Sox9 directs hypertrophic maturation and blocks osteoblast differentiation of growth plate chondrocytes

Stat3 loss in mesenchymal progenitors causes Job syndrome-like skeletal defects by reducing Wnt/β-catenin signaling

Job syndrome is a rare genetic disorder caused by STAT3 mutations and primarily characterized by immune dysfunction along with comorbid skeleton developmental abnormalities including osteopenia, recurrent fracture of long bones, and scoliosis. So far, there is no definitive cure for the skeletal defects in Job syndrome, and treatments are limited to management of clinical symptoms only. Here, we have investigated the molecular mechanism whereby Stat3 regulates skeletal development and osteoblast differentiation. We showed that removing Stat3 function in the developing limb mesenchyme or osteoprogenitor cells in mice resulted in shortened and bow limbs with multiple fractures in long bones that resembled the skeleton symptoms in the Job Syndrome. However, Stat3 loss did not alter chondrocyte differentiation and hypertrophy in embryonic development, while osteoblast differentiation was severely reduced. Genome-wide transcriptome analyses as well as biochemical and histological studies showed that Stat3 loss resulted in down-regulation of Wnt/β-catenin signaling. Restoration of Wnt/β-catenin signaling by injecting BIO, a small molecule inhibitor of GSK3, or crossing with a Lrp5 gain of function (GOF) allele, rescued the bone reduction phenotypes due to Stat3 loss to a great extent. These studies uncover the essential functions of Stat3 in maintaining Wnt/β-catenin signaling in early mesenchymal or osteoprogenitor cells and provide evidence that bone defects in the Job Syndrome are likely caused by Wnt/β-catenin signaling reduction due to reduced STAT3 activities in bone development. Enhancing Wnt/β-catenin signaling could be a therapeutic approach to reduce bone symptoms of Job syndrome patients.

Regulation and Role of Transcription Factors in Osteogenesis

Bone is a dynamic tissue constantly responding to environmental changes such as nutritional and mechanical stress. Bone homeostasis in adult life is maintained through bone remodeling, a controlled and balanced process between bone-resorbing osteoclasts and bone-forming osteoblasts. Osteoblasts secrete matrix, with some being buried within the newly formed bone, and differentiate to osteocytes. During embryogenesis, bones are formed through intramembraneous or endochondral ossification. The former involves a direct differentiation of mesenchymal progenitor to osteoblasts, and the latter is through a cartilage template that is subsequently converted to bone. Advances in lineage tracing, cell sorting, and single-cell transcriptome studies have enabled new discoveries of gene regulation, and new populations of skeletal stem cells in multiple niches, including the cartilage growth plate, chondro-osseous junction, bone, and bone marrow, in embryonic development and postnatal life. Osteoblast differentiation is regulated by a master transcription factor RUNX2 and other factors such as OSX/SP7 and ATF4. Developmental and environmental cues affect the transcriptional activities of osteoblasts from lineage commitment to differentiation at multiple levels, fine-tuned with the involvement of co-factors, microRNAs, epigenetics, systemic factors, circadian rhythm, and the microenvironments. In this review, we will discuss these topics in relation to transcriptional controls in osteogenesis.

Signaling Pathways in Bone Development and Their Related Skeletal Dysplasia

Bone development is a tightly regulated process. Several integrated signaling pathways including HH, PTHrP, WNT, NOTCH, TGF-β, BMP, FGF and the transcription factors SOX9, RUNX2 and OSX are essential for proper skeletal development. Misregulation of these signaling pathways can cause a large spectrum of congenital conditions categorized as skeletal dysplasia. Since the signaling pathways involved in skeletal dysplasia interact at multiple levels and have a different role depending on the time of action (early or late in chondrogenesis and osteoblastogenesis), it is still difficult to precisely explain the physiopathological mechanisms of skeletal disorders. However, in recent years, significant progress has been made in elucidating the mechanisms of these signaling pathways and genotype-phenotype correlations have helped to elucidate their role in skeletogenesis. Here, we review the principal signaling pathways involved in bone development and their associated skeletal dysplasia.

Dmrt2 promotes transition of endochondral bone formation by linking Sox9 and Runx2

Endochondral bone formation is fundamental for skeletal development. During this process, chondrocytes undergo multiple steps of differentiation and coordinated transition from a proliferating to a hypertrophic stage, which is critical to advance skeletal development. Here, we identified the transcription factor Dmrt2 (double-sex and mab-3 related transcription factor 2) as a Sox9-inducible gene that promotes chondrocyte hypertrophy in pre-hypertrophic chondrocytes. Epigenetic analysis further demonstrated that Sox9 regulates Dmrt2 expression through an active enhancer located 18 kb upstream of the Dmrt2 gene and that this enhancer's chromatin status is progressively activated through chondrocyte differentiation. Dmrt2-knockout mice exhibited a dwarf phenotype with delayed initiation of chondrocyte hypertrophy. Dmrt2 augmented hypertrophic chondrocyte gene expression including Ihh through physical and functional interaction with Runx2. Furthermore, Dmrt2 deficiency reduced Runx2-dependent Ihh expression. Our findings suggest that Dmrt2 is critical for sequential chondrocyte differentiation during endochondral bone formation and coordinates the transcriptional network between Sox9 and Runx2.

EGR1 promotes stemness and predicts a poor outcome of uterine cervical cancer by inducing SOX9 expression

BACKGROUND

Early growth response-1 (EGR1) is a transcription factor involved in the progression of several cancer types. However, the expression and clinical significance of EGR1 in uterine cervical cancer (CC) have not been elucidated.

OBJECTIVE

To investigate the expression, clinical significance and prognostic value of EGR1 in CC.

METHODS

The expression of EGR1 was detected in 13 CCs and paired adjacent tissues with qRT-PCR and in 144 CC tissues with immunohistochemistry (IHC). The IHC scores were used to divide the patients into subsets with low and high EGR1 expression. The correlations between the EGR1 expression and clinicopathological factors were analyzed with the chi-square test, and the prognostic significance of EGR1 expression was evaluated with univariate and multivariate analyses. The functions of EGR1 in the proliferation, invasion and stemness of CC cells were investigated, and the molecular mechanism was assessed by in vitro experiments.

RESULTS

High expression of EGR1 was significantly associated with low survival rates of CC. EGR1 is an independent prognostic biomarker of CC, and its high expression predicted a poor outcome. EGR1 facilitated stemness and thus promoted proliferation and invasion of CC cells. SOX9 played an essential role in the EGR1-induced progression of CC cells.

CONCLUSIONS

EGR1 is an independent prognostic biomarker of CC. High EGR1 expression promoted proliferation, invasion and stemness by increasing SOX9 expression in CC cells. Our results suggested that the EGR1-SOX9 axis may be a potential drug target and that blocking the EGR1-SOX9 axis may be a possible approach to treating CC.

SOX9 modulates cancer biomarker and cilia genes in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive form of cancer with high mortality. The cellular origins of PDAC are largely unknown; however, ductal cells, especially centroacinar cells (CACs), have several characteristics in common with PDAC, such as expression of SOX9 and components of the Notch-signaling pathway. Mutations in KRAS and alterations to Notch signaling are common in PDAC, and both these pathways regulate the transcription factor SOX9. To identify genes regulated by SOX9, we performed siRNA knockdown of SOX9 followed by RNA-seq in PANC-1s, a human PDAC cell line. We report 93 differentially expressed (DE) genes, with convergence on alterations to Notch-signaling pathways and ciliogenesis. These results point to SOX9 and Notch activity being in a positive feedback loop and SOX9 regulating cilia production in PDAC. We additionally performed ChIP-seq in PANC-1s to identify direct targets of SOX9 binding and integrated these results with our DE gene list. Nine of the top 10 downregulated genes have evidence of direct SOX9 binding at their promoter regions. One of these targets was the cancer stem cell marker EpCAM. Using whole-mount in situ hybridization to detect epcam transcript in zebrafish larvae, we demonstrated that epcam is a CAC marker and that Sox9 regulation of epcam expression is conserved in zebrafish. Additionally, we generated an epcam null mutant and observed pronounced defects in ciliogenesis during development. Our results provide a link between SOX9, EpCAM and ciliary repression that can be exploited in improving our understanding of the cellular origins and mechanisms of PDAC.

Ultra-processed food targets bone quality via endochondral ossification

Ultra-processed foods have known negative implications for health; however, their effect on skeletal development has never been explored. Here, we show that young rats fed ultra-processed food rich in fat and sugar suffer from growth retardation due to lesions in their tibial growth plates. The bone mineral density decreases significantly, and the structural parameters of the bone deteriorate, presenting a sieve-like appearance in the cortices and poor trabecular parameters in long bones and vertebrae. This results in inferior mechanical performance of the entire bone with a high fracture risk. RNA sequence analysis of the growth plates demonstrated an imbalance in extracellular matrix formation and degradation and impairment of proliferation, differentiation and mineralization processes. Our findings highlight, for the first time, the severe impact of consuming ultra-processed foods on the growing skeleton. This pathology extends far beyond that explained by the known metabolic effects, highlighting bone as a new target for studies of modern diets.

SOX9 keeps growth plates and articular cartilage healthy by inhibiting chondrocyte dedifferentiation/osteoblastic redifferentiation

Cartilage is essential throughout vertebrate life. It starts developing in embryos when osteochondroprogenitor cells commit to chondrogenesis, activate a pancartilaginous program to form cartilaginous skeletal primordia, and also embrace a growth-plate program to drive skeletal growth or an articular program to build permanent joint cartilage. Various forms of cartilage malformation and degeneration diseases afflict humans, but underlying mechanisms are still incompletely understood and treatment options suboptimal. The transcription factor SOX9 is required for embryonic chondrogenesis, but its postnatal roles remain unclear, despite evidence that it is down-regulated in osteoarthritis and heterozygously inactivated in campomelic dysplasia, a severe skeletal dysplasia characterized postnatally by small stature and kyphoscoliosis. Using conditional knockout mice and high-throughput sequencing assays, we show here that SOX9 is required postnatally to prevent growth-plate closure and preosteoarthritic deterioration of articular cartilage. Its deficiency prompts growth-plate chondrocytes at all stages to swiftly reach a terminal/dedifferentiated stage marked by expression of chondrocyte-specific (Mgp) and progenitor-specific (Nt5e and Sox4) genes. Up-regulation of osteogenic genes (Runx2, Sp7, and Postn) and overt osteoblastogenesis quickly ensue. SOX9 deficiency does not perturb the articular program, except in load-bearing regions, where it also provokes chondrocyte-to-osteoblast conversion via a progenitor stage. Pathway analyses support roles for SOX9 in controlling TGFβ and BMP signaling activities during this cell lineage transition. Altogether, these findings deepen our current understanding of the cellular and molecular mechanisms that specifically ensure lifelong growth-plate and articular cartilage vigor by identifying osteogenic plasticity of growth-plate and articular chondrocytes and a SOX9-countered chondrocyte dedifferentiation/osteoblast redifferentiation process.

Piezo1 Inactivation in Chondrocytes Impairs Trabecular Bone Formation

The skeleton is a dynamic tissue continuously adapting to mechanical stimuli. Although matrix-embedded osteocytes are considered as the key mechanoresponsive bone cells, all other skeletal cell types are principally exposed to macroenvironmental and microenvironmental mechanical influences that could potentially affect their activities. It was recently reported that Piezo1, one of the two mechanically activated ion channels of the Piezo family, functions as a mechanosensor in osteoblasts and osteocytes. Here we show that Piezo1 additionally plays a critical role in the process of endochondral bone formation. More specifically, by targeted deletion of Piezo1 or Piezo2 in either osteoblast (Runx2Cre) or osteoclast lineage cells (Lyz2Cre), we observed severe osteoporosis with numerous spontaneous fractures specifically in Piezo1^Runx2Cre mice. This phenotype developed at an early postnatal stage and primarily affected the formation of the secondary spongiosa. The presumptive Piezo1^Runx2Cre osteoblasts in this region displayed an unusual flattened appearance and were positive for type X collagen. Moreover, transcriptome analyses of primary osteoblasts identified an unexpected induction of chondrocyte-related genes in Piezo1^Runx2Cre cultures. Because Runx2 is not only expressed in osteoblast progenitor cells, but also in prehypertrophic chondrocytes, these data suggested that Piezo1 functions in growth plate chondrocytes to ensure trabecular bone formation in the process of endochondral ossification. To confirm this hypothesis, we generated mice with Piezo1 deletion in chondrocytes (Col2a1Cre). These mice essentially recapitulated the phenotype of Piezo1^Runx2Cre animals, because they displayed early-onset osteoporosis with multiple fractures, as well as impaired formation of the secondary spongiosa with abnormal osteoblast morphology. Our data identify a previously unrecognized key function of Piezo1 in endochondral ossification, which, together with its role in bone remodeling, suggests that Piezo1 represents an attractive target for the treatment of skeletal disorders. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Repression of Wnt/β-catenin signaling by SOX9 and Mastermind-like transcriptional coactivator 2

Wnt/β-catenin signaling requires inhibition of a multiprotein destruction complex that targets β-catenin for proteasomal degradation. SOX9 is a potent antagonist of the Wnt pathway and has been proposed to act through direct binding to β-catenin or the β-catenin destruction complex. Here, we demonstrate that SOX9 promotes turnover of β-catenin in mammalian cell culture, but this occurs independently of the destruction complex and the proteasome. This activity requires SOX9's ability to activate transcription. Transcriptome analysis revealed that SOX9 induces the expression of the Notch coactivator Mastermind-like transcriptional activator 2 (MAML2), which is required for SOX9-dependent Wnt/β-catenin antagonism. MAML2 promotes β-catenin turnover independently of Notch signaling, and MAML2 appears to associate directly with β-catenin in an in vitro binding assay. This work defines a previously unidentified pathway that promotes β-catenin degradation, acting in parallel to established mechanisms. SOX9 uses this pathway to restrict Wnt/β-catenin signaling.

Foxc1 establishes enhancer accessibility for craniofacial cartilage differentiation

The specification of cartilage requires Sox9, a transcription factor with broad roles for organogenesis outside the skeletal system. How Sox9 and other factors gain access to cartilage-specific cis-regulatory regions during skeletal development was unknown. By analyzing chromatin accessibility during the differentiation of neural crest cells into chondrocytes of the zebrafish head, we find that cartilage-associated chromatin accessibility is dynamically established. Cartilage-associated regions that become accessible after neural crest migration are co-enriched for Sox9 and Fox transcription factor binding motifs. In zebrafish lacking Foxc1 paralogs, we find a global decrease in chromatin accessibility in chondrocytes, consistent with a later loss of dorsal facial cartilages. Zebrafish transgenesis assays confirm that many of these Foxc1-dependent elements function as enhancers with region- and stage-specific activity in facial cartilages. These results show that Foxc1 promotes chondrogenesis in the face by establishing chromatin accessibility at a number of cartilage-associated gene enhancers.

Intermittent hypoxia inhibits mandibular cartilage growth with reduced TGF-β and SOX9 expressions in neonatal rats

Intermittent hypoxia (IH) has been associated with skeletal growth. However, the influence of IH on cartilage growth and metabolism is unknown. We compared the effects of IH on chondrocyte proliferation and maturation in the mandibular condyle fibrocartilage and tibial hyaline cartilage of 1-week-old male Sprague-Dawley rats. The rats were exposed to normoxic air (n = 9) or IH at 20 cycles/h (nadir, 4% O₂; peak, 21% O₂; 0% CO₂) (n = 9) for 8 h each day. IH impeded body weight gain, but not tibial elongation. IH also increased cancellous bone mineral and volumetric bone mineral densities in the mandibular condylar head. The mandibular condylar became thinner, but the tibial cartilage did not. IH reduced maturative and increased hypertrophic chondrocytic layers of the middle and posterior mandibular cartilage. PCR showed that IH shifted proliferation and maturation in mandibular condyle fibrocartilage toward hypertrophic differentiation and ossification by downregulating TGF-β and SOX9, and upregulating collagen X. These effects were absent in the tibial growth plate hyaline cartilage. Our results showed that neonatal rats exposed to IH displayed underdeveloped mandibular ramus/condyles, while suppression of chondrogenesis marker expression was detected in the growth-restricted condylar cartilage.

Kindlin-3 mutation in mesenchymal stem cells results in enhanced chondrogenesis

Identifying patient mutations driving skeletal development disorders has driven our understanding of bone development. Integrin adhesion deficiency disease is caused by a Kindlin-3 (fermitin family member 3) mutation, and its inactivation results in bleeding disorders and osteopenia. In this study, we uncover a role for Kindlin-3 in the differentiation of bone marrow mesenchymal stem cells (BMSCs) down the chondrogenic lineage. Kindlin-3 expression increased with chondrogenic differentiation, similar to RUNX2. BMSCs isolated from a Kindlin-3 deficient patient expressed chondrocyte markers, including SOX9, under basal conditions, which were further enhanced with chondrogenic differentiation. Rescue of integrin activation by a constitutively activated β₃ integrin construct increased adhesion to multiple extracellular matrices and reduced SOX9 expression to basal levels. Growth plates from mice expressing a mutated Kindlin-3 with the integrin binding site ablated demonstrated alterations in chondrocyte maturation similar to that seen with the human Kindlin-3 deficient BMSCs. These findings suggest that Kindlin-3 expression mirrors RUNX2 during chondrogenesis.

GnRHa/Stanozolol Combined Therapy Maintains Normal Bone Growth in Central Precocious Puberty

BACKGROUND

Gonadotropin-releasing hormone agonist (GnRHa) is the gold standard in the treatment of Central Precocious Puberty (CPP) with progressive puberty and accelerative growth. However, GnRHa treatment is reported to result in growth deceleration and prevents growth plate development which leads to a reduction in height velocity. Stanozolol (ST) has been used to stimulate growth in patients with delayed growth and puberty, nevertheless, the effects and mechanisms of ST on CPP with GnRHa treatment are currently unclear.

METHODS AND RESULTS

In the current study, we recorded the following vital observations that provided insights into ST induced chondrogenic differentiation and the maintenance of normal growth plate development: (1) ST efficiently prevented growth deceleration and maintained normal growth plate development in rats undergoing GnRHa treatment; (2) ST suppressed the inhibitory effect of GnRHa to promote chondrogenic differentiation; (3) ST induced chondrogenic differentiation through the activation of the JNK/c-Jun/Sox9 signaling pathway; (4) ST promoted chondrogenic differentiation and growth plate development through the JNK/Sox9 signaling pathway in vivo.

CONCLUSIONS

ST mitigated the inhibitory effects of GnRHa and promoted growth plate development in rats. ST induced the differentiation of chondrocytes and maintained normal growth plate development through the activation of JNK/c-Jun/Sox9 signaling. These novel findings indicated that ST could be a potential agent for maintaining normal bone growth in cases of CPP undergoing GnRHa treatment.

Making and shaping endochondral and intramembranous bones

Skeletal elements have a diverse range of shapes and sizes specialized to their various roles including protecting internal organs, locomotion, feeding, hearing, and vocalization. The precise positioning, size, and shape of skeletal elements is therefore critical for their function. During embryonic development, bone forms by endochondral or intramembranous ossification and can arise from the paraxial and lateral plate mesoderm or neural crest. This review describes inductive mechanisms to position and pattern bones within the developing embryo, compares and contrasts the intrinsic vs extrinsic mechanisms of endochondral and intramembranous skeletal development, and details known cellular processes that precisely determine skeletal shape and size. Key cellular mechanisms are employed at distinct stages of ossification, many of which occur in response to mechanical cues (eg, joint formation) or preempting future load‐bearing requirements. Rapid shape changes occur during cellular condensation and template establishment. Specialized cellular behaviors, such as chondrocyte hypertrophy in endochondral bone and secondary cartilage on intramembranous bones, also dramatically change template shape. Once ossification is complete, bone shape undergoes functional adaptation through (re)modeling. We also highlight how alterations in these cellular processes contribute to evolutionary change and how differences in the embryonic origin of bones can influence postnatal bone repair.

Compares and contrasts Endochondral and intramembranous bone development

Reviews embryonic origins of different bones

Describes the cellular and molecular mechanisms of positioning skeletal elements.

Describes mechanisms of skeletal growth with a focus on the generation of skeletal shape

Laser Capture Microdissection of Mouse Growth Plate Cartilage

The ability to identify, isolate, and study pure populations of cells is critical for understanding normal physiology in organs and tissues, which involves spatial regulation of signaling pathways and interactions between cells with different functions, expression profiles, and lineages. Here, we focus on assessing the growth plate cartilage, composed of multiple functionally and histologically distinct zones, to investigate temporally and spatially dependent gene expression differences. In this chapter, we describe the method of laser capture microdissection to isolate chondrocytes from different zones of differentiation in the mouse growth plate cartilage for RNA isolation, and subsequent downstream applications, such as RNA-sequencing and quantitative real-time PCR. We also provide an assessment of different factors contributing to the integrity of the isolated RNA, such as staining methods and procedures in RNA isolation.

Chondrogenesis of cocultures of mesenchymal stem cells and articular chondrocytes in poly(l-lysine)-loaded hydrogels

This work investigated the effect of poly(l-lysine) (PLL) molecular weight and concentration on chondrogenesis of cocultures of mesenchymal stem cells (MSCs) and articular chondrocytes (ACs) in PLL-loaded hydrogels. An injectable dual-network hydrogel composed of a poly(N-isopropylacrylamide)-based synthetic thermogelling macromer and a chondroitin sulfate-based biological network was leveraged as a model to deliver PLL and encapsulate the two cell populations. Incorporation of PLL into the hydrogel did not affect the hydrogel's swelling properties and degradation characteristics, nor the viability of encapsulated cells. Coculture groups demonstrated higher type II collagen expression compared to the MSC monoculture group. Expression of hypertrophic phenotype was also limited in the coculture groups. Histological analysis indicated that the ratio of MSCs to ACs was an accurate predictor of the degree of long-term chondrogenesis, while the presence of PLL was shown to have a more substantial short-term effect. Altogether, this study demonstrates that coculturing MSCs with ACs can greatly enhance the chondrogenicity of the overall cell population and offers a platform to further elucidate the short- and long-term effect of polycationic factors on the chondrogenesis of MSC and AC cocultures.

Stable Reference Genes for qPCR Analysis in BM-MSCs Undergoing Osteogenic Differentiation within 3D Hyaluronan-Based Hydrogels

Reverse transcription quantitative polymerase chain reaction (RT-qPCR) enables the monitoring of changes in cell phenotype via the high-throughput screening of numerous genes. RT-qPCR is a fundamental approach in numerous research fields, including biomaterials, yet little attention has been given to the potential impact of 3D versus monolayer (2D) cell culture and to the requirement for a constant validation of the multiple steps of gene expression analysis. The aim of this study is to use high-quality RNA to identify the most suitable reference genes for RT-qPCR analysis during the osteogenic differentiation of human bone marrow mesenchymal stem/stromal cells (BM-MSCs). BM-MSCs are cultured under osteogenic conditions for 28 days in 2D or within hyaluronic acid hydrogels (3D). RNA is subject to quality controls and is then used to identify the most stable reference genes using geNorm, NormFinder, and the ∆Cq method. The effect of the reverse transcriptase is investigated, as well as the expression of osteogenic-related markers. This study shows marked differences in the stability of reference genes between 2D (RPLP0/GAPDH) and 3D (OAZ1/PPIA) culture, suggesting that it is critical to choose appropriate reference genes for 3D osteogenic cell cultures. Thus, a thorough validation under specific experimental settings is essential to obtain meaningful gene expression results.

Genomic characterization of the adolescent idiopathic scoliosis-associated transcriptome and regulome

Adolescent idiopathic scoliosis (AIS), a sideways curvature of the spine, is the most common pediatric musculoskeletal disorder, affecting ~3% of the population worldwide. However, its genetic bases and tissues of origin remain largely unknown. Several genome-wide association studies (GWAS) have implicated nucleotide variants in non-coding sequences that control genes with important roles in cartilage, muscle, bone, connective tissue and intervertebral disks (IVDs) as drivers of AIS susceptibility. Here, we set out to define the expression of AIS-associated genes and active regulatory elements by performing RNA-seq and chromatin immunoprecipitation-sequencing against H3 lysine 27 acetylation in these tissues in mouse and human. Our study highlights genetic pathways involving AIS-associated loci that regulate chondrogenesis, IVD development and connective tissue maintenance and homeostasis. In addition, we identify thousands of putative AIS-associated regulatory elements which may orchestrate tissue-specific expression in musculoskeletal tissues of the spine. Quantification of enhancer activity of several candidate regulatory elements from our study identifies three functional enhancers carrying AIS-associated GWAS SNPs at the ADGRG6 and BNC2 loci. Our findings provide a novel genome-wide catalog of AIS-relevant genes and regulatory elements and aid in the identification of novel targets for AIS causality and treatment.

In vitro temporal HIF-mediated deposition of osteochondrogenic matrix governed by hypoxia and osteogenic factors synergy

Musculoskeletal interfaces are naturally hypoxic. An understanding of key interactions occurring between different cell populations and their environment is critical for native tissue recapitulation. Here, an enthesis coculture model (preosteoblasts and tendon cells) was used to understand the influence of hypoxia (5% O₂ ) and osteogenic medium (OM) supplementation in cells' phenotype modulation. In single cultures, preosteoblasts were found to undergo osteogenic impairment, while tendon cells underwent a maturation process through extracellular matrix (ECM) rescue. When in co-culture, hypoxia and osteoinduction promoted a temporal chondro/osteogenic pathway activation, as observed by an early deposition of cartilaginous ECM associated with HIF1A stabilization and RUNX2 activation, and later hypertrophic differentiation resulting from HIF2A translocation and SOX9 activation. Moreover, the presence of OM under hypoxia was shown to influence the extracellular ROS/HIF1A interplay. Overall, this study revealed a link between biochemical factors and cell-cell crosstalk, providing a molecular framework for hypoxic control and modulation of cells' fate toward enthesis-like phenotypes.

TET1 Directs Chondrogenic Differentiation by Regulating SOX9 Dependent Activation of Col2a1 and Acan In Vitro

Skeletal development is a tightly orchestrated process in which cartilage and bone differentiation are intricately intertwined. Recent studies have highlighted the contribution of epigenetic modifications and their writers to skeletal development. Methylated cytosine (5mC) can be oxidized to 5-hydroxymethylcytosine (5hmC) by the Ten-eleven-translocation (TET) enzymes leading to demethylation. We have previously demonstrated that 5hmC is stably accumulated on lineage-specific genes that are activated during in vitro chondrogenesis in the ATDC5 chondroprogenitors. Knockdown (KD) of Tet1 via short-hairpin RNAs blocked ATDC5 chondrogenic differentiation. Here, we aimed to provide the mechanistic basis for TET1 function during ATDC5 differentiation. Transcriptomic analysis of Tet1 KD cells demonstrated that 54% of downregulated genes were SOX9 targets, suggesting a role for TET1 in mediating activation of a subset of the SOX9 target genes. Using genome-wide mapping of 5hmC during ATDC5 differentiation, we found that 5hmC is preferentially accumulated at chondrocyte-specific class II binding sites for SOX9, as compared with the tissue-agnostic class I sites. Specifically, we find that SOX9 is unable to bind to Col2a1 and Acan after Tet1 KD, despite no changes in SOX9 levels. Finally, we compared this KD scenario with the genetic loss of TET1 in the growth plate using Tet1 ^-/- embryos, which are approximately 10% smaller than their WT counterparts. In E17.5 Tet1 ^-/- embryos, loss of SOX9 target gene expression is more modest than upon Tet1 KD in vitro. Overall, our data suggest a role for TET1-mediated 5hmC deposition in partly shaping an epigenome conducive for SOX9 function. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Involvement of Bmal1 and circadian clock signaling in chondrogenic differentiation of ATDC5 cells by fluoride

Skeletal fluorosis causes growth plate impairment and growth retardation during bone development. However, the mechanism of how fluoride impairs chondrocyte is unclear. To explore the effect of fluoride on chondrocyte differentiation and the regulation of circadian clock signaling pathway during chondrogenesis, we treated ATDC5 cells with fluoride and carried out a series of experiments. 10^-3 M fluoride inhibited cell viability and significantly decreased the expression of Sox9 and Col2a1 (P < 0.05). Fluoride inhibited proteoglycan synthesis and decreased significantly the expression of Aggrecan, Ihh and Col10a1 (P < 0.05). Meanwhile, fluoride significantly inhibited the expression of Bmal1 and disrupted circadian clock signaling pathway (P < 0.05). Furthermore, fluoride disrupted the time-dependent expression of circadian clock molecules and stage-specific differentiation markers. Overexpression of Bmal1 by lentivirus reversed the adverse effects of fluoride on chondrogenesis. These results suggested that fluoride inhibited chondrocyte viability and delayed chondrocyte differentiation. Fluoride delayed chondrogenesis partly via interfering with Bmal1 and circadian clock signaling pathway. Nevertheless, the specific mechanism of circadian clock in fluoride-induced cartilage damage needs to be further studied.

New insights on the reparative cells in bone regeneration and repair

Bone possesses a remarkable repair capacity to regenerate completely without scar tissue formation. This unique characteristic, expressed during bone development, maintenance and injury (fracture) healing, is performed by the reparative cells including skeletal stem cells (SSCs) and their descendants. However, the identity and functional roles of SSCs remain controversial due to technological difficulties and the heterogeneity and plasticity of SSCs. Moreover, for many years, there has been a biased view that bone marrow is the main cell source for bone repair. Together, these limitations have greatly hampered our understanding of these important cell populations and their potential applications in the treatment of fractures and skeletal diseases. Here, we reanalyse and summarize current understanding of the reparative cells in bone regeneration and repair and outline recent progress in this area, with a particular emphasis on the temporal and spatial process of fracture healing, the sources of reparative cells, an updated definition of SSCs, and markers of skeletal stem/progenitor cells contributing to the repair of craniofacial and long bones, as well as the debate between SSCs and pericytes. Finally, we also discuss the existing problems, emerging novel technologies and future research directions in this field.

Serum NT/CT SIRT1 ratio reflects early osteoarthritis and chondrosenescence

OBJECTIVE

Previous work has established that the deacetylase sirtuin-1 (SIRT1) is cleaved by cathepsin B in chondrocytes subjected to proinflammatory stress, yielding a stable but inactive N-terminal (NT) polypeptide (75SIRT1) and a C-terminal (CT) fragment. The present work examined if chondrocyte-derived NT-SIRT1 is detected in serum and may serve as an investigative and exploratory biomarker of osteoarthritis (OA).

METHODS

We developed a novel ELISA assay to measure the ratio of NT to CT of SIRT1 in the serum of human individuals and mice subjected to post-traumatic OA (PTOA) or age-dependent OA (ADOA). We additionally monitored NT/CT SIRT1 in mice subject to ADOA/PTOA followed by senolytic clearance. Human chondrosenescent and non-senescent chondrocytes were exposed to cytokines and analysed for apoptosis and NT/CT SIRT1 ratio in conditioned medium.

RESULTS

Wild-type mice with PTOA or ADOA of moderate severity exhibited increased serum NT/CT SIRT1 ratio. In contrast, this ratio remained low in cartilage-specific Sirt1 knockout mice despite similar or increased PTOA and ADOA severity. Local clearance of senescent chondrocytes from old mice with post-traumatic injury resulted in a lower NT/CT ratio and reduced OA severity. While primary chondrocytes exhibited NT/CT ratio increased in conditioned media after prolonged cytokine stimulation, this increase was not evident in cytokine-stimulated chondrosenescent cells. Finally, serum NT/CT ratio was elevated in humans with early-stage OA.

CONCLUSIONS

Increased levels of serum NT/CT SIRT1 ratio correlated with moderate OA in both mice and humans, stemming at least in part from non-senescent chondrocyte apoptosis, possibly a result of prolonged inflammatory insult.

Changes in DNA methylation accompany changes in gene expression during chondrocyte hypertrophic differentiation in vitro

During osteoarthritis (OA), articular chondrocytes undergo phenotypic changes that resemble developmental patterns characteristic of growth plate chondrocytes. These phenotypic alterations lead to a hypertrophy-like phenotype characterized by altered production of extracellular matrix constituents and increased collagenase activity, which, in turn, results in cartilage destruction in OA disease. Recent studies have shown that the phenotypic instability and dysregulated gene expression in OA are associated with changes in DNA methylation patterns. Subsequent efforts have aimed to identify changes in DNA methylation with functional impact in OA disease, to potentially uncover therapeutic targets. Here, we paired an in vitro 3D/pellet culture system that mimics chondrocyte hypertrophy with RNA sequencing (RNA-Seq) and enhanced reduced representation of bisulfite sequencing (ERRBS) to identify transcriptomic and epigenomic changes in murine primary articular chondrocytes undergoing hypertrophy-like differentiation. We identified hypertrophy-associated changes in DNA methylation patterns in vitro. Integration of RNA-Seq and ERRBS datasets identified associations between changes in methylation and gene expression. Our integrative analyses showed that hypertrophic differentiation of articular chondrocytes is accompanied by transcriptomic and epigenomic changes in vitro. We believe that our integrative approaches have the potential to uncover new targets for therapeutic intervention.

Extracellular Matrix and Cellular Plasticity in Musculoskeletal Development

Cellular plasticity refers to the ability of cell fates to be reprogrammed given the proper signals, allowing for dedifferentiation or transdifferentiation into different cell fates. In vitro, this can be induced through direct activation of gene expression, however this process does not naturally occur in vivo. Instead, the microenvironment consisting of the extracellular matrix (ECM) and signaling factors, directs the signals presented to cells. Often the ECM is involved in regulating both biochemical and mechanical signals. In stem cell populations, this niche is necessary for maintenance and proper function of the stem cell pool. However, recent studies have demonstrated that differentiated or lineage restricted cells can exit their current state and transform into another state under different situations during development and regeneration. This may be achieved through (1) cells responding to a changing niche; (2) cells migrating and encountering a new niche; and (3) formation of a transitional niche followed by restoration of the homeostatic niche to sequentially guide cells along the regenerative process. This review focuses on examples in musculoskeletal biology, with the concept of ECM regulating cells and stem cells in development and regeneration, extending beyond the conventional concept of small population of progenitor cells, but under the right circumstances even “lineage-restricted” or differentiated cells can be reprogrammed to enter into a different fate.

Transcriptional, epigenetic and microRNA regulation of growth plate

Endochondral ossification is a critical event in bone formation, particularly in long shaft bones. Many cellular differentiation processes work in concert to facilitate the generation of cartilage primordium to formation of trabecular structures, all of which occur within the growth plate. Previous studies have revealed that the growth plate is tightly regulated by various transcription factors, epigenetic systems, and microRNAs. Hence, understanding these mechanisms that regulate the growth plate is crucial to furthering the current understanding on skeletal diseases, and in formulating effective treatment strategies. In this review, we focus on describing the function and mechanisms of the transcription factors, epigenetic systems, and microRNAs known to regulate the growth plate.

What do we know about bone morphogenetic proteins and osteochondroprogenitors in inflammatory conditions?

Osteochondroprogenitors are crucial for embryonic bone development and postnatal processes such as bone repair in response to fracture injury, and their dysfunction may contribute to insufficient repair of structural damage in inflammatory arthritides. In the fracture healing, the early inflammatory phase is crucial for normal callus development and new bone formation. This process involves a complex interplay of many molecules and cell types, responsible for recruitment, expansion and differentiation of osteochondroprogenitor populations. In inflammatory arthritides, inflammation induces bone resorption and causes insufficient bone formation, which leads to local and systemic bone loss. While bone loss is a predominant feature in rheumatoid arthritis, inflammation also induces pathologic bone formation at enthesial sites in seronegative spondyloarthropathies. Bone morphogenetic proteins (BMP) are involved in cell proliferation, differentiation and apoptosis, and have fundamental roles in maintenance of postnatal bone homeostasis. They are crucial regulators of the osteochondroprogenitor pool and drive their proliferation, differentiation, and lifespan during bone regeneration. In this review, we summarize the effects of inflammation on osteochondroprogenitor populations during fracture repair and in inflammatory arthritides, with special focus on inflammation-mediated modulation of BMP signaling. We also present data in which we describe a population of murine synovial osteochondroprogenitor cells, which are reduced in arthritis, and characterize their expression of genes involved in regulation of bone homeostasis, emphasizing the up-regulation of BMP pathways in early progenitor subset. Based on the presented data, it may be concluded that during an inflammatory response, innate immune cells induce osteochondroprogenitors by providing signals for their recruitment, by producing BMPs and other osteogenic factors for paracrine effects, and by secreting inflammatory cytokines that may positively regulate osteogenic pathways. On the other hand, inflammatory cells may secrete cytokines that interfere with osteogenic pathways, proapoptotic factors that reduce the pool of osteochondroprogenitor cells, as well as BMP and Wnt antagonists. The net effect is strongly context-dependent and influenced by the local milieu of cells, cytokines, and growth factors. Further elucidation of the interplay between inflammatory signals and BMP-mediated bone formation may provide valuable tools for therapeutic targeting.

The KMT2D Kabuki syndrome histone methylase controls neural crest cell differentiation and facial morphology

Kabuki syndrome (KS) is a congenital craniofacial disorder resulting from mutations in the KMT2D histone methylase (KS1) or the UTX histone demethylase (KS2). With small cohorts of KS2 patients, it is not clear whether differences exist in clinical manifestations relative to KS1. We mutated KMT2D in neural crest cells (NCCs) to study cellular and molecular functions in craniofacial development with respect to UTX. Similar to UTX, KMT2D NCC knockout mice demonstrate hypoplasia with reductions in frontonasal bone lengths. We have traced the onset of KMT2D and UTX mutant NCC frontal dysfunction to a stage of altered osteochondral progenitor differentiation. KMT2D NCC loss-of-function does exhibit unique phenotypes distinct from UTX mutation, including fully penetrant cleft palate, mandible hypoplasia and deficits in cranial base ossification. KMT2D mutant NCCs lead to defective secondary palatal shelf elevation with reduced expression of extracellular matrix components. KMT2D mutant chondrocytes in the cranial base fail to properly differentiate, leading to defective endochondral ossification. We conclude that KMT2D is required for appropriate cranial NCC differentiation and KMT2D-specific phenotypes may underlie differences between Kabuki syndrome subtypes.

Extracellular matrix mechanics regulate transfection and SOX9-directed differentiation of mesenchymal stem cells

Gene delivery within hydrogel matrices can potentially direct mesenchymal stem cells (MSCs) towards a chondrogenic fate to promote regeneration of cartilage. Here, we investigated whether the mechanical properties of the hydrogel containing the gene delivery systems could enhance transfection and chondrogenic programming of primary human bone marrow-derived MSCs. We developed collagen-I-alginate interpenetrating polymer network hydrogels with tunable stiffness and adhesion properties. The hydrogels were activated with nanocomplexed SOX9 polynucleotides to direct chondrogenic differentiation of MSCs. MSCs transfected within the hydrogels showed higher expression of chondrogenic markers compared to MSCs transfected in 2D prior to encapsulation. The nanocomplex uptake and resulting expression of transfected SOX9 were jointly enhanced by increased stiffness and cell-adhesion ligand density in the hydrogels. Further, transfection of SOX9 effectively induced MSCs chondrogenesis and reduced markers of hypertrophy compared to control matrices. These findings highlight the importance of matrix stiffness and adhesion as design parameters in gene-activated matrices for regenerative medicine. STATEMENT OF SIGNIFICANCE: Gene-activated matrices (GAMs) are biodegradable polymer networks integrating gene therapies, and they are promising technologies for supporting tissue regeneration. Despite this interest, there is still limited information on how to rationally design these systems. Here, we provide a systematic study of the effect of matrix stiffness and cell adhesion ligands on gene transfer efficiency. We show that high stiffness and the presence of cell-binding sites promote transfection efficiency and that this result is related to more efficient internalization and trafficking of the gene therapies. GAMs with optimized mechanical properties can induce cartilage formation and result in tissues with better characteristics for articular cartilage tissue engineering as compared to previously described standard methods.

Zebrafish models of skeletal dysplasia induced by cholesterol biosynthesis deficiency

Human disorders of the post-squalene cholesterol biosynthesis pathway frequently result in skeletal abnormalities, yet our understanding of the mechanisms involved is limited. In a forward-genetic approach, we have found that a late-onset skeletal mutant, named kolibernu7 , is the result of a cis-acting regulatory mutation leading to loss of methylsterol monooxygenase 1 (msmo1) expression within pre-hypertrophic chondrocytes. Generated msmo1nu81 knockdown mutation resulted in lethality at larval stage. We demonstrated that this is a result of both cholesterol deprivation and sterol intermediate accumulation by creating a mutation eliminating activity of Lanosterol synthase (Lss). Our results indicate that double lssnu60;msmo1nu81 and single lssnu60 mutants survive significantly longer than msmo1nu81 homozygotes. Liver-specific restoration of either Msmo1 or Lss in corresponding mutant backgrounds suppresses larval lethality. Rescued mutants develop dramatic skeletal abnormalities, with a loss of Msmo1 activity resulting in a more-severe patterning defect of a near-complete loss of hypertrophic chondrocytes marked by col10a1a expression. Our analysis suggests that hypertrophic chondrocytes depend on endogenous cholesterol synthesis, and blocking C4 demethylation exacerbates the cholesterol deficiency phenotype. Our findings offer new insight into the genetic control of bone development and provide new zebrafish models for human disorders of the cholesterol biosynthesis pathway.

Ablation of the miRNA Cluster 24 Has Profound Effects on Extracellular Matrix Protein Abundance in Cartilage

MicroRNAs (miRNAs) regulate cartilage differentiation and contribute to the onset and progression of joint degeneration. These small RNA molecules may affect extracellular matrix organization (ECM) in cartilage, but for only a few miRNAs has this role been defined in vivo. Previously, we showed that cartilage-specific genetic ablation of the Mirc24 cluster in mice leads to impaired cartilage development due to increased RAF/MEK/ERK pathway activation. Here, we studied the expression of the cluster in cartilage by LacZ reporter gene assays and determined its role for extracellular matrix homeostasis by proteome and immunoblot analysis. The cluster is expressed in prehypertrophic/hypertrophic chondrocytes of the growth plate and we now show that the cluster is also highly expressed in articular cartilage. Cartilage-specific loss of the cluster leads to increased proteoglycan 4 and matrix metallopeptidase 13 levels and decreased aggrecan and collagen X levels in epiphyseal cartilage. Interestingly, these changes are linked to a decrease in SRY-related HMG box-containing (SOX) transcription factors 6 and 9, which regulate ECM production in chondrocytes. Our data suggests that the Mirc24 cluster is important for ECM homoeostasis and the expression of transcriptional regulators of matrix production in cartilage.

RNA-seq reveals downregulated osteochondral genes potentially related to tibia bacterial chondronecrosis with osteomyelitis in broilers

Background

Bacterial chondronecrosis with osteomyelitis (BCO) develops in the growth plate (GP) of the proximal femur and tibia and is initiated by damage to the less mineralized chondrocytes followed by colonization of opportunistic bacteria. This condition affects approximately 1% of all birds housed, being considered one of the major causes of lameness in fast growing broilers. Although several studies have been previously performed aiming to understand its pathogenesis, the molecular mechanisms involved with BCO remains to be elucidated. Therefore, this study aimed to generate a profile of global differential gene expression involved with BCO in the tibia of commercial broilers, through RNA sequencing analysis to identity genes and molecular pathways involved with BCO in chickens.

Results

Our data showed 192 differentially expressed (DE) genes: 63 upregulated and 129 downregulated in the GP of the tibia proximal epiphysis of BCO-affected broilers. Using all DE genes, six Biological Processes (BP) were associated with bone development (connective tissue development, cartilage development, skeletal system development, organ morphogenesis, system development and skeletal system morphogenesis). The analyses of the upregulated genes did not indicate any significant BP (FDR < 0.05). However, with the downregulated genes, the same BP were identified when using all DE genes in the analysis, with a total of 26 coding genes explaining BCO in the tibia: ACAN, ALDH1A2, CDH7, CHAD, CHADL, COL11A1, COMP, CSGALNACT1, CYR61, FRZB, GAL3ST1, HAPLN1, IHH, KIF26B, LECT1, LPPR1, PDE6B, RBP4A, SERINC5, SFRP1, SOX8, SOX9, TENM2, THBS1, UCHL1 and WFIKKN2. In addition, seven transcription factors were also associated to BCO: NFATC2, MAFB, HIF1A-ARNT, EWSR1-FLI1, NFIC, TCF3 and NF-KAPPAB.

Conclusions

Our data show that osteochondral downregulated genes are potential molecular causes of BCO in broilers, and the bacterial process seems to be, in fact, a secondary condition. Sixteen genes responsible for bone and cartilage formation were downregulated in BCO-affected broilers being strong candidate genes to trigger this disorder.

Regulation and function of SOX9 during cartilage development and regeneration

Chondrogenesis is a highly coordinated event in embryo development, adult homeostasis, and repair of the vertebrate cartilage. Fate decisions and differentiation of chondrocytes accompany differential expression of genes critical for each step of chondrogenesis. SOX9 is a master transcription factor that participates in sequential events in chondrogenesis by regulating a series of downstream factors in a stage-specific manner. SOX9 either works alone or in combination with downstream SOX transcription factors, SOX5 and SOX6 as chondrogenic SOX Trio. SOX9 is reduced in the articular cartilage of patients with osteoarthritis while highly maintained during tumorigenesis of cartilage and bone. Gene therapy using viral and non-viral vectors accompanied by tissue engineering (scaffolds) is a promising tool to regenerate impaired cartilage. Delivery of SOX9 or chondrogenic SOX Trio into cells produces efficient therapeutic effects on chondrogenesis and this event is facilitated by scaffolds. Non-viral vector-guided delivery systems encapsulated or loaded in mechanically stable solid scaffolds are useful for the regeneration of articular cartilage. Here we review major milestones and most recent studies focusing on regulation and function of chondrogenic SOX Trio, during chondrogenesis and cartilage regeneration, and on the development of advanced technologies in gene delivery with tissue engineering to improve efficiency of cartilage repair process.

Angelicin-A Furocoumarin Compound With Vast Biological Potential

Angelicin, a member of the furocoumarin group, is related to psoralen which is well known for its effectiveness in phototherapy. The furocoumarins as a group have been studied since the 1950s but only recently has angelicin begun to come into its own as the subject of several biological studies. Angelicin has demonstrated anti-cancer properties against multiple cell lines, exerting effects via both the intrinsic and extrinsic apoptotic pathways, and also demonstrated an ability to inhibit tubulin polymerization to a higher degree than psoralen. Besides that, angelicin too demonstrated anti-inflammatory activity in inflammatory-related respiratory and neurodegenerative ailments via the activation of NF-κB pathway. Angelicin also showed pro-osteogenesis and pro-chondrogenic effects on osteoblasts and pre-chondrocytes respectively. The elevated expression of pro-osteogenic and chondrogenic markers and activation of TGF-β/BMP, Wnt/β-catenin pathway confirms the positive effect of angelicin bone remodeling. Angelicin also increased the expression of estrogen receptor alpha (ERα) in osteogenesis. Other bioactivities, such as anti-viral and erythroid differentiating properties of angelicin, were also reported by several researchers with the latter even displaying an even greater aptitude as compared to the commonly prescribed drug, hydroxyurea, which is currently on the market. Apart from that, recently, a new application for angelicin against periodontitis had been studied, where reduction of bone loss was indirectly caused by its anti-microbial properties. All in all, angelicin appears to be a promising compound for further studies especially on its mechanism and application in therapies for a multitude of common and debilitating ailments such as sickle cell anaemia, osteoporosis, cancer, and neurodegeneration. Future research on the drug delivery of angelicin in cancer, inflammation and erythroid differentiation models would aid in improving the bioproperties of angelicin and efficacy of delivery to the targeted site. More in-depth studies of angelicin on bone remodeling, the pro-osteogenic effect of angelicin in various bone disease models and the anti-viral implications of angelicin in periodontitis should be researched. Finally, studies on the binding of angelicin toward regulatory genes, transcription factors, and receptors can be done through experimental research supplemented with molecular docking and molecular dynamics simulation.

Detection of a 4 bp Mutation in the 3'UTR Region of Goat Sox9 Gene and Its Effect on the Growth Traits

Simple Summary

The sex determining region Y (SRY)-type high mobility group (HMG) box 9 (Sox9) gene is critically important in the formation and development of cartilage and is considered the “main regulator” of chondrogenesis. Additionally, a large number of studies have shown that mutations in a single allele of human Sox9 can lead to campomelic dysplasia syndrome. Therefore, the mutations of Sox9 have been the subject of increasing interest among researchers. However, no studies to date have examined the association between Sox9 gene variants and growth traits in goats. Here, we detected a 4 bp indel in the 3′Untranslated Regions (3′UTR) region of Sox9 in Shaanbei white cashmere (SBWC) goats (n = 1109) and studied the association between this indel and growth traits. The 4 bp indel of Sox9 was significantly associated with body length, heart girth, hip width, and all body measurement indexes (p < 0.05) in SBWC goats. Thus, this deletion could be used as an effective molecular marker for maximizing the growth traits of goats in breeding programs.

Abstract

The SRY-type HMG box 9 (Sox9) gene plays an important role in chondrocyte development as well as changes in hypertrophic chondrocytes, indicating that Sox9 can regulate growth in animals. However, no studies to date have examined the correlation between variations in Sox9 and growth traits in goats. Here, we found a 4 bp indel in the 3′UTR of Sox9 and verified its association with growth traits in Shaanbei white cashmere goats (n = 1109). The frequencies of two genotypes (ID and II) were 0.397 and 0.603, respectively, and polymorphic information content (PIC) values showed that the indel had a medium PIC (PIC > 0.25). The 4 bp indel was significantly correlated with body length (p = 0.006), heart girth (p = 0.001), and hip width (p = 4.37 × 10 ⁻⁴). Notably, individuals with the ID genotype had significantly superior phenotypic traits compared with individuals bearing the II genotype. Hence, we speculated that the 4 bp indel is an important mutation affecting growth traits in goat, and may serve as an effective DNA molecular marker for marker-assisted selection in goat breeding programs.

Asporin Reduces Adult Aortic Valve Interstitial Cell Mineralization Induced by Osteogenic Media and Wnt Signaling Manipulation In Vitro

Worldwide, calcific aortic valve disease is one of the leading causes of morbidity and mortality among patients with cardiac abnormalities. Aortic valve mineralization and calcification are the key events of adult calcific aortic valve disease manifestation and functional insufficiency. Due to heavy mineralization and calcification, adult aortic valvular cusps show disorganized and dispersed stratification concomitant with deposition of calcific nodules with severely compromised adult valve function. Interestingly, shared gene regulatory pathways are identified between bone-forming cells and heart valve cells during development. Asporin, a small leucine-rich proteoglycan (43 kDa), acts to inhibit mineralization in periodontal ligament cells and is also detected in normal murine adult aortic valve leaflets with unknown function. Therefore, to understand the Asporin function in aortic cusp mineralization and calcification, adult avian aortic valvular interstitial cell culture system is established and osteogenesis has been induced in these cells successfully. Upon induction of osteogenesis, reduced expression of Asporin mRNA and increased expression of bone and osteogenesis markers are detected compared to cells maintained without osteogenic induction. Importantly, treatment with human recombinant Asporin protein reduces the mineralization level in osteogenic media-induced aortic valvular interstitial cells with the concomitant decreased level of Wnt/β-catenin signaling. Overall, all these data are highly indicative that Asporin might be a novel biomolecular target to treat patients of calcific aortic valve disease over current cusp replacement surgery.

Cellular Plasticity in Musculoskeletal Development, Regeneration, and Disease

In this review, we highlight themes from a recent workshop focused on "Plasticity of Cell Fate in Musculoskeletal Tissues" held at the Orthopaedic Research Society's 2019 annual meeting. Experts in the field provided examples of mesenchymal cell plasticity during normal musculoskeletal development, regeneration, and disease. A thorough understanding of the biology underpinning mesenchymal cell plasticity may offer a roadmap for promoting regeneration while attenuating pathologic differentiation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:708-718, 2020.

SOX9: The master regulator of cell fate in breast cancer

SRY-related high-mobility group box 9 (SOX9) is an indispensable transcription factor that regulates multiple developmental pathways related to stemness, differentiation, and progenitor development. Previous studies have demonstrated that the SOX9 protein directs pathways involved in tumor initiation, proliferation, migration, chemoresistance, and stem cell maintenance, thereby regulating tumorigenesis as an oncogene. SOX9 overexpression is a frequent event in breast cancer (BC) subtypes. Of note, the molecular mechanisms and functional regulation underlying SOX9 upregulation during BC progression are still being uncovered. The focus of this review is to appraise recent advances regarding the involvement of SOX9 in BC pathogenesis. First, we provide a general overview of SOX9 structure and function, as well as its involvement in various kinds of cancer. Next, we discuss pathways of SOX9 regulation, particularly its miRNA-mediated regulation, in BC. Finally, we describe the involvement of SOX9 in BC pathogenesis via its regulation of pathways involved in regulating cancer hallmarks, as well as its clinical and therapeutic importance. In general, this review article aims to serve as an ample source of knowledge on the involvement of SOX9 in BC progression. Targeting SOX9 activity may improve therapeutic strategies to treat BC, but precisely inhibiting SOX9 using drugs and/or small peptides remains a huge challenge for forthcoming cancer research.

Hypoxia Inducible Factor-1α in Osteochondral Tissue Engineering

Damage to osteochondral (OC) tissues can lead to pain, loss of motility, and progress to osteoarthritis. Tissue engineering approaches offer the possibility of replacing damaged tissues and restoring joint function; however, replicating the spatial and functional heterogeneity of native OC tissue remains a pressing challenge. Chondrocytes in healthy cartilage exist in relatively low-oxygen conditions, while osteoblasts in the underlying bone experience higher oxygen pressures. Such oxygen gradients also exist in the limb bud, where they influence OC tissue development. The cellular response to these spatial variations in oxygen pressure, which is mediated by the hypoxia inducible factor (HIF) pathway, plays a central role in regulating osteo- and chondrogenesis by directing progenitor cell differentiation and promoting and maintaining appropriate extracellular matrix production. Understanding the role of the HIF pathway in OC tissue development may enable new approaches to engineer OC tissue. In this review, we discuss strategies to spatially and temporarily regulate the HIF pathway in progenitor cells to create functional OC tissue for regenerative therapies. Impact statement Strategies to engineer osteochondral (OC) tissue are limited by the complex and varying microenvironmental conditions in native bone and cartilage. Indeed, native cartilage experiences low-oxygen conditions, while the underlying bone is relatively normoxic. The cellular response to these low-oxygen conditions, which is mediated through the hypoxia inducible factor (HIF) pathway, is known to promote and maintain the chondrocyte phenotype. By using tissue engineering scaffolds to spatially and temporally harness the HIF pathway, it may be possible to improve OC tissue engineering strategies for the regeneration of damaged cartilage and its underlying subchondral bone.

mRNA-activated matrices encoding transcription factors as primers of cell differentiation in tissue engineering

Gene-activated matrices (GAMs) encoding pivotal transcription factors (TFs) represent a powerful tool to direct stem cell specification for tissue engineering applications. However, current TF-based GAMs activated with pDNA, are challenged by their low transfection efficiency and delayed transgene expression. Here, we report a GAM technology activated with mRNAs encoding TFs SOX9 (cartilage) and MYOD (muscle). We find that these mRNA-GAMs induce a higher and faster TF expression compared to pDNA-GAMs, especially in the case of RNase resistant mRNA sequences. This potent TF expression was translated into a high synthesis of cartilage- and muscle-specific markers, and ultimately, into successful tissue specification in vitro. Additionally, we show that the expression of tissue-specific markers can be further modulated by altering the properties of the mRNA-GAM environment. These results highlight the value of this GAM technology for priming cell lineage specification, a key centerpiece for future tissue engineering devices.

Targeted and sustained Sox9 expression in mouse hypertrophic chondrocytes causes severe and spontaneous osteoarthritis by perturbing cartilage homeostasis

Sox9 is the master transcription factor essential for cartilage development and homeostasis. To investigate the specific role of Sox9 during chondrocyte hypertrophy, we generated a novel Col10a1-Sox9 transgenic mouse model, in which Sox9 is specifically expressed in hypertrophic chondrocytes driven by a well-characterized 10-kb Col10a1 promoter. These mice were viable and fertile, and appeared normal at birth. However, they developed dwarfism by ten weeks of age. The histological analysis of the growth plates from these transgenic mice demonstrated an abnormal growth plate architecture and a significantly reduced amount of trabecular bone and mineral content in the primary spongiosa. Real-time qPCR analysis revealed the reduced expression of Col10a1, and increased expressions of adipogenic differentiation markers in primary hypertrophic chondrocytes isolated from transgenic mice. Concomitantly, the transgenic mouse chondrocyte cultures had increased lipid droplet accumulation. Unexpectedly, we also observed an increased incidence of spontaneous osteoarthritis (OA) development in the transgenic mice by X-ray analysis, micro-computed tomography scanning, and histological examination of knee joints. The manifestation of OA in Col10a1-Sox9 transgenic mice began by six-months of age, and worsened by eleven-months of age. In conclusion, we provide strong evidence that the proper spatiotemporal expression of Sox9 is necessary for normal adult hypertrophic cartilage homeostasis, and that the aberrant expression of Sox9 might lead to spontaneous OA development.

Significance of MEF2C and RUNX3 Regulation for Endochondral Differentiation of Human Mesenchymal Progenitor Cells

Guiding progenitor cell development between chondral versus endochondral pathways is still an unachieved task of cartilage neogenesis, and human mesenchymal progenitor cell (MPC) chondrogenesis is considered as a valuable model to better understand hypertrophic development of chondrocytes. Transcription factors Runx2, Runx3, and Mef2c play prominent roles for chondrocyte hypertrophy during mouse development, but little is known on the importance of these key fate-determining factors for endochondral development of human MPCs. The aim of this study was to unravel the regulation of RUNX2, RUNX3, and MEF2C during MPC chondrogenesis, the pathways driving their expression, and the downstream hypertrophic targets affected by their regulation. RUNX2, RUNX3, and MEF2C gene expression was differentially regulated during chondrogenesis of MPCs, but remained low and unregulated when non-hypertrophic articular chondrocytes were differentiated under the same conditions. RUNX3 and MEF2C mRNA and protein levels rose in parallel to hypertrophic marker upregulation, but surprisingly, RUNX2 gene expression changed only by trend and RUNX2 protein remained undetectable. While RUNX3 expression was driven by TGF-β and BMP signaling, MEF2C responded to WNT-, BMP-, and Hedgehog-pathway inhibition. MEF2C but not RUNX3 levels correlated significantly with COL10A1, IHH, and IBSP gene expression when hypertrophy was attenuated. IBSP was a downstream target of RUNX3 and MEF2C but not RUNX2 in SAOS-2 cells, underlining the capacity of RUNX3 and MEF2C to stimulate osteogenic marker expression in human cells. Conclusively, RUNX3 and MEF2C appeared more important than RUNX2 for human endochondral MPC chondrogenesis. Pathways altering the speed of chondrogenesis (FGF, TGF-β, BMP) affected RUNX2 or RUNX3, while pathways changing hypertrophy (WNT, PTHrP/HH) regulated mainly MEF2C. Taken together, reduction of MEF2C levels is a new goal to shift human cartilage neogenesis toward the chondral pathway.

Arp2/3 inactivation causes intervertebral disc and cartilage degeneration with dysregulated TonEBP-mediated osmoadaptation

Extracellular matrix and osmolarity influence the development and homeostasis of skeletal tissues through Rho GTPase-mediated alteration of the actin cytoskeleton. This study investigated whether the actin-branching Arp2/3 complex, a downstream effector of the Rho GTPases Cdc42 and Rac1, plays a critical role in maintaining the health of matrix-rich and osmotically loaded intervertebral discs and cartilage. Mice with constitutive intervertebral disc- and cartilage-specific deletion of the critical Arp2/3 subunit Arpc2 (Col2-Cre; Arpc2fl/fl) developed chondrodysplasia and spinal defects. Since these mice did not survive to adulthood, we generated mice with inducible Arpc2 deletion in disc and cartilage (Acan-CreERT2; Arpc2fl/fl). Inactivation of Arp2/3 at skeletal maturity resulted in growth plate closure, loss of proteoglycan content in articular cartilage, and degenerative changes in the intervertebral disc at 1 year of age. Chondrocytes with Arpc2 deletion showed compromised cell spreading on both collagen and fibronectin. Pharmacological inhibition of Cdc42 and Arp2/3 prevented the osmoadaptive transcription factor TonEBP/NFAT5 from recruiting cofactors in response to a hyperosmolarity challenge. Together, these findings suggest that Arp2/3 plays a critical role in cartilaginous tissues through the regulation of cell-extracellular matrix interactions and modulation of TonEBP-mediated osmoadaptation.

Reduced hypertrophy in vitro after chondrogenic differentiation of adult human mesenchymal stem cells following adenoviral SOX9 gene delivery

Background

Mesenchymal stem cell (MSC) based-treatments of cartilage injury are promising but impaired by high levels of hypertrophy after chondrogenic induction with several bone morphogenetic protein superfamily members (BMPs). As an alternative, this study investigates the chondrogenic induction of MSCs via adenoviral gene-delivery of the transcription factor SOX9 alone or in combination with other inducers, and comparatively explores the levels of hypertrophy and end stage differentiation in a pellet culture system in vitro.

Methods

First generation adenoviral vectors encoding SOX9, TGFB1 or IGF1 were used alone or in combination to transduce human bone marrow-derived MSCs at 5 × 10² infectious particles/cell. Thereafter cells were placed in aggregates and maintained for three weeks in chondrogenic medium. Transgene expression was determined at the protein level (ELISA/Western blot), and aggregates were analysed histologically, immunohistochemically, biochemically and by RT-PCR for chondrogenesis and hypertrophy.

Results

SOX9 cDNA was superior to that encoding TGFB1, the typical gold standard, as an inducer of chondrogenesis in primary MSCs as evidenced by improved lacuna formation, proteoglycan and collagen type II staining, increased levels of GAG synthesis, and expression of mRNAs associated with chondrogenesis. Moreover, SOX9 modified aggregates showed a markedly lower tendency to progress towards hypertrophy, as judged by expression of the hypertrophy markers alkaline phosphatase, and collagen type X at the mRNA and protein levels.

Conclusion

Adenoviral SOX9 gene transfer induces chondrogenic differentiation of human primary MSCs in pellet culture more effectively than TGFB1 gene transfer with lower levels of chondrocyte hypertrophy after 3 weeks of in vitro culture. Such technology might enable the formation of more stable hyaline cartilage repair tissues in vivo.

Scaffold- and serum-free hypertrophic cartilage tissue engineering as an alternative approach for bone repair

Human adipose stem/stromal cell (ASC) spheroids were used as a serum-free in vitro model to recapitulate the molecular events and extracellular matrix organization that orchestrate a hypertrophic cartilage phenotype. Induced-ASC spheroids (ø = 450 µm) showed high cell viability throughout the period of culture. The expression of collagen type X alpha 1 chain (COLXA1) and matrix metallopeptidase 13 (MMP-13) was upregulated at week 2 in induced-ASC spheroids compared with week 5 (P < .001) evaluated by quantitative real-time PCR. In accordance, secreted levels of IL-6 (P < .0001), IL-8 (P < .0001), IL-10 (P < .0001), bFGF (P < .001), VEGF (P < .0001), and RANTES (P < .0001) were the highest at week 2. Strong in situ staining for collagen type X and low staining for TSP-1 was associated with the increase of hypertrophic genes expression at week 2 in induced-ASC spheroids. Collagen type I, osteocalcin, biglycan, and tenascin C were detected at week 5 by in situ staining, in accordance with the highest expression of alkaline phosphatase (ALPL) gene and the presence of calcium deposits as evaluated by Alizarin Red O staining. Induced-ASC spheroids showed a higher force required to compression at week 2 (P < .0001). The human ASC spheroids under serum-free inducer medium and normoxic culture conditions were induced to a hypertrophic cartilage phenotype, opening a new perspective to recapitulate endochondral ossification in vivo.

Regulation of terminal hypertrophic chondrocyte differentiation in Prmt5 mutant mice modeling infantile idiopathic scoliosis

Idiopathic scoliosis (IS) is the most common type of musculoskeletal defect affecting children worldwide, and is classified by age of onset, location and degree of spine curvature. Although rare, IS with onset during infancy is the more severe and rapidly progressive form of the disease, associated with increased mortality due to significant respiratory compromise. The pathophysiology of IS, in particular for infantile IS, remains elusive. Here, we demonstrate the role of PRMT5 in the infantile IS phenotype in mouse. Conditional genetic ablation of PRMT5 in osteochondral progenitors results in impaired terminal hypertrophic chondrocyte differentiation and asymmetric defects of endochondral bone formation in the perinatal spine. Analysis of these several markers of endochondral ossification revealed increased type X collagen (COLX) and Ihh expression, coupled with a dramatic reduction in Mmp13 and RUNX2 expression, in the vertebral growth plate and in regions of the intervertebral disc in the Prmt5 conditional mutant mice. We also demonstrate that PRMT5 has a continuous role in the intervertebral disc and vertebral growth plate in adult mice. Altogether, our results establish PRMT5 as a critical promoter of terminal hypertrophic chondrocyte differentiation and endochondral bone formation during spine development and homeostasis.

This article has an associated First Person interview with the first author of the paper.

Summary: Loss of Prmt5 in osteochondral progenitors impairs terminal hypertrophic chondrocyte differentiation, leading to defects in endochondral bone formation and models infantile idiopathic scoliosis in mouse.

Insights into Gene Regulatory Networks in Chondrocytes

Chondrogenesis is a key developmental process that molds the framework of our body and generates the skeletal tissues by coupling with osteogenesis. The developmental processes are well-coordinated by spatiotemporal gene expressions, which are hardwired with gene regulatory elements. Those elements exist as thousands of modules of DNA sequences on the genome. Transcription factors function as key regulatory proteins by binding to regulatory elements and recruiting cofactors. Over the past 30 years, extensive attempts have been made to identify gene regulatory mechanisms in chondrogenesis, mainly through biochemical approaches and genetics. More recently, newly developed next-generation sequencers (NGS) have identified thousands of gene regulatory elements on a genome scale, and provided novel insights into the multiple layers of gene regulatory mechanisms, including the modes of actions of transcription factors, post-translational histone modifications, chromatin accessibility, the concept of pioneer factors, and three-dimensional chromatin architecture. In this review, we summarize the studies that have improved our understanding of the gene regulatory mechanisms in chondrogenesis, from the historical studies to the more recent works using NGS. Finally, we consider the future perspectives, including efforts to improve our understanding of the gene regulatory landscape in chondrogenesis and potential applications to the treatment of chondrocyte-related diseases.

EFTUD2 gene deficiency disrupts osteoblast maturation and inhibits chondrocyte differentiation via activation of the p53 signaling pathway

Background

Mandibulofacial dysostosis with microcephaly (MFDM) is characteristic of multiple skeletal anomalies comprising craniofacial anomalies/dysplasia, microcephaly, dysplastic ears, choanal atresia, and short stature. Heterozygous loss of function variants of EFTUD2 was previously reported in MFDM; however, the mechanism underlying EFTUD2-associated skeletal dysplasia remains unclear.

Results

We identified a novel frameshift variant of EFTUD2 (c.1030_1031delTG, p.Trp344fs*2) in an MFDM Chinese patient with craniofacial dysmorphism including ear canal structures and microcephaly, mild intellectual disability, and developmental delay. We generated a zebrafish model of eftud2 deficiency, and a consistent phenotype consisting of mandibular bone dysplasia and otolith loss was observed. We also showed that EFTUD2 deficiency significantly inhibited proliferation, differentiation, and maturation in human calvarial osteoblast (HCO) and human articular chondrocyte (HC-a) cells. RNA-Seq analysis uncovered activated TP53 signaling with increased phosphorylation of the TP53 protein and upregulation of five TP53 downstream target genes (FAS, STEAP3, CASP3, P21, and SESN1) both in HCO and in eftud2−/− zebrafish. Additionally, inhibition of p53 by morpholino significantly reduced the mortality of eftud2−/− larvae.

Conclusions

Our results confirm a novel de novo variant of the EFTUD2 gene and suggest that EFTUD2 may participate in the maturation and differentiation of osteoblasts and chondrocytes, possibly via activation of the TP53 signaling pathway. Thus, mutations in this gene may lead to skeletal anomalies in vertebrates.

Tankyrase inhibition preserves osteoarthritic cartilage by coordinating cartilage matrix anabolism via effects on SOX9 PARylation

Osteoarthritis (OA) is a prevalent degenerative disease, which involves progressive and irreversible destruction of cartilage matrix. Despite efforts to reconstruct cartilage matrix in osteoarthritic joints, it has been a difficult task as adult cartilage exhibits marginal repair capacity. Here we report the identification of tankyrase as a regulator of the cartilage anabolism axis based on systems-level factor analysis of mouse reference populations. Tankyrase inhibition drives the expression of a cartilage-signature matrisome and elicits a transcriptomic pattern that is inversely correlated with OA progression. Furthermore, tankyrase inhibitors ameliorate surgically induced OA in mice, and stem cell transplantation coupled with tankyrase knockdown results in superior regeneration of cartilage lesions. Mechanistically, the pro-regenerative features of tankyrase inhibition are mainly triggered by uncoupling SOX9 from a poly(ADP-ribosyl)ation (PARylation)-dependent protein degradation pathway. Our findings provide insights into the development of future OA therapies aimed at reconstruction of articular cartilage.

Osteoarthritis results from the progressive destruction of cartilage matrix. Here, Kim et al. identify tankyrase as a regulator of cartilage matrix anabolism, and find that tankyrase inhibition, by preventing SOX9 PARylation, protects from cartilage destruction in a mouse model of osteoarthritis.