Regulation of Heat Shock Gene Expression During Xenopus Development

Abstract

Heat shock-induced activation of the expression of a set of heat shock protein (hsp) genes giving rise to the accumulation of hsp mRNA and the synthesis of Hsps is characteristic of essentially all prokaryotic and eukaryotic cells (reviewed by Nover 1984: Atkinson and Walden 1985; Craig 1985; Lindquist 1986). However, in early stages of animal embryogenesis a heat shock response is not detectable. For example, during Xenopus laevis development heat shock-induced synthesis of Hsps does not occur in early cleavage stage embryos (Heikkila et al. 1985a; Nickells and Browder 1985; Browder et al. 1989). Only immediately after the midblastula transition (MBT) of development, do embryos respond to heat shock by synthesizing Hsps with molecular weights of 87 (Hsp87) and 70 kd (Hsp70). Interestingly, the acquisition of the ability of the Xenopus embryos to synthesize hsps coincides with a dramatic increase in the acquisition of thermoresistance. This last finding is consistent with numerous studies suggesting that Hsp synthesis may play a role in the development of thermotolerance (Lindquist 1986). Also during Drosophila and sea-urchin development, heat-induced Hsp synthesis is not observed until embryos reach the blastoderm and blastula stage respectively (Dura 1981; Roccheri et al.1981; Heikkila et al. 1985a,b, 1986). Mouse and rabbit embryos are incompetent for heat-induced Hsp synthesis during the early cleavage stages of development but do synthesize Hsp70 at the blastocyst stage (Heikkila and Schultz 1984; Heikkila et al. 1985b). Given the similarity in the timing of the acquisition of the heat shock response in insect, echinoderm, amphibian and mammalian development, it is likely that this phenomenon has been conserved through evolution.