Over-expression of Hoxb3 affects sensory neuron differentiation and inner ear morphogenesis

Abstract

Mutations of several Hox genes, including Hoxa1, Hoxb1 and Hoxa2, have been shown to cause sensorineural deafness and abnormal inner ear development in human and mice. However, the pathogenesis mechanisms and the functions of Hox genes during inner ear morphogenesis remain unclear. Using a Hoxb2 enhancer element to over-express Hoxb3 in the otic vesicle, we generated a transgenic mouse mutant Hoxb3Tg. The Hoxb3Tg transgenic mice were deaf and had abnormal circling behavior. In wildtype embryos, Hoxb3 is expressed in the otic epithelium at E9.5, by E11.5 the expression is restricted to the superior vestibular ganglion. In Hoxb3Tg embryos, we found that as a result of over-expression of Hoxb3, the expression of Hoxb1 was down-regulated. The expression of Ngn1, Six1 and NeuroD which are required for neuroblast specification, delamination and differentiation were reduced in Hoxb3Tg/+ mutants, indicating that Hox genes are involved in sensory neuron specification. The Hoxb3Tg/Tg homozygotes developed dysmorphic inner ears with poor differentiation of sensory epithelium and had almost no sensory neurons, except for a diminished projection to the posterior cristae. The Hoxb3Tg/+ heterozygotes lacked or had a reduced utricle whereas the horizontal and anterior crista and saccule were less affected. These defects correlate with a severe reduction of sensory neurons in the superior vestibular ganglion, specifically the innervation to the utricle. Hoxb3Tg/+ mutants had a shortened cochlea with multiple rows of hair cells and supporting cells and fibers overshooting the organ of Corti. The innervation defects were similar to those of Foxg1, Fgf10, Lmx1A, Ngn1, Gata3 and Eya1/Six1 mutants. The expression of these genes in cochlea was altered and the expression of Sox2 in prosensory domain was reduced in Hoxb3Tg/+ mutants. We show that overexpression of Hoxb3 affects prosensory, sensory neurons, and inner ear development in a dosage dependent fashion.