Ephrin-B2 governs morphogenesis of endolymphatic sac and duct epithelia in the mouse inner ear

Abstract

Control over ionic composition and volume of the inner ear luminal fluid endolymph is essential for normal hearing and balance. Mice deficient in either the EphB2 receptor tyrosine kinase or the cognate transmembrane ligand ephrin-B2 (Efnb2) exhibit background strain-specific vestibular behavioral dysfunction and signs of abnormal endolymph homeostasis. Using various loss-of-function mouse models, we found that Efnb2 is required for growth and morphogenesis of the embryonic endolymphatic epithelium, a precursor of the endolymphatic sac (ES) and duct (ED), which mediate endolymph homeostasis. Conditional inactivation of Efnb2 in early-stage embryonic ear tissues disrupted cell proliferation, cell survival, and epithelial folding at the origin of the endolymphatic epithelium. This correlated with apparent absence of an ED, mis-localization of ES ion transport cells relative to inner ear sensory organs, dysplasia of the endolymph fluid space, and abnormally formed otoconia (extracellular calcite protein composites) at later stages of embryonic development. A comparison of Efnb2 and Notch signaling deficient mutant phenotypes indicated that these two signaling systems have distinct and non overlapping roles in ES/ED development. Homozygous deletion of the Efnb2 C terminus caused abnormalities similar to those found in the conditional Efnb2 null homozygote. Analyses of fetal Efnb2 C-terminus deletion heterozygotes found mis-localized ES ion transport cells only in the genetic background exhibiting vestibular dysfunction. We propose that developmental dysplasias described here are a gene dose sensitive cause of the vestibular dysfunction observed in EphB-Efnb2 signaling-deficient mice.

The mouse Foxi3 transcription factor is necessary for the development of posterior placodes

“…Here, both genes are expressed later in development and appear to regulate genes involved in the function of transport epithelium. In the case of zebrafish, foxi3 regulates the development of epidermal ionocytes (Cruz et al, 2013; Esaki et al, 2009; Hsiao et al, 2007; Janicke et al, 2007; Janicke et al, 2010; Thermes et al, 2010), while Foxi1 regulates genes important for production and homeostasis of endolymphatic fluid, such as SLC26A4 which encodes pendrin (Blomqvist et al, 2004; Hulander et al, 2003; Raft et al, 2014; Vidarsson et al, 2009). …”

Ephrin-B/EphB Signaling Is Required for Normal Innervation of Lingual Gustatory Papillae

“…Although we focused on the potential role of ephrin-B/EphB signaling in axon targeting, we note that ephrin/Eph interactions are also involved in epithelial cell organization [55,56]. In EphB3-rtTA mice, we detected lacZ activity in the mesenchyme adjacent to the epithelium at E13.5–14.5 and, at E17.5, in the apical aspect of the dorsal lingual epithelium as well as just beneath the epithelium (our unpublished observations).…”

The cochlear sensory epithelium derives from Wnt responsive cells in the dorsomedial otic cup

“…Wnt signaling orchestrates the proliferative pressure that drives the upward movement of intestinal epithelial cells and also regulates their migratory properties by controlling the expression of Eph/ephrin signaling components (Batlle et al, 2002). Eph/ephrin signaling is associated with several aspects of inner ear development and hair cell innervation (Cowan et al, 2000; Coate et al, 2012; Defourny et al, 2013; Raft et al, 2014), but a specific role in auditory hair cell formation and/or cochlear morphogenesis has yet to be reported, possibly due to functional redundancy between the large number of family members expressed in the ear (Saeger et al, 2011). It will be interesting to determine whether similar mechanisms act to facilitate the migration of Wnt responsive progenitors in the gut and inner ear.…”