FRAS1

Abstract: Cell adhesion to extracellular matrix (ECM) proteins is crucial for the structural integrity of tissues and epithelial-mesenchymal interactions mediating organ morphogenesis1,2. Here we describe how the loss of a cytoplasmic multi-PDZ scaffolding protein, glutamate receptor interacting protein 1 (GRIP1), leads to the formation of subepidermal hemorrhagic blisters, renal agenesis, syndactyly or polydactyly and permanent fusion of eyelids (cryptophthalmos). Similar malformations are characteristic of individuals with Fraser syndrome and animal models of this human genetic disorder, such as mice carrying the blebbed mutation (bl) in the gene encoding the Fras1 ECM protein3,4. GRIP1 can physically interact with Fras1 and is required for the localization of Fras1 to the basal side of cells. In one animal model of Fraser syndrome, the eye-blebs (eb) mouse, Grip1 is disrupted by a deletion of two coding exons. Our data indicate that GRIP1 is required for normal cell-matrix interactions during early embryonic development and that inactivation of Grip1 causes Fraser syndrome–like defects in mice.

Abstract: An emerging family of extracellular matrix proteins characterized by 12 consecutive CSPG repeats and the presence of Calx-beta motif(s) includes Fras1, QBRICK/Frem1, and Frem2. Mutations in the genes encoding these proteins have been associated with mouse models of Fraser syndrome, which is characterized by subepidermal blistering, cryptophthalmos, syndactyly, and renal dysmorphogenesis. Here, we report that all of these proteins are localized to the basement membrane, and that their basement membrane localization is simultaneously impaired in Fraser syndrome model mice. In Frem2 mutant mice, not only Frem2 but Fras1 and QBRICK/Frem1 were depleted from the basement membrane zone. This coordinated reduction in basement membrane deposition was also observed in another Fraser syndrome model mouse, in which GRIP1, a Fras1- and Frem2-interacting adaptor protein, is primarily affected. Targeted disruption of Qbrick/Frem1 also resulted in diminished expression of Fras1 and Frem2 at the epidermal basement membrane, confirming the reciprocal stabilization of QBRICK/Frem1, Fras1, and Frem2 in this location. When expressed and secreted by transfected cells, these proteins formed a ternary complex, raising the possibility that their reciprocal stabilization at the basement membrane is due to complex formation. Given the close association of Fraser syndrome phenotypes with defective epidermal-dermal interactions, the coordinated assembly of three Fraser syndrome-associated proteins at the basement membrane appears to be instrumental in epidermal-dermal interactions during morphogenetic processes.

Abstract: Fraser syndrome is a rare recessive disorder characterized by cryptophthalmos, syndactyly, renal defects, and a range of other developmental abnormalities. Because of their extensive phenotypic overlap, the mouse blebbing mutants have been considered models of this disorder, and the recent isolation of mutations in Fras1 in both the blebbed mouse and human Fraser patients confirms this hypothesis. Here we report the identification of mutations in an extracellular matrix gene Fras1-related extracellular matrix gene 1 (Frem1) in both the classic head blebs mutant and in an N-ethyl-N-nitrosourea-induced allele. We show that inactivation of the gene results in the formation of in utero epidermal blisters beneath the lamina densa of the basement membrane and also in renal agenesis. Frem1 is expressed widely in the developing embryo in regions of epithelial/mesenchymal interaction and epidermal remodeling. Furthermore, Frem1 appears to act as a dermal mediator of basement membrane adhesion, apparently independently of the other known “blebs” proteins Fras1 and Grip1. Unlike both Fras1 and Grip1 mutants, collagen VI and Fras1 deposition in the basement membrane is normal, indicating that the protein plays an independent role in epidermal differentiation and is required for epidermal adhesion during embryonic development.

Abstract: Using forward genetics, we have identified the genes mutated in two classes of zebrafish fin mutants. The mutants of the first class are characterized by defects in embryonic fin morphogenesis, which are due to mutations in a Laminin subunit or an Integrin alpha receptor, respectively. The mutants of the second class display characteristic blistering underneath the basement membrane of the fin epidermis. Three of them are due to mutations in zebrafish orthologues of FRAS1, FREM1, or FREM2, large basement membrane protein encoding genes that are mutated in mouse bleb mutants and in human patients suffering from Fraser Syndrome, a rare congenital condition characterized by syndactyly and cryptophthalmos. Fin blistering in a fourth group of zebrafish mutants is caused by mutations in Hemicentin1 (Hmcn1), another large extracellular matrix protein the function of which in vertebrates was hitherto unknown. Our mutant and dose-dependent interaction data suggest a potential involvement of Hmcn1 in Fraser complex-dependent basement membrane anchorage. Furthermore, we present biochemical and genetic data suggesting a role for the proprotein convertase FurinA in zebrafish fin development and cell surface shedding of Fras1 and Frem2, thereby allowing proper localization of the proteins within the basement membrane of forming fins. Finally, we identify the extracellular matrix protein Fibrillin2 as an indispensable interaction partner of Hmcn1. Thus we have defined a series of zebrafish mutants modelling Fraser Syndrome and have identified several implicated novel genes that might help to further elucidate the mechanisms of basement membrane anchorage and of the disease's aetiology. In addition, the novel genes might prove helpful to unravel the molecular nature of thus far unresolved cases of the human disease.