Ablation of axial structures activates apoptotic pathways in somite cells of the chick embryo

Abstract

It has been known for some time that ablation of the neural tube and/or the notochord in the chick embryo leads to a massive wave of cell death in the adjacent somites. It is postulated that in the normal embryo, survival signals emanate from the neural tube and/or notochord that suppress apoptosis in the cells of the somites, except for a small population of sclerotome cells that are programmed to die naturally. In this study we show that axial ablation results in the death of sclerotome and not somitic neural crest cells, and we have examined the apoptotic response of these cells to the ablation. We show that several elements of the apoptotic cascade become detectable in somite cells in response to the withdrawal of survival signals. We demonstrate the down-regulation of bcl-2 protein in the somites adjacent to, and caudal to, the site of ablation, corresponding to the region that displays an elevated level of cell death. Although caspase-9 appeared to be activated in somites at all levels of the trunk, caspase-2 showed a clear response to the ablation of the axial structures. Removal of the neural tube and notochord produced an up-regulation of caspase-2 activity in somites in the region of the operation. Cleavage of two down-stream substrates of these caspases was examined. The cleavage of poly (ADP-ribose) polymerase (PARP) was apparent in somites at all levels of the trunk, and showed only a modest up-regulation after ablation. By contrast, the cleavage of DNA fragmentation factor (DFF45) showed a marked up-regulation in response to ablation, suggesting that this is a primary substrate for a caspase-dependent apoptotic mechanism. Evidence was also found for a caspase-independent mechanism, since the expression of apoptosis-inducing factor (AIF) was found to be very sensitive to, and up-regulated in somites by, axial ablation. Because the wave of apoptosis that is precipitated in somites by removal of the axial structures may be mediated by BMP-4, we examined the levels of BMP-4 in somites in response to axial ablation. BMP-4 expression was clearly up-regulated in somites adjacent to, or close to, the site of operation.

Morphological Features of Tail Bud Development in <i>Truncate</i> Mouse Mutants

“…The expression of Pax3 , the marker of the dorsal part of the neural tube, is shifted ventrally in segments of truncate tails without notochord [Dietrich et al, 1993]. Similarly, the development of the somites was comparable to the experiments where the notochord is removed [Teillet and Le Douarin, 1983;Sanders and Parker, 2001], and to the data obtained from different mutants without notochord [Dietrich et al, 1993;Weinstein et al, 1994;Asakura and Tapscott, 1998]. Surprisingly, truncate mutants showed wide variations in the morphology of somites in notochordless segments ranging from normal morphology to somites fused in the median line.…”

Control of the temporal and spatial Uncx4.1 expression in the paraxial mesoderm of avian embryos

“…Ablation of axial structures after somite formation leads to an increase of cell death in the sclerotome, which becomes reduced to the very medial part of the somite (Hirano et al 1995;Sanders et al 2001). A similar reduction in the sclerotome size is observed after ablation of the notochord-floor plate complex (reviewed by .…”

Formation and differentiation of the avian sclerotome

“…The most important signal to prevent cell death in the sclerotome is Shh emanated from the notochord and the floor plate of the neural tube. After extirpation of the axial structures, apoptotic pathways are activated in the sclerotome cells leading to a down-regulation of bcl-2 protein and an upregulation of caspase-2 activity (Sanders and Parker 2001). This high rate of cell death can be prevented by the implantation of a notochord or the application of Shh protein (Teillet et al 1998).…”