No abstract

In many species, the early post-fertilization development of the egg appears to occur mainly under maternal control and does not require transcription of the embryonic genome. In the mouse this situation is restricted to the one-cell stage; activation of the embryonic genome occurs at the late two-cell stage and results in a drastic change in the spectrum of proteins synthesized. This activation is preceded by a decrease in the overall synthesis of proteins at the end of the one-cell stage and the appearance, at the early two-cell stage, of a set of new polypeptides of molecular weight approximately 70,000 (70K) (refs 2, 8, 9). This can be compared with the series of events that occur after hyperthermia in differentiated cells. Heat shock results in an arrest of most transcription and translation; subsequently, expression of a limited set of genes, the heat shock genes, precedes the overall reactivation of cellular genome. Here we show that the 70K early two-cell-specific proteins are identical to two of the mouse heat shock proteins, HSP 68 and HSP 70.

The spontaneous expression of heat shock genes during development is well documented in many animal species, but the mechanisms responsible for this developmental regulation are only poorly understood. In vertebrates, additional heat shock transcription factors, distinct from the heat shock factor 1 (HSF1) involved in the stress response, were suggested to be involved in this developmental control. In particular, the mouse HSF2 has been found to be active in testis and during preimplantation development. However, the role of HSF2 and its mechanism of activation have remained elusive due to the paucity of data on its expression during development. In this study, we have examined HSF2 expression during the postimplantation phase of mouse development. Our data show a developmental regulation of HSF2, which is expressed at least until 15.5 days of embryogenesis. It becomes restricted to the central nervous system during the second half of gestation. It is expressed in the ventricular layer of the neural tube which contains mitotically active cells but not in postmitotic neurons. Parallel results were obtained for mRNA, protein, and activity levels, demonstrating that the main level of control was transcriptional. The detailed analysis of the activity of a luciferase reporter gene under the control of the hsp70.1 promoter, as well as the description of the protein expression patterns of the major heat shock proteins in the central nervous system, show that HSF2 and heat shock protein expression domains do not coincide. This result suggests that HFS2 might be involved in other regulatory developmental pathways and paves the way to new functional approaches.