Myocardin-related transcription factor A (MRTFA) regulates the fate of bone marrow mesenchymal stem cells and its absence in mice leads to osteopenia

Discovery and identification of semaphorin 4D as a bioindicator of high fracture incidence in type 2 diabetic mice with glucose control

INTRODUCTION

Bone fracture is increasing in patients with type 2 diabetes mellitus (T2DM) due to skeletal fragility. Most antidiabetics are expected to reduce the incidence of fracture in patients with T2DM, however the results are disappointing. Metformin and GLP-1 receptor agonists have a neutral or minor positive effect in reducing fractures.

OBJECTIVES

We aim to reveal the mechanism of fracture in patients with T2DM treated with metformin or exendin-4, explore the key regulators responsible for bone fragility in T2DM.

METHODS

Trabecular and cortical masses in mice with T2DM were analyzed using micro-computed tomography. Biomechanical strength of bone was determined according to three-point bending, and the expression of bone-associated factors was examined with enzyme-linked immunosorbent assays. Important proteins and miRNAs were identified using proteomics analysis and deep screening analysis. Lastly, immunoprecipitation-mass spectrometry and dual-luciferase reporter analysis were used to identify key molecular signals.

RESULTS

We found that sermaphorin 4D (Sema4D) is the key regulator of bone fragility in T2DM. Exendin-4 increased the biomechanical properties of bone by decreasing serum Sema4D levels, and metformin has little effect on Sema4D. Anti-sema4D treatment could improve bone strength in T2DM mice compared with metformin or exendin-4. The biomechanical properties of bone were comparable between anti-Sema 4D and the combination of metformin and exendin-4. Exendin-4 promoted osteogenesis of BMSCs by activating CRMP2 to reverse the effect of sema4D. Metformin increased miR-140-3p levels, which decreased plexin B1 expression in bone mesenchymal stem cells. Metformin increased the effect of exendin-4 with more GLP-1 receptor expression to increase the biomechanical strength of bone via miR-140-3p-STAT3-miR-3657 signaling.

CONCLUSION

Blood glucose level is not the major factor contributing to impairment in bone remodeling. Sema4D is responsible for the increase in the incidence of bone fractures in T2DM. Accordingly, we proposed an effective therapeutic strategy to eliminate the effect of sema4D.

Single-cell RNA transcriptomics reveals Du-Zhong-Wan promotes osteoporotic fracture healing via YAP/β-catenin/VEGF axis in BMSCs

BACKGROUND

Our previous study demonstrated that Du-Zhong-Wan (DZW) promoted osteoporotic fracture (OPF) healing by enhancing osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) and angiogenesis of endothelial cells (ECs). However, the heterogeneity of BMSCs and ECs, as well as the specific molecular mechanism underlying these effects, still require further evaluation.

PURPOSE

The primary objective of this study was to elucidate the heterogeneity of BMSCs and ECs, as well as the cellular-level mechanism of DZW against OPF through single-cell RNA sequencing.

METHODS

In this study, we presented a single-cell atlas of mouse femoral callus, comparing samples with and without DZW treatment, utilizing single-cell RNA sequencing. Variable genes were identified using the FindVariableGenes (FVG) and principal component analysis (PCA) analysis. Additionally, uniform manifold approximation and projection (U-MAP) was employed to reduce and visualize the distinct subclusters. The CellPhoneDB2 method was employed to analyze intercellular communication and quantify the interaction between ligands and receptors within distinct cell clusters. The osteogenic differentiation capacity of BMSCs was assessed by micro-CT, alkaline phosphatase (ALP), and alizarin red S (ARS) assay. The scratch wound assay and tube formation assay were utilized to assess the angiogenic capabilities of ECs in vitro. Additionally, western blot and immunofluorescence experiments were utilized to elucidate the related protein expression.

RESULTS

Consistent with our previous studies, DZW obviously promoted osteoporotic fracture healing. Moreover, this study discovered 14 cell clusters at the femoral fracture callus, where the BMSCs most actively interacted with ECs, through single-cell sequencing. Notably, DZW significantly elevated the proportion of Lepr+ BMSCs and Podxl+ ECs subgroup, which were respectively considered essential cells for osteoblastogenesis and angiogenesis of arteriolar vessels. The increased proportion of Podxl+ ECs was partially attributed to vascular endothelial growth factor (VEGF), secreted by BMSCs, which were able to be reversed by YAP pharmacological inhibitor verteporfin. Furthermore, the western blot assay revealed elevated expression levels of YAP/β-catenin, VEGF, RUNX2, and OCN in BMSCs treated with DZW, which were counteracted by verteporfin.

CONCLUSION

The data above indicates that DZW elevates the proportion of LEPR+ BMSCs and Podxl+ ECs, therefore contributing for the osteogenic ability of BMSCs and BMSCs-mediated angiogenesis via activation of the YAP/β-catenin/VEGF axis, which provides novel potential targets and mechanism for DZW in treating OPF in sub-clusters and molecular level.

Dental pulp stem cells induce anti-inflammatory phenotypic transformation of macrophages to enhance osteogenic potential via IL-6/GP130/STAT3 signaling

Background

Periodontitis is a major oral condition and current treatment outcomes can be unsatisfactory. Macrophages are essential to the regeneration process, so we investigated the influence of human dental pulp stem cells (hDPSCs) on macrophage differentiation and the microenvironment and the underlying mechanism.

Methods

hDPSCs were isolated from healthy third molars extracted from patients undergoing maxillofacial surgery. The surface antigens CD73, CD45, CD90 and CD11b of the hDPSCs were detected using flow cytometry. hDPSCs were induced for osteogenic and adipogenic differentiation, and the outcome was assessed by alizarin red staining or Oil Red O staining. The IL-6 level released by hDPSCs was measured by enzyme linked immunosorbent assay (ELISA). Tohoku Hospital Pediatrics-1 (THP-1) cells were cultured and induced into macrophages by phorbol-12-myristate-13-acetate. After coculture of THP-1-derived macrophages with hDPSCs, interleukin 6 (IL-6), Argininase-1 (Arg-1), Mannose receptor C-1 (Mrc-1), inducible nitric oxide synthase (iNOS), and tumor necrosis factor-α (TNF-α) levels in the medium were measured using ELISA and quantificational RT-PCR (qRT-PCR). The numbers of CD80⁺ and CD163⁺ macrophages were counted by immunofluorescence, and GP130/STAT3 signaling protein expression was detected. After coculturing the culture medium of hDPSCs with human bone marrow stem cells (BMSCs), scratch assays and transwell assays were performed to evaluate cell migration and invasion.

Results

Alkaline phosphatase (ALP) staining, alizarin red staining, and western blots were performed to assess osteoblast differentiation. The hDPSCs were positive for surface antigens CD73 and CD90 and negative for CD45 and CD11b expression. The level of IL-6 secreted by hDPSCs significantly increased the number of CD80⁺ cells as well as the levels of Arg-1 and Mrc-1. It also promoted M2 macrophage polarization and activated GP130/STAT3 signaling. However, the medium cocultured with THP-1-derived macrophages by hDPSCs facilitated the migration, invasion, and osteogenic abilities of human bone marrow-derived stem cells (hBMSCs).

Conclusions

hDPSCs can regulate the periodontal microenvironment through IL-6 by inducing phenotypic transformation of M2 macrophages and stimulating osteogenic differentiation of BMSCs.

Mechanosensation mediates volume adaptation of cardiac cells and spheroids in 3D

With the adoption of 3-dimensional (3D) cell culture for in vitro modelling of cardiac function and regenerative medicine applications, there is an increased need to understand cardiomyocyte mechanosensation in 3D. With existing studies of cardiomyocyte mechanosensation primarily focussed on the behaviour of individual cells in a 2-Dimensional context, it is unclear whether mechanosensation is the same in a 3D, multicellular context. In this study, H9C2 cardiac-derived myoblasts were encapsulated as individual cells and as cell spheroids within stiffness gradient gelatin methacryloyl (GelMA) hydrogels to investigate individual and collective cardiac cell mechanosensation in 3D. Over a 3.68–17.52 ​kPa stiffness range, it was found that H9C2 cells have a limited capacity to adapt their volume to increasing substrate stiffness, demonstrated by the lack of changes in cell volume and shape across the stiffness gradient. Morphological trends were reflected by the expression of the mechanomarkers YAP, MRTF-A and Lamin-A, which were better correlated with cell and nuclear volume than with substrate stiffness. The localisation of YAP and MRTF-A were dependent on the relative volumes of the cytoplasm and nucleus while Lamin-A expression was elevated with increasing cytoplasmic and nuclear volumes. When cultured as spheroids rather than as individual cells, H9C2 cells adopted a distinct morphology with comparably smaller nuclei than individually cultured cells, while retaining the same overall cell volume. As spheroids, H9C2 cells were sensitive to stiffness cues, shown by decreasing YAP and MRTF-A nuclear localisation, increasing Lamin-A expression, and increasing vinculin expression with increasing substrate stiffness. Like the individually cultured H9C2 cells, mechanomarker expression was correlated to volume adaptation. With increasing cytoplasmic volume, YAP and MRTF-A became less nuclear localised, vinculin expression was increased, and with increasing nuclear volume, the Lamin-A expression fincreased. Together, these data suggest that cardiac cell volume adaptation may be enhanced by cell-cell interactions.

Arp2/3 complex activity is necessary for mouse ESC differentiation, times formative pluripotency, and enables lineage specification

Mouse embryonic stem cells (mESCs), a model for differentiation into primed epiblast-like cells (EpiLCs), have revealed transcriptional and epigenetic control of early embryonic development. The control and significance of morphological changes, however, remain less defined. We show marked changes in morphology and actin architectures during differentiation that depend on Arp2/3 complex but not formin activity. Inhibiting Arp2/3 complex activity pharmacologically or genetically does not block exit from naive pluripotency, but attenuates increases in EpiLC markers. We find that inhibiting Arp2/3 complex activity delays formative pluripotency and causes globally defective lineage specification as indicated by RNA sequencing, with significant effects on TBX3-depedendent transcriptional programs. We also identify two previously unreported indicators of mESC differentiation, namely, MRTF and FHL2, which have inverse Arp2/3 complex-dependent nuclear translocation. Our findings on Arp2/3 complex activity in differentiation and the established role of formins in EMT indicate that these two actin nucleators regulate distinct modes of epithelial plasticity.

Attenuated clinical and osteoclastic phenotypes of Paget's disease of bone linked to the p.Pro392Leu/SQSTM1 mutation by a rare variant in the DOCK6 gene

Background

We identified two families with Paget's disease of bone (PDB) linked to the p.Pro392Leu mutation within the SQSTM1 gene displaying a possible digenism. This study aimed at identifying this second genetic variant cosegregating with the p.Pro392Leu mutation and at characterizing its impact on the clinical and cellular phenotypes of PDB.

Methods

Whole exome sequencing was performed in one patient per family and two healthy controls. We compared clinical characteristics of PDB in 14 relatives from the two families. The osteoclastic phenotype was compared in in vitro differentiated osteoclasts from 31 participants carrying the DOCK6 and/or SQSTM1 variants. Tridimensional models of SQSTM1 and DOCK6 proteins were generated to evaluate the impact of these variants on their stability and flexibility. Statistical analyses were performed with Graphpad prism.

Results

Whole-exome sequencing allowed us to identify the p.Val45Ile missense variant in the DOCK6 gene in patients. In both families, the mean age at PDB diagnosis was delayed in pagetic patients carrier of the p.Val45Ile variant alone compared to those carrying the p.Pro392Leu mutation alone (67 vs. 44 years, P = 0.03). Although both p.Val45Ile and p.Pro392Leu variants gave rise to a pagetic phenotype of osteoclast versus healthy controls, the p.Val45Ile variant was found to attenuate the severity of the osteoclastic phenotype of PDB caused by the p.Pro392Leu mutation when both variants were present. The DOCK6 mRNA expression was higher in carriers of the p.Val45Ile variant than in pagetic patients without any mutations and healthy controls. Structural bioinformatics analyses suggested that the p.Pro392Leu mutation might rigidify the UBA domain and thus decrease its possible intramolecular interaction with a novel domain, the serum response factor–transcription factor (SRF-TF)-like domain, whereas the p.Val45Ile variant may decrease SRF-TF-like activity.

Conclusion

The p.Val45Ile variant may attenuate the severity of the clinical phenotype of PDB in patient carriers of both variants. In vitro, the rare variant of the DOCK6 may have a modifier effect on the p.Pro392Leu mutation, possibly via its effect on the SRF-TF-like.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12920-022-01198-9.

Insight Into Rho Kinase Isoforms in Obesity and Energy Homeostasis

Obesity and associated complications increasingly jeopardize global health and contribute to the rapidly rising prevalence of type 2 diabetes mellitus and obesity-related diseases. Developing novel methods for the prevention and treatment of excess body adipose tissue expansion can make a significant contribution to public health. Rho kinase is a Rho-associated coiled-coil-containing protein kinase (Rho kinase or ROCK). The ROCK family including ROCK1 and ROCK2 has recently emerged as a potential therapeutic target for the treatment of metabolic disorders. Up-regulated ROCK activity has been involved in the pathogenesis of all aspects of metabolic syndrome including obesity, insulin resistance, dyslipidemia and hypertension. The RhoA/ROCK-mediated actin cytoskeleton dynamics have been implicated in both white and beige adipogenesis. Studies using ROCK pan-inhibitors in animal models of obesity, diabetes, and associated complications have demonstrated beneficial outcomes. Studies via genetically modified animal models further established isoform-specific roles of ROCK in the pathogenesis of metabolic disorders including obesity. However, most reported studies have been focused on ROCK1 activity during the past decade. Due to the progress in developing ROCK2-selective inhibitors in recent years, a growing body of evidence indicates more attention should be devoted towards understanding ROCK2 isoform function in metabolism. Hence, studying individual ROCK isoforms to reveal their specific roles and principal mechanisms in white and beige adipogenesis, insulin sensitivity, energy balancing regulation, and obesity development will facilitate significant breakthroughs for systemic treatment with isoform-selective inhibitors. In this review, we give an overview of ROCK functions in the pathogenesis of obesity and insulin resistance with a particular focus on the current understanding of ROCK isoform signaling in white and beige adipogenesis, obesity and thermogenesis in adipose tissue and other major metabolic organs involved in energy homeostasis regulation.

Interrogating cardiac muscle cell mechanobiology on stiffness gradient hydrogels

Extracellular matrix (ECM) remodeling is a major facet of cardiac development and disease, yet our understanding of cardiomyocyte mechanotransduction remains limited. To enhance our understanding of cardiomyocyte mechanosensation, we studied stiffness-driven changes to cell morphology and mechanomarker expression in H9C2 cells and neonatal rat cardiomyocytes (NRCMs). Linear stiffness gradient polyacrylamide hydrogels (2-33 kPa) coated with ECM proteins including Collagen I (Col), Fibronectin (Fn) or Laminin (Ln) were used to represent necrotic, healthy, and infarcted cardiac tissue on a continuous stiffness gradient. Cell size, cell shape and nuclear size were found to be mechanosensitive in H9C2 cells, as was the expression or nuclear translocalization of the mechanomarkers Lamin-A, YAP, and MRTF-A. Minor differences were observed between the different ECM coatings, with the same overarching stiffness-dependent trends being observed across Col, Fn and Ln coated hydrogels. Inhibition of mechanotransduction in H9C2 cells using blebbistatin or Y27632 resulted in disruptions to cell shape, nuclear shape, and nuclear size, however, trends in cell size and mechanomarker expression were not significantly attenuated. Mechanosensation in NRCMs was much less marked, with no significant changes in cell morphology being detected, although YAP did become increasingly nuclear localized with increasing stiffness. In α-actinin positive cells, striations formed with regular structure and frequency at all stiffnesses for Col and Fn coated hydrogels, but not Ln coated gels. In this study, we used our stiffness gradient hydrogels to comprehensively map the relationship between ECM stiffness and cardiac cell phenotype and found that less mature H9C2 cardiac cells are more sensitive to ECM changes than the more developed neonatal cardiomyocytes.

Steering cell behavior through mechanobiology in 3D: A regenerative medicine perspective

Mechanobiology, translating mechanical signals into biological ones, greatly affects cellular behavior. Steering cellular behavior for cell-based regenerative medicine approaches requires a thorough understanding of the orchestrating molecular mechanisms, among which mechanotransducive ones are being more and more elucidated. Because of their wide use and highly mechanotransduction dependent differentiation, this review focuses on mesenchymal stromal cells (MSCs), while also briefly relating the discussed results to other cell types. While the mechanotransduction pathways are relatively well-studied in 2D, much remains unknown of the role and regulation of these pathways in 3D. Ultimately, cells need to be cultured in a 3D environment to create functional de novo tissue. In this review, we explore the literature on the roles of different material properties on cellular behavior and mechanobiology in 2D and 3D. For example, while stiffness plays a dominant role in 2D MSCs differentiation, it seems to be of subordinate importance in 3D MSCs differentiation, where matrix remodeling seems to be key. Also, the role and regulation of some of the main mechanotransduction players are discussed, focusing on MSCs. We have only just begun to fundamentally understand MSCs and other stem cells behavior in 3D and more fundamental research is required to advance biomaterials able to replicate the stem cell niche and control cell activity. This better understanding will contribute to smarter tissue engineering scaffold design and the advancement of regenerative medicine.

Yes-Associated Protein 1: Role and Treatment Prospects in Orthopedic Degenerative Diseases

The Hippo/yes-associated protein 1 signaling pathway is an evolutionarily conserved signaling pathway. This signaling pathway is primarily involved in the regulation of stem cell self-renewal, organ size and tissue regeneration by regulating cell proliferation, differentiation and apoptosis. It plays an important role in embryonic development and tissue organ formation. Yes-associated protein 1 (YAP1) is a key transcription factor in the Hippo signaling pathway and is negatively regulated by this pathway. Changes in YAP1 expression levels affect the occurrence and development of a variety of tumors, but the specific mechanism associated with this phenomenon has not been thoroughly studied. Recently, several studies have described the role of YAP1 in osteoarthritis (OA). Indeed, YAP1 is involved in orthopedic degenerative diseases such as osteoporosis (OP) in addition to OA. In this review, we will summarize the significance of YAP1 in orthopedic degenerative diseases and discuss the potential of the targeted modulation of YAP1 for the treatment of these diseases.

Skeletal glucocorticoid signalling determines leptin resistance and obesity in aging mice

Objective

Aging and chronic glucocorticoid excess share a number of critical features, including the development of central obesity, insulin resistance and osteoporosis. Previous studies have shown that skeletal glucocorticoid signalling increases with aging and that osteoblasts mediate the detrimental skeletal and metabolic effects of chronic glucocorticoid excess. Here, we investigated whether endogenous glucocorticoid action in the skeleton contributes to metabolic dysfunction during normal aging.

Methods

Mice lacking glucocorticoid signalling in osteoblasts and osteocytes (HSD2^OB/OCY-tg mice) and their wild-type littermates were studied until 3, 6, 12 and 18 months of age. Body composition, adipose tissue morphology, skeletal gene expression and glucose/insulin tolerance were assessed at each timepoint. Leptin sensitivity was assessed by arcuate nucleus STAT3 phosphorylation and inhibition of feeding following leptin administration. Tissue-specific glucose uptake and adipose tissue oxygen consumption rate were also measured.

Results

As they aged, wild-type mice became obese and insulin-resistant. In contrast, HSD2^OB/OCY-tg mice remained lean and insulin-sensitive during aging. Obesity in wild-type mice was due to leptin resistance, evidenced by an impaired ability of exogenous leptin to suppress food intake and phosphorylate hypothalamic STAT3, from 6 months of age onwards. In contrast, HSD2^OB/OCY-tg mice remained leptin-sensitive throughout the study. Compared to HSD2^OB/OCY-tg mice, leptin-resistant wild-type mice displayed attenuated sympathetic outflow, with reduced tyrosine hydroxylase expression in both the hypothalamus and thermogenic adipose tissues. Adipose tissue oxygen consumption rate declined progressively in aging wild-type mice but was maintained in HSD2^OB/OCY-tg mice. At 18 months of age, adipose tissue glucose uptake was increased 3.7-fold in HSD2^OB/OCY-tg mice, compared to wild-type mice.

Conclusions

Skeletal glucocorticoid signalling is critical for the development of leptin resistance, obesity and insulin resistance during aging. These findings underscore the skeleton's importance in the regulation of body weight and implicate osteoblastic/osteocytic glucocorticoid signalling in the aetiology of aging-related obesity and metabolic disease.

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As they aged, wild-type CD1 mice became hyperphagic, obese and insulin-resistant.

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Mice lacking skeletal glucocorticoid signalling (HSD2^OB/OCY-tg) were lean and healthy.

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Unlike wild-type mice, HSD2^OB/OCY-tg mice remained leptin-sensitive during aging.

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Adipose tissue sympathetic outflow was maintained in aging HSD2^OB/OCY-tg mice.

Identification of potential specific biomarkers and key signaling pathways between osteogenic and adipogenic differentiation of hBMSCs for osteoporosis therapy

Background

The differentiation of bone mesenchymal stem cells (BMSCs) into adipogenesis (AD) rather than osteogenesis (OS) is an important pathological feature of osteoporosis. Illuminating the detailed mechanisms of the differentiation of BMSCs into OS and AD would contribute to the interpretation of osteoporosis pathology.

Methods

To identify the regulated mechanism in lineage commitment of the BMSCs into OS and AD in the early stages, the gene expression profiles with temporal series were downloaded to reveal the distinct fates when BMSCs adopt a committed lineage. For both OS and AD lineages, the profiles of days 2–4 were compared with day 0 to screen the differentially expressed genes (DEGs), respectively. Next, the functional enrichment analysis was utilized to find out the biological function, and protein-protein interaction network to predict the central genes. Finally, experiments were performed to verify our finding.

Results

FoxO signaling pathway with central genes like FoxO3, IL6, and CAT is the crucial mechanism of OS, while Rap1 signaling pathway of VEGFA and FGF2 enrichment is more significant for AD. Besides, PI3K-Akt signaling pathway might serve as the latent mechanism about the initiation of differentiation of BMSCs into multiple lineages.

Conclusion

Above hub genes and early-responder signaling pathways control osteogenic and adipogenic fates of BMSCs, which maybe mechanistic models clarifying the changes of bone metabolism in the clinical progress of osteoporosis. The findings provide a crucial reference for the prevention and therapy of osteoporosis.

F-actin Regulates Osteoblastic Differentiation of Mesenchymal Stem Cells on TiO2 Nanotubes Through MKL1 and YAP/TAZ

Titanium and titanium alloys are widely used in orthopedic implants. Modifying the nanotopography provides a new strategy to improve osseointegration of titanium substrates. Filamentous actin (F-actin) polymerization, as a mechanical loading structure, is generally considered to be involved in cell migration, endocytosis, cell division, and cell shape maintenance. Whether F-actin is involved and how it functions in nanotube-induced osteogenic differentiation of mesenchymal stem cells (MSCs) remain to be elucidated. In this study, we fabricated TiO₂ nanotubes on the surface of a titanium substrate by anodic oxidation and characterized their features by scanning electron microscopy (SEM), X-ray energy dispersive analysis (EDS), and atomic force microscopy (AFM). Alkaline phosphatase (ALP) staining, Western blotting, qRT-PCR, and immunofluorescence staining were performed to explore the osteogenic potential, the level of F-actin, and the expression of MKL1 and YAP/TAZ. Our results showed that the inner diameter and roughness of TiO₂ nanotubes increased with the increase of the anodic oxidation voltage from 30 to 70 V, while their height was 2 μm consistently. Further, the larger the tube diameter, the stronger the ability of TiO₂ nanotubes to promote osteogenic differentiation of MSCs. Inhibiting F-actin polymerization by Cyto D inhibited osteogenic differentiation of MSCs as well as the expression of proteins contained in focal adhesion complexes such as vinculin (VCL) and focal adhesion kinase (FAK). In contrast, after Jasp treatment, polymerization of F-actin enhanced the expression of RhoA and transcription factors YAP/TAZ. Based on these data, we concluded that TiO₂ nanotubes facilitated the osteogenic differentiation of MSCs, and this ability was enhanced with the increasing diameter of the nanotubes within a certain range (30-70 V). F-actin mediated this process through MKL1 and YAP/TAZ.

Myocardin-Related Transcription Factor A (MRTF-A) Regulates the Balance between Adipogenesis and Osteogenesis of Human Adipose Stem Cells

Previous studies have demonstrated that myocardin-related transcription factor A (MRTF-A) generates a link between the dynamics of the actin cytoskeleton and gene expression with its coregulator, serum response factor (SRF). MRTF-A has also been suggested as a regulator of stem cell differentiation. However, the role of MRTF-A in human mesenchymal stem cell differentiation remains understudied. We aimed to elucidate whether MRTF-A is a potential regulator of human adipose stem cell (hASC) differentiation towards adipogenic and osteogenic lineages. To study the role of MRTF-A activity in the differentiation process, hASCs were cultured in adipogenic and osteogenic media supplemented with inhibitor molecules CCG-1423 or CCG-100602 that have been shown to block the expression of MRTF-A/SRF-activated genes. Our results of image-based quantification of Oil Red O stained lipid droplets and perilipin 1 staining denote that MRTF-A inhibition enhanced the adipogenic differentiation. On the contrary, MRTF-A inhibition led to diminished activity of an early osteogenic marker alkaline phosphatase, and export of extracellular matrix (ECM) proteins collagen type I and osteopontin. Also, quantitative Alizarin Red staining representing ECM mineralization was significantly decreased under MRTF-A inhibition. Image-based analysis of Phalloidin staining revealed that MRTF-A inhibition reduced the F-actin formation and parallel orientation of the actin filaments. Additionally, MRTF-A inhibition affected the protein amounts of α-smooth muscle actin (α-SMA), myosin light chain (MLC), and phosphorylated MLC suggesting that MRTF-A would regulate differentiation through SRF activity. Our results strongly indicate that MRTF-A is an important regulator of the balance between osteogenesis and adipogenesis of hASCs through its role in mediating the cytoskeletal dynamics. These results provide MRTF-A as a new interesting target for guiding the stem cell differentiation in tissue engineering applications for regenerative medicine.

Upregulation of Parkin Accelerates Osteoblastic Differentiation of Bone Marrow-Derived Mesenchymal Stem Cells and Bone Regeneration by Enhancing Autophagy and β-Catenin Signaling

Osteogenic differentiation of bone marrow-derived mesenchymal stem cells (BMSCs) plays a key role in bone formation. Parkin, an E3 ubiquitin ligase, related to Parkinson’s disease and aging. Previous studies have indicated that Parkinson’s disease have a higher risk of osteoporotic fracture. To investigate the effects and underlying mechanism of Parkin in the osteogenic differentiation of BMSCs, osteogenic differentiation was analyzed following upregulation or downregulation of Parkin. We found that Parkin was increased during differentiation. Parkin overexpression enhanced osteo-specific markers, and downregulation of Parkin mitigated osteo-specific markers. Moreover, upregulation of Parkin promoted β-catenin expression and autophagy and vice versa. The upregulation of β-catenin enhanced autophagy, and the activation of autophagy also increased the expression of β-catenin in Parkin-downregulated BMSCs. Parkin-overexpressed cell sheets accelerated bone healing in a tibial fracture model. Based on these results, we concluded that Parkin meditates osteoblastic differentiation of BMSCs via β-catenin and autophagy signaling.

BMP signalling in a mechanical context - Implications for bone biology

Bone Morphogenetic Proteins (BMPs) are extracellular multifunctional signalling cytokines and members of the TGFβ super family. These pleiotropic growth factors crucially promote bone formation, remodeling and healing after injury. Additionally, bone homeostasis is systematically regulated by mechanical inputs from the environment, which are incorporated into the bone cells' biochemical response. These inputs range from compression and tension induced by the movement of neighboring muscle, to fluid shear stress induced by interstitial fluid flow in the canaliculi and in the vascular system. Although BMPs are widely applied in a clinic context to promote fracture healing, it is still elusive how mechanical inputs modulate this signalling pathway, hindering an efficient and side-effect free application of these ligands in bone healing. This review aims to summarize the current understanding in how mechanical cues (tension, compression, shear force and hydrostatic pressure) and substrate stiffness modulate BMP signalling. We highlight the time-dependent effects in modulating immediate early up to long-term effects of mechano-BMP crosstalk during bone formation and remodeling, considering the interplay with other already established mechanosensitive pathways, such as MRTF/SRF and Hippo signalling.

M2 Macrophagy-derived exosomal miRNA-5106 induces bone mesenchymal stem cells towards osteoblastic fate by targeting salt-inducible kinase 2 and 3

BACKGROUND

Osteoblast differentiation is a vital process for fracture healing, and exosomes are nanosized membrane vesicles that can deliver therapeutic drugs easily and safely. Macrophages participate in the regulation of various biological processes in vivo, and macrophage-derived exosomes (MD-Exos) have recently been a topic of increasing research interest. However, few study has explored the link between MD-Exos and osteoblast differentiation. Herein, we sought to identify miRNAs differentially expressed between M1 and M2 macrophage-derived exosomes, and to evaluate their roles in the context of osteoblast differentiation.

RESULTS

We found that microRNA-5106 (miR-5106) was significantly overexpressed in M2 macrophage-derived exosomes (M2D-Exos), while its expression was decreased in M1 macrophage-derived exosomes (M1D-Exos), and we found that this exosomal miRNA can induce bone mesenchymal stem cell (BMSC) osteogenic differentiation via directly targeting the Salt-inducible kinase 2 and 3 (SIK2 and SIK3) genes. In addition, the local injection of both a miR-5106 agonist or M2D-Exos to fracture sites was sufficient to accelerate healing in vivo.

CONCLUSIONS

Our study demonstrates that miR-5106 is highly enriched in M2D-Exos, and that it can be transferred to BMSCs wherein it targets SIK2 and SIK3 genes to promote osteoblast differentiation.

Mechanosensing of Mechanical Confinement by Mesenchymal-Like Cells

Mesenchymal stem cells (MSCs) and tumor cells have the unique capability to migrate out of their native environment and either home or metastasize, respectively, through extremely heterogeneous environments to a distant location. Once there, they can either aid in tissue regrowth or impart an immunomodulatory effect in the case of MSCs, or form secondary tumors in the case of tumor cells. During these journeys, cells experience physically confining forces that impinge on the cell body and the nucleus, ultimately causing a multitude of cellular changes. Most drastically, confining individual MSCs within hydrogels or confining monolayers of MSCs within agarose wells can sway MSC lineage commitment, while applying a confining compressive stress to metastatic tumor cells can increase their invasiveness. In this review, we seek to understand the signaling cascades that occur as cells sense confining forces and how that translates to behavioral changes, including elongated and multinucleated cell morphologies, novel migrational mechanisms, and altered gene expression, leading to a unique MSC secretome that could hold great promise for anti-inflammatory treatments. Through comparison of these altered behaviors, we aim to discern how MSCs alter their lineage selection, while tumor cells may become more aggressive and invasive. Synthesizing this information can be useful for employing MSCs for therapeutic approaches through systemic injections or tissue engineered grafts, and developing improved strategies for metastatic cancer therapies.

Myocardin-Related Transcription Factor A Promotes Recruitment of ITGA5+ Profibrotic Progenitors during Obesity-Induced Adipose Tissue Fibrosis

Adipose tissue fibrosis is associated with inflammation and insulin resistance in human obesity. In particular, visceral fat fibrosis is correlated with hyperlipidemia and ectopic fat accumulation. Myocardin-related transcription factor A (MRTFA) is an important coactivator that mediates the transcription of extracellular matrix and other fibrogenic genes. Here, we examine the role of MRTFA in the development of adipose tissue fibrosis and identify a signaling pathway that regulates the fate of vascular progenitors. We demonstrate that obesity induces the formation of Sca1^-, Sma⁺, ITGA5⁺ fibrogenic progenitor cells (FPCs) in adipose tissue. MRTFA deficiency in mice shifts the fate of perivascular progenitors from FPCs to adipocyte precursor cells and protects against chronic obesity-induced fibrosis and accompanying metabolic dysfunction, without a shift in energy expenditure. Our findings highlight the ITGA5-MRTFA pathway as a potential target to ameliorate obesity-associated metabolic disease.

Cell-Matrix Interactions and Matricrine Signaling in the Pathogenesis of Vascular Calcification

Vascular calcification is a complex pathological process occurring in patients with atherosclerosis, type 2 diabetes, and chronic kidney disease. The extracellular matrix, via matricrine-receptor signaling plays important roles in the pathogenesis of calcification. Calcification is mediated by osteochondrocytic-like cells that arise from transdifferentiating vascular smooth muscle cells. Recent advances in our understanding of the plasticity of vascular smooth muscle cell and other cells of mesenchymal origin have furthered our understanding of how these cells transdifferentiate into osteochondrocytic-like cells in response to environmental cues. In the present review, we examine the role of the extracellular matrix in the regulation of cell behavior and differentiation in the context of vascular calcification. In pathological calcification, the extracellular matrix not only provides a scaffold for mineral deposition, but also acts as an active signaling entity. In recent years, extracellular matrix components have been shown to influence cellular signaling through matrix receptors such as the discoidin domain receptor family, integrins, and elastin receptors, all of which can modulate osteochondrocytic differentiation and calcification. Changes in extracellular matrix stiffness and composition are detected by these receptors which in turn modulate downstream signaling pathways and cytoskeletal dynamics, which are critical to osteogenic differentiation. This review will focus on recent literature that highlights the role of cell-matrix interactions and how they influence cellular behavior, and osteochondrocytic transdifferentiation in the pathogenesis of cardiovascular calcification.

Cell fate decisions: emerging roles for metabolic signals and cell morphology

Understanding how cell fate decisions are regulated is a fundamental goal of developmental and stem cell biology. Most studies on the control of cell fate decisions address the contributions of changes in transcriptional programming, epigenetic modifications, and biochemical differentiation cues. However, recent studies have found that other aspects of cell biology also make important contributions to regulating cell fate decisions. These cues can have a permissive or instructive role and are integrated into the larger network of signaling, functioning both upstream and downstream of developmental signaling pathways. Here, we summarize recent insights into how cell fate decisions are influenced by four aspects of cell biology: metabolism, reactive oxygen species (ROS), intracellular pH (pHi), and cell morphology. For each topic, we discuss how these cell biological cues interact with each other and with protein-based mechanisms for changing gene transcription. In addition, we highlight several questions that remain unanswered in these exciting and relatively new areas of the field.

TGF-β1 regulates the expression and transcriptional activity of TAZ protein via a Smad3-independent, myocardin-related transcription factor-mediated mechanism

Hippo pathway transcriptional coactivators TAZ and YAP and the TGF-β1 (TGFβ) effector Smad3 regulate a common set of genes, can physically interact, and exhibit multilevel cross-talk regulating cell fate-determining and fibrogenic pathways. However, a key aspect of this cross-talk, TGFβ-mediated regulation of TAZ or YAP expression, remains uncharacterized. Here, we show that TGFβ induces robust TAZ but not YAP protein expression in both mesenchymal and epithelial cells. TAZ levels, and to a lesser extent YAP levels, also increased during experimental kidney fibrosis. Pharmacological or genetic inhibition of Smad3 did not prevent the TGFβ-induced TAZ up-regulation, indicating that this canonical pathway is dispensable. In contrast, inhibition of p38 MAPK, its downstream effector MK2 (e.g. by the clinically approved antifibrotic pirferidone), or Akt suppressed the TGFβ-induced TAZ expression. Moreover, TGFβ elevated TAZ mRNA in a p38-dependent manner. Myocardin-related transcription factor (MRTF) was a central mediator of this effect, as MRTF silencing/inhibition abolished the TGFβ-induced TAZ expression. MRTF overexpression drove the TAZ promoter in a CC(A/T-rich)₆GG (CArG) box-dependent manner and induced TAZ protein expression. TGFβ did not act by promoting nuclear MRTF translocation; instead, it triggered p38- and MK2-mediated, Nox4-promoted MRTF phosphorylation and activation. Functionally, higher TAZ levels increased TAZ/TEAD-dependent transcription and primed cells for enhanced TAZ activity upon a second stimulus (i.e. sphingosine 1-phosphate) that induced nuclear TAZ translocation. In conclusion, our results uncover an important aspect of the cross-talk between TGFβ and Hippo signaling, showing that TGFβ induces TAZ via a Smad3-independent, p38- and MRTF-mediated and yet MRTF translocation-independent mechanism.