HSP GENE-EXPRESSION AND HSF2 IN MOUSE DEVELOPMENT

During the pre-implantation phase of development, the mouse embryo syn thesizes HSC70, and HSP90 alpha and beta at a very high rate. After im plantation, the expression of HSPs appears non-coordinated and is not uniform in the different tissues. The expression of inducible HSPs app ears later in development than that of constitutive members of the fam ily. HSP25 is highly expressed early in heart and muscle development, but also in some structure of the central nervous system. HSC70 and HS P90 beta are expressed ubiquitously, but their expression reaches very high levels in the nervous system (neural tracks) and during bone mor phogenesis (in the hypertrophic chondrocytes). The mechanisms involved in HSP expression during mouse embryogenesis are probably diverse, in volving tissue-specific sequences. Although the DNA-binding activity a nd expression of the second heat shock transcription factor, HSF2, see ms to be developmentally regulated, becoming detectable at the blastoc yst stage and reaching a peak at day 10 of development, there is no ob vious correlation between the level of this factor and the expression of HSPs. HSF2 might be involved in the onset of expression of HSPs, re gulate (inhibit) their expression, or control the expression of other developmental genes yet to be discovered.