Distinct roles for Hedgehog and canonical Wnt signaling in specification, differentiation and maintenance of osteoblast progenitors

Effects of alpelisib treatment on murine Pten-deficient lipomas

 Phosphatase and tensin homolog (PTEN) hamartoma tumour syndrome (PHTS) is a rare disorder caused by germline mutations in the tumour suppressor gene PTEN, a key negative regulator of phosphatidylinositol 3-kinase (PI3K)/AKT signalling. Children with PHTS often develop lipomas, for which only surgical resection is available as treatment. We investigated the effects of the selective PI3K-inhibitor alpelisib on Pten-deficient lipomas. After incubation with alpelisib or the non-selective PI3K inhibitor wortmannin, we analysed histology, gene expression, and Pi3k pathway in lipoma and control epididymal adipose tissue (epiWAT). Alpelisib increased adipocyte area in lipomas compared to epiWAT. Baseline gene expression showed higher levels of markers for proliferation (Pcna), fibrosis (Tgfb1), and adipogenesis (Pparg) in lipomas, while hormone-sensitive lipase expression was lower than in epiWAT. Following alpelisib incubation, target genes of Pi3k signalling and extracellular matrix factors were reduced. We confirmed Pi3k inhibition through detecting decreased Akt levels compared to control treatment. Human lipoma samples treated with alpelisib showed variable lipolysis responses, suggesting variability in therapeutic outcomes. We established an ex vivo model to study alpelisib effects on Pten-deficient lipomas. These results underscore the therapeutic potential of targeted PI3K inhibition in the treatment of PHTS-associated lipomas, particularly in cases that are inoperable.

Novel LBR pathogenic variants with loss of sterol reductase activity participate in the pathogenesis of skeletal dysplasia via dysregulating canonical Wnt pathway

Biallelic pathogenic variants in the lamin B receptor (LBR) with impaired sterol reductase function are associated with the development of perinatal lethal Greenberg dysplasia (GRBGD) and mild nonfatal skeletal dysplasia with or without Pelger-Huet anomaly (PHASK), as well as other related hereditary skeletal dysplasia. However, the underlying molecular mechanism remains unclear. In this study, we found two novel pathogenic variants of LBR, namely missense mutation (c.1011 T > G, NM_002296.4; p.Cys337Trp, NP_002287.2) and LBR gene deletion (Chr1q42.12 (225,515,082-225,633,464), NC_000001.10). LBR is a novel substrate of FBW7, which is degraded by GSK3β/FBW7-mediated proteasome pathway and whose C337W mutation promotes its degradation through enhanced interaction with FBW7. Wild-type but not C337W mutant LBR is upregulated by WNT3A-mediated inactivation of GSK3β/FBW7 axis and then participated in WNT3A-activated Wnt pathway through its mediated cholesterol synthesis. MC3T3-E1 cells with Lbr knockdown or cholesterol removal exhibited reduced mineralized nodules in the presence of WNT3A, but addition of cholesterol in the culture medium reversed this phenotype. Collectively, we detected two novel variants in LBR and our study revealed for the first time that disruption of cholesterol synthesis by LBR impairs Wnt pathway and thus disrupts the cell osteogenic differentiation, providing new insights into the pathogenesis of skeletal dysplasia caused by LBR variation.

Self-propagating wave drives morphogenesis of skull bones in vivo

Cellular motion is a key feature of tissue morphogenesis and is often driven by migration. However, migration need not explain cell motion in contexts where there is little free space or no obvious substrate, such as those found during organogenesis of mesenchymal organs including the embryonic skull. Through ex vivo imaging, biophysical modeling, and perturbation experiments, we find that mechanical feedback between cell fate and stiffness drives bone expansion and controls bone size in vivo. This mechanical feedback system is sufficient to propagate a wave of differentiation that establishes a collagen gradient which we find sufficient to describe patterns of osteoblast motion. Our work provides a mechanism for coordinated motion that may not rely upon cell migration but on emergent properties of the mesenchymal collective. Identification of such alternative mechanisms of mechanochemical coupling between differentiation and morphogenesis will help in understanding how directed cellular motility arises in complex environments with inhomogeneous material properties.

Vitamin D receptor in osteoblast lineage cells mediates increased sclerostin circulation and decreased bone formation in hypervitaminosis D

Hypervitaminosis D is induced iatrogenically or endogenously. We previously reported that the vitamin D receptor (VDR) in osteoblast lineage cells mediates bone resorption and soft-tissue calcification in hypervitaminosis D. However, bone formation in hypervitaminosis D remains understudied. Here, we show that abundant 1α,25-dihydroxyvitamin D3 [1,25(OH)2D3] suppresses bone formation through VDR in osteoblast lineage cells. High-dose 1,25(OH)2D3 suppressed bone formation and increased serum sclerostin, a bone formation inhibitor, in Control but not osteoblast lineage-specific VDR-cKO [Osterix (Osx)-VDR-cKO] mice. However, Sost mRNA expression in bone was downregulated by 1,25(OH)2D3 in Control but not Osx-VDR-cKO mice. Meanwhile, mRNA expression of β-1,4-N-acetyl-galactosaminyltransferase 3 (B4GALNT3), whose function is reported to decrease circulating sclerostin, was suppressed by 1,25(OH)2D3 in bone in Control but not Osx-VDR-cKO mice. Overexpressed B4galnt3 in rodent osteoblast-lineage cell lines increased GalNAcβ1→4GlcNAc- (LDN-) glycosylated sclerostin, suggesting that this modification can explain the discordance between serum sclerostin levels and mRNA in bone. Although excessive 1,25(OH)2D3 increased mRNA levels of Fibroblast growth factor 23 (Fgf23), another osteotropic factor, by 10-fold through VDR in osteoblast lineage cells, it was previously shown to increase serum FGF23 levels by several hundred-fold. 1,25(OH)2D3-induced changes of FGF23-degradation regulators, such as furin, polypeptide N-acetylgalactosaminyltransferase 3 (GALNT3), and family with sequence similarity member 20 C (FAM20C), did not match the markedly high FGF23 levels, suggesting the existence of other regulators of FGF23. These findings suggest that VDR plays pivotal roles in the suppression of bone formation in hypervitaminosis D, possibly by increasing circulations of sclerostin and FGF23 through post-translational or post-transcriptional mechanisms.

Universal fibroblasts across tissues can differentiate into niche cells for hematopoietic stem cells

Hematopoietic stem cells (HSCs) generating all blood cells are maintained by their niche cells, termed CXCL12-abundant reticular (CAR) cells, which strongly overlap with leptin-receptor-expressing (LepR+) cells in the bone marrow. A meta-analysis of single-cell RNA sequencing datasets across tissues hypothesized that universal fibroblasts present in all organs give rise to distinct tissue-specific fibroblast subsets designated as specialized fibroblasts, including CAR/LepR+ cells. However, there is no direct evidence that universal fibroblasts can differentiate into specialized fibroblasts at a distant location. Here, we demonstrated that CD248+ universal fibroblasts from the lung and colon outside the skeletal system, as well as from muscle, generated CAR/LepR+ cells characterized by HSC niche functions and expression of cytokines and transcription factors essential for HSC maintenance during ectopic bone formation or after intra-bone marrow transplantation. These results demonstrate that universal fibroblasts with the potential to differentiate into bone marrow-specific HSC niche cells are scattered throughout the entire body.

Crosstalk Between Wnt/β-Catenin and Hedgehog Supports Gli1+ Lineage Osteogenesis in Cranial Sutures

Sutures such as fibrous joints in craniofacial bones provide a niche for Gli1+ mesenchymal stem cells (MSCs) in promoting calvarial bone development and growth. However, the underlying molecular mechanism behind the fate of the Wnt/β-catenin regulation of Gli1+ MSCs during calvarial bone formation remains unclear. Here, we showed that β-catenin was colocalized with Gli1+ lineage cells near the osteogenic front within a suture, and postnatal skull development was delayed via a conditional knockout of Ctnnb1 in Gli1+ MSCs. Calcein-Alizarin Red dual staining revealed that Wnt/β-catenin signal inhibition impaired the rate of bone formation. Furthermore, immunofluorescent staining indicated that Wnt/β-catenin signaling was crucial in facilitating the proliferative capacity of Gli1+ MSCs and their commitment to the osteogenic lineage. Notably, activating hedgehog (Hh) signaling partially restored the suture morphology in Ctnnb1 knockout mice. Collectively, our findings revealed the crosstalk between Wnt and Hh signaling modulates the fate of Gli1+ MSCs during calvarial bone formation.

Mechanisms of mitochondrial reactive oxygen species action in bone mesenchymal cells

Mitochondrial (mt)ROS, insufficient NAD+, and cellular senescence all contribute to the decrease in bone formation with aging. ROS can cause senescence and decrease NAD+, but it remains unknown whether these mechanisms mediate the effects of ROS in vivo. Here, we generated mice lacking the mitochondrial antioxidant enzyme Sod2 in osteoblast lineage cells targeted by Osx1-Cre and showed that Sod2ΔOsx1 mice had low bone mass. Osteoblastic cells from these mice had impaired mitochondrial respiration and attenuated NAD+ levels. Administration of an NAD+ precursor improved mitochondrial function in vitro but failed to rescue the low bone mass of Sod2ΔOsx1 mice. Single-cell RNA-sequencing of bone mesenchymal cells indicated that ROS had no significant effects on markers of senescence but disrupted parathyroid hormone signaling, iron metabolism, and proteostasis. Our data supports the rationale that treatment combinations aimed at decreasing mtROS and senescent cells and increasing NAD+ should confer additive effects in delaying age-associated osteoporosis.

Mitochondrial oxidative stress or decreased autophagy in osteoblast lineage cells is not sufficient to mimic the deleterious effects of aging on bone mechanoresponsiveness

Exercise-induced mechanical load stimulates bone cells, including osteocytes, to promote bone formation. The bone response to loading is less effective with aging, but the cellular and molecular mechanisms responsible for the impaired mechanoresponsiveness remain unclear. Excessive mitochondrial reactive oxygen species (mtROS) and deficient autophagy are common aging mechanisms implicated in decreased bone formation in old mice. Here, we confirmed that the osteogenic effects of tibia compressive loading are lower in old versus young female mice. We also examined whether an increase in mtROS or decreased autophagy in osteoblast-lineage cells of adult female mice could mimic the deleterious effects of aging. To this end, we loaded mice lacking the antioxidant enzyme superoxide dismutase 2 (Sod2) or autophagy-related 7 (Atg7) in cells targeted by Osterix1 (Osx1)-Cre. Osteocytes in Atg7ΔOsx1 exhibited altered morphology and decreased osteocyte dendrite projections. Two weeks of loading increased cortical bone mass and bone formation rate at both periosteal and endosteal surfaces of Osx1-Cre control mice. Nonetheless, in both Atg7ΔOsx1 and Sod2ΔOsx1 mice the response to loading was identical to that observed in control mice, indicating that compromised Atg7-dependent autophagy or excessive mtROS are not sufficient to impair the bone response to tibial compressive loading. Thus, alternative mechanisms of aging might be responsible for the decreased response of the aged skeleton to mechanical stimuli. These findings also suggest that an intact osteocyte dendrite network is not required for the osteogenic response in this model of bone loading.

A trend of osteocalcin in diabetes mellitus research: bibliometric and visualization analysis

Background

Osteocalcin has attracted attention for its potential role in diabetes management. However, there has been no bibliometric assessment of scientific progress in this field.

Methods

We analysed 1680 articles retrieved from the Web of Science Core Collection (WoSCC) between 1 January 1986 and 10 May 2024 using various online tools.

Result

These papers accumulated 42,714 citations,with an average of 25.43 citations per paper. Publication output increased sharply from 1991 onwards. The United States and China are at the forefront of this research area.

Discussion

The keywords were grouped into four clusters: 'Differential and functional osteocalcin genes', 'Differential expression of osteocalcin genes in relation to diabetes mellitus', 'Role of osteocalcin in the assessment of osteoporosis and diabetes mellitus', and 'Indirect involvement of osteocalcin in metabolic processes'. Analysis using the VoS viewer suggests a shift in research focus towards the correlation between osteocalcin levels and diabetic complications, the clinical efficacy of therapeutic agents or vitamins in the treatment of osteoporosis in diabetic patients, and the mechanisms by which osteocalcin modulates insulin action. The proposed focus areas are "osteocalcin genes", "insulin regulation and osteoporosis ", "different populations", "diabetes-related complications" and "type 2 diabetes mellitus","effect of osteocalcin expression on insulin sensitivity as well as secretion","osteocalcin expression in different populations of diabetic patients and treatment-related studies".

Fam102a translocates Runx2 and Rbpjl to facilitate Osterix expression and bone formation

Bone remodeling maintains the robustness of the bone tissue by balancing bone resorption by osteoclasts and bone formation by osteoblasts. Although these cells together play a crucial role in bone remodeling, only a few reports are available on the common factors involved in the differentiation of the two types of cells. Here, we show family with sequence similarity 102 member A (Fam102a) as a bone-remodeling factor that positively regulates both osteoclast and osteoblast differentiation. Fam102a regulates osteoblast differentiation by controlling recombination signal binding protein for immunoglobulin κ J region-like (Rbpjl). The Fam102a-Rbpjl axis promotes the nuclear translocation of transcription factors and enhances the expression of Osterix, a transcription factor essential for osteoblast differentiation. The deletion of Fam102a or a functional mutation in Rbpjl leads to osteopenia accompanied by reduced osteoblastic bone formation. Thus, the Fam102a-Rbpjl axis plays an important role in osteoblasts and this finding provides insights into bone remodeling.

Influence of bone morphogenetic protein (BMP) signaling and masticatory load on morphological alterations of the mouse mandible during postnatal development

OBJECTIVE

Bone homeostasis relies on several contributing factors, encompassing growth factors and mechanical stimuli. While bone morphogenetic protein (BMP) signaling is acknowledged for its essential role in skeletal development, its specific impact on mandibular morphogenesis remains unexplored. Here, we investigated the involvement of BMP signaling and mechanical loading through mastication in postnatal mandibular morphogenesis.

DESIGN

We employed conditional deletion of Bmpr1a in osteoblasts and chondrocytes via Osterix-Cre. Cre activity was induced at birth for the 3-week group and at three weeks for the 9-week and 12-week groups, respectively. The conditional knockout (cKO) and control mice were given either a regular diet (hard diet, HD) or a powdered diet (soft diet, SD) from 3 weeks until sample collection, followed by micro-CT and histological analysis.

RESULTS

The cKO mice exhibited shorter anterior lengths and a posteriorly inclined ramus across all age groups compared to the control mice. The cKO mice displayed an enlarged hypertrophic cartilage area along with fewer osteoclast numbers in the subchondral bone of the condyle compared to the control group at three weeks, followed by a reduction in the cartilage area in the posterior region at twelve weeks. Superimposed imaging and histomorphometrical analysis of the condyle revealed that BMP signaling primarily affects the posterior part of the condyle, while mastication affects the anterior part.

CONCLUSIONS

Using 3D landmark-based geometric morphometrics and histological assessments of the mandible, we demonstrated that BMP signaling and mechanical loading reciprocally contribute to the morphological alterations of the mandible and condyle during postnatal development.

Bone development by Hedgehog and Wnt signaling, Runx2, and Sp7

Hedgehog and canonical Wnt signaling pathways and the transcription factors Runx2 and Sp7 are essential for osteoblast differentiation. Ihh is necessary for the commitment of perichondrial mesenchymal cells to Runx2+ osteoprogenitors and for the formation of the bone collar and primary spongiosa. Runx2 is needed for osteoblast differentiation during both endochondral and intramembranous ossification. It regulates the commitment of mesenchymal cells to osteoblast-lineage cells and their proliferation by inducing the expression of Hedgehog, Fgf, Wnt, Pthlh signaling pathway genes, and Dlx5. The Runx2-induced expression of Fgfr2 and Fgfr3 is important for the proliferation of osteoblast-lineage cells. Runx2 induces Sp7 expression and Runx2+ osteoprogenitors become Runx2+Sp7+ preosteoblasts. Runx2, Sp7, and canonical Wnt signaling induce the differentiation of preosteoblasts into osteoblasts. Canonical Wnt signaling, but not Sp7, enhances the proliferation of osteoblast-lineage cells. In mature osteoblasts, Runx2 plays an important role in the expression of major bone matrix protein genes, including Col1a1, Col1a2, Spp1, Ibsp, and Bglap/Bglap2. The canonical Wnt signaling pathway is also crucial for bone formation by mature osteoblasts. Sp7 is needed for osteocytes to acquire a sufficient number of processes and a reduction in these processes results in osteocyte apoptosis and cortical porosity.

Cracking the code: Understanding ESWT's role in bone fracture healing

Bone non-union has always been a research hotspot in the field of orthopedics. Non-unions are often accompanied by symptoms such as pain, deformity, and dysfunction, which can significantly affect patients' quality of life and cause related socioeconomic problems. Clinically, there are various treatments available for non-unions, and the main treatment methods are divided into surgical and non-surgical treatments. At present, surgery is the most widely used treatment for bone non-unions and has a high healing rate. However, even after surgery, some patients still face the problem of bone non-union. Furthermore, a small number of patients have surgical contraindications and could not tolerate surgery. Therefore, alternative treatments are needed to improve outcomes for patients with bone fractures. Extracorporeal shock wave therapy (ESWT) is a non-invasive treatment method with similar efficacy and better safety compared with surgery. Nevertheless, the exact mechanism for ESWT to treat patients with bone non-union are still not well understood. This article reviews the mechanisms of ESWT in promoting bone fracture healing by regulating osteoblasts and osteoclasts, providing a theoretical foundation for the clinical application of ESWT. The Translational Potential of this Article: This review provides a comprehensive overview of the mechanisms underlying ESWT on promoting bone fracture healing by regulating osteoblasts and osteoclasts. The information provided in this article can offer a novel non-invasive method for clinicians to treat bone non-union.

A new effLuc/Kate dual reporter allele for tumor imaging in mice

Genetically engineered mouse models (GEMMs) are instrumental for modelling local and systemic features of complex diseases, such as cancer. Non-invasive, longitudinal cell detection and monitoring in tumors, metastases and/or the micro-environment is paramount to achieve a better spatiotemporal understanding of cancer progression and to evaluate therapies in preclinical studies. Bioluminescent and fluorescent reporters marking tumor cells or their microenvironment are valuable for non-invasive cell detection and monitoring in vivo. Here, we report the generation of a dual reporter allele allowing simultaneous bioluminescence and fluorescence detection of cells that have undergone Cre-Lox recombination in mice. The single copy knock-in allele in the permissive collagen I locus was evaluated in the context of several cancer GEMMs, where Cre expression was achieved genetically or by ectopic virus-mediated delivery. The new reporter allele was also combined with gene-targeted alleles widely used in bone, prostate, brain and pancreas cancer research, as well as with alleles inserted into the commonly used Rosa26 and collagen I loci. This allele is, therefore, a useful addition to the portfolio of reporters to help advance preclinical research.

YAP regulates periosteal expansion in fracture repair

Bone fracture repair initiates by periosteal expansion. The periosteum is typically quiescent, but upon fracture, periosteal cells proliferate and contribute to bone fracture repair. The expansion of the periosteum is regulated by gene transcription; however, the molecular mechanisms behind periosteal expansion are unclear. Here, we show that Yes-Associated Protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) mediate periosteal expansion and periosteal cell proliferation. Bone fracture increases the number of YAP-expressing periosteal cells, and deletion of YAP and TAZ from Osterix (Osx) expressing cells impairs early periosteal expansion. Mechanistically, YAP regulates both 'cell-intrinsic' and 'cell-extrinsic' factors that allow for periosteal expansion. Specifically, we identified Bone Morphogenetic Protein 4 (BMP4) as a cell extrinsic factor regulated by YAP, that rescues the impairment of periosteal expansion upon YAP/TAZ deletion. Together, these data establish YAP mediated transcriptional mechanisms that induce periosteal expansion in the early stages of fracture repair and provide new putative targets for therapeutic interventions.

Dehydroepiandrosterone and Bone Health: Mechanisms and Insights

BACKGROUND/OBJECTIVES

Dehydroepiandrosterone (DHEA), a steroid hormone produced by the adrenal glands, plays a key role in various physiological processes, including bone health. Its age-related decline is linked to reduced bone density, though the mechanisms by which DHEA affects bone metabolism remain complex. This review summarises the diverse effects of DHEA on bone metabolism and density, highlighting its therapeutic potential; Methods: A literature search on the effects of DHEA on bone-related parameters was conducted from PubMed and Scopus using a specific search string, and after removing duplicates and irrelevant articles, 36 relevant full-text studies were included; Results: DHEA promotes osteoblast differentiation and proliferation, regulates the RANKL/OPG ratio, and inhibits osteoclastogenesis and bone resorption. Its osteogenic effects are mediated through multiple signalling pathways. In ovariectomised rat models, DHEA enhances trabecular bone volume, stimulates osteoblast proliferation, and increases oestradiol production and aromatase activity. In elderly individuals with low androgen levels, DHEA supplementation increases sulphated DHEA and oestradiol levels and improves bone mineral density, particularly in the ultra-distal radius of women and the femoral neck of men. However, the clinical use of DHEA remains debated due to inconsistent study results. Its effects on bone health may vary based on factors such as age, gender, and health conditions, emphasising the need for further research to clarify its mechanisms and optimise its use; Conclusions: In conclusion, while DHEA shows potential as a modulator of bone health, comprehensive clinical trials are required to assess its efficacy and safety, particularly in at-risk populations.

Loss of Hdac4 in osteoprogenitors impairs postnatal trabecular and cortical bone formation, resulting in a dwarfism and osteopenia phenotype in mice

HDAC4 is a class II histone deacetylation protein with a well-characterized role in chondrocyte differentiation and skeletal development, and dysregulated expression or haploinsufficiency of Hdac4 leads to skeletal formation and malformation disorders. The early lethality of Hdac4 ablation mice hindered further investigation of its role in postnatal bone growth and development. Therefore, this study aims to investigate the significant role of Hdac4 in postnatal endochondral bone development using two mouse models with conditional deletion of Hdac4 in Sp7-expressing osteoprogenitors or chondrocytes and monitored postnatal bone development. The phenotype of Acan-CreERT2; Hdac4fl/fl mice largely resembled that of conventional Hdac4-/- mice. But phenotypic characterizations of mice with Hdac4 inactivation in Sp7-expressing osteoprogenitors (Sp7-Cre; Hdac4fl/fl) showed dwarfism with body and limb shortening and remarkable skeletal defects. Microcomputed tomography analysis of tibias further demonstrated that loss of Hdac4 expression impaired bone formation and microarchitecture, mainly characterized by dysplasia of trabecular and cortical bone in young mice. Our in vivo and in vitro data support a crucial role for Hdac4 in regulating osteoblast proliferation and differentiation, bone matrix protein production, angiogenesis, and ultimately trabecular and cortical bone formation. Moreover, RNA-seq analysis implicated Hdac4 in the regulation of key genes and pathways necessary to affect the accumulation of extracellular matrix, biological processes related to signal transduction, and skeletal growth. Collectively, our data show that postnatal expression of Hdac4 in Sp7-expressing osteoprogenitors provides essential regulatory oversight of endochondral bone formation, bone morphology, and homeostasis.

Metabolic reprogramming in skeletal cell differentiation

The human skeleton is a multifunctional organ made up of multiple cell types working in concert to maintain bone and mineral homeostasis and to perform critical mechanical and endocrine functions. From the beginning steps of chondrogenesis that prefigures most of the skeleton, to the rapid bone accrual during skeletal growth, followed by bone remodeling of the mature skeleton, cell differentiation is integral to skeletal health. While growth factors and nuclear proteins that influence skeletal cell differentiation have been extensively studied, the role of cellular metabolism is just beginning to be uncovered. Besides energy production, metabolic pathways have been shown to exert epigenetic regulation via key metabolites to influence cell fate in both cancerous and normal tissues. In this review, we will assess the role of growth factors and transcription factors in reprogramming cellular metabolism to meet the energetic and biosynthetic needs of chondrocytes, osteoblasts, or osteoclasts. We will also summarize the emerging evidence linking metabolic changes to epigenetic modifications during skeletal cell differentiation.

O-GlcNAcylation mediates Wnt-stimulated bone formation by rewiring aerobic glycolysis

Wnt signaling is an important target for anabolic therapies in osteoporosis. A sclerostin-neutralizing antibody (Scl-Ab), that blocks the Wnt signaling inhibitor (sclerostin), has been shown to promote bone mass in animal models and clinical studies. However, the cellular mechanisms by which Wnt signaling promotes osteogenesis remain to be further investigated. O-GlcNAcylation, a dynamic post-translational modification of proteins, controls multiple critical biological processes including transcription, translation, and cell fate determination. Here, we report that Wnt3a either induces O-GlcNAcylation rapidly via the Ca2+-PKA-Gfat1 axis, or increases it in a Wnt-β-catenin-dependent manner following prolonged stimulation. Importantly, we find O-GlcNAcylation indispensable for osteoblastogenesis both in vivo and in vitro. Genetic ablation of O-GlcNAcylation in the osteoblast-lineage diminishes bone formation and delays bone fracture healing in response to Wnt stimulation in vivo. Mechanistically, Wnt3a induces O-GlcNAcylation at Serine 174 of PDK1 to stabilize the protein, resulting in increased glycolysis and osteogenesis. These findings highlight O-GlcNAcylation as an important mechanism regulating Wnt-induced glucose metabolism and bone anabolism.

CRISPR activation of Tfeb , a master regulator of autophagy and lysosomal biogenesis, in osteoblast lineage cells increases bone mass and strength

Autophagy is a recycling pathway in which damaged or dysfunctional proteins, protein aggregates, and organelles are delivered to lysosomes for degradation. Insufficiency of autophagy is thought to contribute to several age-related diseases including osteoporosis. Consistent with this, elimination of autophagy from the osteoblast lineage reduces bone formation and causes low bone mass. However, whether increasing autophagy would benefit bone health is unknown. Here, we increased expression of the endogenous Transcription Factor EB gene ( Tfeb ) in osteoblast lineage cells in vivo via CRISPR activation. Tfeb overexpression stimulated autophagy and lysosomal biogenesis in osteoblasts. Tfeb overexpressing male mice displayed a robust increase in femoral and vertebral cortical thickness at 4.5 months of age. Histomorphometric analysis revealed that the increase in femoral cortical thickness was due to increased bone formation at the periosteal surface. Tfeb overexpression also increased femoral trabecular bone volume. Consistent with these results, bone strength was increased in Tfeb overexpressing mice. Female Tfeb overexpressing mice also displayed a progressive increase in bone mass over time and at 12 months of age had high cortical thickness and trabecular bone volume. This increase in vertebral trabecular bone volume was due to elevated bone formation. Osteoblastic cultures showed that Tfeb overexpression increased proliferation and osteoblast formation. Overall, these results demonstrate that stimulation of autophagy in osteoblast lineage cells promotes bone formation and strength and may represent an effective approach to combat osteoporosis.

Regulation of Skeletal Development and Maintenance by Runx2 and Sp7

Runx2 (runt related transcription factor 2) and Sp7 (Sp7 transcription factor 7) are crucial transcription factors for bone development. The cotranscription factor Cbfb (core binding factor beta), which enhances the DNA-binding capacity of Runx2 and stabilizes the Runx2 protein, is necessary for bone development. Runx2 is essential for chondrocyte maturation, and Sp7 is partly involved. Runx2 induces the commitment of multipotent mesenchymal cells to osteoblast lineage cells and enhances the proliferation of osteoprogenitors. Reciprocal regulation between Runx2 and the Hedgehog, fibroblast growth factor (Fgf), Wnt, and parathyroid hormone-like hormone (Pthlh) signaling pathways and Dlx5 (distal-less homeobox 5) plays an important role in these processes. The induction of Fgfr2 (Fgf receptor 2) and Fgfr3 expression by Runx2 is important for the proliferation of osteoblast lineage cells. Runx2 induces Sp7 expression, and Runx2+ osteoprogenitors become Runx2+Sp7+ preosteoblasts. Sp7 induces the differentiation of preosteoblasts into osteoblasts without enhancing their proliferation. In osteoblasts, Runx2 is required for bone formation by inducing the expression of major bone matrix protein genes, including Col1a1 (collagen type I alpha 1), Col1a2, Spp1 (secreted phosphoprotein 1), Ibsp (integrin binding sialoprotein), and Bglap (bone gamma carboxyglutamate protein)/Bglap2. Bglap/Bglap2 (osteocalcin) regulates the alignment of apatite crystals parallel to collagen fibrils but does not function as a hormone that regulates glucose metabolism, testosterone synthesis, and muscle mass. Sp7 is also involved in Co1a1 expression and regulates osteoblast/osteocyte process formation, which is necessary for the survival of osteocytes and the prevention of cortical porosity. SP7 mutations cause osteogenesis imperfecta in rare cases. Runx2 is an important pathogenic factor, while Runx1, Runx3, and Cbfb are protective factors in osteoarthritis development.

Inflammation perturbs hematopoiesis by remodeling specific compartments of the bone marrow niche

Hematopoietic stem and progenitor cells (HSPC) are regulated by interactions with stromal cells in the bone marrow (BM) cavity, which can be segregated into two spatially defined central marrow (CM) and endosteal (Endo) compartments. However, the importance of this spatial compartmentalization for BM responses to inflammation and neoplasia remains largely unknown. Here, we extensively validate a combination of scRNA-seq profiling and matching flow cytometry isolation that reproducibly identifies 7 key CM and Endo populations across mouse strains and accurately surveys both niche locations. We demonstrate that different perturbations exert specific effects on different compartments, with type I interferon responses causing CM mesenchymal stromal cells to adopt an inflammatory phenotype associated with overproduction of chemokines modulating local monocyte dynamics in the surrounding microenvironment. Our results provide a comprehensive method for molecular and functional stromal characterization and highlight the importance of altered stomal cell activity in regulating hematopoietic responses to inflammatory challenges.

Sex-specific trisomic Dyrk1a-related skeletal phenotypes during development in a Down syndrome model

Skeletal insufficiency affects all individuals with Down syndrome (DS) or trisomy 21 and may alter bone strength throughout development due to a reduced period of bone formation and early attainment of peak bone mass compared to those in typically developing individuals. Appendicular skeletal deficits also appear in males before females with DS. In femurs of male Ts65Dn DS model mice, cortical deficits were pronounced throughout development, but trabecular deficits and Dyrk1a overexpression were transitory until postnatal day (P) 30, when there were persistent trabecular and cortical deficits and Dyrk1a was trending toward overexpression. Correction of DS-related skeletal deficits by a purported DYRK1A inhibitor or through genetic means beginning at P21 was not effective at P30, but germline normalization of Dyrk1a improved male bone structure by P36. Trabecular and cortical deficits in female Ts65Dn mice were evident at P30 but subsided by P36, typifying periodic developmental skeletal normalizations that progressed to more prominent bone deficiencies. Sex-dependent differences in skeletal deficits with a delayed impact of trisomic Dyrk1a are important to find temporally specific treatment periods for bone and other phenotypes associated with trisomy 21.

Osterix-driven LINC complex disruption in vivo diminishes osteogenesis at 8 weeks but not at 15 weeks

The Linker of Nucleoskeleton and Cytoskeleton (LINC) complex is a crucial connective component between the nuclear envelope and the cytoskeleton involving various cellular processes including nuclear positioning, nuclear architecture, and mechanotransduction. How LINC complexes regulate bone formation in vivo, however, is not well understood. To start bridging this gap, here we created a LINC disruption murine model using transgenic mice expressing Cre recombinase enzyme under the control of the Osterix (Osx-Cre) which is primarily active in pre-osteoblasts and floxed Tg(CAG-LacZ/EGFP-KASH2) mice. Tg(CAG-LacZ/EGFP-KASH2) mice contain a lox-STOP-lox flanked LacZ gene which is deleted upon cre recombination allowing for the overexpression of an EGFP-KASH2 fusion protein. This overexpressed protein disrupts endogenous Nesprin-Sun binding leading to disruption of LINC complexes. Thus, crossing these two lines results in an  Osx- driven  LINC  disruption (ODLD) specific to pre-osteoblasts. In this study, we investigated how this LINC disruption affects exercise-induced bone accrual. ODLD cells had decreased osteogenic and adipogenic potential in vitro compared to non-disrupted controls and sedentary ODLD mice showed decreased bone quality at 8 weeks. Upon access to a voluntary running wheel, ODLD animals showed increased running time and distance; however, our 6-week exercise intervention did not significantly affect bone microarchitecture and bone mechanical properties.

Hdac3-deficiency increases senescence-associated distention of satellite DNA and telomere-associated foci in osteoprogenitor cells

Histone deacetylase 3 (Hdac3) is an epigenetic regulator of gene expression and interacts with skeletal transcription factors such as Runx2. We previously reported that conditional deletion of Hdac3 in Osterix-Cre recombinase-expressing osteoprogenitor cells (Hdac3 CKOOsx) caused osteopenia and increased marrow adiposity, both hallmarks of skeletal aging. We also showed that Runx2+ cells within osteogenic cultures of Hdac3-depleted bone marrow stromal cells (BMSCs) contain lipid droplets (LDs). Cellular senescence, a nonproliferative metabolically active state, is associated with increased marrow adiposity, bone loss, and aging. In this study, we sought to determine if Hdac3 depleted Runx2+ pre-osteoblasts from young mice exhibit chromatin changes associated with early cellular senescence and how these events correlate with the appearance of LDs. We first confirmed that BMSCs from Hdac3 CKOOsx mice have more Runx2 + LD+ cells compared with controls under osteogenic conditions. We then measured senescence-associated distention of satellite (SADS) DNA and telomere-associated foci (TAFs) in Hdac3 CKOOsx and control BMSCs. In situ, Runx2+ cells contained more SADS per nuclei in Hdac3 CKOOsx femora than in controls. Runx2+ BMSCs from Hdac3 CKOOsx mice also contained more SADS and TAFs per nuclei than Runx2+ cells from age-matched control mice in vitro. SADs and TAFs were present at similar levels in Runx2 + LD+ cells and Runx2 + LD- cells from Hdac3 CKOOsx mice. Hdac inhibitors also increased the number of SADS in Runx2 + LD+ and Runx2 + LD- WT BMSCs. Senolytics reduced viable cell numbers in Hdac3 CKOOsx BMSC cultures. These data demonstrate that the depletion of Hdac3 in osteochondral progenitor cells triggers LD formation and early events in cellular senescence in Runx2+ BMSCs through mutually exclusive mechanisms.

Wnt/β-catenin signaling components and mechanisms in bone formation, homeostasis, and disease

Wnts are secreted, lipid-modified proteins that bind to different receptors on the cell surface to activate canonical or non-canonical Wnt signaling pathways, which control various biological processes throughout embryonic development and adult life. Aberrant Wnt signaling pathway underlies a wide range of human disease pathogeneses. In this review, we provide an update of Wnt/β-catenin signaling components and mechanisms in bone formation, homeostasis, and diseases. The Wnt proteins, receptors, activators, inhibitors, and the crosstalk of Wnt signaling pathways with other signaling pathways are summarized and discussed. We mainly review Wnt signaling functions in bone formation, homeostasis, and related diseases, and summarize mouse models carrying genetic modifications of Wnt signaling components. Moreover, the therapeutic strategies for treating bone diseases by targeting Wnt signaling, including the extracellular molecules, cytosol components, and nuclear components of Wnt signaling are reviewed. In summary, this paper reviews our current understanding of the mechanisms by which Wnt signaling regulates bone formation, homeostasis, and the efforts targeting Wnt signaling for treating bone diseases. Finally, the paper evaluates the important questions in Wnt signaling to be further explored based on the progress of new biological analytical technologies.

Conditional loss of CaMKK2 in Osterix-positive osteoprogenitors enhances osteoblast function in a sex-divergent manner

Ca2+/calmodulin-dependent protein kinase kinase 2 (CaMKK2) is a multi-functional, serine/threonine protein kinase with predominant roles in inflammation, systemic energy metabolism, and bone remodeling. We previously reported that global ablation of CaMKK2 or its systemic pharmacological inhibition led to bone mass accrual in mice by stimulating osteoblasts and inhibiting osteoclasts. However, a direct, cell-intrinsic role for the kinase in the osteoblast lineage has not been established. Here we report that conditional deletion of CaMKK2 from osteoprogenitors, using the Osterix 1 (Osx1) - GFP::Cre (tetracycline-off) mouse line, resulted in increased trabecular bone mass due to an acute stimulation of osteoblast function in male and female mice. The acute simulation of osteoblasts and bone formation following conditional ablation of osteoprogenitor-derived CaMKK2 was sustained only in female mice. Periosteal bone formation at the cortical bone was enhanced only in male conditional knockout mice without altering cortical bone mass or strength. Prolonged deletion of CaMKK2 in early osteoblasts was accompanied by a stimulation of osteoclasts in both sexes, indicating a coupling effect. Notably, alterations in trabecular and cortical bone mass were absent in the doxycycline-removed "Cre-only" Osx1-GFP::Cre mice. Thus, the increase in osteoblast function at the trabecular and cortical bone surfaces following the conditional deletion of CaMKK2 in osteoprogenitors is indicative of a direct but sex-divergent role for the kinase in osteoblasts.

Loss of signal transducer and activator of transcription 3 in osteoblasts impaired the bone healing in inflammatory microenvironment

INTRODUCTION

This study aimed to investigate the effect of Stat3 on the osteoblast-mediated bone healing in the inflammatory lesion.

METHODS

The conditional knockout of Stat3 in osteoblasts (Stat3 CKO) was generated via the Cre-loxP recombination system using Osterix-Cre transgenic mice. The calvarial bone inflammatory lesions were established on both Stat3 CKO and wild-type mice, then harvested to assess the bone healing. In response to lipopolysaccharide (LPS) stimulation, osteoblasts from Stat3 CKO and wild-type mice were subjected to examine the formation of calcium deposits, the expression of osteogenic markers (i.e., Runx2, OPN, COL1A1), and osteoclast-related markers (i.e., RANKL, OPG). The EdU and transwell assays were performed to assess the proliferation and migration of the cells.

RESULTS

A decrease in bone mass and an increase in osteolysis were found in the inflammatory lesions on Stat3 CKO mice when compared with the control. More osteoclastic-like cells and an increased expression of RANKL were observed in Stat3 CKO mice. Both mRNA and protein expressions of Stat3 and osteogenic markers in the lesions were significantly decreased in Stat3 CKO mice. After co-cultured with osteogenic medium, the Stat3-deficient osteoblasts were found with a significant decrease in calcium deposits and the expression of osteogenic markers, and with a significant increased expression of RANKL. The impaired ossification of Stat3-deficient osteoblasts was even more pronounced with the presence of lipopolysaccharides in vitro. The most decrease in cell proliferation and migration was found in Stat3-deficient osteoblasts in response to LPS.

CONCLUSIONS

Loss of Stat3 in osteoblasts impaired bone healing in an inflammatory microenvironment.

Sex specific emergence of trisomic Dyrk1a-related skeletal phenotypes in the development of a Down syndrome mouse model

Skeletal insufficiency affects all individuals with Down syndrome (DS) or Trisomy 21 (Ts21) and may alter bone strength throughout development due to a reduced period of bone formation and early attainment of peak bone mass compared to typically developing individuals. Appendicular skeletal deficits also appear in males before females with DS. In femurs of male Ts65Dn DS model mice, cortical deficits were pronounced throughout development, but trabecular deficits and Dyrk1a overexpression were transitory until postnatal day (P) 30 when there were persistent trabecular and cortical deficits and Dyrk1a was trending overexpression. Correction of DS-related skeletal deficits by a purported DYRK1A inhibitor or through genetic means beginning at P21 was not effective at P30, but germline normalization of Dyrk1a improved male bone structure by P36. Trabecular and cortical deficits in female Ts65Dn mice were evident at P30 but subsided by P36, typifying periodic developmental skeletal normalizations that progressed to more prominent bone deficiencies. Sex-dependent differences in skeletal deficits with a delayed impact of trisomic Dyrk1a are important to find temporally specific treatment periods for bone and other phenotypes associated with Ts21.

Hyperthyroidism-driven bone loss depends on BMP receptor Bmpr1a expression in osteoblasts

Hyperthyroidism is a well-known trigger of high bone turnover that can lead to the development of secondary osteoporosis. Previously, we have shown that blocking bone morphogenetic protein (BMP) signaling systemically with BMPR1A-Fc can prevent bone loss in hyperthyroid mice. To distinguish between bone cell type-specific effects, conditional knockout mice lacking Bmpr1a in either osteoclast precursors (LysM-Cre) or osteoprogenitors (Osx-Cre) were rendered hyperthyroid and their bone microarchitecture, strength and turnover were analyzed. While hyperthyroidism in osteoclast precursor-specific Bmpr1a knockout mice accelerated bone resorption leading to bone loss just as in wildtype mice, osteoprogenitor-specific Bmpr1a deletion prevented an increase of bone resorption and thus osteoporosis with hyperthyroidism. In vitro, wildtype but not Bmpr1a-deficient osteoblasts responded to thyroid hormone (TH) treatment with increased differentiation and activity. Furthermore, we found an elevated Rankl/Opg ratio with TH excess in osteoblasts and bone tissue from wildtype mice, but not in Bmpr1a knockouts. In line, expression of osteoclast marker genes increased when osteoclasts were treated with supernatants from TH-stimulated wildtype osteoblasts, in contrast to Bmpr1a-deficient cells. In conclusion, we identified the osteoblastic BMP receptor BMPR1A as a main driver of osteoporosis in hyperthyroid mice promoting TH-induced osteoblast activity and potentially its coupling to high osteoclastic resorption.

Fibrous periosteum repairs bone fracture and maintains the healed bone throughout mouse adulthood

Bone is regarded as one of few tissues that heals without fibrous scar. The outer layer of the periosteum is covered with fibrous tissue, whose function in bone formation is unknown. We herein developed a system to distinguish the fate of fibrous-layer periosteal cells (FL-PCs) from the skeletal stem/progenitor cells (SSPCs) in the cambium-layer periosteum and bone marrow in mice. We showed that FL-PCs did not participate in steady-state osteogenesis, but formed the main body of fibrocartilaginous callus during fracture healing. Moreover, FL-PCs invaded the cambium-layer periosteum and bone marrow after fracture, forming neo-SSPCs that continued to maintain the healed bones throughout adulthood. The FL-PC-derived neo-SSPCs expressed lower levels of osteogenic signature genes and displayed lower osteogenic differentiation activity than the preexisting SSPCs. Consistent with this, healed bones were thinner and formed more slowly than normal bones. Thus, the fibrous periosteum becomes the cellular origin of bones after fracture and alters bone properties permanently.

The evolving roles of Wnt signaling in stem cell proliferation and differentiation, the development of human diseases, and therapeutic opportunities

The evolutionarily conserved Wnt signaling pathway plays a central role in development and adult tissue homeostasis across species. Wnt proteins are secreted, lipid-modified signaling molecules that activate the canonical (β-catenin dependent) and non-canonical (β-catenin independent) Wnt signaling pathways. Cellular behaviors such as proliferation, differentiation, maturation, and proper body-axis specification are carried out by the canonical pathway, which is the best characterized of the known Wnt signaling paths. Wnt signaling has emerged as an important factor in stem cell biology and is known to affect the self-renewal of stem cells in various tissues. This includes but is not limited to embryonic, hematopoietic, mesenchymal, gut, neural, and epidermal stem cells. Wnt signaling has also been implicated in tumor cells that exhibit stem cell-like properties. Wnt signaling is crucial for bone formation and presents a potential target for the development of therapeutics for bone disorders. Not surprisingly, aberrant Wnt signaling is also associated with a wide variety of diseases, including cancer. Mutations of Wnt pathway members in cancer can lead to unchecked cell proliferation, epithelial-mesenchymal transition, and metastasis. Altogether, advances in the understanding of dysregulated Wnt signaling in disease have paved the way for the development of novel therapeutics that target components of the Wnt pathway. Beginning with a brief overview of the mechanisms of canonical and non-canonical Wnt, this review aims to summarize the current knowledge of Wnt signaling in stem cells, aberrations to the Wnt pathway associated with diseases, and novel therapeutics targeting the Wnt pathway in preclinical and clinical studies.

SOXC are critical regulators of adult bone mass

Pivotal in many ways for human health, the control of adult bone mass is governed by complex, incompletely understood crosstalk namely between mesenchymal stem cells, osteoblasts and osteoclasts. The SOX4, SOX11 and SOX12 (SOXC) transcription factors were previously shown to control many developmental processes, including skeletogenesis, and SOX4 was linked to osteoporosis, but how SOXC control adult bone mass remains unknown. Using SOXC loss- and gain-of-function mouse models, we show here that SOXC redundantly promote prepubertal cortical bone mass strengthening whereas only SOX4 mitigates adult trabecular bone mass accrual in early adulthood and subsequent maintenance. SOX4 favors bone resorption over formation by lowering osteoblastogenesis and increasing osteoclastogenesis. Single-cell transcriptomics reveals its prevalent expression in Lepr+ mesenchymal cells and ability to upregulate genes for prominent anti-osteoblastogenic and pro-osteoclastogenic factors, including interferon signaling-related chemokines, contributing to these adult stem cells' secretome. SOXC, with SOX4 predominantly, are thus key regulators of adult bone mass.

Refining the identity of mesenchymal cell types associated with murine periosteal and endosteal bone

Single-cell RNA-seq has led to novel designations for mesenchymal cells associated with bone as well as multiple designations for what appear to be the same cell type. The main goals of this study were to increase the amount of single-cell RNA sequence data for osteoblasts and osteocytes, to compare cells from the periosteum to those inside bone, and to clarify the major categories of cell types associated with murine bone. We created an atlas of murine bone-associated cells by harmonizing published datasets with in-house data from cells targeted by Osx1-Cre and Dmp1-Cre driver strains. Cells from periosteal bone were analyzed separately from those isolated from the endosteum and trabecular bone. Over 100,000 mesenchymal cells were mapped to reveal 11 major clusters designated fibro-1, fibro-2, chondrocytes, articular chondrocytes, tenocytes, adipo-Cxcl12 abundant reticular (CAR), osteo-CAR, preosteoblasts, osteoblasts, osteocytes, and osteo-X, the latter defined in part by periostin expression. Osteo-X, osteo-CAR, and preosteoblasts were closely associated with osteoblasts at the trabecular bone surface. Wnt16 was expressed in multiple cell types from the periosteum but not in cells from endocortical or cancellous bone. Fibro-2 cells, which express markers of stem cells, localized to the periosteum but not trabecular bone in adult mice. Suppressing bone remodeling eliminated osteoblasts and altered gene expression in preosteoblasts but did not change the abundance or location of osteo-X or osteo-CAR cells. These results provide a framework for identifying bone cell types in murine single-cell RNA-seq datasets and suggest that osteoblast progenitors reside near the surface of remodeling bone.

KLF transcription factors in bone diseases

Krüppel-like factors (KLFs) are crucial in the development of bone disease. They are a family of zinc finger transcription factors that are unusual in containing three highly conserved zinc finger structural domains interacting with DNA. It has been discovered that it engages in various cell functions, including proliferation, apoptosis, autophagy, stemness, invasion and migration, and is crucial for the development of human tissues. In recent years, the role of KLFs in bone physiology and pathology has received adequate attention. In addition to regulating the normal growth and development of the musculoskeletal system, KLFs participate in the pathological process of the bones and joints and are intimately linked to several skeletal illnesses, such as osteoarthritis (OA), rheumatoid arthritis (RA), osteoporosis (OP) and osteosarcoma (OS). Consequently, targeting KLFs has emerged as a promising therapeutic approach for an array of bone disorders. In this review, we summarize the current literature on the importance of KLFs in the emergence and regulation of bone illnesses, with a particular emphasis on the pertinent mechanisms by which KLFs regulate skeletal diseases. We also discuss the need for KLFs-based medication-targeted treatment. These endeavours offer new perspectives on the use of KLFs in bone disorders and provide prognostic biomarkers, therapeutic targets and possible drug candidates for bone diseases.

Effect of Lithium on Orthodontic Tooth Movement: a Systematic Review of Animal Studies

Objective

This study aimed to systematically review the effect of lithium on orthodontic tooth movement (OTM).

Methods

The focus question was "does lithium have an effect on OTM?" A systematic search was conducted using indexed databases and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed. The quality assessment of the selected studies was performed according to the systematic review center for laboratory animal experimentation.

Results

Five of the initially identified 656 articles fulfilled the eligibility criteria and were selected for this review. The studies reported that lithium administration lowered the rate of OTM by inducing a reduction in the number of osteoclasts and possibly inhibiting osteoclastogenesis. These studies further showed an increase in bone density and bone volume by promoting the Wnt/ß-catenin signaling pathway and osteoblastogenesis. It was also noted that lithium reduced orthodontically induced root resorption during experimental OTM. Further, standardized studies are warranted to understand the impact of lithium in OTM. Overall, the risk of bias for 3 studies was very high, high in 1 study, and moderate in 1 study.

Conclusion

On an experimental level in animals, lithium decreased the rate of OTM during the active treatment phase by increasing bone density and bone volume and reducing root resorption. In addition, lithium may enhance alveolar bone formation during orthodontic retention. Clinically, this may impact the orthodontic treatment duration in patients receiving lithium, and further studies are needed to understand the true impact of lithium on OTM.

Forkhead box O proteins: steering the course of stem cell fate

Stem cells are pivotal players in the intricate dance of embryonic development, tissue maintenance, and regeneration. Their behavior is delicately balanced between maintaining their pluripotency and differentiating as needed. Disruptions in this balance can lead to a spectrum of diseases, underscoring the importance of unraveling the complex molecular mechanisms that govern stem cell fate. Forkhead box O (FOXO) proteins, a family of transcription factors, are at the heart of this intricate regulation, influencing a myriad of cellular processes such as survival, metabolism, and DNA repair. Their multifaceted role in steering the destiny of stem cells is evident, as they wield influence over self-renewal, quiescence, and lineage-specific differentiation in both embryonic and adult stem cells. This review delves into the structural and regulatory intricacies of FOXO transcription factors, shedding light on their pivotal roles in shaping the fate of stem cells. By providing insights into the specific functions of FOXO in determining stem cell fate, this review aims to pave the way for targeted interventions that could modulate stem cell behavior and potentially revolutionize the treatment and prevention of diseases.

Pten knockout in mouse preosteoblasts leads to changes in bone turnover and strength

Bone development and remodeling are controlled by the phosphoinositide-3-kinase (Pi3k) signaling pathway. We investigated the effects of downregulation of phosphatase and tensin homolog (Pten), a negative regulator of Pi3k signaling, in a mouse model of Pten deficiency in preosteoblasts. We aimed to identify mechanisms that are involved in the regulation of bone turnover and are linked to bone disorders. Femora, tibiae, and bone marrow stromal cells (BMSCs) isolated from mice with a conditional deletion of Pten (Pten cKO) in Osterix/Sp7-expressing osteoprogenitor cells were compared to Cre-negative controls. Bone phenotyping was performed by μCT measurements, bone histomorphometry, quantification of bone turnover markers CTX and procollagen type 1 N propeptide (P1NP), and three-point bending test. Proliferation of BMSCs was measured by counting nuclei and Ki-67-stained cells. In vitro, osteogenic differentiation capacity was determined by ALP staining, as well as by detecting gene expression of osteogenic markers. BMSCs from Pten cKO mice were functionally different from control BMSCs. Osteogenic markers were increased in BMSCs derived from Pten cKO mice, while Pten protein expression was lower and Akt phosphorylation was increased. We detected a higher trabecular bone volume and an altered cortical bone morphology in Pten cKO bones with a progressive decrease in bone and tissue mineral density. Pten cKO bones displayed fewer osteoclasts and more osteoblasts (P = .00095) per trabecular bone surface and a higher trabecular bone formation rate. Biomechanical analysis revealed a significantly higher bone strength (P = .00012 for males) and elasticity of Pten cKO femora. On the cellular level, both proliferation and osteogenic differentiation capacity of Pten cKO BMSCs were significantly increased compared to controls. Our findings suggest that Pten knockout in osteoprogenitor cells increases bone stability and elasticity by increasing trabecular bone mass and leads to increased proliferation and osteogenic differentiation of BMSCs.

YAP and TAZ couple osteoblast precursor mobilization to angiogenesis and mechanoregulation in murine bone development

Fetal bone development occurs through the conversion of avascular cartilage to vascularized bone at the growth plate. This requires coordinated mobilization of osteoblast precursors with blood vessels. In adult bone, vessel-adjacent osteoblast precursors are maintained by mechanical stimuli; however, the mechanisms by which these cells mobilize and respond to mechanical cues during embryonic development are unknown. Here, we show that the mechanoresponsive transcriptional regulators Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) spatially couple osteoblast precursor mobilization to angiogenesis, regulate vascular morphogenesis to control cartilage remodeling, and mediate mechanoregulation of embryonic murine osteogenesis. Mechanistically, YAP and TAZ regulate a subset of osteoblast-lineage cells, identified by single-cell RNA sequencing as vessel-associated osteoblast precursors, which regulate transcriptional programs that direct blood vessel invasion through collagen-integrin interactions and Cxcl12. Functionally, in 3D human cell co-culture, CXCL12 treatment rescues angiogenesis impaired by stromal cell YAP/TAZ depletion. Together, these data establish functions of the vessel-associated osteoblast precursors in bone development.

In Vivo Optical Interrogation of Neuronal Responses to Genetic, Cell Type-Specific Silencing

We established a low background, Cre-dependent version of the inducible Tet-On system for fast, cell type-specific transgene expression in vivo Coexpression of a constitutive, Cre-dependent fluorescent marker selectively allowed single-cell analyses before and after inducible, Tet-dependent transgene expression. Here, we used this method for precise, acute manipulation of neuronal activity in the living brain. The goal was to study neuronal network homeostasis at cellular resolution. Single induction of the potassium channel Kir2.1 produced cell type-specific silencing within hours that lasted for at least 3 d. Longitudinal in vivo imaging of spontaneous calcium transients and neuronal morphology demonstrated that prolonged silencing did not alter spine densities or synaptic input strength. Furthermore, selective induction of Kir2.1 in parvalbumin interneurons increased the activity of surrounding neurons in a distance-dependent manner. This high-resolution, inducible interference and interval imaging of individual cells (high I5, HighFive) method thus allows visualizing temporally precise, genetic perturbations of defined cells.SIGNIFICANCE STATEMENT Gene function is studied by KO or overexpression of a specific gene followed by analyses of phenotypic changes. However, being able to predict and analyze exactly those cells in which genetic manipulation will occur is not possible. We combined two prominent transgene overexpression methods to fluorescently highlight the targeted cells appropriately before cell type-specific transgene induction. By inducing a potassium channel that decreases neuronal firing, we investigated how neuronal networks in the living mouse brain possibly compensate swift changes in cellular activities. Unlike in vitro, known compensatory homeostatic mechanisms, such as changes in synapses, were not observed in vivo Overall, we demonstrated with our method rapid genetic manipulation and analysis of neuronal activities as well as precision transgene expression.

Trametinib inhibits lymphatic vessel invasion of bone in a mouse model of Gorham-Stout disease

Objective

Gorham-Stout disease (GSD) is a rare lymphatic anomaly that can be caused by somatic activating mutations in KRAS. This discovery has led investigators to suggest that MEK inhibitors could be a novel treatment for GSD. However, the effect of MEK inhibitors on bone disease in animal models of GSD has not been investigated. We recently reported that Osx-tTA;TetO-Vegfc mice exhibit a phenotype that resembles GSD. Osx-tTA;TetO-Vegfc mice overexpress VEGF-C in bone, which stimulates the development of lymphatic vessels in bone and the gradual loss of cortical bone. The objective of this study was to characterize the effect of trametinib, an FDA-approved MEK1/2 inhibitor, on lymphangiogenesis and osteolysis in Osx-tTA;TetO-Vegfc mice.

Methods

Immunoblotting was performed to assess the effect of trametinib on VEGF-C-induced phosphorylation of ERK1/2, AKT, and S6 in primary human lymphatic endothelial cells (LECs). Prevention and intervention experiments were performed to determine the effect of trametinib on lymphangiogenesis and osteolysis in Osx-tTA;TetO-Vegfc mice.

Results

We found that trametinib blocked VEGF-C-induced phosphorylation of ERK1/2 in primary human LECs. We also found that trametinib prevented VEGF-C-induced lymphatic invasion of bone and cortical bone loss in Osx-tTA;TetO-Vegfc mice. Additionally, trametinib slowed the progression of disease in Osx-tTA;TetO-Vegfc mice with established disease. However, it did not reverse disease in Osx-tTA;TetO-Vegfc mice.

Conclusion

Our results show trametinib impacts bone disease in Osx-tTA;TetO-Vegfc mice. These findings further support the testing of MEK inhibitors in patients with GSD and other RAS pathway-driven complex lymphatic anomalies with bone involvement.

A framework for defining mesenchymal cell types associated with murine periosteal and endosteal bone

Single-cell RNA sequencing has led to numerous novel designations for mesenchymal cell types associated with bone. Consequently, there are now multiple designations for what appear to be the same cell type. In addition, existing datasets contain relatively small numbers of mature osteoblasts and osteocytes and there has been no comparison of periosteal bone cells to those at the endosteum and trabecular bone. The main goals of this study were to increase the amount of single cell RNA sequence data for osteoblasts and osteocytes, to compare cells from the periosteum to those inside bone, and to clarify the major categories of cell types associated with murine bone. To do this, we created an atlas of murine bone-associated cells by harmonizing published datasets with in-house data from cells targeted by Osx1-Cre and Dmp1-Cre driver strains. Cells from periosteal bone were analyzed separately from those isolated from the endosteum and trabecular bone. Over 100,000 mesenchymal cells were mapped to reveal 11 major clusters designated fibro-1, fibro-2, chondrocytes, articular chondrocytes, tenocytes, adipo-CAR, osteo-CAR, pre-osteoblasts, osteoblasts, osteocytes, and osteo-X, the latter defined in part by Postn expression. Osteo-X, osteo-CAR, and pre-osteoblasts were closely associated with osteoblasts at the trabecular bone surface. Wnt16 was expressed in multiple cell types from the periosteum but not in any cells from endocortical or cancellous bone. Fibro-2 cells, which express markers of skeletal stem cells, localized to the periosteum but not trabecular bone in adult mice. Suppressing bone remodeling eliminated osteoblasts and altered gene expression in pre-osteoblasts but did not change the abundance or location of osteo-X or osteo-CAR cells. These results provide a framework for identifying bone cell types in murine single cell RNA sequencing datasets and suggest that osteoblast progenitors reside near the surface of remodeling bone.

The bone marrow stroma in human myelodysplastic syndrome reveals alterations that regulate disease progression

Myelodysplastic syndromes (MDSs) are a heterogenous group of diseases affecting the hematopoietic stem cell that are curable only by stem cell transplantation. Both hematopoietic cell intrinsic changes and extrinsic signals from the bone marrow (BM) niche seem to ultimately lead to MDS. Animal models of MDS indicate that alterations in specific mesenchymal progenitor subsets in the BM microenvironment can induce or select for abnormal hematopoietic cells. Here, we identify a subset of human BM mesenchymal cells marked by the expression of CD271, CD146, and CD106. This subset of human mesenchymal cells is comparable with mouse mesenchymal cells that, when perturbed, result in an MDS-like syndrome. Its transcriptional analysis identified Osteopontin (SPP1) as the most overexpressed gene. Selective depletion of Spp1 in the microenvironment of the mouse MDS model, Vav-driven Nup98-HoxD13, resulted in an accelerated progression as demonstrated by increased chimerism, higher mutant myeloid cell burden, and a more pronounced anemia when compared with that in wild-type microenvironment controls. These data indicate that molecular perturbations can occur in specific BM mesenchymal subsets of patients with MDS. However, the niche adaptations to dysplastic clones include Spp1 overexpression that can constrain disease fitness and potentially progression. Therefore, niche changes with malignant disease can also serve to protect the host.

LepR+ niche cell-derived AREG compromises hematopoietic stem cell maintenance under conditions of DNA repair deficiency and aging

The cross talk between extrinsic niche-derived and intrinsic hematopoietic stem cell (HSC) factors controlling HSC maintenance remains elusive. Here, we demonstrated that amphiregulin (AREG) from bone marrow (BM) leptin receptor (LepR+) niche cells is an important factor that mediates the cross talk between the BM niche and HSCs in stem cell maintenance. Mice deficient of the DNA repair gene Brca2, specifically in LepR+ cells (LepR-Cre;Brca2fl/fl), exhibited increased frequencies of total and myeloid-biased HSCs. Furthermore, HSCs from LepR-Cre;Brca2fl/fl mice showed compromised repopulation, increased expansion of donor-derived, myeloid-biased HSCs, and increased myeloid output. Brca2-deficient BM LepR+ cells exhibited persistent DNA damage-inducible overproduction of AREG. Ex vivo treatment of wild-type HSCs or systemic treatment of C57BL/6 mice with recombinant AREG impaired repopulation, leading to HSC exhaustion. Conversely, inhibition of AREG by an anti-AREG-neutralizing antibody or deletion of the Areg gene in LepR-Cre;Brca2fl/fl mice rescued HSC defects caused by AREG. Mechanistically, AREG activated the phosphoinositide 3-kinases (PI3K)/AKT/mammalian target of rapamycin (mTOR) pathway, promoted HSC cycling, and compromised HSC quiescence. Finally, we demonstrated that BM LepR+ niche cells from other DNA repair-deficient and aged mice also showed persistent DNA damage-associated overexpression of AREG, which exerts similar negative effects on HSC maintenance. Therefore, we identified an important factor that regulates HSCs function under conditions of DNA repair deficiency and aging.

Ligand-dependent hedgehog signaling maintains an undifferentiated, malignant osteosarcoma phenotype

TP53 and RB1 loss-of-function mutations are common in osteosarcoma. During development, combined loss of TP53 and RB1 function leads to downregulation of autophagy and the aberrant formation of primary cilia, cellular organelles essential for the transmission of canonical Hedgehog (Hh) signaling. Excess cilia formation then leads to hypersensitivity to Hedgehog (Hh) ligand signaling. In mouse and human models, we now show that osteosarcomas with mutations in TP53 and RB1 exhibit enhanced ligand-dependent Hh pathway activation through Smoothened (SMO), a transmembrane signaling molecule required for activation of the canonical Hh pathway. This dependence is mediated by hypersensitivity to Hh ligand and is accompanied by impaired autophagy and increased primary cilia formation and expression of Hh ligand in vivo. Using a conditional genetic mouse model of Trp53 and Rb1 inactivation in osteoblast progenitors, we further show that deletion of Smo converts the highly malignant osteosarcoma phenotype to benign, well differentiated bone tumors. Conversely, conditional overexpression of SHH ligand, or a gain-of-function SMO mutant in committed osteoblast progenitors during development blocks terminal bone differentiation. Finally, we demonstrate that the SMO antagonist sonidegib (LDE225) induces growth arrest and terminal differentiation in vivo in osteosarcomas that express primary cilia and Hh ligand combined with mutations in TP53. These results provide a mechanistic framework for aberrant Hh signaling in osteosarcoma based on defining mutations in the tumor suppressor, TP53.

Analysis of Cellular Crosstalk and Molecular Signal between Periosteum-Derived Precursor Cells and Peripheral Cells During Bone Healing Process Using a Paper-Based Osteogenesis-On-A-Chip Platform

Periosteum-derived progenitor cells (PDPCs) are highly promising cell sources that are indispensable in the bone healing process. Adipose-derived stem cells (ADSCs) are physiologically close to periosteum tissue and release multiple growth factors to promote the bone healing process. Co-culturing PDPCs and ADSCs can construct periosteum-bone tissue microenvironments for the study of cellular crosstalk and molecular signal in the bone healing process. In the current work, a paper-based osteogenesis-on-a-chip platform was successfully developed to provide an in vitro three-dimensional coculture model. The platform was a paper substrate sandwiched between PDPC-hydrogel and ADSC-hydrogel suspensions. Cell secretion could be transferred through the paper substrate from one side to another side. Growth factors including BMP2, TGF-β, POSTN, Wnt proteins, PDGFA, and VEGFA were directly analyzed by a paper-based immunoassay. Cellular crosstalk was studied by protein expression on the paper substrate. Moreover, osteogenesis of PDPCs was investigated by examining the mRNA expressions of PDPCs after culture. Neutralizing and competitive assays were conducted to understand the correlation between growth factors secreted from ADSCs and the osteogenesis of PDPCs. In vitro periosteum-bone tissue microenvironment was established by the paper-based osteogenesis-on-a-chip platform. The proposed approach provides a promising assay of cellular crosstalk and molecular signal in 3D coculture microenvironment that may potentially lead to the development of effective bone regeneration therapy.

GNAS locus: bone related diseases and mouse models

GNASis a complex locus characterized by multiple transcripts and an imprinting effect. It orchestrates a variety of physiological processes via numerous signaling pathways. Human diseases associated with the GNAS gene encompass fibrous dysplasia (FD), Albright's Hereditary Osteodystrophy (AHO), parathyroid hormone(PTH) resistance, and Progressive Osseous Heteroplasia (POH), among others. To facilitate the study of the GNAS locus and its associated diseases, researchers have developed a range of mouse models. In this review, we will systematically explore the GNAS locus, its related signaling pathways, the bone diseases associated with it, and the mouse models pertinent to these bone diseases.

Runt-related Transcription Factors and Gene Regulatory Mechanisms in Skeletal Development and Diseases

PURPOSE OF REVIEW

Runt-related transcription factors (RUNX) play critical roles in skeletal development, metabolism, and diseases. In mammals, three RUNX members, namely RUNX1, RUNX2, and RUNX3, play distinct and redundant roles, although RUNX2 is a dominant factor in skeletal development and several skeletal diseases. This review is to provide an overview of the current understanding of RUNX-mediated transcriptional regulation in different skeletal cell types.

RECENT FINDINGS

Advances in chromatin immunoprecipitation and next-generation sequencing (ChIP-seq) have revealed genome-wide RUNX-mediated gene regulatory mechanisms, including their association with cis-regulatory elements and putative target genes. Further studies with genome-wide analysis and biochemical assays have shed light on RUNX-mediated pioneering action and involvements of RUNX2 in lipid-lipid phase separation. Emerging multi-layered mechanisms of RUNX-mediated gene regulations help us better understanding of skeletal development and diseases, which also provides clues to think how genome-wide studies can help develop therapeutic strategies for skeletal diseases.

Single-Cell Integration of BMD GWAS Results Prioritize Candidate Genes Influencing Age-Related Bone Loss

The regulation of bone mineral density (BMD) is highly influenced by genetics and age. Although genome-wide association studies (GWAS) for BMD have uncovered many genes through their proximity to associated variants (variant nearest-neighbor [VNN] genes), the cell-specific mechanisms of each VNN gene remain unclear. This is primarily due to the inability to prioritize these genes by cell type and age-related expression. Using age-related transcriptomics, we found that the expression of many VNN genes was upregulated in the bone and marrow from aged mice. Candidate genes from GWAS were investigated using single-cell RNA-sequencing (scRNA-seq) datasets to enrich for cell-specific expression signatures. VNN candidate genes are highly enriched in osteo-lineage cells, osteocytes, hypertrophic chondrocytes, and Lepr+ mesenchymal stem cells. These data were used to generate a "blueprint" for Cre-loxp mouse line selection for functional validation of candidate genes and further investigation of their role in BMD maintenance throughout aging. In VNN-gene-enriched cells, Sparc, encoding the extracellular matrix (ECM) protein osteonectin, was robustly expressed. This, along with expression of numerous other ECM genes, indicates that many VNN genes likely have roles in ECM deposition by osteoblasts. Overall, we provide data supporting streamlined translation of GWAS candidate genes to potential novel therapeutic targets for the treatment of osteoporosis. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Perivascular niche cells sense thrombocytopenia and activate hematopoietic stem cells in an IL-1 dependent manner

Hematopoietic stem cells (HSCs) residing in specialized niches in the bone marrow are responsible for the balanced output of multiple short-lived blood cell lineages in steady-state and in response to different challenges. However, feedback mechanisms by which HSCs, through their niches, sense acute losses of specific blood cell lineages remain to be established. While all HSCs replenish platelets, previous studies have shown that a large fraction of HSCs are molecularly primed for the megakaryocyte-platelet lineage and are rapidly recruited into proliferation upon platelet depletion. Platelets normally turnover in an activation-dependent manner, herein mimicked by antibodies inducing platelet activation and depletion. Antibody-mediated platelet activation upregulates expression of Interleukin-1 (IL-1) in platelets, and in bone marrow extracellular fluid in vivo. Genetic experiments demonstrate that rather than IL-1 directly activating HSCs, activation of bone marrow Lepr+ perivascular niche cells expressing IL-1 receptor is critical for the optimal activation of quiescent HSCs upon platelet activation and depletion. These findings identify a feedback mechanism by which activation-induced depletion of a mature blood cell lineage leads to a niche-dependent activation of HSCs to reinstate its homeostasis.