SCLERAXIS - A BASIC HELIX-LOOP-HELIX PROTEIN THAT PREFIGURES SKELETALFORMATION DURING MOUSE EMBRYOGENESIS

Members of the basic helix-loop-helix (bHLH) family of transcription f actors have been shown to regulate growth and differentiation of numer ous cell types. Cell-type-specific bHLH proteins typically form hetero dimers with ubiquitous bHLH proteins, such as E12, and bind a DNA cons ensus sequence known as an E-box, We used the yeast two-hybrid system to screen mouse embryo cDNA libraries for cDNAs encoding novel cell-ty pe-specific bHLH proteins that dimerize with E12. One of the cDNAs iso lated encoded a novel bHLH protein, called scleraxis. During mouse emb ryogenesis, scleraxis transcripts were first detected between day 9.5 and 10.5 post coitum (p.c.) in the sclerotome of the somites and in me senchymal cells in the body wall and limb buds, Subsequently, scleraxi s was expressed at high levels within mesenchymal precursors of the ax ial and appendicular skeleton and in cranial mesenchyme in advance of chondrogenesis; its expression pattern in these cell types foreshadowe d the developing skeleton, Prior to formation of the embryonic cartila ginous skeleton, scleraxis expression declined to low levels, As devel opment proceeded, high levels of scleraxis expression became restricte d to regions where cartilage and connective tissue formation take plac e, Scleraxis bound the E-box consensus sequence as a heterodimer with E12 and activated transcription of a reporter gene linked to its DNA-b inding site. The expression pattern, DNA-binding properties and transc riptional activity of scleraxis suggest that it is a regulator of gene expression within mesenchymal cell lineages that give rise to cartila ge and connective tissue.