Myoblast sensitivity and fibroblast insensitivity to osteogenic conversion by BMP-2 correlates with the expression of Bmpr-1a.
BACKGROUND
Osteoblasts are considered to primarily arise from osseous progenitors within the periosteum or bone marrow. We have speculated that cells from local soft tissues may also take on an osteogenic phenotype. Myoblasts are known to adopt a bone gene program upon treatment with the osteogenic bone morphogenetic proteins (BMP-2,-4,-6,-7,-9), but their osteogenic capacity relative to other progenitor types is unclear. We further hypothesized that the sensitivity of cells to BMP-2 would correlate with BMP receptor expression.


METHODS
We directly compared the BMP-2 sensitivity of myoblastic murine cell lines and primary cells with osteoprogenitors from osseous tissues and fibroblasts. Fibroblasts forced to undergo myogenic conversion by transduction with a MyoD-expressing lentiviral vector (LV-MyoD) were also examined. Outcome measures included alkaline phosphatase expression, matrix mineralization, and expression of osteogenic genes (alkaline phosphatase, osteocalcin and bone morphogenetic protein receptor-1A) as measured by quantitative PCR.


RESULTS
BMP-2 induced a rapid and robust osteogenic response in myoblasts and osteoprogenitors, but not in fibroblasts. Myoblasts and osteoprogenitors grown in osteogenic media rapidly upregulated Bmpr-1a expression. Chronic BMP-2 treatment resulted in peak Bmpr-1a expression at day 6 before declining, suggestive of a negative feedback mechanism. In contrast, fibroblasts expressed low levels of Bmpr-1a that was only weakly up-regulated by BMP-2 treatment. Bioinformatics analysis confirmed the presence of myogenic responsive elements in the proximal promoter region of human and murine BMPR-1A/Bmpr-1a. Forced myogenic gene expression in fibroblasts was associated with a significant increase in Bmpr-1a expression and a synergistic increase in the osteogenic response to BMP-2.


CONCLUSIONS
These data demonstrate the osteogenic sensitivity of muscle progenitors and provide a mechanistic insight into the variable response of different cell lineages to BMP-2.
Liu
Renjing
R
Orthopaedic Research & Biotechnology Unit, The Children's Hospital at Westmead, Sydney, NSW, Australia. renjingl@chw.edu.au
Ginn
Samantha L
SL
Lek
Monkol
M
North
Kathryn N
KN
Alexander
Ian E
IE
Little
David G
DG
Schindeler
Aaron
A
D000818:Animals; D015415:Biomarkers / Q000032:analysis / Q000378:metabolism; D055396:Bone Morphogenetic Protein 2 / Q000378:metabolism / Q000494:pharmacology*; D052004:Bone Morphogenetic Protein Receptors / Q000187:drug effects / Q000378:metabolism; D052005:Bone Morphogenetic Protein Receptors, Type I / Q000235:genetics / Q000378:metabolism*; D002113:Calcification, Physiologic / Q000187:drug effects / Q000502:physiology; D002454:Cell Differentiation / Q000187:drug effects* / Q000502:physiology; D019070:Cell Lineage / Q000187:drug effects / Q000502:physiology; D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D025461:Feedback, Physiological / Q000187:drug effects / Q000502:physiology; D005347:Fibroblasts / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D017570:MyoD Protein / Q000235:genetics; D032446:Myoblasts / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D041681:NIH 3T3 Cells; D010012:Osteogenesis / Q000187:drug effects / Q000502:physiology; D013234:Stem Cells / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D014161:Transduction, Genetic / Q000379:methods; D015854:Up-Regulation / Q000187:drug effects / Q000502:physiology
D015415:Biomarkers; C525719:Bmp2 protein, mouse; D055396:Bone Morphogenetic Protein 2; D017570:MyoD Protein; C057363:MyoD1 myogenic differentiation protein; C498212:Bmpr1a protein, mouse; D052004:Bone Morphogenetic Protein Receptors; D052005:Bone Morphogenetic Protein Receptors, Type I