A Wnt-mediated transformation of the bone marrow stromal cell identity orchestrates skeletal regeneration

Abstract

Bone marrow stromal cells (BMSCs) are versatile mesenchymal cell populations underpinning the major functions of the skeleton, a majority of which adjoin sinusoidal blood vessels and express C-X-C motif chemokine ligand 12 (CXCL12). However, how these cells are activated during regeneration and facilitate osteogenesis remains largely unknown. Celllineage analysis using Cxcl12-creER mice reveals that quiescent Cxcl12-creER + perisinusoidal BMSCs differentiate into cortical bone osteoblasts solely during regeneration. A combined single cell RNA-seq analysis demonstrate that these cells convert their identity into a skeletal stem cell-like state in response to injury, associated with upregulation of osteoblast-signature genes and activation of canonical Wnt signaling components along the single-cell trajectory. β-catenin deficiency in these cells indeed causes insufficiency in cortical bone regeneration.Therefore, quiescent Cxcl12-creER + BMSCs transform into osteoblast precursor cells in a manner mediated by canonical Wnt signaling, highlighting a unique mechanism by which dormant stromal cells are enlisted for skeletal regeneration.

Heterogeneity of murine periosteum progenitors involved in fracture healing

“…Axin2+ and Gli1+ cells contribute to both growth and healing, however Prx1CreER labels a more restricted population only involved in healing (51)(52)(53). Cxcl12+ cells in the central bone marrow do not contribute osteoblast formation under homeostatic conditions, but are activated and recruiting following injury involving the bone marrow (44). αSMA+ cells in the periosteum appear to represent a population involved primarily in response to injury, or increased need for bone formation.…”

“…The contribution of Gli1+ cells in the fibrous layer of the periosteum, or from non-periosteal sources could explain these observations. Cxcl12-CreER also appears to identify cells that form mainly chondrocytes in a periosteal fracture callus, however the presence of labeled cells outside the bone marrow compartment was not evaluated making it difficult to identify the tissue where these cells originated (44).…”

How faithfully does intramembranous bone regeneration recapitulate embryonic skeletal development?

“…182 Cxcl12-abundant reticular cells have also been shown to promote intramembranous bone regeneration following both creation of a cortical bone defect and bone marrow ablation. 183 This intramembranous bone regeneration process was impaired by the Cxcl12-abundant reticular cell specific deletion of β-catenin, suggesting Wnt signaling mediates differentiation of bone marrow skeletal stem cells to osteoblasts. In addition, administration of Wnt proteins accelerated intramembranous bone regeneration following the creation of a cortical bone defect.…”

Skeletal stem cells: insights into maintaining and regenerating the skeleton

“…When Ebf3 is deleted in Lepr + stromal cells (using Lepr-Cre), cells lose their HSC-supportive stromal function and prematurely differentiate into osteoblasts, suggesting that Ebf3 functions to maintain an immature 'stromal' phenotype in a subset of Lepr + BMSCs (Seike et al, 2018). Further, a recent study specifically tracked the perisinusoidal Cxcl12-expressing cells using a Cxcl12-CreER mouse line, and saw that, although normally quiescent, these cells participated in repair after a drill hole injury or fracture (Matsushita et al, 2020). The timing, contributions and requirements of these and other BMSCs marked by various genes are summarized in Tables 1 and 2.…”