Differential expression of B-crystallin and Hsp27 in skeletal muscle during continuous contractile activity. Relationship to myogenic regulatory factors.

AlphaB-crystallin (alphaBC) is a major structural protein (22 kDa) of the ocular lens as well as a bona fide heat shock protein in non-lens tissue. The alphaBC gene is abundantly expressed in tissues with high oxidative capacity, including the heart and type I skeletal muscle fibers, and is regulated by the MyoD family of basic helix-loop-helix transcription factors during myogenesis. To test the hypothesis that alphaBC expression may be directly regulated by the demand for oxidative metabolism, we examined the expression of alphaBC and the ancestral-related Hsp27 in rabbit tibialis anterior muscle subjected to continuous low frequency motor nerve stimulation (3 V, 10 Hz). alphaBC mRNA and protein increased within the 1st day of continuous contractile activity (5- and 2.5-fold, respectively) and achieved maximum levels (>20-and 4-fold, respectively) after 21 d of stimulation. Hsp27 mRNA and protein levels also increased with stimulation, but with a less specific and dramatic induction pattern. In agreement with the Northern analysis, in situ hybridization performed on cross sections from tibialis anterior muscle revealed progressively increasing alphaBC transcript signal, localized in a ringlet pattern, from 1 through 21 days of stimulation. Serial sections subjected to myosin immunohistochemistry revealed that alphaBC expression was confined to slow-twitch type I and a subpopulation of fast twitch type II fibers after 1 day but present in nearly all fibers after 21 days of stimulation. Transcript levels of all four myogenic regulatory factors (MyoD, myogenin, myf-5, and MRF4) also increased with stimulation in a pattern temporally similar with alphaBC, suggesting that expression of alphaBC in response to stimulation may, in part, be regulated through myogenic regulatory factor(s) interaction with the canonical E-box element located within the alphaBC promotor. These data demonstrate that expression of the small heat shock protein, alphaBC, is rapidly induced independent of the ancestrally related Hsp27 in a fiber type specific pattern in skeletal muscle subjected to the oxidative stress imposed by continuous contractile activity.