Schnurri-3 controls osteogenic fate of Adipoq-lineage progenitors in bone marrow

Recently, the osteogenic potential of Adiponectin-labeled adipogenic lineage progenitors (Adipoq-lineage progenitors) in bone marrow has been observed to support bone maintenance and repair. However, little is known about the function of Schnurri-3 (SHN3, also known as HIVEP3) in other mesenchymal lineage cells, apart from its negative regulation of bone formation on osteoblasts. In this study, we used single-cell RNA sequencing (scRNA-seq) profiling to demonstrate that Adipoq-lineage progenitors express higher levels of Shn3 compared to other mesenchymal cell populations in mice and humans. To investigate the role of SHN3 in Adipoq-lineage progenitors, we generated a murine model specifically harboring a Shn3-deficient allele in Adipoq-expressing cells. Information of mice body weight was collected weekly to generate body weight curve. Bone phenotype was analyzed using micro-CT and histomorphometric studies. To eliminate the role of peripheral adipose tissue on bone, we collected adipose wet weight, performed intraperitoneal glucose tolerance tests and intraperitoneal insulin tolerance tests, and conducted a fat-transplantation study. Osteoblast and osteoclast functions were assessed through toluidine blue staining and TRAP staining, respectively. We further investigated the effect of Shn3 depletion on the differentiation of Adipoq-lineage progenitors through immunostaining and in vitro differentiation assays. Finally, we evaluated whether Shn3 deficiency in Adipoq-lineage progenitors affects the fracture healing process by generating bi-cortical femoral fracture models. Depletion of Shn3 in Adipoq-lineage progenitors resulted in a significant increase in trabecular bone mass and bone formation in vivo, without disrupting whole-body energy metabolism and skeletal development. Consistent with these findings, both cell-lineage tracing and functional assays revealed that Shn3 ablation effectively shifted the cell fate of Adipoq-lineage progenitors towards an osteogenic phenotype in the bone marrow. Furthermore, in vivo studies demonstrated that the lack of Shn3 in Adipoq-lineage progenitors also enhanced bone fracture healing under pathological conditions. Overall, our findings provide a novel strategy for targeting the osteoanabolic potential of bone marrow Adipoq-lineage progenitors as a potential treatment for bone loss-related disorders. We have identified a novel gene target that directs the cell fate of a previously identified non-osteogenic cell population under physiological conditions. This study not only expands the therapeutic value of Shn3 ablation in treating osteoporotic or traumatic bone diseases but also provides new insights into the contribution of bone marrow Adipoq-lineage progenitors to osteogenesis. Thus, this article further supports Shn3 silencing as a valuable approach to treat osteopenia and accelerate fracture healing (see graphical abstract)