Frizzled Receptors in Development and Disease

Abstract

Frizzled proteins are the principal receptors for the Wnt family of ligands. They mediate canonical Wnt signaling together with Lrp5 and Lrp6 coreceptors. In conjunction with Celsr, Vangl, and a small number of additional membrane and membrane-associated proteins, they also play a central role in tissue polarity/planar cell polarity (PCP) signaling. Targeted mutations in 9 of the 10 mammalian Frizzled genes have revealed their roles in an extraordinarily diverse set of developmental and homeostatic processes, including morphogenetic movements responsible for palate, ventricular septum, ocular furrow, and neural tube closure; survival of thalamic neurons; bone formation; central nervous system (CNS) angiogenesis and blood-brain barrier formation and maintenance; and a wide variety of processes that orient subcellular, cellular, and multicellular structures relative to the body axes. The last group likely reflects the mammalian equivalent of tissue polarity/PCP signaling, as defined in Drosophila, and it includes CNS axon guidance, hair follicle and tongue papilla orientation, and inner ear sensory hair bundle orientation. Frizzled receptors are ubiquitous among multicellular animals and, with other signaling molecules, they very likely evolved to permit the development of the complex tissue architectures that provide multicellular animals with their enormous selective advantage.

Keywords: Angiogenesis; Axon guidance; Blood–brain barrier; Hair follicles; Planar cell polarity; Tissue polarity.

Figure 1

Evolutionary relationships among mammalian Frizzled proteins and genes. (A) Dendrogram showing the relatedness of the 10 mammalian Frizzled amino acid sequences. (B) Intron–exon structures of mammalian Frizzled genes, with coding regions depicted schematically as the same length. Only Fz3, Fz4, and Fz6 have introns within the coding region. The five coding introns in the Fz3 and Fz6 genes are located at identical positions. From Hua, Chang, Wang, Smallwood, and Nathans (2014).

Figure 2

Palate closure defect in Fz1^−/−;Fz2^−/− fetuses. (A and B) Scanning electron microscopy of the palate at E14. PS, palatal shelf. (C and D) Coronal section through the head at E17, stained with hematoxylin and eosin. N, nasal septum; T, tongue; Yellow asterisks, palatal shelves. Scale bars: B, 200 μm; D, 500 μm. From Yu et al. (2010).

Figure 3

Convergent extension, neural tube closure, and cardiac asymmetry defects in Fz2^−/−;Fz7^−/− embryos. E8.5 embryos are shown intact in a dorsal view (left) and in paraffin sections stained with hematoxylin and eosin (right). Left panels, the Fz2^−/−;Fz7^−/− embryo is shorter and wider than the Fz2^+/+;Fz7^−/− control due to a failure of convergent extension. Right panels, in cross section, the Fz2^−/−;Fz7^−/− embryo shows an open neural tube (red arrows) and symmetric cardiac chambers (green arrowheads). The Fz2^+/+;Fz7^−/− embryo is phenotypically WT. Scale bars: left panels, 1 mm; right panels, 200 μm. From Yu et al. (2012).

Figure 4

Reduced bone density in Fz9^−/− mice. The vertebral body shows reduced bone density in a 52-week-old Fz9^−/− mouse (right) compared to an age- and sex-matched control (left). Scale bar, 250 μm. From Albers et al. (2011).

Figure 5

Defective retinal vascularization in Ndp^KO mice. Z-stacked confocal images of flat mount adult WT and Ndp^KO retinas, with ECs visualized with GS-lectin. The depth of the different vascular structures has been color coded as indicated at left. The Ndp^KO retina has only a superficial vascular plexus from which numerous EC clusters invade a short distance into the retina. IPL, inner plexiform layer; OPL, outer plexiform layer. Scale bar, 100 μm. From Wang et al. (2012).

Figure 6

Redundant actions of Gpr124 and Norrin in maintaining BBB integrity. Coronal sections through the cortex and thalamus of postnatal day (P)6 mice with the indicated genotypes. Pdgfb-CreER recombines specifically in ECs. 4-Hydroxytamoxifen (4HT) was delivered by intraperitoneal injection at P3–P4. Intravascular Sulfo-NHS-biotin leaks from the choroid plexus in the lateral ventricles in all brain samples (upper panels), but it leaks extensively into the brain parenchyma only in the sample that is deleted for both Gpr124 and Ndp (upper right panel). Intracerebral capillaries in the WT brain (lower left) and in the brains from mice with either one functional copy of Gpr124 or one functional copy ofNdp (lower middletwo panels) expressclaudin5(a hallmark ofCNS vasculature) and suppress plasmalemma vesicle-associated protein (PLVAP; a hallmark of peripheral vasculature). Deletion of both Gpr124 and Ndp reverses this expression pattern (lower right panel). Scale bar, 200 μm. From Zhou and Nathans (2014).

Figure 7

Schematic of ligand and receptor components involved in canonical Wnt signaling in CNS ECs. Horizontal black lines represent the plasma membrane; extracellular space is above and cytoplasm is below. Additional components, such as Lrp6, other Frizzleds, and other Wnts, are also involved, but their roles are not yet well defined. Modified from Zhou and Nathans (2014).

Figure 8

Hair patterns in a Fz6^−/−;Vangl2^Lp/+ back skin at P8. (A) Vector map showing hair follicle orientations over the head and back in a flat-mount skin from a P8 Fz6^−/−; Vangl2^Lp/+ mouse. Anterior is down; posterior is up. The small ovals are the eye positions; the larger ovals are the ear positions. Four patterning centers are present along the midline. (B–I) The central regions of the two most posterior patterning centers are enlarged in panels B, C, F, and G, and further enlarged in panels D, E, H, and I, as indicated by the rectangles in panels A, B, C, F, and G. Spatial order among follicles is evident at long range (~1 cm) and short range (e.g., between neighboring follicles; tens of microns). Scale bars: A, 1 cm; B and F, 2 mm; D and H, 300 μm. From Wang, Chang, and Nathans (2010).

Figure 9

Loss of polarity in the arrangement of Merkel cells in Fz6^−/− mice. (A) Vertical section of P1 back skin from a phenotypically WT Fz6^+/− mouse. Merkel cells (MC) are visualized with anticytokeratin8 (CK8) immunostaining and afferent sensory nerve fibers (AF) are visualized with antineurofilament (NFL) immunostaining. Ep, epidermis; Der, dermis. (B–D) Flat mounts of P1 skin in which Merkel cells are visualized with CK8 and the central guard hair follicle is visualized with a Keratin (K)17-GFP transgene. Anterior is to the left. The phenotypically WT Fz6^+/− Merkel cells form a semicircle that opens toward the anterior. The Fz6^−/− Merkel cells form a closed circle. Scale bar, 50 μm. From Chang and Nathans (2013).

Figure 10

Thalamocortical axon guidance defects in the Fz3^−/− forebrain at E18.5. Serial coronal sections through the forebrains of E18.5 Fz3^−/− and Fz3^+/+ fetuses stained with anti-neurofilament antibodies. The midline of each hemibrain image is at the right border of each panel. The sections are arranged from rostral (A and A′) to caudal (E and E′). In the Fz3^+/+ brain, arrows in (A–D) trace the thalamocortical projections originating in the thalamus (D) and projecting via the internal capsule (A–C) to the cortex (A–D). In the Fz3^−/− brain, thalamic axons track inferiorly with most crossing the midline to the contralateral thalamus (D′) and a minority projecting to the inferior border of the cortex (C′). Additional axon guidance defects are indicated by arrowheads in (A′–E′); see Hua, Jeon, et al. (2014) for details. GP, globus pallidus. Scale bar, 1 mm. From Hua, Jeon, et al. (2014).