Spatial and temporal changes in myogenic protein expression by the microenvironment after freeze injury.

Skeletal muscle has the remarkable capability to regenerate itself following injury. Adult myogenic stem cells (MSCs) are responsible for the repair and regeneration, and their activity is controlled by intrinsic and extrinsic factors. The aim of this study was to examine and compare the expression levels of Pax3, Pax7, MRF and p38 proteins during the course of regeneration and in different areas of the focal freeze-lesion damaged adult rat TA muscle. Using the focal freeze injury model, immunohistochemistry, laser-capture micro-dissection and Western blot analysis were performed. The results show that (1) in the severely damaged area, the focal freeze-lesion injury significantly activated Pax7 and myogenin expression within 7 days and down-regulated Pax3, MyoD and Myf-5 within 1 or 3 days, and (2) the level of the p38 protein was strongly and transiently up-regulated in the whole muscle on day 7 following injury, whereas the level of the pp38 protein was down-regulated within 3 days in the severely damaged and non-damaged areas. These findings indicate that the temporal (e.g. the time course of regeneration) and spatial (e.g. three zones created by the focal freeze-lesion) cues in a regenerating muscle have a significant impact on the activity of the adult MSCs.