LGR4 and LGR5 Regulate Hair Cell Differentiation in the Sensory Epithelium of the Developing Mouse Cochlea: Lgr4/5 in the developing cochlea

In the developing cochlea, Wnt/β-catenin signaling positively regulates the proliferationof precursors and promotes the formation of hair cells by up-regulating Atoh1expression. Not much, however, is known about the regulation of Wnt/β-catenin activityin the cochlea. In multiple tissues, the activity of Wnt/β-catenin signaling is modulated byan interaction between LGR receptors and their ligands from the R-spondin family. Thedeficiency in Lgr4 and Lgr5 genes leads to developmental malformations and lethality.Using the Lgr5 knock-in mouse line we show that loss of LGR5 function increasesWnt/β-catenin activity in the embryonic cochlea, resulting in a mild overproduction ofinner and outer hair cells (OHC). Supernumerary hair cells are likely formed due toan up-regulation of the “pro-hair cell” transcription factors Atoh1, Nhlh1, and Pou4f3.Using a hypomorphic Lgr4 mouse model we showed a mild overproduction of OHCs inthe heterozygous and homozygous Lgr4 mice. The loss of LGR4 function prolongedthe proliferation in the mid-basal turn of E13 cochleae, causing an increase in thenumber of SOX2-positive precursor cells within the pro-sensory domain. The prematuredifferentiation of hair cells progressed in a medial to lateral gradient in Lgr4 deficientembryos. No significant up-regulation of Atoh1 was observed following Lgr4 deletion.Altogether, our findings suggest that LGR4 and LGR5 play an important role in theregulation of hair cell differentiation in the embryonic cochlea.