Wnt signaling controls the fate of mesenchymal stem cells

Role of Wnt/β-catenin pathway in activation and proliferation of hepatic stellate cells

Liver fibrosis refers to a pathological state in which a variety of pathogenic factors lead to hepatocyte inflammation and necrosis, the imbalance between degradation and deposition of collagen and other extracellular matrix (ECM) molecules, and the abnormal proliferation of liver connective tissue. In the process of the formation of hepatic fibrosis, hepatic stellate cells are the major contributor. Activated hepatic stellate cells secrete extracellular matrix components, and excessive deposition of extracellular matrix is the central event in liver fibrosis. Currently, there have been many articles reporting that the Wnt/β-catenin signaling pathway is involved in organ fibrosis and liver fibrosis, but the underlying mechanism has not been clearly clarified. Some studies have also confirmed that there is a certain relationship between the Wnt/β-catenin pathway and activation of hepatic stellate cells in liver fibrosis.

Periodontal-ligament-derived stem cells exhibit the capacity for long-term survival, self-renewal, and regeneration of multiple tissue types in vivo

Primary periodontal ligament stem cells (PDLSCs) are known to possess multidifferentiation potential and exhibit an immunophenotype similar to that described for bone-marrow-derived mesenchymal stem cells. In the present study, bromo-deoxyuridine (BrdU)-labeled ovine PDLSCs implanted into immunodeficient mice survived after 8 weeks post-transplantation and exhibited the capacity to form bone/cementum-like mineralized tissue, ligament structures similar to Sharpey's fibers with an associated vasculature. To evaluate self-renewal potential, PDLSCs were recovered from harvested primary transplants 8 weeks post-transplantation that exhibit an immunophenotype and multipotential capacity comparable to primary PDLSCs. The re-derived PDLSCs isolated from primary transplants were implanted into secondary ectopic xenogeneic transplants. Histomorphological analysis demonstrated that four out of six donor re-derived PDLSC populations displayed a capacity to survive and form fibrous ligament structures and mineralized tissues associated with vasculature in vivo, although at diminished levels in comparison to primary PDLSCs. Further, the capacity for long-term survival and the potential role of PDLSCs in dental tissue regeneration were determined using an ovine preclinical periodontal defect model. Autologous ex vivo-expanded PDLSCs that were prelabeled with BrdU were seeded onto Gelfoam(®) scaffolds and then transplanted into fenestration defects surgically created in the periodontium of the second premolars. Histological assessment at 8 weeks post-implantation revealed surviving BrdU-positive PDLSCs associated with regenerated periodontium-related tissues, including cementum and bone-like structures. This is the first report to demonstrate the self-renewal capacity of PDLSCs using serial xenogeneic transplants and provides evidence of the long-term survival and tissue contribution of autologous PDLSCs in a preclinical periodontal defect model.

Effects of glutamine on proliferation, migration, and differentiation of human dental pulp cells

INTRODUCTION

Although glutamine (Gln) is mitogenic in various cell types, little is known about its role in human dental pulp cells (HDPCs). This study investigated the effects of Gln on proliferation, migration, and odontoblastic differentiation of HDPCs and the underlying signal pathway mechanisms.

METHODS

Growth and migration were assessed by cell counting and colorimetric cell migration kits. Differentiation was measured as alkaline phosphatase activity, calcified nodule formation by alizarin red staining, and marker mRNA expression by reverse transcriptase-polymerase chain reaction (RT-PCR). Chemokine expression was also evaluated by RT-PCR. Signal transduction pathways were examined by RT-PCR and Western blot analysis.

RESULTS

Gln dose-dependently increased proliferation, migration, alkaline phosphatase activity, mineralized nodule formation, and odontoblast-marker mRNA of HDPCs. Gln also up-regulated expression of interleukin-6, interleukin-8, MCP-1, MIP-3α, CCL2, CCL20, and CXCL1. Gln increased BMP-2 and BMP-4 mRNA, phosphorylation of Smad 1/5/8, β-catenin, and key proteins of the Wnt signaling pathway. Furthermore, Gln resulted in up-regulation of extracellular signal-regulated kinase, p38, and c-Jun N-terminal kinase. In addition, noggin, DKK1, inhibitors of p38, ERK, and JNK significantly attenuatted Gln-induced growth, migration, and odontoblastic differentiation.

CONCLUSIONS

Collectively, this study demonstrated that Gln promoted growth, migration, and differentiation in HDPCs through the BMP-2, Wnt, and MAPK pathways, leading to improved pulp repair and regeneration.

Transforming growth factor β1 signal is crucial for dedifferentiation of cancer cells to cancer stem cells in osteosarcoma

Human osteosarcoma harbors a small subpopulation of cancer stem cells (CSCs) that is believed to be associated with tumor metastasis, radioresistance/chemoresistance, local invasion, and poor clinical outcome. In this study, we found that transforming growth factor β1 (TGF-β1) signaling and a hypoxic environment dramatically induced self-renewal capacity in non-stem osteosarcoma cells, which in turn promoted chemoresistance, tumorigenicity, neovasculogenesis, and metastatic potential. Furthermore, blocking the TGF-β1 signaling pathway resulted in the inhibition of the dedifferentiation and clonogenicity of osteosarcoma cells, and the reduction of CSC self-renewal capacity and hypoxia-mediated dedifferentiation. These findings demonstrate that stem cells and non-stem cells exist in a dynamic equilibrium within the osteosarcoma cell population, and that CSCs may develop de novo from differentiated cancer cells. Hierarchical models of mammalian CSCs, therefore, should be considered to serve as bidirectional interconversion between the stem and non-stem cell components of the tumor.

Akt- and Erk-mediated regulation of proliferation and differentiation during PDGFRβ-induced MSC self-renewal

Understanding the mechanisms that direct mesenchymal stem cell (MSC) self-renewal fate decisions is a key to most tissue regenerative approaches. The aim of this study here was to investigate the mechanisms of action of platelet-derived growth factor receptor β (PDGFRβ) signalling on MSC proliferation and differentiation. MSC were cultured and stimulated with PDGF-BB together with inhibitors of second messenger pathways. Cell proliferation was assessed using ethynyl-2′-deoxyuridine and phosphorylation status of signalling molecules assessed by Western Blots. To assess differentiation potentials, cells were transferred to adipogenic or osteogenic media, and differentiation assessed by expression of differentiation association genes by qRT-PCR, and by long-term culture assays. Our results showed that distinct pathways with opposing actions were activated by PDGF. PI3K/Akt signalling was the main contributor to MSC proliferation in response to activation of PDGFRβ. We also demonstrate a negative feedback mechanism between PI3K/Akt and PDGFR-β expression. In addition, PI3K/Akt downstream signal cascades, mTOR and its associated proteins p70S6K and 4E-BP1 were involved. These pathways induced the expression of cyclin D1, cyclin D3 and CDK6 to promote cell cycle progression and MSC proliferation. In contrast, activation of Erk by PDGFRβ signalling potently inhibited the adipocytic differentiation of MSCs by blocking PPARγ and CEBPα expression. The data suggest that PDGFRβ-induced Akt and Erk pathways regulate opposing fate decisions of proliferation and differentiation to promote MSC self-renewal. Thus, activation of multiple intracellular cascades is required for successful and sustainable MSC self-renewal strategies.

Hyperbaric oxygen promotes osteogenic differentiation of bone marrow stromal cells by regulating Wnt3a/β-catenin signaling--an in vitro and in vivo study

We hypothesized that the effect of hyperbaric oxygen (HBO) on bone formation is increased via osteogenic differentiation of bone marrow stromal cells (BMSCs), which is regulated by Wnt3a/β-catenin signaling. Our in vitro data showed that HBO increased cell proliferation, Wnt3a production, LRP6 phosphorylation, and cyclin D1 expression in osteogenically differentiated BMSCs. The mRNA and protein levels of Wnt3a, β-catenin, and Runx2 were upregulated while those of GSK-3β were downregulated after HBO treatment. The relative density ratio (phospho-protein/protein) of Akt and GSK-3β was both up-regulated while that of β-catenin was down-regulated after HBO treatment. We next investigated whether HBO affects the accumulation of β-catenin. Our Western blot analysis showed increased levels of translocated β-catenin that stimulated the expression of target genes after HBO treatment. HBO increased TCF-dependent transcription, Runx2 promoter/Luc gene activity, and the expression of osteogenic markers of BMSCs, such as alkaline phosphatase activity, type I collagen, osteocalcin, calcium, and the intensity of Alizarin Red staining. HBO dose dependently increased the bone morphogenetic protein (BMP2) and osterix production. We further demonstrated that HBO increased the expression of vacuolar-ATPases, which stimulated Wnt3a secretion from BMSCs. Finally, we showed that the beneficial effects of HBO on bone formation were related to Wnt3a/β-catenin signaling in a rabbit model by histology, mechanical testing, and immunohistochemical assays. Accordingly, we concluded that HBO increased the osteogenic differentiation of BMSCs by regulating Wnt3a secretion and signaling.

Roles of brain and muscle ARNT-like 1 and Wnt antagonist Dkk1 during osteogenesis of bone marrow stromal cells

OBJECTIVES

Many studies have demonstrated that the clock gene, brain and muscle ARNT-like 1 (Bmal1), is directly related to bone ageing by affecting age-related changes to mesenchymal stem cells (MSCs). As a main developmental signal, Wnt may play an important role in this process. Here, we have aimed to elucidate whether Bmal1 positively regulates osteogenesi via Wnt pathways.

MATERIALS AND METHODS

Bone marrow stromal cells were cultured in basic and in osteo-induction medium with Wnt signalling inhibitor Dkk1 and Bmal1 transfection. Proliferation and osteogenesis of MSCs, expression of Bmal1 and activation of Wnt signalling were investigated by flow cytometry, senescence-associated β-galactosidase (SA-β-gal) staining, real-time quantitative PCR and western blot analysis.

RESULTS

Expression of Bmal1 (specially after 7 days osteo-induction), activation of Wnt signalling and osteo-related factors fell significantly during osteo-induction after Dkk1 addition. When cellular Bmal1 was increased by transfection, osteogenesis inhibition by Dkk1 was rescued to a certain extent with activation of Wnt signalling. However, Dkk1 did not significantly affect proliferation or senescence of MSCs during early periods of culture.

CONCLUSION

These findings demonstrated that Bmal1 and Wnt signalling may have a synergistic effect at a particular stage of osteogenesis. Inhibition of Wnt signalling did not greatly affect ageing of MSCs through early passages.

Can mesenchymal stem cells and novel gabapentin-lactam enhance maxillary bone formation?

PURPOSE

Novel gabapentin-lactam (GBP-L) has shown its potency in enhancing new bone formation (NBF) in vitro. The objective of the present preclinical trial was to investigate the in vivo performance of GBP-L.

MATERIALS AND METHODS

Bilateral sinus floor augmentations in 10 adult sheep were conducted. Bovine bone mineral (BBM) and mesenchymal stem cells (MSCs) combined with novel GBP-L were placed into the test sinus of each sheep. The BBM and MSCs alone served as the control on the contralateral side. Simultaneously, 3 dental implants were inserted in each maxillary sinus. The animals were sacrificed after 8 and 16 weeks, and the amount of NBF was analyzed using histomorphometry. The osteogenic potency of the MSCs was demonstrated using the colony-forming unit and differentiation assay. Statistical evaluation was performed using the Wilcoxon signed rank test and 3-factorial nonparametric analysis of variance.

RESULTS

The histologic examination showed NBF in tight contact with the original bone in the control and test groups. The NBF was not significantly different between the test and control sites (P > .05). However, a highly significant difference in NBF between the apical and coronal sites in the specimens from the control and test groups was detected (P < .05). GBP-L did not alter the multipotency of the MSCs or impair NBF.

CONCLUSIONS

Bone formation is initiated from the residual alveolar crest and along the implant. The elected mode of GBP-L application did not induce faster NBF. Alternate forms of application (eg, slow release or systemic administration) might clarify the controversial in vitro findings.

The evolutionary analysis reveals domain fusion of proteins with Frizzled-like CRD domain

Frizzleds (FZDs) are transmembrane receptors in the Wnt signaling pathway and they play pivotal roles in developments. The Frizzled-like extracellular Cysteine-rich domain (Fz-CRD) has been identified in FZDs and other proteins. The origin and evolution of these proteins with Fz-CRD is the main interest of this study. We found that the Fz-CRD exists in FZD, SFRP, RTK, MFRP, CPZ, CORIN, COL18A1 and other proteins. Our systematic analysis revealed that the Fz-CRD domain might have originated in protists and then fused with the Frizzled-like seven-transmembrane domain (7TM) to form the FZD receptors, which duplicated and diversified into about 11 members in Vertebrates. The SFRPs and RTKs with the Fz-CRD were found in sponge and expanded in Vertebrates. Other proteins with Fz-CRD may have emerged during Vertebrate evolution through domain fusion. Moreover, we found a glycosylation site and several conserved motifs in FZDs, which may be related to Wnt interaction. Based on these results, we proposed a model showing that the domain fusion and expansion of Fz-CRD genes occurred in Metazoa and Vertebrates. Our study may help to pave the way for further research on the conservation and diversification of Wnt signaling functions during evolution.

PML overexpression inhibits proliferation and promotes the osteogenic differentiation of human mesenchymal stem cells

The promyelocytic leukemia (PML) gene, as an important tumor-suppressor, has been proven to regulate stem cell function in multiple tissues; however its role in human mesenchymal stem cells (hMSCs) remains unclear. In the present study, the effect of PML on regulating the proliferation and osteogenic differentiation of hMSCs was explored. New downstream genes that may be responsible for the regulation of PML were found, and possible mechanisms were analyzed. The lentiviral vector which encodes full-length human PML cDNA or shRNA against PML was transfected into hMSCs. RT-PCR and western blotting were used to detect mRNA and protein expression. Flow cytometry was used to analyze apoptosis and the cell cycle distribution. Osteogenic differentiation of hMSCs was induced by osteo-inductive medium for 7 to 14 days. cDNA microarray was used to scan the gene expression profile and to identify significant changes in gene expression. In the present study, we found that PML was stably expressed in hMSCs, and the expression was increased time-dependently along with cell osteogenic differentiation. Overexpression of PML inhibited hMSC proliferation by inducing apoptosis and arresting the cell cycle. However, PML enhanced the osteoblast differentiation potential of hMSCs. PML-overexpressing hMSCs had a significant increase in mineralized matrix production and ALP activity on day 7 under osteogenic or non-osteogenic differentiation conditions. Upregulation of integrin-binding sialoprotein (IBSP, bone sialoprotein) induced by PML overexpression was found. Our data indicate that PML regulates hMSCs as an inhibitor of cell proliferation but a promoter of osteogenic differentiation.

Alternative splicing targeting the hTAF4-TAFH domain of TAF4 represses proliferation and accelerates chondrogenic differentiation of human mesenchymal stem cells

Transcription factor IID (TFIID) activity can be regulated by cellular signals to specifically alter transcription of particular subsets of genes. Alternative splicing of TFIID subunits is often the result of external stimulation of upstream signaling pathways. We studied tissue distribution and cellular expression of different splice variants of TFIID subunit TAF4 mRNA and biochemical properties of its isoforms in human mesenchymal stem cells (hMSCs) to reveal the role of different isoforms of TAF4 in the regulation of proliferation and differentiation. Expression of TAF4 transcripts with exons VI or VII deleted, which results in a structurally modified hTAF4-TAFH domain, increases during early differentiation of hMSCs into osteoblasts, adipocytes and chondrocytes. Functional analysis data reveals that TAF4 isoforms with the deleted hTAF4-TAFH domain repress proliferation of hMSCs and preferentially promote chondrogenic differentiation at the expense of other developmental pathways. This study also provides initial data showing possible cross-talks between TAF4 and TP53 activity and switching between canonical and non-canonical WNT signaling in the processes of proliferation and differentiation of hMSCs. We propose that TAF4 isoforms generated by the alternative splicing participate in the conversion of the cellular transcriptional programs from the maintenance of stem cell state to differentiation, particularly differentiation along the chondrogenic pathway.

Premature lung aging and cellular senescence in the pathogenesis of idiopathic pulmonary fibrosis and COPD/emphysema

Different anatomic and physiological changes occur in the lung of aging people that can affect pulmonary functions, and different pulmonary diseases, including deadly diseases such as chronic obstructive pulmonary disease (COPD)/emphysema and idiopathic pulmonary fibrosis (IPF), can be related to an acceleration of the aging process. The individual genetic background, as well as exposure to a variety of toxic substances (cigarette smoke in primis) can contribute significantly to accelerating pulmonary senescence. Premature aging can impair lung function by different ways: by interfering specifically with tissue repair mechanisms after damage, thus perturbing the correct crosstalk between mesenchymal and epithelial components; by inducing systemic and/or local alteration of the immune system, thus impairing the complex mechanisms of lung defense against infections; and by stimulating a local and/or systemic inflammatory condition (inflammaging). According to recently proposed pathogenic models in COPD and IPF, premature cellular senescence likely affects distinct progenitors cells (mesenchymal stem cells in COPD, alveolar epithelial precursors in IPF), leading to stem cell exhaustion. In this review, the large amount of data supporting this pathogenic view are discussed, with emphasis on the possible molecular and cellular mechanisms leading to the severe parenchymal remodeling that characterizes, in different ways, these deadly diseases.

Expression of osteoblast and osteoclast regulatory genes in the bone marrow microenvironment in multiple myeloma: only up-regulation of Wnt inhibitors SFRP3 and DKK1 is associated with lytic bone disease

Multiple myeloma (MM) lytic bone disease (LBD) is caused by osteoclast activation and osteoblast inhibition. RANK/RANKL/OPG play central roles in osteoclast activation and Wnt inhibitor DKK1 in osteoblast inhibition. The role of other Wnt inhibitors is less clear. We evaluated gene expression of osteoclast regulators (RANK, RANKL, OPG, TRAIL, MIP1A), Wnt inhibitors (DKK1, SFRP2, SFRP3, sclerostin, WIF1) and osteoblast transcription factors (RUNX2, osterix) by quantitative reverse transcriptase polymerase chain reaction (RT-PCR) in the bone marrow (BM) microenvironment using snap-frozen BM biopsies, thereby achieving minimal post-sampling manipulation, and gene expression profiling (GEP) data, reflecting the in vivo situation. We analyzed 110 biopsies from newly diagnosed patients with MM and monoclonal gammopathy of unknown significance (MGUS) and healthy volunteers. LBD was evaluated using standard radiographs and the bone resorption marker CTX-1. Protein levels were evaluated by enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry. Among Wnt inhibitors, only SFRP3 and DKK1 were significantly overexpressed in advanced LBD, correlating with protein levels. SFRP3 correlated with CTX-1. Our findings support osteoblast inhibition as the driving force behind MM LBD.

Enhancing osteoconductivity of fibrin gels with apatite-coated polymer microspheres

Fibrin gels are a promising material for use in promoting bone repair and regeneration due to their ease of implant formation, tailorability, biocompatibility, and degradation by natural processes. However, these materials lack necessary osteoconductivity to nucleate calcium, integrate with surrounding bone, and promote bone formation. Polymeric substrata formed from poly(lactide-co-glycolide) (PLG) are widely used in bone tissue engineering. A carbonated apatite layer of bone-like mineral can be successfully grown on the surface of PLG microspheres after a multiday incubation process in modified simulated body fluid. Such coatings improve the osteoconductivity of the polymer, provide nucleation sites for cell-secreted calcium, and enhance the potential osseointegration with host tissue. We examined the capacity of mineralized polymeric microspheres suspended within fibrin hydrogels to enhance the osteoconductivity of fibrin gels and increase the osteogenic potential of these materials. The inclusion of microparticles, both nonmineralized and mineralized, reduced the capacity of mesenchymal stem cells (MSCs) to contract the gel. When cultured in osteogenic media, we detected a near linear increase in both calcium and phosphate incorporation in gels containing mineralized microspheres and entrapped MSCs. The osteoconductivity of acellular fibrin gels with mineralized and nonmineralized microspheres was assessed in a rodent calvarial bone defect over 12 weeks. Compared to untreated rodent calvarial bone defects, we detected significant increases in early vascularization when treated with fibrin gels, with greater vascularization, on average, occurring with gels containing microspheres. We detected a trend for increased bone mineral density in gels containing mineralized microspheres after 12 weeks. These findings demonstrate that the osteoconductivity of fibrin gels can be increased by inclusion of mineralized microspheres, but additional signals may be required to rapidly accelerate bone repair.

Priming 3D cultures of human mesenchymal stromal cells toward cartilage formation via developmental pathways

The field of regenerative medicine has increasingly recognized the importance to be inspired by developmental processes to identify signaling pathways crucial for 3D organogenesis and tissue regeneration. Here, we aimed at recapitulating the first events occurring during limb development (ie, cell condensation and expansion of an undifferentiated mesenchymal cell population) to prime 3D cultures of human bone marrow-derived mesenchymal stromal/stem cells (hBM-MSC) toward the chondrogenic route. Based on embryonic development studies, we hypothesized that Wnt3a and fibroblast growth factor 2 (FGF2) induce hBM-MSC to proliferate in 3D culture as an undifferentiated pool of progenitors (defined by clonogenic capacity and expression of typical markers), retaining chondrogenic potential upon induction by suitable morphogens. hBM-MSC were responsive to Wnt signaling in 3D pellet culture, as assessed by significant upregulation of main target genes and increase of unphosphorylated β-catenin levels. Wnt3a was able to induce a five-fold increase in the number of proliferating hBM-MSC (6.4% vs. 1.3% in the vehicle condition), although total DNA content of the 3D construct was decreasing over time. Preconditioning with Wnt3a improved transforming growth factor-β1 mediated chondrogenesis (30% more glycosaminoglycans/cell in average). In contrast to developmental and 2D MSC culture models, FGF2 antagonized the Wnt-mediated effects. Interestingly, the CD146⁺ subpopulation was found to be more responsive to Wnt3a. The presented data indicate a possible strategy to prime 3D cultures of hBM-MSC by invoking a "developmental engineering" approach. The study also identifies some opportunities and challenges to cross-fertilize skeletal development models and 3D hBM-MSC culture systems.

A role for matrix metalloproteinases in regulating mammary stem cell function via the Wnt signaling pathway

The microenvironment provides cues that control the behavior of epithelial stem and progenitor cells. Here, we identify matrix metalloproteinase-3 (MMP3) as a regulator of Wnt signaling and mammary stem cell (MaSC) activity. We show that MMP3 overexpression promotes hyperplastic epithelial growth, surprisingly, in a nonproteolytic manner via its hemopexin (HPX) domain. We demonstrate that MMP3-HPX specifically binds and inactivates Wnt5b, a noncanonical Wnt ligand that inhibits canonical Wnt signaling and mammary epithelial outgrowth in vivo. Indeed, transplants overexpressing MMP3 display increased canonical Wnt signaling, demonstrating that MMP3 is an extracellular regulator of the Wnt signaling pathway. MMP3-deficient mice exhibit decreased MaSC populations and diminished mammary-reconstituting activity, whereas MMP3 overexpression elevates MaSC function, indicating that MMP3 is necessary for the maintenance of MaSCs. Our study reveals a mechanism by a microenvironmental protease that regulates Wnt signaling and impacts adult epithelial stem cell function.

Differential regulation of stiffness, topography, and dimension of substrates in rat mesenchymal stem cells

The physiological microenvironment of the stem cell niche, including the three factors of stiffness, topography, and dimension, is crucial to stem cell proliferation and differentiation. Although a growing body of evidence is present to elucidate the importance of these factors individually, the interaction of the biophysical parameters of the factors remains insufficiently characterized, particularly for stem cells. To address this issue fully, we applied a micro-fabricated polyacrylamide hydrogel substrate with two elasticities, two topographies, and three dimensions to systematically test proliferation, morphology and spreading, differentiation, and cytoskeletal re-organization of rat bone marrow mesenchymal stem cells (rBMSCs) on twelve cases. An isolated but not combinatory impact of the factors was found regarding the specific functions. Substrate stiffness or dimension is predominant in regulating cell proliferation by fostering cell growth on stiff, unevenly dimensioned substrate. Topography is a key factor for manipulating cell morphology and spreading via the formation of a large spherical shape in a pillar substrate but not in a grooved substrate. Although stiffness leads to osteogenic or neuronal differentiation of rBMSCs on a stiff or soft substrate, respectively, topography or dimension also plays a lesser role in directing cell differentiation. Neither an isolated effect nor a combinatory effect was found for actin or tubulin expression, whereas a seemingly combinatory effect of topography and dimension was found in manipulating vimentin expression. These results further the understandings of stem cell proliferation, morphology, and differentiation in a physiologically mimicking microenvironment.

Mesenchymal stem cell mechanobiology and emerging experimental platforms

Experimental control over progenitor cell lineage specification can be achieved by modulating properties of the cell's microenvironment. These include physical properties of the cell adhesion substrate, such as rigidity, topography and deformation owing to dynamic mechanical forces. Multipotent mesenchymal stem cells (MSCs) generate contractile forces to sense and remodel their extracellular microenvironments and thereby obtain information that directs broad aspects of MSC function, including lineage specification. Various physical factors are important regulators of MSC function, but improved understanding of MSC mechanobiology requires novel experimental platforms. Engineers are bridging this gap by developing tools to control mechanical factors with improved precision and throughput, thereby enabling biological investigation of mechanics-driven MSC function. In this review, we introduce MSC mechanobiology and review emerging cell culture platforms that enable new insights into mechanobiological control of MSCs. Our main goals are to provide engineers and microtechnology developers with an up-to-date description of MSC mechanobiology that is relevant to the design of experimental platforms and to introduce biologists to these emerging platforms.

Mesenchymal Stem Cells in Bone Regeneration

SIGNIFICANCE

Mesenchymal stem cells (MSCs) play a key role in fracture repair by differentiating to become bone-forming osteoblasts and cartilage-forming chondrocytes. Cartilage then serves as a template for additional bone formation through the process of endochondral ossification.

RECENT ADVANCES

Endogenous MSCs that contribute to healing are primarily derived from the periosteum, endosteum, and marrow cavity, but also may be contributed from the overlying muscle or through systemic circulation, depending on the type of injury. A variety of growth factor signaling pathways, including BMP, Wnt, and Notch signaling, influence MSC proliferation and differentiation. These MSCs can be therapeutically manipulated to promote differentiation. Furthermore, MSCs can be harvested, cultivated, and delivered to promote bone healing.

CRITICAL ISSUES

Pharmacologically manipulating the number and differentiation capacity of endogenous MSCs is one potential therapeutic approach to improve healing; however, ideal agents to influence signaling pathways need to be developed and additional therapeutics that activate endogenous MSCs are needed. Whether isolated and purified, MSCs participate directly in the healing process or serve a bystander effect and indirectly influence healing is not well defined.

FUTURE DIRECTIONS

Studies must focus on better understanding the regulation of endogenous MSCs durings fracture healing. This will reveal novel molecules and pathways to therapeutically target. Similarly, while animal models have demonstrated efficacy in the delivery of MSCs to promote healing, more research is needed to understand ideal donor cells, cultivation methods, and delivery before stem cell therapy approaches can be utilized to repair bone.

Expression of Wnt pathway mediators in metaplasic tissue in animal model and clinical samples of tendinopathy

OBJECTIVE

Tissue metaplasia is observed in both ossified failed healing animal model and clinical samples of tendinopathy. The Wnt signalling pathway plays a vital role in pathological calcification. We hypothesized that the Wnt signalling pathway might contribute to tissue metaplasia and failed healing in tendinopathy. This study aimed to examine the spatial-temporal expression of Wnt pathway mediators in an ossified failed tendon healing animal model and clinical samples of tendinopathy. The effect of Wnt3a on the osteogenic differentiation of tendon-derived stem cells (TDSCs) was also examined.

METHODS

Ossified failed tendon healing was induced by the injection of collagenase into the patellar tendon of rats. At various times the tendons were harvested for immunohistochemical staining of Wnt3a, β-catenin, Lrp5 and Tcf1. Patellar tendon samples were obtained from 13 patients with patellar tendinopathy (11 unossified and 2 ossified) and 10 controls. Immunohistochemical staining of Wnt3a, β-catenin, Lrp5 and Tcf1 was similarly performed. Rat patellar TDSCs were treated with Wnt3a. The osteogenic differentiation of TDSCs was examined by ALP activity, alizarin red S staining and mRNA expression of osteogenic markers.

RESULTS

There was increased expression of Wnt3a, β-catenin, Lrp5 and Tcf1 in the healing fibroblast-like cells, chondrocyte-like cells and ossified deposits in the animal model and in some clinical samples of tendinopathy. Wnt3a increased ALP activity, calcium nodule formation and expression of osteogenic markers in TDSCs.

CONCLUSION

Activation of the Wnt signalling pathway and its effect on TDSCs might contribute to tissue metaplasia and failed healing in some cases of tendinopathy.

Global proteomic signature of undifferentiated human bone marrow stromal cells: evidence for donor-to-donor proteome heterogeneity

The clinical application of human bone marrow stromal cells (hBMSCs) largely depends on their capacity to expand in vitro. We have conducted a comprehensive comparative proteomic analysis of culture-expanded hBMSCs obtained from different human donors. The data reveal extensive donor-to-donor proteomic heterogeneity. Processing and database-searching of the tandem MS data resulted in a most comprehensive to date proteomic dataset for hBMSC. A total of 7753 proteins including 712 transcription and translation regulators, 384 kinases, 248 receptor proteins, and 29 cytokines were confidently identified. The proteins identified are mainly nuclear (43.2%) and the share of proteins assigned to more than one subcellular location constitutes 10% of the identified proteome. Bioinformatics tools (IPA, DAVID, and PANTHER) were used to annotate proteins with respect to cellular locations, functions, and other physicochemical characteristics. We also compared the proteomic profile of hBMSCs to recently compiled datasets for human and mouse pluripotent stem cells. The result shows the extent of similarity between the three cell populations and also identified 253 proteins expressed uniformly by all lines of hBMSCs but not reported in the proteomic datasets of the two pluripotent stem cells. Overall, the proteomic database reported in this paper can serve as a reference map for extensive evaluation of hBMSC to explain their biology as well as identify possible marker candidates for further evaluation.

New insights into osteogenic and chondrogenic differentiation of human bone marrow mesenchymal stem cells and their potential clinical applications for bone regeneration in pediatric orthopaedics

Human mesenchymal stem cells (hMSCs) are pluripotent adult stem cells capable of being differentiated into osteoblasts, adipocytes, and chondrocytes. The osteogenic differentiation of hMSCs is regulated either by systemic hormones or by local growth factors able to induce specific intracellular signal pathways that modify the expression and activity of several transcription factors. Runt-related transcription factor 2 (Runx2) and Wnt signaling-related molecules are the major factors critically involved in the osteogenic differentiation process by hMSCs, and SRY-related high-mobility-group (HMG) box transcription factor 9 (SOX9) is involved in the chondrogenic one. hMSCs have generated a great interest in the field of regenerative medicine, particularly in bone regeneration. In this paper, we focused our attention on the molecular mechanisms involved in osteogenic and chondrogenic differentiation of hMSC, and the potential clinical use of hMSCs in osteoarticular pediatric disease characterized by fracture nonunion and pseudarthrosis.

Brain and muscle Arnt-like 1 is a key regulator of myogenesis

The circadian clock network is an evolutionarily conserved mechanism that imparts temporal regulation to diverse biological processes. Brain and muscle Arnt-like 1 (Bmal1), an essential transcriptional activator of the clock, is highly expressed in skeletal muscle. However, whether this key clock component impacts myogenesis, a temporally regulated event that requires the sequential activation of myogenic regulatory factors, is not known. Here we report a novel function of Bmal1 in controlling myogenic differentiation through direct transcriptional activation of components of the canonical Wnt signaling cascade, a major inductive signal for embryonic and postnatal muscle growth. Genetic loss of Bmal1 in mice leads to reduced total muscle mass and Bmal1-deficient primary myoblasts exhibit significantly impaired myogenic differentiation accompanied by markedly blunted expression of key myogenic regulatory factors. Conversely, forced expression of Bmal1 enhances differentiation of C2C12 myoblasts. This cell-autonomous effect of Bmal1 is mediated by Wnt signaling as both expression and activity of Wnt components are markedly attenuated by inhibition of Bmal1, and activation of the Wnt pathway partially rescues the myogenic defect in Bmal1-deficient myoblasts. We further reveal direct association of Bmal1 with promoters of canonical Wnt pathway genes, and as a result of this transcriptional regulation, Wnt signaling components exhibit intrinsic circadian oscillation. Collectively, our study demonstrates that the core clock gene, Bmal1, is a positive regulator of myogenesis, which may represent a temporal regulatory mechanism to fine-tune myocyte differentiation.

Dysfunctional resident lung mesenchymal stem cells contribute to pulmonary microvascular remodeling

Pulmonary vascular remodeling and oxidative stress are common to many adult lung diseases. However, little is known about the relevance of lung mesenchymal stem cells (MSCs) in these processes. We tested the hypothesis that dysfunctional lung MSCs directly participate in remodeling of the microcirculation. We employed a genetic model to deplete extracellular superoxide dismutase (EC-SOD) in lung MSCs coupled with lineage tracing analysis. We crossed ^floxpsod3 and mT/mG reporter mice to a strain expressing Cre recombinase under the control of the ABCG2 promoter. We demonstrated In vivo that depletion of EC-SOD in lung MSCs resulted in their contribution to microvascular remodeling in the smooth muscle actin positive layer. We further characterized lung MSCs to be multipotent vascular precursors, capable of myofibroblast, endothelial and pericyte differentiation in vitro. EC-SOD deficiency in cultured lung MSCs accelerated proliferation and apoptosis, restricted colony-forming ability, multilineage differentiation potential and promoted the transition to a contractile phenotype. Further studies correlated cell dysfunction to alterations in canonical Wnt/β-catenin signaling, which were more evident under conditions of oxidative stress. Our data establish that lung MSCs are a multipotent vascular precursor population, a population which has the capacity to participate in vascular remodeling and their function is likely regulated in part by the Wnt/β-catenin signaling pathway. These studies highlight an important role for microenviromental regulation of multipotent MSC function as well as their potential to contribute to tissue remodeling.

Delivering bioactive molecules as instructive cues to engineered tissues

INTRODUCTION

Growth factors and other bioactive molecules play a crucial role in the creation of functional engineered tissues from dissociated cells.

AREAS COVERED

This review discusses the delivery of bioactive molecules - particularly growth factors - to affect cellular function in the context of tissue engineering. We discuss the primary biological themes that are addressed by delivering bioactives, the types of molecules that are to be delivered, the major materials used in producing scaffolds and/or drug delivery systems, and the principal drug delivery strategies.

EXPERT OPINION

Drug delivery systems have allowed the sustained release of bioactive molecules to engineered tissues, with marked effects on tissue function. Sophisticated drug delivery techniques will allow precise recapitulation of developmental milestones by providing temporally distinct patterns of release of multiple bioactives. High-resolution patterning techniques will allow tissue constructs to be designed with precisely defined areas where bioactives can act. New biological discoveries, just as the development of small molecules with potent effects on cell differentiation, will likely have a marked impact on the field.

Inner ear progenitor cells can be generated in vitro from human bone marrow mesenchymal stem cells

AIM

Mouse mesenchymal stem cells (MSCs) can generate sensory neurons and produce inner ear hair cell-like cells. An equivalent source from humans is highly desirable, given their potential application in patient-specific regenerative therapies for deafness. In this study, we explored the ability of human MSCs (hMSCs) to differentiate into otic lineages.

MATERIALS & METHODS

hMSCs were exposed to culture media conditioned by human fetal auditory stem cells.

RESULTS

Conditioned media induced the expression of otic progenitor markers PAX8, PAX2, GATA3 and SOX2. After 4 weeks, cells coexpressed ATOH1, MYO7A and POU4F3 (indicators of hair cell lineage) or neuronal markers NEUROG1, POU4F1 and NEFH. Inhibition of WNT signaling prevented differentiation into otic progenitors, while WNT activation partially phenocopied results seen with the conditioned media.

CONCLUSION

This study demonstrates that hMSCs can be driven to express key genes found in the otic lineages and thereby promotes their status as candidates for regenerative therapies for deafness.

Microfluidic artificial "vessels" for dynamic mechanical stimulation of mesenchymal stem cells

Cells in the cardiovascular system are constantly exposed to complex mechanical stimulation due to the pulsatile nature of blood flow and the haemodynamic forces that are key to the regulation of vascular development, remodeling and pathophysiology. Mechanical stretch can also modulate the differentiation of stem cells toward vascular cell lineages (i.e., vascular smooth muscle cells), and represent a critical factor in vascular tissue engineering. Here we report on the development of a microchip platform that can emulate several key aspects of the vascular mechanical environment, such as cyclic stimulation and circumferential strain. This chip consists of an array of microfluidic channels with widths ranging from 20 to 500 micrometers. These channels are covered by suspended deformable membranes, on which cells are cultured and stimulated by cyclic circumferential strain of up to 20% via hydrodynamic actuation of the fluid in the microfluidic channels, thereby mimicking the biomechanical conditions of small blood vessels. We show that human mesenchymal stem cells (MSCs) can be cultured and continuously stimulated by cyclic stretch over a period of 7 days with no evidence of device fatigue or performance degradation. We observed localization and alignment of MSCs when mechanical stretch is larger than 10%, indicating the importance of mechanical stimulation in modulating cellular behavior. We further demonstrated simultaneous detection of proteins in multiple signaling pathways, including SMAD1/SMAD2 and canonical Wnt/β-catenin. This microchip represents a generic and versatile platform for high-throughput and rapid screening of cellular responses, including signal transduction cascades, in response to mechanical cues. The system emulates the physiological conditions of blood vessels and other tissues that are subject to cyclic strain, and may have a wide range of applications in the fields of stem cell mechanobiology, vascular tissue engineering, and other areas of regenerative medicine.

Activation of canonical wnt pathway promotes differentiation of mouse bone marrow-derived MSCs into type II alveolar epithelial cells, confers resistance to oxidative stress, and promotes their migration to injured lung tissue in vitro

The differentiation of mesenchymal stem cells (MSCs) into type II alveolar epithelial (AT II) cells in vivo and in vitro, is critical for reepithelization and recovery in acute lung injury (ALI), but the mechanisms responsible for differentiation are unclear. In the present study, we investigated the role of the canonical wnt pathway in the differentiation of mouse bone marrow-derived MSCs (mMSCs) into AT II cells. Using a modified co-culture system with murine lung epithelial-12 (MLE-12) cells and small airway growth media (SAGM) to efficiently drive mMSCs differentiation, we found that GSK 3β and β-catenin in the canonical wnt pathway were up-regulated during differentiation. The levels of surfactant protein (SP) C, SPB, and SPD, the specific markers of AT II cells, correspondingly increased in mMSCs when Wnt3a or LiCl was added to the co-culture system to activate wnt/β-catenin signaling. The expression of these factors was depressed to some extent by inhibiting the pathway with the addition of DKK 1. The differentiation rate of mMSCs also depends on their abilities to accumulate and survive in inflammatory tissue. Our results suggested that the activation of wnt/β-catenin signaling promoted mMSCs migration towards ALI mouse-derived lung tissue in a Transwell assay, and ameliorated the cell death and the reduction of Bcl-2/Bax induced by H(2) O(2), which simultaneously caused reduced GSK 3β and β-catenin in mMSCs. These data supports a potential mechanism for the differentiation of mMSCs into AT II cells involving canonical wnt pathway activation, which may be significant to their application in ALI.

Apc deficiency alters pulmonary epithelial cell fate and inhibits Nkx2.1 via triggering TGF-beta signaling

Wnt signaling is critical for cell fate specification and cell differentiation in many organs, but its function in pulmonary neuroendocrine cell (PNEC) differentiation has not been fully addressed. In this study, we examined the role of canonical Wnt signaling by targeting the gene for Adenomatous Polyposis Coli (Apc), which controls Wnt signaling activity via mediating phosphorylation of beta-catenin (Ctnnb). Targeting the Apc gene in lung epithelial progenitors by Nkx2.1-cre stabilized Ctnnb and activated canonical Wnt signaling. Apc deficiency altered lung epithelial cell fate by inhibiting Clara and ciliated cell differentiation and activating Uchl1, a marker of neuroendocrine cells. Similar to PNEC in normal lung, Uchl1(positive) cells were innervated. In mice with targeted inactivation of Ctnnb by Nkx2.1-cre, PNEC differentiation was not interrupted. These indicate that, after lung primordium formation, Wnt signaling is not essential for PNEC differentiation; however, its over-activation promotes PNEC features. Interestingly, Nkx2.1 was extinguished in Apc deficient epithelial progenitors before activation of Uchl1. Examination of Nkx2.1 null lungs suggested that early deletion of Nkx2.1 inhibits PNEC differentiation, while late repression does not. Nkx2.1 was specifically inhibited in Apc deficient lungs but not in Ctnnb gain-of-function lungs indicating a functional difference between Apc deletion and Ctnnb stabilization, both of which activate Wnt signaling. Further analysis revealed that Apc deficiency led to increased TGF-beta signaling, which inhibited Nkx2.1 in cultured lung endodermal explants. In contrast, TGF-beta activity was not increased in Ctnnb gain-of-function lungs. Therefore, our studies revealed an important mechanism involving Apc and TGF-beta signaling in regulating the key transcriptional factor, Nkx2.1, for lung epithelial progenitor cell fate determination.

TCF3, a novel positive regulator of osteogenesis, plays a crucial role in miR-17 modulating the diverse effect of canonical Wnt signaling in different microenvironments

Wnt signaling pathways are a highly conserved pathway, which plays an important role from the embryonic development to bone formation. The effect of Wnt pathway on osteogenesis relies on their cellular environment and the expression of target genes. However, the molecular mechanism of that remains unclear. On the basis of the preliminary results, we observed the contrary effect of canonical Wnt signaling on osteogenic differentiation of periodontal ligament stem cells (PDLSCs) in the different culture environment. Furthermore, we found that the expression level of miR-17 was also varied with the change in the culture environment. Therefore, we hypothesized that miR-17 and canonical Wnt signaling may have potential interactions, particularly the inner regulation relationship in different microenvironments. In this paper, we observed that canonical Wnt signaling promoted osteogenesis of PDLSCs in the fully culture medium, while inhibited it in the osteogenic differentiation medium. Interestingly, alteration in the expression level of endogenous miR-17 could partially reverse the different effect of canonical Wnt signaling. Furthermore, the role of miR-17 was because of its target gene TCF3 (transcription factor 3), a key transcription factor of canonical Wnt pathway. Overexpression of TCF3 attenuated the effect of miR-17 on modulating canonical Wnt signaling. Finally, we elucidated that TCF3 enhanced osteogenesis both in vitro and in vivo. In brief, the different level of miR-17 was the main cause of the different effect of canonical Wnt signaling, and TCF3 was the crucial node of miR-17–canonial Wnt signaling regulation loop. This understanding of microRNAs regulating signaling pathways in different microenvironments may pave the way for fine-tuning the process of osteogenesis in bone-related disorders.

Isolation and characterization of equine amniotic membrane-derived mesenchymal stem cells

Recent studies have shown that mesenchymal stem cells (MSCs) are able to differentiate into multi-lineage cells such as adipocytes, chondroblasts, and osteoblasts. Amniotic membrane from whole placenta is a good source of stem cells in humans. This membrane can potentially be used for wound healing and corneal surface reconstruction. Moreover, it can be easily obtained after delivery and is usually discarded as classified waste. In the present study, we successfully isolated and characterized equine amniotic membrane-derived mesenchymal stem cells (eAM-MSCs) that were cultured and maintained in low glucose Dulbecco's modified Eagle's medium. The proliferation of eAM-MSCs was measured based on the cumulative population doubling level (CPDL). Immunophenotyping of eAM-MSCs by flow cytometry showed that the major population was of mesenchymal origin. To confirm differentiation potential, a multi-lineage differentiation assay was conducted. We found that under appropriate conditions, eAM-MSCs are capable of multi-lineage differentiation. Our results indicated that eAM-MSCs may be a good source of stem cells, making them potentially useful for veterinary regenerative medicine and cell-based therapy.

SIRT1 regulates differentiation of mesenchymal stem cells by deacetylating β-catenin

Mesenchymal stem cells (MSCs) are multi-potent cells that can differentiate into osteoblasts, adipocytes, chondrocytes and myocytes. This potential declines with aging. We investigated whether the sirtuin SIRT1 had a function in MSCs by creating MSC specific SIRT1 knock-out (MSCKO) mice. Aged MSCKO mice (2.2 years old) showed defects in tissues derived from MSCs; i.e. a reduction in subcutaneous fat, cortical bone thickness and trabecular volume. Young mice showed related but less pronounced effects. MSCs isolated from MSCKO mice showed reduced differentiation towards osteoblasts and chondrocytes in vitro, but no difference in proliferation or apoptosis. Expression of β-catenin targets important for differentiation was reduced in MSCKO cells. Moreover, while β-catenin itself (T41A mutant resistant to cytosolic turnover) accumulated in the nuclei of wild-type MSCs, it was unable to do so in MSCKO cells. However, mutating K49R or K345R in β-catenin to mimic deacetylation restored nuclear localization and differentiation potential in MSCKO cells. We conclude that SIRT1 deacetylates β-catenin to promote its accumulation in the nucleus leading to transcription of genes for MSC differentiation.

Interleukin-1β induces differentiation of human mesenchymal stem cells into osteoblasts via the Wnt-5a/receptor tyrosine kinase-like orphan receptor 2 pathway

OBJECTIVE

Mesenchymal stem cells (MSCs) are considered to be a novel tool for the treatment of rheumatoid arthritis (RA) because of their multipotency to differentiate into osteoblasts and chondrocytes, their immunosuppressive effects, and availability. The aim of this study was to assess the mechanisms of human MSC differentiation into osteoblasts under inflammatory conditions.

METHODS

Human MSCs were cultured in commercialized osteogenic induction medium with inflammatory cytokines for up to 10 days. Osteoblast differentiation was detected by alkaline phosphatase staining and messenger RNA (mRNA) expression of multiple osteoblast markers. Mineralization was assessed by alizarin red S staining.

RESULTS

Among the various cytokines tested, interleukin-1β (IL-1β) induced differentiation of human MSCs into osteoblasts, which was confirmed by alkaline phosphatase activity, expression of RUNX2 mRNA, and strong alizarin red S staining. Among various molecules of the Wnt family, Wnt-5a and receptor tyrosine kinase-like orphan receptor 2 (Ror2), a major receptor of Wnt-5a, were significantly induced in human MSCs by IL-1β. Silencing of either WNT5A or ROR2 by small interfering RNA with 2 different sequences reduced alkaline phosphatase activity, RUNX2 expression, and alizarin red S staining of human MSCs induced by IL-1β.

CONCLUSION

IL-1β effectively and rapidly induced human MSC differentiation into osteoblasts and mineralization, mainly through the noncanonical Wnt-5a/Ror2 pathway. These results suggest potential benefits of IL-1β-treated human MSCs in the treatment of damaged bone as well as in the induction of self-renewal and self-repair of damaged tissue, including osseous tissue.

Curcumin and obesity

Turmeric has been long recognized for its anti-inflammatory and health-promoting properties. Curcumin is one of the principal anti-inflammatory and healthful components of turmeric comprising 2-8% of most turmeric preparations. Experimental evidence supports the activity of curcumin in promoting weight loss and reducing the incidence of obesity-related diseases. With the discovery that obesity is characterized by chronic low-grade metabolic inflammation, phytochemicals like curcumin which have anti-inflammatory activity are being intensely investigated. Recent scientific research reveals that curcumin directly interacts with white adipose tissue to suppress chronic inflammation. In adipose tissue, curcumin inhibits macrophage infiltration and nuclear factor κB (NF-κB) activation induced by inflammatory agents. Curcumin reduces the expression of the potent proinflammatory adipokines tumor necrosis factor-α (TNFα), monocyte chemoattractant protein-1 (MCP-1), and plasminogen activator inhibitor type-1 (PAI-1), and it induces the expression of adiponectin, the principal anti-inflammatory agent secreted by adipocytes. Curcumin also has effects to inhibit adipocyte differentiation and to promote antioxidant activities. Through these diverse mechanisms curcumin reduces obesity and curtails the adverse health effects of obesity.

Mesenchymal stem cells improve cardiac conduction by upregulation of connexin 43 through paracrine signaling

Mesenchymal stem cells (MSCs) were shown to improve cell survival and alleviate cardiac arrhythmias when transplanted into cardiac tissue; however, little is known about the mechanism by which MSCs modify the electrophysiological properties of cardiac tissue. We aimed to distinguish the influence of cell-cell coupling between myocytes and MSCs from that of MSC-derived paracrine factors on the spontaneous activity and conduction velocity (θ) of multicellular cardiomyocyte preparations. HL-1 cells were plated on microelectrode arrays and their spontaneous activity and θ was determined from field potential recordings. In heterocellular cultures of MSCs and HL-1 cells the beating frequency was attenuated (t(0h): 2.26 ± 0.18 Hz; t(4h): 1.98 ± 0.26 Hz; P < 0.01) concomitant to the intercellular coupling between MSCs and cardiomyocytes. In HL-1 monolayers supplemented with MSC conditioned media (ConM) or tyrode (ConT) θ significantly increased in a time-dependent manner (ConT: t(0h): 2.4 cm/s ± 0.2; t(4h): 3.1 ± 0.4 cm/s), whereas the beating frequency remained constant. Connexin (Cx)43 mRNA and protein expression levels also increased after ConM or ConT treatment over the same time period. Enhanced low-density lipoprotein receptor-related protein 6 (LRP6) phosphorylation after ConT treatment implicates the Wnt signaling pathway. Suppression of Wnt secretion from MSCs (IWP-2; 5 μmol/l) reduced the efficacy of ConT to induce phospho-LRP6 and to increase θ. Inhibition of β-catenin (cardamonin; 10 μmol/l) or GSK3-α/β (LiCl; 5 mmol/l) also suppressed changes in θ, further supporting the hypothesis that MSC-mediated Cx43 upregulation occurs in part through secreted Wnt ligands and activation of the canonical Wnt signaling pathway.