Expression of transcription factor c-Rel and apoptosis occurrence in polydactylous and syndactylous limb buds of the talpid3 mutant chick embryo

The chicken proto-oncogene c-rel encodes a transcription factor of the Rel/ NF-kappa B family. We have previously shown that c-rel mRNAs accumulate in different types of apoptotic cells of the chick embryo, especially in mesen chymal cells within the four cell death areas of the limb bud: the anterior and posterior necrotic zones, the opaque patch and the interdigital necrot ic zones. This study aimed to further establish the involvement of c-Rel in apoptosis of the developing limb by investigating its expression in the ta lpid3 mutant which was originally shown to be defective in apoptosis. Howev er, our preliminary examinations highlighted the apparent presence of apopt otic cells in talpid3 embryos. Hence, we performed a systematic study of th e occurence of apoptosis in mutant and control embryos by the TUNEL method. The results revealed that apoptosis does occur in talpid3 embryos but with altered spatial and temporal patterns. This suggests that the talpid3 muta tion does not affect a gene involved in apoptosis per se but rather in the determination of the pattern of apoptosis. Neither the expression of c-Rel nor that of its I kappa B alpha inhibitor are grossly modified in talpid3 l imb buds, suggesting that the talpid3 mutation does not affect any of these genes. They are mostly expressed in epidermal, endodermal and striated mus cle cells in control and in talpid3 limb buds as well. C-Rel was also detec ted in some scarce mesenchymal cells that could be apoptotic, in both contr ol and mutant embryos. The only slight difference between control and talpi d3 limbs lies in the perichondrium which is not fully differentiated in tal pid3 embryos: c-Rel and I kappa B alpha are only faintly expressed in talpi d3 perichondrial cells, whereas they are both detected in control perichond rial cells. Taken together, these results suggest that c-Rel could particip ate in several developmental processes, especially in the differentiation o f perichondrial cells, besides its already documented involvement in apopto sis and haematopoeisis.