Oscillating expression of c-Hey2 in the presomitic mesoderm suggests that the segmentation clock may use combinatorial signaling through multiple interacting bHLH factors

Vertebrate somitogenesis comprises the generation of a temporal periodicity , the establishment of anteroposterior compartment identity, and the transl ation of the temporal periodicity into the metameric pattern of somites. Mo lecular players at each of these steps are beginning to be identified. Espe cially, members of the Notch signaling cascade appear to be involved in set ting up the somitogenesis clock and subsequent events. We had previously de monstrated specific expression of the mHey1 and mHey2 basic helix-loop-heli x (bHLH) factors during somitogenesis. Here we show that perturbed Notch si gnaling in Dll1 and Notch1 knockout mutants affects this expression in the presomitic mesoderm (PSM) and the somites. In the caudal PSM, however, mHey 2 expression is maintained and thus is likely to be independent of Notch si gnaling. Furthermore, we analysed the dynamic expression of the respective chicken c-Hey1 and c-Hey2 genes during somitogenesis. Not only is c-Hey2 rh ythmically expressed across the chicken presomitic mesoderm like c-hairy1, but its transcription is similarly independent of de novo protein synthesis . In contrast, the dynamic expression of c-Hey1 is restricted to the anteri or segmental plate. Both c-Hey genes are coexpressed with c-hairy1 in the p osterior somite half. further in vitro and in vivo interaction assays demon strated direct homo- and heterodimerisation between these hairy-related bHL H proteins, suggesting a combinatorial action in both the generation of a t emporal periodicity and the anterior-posterior somite compartmentalisation. (C) 2000 Academic Press.