Expression of stress proteins and mitochondrial chaperonins in chronically stimulated skeletal muscle

Molecular chaperones and cytosolic stress proteins are actively involved in the stabilization, import and refolding of precursor proteins into mitochondria. The purpose of the present study was to evaluate the relationship between mitochondrial content under steady-state conditions, and during the induction of organelle biogenesis, with the expression of stress proteins and mitochondrial chaperonins. A comparison of steady-state levels of mitochondrial enzyme activity [cytochrome c oxidase (CYTOX)] with chaperonin levels [the heat-shock protein HSP60, the glucose-regulated protein GRP75 (mtHSP70)] in striated muscles possessing a wide range of oxidative capacities revealed a proportional expression between the two. This relationship was disrupted by chronic contractile activity brought about by 10 days of 10 Hz stimulation of the tibialis anterior (TA) muscle, which induced 2.4-fold increases in CYTOX activity, but 3.2- and 9.3-fold increases in HSP60 and GRP75 respectively. The inducible stress protein HSP70i was detected at low levels in control TA muscle, and was increased 9.6-fold by chronic contractile activity, to values comparable with those found in the unstressed soleus muscle. This increase occurred in the absence of changes in type I MHC levels, indicating independent regulation of these genes. Despite the increases in HSP60 and HSP70i proteins, contractile activity did not alter their respective mRNA levels, illustrating post-transcriptional mechanisms of gene regulation during contractile activity. In contrast, the mRNA levels encoding the co-chaperonin CPN10 were increased 3.3-fold by contractile activity. Thus, the expression of individual mitochondrial chaperonins is independently regulated and uncoordinated. The extent of the induction of these stress proteins and chaperonins by contractile activity exceeded that of membrane enzymes (e.g. CYTOX). It remains to be determined whether this marked induction of proteins comprising part of the protein import machinery is beneficial for the translocation of enzyme precursors into the mitochondria during conditions of accelerated biogenesis.