TM9SF4 is a novel regulator in lineage commitment of bone marrow mesenchymal stem cells to either osteoblasts or...

Osteoporosis is a common bone disease in elderly population caused by imbalanced bone formation and bone resorption. Mesenchymal stem cells (MSCs) are responsible for maintaining this bone homeostasis. The phenotype of transmembrane 9 superfamily 4 (TM9SF4) knockout mice suggests a relationship between TM9SF4 proteins and bone homeostasis. But the effect of TM9SF4 in osteology has never been reported. In the present study, we investigated the function of TM9SF4 in MSC differentiation commitment, as well as its role in osteoporosis.

Primary bone marrow MSCs, isolated from TM9SF4 wildtype (TM9SF4^+/+) and knockout (TM9SF4^-/-) mice, were induced to differentiate into osteoblasts or adipocytes, respectively. The osteogenesis was examined by qRT-PCR detection of osteogenic markers, ALP staining and Alizarin Red S staining. The adipogenesis was tested by qRT-PCR quantification of adipogenic markers and Oil Red O staining. The cytoskeletal organization of MSCs was observed under confocal microscope. The osteoporotic model was induced by ovariectomy in TM9SF4^+/+ and TM9SF4^-/- mice, followed by Toluidine blue and H&E staining to assess lipid accumulation in trabecular bones, as well as micro-computed tomography scanning and immunohistochemistry staining for bone mass density assessment. The experiments on signaling pathways were conducted using qRT-PCR, Western blot and Alizarin Red S staining.

We determined the role of TM9SF4 in MSC differentiation and found that TM9SF4^-/- MSCs had higher potential to differentiate into osteoblasts and lower capability into adipocytes, without affecting osteoclastogenesis in vitro. In ovariectomy-induced osteoporotic model, TM9SF4^-/- mice retained higher bone mass and less lipid accumulation in trabecular bones, indicating an important role of TM9SF4 in the regulation of osteoporosis. Mechanistically, TM9SF4-depleted cells showed elongated actin fibers, which may act through mTORC2/Akt/β-catenin pathway to promote their commitment into osteoblasts. Furthermore, TM9SF4-depleted cells showed higher activity of canonical Wnt pathway, suggesting the participation of Wnt/β-catenin during TM9SF4-regulated osteogenesis.

Our study demonstrates TM9SF4 as a novel regulator for MSC lineage commitment. Depletion of TM9SF4 preferentially drives MSCs into osteoblasts instead of adipocytes. Furthermore, TM9SF4^-/- mice show delayed bone loss and reduced lipid accumulation during ovariectomy-induced osteoporosis. Our results indicate TM9SF4 as a promising target for the future clinical osteoporotic treatment.