Satellite Cells• Define the myonuclear domain• Describe the satellite cell niche• Describe the signaling networks controlling SC
proliferation and fusion– HGF, FGF, myostatin– MRFs, pax
Satellite cell• Anatomical definition
– Mononucleated cells in muscle– Oustide the fiber plasma membrane– Inside the basal lamina
EM of SC on muscle fiber; Dark thymidine-labeled new DNA. Moss & Leblond, 1971
Satellite Cell: Molecular definition• Unique
– c-Met (HGF receptor)– M-cadherin (adhesion molecule)– Pax3/7 (transcription factor)
• Overlapping– CD34 (hematopoetic)– Integrin-a7 (adhesion molecule)– Myf5 (transcription factor)– Caveolin-1 Laminin
M-cadherinNucelusIrintchev & al 1994
SC Function• Quiescent: withdrawn from cell cycle• Regeneration
– SCs “activated” by damage– Re-enter cell cycle– Fully differentiate– Fusemyotubemuscle fiber
• Hypertrophy/growth– Re-enter cell cycle(?)– Fusemuscle fiber
Myonuclear domain• Volume of cytoplasm supported by one nucleus
– Conserved during growth– Consistent across body size
• DNA contentmay limitRNA synthesisand proteincontent
Liu & al., 2008
SC contribution to hypertrophy• Adams & al., 2002• Irradiation to silence SC• Synergist ablation• Irradiation blocks
substantial growth• MN domain conserved
Signaling during Ir-hypertrophy• Early (0-3 days)
– Muscle IGF-1– Myogenin
• Persistent– MGF/myogenin– mTOR effectors
Both groups start the same, but Ir deviates after a few days. Fails to maintain trajectory
Another model of SC ablation• McCarthy & al 2011• SC-specific, Tamoxifen-inducible diptheria toxin
– “Cre-Lox” recombination• Cre recombinase targets Lox sequences for recombination• Tamoxifen-dependent
– Rosa26/DTA: diptheria toxin preceded by floxed transcription stop sequence
– Pax7/Cre: satellite-cell specific Cre
eGFP DTAStart Stop StopLox Lox
DTAStart StopLox
Pax7/DTA disrupts SC-function• BaCl2 kills myofibersregeneration• SC-mediated repair is disrupted in pax7/DTA
Healthy muscleInjured & repaired muscle (centrally-located nuclei)Injured & not repaired muscle (unstructured fibrosis)
Irradiation vs DTA questions• Irradiation kills both SC and bone marrow• Is the initial FO response to the injury or to
the overload?• Most clear DTA results at 2 weeks, when
edema dominates protein accretion
Satellite cell niche• Niche: physical environment
– Circulating growth factors are only part– Physical contacts: myofiber and ECM– Paracrine factors: fiber, inflammatory cells– Mechanical factors
• Basal Lamina/ECM– Collagen/laminin– Heparin sulfate proteoglycans
• Adhesive substrates• Growth factor chelators• Receptor cofactors
Contact control of SC• Contact with fiber blocks proliferation• Contact with BL
facilitates proliferation• Ground-up muscle helps
Dissociate muscle into Fiber-BL chunksKill fiber with marcaineCount SC (Bischoff, 1990)
Killed fiber clotBasal Lamina shell
Satellite cell
Regulatory control of fate• Proliferation
– Cell cycle progression– HGF/FGF– MyoD/Myf5
• Differentiation– IGF-1/PGF2a– Myogenin/MRF4– Loss of Pax3/7
Charge & Rudnicki, 2004
Activation of quiescent SCs• Some signal is released from damaged muscle
– bFGF? HGF?
• Cell cycle re-entry (SCadult myoblast)– PI3K-mTORgrow the cell– ERKcell cycle progression– MyoD/myf5be a muscle
• Negative controls– Inhibit differentiation– TGF- family (TGF-, BMP)– Myostatin
Proliferation• Mitogens
– FGF, EGF, HGF
• DNA synthesis– CyD/CDK4 accumulation– Degradation of Rb– Activation of E2F
• Cell size integration– GSK3 inhibits CDK4– GSK3 inhibits b-Catenin
GF-R(c-Met, FGFR)
Ras-raf-MEK-ERK
Rb
E2F
Cyclin-ADNA-pol’ase
CDK1
DNA synthesis
Starvation
GSK3
Differentiation• Cell cycle withdrawal
– M-cadherinb-catenin/TCF– MYf5/myogeninterminal differentiation
• Fusion or quiescence– Pax3/7MRF suppressor– Cytoskeletal rearrangement– Contact recognition of fusion partner
Differentiation• bHLH transcription factors
– Class A/general• TCF
– Class B/Tissue specific• MyoD
• b-Catenin: bHLH-HDAC deactivator• Inhibitor of differentiation (Id)
– HLH, no b– No b = no DNA binding
Wnt receptor
Dishevelled
GSK3b
B-catenin
TCF/LEF
DNA synthesis
MyoD
Myogenin/MHCMyostatin Smad2/3 Id3
M-cadherin
Muscle and other stem cells• Pluripotency• The Side Population
– Exclude Hoechst dyes viaactive ABC transporters
– Subset of many stem-likecell populations
• Muscle SP cells– CD45/Sca1 positive– Pluripotent
• Muscle MP cells– Unipotent
Asakura & al, 2002
Muscle and other stem cells• Hematopoetic stem cells
– Bone marrow derived– Mostly WBC– 0.2% SP, pluripotent– May contribute to regen in many tissues
• eg: chimeric neurons after BMT
• Are muscle SP cells really satellite cells?• Do MP and SP cells contribute equally to
regeneration? Hypertrophy?
Satellite cell therapies• Muscle normally
incorporates new genetic material from proliferating cells
• Grow (and engineer) myoblast population
• Inject & allow fusion• None of them work, yet.
– Some progress in animal models
Skuk & Tremblay, 2003)
Myoblast transplantation (DMD)• Correction of genetic defects (Dystrophin)
– 1e7 donor SC in 100 injections to one muscle– CsA as immunosuppressant– 6 month follow-up– Force gain (CsA)– No dystrophin
Miller & al., 1997