Epithelial Planar Bipolarity Emerges from Notch-Mediated Asymmetric Inhibition of Emx2

• Published in: Current biology 2020-03, Vol.30 (6), p.1142-1151.e6
• Main Authors: Kozak, Eva L., Palit, Subarna, Miranda-Rodríguez, Jerónimo R., Janjic, Aleksandar, Böttcher, Anika, Lickert, Heiko, Enard, Wolfgang, Theis, Fabian J., López-Schier, Hernán
• Format: Article
• Language: English
• Subjects: diffusion pseudotime; hair cells; planar polarity; single-cell RNA sequencing
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2020.01.027

Most plane-polarized tissues are formed by identically oriented cells [1, 2]. A notable exception occurs in the vertebrate vestibular system and lateral-line neuromasts, where mechanosensory hair cells orient along a single axis but in opposite directions to generate bipolar epithelia [3–5]. In zebrafish neuromasts, pairs of hair cells arise from the division of a non-sensory progenitor [6, 7] and acquire opposing planar polarity via the asymmetric expression of the polarity-determinant transcription factor Emx2 [8–11]. Here, we reveal the initial symmetry-breaking step by decrypting the developmental trajectory of hair cells using single-cell RNA sequencing (scRNA-seq), diffusion pseudotime analysis, lineage tracing, and mutagenesis. We show that Emx2 is absent in non-sensory epithelial cells, begins expression in hair-cell progenitors, and is downregulated in one of the sibling hair cells via signaling through the Notch1a receptor. Analysis of Emx2-deficient specimens, in which every hair cell adopts an identical direction, indicates that Emx2 asymmetry does not result from auto-regulatory feedback. These data reveal a two-tiered mechanism by which the symmetric monodirectional ground state of the epithelium is inverted by deterministic initiation of Emx2 expression in hair-cell progenitors and a subsequent stochastic repression of Emx2 in one of the sibling hair cells breaks directional symmetry to establish planar bipolarity. [Display omitted] •Unipotent hair-cell progenitors (UHCPs) initiate Emx2 expression•Emx2 expression symmetry is broken between sibling hair cells•Notch1a-activity-mediated symmetry breaking is stochastic•Anatomical and genetic asymmetries are separable Kozak et al. combine single-cell RNA sequencing, pseudotime trajectory analysis, and mutagenesis to reveal the initial symmetry-breaking step that results in the establishment of planar bipolarity in a sensory epithelium.