DISTINCT TEMPORAL EXPRESSION OF MOUSE NKX-5.1 AND NKX-5.2 HOMEOBOX GENES DURING BRAIN AND EAR DEVELOPMENT

The mouse Nkx-5.1 and Nkx-5.2 genes have been identified by sequence h omology to Drosophila NK genes within the homeobox domain. Here, we re port the isolation pf the Nkx-5.2 cDNA and a detailed comparative anal ysis of the spatio-temporal expression patterns for Nkx-5.1 and Nkx-5. 2 genes. Nkx-5.2 transcripts are first detected in E13.5 embryos where they colocalize with Nkx-5.1 mRNA in the developing central nervous s ystem and the inner ear. However, the onset of Nkx-5.1 transcription b egins much earlier in 10 somite stage embryos (E8.5) in the otic placo de and the branchial region. Nkx-5.1 expression in the ear persists un til birth, whereas in branchial arches it is transient between E8.5 to E11.5. Transcript distribution appears regionalized in the otic vesic le concentrating at the anterior and posterior margin and later at the dorsal side of the otocyst. These domains are distinct from regions e xpressing Pax-2 and sek, two other early markers for otic development. From E11.5 to birth several Nkx-5.1 expression domains appear in the brain between the ventral diencephalon and the myelencephalon. The sam e expression domains also exist for Nkx-5.2 beginning at E13.5. The re gionally restricted expression pattern of both Nkx-5 genes during mous e development suggests their involvement in cell type specification of neuronal cells.