Periosteal Skeletal Stem Cells Can Migrate into the Bone Marrow and Support Hematopoietic Regeneration

Hematopoietic stem cells (HSC) reside within the bone marrow microenvironment, coined the HSC niche, and are supported by various cell types, including bone marrow mesenchymal stem cells (BM-MSCs). BM-MSCs are multipotent stromal cells capable of self-renewal and trilineage differentiation into chondrocytes, osteoblasts, and adipocytes. BM-MSCs secrete factors important for hematopoiesis, such as C-X-C motif ligand 12 (CXCL12) and stem cell factor (SCF). Functional stromal cells are important for bone marrow regeneration after chemotherapy or radiation-induced injury to avoid myelosuppression, a common side effect of these treatments. However, it is not known what governs bone marrow regeneration and how the cells within the bone marrow are repopulated post-injury. Here we describe a novel model of whole bone transplantation to study hematopoietic regeneration and identify periosteal skeletal stem cells (P-SSCs) as key contributors to stromal recovery following bone marrow injury. Skeletal stem cells are skeletal lineage cells that can be found in multiple locations within the body, including the periosteum. The periosteum is a thin, two-layered membrane that covers the surface of bones and has been well studied in the context of bone fracture. P-SSCs can be found in the inner layer of the periosteum and contribute to bone development, regeneration and homeostasis. However, despite the close relationship between bone and bone marrow physiology, the potential role of P-SSCs in hematopoiesis and within the bone marrow has yet to be explored.

In this study, we have developed a bone transplantation model that involves isolating a femur from a donor mouse and subcutaneously transplanting it into a recipient mouse (Figure 1A). This model mimics the initial bone marrow cellular necrosis and fatty infiltration seen after bone marrow injury and the subsequent regeneration over time. We discovered that over 99% of the BM-MSCs (CD45^-Ter119^-CD31^-CD51⁺CD200⁺) within the engrafted femur were derived from the graft, while over 98% of the hematopoietic cells (CD45⁺Ter119⁺) within the engrafted femur were derived from the hosts. Notably, however, we observed that P-SSCs were migrating from the periosteum and into the bone marrow, aiding in stromal regeneration. To assess the contribution of P-SSCs to the recovering bone marrow stroma, we crossed Periostin (Postn)-Cre^ER mice with a tdTomato (Tmt) line. After 5 months, at which point there is no significant difference between host femur and graft femur cellularity, we observed that 83.9% (± 7.18%) of the BM-MSCs within the engrafted femur were Tmt⁺, indicating periosteal origin (Figure 1B). Once in the bone marrow, these P-SSCs are reprogrammed into BM-MSC-like cells and facilitate hematopoietic recovery, as shown by expression levels of HSC maintenance factors. Compared to steady state P-SSCs, Tmt⁺ graft BM-MSCs increased CXCL12 and SCF expression by 6.92-fold (p<0.0001) and 5.74-fold (p<0.0001), respectively. Additionally, unlike BM-MSCs, P-SSCs are resistant to transplantation stress, and actively proliferate in the early days following bone transplantation. Using RNA sequencing, qPCR, and flow cytometric analyses, we were able to show that at steady state, P-SSCs appear to be even more quiescent than BM-MSCs, potentially contributing to their stress resistance.

Our bone transplantation model has enabled us to study the bone marrow regeneration process using genetic mouse models and led us to uncover a role for P-SSCs in supporting hematopoiesis. Additionally, we have assessed the differential stress response of P-SSCs and BM-MSCs after transplantation. This study highlights a novel bone transplantation model to investigate bone marrow injury and regeneration. It also reveals a critical role of P-SSCs in stromal regeneration and hematopoietic recovery.