The protective roles of phosphorylated heat shock protein 27 in human cells harboring myoclonus epilepsy with ragged-red fibers A8344G mtDNA mutation

Mitochondrial DNA (mtDNA) mutations are associated with a large number of neuromuscular diseases. Myoclonus epilepsy with ragged-red fibers (MERRF) syndrome is a mitochondrial disease inherited through the maternal lineage. The most common mutation in MERRF syndrome, the A8344G mutation of mtDNA, is associated with severe defects in mitochondrial protein synthesis, which impair the assembly and function of the respiratory chain. We have previously shown that there is a decreased level of heat shock protein 27 (HSP27) in lymphoblastoid cells derived from a MERRF patient and in cytoplasmic hybrids (cybrids) harboring the A8344G mutation of mtDNA. In the present study, we found a dramatic decrease in the level of phosphorylated HSP27 (p-HSP27) in the mutant cybrids. Even though the steady-state level of p-HSP27 was reduced in the mutant cybrids, normal phosphorylation and dephosphorylation were observed upon exposure to stress, indicating normal kinase and phosphatase activities. To explore the roles that p-HSP27 may play, transfection experiments with HSP27 mutants, in which three specific serines were replaced with alanine or aspartic acid, showed that the phosphomimicking HSP27 desensitized mutant cybrids to apoptotic stress induced by staurosporine (STS). After heat shock stress, p-HSP27 was found to enter the nucleus immediately, and with a prolonged interval of recovery, p-HSP27 returned to the cytoplasm in wild-type cybrids but not in mutant cybrids. The translocation of p-HSP27 was correlated with cell viability, as shown by the increased number of apoptotic cells after p-HSP27 returned to the cytoplasm. In summary, our results demonstrate that p-HSP27 provides significant protection when cells are exposed to different stresses in the cell model of MERRF syndrome. Therapeutic agents targeting anomalous HSP27 phosphorylation might represent a potential treatment for mitochondrial diseases.