Expression of hsp90a and hsp90b during Xenopus laevis Embryonic Development.

Members of the eukaryotic Hsp90 family function as important molecular chaperones in the assembly, folding and activation of cellular signaling in development. Two hsp90 genes, hsp90 alpha and hsp90 beta, have been identified in fish and homeothermic vertebrates but not in poikilothermic vertebrates. In the present study, the expression of hsp90 alpha and hsp90 beta genes in Xenopus laevis, which is phylogenetically positioned between zebrafish and mammals, has been addressed. Partial Xenopus hsp90 alpha and hsp90 beta cDNA were identified and isolated using RT-PCR, and a full-length Xenopus hsp90 beta cDNA was isolated from an embryonic cDNA library. Northern-blot analysis was used to study the expression of hsp90 alpha and hsp90 beta genes in total RNA of the embryos and in situ hybridization was used to compare the expression of these genes with that of hsp70 and MyoD genes in Xenopus embryogenesis. Northern-blot analysis revealed that the hsp90 beta gene was strongly expressed constitutively at all stages of embryogenesis, but weakly induced following the heat shock. In contrast, the hsp90 alpha gene was weakly expressed in embryos at control temperature, but strongly up-regulated following heat shock. In situ hybridization results showed that hsp90 alpha gene was observed predominantly in cells of the developing somite. Microscopic sections showed that hsp90 alpha and MyoD mRNA are expressed in similar regions in somite and this pattern was distinct from that of hsp70 and hsp90 beta. These data support the hypothesis that the presence of hsp90 alpha and hsp90 beta genes is conserved among vertebrates, and these genes are differentially regulated in a tissue, stress, and development stage-specific manner.