Expression and splicing of the insulin-like growth factor gene in rodent muscle is associated with muscle satellite (stem) cell activation following local tissue damage

Muscle satellite cells are mononuclear cells that remain in a quiescent state until activated when they proliferate and fuse with muscle fibres to donate nuclei, a process necessary for post-embryonic growth, hypertrophy and tissue repair in this post-mitotic tissue. These processes have been associated with expression of the insulin-like growth factor (IGF-I) gene that can undergo alternative splicing to generate different gene products with varying functions. To gain insight into the cellular mechanisms involved in local tissue repair, the time courses of expression of two IGF-I splice variants produced in muscle were determined together with marker genes for satellite cell activation following local muscle damage. Using real-time RT-PCR with specific primers, the mRNA transcripts in rat tibialis anterior muscles were measured at different time intervals following either mechanical damage imposed by electrical stimulation of the stretched muscle or damage caused by injection with bupivacaine. It was found that the autocrine splice variant mechano growth factor (MGF) was rapidly expressed and then declined within a few days following both types of damage. Systemic IGF-IEa was more slowly upregulated and its increase was commensurate with the rate of decline in MGF expression. Satellite cell activation as measured by M-cadherin and one of the muscle regulatory factors MyoD and the sequence of expression suggests that the initial pulse of MGF is responsible for satellite cell activation, as the systemic IGF-IEa mRNA expression peaks after the expression of these markers, including M-cadherin protein. Later splicing of the IGF-I gene away from MGF but towards IGF-IEa seems physiologically appropriate as IGF-IEa is the main source of mature IGF-I for upregulation of protein synthesis required to complete the repair.