Frizzled receptors signal through G proteins

Frizzled receptors (FZDs) in Wnt signaling: potential therapeutic targets for human cancers

Frizzled receptors (FZDs) are key contributors intrinsic to the Wnt signaling pathway, activation of FZDs triggering the Wnt signaling cascade is frequently observed in human tumors and intimately associated with an aggressive carcinoma phenotype. It has been shown that the abnormal expression of FZD receptors contributes to the manifestation of malignant characteristics in human tumors such as enhanced cell proliferation, metastasis, chemotherapy resistance as well as the acquisition of cancer stemness. Given the essential roles of FZD receptors in the Wnt signaling in human tumors, this review aims to consolidate the prevailing knowledge on the specific status of FZD receptors (FZD1-10) and elucidate their respective functions in tumor progression. Furthermore, we delineate the structural basis for binding of FZD and its co-receptors to Wnt, and provide a better theoretical foundation for subsequent studies on related mechanisms. Finally, we describe the existing biological classes of small molecule-based FZD inhibitors in detail in the hope that they can provide useful assistance for design and development of novel drug candidates targeted FZDs.

A framework for Frizzled-G protein coupling and implications to the PCP signaling pathways

The ten Frizzled receptors (FZDs) are essential in Wnt signaling and play important roles in embryonic development and tumorigenesis. Among these, FZD6 is closely associated with lens development. Understanding FZD activation mechanism is key to unlock these emerging targets. Here we present the cryo-EM structures of FZD6 and FZD3 which are known to relay non-canonical planar cell polarity (PCP) signaling pathways as well as FZD1 in their G protein-coupled states and in the apo inactive states, respectively. Comparison of the three inactive/active pairs unveiled a shared activation framework among all ten FZDs. Mutagenesis along with imaging and functional analysis on the human lens epithelial tissues suggested potential crosstalk between the G-protein coupling of FZD6 and the PCP signaling pathways. Together, this study provides an integrated understanding of FZD structure and function, and lays the foundation for developing therapeutic modulators to activate or inhibit FZD signaling for a range of disorders including cancers and cataracts.

Wnt Signaling Inhibitors and Their Promising Role in Tumor Treatment

In a continuous search for the improvement of antitumor therapies, the inhibition of the Wnt signaling pathway has been recognized as a promising target. The altered functioning of the Wnt signaling in human tumors points to the strategy of the inhibition of its activity that would impact the clinical outcomes and survival of patients. Because the Wnt pathway is often mutated or epigenetically altered in tumors, which promotes its activation, inhibitors of Wnt signaling are being intensively investigated. It has been shown that knocking down specific components of the Wnt pathway has inhibitory effects on tumor growth in vivo and in vitro. Thus, similar effects are expected from the application of Wnt inhibitors. In the last decades, molecules acting as inhibitors on the pathway’s specific molecular levels have been identified and characterized. This review will discuss the inhibitors of the canonical Wnt pathway, summarize knowledge on their effectiveness as therapeutics, and debate their side effects. The role of the components frequently mutated in various tumors that are principal targets for Wnt inhibitors is also going to be brought to the reader’s attention. Some of the molecules identified as Wnt pathway inhibitors have reached early stages of clinical trials, and some have only just been discovered. All things considered, inhibition of the Wnt signaling pathway shows potential for the development of future therapies.

Frizzled receptors and SFRP5 in lipid metabolism: Current findings and potential applications

Lipid metabolism plays a very important role as the central metabolic process of the body. Lipid metabolism interruptions may cause many chronic diseases, for example, non-alcoholic fatty liver disease (NAFLD), diabetes, and obesity. Secreted Frizzled Related Protein 5 (SFRP5) and Frizzled receptors (FZD) are two newly discovered adipokines that are involved in lipid metabolism as well as lipogenesis. Both of these adipokines affect lipid metabolism and adipogenesis through three WNT signaling pathways (WNTSP): WNT/β-catenin, WNT/Ca²⁺, and WNT/JNK. FZD consists of 10 species, which have a cysteine-rich domain (CRD) to bind to the WNT protein for signal transduction. Depending on the type of ligand or co-receptor, they can stimulate or inhibit adipogenesis. In lipid metabolism, they play a role in recognizing fatty acids. In obesity, gene expression of the WNT/FZD receptors is significantly increased. In contrast, SFPR5 serves as an antagonist that can compete with FZD for inhibition of WNTSP. It is believed to have anti-inflammatory potential in obesity and diseases related to abnormal lipid metabolism. In these cases, the expression of SFRP5 is found to be very low leading to the promoted production of proinflammatory cytokines (PICS). Some methods that include using recombinant SFRP5 to improve non-alcoholic steatohepatitis (NASH), using secreted Ly-6/uPAR-related protein 1 (Slurp1) to regulate fat accumulation in the liver through SFRP5, and dietary and lifestyle interventions to improve overweight/obesity have been studied. However, understandings of the molecular mechanisms of these two adipokines and their interactions are very limited. Therefore, more in-depth studies are needed in the future.

Frizzled receptors in melanomagenesis: From molecular interactions to target identification

Frizzled (FZD) proteins are receptors for the WNT family ligands. Inherited human diseases and genetic experiments using knockout mice have revealed a central role of FZDs in multiple aspects of embryonic development and tissue homeostasis. Misregulated FZD signaling has also been found in many cancers. Recent studies on three out of the ten mammalian FZDs in melanoma have shown that they promote tumor cell proliferation and invasion, via the activation of the canonical WNT/β-catenin or non-canonical PCP signaling pathway. In this concise review, we summarize our current knowledge of individual FZDs in melanoma, discuss the involvement of both the canonical and non-canonical pathways, and describe ongoing efforts to target the FZD receptors for melanoma treatment.

The Association of RGS2 and Slug in the Androgen-induced Acquisition of Mesenchymal Features of Breast MDA-MB-453 Cancer Cells

BACKGROUND

Epithelial-mesenchymal transition (EMT) of tumor cells is a prerequisite to cancer cell invasion and metastasis. This process involves a network of molecular alterations. Androgen receptor (AR) plays an important role in the biology of breast cancers, particularly those dependent on AR expression like luminal AR (LAR) breast cancer subtype. We have recently reported that the AR agonist, dihydrotestosterone (DHT), induces a mesenchymal transition of MDA-MB-453 cells, concomitant with transcriptional up-regulation of Slug and regulator of G protein signaling 2 (RGS2).

OBJECTIVE

The role of Slug and RGS2 in mediating the DHT-induced effects in these cells was investigated.

METHODS

MDA-MB-453 cells were used as a model system of LAR breast cancer. Immunofluorescence was used to examine cell morphology and protein localization. Protein expression was analyzed by immunoblotting. Protein localization was confirmed by cell fractionation followed by immunoblotting. Protein-protein interaction was confirmed by co-immunoprecipitation followed by immunoblotting. Transwell membranes were used to assess cell migration. Transfection of cells with siRNA molecules that target Slug and RGS2 mRNA was utilized to delineate the modes of action of these two molecules.

RESULTS

Treatment of MDA-MB-453 cells with DHT induced the expression of both proteins. In addition, AR-Slug, AR-RGS2, and Slug-RGS2 interactions were observed shortly after AR activation. Knocking down Slug abrogated the basal, but not the DHT-induced, cell migration and blocked DHT-induced mesenchymal transition. On the other hand, RGS2 knocked-down cells had an increased level of Slug protein and assumed mesenchymal cell morphology with induced migration, and the addition of DHT further elongated cell morphology and stimulated their migration. Inhibition of AR or β-catenin reverted the RGS2 knocked-down cells to the epithelial phenotype, but only inhibition of AR blocked their DHT-induced migration.

CONCLUSIONS

These results suggest the involvement of RGS2 and Slug in a complex molecular network regulating the DHT-induced mesenchymal features in MDA-MB-453 cells. The study may offer a better understanding of the biological role of AR in breast cancer toward devising AR-based therapeutic strategies.

Wnt signaling promotes tumor development in part through phosphofructokinase 1 platelet isoform upregulation

The activation of Wnt signaling has been detected in various types of human cancer and has been shown to be associated with cancer development. In the present study, it was revealed that Wnt signaling induced the expression of phosphofructokinase 1 platelet isoform (PFKP), which has been reported to catalyze a rate‑limiting reaction in glycolysis and is important for the Warburg effect, proliferation, colony formation and cancer cell migration. Moreover, it was demonstrated that Wnt3A induced PFKP expression in a β‑catenin‑independent manner, resulting in increased PFK enzyme activity. Wnt3A‑induced epidermal growth factor receptor transactivation activated PI3K/AKT, which stabilized PFKP through PFKP S386 phosphorylation and subsequent PFKP upregulation. Wnt3A‑induced PFKP S386 phosphorylation increased PFKP expression and promoted the Warburg effect, cell proliferation, colony formation and the migratory ability of cancer cells. On the whole, the findings of the present study underscore the potential role of PFKP in Wnt signaling‑induced tumor development.

A Role for Frizzled and Their Post-Translational Modifications in the Mammalian Central Nervous System

The Wnt pathway is a key signalling cascade that regulates the formation and function of neuronal circuits. The main receptors for Wnts are Frizzled (Fzd) that mediate diverse functions such as neurogenesis, axon guidance, dendritogenesis, synapse formation, and synaptic plasticity. These processes are crucial for the assembly of functional neuronal circuits required for diverse functions ranging from sensory and motor tasks to cognitive performance. Indeed, aberrant Wnt-Fzd signalling has been associated with synaptic defects during development and in neurodegenerative conditions such as Alzheimer's disease. New studies suggest that the localisation and stability of Fzd receptors play a crucial role in determining Wnt function. Post-translational modifications (PTMs) of Fzd are emerging as an important mechanism that regulates these Wnt receptors. However, only phosphorylation and glycosylation have been described to modulate Fzd function in the central nervous system (CNS). In this review, we discuss the function of Fzd in neuronal circuit connectivity and how PTMs contribute to their function. We also discuss other PTMs, not yet described in the CNS, and how they might modulate the function of Fzd in neuronal connectivity. PTMs could modulate Fzd function by affecting Fzd localisation and stability at the plasma membrane resulting in local effects of Wnt signalling, a feature particularly important in polarised cells such as neurons. Our review highlights the importance of further studies into the role of PTMs on Fzd receptors in the context of neuronal connectivity.

Wnts regulate planar cell polarity via heterotrimeric G protein and PI3K signaling

In the mammalian cochlea, the planar cell polarity (PCP) pathway aligns hair cell orientation along the plane of the sensory epithelium. Concurrently, multiple cell intrinsic planar polarity (referred to as iPCP) modules mediate planar polarization of the hair cell apical cytoskeleton, including the kinocilium and the V-shaped hair bundle essential for mechanotransduction. How PCP and iPCP are coordinated during development and the roles of Wnt ligands in this process remain unresolved. Here we show that genetic blockade of Wnt secretion in the cochlear epithelium resulted in a shortened cochlear duct and misoriented and misshapen hair bundles. Mechanistically, Wnts stimulate Gi activity by regulating the localization of Daple, a guanine nucleotide exchange factor (GEF) for Gαi. In turn, the Gβγ complex signals through phosphoinositide 3-kinase (PI3K) to regulate kinocilium positioning and asymmetric localizations of a subset of core PCP proteins, thereby coordinating PCP and iPCP. Thus, our results identify a putative Wnt/heterotrimeric G protein/PI3K pathway for PCP regulation.

Regulatory roles of G-protein coupled receptors in adipose tissue metabolism and their therapeutic potential

The high incidence of obesity has increased the need to discover new therapeutic targets to combat obesity and obesity-related metabolic diseases. Obesity is defined as an abnormal accumulation of adipose tissue, which is one of the major metabolic organs that regulate energy homeostasis. However, there are currently no approved anti-obesity therapeutics that directly target adipose tissue metabolism. With recent advances in the understanding of adipose tissue biology, molecular mechanisms involved in brown adipose tissue expansion and metabolic activation have been investigated as potential therapeutic targets to increase energy expenditure. This review focuses on G-protein coupled receptors (GPCRs) as they are the most successful class of druggable targets in human diseases and have an important role in regulating adipose tissue metabolism. We summarize recent findings on the major GPCR classes that regulate thermogenesis and mitochondrial metabolism in adipose tissue. Improved understanding of GPCR signaling pathways that regulate these processes could facilitate the development of novel pharmacological approaches to treat obesity and related metabolic disorders.

The essential role of primary cilia in cerebral cortical development and disorders

Primary cilium, first described in the 19th century in different cell types and organisms by Alexander Ecker, Albert Kolliker, Aleksandr Kowalevsky, Paul Langerhans, and Karl Zimmermann (Ecker, 1844; Kolliker, 1854; Kowalevsky, 1867; Langerhans, 1876; Zimmermann, 1898), play an essential modulatory role in diverse aspects of nervous system development and function. The primary cilium, sometimes referred to as the cell's 'antennae', can receive wide ranging inputs from cellular milieu, including morphogens, growth factors, neuromodulators, and neurotransmitters. Its unique structural and functional organization bequeaths it the capacity to hyper-concentrate signaling machinery in a restricted cellular domain approximately one-thousandth the volume of cell soma. Thus enabling it to act as a signaling hub that integrates diverse developmental and homestatic information from cellular milieu to regulate the development and function of neural cells. Dysfunction of primary cilia contributes to the pathophysiology of several brain malformations, intellectual disabilities, epilepsy, and psychiatric disorders. This review focuses on the most essential contributions of primary cilia to cerebral cortical development and function, in the context of neurodevelopmental disorders and malformations. It highlights the recent progress made in identifying the mechanisms underlying primary cilia's role in cortical progenitors, neurons and glia, in health and disease. A future challenge will be to translate these insights and advances into effective clinical treatments for ciliopathies.

The Ion Channel and GPCR Toolkit of Brain Capillary Pericytes

Brain pericytes reside on the abluminal surface of capillaries, and their processes cover ~90% of the length of the capillary bed. These cells were first described almost 150 years ago (Eberth, 1871; Rouget, 1873) and have been the subject of intense experimental scrutiny in recent years, but their physiological roles remain uncertain and little is known of the complement of signaling elements that they employ to carry out their functions. In this review, we synthesize functional data with single-cell RNAseq screens to explore the ion channel and G protein-coupled receptor (GPCR) toolkit of mesh and thin-strand pericytes of the brain, with the aim of providing a framework for deeper explorations of the molecular mechanisms that govern pericyte physiology. We argue that their complement of channels and receptors ideally positions capillary pericytes to play a central role in adapting blood flow to meet the challenge of satisfying neuronal energy requirements from deep within the capillary bed, by enabling dynamic regulation of their membrane potential to influence the electrical output of the cell. In particular, we outline how genetic and functional evidence suggest an important role for G_s-coupled GPCRs and ATP-sensitive potassium (K_ATP) channels in this context. We put forth a predictive model for long-range hyperpolarizing electrical signaling from pericytes to upstream arterioles, and detail the TRP and Ca²⁺ channels and G_q, G_i/o, and G_12/13 signaling processes that counterbalance this. We underscore critical questions that need to be addressed to further advance our understanding of the signaling topology of capillary pericytes, and how this contributes to their physiological roles and their dysfunction in disease.

The Canonical Wnt Signaling (Wnt/β-Catenin Pathway): A Potential Target for Cancer Prevention and Therapy

Precise regulation of signal transduction pathways is crucial for normal animal development and for maintaining cellular and tissue homeostasis in adults. The Wnt/Frizzled-mediated signaling includes canonical and non-canonical signal transduction pathways. Upregulation or downregulation of the canonical Wnt signaling (or the Wnt/β-Catenin signal transduction) leads to a variety of human diseases, including cancers, neurodegenerative disorders, skin and bone diseases, and heart deficiencies. Therefore, Wnt/β-Catenin signal transduction is a potential clinical target for the treatment of not only human cancers but also some other human chronic diseases. Here, some recent results including those from my laboratory highlighting the role of Wnt/β-Catenin signal transduction in human cancers will be reviewed. After a brief overview on canonical Wnt signaling and introducing some critical β-Catenin/T-cell factor-target genes, the interaction of canonical Wnt signaling with some common human cancers will be discussed. In the end, the different segments of the aforesaid signaling pathway, which have been considered as targets for clinical purposes, will be scrutinized.

Primary Cilia Signaling Promotes Axonal Tract Development and Is Disrupted in Joubert Syndrome-Related Disorders Models

Appropriate axonal growth and connectivity are essential for functional wiring of the brain. Joubert syndrome-related disorders (JSRD), a group of ciliopathies in which mutations disrupt primary cilia function, are characterized by axonal tract malformations. However, little is known about how cilia-driven signaling regulates axonal growth and connectivity. We demonstrate that the deletion of related JSRD genes, Arl13b and Inpp5e, in projection neurons leads to de-fasciculated and misoriented axonal tracts. Arl13b deletion disrupts the function of its downstream effector, Inpp5e, and deregulates ciliary-PI3K/AKT signaling. Chemogenetic activation of ciliary GPCR signaling and cilia-specific optogenetic modulation of downstream second messenger cascades (PI3K, AKT, and AC3) commonly regulated by ciliary signaling receptors induce rapid changes in axonal dynamics. Further, Arl13b deletion leads to changes in transcriptional landscape associated with dysregulated PI3K/AKT signaling. These data suggest that ciliary signaling acts to modulate axonal connectivity and that impaired primary cilia signaling underlies axonal tract defects in JSRD.

Low-density lipoprotein receptor-related protein 6-mediated signaling pathways and associated cardiovascular diseases: diagnostic and therapeutic opportunities

Low-density lipoprotein receptor-related protein 6 (LRP6) is a member of the low-density lipoprotein receptors (LDLRs) family and accumulating evidence points to the critical role of LRP6 in cardiovascular health and homeostasis. In addition to presenting the well-appreciated roles in canonical signaling regulating blood pressure, blood glucose, lipid metabolism, atherosclerosis, cardiac valve disease, cardiac development, Alzheimer's disease and tumorigenesis, LRP6 also inhibits non-canonical Wnt signals that promote arterial smooth muscle cell proliferation and vascular calcification. Noticeably, the role of LRP6 is displayed in cardiometabolic disease, an increasingly important clinical burden with aging and obesity. The prospect for cardiovascular diseases treatment via targeting LRP6-mediated signaling pathways may improve central blood pressure and lipid metabolism, and reduce neointima formation and myocardial ischemia-reperfusion injury. Thus, a deep and comprehensive understanding of LRP6 structure, function and signaling pathways will contribute to clinical diagnosis, therapy and new drug development for LRP6-related cardiovascular diseases.

Mechanisms of Wnt signaling and control

The Wnt signaling pathway is a highly conserved system that regulates complex biological processes across all metazoan species. At the cellular level, secreted Wnt proteins serve to break symmetry and provide cells with positional information that is critical to the patterning of the entire body plan. At the organismal level, Wnt signals are employed to orchestrate fundamental developmental processes, including the specification of the anterior-posterior body axis, induction of the primitive streak and ensuing gastrulation movements, and the generation of cell and tissue diversity. Wnt functions extend into adulthood where they regulate stem cell behavior, tissue homeostasis, and damage repair. Disruption of Wnt signaling activity during embryonic development or in adults results in a spectrum of abnormalities and diseases, including cancer. The molecular mechanisms that underlie the myriad of Wnt-regulated biological effects have been the subject of intense research for over three decades. This review is intended to summarize our current understanding of how Wnt signals are generated and interpreted. This article is categorized under: Biological Mechanisms > Cell Signaling Developmental Biology > Stem Cell Biology and Regeneration.

Frizzleds as GPCRs - More Conventional Than We Thought!

For more than 30 years, WNT/β-catenin and planar cell polarity signaling has formed the basis for what we understand to be the primary output of the interaction between WNTs and their cognate receptors known as Frizzleds (FZDs). In the shadow of these pathways, evidence for the involvement of heterotrimeric G proteins in WNT signaling has grown substantially over the years - redefining the complexity of the WNT signaling network. Moreover, the distinct characteristics of FZD paralogs are becoming better understood, and we can now apply concepts valid for classical GPCRs to grasp FZDs as molecular machines at the interface of ligand binding and intracellular effects. This review discusses recent developments in the field of WNT/FZD signaling in the context of GPCR pharmacology, and identifies remaining mysteries with an emphasis on structural and kinetic components that support this dogma shift.

Identification of Proteins Required for Precise Positioning of Apc2 in Dendrites

In Drosophila neurons, uniform minus-end-out polarity in dendrites is maintained in part by kinesin-2-mediated steering of growing microtubules at branch points. Apc links the kinesin motor to growing microtubule plus ends and Apc2 recruits Apc to branch points where it functions. Because Apc2 acts to concentrate other steering proteins to branch points, we wished to understand how Apc2 is targeted. From an initial broad candidate RNAi screen, we found Miro (a mitochondrial transport protein), Ank2, Axin, spastin and Rac1 were required to position Apc2-GFP at dendrite branch points. YFP-Ank2-L8, Axin-GFP and mitochondria also localized to branch points suggesting the screen identified relevant proteins. By performing secondary screens, we found that energy production by mitochondria was key for Apc2-GFP positioning and spastin acted upstream of mitochondria. Ank2 seems to act independently from other players, except its membrane partner, Neuroglian (Nrg). Rac1 likely acts through Arp2/3 to generate branched actin to help recruit Apc2-GFP. Axin can function in a variety of wnt signaling pathways, one of which includes heterotrimeric G proteins and Frizzleds. Knockdown of Gαs, Gαo, Fz and Fz2, reduced targeting of Apc2 and Axin to branch points. Overall our data suggest that mitochondrial energy production, Nrg/Ank2, branched actin generated by Arp2/3 and Fz/G proteins/Axin function as four modules that control localization of the microtubule regulator Apc2 to its site of action in dendrite branch points.

The curious case of Gαs gain-of-function in neoplasia

BACKGROUND

Mutations activating the α subunit of heterotrimeric Gs protein are associated with a number of highly specific pathological molecular phenotypes. One of the best characterized is the McCune Albright syndrome. The disease presents with an increased incidence of neoplasias in specific tissues.

MAIN BODY

A similar repertoire of neoplasms can develop whether mutations occur spontaneously in somatic tissues during fetal development or after birth. Glands are the most "permissive" tissues, recently found to include the entire gastrointestinal tract. High frequency of activating Gαs mutations is associated with precise diagnoses (e.g., IPMN, Pyloric gland adenoma, pituitary toxic adenoma). Typically, most neoplastic lesions, from thyroid to pancreas, remain well differentiated but may be a precursor to aggressive cancer.

CONCLUSIONS

Here we propose the possibility that gain-of-function mutations of Gαs interfere with signals in the microenvironment of permissive tissues and lead to a transversal neoplastic phenotype.

Deregulation of Frizzled Receptors in Hepatocellular Carcinoma

G protein-coupled receptors (GPCRs) have a substantial role in tumorigenesis and are described as a “cancer driver”. Aberrant expression or activation of GPCRs leads to the deregulation of downstream signaling pathways, thereby promoting cancer progression. In hepatocellular carcinoma (HCC), the Wnt signaling pathway is frequently activated and it is associated with an aggressive HCC phenotype. Frizzled (FZD) receptors, a family member of GPCRs, are known to mediate Wnt signaling. Accumulating findings have revealed the deregulation of FZD receptors in HCC and their functional roles have been implicated in HCC progression. Given the important role of FZD receptors in HCC, we summarize here the expression pattern of FZD receptors in HCC and their corresponding functional roles during HCC progression. We also further review and highlight the potential targeting of FZD receptors as an alternative therapeutic strategy in HCC.

Lipoprotein Receptor-related Protein 6 Signaling is Necessary for Vasculogenic Differentiation of Human Dental Pulp Stem Cells

The aim of this study was to evaluate the effects of Wnt signaling through lipoprotein receptor-related protein 6 (LRP6) and Frizzled6 on the endothelial differentiation of dental pulp stem cells (DPSCs). DPSCs were stably transduced with enhanced green fluorescent protein (EGFP)-tagged lentiviral vectors (short hairpin RNA-LRP6, short hairpin RNA-Frizzled6, or empty vector controls). We evaluated the effects of LRP6 and Frizzled6 on expression of endothelial markers and on capillary tube formation mediated by DPSCs induced with recombinant human Wnt1 (rhWnt1) and/or recombinant human vascular endothelial growth factor₁₆₅ (rhVEGF₁₆₅). In vivo, tooth slices/scaffolds were seeded with LRP6-silenced, Frizzled6-silenced, or vector control DPSC cells and transplanted into immunodeficient mice. The density of blood vessels generated by DPSCs differentiated into vascular endothelial cells was analyzed by immunohistochemistry for EGFP. The rhWnt1 and rhVEGF₁₆₅ induced expression of active β-catenin in control DPSCs and in Frizzled6-silenced DPSCs, but not in LRP6-silenced DPSCs. Furthermore, VEGF and interleukin-8 were downregulated in LRP6-silenced DPSCs, but not in control DPSCs or in Frizzled6-silenced DPSCs (P < .05). Likewise, rhWnt1 and rhVEGF₁₆₅ induced expression of the endothelial marker VEGF receptor-2 in control DPSCs and in Frizzled6-silenced DPSCs, but not in LRP6-silenced DPSCs. These data correlated with a trend for lower density of capillary sprouts generated by LRP6-silenced DPSCs when compared with control DPSCs in Matrigel. In vivo, tooth slice/scaffolds seeded with DPSC-short hairpinRNA-LRP6 cells showed lower density of human blood vessels (ie, EGFP-positive blood vessels), when compared with tooth slice/scaffolds seeded with vector control cells (P < .05). Collectively, these data demonstrated that LRP6 signaling is necessary for the vasculogenic differentiation of human DPSCs.

Frizzled7: A Promising Achilles' Heel for Targeting the Wnt Receptor Complex to Treat Cancer

Frizzled7 is arguably the most studied member of the Frizzled family, which are the cognate Wnt receptors. Frizzled7 is highly conserved through evolution, from Hydra through to humans, and is expressed in diverse organisms, tissues and human disease contexts. Frizzled receptors can homo- or hetero-polymerise and associate with several co-receptors to transmit Wnt signalling. Notably, Frizzled7 can transmit signalling via multiple Wnt transduction pathways and bind to several different Wnt ligands, Frizzled receptors and co-receptors. These promiscuous binding and functional properties are thought to underlie the pivotal role Frizzled7 plays in embryonic developmental and stem cell function. Recent studies have identified that Frizzled7 is upregulated in diverse human cancers, and promotes proliferation, progression and invasion, and orchestrates cellular transitions that underscore cancer metastasis. Importantly, Frizzled7 is able to regulate Wnt signalling activity even in cancer cells which have mutations to down-stream signal transducers. In this review we discuss the various aspects of Frizzled7 signalling and function, and the implications these have for therapeutic targeting of Frizzled7 in cancer.

Heterotrimeric G protein signaling in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a signalopathy of renal tubular epithelial cells caused by naturally occurring mutations in two distinct genes, polycystic kidney disease 1 (PKD1) and 2 (PKD2). Genetic variants in PKD1, which encodes the polycystin-1 (PC-1) protein, remain the predominant factor associated with the pathogenesis of nearly two-thirds of all patients diagnosed with PKD. Although the relationship between defective PC-1 with renal cystic disease initiation and progression remains to be fully elucidated, there are numerous clinical studies that have focused upon the control of effector systems involving heterotrimeric G protein regulation. A major regulator in the activation state of heterotrimeric G proteins are G protein-coupled receptors (GPCRs), which are defined by their seven transmembrane-spanning regions. PC-1 has been considered to function as an unconventional GPCR, but the mechanisms by which PC-1 controls signal processing, magnitude, or trafficking through heterotrimeric G proteins remains to be fully known. The diversity of heterotrimeric G protein signaling in PKD is further complicated by the presence of non-GPCR proteins in the membrane or cytoplasm that also modulate the functional state of heterotrimeric G proteins within the cell. Moreover, PC-1 abnormalities promote changes in hormonal systems that ultimately interact with distinct GPCRs in the kidney to potentially amplify or antagonize signaling output from PC-1. This review will focus upon the canonical and noncanonical signaling pathways that have been described in PKD with specific emphasis on which heterotrimeric G proteins are involved in the pathological reorganization of the tubular epithelial cell architecture to exacerbate renal cystogenic pathways.

G-protein Coupled Receptor Signaling in Pluripotent Stem Cell-derived Cardiovascular Cells: Implications for Disease Modeling

Human pluripotent stem cell derivatives show promise as an in vitro platform to study a range of human cardiovascular diseases. A better understanding of the biology of stem cells and their cardiovascular derivatives will help to understand the strengths and limitations of this new model system. G-protein coupled receptors (GPCRs) are key regulators of stem cell maintenance and differentiation and have an important role in cardiovascular cell signaling. In this review, we will therefore describe the state of knowledge concerning the regulatory role of GPCRs in both the generation and function of pluripotent stem cell derived-cardiomyocytes, -endothelial, and -vascular smooth muscle cells. We will consider how far the in vitro disease models recapitulate authentic GPCR signaling and provide a useful basis for discovery of disease mechanisms or design of therapeutic strategies.

Outside-in signaling--a brief review of GPCR signaling with a focus on the Drosophila GPCR family

G-protein-coupled receptors (GPCRs) are the largest family of receptors in many organisms, including worms, mice and humans. GPCRs are seven-transmembrane pass proteins that are activated by binding a stimulus (or ligand) in the extracellular space and then transduce that information to the inside of the cell through conformational changes. The conformational changes activate heterotrimeric G-proteins, which execute the downstream signaling pathways through the recruitment and activation of cellular enzymes. The highly specific ligand-GPCR interaction prompts an efficient cellular response, which is vital for the health of the cell and organism. In this Commentary, we review general features of GPCR signaling and then focus on the Drosophila GPCRs, which are not as well-characterized as their worm and mammalian counterparts. We discuss findings that the Drosophila odorant and gustatory receptors are not bona fide GPCRs as is the case for their mammalian counterparts. We also present here a phylogenetic analysis of the bona fide Drosophila GPCRs that suggest potential roles for several family members. Finally, we discuss recently discovered roles of GPCRs in Drosophila embryogenesis, a field we expect will uncover many previously unappreciated functions for GPCRs.

Structure and functional properties of Norrin mimic Wnt for signalling with Frizzled4, Lrp5/6, and proteoglycan

Wnt signalling regulates multiple processes including angiogenesis, inflammation, and tumorigenesis. Norrin (Norrie Disease Protein) is a cystine-knot like growth factor. Although unrelated to Wnt, Norrin activates the Wnt/β-catenin pathway. Signal complex formation involves Frizzled4 (Fz4), low-density lipoprotein receptor related protein 5/6 (Lrp5/6), Tetraspanin-12 and glycosaminoglycans (GAGs). Here, we report crystallographic and small-angle X-ray scattering analyses of Norrin in complex with Fz4 cysteine-rich domain (Fz4CRD), of this complex bound with GAG analogues, and of unliganded Norrin and Fz4CRD. Our structural, biophysical and cellular data, map Fz4 and putative Lrp5/6 binding sites to distinct patches on Norrin, and reveal a GAG binding site spanning Norrin and Fz4CRD. These results explain numerous disease-associated mutations. Comparison with the Xenopus Wnt8-mouse Fz8CRD complex reveals Norrin mimics Wnt for Frizzled recognition. The production and characterization of wild-type and mutant Norrins reported here open new avenues for the development of therapeutics to combat abnormal Norrin/Wnt signalling.

Daple is a novel non-receptor GEF required for trimeric G protein activation in Wnt signaling

Wnt signaling is essential for tissue homeostasis and its dysregulation causes cancer. Wnt ligands trigger signaling by activating Frizzled receptors (FZDRs), which belong to the G-protein coupled receptor superfamily. However, the mechanisms of G protein activation in Wnt signaling remain controversial. In this study, we demonstrate that FZDRs activate G proteins and trigger non-canonical Wnt signaling via the Dishevelled-binding protein, Daple. Daple contains a Gα-binding and activating (GBA) motif, which activates Gαi proteins and an adjacent domain that directly binds FZDRs, thereby linking Wnt stimulation to G protein activation. This triggers non-canonical Wnt responses, that is, suppresses the β-catenin/TCF/LEF pathway and tumorigenesis, but enhances PI3K-Akt and Rac1 signals and tumor cell invasiveness. In colorectal cancers, Daple is suppressed during adenoma-to-carcinoma transformation and expressed later in metastasized tumor cells. Thus, Daple activates Gαi and enhances non-canonical Wnt signaling by FZDRs, and its dysregulation can impact both tumor initiation and progression to metastasis.

Regulation of the oncoprotein Smoothened by small molecules

The Hedgehog pathway is critical for animal development and has been implicated in multiple human malignancies. Despite great interest in targeting the pathway pharmacologically, many of the principles underlying the signal transduction cascade remain poorly understood. Hedgehog ligands are recognized by a unique receptor system that features the transporter-like protein Patched and the G protein-coupled receptor (GPCR)-like Smoothened (SMO). The biochemical interaction between these transmembrane proteins is the subject of intensive efforts. Recent structural and functional studies have provided great insight into the small-molecule regulation of SMO through identification of two distinct ligand-binding sites. In this Perspective, we review these recent findings and relate them to potential mechanisms for the endogenous regulation of SMO.

Heterotrimeric Go protein links Wnt-Frizzled signaling with ankyrins to regulate the neuronal microtubule cytoskeleton

Drosophila neuromuscular junctions (NMJs) represent a powerful model system with which to study glutamatergic synapse formation and remodeling. Several proteins have been implicated in these processes, including components of canonical Wingless (Drosophila Wnt1) signaling and the giant isoforms of the membrane-cytoskeleton linker Ankyrin 2, but possible interconnections and cooperation between these proteins were unknown. Here, we demonstrate that the heterotrimeric G protein Go functions as a transducer of Wingless-Frizzled 2 signaling in the synapse. We identify Ankyrin 2 as a target of Go signaling required for NMJ formation. Moreover, the Go-ankyrin interaction is conserved in the mammalian neurite outgrowth pathway. Without ankyrins, a major switch in the Go-induced neuronal cytoskeleton program is observed, from microtubule-dependent neurite outgrowth to actin-dependent lamellopodial induction. These findings describe a novel mechanism regulating the microtubule cytoskeleton in the nervous system. Our work in Drosophila and mammalian cells suggests that this mechanism might be generally applicable in nervous system development and function.

β-Arrestin interacts with the beta/gamma subunits of trimeric G-proteins and dishevelled in the Wnt/Ca(2+) pathway in xenopus gastrulation

β-Catenin independent, non-canonical Wnt signaling pathways play a major role in the regulation of morphogenetic movements in vertebrates. The term non-canonical Wnt signaling comprises multiple, intracellularly divergent, Wnt-activated and β-Catenin independent signaling cascades including the Wnt/Planar Cell Polarity and the Wnt/Ca²⁺ cascades. Wnt/Planar Cell Polarity and Wnt/Ca²⁺ pathways share common effector proteins, including the Wnt ligand, Frizzled receptors and Dishevelled, with each other and with additional branches of Wnt signaling. Along with the aforementioned proteins, β-Arrestin has been identified as an essential effector protein in the Wnt/β-Catenin and the Wnt/Planar Cell Polarity pathway. Our results demonstrate that β-Arrestin is required in the Wnt/Ca²⁺ signaling cascade upstream of Protein Kinase C (PKC) and Ca²⁺/Calmodulin-dependent Protein Kinase II (CamKII). We have further characterized the role of β-Arrestin in this branch of non-canonical Wnt signaling by knock-down and rescue experiments in Xenopus embryo explants and analyzed protein-protein interactions in 293T cells. Functional interaction of β-Arrestin, the β subunit of trimeric G-proteins and Dishevelled is required to induce PKC activation and membrane translocation. In Xenopus gastrulation, β-Arrestin function in Wnt/Ca²⁺ signaling is essential for convergent extension movements. We further show that β-Arrestin physically interacts with the β subunit of trimeric G-proteins and Dishevelled, and that the interaction between β-Arrestin and Dishevelled is promoted by the beta/gamma subunits of trimeric G-proteins, indicating the formation of a multiprotein signaling complex.

β-Catenin-dependent pathway activation by both promiscuous "canonical" WNT3a-, and specific "noncanonical" WNT4- and WNT5a-FZD receptor combinations with strong differences in LRP5 and LRP6 dependency

The WNT/β-catenin signalling cascade is the best-investigated frizzled receptor (FZD) pathway, however, whether and how specific combinations of WNT/FZD and co-receptors LRP5 and LRP6 differentially affect this pathway are not well understood. This is mostly due to the fact that there are 19 WNTs, 10 FZDs and at least two co-receptors. In our attempt to identify the signalling capabilities of specific WNT/FZD/LRP combinations we made use of our previously reported TCF/LEF Gaussia luciferase reporter gene HEK293 cell line (Ring et al., 2011). Generation of WNT/FZD fusion constructs - but not their separate transfection - without or with additional isogenic overexpression of LRP5 and LRP6 in our reporter cells permitted the investigation of specific WNT/FZD/LRP combinations. The canonical WNT3a in fusion to almost all FZDs was able to induce β-catenin-dependent signalling with strong dependency on LRP6 but not LRP5. Interestingly, noncanonical WNT ligands, WNT4 and WNT5a, were also able to act "canonically" but only in fusion with specific FZDs and with selective dependence on LRP5 or LRP6. These data and extension of this experimental setup to the poorly characterized other WNTs should facilitate deeper insight into the complex WNT/FZD signalling system and its function.