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

Cellular localization of receptors using antibodies visualized by light and dual labeling confocal microscopy

Immunocytochemistry can be used to visualize the binding of specific site-directed antisera to receptors in tissue sections and permits the precise identification of cell types expressing a particular receptor when viewed using a conventional light microscope or by confocal microscopy. Protocols are also described for the dual labeling of cells in the same section using primary antisera raised in two different species (one to the receptor of interest, the second to an immunogen such as a cell-specific marker or the endogenous ligand) with the corresponding secondary antisera conjugated to different fluorescent dyes.The technique has a range of applications. Subtypes of receptors can be identified and distinguished prior to the development of selective agonists or antagonists, which is particularly important for mapping orphan receptors, where the identity of the endogenous ligand in not yet known. The deletion of genes encoding receptors, particularly in mice, has emerged as a powerful tool in understanding the role of a specific receptor in physiological processes. Receptor immunocytochemistry can be used to analyze the resulting phenotype in whole body sections of mice without preselection of the tissue to be studied.

Dishevelled C-terminus: prolyl and histidinyl motifs

The phosphoprotein scaffold Dishevelled is an essential component of both Wnt signalling and of the signalsome that constitutes the supermolecular 'punctae' of assembled proteins often observed in fluorescence microscopy. The C-terminal region beyond the DEP domain displays unique and interesting character, exploited herein by careful analysis of the primary structure. Human Dishevelled-1, -2, -3 and fly Dishevelled (Dsh) sequences were downloaded and interrogated in silico. The C-terminus of Dishevelled-3 is revealed by FoldIndex(®) to be rich in ordered structure. It displays primary sequence that is unique and divergent in important ways from vertebrate isoforms as well as from the fly Dsh. The region is amphipathic, high in prolyl content, and harbours polyprolines. Dishevelled-3 displays some regions, where the proline content is >40%. Polyprolyl sequences (2-4 residues) likely constitute important sites of interaction with other Dishevelled isoforms. Several histidine-single amino acid repeats are notable. The 637,638/647,648 repeats of Dvl3 are essential for Wnt non-canonical, but not canonical signalling. Mutagenesis reveals that the C-terminal sequence is essential for the formation of punctae, made visible by fluorescence microscopy. These Dvl3-based signalsomes are very large (25-35 MDa-MW), supermolecular complexes that display dynamic reorganization in response to Wnt stimulation. Dishevelled-3 C-terminus is rich in structure and unique motifs, worthy of detailed analysis with modern molecular tools.

Radioligand binding assays and their analysis

Radioligand binding is widely used to characterize receptors and determine their anatomical distribution, particularly the superfamily of seven transmembrane-spanning G protein-coupled receptors for both established transmitters such as endothelin-1 and an increasing number of orphan receptors recently paired with their cognate ligands. Three types of assay are described. In saturation experiments, tissue sections, cultured cells, or homogenates are incubated with an increasing concentration of a radiolabeled ligand, which can be a labeled analog of a naturally occurring transmitter, hormone, or synthetic drug. Analysis using iterative nonlinear curve-fitting programs, such as KELL, measures the affinity of the labeled ligand for a receptor (equilibrium dissociation constant, K ( D )), receptor density (B (max)), and Hill slope (nH). The affinity and selectivity of an unlabeled ligand to compete for the binding of a fixed concentration of a radiolabeled ligand to a receptor are determined using a competition binding assay. Kinetic assays measure the rate of association to or dissociation from a receptor from which a kinetic K ( D ) may be derived. Quantitative autoradiography and image analysis is a sensitive technique to detect low levels of radiolabeled ligands and determine the anatomical distribution of receptors in sections that retain the morphology of the tissue. The measurement of bound radioligand within discrete regions of autoradiographical images using -computer-assisted image analysis is described.

Feeding and the rhodopsin family g-protein coupled receptors in nematodes and arthropods

In vertebrates, receptors of the rhodopsin G-protein coupled superfamily (GPCRs) play an important role in the regulation of feeding and energy homeostasis and are activated by peptide hormones produced in the brain-gut axis. These peptides regulate appetite and energy expenditure by promoting or inhibiting food intake. Sequence and function homologs of human GPCRs involved in feeding exist in the nematode roundworm, Caenorhabditis elegans (C. elegans), and the arthropod fruit fly, Drosophila melanogaster (D. melanogaster), suggesting that the mechanisms that regulate food intake emerged early and have been conserved during metazoan radiation. Nematodes and arthropods are the most diverse and successful animal phyla on Earth. They can survive in a vast diversity of environments and have acquired distinct life styles and feeding strategies. The aim of the present review is to investigate if this diversity has affected the evolution of invertebrate GPCRs. Homologs of the C. elegans and D. melanogaster rhodopsin receptors were characterized in the genome of other nematodes and arthropods and receptor evolution compared. With the exception of bombesin receptors (BBR) that are absent from nematodes, a similar gene complement was found. In arthropods, rhodopsin GPCR evolution is characterized by species-specific gene duplications and deletions and in nematodes by gene expansions in species with a free-living stage and gene deletions in representatives of obligate parasitic taxa. Based upon variation in GPCR gene number and potentially divergent functions within phyla we hypothesize that life style and feeding diversity practiced by nematodes and arthropods was one factor that contributed to rhodopsin GPCR gene evolution. Understanding how the regulation of food intake has evolved in invertebrates will contribute to the development of novel drugs to control nematodes and arthropods and the pests and diseases that use them as vectors.

Negative cross-talk between calcium-sensing receptor and β-catenin signaling systems in colonic epithelium

Here, we examined the role of the extracellular Ca(2+)-sensing receptor (CaSR) in the control of colonic epithelial cell proliferation in vivo and changes in β-catenin triggered by CaSR stimulation in human colonic epithelial cells in vitro. The in vivo studies, using a novel Casr intestinal-specific knock-out mouse, indicate that the genetic ablation of the Casr leads to hyperproliferation of colonic epithelial cells, expansion of the proliferative zone, changes in crypt structure, and enhanced β-catenin nuclear localization. The in vitro results indicate that stimulation of the CaSR, by Ca(2+) or by the calcimimetic R-568, produced a striking and time-dependent decrease in the phosphorylation of β-catenin at Ser-552 and Ser-675, two amino acid residues that promote β-catenin transcriptional activity. The reduced phosphorylation of β-catenin coincided with a decline in its nuclear localization and a marked redistribution to the plasma membrane. Furthermore, CaSR stimulation promoted a down-regulation of β-catenin-mediated transcriptional activation. These studies demonstrate that signaling pathways emanating from the CaSR control colonic epithelial cell proliferation in vivo and suggest that the mechanism involves regulation of β-catenin phosphorylation.

Recombinant WNTs differentially activate β-catenin-dependent and -independent signalling in mouse microglia-like cells

AIM

The objective of this study was to compare the efficacy of different recombinant, commercially available Wingless/Int-1 (WNTs) with regard to WNT/β-catenin signalling, dishevelled (DVL) and G protein activation and the induction of cell proliferation in a microglia-like cell line called N13.

METHODS

For detection of activated signalling molecules, cell lysates are analysed by immunoblotting. Furthermore, we used a [γ(35)S] GTP binding assay to monitor the exchange of GDP for GTP in heterotrimeric G proteins in N13 membrane preparations. Cell proliferation was assessed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay measuring mitochondrial function, which is proportional to the amount of viable cells.

RESULTS

Of the WNTs tested (WNT-3A, -4, -5A, -5B, -7A,-9B), only WNT-3A activated WNT/β-catenin signalling in N13 cells. All WNTs induced the formation of phosphorylated and shifted DVL (PS-DVL) and the activation of heterotrimeric G proteins with variable efficacies. WNT-5A and WNT-9B, which had the highest efficacy in the G protein assay, also induced N13 cell proliferation.

CONCLUSION

WNTs show significant differences in their efficacy to activate β-catenin-dependent and -independent signalling. The WNTs tested are present during maturation of the central nervous system and/or in the adult brain and are thus potential regulators of microglia-mediated neuroinflammation.

Secreted Wnt "inhibitors" are not just inhibitors: regulation of extracellular Wnt by secreted Frizzled-related proteins

Gradient formation and signaling ranges of secreted proteins are crucial problems to understand how morphogens work for positional information and patterning in animal development. Yet, extracellular behaviors of secreted signaling molecules remain unexplored compared to their downstream pathways inside the cell. Recent advances in bioimaging make it possible to directly visualize morphogen molecules, and this simple strategy has, at least partly, succeeded in uncovering molecular behaviors of morphogens, such as Wnt (wingless-type MMTV integration site family member) and BMP (bone morphogenetic protein) as well as secreted Wnt binding proteins, sFRPs (secreted Frizzled-related proteins), in embryonic tissues. Here, we review the regulation of Wnt signaling by sFRPs, focusing on extracellular regulation of Wnt ligands in comparison with other morphogens. We also discuss evolutionary aspects with comprehensive syntenic and phylogenetic information about vertebrate sfrp genes. We newly annotated several sfrp genes including sfrp2-like 1 (sfrp2l1) in frogs and fishes and crescent in mammals.

Reporter gene HEK 293 cells and WNT/Frizzled fusion proteins as tools to study WNT signaling pathways

WNT/Frizzled receptor (FZD) signaling pathways are pivotal for physiological and pathophysiological processes. In humans, the complexity of WNT/FZD signaling is based on 19 WNTs, 10 FZDs and at least two (co)receptors (LRP5/6) mediating supposably four different signaling cascades. The detailed investigation of the specific function of the different initiating components is primarily hampered by the lack of most WNT proteins in a purified form. Therefore, we constructed and examined a chimeric protein of WNT3a and FZD4 as a suitable approach to overcome this obstacle for future studies of the specificity of other WNT/FZD combinations. Furthermore, we produced four different reporter HEK 293 cell lines to quantify the induced activation of the proposed signaling cascades, the β-catenin-, the NFAT-, the AP-1- and the CRE-regulated pathways. The chimera WNT3aFZD4 efficiently induced β-catenin-mediated luciferase activity. This activity was increased 40-fold compared with basal when LRP6 was stably cotransfected, proving that the chimera WNT3aFZD4 can also interact efficiently with LRP6. Our results demonstrate that the approach of using reporter gene cell lines in combination with WNT/FZD chimeras is efficient to study the β-catenin-mediated pathway and should also allow clarifying the specificity of WNT/FZD combinations in the activation of the other pathways.

Mutations in Frizzled 6 cause isolated autosomal-recessive nail dysplasia

Inherited and isolated nail malformations are rare and heterogeneous conditions. We identified two consanguineous pedigrees in which some family members were affected by isolated nail dysplasia that suggested an autosomal-recessive inheritance pattern and was characterized by claw-shaped nails, onychauxis, and onycholysis. Genome-wide SNP array analysis of affected individuals from both families showed an overlapping and homozygous region of 800 kb on the long arm of chromosome 8. The candidate region spans eight genes, and DNA sequence analysis revealed homozygous nonsense and missense mutations in FZD(6), the gene encoding Frizzled 6. FZD(6) belongs to a family of highly conserved membrane-bound WNT receptors involved in developmental processes and differentiation through several signaling pathways. We expressed the FZD(6) missense mutation and observed a quantitative shift in subcellular distribution from the plasma membrane to the lysosomes, where the receptor is inaccessible for signaling and presumably degraded. Analysis of human fibroblasts homozygous for the nonsense mutation showed an aberrant response to both WNT-3A and WNT-5A stimulation; this response was consistent with an effect on both canonical and noncanonical WNT-FZD signaling. A detailed analysis of the Fzd(6)(-/-) mice, previously shown to have an altered hair pattern, showed malformed claws predominantly of the hind limbs. Furthermore, a transient Fdz6 mRNA expression was observed in the epidermis of the digital tips at embryonic day 16.5 during early claw morphogenesis. Thus, our combined results show that FZD6 mutations can result in severe defects in nail and claw formation through reduced or abolished membranous FZD(6) levels and several nonfunctional WNT-FZD pathways.

Wnt signalling: the case of the 'missing' G-protein

Wnt signalling remains a hot topic for cell signalling sleuthhounds. The trail of signalling downstream of the seven-transmembrane segment Frizzleds, which bind Wnt ligands, is replete of clues [e.g. LPR5/6 (lipoprotein receptor-related protein 5/6), G-proteins or Dishevelled] and yet remains the 'final problem'. Although the heptahelical nature of Frizzleds places them well within a populous family of G-protein-coupled receptors, resistance to this theme has waxed and waned amid increasing demands for 'proof'. The Wnt Homepage (http://www.stanford.edu/group/nusselab/cgi-bin/wnt/) has acted as a dynamic real-time arbiter of the controversy, highlighted by the appearance and later the disappearance of the G-protein from its central diagramming and tabulations. A recent publication in this issue of the Biochemical Journal offers a solution to the 'final problem', demonstrating under native conditions that Frizzleds expressed in mammalian brain preparations act functionally to catalyse guanine-nucleotide exchange in response to stimulation with Wnt3a. Lensed from the fictional character of Sherlock Holmes, The Case of the Missing G-Protein is investigated.

WNT-5A stimulates the GDP/GTP exchange at pertussis toxin-sensitive heterotrimeric G proteins

The lipoglycoproteins of the WNT family act on seven transmembrane-spanning Class Frizzled receptors. Here, we show that WNT-5A evokes a proliferative response in a mouse microglia-like cell line (N13), which is sensitive to pertussis toxin, thus implicating the involvement of heterotrimeric G proteins of the G(i/o) family. We continue to show that WNT-5A stimulation of N13 membranes and permeabilized cells evokes the exchange of GDP for GTP of pertussis toxin-sensitive G proteins employing [γ-(35)S]GTP assay and activity state-specific antibodies to GTP-bound G(i) proteins. Our functional analysis of the PTX-sensitivity of WNT-induced G protein activation and PCR analysis of G protein and FZD expression patterns suggest that WNT-5A stimulation leads to the activation of G(i2/3) proteins in N13 cells possibly mediated by FZD(5), the predominant FZD expressed. In summary, we provide for the first time molecular proof that WNT-5A stimulation results in the activation of heterotrimeric G(i2/3) proteins in mammalian cells with physiological protein stochiometry.

WNT signaling in activated microglia is proinflammatory

Microglia activation is central to the neuroinflammation associated with neurological and neurodegenerative diseases, particularly because activated microglia are often a source of proinflammatory cytokines. Despite decade-long research, the molecular cascade of proinflammatory transformation of microglia in vivo remains largely elusive. Here, we report increased β-catenin expression, a central intracellular component of WNT signaling, in microglia undergoing a proinflammatory morphogenic transformation under pathogenic conditions associated with neuroinflammation such as Alzheimer's disease. We substantiate disease-associated β-catenin signaling in microglia in vivo by showing age-dependent β-catenin accumulation in mice with Alzheimer's-like pathology (APdE9). In cultured mouse microglia expressing the WNT receptors Frizzled FZD(4,5,7,8) and LDL receptor-related protein 5/6 (LRP5/6), we find that WNT-3A can stabilize β-catenin. WNT-3A dose dependently induces LRP6 phosphorylation with downstream activation of disheveled, β-catenin stabilization, and nuclear import. Gene-expression profiling reveals that WNT-3A stimulation specifically increases the expression of proinflammatory immune response genes in microglia and exacerbates the release of de novo IL-6, IL-12, and tumor necrosis factor α. In summary, our data suggest that the WNT family of lipoglycoproteins can instruct proinflammatory microglia transformation and emphasize the pathogenic significance of β-catenin-signaling networks in this cell type.