title: TLR4 as receptor for HMGB1 induced muscle dysfunction in myositis.
abstract: OBJECTIVE
  Polymyositis and dermatomyositis are characterised by muscle weakness and fatigue even in patients with normal muscle histology via unresolved pathogenic mechanisms. In this study, we investigated the mechanisms by which high mobility group box protein 1 (HMGB1) acts to accelerate muscle fatigue development.
  
  
  METHODS
  Intact single fibres were dissociated from flexor digitorum brevis (FDB) of wild type, receptor for advanced glycation endproduct (RAGE) knockout and toll like receptor 4 (TLR4) knockout mice and cultured in the absence or presence of recombinant HMGB1. A decrease in sarcoplasmic reticulum Ca(2+) release during a series of 300 tetanic contractions, which reflects the development of muscle fatigue, was determined by measuring myoplasmic free tetanic Ca(2+). TLR4 and major histocompatibility complex (MHC)-class I expression in mouse FDB fibres were investigated by immunofluorescence and confocal microscopy. Immunohistochemistry was used to investigate TLR4, MHC-class I and myosin heavy chain expression in muscle fibres of patients.
  
  
  RESULTS
  Our results demonstrate that TLR4 is expressed in human and mouse skeletal muscle fibres, and coexpressed with MHC-class I in muscle fibres of patients with myositis. Furthermore, we show that HMGB1 acts via TLR4 but not RAGE to accelerate muscle fatigue and to induce MHC-class I expression in vitro. In order to bind and signal via TLR4, HMGB1 must have a reduced cysteine 106 and a disulphide linkage between cysteine 23 and 45.
  
  
  CONCLUSIONS
  The HMGB1-TLR4 pathway may play an important role in causing muscle fatigue in patients with polymyositis or dermatomyositis and thus is a potential novel target for future therapy.
authors.affiliation: Rheumatology Unit, Department of Medicine, Karolinska University Hospital, Solna, Karolinska Institutet, Stockholm, Sweden. mei.zong@ki.se
authors.forename: Mei
authors.initials: M
authors.lastname: Zong
authors.forename: Joseph D
authors.initials: JD
authors.lastname: Bruton
authors.forename: Cecilia
authors.initials: C
authors.lastname: Grundtman
authors.forename: Huan
authors.initials: H
authors.lastname: Yang
authors.forename: Jian Hua
authors.initials: JH
authors.lastname: Li
authors.forename: Helene
authors.initials: H
authors.lastname: Alexanderson
authors.forename: Karin
authors.initials: K
authors.lastname: Palmblad
authors.forename: Ulf
authors.initials: U
authors.lastname: Andersson
authors.forename: Helena E
authors.initials: HE
authors.lastname: Harris
authors.forename: Ingrid E
authors.initials: IE
authors.lastname: Lundberg
authors.forename: Håkan
authors.initials: H
authors.lastname: Westerblad
mesh_terms: D000328:Adult; D000368:Aged; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D005260:Female; D024243:HMGB1 Protein / Q000494:pharmacology*; D006801:Humans; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008875:Middle Aged; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology; D018763:Muscle Fatigue / Q000187:drug effects*; D018485:Muscle Fibers, Skeletal; D018995:Myosin Heavy Chains / Q000378:metabolism; D009220:Myositis / Q000378:metabolism* / Q000473:pathology; D000067759:Receptor for Advanced Glycation End Products; D011971:Receptors, Immunologic / Q000172:deficiency / Q000378:metabolism; D011994:Recombinant Proteins; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D051197:Toll-Like Receptor 4 / Q000172:deficiency / Q000378:metabolism*
chemical_list: D024243:HMGB1 Protein; D000067759:Receptor for Advanced Glycation End Products; D011971:Receptors, Immunologic; D011994:Recombinant Proteins; C493487:Tlr4 protein, mouse; D051197:Toll-Like Receptor 4; D018995:Myosin Heavy Chains; D002118:Calcium