Insights into regulation and function of the major stress-induced hsp70 molecular chaperone in vivo: Analysis of mice with targeted gene disruption of the hsp70.1 or hsp70.3 gene

The murine hsp70 gene family includes the evolutionarily conserved hsp70.1 and hsp70.3 genes, which are the major proteins induced by heat and other s tress stimuli. hsp70.1 and hsp70.3 encode identical proteins which protect cells and facilitate their recovery from stress-induced damage. While the h sp70 gene family has been widely studied and the roles of the proteins it e ncodes as molecular chaperones in a range of human pathologies are apprecia ted, little is known about the developmental regulation of hsp70.1 and hsp7 0.3 expression and the in vivo biological function of their products. To di rectly study the physiological role of these proteins in vivo, we have gene rated mice deficient in heat shock protein 70 (hsp70) by replacing the hsp7 0.1 or hsp70.3 gene with an in-frame beta -galactosidase sequence. We repor t here that the expression of hsp70.1 and hsp70.3 is developmentally regula ted at the transcriptional level, and an overlapping expression pattern for both genes is observed during embryo development and in the tissues of adu lt mice. hsp70.1(-/-) or hsp70.3(-/-) mice are viable and fertile, with no obvious morphological abnormalities. In late embryonic stage and adult mice , both genes are expressed constitutively in tissues exposed directly to th e environment (the epidermis and cornea) and in certain internal organs (th e epithelium of the tongue, esophagus, and forestomach, and the kidney, bla dder, and hippocampus). Exposure of mice to thermal stress results in the r apid induction and expression of hsp70, especially in organs not constituti vely expressing hsp70 (the liver, pancreas, heart, lung, adrenal cortex, an d intestine). Despite functional compensation in the single-gene-deficient mice by the intact homologous gene (i.e., hsp70.3 in hsp70.1(-/-) mice and vice versa), a marked reduction in hsp70 protein expression was observed in tissues under both normal and heat stress conditions. At the cellular leve l, inactivation of hsp70.1 or hsp70.3 resulted in deficient maintenance of acquired thermotolerance and increased sensitivity to heat stress-induced a poptosis. The additive or synergistic effects exhibited by coexpression of both hsp70 genes, and the evolutionary significance of the presence of both hsp70 genes, is hence underlined.