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In central nervous system vascular endothelial cells, signaling via the partially redundant ligands WNT7A and WNT7B requires two co-activator proteins, GPR124 and RECK. WNT7A and RECK have been shown previously to play a role in limb development, but the mechanism of RECK action in this context is unknown. The roles of WNT7B and GPR124 in limb development have not been investigated. Using combinations of conventional and/or conditional loss-of-function alleles for mouse Wnt7a, Wnt7b, Gpr124 and Reck, including a Reck allele that codes for a protein that is specifically defective in WNT7A/WNT7B signaling, we show that reductions in ligand and/or co-activator function synergize to cause reduced and dysmorphic limb bone growth. Two additional limb phenotypes - loss of distal Lmx1b expression and ectopic growth of nail-like structures - occur with reduced Wnt7a/Wnt7b gene copy number and, respectively, with Reck mutations and with combined Reck and Gpr124 mutations. A third limb phenotype - bleeding into a digit - occurs with the most severe combinations of Wnt7a/Wnt7b, Reck and Gpr124 mutations. These data imply that the WNT7A/WNT7B-FRIZZLED-LRP5/LRP6-GPR124-RECK signaling system functions as an integral unit in limb development., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2022. Published by The Company of Biologists Ltd.)

In mammals, a set of anatomically diverse polarity processes - including axon growth and guidance, hair follicle orientation, and stereociliary bundle orientation in inner ear sensory hair cells - appear to be mechanistically related, as judged by their dependence on vertebrate homologues of core tissue polarity/planar cell polarity (PCP) genes in Drosophila. To explore more deeply the mechanistic similarities between different polarity processes, we have determined the extent to which frizzled 3 (Fz3) can rescue the hair follicle and Merkel cell polarity defects in frizzled 6-null (Fz6(-/-)) mice, and, reciprocally, the extent to which Fz6 can rescue the axon growth and guidance defects in Fz3(-/-) mice. These experiments reveal full rescue of the Fz6(-/-) phenotype by Fz3 and partial rescue of the Fz3(-/-) phenotype by Fz6, implying that these two proteins are likely to act in a conserved manner in these two contexts. Stimulated by these observations, we searched for additional anatomical structures that exhibit macroscopic polarity and that might plausibly use Fz3 and/or Fz6 signaling. This search has revealed a hitherto unappreciated pattern of papillae on the dorsal surface of the tongue that depends, at least in part, on redundant signaling by Fz3 and Fz6. Taken together, these experiments provide compelling evidence for a close mechanistic relationship between multiple anatomically diverse polarity processes., (© 2014. Published by The Company of Biologists Ltd.)

Frizzled (Fz) 2 and Fz7, together with Fz1, form a distinct subfamily within the Frizzled family of Wnt receptors. Using targeted gene deletion, we show that: Fz7(-/-) mice exhibit tail truncation and kinking with 100% penetrance and ventricular septal defects (VSDs) with ~15% penetrance; Fz2(+/-);Fz7(-/-) mice exhibit VSDs with ~50% penetrance and cleft palate with less than 10% penetrance; and Fz2(-/-);Fz7(-/-) mice exhibit convergent extension defects and mid-gestational lethality with 100% penetrance. When Fz2 and/or Fz7 mutations are combined with mutations in Vangl2, Dvl3, Wnt3a, Wnt5a or Wnt11, an increased frequency of VSDs is observed with Dvl3, Wnt3a and Wnt11; an increased frequency of palate closure defects is observed with Vangl2; and early lethality and enhanced tail shortening are observed with Wnt5a. To assess the signaling pathways that underlie these and other Frizzled-mediated genetic interactions, we used transfected mammalian cells to analyze (1) canonical Wnt signaling induced by all pairwise combinations of the ten mouse Frizzleds and the 19 mouse Wnts and (2) localization of each Frizzled at cell-cell junctional complexes formed by mouse Celsr1, a likely indicator of competence for planar cell polarity signaling. These in vitro experiments indicate that Fz2 and Fz7 are competent to signal via the canonical pathway. Taken together, the data suggest that genetic interactions between Fz2, Fz7 and Vangl2, Dvl3 and Wnt genes reflect interactions among different signaling pathways in developmental processes that are highly sensitive to perturbations in Frizzled signaling.

The developing mammalian kidney is an attractive system in which to study the control of organ growth. Targeted mutations in the Wnt receptors frizzled (Fz) 4 and Fz8 lead to reduced ureteric bud growth and a reduction in kidney size, a phenotype previously reported for loss of Wnt11. In cell culture, Fz4 and Fz8 can mediate noncanonical signaling stimulated by Wnt11, but only Fz4 mediates Wnt11-stimulated canonical signaling. In genetically mosaic mouse ureteric buds, competition between phenotypically mutant Fz4(-/-) or Fz4(-/-);Fz8(-/-) cells and adjacent phenotypically wild-type Fz4(+/-) or Fz4(+/-);Fz8(-/-) cells results in under-representation of the mutant cells to an extent far greater than would be predicted from the size reduction of homogeneously mutant kidneys. This discrepancy presumably reflects the compensatory action of a network of growth regulatory systems that minimize developmental perturbations. The present work represents the first description of a kidney phenotype referable to one or more Wnt receptors and demonstrates a general strategy for revealing the contribution of an individual growth regulatory pathway when it is part of a larger homeostatic network.

Surface appendages such as bristles, feathers and hairs exhibit both long- and short-range order. In the frizzled 6 null (Fz6(-/-)) mouse the orientations of the earliest born hair follicles are uncorrelated, but over time the follicles reorient to create patterns that are characterized by a high degree of local order. By quantifying follicle orientations over time, in both living and fixed tissues, we define the time course of local hair follicle refinement and the resulting evolution of a montage of competing patterns in Fz6(-/-) skin. We observe an apparently local process that within one week can organize a field of many tens of thousands of follicles, generating long-range order that extends over distances of more than one centimeter. Physical systems that undergo an analogous ordering of vector components suggest potential mechanisms that might apply to the patterning of hair follicles and related biological structures.

The closure of an open anatomical structure by the directed growth and fusion of two tissue masses is a recurrent theme in mammalian embryology, and this process plays an integral role in the development of the palate, ventricular septum, neural tube, urethra, diaphragm and eye. In mice, targeted mutations of the genes encoding frizzled 1 (Fz1) and frizzled 2 (Fz2) show that these highly homologous integral membrane receptors play an essential and partially redundant role in closure of the palate and ventricular septum, and in the correct positioning of the cardiac outflow tract. When combined with a mutant allele of the planar cell polarity gene Vangl2 (Vangl2(Lp)), Fz1 and/or Fz2 mutations also cause defects in neural tube closure and misorientation of inner ear sensory hair cells. These observations indicate that frizzled signaling is involved in diverse tissue closure processes, defects in which account for some of the most common congenital anomalies in humans.

Microphthalmia, coloboma and persistent fetal vasculature within the vitreous cavity are among the most common human congenital ocular anomalies, and each has been associated with a variety of genetic disorders. Here we show that, in the mouse, loss of frizzled 5 (Fz5) - a putative Wnt receptor expressed in the eye field, optic cup and retina - causes all of these defects with high penetrance. In the developing Fz5(-/-) eye, the sequence of defects, in order of appearance, is: increased cell death in the ventral retina, delayed and/or incomplete closure of the ventral fissure, an excess of mesenchymal cells in the vitreous cavity, an excess of retinal astrocyte precursors and mature astrocytes, and persistence of the hyaloid vasculature in association with a large number of pigment cells. Fz5(-/-) mice also exhibit a late-onset progressive retinal degeneration by approximately 6 months of age, which might be related to the expression of Fz5 in Müller glia in the adult retina. These results demonstrate a central role for frizzled signaling in mammalian eye development and are likely to be relevant to the etiology of congenital human ocular anomalies.

This review focuses on the tissue/planar cell polarity (PCP) pathway and its role in generating spatial patterns in vertebrates. Current evidence suggests that PCP integrates both global and local signals to orient diverse structures with respect to the body axes. Interestingly, the system acts on both subcellular structures, such as hair bundles in auditory and vestibular sensory neurons, and multicellular structures, such as hair follicles. Recent work has shown that intriguing connections exist between the PCP-based orienting system and left-right asymmetry, as well as between the oriented cell movements required for neural tube closure and tubulogenesis. Studies in mice, frogs and zebrafish have revealed that similarities, as well as differences, exist between PCP in Drosophila and vertebrates.