Regulation of Spemann organizer formation by the intracellular kinase Xgsk-3

Repeated Decision Stumping Distils Simple Rules from Single-Cell Data

Single-cell data afford unprecedented insights into molecular processes. But the complexity and size of these data sets have proved challenging and given rise to a large armory of statistical and machine learning approaches. The majority of approaches focuses on either describing features of these data, or making predictions and classifying unlabeled samples. In this study, we introduce repeated decision stumping (ReDX) as a method to distill simple models from single-cell data. We develop decision trees of depth one-hence "stumps"-to identify in an inductive manner, gene products involved in driving cell fate transitions, and in applications to published data we are able to discover the key players involved in these processes in an unbiased manner without prior knowledge. Our algorithm is deliberately targeting the simplest possible candidate hypotheses that can be extracted from complex high-dimensional data. There are three reasons for this: (1) the predictions become straightforwardly testable hypotheses; (2) the identified candidates form the basis for further mechanistic model development, for example, for engineering and synthetic biology interventions; and (3) this approach complements existing descriptive modeling approaches and frameworks. The approach is computationally efficient, has remarkable predictive power, including in simulation studies where the ground truth is known, and yields robust and statistically stable predictors; the same set of candidates is generated by applying the algorithm to different subsamples of experimental data.

Lateral thinking in syndromic congenital cardiovascular disease

Syndromic birth defects are rare diseases that can present with seemingly pleiotropic comorbidities. Prime examples are rare congenital heart and cardiovascular anomalies that can be accompanied by forelimb defects, kidney disorders and more. Whether such multi-organ defects share a developmental link remains a key question with relevance to the diagnosis, therapeutic intervention and long-term care of affected patients. The heart, endothelial and blood lineages develop together from the lateral plate mesoderm (LPM), which also harbors the progenitor cells for limb connective tissue, kidneys, mesothelia and smooth muscle. This developmental plasticity of the LPM, which founds on multi-lineage progenitor cells and shared transcription factor expression across different descendant lineages, has the potential to explain the seemingly disparate syndromic defects in rare congenital diseases. Combining patient genome-sequencing data with model organism studies has already provided a wealth of insights into complex LPM-associated birth defects, such as heart-hand syndromes. Here, we summarize developmental and known disease-causing mechanisms in early LPM patterning, address how defects in these processes drive multi-organ comorbidities, and outline how several cardiovascular and hematopoietic birth defects with complex comorbidities may be LPM-associated diseases. We also discuss strategies to integrate patient sequencing, data-aggregating resources and model organism studies to mechanistically decode congenital defects, including potentially LPM-associated orphan diseases. Eventually, linking complex congenital phenotypes to a common LPM origin provides a framework to discover developmental mechanisms and to anticipate comorbidities in congenital diseases affecting the cardiovascular system and beyond.

The Organizer and Its Signaling in Embryonic Development

Germ layer specification and axis formation are crucial events in embryonic development. The Spemann organizer regulates the early developmental processes by multiple regulatory mechanisms. This review focuses on the responsive signaling in organizer formation and how the organizer orchestrates the germ layer specification in vertebrates. Accumulated evidence indicates that the organizer influences embryonic development by dual signaling. Two parallel processes, the migration of the organizer’s cells, followed by the transcriptional activation/deactivation of target genes, and the diffusion of secreting molecules, collectively direct the early development. Finally, we take an in-depth look at active signaling that originates from the organizer and involves germ layer specification and patterning.

Mutagenesis of a Lotus japonicus GSK3β/Shaggy-like kinase reveals functionally conserved regulatory residues

The glycogen synthase kinases 3 family (GSK3s/SKs; serine/threonine protein kinases) is conserved throughout eukaryotic evolution from yeast to plants and mammals. We studied a plant SK kinase from Lotus japonicus (LjSK1), previously implicated in nodule development, by enzyme kinetics and mutagenesis studies to compare it to mammalian homologues. Using a phosphorylated peptide as substrate, LjSK1 displays optimum kinase activity at pH 8.0 and 20 °C following Michaelis-Menten kinetics with K_m and Vmax values of 48.2 μM and 111.6 nmol/min/mg, respectively, for ATP. Mutation of critical residues, as inferred by sequence comparison to the human homologue GSK3β and molecular modeling, showed a conserved role for Lys167, while residues conferring substrate specificity in the human enzyme are not as significant in modulating LjSK1 substrate specificity. Mutagenesis studies also indicate a regulation mechanism for LjSK1 via proteolysis since removal of a 98 residue long N-terminal segment increases its catalytic efficiency by almost two-fold. In addition, we evaluated the alteration of LjSK1 kinase activity in planta, by overexpressing the mutant variants in hairy-roots and a phenotype in nodulation and lateral root development was verified.

Hierarchical pattern formation during amphibian limb regeneration

In 1901 T.H. Morgan proposed in "Regeneration" that pattern formation in amphibian limb regeneration is a stepwise process. Since, biologist have continued to piece together the molecular components of this process to better understand the "patterning code" responsible for regenerate formation. Within this context, several different models have been proposed; however, all are based on one of two underlying hypotheses. The first is the "morphogen hypothesis" that dictates that pattern emerges from localized expression of signaling molecules, which produce differing position-specific cellular responses in receptive cells depending on the intensity of the signal. The second hypothesis is that cells in the remaining tissues retain memory of their patterning information, and use this information to generate new cells with the missing positional identities. A growing body of evidence supports the possibility that these two mechanisms are not mutually exclusive. Here, we propose our theory of hierarchical pattern formation, which consists of 4 basic steps. The first is the existence of cells with positional memory. The second is the communication of positional information through cell-cell interactions in a regeneration-permissive environment. The third step is the induction of molecular signaling centers. And the last step is the interpretation of these signals by specialized cell types to ultimately restore the limb in its entirety. Biological codes are intertwined throughout this model, and we will discuss their multiple roles and mechanisms.

PAWS1 controls Wnt signalling through association with casein kinase 1α

The BMP and Wnt signalling pathways determine axis specification during embryonic development. Our previous work has shown that PAWS1 (also known as FAM83G) interacts with SMAD1 and modulates BMP signalling. Here, surprisingly, we show that overexpression of PAWS1 in Xenopus embryos activates Wnt signalling and causes complete axis duplication. Consistent with these observations in Xenopus, Wnt signalling is diminished in U2OS osteosarcoma cells lacking PAWS1, while BMP signalling is unaffected. We show that PAWS1 interacts and co-localises with the α isoform of casein kinase 1 (CK1), and that PAWS1 mutations incapable of binding CK1 fail both to activate Wnt signalling and to elicit axis duplication in Xenopus embryos.

Glycogen synthase kinase 3 controls migration of the neural crest lineage in mouse and Xenopus

Neural crest migration is critical to its physiological function. Mechanisms controlling mammalian neural crest migration are comparatively unknown, due to difficulties accessing this cell population in vivo. Here we report requirements of glycogen synthase kinase 3 (GSK3) in regulating the neural crest in Xenopus and mouse models. We demonstrate that GSK3 is tyrosine phosphorylated (pY) in mouse neural crest cells and that loss of GSK3 leads to increased pFAK and misregulation of Rac1 and lamellipodin, key regulators of cell migration. Genetic reduction of GSK3 results in failure of migration. We find that pY-GSK3 phosphorylation depends on anaplastic lymphoma kinase (ALK), a protein associated with neuroblastoma. Consistent with this, neuroblastoma cells with increased ALK activity express high levels of pY-GSK3, and blockade of GSK3 or ALK can affect migration of these cells. Altogether, this work identifies a role for GSK3 in cell migration during neural crest development and cancer.

Defects in neural crest development cause neurocristopathies and cancer, but what regulates this is unclear. Here, the authors show that glycogen synthase kinase 3 (GSK3) regulates migration of neural crest cells, as shown on genetic deletion of GSK3 in the mouse, and that this acts via anaplastic lymphoma kinase.

Reversal of hyperactive Wnt signaling-dependent adipocyte defects by peptide boronic acids

Deregulated Wnt signaling and altered lipid metabolism have been linked to obesity, diabetes, and various cancers, highlighting the importance of identifying inhibitors that can modulate Wnt signaling and aberrant lipid metabolism. We have established a Drosophila model with hyperactivated Wnt signaling caused by partial loss of axin, a key component of the Wnt cascade. The Axin mutant larvae are transparent and have severe adipocyte defects caused by up-regulation of β-catenin transcriptional activities. We demonstrate pharmacologic mitigation of these phenotypes in Axin mutants by identifying bortezomib and additional peptide boronic acids. We show that the suppressive effect of peptide boronic acids on hyperactive Wnt signaling is dependent on α-catenin; the rescue effect is completely abolished with the depletion of α-catenin in adipocytes. These results indicate that rather than targeting the canonical Wnt signaling pathway directly, pharmacologic modulation of β-catenin activity through α-catenin is a potentially attractive approach to attenuating Wnt signaling in vivo.

Fezf2 promotes neuronal differentiation through localised activation of Wnt/β-catenin signalling during forebrain development

Brain regionalisation, neuronal subtype diversification and circuit connectivity are crucial events in the establishment of higher cognitive functions. Here we report the requirement for the transcriptional repressor Fezf2 for proper differentiation of neural progenitor cells during the development of the Xenopus forebrain. Depletion of Fezf2 induces apoptosis in postmitotic neural progenitors, with concomitant reduction in forebrain size and neuronal differentiation. Mechanistically, we found that Fezf2 stimulates neuronal differentiation by promoting Wnt/β-catenin signalling in the developing forebrain. In addition, we show that Fezf2 promotes activation of Wnt/β-catenin signalling by repressing the expression of two negative regulators of Wnt signalling, namely lhx2 and lhx9. Our findings suggest that Fezf2 plays an essential role in controlling when and where neuronal differentiation occurs within the developing forebrain and that it does so by promoting local Wnt/β-catenin signalling via a double-repressor model.

Phosphorylation-dependent ubiquitination of paraxial protocadherin (PAPC) controls gastrulation cell movements

Paraxial protocadherin (PAPC) has been shown to be involved in gastrulation cell movements during early embryogenesis. It is first expressed in the dorsal marginal zone at the early gastrula stage and subsequently restricted to the paraxial mesoderm in Xenopus and zebrafish. Using Xenopus embryos, we found that PAPC is also regulated at the protein level and is degraded and excluded from the plasma membrane in the axial mesoderm by the late gastrula stage. Regulation of PAPC requires poly-ubiquitination that is dependent on phosphorylation. PAPC is phosphorylated by GKS3 in the evolutionarily conserved cytoplasmic domain, and this in turn is necessary for poly-ubiquitination by an E3 ubiquitin ligase β-TrCP. We also show that precise control of PAPC by phosphorylation/ubiquitination is essential for normal Xenopus gastrulation cell movements. Taken together, our findings unveil a novel mechanism of regulation of a cell adhesion protein and show that this system plays a crucial role in vertebrate embryogenesis.

Wnt activation of immortalized brain endothelial cells as a tool for generating a standardized model of the blood brain barrier in vitro

Reproducing the characteristics and the functional responses of the blood–brain barrier (BBB) in vitro represents an important task for the research community, and would be a critical biotechnological breakthrough. Pharmaceutical and biotechnology industries provide strong demand for inexpensive and easy-to-handle in vitro BBB models to screen novel drug candidates. Recently, it was shown that canonical Wnt signaling is responsible for the induction of the BBB properties in the neonatal brain microvasculature in vivo. In the present study, following on from earlier observations, we have developed a novel model of the BBB in vitro that may be suitable for large scale screening assays. This model is based on immortalized endothelial cell lines derived from murine and human brain, with no need for co-culture with astrocytes. To maintain the BBB endothelial cell properties, the cell lines are cultured in the presence of Wnt3a or drugs that stabilize β-catenin, or they are infected with a transcriptionally active form of β-catenin. Upon these treatments, the cell lines maintain expression of BBB-specific markers, which results in elevated transendothelial electrical resistance and reduced cell permeability. Importantly, these properties are retained for several passages in culture, and they can be reproduced and maintained in different laboratories over time. We conclude that the brain-derived endothelial cell lines that we have investigated gain their specialized characteristics upon activation of the canonical Wnt pathway. This model may be thus suitable to test the BBB permeability to chemicals or large molecular weight proteins, transmigration of inflammatory cells, treatments with cytokines, and genetic manipulation.

Xenopus Nkx6.1 and Nkx6.2 are required for mid-hindbrain boundary development

The mid-hindbrain boundary (MHB) organizer is among the best studied local organizers that patterns the midbrain and cerebellum in vertebrates and is established by a regulatory network of several transcription factors and signals, including Wnt signaling. In the present study, we found that Xenopus Nkx6.1 and Nkx6.2, two transcription factors of the Nkx homeobox family, are required for MHB formation. In Xenopus embryos, nkx6.1 and nkx6.2 are expressed in the MHB, and knockdown of either nkx6.1 or nkx6.2 by specific morpholinos disrupts the MHB expression of en2 as well as wnt1, a key regulator for vertebrate MHB formation. In the nkx6.1/nkx6.2 morphants, co-injection of wnt1 or dngsk3β mRNA, which activates Wnt signaling, rescues the expression of en2. Nkx6.1 and Nkx6.2 activate canonical Wnt signaling in reporter assays in both cultured mammalian cells and Xenopus embryos. An Nkx6.2 deletion construct, which inhibits the ability of wild type Nkx6.2 to activate Wnt signaling, also reduces the MHB expression of en2 in Xenopus embryos. These results suggest that Nkx6.1 and Nkx6.2 are involved in MHB formation in Xenopus embryos, likely by modulating wnt1 expression.

The C. elegans rab family: identification, classification and toolkit construction

Rab monomeric GTPases regulate specific aspects of vesicle transport in eukaryotes including coat recruitment, uncoating, fission, motility, target selection and fusion. Moreover, individual Rab proteins function at specific sites within the cell, for example the ER, golgi and early endosome. Importantly, the localization and function of individual Rab subfamily members are often conserved underscoring the significant contributions that model organisms such as Caenorhabditis elegans can make towards a better understanding of human disease caused by Rab and vesicle trafficking malfunction. With this in mind, a bioinformatics approach was first taken to identify and classify the complete C. elegans Rab family placing individual Rabs into specific subfamilies based on molecular phylogenetics. For genes that were difficult to classify by sequence similarity alone, we did a comparative analysis of intron position among specific subfamilies from yeast to humans. This two-pronged approach allowed the classification of 30 out of 31 C. elegans Rab proteins identified here including Rab31/Rab50, a likely member of the last eukaryotic common ancestor (LECA). Second, a molecular toolset was created to facilitate research on biological processes that involve Rab proteins. Specifically, we used Gateway-compatible C. elegans ORFeome clones as starting material to create 44 full-length, sequence-verified, dominant-negative (DN) and constitutive active (CA) rab open reading frames (ORFs). Development of this toolset provided independent research projects for students enrolled in a research-based molecular techniques course at California State University, East Bay (CSUEB).

Glycogen synthase kinase-3 is essential for β-arrestin-2 complex formation and lithium-sensitive behaviors in mice

Lithium is the first-line therapy for bipolar disorder. However, its therapeutic target remains controversial. Candidates include inositol monophosphatases, glycogen synthase kinase-3 (GSK-3), and a β-arrestin-2/AKT/protein phosphatase 2A (β-arrestin-2/AKT/PP2A) complex that is known to be required for lithium-sensitive behaviors. Defining the direct target(s) is critical for the development of new therapies and for elucidating the molecular pathogenesis of this major psychiatric disorder. Here, we show what we believe to be a new link between GSK-3 and the β-arrestin-2 complex in mice and propose an integrated mechanism that accounts for the effects of lithium on multiple behaviors. GSK-3β (Gsk3b) overexpression reversed behavioral defects observed in lithium-treated mice and similar behaviors observed in Gsk3b+/- mice. Furthermore, immunoprecipitation of striatial tissue from WT mice revealed that lithium disrupted the β-arrestin-2/Akt/PP2A complex by directly inhibiting GSK-3. GSK-3 inhibitors or loss of one copy of the Gsk3b gene reduced β-arrestin-2/Akt/PP2A complex formation in mice, while overexpression of Gsk3b restored complex formation in lithium-treated mice. Thus, GSK-3 regulates the stability of the β-arrestin-2/Akt/PP2A complex, and lithium disrupts the complex through direct inhibition of GSK-3. We believe these findings reveal a new role for GSK-3 within the β-arrestin complex and demonstrate that GSK-3 is a critical target of lithium in mammalian behaviors.

Anterior neural development requires Del1, a matrix-associated protein that attenuates canonical Wnt signaling via the Ror2 pathway

During early embryogenesis, the neural plate is specified along the anterior-posterior (AP) axis by the action of graded patterning signals. In particular, the attenuation of canonical Wnt signals plays a central role in the determination of the anterior brain region. Here, we show that the extracellular matrix (ECM) protein Del1, expressed in the anterior neural plate, is essential for forebrain development in the Xenopus embryo. Overexpression of Del1 expands the forebrain domain and promotes the formation of head structures, such as the eye, in a Chordin-induced secondary axis. Conversely, the inhibition of Del1 function by a morpholino oligonucleotide (MO) represses forebrain development. Del1 also augments the expression of forebrain markers in neuralized animal cap cells, whereas Del1-MO suppresses them. We previously reported that Del1 interferes with BMP signaling in the dorsal-ventral patterning of the gastrula marginal zone. By contrast, we demonstrate here that Del1 function in AP neural patterning is mediated mainly by the inhibition of canonical Wnt signaling. Wnt-induced posteriorization of the neural plate is counteracted by Del1, and the Del1-MO phenotype (posteriorization) is reversed by Dkk1. Topflash reporter assays show that Del1 suppresses luciferase activities induced by Wnt1 and beta-catenin. This inhibitory effect of Del1 on canonical Wnt signaling, but not on BMP signaling, requires the Ror2 pathway, which is implicated in non-canonical Wnt signaling. These findings indicate that the ECM protein Del1 promotes forebrain development by creating a local environment that attenuates the cellular response to posteriorizing Wnt signals via a unique pathway.

A noncatalytic domain of glycogen synthase kinase-3 (GSK-3) is essential for activity

Glycogen synthase kinase-3 (GSK-3) isoforms, GSK-3alpha and GSK-3beta, are serine/threonine kinases involved in numerous cellular processes and diverse diseases, including Alzheimer disease, cancer, and diabetes. GSK-3 isoforms function redundantly in some settings, while, in others, they exhibit distinct activities. Despite intensive investigation into the physiological roles of GSK-3 isoforms, the basis for their differential activities remains unresolved. A more comprehensive understanding of the mechanistic basis for GSK-3 isoform-specific functions could lead to the development of isoform-specific inhibitors. Here, we describe a structure-function analysis of GSK-3alpha and GSK-3beta in mammalian cells. We deleted the noncatalytic N and C termini in both GSK-3 isoforms and generated point mutations of key regulatory residues. We examined the effect of these mutations on GSK-3 activity toward Tau, activity in Wnt signaling, interaction with Axin, and GSK-3alpha/beta Tyr(279/216) phosphorylation. We found that the N termini of both GSK-3 isoforms were dispensable, whereas progressive C-terminal deletions resulted in protein misfolding exhibited by deficient activity, impaired ability to interact with Axin, and a loss of Tyr(279/216) phosphorylation. Our data predict that small molecules targeting the divergent C terminus may lead to isoform-specific GSK-3 inhibition through destabilization of the GSK-3 structure.

GSK3: a multifaceted kinase in Wnt signaling

GSK3 is one of the few signaling mediators that play central roles in a diverse range of signaling pathways, including those activated by Wnts, hedgehog, growth factors, cytokines, and G protein-coupled ligands. Although the inhibition of GSK3-mediated beta-catenin phosphorylation is known to be the key event in Wnt-beta-catenin signaling, the mechanisms that underlie this event remain incompletely understood. The recent demonstration of GSK3 involvement in Wnt receptor phosphorylation illustrates the multifaceted roles that GSK3 plays in Wnt-beta-catenin signaling. In this review, we will summarize these recent results and offer explanations, hypotheses, and models to reconcile some of these observations.

Inhibition of GSK3 phosphorylation of beta-catenin via phosphorylated PPPSPXS motifs of Wnt coreceptor LRP6

The Wnt/β-catenin signaling pathway plays essential roles in cell proliferation and differentiation, and deregulated β-catenin protein levels lead to many types of human cancers. On activation by Wnt, the Wnt co-receptor LDL receptor related protein 6 (LRP6) is phosphorylated at multiple conserved intracellular PPPSPXS motifs by glycogen synthase kinase 3 (GSK3) and casein kinase 1 (CK1), resulting in recruitment of the scaffolding protein Axin to LRP6. As a result, β-catenin phosphorylation by GSK3 is inhibited and β-catenin protein is stabilized. However, how LRP6 phosphorylation and the ensuing LRP6-Axin interaction lead to the inhibition of β-catenin phosphorylation by GSK3 is not fully understood. In this study, we reconstituted Axin-dependent β-catenin phosphorylation by GSK3 and CK1 in vitro using recombinant proteins, and found that the phosphorylated PPPSPXS peptides directly inhibit β-catenin phosphorylation by GSK3 in a sequence and phosphorylation-dependent manner. This inhibitory effect of phosphorylated PPPSPXS motifs is direct and specific for GSK3 phosphorylation of β-catenin at Ser33/Ser37/Thr41 but not for CK1 phosphorylation of β-catenin at Ser45, and is independent of Axin function. We also show that a phosphorylated PPPSPXS peptide is able to activate Wnt/β-catenin signaling and to induce axis duplication in Xenopus embryos, presumably by inhibition of GSK3 in vivo. Based on these observations, we propose a working model that Axin recruitment to the phosphorylated LRP6 places GSK3 in the vicinity of multiple phosphorylated PPPSPXS motifs, which directly inhibit GSK3 phosphorylation of β-catenin. This model provides a possible mechanism to account, in part, for inhibition of β-catenin phosphorylation by Wnt-activated LRP6.

Identification of targets of the Wnt pathway destruction complex in addition to beta-catenin

The proteasomal degradation of beta-catenin mediated by the glycogen synthase kinase 3beta (GSK3beta) and destruction complex is the central step in the canonical Wnt signaling pathway. However, that there are branches of Wnt signaling pathways that do not depend on beta-catenin/Tcf-mediated transcription activation has long been understood. In this study, we hypothesized that there are many more GSK3 and destruction complex-dependent proteolytic target proteins that mediate Wnt signaling in the cell. To test this hypothesis, we have developed and carried out a screen for such candidate proteins using an in vitro expression cloning technique and biochemical reconstitution of Wnt signaling in Xenopus egg cytoplasmic extracts. Forty-two proteins have been identified as potential candidates for GSK3-regulated phosphorylation, proteasomal degradation, or both, of which 12 are strong candidates for Wnt-pathway-regulated degradation. Some of them have been reported to interact with beta-catenin and implicated in the canonical Wnt signaling pathway, and other targets identified include proteins with various cellular functions such as RNA processing, cytoskeletal dynamics, and cell metabolism. Thus, we propose that Wnt/GSK3/destruction complex signaling regulates multiple target proteins to control a broad range of cellular activities in addition to beta-catenin-mediated transcription activation.

Lithium regulates adult hippocampal progenitor development through canonical Wnt pathway activation

Neural stem cells give rise to new hippocampal neurons throughout adulthood, and defects in neurogenesis may predispose an individual to mood disorders, such as major depression. Our understanding of the signals controlling this process is limited, so we explored potential pathways regulating adult hippocampal progenitor (AHP) proliferation and neuronal differentiation. We demonstrate that the mood stabilizer lithium directly expands pools of AHPs in vitro, and induces them to become neurons at therapeutically relevant concentrations. We show that these effects are independent of inositol monophosphatase, but dependent on Wnt pathway components. Both downregulation of glycogen synthase kinase-3beta, a lithium-sensitive component of the canonical Wnt signaling pathway, and elevated beta-catenin, a downstream component of the same pathway produce effects similar to lithium. In contrast, RNAi-mediated inhibition of beta-catenin abolishes the proliferative effects of lithium, suggesting that Wnt signal transduction may underlie lithium's therapeutic effect. Together, these data strengthen the connection between psychopharmacologic treatment and the process of adult neurogenesis, while also suggesting the pursuit of modulators of Wnt signaling as a new class of more effective mood stabilizers/antidepressants.

Cdc42 is crucial for the establishment of epithelial polarity during early mammalian development

To study the role of Cdc42 in the establishment of epithelial polarity during mammalian development, we generated murine Cdc42-null embryonic stem cells and analyzed peri-implantation development using embryoid bodies (EBs). Mutant EBs developed endoderm and underlying basement membrane, but exhibited defects of cell polarity, cell-cell junctions, survival, and cavitation. These defects corresponded to a decreased phosphorylation and membrane localization of aPKC, a reduced phosphorylation of GSK3beta, and a diminished activity of Rac1. However, neither Rac1 nor the kinase function of GSK3beta seem to contribute to cell polarization and cell-cell contacts. In contrast, EBs expressing dominant-negative (dn) PKCzeta mimicked well the phenotype of Cdc42-null EBs, suggesting a major role of aPKC in mediating cell polarization downstream of Cdc42. Finally, aggregation experiments with endodermal cell lines suggested that Cdc42 might affect formation of adherens and tight junctions by PKCzeta-dependent regulation of the protein levels of p120 catenin and E-cadherin.

The Xenopus Nieuwkoop center and Spemann-Mangold organizer share molecular components and a requirement for maternal Wnt activity

In Xenopus embryos, the dorso-ventral and antero-posterior axes are established by the Spemann-Mangold organizer. According to the prevalent model of early development, the organizer is induced by the dorsalizing Nieuwkoop signal, which is secreted by the Nieuwkoop center. Formation of the center requires the maternal Wnt pathway, which is active on the dorsal side of embryos. Nevertheless, the molecular nature of the Nieuwkoop signal remains unclear. Since the Nieuwkoop center and the organizer both produce dorsalizing signals in vitro, we asked if they might share molecular components. We find that vegetal explants, the source of Nieuwkoop signal in recombination assays, express a number of organizer genes. The product of one of these genes, chordin, is required for signaling, suggesting that the organizer and the center share at least some molecular components. Furthermore, experiments with whole embryos show that maternal Wnt activity is required in the organizer just as it is needed in the Nieuwkoop center in vitro. We conclude that the maternal Wnt pathway generates the Nieuwkoop center in vitro and the organizer in vivo by activating a common set of genes, without the need of an intermediary signaling step.

Increased nuclear beta-catenin in suprabasal involved psoriatic epidermis

BACKGROUND

Psoriasis is a common inflammatory skin disease characterized by abnormal keratinocyte proliferation and differentiation, increased angiogenesis and inflammation. There is evidence that some keratinocyte differentiation events are controlled by changes in cell-cell adhesion. beta-catenin is a 94-kDa protein which has a dual function as a component of intercellular adherens junctions and also as a transcription factor as part of the Wnt signalling pathway. beta-catenin is not required for keratinocyte proliferation but has been shown to regulate keratinocyte stem cells and hair follicle morphogenesis.

OBJECTIVES

To investigate the distribution and function of beta-catenin in involved psoriatic epidermis and in epidermal keratinocytes.

METHODS

Biopsies were obtained from patients with psoriasis and from normal controls. The distribution of beta-catenin was investigated using antibodies to both total and unphosphorylated active beta-catenin. Luciferase assays were used to measure transcriptional activation of transglutaminase 1 (TGase 1) and involucrin and to investigate the functional role of beta-catenin in interfollicular keratinocytes.

RESULTS

Increased nuclear beta-catenin was seen in lesional suprabasal psoriatic epidermis compared with uninvolved or normal skin. Increased active unphosphorylated beta-catenin was also detected within the differentiating compartment of involved psoriatic epidermis. Expression of TGase 1 overlapped with beta-catenin in suprabasal lesional psoriasis. The TGase 1 promoter was positively regulated by activated beta-catenin and by the glycogen synthase kinase binding protein, suggesting that beta-catenin and glycogen synthase kinase 3beta may regulate TGase 1 expression.

CONCLUSIONS

This is the first report to convincingly demonstrate increased beta-catenin in involved psoriasis and to implicate beta-catenin in the regulation of TGase 1. This evidence suggests a role for beta-catenin signalling in regulating keratinocyte differentiation in interfollicular skin in addition to previously reported functions in stem cell fate determination, hair follicle regulation and skin tumorigenesis.

Glycogen synthase kinase 3 alpha and 3 beta have distinct functions during cardiogenesis of zebrafish embryo

Background

Glycogen synthase kinase 3 (GSK3) encodes a serine/threonine protein kinase, is known to play roles in many biological processes. Two closely related GSK3 isoforms encoded by distinct genes: GSK3α (51 kDa) and GSK3β (47 kDa). In previously studies, most GSK3 inhibitors are not only inhibiting GSK3, but are also affecting many other kinases. In addition, because of highly similarity in amino acid sequence between GSK3α and GSK3β, making it difficult to identify an inhibitor that can be selective against GSK3α or GSK3β. Thus, it is relatively difficult to address the functions of GSK3 isoforms during embryogenesis. At this study, we attempt to specifically inhibit either GSK3α or GSK3β and uncover the isoform-specific roles that GSK3 plays during cardiogenesis.

Results

We blocked gsk3α and gsk3β translations by injection of morpholino antisense oligonucleotides (MO). Both gsk3α- and gsk3β-MO-injected embryos displayed similar morphological defects, with a thin, string-like shaped heart and pericardial edema at 72 hours post-fertilization. However, when detailed analysis of the gsk3α- and gsk3β-MO-induced heart defects, we found that the reduced number of cardiomyocytes in gsk3α morphants during the heart-ring stage was due to apoptosis. On the contrary, gsk3β morphants did not exhibit significant apoptosis in the cardiomyocytes, and the heart developed normally during the heart-ring stage. Later, however, the heart positioning was severely disrupted in gsk3β morphants. bmp4 expression in gsk3β morphants was up-regulated and disrupted the asymmetry pattern in the heart. The cardiac valve defects in gsk3β morphants were similar to those observed in axin1 and apc^mcr mutants, suggesting that GSK3β might play a role in cardiac valve development through the Wnt/β-catenin pathway. Finally, the phenotypes of gsk3α mutant embryos cannot be rescued by gsk3β mRNA, and vice versa, demonstrating that GSK3α and GSK3β are not functionally redundant.

Conclusion

We conclude that (1) GSK3α, but not GSK3β, is necessary in cardiomyocyte survival; (2) the GSK3β plays important roles in modulating the left-right asymmetry and affecting heart positioning; and (3) GSK3α and GSK3β play distinct roles during zebrafish cardiogenesis.

Downregulation of the upstream binding factor1 by glycogen synthase kinase3beta in myeloid cells induced to differentiate

The upstream binding factor 1 (UBF1), one of the proteins that regulate the activity of RNA polymerase I, is downregulated in 32D myeloid cells induced to differentiate into granulocytes, either by the type 1 insulin-like growth factor (IGF-1) or the granulocytic colony stimulating factor (G-CSF). Downregulation of UBF1 is largely due to protein degradation, while mRNA levels are not affected. Inhibition of UBF1 degradation by lithium chloride (LiCl)and lactacystin suggest a role of glycogen synthase kinase beta (GSK3beta) in a proteasome-dependent degradation of UBF. GSK3beta phosphorylates in vitro and in vivo the UBF protein, which has five putative motifs for phosphorylation by GSK3beta. Elimination and/or mutations of these motifs stabilize the UBF1 protein even in cells induced to differentiate. Conversely, a stably transfected, constitutively active GSK3beta accelerates the downregulation of UBF1. We show further that activation of the differentiating protein C/EPBalpha in 32D cells transformed by the oncogenic BCR/ABL protein causes downregulation of UBF1. Finally, inhibition of differentiation of myeloid cells by a dominant negative mutant of Stat3 stabilizes the UBF1 protein, while rapamycin-induced differentiation of myeloid cells downregulates UBF1 levels. Taken together, our results indicate that the induction of granulocytic differentiation in 32D murine myeloid cells causes the degradation of UBF1, via GSK3beta and the proteasome pathway.

The Arabidopsis thaliana GSK3/Shaggy like kinase AtSK3-2 modulates floral cell expansion

The GSK3/Shaggy family of serine/threonine protein kinases is involved in a series of biological processes in animals, plants and yeast [Charrier et al. (2002) Plant Physiol 130:577-590; Jope and Johnson (2004) Trends Biochem Sci 29:95-102; Li and Nam (2002) Science 295:1299-1301; Piao et al. (2001) Plant J 27:305-314]. In Arabidopsis thaliana, out of the 10 members of the GSK3/Shaggy-like gene family (AtSKs), a biological function has been assigned to only 1 member (AtSK2-1) by mutation. In the present work, a study was undertaken to elucidate the function of AtSK3-2. We have generated mutated versions of the A. thaliana Shaggy-like kinase 3-2 (AtSK3-2), in which Lys(167) and Arg(178), respectively homologues to Lys(85) and Arg(96) of the mammal GSK3beta, were modified into Ala by site-directed mutagenesis. In vitro kinase activity assays of the mutated recombinant protein AtSK3-2-R178A showed that the "primed activity" of the mutated kinase was reduced by 90% while the "non-primed" activity was only 20% reduced compared to the wild-type protein kinase. However, the mutant protein AtSK3-2-K167A showed no activity. Arabidopsis transgenic lines over-expressing AtSK3-2-R178A displayed smaller floral organs, namely pedicels, sepals and petals. Conversely, over-expression of both the wild-type AtSK3-2 protein and the AtSK3-2-K167A mutated version, displayed no altered morphogenesis. Scanning electron microscopic analyses of the AtSK3-2-R178A transgenic plants clearly showed a reduced cell size in flower organs, in which quantitative RT-PCR expression analyses of cell wall expansion enzymes showed reduced transcript levels of three xyloglucan endotransglycosylases (XET), namely XTH22 (TCH4), XTH23 (XTR6) and XTH30 (XTR4). Our data show that AtSK3-2 plays an important role in the control of cell elongation in flower development.

Glycogen synthase kinase-3 regulation of chromatin segregation and cytokinesis in mouse preimplantation embryos

Glycogen synthase kinase-3 (GSK-3) is a highly conserved serine/threonine protein kinase implicated in diverse cellular processes. Activity of GSK-3 is essential for meiotic chromatin segregation in oocytes, yet expression and/or function of GSK-3 have not been reported in mammalian preimplantation embryos. Objectives of this study were to characterize GSK-3 protein expression/phosphorylation in mouse preimplantation embryos, to assess the effect of GSK-3 activity inhibition on early mitotic events, and to differentiate nuclear and cytoplasmic anomalies in GSK-3 inhibited embryos. Both GSK-3 isoforms were expressed during embryo development, with a differential expression of alpha versus beta. Phosphorylation of GSK-3alpha/beta at residues Y279/Y216 indicated constitutive activation throughout preimplantation development. Phosphorylation at N-terminal residues S21/S9 indicated inhibition of GSK-3alpha/beta activity that was differentially regulated during early development; both alpha and beta isoforms were phosphorylated during early divisions, whereas at the blastocyst stage, only beta was phosphorylated. Cytoplasmic microinjection of zygotes with anti-GSK-3alpha/beta antibody significantly compromised embryonic development past the two-cell stage compared to controls. Reversibility of developmental block was tested via pharmacological inhibitors of GSK-3, lithium chloride (LiCl) and alsterpaullone. Similar to immunoneutralization, significantly fewer zygotes cultured with either LiCl or alsterpaullone developed past the two-cell stage compared to controls and this mitotic block was not reversible. Inhibition of GSK-3 activity significantly compromised timing of pronuclear membrane breakdown and mitosis initiation, nuclear development, and cytokinesis. Inhibition of GSK-3 also resulted in abnormal chromatin segregation, evidenced by incomplete karyokinesis and micronuclei formation. These results suggest that GSK-3 activity is critical for early preimplantation embryonic development.

Glial cell line-derived neurotrophic factor-mediated enteric neuronal survival involves glycogen synthase kinase-3beta phosphorylation and coupling with 14-3-3

Glial cell line-derived neurotrophic factor (GDNF) promotes the growth and survival of enteric neurons, but the mechanisms involved are poorly understood. GDNF is known to promote the survival of enteric neurons through activation of the PI3-Kinase/Akt signaling pathway. We investigated the role of glycogen synthase kinase-3beta (GSK-3beta) in enteric neuronal survival, and the ability of GDNF to regulate the activity of GSK-3beta using primary rat embryonic enteric neurons. GDNF, through activation of the PI3-kinase pathway enhanced the phosphorylation of GSK-3beta at its N-terminal serine-9 residue, and promoted the association of GSK-3beta with 14-3-3. Transfection of a constitutively active S9A-GSK-3beta mutant prevented the survival effects of GDNF, whereas a dominant negative GSK-3beta construct prevented GDNF withdrawal-induced cell death. Increased GSK-3beta activity was associated with an increase in tau phosphorylation. Thus, GDNF promotes enteric neuronal survival by modulating GSK-3beta and its downstream target tau. Inhibitors of GSK-3beta activity may have therapeutic potential in improving enteric neuronal survival.

hecate, a zebrafish maternal effect gene, affects dorsal organizer induction and intracellular calcium transient frequency

A zebrafish maternal effect mutation, in the gene hecate, results in embryos that have defects in the formation of dorsoanterior structures and altered calcium release. hecate mutant embryos lack nuclear accumulation of beta-catenin and have reduced expression of genes specific to the dorsal organizer. We found that hecate mutant embryos exhibit a nearly 10-fold increase in the frequency of intracellular Ca2+ transients normally present in the enveloping layer during the blastula stages. Inhibition of Ca2+ release leads to ectopic expression of dorsal genes in mutant embryos suggesting that Ca2+ transients are important in mediating dorsal gene expression. Inhibition of Ca2+ release also results in the expression of dorsal-specific genes in the enveloping layer in a beta-catenin-independent manner, which suggests an additional function for the Ca2+ transients in this cellular layer. The mutant phenotype can be reversed by the expression of factors that activate Wnt/beta-catenin signaling, suggesting that the Wnt/beta-catenin pathway, at least as activated by an exogenous Wnt ligand, is intact in hec mutant embryos. Our results are consistent with a role for the hecate gene in the regulation of Ca2+ release during the cleavage stages, which in turn influences dorsal gene expression in both marginal cells along the dorsoventral axis and in the enveloping layer.

LZIC regulates neuronal survival during zebrafish development

Development of the brain and central nervous system is a complex process involving localized gene expression and regulated cell death and proliferation. Here, we describe a gene involved in neuronal survival, the zebrafish ortholog of the human lzic gene. Zebrafish lzic is expressed ubiquitously during early development and later becomes enriched in the developing brain. Using antisense morpholino oligonucleotides, we demonstrate that zebrafish lzic is required zygotically for the survival of distinct neuronal populations. LZIC is closely related to ICAT, a physiological inhibitor of the Wnt pathway that interacts physically and functionally with beta-catenin to prevent the transcription of Wnt target genes. LZIC's ICAT-homologous region is highly similar to ICAT with particular conservation of residues that are used by ICAT for beta-catenin-binding. Surprisingly, despite this high similarity, LZIC does not interact with beta-catenin in vitro or in vivo. Our results reveal that LZIC, a protein conserved in vertebrates, is required for neuronal survival in zebrafish.

Kinase-inactive glycogen synthase kinase 3beta promotes Wnt signaling and mammary tumorigenesis

Recent studies have implicated ectopic activation of the Wnt pathway in many human cancers, including breast cancer. beta-catenin is a critical coactivator in this signaling pathway and is regulated in a complex fashion by phosphorylation, degradation, and nuclear translocation. Glycogen synthase kinase 3beta (GSK3beta) phosphorylation of the NH2-terminal domain of beta-catenin targets it for ubiquitination and proteosomal degradation. We hypothesized that expression of kinase-inactive GSK3beta (KI-GSK3beta) in mammary glands would function in a dominant-negative fashion by antagonizing the endogenous activity of GSK3beta and promoting breast cancer development. Consistent with this, we find that KI-GSK3beta stabilizes beta-catenin expression, catalyzes its localization to the nucleus, and up-regulates the downstream target gene, cyclin D1, in vitro. In vivo, transgenic mice overexpressing the KI-GSK3beta under the control of the mouse mammary tumor virus-long terminal repeat develop mammary tumors with overexpression of beta-catenin and cyclin D1. Thus, antagonism of GSK3beta activity is oncogenic in the mammary epithelium; mutation or pharmacologic down-regulation of GSK3beta could promote mammary tumors.

xBtg-x regulates Wnt/beta-Catenin signaling during early Xenopus development

In Xenopus, two signaling systems, maternal beta-Catenin and Nodal-related, are required for induction of the Spemann organizer and establishment of the body plan. By screening cDNA macroarrays for genes activated by these two signaling pathways, we identified Xenopus xBtg-x, a novel member of the Btg/Tob gene family of antiproliferative proteins. We show that xBtg-x is expressed in the dorsal mesendoderm (Spemann organizer tissue) of gastrula stage embryos and that its expression is regulated by both beta-Catenin and Nodal-related signals. Microinjection of synthetic xBtg-x mRNA into Xenopus embryos induced axis duplication and completely rescued the ventralizing effects of UV irradiation through the activation of the canonical Wnt/beta-Catenin signaling pathway. Interestingly, xBtg-x stimulated beta-Catenin-dependent transcription without affecting the stability of beta-Catenin protein. These data suggest that xBtg-x is a novel component of the Wnt/beta-Catenin signaling pathway regulating early embryonic patterning.

A role for CK2α/β in Xenopus early embryonic development

CK2 is expressed widely in early embryonic development in several animal models, however its developmental role is unclear. One of the substrates of CK2 that is important in embryonic development is beta-catenin, the transcriptional co-activator of the canonical Wnt signaling pathway. This pathway has been implicated in diverse aspects of embryonic development, including one of the earliest events in embryonic development, the establishment of the dorso-ventral embryonic axis. In Xenopus laevis, dorso-ventral axis formation is dependent upon stabilization of beta-catenin in the future dorsal side of the embryo. Since CK2 phosphorylation of beta-catenin stabilizes it, we hypothesized that CK2 might be critical to upregulation of beta-catenin in Xenopus embryos and to the process of axis establishment. Our results demonstrate that CK2 is required for dorsal axis formation and is for normal upregulation of Wnt signaling genes and targets. Thus, CK2 is a regulator of endogenous axis formation in vertebrates.

Glycogen synthase kinase 3 has a proapoptotic function in Hydra gametogenesis

In an approach to study the evolutionary conservation of molecules of the Wnt signal transduction pathway, we analyzed the function of the major negative regulator of this pathway, GSK3 (glycogen synthase kinase 3), in the basal metazoan Hydra. Microinjection assays reveal that HyGSK3 inhibits beta-catenin in zebrafish embryos, indicating that the function of GSK3 in the canonical Wnt signaling pathway is evolutionarily conserved. In Hydra, HyGSK3 transcripts are strongly upregulated during gametogenesis. Treatment of female polyps with the GSK3 inhibitors lithium and alsterpaullone prevents the differentiation of nurse cells and subsequent oocyte formation. Our data indicate that GSK3 is required for the early induction of apoptosis in germline cells, which has been shown to be an initial step in Hydra gametogenesis. Our experiments show that main functions of GSK3 are evolutionarily conserved and unique to multicellular animals, a conclusion which is additionally supported by the presence of specific regulatory domains in the HyGSK3 protein.

Xenopus ILK (integrin-linked kinase) is required for morphogenetic movements during gastrulation

It has been suggested that ILK (integrin-linked kinase) participates in integrin- and growth factor-mediated signaling pathways and also functions as a scaffold protein at cell-extracellular matrix (ECM) adhesion sites. As the recently reported ILK knockout mice were found to die at the peri-implantation stage, the stage specific to mammals, little is known about the function of ILK in early developmental processes common to every vertebrate. To address this, we isolated a Xenopus ortholog of ILK (XeILK) and characterized its role in early Xenopus embryogenesis. XeILK was expressed constitutively and ubiquitously throughout the early embryogenesis. Depletion of XeILK with morpholino oligonucleotides (XeILK MO) caused severe defects in blastopore closure and axis elongation without affecting the mesodermal specification. Furthermore, XeILK MO was found to interfere with cell-cell and cell-ECM adhesions in dorsal marginal zone explants and to result in a significant loss of cell-ECM adhesions in activin-treated dissociated animal cap cells. These results thus indicate that XeILK plays an essential role in morphogenetic movements during gastrulation.

Genetic evidence for involvement of maternally derived Wnt canonical signaling in dorsal determination in zebrafish

In zebrafish, the program for dorsal specification begins soon after fertilization. Dorsal determinants are localized initially to the vegetal pole, then transported to the blastoderm, where they are thought to activate the canonical Wnt pathway, which induces the expression of dorsal-specific genes. We identified a novel maternal-effect recessive mutation, tokkaebi (tkk), that affects formation of the dorsal axis. Severely ventralized phenotypes, including a lack of dorso-anterior structures, were seen in 5-100% of the embryos obtained from tkk homozygous transmitting females. tkk embryos displayed defects in the nuclear accumulation of beta-catenin on the dorsal side, and reduced or absent expression of dorsal-specific genes. Mesoderm and endoderm formation outside the dorsal axis was not significantly affected. Injection of RNAs for activated beta-catenin, dominant-negative forms of Axin1 and GSK3beta, and wild-type Dvl3, into the tkk embryos suppressed the ventralized phenotypes and/or dorsalized the embryos, and restored or induced an ectopic and expanded expression of bozozok/dharma and goosecoid. However, dorsalization by wnt RNAs was affected in the tkk embryos. Inhibition of cytoplasmic calcium release elicited an ectopic and expanded expression of chordin in the wild-type, but did not restore chordin expression efficiently in the tkk embryos. These data indicate that the tkk gene product functions upstream of or parallel to the beta-catenin-degradation machinery to control the stability of beta-catenin. The tkk locus was mapped to chromosome 16. These data provide genetic evidence that the maternally derived canonical Wnt pathway upstream of beta-catenin is involved in dorsal axis formation in zebrafish.

Protein kinase CK2 is required for dorsal axis formation in Xenopus embryos

Dorsal axis formation in Xenopus embryos is dependent upon asymmetrical localization of beta-catenin, a transducer of the canonical Wnt signaling pathway. Recent biochemical experiments have implicated protein kinase CK2 as a regulator of members of the Wnt pathway including beta-catenin. Here, we have examined the role of CK2 in dorsal axis formation. CK2 was present in the developing embryo at an appropriate time and place to participate in dorsal axis formation. Overexpression of mRNA encoding CK2 in ventral blastomeres was sufficient to induce a complete ectopic axis, mimicking Wnt signaling. A kinase-inactive mutant of CK2alpha was able to block ectopic axis formation induced by XWnt8 and beta-catenin and was capable of suppressing endogenous axis formation when overexpressed dorsally. Taken together, these studies demonstrate that CK2 is a bona fide member of the Wnt pathway and has a critical role in the establishment of the dorsal embryonic axis.

Homology modeling and molecular dynamics study of GSK3/SHAGGY-like kinase

Although the GSK3/SHAGGY-like kinase is a highly conserved serine/threonine kinase implicated in many signaling pathways in eukaryotes, the lack of knowledge of its three-dimensional (3D) structure has hindered efforts to understand the binding specificities of substrate and catalytic mechanism. To understand the structure-activity relationships, the protein 3D structure was built by using homology modeling based on the known X-ray diffraction structure of Glycogen synthase kinase-3beta (Gsk3beta) and the model structure was further refined using unrestrained molecular dynamics simulations. The research indicates that the general 3D organization of the GSK3/SHAGGY-like kinase is a typical kinase family and comprises an N-terminal domain of beta-sheet and a larger C-terminal domain mainly constituted by alpha-helix. In order to understand the molecular interactions between the natural substrate-ATP and GSK3/SHAGGY-like kinase, a 3D model of the complex ATP-GSK3/SHAGGY-like kinase is developed by molecular docking program, which is helpful to guide the experimental realization and the new mutant designs as well. One important finding is that the identification of the key binding-site residue of Lys69 which plays an important role in the catalysis of GSK3/SHAGGY-like kinase and this is in consistent with experimental observation.

Sequential antagonism of early and late Wnt-signaling by zebrafish colgate promotes dorsal and anterior fates

The establishment of the vertebrate body plan involves patterning of the ectoderm, mesoderm, and endoderm along the dorsoventral and antero-posterior axes. Interactions among numerous signaling molecules from several multigene families, including Wnts, have been implicated in regulating these processes. Here we provide evidence that the zebrafish colgate(b382) (col) mutation results in increased Wnt signaling that leads to defects in dorsal and anterior development. col mutants display early defects in dorsoventral patterning manifested by a decrease in the expression of dorsal shield-specific markers and ectopic expression of ventrolaterally expressed genes during gastrulation. In addition to these early patterning defects, col mutants display a striking regional posteriorization within the neuroectoderm, resulting in a reduction in anterior fates and an expansion of posterior fates within the forebrain and midbrain-hindbrain regions. We are able to correlate these phenotypes to the overactivation of Wnt signaling in col mutants. The early dorsal and anterior patterning phenotypes of the col mutant embryos are selectively rescued by inactivation of Wnt8 function by morpholino translational interference. In contrast, the regionalized neuroectoderm posterioriorization phenotype is selectively rescued by morpholino-mediated inactivation of Wnt8b. These results suggest that col-mediated antagonism of early and late Wnt-signaling activity during gastrulation is normally required sequentially for both early dorsoventral patterning and the specification and patterning of regional fates within the anterior neuroectoderm.

Progesterone and insulin stimulation of CPEB-dependent polyadenylation is regulated by Aurora A and glycogen synthase kinase-3

Progesterone stimulation of Xenopus oocyte maturation requires the cytoplasmic polyadenylation-induced translation of mos and cyclin B mRNAs. One cis element that drives polyadenylation is the CPE, which is bound by the protein CPEB. Polyadenylation is stimulated by Aurora A (Eg2)-catalyzed CPEB serine 174 phosphorylation, which occurs soon after oocytes are exposed to progesterone. Here, we show that insulin also stimulates Aurora A-catalyzed CPEB S174 phosphorylation, cytoplasmic polyadenylation, translation, and oocyte maturation. However, these insulin-induced events are uniquely controlled by PI3 kinase and PKC-zeta, which act upstream of Aurora A. The intersection of the progesterone and insulin signaling pathways occurs at glycogen synthase kinase 3 (GSK-3), which regulates the activity of Aurora A. GSK-3 and Aurora A interact in vivo, and overexpressed GSK-3 inhibits Aurora A-catalyzed CPEB phosphorylation. In vitro, GSK-3 phosphorylates Aurora A on S290/291, the result of which is an autophosphorylation of serine 349. GSK-3 phosphorylated Aurora A, or Aurora A proteins with S290/291D or S349D mutations, have reduced or no capacity to phosphorylate CPEB. Conversely, Aurora A proteins with S290/291A or S349A mutations are constitutively active. These results suggest that the progesterone and insulin stimulate maturation by inhibiting GSK-3, which allows Aurora A activation and CPEB-mediated translation.

Thyroid-stimulating hormone/cAMP and glycogen synthase kinase 3beta elicit opposing effects on Rap1GAP stability

Beyond regulating Rap activity, little is known regarding the regulation and function of the Rap GTPase-activating protein Rap1GAP. Tuberin and E6TP1 protein levels are tightly regulated through ubiquitin-mediated proteolysis. A role for these RapGAPs, along with SPA-1, as tumor suppressors has been demonstrated. Whether Rap1GAP performs a similar role was investigated. We now report that Rap1GAP protein levels are dynamically regulated in thyroid-stimulating hormone (TSH)-dependent thyroid cells. Upon TSH withdrawal, Rap1GAP undergoes a net increase in phosphorylation followed by proteasome-mediated degradation. Sequence analysis identified two putative destruction boxes in the Rap1GAP C-terminal domain. Glycogen synthase kinase 3beta (GSK3beta) phosphorylated Rap1GAP immunoprecipitated from thyroid cells, and GSK3beta inhibitors prevented phosphorylation and degradation of endogenous Rap1GAP. Co-expression of GSK3beta and Rap1GAP in human embryonic kidney 293 cells stimulated proteasome-dependent Rap1GAP turnover. Mutational analysis established a role for serine 525 in the regulation of Rap1GAP stability. Overexpression of Rap1GAP in thyroid cells impaired TSH/cAMP-stimulated p70S6 kinase activity and cell proliferation. These data are the first to show that Rap1GAP protein levels are tightly regulated and are the first to support a role for Rap1GAP as a tumor suppressor.

Cdc42 regulates GSK-3beta and adenomatous polyposis coli to control cell polarity

Cell polarity is a fundamental property of all cells. In higher eukaryotes, the small GTPase Cdc42, acting through a Par6-atypical protein kinase C (aPKC) complex, is required to establish cellular asymmetry during epithelial morphogenesis, asymmetric cell division and directed cell migration. However, little is known about what lies downstream of this complex. Here we show, through the use of primary rat astrocytes in a cell migration assay, that Par6-PKCzeta interacts directly with and regulates glycogen synthase kinase-3beta (GSK-3beta) to promote polarization of the centrosome and to control the direction of cell protrusion. Cdc42-dependent phosphorylation of GSK-3beta occurs specifically at the leading edge of migrating cells, and induces the interaction of adenomatous polyposis coli (Apc) protein with the plus ends of microtubules. The association of Apc with microtubules is essential for cell polarization. We conclude that Cdc42 regulates cell polarity through the spatial regulation of GSK-3beta and Apc. This role for Apc may contribute to its tumour-suppressor activity.

Endostatin is a potential inhibitor of Wnt signaling

Endostatin (ES) is a fragment of collagen XVIII that possesses antiangiogenic activity. To gain insight into ES-mediated signaling, we studied the effects of ES RNA on Xenopus embryogenesis and observed developmental abnormalities consistent with impaired Wnt signaling. ES RNA blocked the axis duplication induced by beta-catenin, partially suppressed Wnt-dependent transcription, and stimulated degradation of both wild-type and "stabilized" forms of beta-catenin, the latter suggesting that ES signaling does not involve glycogen synthase kinase 3. Moreover, ES uses a pathway independent of the Siah1 protein in targeting beta-catenin for proteasome-mediated degradation. ES failed to suppress the effects of T cell-specific factor (TCF)-VP16 (TVP), a constitutive downstream transcriptional activator that acts independently of beta-catenin. Importantly, these data were replicated in endothelial cells and also in the DLD-1 colon carcinoma cells with the mutated adenomatous polyposis coli protein. Finally, suppression of endothelial cell migration and inhibition of cell cycle by ES were reversed by TVP. Though high levels of ES were used in both the Xenopus and endothelial cell studies and the effects on beta-catenin signaling were modest, these data argue that at pharmacological concentrations ES may impinge on Wnt signaling and promote beta-catenin degradation.

Expression and characterization of GSK-3 mutants and their effect on beta-catenin phosphorylation in intact cells

Glycogen synthase kinase-3 (GSK-3) is a serine-threonine kinase that is involved in multiple cellular signaling pathways, including the Wnt signaling cascade where it phosphorylates beta-catenin, thus targeting it for proteasome-mediated degradation. Unlike phosphorylation of glycogen synthase, phosphorylation of beta-catenin by GSK-3 does not require priming in vitro, i.e. it is not dependent on the presence of a phosphoserine, four residues C-terminal to the GSK-3 phosphorylation site. Recently, a means of dissecting GSK-3 activity toward primed and non-primed substrates has been made possible by identification of the R96A mutant of GSK-3beta. This mutant is unable to phosphorylate primed but can still phosphorylate unprimed substrates (Frame, S., Cohen, P., and Biondi R. M. (2001) Mol. Cell 7, 1321-1327). Here we have investigated whether phosphorylation of Ser(33), Ser(37), and Thr(41) in beta-catenin requires priming through prior phosphorylation at Ser(45) in intact cells. We have shown that the Arg(96) mutant does not induce beta-catenin degradation but instead stabilizes beta-catenin, indicating that it is unable to phosphorylate beta-catenin in intact cells. Furthermore, if Ser(45) in beta-catenin is mutated to Ala, beta-catenin is markedly stabilized, and phosphorylation of Ser(33), Ser(37), and Thr(41) in beta-catenin by wild type GSK-3beta is prevented in intact cells. In addition, we have shown that the L128A mutant, which is deficient in phosphorylating Axin in vitro, is still able to phosphorylate beta-catenin in intact cells although it has reduced activity. Mutation of Tyr(216) to Phe markedly reduces the ability of GSK-3beta to phosphorylate and down-regulate beta-catenin. In conclusion, we have found that the Arg(96) mutant has a dominant-negative effect on GSK-3beta-dependent phosphorylation of beta-catenin and that targeting of beta-catenin for degradation requires prior priming through phosphorylation of Ser(45).

Glycogen synthase kinase-3 beta mutagenesis identifies a common binding domain for GBP and Axin

Glycogen synthase kinase-3 beta (GSK-3) is a key downstream target of Wnt signaling and is regulated by its interactions with activating and inhibitory proteins. We and others have shown that GSK-3 activity toward non-primed substrates is regulated in part through a competition between its activating (Axin) and inhibitory (GBP/FRAT) binding partners. Here we use a reverse two-hybrid screen to identify mutations in GSK-3 that alter binding to GBP and Axin. We find that these mutations overlap and propose that GBP and Axin compete for binding to the same region of GSK-3. We use these mutations to examine the ability of GSK-3 to block eye development in Xenopus embryos and suggest that GSK-3 regulates eye development through a non-Wnt pathway.

An inactive pool of GSK-3 at the leading edge of growth cones is implicated in Semaphorin 3A signaling

Glycogen synthase kinase (GSK)-3 is a serine/threonine kinase that has been implicated in several aspects in embryonic development and several growth factor signaling cascades. We now report that an inactive phosphorylated pool of the enzyme colocalizes with F-actin in both neuronal and nonneuronal cells. Semaphorin 3A (Sema 3A), a molecule that inhibits axonal growth, activates GSK-3 at the leading edge of neuronal growth cones and in Sema 3A-responsive human breast cancer cells, suggesting that GSK-3 activity might play a role in coupling Sema 3A signaling to changes in cell motility. We show that three different GSK-3 antagonists (LiCl, SB-216763, and SB-415286) can inhibit the growth cone collapse response induced by Sema 3A. These studies reveal a novel compartmentalization of inactive GSK-3 in cells and demonstrate for the first time a requirement for GSK-3 activity in the Sema 3A signal transduction pathway.

Posttranslational mechanisms control the timing of bHLH function and regulate retinal cell fate

During central nervous system development, neurons are often born in a precise temporal sequence. Basic helix-loop-helix (bHLH) transcription factors are required for the development of specific subpopulations of neurons, but how they contribute to their ordered genesis is unclear. We show that the ability of bHLH factors to regulate the development of distinct neuronal subtypes in the Xenopus retina depends upon the timing of their function. In addition, we find that the timing of bHLH function can be regulated posttranslationally, so that bHLH factors with overlapping expression can function independently. Specifically, XNeuroD function in the retina can be inhibited by glycogen synthase kinase 3beta (GSK3beta), while Xath5 function can be inhibited by Notch. Thus, the potential of bHLH factors to regulate the development of neuronal subtypes depends upon the context in which they function.

Convergence of multiple signaling cascades at glycogen synthase kinase 3: Edg receptor-mediated phosphorylation and inactivation by lysophosphatidic acid through a protein kinase C-dependent intracellular pathway

Lysophosphatidic acid (LPA) is a natural phospholipid with multiple biological functions. We show here that LPA induces phosphorylation and inactivation of glycogen synthase kinase 3 (GSK-3), a multifunctional serine/threonine kinase. The effect of LPA can be reconstituted by expression of Edg-4 or Edg-7 in cells lacking LPA responses. Compared to insulin, LPA stimulates only modest phosphatidylinositol 3-kinase (PI3K)-dependent activation of protein kinase B (PKB/Akt) that does not correlate with the magnitude of GSK-3 phosphorylation induced by LPA. PI3K inhibitors block insulin- but not LPA-induced GSK-3 phosphorylation. In contrast, the effect of LPA, but not that of insulin or platelet-derived growth factor (PDGF), is sensitive to protein kinase C (PKC) inhibitors. Downregulation of endogenous PKC activity selectively reduces LPA-mediated GSK-3 phosphorylation. Furthermore, several PKC isotypes phosphorylate GSK-3 in vitro and in vivo. To confirm a specific role for PKC in regulation of GSK-3, we further studied signaling properties of PDGF receptor beta subunit (PDGFRbeta) in HEK293 cells lacking endogenous PDGF receptors. In clones expressing a PDGFRbeta mutant wherein the residues that couple to PI3K and other signaling functions are mutated with the link to phospholipase Cgamma (PLCgamma) left intact, PDGF is fully capable of stimulating GSK-3 phosphorylation. The process is sensitive to PKC inhibitors in contrast to the response through the wild-type PDGFRbeta. Therefore, growth factors, such as PDGF, which control GSK-3 mainly through the PI3K-PKB/Akt module, possess the ability to regulate GSK-3 through an alternative, redundant PLCgamma-PKC pathway. LPA and potentially other natural ligands primarily utilize a PKC-dependent pathway to modulate GSK-3.

Control of beta-catenin phosphorylation/degradation by a dual-kinase mechanism

Wnt regulation of beta-catenin degradation is essential for development and carcinogenesis. beta-catenin degradation is initiated upon amino-terminal serine/threonine phosphorylation, which is believed to be performed by glycogen synthase kinase-3 (GSK-3) in complex with tumor suppressor proteins Axin and adnomatous polyposis coli (APC). Here we describe another Axin-associated kinase, whose phosphorylation of beta-catenin precedes and is required for subsequent GSK-3 phosphorylation of beta-catenin. This "priming" kinase is casein kinase Ialpha (CKIalpha). Depletion of CKIalpha inhibits beta-catenin phosphorylation and degradation and causes abnormal embryogenesis associated with excessive Wnt/beta-catenin signaling. Our study uncovers distinct roles and steps of beta-catenin phosphorylation, identifies CKIalpha as a component in Wnt/beta-catenin signaling, and has implications to pathogenesis/therapeutics of human cancers and diabetes.