Transient induction of cyclin A in loaded chicken skeletal muscle.

Cell proliferation is believed to contribute to the increased synthesis rate during load-induced growth of avian anterior latissimus dorsi (ALD) skeletal muscle, but the relative contribution of different cell types to this proliferative response and the time course of cell activation are not well documented. The present investigation measured the abundance and localization of cyclin A protein, which is uniquely present in proliferating cells and required for the entry of vertebrate cells into the DNA synthesis phase during the time course of chicken ALD loading. Total protein content in 1.5-, 7-, and 13-day loaded ALD increased by 60, 191, and 294%, respectively. Immunoblotting analysis identified that cyclin A protein per total protein was dramatically increased in ALD muscle after 1.5 days of loading but returned to control level at 7 days. In vitro kinase assays demonstrated a corresponding massive activation of the cyclin A-regulated, cyclin-dependent kinase 2 but not of cyclin-dependent kinase 2 protein level in muscle homogenates after 1.5 days of muscle loading. Immunofluorescence experiments demonstrated that the increase of cyclin A in 1.5 days of loaded ALD was primarily confined to nuclei of interstitial cells (92%) but was also found in fiber-associated cells (8%). In situ hybridization demonstrated an increased number of nuclei of interstitial cells expressing collagen I transcripts after 1.5 days of loading. These data show that the cell cycle protein cyclin A is induced in fiber-associated cells during the early growth response in loaded ALD but also implicate an activation of interstitial cells as playing an early role in this model for muscle growth.