IL-6 - a hypertrophic factor 3 rd CIM PhD Course Copenhagen, April 30 th, 2009 Pompeu Fabra University Department of Experimental and Health Sciences Barcelona, Spain
Myogenesis MRF: muscle regulatory factor
p38 MAPK How? Myoblast Myofiber
p38α controls myogenesis in vitro at two stages: proliferation and differentiation 1 2
Signaling pathways regulating skeletal muscle differentiation NF-êB p38 Calcineurin IGF-1 SRF E protein E Box MyoD NF-êB TGFâ TNFá FGF
The expresion of IL-6 is controlled by the p38/nf-kb pathway during myoblast differentiation DM+SB DM DM+PDTC IL-6 MKK6 MIAP-1 SOD MCP-1 p38 NF-κB C-FLIP GAPDH p65 p50 18S IL-6
IL-6 and skeletal muscle Inflammation-induced (overexpression, systemic, persistent) Cachexia / Muscle atrophy Deleterious effect on muscle IL-6 Exercise-induced (local and transient expression) Beneficial effects on different organs B. Pedersen and M. Febbraio CONCEPT IL-6 as a myokine
? IL-6 What is the effect of muscle-produced IL-6 on the muscle itself? Does it play a beneficial or detrimental function on muscle growth?
Compensatory hypertrophic muscle growth Overloading Incapacitation of gastrocnemious Growth of plantaris Myofiber Hypertrophied myofiber
IL-6 expression is induced during hypertrophic muscle growth
IL-6 deficiency blunts hypertrophic myofiber growth
IL-6 is expressed by both hypertrophying myofibers and associated satellite cells
AKT-mTOR Protein synthesis IL-6 is not required for the activation of the protein synthesis/growth-associated PI3K/AKT pathway during CH
Hypertrophic myofiber growth AKT-mTOR Protein synthesis Myonuclear domain is not maintained Satellite cell-mediated myonuclei accretion The factors/mechanisms regulating myonuclei accretion during CH are not well known How is myonuclear accretion in IL-6 -/- overloaded muscles?
IL-6-deficient myofibers exhibit impaired myonuclear accretion after overloading
Satellite cell activation is not affected by IL-6 deficiency after overloading 3 days after overloading
Loss of IL-6 reduces satellite cell proliferation after muscle overloading 3 days after overloading
The number of BrdU-positive cells is reduced in IL-6-deficient muscles after overloading 3 days after overloading
Summary IL-6 is necessary for myofiber compensatory growth (compensatory hypertrophy = CH) IL-6 is produced by growing myofibers and satellite cells during CH IL-6 is dispensable for satellite cell activation, but it is critical for satellite cell proliferation during CH.hence, the impaired proliferation may impede new myonuclei accretion to growing myofibers in the absence of IL-6
Is the growth defect in the absence of IL-6 muscle-cell intrinsic?
IL-6 is produced by primary myoblasts and growing myotubes
IL-6 is necessary for adequate myoblast proliferation in vitro Exogenous IL-6 (recombinant and from CM of WT myotubes) rescues the proliferation defect of IL-6 -/- myoblasts
Main signaling pathways activated by IL-6 IL-6 JAK Stat3 p p Stat3 p Stat3 STAT3 gp130 JAK IL-6Rá GAB PI3K SHP2 gp130 p Grb2/Sos Ras Raf MEK ERK p AKT PI3K/AKT Reduced activation of STAT3 in myoblast lacking IL-6
Inhibition of the JAK/ STAT3 pathway reduces myoblast proliferation, and exogenous IL-6-rescued proliferation of IL-6 -/- myoblasts
Reduced activation of STAT3 underlies the defective myoblast proliferation in the absence of IL-6
The IL-6/STAT3 axis controls the expression of proliferation genes in myoblasts: cyclin D1 and c-myc
Is STAT3 activation reduced in IL-6-deficient muscles during CH?
Loss of IL-6 reduces STAT3 activation in satellite cells after muscle overloading 3 days after overloading
Satellite cells are adequately activated early after overloading, but they fail to proliferate during the CH process
The number of myogenin- and emhc-positive cells is reduced in overloaded IL-6 -/- muscles 3 days after overloading
The progression of satellite cells to late differentiation stages is compromised in overloaded IL-6 -/- muscles
The deficient satellite cell differentiation after overloading in the absence of IL-6 may be consequence of the proliferation block, resulting in impaired myofiber growth.
Hypertrophic stimulus (overloading) AKT-mTOR protein synthesis IL-6 STAT3 SC proliferation Myonuclei accretion IL-6 is locally and transiently produced in overloaded adult muscles by myofibers and associated satellite cells. Muscle-produced IL-6 induces satellite cell proliferation via STAT3 activation. The IL-6/STAT3 axis is required for myonuclei accretion by the preexisting myofiber.
How does skeletal muscle grow? 1. New myofiber formation Development Muscle repair Muscle pathology Myoblast Myofiber 2. Growth of preexisting myofibers Hypertrophic stimuli Myofiber Hypertrophied myofiber
Myogenesis Myoblast Myofiber Proliferation Migration Diferentiation Fusion IL-6
IL-6 is required for myoblast proliferation and migration
IL-6 loss attenuates myoblast differentiation and fusion
IL-6 loss attenuates myoblast fusion
How does skeletal muscle grow? 1. New myofiber formation Development Muscle repair Muscle pathology Myoblast 2. Growth of preexisting myofibers Hypertrophic stimuli IL-6 Myofiber Myofiber Hypertrophied myofiber
Impaired skeletal muscle regeneration in IL-6-deficient mice WT IL-6 -/- 1 week after CTX injury emhc Two possible actions of IL-6 in regenerating muscle: a) inflammation (+?) b) new myofiber formation (+?)
The path to muscle repair after injury. Acute injury Degeneration Inflammatory response macrophages Regeneration Full repair Connective tissue deposition (transient) (positive role)
Inflammation-mediated muscle fibrosis Persistent damage (dystrophic muscle) Degeneration / Regeneration Fibrosis (months/years) Chronic inflammation Macrophages (negative role)
Antonio Serrano Eusebio Perdiguero Vanessa Ruiz-Bonilla Pedro Sousa-Victor Mercè Jardí Esther Ardite Berta Vidal Roberta De Mori Mónica Zamora A. Bernad (CNIC, Madrid) Bernat Baeza-Raja (UCSF)