DIFFERENTIAL EXPRESSION OF ALPHA-B-CRYSTALLIN AND HSP27 IN SKELETAL-MUSCLE DURING CONTINUOUS CONTRACTILE ACTIVITY - RELATIONSHIP TO MYOGENIC REGULATORY FACTORS

alpha B-crystallin (alpha BC) is a major structural protein (22 kDa) o f the ocular lens as well as a bona fide heat shock protein in non-len s tissue. The alpha BC gene is abundantly expressed in tissues with hi gh 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 a lpha BC expression may be directly regulated by the demand for oxidati ve metabolism, we examined the expression of alpha BC and the ancestra l-related Hsp27 in rabbit tibialis anterior muscle subjected to contin uous low frequency motor nerve stimulation (3 V, 10 Hz). alpha BC mRNA and protein increased within the 1st day of continuous contractile ac tivity (5- and 2.5-fold, respectively) and achieved maximum levels (>2 0- and 4-fold, respectively) after 21 d of stimulation, Hsp27 mRNA and protein levels also increased with stimulation, but with a less speci fic and dramatic induction pattern. In agreement with the Northern ana lysis, in situ hybridization performed on cross sections from tibialis anterior muscle revealed progressively increasing alpha BC transcript signal, localized in a ringlet pattern, from 1 through 21 days of sti mulation, Serial sections subjected to myosin immunohistochemistry rev ealed that alpha BC 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 o f all four myogenic regulatory factors (MyoD, myogenin, myf-5, and MRF 4) also increased with stimulation in a pattern temporally similar wit h alpha BC, suggesting that expression of alpha BC in response to stim ulation may, in part, be regulated through myogenic regulatory factor( s) interaction with the canonical E-box element located within the alp ha BC promotor. These data demonstrate that expression of the small he at shock protein, alpha BC, is rapidly induced independent of the ance strally related Hsp27 in a fiber type specific pattern in skeletal mus cle subjected to the oxidative stress imposed by continuous contractil e activity.