Distinct calcineurin‐related transgenic approaches rescue or exacerbate the dystrophic phenotype in fibers from crossbred mdx mice despite constant HSP70 expression

Abstract

We have shown the dystrophic phenotype to be rescued by driving the slower oxidative myogenic program via calcineurin/NFAT (Cn/NFAT) signaling together with an increase in utrophin A expression (Chakkalakal et al 2004, 2006). However, a recent study suggests an alternate strategy in the rescue of the dystrophic phenotype by overexpressing HSP70 without necessary changes in utrophin A expression (Gehrig et al 2012). We thus set out to examine HSP70 expression in muscles of dystrophic mdx mice crossbred with mice expressing transgenes (Tg) that manipulate the Cn/NFAT pathway. From WBs we found upregulation of utrophin A driven by the Tg overexpression of Cn and its downregulation after the Tg expression of either parvalbumin or calmodulin binding protein. These changes in utrophin A expression occurred without changes in HSP70 levels, indicating that regulation of these two proteins is not correlated. Further, IF experiments showed a robust and rigid co‐localization of HSP70 with MyHC I in Tg‐modified mdx slow fibers displaying impaired Cn/NFAT signaling together with decreased utrophin A expression and a clear exacerbation of the dystrophic phenotype. These results not only underscore the therapeutic potential of targeting Ca+2/Cn‐based signaling intermediates as effective countermeasures for DMD, but also raise questions to the role of HSP70 in rescuing the dystrophic pathology. Funded by CIHR, NSERC and CRC.