Regulation of phosphatidylinositol kinases and metabolism by Wnt3a and Dvl

PI(4,5)P2-stimulated positive feedback drives the recruitment of Dishevelled to Frizzled in Wnt-β-catenin signaling

In the Wnt-β-catenin pathway, Wnt binding to Frizzled (Fzd) and LRP5 or LRP6 (LRP5/6) co-receptors inhibits the degradation of the transcriptional coactivator β-catenin by recruiting the cytosolic effector Dishevelled (Dvl). Polymerization of Dvl at the plasma membrane recruits the β-catenin destruction complex, enabling the phosphorylation of LRP5/6, a key step in inhibiting β-catenin degradation. Using purified Fzd proteins reconstituted in lipid nanodiscs, we investigated the factors that promote the recruitment of Dvl to the plasma membrane. We found that the affinity of Fzd for Dvl was not affected by Wnt ligands, in contrast to other members of the GPCR superfamily for which the binding of extracellular ligands affects the affinity for downstream transducers. Instead, Fzd-Dvl binding was enhanced by increased concentration of the lipid PI(4,5)P₂, which is generated by Dvl-associated lipid kinases in response to Wnt and which is required for LRP5/6 phosphorylation. Moreover, binding to Fzd did not promote Dvl DEP domain dimerization, which has been proposed to be required for signaling downstream of Fzd. Our findings suggest a positive feedback loop in which Wnt-stimulated local PI(4,5)P₂ production enhances Dvl recruitment and further PI(4,5)P₂ production to support Dvl polymerization, LRP5/6 phosphorylation, and β-catenin stabilization.

Biophysical physiology of phosphoinositide rapid dynamics and regulation in living cells

Phosphoinositide membrane lipids are ubiquitous low-abundance signaling molecules. They direct many physiological processes that involve ion channels, membrane identification, fusion of membrane vesicles, and vesicular endocytosis. Pools of these lipids are continually broken down and refilled in living cells, and the rates of some of these reactions are strongly accelerated by physiological stimuli. Recent biophysical experiments described here measure and model the kinetics and regulation of these lipid signals in intact cells. Rapid on-line monitoring of phosphoinositide metabolism is made possible by optical tools and electrophysiology. The experiments reviewed here reveal that as for other cellular second messengers, the dynamic turnover and lifetimes of membrane phosphoinositides are measured in seconds, controlling and timing rapid physiological responses, and the signaling is under strong metabolic regulation. The underlying mechanisms of this metabolic regulation remain questions for the future.

High expression of PI4K2A predicted poor prognosis of colon adenocarcinoma (COAD) and correlated with immunity

BACKGROUND

PI4K2A has been found to have a tumor-promoting role in various solid tumors and be involved in various biological procedures. In this article, we aim to investigate the prognostic values of PI4K2A and provide new insights in colon adenocarcinoma (COAD).

METHODS

The Cancer Genome Atlas (TCGA) database, Human Protein Atlas online database, and UALCAN database were used to analyze the expression of PI4K2A in COAD and the survival of patients. Univariate and multifactorial Cox regression analyses were used to assess the prognosis of PI4K2A on COAD. GSEA was used to explore PI4K2A-related signaling pathways. In addition, the effect of PI4K2A on immune checkpoint inhibitors (ICIs) treatment was investigated by constructing a TIDE model and predicting the association between PI4K2A and anticancer drug sensitivity through the CellMiner database.

RESULTS

In the TCGA database, PI4K2A was highly expressed in COAD and the similar results were verified by qRT-PCR. Survival analysis, utilizing Kaplan-Meier curves, revealed that COAD patients with high PI4K2A expression had a worse prognosis. In addition, PI4K2A expression was discovered to have been associated with T-stage, N-stage, and pathological stage by logistic analysis. Next, we utilized univariate and multifactorial Cox regression analyses to identify PI4K2A as an independent predictor. Additionally, GSEA analysis indicates that PI4K2A is enriched in MAPK signaling pathway, Toll-like receptor signaling pathway, etc. In COAD, PI4K2A was remarkably associated with the tumor immune microenvironment. In addition, by constructing a TIDE model, we discovered that COAD patients in the PI4K2A low-expression cohort were better treated with ICI. Finally, analysis of the CellMiner database predicted that PI4K2A was adversely correlated with the sensitivity of various anticancer drugs.

CONCLUSIONS

Our study suggests that PI4K2A may be a potential predictor of poor prognosis in COAD and a potential biomarker for early diagnosis, prognosis, and treatment.

Dishevelled coordinates phosphoinositide kinases PI4KIIIα and PIP5KIγ for efficient PtdInsP2 synthesis

Phosphatidylinositol(4,5)-bisphosphate (PtdInsP2) is an important modulator of many cellular processes, and its abundance in the plasma membrane is closely regulated. We examined the hypothesis that members of the Dishevelled scaffolding protein family can bind the lipid kinases phosphatidylinositol 4-kinase (PI4K) and phosphatidylinositol 4-phosphate 5-kinase (PIP5K), facilitating synthesis of PtdInsP2 directly from phosphatidylinositol. We used several assays for PtdInsP2 to examine the cooperative function of phosphoinositide kinases and the Dishevelled protein Dvl3 in the context of two receptor signaling cascades. Simultaneous overexpression of PI4KIIIα (also known as PI4KA) and PIP5KIγ (also known as PIP5K1C) had a synergistic effect on PtdInsP2 synthesis that was recapitulated by overexpression of Dvl3. Increasing the activity of Dvl3 by overexpression increased resting plasma membrane PtdInsP2. Knockdown of Dvl3 reduced resting plasma membrane PtdInsP2 and slowed PtdInsP2 resynthesis following receptor activation. We confirm that Dvl3 promotes coupling of PI4KIIIα and PIP5KIγ and show that this interaction is essential for efficient resynthesis of PtdInsP2 following receptor activation.

Phospholipase Cγ1 represses colorectal cancer growth by inhibiting the Wnt/β-catenin signaling axis

As essential phospholipid signaling regulators, phospholipase C (PLC)s are activated by various extracellular ligands and mediate intracellular signal transduction. PLCγ1 is involved in regulating various cancer cell functions. However, the precise in vivo link between PLCγ1 and cancer behavior remains undefined. To investigate the role of PLCγ1 in colorectal carcinogenesis, we generated an intestinal tissue-specific Plcg1 knock out (KO) in adenomatous polyposis coli (Apc) Min/+ mice. Plcg1 deficiency in ApcMin/+ mice showed earlier death, with a higher colorectal tumor incidence in both number and size than in wild-type mice. Mechanistically, inhibition of PLCγ1 increased the levels of its substrate phosphoinositol 4,5-bisphosphate (PIP2) at the plasma membrane and promoted the activation of Wnt receptor low-density lipoprotein receptor-related protein 6 (LRP6) by glycogen synthase kinase 3β (GSK3β) to enhance β-catenin signaling. Enhanced cell proliferation and Wnt/β-catenin signaling were observed in colon tumors from Plcg1 KO mice. Furthermore, low PLCγ1 expression was associated with a poor prognosis of colon cancer patients. Collectively, we demonstrated the role of PLCγ1 in vivo as a tumor suppressor relationship between the regulation of the PIP2 level and Wnt/β-catenin-dependent intestinal tumor formation.

Control of Neuronal Excitability by Cell Surface Receptor Density and Phosphoinositide Metabolism

Phosphoinositides are members of a family of minor phospholipids that make up about 1% of all lipids in most cell types. Despite their low abundance they have been found to be essential regulators of neuronal activities such as action potential firing, release and re-uptake of neurotransmitters, and interaction of cytoskeletal proteins with the plasma membrane. Activation of several different neurotransmitter receptors can deplete phosphoinositide levels by more than 90% in seconds, thereby profoundly altering neuronal behavior; however, despite the physiological importance of this mechanism we still lack a profound quantitative understanding of the connection between phosphoinositide metabolism and neuronal activity. Here, we present a model that describes phosphoinositide metabolism and phosphoinositide-dependent action potential firing in sympathetic neurons. The model allows for a simulation of activation of muscarinic acetylcholine receptors and its effects on phosphoinositide levels and their regulation of action potential firing in these neurons. In this paper, we describe the characteristics of the model, its calibration to experimental data, and use the model to analyze how alterations of surface density of muscarinic acetylcholine receptors or altered activity levels of a key enzyme of phosphoinositide metabolism influence action potential firing of sympathetic neurons. In conclusion, the model provides a comprehensive framework describing the connection between muscarinic acetylcholine signaling, phosphoinositide metabolism, and action potential firing in sympathetic neurons which can be used to study the role of these signaling systems in health and disease.

Daam2 couples translocation and clustering of Wnt receptor signalosomes through Rac1

Wnt signaling plays a critical role in development across species and is dysregulated in a host of human diseases. A key step in signal transduction is the formation of Wnt receptor signalosomes, during which a large number of components translocate to the membrane, cluster together and amplify downstream signaling. However, the molecular processes that coordinate these events remain poorly defined. Here, we show that Daam2 regulates canonical Wnt signaling via the PIP₂-PIP5K axis through its association with Rac1. Clustering of Daam2-mediated Wnt receptor complexes requires both Rac1 and PIP5K, and PIP5K promotes membrane localization of these complexes in a Rac1-dependent manner. Importantly, the localization of Daam2 complexes and Daam2-mediated canonical Wnt signaling is dependent upon actin polymerization. These studies - in chick spinal cord and human and monkey cell lines - highlight novel roles for Rac1 and the actin cytoskeleton in the regulation of canonical Wnt signaling and define Daam2 as a key scaffolding hub that coordinates membrane translocation and signalosome clustering.

HECT (Homologous to the E6-AP Carboxyl Terminus)-Type Ubiquitin E3 Ligase ITCH Attenuates Cardiac Hypertrophy by Suppressing the Wnt/β-Catenin Signaling Pathway

The HECT (homologous to the E6-AP carboxyl terminus)-type ubiquitin E3 ligase ITCH is an enzyme that plays an important role in ubiquitin-proteasomal protein degradation. Disheveled proteins (Dvl1 [disheveled protein 1], Dvl2, and Dvl3) are the main components of the Wnt/β-catenin signaling pathway, which is involved in cardiac hypertrophy. The aim of this study was to examine the role of ITCH during development of cardiac hypertrophy. Thoracic transverse aortic constriction (TAC) was performed in transgenic mice with cardiac-specific overexpression of ITCH (ITCH-Tg) and wild-type mice. Cardiac hypertrophy after TAC was attenuated in ITCH-Tg mice, and the survival rate was higher for ITCH-Tg mice than for wild-type mice. Protein interaction between ITCH and Dvls was confirmed with immunoprecipitation in vivo and in vitro. Expression of key molecules of the Wnt/β-catenin signaling pathway (Dvl1, Dvl2, GSK3β [glycogen synthase kinase 3β], and β-catenin) was inhibited in ITCH-Tg mice compared with wild-type mice. Notably, the ubiquitination level of Dvl proteins increased in ITCH-Tg mice. Protein and mRNA expression levels of ITCH increased in response to Wnt3a stimulation in neonatal rat cardiomyocytes. Knockdown of ITCH using small-interfering RNA increased cardiomyocyte size and augmented protein expression levels of Dvl proteins, phospho-GSK3β, and β-catenin after Wnt3a stimulation in cardiomyocytes. Conversely, overexpression of ITCH attenuated cardiomyocyte hypertrophy and decreased protein expression levels of Dvl proteins, phospho-GSK3β and β-catenin. In conclusion, ITCH targets Dvl proteins for ubiquitin-proteasome degradation in cardiomyocytes and attenuates cardiac hypertrophy by suppressing the Wnt/β-catenin signaling pathway.

The Daam2-VHL-Nedd4 axis governs developmental and regenerative oligodendrocyte differentiation

Here, Ding et al. sought to understand whether and how the ubiquitin–proteasomal system (UPS) contributes to oligodendrocyte dysfunction and repair after white matter injury (WMI). They demonstrate that the E3 ligase VHL interacts with Daam2 and their mutual antagonism regulates oligodendrocyte differentiation during development.

Dysregulation of the ubiquitin–proteasomal system (UPS) enables pathogenic accumulation of disease-driving proteins in neurons across a host of neurological disorders. However, whether and how the UPS contributes to oligodendrocyte dysfunction and repair after white matter injury (WMI) remains undefined. Here we show that the E3 ligase VHL interacts with Daam2 and their mutual antagonism regulates oligodendrocyte differentiation during development. Using proteomic analysis of the Daam2–VHL complex coupled with conditional genetic knockout mouse models, we further discovered that the E3 ubiquitin ligase Nedd4 is required for developmental myelination through stabilization of VHL via K63-linked ubiquitination. Furthermore, studies in mouse demyelination models and white matter lesions from patients with multiple sclerosis corroborate the function of this pathway during remyelination after WMI. Overall, these studies provide evidence that a signaling axis involving key UPS components contributes to oligodendrocyte development and repair and reveal a new role for Nedd4 in glial biology.

TLR4 induced Wnt3a-Dvl3 restrains the intensity of inflammation and protects against endotoxin-driven organ failure through GSK3β/β-catenin signaling

BACKGROUND

Accumulating evidence suggests a regulatory role of Wnt proteins in innate immune responses. However, the effects of Wnt3a signaling on TLR4-mediated inflammatory responses are controversial and the signaling crosstalk between TLR4 and Wnt3a remains uncertain.

METHODS

Gain- and Loss- of function approaches were utilized to determine the function of Wnt3a signaling in TLR4-mediated inflammatory responses. Cytokine production at protein and mRNA levels and phosphorylation of signaling molecules were measured by ELISA, qRT-PCR, and Western Blot, respectively. Endotoxemia mouse model was employed to assess the effect of Wnt3a on systemic inflammatory cytokine levels and neutrophil infiltration.

RESULTS

LPS stimulation leads to an increase of Wnt3a expression and its downstream molecule, Dvl3, in primary monocytes. Inhibition or silence of Wnt3a or Dvl3 significantly increases the production of pro-inflammatory cytokines (IL-12, IL-6, TNFα), robustly reduces β-catenin accumulation, and enhances the phosphorylation of NF-κB P65 and its DNA binding activity. These results were confirmed by multiple gain- and loss- of function approaches including specific siRNA and ectopic expression of Dvl3, GSK3β, and β-catenin in monocytes. Moreover, in vivo relevance was established in a murine endotoxin model, in which Wnt3a inhibition enhances the inflammatory responses by augmenting the systemic pro-inflammatory cytokine levels and neutrophil infiltration.

CONCLUSIONS

TLR4 activation promotes Wnt3a-Dvl3 signaling, which acts as rheostats to restrain the intensity of inflammation through regulating GSK3β-β-catenin signaling and NF-κB activity.

GENERAL SIGNIFICANCE

Wnt3a-Dvl3-β-catenin signaling axis could be a potential interventional target for manipulating the direction and intensity of inflammatory responses.

Plasma membrane processes are differentially regulated by type I phosphatidylinositol phosphate 5-kinases and RASSF4

Phosphoinositide lipids regulate many cellular processes and are synthesized by lipid kinases. Type I phosphatidylinositol phosphate 5-kinases (PIP5KIs) generate phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P ₂]. Several phosphoinositide-sensitive readouts revealed the nonequivalence of overexpressing PIP5KIβ, PIP5KIγ or Ras association domain family 4 (RASSF4), believed to activate PIP5KIs. Mass spectrometry showed that each of these three proteins increased total cellular phosphatidylinositol bisphosphates (PtdInsP ₂) and trisphosphates (PtdInsP ₃) at the expense of phosphatidylinositol phosphate (PtdInsP) without changing lipid acyl chains. Analysis of KCNQ2/3 channels and PH domains confirmed an increase in plasma membrane PtdIns(4,5)P ₂ in response to PIP5KIβ or PIP5KIγ overexpression, but RASSF4 required coexpression with PIP5KIγ to increase plasma membrane PtdIns(4,5)P ₂ Effects on the several steps of store-operated calcium entry (SOCE) were not explained by plasma membrane phosphoinositide increases alone. PIP5KIβ and RASSF4 increased STIM1 proximity to the plasma membrane, accelerated STIM1 mobilization and speeded onset of SOCE; however, PIP5KIγ reduced STIM1 recruitment but did not change induced Ca²⁺ entry. These differences imply actions through different segregated pools of phosphoinositides and specific protein-protein interactions and targeting.This article has an associated First Person interview with the first author of the paper.

The Great Escape: how phosphatidylinositol 4-kinases and PI4P promote vesicle exit from the Golgi (and drive cancer)

Phosphatidylinositol 4-phosphate (PI4P) is a membrane glycerophospholipid and a major regulator of the characteristic appearance of the Golgi complex as well as its vesicular trafficking, signalling and metabolic functions. Phosphatidylinositol 4-kinases, and in particular the PI4KIIIβ isoform, act in concert with PI4P to recruit macromolecular complexes to initiate the biogenesis of trafficking vesicles for several Golgi exit routes. Dysregulation of Golgi PI4P metabolism and the PI4P protein interactome features in many cancers and is often associated with tumour progression and a poor prognosis. Increased expression of PI4P-binding proteins, such as GOLPH3 or PITPNC1, induces a malignant secretory phenotype and the release of proteins that can remodel the extracellular matrix, promote angiogenesis and enhance cell motility. Aberrant Golgi PI4P metabolism can also result in the impaired post-translational modification of proteins required for focal adhesion formation and cell–matrix interactions, thereby potentiating the development of aggressive metastatic and invasive tumours. Altered expression of the Golgi-targeted PI 4-kinases, PI4KIIIβ, PI4KIIα and PI4KIIβ, or the PI4P phosphate Sac1, can also modulate oncogenic signalling through effects on TGN-endosomal trafficking. A Golgi trafficking role for a PIP 5-kinase has been recently described, which indicates that PI4P is not the only functionally important phosphoinositide at this subcellular location. This review charts new developments in our understanding of phosphatidylinositol 4-kinase function at the Golgi and how PI4P-dependent trafficking can be deregulated in malignant disease.

PLEKHA4/kramer Attenuates Dishevelled Ubiquitination to Modulate Wnt and Planar Cell Polarity Signaling

Wnt signaling pathways direct key physiological decisions in development. Here, we establish a role for a pleckstrin homology domain-containing protein, PLEKHA4, as a modulator of signaling strength in Wnt-receiving cells. PLEKHA4 oligomerizes into clusters at PI(4,5)P2-rich regions of the plasma membrane and recruits the Cullin-3 (CUL3) E3 ubiquitin ligase substrate adaptor Kelch-like protein 12 (KLHL12) to these assemblies. This recruitment decreases CUL3-KLHL12-mediated polyubiquitination of Dishevelled, a central intermediate in canonical and non-canonical Wnt signaling. Knockdown of PLEKHA4 in mammalian cells demonstrates that PLEKHA4 positively regulates canonical and non-canonical Wnt signaling via these effects on the Dishevelled polyubiquitination machinery. In vivo knockout of the Drosophila melanogaster PLEKHA4 homolog, kramer, selectively affects the non-canonical, planar cell polarity (PCP) signaling pathway. We propose that PLEKHA4 tunes the sensitivities of cells toward the stimulation of Wnt or PCP signaling by sequestering a key E3 ligase adaptor controlling Dishevelled polyubiquitination within PI(4,5)P2-rich plasma membrane clusters.

Silencing of type II phosphatidylinositol 4-kinase β stabilizes prostate apoptosis response-4 and induces apoptosis in cancer cells

Type II phosphatidylinositol 4-kinase β (PtdIns 4-kinase II β) is an enigma among the phosphatidylinositol 4-kinase family. The role of PtdIns 4-kinase II β in MCF-7 cells was addressed with the help of short hairpin RNA (shRNA). PtdIns 4-kinase II β shRNA transfection increased pan-caspase activity and induced apoptosis in cancerous MCF-7 cells. Non-cancerous MCF-10A cells were resistant to PtdIns 4-kinase II β shRNA-induced apoptosis. Caspase 8 and 9 inhibitors rescued MCF-7 cells from apoptosis. Shotgun proteomic studies with Flag-tagged PtdIns 4-kinase II β immunoprecipitates showed tumor suppressor prostate apoptosis response-4 (Par-4) as one of the interacting proteins in HEK293 cells. In reciprocal experiments, Par-4 antibodies co-precipitated PtdIns 4-kinase II β from MCF-7 cells. Deletion of membrane localization motif (ΔCCPCC) or a mutation in ATP-binding region (D304A) of PtdIns 4-kinase II β did not affect its interaction with Par-4. Pull-down assays with GST-PtdIns 4-kinase II β-truncated mutants showed that the region between 101 and 215 amino acid residues is essential for interaction with Par-4. At molecular level, PtdIns 4-kinase II β shRNA transfection increased Par-4 stability, its nuclear localization and inhibition of NF-κB binding to target DNA. Knocking down of Par-4 with siRNA (small interfering RNA) rescued MCF-7 cells from PtdIns 4-kinase II β shRNA-induced apoptosis. These results suggest that PtdIns 4-kinase II β may be a novel regulator of Par-4 through protein–protein interactions. These studies have potential implications in cancer therapy.

A complex between phosphatidylinositol 4-kinase IIα and integrin α3β1 is required for N-glycan sialylation in cancer cells

Aberrant N-glycan sialylation of glycoproteins is closely associated with malignant phenotypes of cancer cells and metastatic potential, which includes cell adhesion, migration, and growth. Recently, phosphatidylinositol 4-kinase IIα (PI4KIIα), which is localized to the trans-Golgi network, was identified as a regulator of Golgi phosphoprotein 3 (GOLPH3) and of vesicle transport in the Golgi apparatus. GOLPH3 is a target of PI4KIIα and helps anchor sialyltransferases and thereby regulates sialylation of cell surface receptors. However, how PI4KIIα-mediated sialyation of cell surface proteins is regulated remains unclear. In this study, using several cell lines, CRISPR/Cas9-based gene knockout and short hairpin RNA-mediated silencing, RT-PCR, lentivirus-mediated overexpression, and immunoblotting methods, we confirmed that PI4KIIα knockdown suppresses the sialylation of N-glycans on the cell surface, in Akt phosphorylation and activation, and integrin α3-mediated cell migration of MDA-MB-231 breast cancer cells. Interestingly, both integrin α3β1 and PI4KIIα co-localized to the trans-Golgi network, where they physically interacted with each other, and PI4KIIα specifically associated with integrin α3 but not α5. Furthermore, overexpression of both integrin α3β1 and PI4KIIα induced hypersialylation. Conversely, integrin α3 knockout significantly inhibited the sialylation of membrane proteins, such as the epidermal growth factor receptor, as well as in total cell lysates. These findings suggest that the malignant phenotype of cancer cells is affected by a sialylation mechanism that is regulated by a complex between PI4KIIα and integrin α3β1.

A large scale high-throughput screen identifies chemical inhibitors of phosphatidylinositol 4-kinase type II alpha

The minor phospholipid, phosphatidylinositol 4-phosphate (PI4P), is emerging as a key regulator of lipid transfer in ER-membrane contact sites. Four different phosphatidylinositol 4-kinase (PI4K) enzymes generate PI4P in different membrane compartments supporting distinct cellular processes, many of which are crucial for the maintenance of cellular integrity but also hijacked by intracellular pathogens. While type III PI4Ks have been targeted by small molecular inhibitors, thus helping decipher their importance in cellular physiology, no inhibitors are available for the type II PI4Ks, which hinders investigations into their cellular functions. Here, we describe the identification of small molecular inhibitors of PI4K type II alpha (PI4K2A) by implementing a large scale small molecule high-throughput screening. A novel assay was developed that allows testing of selected inhibitors against PI4K2A in intact cells using a bioluminescence resonance energy transfer approach adapted to plate readers. The compounds disclosed here will pave the way to the optimization of PI4K2A inhibitors that can be used in cellular and animal studies to better understand the role of this enzyme in both normal and pathological states.

Ethanol targets nucleoredoxin/dishevelled interactions and stimulates phosphatidylinositol 4-phosphate production in vivo and in vitro

Nucleoredoxin (NXN) is a redox-regulating protein potentially targeted by reactive oxygen species (ROS). It regulates molecular pathways that participate in several key cellular processes. However, the role of NXN in the alcohol liver disease (ALD) redox regulation has not been fully understood. Here, we investigated the effects of ethanol and ethanol plus lipopolysaccharide, a two-hit liver injury model (Ethanol/LPS), on NXN/dishevelled (DVL) interaction and on DVL-dependent phosphoinositides production both in mouse liver and in a co-culture system consisting of human hepatic stellate cells (HSC) and ethanol metabolizing-VL17A human hepatocyte cells. Ethanol and two-hit model increased Nxn protein and mRNA expression, and 4-hydroxynonenal adducts. Two-hit model promoted Nxn nuclear translocation and Dvl/Phosphatidylinositol 4-kinase type-IIα (Pi4k2a) interaction ratio but surprisingly decreased Dvl protein and mRNA levels and reverted ethanol-induced Nxn/Dvl and Dvl/frizzled (Fzd) interaction ratios. Ethanol resulted in a significant increase of Dvl protein and mRNA expression, and decreased Nxn/Dvl interaction ratio but promoted the interaction of Dvl with Fzd and Pi4k2a; formation of this complex induced phosphatidylinositol 4-phosphate [PI(4)P] production. Ethanol and LPS treatments provoked similar alterations on NXN/DVL interaction and its downstream effect in HSC/VL17A co-culture system. Interestingly, ROS and glutathione levels as well as most of ethanol-induced alterations were modified by NXN overexpression in the co-culture system. In conclusion, two-hit model of ethanol exposure disrupts NXN/DVL homeostatic status to allow DVL/FZD/PI4K2A complex formation and stimulates PI(4)P production. These results provide a new mechanism showing that NXN also participates in the regulation of phosphoinositides production that is altered by ethanol during alcoholic liver disease progression.

Tissue-selective alteration of ethanolamine plasmalogen metabolism in dedifferentiated colon mucosa

Human colon lipid analysis by imaging mass spectrometry (IMS) demonstrates that the lipid fingerprint is highly sensitive to a cell's pathophysiological state. Along the colon crypt axis, and concomitant to the differentiation process, certain lipid species tightly linked to signaling (phosphatidylinositols and arachidonic acid (AA)-containing diacylglycerophospholipids), change following a rather simple mathematical expression. We extend here our observations to ethanolamine plasmalogens (PlsEtn), a unique type of glycerophospholipid presenting a vinyl ether linkage at sn-1 position. PlsEtn distribution was studied in healthy, adenomatous, and carcinomatous colon mucosa sections by IMS. In epithelium, 75% of PlsEtn changed in a highly regular manner along the crypt axis, in clear contrast with diacyl species (67% of which remained constant). Consistently, AA-containing PlsEtn species were more abundant at the base, where stem cells reside, and decreased while ascending the crypt. In turn, mono-/diunsaturated species experienced the opposite change. These gradients were accompanied by a gradual expression of ether lipid synthesis enzymes. In lamina propria, 90% of stromal PlsEtn remained unchanged despite the high content of AA and the gradient in AA-containing diacylglycerophospholipids. Finally, both lipid and protein gradients were severely affected in polyps and carcinoma. These results link PlsEtn species regulation to cell differentiation for the first time and confirm that diacyl and ether species are differently regulated. Furthermore, they reaffirm the observations on cell lipid fingerprint image sensitivity to predict cell pathophysiological status, reinforcing the translational impact both lipidome and IMS might have in clinical research.

Sonic Hedgehog Activates Phospholipase A2 to Enhance Smoothened Ciliary Translocation

The G protein-coupled receptor Smoothened (Smo) is the signal transducer of the Sonic Hedgehog (Shh) pathway. Smo signals through G protein-dependent and -independent routes, with G protein-independent canonical signaling to Gli effectors requiring Smo accumulation in the primary cilium. The mechanisms controlling Smo activation and trafficking are not yet clear but likely entail small-molecule binding to pockets in its extracellular cysteine-rich domain (CRD) and/or transmembrane bundle. Here, we demonstrate that the cytosolic phospholipase cPLA2α is activated through Gβγ downstream of Smo to release arachidonic acid. Arachidonic acid binds Smo and synergizes with CRD-binding agonists, promoting Smo ciliary trafficking and high-level signaling. Chemical or genetic cPLA2α inhibition dampens Smo signaling to Gli, revealing an unexpected contribution of G protein-dependent signaling to canonical pathway activity. Arachidonic acid displaces the Smo transmembrane domain inhibitor cyclopamine to rescue CRD agonist-induced signaling, suggesting that arachidonic acid may target the transmembrane bundle to allosterically enhance signaling by CRD agonist-bound Smo.

A Mathematical Model of the Phosphoinositide Pathway

Phosphoinositides are signalling lipids that constitute a complex network regulating many cellular processes. We propose a computational model that accounts for all species of phosphoinositides in the plasma membrane of mammalian cells. The model replicates the steady-state of the pathway and most known dynamic phenomena. Sensitivity analysis demonstrates model robustness to alterations in the parameters. Model analysis suggest that the greatest contributor to phosphatidylinositol 4,5-biphosphate (PI(4,5)P₂) production is a flux representing the direct transformation of PI into PI(4,5)P₂, also responsible for the maintenance of this pool when phosphatidylinositol 4-phosphate (PI(4)P) is decreased. PI(5)P is also shown to be a significant source for PI(4,5)P₂ production. The model was validated with siRNA screens that knocked down the expression of enzymes in the pathway. The screen monitored the activity of the epithelium sodium channel (ENaC), which is activated by PI(4,5)P₂. While the model may deepen our understanding of other physiological processes involving phosphoinositides, we highlight therapeutic effects of ENaC modulation in Cystic Fibrosis (CF). The model suggests control strategies where the activities of the enzyme phosphoinositide 4-phosphate 5-kinase I (PIP5KI) or the PI4K + PIP5KI + DVL protein complex are decreased and cause an efficacious reduction in PI(4,5)P₂ levels while avoiding undesirable alterations in other phosphoinositide pools.

The Many Roles of Type II Phosphatidylinositol 4-Kinases in Membrane Trafficking: New Tricks for Old Dogs

The type II phosphatidylinositol 4-kinases (PI4KIIs) produce the lipid phosphatidylinositol 4-phosphate (PtdIns4P) and participate in a confusing variety of membrane trafficking and signaling roles. This review argues that both historical and contemporary evidence supports the function of the PI4KIIs in numerous trafficking pathways, and that the key to understanding the enzymatic regulation is through membrane interaction and the intrinsic membrane environment. By summarizing new research and examining the trafficking roles of the PI4KIIs in the context of recently solved molecular structures, I highlight how mechanisms of PI4KII function and regulation are providing insights into the development of cancer and in neurological disease. I present an integrated view connecting the cell biology, molecular regulation, and roles in whole animal systems of these increasingly important proteins.

Complexity of the Wnt/β‑catenin pathway: Searching for an activation model

Wnt signaling refers to a conserved signaling pathway, widely studied due to its roles in cellular communication, cell fate decisions, development and cancer. However, the exact mechanism underlying inhibition of the GSK phosphorylation towards β-catenin and activation of the pathway after biding of Wnt ligand to its cognate receptors at the plasma membrane remains unclear. Wnt target genes are widely spread over several animal phyla. They participate in a plethora of functions during the development of an organism, from axial specification, gastrulation and organogenesis all the way to regeneration and repair in adults. Temporal and spatial oncogenetic re-activation of Wnt signaling almost certainly leads to cancer. Wnt signaling components have been extensively studied as possible targets in anti-cancer therapies. In this review we will discuss one of the most intriguing questions in this field, that is how β-catenin, a major component in this pathway, escapes the destruction complex, gets stabilized in the cytosol and it is translocated to the nucleus where it acts as a co-transcription factor. Four major models have evolved during the past 20years. We dissected each of them along with current views and future perspectives on this pathway. This review will focus on the molecular mechanisms by which Wnt proteins modulate β-catenin cytoplasmic levels and the relevance of this pathway for the development and cancer.

Lipid fingerprint image accurately conveys human colon cell pathophysiologic state: A solid candidate as biomarker

Membrane lipids are gaining increasing attention in the clinical biomarker field, as they are associated with different pathologic processes such as cancer or neurodegenerative diseases. Analyzing human colonoscopic sections by matrix assisted laser/desorption ionization (MALDI) mass spectrometry imaging techniques, we identified a defined number of lipid species changing concomitant to the colonocyte differentiation and according to a quite simple mathematical expression. These species felt into two lipid families tightly associated in signaling: phosphatidylinositols and arachidonic acid-containing lipids. On the other hand, an opposed pattern was observed in lamina propria for AA-containing lipids, coinciding with the physiological distribution of the immunological response cells in this tissue. Importantly, the lipid gradient was accompanied by a gradient in expression of enzymes involved in lipid mobilization. Finally, both lipid and protein gradients were lost in adenomatous polyps. The latter allowed us to assess how different a single lipid species is handled in a pathological context depending on the cell type. The strict patterns of distribution in lipid species and lipid enzymes described here unveil the existence of fine regulatory mechanisms orchestrating the lipidome according to the physiological state of the cell. In addition, these results provide solid evidence that the cell lipid fingerprint image can be used to predict precisely the physiological and pathological status of a cell, reinforcing its translational impact in clinical research.

PTEN regulates IGF-1R-mediated therapy resistance in melanoma

Inhibition of the mitogen-activated protein kinase (MAPK) pathway is a major advance in the treatment of metastatic melanoma. However, its therapeutic success is limited by the rapid emergence of drug resistance. The insulin-like growth factor-1 receptor (IGF-1R) is overexpressed in melanomas developing resistance toward the BRAF(V) (600) inhibitor vemurafenib. Here, we show that hyperactivation of BRAF enhances IGF-1R expression. In addition, the phosphatase activity of PTEN as well as heterocellular contact to stromal cells increases IGF-1R expression in melanoma cells and enhances resistance to vemurafenib. Interestingly, PTEN-negative melanoma cells escape IGF-1R blockade by decreased expression of the receptor, implicating that only in melanoma patients with PTEN-positive tumors treatment with IGF-1R inhibitors would be a suitable strategy to combat therapy resistance. Our data emphasize the crosstalk and therapeutic relevance of microenvironmental and tumor cell-autonomous mechanisms in regulating IGF-1R expression and by this sensitivity toward targeted therapies.

Type II PI4-kinases control Weibel-Palade body biogenesis and von Willebrand factor structure in human endothelial cells

Weibel-Palade bodies (WPBs) are endothelial storage organelles that mediate the release of molecules involved in thrombosis, inflammation and angiogenesis, including the pro-thrombotic glycoprotein von Willebrand factor (VWF). Although many protein components required for WPB formation and function have been identified, the role of lipids is almost unknown. We examined two key phosphatidylinositol kinases that control phosphatidylinositol 4-phosphate levels at the trans-Golgi network, the site of WPB biogenesis. RNA interference of the type II phosphatidylinositol 4-kinases PI4KIIα and PI4KIIβ in primary human endothelial cells leads to formation of an increased proportion of short WPB with perturbed packing of VWF, as exemplified by increased exposure of antibody-binding sites. When stimulated with histamine, these cells release normal levels of VWF yet, under flow, form very few platelet-catching VWF strings. In PI4KIIα-deficient mice, immuno-microscopy revealed that VWF packaging is also perturbed and these mice exhibit increased blood loss after tail cut compared to controls. This is the first demonstration that lipid kinases can control the biosynthesis of VWF and the formation of WPBs that are capable of full haemostatic function.

Identification of the Interactome of a Palmitoylated Membrane Protein, Phosphatidylinositol 4-Kinase Type II Alpha

Phosphatidylinositol 4-kinases (PI4K) are enzymes responsible for the production of phosphatidylinositol 4-phosphates, important intermediates in several cell signaling pathways. PI4KIIα is the most abundant membrane-associated kinase in mammalian cells and is involved in a variety of essential cellular functions. However, the precise role(s) of PI4KIIα in the cell is not yet completely deciphered. Here we present an experimental protocol that uses a chemical cross-linker, DSP, combined with immunoprecipitation and immunoaffinity purification to identify novel PI4KIIα interactors. As predicted, PI4KIIα participates in transient, low-affinity interactions that are stabilized by the use of DSP. Using this optimized protocol we have successfully identified actin cytoskeleton regulators-the WASH complex and RhoGEF1, as major novel interactors of PI4KIIα. While this chapter focuses on the PI4KIIα interactome, this protocol can and has been used to generate other membrane interactome networks.

PI(4)P homeostasis: Who controls the controllers?

During recent decades, PI(4)P (phosphoinositol-4-phosphate) has been described as a key regulator of a wide range of cellular functions such as organelle biogenesis, lipid metabolism and distribution, membrane trafficking, ion channels, pumps, and transporter activities. In this review we will focus on the multiple mechanisms that regulate PI(4)P homeostasis ranging from those responsible for the spatial distribution of the PI4 kinases and PI(4)P phosphatase to those controlling their enzymatic activity or the delivery/presentation of the substrate, i.e. PI or PI(4)P, to the PI4Ks or PI(4)P phosphatase, respectively. We will also highlight the open questions in the field mainly dealing with the existence and mode of action of PI(4)P sensors that monitor its amount and can act as a rheostat tuning PI(4)P levels in different compartments and adapting them to the different needs of the cell.

Resolution of structure of PIP5K1A reveals molecular mechanism for its regulation by dimerization and dishevelled

Type I phosphatidylinositol phosphate kinase (PIP5K1) phosphorylates the head group of phosphatidylinositol 4-phosphate (PtdIns4P) to generate PtdIns4,5P₂, which plays important roles in a wide range of cellular functions including Wnt signalling. However, the lack of its structural information has hindered the understanding of its regulation. Here we report the crystal structure of the catalytic domain of zebrafish PIP5K1A at 3.3 Å resolution. This molecule forms a side-to-side dimer. Mutagenesis study of PIP5K1A reveals two adjacent interfaces for the dimerization and interaction with the DIX domain of the Wnt signalling molecule dishevelled. Although these interfaces are located distally to the catalytic/substrate-binding site, binding to these interfaces either through dimerization or the interaction with DIX stimulates PIP5K1 catalytic activity. DIX binding additionally enhances PIP5K1 substrate binding. Thus, this study elucidates regulatory mechanisms for this lipid kinase and provides a paradigm for the understanding of PIP5K1 regulation by their interacting molecules.

Type I phosphatidylinositol phosphate kinase is an important component of many cellular pathways, including Wnt signalling. Here the authors report the crystal structure of the zebrafish protein along with in vitro assays that help to elucidate the regulation and function of this kinase.

The high-resolution crystal structure of phosphatidylinositol 4-kinase IIβ and the crystal structure of phosphatidylinositol 4-kinase IIα containing a nucleoside analogue provide a structural basis for isoform-specific inhibitor design

Phosphatidylinositol 4-phosphate (PI4P) is the most abundant monophosphoinositide in eukaryotic cells. Humans have four phosphatidylinositol 4-kinases (PI4Ks) that synthesize PI4P, among which are PI4K IIβ and PI4K IIα. In this study, two crystal structures are presented: the structure of human PI4K IIβ and the structure of PI4K IIα containing a nucleoside analogue. The former, a complex with ATP, is the first high-resolution (1.9 Å) structure of a PI4K. These structures reveal new details such as high conformational heterogeneity of the lateral hydrophobic pocket of the C-lobe and together provide a structural basis for isoform-specific inhibitor design.

Wnt/β-catenin signaling pathway and colorectal cancer

The Wnt/β-catenin signaling pathway is an extensively studied Wnt signaling pathway. It plays a critical role in embryogenesis, organogenesis and tumorigenesis. Colorectal cancer has high rates of incidence and mortality, and it is critical to elucidate the mechanisms responsible for the pathogenesis of colorectal cancer. Molecular studies indicate that aberrant activation of the Wnt/β-catenin signaling pathway occurs in colorectal cancer frequently, and this signaling pathway may provide new potential targets for the treatment of colorectal cancer. In this article, we give an overview of the Wnt/β-catenin signaling pathway and discuss its relationship with colorectal cancer and its potential use as therapeutic targets for this malignancy.

Daam2-PIP5K is a regulatory pathway for Wnt signaling and therapeutic target for remyelination in the CNS

Wnt signaling plays an essential role in developmental and regenerative myelination of the CNS; however, contributions of proximal regulators of the Wnt receptor complex to these processes remain undefined. To identify components of the Wnt pathway that regulate these processes, we applied a multifaceted discovery platform and found that Daam2-PIP5K comprise a novel pathway regulating Wnt signaling and myelination. Using dorsal patterning of the chick spinal cord we found that Daam2 promotes Wnt signaling and receptor complex formation through PIP5K-PIP2. Analysis of Daam2 function in oligodendrocytes (OLs) revealed that it suppresses OL differentiation during development, after white matter injury (WMI), and is expressed in human white matter lesions. These findings suggest a pharmacological strategy to inhibit Daam2-PIP5K function, application of which stimulates remyelination after WMI. Put together, our studies integrate information from multiple systems to identify a novel regulatory pathway for Wnt signaling and potential therapeutic target for WMI.

Chromosomal Instability and Phosphoinositide Pathway Gene Signatures in Glioblastoma Multiforme

Structural rearrangements of chromosome 10 are frequently observed in glioblastoma multiforme and over 80 % of tumour samples archived in the catalogue of somatic mutations in cancer database had gene copy number loss for PI4K2A which encodes phosphatidylinositol 4-kinase type IIalpha. PI4K2A loss of heterozygosity mirrored that of PTEN, another enzyme that regulates phosphoinositide levels and also PIK3AP1, MINPP1, INPP5A and INPP5F. These results indicated a reduction in copy number for a set of phosphoinositide signalling genes that co-localise to chromosome 10q. This analysis was extended to a panel of phosphoinositide pathway genes on other chromosomes and revealed a number of previously unreported associations with glioblastoma multiforme. Of particular note were highly penetrant copy number losses for a group of X-linked phosphoinositide phosphatase genes OCRL, MTM1 and MTMR8; copy number amplifications for the chromosome 19 genes PIP5K1C, AKT2 and PIK3R2, and also for the phospholipase C genes PLCB1, PLCB4 and PLCG1 on chromosome 20. These mutations are likely to affect signalling and trafficking functions dependent on the PI(4,5)P2, PI(3,4,5)P3 and PI(3,5)P2 lipids as well as the inositol phosphates IP3, IP5 and IP6. Analysis of flanking genes with functionally unrelated products indicated that chromosomal instability as opposed to a phosphoinositide-specific process underlay this pattern of copy number variation. This in silico study suggests that in glioblastoma multiforme, karyotypic changes have the potential to cause multiple abnormalities in sets of genes involved in phosphoinositide metabolism and this may be important for understanding drug resistance and phosphoinositide pathway redundancy in the advanced disease state.

Dual inhibition of EGFR at protein and activity level via combinatorial blocking of PI4KIIα as anti-tumor strategy

Our previous studies indicate that phosphatidylinositol 4-kinase IIα can promote the growth of multi-malignant tumors via HER-2/PI3K and MAPK pathways. However, the molecular mechanisms of this pathway and its potential for clinical application remain unknown. In this study, we found that PI4KIIα could be an ideal combinatorial target for EGFR treatment via regulating EGFR degradation. Results showed that PI4KIIα knockdown reduced EGFR protein level, and the expression of PI4KIIα shows a strong correlation with EGFR in human breast cancer tissues (r = 0.77, P < 0.01). PI4KIIα knockdown greatly prolonged the effects and decreased the effective dosage of AG-1478, a specific inhibitor of EGFR. In addition, it significantly enhanced AG1478-induced inhibition of tumor cell survival and strengthened the effect of the EGFR-targeting anti-cancer drug Iressa in xenograft tumor models. Mechanistically, we found that PI4KIIα suppression increased EGFR ligand-independent degradation. Quantitative proteomic analysis by stable isotope labeling with amino acids in cell culture (SILAC) and LC-MS/MS suggested that HSP90 mediated the effect of PI4KIIα on EGFR. Furthermore, we found that combined inhibition of PI4KIIα and EGFR suppressed both PI3K/AKT and MAPK/ERK pathways, and resulted in downregulation of multiple oncogenes like PRDX2, FASN, MTA2, ultimately leading to suppression of tumor growth. Therefore, we conclude that combined inhibition of PI4KIIα and EGFR exerts a multiple anti-tumor effect. Dual inhibition of EGFR at protein and activity level via combinatorial blocking of PI4KIIα presents a novel strategy to combat EGFR-dependent tumors.

Wnt and lithium: a common destiny in the therapy of nervous system pathologies?

Wnt signaling is required for neurogenesis, the fate of neural progenitors, the formation of neuronal circuits during development, neuron positioning and polarization, axon and dendrite development and finally for synaptogenesis. This signaling pathway is also implicated in the generation and differentiation of glial cells. In this review, we describe the mechanisms of action of Wnt signaling pathways and their implication in the development and correct functioning of the nervous system. We also illustrate how a dysregulated Wnt pathway could lead to psychiatric, neurodegenerative and demyelinating pathologies. Lithium, used for the treatment of bipolar disease, inhibits GSK3β, a central enzyme of the Wnt/β-catenin pathway. Thus, lithium could, to some extent, mimic Wnt pathway. We highlight the possible dialogue between lithium therapy and modulation of Wnt pathway in the treatment of the diseases of the nervous system.

β-arrestin promotes Wnt-induced low density lipoprotein receptor-related protein 6 (Lrp6) phosphorylation via increased membrane recruitment of Amer1 protein

β-Arrestin is a scaffold protein that regulates signal transduction by seven transmembrane-spanning receptors. Among other functions it is also critically required for Wnt/β-catenin signal transduction. In the present study we provide for the first time a mechanistic basis for the β-arrestin function in Wnt/β-catenin signaling. We demonstrate that β-arrestin is required for efficient Wnt3a-induced Lrp6 phosphorylation, a key event in downstream signaling. β-Arrestin regulates Lrp6 phosphorylation via a novel interaction with phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2)-binding protein Amer1/WTX/Fam123b. Amer1 has been shown very recently to bridge Wnt-induced and Dishevelled-associated PtdIns(4,5)P2 production to the phosphorylation of Lrp6. Using fluorescence recovery after photobleaching we show here that β-arrestin is required for the Wnt3a-induced Amer1 membrane dynamics and downstream signaling. Finally, we show that β-arrestin interacts with PtdIns kinases PI4KIIα and PIP5KIβ. Importantly, cells lacking β-arrestin showed higher steady-state levels of the relevant PtdInsP and were unable to increase levels of these PtdInsP in response to Wnt3a. In summary, our data show that β-arrestins regulate Wnt3a-induced Lrp6 phosphorylation by the regulation of the membrane dynamics of Amer1. We propose that β-arrestins via their scaffolding function facilitate Amer1 interaction with PtdIns(4,5)P2, which is produced locally upon Wnt3a stimulation by β-arrestin- and Dishevelled-associated kinases.

Cinderella story: PI4P goes from precursor to key signaling molecule

Phosphatidylinositol lipids are signaling molecules involved in nearly all aspects of cellular regulation. Production of phosphatidylinositol 4-phosphate (PI4P) has long been recognized as one of the first steps in generating poly-phosphatidylinositol phosphates involved in actin organization, cell migration, and signal transduction. In addition, progress over the last decade has brought to light independent roles for PI4P in membrane trafficking and lipid homeostasis. Here, we describe recent advances that reveal the breadth of processes regulated by PI4P, the spectrum of PI4P effectors, and the mechanisms of spatiotemporal control that coordinate crosstalk between PI4P and cellular signaling pathways.

Modeling anterior development in mice: diet as modulator of risk for neural tube defects

Head morphogenesis is a complex process that is controlled by multiple signaling centers. The most common defects of cranial development are craniofacial defects, such as cleft lip and cleft palate, and neural tube defects, such as anencephaly and encephalocoele in humans. More than 400 genes that contribute to proper neural tube closure have been identified in experimental animals, but only very few causative gene mutations have been identified in humans, supporting the notion that environmental influences are critical. The intrauterine environment is influenced by maternal nutrition, and hence, maternal diet can modulate the risk for cranial and neural tube defects. This article reviews recent progress toward a better understanding of nutrients during pregnancy, with particular focus on mouse models for defective neural tube closure. At least four major patterns of nutrient responses are apparent, suggesting that multiple pathways are involved in the response, and likely in the underlying pathogenesis of the defects. Folic acid has been the most widely studied nutrient, and the diverse responses of the mouse models to folic acid supplementation indicate that folic acid is not universally beneficial, but that the effect is dependent on genetic configuration. If this is the case for other nutrients as well, efforts to prevent neural tube defects with nutritional supplementation may need to become more specifically targeted than previously appreciated. Mouse models are indispensable for a better understanding of nutrient-gene interactions in normal pregnancies, as well as in those affected by metabolic diseases, such as diabetes and obesity.

Mammalian phosphatidylinositol 4-kinases as modulators of membrane trafficking and lipid signaling networks

The four mammalian phosphatidylinositol 4-kinases modulate inter-organelle lipid trafficking, phosphoinositide signalling and intracellular vesicle trafficking. In addition to catalytic domains required for the synthesis of PI4P, the phosphatidylinositol 4-kinases also contain isoform-specific structural motifs that mediate interactions with proteins such as AP-3 and the E3 ubiquitin ligase Itch, and such structural differences determine isoform-specific roles in membrane trafficking. Moreover, different permutations of phosphatidylinositol 4-kinase isozymes may be required for a single cellular function such as occurs during distinct stages of GPCR signalling and in Golgi to lysosome trafficking. Phosphatidylinositol 4-kinases have recently been implicated in human disease. Emerging paradigms include increased phosphatidylinositol 4-kinase expression in some cancers, impaired functioning associated with neurological pathologies, the subversion of PI4P trafficking functions in bacterial infection and the activation of lipid kinase activity in viral disease. We discuss how the diverse and sometimes overlapping functions of the phosphatidylinositol 4-kinases present challenges for the design of isoform-specific inhibitors in a therapeutic context.

Frizzled and LRP5/6 receptors for Wnt/β-catenin signaling

Frizzled and LRP5/6 are Wnt receptors that upon activation lead to stabilization of cytoplasmic β-catenin. In this study, we review the current knowledge of these two families of receptors, including their structures and interactions with Wnt proteins, and signaling mechanisms from receptor activation to the engagement of intracellular partners Dishevelled and Axin, and finally to the inhibition of β-catenin phosphorylation and ensuing β-catenin stabilization.

Phosphatidylinositol 4-kinase IIα function at endosomes is regulated by the ubiquitin ligase Itch

Phosphatidylinositol (PI) 4-phosphate (PI(4)P) and its metabolizing enzymes serve important functions in cell signalling and membrane traffic. PI 4-kinase type IIα (PI4KIIα) regulates Wnt signalling, endosomal sorting of signalling receptors, and promotes adaptor protein recruitment to endosomes and the trans-Golgi network. Here we identify the E3 ubiquitin ligase Itch as binding partner and regulator of PI4KIIα function. Itch directly associates with and ubiquitinates PI4KIIα, and both proteins colocalize on endosomes containing Wnt-activated frizzled 4 (Fz4) receptor. Depletion of PI4KIIα or Itch regulates Wnt signalling with corresponding changes in Fz4 internalization and degradative sorting. These findings unravel a new molecular link between phosphoinositide-regulated endosomal membrane traffic, ubiquitin and the modulation of Wnt signalling.

On the role of Wnt/β-catenin signaling in stem cells

BACKGROUND

Stem cells are mainly characterized by two properties: self-renewal and the potency to differentiate into diverse cell types. These processes are regulated by different growth factors including members of the Wnt protein family. Wnt proteins are secreted glycoproteins that can activate different intracellular signaling pathways.

SCOPE OF REVIEW

Here we summarize our current knowledge on the role of Wnt/β-catenin signaling with respect to these two main features of stem cells.

MAJOR CONCLUSIONS

A particular focus is given on the function of Wnt signaling in embryonic stem cells. Wnt signaling can also improve reprogramming of somatic cells towards iPS cells highlighting the importance of this pathway for self-renewal and pluripotency. As an example for the role of Wnt signaling in adult stem cell behavior, we furthermore focus on intestinal stem cells located in the crypts of the small intestine.

GENERAL SIGNIFICANCE

A broad knowledge about stem cell properties and the influence of intrinsic and extrinsic factors on these processes is a requirement for the use of these cells in regenerative medicine in the future or to understand cancer development in the adult. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

The E3 ubiquitin ligase ITCH negatively regulates canonical Wnt signaling by targeting dishevelled protein

Dishevelled (Dvl) is a key component in the canonical Wnt signaling pathway and becomes hyperphosphorylated upon Wnt stimulation. Dvl is required for LRP6 phosphorylation, which is essential for subsequent steps of signal transduction, such as Axin recruitment and cytosolic β-catenin stabilization. Here, we identify the HECT-containing Nedd4-like ubiquitin E3 ligase ITCH as a new Dvl-binding protein. ITCH ubiquitinates the phosphorylated form of Dvl and promotes its degradation via the proteasome pathway, thereby inhibiting canonical Wnt signaling. Knockdown of ITCH by RNA interference increased the stability of phosphorylated Dvl and upregulated Wnt reporter gene activity as well as endogenous Wnt target gene expression induced by Wnt stimulation. In addition, we found that both the PPXY motif and the DEP domain of Dvl are critical for its interaction with ITCH, as mutation in the PPXY motif (Dvl2-Y568F) or deletion of the DEP domain led to reduced affinity for ITCH. Consistently, overexpression of ITCH inhibited wild-type Dvl2-induced, but not Dvl2-Y568F mutant-induced, Wnt reporter activity. Moreover, the Y568F mutant, but not wild-type Dvl2, can reverse the ITCH-mediated inhibition of Wnt-induced reporter activity. Collectively, these results indicate that ITCH plays a negative regulatory role in modulating canonical Wnt signaling by targeting the phosphorylated form of Dvl.

Phosphatidylinositol 4-kinases, phosphatidylinositol 4-phosphate and cancer

This article focuses on the emerging roles for phosphatidylinositol 4-phosphate and the phosphatidylinositol 4-kinases in cancer. Phosphatidylinositol 4-phosphate is a common substrate for both the phosphatidylinositol 3-kinase and phospholipase C pathways, and has been implicated in the membrane targeting of proteins such as Girdin/GIV and OSBP. Alterations to phosphatidylinositol 4-kinase expression levels can modulate MAP kinase and Akt signalling, and are important for chemoresistance, tumour angiogenesis and the suppression of apoptosis and metastases. Recent improvements in high-throughput screening assays, and the discoveries that some anti-viral molecules are isoform selective phosphatidylinositol 4-kinase inhibitors have advanced the drugability of these enzymes.

The phosphatidylinositol 4-kinases: don't call it a comeback

Phosphatidylinositol 4-phosphate (PtdIns4P) is a quantitatively minor membrane phospholipid which is the precursor of PtdIns(4,5)P (2) in the classical agonist-regulated phospholipase C signalling pathway. However, PtdIns4P also governs the recruitment and function of numerous trafficking molecules, principally in the Golgi complex. The majority of phosphoinositides (PIs) phosphorylated at the D4 position of the inositol headgroup are derived from PtdIns4P and play roles in a diverse array of fundamental cellular processes including secretion, cell migration, apoptosis and mitogenesis; therefore, PtdIns4P biosynthesis can be regarded as key point of regulation in many PI-dependent processes.Two structurally distinct sequence families, the type II and type III PtdIns 4-kinases, are responsible for PtdIns4P synthesis in eukaryotic organisms. These important proteins are differentially expressed, localised and regulated by distinct mechanisms, indicating that the enzymes perform non-redundant roles in trafficking and signalling. In recent years, major advances have been made in our understanding of PtdIns4K biology and here we summarise current knowledge of PtdIns4K structure, function and regulation.

Implication of Wnt signaling in neuronal polarization

Wnt ligands comprise a large family of secreted glycoproteins that control a variety of developmental processes including cell polarization in diverse organisms. Through various receptors present on receiving cells, Wnts initiate intracellular signaling cascades resulting in changes in gene transcription or cytoskeleton reorganization. Recently, several lines of evidence have suggested the role of Wnt signaling in establishing axon-dendrite polarity in developing neurons. In this review, we summarize the recent results related with the role of Wnt signaling in neuronal polarization.

International Union of Basic and Clinical Pharmacology. LXXX. The class Frizzled receptors

The receptor class Frizzled, which has recently been categorized as a separate group of G protein-coupled receptors by the International Union of Basic and Clinical Pharmacology, consists of 10 Frizzleds (FZD(1-10)) and Smoothened (SMO). The FZDs are activated by secreted lipoglycoproteins of the Wingless/Int-1 (WNT) family, whereas SMO is indirectly activated by the Hedgehog (HH) family of proteins acting on the transmembrane protein Patched (PTCH). Recent years have seen major advances in our knowledge about these seven-transmembrane-spanning proteins, including: receptor function, molecular mechanisms of signal transduction, and the receptor's role in embryonic patterning, physiology, cancer, and other diseases. Despite intense efforts, many question marks and challenges remain in mapping receptor-ligand interaction, signaling routes, mechanisms of specificity and how these molecular details underlie disease and also the receptor's important role in physiology. This review therefore focuses on the molecular aspects of WNT/FZD and HH/SMO signaling discussing receptor structure, mechanisms of signal transduction, accessory proteins, receptor dynamics, and the possibility of targeting these signaling pathways pharmacologically.

DEEP Insights through the DEP Domain

In this issue, Yu et al. (2010) provide a crystal structure for the bipartite interface between the μ2 subunit of the adaptor protein AP-2 complex and Dishevelled, a key component for Wnt signaling.

Regulation of PIP5K activity by Arf6 and its physiological significance

The phospholipid kinase phosphatidylinositol 4-phosphate 5-kinase (PIP5K) catalyzes the phosphorylation of the membrane phospholipid phosphatidylinositol 4-phosphate to generate the pleiotropic phospholipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2) ]. To date, three mammalian PIP5K isozymes, α, β, and γ, and several splicing variants of the γ isozyme have been identified. These PIP5K isozymes and PIP5Kγ variants play critical roles in various cellular functions through their product PI(4,5)P(2) . The small GTPase Arf6 is one of the key activators of PIP5K. Increasing evidence suggests that PIP5K functions as a downstream effector of Arf6 to regulate a wide variety of cellular functions, such as exocytosis, endocytosis, endosomal recycling, membrane ruffle formation, immune response, and bacterial invasion. In this review, we place our focus on the recent advances in Arf6/PIP5K signaling and its linkage to cellular functions.