CD44 and hyaluronan promote the bone morphogenetic protein 7 signaling response in murine chondrocytes.
OBJECTIVE
Cell-matrix interactions promote cartilage homeostasis. We previously found that Smad1, the transcriptional modulator of the canonical bone morphogenetic protein 7 (BMP-7) pathway, interacted with the cytoplasmic domain of CD44, the principal hyaluronan receptor on chondrocytes. To elucidate the physiologic function of CD44-Smad1 interactions, as well as the role of hyaluronan, we studied the response of chondrocytes isolated from CD44(-/-) and BALB/c (wild-type [WT]) mice to stimulation with BMP-7.


METHODS
In primary murine chondrocytes, CD44 expression was decreased by small interfering RNA (siRNA) transfection or was enhanced by plasmid transfection. Pericellular hyaluronan was removed by hyaluronidase treatment, or its endogenous synthesis was inhibited. Changes in response to BMP-7 stimulation were evaluated by Western blotting of Smad1 phosphorylation and aggrecan messenger RNA (mRNA) expression.


RESULTS
Chondrocytes from CD44(-/-) mice and WT mice transfected with CD44 siRNA were less responsive than untransfected chondrocytes from WT mice to BMP-7. CD44(-/-) mouse chondrocytes transfected with pCD44 showed increased sensitivity to BMP-7. Significant increases in aggrecan mRNA were observed in WT mouse chondrocytes in response to 10 ng/ml of BMP-7, whereas at least 100 ng/ml of BMP-7 was required for CD44(-/-) mouse chondrocytes. However, in chondrocytes from CD44(-/-) and WT mice, hyaluronidase treatment decreased cellular responses to BMP-7. Treatment of both bovine and murine chondrocytes with 4-methylumbelliferone to reduce the synthesis of endogenous hyaluronan confirmed that hyaluronan promoted BMP-7 signaling.


CONCLUSIONS
Taken together, these investigations into the mechanisms underlying BMP-7 signaling in chondrocytes revealed that while hyaluronan-dependent pericellular matrix is critical for BMP-7 signaling, the expression of CD44 promotes the cellular response to lower concentrations of BMP-7.
East Carolina University, Brody School of Medicine, Greenville, North Carolina; Nankai University School of Medicine, Tianjin, China.
Na
N
Luo
Warren
W
Knudson
Emily B
EB
Askew
Roberta
R
Veluci
Cheryl B
CB
Knudson
D053674:Aggrecans / Q000378:metabolism; D000818:Animals; D055419:Bone Morphogenetic Protein 7 / Q000494:pharmacology*; D002478:Cells, Cultured; D019902:Chondrocytes / Q000187:drug effects* / Q000378:metabolism / Q000473:pathology; D018960:Hyaluronan Receptors / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D006820:Hyaluronic Acid / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006821:Hyaluronoglucosaminidase / Q000494:pharmacology; D051379:Mice; D008807:Mice, Inbred BALB C; D018345:Mice, Knockout; D034741:RNA, Small Interfering / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects*; D051898:Smad1 Protein / Q000378:metabolism; D014162:Transfection
D053674:Aggrecans; D055419:Bone Morphogenetic Protein 7; D018960:Hyaluronan Receptors; D034741:RNA, Small Interfering; D051898:Smad1 Protein; C497401:Smad1 protein, mouse; D006820:Hyaluronic Acid; D006821:Hyaluronoglucosaminidase