Mouse models for dissecting vertebrate planar cell polarity signaling in the inner ear

Abstract

Planar cell polarity (PCP) refers to coordinated polarization of cells in the plane of a cell sheet. In Drosophila, the stereotypical arrangement of the eight photoreceptor cells in each of the ommatidia of the fly compound eye and the uniform orientation of the hairs in all the wing cells are two representative forms of PCP. Using these powerful Drosophila model systems, a set of genes was identified to constitute the invertebrate PCP signaling pathway. In vertebrates, the inner ear sensory organs display distinctive forms of PCP. In particular, the auditory sensory organ in the cochlea, adorned with precisely patterned sensory hair cell arrays and uniformly oriented hair bundles, has served as an excellent model system to complement other vertebrate PCP models and has illustrated a genetic pathway that consists of genes conserved from the Drosophila model as well as genes uniquely required for vertebrate PCP regulation. This review will focus on the mouse models that have made valuable contributions to our current understanding of PCP signaling in the vertebrates.

Fig. 1

The planar cell polarity of the inner ear sensory organs. (A) Inner ear isolated from an E18.5 mouse embryo. The embryo expresses GFP under the control of Math1 enhancers in hair cells allowing visualization of the 6 inner ear sensory epithelia: the organ of Corti of the cochlea (CO), the maculae of the saccule (SA) and utricle (UT), and the three orthogonally positioned cristae: the anterior crista (AC), posterior crista (PC) and lateral crista (LC). The white tracing outlines the fluid-filled labyrinth of the inner ear that includes both the cochlear and vestibular systems. (B) Confocal image of the organ of Corti viewed from the apical surface. The actin-rich stereociliary bundles (green) of the 1 row of inner (toward the medial side) and 3 rows of outer hair cells form a “V”-shaped structure and all the “Vs” are uniformly aligned (white arrows, lower panel) with the vertices pointing away from the center, or the medial side, of the spiraling cochlea, showing the distinctive planar polarity of this sensory epithelium. (C) Planar polarity in an isolated utricle from an E18.5 mouse embryo. The panels show a confocal scan of the utricle at a level just below the apical surface. Spectrin staining (red) permits easy visualization of the intrinsic polarity of each hair cell (white arrows, right panel) while cell membranes can be seen by staining for actin (green). The white arrows in the right panel indicate the orientation of hair cells. Hair cells on the two sides of the line of polarity reversal (pink line) are oriented towards each other.

Fig. 2

PCP signaling in the development of the organ of Corti. (A) Diagram of an apical view of the developing organ of Corti at embryonic day 13 (E13) in mice. The precursor cells of the organ of Corti exit cell cycle around E13 in mice. Cells are irregularly shaped at this stage. No molecular or morphologic differentiation of hair cells is observed at this stage. (B) About one day later, cells become more regularly-shaped under the influence of multiple signals. Wnt molecules may be one class of signaling molecules involved and may utilize co-receptor Ror to activate PCP-specific downstream events. The diagram here shows an apical view of the developing organ of Corti around E14–E15. At this stage, hair cells have just started to express the earliest hair cell-specific markers, and no apical morphologic differentiation is detectable. Notably, membrane PCP proteins, Vangl2 (magenta) and Fz3/Fz6 (blue), display asymmetric localizations that are polarized along the future PCP axis while the primary cilium (the black apical ciliary projection) and its associated basal body (red doublets) of the cell is centrally positioned. It is thought that both Celsr1 (orange) and Scrb1 facilitate the polarized sorting of Vangl2 and Fz3/6 and/or their recruitment of cytoplasmic mediators, such as Dvl (green), Diversin (Di), and Pk. Note that only the localization of Vangl2, Dvl2/3, and Fz3/6 in the mouse organ of Corti has been reported. The localization of Celsr1 in the organ of Corti is assigned based on its chicken homology localization in the chicken cochlea and its Drosophila homolog localization. (C) By E18, the morphologic polarization of hair cells is reached. This diagram depicts the apical polarity characteristics of hair cells. Apical hair bundles, consisting of the staircase of stereocilia (dark green apical projections) and a single primary cilium (kinocilium, the black ciliary projection), are uniformed oriented across the organ of Corti by this stage. This is achieved likely though a concerted action of polarized membrane-associated PCP complexes and the primary cilium/basal body) on cytoskeleton. The Wnt co-receptor Ror may have a direct role in morphologic polarization since it can activate cytoskeleton regulator JNK. It is worth noting that the space among hair cells is occupied by nonsensory supporting cells (not depicted), and the absolute subcellular localization of PCP proteins, e.g. on hair cell and/or supporting cell membrane, is not determined. The molecular role for PTK7 is not clear. Bb: basal body (Modified from a figure in Jones et al., 2008).