EXPRESSION OF THE NOVEL MURINE HOMEOBOX GENE SAX-1 IN THE DEVELOPING NERVOUS-SYSTEM

We have isolated the novel murine Sax-1 gene, a member of the NK-I cla ss of homeobox genes, and report its expression pattern in the develop ing central nervous system (CNS) in comparison to two other homeobox g enes, Evx-1 and Pax-6. Sax-1 was found to be transiently expressed in the developing posterior CNS. First seen in the ectoderm lateral to th e primitive streak, the signal later encompassed the neural plate. Pos teriorly, the expression domain overlapped with the Evx-1 expression i n the streak, while anteriorly it was delimited by the Pax-6 signal in the neural tube. This early phase of Sax-1 expression suggests a role during the early determinative events in the formation of hindbrain a nd spinal cord. In a second phase starting at day 9.5 pc, Sax-1 was ex pressed in distinct areas of spinal cord, hindbrain, midbrain and fore brain. Particularly strong signals were detected in rhombomere 1 and i n the pretectum. In these areas, subsets of neurons may be marked and specified. In addition to the normal pattern of Sax-1 during developme nt, the expression in different mouse mutants was analysed. In Brachyu ry curtailed homozygotes, the expression of Sax-1 was found to be redu ced during neurulation and even lost at day 9.0 pc. Ventral shift and finally loss of the signal in the ventral spinal cord was observed in Danforth's short tail homozygotes.