Negative regulation of LRP6 function by casein kinase I epsilon phosphorylation

Substrate displacement of CK1 C-termini regulates kinase specificity

CK1 kinases participate in many signaling pathways, and their regulation is of meaningful biological consequence. CK1s autophosphorylate their C-terminal noncatalytic tails, and eliminating these tails increases substrate phosphorylation in vitro, suggesting that the autophosphorylated C-termini act as inhibitory pseudosubstrates. To test this prediction, we comprehensively identified the autophosphorylation sites on Schizosaccharomyces pombe Hhp1 and human CK1ε. Phosphoablating mutations increased Hhp1 and CK1ε activity toward substrates. Peptides corresponding to the C-termini interacted with the kinase domains only when phosphorylated, and substrates competitively inhibited binding of the autophosphorylated tails to the substrate binding grooves. Tail autophosphorylation influenced the catalytic efficiency with which CK1s targeted different substrates, and truncating the tail of CK1δ broadened its linear peptide substrate motif, indicating that tails contribute to substrate specificity as well. Considering autophosphorylation of both T220 in the catalytic domain and C-terminal sites, we propose a displacement specificity model to describe how autophosphorylation modulates substrate specificity for the CK1 family.

Exosomal miR-122-3p represses the growth and metastasis of MCF-7/ADR cells by targeting GRK4-mediated activation of the Wnt/β-catenin pathway

Breast cancer (BC) is a common cancer whose incidence continues to grow while its medical progress has stagnated. miRNAs are vital messengers that facilitate communications among different cancer cells. This study was to reveal the correlation of miR-122-3p expression with BC metastasis and Adriamycin (ADM) resistance and its mechanism of inhibiting BC metastasis. We found that expression of miR-122-3p is negatively correlated with BC metastasis and is lower in MCF-7/ADR cells. Overexpression of miR-122-3p in MCF-7/ADR cancer cells impairs their ability to migrate, invade, and stimulate blood vessel formation. Further research found that miR-122-3p directly binds to the 3' UTR of GRK4, reducing the phosphorylation of LRP6, which activates the Wnt/β-catenin signaling pathway, facilitating BC development and metastasis. In addition, we observed that miR-122-3p is present in MCF-7  cells, and treatment of MCF-7/ADR cells with MCF-7-derived exosomes, but not with exosomes from miR-122-3p-deficient MCF-7 cells, has identical effects to miR-122-3p overexpression. Data from xenograft experiments further suggest that excess miR-122-3p and MCF-7-derived exosomes inhibit the growth and metastasis of MCF-7/ADR cancer cells in vivo. In conclusion our data reveal that exosomal miR-122-3p may negatively regulate BC growth and metastasis, potentially serving as a diagnostic and druggable target for BC treatment.

Substrate displacement of CK1 C-termini regulates kinase specificity

CK1 kinases participate in many signaling pathways; how these enzymes are regulated is therefore of significant biological consequence. CK1s autophosphorylate their C-terminal non-catalytic tails, and eliminating these modifications increases substrate phosphorylation in vitro, suggesting that the autophosphorylated C-termini act as inhibitory pseudosubstrates. To test this prediction, we comprehensively identified the autophosphorylation sites on Schizosaccharomyces pombe Hhp1 and human CK1ε. Peptides corresponding to the C-termini interacted with the kinase domains only when phosphorylated, and phosphoablating mutations increased Hhp1 and CK1ε activity towards substrates. Interestingly, substrates competitively inhibited binding of the autophosphorylated tails to the substrate binding grooves. The presence or absence of tail autophosphorylation influenced the catalytic efficiency with which CK1s targeted different substrates, indicating that tails contribute to substrate specificity. Combining this mechanism with autophosphorylation of the T220 site in the catalytic domain, we propose a displacement specificity model to describe how autophosphorylation regulates substrate specificity for the CK1 family.

The Wnt pathway protein Dvl1 targets somatostatin receptor 2 for lysosome-dependent degradation

The Somatostatin receptor 2 (Sstr2) is a heterotrimeric G protein-coupled receptor that is highly expressed in neuroendocrine tumors and is a common pharmacological target for intervention. Unfortunately, not all neuroendocrine tumors express Sstr2, and Sstr2 expression can be downregulated with prolonged agonist use. Sstr2 is rapidly internalized following agonist stimulation and, in the short term, is quantitatively recycled back to the plasma membrane. However, mechanisms controlling steady state expression of Sstr2 in the absence of agonist are less well described. Here, we show that Sstr2 interacts with the Wnt pathway protein Dvl1 in a ligand-independent manner to target Sstr2 for lysosomal degradation. Interaction of Sstr2 with Dvl1 does not affect receptor internalization, recycling, or signaling to adenylyl cyclase but does suppress agonist-stimulated ERK1/2 activation. Importantly, Dvl1-dependent degradation of Sstr2 can be stimulated by overexpression of Wnts and treatment of cells with Wnt pathway inhibitors can boost Sstr2 expression in neuroendocrine tumor cells. Taken together, this study identifies for the first time a mechanism that targets Sstr2 for lysosomal degradation that is independent of Sstr2 agonist and can be potentiated by Wnt ligand. Intervention in this signaling mechanism has the potential to elevate Sstr2 expression in neuroendocrine tumors and enhance Sstr2-directed therapies.

Single-cell RNA sequencing reveals abnormal fluctuations in human eight-cell embryos associated with blastocyst formation failure

Infertility has become a global health issue, with the number of people suffering from the disease increasing year by year, and ART offering great promise for infertility treatment. However, the regulation of early embryonic development is complicated and a series of processes takes place, including the maternal-to-zygotic transition. In addition, developmental arrest is frequently observed during human early embryonic development. In this study, we performed single-cell RNA sequencing on a biopsied blastomere from human eight-cell embryos and tracked the developmental potential of the remaining cells. To compare the sequencing results between different eight-cell embryos, we have combined the research data of this project with the data previously shared in the database and found that cells from the same embryo showed a higher correlation. Additionally, the transcriptome of embryos with blastocyst formation failure was significantly different from developed embryos, and the gene expression as well as cell signaling pathways related to embryonic development were also altered. In particular, the expression of some maternal and zygotic genes in the failed blastocyst formation group was significantly altered: the overall expression level of maternal genes was significantly higher in the failed blastocyst than the developed blastocyst group. In general, these findings provide clues for the causes of human embryonic arrest after the eight-cell stage, and they also provide new ideas for improving the success rate of ART in clinical practice.

Transmembrane protein 97 exhibits oncogenic properties via enhancing LRP6-mediated Wnt signaling in breast cancer

Upregulation of transmembrane protein 97 (TMEM97) has been associated with progression and poor outcome in multiple human cancers, including breast cancer. Recent studies suggest that TMEM97 may be involved in the activation of the Wnt/β-catenin pathway. However, the molecular mechanism of TMEM97 action on Wnt/β-catenin signaling is completely unclear. In the current study, TMEM97 was identified as an LRP6-interacting protein. TMEM97 could interact with LRP6 intracellular domain and enhance LRP6-mediated Wnt signaling in a CK1δ/ε-dependent manner. The binding of TMEM97 to LRP6 facilitated the recruitment of CK1δ/ε to LRP6 complex, resulting in LRP6 phosphorylation at Ser 1490 and the stabilization of β-catenin. In breast cancer cells, knockout of TMEM97 attenuated the Wnt/β-catenin signaling cascade via regulating LRP6 phosphorylation, leading to a decrease in the expression of Wnt target genes AXIN2, LEF1, and survivin. TMEM97 deficiency also suppressed cell viability, proliferation, colony formation, migration, invasion, and stemness properties in breast cancer cells. Importantly, TMEM97 knockout suppressed tumor growth through downregulating the Wnt/β-catenin signaling pathway in a breast cancer xenograft model. Taken together, our results revealed that TMEM97 is a positive modulator of canonical Wnt signaling. TMEM97-mediated Wnt signaling is implicated in the tumorigenesis of breast cancer, and its targeted inhibition may be a promising therapeutic strategy for breast cancer.

Regulation of the Low-Density Lipoprotein Receptor-Related Protein LRP6 and Its Association With Disease: Wnt/β-Catenin Signaling and Beyond

Wnt signaling plays crucial roles in development and tissue homeostasis, and its dysregulation leads to various diseases, notably cancer. Wnt/β-catenin signaling is initiated when the glycoprotein Wnt binds to and forms a ternary complex with the Frizzled and low-density lipoprotein receptor-related protein 5/6 (LRP5/6). Despite being identified as a Wnt co-receptor over 20 years ago, the molecular mechanisms governing how LRP6 senses Wnt and transduces downstream signaling cascades are still being deciphered. Due to its role as one of the main Wnt signaling components, the dysregulation or mutation of LRP6 is implicated in several diseases such as cancer, neurodegeneration, metabolic syndrome and skeletal disease. Herein, we will review how LRP6 is activated by Wnt stimulation and explore the various regulatory mechanisms involved. The participation of LRP6 in other signaling pathways will also be discussed. Finally, the relationship between LRP6 dysregulation and disease will be examined in detail.

Single-Nucleotide Polymorphisms Associated with the Senescence-Accelerated Phenotype of OXYS Rats: A Focus on Alzheimer's Disease-Like and Age-Related-Macular-Degeneration-Like Pathologies

Alzheimer's disease (AD) and age-related macular degeneration (AMD) are two complex incurable neurodegenerative disorders the common pathogenesis of which is actively discussed. There are overlapping risk factors and molecular mechanisms of the two diseases; at the same time, there are arguments in favor of the notion that susceptibility to each of these diseases is associated with a distinct genetic background. Here we identified single-nucleotide polymorphisms (SNPs) that are specific for senescence-accelerated OXYS rats, which simulate key characteristics of both sporadic AD and AMD. Transcriptomes of the hippocampus, prefrontal cortex, and retina (data of RNA-Seq) were analyzed. We detected SNPs in genes Rims2, AABR07072639.2, Lemd2, and AABR07045405.1, which thus can express significantly truncated proteins lacking functionally important domains. Additionally, 33 mutations in genes-which are related to various metabolic and signaling pathways-cause nonsynonymous amino acid substitutions presumably leading to disturbances in protein structure or functions. Some of the genes carrying these SNPs are associated with aging, neurodegenerative, and mental diseases. Thus, we revealed the SNPs can lead to abnormalities in protein structure or functions and affect the development of the senescence-accelerated phenotype of OXYS rats. Our data are consistent with the latest results of genome-wide association studies that highlight the importance of multiple pathways for the pathogenesis of AD and AMD. Identified SNPs can serve as promising research objects for further studies on the molecular mechanisms underlying this particular rat model as well as for the prediction of potential biomarkers of AD and AMD.

Achieving effective and selective CK1 inhibitors through structure modification

Casein kinase 1 (CK1) is an extensively expressed serine/threonine kinase family, with six highly conserved isoforms of human CK1. Due to its involvement in many biological processes, CK1 is a promising target for several pathological states, including circadian sleep disorder, neurodegenerative diseases, cancer and inflammation. However, due to the structural similarities between the six CK1 members, the design of CK1 inhibitors is intricate. So far, no effective CK1 inhibitors are reported to reach clinical trials; thus, approaches to obtaining both selective and effective CK1 inhibitors are in great demand. Here we analyze several CK1 inhibitors that provide successful experience for structure-based drug design and rational structure modification, which could provide references for further drug design.

Structural Basis of WLS/Evi-Mediated Wnt Transport and Secretion

Wnts are evolutionarily conserved ligands that signal at short range to regulate morphogenesis, cell fate, and stem cell renewal. The first and essential steps in Wnt secretion are their O-palmitoleation and subsequent loading onto the dedicated transporter Wntless/evenness interrupted (WLS/Evi). We report the 3.2 Å resolution cryogenic electron microscopy (cryo-EM) structure of palmitoleated human WNT8A in complex with WLS. This is accompanied by biochemical experiments to probe the physiological implications of the observed association. The WLS membrane domain has close structural homology to G protein-coupled receptors (GPCRs). A Wnt hairpin inserts into a conserved hydrophobic cavity in the GPCR-like domain, and the palmitoleate protrudes between two helices into the bilayer. A conformational switch of highly conserved residues on a separate Wnt hairpin might contribute to its transfer to receiving cells. This work provides molecular-level insights into a central mechanism in animal body plan development and stem cell biology.

A de novo pathogenic CSNK1E mutation identified by exome sequencing in family trios with epileptic encephalopathy

Recent whole-exome sequencing (WES) studies have demonstrated the contribution of de novo mutations (DNMs) to epileptic encephalopathies (EEs). Here, we performed WES on four trios with West syndrome and identified three loss-of-function DNMs in both CSNK1E (c.885+1G>A) and STXBP1 (splicing, c.1111-2A>G; nonsense, p.(Y519X)). The splicing mutation in CSNK1E creates insertion of 116 new amino acids at position 246 followed by a premature stop codon. Both CSNK1E and STXBP1 showed a closer coexpression relationship with epilepsy candidate genes beyond that expected by chance. In addition, genes coexpressed with CSNK1E were enriched in early prenatal stages across multiple brain regions. We also found that 60 CSNK1E-interacting genes share an association with multiple neuropsychiatric disorders, and these genes formed a significant interconnected interaction network with roles in the midbrain development. Our study supported the potential role of CSNK1E variants in EE susceptibility and expanded the phenotypic spectrum associated with CSNK1E variation.

Discovery of Inhibitor of Wnt Production 2 (IWP-2) and Related Compounds As Selective ATP-Competitive Inhibitors of Casein Kinase 1 (CK1) δ/ε

Inhibitors of Wnt production (IWPs) are known antagonists of the Wnt pathway, targeting the membrane-bound O-acyltransferase porcupine (Porcn) and thus preventing a crucial Wnt ligand palmitoylation. Since IWPs show structural similarities to benzimidazole-based CK1 inhibitors, we hypothesized that IWPs could also inhibit CK1 isoforms. Molecular modeling revealed a plausible binding mode of IWP-2 in the ATP binding pocket of CK1δ which was confirmed by X-ray analysis. In vitro kinase assays demonstrated IWPs to be ATP-competitive inhibitors of ^wtCK1δ. IWPs also strongly inhibited the gatekeeper mutant ^M82FCK1δ. When profiled in a panel of 320 kinases, IWP-2 specifically inhibited CK1δ. IWP-2 and IWP-4 also inhibited the viability of various cancer cell lines. By a medicinal chemistry approach, we developed improved IWP-derived CK1 inhibitors. Our results suggest that the effects of IWPs are not limited to Porcn, but also might influence CK1δ/ε-related pathways.

Casein kinase 1-epsilon or 1-delta required for Wnt-mediated intestinal stem cell maintenance

The intestinal epithelium holds an immense regenerative capacity mobilized by intestinal stem cells (ISCs), much of it supported by Wnt pathway activation. Several unique regulatory mechanisms ensuring optimal levels of Wnt signaling have been recognized in ISCs. Here, we identify another Wnt signaling amplifier, CKIε, which is specifically upregulated in ISCs and is essential for ISC maintenance, especially in the absence of its close isoform CKIδ. Co-ablation of CKIδ/ε in the mouse gut epithelium results in rapid ISC elimination, with subsequent growth arrest, crypt-villous shrinking, and rapid mouse death. Unexpectedly, Wnt activation is preserved in all CKIδ/ε-deficient enterocyte populations, with the exception of Lgr5⁺ ISCs, which exhibit Dvl2-dependent Wnt signaling attenuation. CKIδ/ε-depleted gut organoids cease proliferating and die rapidly, yet survive and resume self-renewal upon reconstitution of Dvl2 expression. Our study underscores a unique regulation mode of the Wnt pathway in ISCs, possibly providing new means of stem cell enrichment for regenerative medicine.

Intrinsic disorder in spondins and some of their interacting partners

Spondins, which are proteins that inhibit and promote adherence of embryonic cells so as to aid axonal growth are part of the thrombospondin-1 family. Spondins function in several important biological processes, such as apoptosis, angiogenesis, etc. Spondins constitute a thrombospondin subfamily that includes F-spondin, a protein that interacts with Aβ precursor protein and inhibits its proteolytic processing; R-spondin, a 4-membered group of proteins that regulates Wnt pathway and have other functions, such as regulation of kidney proliferation, induction of epithelial proliferation, the tumor suppressant action; M-spondin that mediates mechanical linkage between the muscles and apodemes; and the SCO-spondin, a protein important for neuronal development. In this study, we investigated intrinsic disorder status of human spondins and their interacting partners, such as members of the LRP family, LGR family, Frizzled family, and several other binding partners in order to establish the existence and importance of disordered regions in spondins and their interacting partners by conducting a detailed analysis of their sequences, finding disordered regions, and establishing a correlation between their structure and biological functions.

Distinct Signaling Requirements for the Establishment of ESC Pluripotency in Late-Stage EpiSCs

It has previously been reported that mouse epiblast stem cell (EpiSC) lines comprise heterogeneous cell populations that are functionally equivalent to cells of either early- or late-stage postimplantation development. So far, the establishment of the embryonic stem cell (ESC) pluripotency gene regulatory network through the widely known chemical inhibition of MEK and GSK3beta has been impractical in late-stage EpiSCs. Here, we show that chemical inhibition of casein kinase 1alpha (CK1alpha) induces the conversion of recalcitrant late-stage EpiSCs into ESC pluripotency. CK1alpha inhibition directly results in the simultaneous activation of the WNT signaling pathway, together with inhibition of the TGFbeta/SMAD2 signaling pathway, mediating the rewiring of the gene regulatory network in favor of an ESC-like state. Our findings uncover a molecular mechanism that links CK1alpha to ESC pluripotency through the direct modulation of WNT and TGFbeta signaling.

•

Inhibition of CK1alpha induces ESC conversion in EpiSCs recalcitrant to 2i/LIF

•

The ESC conversion acts via WNT activation and TGFbeta/SMAD2 inhibition

•

MEK inhibition stabilizes the conversion and restores germline competence

•

CK1 inhibition promotes activation and maintenance of the pluripotency network

Updating the Wnt pathways

In the three decades since the discovery of the Wnt1 proto-oncogene in virus-induced mouse mammary tumours, our understanding of the signalling pathways that are regulated by the Wnt proteins has progressively expanded. Wnts are involved in an complex signalling network that governs multiple biological processes and cross-talk with multiple additional signalling cascades, including the Notch, FGF (fibroblast growth factor), SHH (Sonic hedgehog), EGF (epidermal growth factor) and Hippo pathways. The Wnt signalling pathway also illustrates the link between abnormal regulation of the developmental processes and disease manifestation. Here we provide an overview of Wnt-regulated signalling cascades and highlight recent advances. We focus on new findings regarding the dedicated Wnt production and secretion pathway with potential therapeutic targets that might be beneficial for patients with Wnt-related diseases.

Casein kinase 1 and Wnt/β-catenin signaling

Casein kinase 1 (CK1) members play a critical and evolutionary conserved role in Wnt/β-catenin signaling. They phosphorylate several pathway components and exert a dual function, acting as both Wnt activators and Wnt inhibitors. Recent discoveries suggest that CK1 members act in a coordinated manner to regulate early responses to Wnt and notably that their enzymatic activity is regulated. Here, I provide a brief update of CK1 function and regulation in Wnt/β-catenin signaling.

Microtubules depolymerization caused by the CK1 inhibitor IC261 may be not mediated by CK1 blockage

The ubiquitously expressed serine/threonine specific casein kinase 1 (CK1) family plays important roles in the regulation of various physiological processes. Small-molecule inhibitors, such as the CK1δ/ε selectively inhibitor IC261, have been used to antagonize CK1 phosphorylation events in cells in many studies. Here we present data to show that, similarly to the microtubule destabilizing agent nocodazole, IC261 depolymerizes microtubules in interphase cells. IC261 treatment of interphase cells affects the morphology of the TGN and Golgi apparatus as well as the localization of CK1δ, which co-localizes with COPI positive membranes. IC261-induced depolymerization of microtubules is rapid, reversible and can be antagonized by pre-treatment of cells with taxol. At lower concentrations of IC261, mitotic spindle microtubule dynamics are affected; this leads to cell cycle arrest and, depending on the cellular background, to apoptosis in a dose-dependent manner. In addition, FACS analysis revealed that IC261 could induce apoptosis independent of cell cycle arrest. In summary this study provides additional and valuable information about various IC261-induced effects that could be caused by microtubule depolymerization rather than by inhibition of CK1. Data from studies that have used IC261 as an inhibitor of CK1 should be interpreted in light of these observations.

The CK1 Family: Contribution to Cellular Stress Response and Its Role in Carcinogenesis

Members of the highly conserved and ubiquitously expressed pleiotropic CK1 family play major regulatory roles in many cellular processes including DNA-processing and repair, proliferation, cytoskeleton dynamics, vesicular trafficking, apoptosis, and cell differentiation. As a consequence of cellular stress conditions, interaction of CK1 with the mitotic spindle is manifold increased pointing to regulatory functions at the mitotic checkpoint. Furthermore, CK1 is able to alter the activity of key proteins in signal transduction and signal integration molecules. In line with this notion, CK1 is tightly connected to the regulation and degradation of β-catenin, p53, and MDM2. Considering the importance of CK1 for accurate cell division and regulation of tumor suppressor functions, it is not surprising that mutations and alterations in the expression and/or activity of CK1 isoforms are often detected in various tumor entities including cancer of the kidney, choriocarcinomas, breast carcinomas, oral cancer, adenocarcinomas of the pancreas, and ovarian cancer. Therefore, scientific effort has enormously increased (i) to understand the regulation of CK1 and its involvement in tumorigenesis- and tumor progression-related signal transduction pathways and (ii) to develop CK1-specific inhibitors for the use in personalized therapy concepts. In this review, we summarize the current knowledge regarding CK1 regulation, function, and interaction with cellular proteins playing central roles in cellular stress-responses and carcinogenesis.

Integrated genetic approaches identify the molecular mechanisms of Sox4 in early B-cell development: intricate roles for RAG1/2 and CK1ε

Commitment of hematopoietic stem cells to B lineage precursors and subsequent development of B lineage precursors into mature B cells is stringently controlled by stage-specific transcription factors. In this study, we used integrated genetic approaches and systematically determined the role of Sry-related high mobility group box (Sox) 4 and the underlying molecular mechanisms in early B-cell development. We found that Sox4 coordinates multilevel controls in the differentiation of early stage B cells. At the molecular level, Sox4 orchestrates a unique gene regulatory program, and its function was predominantly mediated through a conventional Sox4-binding motif as well as an unconventional GA-binding protein α chain binding motif. Our integrated gene network and functional analysis indicated that Sox4 functions as a bimodular transcription factor and ensures B lineage precursor differentiation through 2 distinct mechanisms. It positively induces gene rearrangements at immunoglobulin heavy chain gene loci by transcriptionally activating the Rag1 and Rag2 genes and negatively regulates Wnt signaling, which is critical for self-renewal, by inducing the expression of casein kinase 1 ε. Our findings illustrate that Sox4 mediates critical fine-tuning of the 2 opposing forces in early B-cell development and also set forth a model for characterization of critical genes whose deficiency, like Sox4 deficiency, is detrimental to this process.

Regulation of Wnt/β-catenin signaling by posttranslational modifications

The canonical Wnt signaling pathway (or Wnt/β-catenin pathway) plays a pivotal role in embryonic development and adult homeostasis; deregulation of the Wnt pathway contributes to the initiation and progression of human diseases including cancer. Despite its importance in human biology and disease, how regulation of the Wnt/β-catenin pathway is achieved remains largely undefined. Increasing evidence suggests that post-translational modifications (PTMs) of Wnt pathway components are essential for the activation of the Wnt/β-catenin pathway. PTMs create a highly dynamic relay system that responds to Wnt stimulation without requiring de novo protein synthesis and offer a platform for non-Wnt pathway components to be involved in the regulation of Wnt signaling, hence providing alternative opportunities for targeting the Wnt pathway. This review highlights the current status of PTM-mediated regulation of the Wnt/β-catenin pathway with a focus on factors involved in Wnt-mediated stabilization of β-catenin.

CK1 is required for a mitotic checkpoint that delays cytokinesis

Failure to accurately partition genetic material during cell division causes aneuploidy and drives tumorigenesis. Cell-cycle checkpoints safeguard cells from such catastrophes by impeding cell-cycle progression when mistakes arise. FHA-RING E3 ligases, including human RNF8 and CHFR and fission yeast Dma1, relay checkpoint signals by binding phosphorylated proteins via their FHA domains and promoting ubiquitination of downstream targets. Upon mitotic checkpoint activation, S. pombe Dma1 concentrates at spindle pole bodies (SPBs) in an FHA-dependent manner and ubiquitinates Sid4, a scaffold of Polo kinase, to suspend cytokinesis. However, the kinase or kinases that phosphoprime Sid4 for Dma1-mediated ubiquitination are unknown. Here, we report that the highly conserved protein kinase CK1 transmits the signal necessary to stall cytokinesis by phosphopriming Sid4 for Dma1-mediated ubiquitination. Like Dma1, CK1 accumulates at SPBs during a mitotic arrest and associates stably with SPB components, including Sid4. Our results establish CK1 as an integral component of a mitotic, ubiquitin-mediated checkpoint pathway.

CK1δ kinase activity is modulated by Chk1-mediated phosphorylation

CK1δ, a member of the casein kinase 1 family, is involved in the regulation of various cellular processes and has been associated with the pathophysiology of neurodegenerative diseases and cancer. Therefore recently, interest in generating highly specific inhibitors for personalized therapy has increased enormously. However, the efficacy of newly developed inhibitors is affected by the phosphorylation state of CK1δ. Cellular kinases phosphorylating CK1δ within its C-terminal domain have been identified but still more information regarding the role of site-specific phosphorylation in modulating the activity of CK1δ is required. Here we show that Chk1 phosphorylates rat CK1δ at serine residues 328, 331, 370, and threonine residue 397 as well as the human CK1δ transcription variants 1 and 2. CK1δ mutant proteins bearing one, two or three mutations at these identified phosphorylation sites exhibited significant differences in their kinetic properties compared to wild-type CK1δ. Additionally, CK1δ co-precipitates with Chk1 from HT1080 cell extracts and activation of cellular Chk1 resulted in a significant decrease in cellular CK1δ kinase activity. Taken together, these data point towards a possible regulatory relationship between Chk1 and CK1δ.

A screen for X-linked mutations affecting Drosophila photoreceptor differentiation identifies Casein kinase 1α as an essential negative regulator of wingless signaling

The Wnt and Hedgehog signaling pathways are essential for normal development and are misregulated in cancer. The casein kinase family of serine/threonine kinases regulates both pathways at multiple levels. However, it has been difficult to determine whether individual members of this family have distinct functions in vivo, due to their overlapping substrate specificities. In Drosophila melanogaster, photoreceptor differentiation is induced by Hedgehog and inhibited by Wingless, providing a sensitive system in which to identify regulators of each pathway. We used a mosaic genetic screen in the Drosophila eye to identify mutations in genes on the X chromosome required for signal transduction. We recovered mutations affecting the transcriptional regulator CREB binding protein, the small GTPase dynamin, the cytoskeletal regulator Actin-related protein 2, and the protein kinase Casein kinase 1α. Consistent with its reported function in the β-Catenin degradation complex, Casein Kinase 1α mutant cells accumulate β-Catenin and ectopically induce Wingless target genes. In contrast to previous studies based on RNA interference, we could not detect any effect of the same Casein Kinase 1α mutation on Hedgehog signaling. We thus propose that Casein kinase 1α is essential to allow β-Catenin degradation and prevent inappropriate Wingless signaling, but its effects on the Hedgehog pathway are redundant with other Casein kinase 1 family members.

Neurite outgrowth of mature retinal ganglion cells and PC12 cells requires activity of CK1δ and CK1ε

Mature retinal ganglion cells (RGCs) do not normally regenerate severed axons after optic nerve injury and show only little neurite outgrowth in culture. However, RGCs can be transformed into an active regenerative state after lens injury (LI) enabling these neurons to regrow axons in vitro and in vivo. In the current study we investigated the role of CK1δ and CK1ε activity in neurite outgrowth of LI stimulated RGCs and nerve growth factor (NGF) stimulated PC12 cells, respectively. In both cell types CK1δ and ε were localized in granular particles aligned at microtubules in neurites and growth cones. Although LI treatment did not measurably affect the expression of CK1δ and ε, it significantly elevated the specific kinase activity in the retina. Similarly, CK1δ/ε specific kinase activity was also elevated in NGF treated PC12 cells compared with untreated controls. Neurite extension in PC12 cells was associated with a change in the activity of CK1δ C-terminal targeting kinases, suggesting that activity of these kinases might be necessary for neurite outgrowth. Pharmacological inactivation of CK1δ and ε markedly compromised neurite outgrowth of both, PC12 cells and LI stimulated RGCs in a concentration dependent manner. These data provide evidence for a so far unknown, but essential role of CK1 isoforms in neurite growth.

WLS-dependent secretion of WNT3A requires Ser209 acylation and vacuolar acidification

Wnt proteins are secreted post-translationally modified proteins that signal locally to regulate development and proliferation. The production of bioactive Wnts requires a number of dedicated factors in the secreting cell whose coordinated functions are not fully understood. A screen for small molecules identified inhibitors of vacuolar acidification as potent inhibitors of Wnt secretion. Inhibition of the V-ATPase or disruption of vacuolar pH gradients by diverse drugs potently inhibited Wnt/β-catenin signaling both in cultured human cells and in vivo, and impaired Wnt-regulated convergent extension movements in Xenopus embryos. WNT secretion requires its binding to the carrier protein wntless (WLS); we find that WLS is ER-resident in human cells and WNT3A binding to WLS requires PORCN-dependent lipid modification of WNT3A at serine 209. Inhibition of vacuolar acidification results in accumulation of the WNT3A-WLS complex both in cells and at the plasma membrane. Modeling predictions suggest that WLS has a lipid-binding β-barrel that is similar to the lipocalin-family fold. We propose that WLS binds Wnts in part through a lipid-binding domain, and that vacuolar acidification is required to release palmitoylated WNT3A from WLS in secretory vesicles, possibly to facilitate transfer of WNT3A to a soluble carrier protein.

Regulation of Lrp6 phosphorylation

The Wnt/beta-catenin signaling pathway plays important roles in embryonic development and tissue homeostasis, and is implicated in human disease. Wnts transduce signals via transmembrane receptors of the Frizzled (Fzd/Fz) family and the low density lipoprotein receptor-related protein 5/6 (Lrp5/6). A key mechanism in their signal transduction is that Wnts induce Lrp6 signalosomes, which become phosphorylated at multiple conserved sites, notably at PPSPXS motifs. Lrp6 phosphorylation is crucial to beta-catenin stabilization and pathway activation by promoting Axin and Gsk3 recruitment to phosphorylated sites. Here, we summarize how proline-directed kinases (Gsk3, PKA, Pftk1, Grk5/6) and non-proline-directed kinases (CK1 family) act upon Lrp6, how the phosphorylation is regulated by ligand binding and mitosis, and how Lrp6 phosphorylation leads to beta-catenin stabilization.

Isoform specific phosphorylation of p53 by protein kinase CK1

The ability of three isoforms of protein kinase CK1 (alpha, gamma(1), and delta) to phosphorylate the N-terminal region of p53 has been assessed using either recombinant p53 or a synthetic peptide reproducing its 1-28 sequence. Both substrates are readily phosphoylated by CK1delta and CK1alpha, but not by the gamma isoform. Affinity of full size p53 for CK1 is 3 orders of magnitude higher than that of its N-terminal peptide (K (m) 0.82 muM vs 1.51 mM). The preferred target is S20, whose phosphorylation critically relies on E17, while S6 is unaffected despite displaying the same consensus (E-x-x-S). Our data support the concept that non-primed phosphorylation of p53 by CK1 is an isoform-specific reaction preferentially affecting S20 by a mechanism which is grounded both on a local consensus and on a remote docking site mapped to the K(221)RQK(224) loop according to modeling and mutational analysis.

Analysis of cell type-specific expression of CK1 epsilon in various tissues of young adult BALB/c Mice and in mammary tumors of SV40 T-Ag-transgenic mice

Casein kinase 1 epsilon (CK1epsilon) is involved in various cellular processes, including cell growth, differentiation, and apoptosis, vesicle transport, and control of the circadian rhythm. Deregulation of CK1epsilon has been linked to neurodegenerative diseases and cancer. To better understand the cell type-specific functions of CK1epsilon, we determined its localization by immunhistochemistry in tissues of healthy, young adult BALB/c mice and in mammary tumors of SV40 T-antigen-transgenic mice. CK1epsilon expression was found to be highly regulated in normal tissues of endodermal, mesodermal, and ectodermal origin and in neoplastic tissue of mammary cancer. The data presented here give an overview of CK1epsilon reactivity in different organs under normal conditions and outline changes in its expression in mammary carcinomas. Our data suggest a cell/organ type-specific function of CK1epsilon and indicate that tumorigenic conversion of mammary glands in SV40 T-antigen-transgenic mice leads to downregulation of CK1epsilon. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

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.

Immunohistochemical characterisation of cell-type specific expression of CK1delta in various tissues of young adult BALB/c mice

BACKGROUND

Casein kinase 1 delta (CK1delta) phosphorylates many key proteins playing important roles in such biological processes as cell growth, differentiation, apoptosis, circadian rhythm and vesicle transport. Furthermore, deregulation of CK1delta has been linked to neurodegenerative diseases and cancer. In this study, the cell specific distribution of CK1delta in various tissues and organs of young adult BALB/c mice was analysed by immunohistochemistry.

METHODOLOGY/PRINCIPAL FINDINGS

Immunohistochemical staining of CK1delta was performed using three different antibodies against CK1delta. A high expression of CK1delta was found in a variety of tissues and organ systems and in several cell types of endodermal, mesodermal and ectodermal origin.

CONCLUSIONS

These results give an overview of the cell-type specific expression of CK1delta in different organs under normal conditions. Thus, they provide evidence for possible cell-type specific functions of CK1delta, where CK1delta can interact with and modulate the activity of key regulator proteins by site directed phosphorylation. Furthermore, they provide the basis for future analyses of CK1delta in these tissues.

The Frizzled family of unconventional G-protein-coupled receptors

The Frizzled (FZD) family of receptors is critically involved in embryogenesis, and there is substantial evidence that members of this family also regulate tissue homeostasis in many different organs in the adult. FZD receptors have seven transmembrane-spanning domains and are activated by the WNT family of lipoglycoproteins. Many aspects of FZD signal transduction and pharmacology are still unclear. In this review, we summarize recent advances and some of the key questions about the molecular pharmacology of FZDs, FZD-associated proteins and signal transduction. We also discuss what little is known about the pharmacological binding profiles and the degree of selectivity of WNTs and other extracellular ligands for FZDs. Finally, we focus on signaling events that occur as a direct consequence of FZD activation, signaling via the central phosphoprotein Dishevelled (DVL) and FZD coupling to heterotrimeric G proteins. Here, we outline the current state of knowledge on FZDs and FZD signal transduction and pinpoint aspects of debate and future directions.

Quantitative phosphoproteome profiling of Wnt3a-mediated signaling network: indicating the involvement of ribonucleoside-diphosphate reductase M2 subunit phosphorylation at residue serine 20 in canonical Wnt signal transduction

The complexity of canonical Wnt signaling comes not only from the numerous components but also from multiple post-translational modifications. Protein phosphorylation is one of the most common modifications that propagates signals from extracellular stimuli to downstream effectors. To investigate the global phosphorylation regulation and uncover novel phosphoproteins at the early stages of canonical Wnt signaling, HEK293 cells were metabolically labeled with two stable isotopic forms of lysine and were stimulated for 0, 1, or 30 min with purified Wnt3a. After phosphoprotein enrichment and LC-MS/MS analysis, 1057 proteins were identified in all three time points. In total 287 proteins showed a 1.5-fold or greater change in at least one time point. In addition to many known Wnt signaling transducers, other phosphoproteins were identified and quantitated, implicating their involvement in canonical Wnt signaling. k-Means clustering analysis showed dynamic patterns for the differential phosphoproteins. Profile pattern and interaction network analysis of the differential phosphoproteins implicated the possible roles for those unreported components in Wnt signaling. Moreover 100 unique phosphorylation sites were identified, and 54 of them were quantitated in the three time points. Site-specific phosphopeptide quantitation revealed that Ser-20 phosphorylation on RRM2 increased upon 30-min Wnt3a stimulation. Further studies with mutagenesis, the Wnt reporter gene assay, and RNA interference indicated that RRM2 functioned downstream of beta-catenin as an inhibitor of Wnt signaling and that Ser-20 phosphorylation of RRM2 counteracted its inhibition effect. Our systematic profiling of dynamic phosphorylation changes responding to Wnt3a stimulation not only presented a comprehensive phosphorylation network regulated by canonical Wnt signaling but also found novel molecules and phosphorylation involved in Wnt signaling.

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.

The first armadillo repeat is involved in the recognition and regulation of beta-catenin phosphorylation by protein kinase CK1

Multiple phosphorylation of beta-catenin by glycogen synthase kinase 3 (GSK3) in the Wnt pathway is primed by CK1 through phosphorylation of Ser-45, which lacks a typical CK1 canonical sequence. Synthetic peptides encompassing amino acids 38-64 of beta-catenin are phosphorylated by CK1 on Ser-45 with low affinity (K(m) approximately 1 mM), whereas intact beta-catenin is phosphorylated at Ser-45 with very high affinity (K(m) approximately 200 nM). Peptides extended to include a putative CK1 docking motif (FXXXF) at 70-74 positions or a F74AA mutation in full-length beta-catenin had no significant effect on CK1 phosphorylation efficiency. beta-Catenin C-terminal deletion mutants up to residue 181 maintained their high affinity, whereas removal of the 131-181 fragment, corresponding to the first armadillo repeat, was deleterious, resulting in a 50-fold increase in K(m) value. Implication of the first armadillo repeat in beta-catenin targeting by CK1 is supported in that the Y142E mutation, which mimics phosphorylation of Tyr-142 by tyrosine kinases and promotes dissociation of beta-catenin from alpha-catenin, further improves CK1 phosphorylation efficiency, lowering the K(m) value to <50 nM, approximating the physiological concentration of beta-catenin. In contrast, alpha-catenin, which interacts with the N-terminal region of beta-catenin, prevents Ser-45 phosphorylation of CK1 in a dose-dependent manner. Our data show that the integrity of the N-terminal region and the first armadillo repeat are necessary and sufficient for high-affinity phosphorylation by CK1 of Ser-45. They also suggest that beta-catenin association with alpha-catenin and beta-catenin phosphorylation by CK1 at Ser-45 are mutually exclusive.

Caveolin is necessary for Wnt-3a-dependent internalization of LRP6 and accumulation of beta-catenin

beta-catenin-mediated Wnt signaling is critical in animal development and tumor progression. The single-span transmembrane Wnt receptor, low-density lipoprotein receptor-related protein 6 (LRP6), interacts with Axin to promote the Wnt-dependent accumulation of beta-catenin. However, the molecular mechanism of receptor internalization and its impact on signaling are unclear. Here, we present evidence that LRP6 is internalized with caveolin and that the components of this endocytic pathway are required not only for Wnt-3a-induced internalization of LRP6 but also for accumulation of beta-catenin. Overall, our data suggest that Wnt-3a triggers the interaction of LRP6 with caveolin and promotes recruitment of Axin to LRP6 phosphorylated by glycogen synthase kinase-3beta and that caveolin thereby inhibits the binding of beta-catenin to Axin. Thus, caveolin plays critical roles in inducing the internalization of LRP6 and activating the Wnt/beta-catenin pathway. We also discuss the idea that distinct endocytic pathways correlate with the specificity of Wnt signaling events.

Casein kinase 2-dependent serine phosphorylation of MuSK regulates acetylcholine receptor aggregation at the neuromuscular junction

The release of Agrin by motoneurons activates the muscle-specific receptor tyrosine kinase (MuSK) as the main organizer of subsynaptic specializations at the neuromuscular junction. MuSK downstream signaling is largely undefined. Here we show that protein kinase CK2 interacts and colocalizes with MuSK at post-synaptic specializations. We observed CK2-mediated phosphorylation of serine residues within the kinase insert (KI) of MuSK. Inhibition or knockdown of CK2, or exchange of phosphorylatable serines by alanines within the KI of MuSK, impaired acetylcholine receptor (AChR) clustering, whereas their substitution by residues that imitate constitutive phosphorylation led to aggregation of AChRs even in the presence of CK2 inhibitors. Impairment of AChR cluster formation after replacement of MuSK KI with KIs of other receptor tyrosine kinases correlates with potential CK2-dependent serine phosphorylation within KIs. MuSK activity was unchanged but AChR stability decreased in the presence of CK2 inhibitors. Muscle-specific CK2beta knockout mice develop a myasthenic phenotype due to impaired muscle endplate structure and function. This is the first description of a regulatory cross-talk between MuSK and CK2 and of a role for the KI of the receptor tyrosine kinase MuSK for the development of subsynaptic specializations.