EXPRESSION OF MYOSIN VIIA DURING MOUSE EMBRYOGENESIS

The gene encoding myosin VIIA is responsible for the mouse shaker-1 ph enotype, which consists of deafness and balance deficiency related to cochlear and vestibular neuroepithelial defects. In humans, a defectiv e myosin VIIA gene is responsible for Usher syndrome type IB, which as sociates congenital deafness, vestibular dysfunction and retinitis pig mentosa. In an attempt to progress in the understanding of the functio n(s) of myosin VIIA, we studied the expression of the myosin VIIA gene during mouse embryonic development. Embryos from day 9 (E9) to E18 we re analyzed by in situ hybridization and immunohistofluorescence. The myosin VIIA mRNA and protein were consistently detected in the same em bryonic tissues throughout development. Myosin VIIA was first observed in the otic vesicle at E9, and later in a variety of tissues. The olf actory epithelium and the liver express it as early as E10. In the ret inal pigment epithelium, choroid plexus, adrenal gland and tongue, exp ression begins at E12 and in the testis and the adenohypophysis at E13 . In the small intestine, kidney and hair follicles of the vibrissae, expression of myosin VIIA starts only at E15. Myosin VIIA expression w as observed only in epithelial cell types, most of which possess micro villi or cilia. Interestingly, myosin VIIA expression seems to be conc omitant with the appearance of these structures in the epithelial cell s, suggesting a role for this myosin in their morphogenesis. The cellu lar location of myosin VIIA within sensory hair cells and olfactory re ceptor neurons also argues for a role of this system vesicle trafficki ng.