Cell death in the avian blastoderm: resistance to stress-induced apoptosis and expression of anti-apoptotic genes
We have examined the expression of TNF-α and its receptors, TNFR1 and TNFR2, during gastrulation in the chick embryo, and have investigated the possible role of this factor in the control of cell death at this early stage of development. TNF-α, immunoreactive at approximately 17 kD, was found both in vivo and in vitro, most intensely associated with the cell surface and cytoskeleton of endoderm cells. TNFR2 was especially immunoreactive in endoderm cells of the marginal zone. TNFR1 was found in nuclei throughout the embryo. Embryos also showed widespread expression of both the bcl-2 and Bax gene products, which are both associated with cell death pathways. Intact embryos in culture were sensitive to the addition of TNF-α (approx. 110 ng/ml), responding by significantly increasing the incidence of DNA fragmentation in cells from all tissues of the embryo. This effect was abrogated by immunological pre-absorption of the cytokine. Cultured cells from these embryos also responded to the addition of agonistic antibodies to TNF-α receptors by increasing DNA fragmentation. A similar response to TNF-α antiserum by cultured cells appeared to be related to a concomitant decrease in cell-substratum adhesion caused by the antibody. Decreased cell adhesion, induced non-specifically with anti-integrin antiserum, also resulted in increased DNA fragmentation. TNF-α, synthesized and secreted by the embryo itself, may be able to exert a paracrine effect on the incidence of cell death in tissues of the embryo, and the cell death process may be related to the expression of bcl-2 and Bax gene products. The influence of TNF-α may be exerted by the activation of cell death signalling pathways directly, or indirectly through perturbation of the cytoskeleton or of integrin-mediated cell adhesion.