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Excessive transforming growth factor-β signaling is a common mechanism in osteogenesis imperfecta.

Grafe I, Yang T, Alexander S, Homan EP, Lietman C, Jiang MM, Bertin T, Munivez E, Chen Y, Dawson B, Ishikawa Y, Weis MA, Sampath TK, Ambrose C, Eyre D, Bächinger HP, Lee B - Nat. Med. (2014)

Bottom Line: Notably, the clinical overlap between dominant and recessive forms of OI suggests common molecular pathomechanisms.In the skeleton, we find higher expression of TGF-β target genes, higher ratio of phosphorylated Smad2 to total Smad2 protein and higher in vivo Smad2 reporter activity.Hence, altered TGF-β matrix-cell signaling is a primary mechanism in the pathogenesis of OI and could be a promising target for the treatment of OI.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas, USA.

ABSTRACT
Osteogenesis imperfecta (OI) is a heritable disorder, in both a dominant and recessive manner, of connective tissue characterized by brittle bones, fractures and extraskeletal manifestations. How structural mutations of type I collagen (dominant OI) or of its post-translational modification machinery (recessive OI) can cause abnormal quality and quantity of bone is poorly understood. Notably, the clinical overlap between dominant and recessive forms of OI suggests common molecular pathomechanisms. Here, we show that excessive transforming growth factor-β (TGF-β) signaling is a mechanism of OI in both recessive (Crtap(-/-)) and dominant (Col1a2(tm1.1Mcbr)) OI mouse models. In the skeleton, we find higher expression of TGF-β target genes, higher ratio of phosphorylated Smad2 to total Smad2 protein and higher in vivo Smad2 reporter activity. Moreover, the type I collagen of Crtap(-/-) mice shows reduced binding to the small leucine-rich proteoglycan decorin, a known regulator of TGF-β activity. Anti-TGF-β treatment using the neutralizing antibody 1D11 corrects the bone phenotype in both forms of OI and improves the lung abnormalities in Crtap(-/-) mice. Hence, altered TGF-β matrix-cell signaling is a primary mechanism in the pathogenesis of OI and could be a promising target for the treatment of OI.

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Reduced decorin binding to type I collagen of Crtap−/− mice. (a) Quantitative RTPCR of calvarial bone of P3 mice for the small leucine-rich proteoglycans decorin (Dcn), biglycan (Bgn), and asporin (Aspn) in WT and Crtap−/− mice. Results given as fold change of the mean of WT group±SD, n=5 per group. NS, not significant. (b) Surface plasmon resonance analysis measuring the binding of recombinant decorin core protein to type I collagen of WT and Crtap−/− mice. Three technical replicates at each of the indicated concentrations of decorin were performed in two independent biological replicates (◆ replicate 1, ▲ replicate 2). Results are shown as the percentage of the mean of WT (bars indicate mean per group).
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Figure 3: Reduced decorin binding to type I collagen of Crtap−/− mice. (a) Quantitative RTPCR of calvarial bone of P3 mice for the small leucine-rich proteoglycans decorin (Dcn), biglycan (Bgn), and asporin (Aspn) in WT and Crtap−/− mice. Results given as fold change of the mean of WT group±SD, n=5 per group. NS, not significant. (b) Surface plasmon resonance analysis measuring the binding of recombinant decorin core protein to type I collagen of WT and Crtap−/− mice. Three technical replicates at each of the indicated concentrations of decorin were performed in two independent biological replicates (◆ replicate 1, ▲ replicate 2). Results are shown as the percentage of the mean of WT (bars indicate mean per group).

Mentions: Hence, we hypothesize that decorin binding to collagen is critical for TGFβ regulation and that this binding is disrupted with altered collagen structure in OI. We found that while loss of Crtap did not alter the expression of decorin and other SLRPs in calvarial bone (Fig. 3a) nor the qualitative abundance of decorin in trabecular bone (Supplementary Fig.4), it did reduce binding of decorin to type I collagen (Fig. 3b; mean reductions of decorin binding to Crtap−/− vs. WT type I collagen at 3, 5 and 12 μM of decorin were 28.5%, 33.5% and 38.1%, respectively). Based on the requirement of collagen binding for decorin to effectively reduce TGFβ bioactivity3, it is possible that this altered binding in OI affects decorin's ability to sequester mature TGFβ in the matrix and modulate TGFβ function. This could contribute to dysregulated TGFβ signaling, even if no major changes in absolute TGFβ levels are present (Supplementary Fig.5 and 6). This notion is supported by the finding that COL1A1 and COL1A2 mutations in severe forms of dominant OI cluster in regions that are known to bind proteoglycans33, further supporting the relevance of proteoglycan-collagen interactions for normal bone homeostasis. This implies that other proteoglycans that are competing with decorin for the collagen binding site34 may also contribute to dysregulated TGFβ activity, and that additional signaling pathways could be altered35.


Excessive transforming growth factor-β signaling is a common mechanism in osteogenesis imperfecta.

Grafe I, Yang T, Alexander S, Homan EP, Lietman C, Jiang MM, Bertin T, Munivez E, Chen Y, Dawson B, Ishikawa Y, Weis MA, Sampath TK, Ambrose C, Eyre D, Bächinger HP, Lee B - Nat. Med. (2014)

Reduced decorin binding to type I collagen of Crtap−/− mice. (a) Quantitative RTPCR of calvarial bone of P3 mice for the small leucine-rich proteoglycans decorin (Dcn), biglycan (Bgn), and asporin (Aspn) in WT and Crtap−/− mice. Results given as fold change of the mean of WT group±SD, n=5 per group. NS, not significant. (b) Surface plasmon resonance analysis measuring the binding of recombinant decorin core protein to type I collagen of WT and Crtap−/− mice. Three technical replicates at each of the indicated concentrations of decorin were performed in two independent biological replicates (◆ replicate 1, ▲ replicate 2). Results are shown as the percentage of the mean of WT (bars indicate mean per group).
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Figure 3: Reduced decorin binding to type I collagen of Crtap−/− mice. (a) Quantitative RTPCR of calvarial bone of P3 mice for the small leucine-rich proteoglycans decorin (Dcn), biglycan (Bgn), and asporin (Aspn) in WT and Crtap−/− mice. Results given as fold change of the mean of WT group±SD, n=5 per group. NS, not significant. (b) Surface plasmon resonance analysis measuring the binding of recombinant decorin core protein to type I collagen of WT and Crtap−/− mice. Three technical replicates at each of the indicated concentrations of decorin were performed in two independent biological replicates (◆ replicate 1, ▲ replicate 2). Results are shown as the percentage of the mean of WT (bars indicate mean per group).
Mentions: Hence, we hypothesize that decorin binding to collagen is critical for TGFβ regulation and that this binding is disrupted with altered collagen structure in OI. We found that while loss of Crtap did not alter the expression of decorin and other SLRPs in calvarial bone (Fig. 3a) nor the qualitative abundance of decorin in trabecular bone (Supplementary Fig.4), it did reduce binding of decorin to type I collagen (Fig. 3b; mean reductions of decorin binding to Crtap−/− vs. WT type I collagen at 3, 5 and 12 μM of decorin were 28.5%, 33.5% and 38.1%, respectively). Based on the requirement of collagen binding for decorin to effectively reduce TGFβ bioactivity3, it is possible that this altered binding in OI affects decorin's ability to sequester mature TGFβ in the matrix and modulate TGFβ function. This could contribute to dysregulated TGFβ signaling, even if no major changes in absolute TGFβ levels are present (Supplementary Fig.5 and 6). This notion is supported by the finding that COL1A1 and COL1A2 mutations in severe forms of dominant OI cluster in regions that are known to bind proteoglycans33, further supporting the relevance of proteoglycan-collagen interactions for normal bone homeostasis. This implies that other proteoglycans that are competing with decorin for the collagen binding site34 may also contribute to dysregulated TGFβ activity, and that additional signaling pathways could be altered35.

Bottom Line: Notably, the clinical overlap between dominant and recessive forms of OI suggests common molecular pathomechanisms.In the skeleton, we find higher expression of TGF-β target genes, higher ratio of phosphorylated Smad2 to total Smad2 protein and higher in vivo Smad2 reporter activity.Hence, altered TGF-β matrix-cell signaling is a primary mechanism in the pathogenesis of OI and could be a promising target for the treatment of OI.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas, USA.

ABSTRACT
Osteogenesis imperfecta (OI) is a heritable disorder, in both a dominant and recessive manner, of connective tissue characterized by brittle bones, fractures and extraskeletal manifestations. How structural mutations of type I collagen (dominant OI) or of its post-translational modification machinery (recessive OI) can cause abnormal quality and quantity of bone is poorly understood. Notably, the clinical overlap between dominant and recessive forms of OI suggests common molecular pathomechanisms. Here, we show that excessive transforming growth factor-β (TGF-β) signaling is a mechanism of OI in both recessive (Crtap(-/-)) and dominant (Col1a2(tm1.1Mcbr)) OI mouse models. In the skeleton, we find higher expression of TGF-β target genes, higher ratio of phosphorylated Smad2 to total Smad2 protein and higher in vivo Smad2 reporter activity. Moreover, the type I collagen of Crtap(-/-) mice shows reduced binding to the small leucine-rich proteoglycan decorin, a known regulator of TGF-β activity. Anti-TGF-β treatment using the neutralizing antibody 1D11 corrects the bone phenotype in both forms of OI and improves the lung abnormalities in Crtap(-/-) mice. Hence, altered TGF-β matrix-cell signaling is a primary mechanism in the pathogenesis of OI and could be a promising target for the treatment of OI.

Show MeSH
Related in: MedlinePlus