title: OPN-a induces muscle inflammation by increasing recruitment and activation of pro-inflammatory macrophages.
abstract: What is the central question of this study? What is the functional relevance of OPN isoform expression in muscle pathology? What is the main finding and its importance? The full-length human OPN-a isoform is the most pro-inflammatory isoform in the muscle microenvironment, acting on macrophages and myoblasts in an RGD-integrin-dependent manner. OPN-a upregulates expression of tenascin-C (TNC), a known Toll-like receptor 4 (TLR4) agonist. Blocking TLR4 signalling inhibits the pro-inflammatory effects of OPN-a, suggesting that a potential mechanism of OPN action is by promoting TNC-TLR4 signalling. Although osteopontin (OPN) is an important mediator of muscle remodelling in health and disease, functional differences in human spliced OPN variants in the muscle microenvironment have not been characterized. We thus sought to define the pro-inflammatory activities of human OPN isoforms (OPN-a, OPN-b and OPN-c) on cells present in regenerating muscle. OPN transcripts were quantified in normal and dystrophic human and dog muscle. Human macrophages and myoblasts were stimulated with recombinant human OPN protein isoforms, and cytokine mRNA and protein induction was assayed. OPN isoforms were greatly increased in dystrophic human (OPN-a > OPN-b > OPN-c) and dog muscle (OPN-a = OPN-c). In healthy human muscle, mechanical loading also upregulated OPN-a expression (eightfold; P < 0.01), but did not significantly upregulate OPN-c expression (twofold; P > 0.05). In vitro, OPN-a displayed the most pronounced pro-inflammatory activity among isoforms, acting on both macrophages and myoblasts. In vitro and in vivo data revealed that OPN-a upregulated tenascin-C (TNC), a known Toll-like receptor 4 (TLR4) agonist. Inhibition of TLR4 signalling attenuated OPN-mediated macrophage cytokine production. In summary, OPN-a is the most abundant and functionally active human spliced isoform in the skeletal muscle microenvironment. Here, OPN-a promotes pro-inflammatory signalling in both macrophages and myoblasts, possibly through induction of TNC-TLR4 signalling. Together, our findings suggest that specific targeting of OPN-a and/or TNC signalling in the damaged muscle microenvironment may be of therapeutic relevance.
authors.affiliation: Research Center for Genetic Medicine, Children's National Medical Center, Washington, DC, USA.|Department of Integrative Systems Biology, George Washington University School of Medicine & Health Sciences, Washington, DC, USA.|Department of Cell, Developmental and Integrative Biology, University of Alabama Birmingham, Birmingham, AL, USA.
authors.forename: Gina M
authors.initials: GM
authors.lastname: Many
authors.affiliation: Hiroshima University, Minamiku, Hiroshima, Japan.
authors.forename: Yasuyuki
authors.initials: Y
authors.lastname: Yokosaki
authors.affiliation: Research Center for Genetic Medicine, Children's National Medical Center, Washington, DC, USA.
authors.forename: Kitipong
authors.initials: K
authors.lastname: Uaesoontrachoon
authors.affiliation: Research Center for Genetic Medicine, Children's National Medical Center, Washington, DC, USA.|Department of Integrative Systems Biology, George Washington University School of Medicine & Health Sciences, Washington, DC, USA.|Department of Veterinary Integrative Biosciences, Texas A&M University, College Station, TX, USA.
authors.forename: Peter P
authors.initials: PP
authors.lastname: Nghiem
authors.affiliation: Research Center for Genetic Medicine, Children's National Medical Center, Washington, DC, USA.
authors.forename: Luca
authors.initials: L
authors.lastname: Bello
authors.affiliation: Research Center for Genetic Medicine, Children's National Medical Center, Washington, DC, USA.
authors.forename: Sherry
authors.initials: S
authors.lastname: Dadgar
authors.affiliation: National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD, USA.
authors.forename: Ying
authors.initials: Y
authors.lastname: Yin
authors.affiliation: Research Center for Genetic Medicine, Children's National Medical Center, Washington, DC, USA.|ReveraGen BioPharma, Rockville, MD, USA.
authors.forename: Jesse M
authors.initials: JM
authors.lastname: Damsker
authors.affiliation: Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, MD, USA.
authors.forename: Heather B
authors.initials: HB
authors.lastname: Cohen
authors.affiliation: Department of Veterinary Integrative Biosciences, Texas A&M University, College Station, TX, USA.
authors.forename: Joe N
authors.initials: JN
authors.lastname: Kornegay
authors.affiliation: Department of Cell, Developmental and Integrative Biology, University of Alabama Birmingham, Birmingham, AL, USA.
authors.forename: Marcas M
authors.initials: MM
authors.lastname: Bamman
authors.affiliation: Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, MD, USA.
authors.forename: David M
authors.initials: DM
authors.lastname: Mosser
authors.affiliation: Research Center for Genetic Medicine, Children's National Medical Center, Washington, DC, USA.|Department of Integrative Systems Biology, George Washington University School of Medicine & Health Sciences, Washington, DC, USA.
authors.forename: Kanneboyina
authors.initials: K
authors.lastname: Nagaraju
authors.affiliation: Research Center for Genetic Medicine, Children's National Medical Center, Washington, DC, USA. ehoffman@binghamton.edu.|Department of Integrative Systems Biology, George Washington University School of Medicine & Health Sciences, Washington, DC, USA. ehoffman@binghamton.edu.
authors.forename: Eric P
authors.initials: EP
authors.lastname: Hoffman
mesh_terms: D000328:Adult; D000818:Animals; D002478:Cells, Cultured; D016207:Cytokines / Q000378:metabolism; D004285:Dogs; D005260:Female; D006801:Humans; D007249:Inflammation / Q000378:metabolism*; D008264:Macrophages / Q000378:metabolism*; D008297:Male; D051379:Mice; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000378:metabolism*; D032446:Myoblasts / Q000378:metabolism; D053495:Osteopontin / Q000378:metabolism*; D020033:Protein Isoforms / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology; D051197:Toll-Like Receptor 4 / Q000378:metabolism; D015854:Up-Regulation / Q000502:physiology
chemical_list: D016207:Cytokines; D020033:Protein Isoforms; D012333:RNA, Messenger; D051197:Toll-Like Receptor 4; D053495:Osteopontin
keywords: Duchenne muscular dystrophy; OPN-a; TNC; muscle inflammation; osteopontin; skeletal muscle