Phosphorylation by double-time/CKIepsilon and CKIalpha targets cubitus interruptus for Slimb/beta-TRCP-mediated proteolytic processing

Suppression of Hedgehog signaling by Cul3 ligases in proliferation control of retinal precursors

Cullin-RING ubiquitin ligases ubiquitinate protein substrates and control their levels through degradation. Here we show that cullin3 (Cul3) suppresses Hedgehog (Hh) signaling through downregulating the level of the signaling pathway effector cubitus interruptus (Ci). High-level Hh signaling promotes Cul3-dependent Ci degradation, leading to the downregulation of Hh signaling. This process is manifested in controlling cell proliferation during Drosophila retinal development. In Cul3 mutants, the population of interommatidial cells is increased, which can be mimicked by overexpression of Ci and suppressed by depleting endogenous Ci. Hh also regulates the population of interommatidial cells in the pupal stage. Alterations in the interommatidial cell population correlate with alterations in precursor proliferation in the second mitotic wave of larval eye discs. Taken together, these results suggest that Cul3 downregulates Ci levels to modulate Hh signaling activity, thus ensuring proper cell proliferation during retinal development.

beta-Trcp mediates ubiquitination and degradation of the erythropoietin receptor and controls cell proliferation

Control of intensity and duration of erythropoietin (Epo) signaling is necessary to tightly regulate red blood cell production. We have recently shown that the ubiquitin/proteasome system plays a major role in the control of Epo-R signaling. Indeed, after Epo stimulation, Epo-R is ubiquitinated and its intracellular part is degraded by the proteasome, preventing further signal transduction. The remaining part of the receptor and associated Epo are internalized and degraded by the lysosomes. We show that beta-Trcp is responsible for Epo-R ubiquitination and degradation. After Epo stimulation, beta-Trcp binds to the Epo-R. This binding, like Epo-R ubiquitination, requires Jak2 activation. The Epo-R contains a typical DSG binding sequence for beta-Trcp that is highly conserved among species. Interestingly, this sequence is located in a region of the Epo-R that is deleted in patients with familial polycythemia. Mutation of the serine residue of this motif to alanine (Epo-RS462A) abolished beta-Trcp binding, Epo-R ubiquitination, and degradation. Epo-RS462A activation was prolonged and BaF3 cells expressing this receptor are hypersensitive to Epo, suggesting that part of the hypersensitivity to Epo in familial polycythemia could be the result of the lack of beta-Trcp recruitment to the Epo-R.

Shifting paradigms in Hedgehog signaling

Hedgehog (Hh) signaling proteins regulate multiple developmental and adult homeostatic processes. A defining feature of Hh signaling is that relatively small changes in the concentration of Hh ligand elicit dramatically different cellular responses. As a result, the processing, release and trafficking of Hh ligands must be tightly regulated to ensure proper signaling. In addition, sensitive and specific intracellular signaling cascades are needed to interpret subtle differences in the level of Hh signal to execute an appropriate response. A detailed understanding of the mechanisms that regulate these responses is critical to shaping our view of this key regulatory system.

A novel protein-processing domain in Gli2 and Gli3 differentially blocks complete protein degradation by the proteasome

The proteasome usually completely degrades its target proteins, but it can also degrade a handful of proteins in a limited and site-specific manner. The molecular mechanism for such limited degradation is unknown. The repressor forms of Gli2 and Gli3 transcription factors are generated from their full-length proteins through limited proteasome-mediated protein degradation. In this study, we have taken advantage of the fact that Gli3 is efficiently processed, whereas Gli2 is not, and identified a region of approximately 200 residues in their C termini that determine differential processing of the two proteins. This region, named processing determinant domain, functions as a signal for protein processing in the context of not only Gli2 and Gli3 protein sequences but also a heterologous hybrid protein, which would otherwise be completely degraded by the proteasome. Thus, the processing determinant domain constitutes a novel domain that functions independently. Our findings explain, at the molecular level, why Gli2 and Gli3 are differentially processed and, more importantly, may help understand a probably general mechanism by which the proteasome degrades some of its target proteins partially rather than completely.

Using immunoprecipitation to study protein-protein interactions in the Hedgehog-signaling pathway

The Hedgehog (Hh)-signaling pathway has been intensively studied in the past decade. Increasing evidence suggests that dynamic formation of protein complexes plays a critical role in the organization and regulation of Hh signaling. Immunoprecipitation (IP) is a powerful tool to study protein-protein interactions and has provided important insights into the regulation of Hh signal transduction. Here, we show how to use IP to study protein-protein interactions in the Drosophila Hh-signaling pathway.

Delineation of a Fat tumor suppressor pathway

Recent studies in Drosophila melanogaster of the protocadherins Dachsous and Fat suggest that they act as ligand and receptor, respectively, for an intercellular signaling pathway that influences tissue polarity, growth and gene expression, but the basis for signaling downstream of Fat has remained unclear. Here, we characterize functional relationships among D. melanogaster tumor suppressors and identify the kinases Discs overgrown and Warts as components of a Fat signaling pathway. fat, discs overgrown and warts regulate a common set of downstream genes in multiple tissues. Genetic experiments position the action of discs overgrown upstream of the Fat pathway component dachs, whereas warts acts downstream of dachs. Warts protein coprecipitates with Dachs, and Warts protein levels are influenced by fat, dachs and discs overgrown in vivo, consistent with its placement as a downstream component of the pathway. The tumor suppressors Merlin, expanded, hippo, salvador and mob as tumor suppressor also share multiple Fat pathway phenotypes but regulate Warts activity independently. Our results functionally link what had been four disparate groups of D. melanogaster tumor suppressors, establish a basic framework for Fat signaling from receptor to transcription factor and implicate Warts as an integrator of multiple growth control signals.

Planar polarity is positively regulated by casein kinase Iepsilon in Drosophila

Members of the casein kinase I (CKI) family have been implicated in regulating canonical Wnt/Wingless (Wg) signaling by phosphorylating multiple pathway components. Overexpression of CKI in vertebrate embryos activates Wg signaling, and one target is thought to be the cytoplasmic effector Dishevelled (Dsh), which is an in vitro target of CKI phosphorylation. Phosphorylation of Dsh by CKI has also been suggested to switch its activity from noncanonical to canonical Wingless signaling. However, in vivo loss-of-function experiments have failed to identify a clear role for CKI in positive regulation of Wg signaling. By examining hypomorphic mutations of the Drosophila CKIepsilon homolog discs overgrown (dco)/double-time, we now show that it is an essential component of the noncanonical/planar cell polarity pathway. Genetic interactions indicate that dco acts positively in planar polarity signaling, demonstrating that it does not act as a switch between canonical and noncanonical pathways. Mutations in dco result in a reduced level of Dishevelled phosphorylation in vivo. Furthermore, in these mutants, Dishevelled fails to adopt its characteristic asymmetric subcellular localisation at the distal end of pupal wing cells, and the site of hair outgrowth is disrupted. Finally, we also find that dco function in polarity is partially redundant with CKIalpha.

Regulation of wingless signaling by the CKI family in Drosophila limb development

The Wingless (Wg)/Wnt signaling pathway regulates a myriad of developmental processes and its malfunction leads to human disorders including cancer. Recent studies suggest that casein kinase I (CKI) family members play pivotal roles in the Wg/Wnt pathway. However, genetic evidence for the involvement of CKI family members in physiological Wg/Wnt signaling events is lacking. In addition, there are conflicting reports regarding whether a given CKI family member functions as a positive or negative regulator of the pathway. Here we examine the roles of seven CKI family members in Wg signaling during Drosophila limb development. We find that increased CKIepsilon stimulates whereas dominant-negative or a null CKIepsilon mutation inhibits Wg signaling. In contrast, inactivation of CKIalpha by RNA interference (RNAi) leads to ectopic Wg signaling. Interestingly, hypomorphic CKIepsilon mutations synergize with CKIalpha RNAi to induce ectopic Wg signaling, revealing a negative role for CKIepsilon. Conversely, CKIalpha RNAi enhances the loss-of-Wg phenotypes caused by CKIepsilon null mutation, suggesting a positive role for CKIalpha. While none of the other five CKI isoforms can substitute for CKIalpha in its inhibitory role in the Wg pathway, several CKI isoforms including CG12147 exhibit a positive role based on overexpression. Moreover, loss of Gilgamesh (Gish)/CKIgamma attenuates Wg signaling activity. Finally, we provide evidence that several CKI isoforms including CKIalpha and Gish/CKIgamma can phosphorylate the Wg coreceptor Arrow (Arr), which may account, at least in part, for their positive roles in the Wg pathway.

A hedgehog-induced BTB protein modulates hedgehog signaling by degrading Ci/Gli transcription factor

The Ci/Gli family of transcription factors mediates Hedgehog (Hh) signaling in many key developmental processes. Here we identify a Hh-induced MATH and BTB domain containing protein (HIB) as a negative regulator of the Hh pathway. Overexpressing HIB down regulates Ci and blocks Hh signaling, whereas inactivating HIB results in Ci accumulation and enhanced pathway activity. HIB binds the N- and C-terminal regions of Ci, both of which mediate Ci degradation. HIB forms a complex with Cul3, a scaffold for modular ubiquitin ligases, and promotes Ci ubiquitination and degradation through Cul3. Furthermore, HIB-mediated Ci degradation is stimulated by Hh and inhibited by Suppressor of Fused (Sufu). The mammalian homolog of HIB, SPOP, can functionally substitute for HIB, and Gli proteins are degraded by HIB/SPOP in Drosophila. We provide evidence that HIB prevents aberrant Hh signaling posterior to the morphogenic furrow, which is essential for normal eye development.

Multisite protein kinase A and glycogen synthase kinase 3beta phosphorylation leads to Gli3 ubiquitination by SCFbetaTrCP

Gli3 is a zinc finger transcription factor proteolytically processed into a truncated repressor lacking C-terminal activation domains. Gli3 processing is stimulated by protein kinase A (PKA) and inhibited by Hedgehog signaling, a major signaling pathway in vertebrate development and disease. We show here that multisite glycogen synthase kinase 3beta (GSK3beta) phosphorylation and ubiquitination by SCFbetaTrCP are required for Gli3 processing. We identified multiple betaTrCP-binding sites related to the DSGX2-4S motif in Gli3, which are intertwined with PKA and GSK3beta sites, and SCFbetaTrCP target lysines that are essential for processing. Our results support a simple model whereby PKA triggers a cascade of Gli3 phosphorylation by GSK3beta and CK1 that leads to direct betaTrCP binding and ubiquitination by SCFbetaTrCP. Binding of betaTrCP to Gli3 N- and C-terminal domains lacking DSGX2-4S-related motifs was also observed, which could reflect indirect interaction via other components of Hedgehog signaling, such as the tumor suppressor Sufu. Gli3 therefore joins a small set of transcription factors whose processing is regulated by the ubiquitin-proteasome pathway. Our study sheds light on the role of PKA phosphorylation in Gli3 processing and will help to analyze how dose-dependent tuning of Gli3 processing is achieved by Hedgehog signaling.

The contributions of protein kinase A and smoothened phosphorylation to hedgehog signal transduction in Drosophila melanogaster

Protein kinase A (PKA) silences the Hedgehog (Hh) pathway in Drosophila in the absence of ligand by phosphorylating the pathway's transcriptional effector, Cubitus interruptus (Ci). Smoothened (Smo) is essential for Hh signal transduction but loses activity if three specific PKA sites or adjacent PKA-primed casein kinase 1 (CK1) sites are replaced by alanine residues. Conversely, Smo becomes constitutively active if acidic residues replace those phosphorylation sites. These observations suggest an essential positive role for PKA in responding to Hh. However, direct manipulation of PKA activity has not provided strong evidence for positive effects of PKA, with the notable exception of a robust induction of Hh target genes by PKA hyperactivity in embryos. Here we show that the latter response is mediated principally by regulatory elements other than Ci binding sites and not by altered Smo phosphorylation. Also, the failure of PKA hyperactivity to induce Hh target genes strongly through Smo phosphorylation cannot be attributed to the coincident phosphorylation of PKA sites on Ci. Finally, we show that Smo containing acidic residues at PKA and CK1 sites can be stimulated further by Hh and acts through Hh pathways that both stabilize Ci-155 and use Fused kinase activity to increase the specific activity of Ci-155.

CKI, there's more than one: casein kinase I family members in Wnt and Hedgehog signaling

Multiple members of the casein kinase I family of serine/threonine protein kinases are involved in positive and negative roles in Wnt and Hedgehog signaling. Here I review these roles, including recent results on casein kinase I (CKI) phosphorylation and activation of LRP6, and CKI phosphorylation of Ci and mediation of Ci-Slimb/beta-TrCP binding.

Sonic hedgehog signaling regulates Gli2 transcriptional activity by suppressing its processing and degradation

Gli2 and Gli3 are the primary transcription factors that mediate Sonic hedgehog (Shh) signals in the mouse. Gli3 mainly acts as a transcriptional repressor, because the majority of full-length Gli3 protein is proteolytically processed. Gli2 is mostly regarded as a transcriptional activator, even though it is also suggested to have a weak repressing activity. What the molecular basis for its possible dual function is and how its activity is regulated by Shh signaling are largely unknown. Here we demonstrate that unlike the results seen with Gli3 and Cubitus Interruptus, the fly homolog of Gli, only a minor fraction of Gli2 is proteolytically processed to form a transcriptional repressor in vivo and that in addition to being processed, Gli2 full-length protein is readily degraded. The degradation of Gli2 requires the phosphorylation of a cluster of numerous serine residues in its carboxyl terminus by protein kinase A and subsequently by casein kinase 1 and glycogen synthase kinase 3. The phosphorylated Gli2 interacts directly with betaTrCP in the SCF ubiquitin-ligase complex through two binding sites, which results in Gli2 ubiquitination and subsequent degradation by the proteasome. Both processing and degradation of Gli2 are suppressed by Shh signaling in vivo. Our findings provide the first demonstration of a molecular mechanism by which the Gli2 transcriptional activity is regulated by Shh signaling.