RSPO3-LGR4 Regulates Osteogenic Differentiation Of Human Adipose-Derived Stem Cells Via ERK/FGF Signalling

LGR4 (GPR48): The Emerging Inter-Bridge in Osteoimmunology

Leucine-rich repeat-containing G-protein-coupled receptor 4 (LGR4), a member of the G-protein-coupled receptor (GPCR) family, has been implicated in various regulatory functions across multiple differentiation stages and numerous target sites in bone diseases. Therefore, LGR4 is a potential regulator of nuclear factor-κB ligand (RANKL) during osteoclast differentiation. However, a comprehensive investigation of its functions and applications in bone immunology is lacking. This review discusses the molecular characteristics, signaling pathways, and role of LGR4 in osteoimmunology, with a particular focus on its interactions with RANKL during osteoclast differentiation, while identifying gaps that warrant further research.

Early concentrate starter introduction induces rumen epithelial parakeratosis by blocking keratinocyte differentiation with excessive ruminal butyrate accumulation

INTRODUCTION

Rumen epithelial parakeratosis, a common disease in ruminants caused by abnormalities in the ruminal stratified squamous epithelial keratinization process, negatively impacts ruminant health and performance. However, we still lack a comprehensive perception of the underlying mechanisms and the predisposing factors for this disorder.

OBJECTIVES

Here, we investigated rumen epithelial cell heterogeneity, differentiation trajectories, and cornification to clarify the rumen epithelial keratinization process and discern the key ruminal metabolites contributing to rumen epithelial parakeratosis.

METHODS

Twenty-four 14-day-old lambs were divided into three groups, including only milk feeding, milk plus alfalfa hay feeding, and milk plus corn-soybean concentrate starter feeding. At 42 days of age, the lambs were slaughtered, and rumen tissues were collected for single-cell RNA-sequencing (scRNA-seq), immunofluorescence, and quantitative real-time PCR (qRT-PCR) analyses. Ruminal fluid samples were collected for metabolomic analyses. Rumen epithelial organoid was used to verify the key ruminal metabolites contributing to parakeratosis.

RESULTS

As expected, we observed that concentrate starter introduction resulted in rumen epithelial parakeratosis. Moreover, scRNA-seq analysis revealed a developmental impediment in the transition from differentiated keratinocytes to terminally differentiated keratinocytes (TDK) in lambs with concentrate starter introduction. Immunofluorescence and qRT-PCR analyses further verified the location and expression of marker genes of TDK. Metabolomic analysis showed a robust positive correlation between ruminal butyrate levels and rumen epithelial keratinization. More importantly, we successfully established a rumen organoid model capable of facilitating the study of the keratinization process in the rumen epithelia and further confirmed that high dose butyrate indeed contributed to rumen epithelial parakeratosis.

CONCLUSION

Collectively, concentrate starter introduction induces ruminal epithelial parakeratosis by blocking keratinocyte differentiation with excessive ruminal butyrate accumulation in a neonatal lamb model. These findings enhance our understanding of rumen epithelial keratinization and provide valuable insights for addressing rumen epithelial parakeratosis using early nutritional intervention strategies.

Jawbone periosteum-derived cells with high osteogenic potential controlled by R-spondin 3

The jawbone periosteum, the easily accessible tissue responding to bone repair, has been overlooked in the recent development of cell therapy for jawbone defect reconstruction. Therefore, this study aimed to elucidate the in vitro and in vivo biological characteristics of jawbone periosteum-derived cells (jb-PDCs). For this purpose, we harvested the jb-PDCs from 8-week-old C57BL/6 mice. The in vitro cultured jb-PDCs (passages 1 and 3) contained skeletal stem/progenitor cells and exhibited clonogenicity and tri-lineage differentiation capacity. When implanted in vivo, the jb-PDCs (passage 3) showed evident ectopic bone formation after 4-week subcutaneous implantation, and active contribution to repair the critical-size jawbone defects in mice. Molecular profiling suggested that R-spondin 3 was strongly associated with the superior in vitro and in vivo osteogenic potentials of jb-PDCs. Overall, our study highlights the significance of comprehending the biological characteristics of the jawbone periosteum, which could pave the way for innovative cell-based therapies for the reconstruction of jawbone defects.

R-spondin family biology and emerging linkages to cancer

The R-spondin protein family comprises four members (RSPO1-4), which are agonists of the canonical Wnt/β-catenin pathway. Emerging evidence revealed that RSPOs should not only be viewed as agonists of the Wnt/β-catenin pathway but also as regulators for tumor development and progression. Aberrant expression of RSPOs is related to tumorigenesis and tumor development in multiple cancers and their expression of RSPOs has also been correlated with anticancer immune cell signatures. More importantly, the role of RSPOs as potential target therapies and their implication in cancer progressions has been studied in the preclinical and clinical settings. These findings highlight the possible therapeutic value of RSPOs in cancer medicine. However, the expression pattern, effects, and mechanisms of RSPO proteins in cancer remain elusive. Investigating the many roles of RSPOs is likely to expand and improve our understanding of the oncogenic mechanisms mediated by RSPOs. Here, we reviewed the recent advances in the functions and underlying molecular mechanisms of RSPOs in tumor development, cancer microenvironment regulation, and immunity, and discussed the therapeutic potential of targeting RSPOs for cancer treatment. In addition, we also explored the biological feature and clinical relevance of RSPOs in cancer mutagenesis, transcriptional regulation, and immune correlation by bioinformatics analysis.KEY MESSAGESAberrant expressions of RSPOs are detected in various human malignancies and are always correlated with oncogenesis.Although extensive studies of RSPOs have been conducted, their precise molecular mechanism remains poorly understood.Bioinformatic analysis revealed that RSPOs may play a part in the development of the immune composition of the tumor microenvironment.

Endothelial cell-derived RSPO3 activates Gαi1/3-Erk signaling and protects neurons from ischemia/reperfusion injury

The current study explores the potential function and the underlying mechanisms of endothelial cell-derived R-spondin 3 (RSPO3) neuroprotection against ischemia/reperfusion-induced neuronal cell injury. In both neuronal cells (Neuro-2a) and primary murine cortical neurons, pretreatment with RSPO3 ameliorated oxygen and glucose deprivation (OGD)/re-oxygenation (OGD/R)-induced neuronal cell death and oxidative injury. In neurons RSPO3 activated the Akt, Erk and β-Catenin signaling cascade, but only Erk inhibitors reversed RSPO3-induced neuroprotection against OGD/R. In mouse embryonic fibroblasts (MEFs) and neuronal cells, RSPO3-induced LGR4-Gab1-Gαi1/3 association was required for Erk activation, and either silencing or knockout of Gαi1 and Gαi3 abolished RSPO3-induced neuroprotection. In mice, middle cerebral artery occlusion (MCAO) increased RSPO3 expression and Erk activation in ischemic penumbra brain tissues. Endothelial knockdown or knockout of RSPO3 inhibited Erk activation in the ischemic penumbra brain tissues and increased MCAO-induced cerebral ischemic injury in mice. Conversely, endothelial overexpression of RSPO3 ameliorated MCAO-induced cerebral ischemic injury. We conclude that RSPO3 activates Gαi1/3-Erk signaling to protect neuronal cells from ischemia/reperfusion injury.

Wnt-associated adult stem cell marker Lgr6 is required for osteogenesis and fracture healing

Despite the remarkable regenerative capacity of skeletal tissues, nonunion of bone and failure of fractures to heal properly presents a significant clinical concern. Stem and progenitor cells are present in bone and become activated following injury; thus, elucidating mechanisms that promote adult stem cell-mediated healing is important. Wnt-associated adult stem marker Lgr6 is implicated in the regeneration of tissues with well-defined stem cell niches in stem cell-reliant organs. Here, we demonstrate that Lgr6 is dynamically expressed in osteoprogenitors in response to fracture injury. We used an Lgr6-null mouse model and found that Lgr6 expression is necessary for maintaining bone volume and efficient postnatal bone regeneration in adult mice. Skeletal progenitors isolated from Lgr6-null mice have reduced colony-forming potential and reduced osteogenic differentiation capacity due to attenuated cWnt signaling. Lgr6-null mice consist of a lower proportion of self-renewing stem cells. In response to fracture injury, Lgr6-null mice have a deficiency in the proliferation of periosteal progenitors and reduced ALP activity. Further, analysis of the bone regeneration phase and remodeling phase of fracture healing in Lgr6-null mice showed impaired endochondral ossification and decreased mineralization. We propose that in contrast to not being required for successful skeletal development, Lgr6-positive cells have a direct role in endochondral bone repair.

Properties and Printability of the Synthesized Hydrogel Based on GelMA

Gelatin methacryloyl (GelMA) has recently attracted increasing attention. Unlike other hydrogels, it allows for the adjustment of the mechanical properties using such factors as degree of functionalization, concentration, and photocrosslinking parameters. In this study, GelMA with a high degree of substitution (82.75 ± 7.09%) was synthesized, and its suitability for extrusion printing, cytocompatibility, and biocompatibility was studied. Satisfactory printing quality was demonstrated with the 15% concentration hydrogel. The high degree of functionalization led to a decrease in the ability of human adipose-derived stem cells (ADSCs) to adhere to the GelMA surface. During the first 3 days after sowing, proliferation was observed. Degradation in animals after subcutaneous implantation was slowed down.

Nidogen-2 is a Novel Endogenous Ligand of LGR4 to Inhibit Vascular Calcification

BACKGROUND

Vascular calcification is closely related to the all-cause mortality of cardiovascular events. Basement membrane protein nidogen-2 is a key component of the vascular extracellular matrix microenvironment and we recently found it is pivotal for the maintenance of contractile phenotype in vascular smooth muscle cells (VSMCs). However, whether nidogen-2 is involved in VSMCs osteochondrogenic transition and vascular calcification remains unclear.

METHODS

VSMCs was treated with high-phosphate to study VSMC calcification in vitro. Three different mice models (5/6 nephrectomy-induced chronic renal failure, cholecalciferol-overload, and periadventitially administered with CaCl₂) were used to study vascular calcification in vivo. Membrane protein interactome, coimmunoprecipitation, flow cytometric binding assay, surface plasmon resonance, G protein signaling, VSMCs calcium assays were performed to clarify the phenotype and elucidate the molecular mechanisms.

RESULTS

Nidogen-2 protein levels were significantly reduced in calcified VSMCs and aortas from mice in different vascular calcification model. Nidogen-2 deficiency exacerbated high-phosphate-induced VSMC calcification, whereas the addition of purified nidogen-2 protein markedly alleviated VSMC calcification in vitro. Nidogen-2^-/- mice exhibited aggravated aorta calcification compared to wild-type (WT) mice in response to 5/6 nephrectomy, cholecalciferol-overload, and CaCl₂ administration. Further unbiased coimmunoprecipitation and interactome analysis of purified nidogen-2 and membrane protein in VSMCs revealed that nidogen-2 directly binds to LGR4 (leucine-rich repeat G-protein-coupled receptor 4) with K_D value 26.77 nM. LGR4 deficiency in VSMCs in vitro or in vivo abolished the protective effect of nidogen-2 on vascular calcification. Of interest, nidogen-2 biased activated LGR4-Gαq-PKCα (protein kinase Cα)-AMPKα1 (AMP-activated protein kinase α1) signaling to counteract VSMCs osteogenic transition and mineralization.

CONCLUSIONS

Nidogen-2 is a novel endogenous ligand of LGR4 that biased activated Gαq- PKCα-AMPKα1 signaling and inhibited vascular calcification.

MiR-137-mediated negative relationship between LGR4 and RANKL modulated osteogenic differentiation of human adipose-derived mesenchymal stem cells

MicroRNA-137 (miR-137) has recently emerged as an osteogenic regulator in several cell lines. This study aimed to identify the function of miR-137 on the crosstalk between leucine rich repeat containing G protein-coupled receptor 4 (LGR4) and receptor activator of nuclear factor-κB ligand (RANKL), thus unveiling the critical role of LGR4-RANKL interplay in the osteogenic differentiation of human adipose-derived mesenchymal stem cells (hASCs). By examining the osteogenic capacity and possible downstream genes expression with miR-137 overexpression/knockdown, we found that miR-137 downregulated LGR4 while upregulating RANKL. According to the results of dual-luciferase reporter assay, LGR4 was validated as a direct target of miR-137. Surprisingly, a negative relationship between LGR4 and RANKL was confirmed by the knockdown of these two genes. Furthermore, RANKL inhibitor could alleviate or reverse the inhibitory effects on osteogenesis generated by LGR4 knockdown. Collectively, this study indicated that miR-137-induced a negative crosstalk between LGR4 and RANKL that could contribute to the osteogenic regulation of hASCs and provide more systematic and in-depth understanding of epigenetic modulation by miR-137.

Low levels of cadmium exposure affect bone by inhibiting Lgr4 expression in osteoblasts and osteoclasts

BACKGROUND

Cadmium exposure is associated with bone loss. However, the mechanisms involved have not yet been fully understood. Leucine-rich repeat containing GPCR-4 (LGR4) can bind with the receptor activator of nuclear factors κB ligand (RANKL) and inhibit osteoclast formation. In addition, Lgr4 plays an important role in maintaining osteoblast activity. In the present study the effect of cadmium exposure on bone was investigated in terms of Lgr4 expression.

METHODS

Raw 264.7 cells and primary osteoblasts were exposed to cadmium (0-60 nM/L). The effects of cadmium on osteoclast formation and osteoblast activity were investigated. Osteoclast differentiation-related (Traf6, NFATc1) and osteoblast-related (RANKL; osteoprotegerin, OPG) gene and protein expression were determined. Lgr4 expression in osteoclasts and osteoblasts were also determined. A rat model was established to show the effects of cadmium (50 mg/L) on bone loss and Lgr4 expression in vivo.

RESULTS

Cadmium exposure inhibited osteoblast activities and stimulated osteoclast formation. Cadmium exposure also inhibited Lgr4 expression in both osteoclasts and osteoblasts. Low dose of RANKL added to the culture medium could promote osteoclast formation in cadmium-pretreated RAW264.7 cells. Blocking Lgr4 in osteoclasts only slightly inhibited cadmium-induced osteoclast formation in cadmium-pretreated RAW264.7 cells. Cadmium significantly upregulated the AKT/ERK signaling pathway. An in vivo study showed that cadmium exposure promoted osteoclast formation and inhibited Lgr4 expression.

CONCLUSIONS

Our data indicates that cadmium may induce bone loss by inhibiting Lgr4-related bone formation and promoting Lgr4-related osteoclast formation.

The role of roof plate-specific spondins in liver homeostasis and disease

As evolutionarily conserved signals, roof plate-specific spondins (R-spondins; RSPOs) are a family with four members (RSPO1-4) exerting distinctly different functions. RSPOs have five receptors and correlate with different signaling pathways through these receptors and then perform various functions. Moreover, their best-known molecular function is the capacity to enhance WNT signaling pathways, which play critical roles in several processes. A recent study shows that RSPOs not only potentiate the WNT/beta (β)-catenin signaling pathway but are also involved in the WNT/planar cell polarity signaling pathway. RSPOs influence liver homeostasis and the development of multiple liver diseases. RSPO1 increases cell proliferation, protects hepatocytes from injury, improves liver regenerative potential, and affects liver metabolic zonation. RSPO2 not only regulates proliferation-associated genes and promotes differentiation in the liver but also participates in liver fibrosis through the WNT/β-catenin signaling pathway. RSPO3 is a key determinant of proper liver function, such as promoting hepatocyte regeneration and maintaining liver zonation. RSPO3 is upregulated in liver fibrosis and livers of patients with non-alcoholic steatohepatitis. Besides, RSPO2 and RSPO3 are confirmed as oncogenes and involved in the occurrence of liver cancer. The role of RSPO4 in the liver remains unclear. In this review, the structural and biochemical properties of RSPOs and their receptors and their roles in liver homeostasis and disease are summarized.

Identification of potential miRNA-mRNA regulatory network and the key miRNAs in intramuscular and subcutaneous adipose

Intramuscular fat (IMF) is an important indicator for evaluating meat quality. Breeds with high IMF content are often accompanied by high subcutaneous fat (SCF), severely affecting the meat rate of pigs. Studying the mechanisms of miRNAs related to lipogenesis and lipid metabolism has important implications for pig breeding. We constructed two small RNA libraries from intramuscular and subcutaneous fat to evaluate the patterns of lipogenesis in Laiwu pig, a Chinese breed. A total of 286 differentially expressed miRNAs (DEmiRNAs), including 193 known miRNA and 93 novel miRNAs, were identified from two types of adipose. GO and KEGG enrichment analysis for DEmiRNAs showed that their target genes involved in many adipogenesis and lipid metabolism biological processes and signaling pathways, such as Wnt signaling pathway,MAPK signaling pathway, Hippo signaling pathway, PI3K-Akt signaling pathway, Melanogenesis, Signaling pathways regulating pluripotency of stem cells and so on. Then, we constructed a miRNA-mRNA interaction network to find out which miRNAs were the key miRNAs of regulation in Wnt signaling pathway. In this pathway, miR-331-3p, miR-339-5p, miR-874 and novel346_mature target PPARD, WNT10B, RSPO3, WNT2B. This study provides a theoretical basis for further understanding the post-transcriptional regulation mechanism of meat quality formation and predicting and treating diseases caused by ectopic fat.

Identifying Causes of Fracture Beyond Bone Mineral Density: Evidence From Human Genetics

New therapies may help to prevent osteoporotic fractures other than through increasing bone mineral density (BMD). Because fracture risk has an important genetic component, we aim to identify loci increasing fracture risk that do not decrease BMD, using a recently-proposed structural equation model adapted to remove genetic influences of BMD on fracture risk. We used summary statistics of the largest genome-wide association studies (GWASs) for BMD and for fracture in these analyses. We next estimated the genetic correlation between the non-BMD or BMD-related genetic effects and other clinical risk factors for fracture. Last, based on white British participants in the UK Biobank, we conducted genetic risk score analyses to assess whether the aggregated genetic effects conferred increased major osteoporotic fracture risk. We found that only three loci affecting fracture risk exhibited genetic effects not mediated by BMD: SOST, CPED1-WNT16, and RSPO3, while these three loci simultaneously conferred BMD-related effects. No strong genetic associations between non-BMD or BMD-related effects and 16 clinical risk factors were observed. However, non-BMD effects might be genetic correlated with hip bone size. In the UK Biobank, a 1 standard deviation (1-SD) increase in the non-BMD genetic risk score conferred an odds ratio of 1.17 for incident major osteoporotic fracture, compared to 1.29 by a BMD-related genetic risk score. Our study suggests that the majority of common genetic predisposition toward fracture risk acts upon BMD. Although non-BMD genetic effects may exist, they are not strongly correlated with most traditional clinical risk factors. Risk loci harboring non-BMD genetic effects may influence other perspectives of bone quality, or confer effects that existing GWASs fail to capture, but they demonstrate weaker impact on fracture risk than BMD-related genetic effects. These findings suggest that most successful drug development programs for osteoporosis should focus on pathways identified through BMD-associated loci. © 2022 American Society for Bone and Mineral Research (ASBMR).

Integrated single-cell analyses decode the developmental landscape of the human fetal spine

The spine has essential roles in supporting body weight, and passaging the neural elements between the body and the brain. In this study, we used integrated single-cell RNA sequencing and single-cell transposase-accessible chromatin sequencing analyses to reveal the cellular heterogeneity, lineage, and transcriptional regulatory network of the developing human spine. We found that EPYC + HAPLN1+ fibroblasts with stem cell characteristics could differentiate into chondrocytes by highly expressing the chondrogenic markers SOX9 and MATN4. Neurons could originate from neuroendocrine cells, and MEIS2 may be an essential transcription factor that promotes spinal neural progenitor cells to selectively differentiate into neurons during early gestation. Furthermore, the interaction of NRP2_SEMA3C and CD74_APP between macrophages and neurons may be essential for spinal cord development. Our integrated map provides a blueprint for understanding human spine development in the early and midgestational stages at single-cell resolution and offers a tool for investigating related diseases.

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scRNA-seq and scATAC-seq analyses reveal the developmental landscape of the fetal spine

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Chondrocytes may originate from EPYC + HAPLN1+ fibroblasts with stem cell characteristics

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Neurons may originate from neuroendocrine cells with regulation by MEIS2

Cartilage Formation In Vivo Using High Concentration Collagen-Based Bioink with MSC and Decellularized ECM Granules

The aim of this study was to verify the applicability of high-concentration collagen-based bioink with MSC (ADSC) and decellularized ECM granules for the formation of cartilage tissue de novo after subcutaneous implantation of the scaffolds in rats. The printability of the bioink (4% collagen, 2.5% decellularized ECM granules, derived via 280 μm sieve) was shown. Three collagen-based compositions were studied: (1) with ECM; (2) with MSC; (3) with ECM and MSC. It has been established that decellularized ECM granules are able to stimulate chondrogenesis both in cell-free and MSC-laden scaffolds. Undesirable effects have been identified: bone formation as well as cartilage formation outside of the scaffold area. The key perspectives and limitations of ECM granules (powder) application have been discussed.

Emerging Roles for LGR4 in Organ Development, Energy Metabolism and Carcinogenesis

The leucine-rich repeats containing G protein-coupled receptor 4 (LGR4) belonging to G protein-coupled receptors (GPCRs) family, had various regulatory roles at multiple cellular types and numerous targeting sites, and aberrant LGR4 signaling played crucial roles in diseases and carcinogenesis. On the basis of these facts, LGR4 may become an appealing therapeutic target for the treatment of diseases and tumors. However, a comprehensive investigation of its functions and applications was still lacking. Hence, this paper provided an overview of the molecular characteristics and signaling mechanisms of LGR4, its involvement in multiple organ development and participation in the modulation of immunology related diseases, metabolic diseases, and oxidative stress damage along with cancer progression. Given that GPCRs accounted for almost a third of current clinical drug targets, the in-depth understanding of the sophisticated connections of LGR4 and its ligands would not only enrich their regulatory networks, but also shed new light on designing novel molecular targeted drugs and small molecule blockers for revolutionizing the treatment of various diseases and tumors.

Wnt Pathway Extracellular Components and Their Essential Roles in Bone Homeostasis

The Wnt pathway is involved in several processes essential for bone development and homeostasis. For proper functioning, the Wnt pathway is tightly regulated by numerous extracellular elements that act by both activating and inhibiting the pathway at different moments. This review aims to describe, summarize and update the findings regarding the extracellular modulators of the Wnt pathway, including co-receptors, ligands and inhibitors, in relation to bone homeostasis, with an emphasis on the animal models generated, the diseases associated with each gene and the bone processes in which each member is involved. The precise knowledge of all these elements will help us to identify possible targets that can be used as a therapeutic target for the treatment of bone diseases such as osteoporosis.

Receptor control by membrane-tethered ubiquitin ligases in development and tissue homeostasis

Paracrine cell-cell communication is central to all developmental processes, ranging from cell diversification to patterning and morphogenesis. Precise calibration of signaling strength is essential for the fidelity of tissue formation during embryogenesis and tissue maintenance in adults. Membrane-tethered ubiquitin ligases can control the sensitivity of target cells to secreted ligands by regulating the abundance of signaling receptors at the cell surface. We discuss two examples of this emerging concept in signaling: (1) the transmembrane ubiquitin ligases ZNRF3 and RNF43 that regulate WNT and bone morphogenetic protein receptor abundance in response to R-spondin ligands and (2) the membrane-recruited ubiquitin ligase MGRN1 that controls Hedgehog and melanocortin receptor abundance. We focus on the mechanistic logic of these systems, illustrated by structural and protein interaction models enabled by AlphaFold. We suggest that membrane-tethered ubiquitin ligases play a widespread role in remodeling the cell surface proteome to control responses to extracellular ligands in diverse biological processes.

R-spondin 3 deletion induces Erk phosphorylation to enhance Wnt signaling and promote bone formation in the appendicular skeleton

Activation of Wnt signaling leads to high bone density. The R-spondin family of four secreted glycoproteins (Rspo1-4) amplifies Wnt signaling. In humans, RSPO3 variants are strongly associated with bone density. Here, we investigated the role of Rspo3 in skeletal homeostasis in mice. Using a comprehensive set of mouse genetic and mechanistic studies, we show that in the appendicular skeleton, Rspo3 haplo-insufficiency and Rspo3 targeted deletion in Runx2⁺ osteoprogenitors lead to an increase in trabecular bone mass, with increased number of osteoblasts and bone formation. In contrast and highlighting the complexity of Wnt signaling in the regulation of skeletal homeostasis, we show that Rspo3 deletion in osteoprogenitors results in the opposite phenotype in the axial skeleton, i.e., low vertebral trabecular bone mass. Mechanistically, Rspo3 deficiency impairs the inhibitory effect of Dkk1 on Wnt signaling activation and bone mass. We demonstrate that Rspo3 deficiency leads to activation of Erk signaling which in turn, stabilizes β-catenin and Wnt signaling activation. Our data demonstrate that Rspo3 haplo-insufficiency/deficiency boosts canonical Wnt signaling by activating Erk signaling, to favor osteoblastogenesis, bone formation, and bone mass.

RSPO3 is important for trabecular bone and fracture risk in mice and humans

With increasing age of the population, countries across the globe are facing a substantial increase in osteoporotic fractures. Genetic association signals for fractures have been reported at the RSPO3 locus, but the causal gene and the underlying mechanism are unknown. Here we show that the fracture reducing allele at the RSPO3 locus associate with increased RSPO3 expression both at the mRNA and protein levels, increased trabecular bone mineral density and reduced risk mainly of distal forearm fractures in humans. We also demonstrate that RSPO3 is expressed in osteoprogenitor cells and osteoblasts and that osteoblast-derived RSPO3 is the principal source of RSPO3 in bone and an important regulator of vertebral trabecular bone mass and bone strength in adult mice. Mechanistic studies revealed that RSPO3 in a cell-autonomous manner increases osteoblast proliferation and differentiation. In conclusion, RSPO3 regulates vertebral trabecular bone mass and bone strength in mice and fracture risk in humans.

A NOTCH1/LSD1/BMP2 co-regulatory network mediated by miR-137 negatively regulates osteogenesis of human adipose-derived stem cells

Background

MicroRNAs have been recognized as critical regulators for the osteoblastic lineage differentiation of human adipose-derived stem cells (hASCs). Previously, we have displayed that silencing of miR-137 enhances the osteoblastic differentiation potential of hASCs partly through the coordination of lysine-specific histone demethylase 1 (LSD1), bone morphogenetic protein 2 (BMP2), and mothers against decapentaplegic homolog 4 (SMAD4). However, still numerous molecules involved in the osteogenic regulation of miR-137 remain unknown. This study aimed to further elucidate the epigenetic mechanisms of miR-137 on the osteogenic differentiation of hASCs.

Methods

Dual-luciferase reporter assay was performed to validate the binding to the 3′ untranslated region (3′ UTR) of NOTCH1 by miR-137. To further identify the role of NOTCH1 in miR-137-modulated osteogenesis, tangeretin (an inhibitor of NOTCH1) was applied to treat hASCs which were transfected with miR-137 knockdown lentiviruses, then together with negative control (NC), miR-137 overexpression and miR-137 knockdown groups, the osteogenic capacity and possible downstream signals were examined. Interrelationships between signaling pathways of NOTCH1-hairy and enhancer of split 1 (HES1), LSD1 and BMP2-SMADs were thoroughly investigated with separate knockdown of NOTCH1, LSD1, BMP2, and HES1.

Results

We confirmed that miR-137 directly targeted the 3′ UTR of NOTCH1 while positively regulated HES1. Tangeretin reversed the effects of miR-137 knockdown on osteogenic promotion and downstream genes expression. After knocking down NOTCH1 or BMP2 individually, we found that these two signals formed a positive feedback loop as well as activated LSD1 and HES1. In addition, LSD1 knockdown induced NOTCH1 expression while suppressed HES1.

Conclusions

Collectively, we proposed a NOTCH1/LSD1/BMP2 co-regulatory signaling network to elucidate the modulation of miR-137 on the osteoblastic differentiation of hASCs, thus providing mechanism-based rationale for miRNA-targeted therapy of bone defect.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02495-3.

Rspo2 inhibits TCF3 phosphorylation to antagonize Wnt signaling during vertebrate anteroposterior axis specification

The Wnt pathway activates target genes by controlling the β-catenin-T-cell factor (TCF) transcriptional complex during embryonic development and cancer. This pathway can be potentiated by R-spondins, a family of proteins that bind RNF43/ZNRF3 E3 ubiquitin ligases and LGR4/5 receptors to prevent Frizzled degradation. Here we demonstrate that, during Xenopus anteroposterior axis specification, Rspo2 functions as a Wnt antagonist, both morphologically and at the level of gene targets and pathway mediators. Unexpectedly, the binding to RNF43/ZNRF3 and LGR4/5 was not required for the Wnt inhibitory activity. Moreover, Rspo2 did not influence Dishevelled phosphorylation in response to Wnt ligands, suggesting that Frizzled activity is not affected. Further analysis indicated that the Wnt antagonism is due to the inhibitory effect of Rspo2 on TCF3/TCF7L1 phosphorylation that normally leads to target gene activation. Consistent with this mechanism, Rspo2 anteriorizing activity has been rescued in TCF3-depleted embryos. These observations suggest that Rspo2 is a context-specific regulator of TCF3 phosphorylation and Wnt signaling.

The Role of LGR4 (GPR48) in Normal and Cancer Processes

Leucine-rich repeats containing G protein-coupled receptor 4 (LGR4) is a receptor that belongs to the superfamily of G protein-coupled receptors that can be activated by R-spondins (RSPOs), Norrin, circLGR4, and the ligand of the receptor activator of nuclear factor kappa-B (RANKL) ligands to regulate signaling pathways in normal and pathological processes. LGR4 is widely expressed in different tissues where it has multiple functions such as tissue development and maintenance. LGR4 mainly acts through the Wnt/β-catenin pathway to regulate proliferation, survival, and differentiation. In cancer, LGR4 participates in tumor progression, invasion, and metastasis. Furthermore, recent evidence reveals that LGR4 is essential for the regulation of the cancer stem cell population by controlling self-renewal and regulating stem cell properties. This review summarizes the function of LGR4 and its ligands in normal and malignant processes.

Synergistic roles of Wnt modulators R-spondin2 and R-spondin3 in craniofacial morphogenesis and dental development

Wnt signaling plays a critical role in craniofacial patterning, as well as tooth and bone development. Rspo2 and Rspo3 are key regulators of Wnt signaling. However, their coordinated function and relative requirement in craniofacial development and odontogensis are poorly understood. We showed that in zebrafish rspo2 and rspo3 are both expressed in osteoprogenitors in the embryonic craniofacial skeleton. This is in contrast to mouse development, where Rspo3 is expressed in osteoprogenitors while Rspo2 expression is not observed. In zebrafish, rspo2 and rspo3 are broadly expressed in the pulp, odontoblasts and epithelial crypts. However, in the developing molars of the mouse, Rspo3 is largely expressed in the dental follicle and alveolar mesenchyme while Rspo2 expression is restricted to the tooth germ. While Rspo3 ablation in the mouse is embryonic lethal, zebrafish rspo3-/- mutants are viable with modest decrease in Meckel's cartilage rostral length. However, compound disruption of rspo3 and rspo2 revealed synergistic roles of these genes in cartilage morphogenesis, fin development, and pharyngeal tooth development. Adult rspo3-/- zebrafish mutants exhibit a dysmorphic cranial skeleton and decreased average tooth number. This study highlights the differential functions of Rspo2 and Rspo3 in dentocranial morphogenesis in zebrafish and in mouse.

Impact of Different Proportions of 2D and 3D Scaffolds on the Proliferation and Differentiation of Human Adipose-Derived Stem Cells

PURPOSE

To observe the proliferation and differentiation of human adipose-derived stem cells (hADSCs) on 2D and 3D scaffolds, the sodium alginate and collagen interpenetrating network hydrogel were developed to determine optimal properties for bone tissue engineering.

METHODS

Three groups of scaffold materials were prepared according to the ratio of sodium alginate to collagen: A (4:1), B (2:1), and C (1:1), respectively. For each group, gel beads (3D surfaces) and freeze-dried films (2D surfaces) were respectively prepared. For gel beads, hADSCs were mixed during the preparation of the beads, and then stem cells were applied to the surface of each film after freeze-drying and sterilization during the preparation of the freeze-dried films. Cell proliferation and osteogenic differentiation potential were detected by cell counting kit, viable/dead cell staining kit, quantitative reverse transcription polymerase chain reaction, and immunofluorescent staining, respectively.

RESULTS

Results showed that cell proliferation rate progressively increased with the increase of collagen ratio, with group C of 3D surfaces of gel beads achieving the highest rate. In particular, highest cell viability on the 2D surfaces was achieved in group B. Differences in BGLAP and RUNX2 expression in hADSCs on 2D or 3D surfaces of the 3 groups were statistically significant. Particularly, BGLAP and RUNX2 gene expression levels were highest in group C of freeze-dried films and were highest in group B of gel beads. Furthermore, the trend of immunofluorescence expression of RUNX2 and osteocalcin expression were consistent with the genetic testing results.

CONCLUSIONS

All data indicated that sodium alginate-collagen scaffolding materials had no adverse impact on the proliferation and osteogenic differentiation of hADSCs. Cell differentiation and proliferation of bone tissue engineering can be promoted with the use of sodium alginate and collagen interpenetrating network hydrogel, and the appropriate ratio of sodium alginate and collagen is 2:1.

RSPO3 is a marker candidate for predicting tumor aggressiveness in ovarian cancer

Background

Ovarian cancer, a highly aggressive and heterogeneous gynecological malignancy that has long been difficult for physicians to identify and treat, requires more effective and precise molecular targets. R-spondin 3 (RSPO3) is a secreted protein that plays a tumorigenic role in several human cancers. However, the functional contribution and prognostic role of RSPO3 in ovarian cancer remain unclear.

Methods

RSPO3 expression in ovarian cancer tissues was assessed using western blotting, quantitative real-time polymerase chain reaction (qRT-PCR), and immunohistochemistry, and its relationships to clinicopathological parameters were investigated using the data of 179 ovarian cancer patients. RSPO3’s biological function was evaluated using Cell Counting Kit-8, colony formation, wound healing, and Matrigel transwell assay in RSPO3-knockdown and RSPO3-overexpression ovarian cancer cell lines SKOV3 and OVCAR3. The possible biological processes associated with RSPO3 were identified using functional enrichment analysis based on the transcriptome sequencing data from The Cancer Genome Atlas (TCGA) ovarian cancer cohort and our experimental cells, and further verified using western blotting and immunofluorescence in the ovarian cancer cell model.

Results

The RSPO3 mRNA and protein levels were both upregulated in ovarian cancer tissues. High RSPO3 expression was correlated with lymphovascular space invasion (LVSI), lymph node metastasis, distant metastasis, and advanced tumor stage. Survival analysis showed that RSPO3 is an independent prognostic marker in ovarian cancer. Moreover, in vitro RSPO3 knockdown significantly inhibited the invasion ability of ovarian cancer cells, while overexpression significantly promoted it. Using transcriptome sequencing and pathway validation experiments, we demonstrated for the first time that RSPO3 promotes ovarian cancer invasiveness through activation of the PI3K/AKT pathway and modulation of epithelial-mesenchymal transition (EMT), while the common Wnt/β-catenin signaling pathway was not involved.

Conclusions

RSPO3 plays a definite oncogenic role and promotes tumor aggressiveness in ovarian cancer, which may serve as a potential prognostic marker and therapeutic target for this disease.

LGRs in Skeletal Tissues: An Emerging Role for Wnt-Associated Adult Stem Cell Markers in Bone

Leucine-rich repeat-containing G protein-coupled receptors (LGRs) are adult stem cell markers that have been described across various stem cell niches, and expression of LGRs and their corresponding ligands (R-spondins) has now been reported in multiple bone-specific cell types. The skeleton harbors elusive somatic stem cell populations that are exceedingly compartment-specific and under tight regulation from various signaling pathways. Skeletal progenitors give rise to multiple tissues during development and during regenerative processes of bone, requiring postnatal endochondral and intramembranous ossification. The relevance of LGRs and the LGR/R-spondin ligand interaction in bone and tooth biology is becoming increasingly appreciated. LGRs may define specific stem cell and progenitor populations and their behavior during both development and regeneration, and their role as Wnt-associated receptors with specific ligands poses these proteins as unique therapeutic targets via potential R-spondin agonism. This review seeks to outline the current literature on LGRs in the context of bone and its associated tissues, and points to key future directions for studying the functional role of LGRs and ligands in skeletal biology. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Rspo2 antagonizes FGF signaling during vertebrate mesoderm formation and patterning

R-spondins are a family of secreted proteins that play important roles in embryonic development and cancer. R-spondins have been shown to modulate the Wnt pathway; however, their involvement in other developmental signaling processes have remained largely unstudied. Here, we describe a novel function of Rspo2 in FGF pathway regulation in vivo Overexpressed Rspo2 inhibited elongation of Xenopus ectoderm explants and Erk1 activation in response to FGF. By contrast, the constitutively active form of Mek1 stimulated Erk1 even in the presence of Rspo2, suggesting that Rspo2 functions upstream of Mek1. The observed inhibition of FGF signaling was accompanied by the downregulation of the FGF target genes tbxt/brachyury and cdx4, which mediate anterioposterior axis specification. Importantly, these target genes were upregulated in Rspo2-depleted explants. The FGF inhibitory activity was mapped to the thrombospondin type 1 region, contrasting the known function of the Furin-like domains in Wnt signaling. Further domain analysis revealed an unexpected intramolecular interaction that might control Rspo2 signaling output. We conclude that, in addition to its role in Wnt signaling, Rspo2 acts as an FGF antagonist during mesoderm formation and patterning.

The secreted matrix protein mindin increases prostate tumor progression and tumor-bone crosstalk via ERK 1/2 regulation

Advanced prostate cancer cells preferentially metastasize to bone by acquiring a bone phenotype that allows metastatic cells to thrive in the skeletal environment. Identification of factors that promote the expression of ectopic bone genes-process known as osteomimicry-leading to tumor progression is crucial to prevent and treat metastatic prostate cancer and prolong life expectancy for patients. Here, we identify the extracelular matrix protein mindin in the secretome of prostate adenocarcinoma cells and show that mindin overexpression in human and mouse TRAMP-C1-induced prostate tumors correlates with upregulated levels of bone-related genes in the tumorigenic prostate tissues. Moreover, mindin silencing decreased osteomimicry in adenocarcinoma cells and in the prostate tumor mice model, as well as reduced tumor cell proliferation, migration and adhesion to bone cells. Inhibition of the extracellular signal-regulated kinase 1/2 (ERK 1/2) phosphorylation decreased the proliferative, migratory and pro-adhesion actions of mindin on prostate tumor cells. In addition, conditioned media obtained by crosstalk stimulation of either osteocytes or osteoblasts with the secretome of TRAMP-C1 cells promoted osteomimicry in prostate tumor cells; an effect inhibited by mindin silencing of TRAMP-C1 cells. In vivo, tibiae of primary tumor-bearing mice overexpressed the pro-angiogenic and pro-metastattic factor vascular endothelial growth factor receptor 2 (VEGFR2) in a mindin-dependent manner. Our findings indicate that mindin is a novel regulator of osteomimicry in prostate tumors and potentially mediates tumor-bone cell crosstalk, suggesting its promising role as a target to inhibit bone metastases.

Canonical WNT/β-Catenin Signaling Activated by WNT9b and RSPO2 Cooperation Regulates Facial Morphogenesis in Mice

The R-spondin (RSPO) family of proteins potentiate canonical WNT/β-catenin signaling and may provide a mechanism to fine-tune the strength of canonical WNT signaling. Although several in vitro studies have clearly demonstrated the potentiation of canonical WNT signaling by RSPOs, whether this potentiation actually occurs in normal development and tissue function in vivo still remains poorly understood. Here, we provide clear evidence of the potentiation of canonical WNT signaling by RSPO during mouse facial development by analyzing compound Wnt9b and Rspo2 gene knockout mice and utilizing ex vivo facial explants. Wnt9b;Rspo2 double mutant mice display facial defects and dysregulated gene expression pattern that are significantly more severe than and different from those of Wnt9b or Rspo2 null mutant mice. Furthermore, we found suggestive evidence that the LGR4/5/6 family of the RSPO receptors may play less critical roles in WNT9b:RSPO2 cooperation. Our results suggest that RSPO-induced cooperation is a key mechanism for fine-tuning canonical WNT/β-catenin signaling in mouse facial development.

R-spondin3 promotes the tumor growth of choriocarcinoma JEG-3 cells

R-spondin3 (RSPO3), an activator of Wnt/β-catenin signaling, plays a key role in tumorigenesis of various cancers, but its role in choriocarcinoma remains unknown. To investigate the effect of RSPO3 on the tumor growth of choriocarcinoma JEG-3 cells, the expression of RSPO3 in human term placenta was detected, and a stable RSPO3-overexpressing JEG-3 cell line was established via lentivirus-mediated transduction. The expression of biomarkers involved in tumorigenicity was detected in the RSPO3-overexpressing JEG-3 cells, and cell proliferation, invasion, migration, and apoptosis were investigated. Moreover, soft agar clonogenic assays and xenograft tumorigenicity assays were performed to assess the effect of RSPO3 on tumor growth in vitro and in vivo. The results showed that RSPO3 was widely expressed in human term placenta and overexpression of RSPO3 promoted the proliferation and inhibited the migration, invasion, and apoptosis of the JEG-3 cells. Meanwhile, RSPO3 overexpression promoted tumor growth both in vivo and in vitro. Further investigation showed that the phosphorylation levels of Akt, phosphatidylinositol 3-kinase (PI3K), and ERK as well the expression of β-catenin and proliferating cell nuclear antigen (PCNA) were increased in the RSPO3-overexpressing JEG-3 cells and tumor xenograft. Taken together, these data indicate that RSPO3 promotes the tumor growth of choriocarcinoma via Akt/PI3K/ERK signaling, which supports RSPO3 as an oncogenic driver promoting the progression of choriocarcinoma.

LGR6 promotes osteogenesis by activating the Wnt/β-catenin signaling pathway

Leucine-rich repeat containing G-protein-coupled receptor 6 (LGR6) is a member of the rhodopsin-like 7-transmembrane domain receptor superfamily and has high homology to LGR4 and LGR5. LGR6 is highly expressed in osteoblastic progenitors, and LGR6-deficient mice show nail and bone regeneration defect. However, the effect of LGR6 on the osteogenic differentiation of osteoblastic progenitors and its underlying mechanisms are largely unknown. In this study, we overexpressed and knockdown LGR6 with lentivirus in the preosteoblastic cell MC3T3-E1 to observe the effect of LGR6 on osteogenic differentiation and explore its possible molecular mechanism. LGR6 overexpression promoted osteogenic differentiation and mineralization by stabilizing β-catenin to potentiate the Wnt/β-catenin signaling pathway in MC3T3-E1 cells. Conversely, LGR6 knockdown inhibited osteogenic differentiation and mineralization by enhancing β-catenin degradation to inactivate the Wnt/β-catenin signaling pathway. These results reveal that LGR6 is highly expressed in osteoblastic progenitors, and promotes osteogenesis by enhancing β-catenin stability to strengthen the Wnt signaling pathway. This study provides an important reference into the exact mechanisms of osteogenic differentiation.

Asymmetrical methyltransferase PRMT3 regulates human mesenchymal stem cell osteogenesis via miR-3648

Histone arginine methylation, which is catalyzed by protein arginine methyltransferases (PRMTs), plays a key regulatory role in various biological processes. Several PRMTs are involved in skeletal development; however, their role in the osteogenic differentiation of mesenchymal stem cells (MSCs) is not completely clear. In this study, we aimed to elucidate the function of PRMT3, a type-I PRMT that catalyzes the formation of ω-mono- or asymmetric dimethyl arginine, in MSCs osteogenesis. We found that PRMT3 promoted MSCs osteogenic commitment and bone remodeling. PRMT3 activated the expression of miR-3648 by enhancing histone H4 arginine 3 asymmetric dimethylation (H4R3me2a) levels at promoter region of the gene. Overexpression of miR-3648 rescued impaired osteogenesis in PRMT3-deficient cells. Moreover, administration of Prmt3 shRNA or a chemical inhibitor of PRMT3 (SGC707) caused an osteopenia phenotype in mice. These results indicate that PRMT3 is a potential therapeutic target for the treatment of bone regeneration and osteopenia disorders.

R-spondin signaling as a pivotal regulator of tissue development and homeostasis

R-spondins (Rspos) are cysteine-rich secreted glycoproteins which control a variety of cellular functions and are essential for embryonic development and tissue homeostasis. R-spondins (Rspo1 to 4) have high structural similarity and share 60% sequence homology. It has been shown that their cysteine-rich furin-like (FU) domain and the thrombospondin (TSP) type I repeat domain are essential for initiating downstream signaling cascades and therefore for their biological functions. Although numerous studies have unveiled their pivotal role as critical developmental regulators, the most important finding is that Rspos synergize Wnt signaling. Recent studies have identified novel receptors for Rspos, the Lgr receptors, closely related orphans of the leucin-rich repeat containing G protein-coupled receptors, and proposed that Rspos potentiate canonical Wnt signaling via these receptors. Given that Wnt signaling is one of the most important developmental signaling pathways that controls cell fate decisions and tissue development, growth and homeostasis, Rspos may function as key players for these processes as well as potential therapeutic targets. Here, I recapitulate the Wnt signaling and then outline the biological role of Rspos in tissue development and homeostasis and explore the possibility that Rspos may be used as therapeutic targets.

Identification of key genes and pathways associated with osteogenic differentiation of adipose stem cells

Adipose stem cells (ASCs) are considered a great alternative source of mesenchymal stem cells (MSCs) and have shown great promise on tissue engineering and regenerative medicine applications, including bone repair. However, the underlying mechanisms regulating the osteogenic differentiation of ASCs remain poorly known. Gene expression profiles of GSE63754 and GSE37329 were downloaded from gene expression omnibus database. R software and Bioconductor packages were used to compare and identify the differentially expressed genes (DEGs) before and after ASC osteogenic differentiation. The common significant DEGs between GSE63754 and GSE37329 were then subjected to gene ontology (GO) enrichment analysis, ingenuity pathway analysis (IPA), and protein-protein interactions (PPIs) networks analysis. One of the central node genes FOXO1 was selected for further investigation. A total of 142 up- and 69 downregulated genes were aberrantly expressed in both GSE63754 and GSE37329. GO analysis revealed that these DEGs were associated with extracellular matrix organization, proteinaceous extracellular matrix, and Wnt-protein binding. IPA analysis showed that canonical pathways, such as FXR/RXR activation, adipogenesis pathway, and LXR/RXR activation, were involved in regulating osteogenic differentiation of ASCs. A total of three subnetworks and 39 nodes were identified with PPI network and MCODE plugin. Moreover, suppression of one central node gene FOXO1 inhibited the osteogenic differentiation of ASCs. Our study provides a registry of genes and pathways that play important roles in regulating osteogenic differentiation of ASCs, which might have potential therapeutic applications in bone regeneration and bone tissue engineering.

Wnt signaling suppresses MAPK-driven proliferation of intestinal stem cells

Intestinal homeostasis depends on a slowly proliferating stem cell compartment in crypt cells, followed by rapid proliferation of committed progenitor cells in the transit amplifying (TA) compartment. The balance between proliferation and differentiation in intestinal stem cells (ISCs) is regulated by Wnt/β-catenin signaling, although the mechanism remains unclear. We previously targeted PORCN, an enzyme essential for all Wnt secretion, and demonstrated that stromal production of Wnts was required for intestinal homeostasis. Here, a PORCN inhibitor was used to acutely suppress Wnt signaling. Unexpectedly, the treatment induced an initial burst of proliferation in the stem cell compartment of the small intestine, due to conversion of ISCs into TA cells with a loss of intrinsic ISC self-renewal. This process involved MAPK pathway activation, as the proliferating cells in the base of the intestinal crypt contained phosphorylated ERK1/2, and a MEK inhibitor attenuated the proliferation of ISCs and their differentiation into TA cells. These findings suggest a role for Wnt signaling in suppressing the MAPK pathway at the crypt base to maintain a pool of ISCs. The interaction between Wnt and MAPK pathways in vivo has potential therapeutic applications in cancer and regenerative medicine.

LHX2 Mediates the FGF-to-SHH Regulatory Loop during Limb Development

During limb development, fibroblast growth factors (Fgfs) govern proximal⁻distal outgrowth and patterning. FGFs also synchronize developmental patterning between the proximal⁻distal and anterior⁻posterior axes by maintaining Sonic hedgehog (Shh) expression in cells of the zone of polarizing activity (ZPA) in the distal posterior mesoderm. Shh, in turn, maintains Fgfs in the apical ectodermal ridge (AER) that caps the distal tip of the limb bud. Crosstalk between Fgf and Shh signaling is critical for patterned limb development, but the mechanisms underlying this feedback loop are not well-characterized. Implantation of Fgf beads in the proximal posterior limb bud can maintain SHH expression in the former ZPA domain (evident 3 h after application), while prolonged exposure (24 h) can induce SHH outside of this domain. Although temporally and spatially disparate, comparative analysis of transcriptome data from these different populations accentuated genes involved in SHH regulation. Comparative analysis identified 25 candidates common to both treatments, with eight linked to SHH expression or function. Furthermore, we demonstrated that LHX2, a LIM Homeodomain transcription factor, is an intermediate in the FGF-mediated regulation of SHH. Our data suggest that LHX2 acts as a competency factor maintaining distal posterior SHH expression subjacent to the AER.

Effects of thermal treatment on the adhesion strength and osteoinductive activity of single-layer graphene sheets on titanium substrates

In recent years, dental implants have become the preferred approach for the restoration of missing teeth. At present, most dental implants are made of pure titanium, and are affected by peri-implantitis and bone resorption, which usually start from the implant neck, due to the complex environment in this region. To address these issues, in this study we modified the surface of titanium (Ti) implants to exploit the antibacterial and osteoinductive effects of single-layer graphene sheets. Chemical vapor deposition (CVD)-grown single-layer graphene sheets were transferred to titanium discs, and a method for improving the adhesion strength of graphene on Ti was developed due to compromised adhesion strength between graphene and titanium surface. A thermal treatment of 2 h at 160 °C was found to enhance the adhesion strength of graphene on Ti to facilitate clinical transformation. Graphene coatings of Ti enhanced cell adhesion and osteogenic differentiation, and imparted antibacterial activity to Ti substrate; these favorable effects were not affected by the thermal treatment. In summary, the present study elucidated the effects of a thermal treatment on the adhesion strength and osteoinductive activity of single-layer graphene sheets on titanium substrates.

Protein deubiquitinase USP7 is required for osteogenic differentiation of human adipose-derived stem cells

BACKGROUND

Human adipose-derived stem cells (hASCs) are multipotent progenitor cells with self-renewal capabilities and multilineage differentiation potential, including osteogenesis. Although protein deubiquitinases have been linked to stem cell fate determination, whether protein deubiquitination contributes to lineage commitment during osteogenic differentiation of hASCs remains to be investigated. The objective of this study was to evaluate the effects of the ubiquitin specific protease 7 (USP7) on osteogenic differentiation of hASCs.

METHODS

An osteocalcin promoter driven luciferase reporter system was established to initially discover the potential association between USP7 and hASC osteogenesis. To further characterize the function of USP7 in osteogenic differentiation of hASCs, a combination of in vitro and in vivo experiments were carried out through genetic depletion or overexpression of USP7 using a lentiviral strategy. Moreover, HBX 41,108, a cyanoindenopyrazine-derived deubiquitinase inhibitor of USP7, was utilized at different doses to further examine whether USP7 regulated osteogenic differentiation of hASCs through its enzymatic activity.

RESULTS

We demonstrated that USP7 depletion was associated with remarkable downregulation of the reporter gene activity. Genetic depletion of USP7 by lentiviral RNAi markedly suppressed hASC osteogenesis both in vitro and in vivo, while overexpression of USP7 enhanced the osteogenic differentiation of hASCs. Notably, chemical blockade via the small molecular inhibitor HBX 41,108 could efficiently mimic the effects of USP7 genetic depletion in a dose-dependent manner.

CONCLUSIONS

Taken together, our study revealed that protein deubiquitinase USP7 is an essential player in osteogenic differentiation of hASCs through its catalytic activity, and supported the pursuit of USP7 as a potential target for modulation of hASC-based stem cell therapy and bone tissue engineering.

GSK3 inhibitor AR-A014418 promotes osteogenic differentiation of human adipose-derived stem cells via ERK and mTORC2/Akt signaling pathway

Small molecule-based bone tissue engineering is emerging as a promising strategy for bone defects restoration. In this study, we intended to identify the roles and mechanisms of AR-A014418, a highly selective inhibitor of GSK3, on the osteogenic differentiation. We found that AR-A014418 exhibited a dose-dependent effect on osteogenic differentiation of human adipose-derived stem cells (hASCs). hASCs treated with AR-A014418 showed higher activity of ERK and mTORC2/Akt signaling. Administration of ERK inhibitor U0126 or knockdown of RICTOR by siRNA attenuated AR-A014418 induced osteogenic differentiation of hASCs. Our results suggested that AR-A014418 significantly promoted osteogenic potential of hASCs partially by the activation of ERK and mTORC2/Akt signaling pathway, and might be used for bone tissue engineering as an osteo-inductive factor.