MEF2C transcription factor controls chondrocyte hypertrophy and bone development

Hesperetin Attenuates T-2 Toxin-Induced Chondrocyte Injury by Inhibiting the p38 MAPK Signaling Pathway

BACKGROUND

Hesperetin, a flavonoid derived from citrus fruits, exhibits potent antioxidant and anti-inflammatory activities and has been implicated in cartilage protection. However, its effectiveness against T-2 toxin-induced knee cartilage damage remains unclear.

METHODS

In this study, high-throughput sequencing analysis was employed to identify the key signaling pathways involved in T-2 toxin-induced articular cartilage damage in rats. Animal models were divided into the following groups: control, low-dose T-2 toxin, high-dose T-2 toxin, T-2 toxin + hesperetin, hesperetin, and vehicle. Pathological staining and immunohistochemistry were used to assess pathological changes, as well as the expression levels of the cartilage matrix-related proteins MMP13 and collagen II, along with the activation of the p38 MAPK signaling pathway. Additionally, primary rat chondrocytes were cultured to establish an in vitro model for investigating the underlying mechanism.

RESULTS

High-throughput sequencing analysis revealed the involvement of the MAPK signaling pathway in T-2 toxin-induced articular cartilage damage in rats. Hesperetin intervention in T-2 toxin-exposed rats attenuated pathological cartilage damage. Immunohistochemistry results demonstrated a significant reduction in collagen II protein expression in the high-dose T-2 toxin group (p < 0.01), accompanied by a significant increase in MMP13 protein expression (p < 0.01). In both the articular cartilage and the epiphyseal plate, the T-2 toxin + hesperetin group exhibited significantly higher collagen II protein expression than the high-dose T-2 toxin group (p < 0.05), along with significantly lower MMP13 protein expression (p < 0.05). Hesperetin inhibited the over-activation of the p38/MEF2C signaling axis induced by T-2 toxin in primary rat chondrocytes. Compared to the T-2 toxin group, the T-2 toxin + hesperetin group showed significantly reduced phosphorylation levels of p38 and protein expression levels of MEF2C (p < 0.001 or p < 0.05). Moreover, the T-2 toxin + hesperetin group exhibited a significant decrease in MMP13 protein expression (p < 0.05) and a significant increase in collagen II protein expression (p < 0.01) compared to the T-2 toxin group.

CONCLUSIONS

T-2 toxin activates the p38 MAPK signaling pathway, causing knee cartilage damage in rats. Treatment with hesperetin inhibits the p38/MEF2C signaling axis, regulates collagen II and MMP13 protein expression, and reduces cartilage injury significantly.

The G protein-coupled receptor ADGRG6 maintains mouse growth plate homeostasis through IHH Signaling

The cartilage growth plate is essential for maintaining skeletal growth; however, the mechanisms governing postnatal growth plate homeostasis are still poorly understood. Using approaches of molecular mouse genetics and spatial transcriptomics applied to formalin-fixed, paraffin-embedded (FFPE) tissues, we show that ADGRG6/GPR126, a cartilage-enriched adhesion G protein-coupled receptor (GPCR), is essential for maintaining slow-cycling resting zone cells, appropriate chondrocyte proliferation and differentiation, and growth plate homeostasis in mice. Constitutive ablation of Adgrg6 in osteochondral progenitor cells with Col2a1Cre leads to a shortened resting zone, formation of cell clusters within the proliferative zone, and an elongated hypertrophic growth plate, marked by limited expression of PTHrP but increased IHH signaling throughout the growth plate. Attenuation of Smoothened (SMO)-dependent hedgehog signaling restored the Adgrg6 deficiency-induced expansion of hypertrophic chondrocytes, confirming that IHH signaling can promote chondrocyte hypertrophy in a PTHrP-independent manner. In contrast, postnatal ablation of Adgrg6 in mature chondrocytes with AcanCreERT2, induced after the formation of the resting zone, does not affect PTHrP expression but causes an overall reduction of growth plate thickness marked by increased cell death specifically in the resting zone cells and a general reduction of chondrocyte proliferation and differentiation. Spatial transcriptomics reveals that ADGRG6 is essential for maintaining chondrocyte homeostasis by regulating osteogenic and catabolic genes in all the zones of the postnatal growth plates, potentially through positive regulation of SOX9 expression. Our findings elucidate the essential role of a cartilage-enriched adhesion GPCR in regulating cell proliferation and hypertrophic differentiation by regulation of PTHrP/IHH signaling, maintenance of slow-cycle resting zone chondrocytes, and safeguarding chondrocyte homeostasis in postnatal mouse growth plates.

LMK-235 suppresses osteoclastogenesis and promotes osteoblastogenesis by inhibiting HDAC4

Osteoblasts and osteoclasts play an important role in maintaining the structural integrity of bone tissue, in which osteoclasts degrade bone structure and osteoblasts restore bone tissue. The imbalance of osteoblast and osteoclast function can lead to many bone-related diseases, such as osteoporosis and inflammatory osteolysis. The drug that can both promote bone formation and inhibit bone loss will be able to treat those diseases. In this study, it was found that LMK-235, an selective HDAC4/5 inhibitor, inhibited the differentiation and maturation of osteoclasts by regulating NF-κB and p-Smad2/3 signaling pathways via inhibition of HDAC4. At the same time, we found that LMK-235 promoted osteoblast mineralization by upregulating Runx2 expression via inhibition of HDAC4. In vivo, LMK-235 was able to alleviate lipopolysaccharide (LPS)-induced calvarial osteolysis and promote the repair of bone defects. Taken together, LMK-235 suppresses osteoclast differentiation and promotes osteoblast formation by inhibiting HDAC4. This may provide a valuable treatment for bone diseases caused by abnormal osteoclast bone resorption and osteoblast bone regeneration.

Understanding the roles of salt-inducible kinases in cardiometabolic disease

Salt-inducible kinases (SIKs) are serine/threonine kinases of the adenosine monophosphate-activated protein kinase family. Acting as mediators of a broad array of neuronal and hormonal signaling pathways, SIKs play diverse roles in many physiological and pathological processes. Phosphorylation by the upstream kinase liver kinase B1 is required for SIK activation, while phosphorylation by protein kinase A induces the binding of 14-3-3 protein and leads to SIK inhibition. SIKs are subjected to auto-phosphorylation regulation and their activity can also be modulated by Ca2+/calmodulin-dependent protein kinase in response to cellular calcium influx. SIKs regulate the physiological processes through direct phosphorylation on various substrates, which include class IIa histone deacetylases, cAMP-regulated transcriptional coactivators, phosphatase methylesterase-1, among others. Accumulative body of studies have demonstrated that SIKs are important regulators of the cardiovascular system, including early works establishing their roles in sodium sensing and vascular homeostasis and recent progress in pulmonary arterial hypertension and pathological cardiac remodeling. SIKs also regulate inflammation, fibrosis, and metabolic homeostasis, which are essential pathological underpinnings of cardiovascular disease. The development of small molecule SIK inhibitors provides the translational opportunity to explore their potential as therapeutic targets for treating cardiometabolic disease in the future.

Leveraging single cell multiomic analyses to identify factors that drive human chondrocyte cell fate

Cartilage plays a crucial role in skeletal development and function, and abnormal development contributes to genetic and age-related skeletal disease. To better understand how human cartilage develops in vivo , we jointly profiled the transcriptome and open chromatin regions in individual nuclei recovered from distal femurs at 2 fetal timepoints. We used these multiomic data to identify transcription factors expressed in distinct chondrocyte subtypes, link accessible regulatory elements with gene expression, and predict transcription factor-based regulatory networks that are important for growth plate or epiphyseal chondrocyte differentiation. We developed a human pluripotent stem cell platform for interrogating the function of predicted transcription factors during chondrocyte differentiation and used it to test NFATC2 . We expect new regulatory networks we uncovered using multiomic data to be important for promoting cartilage health and treating disease, and our platform to be a useful tool for studying cartilage development in vitro .

Statement of Significance

The identity and integrity of the articular cartilage lining our joints are crucial to pain-free activities of daily living. Here we identified a gene regulatory landscape of human chondrogenesis at single cell resolution, which is expected to open new avenues of research aimed at mitigating cartilage diseases that affect hundreds of millions of individuals world-wide.

An upstream enhancer and MEF2 transcription factors fine-tune the regulation of the Bdnf gene in cortical and hippocampal neurons

The myocyte enhancer factor (MEF2) family of transcription factors, originally discovered for its pivotal role in muscle development and function, has emerged as an essential regulator in various aspects of brain development and neuronal plasticity. The MEF2 transcription factors are known to regulate numerous important genes in the nervous system, including brain-derived neurotrophic factor (BDNF), a small secreted neurotrophin responsible for promoting the survival, growth, and differentiation of neurons. The expression of the Bdnf gene is spatiotemporally controlled by various transcription factors binding to both its proximal and distal regulatory regions. While previous studies have investigated the connection between MEF2 transcription factors and Bdnf, the endogenous function of MEF2 factors in the transcriptional regulation of Bdnf remains largely unknown. Here, we aimed to deepen the knowledge of MEF2 transcription factors and their role in the regulation of Bdnf comparatively in rat cortical and hippocampal neurons. As a result, we demonstrate that the MEF2 transcription factor-dependent enhancer located at -4.8 kb from the Bdnf gene regulates the endogenous expression of Bdnf in hippocampal neurons. In addition, we confirm neuronal activity-dependent activation of the -4.8 kb enhancer in vivo. Finally, we show that specific MEF2 family transcription factors have unique roles in the regulation of Bdnf, with the specific function varying based on the particular brain region and stimuli. Altogether, we present MEF2 family transcription factors as crucial regulators of Bdnf expression, fine-tuning Bdnf expression through both distal and proximal regulatory regions.

Mapk7 deletion in chondrocytes causes vertebral defects by reducing MEF2C/PTEN/AKT signaling

Mutation of the MAPK7 gene was related to human scoliosis. Mapk7 regulated the development of limb bones and skulls in mice. However, the role of MAPK7 in vertebral development is still unclear. In this study, we constructed Col2a1-cre; Mapk7f/f transgenic mouse model to delete Mapk7 in cartilage, which displayed kyphosis and osteopenia. Mechanistically, Mapk7 loss decreased MEF2C expression and thus activated PTEN to oppose PI3K/AKT signaling in vertebral growth plate chondrocytes, which impaired chondrocyte hypertrophy and attenuated vertebral ossification. In vivo, systemic pharmacological activation of AKT rescued impaired chondrocyte hypertrophy and alleviated mouse vertebral defects caused by Mapk7 deficiency. Our study firstly clarified the mechanism by which MAPK7 was involved in vertebral development, which might contribute to understanding the pathology of spinal deformity and provide a basis for the treatment of developmental disorders of the spine.

Mef2c regulates bone mass through Sost-dependent and -independent mechanisms

Mef2c is a transcription factor that mediates key cellular behaviors that promote endochondral ossification and bone formation. Previously, Mef2c has been shown to regulate Sost transcription via its osteocyte-specific enhancer, ECR5, and conditional deletions of Mef2cfl/fl with either Col1-Cre or Dmp1-Cre produced generalized high bone mass (HBM) consistent with Van Buchem Disease phenotypes. However, Sost-/-; Mef2cfl/fl; Dmp1-Cre mice produced a significantly higher bone mass phenotype that Sost-/- alone suggesting that Mef2c modulates bone mass through additional mechanisms, independent of Sost. To identify new Mef2c transcriptional targets important in bone metabolism, we profiled gene expression by single-cell RNA sequencing in subpopulations of cells isolated from Mef2cfl/fl; Dmp1-Cre and Mef2cfl/fl; Bglap-Cre femurs, both strains exhibiting similar high bone mass phenotypes. However, we found Mef2cfl/fl; Bglap-Cre to also display a growth plate defect characterized by an expansion of several osteoprogenitor subpopulations. Differential gene expression analysis identified a total of 96 up- and 2434 down- regulated genes in Mef2cfl/fl; Bglap-Cre and 176 up- and 1041 down- regulated genes in Mef2cfl/fl; Dmp1-Cre bone cell subpopulations compared to wildtype mice. Mef2c deletion affected the transcriptomes across several cell types including mesenchymal progenitors (MP), osteoprogenitors (OSP), osteoblast (OB), and osteocyte (OCY) subpopulations. Several energy metabolism genes such as Uqcrb, Ndufv2, Ndufs3, Ndufa13, Ndufb9, Ndufb5, Cox6a1, Cox5a, Atp5o, Atp5g2, Atp5b, Atp5 were significantly down regulated in Mef2c-deficient OBs and OCYs, in both strains. Binding motif analysis of promoter regions of differentially expressed genes identified Mef2c binding in Bone Sialoprotein (BSP/Ibsp), a gene known to cause increased trabecular BV/TV in the femurs of Ibsp-/- mice. Immunohistochemical analysis confirmed the absence of Ibsp protein in OBs and OCYs. These findings suggests that the HBM in Sost-/-; Mef2cfl/fl; Dmp1-Cre is caused by a multitude of transcriptional changes in genes that regulate bone formation, two of which are Sost and Ibsp.

Direct cellular reprogramming techniques for cardiovascular regenerative therapeutics

Cardiovascular diseases remain a leading cause of hospitalization affecting approximately 38 million people worldwide. While pharmacological and revascularization techniques can improve the patient's survival and quality of life, they cannot help reversing myocardial infarction injury and heart failure. Direct reprogramming of somatic cells to cardiomyocyte and cardiac progenitor cells offers a new approach to cellular reprogramming and paves the way for translational regenerative medicine. Direct reprogramming can bypass the pluripotent stage with the potential advantage of non-immunogenic cell products, reduced carcinogenic risk, and no requirement for embryonic tissue. The process of directly reprogramming cardiac cells was first achieved through the overexpression of transcription factors such as GATA4, MEF2C, and TBX5. However, over the past decade, significant work has been focused on enhancing direct reprogramming using a mixture of transcription factors, microRNAs, and small molecules to achieve cardiac cell fate. This review discusses the evolution of direct reprogramming, recent progress in achieving efficient cardiac cell fate conversion, and describes the reprogramming mechanisms at a molecular level. We also explore various viral and non-viral delivery methods currently being used to aid in the delivery of reprogramming factors to improve efficiency. However, further studies will be needed to overcome molecular and epigenetic barriers to successfully achieve translational cardiac regenerative therapeutics.

Exploring Omega-3's Impact on the Expression of Bone-Related Genes in Meagre (Argyrosomus regius)

Dietary supplementation with Omega-3 fatty acids seems to promote skeletal health. Therefore, their consumption at imbalanced or excessive levels has offered less beneficial or even prejudicial effects. Fish produced in aquaculture regimes are prone to develop abnormal skeletons. Although larval cultures are usually fed with diets supplemented with Omega-3 Long Chain Polyunsaturated fatty acids (LC-PUFAs), the lack of knowledge about the optimal requirements for fatty acids or about their impact on mechanisms that regulate skeletal development has impeded the design of diets that could improve bone formation during larval stages when the majority of skeletal anomalies appear. In this study, Argyrosomus regius larvae were fed different levels of Omega-3s (2.6% and 3.6% DW on diet) compared to a commercial diet. At 28 days after hatching (DAH), their transcriptomes were analyzed to study the modulation exerted in gene expression dynamics during larval development and identify impacted genes that can contribute to skeletal formation. Mainly, both levels of supplementation modulated bone-cell proliferation, the synthesis of bone components such as the extracellular matrix, and molecules involved in the interaction and signaling between bone components or in important cellular processes. The 2.6% level impacted several genes related to cartilage development, denoting a special impact on endochondral ossification, delaying this process. However, the 3.6% level seemed to accelerate this process by enhancing skeletal development. These results offered important insights into the impact of dietary Omega-3 LC-PUFAs on genes involved in the main molecular mechanism and cellular processes involved in skeletal development.

LncRNA CASC2 Alleviates Renal Interstitial Inflammation and Fibrosis through MEF2C Downregulation-Induced Hinderance of M1 Macrophage Polarization

INTRODUCTION

Long noncoding RNA (lncRNA) cancer susceptibility candidate 2 (CASC2) alleviates the progression of diabetic nephropathy by inhibiting inflammation and fibrosis. This study investigated how CASC2 impacts renal interstitial fibrosis (RIF) through regulating M1 macrophage (M1) polarization.

METHOD

Nine-week-old mice underwent unilateral ureteral obstruction (UUO) establishment. Macrophages were induced toward M1 polarization using lipopolysaccharide (LPS) in vitro and cocultured with fibroblasts to examine how M1 polarization influences RIF. LnCeCell predicted that CASC2 interacted with myocyte enhancer factor 2 C (MEF2C), which was validated by dual-luciferase reporter assay. CASC2/MEF2C overexpression was achieved by lentivirus-expressing lncRNA CASC2 injection in vivo or CASC2 and MEF2C transfection in vitro. Renal injury was evaluated through biochemical analysis and hematoxylin-eosin/Masson staining. Macrophage infiltration and M1 polarization in the kidney and/or macrophages were detected by immunofluorescence, flow cytometry, and/or quantitative reverse transcription polymerase chain reaction (qRT-PCR). Expressions of CASC2, MEF2C, and markers related to inflammation/M1/fibrosis in the kidney/macrophages/fibroblasts were analyzed by qRT-PCR, fluorescence in situ hybridization, enzyme-linked immunosorbent assay, and/or Western blot.

RESULT

In the kidneys of mice, CASC2 was downregulated and macrophage infiltration was promoted time-dependently from days 3 to 14 post-UUO induction; CASC2 overexpression alleviated renal histological abnormalities, hindered macrophage infiltration and M1 polarization, downregulated renal function markers serum creatinine and blood urea nitrogen and inflammation/M1/fibrosis-related makers, and offset UUO-induced MEF2C upregulation. LncRNA CASC2 overexpression inhibited fibroblast fibrosis and M1 polarization in cocultured fibroblasts with LPS-activated macrophages. Also, CASC2 bound to MEF2C and inhibited its expression in LPS-activated macrophages. Furthermore, MEF2C reversed the inhibitory effects of lncRNA CASC2 overexpression.

CONCLUSION

CASC2 alleviates RIF by inhibiting M1 polarization through directly downregulating MEF2C expression. CASC2 might represent a promising value of future investigations on treatment for RIF.

Human Chondrocytes, Metabolism of Articular Cartilage, and Strategies for Application to Tissue Engineering

Hyaline cartilage, which is characterized by the absence of vascularization and innervation, has minimal self-repair potential in case of damage and defect formation in the chondral layer. Chondrocytes are specialized cells that ensure the synthesis of extracellular matrix components, namely type II collagen and aggregen. On their surface, they express integrins CD44, α1β1, α3β1, α5β1, α10β1, αVβ1, αVβ3, and αVβ5, which are also collagen-binding components of the extracellular matrix. This article aims to contribute to solving the problem of the possible repair of chondral defects through unique methods of tissue engineering, as well as the process of pathological events in articular cartilage. In vitro cell culture models used for hyaline cartilage repair could bring about advanced possibilities. Currently, there are several variants of the combination of natural and synthetic polymers and chondrocytes. In a three-dimensional environment, chondrocytes retain their production capacity. In the case of mesenchymal stromal cells, their favorable ability is to differentiate into a chondrogenic lineage in a three-dimensional culture.

Hypertrophic chondrocytes at the junction of musculoskeletal structures

Hypertrophic chondrocytes are found at unique locations at the junction of skeletal tissues, cartilage growth plate, articular cartilage, enthesis and intervertebral discs. Their role in the skeleton is best understood in the process of endochondral ossification in development and bone fracture healing. Chondrocyte hypertrophy occurs in degenerative conditions such as osteoarthritis. Thus, the role of hypertrophic chondrocytes in skeletal biology and pathology is context dependent. This review will focus on hypertrophic chondrocytes in endochondral ossification, in which they exist in a transient state, but acting as a central regulator of differentiation, mineralization, vascularization and conversion to bone. The amazing journey of a chondrocyte from being entrapped in the extracellular matrix environment to becoming proliferative then hypertrophic will be discussed. Recent studies on the dynamic changes and plasticity of hypertrophic chondrocytes have provided new insights into how we view these cells, not as terminally differentiated but as cells that can dedifferentiate to more progenitor-like cells in a transition to osteoblasts and adipocytes, as well as a source of skeletal stem and progenitor cells residing in the bone marrow. This will provide a foundation for studies of hypertrophic chondrocytes at other skeletal sites in development, tissue maintenance, pathology and therapy.

Common features of cartilage maturation are not conserved in an amphibian model

BACKGROUND

Mouse, chick, and zebrafish undergo a highly conserved program of cartilage maturation during endochondral ossification (bone formation via a cartilage template). Standard histological and molecular features of cartilage maturation are chondrocyte hypertrophy, downregulation of the chondrogenic markers Sox9 and Col2a1, and upregulation of Col10a1. We tested whether cartilage maturation is conserved in an amphibian, the western clawed frog Xenopus tropicalis, using in situ hybridization for standard markers and a novel laser-capture microdissection RNAseq data set. We also functionally tested whether thyroid hormone drives cartilage maturation in X tropicalis, as it does in other vertebrates.

RESULTS

The developing frog humerus mostly followed the standard progression of cartilage maturation. Chondrocytes gradually became hypertrophic as col2a1 and sox9 were eventually down-regulated, but col10a1 was not up-regulated. However, the expression levels of several genes associated with the early formation of cartilage, such as acan, sox5, and col9a2, remained highly expressed even as humeral chondrocytes matured. Greater deviances were observed in head cartilages, including the ceratohyal, which underwent hypertrophy within hours of becoming cartilaginous, maintained relatively high levels of col2a1 and sox9, and lacked col10a1 expression. Interestingly, treating frog larvae with thyroid hormone antagonists did not specifically reduce head cartilage hypertrophy, resulting rather in a global developmental delay.

CONCLUSION

These data reveal that basic cartilage maturation features in the head, and to a lesser extent in the limb, are not conserved in X tropicalis. Future work revealing how frogs deviate from the standard cartilage maturation program might shed light on both evolutionary and health studies.

Single-cell transcriptomic analysis identifies a highly replicating Cd168+ skeletal stem/progenitor cell population in mouse long bones

Skeletal stem/progenitor cells (SSPCs) are tissue-specific stem/progenitor cells localized within skeletons and contribute to bone development, homeostasis, and regeneration. However, the heterogeneity of SSPC populations in mouse long bones and their respective regenerative capacity remain to be further clarified. In this study, we perform integrated analysis using single-cell RNA sequencing (scRNA-seq) datasets of mouse hindlimb buds, postnatal long bones, and fractured long bones. Our analyses reveal the heterogeneity of osteochondrogenic lineage cells and recapitulate the developmental trajectories during mouse long bone growth. In addition, we identify a novel Cd168+ SSPC population with highly replicating capacity and osteochondrogenic potential in embryonic and postnatal long bones. Moreover, the Cd168+ SSPCs can contribute to newly formed skeletal tissues during fracture healing. Furthermore, the results of multicolor immunofluorescence show that Cd168+ SSPCs reside in the superficial zone of articular cartilage as well as in growth plates of postnatal mouse long bones. In summary, we identify a novel Cd168+ SSPC population with regenerative potential in mouse long bones, which adds to the knowledge of the tissue-specific stem cells in skeletons.

Conditional Loss of MEF2C Expression in Osteoclasts Leads to a Sex-Specific Osteopenic Phenotype

Myocyte enhancement factor 2C (MEF2C) is a transcription factor studied in the development of skeletal and smooth muscles. Bone resorption studies have exhibited that the reduced expression of MEF2C contributes to osteopetrosis and the dysregulation of pathological bone remodeling. Our current study aims to determine how MEF2C contributes to osteoclast differentiation and to analyze the skeletal phenotype of Mef2c-cKO mice (Cfms-cre; Mef2cfl/fl). qRT-PCR and Western blot demonstrated that Mef2c expression is highest during the early days of osteoclast differentiation. Osteoclast genes, including c-Fos, c-Jun, Dc-stamp, Cathepsin K, and Nfatc1, had a significant reduction in expression, along with a reduction in osteoclast size. Despite reduced CTX activity, female Mef2c cKO mice were osteopenic, with decreased bone formation as determined via a P1NP ELISA, and a reduced number of osteoblasts. There was no difference between male WT and Mef2c-cKO mice. Our results suggest that Mef2c is critical for osteoclastogenesis, and that its dysregulation leads to a sex-specific osteopenic phenotype.

Upregulated ribosome pathway plays a key role in HDAC4, improving the survival rate and biofunction of chondrocytes

Aims

To explore the novel molecular mechanisms of histone deacetylase 4 (HDAC4) in chondrocytes via RNA sequencing (RNA-seq) analysis.

Methods

Empty adenovirus (EP) and a HDAC4 overexpression adenovirus were transfected into cultured human chondrocytes. The cell survival rate was examined by real-time cell analysis (RTCA) and EdU and flow cytometry assays. Cell biofunction was detected by Western blotting. The expression profiles of messenger RNAs (mRNAs) in the EP and HDAC4 transfection groups were assessed using whole-transcriptome sequencing (RNA-seq). Volcano plot, Gene Ontology, and pathway analyses were performed to identify differentially expressed genes (DEGs). For verification of the results, the A289E/S246/467/632 A sites of HDAC4 were mutated to enhance the function of HDAC4 by increasing HDAC4 expression in the nucleus. RNA-seq was performed to identify the molecular mechanism of HDAC4 in chondrocytes. Finally, the top ten DEGs associated with ribosomes were verified by quantitative polymerase chain reaction (QPCR) in chondrocytes, and the top gene was verified both in vitro and in vivo.

Results

HDAC4 markedly improved the survival rate and biofunction of chondrocytes. RNA-seq analysis of the EP and HDAC4 groups showed that HDAC4 induced 2,668 significant gene expression changes in chondrocytes (1,483 genes upregulated and 1,185 genes downregulated, p < 0.05), and ribosomes exhibited especially large increases. The results were confirmed by RNA-seq of the EP versus mutated HDAC4 groups and the validations in vitro and in vivo.

Conclusion

The enhanced ribosome pathway plays a key role in the mechanism by which HDAC4 improves the survival rate and biofunction of chondrocytes.

The Effect of miR-140-5p with HDAC4 towards Growth and Differentiation Signaling of Chondrocytes in Thiram-Induced Tibial Dyschondroplasia

There is evidence to suggest that microRNA-140-5p (miR-140), which acts as a suppressor, is often elevated and has a role in various malignancies. Nevertheless, neither the function nor the mechanisms in chondrocytes linked with bone disorders, e.g., tibial dyschondroplasia (TD), have been satisfactorily established. The purpose of this study was to look into the role of microRNA-140-5p (miR-140) and its interaction with HDAC4 in chondrocytes, as well as the implications for tibial dyschondroplasia (TD), with a particular focus on the relationship between low miR-140 expression and poor pathologic characteristics, as well as its physiological effects on chondrocyte growth, differentiation, and chondrodysplasia. In this investigation, we discovered that TD had a reduced expression level of the miR-140. There was a correlation between low miR-140 expression, poor pathologic characteristics, and the short overall survival of chondrocytes. Our findings show an aberrant reduction in miR-140 expression, and HDAC4 overexpression caused disengagement in resting and proliferation zones. This further resulted in uncontrolled cell proliferation, differentiation, and chondrodysplasia. Mechanistically, HDAC4 inhibited the downstream transcription factors MEF2C and Runx2 and interacted with Col-Ⅱ, Col-X, and COMP. However, miR-140 binding to the 3'-UTR of HDAC4 resulted in the growth and differentiation of chondrocytes. Moreover, the expression of HDAC4 through LMK-235 was significantly decreased, and the expression was significantly increased under ITSA-1, referring to a positive feedback circuit of miR-140 and HDAC4 for endochondral bone ossification. Furthermore, as a prospective treatment, the flavonoids of Rhizoma drynariae (TFRD) therapy increased the expression of miR-140. Compared to the TD group, TFRD treatment increased the expression of growth-promoting and chondrocyte differentiation markers, implying that TFRD can promote chondrocyte proliferation and differentiation in the tibial growth plate. Hence, directing this circuit may represent a promising target for chondrocyte-related bone disorders and all associated pathological bone conditions.

mef2ca and mef2cb Double Mutant Zebrafish Show Altered Craniofacial Phenotype and Motor Behaviour

The transcription factor MEF2C is crucial in neuronal, cardiac, bone and cartilage molecular processes, as well as for craniofacial development. MEF2C was associated with the human disease MRD20, whose patients show abnormal neuronal and craniofacial development. Zebrafish mef2ca;mef2cb double mutants were analysed for abnormalities in craniofacial and behaviour development through phenotypic analysis. Quantitative PCR was performed to investigate the expression levels of neuronal marker genes in mutant larvae. The motor behaviour was analysed by the swimming activity of 6 dpf larvae. We found that mef2ca;mef2cb double mutants display several abnormal phenotypes during early development, including those already described in zebrafish carrying mutations in each paralog, but also (i) a severe craniofacial phenotype (comprising both cartilaginous and dermal bone structures), (ii) developmental arrest due to the disruption of cardiac oedema and (iii) clear alterations in behaviour. We demonstrate that the defects observed in zebrafish mef2ca;mef2cb double mutants are similar to those previously described in MEF2C-null mice and MRD20 patients, confirming the usefulness of these mutant lines as a model for studies concerning MRD20 disease, the identification of new therapeutic targets and screening for possible rescue strategies.

Peptide Regulation of Chondrogenic Stem Cell Differentiation

The search for innovative ways to treat osteoarthritis (OA) is an urgent task for molecular medicine and biogerontology. OA leads to disability in persons of middle and older age, while safe and effective methods of treating OA have not yet been discovered. The directed differentiation of mesenchymal stem cells (MSCs) into chondrocytes is considered one of the possible methods to treat OA. This review describes the main molecules involved in the chondrogenic differentiation of MSCs. The peptides synthesized on the basis of growth factors' structures (SK2.1, BMP, B2A, and SSPEPS) and components of the extracellular matrix of cartilage tissue (LPP, CFOGER, CMP, RDG, and N-cadherin mimetic peptide) offer the greatest promise for the regulation of the chondrogenic differentiation of MSCs. These peptides regulate the WNT, ERK-p38, and Smad 1/5/8 signaling pathways, gene expression, and the synthesis of chondrogenic differentiation proteins such as COL2, SOX9, ACAN, etc.

Nutrient-regulated dynamics of chondroprogenitors in the postnatal murine growth plate

Longitudinal bone growth relies on endochondral ossification in the cartilaginous growth plate, where chondrocytes accumulate and synthesize the matrix scaffold that is replaced by bone. The chondroprogenitors in the resting zone maintain the continuous turnover of chondrocytes in the growth plate. Malnutrition is a leading cause of growth retardation in children; however, after recovery from nutrient deprivation, bone growth is accelerated beyond the normal rate, a phenomenon termed catch-up growth. Although nutritional status is a known regulator of long bone growth, it is largely unknown whether and how chondroprogenitor cells respond to deviations in nutrient availability. Here, using fate-mapping analysis in Axin2CreERT2 mice, we showed that dietary restriction increased the number of Axin2+ chondroprogenitors in the resting zone and simultaneously inhibited their differentiation. Once nutrient deficiency was resolved, the accumulated chondroprogenitor cells immediately restarted differentiation and formed chondrocyte columns, contributing to accelerated growth. Furthermore, we showed that nutrient deprivation reduced the level of phosphorylated Akt in the resting zone and that exogenous IGF-1 restored the phosphorylated Akt level and stimulated differentiation of the pooled chondroprogenitors, decreasing their numbers. Our study of Axin2CreERT2 revealed that nutrient availability regulates the balance between accumulation and differentiation of chondroprogenitors in the growth plate and further demonstrated that IGF-1 partially mediates this regulation by promoting the committed differentiation of chondroprogenitor cells.

O2 variant chip to simulate site-specific skeletogenesis from hypoxic bone marrow

The stemness of bone marrow mesenchymal stem cells (BMSCs) is maintained by hypoxia. The oxygen level increases from vessel-free cartilage to hypoxic bone marrow and, furthermore, to vascularized bone, which might direct the chondrogenesis to osteogenesis and regenerate the skeletal system. Hence, oxygen was diffused from relatively low to high levels throughout a three-dimensional chip. When we cultured BMSCs in the chip and implanted them into the rabbit defect models of low-oxygen cartilage and high-oxygen calvaria bone, (i) the low oxygen level (base) promoted stemness and chondrogenesis of BMSCs with robust antioxidative potential; (ii) the middle level (two times ≥ low) pushed BMSCs to quiescence; and (iii) the high level (four times ≥ low) promoted osteogenesis by disturbing the redox balance and stemness. Last, endochondral or intramembranous osteogenesis upon transition from low to high oxygen in vivo suggests a developmental mechanism-driven solution to promote chondrogenesis to osteogenesis in the skeletal system by regulating the oxygen environment.

Lineage-specific differences and regulatory networks governing human chondrocyte development

To address large gaps in our understanding of the molecular regulation of articular and growth plate cartilage development in humans, we used our directed differentiation approach to generate these distinct cartilage tissues from human embryonic stem cells. The resulting transcriptomic profiles of hESC-derived articular and growth plate chondrocytes were similar to fetal epiphyseal and growth plate chondrocytes, with respect to genes both known and previously unknown to cartilage biology. With the goal to characterize the regulatory landscapes accompanying these respective transcriptomes, we mapped chromatin accessibility in hESC-derived chondrocyte lineages, and mouse embryonic chondrocytes, using ATAC-sequencing. Integration of the expression dataset with the differentially accessible genomic regions revealed lineage-specific gene regulatory networks. We validated functional interactions of two transcription factors (TFs) (RUNX2 in growth plate chondrocytes and RELA in articular chondrocytes) with their predicted genomic targets. The maps we provide thus represent a framework for probing regulatory interactions governing chondrocyte differentiation. This work constitutes a substantial step towards comprehensive and comparative molecular characterizations of distinct chondrogenic lineages and sheds new light on human cartilage development and biology.

Periostin splice variants affect craniofacial growth by influencing chondrocyte hypertrophy

INTRODUCTION

Periostin, an extracellular matrix protein, plays an important role in osteogenesis and is also known to activate several signals that contribute to chondrogenesis. The absence of periostin in periostin knockout mice leads to several disorders such as craniosynostosis and periostitis. There are several splice variants with different roles in heart disease and myocardial infarction. However, little is known about each variant's role in chondrogenesis, followed by bone formation. Therefore, the aim of this study is to investigate the role of several variants in chondrogenesis differentiation and bone formation in the craniofacial region. Periostin splice variants included a full-length variant (Control), a variant lacking exon 17 (ΔEx17), a variant lacking exon 21 (ΔEx21), and another variant lacking both exon 17 and 21 ***(ΔEx17&21).

MATERIALS AND METHODS

We used C56BL6/N mice (n = 6) for the wild type (Control)*** and the three variant type mice (n = 6 each) to identify the effect of each variant morphologically and histologically. Micro-computed tomography demonstrated a smaller craniofacial skeleton in ΔEx17s, ΔEx21s, and ΔEx17&21s compared to Controls, especially the mandibular bone. We, thus, focused on the mandibular condyle.

RESULTS

The most distinctive histological observation was that each defected mouse appeared to have more hypertrophic chondrocytes than Controls. Real-time PCR demonstrated the differences among the group. Moreover, the lack of exon 17 or exon 21 in periostin leads to inadequate chondrocyte differentiation and presents in a diminutive craniofacial skeleton.

DISCUSSION

Therefore, these findings suggested that each variant has a significant role in chondrocyte hypertrophy, leading to suppression of bone formation.

Deciphering postnatal limb development at single-cell resolution

The early postnatal limb developmental progression bridges embryonic and mature stages and mirrors the pathological remodeling of articular cartilage. However, compared with multitudinous research on embryonic limb development, the early postnatal stage seems relatively unnoticed. Here, a systematic work to portray the postnatal limb developmental landscape was carried out by characterization of 19,952 single cells from murine hindlimbs at 4 postnatal stages using single-cell RNA sequencing technique. By delineation of cell heterogeneity, the candidate progenitor sub-clusters marked by Cd34 and Ly6e were discovered in articular cartilage and enthesis, and three cellular developmental branches marked by Col10a1, Spp1, and Tnni2 were reflected in growth plate. The representative transcriptomes and developmental patterns were intensively explored, and the key regulation mechanisms as well as evolvement in osteoarthritis were discussed. Above all, these results expand horizons of postnatal limb developmental biology and reach the interconnections between limb development, remodeling, and regeneration.

Nutrient-regulated dynamics of chondroprogenitors in the postnatal murine growth plate

Longitudinal bone growth relies on endochondral ossification in the cartilaginous growth plate where chondrocytes accumulate and synthesize the matrix scaffold that is replaced by bone. The chondroprogenitors in the resting zone maintain the continuous turnover of chondrocytes in the growth plate. Malnutrition is a leading cause of growth retardation in children; however, after recovery from nutrient deprivation, bone growth is accelerated beyond the normal rate, a phenomenon termed catch-up growth. Though nutritional status is a known regulator of long bone growth, it is largely unknown if and how chondroprogenitor cells respond to deviations in nutrient availability. Here, using fate-mapping analysis in Axin2Cre ^ERT2 mice, we showed that dietary restriction increased the number of Axin2+ chondroprogenitors in the resting zone and simultaneously inhibited their differentiation. Once nutrient deficiency was resolved, the accumulated chondroprogenitor cells immediately restarted differentiation and formed chondrocyte columns, contributing to accelerated growth. Furthermore, we showed that nutrient deprivation reduced the level of phosphorylated Akt in the resting zone, and that exogenous IGF-1 canceled this reduction and stimulated differentiation of the pooled chondroprogenitors, decreasing their numbers. Our study of Axin2Cre ^ERT2 revealed that nutrient availability regulates the balance between accumulation and differentiation of chondroprogenitors in the growth plate, and further demonstrated that IGF-1 partially mediates this regulation by promoting the committed differentiation of the chondroprogenitor cells.

The role of altered protein acetylation in neurodegenerative disease

Acetylation is a key post-translational modification (PTM) involved in the regulation of both histone and non-histone proteins. It controls cellular processes such as DNA transcription, RNA modifications, proteostasis, aging, autophagy, regulation of cytoskeletal structures, and metabolism. Acetylation is essential to maintain neuronal plasticity and therefore essential for memory and learning. Homeostasis of acetylation is maintained through the activities of histone acetyltransferases (HAT) and histone deacetylase (HDAC) enzymes, with alterations to these tightly regulated processes reported in several neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS). Both hyperacetylation and hypoacetylation can impair neuronal physiological homeostasis and increase the accumulation of pathophysiological proteins such as tau, α-synuclein, and Huntingtin protein implicated in AD, PD, and HD, respectively. Additionally, dysregulation of acetylation is linked to impaired axonal transport, a key pathological mechanism in ALS. This review article will discuss the physiological roles of protein acetylation and examine the current literature that describes altered protein acetylation in neurodegenerative disorders.

Rapid induction and long-term self-renewal of neural crest-derived ectodermal chondrogenic cells from hPSCs

Articular cartilage is highly specific and has limited capacity for regeneration if damaged. Human pluripotent stem cells (hPSCs) have the potential to generate any cell type in the body. Here, we report the dual-phase induction of ectodermal chondrogenic cells (ECCs) from hPSCs through the neural crest (NC). ECCs were able to self-renew long-term (over numerous passages) in a cocktail of growth factors and small molecules. The cells stably expressed cranial neural crest-derived mandibular condylar cartilage markers, such as MSX1, FOXC1 and FOXC2. Compared with chondroprogenitors from iPSCs via the paraxial mesoderm, ECCs had single-cell transcriptome profiles similar to condylar chondrocytes. After the removal of the cocktail sustaining self-renewal, the cells stopped proliferating and differentiated into a homogenous chondrocyte population. Remarkably, after transplantation, this cell lineage was able to form cartilage-like structures resembling mandibular condylar cartilage in vivo. This finding provides a framework to generate self-renewing cranial chondrogenic progenitors, which could be useful for developing cell-based therapy for cranial cartilage injury.

Mesenchymal Stem Cell Secreted-Extracellular Vesicles are Involved in Chondrocyte Production and Reduce Adipogenesis during Stem Cell Differentiation

Background:

Extracellular vesicles (EVs) are derived from internal cellular compartments, and have potential as a diagnostic and therapeutic tool in degenerative disease associated with aging. Mesenchymal stem cells (MSCs) have become a promising tool for functional EVs production. This study investigated the efficacy of EVs and its effect on differentiation capacity.

Methods:

The characteristics of MSCs were evaluated by flow cytometry and stem cell differentiation analysis, and a production mode of functional EVs was scaled from MSCs. The concentration and size of EVs were quantitated by Nanoparticle Tracking Analysis (NTA). Western blot analysis was used to assess the protein expression of exosome-specific markers. The effects of MSC-derived EVs were assessed by chondrogenic and adipogenic differentiation analyses and histological observation.

Results:

The range of the particle size of adipose-derived stem cells (ADSCs)- and Wharton’s jelly -MSCs-derived EVs were from 130 to 150 nm as measured by NTA, which showed positive expression of exosomal markers. The chondrogenic induction ability was weakened in the absence of EVs in vitro. Interestingly, after EV administration, type II collagen, a major component in the cartilage extracellular matrix, was upregulated compared to the EV-free condition. Moreover, EVs decreased the lipid accumulation rate during adipogenic induction.

Conclusion:

The results indicated that the production model could facilitate production of effective EVs and further demonstrated the role of MSC-derived EVs in cell differentiation. MSC-derived EVs could be successfully used in cell-free therapy to guide chondrogenic differentiation of ADSC for future clinical applications in cartilage regeneration.

Functional, structural, and molecular characterizations of the leukemogenic driver MEF2D-HNRNPUL1 fusion

Recurrent MEF2D fusions with poor prognosis have been identified in B-cell precursor ALL (BCP-ALL). The molecular mechanisms underlying the pathogenic function of MEF2D fusions are poorly understood. Here, we show that MEF2D-HNRNPUL1 (MH) knock-in mice developed a progressive disease from impaired B-cell development at the pre-pro-B stage to pre-leukemia over 10 to 12 months. When cooperating with NRASG12D, MH drove an outbreak of BCP-ALL, with a more aggressive phenotype than the NRASG12D-induced leukemia. RNA-sequencing identified key networks involved in disease mechanisms. In chromatin immunoprecipitation-sequencing experiments, MH acquired increased chromatin-binding ability, mostly through MEF2D-responsive element (MRE) motifs in target genes, compared with wild-type MEF2D. Using X-ray crystallography, the MEF2D-MRE complex was characterized in atomic resolution, whereas disrupting the MH-DNA interaction alleviated the aberrant target gene expression and the B-cell differentiation arrest. The C-terminal moiety (HNRNPUL1 part) of MH was proven to contribute to the fusion protein's trans-regulatory activity, cofactor recruitment, and homodimerization. Furthermore, targeting MH-driven transactivation of the HDAC family by using the histone deacetylase inhibitor panobinostat in combination with chemotherapy improved the overall survival of MH/NRASG12D BCP-ALL mice. Altogether, these results not only highlight MH as an important driver in leukemogenesis but also provoke targeted intervention against BCP-ALL with MEF2D fusions.

Variable paralog expression underlies phenotype variation

Human faces are variable; we look different from one another. Craniofacial disorders further increase facial variation. To understand craniofacial variation and how it can be buffered, we analyzed the zebrafish mef2ca mutant. When this transcription factor encoding gene is mutated, zebrafish develop dramatically variable craniofacial phenotypes. Years of selective breeding for low and high penetrance of mutant phenotypes produced strains that are either resilient or sensitive to the mef2ca mutation. Here, we compared gene expression between these strains, which revealed that selective breeding enriched for high and low mef2ca paralog expression in the low- and high-penetrance strains, respectively. We found that mef2ca paralog expression is variable in unselected wild-type zebrafish, motivating the hypothesis that heritable variation in paralog expression underlies mutant phenotype severity and variation. In support, mutagenizing the mef2ca paralogs, mef2aa, mef2b, mef2cb, and mef2d demonstrated modular buffering by paralogs. Specifically, some paralogs buffer severity while others buffer variability. We present a novel, mechanistic model for phenotypic variation where variable, vestigial paralog expression buffers development. These studies are a major step forward in understanding the mechanisms of facial variation, including how some genetically resilient individuals can overcome a deleterious mutation.

Strategies to Convert Cells into Hyaline Cartilage: Magic Spells for Adult Stem Cells

Damaged hyaline cartilage gradually decreases joint function and growing pain significantly reduces the quality of a patient's life. The clinically approved procedure of autologous chondrocyte implantation (ACI) for treating knee cartilage lesions has several limits, including the absence of healthy articular cartilage tissues for cell isolation and difficulties related to the chondrocyte expansion in vitro. Today, various ACI modifications are being developed using autologous chondrocytes from alternative sources, such as the auricles, nose and ribs. Adult stem cells from different tissues are also of great interest due to their less traumatic material extraction and their innate abilities of active proliferation and chondrogenic differentiation. According to the different adult stem cell types and their origin, various strategies have been proposed for stem cell expansion and initiation of their chondrogenic differentiation. The current review presents the diversity in developing applied techniques based on autologous adult stem cell differentiation to hyaline cartilage tissue and targeted to articular cartilage damage therapy.

Simultaneous augmentation of muscle and bone by locomomimetism through calcium-PGC-1α signaling

Impaired locomotion has been extensively studied worldwide because those afflicted with it have a potential risk of becoming bedridden. Physical exercise at times can be an effective remedy for frailty, but exercise therapy cannot be applied in all clinical cases. Medication is safer than exercise, but there are no drugs that reinforce both muscle and bone when administered alone. Multiple medications increase the risk of adverse events; thus, there is a need for individual drugs targeting both tissues. To this end, we established a novel sequential drug screening system and identified an aminoindazole derivative, locamidazole (LAMZ), which promotes both myogenesis and osteoblastogenesis while suppressing osteoclastogenesis. Administration of this drug enhanced locomotor function, with muscle and bone significantly strengthened. Mechanistically, LAMZ induced Mef2c and PGC-1α in a calcium signaling-dependent manner. As this signaling is activated upon physical exercise, LAMZ mimics physical exercise. Thus, LAMZ is a promising therapeutic drug for locomotor diseases, including sarcopenia and osteoporosis.

Salt inducible kinases and PTH1R action

Parathyroid hormone is a central regulator of calcium homeostasis. PTH protects the organism from hypocalcemia through its actions in bone and kidney. Recent physiologic studies have revealed key target genes for PTH receptor (PTH1R) signaling in these target organs. However, the complete signal transduction cascade used by PTH1R to accomplish these physiologic actions has remained poorly defined. Here we will review recent studies that have defined an important role for salt inducible kinases downstream of PTH1R in bone, cartilage, and kidney. PTH1R signaling inhibits the activity of salt inducible kinases. Therefore, direct SIK inhibitors represent a promising novel strategy to mimic PTH actions using small molecules. Moreover, a detailed understanding of the molecular circuitry used by PTH1R to exert its biologic effects will afford powerful new models to better understand the diverse actions of this important G protein coupled receptor in health and disease.

Overexpression of transcription factor FoxA2 in the developing skeleton causes an enlargement of the cartilage hypertrophic zone, but it does not trigger ectopic differentiation in immature chondrocytes

We previously found that FoxA factors are necessary for chondrocyte differentiation. To investigate whether FoxA factors alone are sufficient to drive chondrocyte hypertrophy, we build a FoxA2 transgenic mouse in which FoxA2 cDNA is driven by a reiterated Tetracycline Response Element (TRE) and a minimal CMV promoter. This transgenic line was crossed with a col2CRE;Rosa26^rtTA/+ mouse line to generate col2CRE;Rosa26^rtTA/+;TgFoxA2^+/- mice for inducible expression of FoxA2 in cartilage using doxycycline treatment. Ectopic expression of FoxA2 in the developing skeleton reveals skeletal defects and shorter skeletal elements in E17.5 mice. The chondro-osseous border was frequently mis-shaped in mutant mice, with small islands of col.10+ hypertrophic cells extending in the metaphyseal bone. Even though overexpression of FoxA2 causes an accumulation of hypertrophic chondrocytes, it did not trigger ectopic hypertrophy in the immature chondrocytes. This suggests that FoxA2 may need transcriptional co-factors (such as Runx2), whose expression is restricted to the hypertrophic zone, and absent in the immature chondrocytes. To investigate a potential FoxA2/Runx2 interaction in immature chondrocytes versus hypertrophic cells, we separated these two subpopulations by FACS to obtain CD24⁺CD200⁺ hypertrophic chondrocytes and CD24⁺CD200^- immature chondrocytes and we ectopically expressed FoxA2 alone or in combination with Runx2 via lentiviral gene delivery. In CD24⁺CD200⁺ hypertrophic chondrocytes, FoxA2 enhanced the expression of chondrocyte hypertrophic markers collagen 10, MMP13, and alkaline phosphatase. In contrast, in the CD24⁺CD200^- immature chondrocytes, neither FoxA2 nor Runx2 overexpression could induce ectopic expression of hypertrophic markers MMP13, alkaline phosphatase, or PTH/PTHrP receptor. Overall these findings mirror our in vivo data, and suggest that induction of chondrocyte hypertrophy by FoxA2 may require other factors in addition to Runx2 (i.e., Hif2α, MEF2C, or perhaps unknown factors), whose expression/activity is rate-limiting in immature chondrocytes.

Limb Mesoderm and Head Ectomesenchyme Both Express a Core Transcriptional Program During Chondrocyte Differentiation

To explain how cartilage appeared in different parts of the vertebrate body at discrete times during evolution, we hypothesize that different embryonic populations co-opted expression of a core gene regulatory network (GRN) driving chondrocyte differentiation. To test this hypothesis, laser-capture microdissection coupled with RNA-seq was used to reveal chondrocyte transcriptomes in the developing chick humerus and ceratobranchial, which are mesoderm- and neural crest-derived, respectively. During endochondral ossification, two general types of chondrocytes differentiate. Immature chondrocytes (IMM) represent the early stages of cartilage differentiation, while mature chondrocytes (MAT) undergo additional stages of differentiation, including hypertrophy and stimulating matrix mineralization and degradation. Venn diagram analyses generally revealed a high degree of conservation between chondrocyte transcriptomes of the limb and head, including SOX9, COL2A1, and ACAN expression. Typical maturation genes, such as COL10A1, IBSP, and SPP1, were upregulated in MAT compared to IMM in both limb and head chondrocytes. Gene co-expression network (GCN) analyses of limb and head chondrocyte transcriptomes estimated the core GRN governing cartilage differentiation. Two discrete portions of the GCN contained genes that were differentially expressed in limb or head chondrocytes, but these genes were enriched for biological processes related to limb/forelimb morphogenesis or neural crest-dependent processes, respectively, perhaps simply reflecting the embryonic origin of the cells. A core GRN driving cartilage differentiation in limb and head was revealed that included typical chondrocyte differentiation and maturation markers, as well as putative novel "chondrocyte" genes. Conservation of a core transcriptional program during chondrocyte differentiation in both the limb and head suggest that the same core GRN was co-opted when cartilage appeared in different regions of the skeleton during vertebrate evolution.

A high-quality assembly reveals genomic characteristics, phylogenetic status, and causal genes for leucism plumage of Indian peafowl

BACKGROUND

The dazzling phenotypic characteristics of male Indian peafowl (Pavo cristatus) are attractive both to the female of the species and to humans. However, little is known about the evolution of the phenotype and phylogeny of these birds at the whole-genome level. So far, there are no reports regarding the genetic mechanism of the formation of leucism plumage in this variant of Indian peafowl.

RESULTS

A draft genome of Indian peafowl was assembled, with a genome size of 1.05 Gb (the sequencing depth is 362×), and contig and scaffold N50 were up to 6.2 and 11.4 Mb, respectively. Compared with other birds, Indian peafowl showed changes in terms of metabolism, immunity, and skeletal and feather development, which provided a novel insight into the phenotypic evolution of peafowl, such as the large body size and feather morphologies. Moreover, we determined that the phylogeny of Indian peafowl was more closely linked to turkey than chicken. Specifically, we first identified that PMEL was a potential causal gene leading to the formation of the leucism plumage variant in Indian peafowl.

CONCLUSIONS

This study provides an Indian peafowl genome of high quality, as well as a novel understanding of phenotypic evolution and phylogeny of Indian peafowl. These results provide a valuable reference for the study of avian genome evolution. Furthermore, the discovery of the genetic mechanism for the development of leucism plumage is both a breakthrough in the exploration of peafowl plumage and also offers clues and directions for further investigations of the avian plumage coloration and artificial breeding in peafowl.

Characterization of the class IIa histone deacetylases substrate specificity

Class IIa histone deacetylases (HDACs) play critical roles in vertebrate development and physiology, yet direct evidence of their intrinsic deacetylase activity and on substrate specificity regarding the peptide sequence is still missing. In this study, we designed and synthesized a combinatorial peptide library allowing us to profile class IIa HDACs sequence specificity at positions +3 through -3 from the central lysine modified by the well-accepted trifluoroacetyl function. Our data revealed a strong preference for bulky aromatic acids directly flanking the central trifluoroacetyllysine, while all class IIa HDACs disfavor positively charged residues and proline at the +1/-1 positions. The chemical nature of amino acid residues N-terminally to the central trifluoroacetyllysine has a more profound effect on substrate recognition as compared to residues located C-terminally. These findings were validated by designing selected favored and disfavored peptide sequences, with the favored ones are accepted with catalytic efficacy of 75 000 and 525 000 M^-1  s^-1 for HDAC7 and HDAC5, respectively. Results reported here could help in developing class IIa HDACs inhibitors and also in the search for new natural class IIa HDACs substrates.

Potential of Using Infrapatellar–Fat–Pad–Derived Mesenchymal Stem Cells for Therapy in Degenerative Arthritis: Chondrogenesis, Exosomes, and Transcription Regulation

Infrapatellar fat pad–derived mesenchymal stem cells (IPFP-MSCs) are a type of adipose-derived stem cell (ADSC). They potentially contribute to cartilage regeneration and modulation of the immune microenvironment in patients with osteoarthritis (OA). The ability of IPFP-MSCs to increase chondrogenic capacity has been reported to be greater, less age dependent, and less affected by inflammatory changes than that of other MSCs. Transcription-regulatory factors strictly regulate the cartilage differentiation of MSCs. However, few studies have explored the effect of transcriptional factors on IPFP-MSC-based neocartilage formation, cartilage engineering, and tissue functionality during and after chondrogenesis. Instead of intact MSCs, MSC-derived extracellular vesicles could be used for the treatment of OA. Furthermore, exosomes are increasingly being considered the principal therapeutic agent in MSC secretions that is responsible for the regenerative and immunomodulatory functions of MSCs in cartilage repair. The present study provides an overview of advancements in enhancement strategies for IPFP-MSC chondrogenic differentiation, including the effects of transcriptional factors, the modulation of released exosomes, delivery mechanisms for MSCs, and ethical and regulatory points concerning the development of MSC products. This review will contribute to the understanding of the IPFP-MSC chondrogenic differentiation process and enable the improvement of IPFP-MSC-based cartilage tissue engineering.

The Osteogenesis Imperfecta Type V Mutant BRIL/IFITM5 Promotes Transcriptional Activation of MEF2, NFATc, and NR4A in Osteoblasts

BRIL (bone restricted ifitm-like; also known as IFITM5) is a transmembrane protein expressed in osteoblasts. Although its role in skeletal development and homeostasis is unknown, mutations in BRIL result in rare dominant forms of osteogenesis imperfecta. The pathogenic mechanism has been proposed to be a gain-of or neomorphic function. To understand the function of BRIL and its OI type V mutant (MALEP BRIL) and whether they could activate signaling pathways in osteoblasts, we performed a luciferase reporter assay screen based on the activity of 26 transcription factors. When overexpressed in MC3T3-E1 and MLO-A5 cells, the MALEP BRIL activated the reporters dependent on MEF2, NFATc, and NR4A significantly more. Additional co-transfection experiments with MEF2C and NFATc1 and a number of their modulators (HDAC4, calcineurin, RCAN, FK506) confirmed the additive or synergistic activation of the pathways by MALEP, and suggested a coordinated regulation involving calcineurin. Endogenous levels of Nr4a members, as well as Ptgs2, were upregulated by MALEP BRIL. Y2H and co-immunoprecipitation indicated that BRIL interacted with CAML, but its contribution as the most upstream stimulator of the Ca²⁺-calcineurin-MEF2/NFATc cascade was not confirmed convincingly. Altogether the data presented provide the first ever readout to monitor for BRIL activity and suggest a potential gain-of-function causative effect for MALEP BRIL in OI type V, leading to perturbed signaling events and gene expression.

Single-cell transcriptomics of LepR-positive skeletal cells reveals heterogeneous stress-dependent stem and progenitor pools

Leptin receptor (LepR)-positive cells are key components of the bone marrow hematopoietic microenvironment, and highly enrich skeletal stem and progenitor cells that maintain homeostasis of the adult skeleton. However, the heterogeneity and lineage hierarchy within this population has been elusive. Using genetic lineage tracing and single-cell RNA sequencing, we found that Lepr-Cre labels most bone marrow stromal cells and osteogenic lineage cells in adult long bones. Integrated analysis of Lepr-Cre-traced cells under homeostatic and stress conditions revealed dynamic changes of the adipogenic, osteogenic, and periosteal lineages. Importantly, we discovered a Notch3+ bone marrow sub-population that is slow-cycling and closely associated with the vasculatures, as well as key transcriptional networks promoting osteo-chondrogenic differentiation. We also identified a Sca-1+ periosteal sub-population with high clonogenic activity but limited osteo-chondrogenic potential. Together, we mapped the transcriptomic landscape of adult LepR+ stem and progenitor cells and uncovered cellular and molecular mechanisms underlying their maintenance and lineage specification.

Evolution of Somite Compartmentalization: A View From Xenopus

Somites are transitory metameric structures at the basis of the axial organization of vertebrate musculoskeletal system. During evolution, somites appear in the chordate phylum and compartmentalize mainly into the dermomyotome, the myotome, and the sclerotome in vertebrates. In this review, we summarized the existing literature about somite compartmentalization in Xenopus and compared it with other anamniote and amniote vertebrates. We also present and discuss a model that describes the evolutionary history of somite compartmentalization from ancestral chordates to amniote vertebrates. We propose that the ancestral organization of chordate somite, subdivided into a lateral compartment of multipotent somitic cells (MSCs) and a medial primitive myotome, evolves through two major transitions. From ancestral chordates to vertebrates, the cell potency of MSCs may have evolved and gave rise to all new vertebrate compartments, i.e., the dermomyome, its hypaxial region, and the sclerotome. From anamniote to amniote vertebrates, the lateral MSC territory may expand to the whole somite at the expense of primitive myotome and may probably facilitate sclerotome formation. We propose that successive modifications of the cell potency of some type of embryonic progenitors could be one of major processes of the vertebrate evolution.

Transcription factor MEF2D is required for the maintenance of MLL-rearranged acute myeloid leukemia

MEF2D is highly expressed in MLL-rearranged AML and required for leukemia development in vitro and in vivo.

MEF2D suppresses CEBPE-mediated myeloid differentiation in AML.

Acute myeloid leukemia (AML) with MLL-rearrangement (MLL-r) comprises ∼10% of all AML cases and portends poor outcomes. Much remains uncovered on how MLL-r AML drives leukemia development while preventing cells from normal myeloid differentiation. Here, we identified that transcription factor MEF2D is a super-enhancer-associated, highly expressed gene in MLL-r AML. Knockout of MEF2D profoundly impaired leukemia growth, induced myeloid differentiation, and delayed oncogenic progression in vivo. Mechanistically, MEF2D loss led to robust activation of a CEBPE-centered myeloid differentiation program in AML cells. Chromatin profiling revealed that MEF2D binds to and suppresses the chromatin accessibility of CEBPE cis-regulatory regions. In human acute leukemia samples, MEF2D expression showed a strong negative correlation with the expression of CEBPE. Depletion of CEBPE partially rescued the cell growth defect and myeloid cell differentiation induced by the loss of MEF2D. Lastly, we show that MEF2D is positively regulated by HOXA9, and downregulation of MEF2D is an important mechanism for DOT1L inhibitor-induced antileukemia effects. Collectively, our findings suggest that MEF2D plays a critical role in human MLL-r AML and uncover the MEF2D-CEBPE axis as a crucial transcriptional mechanism regulating leukemia cell self-renewal and differentiation block.

Control of osteocyte dendrite formation by Sp7 and its target gene osteocrin

Some osteoblasts embed within bone matrix, change shape, and become dendrite-bearing osteocytes. The circuitry that drives dendrite formation during "osteocytogenesis" is poorly understood. Here we show that deletion of Sp7 in osteoblasts and osteocytes causes defects in osteocyte dendrites. Profiling of Sp7 target genes and binding sites reveals unexpected repurposing of this transcription factor to drive dendrite formation. Osteocrin is a Sp7 target gene that promotes osteocyte dendrite formation and rescues defects in Sp7-deficient mice. Single-cell RNA-sequencing demonstrates defects in osteocyte maturation in the absence of Sp7. Sp7-dependent osteocyte gene networks are associated with human skeletal diseases. Moreover, humans with a SP7R316C mutation show defective osteocyte morphology. Sp7-dependent genes that mark osteocytes are enriched in neurons, highlighting shared features between osteocytic and neuronal connectivity. These findings reveal a role for Sp7 and its target gene Osteocrin in osteocytogenesis, revealing that pathways that control osteocyte development influence human bone diseases.

The Jun-dependent axon regeneration gene program: Jun promotes regeneration over plasticity

The regeneration-associated gene (RAG) expression program is activated in injured peripheral neurons after axotomy and enables long-distance axon re-growth. Over 1000 genes are regulated, and many transcription factors are upregulated or activated as part of this response. However, a detailed picture of how RAG expression is regulated is lacking. In particular, the transcriptional targets and specific functions of the various transcription factors are unclear. Jun was the first-regeneration-associated transcription factor identified and the first shown to be functionally important. Here we fully define the role of Jun in the RAG expression program in regenerating facial motor neurons. At 1, 4 and 14 days after axotomy, Jun upregulates 11, 23 and 44% of the RAG program, respectively. Jun functions relevant to regeneration include cytoskeleton production, metabolic functions and cell activation, and the downregulation of neurotransmission machinery. In silico analysis of promoter regions of Jun targets identifies stronger over-representation of AP1-like sites than CRE-like sites, although CRE sites were also over-represented in regions flanking AP1 sites. Strikingly, in motor neurons lacking Jun, an alternative SRF-dependent gene expression program is initiated after axotomy. The promoters of these newly expressed genes exhibit over-representation of CRE sites in regions near to SRF target sites. This alternative gene expression program includes plasticity-associated transcription factors and leads to an aberrant early increase in synapse density on motor neurons. Jun thus has the important function in the early phase after axotomy of pushing the injured neuron away from a plasticity response and towards a regenerative phenotype.

Short exposure to glyphosate induces locomotor, craniofacial, and bone disorders in zebrafish (Danio rerio) embryos

Glyphosate [N-(phosphonomethyl)glycine] is the active ingredient in widely used broad-spectrum herbicides. Even though the toxicity mechanism of this herbicide in vertebrates is poorly understood, evidence suggests that glyphosate is an endocrine disruptor capable of producing morphological anomalies as well as cardiotoxic and neurotoxic effects. We used the zebrafish model to assess the effects of early life glyphosate exposure on the development of cartilage and bone tissues and organismal responses. We found functional alterations, including a reduction in the cardiac rate, significant changes in the spontaneous tail movement pattern, and defects in craniofacial development. These effects were concomitant with alterations in the level of the estrogen receptor alpha osteopontin and bone sialoprotein. We also found that embryos exposed to glyphosate presented spine deformities as adults. These developmental alterations are likely induced by changes in protein levels related to bone and cartilage formation.

Manganese deficiency induces avian tibial dyschondroplasia by inhibiting chondrocyte proliferation and differentiation

Manganese (Mn) is an essential trace element for bone growth, and its deficiency has been shown to increase the incidence of leg abnormalities in fast-growing broilers, such as tibial dyschondroplasia (TD). Proliferation and differentiation of growth plate chondrocyte are critical for tibia development, but their roles in Mn deficiency-induced TD remains to be elucidated. Thirty 1-day-old Arbor Acres chicks were randomly divided into two groups and fed with control diet (60 mg Mn/kg diet) and Mn-deficiency diet (22 mg Mn/kg diet) for 42 days, respectively. Mn deficiency-induced TD model was successfully established and samples from proximal tibia metaphysis and growth plate were collected for assays. Pathological observation showed that Mn deficiency induced morphological abnormality and irregular arrangement of chondrocytes in proliferative and hypertrophic zone of tibial growth plate. Also, Mn deficiency decreased mRNA and protein expression levels of type II collagen and type X collagen in tibial growth plate, indicating the impairment of proliferating and hypertrophic chondrocytes. Moreover, down-regulated gene expression levels of Sox9, Tgf-β, Ihh, Runx2, Mef2c and Bmp-2 were shown in tibial growth plate of Mn-deficiency group, demonstrating that Mn deficiency inhibited the transcription levels of key regulators to disrupt chondrocyte proliferation and differentiation. Collectively, these findings confirmed that Mn deficiency affected the proliferation and differentiation of chondrocytes in tibial growth plate via inhibiting related regulatory factors, leading to TD in broilers.

Chondrocytes in the resting zone of the growth plate are maintained in a Wnt-inhibitory environment

Chondrocytes in the resting zone of the postnatal growth plate are characterized by slow cell cycle progression, and encompass a population of parathyroid hormone-related protein (PTHrP)-expressing skeletal stem cells that contribute to the formation of columnar chondrocytes. However, how these chondrocytes are maintained in the resting zone remains undefined. We undertook a genetic pulse-chase approach to isolate slow cycling, label-retaining chondrocytes (LRCs) using a chondrocyte-specific doxycycline-controllable Tet-Off system regulating expression of histone 2B-linked GFP. Comparative RNA-seq analysis identified significant enrichment of inhibitors and activators for Wnt signaling in LRCs and non-LRCs, respectively. Activation of Wnt/β-catenin signaling in PTHrP+ resting chondrocytes using Pthlh-creER and Apc-floxed allele impaired their ability to form columnar chondrocytes. Therefore, slow-cycling chondrocytes are maintained in a Wnt-inhibitory environment within the resting zone, unraveling a novel mechanism regulating maintenance and differentiation of PTHrP+ skeletal stem cells of the postnatal growth plate.

The MEK5/ERK5 Pathway in Health and Disease

The MEK5/ERK5 mitogen-activated protein kinases (MAPK) cascade is a unique signaling module activated by both mitogens and stress stimuli, including cytokines, fluid shear stress, high osmolarity, and oxidative stress. Physiologically, it is mainly known as a mechanoreceptive pathway in the endothelium, where it transduces the various vasoprotective effects of laminar blood flow. However, it also maintains integrity in other tissues exposed to mechanical stress, including bone, cartilage, and muscle, where it exerts a key function as a survival and differentiation pathway. Beyond its diverse physiological roles, the MEK5/ERK5 pathway has also been implicated in various diseases, including cancer, where it has recently emerged as a major escape route, sustaining tumor cell survival and proliferation under drug stress. In addition, MEK5/ERK5 dysfunction may foster cardiovascular diseases such as atherosclerosis. Here, we highlight the importance of the MEK5/ERK5 pathway in health and disease, focusing on its role as a protective cascade in mechanical stress-exposed healthy tissues and its function as a therapy resistance pathway in cancers. We discuss the perspective of targeting this cascade for cancer treatment and weigh its chances and potential risks when considering its emerging role as a protective stress response pathway.