Satellite cells (SCs), stem cells in skeletal muscle, are mitotically quiescent in adult mammals until activated for growth or regeneration. In mouse muscle, SCs are activated by nitric oxide (NO), hepatocyte growth factor (HGF) and the mechanically induced NO-HGF signaling cascade. Here, the SC population on fibers from the adult, ectothermic zebrafish and SC responsiveness to activating stimuli were assessed using the model system of isolated fibers cultured at 27 and 21°C. SCs were identified by immunostaining for the HGF receptor, c-met, and activation was determined using bromodeoxyuridine uptake in culture or in vivo. In dose-response studies, SC activation was increased by treatment with the NO-donor drug isosorbide dinitrate (1 mmol l −1 ) or HGF (10 ng ml −1 ) to maximum activation at lower concentrations of both than in previous studies of mouse fibers. HGF-induced activation was blocked by anti-c-met antibody, and reduced by culture at 21°C. The effect of cyclical stretch (3 h at 4 cycles per minute) increased activation and was blocked by nitric oxide synthase inhibition and reduced by culture at 21°C. The number of c-met+ SCs per fiber increased rapidly (by 3 h) after stretching. The character of signaling in SC activation on zebrafish fibers, in particular temperature-dependent responses to HGF and stretch, gives new insights into the influence of ectothermy on regulation of muscle growth in teleosts and suggests the use of the single-fiber model system to explore the basis of fiber hyperplasia and the conservation of regulatory pathways between species. KEY WORDS: Stem cells, Ectothermy, Proliferation, c-met, Hepatocyte growth factor INTRODUCTIONSkeletal muscle function in fish is fundamentally important for survival using high-and low-speed swimming (Fauconneau et al., 1995), and the zebrafish [Danio rerio (Hamilton 1822)], an ectothermic teleost, is an important vertebrate model for studying development, physiology and pathology (Lieschke and Currie, 2007). In teleosts, muscle mass [over 90% white fibers (Wakeling and Johnston, 1999)] grows by increases in fiber number and size (Bird and Mabee, 2003;Buckingham and Vincent, 2009). The combination of hyperplasia and hypertrophy is termed indeterminate growth (Fauconneau and Paboeuf, 2001;Rowlerson and Veggetti, 2001;Johnston, 2006), and the growth potential is strongly influenced by temperature (Steinbacher et al., 2011). By comparison, in postnatal mammals, fibers hypertrophy and show accretion of post-mitotic myonuclei, but fibers do not increase in number (Petrella et al., 2008;O'Connor and Pavlath, 2007). Satellite RESEARCH ARTICLEDepartment of Biological Sciences, University of Manitoba, Winnipeg, MB, Canada R3T 2N2. Anderson and Pilipowicz, 2002;Bischoff, 1975;Bischoff, 1986;Bischoff, 1990) and in vivo (Anderson, 2000;Anderson and Wozniak, 2004;Tatsumi and Allen, 2008). Calcium-dependent stretch-activated signals release NO from mechano-sensitive NOS-1μ, which induces HGF release from the extracellular matrix (Tatsumi et al., 2002;Y...

Satellite cell activation and populations on single muscle‐fiber cultures from adult zebrafish (Danio rerio) Helia Zhang and Judy E Anderson, Department of Biological Sciences, University of Manitoba, Canada Satellite‐stem cells in adult mouse skeletal muscle are activated for growth or regeneration by signaling via nitric oxide, hepatocyte growth factor (HGF) and the c‐met receptor. Here, the regulation of satellite cell activation in adult zebrafish was assessed using the model system of isolated fibers cultured at 27°C and 21°C. Satellite cells were identified c‐met+ staining; activation was determined by uptake of bromodeoxyuridine. Satellite cells were maximally activated by the NO‐donor isosorbide dinitrate (ISDN, 1mM) or HGF (10 ng/mL); levels lower and higher respectively, than in mouse fibers. HGF‐induced activation was blocked by anti‐c‐met antibody, and reduced at 21°C. Cyclical stretch (3 hr at 4 cycles per min) increased activation; NOS inhibition and culture at 21°C reduced stretch activation. Stretching rapidly increased the number of c‐met+ cells/fiber (by 3 hr). Satellite cell activation signaling on zebrafish fibers, in particular temperature‐dependent responses to HGF and stretch, give new insights into the influences of ectothermy in regulating muscle growth and suggest the single‐fiber model system can be used to explore the basis of fiber hyperplasia in teleosts. Grant Funding Source: Supported by Natural Sciences and Engineering Research Council