In a study published in Cell Research, a team led by Prof. ZOU Weiguo from the Center for Excellence in Molecular Cell Science (Shanghai Institute of Biochemistry and Cell Biology) of the Chinese Academy of Sciences, along with Prof. LE Rongrong from Tongji University, identified a fibrous-layer resident subpopulation of P-SSCs labeled by Angptl7. They found that these cells are crucial for bone fracture repair by mediating endochondral ossification, but exhibit minimal osteogenic capacity during postnatal bone development and maintenance.

Bone fracture is a very common injury. Skeletal stem cells (SSCs) contribute to the development, maintenance, and fracture repair of the bone. Studies have shown that SSCs are not a homogeneous population, and different cell types serve as site-specified SSCs, although they express similar surface markers.

Periosteum is enriched with SSCs that labeled by Ctsk. Ctsk-lineage periosteal SSCs (P-SSCs) form bone directly through intramembranous ossification but not through an initial cartilage template as endochondral ossification. P-SSCs can mediate endochondral ossification after fracture, which raises the question whether there are distinct subpopulations of P-SSCs that are separately responsible for fracture repair and steady-state bone formation.

In this study, the researchers generated Angptl7-mScarlet and Angptl7-CreER mouse models. Using these models, they found that Angptl7-lineage cells were specifically present in the periosteum with rare contribution to steady-state bone formation, although they possess the capacity for in vitro colony formation and trilineage differentiation.

Besides, the researchers found that Angptl7-lineage P-SSCs played a clear role in post-injury endochondral ossification. Using lineage depletion and conditional knock-out mouse models, they found that Angptl7-lineage cells were indispensable for mediating bone fracture healing process. Long-term lineage tracing assays showed that Angptl7-lineage cells could regenerate the entire bone architecture including the whole periosteum, cortex, endosteum, and even bone marrow stroma.

Multi-omics approaches revealed an activated inflammatory-responding stage of Angptl7-lineage P-SSCs after fracture. Inflammatory signals, such as TNF-α, could trigger the activation of Angptl7-lineage P-SSCs by up-regulating NF-κB signaling pathway, which subsequently promoted the expression of Cxcl5 in activated Angptl7-lineage cells.

This study demonstrates that Angptl7-related mouse models define absolute specificity of lineage tracing experiments for fracture-repairing P-SSCs, which advances the understanding of the cell basis for bone maintenance and fracture repair. It will also benefit future studies for uncovering the mechanisms underlying clinical fracture non-union events.