| The sense of balance depends on the precise architecture of the inner ear, which contains three semicircular canals oriented in three dimensions of space. Changes in canal shape cause drastic behavioral deficits, highlighting the need to understand cellular and molecular events that ensure perfect formation of these structures. During development, canals are sculpted from otic vesicle epithelium pouches. A key event is fusion of two opposing epithelial walls in the center of each pouch, creating a hollow canal. During the course of a gene trap mutagenesis screen, we discovered that the Ig superfamily protein Lrig3 is required for lateral canal development. The mutant phenotype is due to ectopic expression of the axon guidance molecule Netrin1. Genetic experiments demonstrate that mutually antagonistic interactions between Lrig3 and Netrin1 create complementary expression domains that define the shape of the lateral canal. Thus, the Lrig3/Netrin1 feedback loop dictates when and where fusion occurs, revealing a new mechanism of complex tissue morphogenesis.;The inner ear is an intricate structure whose development is dependent upon precise execution of various signaling pathways. Because modest perturbations in the structure of the canals result in behavioral deficits, our genetic studies revealed the consequences of slight changes in signaling activity that are undetectable by in vitro methods. In both humans and mice, the lateral canal is the most common site of inner ear anomalies, emphasizing the importance of identifying the molecular players that make this canal unusually susceptible to developmental insults.;Based on current understanding of Lrig proteins, it is likely that Lrig3 represses Netrin1 transcription by regulating activity of receptor tyrosine kinases. The best studied family member, Lrig1, modulates ErbB signaling by enhancing receptor degradation. However, our work revealed that blocking ErbB signaling in the chick inner ear has no effect on canal morphogenesis, making it unlikely that Lrig3 regulates Netrin1 expression by regulating EGF signaling. To further elucidate the molecular interactions during canal morphogenesis we conducted Netrin1 structure/function experiments in the chick. Overexpression of full length Netrin1 results in reproducible fusion defects of the three canals. Current work is aimed at identifying Netrin1 domains responsible for these defects. |
