PARAXIS - A BASIC HELIX-LOOP-HELIX PROTEIN EXPRESSED IN PARAXIAL MESODERM AND DEVELOPING SOMITES

During vertebrate embryogenesis, cells from the paraxial mesoderm coal esce in a rostral-to-caudal progression to form the somites. Subsequen t compartmentalization of the somites yields the sclerotome, myotome, and dermatome, which give rise to the axial skeleton, axial musculatur e, and dermis, respectively. Recently, we cloned a novel basic helix-l oop-helix (bHLH) protein, called scleraxis, which is expressed in the sclerotome, in mesenchymal precursors of bone and cartilage, and in co nnective tissues. Here we report the cloning of a bHLH protein, called paraxis, which is nearly identical to scleraxis within the bHLH regio n but diverges in its amino and carboxyl termini. During mouse embryog enesis, paraxis transcripts are first detected at about Day 7.5 postco itum within primitive mesoderm lying posterior to the head and heart p rimordia. Subsequently, paraxis expression progresses caudally through the paraxial mesoderm, immediately preceding somite formation. Paraxi s is expressed at high levels in newly formed somites before the first detectable expression of the myogenic bHLH genes, and as the somite b ecomes compartmentalized, paraxis becomes downregulated in the myotome . Paraxis and scleraxis are coexpressed in the sclerotome, but paraxis expression declines soon after sclerotome formation, whereas scleraxi s expression increases in the sclerotome and its derivatives. The sequ ential expression of paraxis and scleraxis in the paraxial mesoderm an d somites suggests that these bHLH proteins may comprise part of a reg ulatory pathway involved in patterning of the paraxial mesoderm and in the establishment of semitic cell lineages. (C) 1995 Academic Press, Inc.