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

Abstract

Wnt1 and Wnt3a secreted from the dorsal neural tube were previously shown to regulate a gene expression program in the dorsal otic vesicle that is necessary for vestibular morphogenesis (Riccomagno et al., 2005). Unexpectedly, Wnt1−/−; Wnt3a−/− embryos also displayed a pronounced defect in the outgrowth of the ventrally derived cochlear duct. To determine how Wnt signaling in the dorsal otocyst contributes to cochlear development we performed a series of genetic fate mapping experiments using two independent Wnt responsive driver strains (TopCreER and Gbx2CreER) that when crossed to inducible responder lines (RosalacZ or RosazsGreen) permanently labeled dorsomedial otic progenitors and their derivatives. Tamoxifen time course experiments revealed that most vestibular structures showed some degree of labeling when recombination was induced between E7.75 and E12.5, consistent with continuous Wnt signaling activity in this tissue. Remarkably, a population of Wnt responsive cells in the dorsal otocyst was also found to contribute to the sensory epithelium of the cochlear duct, including auditory hair and support cells. Similar results were observed with both TopCreER and Gbx2CreER strains. The ventral displacement of Wnt responsive cells followed a spatiotemporal sequence that initiated in the anterior otic cup at, or immediately prior to, the 17-somite stage (E9) and then spread progressively to the posterior pole of the otic vesicle by the 25-somite stage (E9.5). These lineage-tracing experiments identify the earliest known origin of auditory sensory progenitors within a population of Wnt responsive cells in the dorsomedial otic cup.

Lineage tracing of Sox2-expressing progenitor cells in the mouse inner ear reveals a broad contribution to non-sensory tissues and insights into the origin of the organ of Corti

“…This region was also identified as containing cochlear progenitors in a recent study using Wnt-responsive Cre-ER mice (Brown et al, 2015; Riccomagno et al, 2005). These divisions are not absolute, however, as our data suggests there is some cell mixing between these regions, particularly among supporting cell progenitors (Figure 6A).…”

“…Our Sox2-CreER transgenic mice offer a complementary fate map of the medial otocyst compared to a recently published TOP-CreER mouse study (Brown et al, 2015): between E9.0 and E10.0, Sox2 is expressed in the ventral region of the medial otocyst (Figure 7), whereas the TOP-CreER transgene that marks Wnt-responsive cells is confined to the dorsal half of the medial otocyst (Brown et al, 2015; Riccomagno et al, 2005). Fate-mapping the medial wall of the otocyst with the two mouse lines confirms that much of the organ of Corti derives from a region on either side of the dorso-ventral equator.…”

“…The cochlear duct grows out from the ventral otocyst starting at embryonic day 10.5 (Brown et al, 2015; Li et al, 1978; Riccomagno et al, 2005; Wu and Kelley, 2012). In contrast to the vestibular sensory organs (Morsli et al, 1998; Raft and Groves, 2015; Wu and Kelley, 2012; Wu and Oh, 1996), there is a dearth of fate-mapping or gene expression data that describes the early location of progenitors of the organ of Corti (Wu and Kelley, 2012).…”

“…Studies using Neurog1-CreER transgenic mice show that some cells in the antero-ventral region of the otocyst give rise to non-sensory regions of the cochlear duct, but that almost no Neurog1- expressing progenitors contribute to the organ of Corti itself (Koundakjian et al, 2007; Raft et al, 2007). Fate mapping of Wnt-responsive progenitors using a CreER mouse line driven by Lef/TCF binding sites ( TOP-CreER ), and more recently a Gbx2-CreER mouse show that some Wnt-responsive progenitors in dorsal parts of the medial wall of the otocyst between E9.5 and 11.5 will contribute to sensory epithelium of the cochlear duct (Brown et al, 2015; Riccomagno et al, 2005). One recent study suggested that neural tube-derived cells may contribute to the inner ear, including the cochlea, at very early stages in its development (Freyer et al, 2011).…”

Modulation of Wnt Signaling Enhances Inner Ear Organoid Development in 3D Culture

“…Of these signaling pathways, canonical Wnt signaling cascade appears to be of particular importance in the development of the otic placode in vivo [2, 9–20]. Moreover, inhibiting Wnt signaling with the potent tankyrase inhibitor XAV-939 at differentiation days 8–10 abolishes otic vesicle formation in our 3D culture [1], strongly suggesting that Wnt ligands synthesized in cells within aggregates are essential for otic placode induction in our organoid culture.…”