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Sequential In vivo Imaging of Osteogenic Stem/Progenitor Cells During Fracture Repair
Published on: May 23, 2014
Rescue of the osteopetrotic defect in op/op mice by osteoblast-specific targeting of soluble colony-stimulating
S L Abboud1, K Woodruff, C Liu
1Department of Pathology, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78284, USA. abbouds@uthscsa.edu
Abstract:
Soluble colony-stimulating factor-1 (sCSF-1) and membrane bound CSF-1 are synthesized by osteoblasts and stromal cells. However, the precise role of each form in osteoclastogenesis is unclear. In the op/op mouse, absence of osteoblast-derived CSF-1 leads to decreased osteoclasts and osteopetrosis. To determine whether sCSF-1 gene replacement can cure the osteopetrotic defect, we took advantage of the osteoblast specificity of the osteocalcin promoter to selectively express sCSF-1 in the bone of op/op mice. Transgenic mice harboring the human sCSF-1 cDNA under the control of the osteocalcin promoter were generated and cross-bred with heterozygous op/wt mice to establish op/op mutants expressing the transgene (op/opT). The op/op genotype and transgene expression were confirmed by PCR and Southern blot analysis, respectively. High levels of human sCSF-1 protein were selectively expressed in bone. At 2(1/2) wk, op/opT mice showed normal growth and tooth eruption. Femurs removed at 5 and 14 wk were analyzed by peripheral quantitative computed tomography and histomorphometry. The abnormal bone mineral density, cancellous bone volume, and growth plate width observed in op/op mice was completely reversed in op/opT mice by 5 wk, and this effect persisted at 14 wk, with measurements comparable with wt/wt mice at each time point. Correction of the skeletal abnormalities in the 5-wk-old op/opT mice correlated with a marked increase in the total osteoclast number, and their number per millimeter of bone surface compared with that of op/op mutants. Osteoclast number was maintained at 14 wk in op/opT mice and morphologically resembled wt/wt osteoclasts. These results indicate that sCSF-1 is sufficient to drive normal osteoclast development and that the osteocalcin promoter provides an efficient tool for delivery of exogenous genes to the bone. Moreover, targeting sCSF-1 to osteoblasts in the bone microenvironment may be a potentially useful therapeutic modality for treating bone disorders.
Insights
Soluble colony-stimulating factor-1 (sCSF-1) gene replacement in osteopetrotic mice restored normal bone growth and osteoclast development. This indicates sCSF-1 is sufficient for osteoclastogenesis and offers a potential therapy for bone disorders.
Area of Science:
- Bone Biology
- Genetics
- Cell Biology
Background:
- Osteoclastogenesis, the formation of bone-resorbing cells, is crucial for bone remodeling.
- Soluble colony-stimulating factor-1 (sCSF-1) and membrane-bound CSF-1 are produced by osteoblasts and stromal cells, but their distinct roles in osteoclastogenesis are not fully understood.
- The op/op mouse model exhibits osteopetrosis due to a deficiency in osteoblast-derived CSF-1, resulting in reduced osteoclast numbers.
Purpose of the Study:
- To investigate whether gene replacement therapy using soluble CSF-1 (sCSF-1) can correct the osteopetrotic phenotype in op/op mice.
- To evaluate the efficacy of the osteocalcin promoter for targeted sCSF-1 expression in osteoblasts within the bone microenvironment.
Main Methods:
- Generation of transgenic op/op mice (op/opT) expressing human sCSF-1 cDNA under the control of the osteocalcin promoter.
- Confirmation of genotype and transgene expression using PCR and Southern blot analysis.
- Assessment of skeletal phenotypes using peripheral quantitative computed tomography (pQCT) and histomorphometry at 5 and 14 weeks of age.
Main Results:
- Transgenic op/opT mice exhibited normal growth and tooth eruption, with complete reversal of abnormal bone mineral density, cancellous bone volume, and growth plate width by 5 weeks.
- Skeletal improvements in op/opT mice were sustained at 14 weeks and comparable to wild-type littermates.
- Correction of skeletal defects correlated with a significant increase in osteoclast number and normalized osteoclast morphology in op/opT mice.
Conclusions:
- Soluble CSF-1 (sCSF-1) is sufficient to drive normal osteoclast development and bone remodeling.
- The osteocalcin promoter is an effective tool for targeted gene delivery to osteoblasts in bone.
- Targeting sCSF-1 expression to osteoblasts presents a promising therapeutic strategy for treating bone disorders characterized by impaired osteoclastogenesis.
