Importance of small heat shock protein 20 (hsp20) C-terminal extension in cardioprotection

Recent studies show that overexpression of small heat shock protein 20 (Hsp20) in mouse hearts reduces infarct size and improves cardiac performance. However, it is not known whether Hsp20 exerts its protective action through improved calcium handling or chaperone activity. The C-terminal extensions of small heat shock proteins, such as alphaB-crystallin and Hsp25, are implicated in chaperoning activity. Through adenovirus mediated overexpression of Hsp20 with C-terminal extension substitution, we delineated the mechanism of protection. Neonatal and adult rat cardiomyocytes overexpressing either the full-length Hsp20 or Hsp20 with a C-terminal extension substitution were subjected to simulated ischemia for 14-16 h followed by reperfusion 6-8 h. Overexpressing Hsp20 with a C-terminus extension substitution did not protect against simulated ischemia/reperfusion in either adult (98+/-8.8% LDH release of control) or neonatal cardiomyocytes (103+/-1.8% CK release of control) as measured by creatine kinase (CK) and lactate dehydrogenase (LDH) cell viability assays (n=4, P<0.05). However, this Hsp20 C-terminal substitution mutant increased calcium transients 33+/-11% and cell contraction amplitude 60+/-15% as quantified through epifluorescence microscopy (n=16 to 34 cells per heart from 4 to 5 hearts, P<0.05). In contrast, overexpression of the full-length Hsp20 protected cultured adult (53+/-8.5% LDH release of control) and neonatal rat (57+/-8.3% CK release of control) cardiomyocytes from simulated ischemia/reperfusion injury. This overexpression also increased calcium transients 30+/-10% and cell contraction amplitude 50+/-10%. These novel data suggest that the C-terminal extension of Hsp20 is essential for cardioprotection. Hsp20 renders this protection through its C-terminal extension protein domain, while this part of the protein is not involved in the Hsp20 ability to increase both calcium transients and cell contraction.