Mechanical loading activates the YAP/TAZ pathway and chemokine expression in the MLO-Y4 osteocyte-like cell line.

Osteocytes are mechanosensitive cells that control bone remodeling in response to mechanical loading. To date, specific signaling pathways modulated by mechanical loading in osteocytes are not well understood. Yes associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), the main effectors of the Hippo pathway, are reported to play a role in mechanotransduction and during osteoblastogenesis. Here, we hypothesized that YAP/TAZ signaling mediates osteocyte mechanosensing to target genes of the bone remodeling process. We aimed to investigate the contribution of YAP/TAZ in modulating the gene expression in an osteocyte-like cell line MLO-Y4. We developed a 3D osteocyte compression culture model from an MLO-Y4 osteocyte cell line embedded in concentrated collagen hydrogel. 3D-mechanical loading led to the increased expression of mechanosensitive genes and a subset of chemokines, including M-csf, Cxcl1, Cxcl2, Cxcl3, Cxcl9, and Cxcl10. The transcription regulators YAP and TAZ translocated to the nucleus and upregulated their target genes and proteins. RNAseq analysis revealed that YAP/TAZ knockdown mediated the regulation of several genes including osteocyte dendrite formation. Use of YAP/TAZ knockdown partially blunted the increase in M-csf and Cxcl3 levels in response to MLO-Y4 compression. These findings demonstrate that YAP/TAZ signaling is required for osteocyte-like cell mechano-transduction, regulates the gene expression profiles and controls chemokine expression.

Interaction of HIF1α and ß-catenin inhibits matrix metalloproteinase 13 expression and prevents cartilage damage in mice.

Low oxygen tension (hypoxia) regulates chondrocyte differentiation and metabolism. Hypoxia-inducible factor 1α (HIF1α) is a crucial hypoxic factor for chondrocyte growth and survival during development. The major metalloproteinase matrix metalloproteinase 13 (MMP13) is also associated with chondrocyte hypertrophy in adult articular cartilage, the lack of which protects from cartilage degradation and osteoarthritis (OA) in mice. MMP13 is up-regulated by the Wnt/ß-catenin signaling, a pathway involved in chondrocyte catabolism and OA. We studied the role of HIF1α in regulating Wnt signaling in cartilage and OA. We used mice with conditional knockout of Hif1α (∆Hif1α(chon)) with joint instability. Specific loss of HIF1α exacerbated MMP13 expression and cartilage destruction. Analysis of Wnt signaling in hypoxic chondrocytes showed that HIF1α lowered transcription factor 4 (TCF4)-ß-catenin transcriptional activity and inhibited MMP13 expression. Indeed, HIF1α interacting with ß-catenin displaced TCF4 from MMP13 regulatory sequences. Finally, ΔHif1α(chon) mice with OA that were injected intraarticularly with PKF118-310, an inhibitor of TCF4-ß-catenin interaction, showed less cartilage degradation and reduced MMP13 expression in cartilage. Therefore, HIF1α-ß-catenin interaction is a negative regulator of Wnt signaling and MMP13 transcription, thus reducing catabolism in OA. Our study contributes to the understanding of the role of HIF1α in OA and highlights the HIF1α-ß-catenin interaction, thus providing new insights into the impact of hypoxia in articular cartilage.

N-acetylcysteine treatment ameliorates the skeletal phenotype of a mouse model of diastrophic dysplasia.

Diastrophic dysplasia (DTD) is a recessive chondrodysplasia caused by mutations in SLC26A2, a cell membrane sulfate-chloride antiporter. Sulfate uptake impairment results in low cytosolic sulfate, leading to cartilage proteoglycan (PG) undersulfation. In this work, we used the dtd mouse model to study the role of N-acetyl-l-cysteine (NAC), a well-known drug with antioxidant properties, as an intracellular sulfate source for macromolecular sulfation. Because of the important pre-natal phase of skeletal development and growth, we administered 30 g/l NAC in the drinking water to pregnant mice to explore a possible transplacental effect on the fetuses. When cartilage PG sulfation was evaluated by high-performance liquid chromatography disaccharide analysis in dtd newborn mice, a marked increase in PG sulfation was observed in newborns from NAC-treated pregnancies when compared with the placebo group. Morphometric studies of the femur, tibia and ilium after skeletal staining with alcian blue and alizarin red indicated a partial rescue of abnormal bone morphology in dtd newborns from treated females, compared with pups from untreated females. The beneficial effect of increased macromolecular sulfation was confirmed by chondrocyte proliferation studies in cryosections of the tibial epiphysis by proliferating cell nuclear antigen immunohistochemistry: the percentage of proliferating cells, significantly reduced in the placebo group, reached normal values in dtd newborns from NAC-treated females. In conclusion, NAC is a useful source of sulfate for macromolecular sulfation in vivo when extracellular sulfate supply is reduced, confirming the potential of therapeutic approaches with thiol compounds to improve skeletal deformity and short stature in human DTD and related disorders.

Systemic inhibition of IL-6/Stat3 signalling protects against experimental osteoarthritis.

OBJECTIVE: To investigate the impact of systemic inhibition of interleukin 6 (IL-6) or signal transducer and activator of transcription (Stat3) in an experimental model of osteoarthritis (OA). METHODS: Expression of major catabolic and anabolic factors of cartilage was determined in IL-6-treated mouse chondrocytes and cartilage explants. The anti-IL-6-receptor neutralising antibody MR16-1 was used in the destabilisation of the medial meniscus (DMM) mouse model of OA. Stat3 blockade was investigated by the small molecule Stattic ex vivo and in the DMM model. RESULTS: In chondrocytes and cartilage explants, IL-6 treatment reduced proteoglycan content with increased production of matrix metalloproteinase (MMP-3 and MMP-13) and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS-4 and ADAMTS-5). IL-6 induced Stat3 and extracellular signal-regulated kinase (ERK) 1/2 signalling but not p38, c-Jun N-terminal kinase or Akt. In the DMM model, Stat3 was activated in cartilage, but neither in the synovium nor in the subchondral bone. Systemic blockade of IL-6 by MR16-1 alleviated DMM-induced OA cartilage lesions, impaired the osteophyte formation and the extent of synovitis. In the same model, Stattic had similar beneficial effects on cartilage and osteophyte formation. Stattic, but not an ERK1/2 inhibitor, significantly counteracted the catabolic effects of IL-6 on cartilage explants and suppressed the IL-6-induced chondrocytes apoptosis. CONCLUSION: IL-6 induces chondrocyte catabolism mainly via Stat3 signalling, a pathway activated in cartilage from joint subjected to DMM. Systemic blockade of IL-6 or STAT-3 can alleviate DMM-induced OA in mice.

Runx2 Controls Bone Resorption through the Down-Regulation of the Wnt Pathway in Osteoblasts.

The transcription factor Runx2 and the Wnt/ß-catenin pathway are major regulators of bone formation. Our aim was to assess the interactions between the Wnt/ß-catenin pathway and Runx2 that contribute to bone resorption. Our results indicate that the activity of the canonical Wnt/ß-catenin pathway depends on Runx2. Runx2 overexpression inhibited ß-catenin levels and activity in vitro and in vivo. Inhibition of Gsk3b using lithium chloride in Runx2-overexpressing osteoporotic female mice rescued the Wnt/ß-catenin signaling in vivo and completely restored trabecular bone volume by increasing bone formation and decreasing bone resorption. The activation of Wnt/ß-catenin signaling by lithium chloride treatment reduced the number and activity of bone marrow-derived osteoclast-like cells in vitro, suggesting that the restoration of trabecular bone in vivo was due to decreased bone resorption, consistent with the reduced receptor activator of NF-κB ligand/osteoprotegerin ratio in Runx2-overexpressing osteoblasts. Lithium chloride also increased osteoblast differentiation and activity in vitro in agreement with the increase in mineral apposition rate and osteocalcin expression detected in vivo. Our results indicate that the activity of the canonical Wnt/ß-catenin pathway in osteoblast is modulated by Runx2. To conclude, our in vivo and in vitro results highlight the role of Runx2 as a negative regulator of Wnt/ß-catenin pathway activity in osteoblasts and indicate that the abnormal Wnt/ß-catenin activity seen in Runx2 transgenic mice affects both osteoblast and osteoclast differentiation and activity.

Sclerostin and Bone Aging: A Mini-Review.

Sclerostin, mainly produced by osteocytes, is now considered a major regulator of bone formation. Identified from patients with a low bone mass, sclerostin inhibits the Wnt pathway by binding to LRP5/6 and subsequently increases bone formation. Sclerostin may also play a role in the mediation of systemic and local factors such as calcitriol, PTH, glucocorticoids and tumor necrosis factor-alpha. Circulating sclerostin levels increase with age and with the decline of kidney function. However, they are surprisingly higher in patients with a high bone mineral density, suggesting that sclerostin may be a relevant marker of the pool of mature osteocytes. The anti-anabolic properties lead to the development of anti-sclerostin biotherapies that are under current evaluation. The results of these clinical trials will open new promising opportunities for the treatment of osteoporosis and bone fragility fractures.

Dmp1 Promoter-Driven Diphtheria Toxin Receptor Transgene Expression Directs Unforeseen Effects in Multiple Tissues.

Mice harbouring a dentin matrix protein 1 (Dmp1) promoter-driven human diphtheria toxin (DT) receptor (HDTR) transgene (Tg) have recently been used to attain targeted ablation of osteocytes by diphtheria toxin (DT) treatment in order to define osteocyte function. Use of these Tg mice has asserted mechano- and novel paracrine regulatory osteocyte functions. To explore osteocyte roles fully, we sought to confirm the selectivity of DT effects in these transgenic mice. However, our findings revealed incomplete DT-induced osteocyte ablation, prevalent HDTR misexpression, as well as more prominent histopathological DT-induced changes in multiple organs in Tg than in wild-type (WT) littermate mice. Mechanistic evidence for DT action, via prominent regulation of phosphorylation status of elongation factor-2 (EF-2), was also found in many non-skeletal tissues in Tg mice; indicative of direct "off-target" DT action. Finally, very rapid deterioration in health and welfare status in response to DT treatment was observed in these Tg when compared to WT control mice. Together, these data lead us to conclude that alternative models for osteocyte ablation should be sought and caution be exercised when drawing conclusions from experiments using these Tg mice alone.

Enhanced F508del-CFTR channel activity ameliorates bone pathology in murine cystic fibrosis.

In patients with cystic fibrosis (CF), rib and thoracic vertebral fractures can have adverse effects on lung health because the resulting pain and debilitation can impair airway clearance. The F508del mutation in the CF transmembrane conductance regulator (Cftr) gene induces an osteopenic phenotype in humans and mice. N-butyldeoxynojyrimicin (miglustat), an approved drug for treating type 1 Gaucher disease, was found to normalize CFTR-dependent chloride transport in human F508del CFTR lung cells and in nasal mucosa of F508del CF mice. Herein, we investigated whether targeting F508del-CFTR may rescue the skeletal osteopenic phenotype in murine CF. We found that oral administration of low-dose miglustat (120 mg/kg once a day for 28 days) improved bone mass and microarchitecture in the lumbar spine and femur in F508del mice. The increased bone density was associated with an increased bone formation rate and reduced bone resorption. This effect was associated with increased 17ß-estradiol but not with insulin-like growth factor 1 serum levels in miglustat-treated F508del mice. Exposure of primary F508del osteoblasts to miglustat partially restored the deficient CFTR-dependent chloride transport in these bone-forming cells. This study provides evidence that reversal of CFTR-dependent chloride transport in osteoblasts normalizes bone mass and microarchitecture in murine CF. These findings may provide a potential therapeutic strategy to prevent or correct the bone disease in patients with CF.

N-cadherin/wnt interaction controls bone marrow mesenchymal cell fate and bone mass during aging.

Age-related bone loss is characterized by reduced osteoblastogenesis and excessive bone marrow adipogenesis. The mechanisms governing bone marrow mesenchymal stromal cell (BMSC) differentiation into adipocytes or osteoblasts during aging are unknown. We show here that overexpressing N-cadherin (Cadh2) in osteoblasts increased BMSC adipocyte differentiation and reduced osteoblast differentiation in young transgenic (Tg) mice whereas this phenotype was fully reversed with aging. The reversed phenotype with age was associated with enhanced Wnt5a and Wnt10b expression in osteoblasts and a concomitant increase in BMSC osteogenic differentiation. Consistent with this mechanism, conditioned media from young wild type osteoblasts inhibited adipogenesis and promoted osteoblast differentiation in BMSC from old Cadh2 Tg mice, and this response was abolished by Wnt5a and Wnt10b silencing. Transplantation of BMSC from old Cadh2 Tg mice into young Tg recipients increased Wnt5a and Wnt10b expression and rescued BMSC osteogenic differentiation. In senescent osteopenic mice, blocking the CADH2-Wnt interaction using an antagonist peptide increased Wnt5a and Wnt10b expression, bone formation, and bone mass. The data indicate that Cadh2/Wnt interaction in osteoblasts regulates BMSC lineage determination, bone formation, and bone mass and suggest a therapeutic target for promoting bone formation in the aging skeleton.

Cadherin-mediated cell-cell adhesion and signaling in the skeleton.

Direct cell-to-cell interactions via cell adhesion molecules, in particular cadherins, are critical for morphogenesis, tissue architecture, and cell sorting and differentiation. Partially overlapping, yet distinct roles of N-cadherin (cadherin-2) and cadherin-11 in the skeletal system have emerged from mouse genetics and in vitro studies. Both cadherins are important for precursor commitment to the osteogenic lineage, and genetic ablation of Cdh2 and Cdh11 results in skeletal growth defects and impaired bone formation. While Cdh11 defines the osteogenic lineage, persistence of Cdh2 in osteoblasts in vivo actually inhibits their terminal differentiation and impairs bone formation. The action of cadherins involves both cell-cell adhesion and interference with intracellular signaling, and in particular the Wnt/ß-catenin pathway. Both cadherin-2 and cadherin-11 bind to ß-catenin, thus modulating its cytoplasmic pools and transcriptional activity. Recent data demonstrate that cadherin-2 also interferes with Lrp5/6 signaling by sequestering these receptors in inactive pools via axin binding. These data extend the biologic action of cadherins in bone forming cells, and provide novel mechanisms for development of therapeutic strategies aimed at enhancing bone formation.

The F508del mutation in cystic fibrosis transmembrane conductance regulator gene impacts bone formation.

The F508del mutation in the cystic fibrosis transmembrane conductance regulator (Cftr) gene is believed to be an independent risk factor for cystic fibrosis-related bone disease. In this study, we evaluated the bone mineral density as well as the histomorphometric parameters of bone formation and bone mass in both F508del-Cftr homozygous mice (F508del Cftr(tm1Eur)) and Cftr(+/+) littermate controls at 6 (prepubertal), 10 (pubertal), and 14 (young adult) weeks of age in both sexes. The bone architecture of F508del Cftr(tm1Eur) and wild-type (WT) littermate mice was evaluated by bone densitometry, microcomputed tomography, and analysis of the dynamic parameters of bone formation. Serum levels of both insulin-like growth factor 1 and osteocalcin also were determined. Reduced bone mineral density, lower femoral bone mass, and altered trabecular bone architecture were observed in F508del Cftr(tm1Eur) mice compared with controls at 6, 10, and 14 weeks of age. A decrease in the bone formation rate in F508del Cftr(tm1Eur) mice was shown compared with control mice, independently of age and sex. In addition, we found lower insulin-like growth factor 1 levels in F508del Cftr(tm1Eur) mice compared with age-matched controls, whereas osteocalcin levels were normal. Severe osteopenia and altered bone architecture were found in young and mature adult F508del Cftr(tm1Eur) mice. Our findings show that the F508del mutation in CFTR impacts trabecular bone mass by reducing bone formation.

Lin28a induces SOX9 and chondrocyte reprogramming via HMGA2 and blunts cartilage loss in mice.

Articular cartilage has low regenerative capacity despite permanent stress. Irreversible cartilage lesions characterize osteoarthritis (OA); this is not followed by tissue repair. Lin28a, an RNA binding protein, is detected in damaged cartilage in humans and mice. We investigated the role of LIN28a in cartilage physiology and in osteoarthritis. Lin28a-inducible conditional cartilage deletion up-regulated Mmp13 in intact mice and exacerbated the cartilage destruction in OA mice. Lin28a-specific cartilage overexpression protected mice against cartilage breakdown, stimulated chondrocyte proliferation and the expression of Prg4 and Sox9, and down-regulated Mmp13. Lin28a overexpression inhibited Let-7b and Let-7c miRNA levels while RNA-sequencing analysis revealed five genes of transcriptional factors regulated by Let-7. Moreover, Lin28a overexpression up-regulated HMGA2 and activated SOX9 transcription, a factor required for chondrocyte reprogramming. HMGA2 siRNA knockdown inhibited the cartilage protective effect of Lin28a overexpression. This study provides insights into a new pathway including the Lin28a-Let7 axis, thus promoting chondrocyte anabolism in injured cartilage in mice.

Essential role of nuclear factor of activated T cells (NFAT)-mediated Wnt signaling in osteoblast differentiation induced by strontium ranelate.

The antiosteoporotic treatment strontium ranelate (SrRan) was shown to increase bone mass and strength by dissociating bone resorption and bone formation. To identify the molecular mechanisms of action of SrRan on osteoblasts, we investigated its effects on calcineurin-NFAT (nuclear factor of activated T cells) signaling, an important calcium sensitive pathway controlling bone formation. Using murine MC3T3-E1 and primary murine osteoblasts, we demonstrate that SrRan induces NFATc1 nuclear translocation, as shown by immunocytochemical and Western blot analyses. Molecular analysis showed that SrRan increased NFATc1 transactivation in osteoblasts, an effect that was fully abrogated by the calcineurin inhibitors cyclosporin A or FK506, confirming that SrRan activates NFATc1 signaling in osteoblasts. This has functional implications because calcineurin inhibitors blunted the enhanced osteoblast replication and expression of the osteoblast phenotypic markers Runx2, alkaline phosphatase, and type I collagen induced by SrRan. We further found that SrRan increased the expression of Wnt3a and Wnt5a as well as beta-catenin transcriptional activity in osteoblasts, and these effects were abolished by calcineurin inhibitors. The Wnt inhibitors sFRP1 and DKK1 abolished SrRan-induced osteoblast gene expression. Furthermore, blunting the Wnt5a receptor Ryk or RhoA that acts downstream of Ryk abrogated cell proliferation and osteoblast gene expression induced by SrRan. These results indicate that activation of NFATc1 and downstream canonical and non-canonical Wnt signaling pathways mediate SrRan-induced osteoblastic cell replication and differentiation, which provides novel insights into the mechanisms of action of this antiosteoporotic agent in osteoblastogenesis.

YAP/TAZ in Bone and Cartilage Biology.

YAP and TAZ were initially described as the main regulators of organ growth during development and more recently implicated in bone biology. YAP and TAZ are regulated by mechanical and cytoskeletal cues that lead to the control of cell fate in response to the cellular microenvironment. The mechanical component represents a major signal for bone tissue adaptation and remodelling, so YAP/TAZ contributes significantly in bone and cartilage homeostasis. Recently, mice and cellular models have been developed to investigate the precise roles of YAP/TAZ in bone and cartilage cells, and which appear to be crucial. This review provides an overview of YAP/TAZ regulation and function, notably providing new insights into the role of YAP/TAZ in bone biology.

Effect of Rubus idaeus Extracts in Murine Chondrocytes and Explants.

Osteoarthritis is characterized by cartilage loss resulting from the activation of chondrocytes associated with a synovial inflammation. Activated chondrocytes promote an increased secretion of matrix proteases and proinflammatory cytokines leading to cartilage breakdown. Since natural products possess anti-inflammatory properties, we investigated the direct effect of Rubus idaeus extracts (RIE) in chondrocyte metabolism and cartilage loss. The effect of RIE in chondrocyte metabolism was analyzed in murine primary chondrocytes and cartilage explants. We also assessed the contribution of RIE in an inflammation environment by culturing mice primary chondrocytes with the supernatant of Raw 264.7 macrophage-like cells primed with RIE. In primary chondrocytes, RIE diminished chondrocyte hypertrophy (Col10), while increasing the expression of catabolic genes (Mmp-3, Mmp-13) and reducing anabolic genes (Col2a1, Acan). In cartilage explants, Rubus idaeus prevented the loss of proteoglycan (14.84 ± 3.07% loss of proteoglycans with IL1 alone vs. 3.03 ± 1.86% with IL1 and 100 µg/mL of RIE), as well as the NITEGE neoepitope expression. RIE alone reduced the expression of Il1 and Il6 in macrophages, without changes in Tnf and Cox2 expression. The secretome of macrophages pre-treated with RIE and transferred to chondrocytes decreases the gene and protein expression of Mmp-3 and Cox2. In conclusion, these data suggest that RIE may protect from chondrocyte catabolism and cartilage loss in inflammatory conditions. Further evaluations are need before considering RIE as a candidate for the treatment for osteoarthritis.

Heparan sulfate functions are altered in the osteoarthritic cartilage.

BACKGROUND: Heparan sulfate (HS) proteoglycans (PG) may be found at the chondrocyte surface and in the pericellular cartilage matrix, and are involved in cell-cell and cell-matrix interactions. An important function of HS chains is to regulate cell fate through specific interactions with heparin-binding proteins (HBP) modulated by their complex sulfation pattern. Osteoarthritis (OA) is a joint disorder characterized by the degradation of articular cartilaginous extracellular matrix. The aim of this study was to investigate HS structure and functions in osteoarthritic cartilages compared to normal cartilages (controls). METHODS: Glycosaminoglycans (GAG) were extracted from human macroscopically normal cartilages (controls, n = 7) and (OA cartilages n = 11). HS were isolated and quantified using the DMMB quantification method. Their structure and functions were then compared using respectively a HPLC analysis and HBP binding tests and their phenotypic effects on murine chondrocytes were studied by RQ-PCR. Statistical analyzes were performed using a one-way ANOVA followed by a Dunnett's test or a t test for pairwise comparisons. RESULTS: In OA, HS were characterized by increased sulfation levels compared to controls. Moreover, the capacity of these HS to bind HBP involved in the OA pathophysiological process such as FGF2 and VEGF was reduced. Chondroitin sulfates and keratan sulfates regulated these binding properties. Finally, HS from OA cartilages induced the mRNA levels of catabolic markers such as MMP3, MMP13, and TS4 and inhibited the mRNA levels of anabolic markers such as COL2, ACAN, SOX9, and VEGF in murine articular chondrocytes. CONCLUSION: The sulfation of HS chains was increased in OA cartilages with changes in HBP binding properties and biological effects on chondrocyte phenotypes. Thus, modified HS present in altered cartilages could be a novel therapeutic target in OA.

Calcium sensing receptor-dependent and receptor-independent activation of osteoblast replication and survival by strontium ranelate.

Age-related osteopenia is characterized by a negative balance between bone resorption and formation. The anti-osteoporotic drug strontium ranelate was found to reduce bone resorption and to promote bone formation. Here, we investigated the implication of the calcium-sensing receptor (CaSR) in the response to strontium ranelate using osteoblasts from CaSR knockout [CaSR(-/-)] and wild-type [CaSR(+/+)] mice. We showed that calcium and strontium ranelates increased cell replication in [CaSR(-/-)] and [CaSR(+/+)] osteoblasts. Strontium ranelate rapidly increased ERK1/2 phosphorylation in [CaSR(+/+)] but not in [CaSR(-/-)] osteoblasts, indicating that strontium ranelate can act independent of the CaSR/ERK1/2 cascade to promote osteoblast replication. We also showed that strontium ranelate prevented cell apoptosis induced by serum deprivation or the pro-inflammatory cytokines IL-1beta and TNF-alpha in [CaSR(-/-)] and [CaSR(+/+)] osteoblasts, indicating that CaSR is not the only receptor involved in the protective effect of strontium ranelate on osteoblast apoptosis. Strontium ranelate activated the Akt pro-survival pathway in [CaSR(-/-)] and [CaSR(+/+)] osteoblasts, and pharmacological inhibition of Akt abrogated the anti-apoptotic effect of strontium ranelate. Furthermore, both the proliferative and anti-apoptotic effects of strontium ranelate in [CaSR(-/-)] and [CaSR(+/+)] osteoblasts were abrogated by selective inhibition of COX-2. The results provide genetic and biochemical evidence that the effects of strontium ranelate on osteoblast replication and survival involve ERK1/2 and Akt signalling and PGE2 production, independent of CaSR expression. The finding that CaSR-dependent and CaSR-independent pathways mediate the beneficial effects of strontium ranelate on osteoblasts, provides novel insight into the mechanism of action of this anti-osteoporotic agent on osteoblastogenesis.

FHL2 mediates dexamethasone-induced mesenchymal cell differentiation into osteoblasts by activating Wnt/beta-catenin signaling-dependent Runx2 expression.

The differentiation of bone marrow mesenchymal stem cells (MSCs) into osteoblasts is a crucial step in bone formation. However, the mechanisms involved in the early stages of osteogenic differentiation are not well understood. In this study, we identified FHL2, a member of the LIM-only subclass of the LIM protein superfamily, that is up-regulated during early osteoblast differentiation induced by dexamethasone in murine and human MSCs. Gain-of-function studies showed that FHL2 promotes the expression of the osteoblast transcription factor Runx2, alkaline phosphatase, type I collagen, as well as in vitro extracellular matrix mineralization in murine and human mesenchymal cells. Knocking down FHL2 using sh-RNA reduces basal and dexamethasone-induced osteoblast marker gene expression in MSCs. We demonstrate that FHL2 interacts with beta-catenin, a key player involved in bone formation induced by Wnt signaling. FHL2-beta-catenin interaction potentiates beta-catenin nuclear translocation and TCF/LEF transcription, resulting in increased Runx2 and alkaline phosphatase expression, which was inhibited by the Wnt inhibitor DKK1. Reduction of Runx2 transcriptional activity using a mutant Runx2 results in inhibition of FHL2-induced alkaline phosphatase expression in MSCs. These findings reveal that FHL2 acts as an endogenous activator of mesenchymal cell differentiation into osteoblasts and mediates osteogenic differentiation induced by dexamethasone in MSCs through activation of Wnt/beta-catenin signaling- dependent Runx2 expression.

Correction: Osteoblastic heparan sulfate glycosaminoglycans control bone remodeling by regulating Wnt signaling and the crosstalk between bone surface and marrow cells.

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Calpain-6 controls the fate of sarcoma stem cells by promoting autophagy and preventing senescence.

Sarcomas are still unsolved therapeutic challenges. Cancer stem cells are believed to contribute to sarcoma development, but lack of specific markers prevents their characterization and targeting. Here, we show that calpain-6 expression is associated with cancer stem cell features. In mouse models of bone sarcoma, calpain-6-expressing cells have unique tumor-initiating and metastatic capacities. Calpain-6 levels are especially high in tumors that have been successfully propagated in mouse to establish patient-derived xenografts. We found that calpain-6 levels are increased by hypoxia in vitro and calpain-6 is detected within hypoxic areas in tumors. Furthermore, calpain-6 expression depends on the stem cell transcription network that involves Oct4, Nanog, and Sox2 and is activated by hypoxia. Calpain-6 knockdown blocks tumor development in mouse and induces depletion of the cancer stem cell population. Data from transcriptomic analyses reveal that calpain-6 expression in sarcomas inversely correlates with senescence markers. Calpain-6 knockdown suppresses hypoxia-dependent prevention of senescence entry and also promotion of autophagic flux. Together, our results demonstrate that calpain-6 identifies sarcoma cells with stem-like properties and is a mediator of hypoxia to prevent senescence, promote autophagy, and maintain the tumor-initiating cell population. These findings open what we believe is a novel therapeutic avenue for targeting sarcoma stem cells.