PTPRU, a quiescence-induced receptor tyrosine phosphatase negatively regulates osteogenic differentiation of human mesenchymal stem cells

Bone marrow mesenchymal stem cells (MSCs) are heterogeneous osteo-progenitors that are mainly responsible for bone regeneration and homeostasis. In vivo, a subpopulation of bone marrow MSCs persists in a quiescent state, providing a source of new cells for repair. Previously, we reported that induction of quiescence in hMSCs in vitro skews their differentiation potential in favour of osteogenesis while suppressing adipogenesis. Herein, we uncover a new role for a protein tyrosine phosphatase, receptor type U (PTPRU) in repressing osteogenesis during quiescence. A 75 kD PTPRU protein isoform was found to be specifically induced during quiescence and down-regulated during cell cycle reactivation. Using siRNA-mediated knockdown, we report that in proliferating hMSC, PTPRU preserves self-renewal, while in quiescent hMSC, PTPRU not only maintains reversibility of cell cycle arrest but also suppresses expression of osteogenic lineage genes. Knockdown of PTPRU in proliferating or quiescent hMSC de-represses osteogenic markers, and enhances induced osteogenic differentiation. We also show that PTPRU positively regulates a β-catenin-TCF transcriptional reporter. Taken together, our study suggests a role for a quiescence-induced 75kD PTPRU isoform in modulating bone differentiation in hMSC, potentially involving the Wnt pathway. In proliferating hMSC, moderate levels of PTPRU maintain proliferation and clonogenicity, potentially via β-catenin. In quiescent hMCS, strong induction of PTPRU suppresses terminal differentiation (particularly osteogenic), thus maintaining the reversibility of the cell cycle arrest. Knockdown of PTPRU leads to mild suppression of β-catenin signaling, correlating with reduced induction of pro-proliferative genes, increased expression of anti-proliferative genes, compromised clonogenic self-renewal, and induction of the osteogenic program. [Display omitted] •PTPRU is strongly and specifically induced in reversibly arrested G0 hMSCs.•PTPRU knockdown cells can exit the cell cycle but fail to reactivate.•Compromising PTPRU expression leads to induction of osteogenic but not adipogenic differentiation.•PTPRU plays a role in mitigating osteogenic differentiation in G0, potentially via effects on Wnt/β-catenin signaling.