Summary How are skeletal tissues derived from skeletal stem cells? Here, we map bone, cartilage and stromal development from a population of highly pure, post-natal skeletal stem cells (mouse Skeletal Stem Cell, mSSC) to its downstream progenitors of bone, cartilage and stromal tissue. We then investigated the transcriptome of the stem/progenitor cells for unique gene expression patterns that would indicate potential regulators of mSSC lineage commitment. We demonstrate that mSSC niche factors can be potent inducers of osteogenesis, and several specific combinations of recombinant mSSC niche factors can activate mSSC genetic programs in situ, even in non-skeletal tissues, resulting in de novo formation of cartilage or bone and bone marrow stroma. Inducing mSSC formation with soluble factors and subsequently regulating the mSSC niche to specify its differentiation towards bone, cartilage, or stromal cells could represent a paradigm shift in the therapeutic regeneration of skeletal tissues.

Summary Stem cell regulation and hierarchical organization of human skeletal progenitors remain largely unexplored. Here, we report the isolation of a self-renewing and multipotent human skeletal stem cell (hSSC) that generates progenitors of bone, cartilage, and stroma, but not fat. Self-renewing and multipotent hSSCs are present in fetal and adult bones and can also be derived from BMP2-treated human adipose stroma (B-HAS) and induced pluripotent stem cells (iPSCs). Gene expression analysis of individual hSSCs reveals overall similarity between hSSCs obtained from different sources and partially explains skewed differentiation towards cartilage in fetal and iPSC-derived hSSCs. hSSCs undergo local expansion in response to acute skeletal injury. In addition, hSSC-derived stroma can maintain human hematopoietic stem cells (hHSCs) in serum-free culture conditions. Finally, we combine gene expression and epigenetic data of mouse skeletal stem cells (mSSCs) and hSSCs to identify evolutionarily conserved and divergent pathways driving SSC-mediated skeletogenesis.

Wnts modulate cell proliferation, differentiation and stem cell self-renewal, by inducing β-catenin dependent signaling through Frizzled (Fzd) and Lrp5/6 to regulate cell fate decisions, and the growth and repair of a multitude of tissues1. The 19 mammalian Wnts interact promiscuously with the 10 Fzds, which has complicated the attribution of specific Fzd/Wnt subtype interactions to distinct biological functions. Furthermore, Wnts are post-translationally modified by palmitoylation, which is essential for Wnt secretion and functions as a critical site of interaction with Fzd 2–4. As a result of their acylation, Wnts are very hydrophobic proteins requiring detergents for purification, which presents major obstacles for the preparation and application of recombinant Wnts. This has hindered the delineation of the molecular mechanisms of Wnt signaling activation, understanding of the functional significance of Fzd subtypes, and the use of Wnts as therapeutics. Here we developed surrogate Wnt agonists, water-soluble Fzd-Lrp5/6 heterodimerizers, consisting of Fzd5/8-specific and broadly Fzd-reactive binding domains, that elicit a characteristic β-catenin signaling response in a Fzd-selective fashion, enhance osteogenic lineage commitment of primary mesenchymal stem cells (MSCs), and support the growth of a broad range of primary human organoid cultures comparably to Wnt3a. Furthermore, we demonstrate that the surrogates can be systemically expressed and exhibit Wnt activity in vivo, regulating metabolic liver zonation and promoting hepatocyte proliferation, resulting in hepatomegaly. These surrogates demonstrate that canonical Wnt signaling can be activated simply through bi-specific ligands that induce receptor heterodimerization. Furthermore, these easily produced non-lipidated Wnt surrogate agonists offer a new avenue to facilitate functional studies of Wnt signaling and the exploration of Wnt agonists for translational applications in regenerative medicine.

No abstract

No abstract