Macrophages are requisite for angiogenesis of type H vessels during bone regeneration in mice
Yukihiro Kohara1, Riko Kitazawa2, Ryuma Haraguchi1
1Department of Molecular Pathology, Ehime University Graduate School of Medicine, Shitsukawa, Toon City, Ehime 791-0295, Japan.
Abstract:
Macrophages are progenitors of osteoclasts as well as regulators of bone metabolism. Macrophages mediate not only bone formation by osteoblasts under physiological conditions, but also bone regeneration after fracture. The mechanisms of macrophages regulation of bone formation and regeneration remain unclear, however. Here, we demonstrate that the liposome-encapsulated Clodronate (Clod-lip) injected mouse model with cortical bone defect induced by drill-hole injury and targeted depletion of phagocytic macrophages exhibits impaired angiogenesis of type H vessels that couple angiogenesis and osteogenesis. Moreover, we identify Tgfbi (encoding TGFBI), Plau (encoding uPA) and Tgfb1 (encoding TGF-β1), through RNA-seq analysis, as genes of macrophage-secreted factors mediating angiogenesis and wound healing. The relevant mRNA was highly expressed in bone marrow-derived macrophages among bone cells, as determined through qRT-PCR. Finally, we disclose that treatment with uPA inhibitor or TGF-β receptor I, receptor II inhibitor impairs bone regeneration after injury, confirming the importance of uPA and TGF-β1 during bone regeneration. Our findings reveal a novel mechanism of bone regeneration mediated by macrophages.
Insights
Macrophages regulate bone metabolism and fracture healing. Depleting these cells impairs blood vessel formation crucial for bone regeneration, highlighting macrophage-secreted factors like uPA and TGF-β1 in this process.
Area of Science:
- Cell Biology
- Orthopedics
- Immunology
Background:
- Macrophages are key regulators of bone metabolism and osteoclast progenitors.
- They play a role in bone formation and regeneration after injury.
- The precise mechanisms by which macrophages influence bone regeneration are not fully understood.
Purpose of the Study:
- To investigate the role of phagocytic macrophages in bone regeneration after injury.
- To identify macrophage-secreted factors involved in angiogenesis and osteogenesis coupling.
- To elucidate the mechanisms of macrophage-mediated bone repair.
Main Methods:
- Utilized a mouse model with cortical bone defects induced by drill-hole injury.
- Employed liposome-encapsulated Clodronate (Clod-lip) for targeted depletion of phagocytic macrophages.
- Performed RNA-sequencing (RNA-seq) to identify macrophage-secreted genes.
- Validated gene expression using quantitative reverse transcription PCR (qRT-PCR).
- Assessed the impact of inhibiting specific factors (uPA, TGF-β receptors) on bone regeneration.
Main Results:
- Targeted depletion of phagocytic macrophages impaired angiogenesis of type H vessels, which couple angiogenesis and osteogenesis.
- RNA-seq identified Tgfbi, Plau (uPA), and Tgfb1 (TGF-β1) as key macrophage-secreted factors.
- mRNA for these factors was highly expressed in bone marrow-derived macrophages.
- Inhibition of uPA or TGF-β receptor signaling significantly impaired bone regeneration.
Conclusions:
- Phagocytic macrophages are essential for effective bone regeneration after injury.
- Macrophage-secreted factors, including uPA and TGF-β1, play critical roles in mediating angiogenesis and osteogenesis.
- This study reveals a novel mechanism of macrophage-driven bone regeneration.
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