Integrin and dystrophin associated adhesion protein complexes during regeneration of shearing-type muscle injury

In shearing injury both the myofibres and connective tissue framework are breached and the muscle tendon continuity is disrupted. During regeneration the firm myofibre to extracellular matrix (ECM) adhesion must be re-established. We have analysed the expression of selected molecules implementing this adhesion in regenerating myofibres 2-56 days after transection of rat soleus muscle using quantitative immunohistochemistry and Northern blotting. Beta1 integrin mRNA level and alpha7 integrin and vinculin immunoreactivities were transiently increased in both the intact and regenerating parts of the transected myofibres by day 5-7 with normalization by day 10-14. After day 14, alpha7 integrin and vinculin accumulated at the tips of the regenerating myofibres, indicating formation of new mini-myotendinous junctions (mMTJ). Immunoreactivities for dystrophin and associated proteins as well as merosin appeared in regenerating myotubes by day 3-4 reaching control levels by day 56. Our results suggest that integrin and dystrophin associated molecules are complementary in myofibre-ECM adhesion. During regeneration, ruptured myofibres temporarily reinforce their integrin mediated lateral adhesion until mMTJs are formed. Thereby the load on the newly formed scar and the risk of rerupture are reduced. Dystrophin associated molecules appear later and replace integrin on the lateral aspects, while both complexes are abundant at the mMTJs. These molecular events correspond to our previous results on tensile strength.