The Role of Zic Genes During Inner Ear Development in the Mouse.

Abstract

The Zic (zinc finger of the cerebellum) family of transcription factors is involved in many developmental processes. Zic gene dysfunction causes developmental defects in mouse and man, including neural tube and skeletal defects and abnormal left-right axis formation. We focused on the role of Zic genes in inner ear development and found that Zic1-5 are expressed in the hindbrain adjacent to the ear and in the periotic mesenchyme (POM) in unique but partially overlapping patterns. Although mice lacking both Zic1 and Zic4 have normal ears, partial (Zic2kd/kd) or complete (Zic2Ku/Ku) loss of Zic2 results in severely malformed inner ears. Ears from the Zic2kd/kd mice were normal in size but had malformations of the cochlear duct and semicircular canals. In the Zic2Ku/Ku mice, ears were misoriented and considerably smaller, and the spatial relationship between the ear and hindbrain was altered. We could not assess whether canal formation was affected in the inner ears from the Zic2Ku/Ku mice, as embryonic lethality prevented us from looking at embryos older than embryonic day 12.5 (E12.5) when canal development is initiated. Despite the altered orientation relative to one another, SHH and WNT signaling from the hindbrain to the ear were relatively unaffected in the mutants we analyzed, though we could not conclude the same about BMP signaling due to inconsistent data and low numbers of replicates. Otocyst patterning appeared to be unaffected in the Zic2Ku/Ku mutants, as the regionalized expression of Pax2, Lfng, Dlx5, Gbx2, Tbx1 in the otocyst were relatively unaffected in the Zic2Ku/Ku mutants we analyzed. Outside of the otic epithelium, there appeared to be an increase in the number of Tbx1+ cells in the POM of the Zic2Ku/Ku mice, but we cannot conclude whether this was due to an increase in proliferation, a decrease in apoptosis, or a shift in the localization of POM cells. Based on these data, it is clear that Zic2 has an important function during inner ear development. This function may include regulation of cell proliferation in the otic epithelium or POM, or regulation of otic capsule formation, but further experiments are required to test these hypotheses.