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Sfrp4 is Required to Maintain Ctsk-Lineage Periosteal Stem Cell Niche Function

ABSTRACT: The periosteum contains cells which function as a reservoir of stem cells and progenitors and contribute to cortical expansion during growth, cortical bone homeostasis and repair. However, the local or paracrine factors that govern stem cell renewal and differentiation within the periosteal niche remains elusive. Cathepsin K (Ctsk) together with additional cell surface markers marks a subset of stem cells in the periosteum (PSC) which possess self-renewal ability and inducible multipotency. These PSCs produce osteoblasts mediating periosteal bone formation and fracture repair. Sfrp4 is expressed in periosteal Ctsk-lineage cells and using CtskCre mice that are either wild type or Sfrp4-/-, we report here that Sfrp4 deletion decreases the pool of PSCs, impairs their self-renewal, their ability to give rise to their derivatives and their clonal multipotency for differentiation into osteoblasts and chondrocytes in vitro and formation of bone organoids in vivo. Bulk RNA sequencing analysis in Ctsk-lineage PSCs demonstrated that Sfrp4 deletion leads to downregulation of signaling pathways associated with skeletal development, positive regulation of bone mineralization and wound healing. Sfrp4 deletion hampers the Ctsk-lineage PSC response and recruitment after bone injury and leads to an impaired periosteal response. Periosteal Ctsk-lineage cells respond to PTH(1-34) treatment with an increase in the % of PSCs, a response not seen in the absence of Sfrp4. Importantly, bone histomorphometry analysis showed that in the absence of Sfrp4, PTH(1-34)-dependent increase in cortical thickness, periosteal bone formation is markedly impaired.

ORGANISM(S): Mus musculus

PROVIDER: GSE236686 | GEO | 2024/08/13

REPOSITORIES: GEO

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Project description:The expression of Prx1 has been used as a marker to define the skeletal stem cells (SSC) populations found within the bone marrow and periosteum that contribute to bone regeneration. However, Prx1 expressing SSCs (Prx1-SSCs) are not restricted to the bone compartments, but are also located within the muscle and able to contribute to ectopic bone formation. Little is known however, about the mechanism(s) regulating Prx1-SSCs that reside in muscle and how they participate in bone regeneration. This study compared both the intrinsic and extrinsic factors of the periosteum and muscle derived Prx1-SSCs and analyzed their regulatory mechanisms of activation, proliferation, and skeletal differentiation. There was considerable transcriptomic heterogeneity in the Prx1-SSCs found in muscle or the periosteum however in vitro cells from both tissues show tri-lineage (adipose, cartilage and bone) differentiation. At homeostasis, periosteal derived Prx1 cells were proliferative and low levels of BMP2 were able to promote their differentiation, while the muscle derived Prx1 cells were quiescent and refractory to comparable levels of BMP2 that promoted periosteal cell differentiation. The transplantation of Prx1-SCC from muscle and periosteum into either the same site from which they were isolated, or their reciprocal sites showed that periosteal cell transplanted onto the surface of bone tissues differentiated into bone and cartilage cells but was incapable of similar differentiation when transplanted into muscle. Prx1-SSCs from the muscle showed no ability to differentiate at either site of transplantation. Both fracture and ten times the BMP2 dose was needed to promote muscle-derived cells to rapidly enter the cell cycle as well as undergo skeletal cell differentiation. This study elucidates the diversity of the Prx1-SSC population showing that cells within different tissue sites are intrinsically different. While muscle tissue must have factors that promote Prx1-SSC to remain quiescent, either bone injury or high levels of BMP2 can activate these cells to both proliferate and undergo skeletal cell differentiation. Finally, these studies raise the possibility that muscle SSCs are potential target for skeletal repair and bone diseases.

Project description:The phases of fracture healing have been well characterized. However, the exact source and genetic profile of the skeletal progenitors that participate in bone repair is somewhat unclear. Sox9 expression in skeletal elements precedes bone and cartilage formation and a Sox9+ cell type is retained in the adult periosteum. We hypothesized that Sox9+ periosteal cells are multipotent skeletal progenitors normally participating in fracture repair. To test this hypothesis we used tamoxifen (TM)-mediated lineage tracing of Sox9+ cells in Sox9CreErt2:Td-Tomato mice. TM injection indelibly labels Sox9+ cells and all their descendants with the fluorescent reporter protein Td-Tomato. Intact mouse femora were harvested 2 weeks after TM injection and analyzed by histology and immunostaining and RNA sequencing, to evaluate the skeletal distribution and gene expression profile of Td-Tomato positive cells in the adult femur. To assess the role of these cells in fracture repair, mice underwent a closed mid-diaphyseal femoral fracture and their hind limbs were harvested at 3, 9 or 56 days post-fracture to assess the contribution of Sox9+ and Td-tomato+ cells in the fracture healing process. In the intact adult mouse femur, Td-Tomato-labelled cells were observed in articular and growth plate cartilage where Sox9 is known to be expressed, but also in the primary spongiosa, periosteum, and endosteum. RNA sequencing and subsequent analysis showed that Td-Tomato positive periosteal cells were enriched in Sox9 transcripts, and transcripts for various osteogenic and chondrocyte specific genes. In a femoral fracture model, we showed that pre-labeled Td-Tomato positive descendent cells were mobilized during the fracture repair process, expanding and migrating towards the fracture site 3 days post-fracture. Here, Td-Tomato positive cells differentiate into chondrocytes and osteoblasts in the soft and hard callus, respectively, 9 days post-fracture. By 2 months post-fracture, descendants of the original Sox9 labelled cell population were prevalent in the cortex and periosteum, and amongst the differentiated osteocyte population embedded within the cortical bone. Thus, a Sox9+ progenitor population resides in the adult periosteum. Fate tracing shows that these cells likely play a substantial role in repair of the fractured femur giving rise to chondrocytes, osteoblasts and mature cortical osteocytes. To our knowledge this is the first report of this Sox9+ cell population in the periosteum of the adult long bone. Taken together with developmental studies on skeletal formation, our data suggest that Sox9+ osteochondroprogenitors play a continuous role in skeletal formation throughout life.

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Sfrp4 is Required to Maintain Ctsk-Lineage Periosteal Stem Cell Niche Function

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