A group 3 medulloblastoma stem cell program is maintained by OTX2-mediated alternative splicing.

OTX2 is a transcription factor and known driver in medulloblastoma (MB), where it is amplified in a subset of tumours and overexpressed in most cases of group 3 and group 4 MB. Here we demonstrate a noncanonical role for OTX2 in group 3 MB alternative splicing. OTX2 associates with the large assembly of splicing regulators complex through protein-protein interactions and regulates a stem cell splicing program. OTX2 can directly or indirectly bind RNA and this may be partially independent of its DNA regulatory functions. OTX2 controls a pro-tumorigenic splicing program that is mirrored in human cerebellar rhombic lip origins. Among the OTX2-regulated differentially spliced genes, PPHLN1 is expressed in the most primitive rhombic lip stem cells, and targeting PPHLN1 splicing reduces tumour growth and enhances survival in vivo. These findings identify OTX2-mediated alternative splicing as a major determinant of cell fate decisions that drive group 3 MB progression.

Impaired OTUD7A-dependent Ankyrin regulation mediates neuronal dysfunction in mouse and human models of the 15q13.3 microdeletion syndrome.

Copy number variations (CNVs) are associated with psychiatric and neurodevelopmental disorders (NDDs), and most, including the recurrent 15q13.3 microdeletion disorder, have unknown disease mechanisms. We used a heterozygous 15q13.3 microdeletion mouse model and patient iPSC-derived neurons to reveal developmental defects in neuronal maturation and network activity. To identify the underlying molecular dysfunction, we developed a neuron-specific proximity-labeling proteomics (BioID2) pipeline, combined with patient mutations, to target the 15q13.3 CNV genetic driver OTUD7A. OTUD7A is an emerging independent NDD risk gene with no known function in the brain, but has putative deubiquitinase function. The OTUD7A protein-protein interaction network included synaptic, axonal, and cytoskeletal proteins and was enriched for ASD and epilepsy risk genes (Ank3, Ank2, SPTAN1, SPTBN1). The interactions between OTUD7A and Ankyrin-G (Ank3) and Ankyrin-B (Ank2) were disrupted by an epilepsy-associated OTUD7A L233F variant. Further investigation of Ankyrin-G in mouse and human 15q13.3 microdeletion and OTUD7AL233F/L233F models revealed protein instability, increased polyubiquitination, and decreased levels in the axon initial segment, while structured illumination microscopy identified reduced Ankyrin-G nanodomains in dendritic spines. Functional analysis of human 15q13.3 microdeletion and OTUD7AL233F/L233F models revealed shared and distinct impairments to axonal growth and intrinsic excitability. Importantly, restoring OTUD7A or Ankyrin-G expression in 15q13.3 microdeletion neurons led to a reversal of abnormalities. These data reveal a critical OTUD7A-Ankyrin pathway in neuronal development, which is impaired in the 15q13.3 microdeletion syndrome, leading to neuronal dysfunction. Furthermore, our study highlights the utility of targeting CNV genes using cell type-specific proteomics to identify shared and unexplored disease mechanisms across NDDs.

BMI1 is a therapeutic target in recurrent medulloblastoma.

Medulloblastoma (MB) is the most frequent malignant pediatric brain tumor, representing 20% of newly diagnosed childhood central nervous system malignancies. Although advances in multimodal therapy yielded a 5-year survivorship of 80%, MB still accounts for the leading cause of childhood cancer mortality. In this work, we describe the epigenetic regulator BMI1 as a novel therapeutic target for the treatment of recurrent human Group 3 MB, a childhood brain tumor for which there is virtually no treatment option beyond palliation. Current clinical trials for recurrent MB patients based on genomic profiles of primary, treatment-naive tumors will provide limited clinical benefit since recurrent metastatic MBs are highly genetically divergent from their primary tumor. Using a small molecule inhibitor against BMI1, PTC-028, we were able to demonstrate complete ablation of self-renewal of MB stem cells in vitro. When administered to mice xenografted with patient tumors, we observed significant reduction in tumor burden in both local and metastatic compartments and subsequent increased survival, without neurotoxicity. Strikingly, serial in vivo re-transplantation assays demonstrated a marked reduction in tumor initiation ability of recurrent MB cells upon re-transplantation of PTC-028-treated cells into secondary recipient mouse brains. As Group 3 MB is often metastatic and uniformly fatal at recurrence, with no current or planned trials of targeted therapy, an efficacious targeted agent would be rapidly transitioned to clinical trials.

OTUD7A Regulates Neurodevelopmental Phenotypes in the 15q13.3 Microdeletion Syndrome.

Copy-number variations (CNVs) are strong risk factors for neurodevelopmental and psychiatric disorders. The 15q13.3 microdeletion syndrome region contains up to ten genes and is associated with numerous conditions, including autism spectrum disorder (ASD), epilepsy, schizophrenia, and intellectual disability; however, the mechanisms underlying the pathogenesis of 15q13.3 microdeletion syndrome remain unknown. We combined whole-genome sequencing, human brain gene expression (proteome and transcriptome), and a mouse model with a syntenic heterozygous deletion (Df(h15q13)/+ mice) and determined that the microdeletion results in abnormal development of cortical dendritic spines and dendrite outgrowth. Analysis of large-scale genomic, transcriptomic, and proteomic data identified OTUD7A as a critical gene for brain function. OTUD7A was found to localize to dendritic and spine compartments in cortical neurons, and its reduced levels in Df(h15q13)/+ cortical neurons contributed to the dendritic spine and dendrite outgrowth deficits. Our results reveal OTUD7A as a major regulatory gene for 15q13.3 microdeletion syndrome phenotypes that contribute to the disease mechanism through abnormal cortical neuron morphological development.

IL-17 receptor signaling inhibits C/EBPbeta by sequential phosphorylation of the regulatory 2 domain.

Interleukin-17 (IL-17), the hallmark cytokine of T helper 17 (T(H)17) cells, signals through a distinct receptor subclass, yet little is known about the mechanisms involved. IL-17 activates the expression of target genes through the actions of the transcription factors nuclear factor kappaB (NF-kappaB), CAAT enhancer binding protein delta (C/EBPdelta), and C/EBPbeta. The adaptor proteins tumor necrosis factor receptor-associated factor 6 (TRAF6) and Act1 are upstream of NF-kappaB and C/EBPdelta, but the regulation of C/EBPbeta remains undefined. Here, we show that IL-17 signaling led to phosphorylation of two sites in the regulatory 2 domain of C/EBPbeta in a sequential, interdependent fashion. The first was rapid and dependent on extracellular signal-regulated kinase (ERK), whereas the second was dependent on the activity of glycogen synthase kinase 3beta (GSK-3beta). These pathways were mediated by distinct subdomains within IL-17 receptor A (IL-17RA). Whereas phosphorylation of threonine 188 (Thr188) was mediated by the previously identified SEF/IL-17R homology domain-Toll-IL-1R-like loop (SEFIR-TILL), phosphorylation of Thr179 occurred through a newly characterized motif located in the distal tail of IL-17RA. Phosphorylated C/EBPbeta mediated a negative signal, because blocking ERK and GSK-3beta increased expression of IL-17 target genes and a C/EBPbeta-Thr188 mutant enhanced activation of a C/EBP-dependent reporter. Overexpression of GSK-3beta inhibited IL-17-induced activation of a C/EBP-dependent reporter, and Thr179 of C/EBPbeta was not phosphorylated in GSK-3beta-deficient cells. Thus, IL-17 triggered the dual phosphorylation of C/EBPbeta, which inhibited the expression of proinflammatory genes. This detailed dissection is the first for the IL-17-mediated C/EBP pathway and the first known example of a negative signal mediated by IL-17RA.

Prevention of amino acid conversion in SILAC experiments with embryonic stem cells.

Recent studies using stable isotope labeling with amino acids in culture (SILAC) in quantitative proteomics have made mention of the problematic conversion of isotope-coded arginine to proline in cells. The resulting converted proline peptide divides the heavy peptide ion signal causing inaccuracy when compared with the light peptide ion signal. This is of particular concern as it can effect up to half of all peptides in a proteomic experiment. Strategies to both compensate for and limit the inadvertent conversion have been demonstrated, but none have been shown to prevent it. Additionally, these methods combined with SILAC labeling in general have proven problematic in their large scale application to sensitive cell types including embryonic stem cells (ESCs) from the mouse and human. Here, we show that by providing as little as 200 mg/liter L-proline in SILAC media, the conversion of arginine to proline can be rendered completely undetectable. At the same time, there was no compromise in labeling with isotope-coded arginine, indicating there is no observable back conversion from the proline supplement. As a result, when supplemented with proline, correct interpretation of "light" and "heavy" peptide ratios could be achieved even in the worst cases of conversion. By extending these principles to ESC culture protocols and reagents we were able to routinely SILAC label both mouse and human ESCs in the absence of feeder cells and without compromising the pluripotent phenotype. This study provides the simplest protocol to prevent proline artifacts in SILAC labeling experiments with arginine. Moreover, it presents a robust, feeder cell-free, protocol for performing SILAC experiments on ESCs from both the mouse and the human.

Initiation of Wnt signaling: control of Wnt coreceptor Lrp6 phosphorylation/activation via frizzled, dishevelled and axin functions.

Canonical Wnt/beta-catenin signaling has central roles in development and diseases, and is initiated by the action of the frizzled (Fz) receptor, its coreceptor LDL receptor-related protein 6 (Lrp6), and the cytoplasmic dishevelled (Dvl) protein. The functional relationships among Fz, Lrp6 and Dvl have long been enigmatic. We demonstrated previously that Wnt-induced Lrp6 phosphorylation via glycogen synthase kinase 3 (Gsk3) initiates Wnt/beta-catenin signaling. Here we show that both Fz and Dvl functions are critical for Wnt-induced Lrp6 phosphorylation through Fz-Lrp6 interaction. We also show that axin, a key scaffolding protein in the Wnt pathway, is required for Lrp6 phosphorylation via its ability to recruit Gsk3, and inhibition of Gsk3 at the plasma membrane blocks Wnt/beta-catenin signaling. Our results suggest a model that upon Wnt-induced Fz-Lrp6 complex formation, Fz recruitment of Dvl in turn recruits the axin-Gsk3 complex, thereby promoting Lrp6 phosphorylation to initiate beta-catenin signaling. We discuss the dual roles of the axin-Gsk3 complex and signal amplification by Lrp6-axin interaction during Wnt/beta-catenin signaling.

R-spondin1 is a high affinity ligand for LRP6 and induces LRP6 phosphorylation and beta-catenin signaling.

R-spondin proteins are newly identified secreted molecules that activate beta-catenin signaling. However, the mechanism of R-spondin action and its relationship with Wnt signaling remain unclear. Here we show that human R-spondin1 (hRspo1) is a high affinity ligand for the Wnt co-receptor LRP6 (K(d) = 1.2 nm). hRspo1 induces glycogen synthase kinase 3-dependent phosphorylation and activation of LRP6. DKK1, an LRP6 antagonist, inhibits hRspo1-induced LRP6 phosphorylation. We further demonstrate that hRspo1 synergizes with Frizzled5 in Xenopus axis induction assays and induces the phosphorylation of Dishevelled, a cytoplasmic component downstream of Frizzled function. Our study reveals interesting similarity and distinction between Wnt and R-spondin signaling.

Glycogen synthase kinase 3, circadian rhythms, and bipolar disorder: a molecular link in the therapeutic action of lithium.

BACKGROUND: Bipolar disorder (BPD) is a widespread condition characterized by recurring states of mania and depression. Lithium, a direct inhibitor of glycogen synthase kinase 3 (GSK3) activity, and a mainstay in BPD therapeutics, has been proposed to target GSK3 as a mechanism of mood stabilization. In addition to mood imbalances, patients with BPD often suffer from circadian disturbances. GSK3, an essential kinase with widespread roles in development, cell survival, and metabolism has been demonstrated to be an essential component of the Drosophila circadian clock. We sought to investigate the role of GSK3 in the mammalian clock mechanism, as a possible mediator of lithium's therapeutic effects. METHODS: GSK3 activity was decreased in mouse embryonic fibroblasts (MEFs) genetically and pharmacologically, and changes in the cyclical expression of core clock genes--mPer2 in particular--were examined. RESULTS: We demonstrate that genetic depletion of GSK3 in synchronized oscillating MEFs results in a significant delay in the periodicity of the endogenous clock mechanism, particularly in the cycling period of mPer2. Furthermore, we demonstrate that pharmacological inhibition of GSK3 activity by kenpaullone, a known antagonist of GSK3 activity, as well as by lithium, a direct inhibitor of GSK3 and the most common treatment for BPD, induces a phase delay in mPer2 transcription that resembles the effect observed with GSK3 knockdown. CONCLUSION: These results confirm GSK3 as a plausible target of lithium action in BPD therapeutics, and suggest the circadian clock mechanism as a significant modulator of lithium's clinical benefits.

A dual-kinase mechanism for Wnt co-receptor phosphorylation and activation.

Signalling by the Wnt family of secreted lipoproteins has essential functions in development and disease. The canonical Wnt/beta-catenin pathway requires a single-span transmembrane receptor, low-density lipoprotein (LDL)-receptor-related protein 6 (LRP6), whose phosphorylation at multiple PPPSP motifs is induced upon stimulation by Wnt and is critical for signal transduction. The kinase responsible for LRP6 phosphorylation has not been identified. Here we provide biochemical and genetic evidence for a 'dual-kinase' mechanism for LRP6 phosphorylation and activation. Glycogen synthase kinase 3 (GSK3), which is known for its inhibitory role in Wnt signalling through the promotion of beta-catenin phosphorylation and degradation, mediates the phosphorylation and activation of LRP6. We show that Wnt induces sequential phosphorylation of LRP6 by GSK3 and casein kinase 1, and this dual phosphorylation promotes the engagement of LRP6 with the scaffolding protein Axin. We show further that a membrane-associated form of GSK3, in contrast with cytosolic GSK3, stimulates Wnt signalling and Xenopus axis duplication. Our results identify two key kinases mediating Wnt co-receptor activation, reveal an unexpected and intricate logic of Wnt/beta-catenin signalling, and illustrate GSK3 as a genuine switch that dictates both on and off states of a pivotal regulatory pathway.

Inhibition of TGFbeta signaling potentiates the FGF-2-induced stimulation of cardiomyocyte DNA synthesis.

OBJECTIVE: Added transforming growth factor beta (TGFbeta) inhibits the proliferation of immature cardiomyocytes. We have now examined the hypothesis that suppression of endogenous TGFbeta signaling will boost the proliferative response (DNA synthesis) of cardiac myocytes to serum and/or to the mitogenic factor fibroblast growth factor-2 (FGF-2). METHODS AND RESULTS: Overexpression of a kinase-deficient TGFbeta type II receptor (TGFbetaRIIDeltaKD) resulted in a 2.8-fold increase in cardiomyocyte DNA synthesis in serum-rich cultures, an effect requiring active FGFR-1 since it was not observed in the presence of excess kinase-deficient FGFR-1. This finding suggested that endogenous TGFbeta-TGFbetaRII suppressed endogenous FGFR-1-mediated signals that stimulate or are permissive for DNA synthesis. TGFbeta had no effect, however, on the FGF-2-induced acute stimulation of extracellular signal regulated kinase1/2. FGF-2, added in the absence or presence of TGFbeta inhibition, elicited a 3- or a 13-fold stimulation of DNA synthesis, respectively, pointing to a synergistic effect. CONCLUSION: Inhibition of TGFbetaRII-transduced signaling upregulates the proliferative response of cardiomyocytes to serum, and greatly potentiates the stimulatory effect of FGF-2. A combinatorial strategy including activation of FGF-2 and inhibition of TGFbeta-triggered signal transduction may be required for maximal stimulation of immature cardiomyocyte DNA synthesis.