Failure of human rhombic lip differentiation underlies medulloblastoma formation

Medulloblastoma (MB) comprises a group of heterogeneous paediatric embryonal neoplasms of the hindbrain with strong links to early development of the hindbrain^1-4. Mutations that activate Sonic hedgehog signalling lead to Sonic hedgehog MB in the upper rhombic lip (RL) granule cell lineage^5-8. By contrast, mutations that activate WNT signalling lead to WNT MB in the lower RL^9,10. However, little is known about the more commonly occurring group 4 (G4) MB, which is thought to arise in the unipolar brush cell lineage^3,4. Here we demonstrate that somatic mutations that cause G4 MB converge on the core binding factor alpha (CBFA) complex and mutually exclusive alterations that affect CBFA2T2, CBFA2T3, PRDM6, UTX and OTX2. CBFA2T2 is expressed early in the progenitor cells of the cerebellar RL subventricular zone in Homo sapiens, and G4 MB transcriptionally resembles these progenitors but are stalled in developmental time. Knockdown of OTX2 in model systems relieves this differentiation blockade, which allows MB cells to spontaneously proceed along normal developmental differentiation trajectories. The specific nature of the split human RL, which is destined to generate most of the neurons in the human brain, and its high level of susceptible EOMES⁺KI67⁺ unipolar brush cell progenitor cells probably predisposes our species to the development of G4 MB.

Transcatheter aortic valve implantation (TAVI) for treatment of aortic valve stenosis: an evidence update

BACKGROUND

One-year mortality outcomes in the PARTNER trial showed that transcatheter aortic valve implantation (TAVI) was noninferior to surgical aortic valve replacement (sAVR) in patients who were eligible for sAVR (cohort A), and superior to standard treatment in patients who were ineligible for sAVR (cohort B).

OBJECTIVE

To update a previous report on the safety, effectiveness, and cost-effectiveness of TAVI, published in 2012.

DATA SOURCES

A literature search was performed on September 11, 2012, using OVID MEDLINE, OVID MEDLINE In-Process and Other Non-Indexed Citations, OVID EMBASE, EBSCO Cumulative Index to Nursing and Allied Health Literature (CINAHL), the Wiley Cochrane Library, and the Centre for Reviews and Dissemination database, for studies published from January 1, 2011, until September 11, 2012.

REVIEW METHODS

Randomized controlled trials investigating TAVI in comparison to sAVR or standard treatment were included for analysis. Results were summarized descriptively.

RESULTS

At 2-year follow-up, mortality in cohort A was similar between the TAVI and sAVR groups. Rates of stroke/transient ischemic attack, major vascular complications, and moderate/severe paravalvular aortic regurgitation were significantly higher in the TAVI group, but rate of major bleeding was significantly higher in the sAVR group. Mortality in cohort B was significantly lower with transfemoral (TF) TAVI than with standard treatment, but rate of stroke was significantly higher with TF TAVI. TF TAVI resulted in a more rapid improvement in quality of life scores than sAVR, but this difference was not sustained at 6 and 12 months. Patients who underwent transapical TAVI did not have a greater early improvement in quality of life compared to sAVR patients. Compared to standard treatment, TF TAVI resulted in a greater improvement in quality of life scores at all time points. Incremental cost-effectiveness ratios were in favour of TAVI for inoperable patients in the base-case analysis, but varied widely for operable patients.

CONCLUSIONS

The findings of the 2-year follow-up with respect to mortality and adverse events were consistent with those of the 1-year follow-up. TAVI was also associated with improvement in quality of life, although results varied by cohort. Consistent with the 2012 report, TAVI may be cost-effective for patients who are not candidates for surgery.

Transcatheter aortic valve implantation (TAVI) for treatment of aortic valve stenosis: an evidence-based Analysis (part B)

BACKGROUND

Transcatheter aortic valve implantation (TAVI) has become an alternative to surgical aortic valve replacement (sAVR) for patients at high risk for surgery.

OBJECTIVE

To evaluate the safety, effectiveness, and cost-effectiveness of TAVI for treatment of aortic valve stenosis in symptomatic older adults.

REVIEW METHODS

A literature search was performed on September 6, 2011, for studies published from January 1, 2007, to September 6, 2011. A combined decision tree and Markov model was developed to compare costs, life years, and quality-adjusted life-years (QALYs) of all treatment options in their respective patient populations over a 20-year time horizon.

RESULTS

Two studies from the PARTNER trial were identified. The first study compared TAVI to sAVR in patients who were candidates for sAVR. The second study compared TAVI to standard treatment in patients who were not eligible for sAVR. The first study showed that TAVI and sAVR had similar mortality rates at 1 year. The second study showed a significant improvement in patient survival in those undergoing TAVI. However, in both studies, the TAVI group had significantly higher rates of stroke/transient ischemic attack, and major vascular complications. Rates of major bleeding were significantly higher in sAVR group in the first study and significantly higher in TAVI group in the second study. The base-case cost-effectiveness of TAVI was $48,912 per QALY, but the incremental cost-effectiveness ratio ranged from $36,000 to $291,000 per QALY depending on the assumptions made in the longer-term prediction portion of the model (i.e., beyond the follow-up period of the PARTNER trial).

CONCLUSIONS

TAVI improves survival in patients who cannot undergo surgery. For those who are candidates for surgery, TAVI has a mortality rate similar to sAVR, but it is associated with significant adverse effects. TAVI may be cost-effective for patients who cannot undergo surgery, but is not cost-effective for patients who can.

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.

Glycogen synthase kinase-3 in insulin and Wnt signalling: a double-edged sword?

Glycogen synthase kinase-3 is an unusual protein serine/threonine kinase that, unlike most of its 500-odd relatives in the genome, is active under resting conditions and is inactivated upon cell stimulation. The two mammalian isoforms, GSK-3alpha and beta, play largely overlapping roles and have been implicated in a variety of human pathologies, including Type II diabetes, Alzheimer's disease, bipolar disorder and cancer. Recently, the modes of regulation of this enzyme have been elucidated through a combination of structural and cell biological studies. A series of relatively selective small molecules have facilitated chemical manipulation of the enzyme in intact cells and tissues, and new roles for the protein kinase in embryonic stem cell differentiation and motility have emerged. Despite these advances, the therapeutic value of this enzyme as a drug target remains clouded by uncertainty over the potential of antagonists to promote tumorigenesis. This article describes the state of understanding of this intriguing enzyme, and weighs current evidence regarding whether there is a therapeutic window for amelioration of diseases in which it is implicated, in the absence of inducing new pathologies.