Heat shock 27kDa protein family, member 7 (Hspb7) is implicated in the pathogenesis of cardiomyopathy - emerging evidence for an ischemia-induced Hspb7 gene function

Abstract

Hspb7 is a member of the small heat shock protein family and is highly expressed in the vertebrate heart. Genome-wide association studies suggest single nucleotide polymorphisms close to the locus of heat shock 27kDa protein family, member 7 (Hspb7) to be involved in the pathogenesis of cardiomyopathy. Furthermore, Hspb7 is suggested as a novel marker of acute cardiac ischemia. Therefore, Hspb7 might have a protective function in cardiovascular disease, especially under hypoxic conditions. Potential mechanisms of action of Hspb7 functions are not sufficiently elucidated to date. To assess the involvement of Hspb7 in the pathogenesis of cardiomyopathy in vivo, we identified the Hspb7 gene in zebrafish and conducted gene-specific knock-down experiments by injecting morpholino (Mo)-modified antisense oligonucleotides into one-cell–stage zebrafish embryos. Mo-splice is designed to block splicing at the exon 2–intron 2 boundary, leading either to inclusion of intron 2 or to skipping of exon 2. Mo-start is directed against the translational start site of Hspb7. Treated embryos develop pericardial edema and 88% of the Mo-splice and 92% of the Mo-start injected zebrafish embryos develop severe heart failure. Contractility and heart rate of Hspb7-Mo-injected embryos are progressively and dose-dependently reduced in contrast to control-injected littermates. Fractional shortening is impaired to 20+4% and heart rate to 52+6 beats per minute (bpm) in treated zebrafish in comparison to 45+3% and 153+2 bpm in control-injected littermates at 72 hours post fertilization, respectively. We found normal development, differentiation and pattering of atrial and ventricular myocytes in Hspb7-injected fish. In addition, transmission electron microscopy of cardiomyocytes revealed a reduced amount of myofibrils, while ultrastructure of the sarcomere is conserved. Interestingly, in contrast to other members of the small heat shock protein family the expression of Hspb7 could not be stimulated by increased temperature suggesting an alternative – for example ischemia-induced – way of induction. To address this question, we exposed zebrafish to cobalt chloride, an established inducer of chemical hypoxia and assessed Hif1 alpha, and Hspb7 mRNA expression by qRT-PCR. Strikingly, while heat shock has no effect, we observe an acute stimulation of Hif1 alpha and Hspb7 mRNA expression by cobalt chloride treatment. Our findings provide evidence for a significant role of Hspb7 gene in the pathogenesis of cardiomyopathy and suggest a novel, ischemia-induced biological function of the Hspb7 gene in vivo.