Wolff's law and the Utah Paradigm of skeletal physiology state that bone architecture adapts to mechanical loads. These models predict the existence of a mechanostat that links strain induced by ...mechanical forces to skeletal remodeling. However, how the mechanostat influences bone remodeling remains elusive. Here, we find that Piezo1 deficiency in osteoblastic cells leads to loss of bone mass and spontaneous fractures with increased bone resorption. Furthermore, Piezo1-deficient mice are resistant to further bone loss and bone resorption induced by hind limb unloading, demonstrating that PIEZO1 can affect osteoblast-osteoclast crosstalk in response to mechanical forces. At the mechanistic level, in response to mechanical loads, PIEZO1 in osteoblastic cells controls the YAP-dependent expression of type II and IX collagens. In turn, these collagen isoforms regulate osteoclast differentiation. Taken together, our data identify PIEZO1 as the major skeletal mechanosensor that tunes bone homeostasis.
Bone remodeling is a lifelong process that gives rise to a mature, dynamic bone structure via a balance between bone formation by osteoblasts and resorption by osteoclasts. These opposite processes ...allow the accommodation of bones to dynamic mechanical forces, altering bone mass in response to changing conditions. Mechanical forces are indispensable for bone homeostasis; skeletal formation, resorption, and adaptation are dependent on mechanical signals, and loss of mechanical stimulation can therefore significantly weaken the bone structure, causing disuse osteoporosis and increasing the risk of fracture. The exact mechanisms by which the body senses and transduces mechanical forces to regulate bone remodeling have long been an active area of study among researchers and clinicians. Such research will lead to a deeper understanding of bone disorders and identify new strategies for skeletal rejuvenation. Here, we will discuss the mechanical properties, mechanosensitive cell populations, and mechanotransducive signaling pathways of the skeletal system.
The skeleton is a dynamic organ that is constantly remodeled. Proteins secreted from bone cells, namely osteoblasts, osteocytes, and osteoclasts exert regulation on osteoblastogenesis, ...osteclastogenesis, and angiogenesis in a paracrine manner. Osteoblasts secrete a range of different molecules including RANKL/OPG, M-CSF, SEMA3A, WNT5A, and WNT16 that regulate osteoclastogenesis. Osteoblasts also produce VEGFA that stimulates osteoblastogenesis and angiogenesis. Osteocytes produce sclerostin (SOST) that inhibits osteoblast differentiation and promotes osteoclast differentiation. Osteoclasts secrete factors including BMP6, CTHRC1, EFNB2, S1P, WNT10B, SEMA4D, and CT-1 that act on osteoblasts and osteocytes, and thereby influenceaA osteogenesis. Osteoclast precursors produce the angiogenic factor PDGF-BB to promote the formation of Type H vessels, which then stimulate osteoblastogenesis. Besides, the evidences over the past decades show that at least three hormones or "osteokines" from bone cells have endocrine functions. FGF23 is produced by osteoblasts and osteocytes and can regulate phosphate metabolism. Osteocalcin (OCN) secreted by osteoblasts regulates systemic glucose and energy metabolism, reproduction, and cognition. Lipocalin-2 (LCN2) is secreted by osteoblasts and can influence energy metabolism by suppressing appetite in the brain. We review the recent progresses in the paracrine and endocrine functions of the secretory proteins of osteoblasts, osteocytes, and osteoclasts, revealing connections of the skeleton with other tissues and providing added insights into the pathogenesis of degenerative diseases affecting multiple organs and the drug discovery process.
Multiple distinct types of skeletal progenitors have been shown to contribute to endochondral bone development and maintenance. However, the division of labor and hierarchical relationship between ...different progenitor populations remain undetermined. Here we developed dual-recombinase fate-mapping systems to capture the skeletal progenitor transition during postnatal bone formation. We showed that postnatal osteoblasts arose primarily from chondrocytes before adolescence and from Lepr+ bone marrow stromal cells (BMSCs) after adolescence. This transition occurred in the diaphysis during adolescence and progressively spread to the metaphysis. The osteoblast-forming Lepr+ BMSCs derived primarily from fetal Col2+ cells. Conditional deletion of Runx2 from perinatal chondrocytes and adult Lepr+ BMSCs impaired bone lengthening and thickening, respectively. Forced running increased osteoblast formation by perinatal chondrocytes but not by adult Lepr+ BMSCs. Thus, the short-term developmental skeletal progenitors generated the long-term adult skeletal progenitors. They sequentially control the growth and maintenance of endochondral bones.
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•Bone formation before and after adolescence is controlled by distinct progenitors•Chondrocytes and Lepr+ BMSCs mediate bone lengthening and thickening, respectively•The adult skeletal progenitors derive mainly from developmental skeletal progenitors•Running enhances osteogenesis by developmental but not adult skeletal progenitors
Shu et al. discovered that bone formation before and after adolescence is dominated sequentially by chondrocytes and Lepr+ BMSCs. They regulate bone lengthening and thickening, respectively. This transition explains, from a stem cell perspective, how mammalian limb bones transition from rapid longitudinal growth to slower appositional remodeling after adolescence.
During the aging process, bone marrow mesenchymal stem cells (BMSCs) exhibit declined osteogenesis accompanied by excess adipogenesis, which will lead to osteoporosis. Here, we report that the H3 ...lysine 36 trimethylation (H3K36me3), catalyzed by histone methyltransferase SET-domain-containing 2 (SETD2), regulates lineage commitment of BMSCs. Deletion of Setd2 in mouse bone marrow mesenchymal stem cells (mBMSCs), through conditional Cre expression driven by Prx1 promoter, resulted in bone loss and marrow adiposity. Loss of Setd2 in BMSCs in vitro facilitated differentiation propensity to adipocytes rather than to osteoblasts. Through conjoint analysis of RNA sequencing (RNA-seq) and chromatin immunoprecipitation sequencing (ChIP-seq) data, we identified a SETD2 functional target gene, Lbp, on which H3K36me3 was enriched, and its expression was affected by Setd2 deficiency. Furthermore, overexpression of lipopolysaccharide-binding protein (LBP) could partially rescue the lack of osteogenesis and enhanced adipogenesis resulting from the absence of Setd2 in BMSCs. Further mechanistic studies demonstrated that the trimethylation level of H3K36 could regulate Lbp transcriptional initiation and elongation. These findings suggest that H3K36me3 mediated by SETD2 could regulate the cell fate of mesenchymal stem cells (MSCs) in vitro and in vivo, indicating that the regulation of H3K36me3 level by targeting SETD2 and/or the administration of downstream LBP may represent a potential therapeutic way for new treatment in metabolic bone diseases, such as osteoporosis.
Abstract
Tri-methylation on lysine 40 of α-tubulin (α-TubK40me3) is a recently identified post-translational modification involved in mitosis and cytokinesis. However, knowledge about α-TubK40me3 in ...microtubule function and post-mitotic cells remains largely incomplete. Here, we report that α-TubK40me3 is required for neuronal polarization and migration by promoting microtubule formation. α-TubK40me3 is enriched in mouse cerebral cortex during embryonic day (E)14 to E16. Knockdown of α-tubulin methyltransferase SETD2 at E14 leads to the defects in neuronal migration, which could be restored by overexpressing either a cytoplasm-localized SETD2 truncation or α-TubK40me3-mimicking mutant. Furthermore, α-TubK40me3 is preferably distributed on polymerized microtubules and potently promotes tubulin nucleation. Downregulation of α-TubK40me3 results in reduced microtubule abundance in neurites and disrupts neuronal polarization, which could be rescued by Taxol. Additionally, α-TubK40me3 is increased after losing α-tubulin K40 acetylation (α-TubK40ac) and largely rescues α-TubK40ac function. This study reveals a critical role of α-TubK40me3 in microtubule formation and neuronal development.
Heterotopic ossification (HO) is pathological bone formation characterized by ossification within muscle, tendons, or other soft tissues. However, the cells of origin and mechanisms involved in the ...pathogenesis of HO remain elusive. Here we show that deletion of suppressor of fused (Sufu) in cathepsin K-Cre-expressing (Ctsk-Cre-expressing) cells resulted in spontaneous and progressive ligament, tendon, and periarticular ossification. Lineage tracing studies and cell functional analysis demonstrated that Ctsk-Cre could label a subpopulation of tendon-derived progenitor cells (TDPCs) marked by the tendon marker Scleraxis (Scx). Ctsk+Scx+ TDPCs are enriched for tendon stem cell markers and show the highest self-renewal capacity and differentiation potential. Sufu deficiency caused enhanced chondrogenic and osteogenic differentiation of Ctsk-Cre-expressing tendon-derived cells via upregulation of Hedgehog (Hh) signaling. Furthermore, pharmacological intervention in Hh signaling using JQ1 suppressed the development of HO. Thus, our results show that Ctsk-Cre labels a subpopulation of TDPCs contributing to HO and that their cell-fate changes are driven by activation of Hh signaling.
How lineage specifiers are regulated during development is an outstanding question, and the molecular regulation of osteogenic factor RUNX2 remains to be fully understood. Here we report that the ...Mediator subunit MED23 cooperates with RUNX2 to regulate osteoblast differentiation and bone development. Med23 deletion in mesenchymal stem cells or osteoblast precursors results in multiple bone defects similar to those observed in Runx2(+/-) mice. In vitro, Med23-deficient progenitor cells are refractory to osteoblast differentiation, and Med23 deficiency reduces Runx2-target gene activity without changing Runx2 expression. Mechanistically, MED23 binds to RUNX2 and modulates its transcriptional activity. Moreover, Med23 deficiency in osteoprogenitor cells exacerbates the skeletal abnormalities observed in Runx2(+/-) mice. Collectively, our results establish a genetic and physical interaction between RUNX2 and MED23, suggesting that MED23 constitutes a molecular node in the regulatory network of anabolic bone formation and related diseases.
Abstract
Critical-sized bone defects often lead to non-union and full-thickness defects of the calvarium specifically still present reconstructive challenges. In this study, we show that neurotrophic ...supplements induce robust in vitro expansion of mesenchymal stromal cells, and in situ transplantation of neurotrophic supplements-incorporated 3D-printed hydrogel grafts promote full-thickness regeneration of critical-sized bone defects. Single-cell RNA sequencing analysis reveals that a unique atlas of in situ stem/progenitor cells is generated during the calvarial bone healing in vivo. Notably, we find a local expansion of resident Msx1+ skeletal stem cells after transplantation of the in situ cell culture system. Moreover, the enhanced calvarial bone regeneration is accompanied by an increased endochondral ossification that closely correlates to the Msx1+ skeletal stem cells. Our findings illustrate the time-saving and regenerative efficacy of in situ cell culture systems targeting major cell subpopulations in vivo for rapid bone tissue regeneration.
Mammalian target of rapamycin complex 1 (mTORC1) is involved in anabolic metabolism in both osteoblasts and chondrocytes, but the role of mTORC1 in osteoclast biology in vivo remains to be ...elucidated. In this study, we showed that deletion of regulatory-associated protein of mTOR (Raptor) in osteoclasts led to an increase in bone mass with decreased bone resorption. Raptor-deficient bone marrow-derived macrophages exhibited lower mTORC1-S6K1 signaling and retarded osteoclast differentiation, as determined by the number of osteoclasts, tartrate-resistant acid phosphatase activity, and expression of osteoclast-specific genes. Enforced expression of constitutively active S6K1 rescued the impaired osteoclast differentiation in Raptor-deficient bone marrow-derived macrophages. Furthermore, pharmacological inhibition of mTORC1 signaling by rapamycin could also inhibit osteoclast differentiation and osteoclast-specific gene expression. Taken together, our findings demonstrate that mTORC1 plays a key role in the network of catabolic bone resorption in osteoclasts and may serve as a potential pharmacological target for the regulation of osteoclast activity in bone metabolic disorders.
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