BMP-2 -4 MEDIATE PROGRAMMED CELL-DEATH IN CHICKEN LIMB BUDS/

During limb development, the mesenchymal cells in restricted areas of limb bud, anterior necrotic zone, posterior necrotic zone, opaque zone and interdigital necrotic zones, are eliminated by programmed cell de ath. The transcripts of bone morphogenetic protein (Bmp)-2 and -4 were first detected in the areas where cell death was observed, then showe d overlapping expression with the programmed cell death zones except t he opaque zone. To investigate the function of BMP-2 and BMP-4 during limb pattern formation, the dominant negative form of BMP receptor was overexpressed in chick leg bud via a replication-competent retrovirus to block the endogenous BMP-2/-4 signaling pathway, This resulted in excess web formation at the anterior and posterior regions of limb bud s in addition to marked suppression of the regression of webbing at th e interdigital regions. Significant reductions in the number of apopto tic cells in these three necrotic zones were found in the limb buds wh ich received the virus carrying dominant negative BMP receptor, This i ndicates that extra tissue formation is due to suppression of programm ed cell death in the three necrotic zones. Moreover, BMP-2/-4 protein induced apoptosis of mesenchymal cells isolated from the interdigital region in vitro, Other TGF beta family proteins as TGF beta 1 and Acti vin did not show this effect, These results suggest that BMP-2 and BMP -4 are the apoptotic signal molecules of the programmed cell death pro cess in the chick limb buds.