The casein kinase I family: roles in morphogenesis

Data-independent acquisition-based global phosphoproteomics reveal the diverse roles of casein kinase 1 in plant development

Casein kinase 1 (CK1) is serine/threonine protein kinase highly conserved among eukaryotes, and regulates multiple developmental and signaling events through phosphorylation of target proteins. Arabidopsis early flowering 1 (EL1)-like (AELs) are plant-specific CK1s with varied functions, but identification and validation of their substrates is a major bottleneck in elucidating their physiological roles. Here, we conducted a quantitative phosphoproteomic analysis in data-independent acquisition mode to systematically identify CK1 substrates. We extracted proteins from seedlings overexpressing individual AEL genes (AEL1/2/3/4-OE) or lacking AEL function (all ael single mutants and two triple mutants) to identify the high-confidence phosphopeptides with significantly altered abundance compared to wild-type Col-0. Among these, we selected 3985 phosphopeptides with higher abundance in AEL-OE lines or lower abundance in ael mutants compared with Col-0 as AEL-upregulated phosphopeptides, and defined 1032 phosphoproteins. Eight CK1s substrate motifs were enriched among AEL-upregulated phosphopeptides and verified, which allowed us to predict additional candidate substrates and functions of CK1s. We functionally characterized a newly identified substrate C3H17, a CCCH-type zinc finger transcription factor, through biochemical and genetic analyses, revealing a role for AEL-promoted C3H17 protein stability and transactivation activity in regulating embryogenesis. As CK1s are highly conserved across eukaryotes, we searched the rice, mouse, and human protein databases using newly identified CK1 substrate motifs, yielding many more candidate substrates than currently known, largely expanding our understanding of the common and distinct functions exerted by CK1s in Arabidopsis and humans, facilitating future mechanistic studies of CK1-mediated phosphorylation in different species.

Can We Pharmacologically Target Dishevelled: The Key Signal Transducer in the Wnt Pathways?

Dishevelled (DVL) is the central signal transducer in both Wnt/β-catenin-dependent and independent signalling pathways. DVL is required to connect receptor complexes and downstream effectors. Since proximal Wnt pathway components and DVL itself are upregulated in many types of cancer, DVL represents an attractive therapeutic target in the Wnt-addicted cancers and other disorders caused by aberrant Wnt signalling. Here, we discuss progress in several approaches for the modulation of DVL function and hence inhibition of the Wnt signalling. Namely, we sum up the potential of modulation of enzymes that control post-translational modification of DVL - such as inhibition of DVL kinases or promotion of DVL ubiquitination and degradation. In addition, we discuss research directions that can take advantage of direct interaction with the protein domains essential for DVL function: the inhibition of DIX- and DEP-domain mediated polymerization and interaction of DVL PDZ domain with its ligands.

Casein kinase 1 epsilon facilitates cartilage destruction in osteoarthritis through JNK pathway

Osteoarthritis (OA) is a high-morbidity skeletal disease worldwide and the exact mechanisms underlying OA pathogenesis are not fully understood. Casein kinase 1 epsilon (CK1ε) is a serine/threonine protein kinase, but its relationship with OA is still unknown. We demonstrated that CK1ε was upregulated in articular cartilage of human patients with OA and mice with experimentally induced OA. Activity of CK1ε, demonstrated by analysis of phosphorylated substrates, was significantly elevated in interleukin (IL)-1β-induced OA-mimicking chondrocytes. CK1ε inhibitor or CK1ε short hairpin RNA (shRNA) partially blocked matrix metalloproteinase (MMP) expression by primary chondrocytes induced by IL-1β, and also inhibited cartilage destruction in knee joints of experimental OA model mice. Conversely, overexpression of CK1ε promoted chondrocyte catabolism. Previous studies indicated that CK1ε was involved in canonical Wnt/β-catenin signaling and noncanonical Wnt/c-Jun N-terminal kinase (JNK) signaling pathway. Interestingly, the activity of JNK but not β-catenin decreased after CK1ε knockdown in IL-1β-treated chondrocytes in vitro, and JNK inhibition reduced MMP expression in chondrocytes overexpressing CK1ε, which illustrated that CK1ε-mediated OA was based on JNK pathway. In conclusion, our results demonstrate that CK1ε promotes OA development, and inhibition of CK1ε could be a potential strategy for OA treatment in the future.

MLK1 and MLK2 integrate gibberellins and circadian clock signaling to modulate plant growth

The covalent histone modifications were associated with plant development. However, the function of histone modification factors involved in gibberellins (GAs) signaling pathway remains unclear. In recent study, we reported that histone modification factors MUT9p-LIKE KINASE1 (MLK1) and MLK2 coordinate GA and circadian clock signaling in hypocotyl elongation. MLK1 and MLK2 interact with the DELLA protein REPRESSOR OF ga1-3 (RGA), and antagonize the function of RGA to interact with CIRCADIAN CLOCK ASSOCIATED1 (CCA1), resulting in promoting hypocotyl elongation. In this addendum to the report, we presented and discussed the results related to the function of MLK1 and MLK2 in GA pathway. MLK1 and MLK2 interact with RGA, which is independent on 17-amino acid DELLA, TVHYNP, or Poly S/T/V motif, suggesting that MLK1 and MLK2 might have novel functions beyond the protein degradation.

MLK1 and MLK2 Coordinate RGA and CCA1 Activity to Regulate Hypocotyl Elongation in Arabidopsis thaliana

Gibberellins (GAs) modulate diverse developmental processes throughout the plant life cycle. However, the interaction between GAs and the circadian rhythm remains unclear. Here, we report that MUT9p-LIKE KINASE1 (MLK1) and MLK2 mediate the interaction between GAs and the circadian clock to regulate hypocotyl elongation in Arabidopsis thaliana DELLA proteins function as master growth repressors that integrate phytohormone signaling and environmental pathways in plant development. MLK1 and MLK2 interact with the DELLA protein REPRESSOR OF ga1-3 (RGA). Loss of MLK1 and MLK2 function results in plants with short hypocotyls and hyposensitivity to GAs. MLK1/2 and RGA directly interact with CIRCADIAN CLOCK ASSOCIATED1 (CCA1), which targets the promoter of DWARF4 (DWF4) to regulate its roles in cell expansion. MLK1/2 antagonize the ability of RGA to bind CCA1, and these factors coordinately regulate the expression of DWF4 RGA suppressed the ability of CCA1 to activate expression from the DWF4 promoter, but MLK1/2 reversed this suppression. Genetically, MLK1/2 act in the same pathway as RGA and CCA1 in hypocotyl elongation. Together, our results provide insight into the mechanism by which MLK1 and MLK2 antagonize the function of RGA in hypocotyl elongation and suggest that MLK1/2 coordinately mediate the regulation of plant development by GAs and the circadian rhythm in Arabidopsis.

Planar cell polarity in coordinated and directed movements

Planar cell polarity is a fundamental concept to understanding the coordination of cell movements in the plane of a tissue. Since the planar cell polarity pathway was discovered in mesenchymal tissues involving cell interaction during vertebrate gastrulation, there is an emerging evidence that a variety of mesenchymal and epithelial cells utilize this genetic pathway to mediate the coordination of cells in directed movements. In this review, we focus on how the planar cell polarity pathway is mediated by migrating cells to communicate with one another in different developmental processes.

Ror-family receptor tyrosine kinases in noncanonical Wnt signaling: their implications in developmental morphogenesis and human diseases

The Ror-family receptor tyrosine kinases (RTKs) play crucial roles in the development of various organs and tissues. In mammals, Ror2, a member of the Ror-family RTKs, has been shown to act as a receptor or coreceptor for Wnt5a to mediate noncanonical Wnt signaling. Ror2- and Wnt5a-deficient mice exhibit similar abnormalities during developmental morphogenesis, reflecting their defects in convergent extension movements and planar cell polarity, characteristic features mediated by noncanonical Wnt signaling. Furthermore, mutations within the human Ror2 gene are responsible for the genetic skeletal disorders dominant brachydactyly type B and recessive Robinow syndrome. Accumulating evidence demonstrate that Ror2 mediates noncanonical Wnt5a signaling by inhibiting the beta-catenin-TCF pathway and activating the Wnt/JNK pathway that results in polarized cell migration. In this article, we review recent progress in understanding the roles of noncanonical Wnt5a/Ror2 signaling in developmental morphogenesis and in human diseases, including heritable skeletal disorders and tumor invasion.

Interactions between Casein kinase Iepsilon (CKIepsilon) and two substrates from disparate signaling pathways reveal mechanisms for substrate-kinase specificity

BACKGROUND

Members of the Casein Kinase I (CKI) family of serine/threonine kinases regulate diverse biological pathways. The seven mammalian CKI isoforms contain a highly conserved kinase domain and divergent amino- and carboxy-termini. Although they share a preferred target recognition sequence and have overlapping expression patterns, individual isoforms often have specific substrates. In an effort to determine how substrates recognize differences between CKI isoforms, we have examined the interaction between CKIepsilon and two substrates from different signaling pathways.

METHODOLOGY/PRINCIPAL FINDINGS

CKIepsilon, but not CKIalpha, binds to and phosphorylates two proteins: Period, a transcriptional regulator of the circadian rhythms pathway, and Disheveled, an activator of the planar cell polarity pathway. We use GST-pull-down assays data to show that two key residues in CKIalpha's kinase domain prevent Disheveled and Period from binding. We also show that the unique C-terminus of CKIepsilon does not determine Dishevelled's and Period's preference for CKIepsilon nor is it essential for binding, but instead plays an auxillary role in stabilizing the interactions of CKIepsilon with its substrates. We demonstrate that autophosphorylation of CKIepsilon's C-terminal tail prevents substrate binding, and use mass spectrometry and chemical crosslinking to reveal how a phosphorylation-dependent interaction between the C-terminal tail and the kinase domain prevents substrate phosphorylation and binding.

CONCLUSIONS/SIGNIFICANCE

The biochemical interactions between CKIepsilon and Disheveled, Period, and its own C-terminus lead to models that explain CKIepsilon's specificity and regulation.

Planar cell polarity signaling: from fly development to human disease

Most, if not all, cell types and tissues display several aspects of polarization. In addition to the ubiquitous epithelial cell polarity along the apical-basolateral axis, many epithelial tissues and organs are also polarized within the plane of the epithelium. This is generally referred to as planar cell polarity (PCP; or historically, tissue polarity). Genetic screens in Drosophila pioneered the discovery of core PCP factors, and subsequent work in vertebrates has established that the respective pathways are evolutionarily conserved. PCP is not restricted only to epithelial tissues but is also found in mesenchymal cells, where it can regulate cell migration and cell intercalation. Moreover, particularly in vertebrates, the conserved core PCP signaling factors have recently been found to be associated with the orientation or formation of cilia. This review discusses new developments in the molecular understanding of PCP establishment in Drosophila and vertebrates; these developments are integrated with new evidence that links PCP signaling to human disease.

Beta-arrestin and casein kinase 1/2 define distinct branches of non-canonical WNT signalling pathways

Recent advances in understanding beta-catenin-independent WNT (non-canonical) signalling suggest an increasing complexity, raising the question of how individual non-canonical pathways are induced and regulated. Here, we examine whether intracellular signalling components such as beta-arrestin (beta-arr) and casein kinases 1 and 2 (CK1 and CK2) can contribute to determining signalling specificity in beta-catenin-independent WNT signalling to the small GTPase RAC-1. Our findings indicate that beta-arr is sufficient and required for WNT/RAC-1 signalling, and that casein kinases act as a switch that prevents the activation of RAC-1 and promotes other non-canonical WNT pathways through the phosphorylation of dishevelled (DVL, xDSH in Xenopus). Thus, our results indicate that the balance between beta-arr and CK1/2 determines whether WNT/RAC-1 or other non-canonical WNT pathways are activated.

The Saccharomyces cerevisiae 60 S ribosome biogenesis factor Tif6p is regulated by Hrr25p-mediated phosphorylation

The biosynthesis of 60 S ribosomal subunits in Saccharomyces cerevisiae requires Tif6p, the yeast homologue of mammalian eIF6. This protein is necessary for the formation of 60 S ribosomal subunits because it is essential for the processing of 35 S pre-rRNA to the mature 25 S and 5.8 S rRNAs. In the present work, using molecular genetic and biochemical analyses, we show that Hrr25p, an isoform of yeast casein kinase I, phosphorylates Tif6p both in vitro and in vivo. Tryptic phosphopeptide mapping of in vitro phosphorylated Tif6p by Hrr25p and (32)P-labeled Tif6p isolated from yeast cells followed by mass spectrometric analysis revealed that phosphorylation occurred on a single tryptic peptide at Ser-174. Sucrose gradient fractionation and coimmunoprecipitation experiments demonstrate that a small but significant fraction of Hrr25p is bound to 66 S preribosomal particles that also contain bound Tif6p. Depletion of Hrr25p from a conditional yeast mutant that fails to phosphorylate Tif6p was unable to process pre-rRNAs efficiently, resulting in significant reduction in the formation of 25 S rRNA. These results along with our previous observations that phosphorylatable Ser-174 is required for yeast cell growth and viability, suggest that Hrr25p-mediated phosphorylation of Tif6p plays a critical role in the biogenesis of 60 S ribosomal subunits in yeast cells.

Cell signaling: moving to a Wnt-Rap

Non-canonical, beta-catenin-independent Wnt signaling regulates cell polarization and movements. A recent study reveals that casein kinase Iepsilon mediates an additional novel non-canonical Wnt pathway via the activation of the Rap1 GTPase during vertebrate gastrulation.

A Wnt-CKIvarepsilon-Rap1 pathway regulates gastrulation by modulating SIPA1L1, a Rap GTPase activating protein

Noncanonical Wnt signals control morphogenetic movements during vertebrate gastrulation. Casein kinase I epsilon (CKIvarepsilon) is a Wnt-regulated kinase that regulates Wnt/beta-catenin signaling and has a beta-catenin-independent role(s) in morphogenesis that is poorly understood. Here we report the identification of a CKIvarepsilon binding partner, SIPA1L1/E6TP1, a GAP (GTPase activating protein) of the Rap small GTPase family. We show that CKIvarepsilon phosphorylates SIPA1L1 to reduce its stability and thereby increase Rap1 activation. Wnt-8, which activates CKIvarepsilon, enhances the CKIvarepsilon-dependent phosphorylation and degradation of SIPA1L1. In early Xenopus or zebrafish development, inactivation of the Rap1 pathway results in abnormal gastrulation and a shortened anterior-posterior axis. Although CKIvarepsilon also transduces Wnt/beta-catenin signaling, inhibition of Rap1 does not alter beta-catenin-regulated gene expression. Our data demonstrate a role for CKIvarepsilon in noncanonical Wnt signaling and indicate that Wnt regulates morphogenesis in part through CKIvarepsilon-mediated control of Rap1 signaling.

Frizzled/PCP signalling: a conserved mechanism regulating cell polarity and directed motility

Signalling through Frizzled (Fz)/planar cell polarity (PCP) is a conserved mechanism that polarizes cells along specific axes in a tissue. Genetic screens in Drosophila melanogaster pioneered the discovery of core PCP factors, which regulate the orientation of hairs on wings and facets in eyes. Recent genetic evidence shows that the Fz/PCP pathway is conserved in vertebrates and is crucial for disparate processes as gastrulation and sensory cell orientation. Fz/PCP signalling depends on complex interactions between core components, leading to their asymmetric distribution and ultimately polarized activity in a cell. Whereas several mechanistic aspects of PCP have been uncovered, the global coordination of this polarization remains debated.

Disease-associated casein kinase I delta mutation may promote adenomatous polyps formation via a Wnt/beta-catenin independent mechanism

The Wnt signaling pathway is critical for embryonic development and is dysregulated in multiple cancers. Two closely related isoforms of casein kinase I (CKIdelta and epsilon) are positive regulators of this pathway. We speculated that mutations in the autoinhibitory domain of CKIdelta/epsilon might upregulate CKIdelta/epsilon activity and hence Wnt signaling and increase the risk of adenomatous polyps and colon cancer. Exons encoding the CKIepsilon and CKIdelta regulatory domains were sequenced from DNA obtained from individuals with adenomatous polyps and a family history of colon cancer unaffected by familial adenomatous polyposis or hereditary nonpolyposis colorectal cancer (HNPCC). A CKIdelta missense mutation, changing a highly conserved residue, Arg324, to His (R324H), was found in an individual with large and multiple polyps diagnosed at a relatively young age. Two findings indicate that this mutation is biologically active. First, ectopic ventral expression of CKIdelta(R324H) in Xenopus embryos results in secondary axis formation with an additional distinctive phenotype (altered morphological movements) similar to that seen with unregulated CKIepsilon. Second, CKIdelta(R324H) is more potent than wildtype CKIdelta in transformation of RKO colon cancer cells. Although the R324H mutation does not significantly change CKIdelta kinase activity in an in vitro kinase assay or Wnt/beta-catenin signal transduction as assessed by a beta-catenin reporter assay, it alters morphogenetic movements via a beta-catenin-independent mechanism in early Xenopus development. This novel human CKIdelta mutation may alter the physiological role and enhance the transforming ability of CKIdelta through a Wnt/beta-catenin independent mechanism and thereby influence colonic adenoma development.

Dual alterations in casein kinase I-epsilon and GSK-3beta modulate beta-catenin stability in hyperproliferating colonic epithelia

Casein kinase I (CKI)-epsilon and GSK-3beta phosphorylate beta-catenin at Ser(45) (beta-cat(45)) and Thr(41)/Ser(37,33) (beta-cat(33,37,41)) residues, thereby facilitating its ubiquitination and proteasomal degradation. We used a Citrobacter rodentium-induced transmissible murine colonic hyperplasia (TMCH) model to determine Ser/Thr phosphorylation and biological function of beta-catenin during crypt hyperproliferation. TMCH was associated with 3-fold and 3.3-fold increases in CKI-epsilon cellular abundance and 2-fold and 1.8-fold increase in its activity at 6 and 12 days after infection, respectively. beta-Catenin coimmunoprecipitated with both cellular and nuclear CKI-epsilon and cellular axin at these time points. Cellular beta-catenin was constitutively phosphorylated at Ser(45) and underwent subcellular redistribution to cytoskeletal and nuclear fractions at days 6 and 12 of TMCH, respectively. beta-cat(33,37,41), however, exhibited only subtle changes in either phosphorylation status or subcellular distribution even after blocking proteasomal degradation in vivo. Interestingly, GSK-3beta underwent increased phosphorylation at Ser(9), leading to 40% and 70% decreases in its activity at these time points, respectively. Coimmunoprecipitation studies exhibited strong association of GSK-3beta with PKC-zeta at either time point. Cellular beta-cat(45) stabilized and, along with unphosphorylated beta-catenin, underwent nuclear translocation, associated with nuclear accumulated Tcf-4 and cAMP response element binding protein binding protein, and was significantly acetylated, leading to increases in DNA binding. Priming of beta-catenin at Ser(45) exists in vivo. However, beta-cat(45) does not necessarily enter the degradation pathway. Impairment in linking beta-cat(45) to subsequent GSK-3beta-mediated phosphorylation and degradation may account for increased steady-state levels of both unphosphorylated as well as Ser(45)-phosphorylated beta-catenin, which may be causally linked to increases in cell census during TMCH.

Wnt-5a induces Dishevelled phosphorylation and dopaminergic differentiation via a CK1-dependent mechanism

Previously, we have shown that Wnt-5a strongly regulates dopaminergic neuron differentiation by inducing phosphorylation of Dishevelled (Dvl). Here, we identify additional components of the Wnt-5a-Dvl pathway in dopaminergic cells. Using in vitro gain-of-function and loss-of-function approaches, we reveal that casein kinase 1 (CK1) delta and CK1epsilon are crucial for Dvl phosphorylation by non-canonical Wnts. We show that in response to Wnt-5a, CK1epsilon binds Dvl and is subsequently phosphorylated. Moreover, in response to Wnt-5a or CK1epsilon, the distribution of Dvl changed from punctate to an even appearance within the cytoplasm. The opposite effect was induced by a CK1epsilon kinase-dead mutant or by CK1 inhibitors. As expected, Wnt-5a blocked the Wnt-3a-induced activation of beta-catenin. However, both Wnt-3a and Wnt-5a activated Dvl2 by a CK1-dependent mechanism in a cooperative manner. Finally, we show that CK1 kinase activity is necessary for Wnt-5a-induced differentiation of primary dopaminergic precursors. Thus, our data identify CK1 as a component of Wnt-5a-induced signalling machinery that regulates dopaminergic differentiation, and suggest that CK1delta/epsilon-mediated phosphorylation of Dvl is a common step in both canonical and non-canonical Wnt signalling.

The Drosophila casein kinase Iepsilon/delta Discs overgrown promotes cell survival via activation of DIAP1 expression

The proper number of cells in developing tissues is achieved by coordinating cell division with apoptosis. In Drosophila, the adult wing is derived from wing imaginal discs, which undergo a period of growth and proliferation during larval stages without much programmed cell death. In this report, we demonstrate that the Drosophila casein kinase Iepsilon/delta, known as Discs overgrown (Dco), is required for maintaining this low level of apoptosis. Expression of dco can suppress the apoptotic activity of Head involution defective (Hid) in the developing eye. Loss of dco in the wing disc results in a dramatic reduction in expression of the caspase inhibitor DIAP1 and a concomitant activation of caspases. The regulation of DIAP1 by Dco occurs by a post-transcriptional mechanism that is independent of hid. Mutant clones of dco are considerably smaller than controls even when apoptosis is inhibited, suggesting that Dco promotes cell division/growth in addition to its role in cell survival. The dco phenotype cannot be explained by defects Wingless (Wg) signaling. We propose that Dco coordinates tissue size by stimulating cell division/growth and blocking apoptosis via activation of DIAP1 expression.

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.

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.

Novel Daple-like protein positively regulates both the Wnt/beta-catenin pathway and the Wnt/JNK pathway in Xenopus

Wnt signaling pathways are essential in various developmental processes including differentiation, proliferation, cell migration, and cell polarity. Wnt proteins execute their multiple functions by activating distinct intracellular signaling cascades, although the mechanisms underlying this activation are not fully understood. We identified a novel Daple-like protein in Xenopus and named it xDal (Xenopus Daple-like). As with Daple, xDal contains several leucine zipper-like regions (LZLs) and a putative PDZ domain-binding motif, and can interact directly with the dishevelled protein. In contrast to mDaple, injection of xDal mRNA into the dorso-vegetal blastomere does not induce ventralization and acted synergistically with xdsh in secondary axis induction. XDal also induced expression of siamois and xnr-3, suggesting that XDal functions as a positive regulator of the Wnt/beta-catenin pathway. Injection of xDal mRNA into the dorso-animal blastomere, however, induced gastrulation-defective phenotypes in a dose-dependent manner. In addition, xDal inhibited activin-induced elongation of animal caps and enhanced c-jun phosphorylation. Based on these findings, xDal is also thought to function in the Wnt/JNK pathway. Moreover, functional domain analysis with several deletion mutants indicated that xDal requires both a putative PDZ domain-binding motif and at least one LZL for its activity. These findings with xDal will provide new information on the Wnt signaling pathways.

Distinct PAR-1 Proteins Function in Different Branches of Wnt Signaling during Vertebrate Development

The kinase PAR-1 plays conserved roles in cell polarity. PAR-1 has also been implicated in axis establishment in C. elegans and Drosophila and in Wnt signaling, but its role in vertebrate development is unclear. Here we report that PAR-1 has two distinct and essential roles in axial development in Xenopus mediated by different PAR-1 isoforms. Depletion of PAR-1A or PAR-1BX causes dorsoanterior deficits, reduced Spemann organizer gene expression, and inhibition of canonical Wnt-beta-catenin signaling. By contrast, PAR-1BY depletion inhibits cell movements and localization of Dishevelled protein to the cell cortex, processes associated with noncanonical Wnt signaling. PAR-1 phosphorylation sites in Dishevelled are required for this translocation, but not for canonical Wnt signaling. We conclude that PAR-1BY is required in the PCP branch and mediates Dsh membrane localization while PAR-1A and PAR-1BX are essential for canonical signaling to beta-catenin, possibly via targets other than Dishevelled.

The casein kinase 1 family: participation in multiple cellular processes in eukaryotes

Phosphorylation of serine, threonine and tyrosine residues by cellular protein kinases plays an important role in the regulation of various cellular processes. The serine/threonine specific casein kinase 1 and 2 protein kinase families--(CK1 and CK2)--were among the first protein kinases that had been described. In recent years our knowledge of the regulation and function of mammalian CK1 kinase family members has rapidly increased. Extracellular stimuli, the subcellular localization of CK1 isoforms, their interaction with various cellular structures and proteins, as well as autophosphorylation and proteolytic cleavage of their C-terminal regulatory domains influence CK1 kinase activity. Mammalian CK1 isoforms phosphorylate many different substrates among them key regulatory proteins involved in the control of cell differentiation, proliferation, chromosome segregation and circadian rhythms. Deregulation and/or the incidence of mutations in the coding sequence of CK1 isoforms have been linked to neurodegenerative diseases and cancer. This review will summarize our current knowledge about the function and regulation of mammalian CK1 isoforms.

Casein kinase Iepsilon plays a functional role in the transforming growth factor-beta signaling pathway

The transforming growth factor-beta (TGF-beta) signaling pathway is known to be involved in a wide range of biological events, including development, cellular differentiation, apoptosis, and oncogenesis. The TGF-beta signal is mediated by ligand binding to the type II receptor, leading to the recruitment and activation of the type I receptor, and subsequent activation of a family of intracellular signal transducing proteins called Smads. Here we report a regulatory role for casein kinase Iepsilon (CKIepsilon) in the TGF-beta signaling cascade. We find that CKIepsilon binds to all Smads and the cytoplasmic domains of the type I and type II receptors both in vitro and in vivo. The interaction of CKIepsilon with the type I and type II receptors is independent of TGF-beta stimulation, whereas the CKIepsilon/Smad interaction is transiently disrupted by ligand treatment. Additionally, CKIepsilon is able to phosphorylate the receptor-activated Smads (Smads 1-3 and 5) and the type II receptor in vitro. Transcriptional reporter assays reveal that transient overexpression of wild type CKIepsilon dramatically reduces basal reporter activity but enhances TGF-beta-stimulated transcription. Furthermore, overexpression of a kinase-dead mutant of CKIepsilon inhibits both basal and ligand-induced transcription, whereas inhibition of endogenous CKI catalytic activity with IC261 blocks only TGF-beta-stimulated reporter activity. Finally, knocking down CKIepsilon protein levels results in a significant increase in basal and TGF-beta-induced transcription. These results suggest that CKIepsilon plays a ligand-dependent, differential, and dual regulatory role within the TGF-beta signaling pathway.

Regulation of casein kinase I epsilon activity by Wnt signaling

The Wnt/beta-catenin signaling pathway is important in both development and cancer. Casein kinase Iepsilon (CKIepsilon) is a positive regulator of the canonical Wnt pathway. CKIepsilon itself can be regulated in vitro by inhibitory autophosphorylation, and recent data suggest that in vivo kinase activity can be regulated by extracellular stimuli. We show here that the phosphorylation state and kinase activity of CKIepsilon are directly regulated by Wnt signaling. Coexpression of XWnt-8 or addition of soluble Wnt-3a ligand led to a significant and rapid increase in the activity of endogenous CKIepsilon. The increase in CKIepsilon activity is the result of decreased inhibitory autophosphorylation because it is abolished by preincubation of immunoprecipitated kinase with ATP. Furthermore, mutation of CKIepsilon inhibitory autophosphorylation sites creates a kinase termed CKIepsilon(MM2) that is significantly more active than CKIepsilon and is not activated further upon Wnt stimulation. Autoinhibition of CKIepsilon is biologically relevant because CKIepsilon(MM2) is more effective than CKIepsilon at activating transcription from a Lef1-dependent promoter. Finally, CKIepsilon(MM2) expression in Xenopus embryos induces both axis duplication and additional developmental abnormalities. The data suggest that Wnt signaling activates CKIepsilon by causing transient dephosphorylation of critical inhibitory sites present in the carboxyl-terminal domain of the kinase. Activation of the Wnt pathway may therefore stimulate a cellular phosphatase to dephosphorylate and activate CKIepsilon

Involvement of casein kinase Iepsilon in cytokine-induced granulocytic differentiation

Two closely related casein kinase I (CKI) isoforms, CKIdelta and CKIepsilon, are ubiquitously expressed in many human tissues, but their specific biologic function remains to be clarified. Here, we provide the first evidence that CKIepsilon is involved in hematopoietic cell differentiation. CKIepsilon, but not CKIdelta, was down-regulated along with human granulocytic differentiation. The specific down-regulation was observed in granulocyte colony-stimulating factor (G-CSF)-induced cell differentiation of murine interleukin-3 (IL-3)-dependent myeloid progenitor 32D cells. Introduction of wild-type (WT)-CKIepsilon into 32D cells inhibited the G-CSF-induced cell differentiation, whereas kinase-negative (KN)-CKIepsilon promoted the differentiation. Neither WT- nor KN-CKIepsilon affected IL-3-dependent cell growth. Moreover, introduction of WT- or KN-CKIdelta did not affect the cytokine-induced cell growth and differentiation. While G-CSF-induced activation of signal transducers and activators of transcription 3 (STAT3) was sustained by KN-CKIepsilon, STAT3 activation was attenuated by WT-CKIepsilon. This may be explained by the fact that the suppressor of cytokine signaling 3 (SOCS3) was stabilized by its physical association with CKIepsilon. Such stabilization by CKIepsilon was also seen in IL-3-induced beta-catenin. The stabilization of downstream components of cytokine and Wnt signaling by CKIepsilon might be critical for integration of several intracellular signaling pathways to a cell-specific biologic response in hematopoietic cell self-renewal.

The Effects of Cholesterol-3-sulfate (CH-3S) on the Phosphorylation of Human C3a (hC3a) in Vitro and on the Ability of hC3a to Induce Vascular Permeability in Rats

The phosphorylation of human C3a (hC3a, anaphylatoxin) by two distinct protein kinases (PKA and CK-I) and the effect of cholesterol-3-sulfate (CH-3S) on this phosphorylation were biochemically investigated in vitro. It was found that (i) hC3a functions as a phosphate acceptor for PKA and CK-I, but not for CK-II; (ii) the CK-I-mediated phosphorylation of hC3a requires the presence of 3 microM CH-3S in a manner similar to the phosphorylation of HMG1 (CH-3S-binding protein) by CK-I; and (iii) CH-3S inhibits the PKA-mediated phosphorylation of hC3a in a dose-dependent manner (ID50=approximately 2 microM). As expected, hC3a containing high levels of Arg- and Lys-residues stimulated approx. 3-fold CK-II activity (phosphorylation of alpha-casein) in vitro. However, no significant effect of hC3a on CK-II activity was observed when hC3a was preincubated with CH-3S or fully phosphorylated by PKA in vitro. Furthermore, preincubation of hC3a with CH-3S diminished the ability of hC3a to induce vascular permeability in rats. The results provided here suggest that (i) hC3a is a CH-3S-binding protein; and (ii) CH-3S functions as a potent inhibitor for its physiological activities, including phosphorylation by PKA and CK-I, in vitro.

Runnin' with the Dvl: proteins that associate with Dsh/Dvl and their significance to Wnt signal transduction

Wnt proteins transmit myriad intercellular signals crucial for the development and homeostasis of metazoan animals from Hydra to human. Abnormal Wnt signaling causes a growing number of diseases, including cancer and osteoporosis. Depending on the context, a given Wnt signal may denote: cell proliferation or apoptosis; cell fate determination, differentiation, or stem cell maintenance; a variety of changes in cell behavior; and/or coordinated interactions with its neighbors. Which event(s) occur in Wnt-responsive cells depends critically on the ability of Dishevelled (Dsh)/Dvl proteins to interpret distinct types of intracellular, receptor-generated stimuli and transmit them to at least two distinct sets of effector molecules, all while apparently ignoring a third type of Wnt-generated Ca(2+) signal. The three conserved domains present in Dsh/Dvl proteins uniquely function in each Wnt pathway, in part by association with 18 (and counting) Dsh/Dvl-associated proteins. The latest data suggest that Dsh/Dvl proteins organize dynamic, pathway-specific subcellular signaling complexes that ensure correct information routing, signal amplification, and dynamic control through feedback regulation. The biochemical and cell biological mechanisms by which Dsh/Dvl proteins accomplish these remarkable tasks remain obscure.

Casein Kinase 1 Regulates Connexin-43 Gap Junction Assembly

Phosphorylation of members of the connexin family of gap junction proteins has been correlated with gap junction assembly, but the mechanisms involved remain unclear. We have examined the role of casein kinase 1 (CK1) in connexin-43 (Cx43) gap junction assembly. Cellular co-immunoprecipitation experiments and in vitro CK1 phosphorylation reactions indicate that CK1 interacted with and phosphorylated Cx43, initially on serine(s) 325, 328, or 330. (32)P(i)-Metabolically labeled cells treated with CKI-7, a specific CK1 inhibitor, showed a reduction in Cx43 phosphorylation on site(s) that can be phosphorylated by CK1 in vitro. To examine CK1 function, normal rat kidney cells were treated with CKI-7, and Cx43 content was analyzed by Triton X-100 extraction, cell-surface biotinylation, and immunofluorescence. Western blot analysis indicated a slight increase in total Cx43, whereas gap junctional (Triton-insoluble) Cx43 decreased, and non-junctional plasma membrane Cx43 increased (as detected by cell surface biotinylation). Immunofluorescence experiments in the presence of CK1 inhibitor showed increases in Cx43 plasma membrane localization but not necessarily accumulation at cell-cell interfaces. Decreased gap junctional and phosphorylated Cx43 was also detected when cells were treated with IC261, a CK1 inhibitor specific for delta or epsilon isoforms. These data suggest CK1delta could regulate Cx43 gap junction assembly by directly phosphorylating Cx43.

Casein kinase I regulates membrane binding by ARF GAP1

ARF GAP1, a 415-amino acid GTPase activating protein (GAP) for ADP-ribosylation factor (ARF) contains an amino-terminal 115-amino acid catalytic domain and no other recognizable features. Amino acids 203-334 of ARF GAP1 were sufficient to target a GFP-fusion protein to Golgi membranes in vivo. When overexpressed in COS-1 cells, this protein domain inhibited protein transport between the ER and Golgi and, in vitro, competed with the full-length ARF GAP1 for binding to membranes. Membrane binding by ARF GAP1 in vitro was increased by a factor in cytosol and this increase was inhibited by IC261, an inhibitor selective for casein kinase Idelta (CKIdelta), or when cytosol was treated with antibody to CKIdelta. The noncatalytic domain of ARF GAP1 was phosphorylated both in vivo and in vitro by CKI. IC261 blocked membrane binding by ARF GAP1 in vivo and inhibited protein transport in the early secretory pathway. Overexpression of a catalytically inactive CKIdelta also inhibited the binding of ARF GAP1 to membranes and interfered with protein transport. Thus, a CKI isoform is required for protein traffic through the early secretory pathway and can modulate the amount of ARF GAP1 that can bind to membranes.

Non-canonical Wnt signaling in Xenopus: regulation of axis formation and gastrulation

Wnt proteins form a family of secreted glycoproteins that are involved in different developmental processes such as differentiation, proliferation, cell migration and cell polarity. To exert its function, Wnt proteins activate different intracellular signaling cascades. Whereas the canonical, Wnt/beta-catenin pathway is well characterized, less is known about the function of non-canonical Wnt pathways in vertebrates. I here summarize recent findings implicating important roles for Wnt/Ca(2+) and Wnt/JNK signaling during different aspects of early Xenopus laevis development, namely axis formation and gastrulation movements.

Non-canonical Wnt signalling and regulation of gastrulation movements

Members of the Wnt family have been implicated in a variety of developmental processes including axis formation, patterning of the central nervous system and tissue morphogenesis. Recent studies have shown that a Wnt signalling pathway similar to that involved in the establishment of planar cell polarity in Drosophila regulates convergent extension movements during zebrafish and Xenopus gastrulation. This finding provides a good starting point to dissect the complex cell biology and genetic regulation of vertebrate gastrulation movements.

Casein kinase I and casein kinase II differentially regulate axin function in Wnt and JNK pathways

Axin uses different combinations of functional domains in down-regulation of the Wnt pathway and activation of the MEKK1/JNK pathway. We are interested in the elucidation of the functional switch of Axin. In the present study, we show that the Wnt activator CKIepsilon, but not CKIIalpha, Frat1, LRP5, or LRP6, inhibited Axin-mediated JNK activation. We also found that both CKIalpha and CKIepsilon interacted with Axin, whereas CKIIalpha did not bind to Axin and had no effect on Axin-mediated JNK activity even though CKIIalpha has also been suggested to be an activator for the Wnt pathway. The COOH-terminal region and the MEKK1-interacting domain of Axin are important for CKIalpha-Axin and CKIepsilon-Axin interaction. We further demonstrated that CKIepsilon and CKIalpha binding to Axin excluded MEKK1 binding, indicating that a competitive physical occupancy may underlie the inhibitory effect. Moreover, our data indicated that CKIepsilon kinase activity plays an additive role in this effect. Taken together, we have demonstrated that CKI and CKII exhibit differential effects on Axin-MEKK1 interaction and Axin-mediated JNK activation. Furthermore, our data suggest that CKI may provide a possible switch mechanism for Axin function in the regulation of Wnt and JNK pathways.

Axin-mediated CKI phosphorylation of beta-catenin at Ser 45: a molecular switch for the Wnt pathway

The Wnt pathway controls numerous developmental processes via the beta-catenin-TCF/LEF transcription complex. Deregulation of the pathway results in the aberrant accumulation of beta-catenin in the nucleus, often leading to cancer. Normally, cytoplasmic beta-catenin associates with APC and axin and is continuously phosphorylated by GSK-3beta, marking it for proteasomal degradation. Wnt signaling is considered to prevent GSK-3beta from phosphorylating beta-catenin, thus causing its stabilization. However, the Wnt mechanism of action has not been resolved. Here we study the regulation of beta-catenin phosphorylation and degradation by the Wnt pathway. Using mass spectrometry and phosphopeptide-specific antibodies, we show that a complex of axin and casein kinase I (CKI) induces beta-catenin phosphorylation at a single site: serine 45 (S45). Immunopurified axin and recombinant CKI phosphorylate beta-catenin in vitro at S45; CKI inhibition suppresses this phosphorylation in vivo. CKI phosphorylation creates a priming site for GSK-3beta and is both necessary and sufficient to initiate the beta-catenin phosphorylation-degradation cascade. Wnt3A signaling and Dvl overexpression suppress S45 phosphorylation, thereby precluding the initiation of the cascade. Thus, a single, CKI-dependent phosphorylation event serves as a molecular switch for the Wnt pathway.

Casein kinase I phosphorylates the Armadillo protein and induces its degradation in Drosophila

Casein kinase I (CKI) was recently reported as a positive regulator of Wnt signaling in vertebrates and Caenorhabditis elegans. To elucidate the function of Drosophila CKI in the wingless (Wg) pathway, we have disrupted its function by double-stranded RNA-mediated interference (RNAi). While previous findings were mainly based on CKI overexpression, this is the first convincing loss-of-function analysis of CKI. Surprisingly, CKIalpha- or CKIepsilon-RNAi markedly elevated the Armadillo (Arm) protein levels in Drosophila Schneider S2R+ cells, without affecting its mRNA levels. Pulse-chase analysis showed that CKI-RNAi stabilizes Arm protein. Moreover, Drosophila embryos injected with CKIalpha double-stranded RNA showed a naked cuticle phenotype, which is associated with activation of Wg signaling. These results indicate that CKI functions as a negative regulator of Wg/Arm signaling. Overexpression of CKIalpha induced hyper-phosphorylation of both Arm and Dishevelled in S2R+ cells and, conversely, CKIalpha-RNAi reduced the amount of hyper-modified forms. His-tagged Arm was phosphorylated by CKIalpha in vitro on a set of serine and threonine residues that are also phosphorylated by Zeste-white 3. Thus, we propose that CKI phosphorylates Arm and stimulates its degradation.

The planar polarity gene strabismus regulates convergent extension movements in Xenopus

The signaling mechanisms that specify, guide and coordinate cell behavior during embryonic morphogenesis are poorly understood. We report that a Xenopus homolog of the Drosophila planar cell polarity gene strabismus (stbm) participates in the regulation of convergent extension, a critical morphogenetic process required for the elongation of dorsal structures in vertebrate embryos. Overexpression of Xstbm, which is expressed broadly in early development and subsequently in the nervous system, causes severely shortened trunk structures; a similar phenotype results from inhibiting Xstbm translation using a morpholino antisense oligo. Experiments with Keller explants further demonstrate that Xstbm can regulate convergent extension in both dorsal mesoderm and neural tissue. The specification of dorsal tissues is not affected. The Xstbm phenotype resembles those obtained with several other molecules with roles in planar polarity signaling, including Dishevelled and Frizzled-7 and -8. Unlike these proteins, however, Stbm has little effect on conventional Wnt/beta-catenin signaling in either frog or fly assays. Thus our results strongly support the emerging hypothesis that a vertebrate analog of the planar polarity pathway governs convergent extension movements.

Galectin-3 phosphorylation is required for its anti-apoptotic function and cell cycle arrest

Galectin-3, a beta-galactoside-binding protein, is implicated in cell growth, adhesion, differentiation, and tumor progression by interactions with its ligands. Recent studies have revealed that galectin-3 suppresses apoptosis and anoikis that contribute to cell survival during metastatic cascades. Previously, it has been shown that human galectin-3 undergoes post-translational signaling modification of Ser(6) phosphorylation that acts as an "on/off" switch for its sugar-binding capability. We questioned whether galectin-3 phosphorylation is required for its anti-apoptotic function. Serine to alanine (S6A) and serine to glutamic acid (S6E) mutations were produced at the casein kinase I phosphorylation site in galectin-3. The cDNAs were transfected into a breast carcinoma cell line BT-549 that innately expresses no galectin-3. Metabolic labeling revealed that only wild type galectin-3 undergoes phosphorylation in vivo. Expression of Ser(6) mutants of galectin-3 failed to protect cells from cisplatin-induced cell death and poly(ADP-ribose) polymerase from degradation when compared with wild type galectin-3. The non-phosphorylated galectin-3 mutants failed to protect cells from anoikis with G(1) arrest when cells were cultured in suspension. In response to a loss of cell-substrate interactions, only cells expressing wild type galectin-3 down-regulated cyclin A expression and up-regulated cyclin D(1) and cyclin-dependent kinase inhibitors, i.e. p21(WAF1/CIP1) and p27(KIP1) expression levels. These results demonstrate that galectin-3 phosphorylation regulates its anti-apoptotic signaling activity.