EXPRESSION OF CELL-DEATH REGULATORY GENES AND LIMITED APOPTOSIS INDUCTION IN AVIAN BLASTODERMAL CELLS

Apoptosis is a well-established cellular mechanism for selective cell deletion during development. However, little is known about the expres sion of an apoptotic pathway and its role in determining the relative sensitivity of the early, pre-gastrula, avian embryo to stress-induced cell death. We examined the sensitivity of avian blastodermal cells t o engage in apoptosis upon exposure to etoposide, a topoisomerase Ii-i nhibitor that rapidly and efficiently induces apoptosis in many cell t ypes. We found that while the blastodermal cells are capable of engagi ng in apoptosis, they are highly resistant to such induction with resp ect to both concentration of drug required and length of exposure, eve n when compared to a tumor cell line with a known multi-drug resistant phenotype. Additionally, we assessed the expression of several candid ate regulatory genes in blastodiscs from infertile eggs (i.e., materna l RNA transcripts), blastodermal cells immediately following ovipositi on, and various stages of early development up to gastrulation. This a nalysis revealed that several genes whose products have anti-apoptotic activity, including bcl-2, bcl-x(L), hsp70, grp78 and the glutathione S-transferases, are expressed as early as the stage 1 embryo in the n ewly oviposited egg. These transcripts are also found in the infertile blastodisc, suggesting a role for maternally derived transcripts in t he protection of the oocyte and zygote. Significantly, constitutive le vels of hsp70 mRNA exceeded those of the other anti-apoptotic genes in the blastodermal cells. These results contribute to an emerging pictu re of stress resistance at the earliest stages of avian embryo develop ment which involves multiple anti-apoptotic genes that act at differen t regulatory points in the apoptotic cascade. (C) 1998 Wiley-Liss, Inc .