The regenerative capacity of muscle dramatically decreases with age because old muscle stem cells fail to proliferate in response to tissue damage. p21 protein levels and enhanced cell proliferation. In agreement with the epigenetic silencing of the loci young muscle stem cells do not depend as much as old on ectopic FGF/pERK for their myogenic proliferation. In addition other CDKIs such asp15INK4B and p27KIP1 become elevated in satellite cells with age confirming and explaining the profound regenerative defect of old muscle. This work enhances our understanding of tissue aging promoting strategies for combating age-imposed tissue degeneration. have not been compared prior to our work either in general or in response to a particular biochemical signaling. Considering the highly important topic of age-specific changes in the proliferative capacity of tissue Triacsin C stem cells we studied the molecular mechanism underlying the FGF2/pERK mediated regulation of cell cycle progression in young Hoxd10 and aged satellite cells. Our work focused on muscle stem cells that respond to tissue injury. The age-induced decline in cell proliferation is observed in injury-activated satellite cells but those cells residing in resting muscle are equally quiescent in young and old tissue. Our data reveal that despite the elevation of FGF2 in old damaged muscle no differences in the levels of pERK exist between young and old satellite cells. Aged cells display a profound proliferation failure as compared to young in agreement with consistently higher expression of several CDKIs. Interestingly ectopic FGF2 enhances the proliferation of aged satellite cells by down-regulating p21 expression but there is only a slight trend toward diminishing p16 levels; and aged satellite cells and myofibers express more p16 protein and mRNA than young. Interestingly experimentally activated pERK associates with the chromatin at the p16 and p21 promoters and furthermore does so only in old Triacsin C muscle stem cells but not in young. Such age-specific difference in the permissiveness of pERK is explained by an age-imposed shift toward open chromatin in the p16 and p21 promoters of the injury activated old satellite cells. Moreover our mining of a published database [36] has identified that there is also a shift toward epigenetically open p16 Triacsin C and p21 chromosomal loci in old quiescent satellite cells as compared to young. Materials and Methods Animals Young (2-3 months old) Triacsin C and old (22-24 months old) C57BL/6J mice were purchased from the Jackson Laboratory and National Institute on Aging respectively. All animals were housed at the University of California Berkeley and the animal experimental procedures were performed in accordance with the Guide for Care and Use of Laboratory Animals of the National Institutes of Health and approved by the University of California Berkeley Office of Laboratory Animal Care. Antibodies Antibodies to phosphor-ERK1/2 and total ERK1/2 were purchased from Cell Signaling Technologies. Anti-actin antibody was from Sigma. FGF2 p16 p21 myogenin MyoD and HRP-conjugated secondary antibodies were from Santa Cruz Biotechnology. Pax7 and myosin heavy chain (MHC) antibody were from the Developmental Studies Hybridoma Bank (University of Iowa). GAPDH Ki67 H3K4me3 H3K27me3 antibodies were from Abcam. Fluorophore-conjugated secondary antibodies were purchased from Invitrogen/Life Sciences. Muscle fibers and satellite cell isolation Isoflurane was used to anesthetize Triacsin C the animal during the muscle injury. For satellite cell activation tibialis anterior (TA) and gastrocnemius muscle were injected with cardiotoxin 1 (Sigma-Aldrich 200 μg/ml in PBS) at 10 μg per leg. Muscles were harvested from 3-day-post-injury (3DPI) or uninjured mice and then myofibers and satellite cells were isolated as described [38]. Briefly injured or uninjured TA and gastrocnemius muscles were dissected from young and old mice with fat pad and tendon removed and then incubated at 37 in digestion medium (250U/ml Collagenase type II in DMEM medium buffered with 30 mM HEPES) for 1 hour. Digested muscle was gently triturated and myofibers were collected. Myofibers were further digested with 1 U/ml Dispase and 40 U/ml Collagenase type II to liberate satellite cells. Satellite cells were pre-plated for 30 minutes and then cultured on diluted Matrigel (BD Biosciences) coated plates in DMEM with serum from the same mouse. Immunofluorescence Cells were fixed with 4% PFA for 10 minutes before permeabilization with 0.1% Triton-X 100 for 30 minutes. Then cells were immunostained with primary antibody. Cells.