Specific localization of zebrafishhsp90α mRNA tomyoD-expressing cells suggests a role for hsp90α during normal muscle development

Members of the eukaryotic hsp90 family function as important molecular chaperones in the assembly, folding and activation of a select group of cellular signalling molecules and transcription factors. Several of the molecules with which hsp90 interacts, such as the bHLH transcription factor myoD, are known to be important regulators of developmental events in vertebrates. However, little information is available in support of any specific role for hsp90 in developing embryos in vivo. In this study, we provide the first in vivo evidence that the hsp90a gene may play a role in the process of myogenesis. We show that constitutive hsp90a mRNA in zebrafish embryos is restricted primarily to a subset of cells within the somites and pectoral tin buds which also express myoD. Furthermore, expression of the hsp90a gene is down-regulated along with myoD in differentiated muscles of the trunk at a time when levels of mRNA encoding the muscle structural protein a-tropomyosin remain high. No hsp90~ mRNA is detectable within the CNS at control temperatures. In contrast, heat shock-induced expression of the hsp90a gene occurs throughout the embryo at all stages of development examined. The expression patterns strongly suggest that the hsp90a gene plays a specific role in the normal process of myogenesis in addition to providing protection to all cells of the embryo during periods of environmental stress.