Wnt proteins constitute a large family of secreted molecules that are involved in intercellular signalling during development. The name derives from the first 2 members of the family to be discovered: int-1 (mouse) and wingless (Drosophila) []. It is now recognised that Wnt signalling controls many cell fate decisions in a variety of different organisms, including mammals []. Wnt signalling has been implicated in tumourigenesis, early mesodermal patterning of the embryo, morphogenesis of the brain and kidneys, regulation of mammary gland proliferation and Alzheimer's disease [, ].Wnt-mediated signalling is believed to proceed initially through binding to cell surface receptors of the frizzled family; the signal is subsequently transduced through several cytoplasmic components to B-catenin, which enters the nucleus and activates the transcription of several genes important indevelopment []. Several non-canonical Wnt signalling pathways have also been elucidated that act independently of B-catenin. Canonical and noncanonical Wnt signaling branches are highly interconnected, and cross-regulate each other [].Members of the Wnt gene family are defined by their sequence similarity to mouse Wnt-1 and Wingless in Drosophila. They encode proteins of ~350-400 residues in length, with orthologues identified in several, mostly vertebrate, species. Very little is known about the structure of Wnts as they are notoriously insoluble, but they share the following features characteristics of secretory proteins: a signal peptide, several potential N-glycosylation sites and 22 conserved cysteines []that are probably involved in disulphide bonds. The Wnt proteins seem to adhere to the plasma membrane of the secreting cells and are therefore likely to signal over only few cell diameters. Fifteen major Wnt gene families have been identified in vertebrates, with multiple subtypes within some classes.Wnt5a is required for prostate gland development [], intestinal elongation []and endothelial differentiation of embryonic stem cells [].
Glypicans are heparan sulphate proteoglycans (HSPGs) attached to the cell membrane by a glycosylphosphatidylinositol (GPI) anchor that interact with various extracellular growth factors and receptors. Six members (GPC1-6) are known in vertebrates [].Glypican-6 has a role in endochondral ossification and skeletal growth []. It also enhances migration and invasion of cancer cells through WNT5A signalling [].
The Ror family of receptor tyrosine kinases consists of two structurally related proteins, Ror1 and Ror2. Ror1 is a pseudokinase that acts as a substrate for the oncogenic tyrosine kinase Met []. It is expressed during development []. It shows no significant expression in normal adult tissues, but it is selectively overexpressed in a number of malignancies []. Ror2 functions as a Wnt receptor required to maintain basal NMDAR-mediated synaptic transmission []. For a time its ligand remained elusive, hence the name receptor tyrosine kinase-like orphan receptor-2 (Ror2). It is now established that Wnt5A acts a ligand for Ror2 [].
The cysteine-rich domain (CRD) is an essential part of the crescent protein, a member of the secreted frizzled-related protein (SFRP) family, which regulates convergent extension movements (CEMs) during gastrulation and neurulation. Xenopus laevis crescent efficiently forms inhibitory complexes with Wnt5a and Wnt11, but this effect is cancelled in the presence of another member of the SFRP family, Frzb1. A potential role for Crescent in head formation is to regulate a non-canonical Wnt pathway positively in the adjacent posterior mesoderm, and negatively in the overlying anterior neuroectoderm [].
Wnt proteins constitute a large family of secreted molecules that are involved in intercellular signalling during development. The name derives from the first 2 members of the family to be discovered: int-1 (mouse) and wingless (Drosophila) []. It is now recognised that Wnt signalling controls many cell fate decisions in a variety of different organisms, including mammals []. Wnt signalling has been implicated in tumourigenesis, early mesodermal patterning of the embryo, morphogenesis of the brain and kidneys, regulation of mammary gland proliferation and Alzheimer's disease [, ].Wnt-mediated signalling is believed to proceed initially through binding to cell surface receptors of the frizzled family; the signal is subsequently transduced through several cytoplasmic components to B-catenin, which enters the nucleus and activates the transcription of several genes important indevelopment []. Several non-canonical Wnt signalling pathways have also been elucidated that act independently of B-catenin. Canonical and noncanonical Wnt signaling branches are highly interconnected, and cross-regulate each other [].Members of the Wnt gene family are defined by their sequence similarity to mouse Wnt-1 and Wingless in Drosophila. They encode proteins of ~350-400 residues in length, with orthologues identified in several, mostly vertebrate, species. Very little is known about the structure of Wnts as they are notoriously insoluble, but they share the following features characteristics of secretory proteins: a signal peptide, several potential N-glycosylation sites and 22 conserved cysteines []that are probably involved in disulphide bonds. The Wnt proteins seem to adhere to the plasma membrane of the secreting cells and are therefore likely to signal over only few cell diameters. Fifteen major Wnt gene families have been identified in vertebrates, with multiple subtypes within some classes.Wnt5b might be implicated in human carcinogenesis through activation of the WNT-beta-catenin-TCF signaling pathway, just like Wnt5a [].
