LGR4 and LGR5 are R-spondin receptors mediating Wnt/β-catenin and Wnt/PCP signalling.
TL;DR: Gain‐ and loss‐of‐function experiments in mammalian cells and Xenopus embryos indicate that LGR4 and LGR5 promote R‐spondin‐mediated Wnt/β‐catenin and WNT/PCP signalling, suggesting that internalization has a mechanistic role in R‐ Spondin signalling.
Abstract: R-spondins are secreted Wnt signalling agonists, which regulate embryonic patterning and stem cell proliferation, but whose mechanism of action is poorly understood. Here we show that R-spondins bind to the orphan G-protein-coupled receptors LGR4 and LGR5 by their Furin domains. Gain- and loss-of-function experiments in mammalian cells and Xenopus embryos indicate that LGR4 and LGR5 promote R-spondin-mediated Wnt/β-catenin and Wnt/PCP signalling. R-spondin-triggered β-catenin signalling requires Clathrin, while Wnt3a-mediated β-catenin signalling requires Caveolin-mediated endocytosis, suggesting that internalization has a mechanistic role in R-spondin signalling.
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TL;DR: An update of the core Wnt/β-catenin signaling pathway is provided, how its various components contribute to disease, and outstanding questions to be addressed in the future are discussed.
4,561 citations
Cites background from "LGR4 and LGR5 are R-spondin recepto..."
...Lgr Molecules are R-Spondin Receptors that Enhance Wnt Signaling Recent studies have uncovered a small family of 7-TM receptors, the Lgr5 family, whichmediate Rspo input into the canonical Wnt pathway (Carmon et al., 2011; de Lau et al., 2011; Glinka et al., 2011)....
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...Recent studies have uncovered a small family of 7-TM receptors, the Lgr5 family, whichmediate Rspo input into the canonical Wnt pathway (Carmon et al., 2011; de Lau et al., 2011; Glinka et al., 2011)....
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TL;DR: The core Wnt/β-catenin signaling pathway is described, how it controls stem cells, and contributes to disease, and strategies for Wnt-based therapies are discussed.
2,663 citations
Cites background from "LGR4 and LGR5 are R-spondin recepto..."
...Three members of a small family of 7-TM receptors, Lgr4, Lgr5, and Lgr6 family, bind R-spondins with high affinity and are essential for signal enhancement of low dose Wnt (Carmon et al., 2011; de Lau et al., 2011; Glinka et al., 2011)....
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TL;DR: Current insights into novel components of Wnt pathways are reviewed and how Wnt signaling affects maintenance of cancer stem cells, metastasis and immune control are described.
Abstract: Wnt signaling is one of the key cascades regulating development and stemness, and has also been tightly associated with cancer. The role of Wnt signaling in carcinogenesis has most prominently been described for colorectal cancer, but aberrant Wnt signaling is observed in many more cancer entities. Here, we review current insights into novel components of Wnt pathways and describe their impact on cancer development. Furthermore, we highlight expanding functions of Wnt signaling for both solid and liquid tumors. We also describe current findings how Wnt signaling affects maintenance of cancer stem cells, metastasis and immune control. Finally, we provide an overview of current strategies to antagonize Wnt signaling in cancer and challenges that are associated with such approaches.
1,698 citations
TL;DR: What emerges is an intricate network of receptors that form higher-order ligand–receptor complexes routing downstream signalling that is regulated both extracellularly by agonists such as R-spondin and intracellulary by post-translational modifications such as phosphorylation, proteolytic processing and endocytosis.
Abstract: 30 years after the identification of WNTs, their signal transduction has become increasingly complex, with the discovery of more than 15 receptors and co-receptors in seven protein families. The recent discovery of three receptor classes for the R-spondin family of WNT agonists further adds to this complexity. What emerges is an intricate network of receptors that form higher-order ligand-receptor complexes routing downstream signalling. These are regulated both extracellularly by agonists such as R-spondin and intracellularly by post-translational modifications such as phosphorylation, proteolytic processing and endocytosis.
1,200 citations
TL;DR: The widespread importance of Wnt signaling in driving tissue renewal has been revealed by the identification of Axin2 and Lgr5, genes expressed in cells that are responding to Wnt signals, and this crucial role in stem cell self renewal is reviewed.
Abstract: BACKGROUND In adult mammalian organisms, multiple tissues—including the skin, blood, stomach, and intestines—are entrapped in a state of permanent regeneration; older cells are constantly shed, and the tissue is continuously being regenerated from resident stem cells. This phenomenon of “tissue renewal” was appreciated by Leblond in 1956, but the underlying mechanism has been unclear. It is now evident that a class of extracellular developmental signaling proteins, known as Wnt signals, animate the continued renewal of several mammalian tissues by fuelling stem cell activity. If the Wnt pathway is inhibited, tissue renewal is crippled. This signaling pathway is an ancient evolutionary program dating from when Wnt signals arose in the simplest multicellular organisms, in which Wnts acted as primordial symmetry-breaking signals crucial for the generation of patterned tissues during embryogenesis. In vertebrates, these signals also function in pattern maintenance: They sustain tissue renewal, enabling tissues to be continuously replenished and maintained over a lifetime. Multiple adult organs are in a state of continual regeneration. In tissues such as the skin, intestines, brain, and mammary glands, Wnt signaling proteins sustain this constant regeneration by inducing stem cells (green cells in the illustration) to grow. This leads to the robust supply of new cells (green) in order to replenish and maintain the tissue. [Image credits are available in the full article online.] ADVANCES In contrast to traditional “long-range” developmental signals, Wnts seem to act as short-range intercellular signals—acting mostly between adjacent cells. Lending credence to this notion, a membrane-tethered Wnt protein variant can fulfill most functions of a normal Wnt protein in Drosophila . Likely explaining the short-range nature of these signals, Wnt proteins are attached to a lipid and therefore are hydrophobic; they cannot freely traverse the extracellular space by themselves. This provides insight into how tissue renewal is regulated. It implies that Wnt signals emanating from the stem cell microenvironment (the “niche”) may influence adjacent stem cells without affecting a broad field of cells located farther away. The concept of an external niche, however, may have to be refined because it is clear that stem cells can sometimes act as their own niche and have unexpected developmental self-organizing capacities. Last, the widespread importance of Wnt signaling in driving tissue renewal has been revealed by the identification of Axin2 and Lgr5 , genes expressed in cells that are responding to Wnt signals. Genetically labeling Axin2 + or Lgr5 + cells in a variety of tissues has revealed that such cells fuel tissue renewal in the intestines, mammary gland, skin, and brain, among other organs. OUTLOOK The amazing continuous self-regeneration of various mammalian tissues over years and decades continues to be an enigmatic terra incognita in biology. For instance, visualization of stem cells in real-time in vivo (through intravital microscopy) has shown that when some stem cells are ablated, they are replaced by more differentiated cells that are recalled to the stem cell niche, whereupon they regain stem cell identity to effect tissue repair. Therefore, lineage barriers between stem cell and differentiated fates are not always stringent and can be traversed during times of tissue damage. Reactivated Wnt signals may be instrumental in this process, and perhaps such signals could be exploited in order to enkindle tissue regeneration after injury or disease. From a pragmatic perspective, Wnt signals have already found practical use in manipulating stem cells, enabling propagation of stem cells in vitro as self-renewing cell populations and as organoids.
1,032 citations
References
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TL;DR: Genetic deletion of Lgr4/5 in mouse intestinal crypt cultures phenocopies withdrawal of Rspo1 and can be rescued by Wnt pathway activation, which will guide future studies towards the application of R-spondins for regenerative purposes of tissues expressing Lgr5 homologues.
Abstract: The adult stem cell marker Lgr5 and its relative Lgr4 are often co-expressed in Wnt-driven proliferative compartments. We find that conditional deletion of both genes in the mouse gut impairs Wnt target gene expression and results in the rapid demise of intestinal crypts, thus phenocopying Wnt pathway inhibition. Mass spectrometry demonstrates that Lgr4 and Lgr5 associate with the Frizzled/Lrp Wnt receptor complex. Each of the four R-spondins, secreted Wnt pathway agonists, can bind to Lgr4, -5 and -6. In HEK293 cells, RSPO1 enhances canonical WNT signals initiated by WNT3A. Removal of LGR4 does not affect WNT3A signalling, but abrogates the RSPO1-mediated signal enhancement, a phenomenon rescued by re-expression of LGR4, -5 or -6. Genetic deletion of Lgr4/5 in mouse intestinal crypt cultures phenocopies withdrawal of Rspo1 and can be rescued by Wnt pathway activation. Lgr5 homologues are facultative Wnt receptor components that mediate Wnt signal enhancement by soluble R-spondin proteins. These results will guide future studies towards the application of R-spondins for regenerative purposes of tissues expressing Lgr5 homologues.
1,133 citations
"LGR4 and LGR5 are R-spondin recepto..." refers methods in this paper
...For the Rspo3 internalization assay, hRspo3-DC–HRP (25 U/ml) was used and detected by tyramide signal amplification (TSA; Dubois et al, 2001; Speel et al, 2006)....
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TL;DR: This work has shown that multiple extracellular, cytoplasmic, and nuclear components modulate Wnt signaling, and it is clear that Wnt signals are required for adult tissue maintenance.
Abstract: The highly conserved Wnt secreted proteins are critical mediators of cell-to-cell signaling during development of animals. Recent biochemical and genetic analyses have led to significant insight into understanding how Wnt signals work. The catalogue of Wnt signaling components has exploded. We now realize that multiple extracellular, cytoplasmic, and nuclear components modulate Wnt signaling. Moreover, receptor-ligand specificity and multiple feedback loops determine Wnt signaling outputs. It is also clear that Wnt signals are required for adult tissue maintenance. Perturbations in Wnt signaling cause human degenerative diseases as well as cancer.
982 citations
"LGR4 and LGR5 are R-spondin recepto..." refers background in this paper
...Wnts have a critical role in development and disease, and understanding their complex signalling mechanisms and biological roles is of wide interest (Nusse, 2005; Grigoryan et al, 2008)....
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...INTRODUCTION Wnts have a critical role in development and disease, and understanding their complex signalling mechanisms and biological roles is of wide interest (Nusse, 2005; Grigoryan et al, 2008)....
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TL;DR: It is demonstrated that LGR4 and LGR5 bind the R-spondins with high affinity and mediate the potentiation of Wnt/β-catenin signaling by enhancing Wnt-induced LRP6 phosphorylation, indicating a unique mechanism of action.
Abstract: The Wnt/β-catenin signaling system plays essential roles in embryonic development and in the self-renewal and maintenance of adult stem cells. R-spondins (RSPOs) are a group of secreted proteins that enhance Wnt/β-catenin signaling and have pleiotropic functions in development and stem cell growth. LGR5, an orphan receptor of the G protein-coupled receptor (GPCR) superfamily, is specifically expressed in stem cells of the intestinal crypt and hair follicle. Knockout of LGR5 in the mouse results in neonatal lethality. LGR4, a receptor closely related to LGR5, also has essential roles in development, as its knockout leads to reduced viability and retarded growth. Overexpression of both receptors has been reported in several types of cancer. Here we demonstrate that LGR4 and LGR5 bind the R-spondins with high affinity and mediate the potentiation of Wnt/β-catenin signaling by enhancing Wnt-induced LRP6 phosphorylation. Interestingly, neither receptor is coupled to heterotrimeric G proteins or to β-arrestin when stimulated by the R-spondins, indicating a unique mechanism of action. The findings provide a basis for stem cell-specific effects of Wnt/β-catenin signaling and for the broad range of functions LGR4, LGR5, and the R-spondins have in normal and malignant growth.
787 citations
"LGR4 and LGR5 are R-spondin recepto..." refers background or methods in this paper
...Cell culture, conditioning of media, cell surface binding, Wnt reporter and Xenopus assays were carried out as described (Cruciat et al, 2010; Ohkawara et al, 2011)....
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...RESULTS AND DISCUSSION LGR4 and LGR5 promote R-spondin signalling In a search for an R-spondin receptor we carried out a genomewide siRNA screen (Cruciat et al, 2010)....
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TL;DR: It is reported that Wnt signaling triggers the sequestration of GSK3 from the cytosol into multivesicular bodies (MVBs), so that this enzyme becomes separated from its many cytosolic substrates.
663 citations
"LGR4 and LGR5 are R-spondin recepto..." refers background in this paper
...Our finding that Clathrin is essential for Rspo3triggered b-catenin signalling is consistent with the model that GSK3 bound to LRP6 signalosomes needs to be sequestered in multivesicular bodies to allow accumulation of b-catenin (Taelman et al, 2010)....
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TL;DR: The data show, for the first time, that disruption of a single gene can lead to complete female-to-male sex reversal in the absence of the testis-determining gene, SRY.
Abstract: R-spondins are a recently characterized small family of growth factors. Here we show that human R-spondin1 (RSPO1) is the gene disrupted in a recessive syndrome characterized by XX sex reversal, palmoplantar hyperkeratosis and predisposition to squamous cell carcinoma of the skin. Our data show, for the first time, that disruption of a single gene can lead to complete female-to-male sex reversal in the absence of the testis-determining gene, SRY.
609 citations
"LGR4 and LGR5 are R-spondin recepto..." refers background in this paper
...They are involved in embryonic patterning and differentiation in frogs and mice (Kazanskaya et al, 2004, 2008; Aoki et al, 2006; Blaydon et al, 2006; Parma et al, 2006)....
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...R-spondins are also implicated in human disease and hold therapeutic promise as potent stem cell growth factors (Kim et al, 2005; Blaydon et al, 2006; Parma et al, 2006; Zhao et al, 2009)....
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