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Reduced expression of thrombospondins and craniofacial dysmorphism in mice overexpressing Fra1
Toru Nishiwaki1, Toru Yamaguchi, Chen Zhao
1Department of Orthopaedics, School of Medicine, Keio University, Tokyo, Japan.
Unlabelled:
Fra1 transgenic (Tg) mice develop osteosclerosis and exhibit altered expression of bone matrix proteins. We found that expression of Thbs1 and Thbs2 was reduced in Fra1 Tg osteoblasts. Fra1 Tg and non-osteosclerotic Thbs1-/-Thbs2-/- mice share an edge-to-edge bite. Therefore, reduced expression of thrombospondins may contribute to craniofacial dysmorphism independently of osteosclerosis.
Introduction:
Tg mice overexpressing Fra1, a component of the transcription factor activator protein-1 (AP-1), show progressive osteosclerosis caused by cell autonomous abnormalities in osteoblasts. The expression of several bone matrix proteins, including matrix gla protein, is dysregulated in Fra1 Tg osteoblasts.
Materials And Methods:
In osteoblastogenic cultures, altered bone matrix production by Fra1 overexpression was monitored using Alizarin red staining, quantitative RT-PCR, and Western blotting. Responsiveness to ovariectomy was examined by bone histomorphometry. Craniofacial parameters were measured on radiographs and using CT.
Results:
Thrombospondin-1 (Thbs1) and thrombospondin-2 (Thbs2) were reduced in Fra1 Tg osteoblasts differentiated in vitro and in bones from Fra1 Tg mice. Despite alterations in bone matrix proteins, ovariectomy induces high turnover bone loss in Fra1 Tg mice as in wildtype mice. Fra1 Tg mice, as well as Thbs1-/- Thbs2-/- mice, which do not show osteosclerosis, exhibit an edge-to-edge bite phenotype associated with craniofacial dysmorphism.
Conclusions:
These data suggest that reduced expression of thrombospondins in Fra1 Tg mice underlies craniofacial dysmorphism, independent of osteosclerosis.
Insights
Fra1 transgenic mice show reduced thrombospondins, leading to craniofacial dysmorphism independent of osteosclerosis. This suggests thrombospondins play a key role in skull development.
Area of Science:
- Bone biology
- Genetics
- Craniofacial development
Background:
- Fra1 transgenic mice overexpress Fra1, a transcription factor component, leading to osteosclerosis due to osteoblast abnormalities.
- Dysregulated bone matrix protein expression, including matrix Gla protein, is observed in these osteoblasts.
Purpose of the Study:
- To investigate the role of Fra1 in osteoblast function and its impact on bone matrix protein expression.
- To determine the relationship between Fra1-induced osteosclerosis and craniofacial dysmorphism.
- To explore the contribution of thrombospondins to craniofacial abnormalities.
Main Methods:
- Osteoblastogenic cultures were used to assess bone matrix production via Alizarin red staining, RT-PCR, and Western blotting.
- Bone histomorphometry evaluated responsiveness to ovariectomy.
- Radiographs and CT scans measured craniofacial parameters.
Main Results:
- Thrombospondin-1 (Thbs1) and thrombospondin-2 (Thbs2) expression was reduced in Fra1 transgenic osteoblasts and bones.
- Fra1 transgenic mice and Thbs1-/-Thbs2-/- mice exhibited an edge-to-edge bite phenotype and craniofacial dysmorphism, despite the absence of osteosclerosis in the latter.
- Ovariectomy induced bone loss in Fra1 transgenic mice similarly to wildtype mice.
Conclusions:
- Reduced thrombospondin expression in Fra1 transgenic mice contributes to craniofacial dysmorphism.
- This craniofacial dysmorphism occurs independently of the osteosclerosis phenotype.
- Thrombospondins are implicated in craniofacial development beyond their role in bone density regulation.
