A MOUSE GENE OF THE PAIRED-RELATED HOMEOBOX CLASS EXPRESSED IN THE CAUDAL SOMITE COMPARTMENT AND IN THE DEVELOPING VERTEBRAL COLUMN, KIDNEYAND NERVOUS-SYSTEM

During vertebrate embryonic development, pairs of metameric units, the somites, bud off at the cranial end of the paraxial mesoderm. The som ites soon obtain cranio-caudal and dorso-ventral polarity. Establishme nt of dorso-ventral and medio-lateral polarity depends on multiple sig nals from the notochord, neural tube surface ectoderm and lateral meso derm. The establishment of cranio-caudal polarity in the somite is les s well understood. One molecule involved is the Dll1 gene product. a t ransmembrane protein expressed in the unsegmented paraxial mesoderm an d in the caudal half of the somites. We have identified a gene, Uncx4. 1, expressed in the caudal half of newly formed somites. It encodes a protein belonging to the paired-related class of homeodomain transcrip tion factors. Uncx4.1 expression is first detected in the entire cauda l half of the somites, is later down-regulated in the myotome and derm atome, and is maintained in the caudal sclerotome and its derivatives from which part of the vertebral column will form. Thus, Uncx4.1 may b e involved in the establishment and maintenance of segment polarity an d in vertebral column formation. Uncx4.1 is also expressed in the firs t branchial arch, the meso- and metanephric kidney, the central nervou s system and the first digit of the forelimb, suggesting control funct ions of Uncx4.1 in multiple processes of embryogenesis.