems. (1) Loss-of-function mutations in murine Gli genes are likely to cause phenotypes differently from equivalent mutations in Hedgehog genes. In particular, even a triple knockout of the Gli1, Gli2 and Gli3 genes, will presumably behave different from combined mutations in the Sonic, Indian and Desert hedgehog genes. The main reason for postulating this is the Hh-independent repressor function of Gli proteins, which appears to be primarily associated with Gli3. Lack of Shh signaling may lead to an increase of Gli3 repressor activity, while lack of Gli3 expression has the opposite effect. Hence a double Shh Gli3 mutant may have a considerably milder phenotype than a Shh single mutant animal (Methot, 2001).

(2) Given the conservation of the Hh transduction pathway in different species, it is unlikely that the mammalian Hh pathway contains end points other than Gli proteins. The critical but genetically challenging test will be the generation of Gli triple mutant mice and their comparison to animals that lack in addition the Shh or the Ptc gene (Methot, 2001).

(3) These results challenge several previous studies that claim the existence of Ci-independent outputs of the Hh signaling pathway. Some of these studies were conducted with a ci null allele, which removes both activator and repressor functions of Ci. For the wing imaginal disc, lack of Ci[rep] causes the ectopic expression of certain Hh target genes. Genetic evidence is now provided that this is also the case in embryos. It is surmised that the seemingly Ci-independent expression of Hh-induced target genes may reflect transcriptional derepression, owing to removal of Ci[rep] (Methot, 2001).

Opposing transcriptional outputs of Hedgehog signaling and Engrailed control compartmental cell sorting at the Drosophila A/P boundary

The wing imaginal disc is subdivided into two nonintermingling sets of cells, the anterior (A) and posterior (P) compartments. Anterior cells require reception of the Hedgehog (Hh) signal to segregate from P cells. Evidence is provided that Hh signaling controls A/P cell segregation not by directly modifying structural components but by a Cubitus interruptus (Ci)-mediated transcriptional response. A shift in the balance between repressor and activator forms of Ci toward the activator form is necessary and sufficient to define 'A-type' cell sorting behavior. Moreover, Engrailed (En), in the absence of Ci, is sufficient to specify 'P-type' sorting. It is proposed that the opposing transcriptional activities of Ci and En control cell segregation at the A/P boundary by regulating a single cell adhesion molecule (Dahmann, 2000).

To test the role of En and Hh-signaling components in controlling cell segregation, two experimental assays were applied. Both assays are based on the presumption that cells maximize contact (intermingle) with cells of the same adhesiveness and minimize contact with (sort out from) cells of different adhesiveness. In the 'round-up assay', clones of mutant cells are assayed for their shape. Each clone is analyzed by how circular it is and how smoothly its border interfaces with surrounding tissue. The degree of roundness of the clone and smoothness of its border is taken as a measure for the difference in adhesiveness between cells inside and outside of the clone. In the wild-type wing imaginal disc, cell segregation is confined to the region of the compartment boundaries. Thus, in the more stringent 'choice assay,' clones generated in the vicinity of the A/P boundary are monitored for their sorting behavior. Clones have three choices: they can (1) remain within their compartment of origin; (2) sort completely into the territory of the adjacent compartment defining a straight border with cells of the compartment of origin at the normal position of the A/P boundary, or (3) sort out from cells of both compartments and take up positions overlapping the normal site of the A/P boundary. Depending on the genetic intervention, the compartment of origin of a clone was determined either by the state of the heritable and P-specific expression of an en-lacZ reporter gene or by the position of the 'twin spot' clone, which is composed of sibling wild-type cells. The position of the A/P boundary was inferred from the expression of a hh-lacZ reporter gene expressed exclusively in P cells (Dahmann, 2000).

Two forms of Ci are distinguished, a constitutively active form, Ci[act], and a repressive form, Ci[rep]. Autonomous and direct roles have been established for Ci[act] and En in specifying A and P cell segregation, respectively. Evidence is also provided that Hh signaling is sufficient to specify A-type cell segregation and that it acts by shifting the balance between Ci[rep] and Ci[act] toward low levels of Ci[rep] and high levels of Ci[act]. It is proposed that the opposing transcriptional activities of Ci[act] and Ci[rep]/En lead to differences in the activity of a cell adhesion system at the boundary of A and P cells, thereby preventing these cell populations from intermingling (Dahmann, 2000).

The smooth and straight boundary between compartments has been ascribed to distinct adhesive properties of cells on opposite sides of the boundary causing these cell populations to minimize contact and sort out. In the case of the A/P boundary of the wing, one difference that could account for the distinct sorting behavior is the exclusive presence of two transcription factors, Ci[act] and En in adjacent A and P cells, respectively. For a long time, the view prevailed that En regulates cell segregation by autonomously and directly specifying P, as opposed to A, cell adhesiveness. This hypothesis has recently been challenged by studies indicating that En acts, at least in part, by directing the expression of Hh and that Hh secreted by P cells induces A cells to acquire a distinct cell adhesiveness. These studies, however, provide conflicting results as to whether or not En also has an autonomous, Hh-independent role in specifying cell segregation at the A/P boundary. The same studies further raised, but did not address, the question of whether Hh signaling would specify cell segregation via its normal transduction pathway by leading to a transcriptional output depending on Ci. In various other systems, the activation of signaling receptors can lead to the posttranscriptional activation of small GTPases that can directly, without altering gene transcription, affect cytoskeletal components and thus conceivably cell adhesion. A key tool for addressing these questions is the choice assay. This assay allows for monitoring whether altering the activity of a gene would change a cell's compartmental preference. Using this assay, the above questions have been addressed by systematically considering three distinct situations (Dahmann, 2000 and references therein).

Situation 1: the 'ground state,' where neither Ci nor En is present.

Irrespective of their compartmental origin, clones of cells null mutant for both ci and en take up positions overlapping the normal site of the A/P boundary with smooth borders to wild-type A and P cells. Because En is not required in A cells and because ci minus single mutant A cells behave like ci,en minus double mutant A cells, it is inferred that Ci is required in A cells for their intermingling with other A cells at the compartment boundary. Since Ci acts in these cells as a transcriptional activator, it is concluded that Hh signaling leads to a Ci-dependent transcriptional response in A cells and transcription of the immediate Hh target gene relevant for A segregation is induced, rather than repressed, in anterior boundary cells. The behavior of ci,en minus double mutant clones also clarifies the role of En. Because clones of P cells lacking En and Ci form smooth borders with neighboring wild-type P cells that also lack Ci and, if in contact with A cells, sort partially into A territory, it is inferred that En has a function in specifying P segregation that is independent of Ci. Since Ci is required for all known responses to Hh signaling, it is concluded that En has a Hh-independent role in determining P segregation. The observation that clones of cells mutant for both ci and en occupy A and P territory to a similar extent leads to the conclusion that Ci and En are required for most if not all aspects of the distinct segregation properties of A and P cells, and the difference between the ground state and the 'A state' brought about by Ci[act] is similar to the difference between the ground state and the 'P state' dependent on En (Dahmann, 2000).

Situation 2: Cells expressing En but lacking Ci.

A more direct argument for a Ci/Hh-independent role of En in the specification of cell sorting behavior can be derived from the experiment in which anterior clones were programmed to express low levels of En. Such cells cease to express Ci and take up positions normally occupied only by P cells. The behavior of these cells is different from that of ground state cells that neither express Ci nor En. In contrast to ci,en minus cells, the low level of En-expressing cells of A origin show a complete transgression to P territory, yet they do not intermingle well with P cells. This latter observation is ascribed to the unnaturally low levels of En produced in these cells (several-fold less than in wild-type P cells). These levels may not repress ci completely and might not be sufficient to fully confer P cell adhesiveness (Dahmann, 2000).

Situation 3: Cells expressing Ci but lacking En.

Posterior clones of cells expressing Ci at physiological levels, but lacking En (mutant for enE), take up positions in the territory normally only occupied by A cells and intermingle with A cells. This behavior is dependent on Ci, since ci,en double mutant clones of P origin only partially occupy A territory and sort out from A cells. Furthermore, overexpression of Ci in P cells leads these cells to sort out from neighboring P cells, and, if in contact with A cells, sort into A territory. Together, by comparing situations (1) to (3), it is concluded that Ci is necessary and sufficient to specify A segregation, and, in the absence of Ci, En is necessary and sufficient to specify P segregation (Dahmann, 2000).

Thus En has an autonomous, Hh-independent role in specifying cell segregation. In addition, Ci is necessary and sufficient to specify A segregation. Ci is activated in anterior boundary cells by Hh whose P-specific expression is in turn controlled by En. Thus, En controls cell segregation at the A/P boundary both by a Hh-dependent as well as a Hh-independent pathway. To determine the relative contributions of these two pathways, situations were generated and analyzed in which En activity was altered under conditions of constant Hh signaling, or conversely, situations in which the activity of Hh signal transduction was altered under constant En conditions. From these experiments, it is concluded that for the segregation behavior of wing cells, the state of the Hh pathway prevails over that of En activity. This conclusion is particularly well corroborated by the finding that cells in which both pathways are simultaneously 'on' (P cells expressing Ci), sort with A cells. The behavior of such cells may also explain why the late expression of en in anterior boundary cells has no deleterious effects on the integrity of the compartment boundary. Like the experimental cells, these cells are exposed to the Hh signal, coexpress ci and en, yet associate with other A cells rather than with En-expressing P cells (Dahmann, 2000).

Ci is required in A cells for proper cell segregation at the A/P boundary. Depending on the status of the Hh signaling pathway, Ci can exist in two forms with opposing transcriptional activities (Ci[rep] and Ci[act]). These two forms of Ci regulate the expression of different subsets of Hh target genes, some of which appear to be regulated exclusively by Ci[rep] or Ci[act]. It is argued that the A/P sorting of wing cells is under control of both forms of Ci. This conclusion is based on findings that both Ci[rep] and Ci[act] have a profound influence on the segregation behavior of A cells. Two observations show that Ci[rep] determines a preference for sorting into P territory. (1) A cells expressing Ci[rep] in the absence of Ci[act] or A cells overexpressing Ci[rep] in the presence of Ci[act] both take up positions occupied normally only by P cells. This is in contrast to cells lacking Ci entirely, which take up positions overlapping the normal position of the A/P boundary. (2) P cells lacking En but expressing Ci[rep] are confined to the P compartment, unlike cells that lack En and Ci or cells that only lack En. It is inferred from this that one important function of Hh signaling in its role of specifying A-type segregation properties is to prevent the formation of Ci[rep] in cells close to the A/P boundary (Dahmann, 2000).

The conclusion that not only prevention of Ci[rep] formation but also the induction of Ci[act] plays an important role in A/P sorting is deduced from the observation that cells lacking both forms of Ci do not mingle with wild-type A cells expressing Ci[act] due to their vicinity to the Hh source. Moreover, the addition of Ci to P cells, where Ci is readily converted to Ci[act], programs P cells to segregate with A cells. Because Ci[rep] influences cell segregation, one might have expected that anterior ci minus clones far away from the A/P boundary would sort out from neighboring Ci[rep]-expressing cells. However, ci minus cells intermingle well with neighboring A cells. One likely explanation for this apparent discrepancy is the partial derepression of hh transcription in ci mutant cells. These low Hh levels induce in neighboring cells the formation of some Ci[act] that might neutralize remnant levels of Ci[rep]. In support of this assumption, it has been found that clones of cells double mutant for ci and hh do sort out at anterior positions (Dahmann, 2000).

Ci and En are both DNA-binding proteins known to act as transcription factors, indicating that they control cell segregation by regulating the expression of target genes. By analogy to dpp, a Hh target gene that is also controlled by En and both forms of Ci, a model is proposed illustrating how Ci[rep], Ci[act], and En might shape the expression profile of a putative immediate target gene involved in cell segregation. Since in the absence of Ci and En, cells segregate neither with A nor with P cells, they are likely expressing an intermediate level of this gene that is different from those in A or P cells. Since Ci[rep] can control cell segregation and is present in A cells far away from the boundary, it is proposed that the basal expression of this hypothetical gene is downregulated by Ci[rep] in these cells. In A cells close to the boundary, Hh signaling prevents the formation of Ci[rep] yet causes the formation of Ci[act], from which it is inferred that in these cells the transcription of this target gene is upregulated. In P cells, En may repress this target gene, consistent with its role as a transcriptional repressor. It is proposed that the opposing transcriptional activities of Ci[act] and En lead to a large difference in the expression of this immediate target gene in cells on opposite sides of the A/P boundary (Dahmann, 2000).

In the above model, it is assumed that Ci and En control cell segregation by transcriptionally regulating one and the same gene, although it is also possible that they regulate different genes. While at present these alternatives cannot be distinguised, the simpler model that Ci and En control the same target gene is preferred for two reasons: (1) there is a precedent case for such a gene, dpp, which is known to be regulated by both Ci and En; (2) a difference in the expression level of a single cell adhesion molecule (Shotgun or DE-cadherin) is sufficient for two cell populations to sort out. While it is conceivable that Ci and En directly regulate the expression of cell adhesion molecules like DE-cadherin, it is also possible that they act more indirectly by regulating genes whose products influence the activity of uniformly expressed cell adhesion molecules. Clones of cells lacking detectable amounts of DE-cadherin do sort out from neighboring wing disc cells; they are, however, exclusively confined to the compartment of origin, indicating that DE-cadherin is not required for the separation of cells at the A/P boundary (Dahmann, 2000).

Why does cell segregation at the A/P boundary require two transcription factors with opposing activities? Based on the results presented here, the differential activities of either Ci or En suffices for separating A and P cells. For Ci, this is best illustrated by the key finding that P cells forced to express Ci sort out from wild-type P cells and segregate into A territory. Conversely, in the absence of Ci, expression of En suffices for A cells to sort into P territory. The use of two transcription factors with opposing activities may have the advantage of increasing the fidelity of the sorting process by further contrasting the expression levels of a common putative target gene in cells of opposite sides of the A/P boundary (Dahmann, 2000).

It seems to be a general mechanism that En controls cell segregation both in a Hh-dependent and -independent manner. In the Drosophila abdomen, En has also been implicated to control separation of A and P cells in Hh-dependent and -independent ways. The relative contributions of these two functions of En, however, appear to differ between the wing imaginal discs and the abdomen. While a prevalence of the Hh-dependent pathway is found in the wing disc, the two functions of En seem to contribute equally to the separation of abdominal A and P cells. This difference in dominance of the Hh-signal transduction pathway might be due to a more influential role of Ci[rep] in the sorting of imaginal versus abdominal cells. It is intriguing to notice that the same intricate network that defines the strip of cells expressing Dpp also appears to restrict the activity of a putative cell adhesion molecule to the very same cells. The use of Hh/En signaling for both setting up the Dpp organizer and segregating A and P cells may ensure that the position and shape of the morphogen source that organizes both compartments is stably maintained during development. The prediction of a dpp-like expression pattern provides a novel criterion for the future identification of the elusive molecules conferring cell segregation (Dahmann, 2000).

Signals transmitted along retinal axons in Drosophila: Hedgehog signal reception and the cell circuitry of lamina cartridge assembly

The arrival of retinal axons in the Drosophila brain triggers the assembly of glial and neuronal precursors into a neurocrystalline array of lamina synaptic cartridges. Retinal axons arriving from the eye imaginal disc trigger the assembly of neuronal and glial precursors into precartridge ensembles in the crescent-shaped lamina target field. In the eye disc, photoreceptor cells assemble into ommatidial clusters behind the morphogenetic furrow (mf) as it moves to the anterior. The ommatidial clusters project their axon fascicles into the crescent-shaped lamina. Neuronal precursor cells of the lamina (LPCs) are incorporated into the axon target field at its anterior margin, which is demarcated by a morphological depression known as the lamina furrow. Glia precursor cells (GPCs) are generated in two domains that lie at the dorsal and ventral anterior margins of the prospective lamina. These glial precursors migrate into the lamina along an axis perpendicular to that of LPC entry. Postmitotic LPCs within the lamina axon target field express the nuclear protein Dac, as revealed by anti-Dac antibody staining. Like the eye, lamina differentiation occurs in a temporal progression on the anterioposterior axis. Axon fascicles from new ommatidial R-cell clusters arrive at the anterior margin of the lamina (adjacent to the lamina furrow) and associate with neuronal and glia precursors in a vertical lamina column assembly. At the anterior of the lamina, at the trough of the lamina furrow, LPCs await a retinal axon-mediated signal in G1-phase and enter their terminal S-phase at the posterior margin of the furrow. Postmitotic (Dac-positive) LPCs assemble into columns at the posterior margin of the furrow. In older columns at the posterior of the lamina, a subset of postmitotic LPCs express definitive neuronal markers as they become specified as the lamina neurons L1-L5. Lamina neurons L1-L4 form a stack in a superficial layer, while L5 neurons reside in a medial layer near the R1-R6 axon termini. These neurons arise at cell-type specific positions along the column's vertical axis. Lamina glial cells take up cell-type positions in the precartridge assemblies. Epithelial (E-glia) and marginal (Ma-glia) glia are located above and below the R1-R6 termini, respectively. Satellite glia are interspersed among the neurons of the L1-L4 layer. The Ma-glia and E-glia layers, both located ventral to the neuronal precursor column, sandwich the R1-R6 axon termini. The medulla neuropil serves as the target for R7/8 axons and is separated from the lamina by the medulla glia, situated just below the Ma-glia (Huang, 1998 and references).

Hedgehog, a secreted protein, is an inductive signal delivered by retinal axons for the initial steps of lamina differentiation. In the development of many tissues, Hedgehog acts in a signal relay cascade via the induction of secondary secreted factors. Lamina neuronal precursors respond directly to Hedgehog signal reception by entering S-phase, a step that is controlled by the Hedgehog-dependent transcriptional regulator Cubitus interruptus. The terminal differentiation of neuronal precursors and the migration and differentiation of glia appear to be controlled by other retinal axon-mediated signals. Thus retinal axons impose a program of developmental events on their postsynaptic field utilizing distinct signals for different precursor populations (Huang, 1998).

A hallmark of Hh signal reception in many Drosophila tissues is an increase in immunoreactivity to the C-terminal portion of the protein Ci, a transcriptional mediator of Hh signaling. This enhanced Ci immunoreactivity is due to inhibition of Ci proteolytic processing, a cellular response to Hh signal reception. LPCs posterior to the lamina furrow display the enhanced Ci immunoreactivity that would be predicted for Hh signal reception by LPCs. In animals in which hh- retinal axons innervate the lamina target field, cells posterior to the lamina furrow display a level of Ci immunoreactivity equivalent to the basal level detected anterior to the furrow, indicating that the increased Ci observed in the wild type is Hh-dependent. In smo mosaic animals, smo cells either anterior or posterior to the lamina furrow display a basal level of Ci immunoreactivity, while smo + cells immediately adjacent to the portion of smo clones within the lamina display the high Hh-dependent level (Huang, 1998).

In a number of instances, pattern formation mediated by Hh is accompanied by cell division. The well-defined pattern of Hh-induced cell division in the lamina provides an opportunity to determine the point at which the Hh signal reception engages the cell cycle machinery. LPC cell cycle progression and cell fate determination are jointly controlled by the transcriptional regulator Cubitus interruptus. Biochemical and epistasis experiments have placed the zinc finger molecule Ci downstream of all other hh signaling pathway components. Ci has been shown to bind directly to the regulatory sequences of Hh-responsive genes. Should all Hh-mediated events of LPC maturation be found to depend on Ci function, it could be concluded that, at least with regard to cell proliferation and the expression of differentiation markers, there is no branchpoint within the signaling pathway. To examine the requirement for Ci, two recombinant constructs were used that result in either dominant Ci gain-of-function or loss-of-function phenotypes. Overexpression of the wild-type Ci gene results in a gain-of-function phenotype that mimics activation of the Hh signaling pathway. Expression of an amino terminal fragment of Ci (hereafter referred to as DN-Ci) results in a dominant loss-of-function phenotype, as the normal in vivo function of this portion of the molecule appears to be transcriptional repression of Hh target genes. With either construct, genetically engineered ectopic expression in the lamina region results in the expected phenotype with respect to the lamina differentiation marker Dac. Dac expression in cells posterior of the lamina furrow is strongly reduced or undetectable in cells expressing DN-Ci. Conversely, the ectopic expression of wild-type Ci results in the induction of Dac-positive cells in the lamina target field of animals lacking innervation from the developing eye. The effects observed with either construct are strictly cell autonomous. Thus the results with ectopic Ci and DN-Ci expression are consistent with the expectation that Ci modulates Hh signaling activity directly in LPCs (Huang, 1998).

To determine whether Hh signaling acts via Ci to regulate the G1- to S-phase transition of LPCs at the lamina furrow, the incorporation of BrdU into S-phase cells was examined in animals harboring clones expressing either of the two constructs described above. Cells expressing DN-Ci at the posterior margin of the lamina furrow fail to enter S-phase. Where clones of DN-Ci-expressing cells traversed the lamina furrow, S-phase LPCs are absent, while S-phase LPCs are observed immediately outside of the clone. Moreover, the effect on cell division is limited to the LPCs at the lamina furrow. No defects are observed in other proliferation zones such as the OPC or IPC, the other major proliferation centers of the optic lobe, when they contain DN-Ci-expressing cells. Conversely, the induction of lamina differentiation by ectopic Ci expression in flies lacking retinal input into the lamina is accompanied by the entry of LPCs into S-phase at the lamina furrow. At the point when lamina differentiation is induced in the absence of retinal axons by ectopic Hh expression, ectopic Ci expression triggers a posterior-to-anterior pattern of differentiation such that S-phase LPCs are found at the anterior margin. In sum, these observations indicate that the induction of cell division by Hh occurs via the transcriptional regulation of Hh target genes (Huang, 1998).

Deciphering synergistic and redundant roles of Hedgehog, Decapentaplegic and Delta that drive the wave of differentiation in Drosophila eye development

In Drosophila, a wave of differentiation progresses across the retinal field in response to signals from posterior cells. Hedgehog (Hh), Decapentaplegic (Dpp) and Notch (N) signaling all contribute. Clones of cells mutated for receptors and nuclear effectors of one, two or all three pathways were studied to define systematically the necessary and sufficient roles of each signal. Hh signaling alone is sufficient for progressive differentiation, acting through both the transcriptional activator Ci155 and the Ci75 repressor. In the absence of Ci, Dpp and Notch signaling together provide normal differentiation. Dpp alone suffices for some differentiation, but Notch is not sufficient alone and acts only to enhance the effect of Dpp. Notch acts in part through downregulation of Hairy; Hh signaling downregulates Hairy independently of Notch. One feature of this signaling network is to limit Dpp signaling spatially to a range coincident with Hh (Fu, 2003).

The development of cells mutant for all three transcription factors, Mad, Su(H), and ci is a helpful starting point, since they may reflect a 'ground state' of eye development that requires extracellular signals to differentiate. Mad Su(H) ci cells fail to express the atonal or senseless genes that initiate R8 differentiation, and, consequently, fail to support retinal differentiation. This shows that the absence of Ci75 is not sufficient for differentiation. Dpp alone can induce Ato [e.g., in Su(H) ci clones], but N and Dpp signaling together are required to activate Atonal with normal kinetics, as occurs in ci-mutant cells. N signaling alone (in tkv ci clones) is insufficient. In the presence of Ci, prompt differentiation requires Hh to downregulate Ci75, and differentiation is delayed in Smo clones that lack this input. The normal role of Hh is not just to remove Ci75 thus permitting Dpp and N to work, because Atonal is turned on normally in Mad Su(H) clones that do not respond to Dpp or N signals. Such differentiation depends exclusively on Hh yet progresses normally, except that a neurogenic phenotype reflects dependence of lateral inhibition on Su(H). Hh depends positively on ci to drive differentiation in Mad Su(H) cells and, therefore, requires Ci155. The positive role of ci can also be inferred from the delayed differentiation of Su(H) ci clones in comparison with Su(H) clones (Fu, 2003).

Hairy is downregulated redundantly by Hh and N signaling. Prolonged Hairy expression is not sufficient to block differentiation completely but it does antagonize it (e.g., in Su(H) ci clones). Downregulation of Hairy in response to Hh as well as N explains why both ci and Su(H) mutant clones can differentiate promptly, and why N enhances differentiation in response to Dpp but is not required for differentiation in response to Hh (Fu, 2003).

Comparison between Mad Su(H) ci cells and Su(H) ci cells shows that Dpp signaling is sufficient to initiate eye differentiation in its normal location in the absence of Hh or N signals, but such differentiation is delayed. The normal timing of differentiation is restored by combined Dpp and N signals (in ci clones). This is the basis for the ectopic differentiation on co-expression of Dpp and Dl ahead of the furrow (Fu, 2003).

Superficially, these results differ from previous ectopic expression studies that concluded that Dpp signaling alone was not sufficient to induce ectopic differentiation in all locations. This discrepancy is probably explained by the baseline repressor activity of Su(H) protein. Previous work shows that without N signaling, repressor activity of Su(H) protein retards differentiation. Dpp signaling is sufficient for differentiation in the experiments where the Su(H) gene has been deleted. In the presence of the Su(H) gene, Dpp may be most effective at locations where there is little Su(H) repressor activity, such as close to the morphogenetic furrow where N signaling is active (Fu, 2003).

Comparison between Mad Su(H) ci cells, which do not differentiate, and Mad ci or tkv ci cells, which differentiate slowly or not at all, shows that Notch signaling alone is insufficient for differentiation. Premature differentiation reported when N is activated ectopically ahead of the furrow must reflect endogenous Dpp signaling at such locations (Fu, 2003).

These experiments reveal an outline of the mechanisms of Hh, Dpp and N redundancy. First, the results show that Mad and Ci independently reinforce differentiation, presumably through the transcription of target genes because Mad is sufficient for differentiation in the absence of Ci, and vice versa. The results show unequivocally that the transcriptional activator Ci155 activates differentiation in addition to Ci75 antagonizing differentiation (Fu, 2003).

It was surprising to find that Dpp stabilizes Ci155 in the absence of Smo, which suggests Dpp input into Hh signal transduction. Although the requirement for smo-dependent input through fused makes it unlikely that Ci155 is functional in smo clones, Ci155 accumulation might be associated with reduced Ci75 levels. Ci75 is shown to repress differentiation in smo clones because smo ci clones differentiation normally. Ci155 stabilization cannot be due to an indirect effect of Dpp signaling on Hh, Ptc or Smo expression levels because the effect is detected in the absence of smo, and, therefore, reflects an effect on Hh signal transduction components downstream of Smo. One idea is that Dpp signaling (or Dpp-induced differentiation) may replace SCFSlimb processing of Ci (which cleaves Ci155 to Ci75) with Cullin3-mediated Ci degradation, just as normally occurs posterior to the morphogenetic furrow. In a smo clone, Ci155 would accumulate because Smo is required for Cullin3 to degrade Ci. However, the SCFSlimb-to-Cullin3 switch may not be the only effect of Dpp on Ci processing, because Tkv slightly enhances Ci155 accumulation even when smo is present (Fu, 2003).

Finally, downregulation of Hairy by N requires the Su(H) gene. N also overcomes baseline repressor activity of Su(H) protein to promote progression of differentiation. This role of N must be independent of Hairy (Fu, 2003).

Dl, Hh and Dpp are generally thought to signal over very different distances. How can signals of such different range substitute for one another to permit normal eye development? Dpp is transcribed in response to Hh signaling and is produced where Ci155 levels are highest. Dl is regulated by Hh indirectly through Ato and Ato-dependent Egfr activity in differentiating cells. Hh is expressed most posteriorly of the three, in differentiating photoreceptors (Fu, 2003).

Eye differentiation uses Hh to progress through cells unable to respond to Dpp (tkv, Mad) or N (Su(H)). The range of Hh diffusion depends in part on the shape of the morphogenetic furrow cells. The Dpp that drives differentiation through ci-mutant cells unable to respond to Hh must diffuse from outside the ci clones because Dpp synthesis is Hh dependent. Large ci clones develop normally so Dpp diffusion cannot be limiting (dpp-mutant clones offer no information about the range of Dpp because they express and differentiate in response to Hh). Instead, the rate of progression in response to Dpp is controlled by Dl. Dl signals over (at most) one or two cell diameters at the morphogenetic furrow (Fu, 2003).

The previous view of eye patterning was influenced by the morphogen function of Hh and Dpp in other discs. It was thought that domains of Ato and Hairy expression reflected increasing concentrations of Hh and Dpp. The data shows that, in the eye, the combination of signals is important. Differentiation is triggered where Dl and/or Hh synergize with Dpp, regardless of where the source of Dpp is. The additional requirements limit Dpp to initiating differentiation at the same locations that Hh does (Fu, 2003).

Hedgehog acts as a somatic stem cell factor in the Drosophila ovary

Although Hedgehog proteins most commonly affect cell fate, they can also stimulate cell proliferation. In humans several distinctive cancers, including basal-cell carcinoma, result from mutations that aberrantly activate Hh signal transduction. In Drosophila, Hh directly stimulates proliferation of ovarian somatic cells. Hh acts specifically on stem cells in the Drosophila ovary. These cells cannot proliferate as stem cells in the absence of Hh signaling, whereas excessive Hh signaling produces supernumerary stem cells. It is deduced that Hh is a stem-cell factor and it is suggested that human cancers due to excessive Hh signaling might result from aberrant expansion of stem cell pools (Zhang, 2001).

The effects of Hh signaling on cell fate determination in Drosophila are mediated largely by altering the activity of the transcription factor Cubitus interruptus (Ci). The role of Ci in somatic stem cell proliferation was examined by inducing somatic clones lacking ci activity. As with smo, very few clones were recovered 8-10 d after clone induction and these clones occupied only a small proportion of the ovariole, indicating that stem cells cannot proliferate normally in the absence of ci activity. When the expression of a constitutively active derivative of Ci was induced by heat-shock-induced excision of a transcriptional terminator, ovarioles were recovered showing massive overproliferation of somatic cells, which accumulated between egg chambers as observed for ptc mutant ovarioles. Thus, the activity of Hh as a stem cell factor seems to depend on Ci-mediated regulation of transcription (Zhang, 2001).

The Hedgehog signalling pathway regulates autophagy

Autophagy is a highly conserved degradative process that removes damaged or unnecessary proteins and organelles, and recycles cytoplasmic contents during starvation. Autophagy is essential in physiological processes such as embryonic development but how autophagy is regulated by canonical developmental pathways is unclear. This study shows that the Hedgehog signalling pathway inhibits autophagosome synthesis, both in basal and in autophagy-induced conditions. This mechanism is conserved in mammalian cells and in Drosophila, and requires the orthologous transcription factors Gli2 and Ci, respectively. Furthermore, it was demonstrated activation of the Hedgehog pathway reduces PERK levels, concomitant with a decrease in phosphorylation of the translation initiation factor eukaryotic initiation factor 2alpha, suggesting a novel target of this pathway and providing a possible link between Hedgehog signalling and autophagy (Jimenez-Sanchez, 2012).

In Drosophila, a single Ptch receptor responds to Hh molecules, whereas in mammalian cells Ptch1 and Ptch2 share this function. Although knockdown of Ptch1 did not inhibit autophagy in basal conditions, activation of the Hh pathway was not as efficient with Ptch1 compared with Ptch2 knockdown. Also, Ptch1 is a transcriptional target of the Hh pathway and its expression is expected to increase when the pathway is activated by Ptch1 siRNA treatment. This feedback implies that Ptch1 siRNA may not be efficient enough to completely counteract its own transcriptional activation. Although more detailed experiments would be needed to rule out a differential contribution of these receptors, the data suggest that Shh modulates autophagy through Ptch1 and/or Ptch2 (Jimenez-Sanchez, 2012).

Gli transcription factors have distinct activator and repressor functions and their roles differ during embryonic development. Although a contribution of Gli1 and Gli3 to autophagy regulation cannot be completely excluded, it was confirmed that Gli2 is necessary for the inhibition of autophagy by Hh signalling. The increased LC3-II levels in Gli2-knockout embryos further confirmed the data in a biologically relevant context. These data do not explain the developmental defects in these mice but might suggest that some of their phenotypes could be secondary to autophagy dysregulation. However, almost certainly, factors other than autophagy inhibition will have major contributions to the developmental defect in these Gli2-knockout mice. Equally, it cannot be excluded that other pathways that increase autophagy could be aberrantly activated in these embryos (Jimenez-Sanchez, 2012).

Despite the effects of Gli2 and Ci on autophagy, the role of their upstream activator Smo was less clear-cut. This is not surprising, as it has been shown that Smo levels are not sufficient to stimulate the pathway, but an activation process is required, with Ptch influencing the transition to an active state of Smo in response to Shh. In Drosophila, null mutations in Smo did not influence autophagy whereas Costal2 inhibition did, and it is therefore also possible that the Hh/Ptch complex and Ci may partially bypass Smo in Drosophila, consistent with the effects of this pathway on Costal2 localization to endomembranes. Consequently, the data suggest that canonical and non-canonical Hh mechanisms might be involved in regulating autophagy, or that other factors might be necessary in Hh-mediated regulation of autophagy (Jimenez-Sanchez, 2012).

Hh inhibition has been largely pursued as a therapeutic strategy for types of cancer resulting from a hyperactivation of the pathway, such as basal cell carcinomas or medulloblastoma. Although the roles of autophagy in cancer are controversial and complex, it is interesting to consider whether inhibition of autophagy by Hh would exacerbate or prevent the progression of these tumours when searching for pharmacological strategies. Cyclopamine, which binds and inhibits Smo, is one of the best characterized Hh signalling inhibitors and it has been shown to be protective in Hh-related cancers. As expected, cyclopamine caused LC3-II (microtubule-associated protein 1 light chain 3 lipidated form, a product of autophagy). In the presence of bafA1, the increase was not as large as in non-treated cells. This result was unexpected because bafA1 should exacerbate the changes on LC3-II if cyclopamine is triggering autophagosome synthesis, suggesting that cyclopamine might also affect autophagosome maturation. Consistent with an impairment in autophagosome degradation, cyclopamine and bafA1 both similarly reduced the number of acidic vesicles in mRFP-GFP-LC3 cells. These data suggest that cyclopamine affects autophagosome degradation but that this effect is independent of Hh inhibition, as cyclopamine could increase LC3-II levels even in Gli2-null MEFs. These data suggest that cyclopamine has two opposite effects on autophagy. It increases autophagosome synthesis by inhibiting Smo activity, but it simultaneously impairs autophagosome maturation through an unknown mechanism that is independent of Gli2. Effects of cyclopamine independent of Smo inhibition have been reported in human breast cancer cell lines and in zebrafish, suggesting that this drug has additional molecular targets (Jimenez-Sanchez, 2012).

PRK-like ER kinase (PERK or EIF2AK3) is an ER-resident protein responsible for the phosphorylation of eukaryotic initiation factor 2α (eIF2α), which inhibits protein translation in response to ER stress, In search of potential Hh transcriptional targets that could modulate autophagy, PERK was identified as a consistent potential target and, in agreement with previous observation, PERK is repressed upon Gli1 and Gli2 induction. It has also been suggested that expression of Gli2 repressed a considerably larger number of genes than Gli1, an observation that may be of relevance when considering the more dominant effect of Gli2 on autophagy. Although Gli2 acts mainly as a transcriptional activator upon Hh activation, it can be proteolytically processed into a transcriptional repressor, similar to the dual function of Ci in Drosophila. The autophagy inhibitory action of Gli2 can be explained by either a gain of repressor function towards some genes (such as PERK) in its active form, or by the fact that when activated it leads to increased expression of a second transcriptional repressor. It remains to be elucidated which of these mechanisms is in action in the case of PERK transcription (Jimenez-Sanchez, 2012).

In response to unfolded proteins in the ER lumen, PERK is activated and phosphorylates eIF2α, increasing translation of the transcription factor ATF4. The exact mechanism by which eIF22α controls autophagy is unknown, eIF22α regulates Atg12 levels in the presence of expanded polyglutamines and ATF4 has been suggested to increase transcription of Atg5 and LC3. Indeed PCR Array data show that levels of >Atg5 upon Shh ligand are decreased to 0.8-fold compared with control cells. However, it is unlikely that this is the complete mechanism, as it has yet to be conclusively demonstrated that transcriptional induction of LC3 and Atg5 upregulate autophagy (Jimenez-Sanchez, 2012).

It is also possible that PERK interacts more directly with other signalling pathways involved in autophagy regulation. As an example, the transcription factor Nrf2, known to be involved in autophagy regulation, is phosphorylated by PERK independently of eIF22α in response to ER stress, promoting its import to the nucleus and activation of transcription (Jimenez-Sanchez, 2012).

In conclusion, these data not only provide insights into the connection between Hh and autophagy but also into the potential implications that Hh has in controlling protein homeostasis in physiological and disease conditions (Jimenez-Sanchez, 2012).

cubitus interruptus: Biological Overview | Evolutionary Homologs | Regulation | Targets of Activity | Protein Interactions | Developmental Biology | References

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