Induction of the Sry-related factor SOX6 contributes to bone morphogenetic protein-2-induced chondroblastic differentiation of C3H10T1/2 cells

Effect of a retinoic acid analogue on BMP-driven pluripotent stem cell chondrogenesis

Osteoarthritis is the most common degenerative joint condition, leading to articular cartilage (AC) degradation, chronic pain and immobility. The lack of appropriate therapies that provide tissue restoration combined with the limited lifespan of joint-replacement implants indicate the need for alternative AC regeneration strategies. Differentiation of human pluripotent stem cells (hPSCs) into AC progenitors may provide a long-term regenerative solution but is still limited due to the continued reliance upon growth factors to recapitulate developmental signalling processes. Recently, TTNPB, a small molecule activator of retinoic acid receptors (RARs), has been shown to be sufficient to guide mesodermal specification and early chondrogenesis of hPSCs. Here, we modified our previous differentiation protocol, by supplementing cells with TTNPB and administering BMP2 at specific times to enhance early development (referred to as the RAPID-E protocol). Transcriptomic analyses indicated that activation of RAR signalling significantly upregulated genes related to limb and embryonic skeletal development in the early stages of the protocol and upregulated genes related to AC development in later stages. Chondroprogenitors obtained from RAPID-E could generate cartilaginous pellets that expressed AC-related matrix proteins such as Lubricin, Aggrecan, and Collagen II, but additionally expressed Collagen X, indicative of hypertrophy. This protocol could lay the foundations for cell therapy strategies for osteoarthritis and improve the understanding of AC development in humans.

Optogenetic manipulation of BMP signaling to drive chondrogenic differentiation of hPSCs

Optogenetics is a rapidly advancing technology combining photochemical, optical, and synthetic biology to control cellular behavior. Together, sensitive light-responsive optogenetic tools and human pluripotent stem cell differentiation models have the potential to fine-tune differentiation and unpick the processes by which cell specification and tissue patterning are controlled by morphogens. We used an optogenetic bone morphogenetic protein (BMP) signaling system (optoBMP) to drive chondrogenic differentiation of human embryonic stem cells (hESCs). We engineered light-sensitive hESCs through CRISPR-Cas9-mediated integration of the optoBMP system into the AAVS1 locus. The activation of optoBMP with blue light, in lieu of BMP growth factors, resulted in the activation of BMP signaling mechanisms and upregulation of a chondrogenic phenotype, with significant transcriptional differences compared to cells in the dark. Furthermore, cells differentiated with light could form chondrogenic pellets consisting of a hyaline-like cartilaginous matrix. Our findings indicate the applicability of optogenetics for understanding human development and tissue engineering.

TGFβ3, dibutyryl cAMP and a notch inhibitor modulate phenotype late in stem cell-derived dopaminergic neuron maturation

The generation of midbrain dopaminergic neurons (mDAs) from pluripotent stem cells (hPSC) holds much promise for both disease modelling studies and as a cell therapy for Parkinson's disease (PD). Generally, dopaminergic neuron differentiation paradigms rely on inhibition of smad signalling for neural induction followed by hedgehog signalling and an elevation of β-catenin to drive dopaminergic differentiation. Post-patterning, differentiating dopaminergic neuron cultures are permitted time for maturation after which the success of these differentiation paradigms is usually defined by expression of tyrosine hydroxylase (TH), the rate limiting enzyme in the synthesis of dopamine. However, during maturation, culture media is often supplemented with additives to promote neuron survival and or promote cell differentiation. These additives include dibutyryl cyclic adenosine monophosphate (dbcAMP), transforming growth factor β3 (TGFβ3) and or the γ-secretase inhibitor (DAPT). While these factors are routinely added to cultures, their impact upon pluripotent stem cell-derived mDA phenotype is largely unclear. In this study, we differentiate pluripotent stem cells toward a dopaminergic phenotype and investigate how the omission of dbcAMP, TGFβ3 or DAPT, late in maturation, affects the regulation of multiple dopaminergic neuron phenotype markers. We now show that the removal of dbcAMP or TGFβ3 significantly and distinctly impacts multiple markers of the mDA phenotype (FOXA2, EN1, EN2, FOXA2, SOX6), while commonly increasing both MSX2 and NEUROD1 and reducing expression of both tyrosine hydroxylase and WNT5A. Removing DAPT significantly impacted MSX2, OTX2, EN1, and KCNJ6. In the absence of any stressful stimuli, we suggest that these culture additives should be viewed as mDA phenotype-modifying, rather than neuroprotective. We also suggest that their addition to cultures is likely to confound the interpretation of both transplantation and disease modelling studies.

Role of SMPD3 during Bone Fracture Healing and Regulation of Its Expression

Sphingomyelin phosphodiesterase 3 (SMPD3), a lipid-metabolizing enzyme present in bone and cartilage, has important roles in the developing skeleton. We previously showed that SMPD3 deficiency results in delayed extracellular matrix (ECM) mineralization and severe skeletal deformities in an inducible knockout mouse model, Smpd3^flox/flox ; Osx-Cre mice, in which Smpd3 was ablated in Osx-expressing chondrocytes and osteoblasts during early skeletogenesis. However, as shown in the current study, ablation of Smpd3 postnatally in 3-month-old Smpd3^flox/flox ; Osx-Cre mice resulted in only a mild bone mineralization defect. Interestingly, though, there was a marked increase of unmineralized osteoid in the fractured tibiae of 3-month-old Smpd3^flox/flox ; Osx-Cre mice. As was the case in the embryonic bones, we also observed impaired chondrocyte apoptosis at the fracture sites of Smpd3^flox/flox ; Osx-Cre mice. We further examined how Smpd3 expression is regulated in ATDC5 chondrogenic cells by two major regulators of chondrogenesis, bone morphogenetic protein 2 (BMP-2) and PTHrP. Our data show that BMP-2 positively regulates Smpd3 expression via p38 mitogen-activated protein kinase. Taken together, our findings show that SMPD3 plays a significant role in ECM mineralization and chondrocyte apoptosis during fracture healing. Furthermore, our gene expression analyses suggest that BMP-2 and PTHrP exert opposing effects on the regulation of Smpd3 expression in chondrocytes.

ELF3 modulates type II collagen gene (COL2A1) transcription in chondrocytes by inhibiting SOX9-CBP/p300-driven histone acetyltransferase activity

AIM

We showed previously that E74-like factor 3 (ELF3) protein levels are increased in osteoarthritic (OA) cartilage, that ELF3 accounts for inflammatory cytokine-driven MMP13 gene expression, and that, upon induction by interleukin-1β, ELF3 binds to the COL2A1 promoter and suppresses its activity in chondrocytes. Here, we aimed to further investigate the mechanism/s by which ELF3 represses COL2A1 transcription in chondrocytes.

METHODS AND RESULTS

We report that ELF3 inhibits Sox9-driven COL2A1 promoter activity by interfering with the activator functions of CBP/300 and Sox9. Co-transfection of the pGL2B-COL2A1 (-577/+3428 bp) reporter construct with Sox9 and with Sox5 and/or Sox6 increased COL2A1 promoter activity, and ELF3 overexpression significantly reduced the promoter transactivation. Co-transfection of ELF3 with the pLuc 4x48 enhancer construct, containing the 89-bp COL2A1 promoter and lacking the previously defined ELF3 binding sites, decreased both basal and Sox9-driven promoter activity. Co-transfection of ELF3 with a Gal4 reporter construct also inhibited Gal4-Sox9-driven transactivation, suggesting that ELF3 directly interacts with Sox9. Using truncated Sox9 fragments, we found that ELF3 interacts directly with the HMG domain of Sox9. Importantly, overexpression of ELF3 significantly decreased Sox9/CBP-dependent HAT activity. Finally, we show evidence that increased ELF3 mRNA expression in OA chondrocytes correlates with hypermethylation of the proximal promoter, suggesting that ELF3 transcription is subjected to epigenetic control in OA disease.

CONCLUSION

Our results highlight the contribution of ELF3 to transcriptional regulation of COL2A1 and its potential role in OA disease, and uncover epigenetic mechanisms at play in the regulation of ELF3 and its downstream targets in articular chondrocytes.

Applications of Chondrocyte-Based Cartilage Engineering: An Overview

Chondrocytes are the exclusive cells residing in cartilage and maintain the functionality of cartilage tissue. Series of biocomponents such as different growth factors, cytokines, and transcriptional factors regulate the mesenchymal stem cells (MSCs) differentiation to chondrocytes. The number of chondrocytes and dedifferentiation are the key limitations in subsequent clinical application of the chondrocytes. Different culture methods are being developed to overcome such issues. Using tissue engineering and cell based approaches, chondrocytes offer prominent therapeutic option specifically in orthopedics for cartilage repair and to treat ailments such as tracheal defects, facial reconstruction, and urinary incontinence. Matrix-assisted autologous chondrocyte transplantation/implantation is an improved version of traditional autologous chondrocyte transplantation (ACT) method. An increasing number of studies show the clinical significance of this technique for the chondral lesions treatment. Literature survey was carried out to address clinical and functional findings by using various ACT procedures. The current study was conducted to study the pharmacological significance and biomedical application of chondrocytes. Furthermore, it is inferred from the present study that long term follow-up studies are required to evaluate the potential of these methods and specific positive outcomes.

Bone Morphogenetic Proteins

Bone morphogenetic proteins (BMPs), originally identified as osteoinductive components in extracts derived from bone, are now known to play important roles in a wide array of processes during formation and maintenance of various organs including bone, cartilage, muscle, kidney, and blood vessels. BMPs and the related "growth and differentiation factors" (GDFs) are members of the transforming growth factor β (TGF-β) family, and transduce their signals through type I and type II serine-threonine kinase receptors and their intracellular downstream effectors, including Smad proteins. Furthermore, BMP signals are finely tuned by various agonists and antagonists. Because deregulation of the BMP activity at multiple steps in signal transduction is linked to a wide variety of human diseases, therapeutic use of activators and inhibitors of BMP signaling will provide potential avenues for the treatment of the human disorders that are caused by hypo- and hyperactivation of BMP signals, respectively.

Articular cartilage tissue engineering: the role of signaling molecules

Effective early disease modifying options for osteoarthritis remain lacking. Tissue engineering approach to generate cartilage in vitro has emerged as a promising option for articular cartilage repair and regeneration. Signaling molecules and matrix modifying agents, derived from knowledge of cartilage development and homeostasis, have been used as biochemical stimuli toward cartilage tissue engineering and have led to improvements in the functionality of engineered cartilage. Clinical translation of neocartilage faces challenges, such as phenotypic instability of the engineered cartilage, poor integration, inflammation, and catabolic factors in the arthritic environment; these can all contribute to failure of implanted neocartilage. A comprehensive understanding of signaling molecules involved in osteoarthritis pathogenesis and their actions on engineered cartilage will be crucial. Thus, while it is important to continue deriving inspiration from cartilage development and homeostasis, it has become increasingly necessary to incorporate knowledge from osteoarthritis pathogenesis into cartilage tissue engineering.

Hedgehog-Gli activators direct osteo-chondrogenic function of bone morphogenetic protein toward osteogenesis in the perichondrium

Specification of progenitors into the osteoblast lineage is an essential event for skeletogenesis. During endochondral ossification, cells in the perichondrium give rise to osteoblast precursors. Hedgehog (Hh) and bone morphogenetic protein (BMP) are suggested to regulate the commitment of these cells. However, properties of perichondrial cells and regulatory mechanisms of the specification process are still poorly understood. Here, we investigated the machineries by combining a novel organ culture system and single-cell expression analysis with mouse genetics and biochemical analyses. In a metatarsal organ culture reproducing bone collar formation, activation of BMP signaling enhanced the bone collar formation cooperatively with Hh input, whereas the signaling induced ectopic chondrocyte formation in the perichondrium without Hh input. Similar phenotypes were also observed in compound mutant mice, where signaling activities of Hh and BMP were genetically manipulated. Single-cell quantitative RT-PCR analyses showed heterogeneity of perichondrial cells in terms of natural characteristics and responsiveness to Hh input. In vitro analyses revealed that Hh signaling suppressed BMP-induced chondrogenic differentiation; Gli1 inhibited the expression of Sox5, Sox6, and Sox9 (SRY box-containing gene 9) as well as transactivation by Sox9. Indeed, ectopic expression of chondrocyte maker genes were observed in the perichondrium of metatarsals in Gli1(-/-) fetuses, and the phenotype was more severe in Gli1(-/-);Gli2(-/-) newborns. These data suggest that Hh-Gli activators alter the function of BMP to specify perichondrial cells into osteoblasts; the timing of Hh input and its target populations are critical for BMP function.

Enhanced hyaline cartilage matrix synthesis in collagen sponge scaffolds by using siRNA to stabilize chondrocytes phenotype cultured with bone morphogenetic protein-2 under hypoxia

Cartilage healing by tissue engineering is an alternative strategy to reconstitute functional tissue after trauma or age-related degeneration. However, chondrocytes, the major player in cartilage homeostasis, do not self-regenerate efficiently and lose their phenotype during osteoarthritis. This process is called dedifferentiation and also occurs during the first expansion step of autologous chondrocyte implantation (ACI). To ensure successful ACI therapy, chondrocytes must be differentiated and capable of synthesizing hyaline cartilage matrix molecules. We therefore developed a safe procedure for redifferentiating human chondrocytes by combining appropriate physicochemical factors: hypoxic conditions, collagen scaffolds, chondrogenic factors (bone morphogenetic protein-2 [BMP-2], and insulin-like growth factor I [IGF-I]) and RNA interference targeting the COL1A1 gene. Redifferentiation of dedifferentiated chondrocytes was evaluated using gene/protein analyses to identify the chondrocyte phenotypic profile. In our conditions, under BMP-2 treatment, redifferentiated and metabolically active chondrocytes synthesized a hyaline-like cartilage matrix characterized by type IIB collagen and aggrecan molecules without any sign of hypertrophy or osteogenesis. In contrast, IGF-I increased both specific and noncharacteristic markers (collagens I and X) of chondrocytes. The specific increase in COL2A1 gene expression observed in the BMP-2 treatment was shown to involve the specific enhancer region of COL2A1 that binds the trans-activators Sox9/L-Sox5/Sox6 and Sp1, which are associated with a decrease in the trans-inhibitors of COL2A1, c-Krox, and p65 subunit of NF-kappaB. Our procedure in which BMP-2 treatment under hypoxia is associated with a COL1A1 siRNA, significantly increased the differentiation index of chondrocytes, and should offer the opportunity to develop new ACI-based therapies in humans.

Role of Slug transcription factor in human mesenchymal stem cells

The pathways that control mesenchymal stem cells (MSCs) differentiation are not well understood, and although some of the involved transcription factors (TFs) have been characterized, the role of others remains unclear. We used human MSCs from tibial plateau (TP) trabecular bone, iliac crest (IC) bone marrow and Wharton’s jelly (WJ) umbilical cord demonstrating a variability in their mineral matrix deposition, and in the expression levels of TFs including Runx2, Sox9, Sox5, Sox6, STAT1 and Slug, all involved in the control of osteochondroprogenitors differentiation program. Because we reasoned that the basal expression level of some TFs with crucial role in the control of MSC fate may be correlated with osteogenic potential, we considered the possibility to affect the hMSCs behaviour by using gene silencing approach without exposing cells to induction media. In this study we found that Slug-silenced cells changed in morphology, decreased in their migration ability, increased Sox9 and Sox5 and decreased Sox6 and STAT1 expression. On the contrary, the effect of Slug depletion on Runx2 was influenced by cell type. Interestingly, we demonstrated a direct in vivo regulatory action of Slug by chromatin immunoprecipitation, showing a specific recruitment of this TF in the promoter of Runx2 and Sox9 genes. As a whole, our findings have important potential implication on bone tissue engineering applications, reinforcing the concept that manipulation of specific TF expression levels may elucidate MSC biology and the molecular mechanisms, which promote osteogenic differentiation.

Sox6, jack of all trades: a versatile regulatory protein in vertebrate development

Approximately 20,000 genes are encoded in our genome, one tenth of which are thought to be transcription factors. Considering the complexity and variety of cell types generated during development, many transcription factors likely play multiple roles. Uncovering the versatile roles of Sox6 in vertebrate development sheds some light on how an organism efficiently utilizes the limited resources of transcription factors. The structure of the Sox6 gene itself may dictate its functional versatility. First, Sox6 contains no known regulatory domains; instead, it utilizes various cofactors. Second, Sox6 has a long 3'-UTR that contains multiple microRNA targets, thus its protein level is duly adjusted by cell type-specific microRNAs. Just combining these two characteristics alone makes Sox6 extremely versatile. To date, Sox6 has been reported to regulate differentiation of tissues of mesoderm, ectoderm, and endoderm origins, making Sox6 a truly multifaceted transcription factor.

Differentiation and mineralization of murine mesenchymal C3H10T1/2 cells in micromass culture

The murine mesenchymal cell line, C3H10T1/2 in micromass culture undergoes chondrogenic differentiation with the addition of BMP-2. This study compares the use of BMP-2 vs. insulin, transferrin, and sodium selenite (ITS) to create a chondrogenic micromass cell culture system that models cartilage calcification in the presence of 4mM inorganic phosphate. BMP-2 treated cultures showed more intense alcian blue staining for proteoglycans than ITS treated cultures at early time points. Both ITS and BMP-2 treated cultures showed similar mineral deposition in cultures treated with 4mM phosphate via von Kossa staining, however FTIR spectroscopy of cultures showed different matrix properties. ITS treated cultures produced matrix that more closely resembled mouse calcified cartilage by FTIR analysis. (45)Ca uptake curves showed delayed onset of mineralization in cultures treated with BMP-2, however they had an increased rate of mineralization (initial slope of (45)Ca uptake curve) when compared to the cultures treated with ITS. Immunohistochemistry showed the presence of both collagens type I and type II in BMP-2 and ITS treated control (1mM inorganic phosphate) and mineralizing cultures. BMP-2 treated mineralizing cultures displayed more intense staining for collagen type II than all other cultures. Collagen type X staining was detected at Day 9 only in mineralizing cultures treated with ITS. Western blotting of Day 9 cultures confirmed the presence of collagen type X in the mineralizing ITS cultures, and also showed very small amounts of collagen type X in BMP-2 treated cultures and control ITS cultures. By Day 16 all cultures stained positive for collagen type X. These data suggest that BMP-2 induces a more chondrogenic phenotype, while ITS treatment favors maturation and hypertrophy of the chondrocytes in the murine micromass cultures.

Identification and characterization of the human SOX6 promoter

The present study attempted to identify and characterize the embryonic promoter of Sox6, a determinant regulator of chondrogenic differentiation. A common transcription start region for human and mouse Sox6 was initially identified, which contained a highly conserved sequence, A-box. Tandem repeats of A-box had a strong transcriptional activity both at the basal level and in response to Sox9. Cells carrying the 4xA-box-DsRed2 reporter fluoresced only upon chondrogenic differentiation. The 46-bp core enhancer region (CES6) was then identified in the 3' half of A-box, within which a C/EBP-binding motif was identified. Overexpressed C/EBPbeta activated the Sox6 promoter, and mutant 4xCES6 constructs lacking the C/EBP motif lost their basal activity. CES6 and nuclear extracts formed a specific complex, which was supershifted by anti-C/EBPbeta antibody, and in vitro translated C/EBPbeta specifically bound to CES6. Thus, we successfully identified the Sox6 promoter and its core enhancer and characterized the interactions with regulatory transcription factors.

Stem cell transplantation demonstrates that Sox6 represses εy globin expression in definitive erythropoiesis of adult mice

Sox6, a member of the Sox transcription factor family, is essential for the silencing of epsilon y globin gene expression in definitive erythropoiesis of mice. Homozygous Sox6-null mice are neonatally lethal, precluding analysis at later stages. We created adult mice that are deficient in Sox6 specifically in hematopoietic tissues by transplanting embryonic liver stem cells from Sox6-deficient mice into lethally irradiated congenic wild-type adult mice. The mice receiving mutant stem cells (mutant engrafted) showed high expression levels of epsilon y in bone marrow, spleen, and circulating blood compared with mice receiving wild-type and heterozygous stem cells (control engrafted). The level of expression of epsilon y in circulating blood was directly correlated with the percentage of successful mutant donor cell engraftment. Additionally, the mutant engrafted adult mice showed an increase in erythroid precursor cells in bone marrow, spleen, and blood. Thus, Sox6 continues to function as a major regulator of epsilon y in adult definitive erythropoiesis and is required for normal erythrocyte maturation. Therefore, Sox6 may provide a novel therapeutic target by reactivating epsilon y in patients with hemoglobinopathies such as sickle cell anemia and beta-thalassemia.

HC-gp39 contributes to chondrocyte differentiation by inducing SOX9 and type II collagen expressions

OBJECTIVE

The transcription factor SOX9 has been shown to be linked to chondrocyte differentiation and induction of type II collagen synthesis. Since the chitinase-like protein, human cartilage glycoprotein 39 (HC-gp39), can be expressed by articular chondrocytes and has been shown to enhance chondrocyte mitogenesis through MAP kinase and PI3 kinase-mediated signalling, we hypothesized that it may also promote synthesis of cartilage matrix components through induction of SOX9, utilizing similar signalling pathways.

METHODS

Primary chondrocytes from neonatal mouse rib cartilage were exposed to purified HC-gp39. The response of the cells was evaluated in terms of SOX9 induction and synthesis of type II collagen. Signalling pathways activated following HC-gp9 exposure were analyzed by Western blotting of cell lysates with phosphorylation-specific antibodies as well as by using selective inhibitors.

RESULTS

HC-gp39 induced both SOX9 and type II collagen synthesis. Similar results were observed for IGF-1. This process required signalling through both MAP kinase and PI3 kinase pathways resulting in rapid phosphorylation of ERK1/2 and AKT, respectively. Neither HC-gp39 nor IGF-1 induced activation of SAPK/JNK.

CONCLUSIONS

The effects of HC-gp39 on chondrocyte function suggest that this molecule may promote the maintenance or expression of a chondrocytic phenotype. Its expression in injured or degenerate cartilage could be related to the initial repair-response and increased matrix synthesis observed in osteoarthritic cartilage.

The Chondrocyte: Biology and Clinical Application

Chondrocyte is a unique cell type in articular cartilage tissue and is essential for cartilage formation and functionality. It arises from mesenchymal stem cells (MSCs) and is regulated by a series of cytokine and transcription factor interactions, including the transforming growth factor-beta super family, fibroblast growth factors, and insulin-like growth factor-1. To understand the biomechanisms of the chondrocyte differentiation process, various cellular model systems have been employed, such as primary chondrocyte culture, clonal normal cell lines (HCS-2/8, Ch-1, ATDC5, CFK-2, and RCJ3.1C5.18), and transformed clonal cell lines (T/C-28a2, T/C-28a4, C-28/I2, tsT/AC62, and HPV-16 E6/E7). Additionally, cell culture methods, including conventional monolayer culture, three-dimensional scaffold culture, bioreactor culture, pellet culture, and organ culture, have been established to create stable environments for the expansion, phenotypic maintenance, and subsequent biological study of chondrocytes for clinical application. Knowledge gained through these study systems has allowed for the use of chondrocytes in orthopedics for the treatment of cartilage injury and epiphyseal growth plate defects using tissue-engineering approaches. Furthermore, the potential of chondrocyte implantation for facial reconstruction, the treatment of long segmental tracheal defects, and urinary incontinence and vesicoureteral reflux are being investigated. This review summarizes the present study of chondrocyte biology and the potential uses of this cell in orthopedics and other disciplines.

Differential expression of novel potential regulators in hematopoietic stem cells

The hematopoietic system is an invaluable model both for understanding basic developmental biology and for developing clinically relevant cell therapies. Using highly purified cells and rigorous microarray analysis we have compared the expression pattern of three of the most primitive hematopoietic subpopulations in adult mouse bone marrow: long-term hematopoietic stem cells (HSC), short-term HSC, and multipotent progenitors. All three populations are capable of differentiating into a spectrum of mature blood cells, but differ in their self-renewal and proliferative capacity. We identified numerous novel potential regulators of HSC self-renewal and proliferation that were differentially expressed between these closely related cell populations. Many of the differentially expressed transcripts fit into pathways and protein complexes not previously identified in HSC, providing evidence for new HSC regulatory units. Extending these observations to the protein level, we demonstrate expression of several of the corresponding proteins, which provide novel surface markers for HSC. We discuss the implications of our findings for HSC biology. In particular, our data suggest that cell–cell and cell–matrix interactions are major regulators of long-term HSC, and that HSC themselves play important roles in regulating their immediate microenvironment.

Hematopoietic, or blood-forming, stem cells (HSC) are responsible for the continual replenishment of all blood cells throughout life. This ability to both renew themselves and give rise to expanded populations of differentiating and mature cells is a hallmark of stem cells and is therefore an area of intense research. The rarity of HSC as well as their location in the bone marrow environment has made it difficult to identify the genes that regulate these properties. The earliest stages of blood development begins with the long-term (LT) repopulating HSC that then differentiate into short-term (ST) repopulating HSC and non-self renewing multipotent progenitors (MPP). The authors investigated the gene expression differences in these highly purified populations that differ mainly in their capacity to self renew, and identified a number of genes specific to each of these populations. Intriguingly, many of these genes code for proteins that are involved in cell–cell and cell–matrix interactions that were not previously identified on these populations. These novel discoveries will, together with future experiments, enhance our understanding of the basic biology of stem cells and their clinical uses.

Slow and fast fiber isoform gene expression is systematically altered in skeletal muscle of the Sox6 mutant, p100H

We have previously demonstrated that p100H mutant mice, which lack a functional Sox6 gene, exhibit skeletal and cardiac muscle degeneration and develop cardiac conduction abnormalities soon after birth. To understand the role of Sox6 in skeletal muscle development, we identified muscle-specific genes differentially expressed between wild-type and p100H mutant skeletal muscles and investigated their temporal expression in the mutant muscle. We found that, in the mutant skeletal muscle, slow fiber and cardiac isoform genes are expressed at significantly higher levels, whereas fast fiber isoform genes are expressed at significantly lower levels than wild-type. Onset of this aberrant fiber type-specific gene expression in the mutant coincides with the beginning of the secondary myotube formation, at embryonic day 15-16 in mice. Together with our earlier report, demonstrating early postnatal muscle defects in the Sox6 null-p100H mutant, the present results suggest that Sox6 likely plays an important role in muscle development.

Sox6 directly silences epsilon globin expression in definitive erythropoiesis

Sox6 is a member of the Sox transcription factor family that is defined by the conserved high mobility group (HMG) DNA binding domain, first described in the testis determining gene, Sry. Previous studies have suggested that Sox6 plays a role in the development of the central nervous system, cartilage, and muscle. In the Sox6-deficient mouse, p100H, epsilony globin is persistently expressed, and increased numbers of nucleated red cells are present in the fetal circulation. Transfection assays in GM979 (erythroleukemic) cells define a 36-base pair region of the epsilony proximal promoter that is critical for Sox6 mediated repression. Electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) assays demonstrate that Sox6 acts as a repressor by directly binding to the epsilony promoter. The normal expression of Sox6 in wild-type fetal liver and the ectopic expression of epsilony in p100H homozygous fetal liver demonstrate that Sox6 functions in definitive erythropoiesis. The present study shows that Sox6 is required for silencing of epsilony globin in definitive erythropoiesis and suggests a role for Sox6 in erythroid cell maturation. Thus, Sox6 regulation of epsilony globin might provide a novel therapeutical target in the treatment of hemoglobinopathies such as sickle cell anemia and thalassemia.

Repression of SOX6 transcriptional activity by SUMO modification

SOX6 plays key functions in several developmental processes, including neurogenesis and skeleton formation. In this report, we show that SOX6 is modified in vitro and in vivo by small ubiquitin-related modifier (SUMO) on two distinct sites. Mutation of both sites abolished SOX6 sumoylation and increased SOX6 transcriptional activity. SUMO dependent repression of SOX6 transcription was promoted by UBC9 whereas siRNA to UBC9, cotransfection of inactive UBC9 or a SUMO protease increased SOX6 transcriptional activity. Furthermore, co-expression of SOX6 with SUMO2 results in the appearance of SOX6 in a punctate nuclear pattern that colocalized with promyelocytic leukemia protein, which was partially abolished by mutations in SOX6 sumoylation sites.

BMP signaling in the cartilage growth plate

Transforming growth factor-beta (TGF-beta) superfamily members play diverse roles in all aspects of cartilage development and maintenance. It is well established that TGF-betas and bone morphogenetic proteins (BMPs) play distinct roles in the growth plate. This chapter discusses key experiments and experimental approaches that have revealed these roles, and progress toward the identification of previously unsuspected roles. Current understanding of the mechanisms by which different TGF-beta and BMP pathways exert their functions is discussed. Finally attempts to utilize this information to promote cartilage regeneration, and important issues for future research, are outlined.

Directing stem cell differentiation into the chondrogenic lineage in vitro

A major area in regenerative medicine is the application of stem cells in cartilage tissue engineering and reconstructive surgery. This requires well-defined and efficient protocols for directing the differentiation of stem cells into the chondrogenic lineage, followed by their selective purification and proliferation in vitro. The development of such protocols would reduce the likelihood of spontaneous differentiation of stem cells into divergent lineages upon transplantation, as well as reduce the risk of teratoma formation in the case of embryonic stem cells. Additionally, such protocols could provide useful in vitro models for studying chondrogenesis and cartilaginous tissue biology. The development of pharmacokinetic and cytotoxicity/genotoxicity screening tests for cartilage-related biomaterials and drugs could also utilize protocols developed for the chondrogenic differentiation of stem cells. Hence, this review critically examines the various strategies that could be used to direct the differentiation of stem cells into the chondrogenic lineage in vitro.

Bmpr1a and Bmpr1b have overlapping functions and are essential for chondrogenesis in vivo

Previous studies have demonstrated the ability of bone morphogenetic proteins (BMPs) to promote chondrogenic differentiation in vitro. However, the in vivo role of BMP signaling during chondrogenesis has been unclear. We report here that BMP signaling is essential for multiple aspects of early chondrogenesis. Whereas mice deficient in type 1 receptors Bmpr1a or Bmpr1b in cartilage are able to form intact cartilaginous elements, double mutants develop a severe generalized chondrodysplasia. The majority of skeletal elements that form through endochondral ossification are absent, and the ones that form are rudimentary. The few cartilage condensations that form in double mutants are delayed in the prechondrocytic state and never form an organized growth plate. The reduced size of mutant condensations results from increased apoptosis and decreased proliferation. Moreover, the expression of cartilage-specific extracellular matrix proteins is severely reduced in mutant elements. We demonstrate that this defect in chondrocytic differentiation can be attributed to lack of Sox9, L-Sox5, and Sox6 expression in precartilaginous condensations in double mutants. In summary, our study demonstrates that BMPR1A and BMPR1B are functionally redundant during early chondrogenesis and that BMP signaling is required for chondrocyte proliferation, survival, and differentiation in vivo.

Multiple functions of BMPs in chondrogenesis

The ability of bone morphogenetic proteins (BMPs) to promote chondrogenesis has been investigated extensively over the past two decades. Although BMPs promote almost every aspect of chondrogenesis, from commitment to terminal differentiation is well known, the mechanisms of BMP action in discrete aspects of endochondral bone formation have only recently begun to be investigated. In this review, we focus on in vivo studies that have identified interactions between BMP signaling pathways and key downstream targets of BMP action in chondrogenesis. We also discuss evidence regarding the potential roles of BMP receptors in mediating distinct aspects of chondrogenesis, and studies investigating the intersection of BMP pathways with other pathways known to coordinate the progression of chondrocytes through the growth plate. These studies indicate that both Smad-dependent and -independent BMP pathways are required for chondrogenesis, and that BMPs exert essential roles via regulation of the Indian hedgehog (IHH)/parathyroid hormone-related protein (PTHrP) and fibroblast growth factor (FGF) pathways in the growth plate.

BMP receptor signaling is required for postnatal maintenance of articular cartilage

Articular cartilage plays an essential role in health and mobility, but is frequently damaged or lost in millions of people that develop arthritis. The molecular mechanisms that create and maintain this thin layer of cartilage that covers the surface of bones in joint regions are poorly understood, in part because tools to manipulate gene expression specifically in this tissue have not been available. Here we use regulatory information from the mouse Gdf5 gene (a bone morphogenetic protein [BMP] family member) to develop new mouse lines that can be used to either activate or inactivate genes specifically in developing joints. Expression of Cre recombinase from Gdf5 bacterial artificial chromosome clones leads to specific activation or inactivation of floxed target genes in developing joints, including early joint interzones, adult articular cartilage, and the joint capsule. We have used this system to test the role of BMP receptor signaling in joint development. Mice with null mutations in Bmpr1a are known to die early in embryogenesis with multiple defects. However, combining a floxed Bmpr1a allele with the Gdf5-Cre driver bypasses this embryonic lethality, and leads to birth and postnatal development of mice missing the Bmpr1a gene in articular regions. Most joints in the body form normally in the absence of Bmpr1a receptor function. However, articular cartilage within the joints gradually wears away in receptor-deficient mice after birth in a process resembling human osteoarthritis. Gdf5-Cre mice provide a general system that can be used to test the role of genes in articular regions. BMP receptor signaling is required not only for early development and creation of multiple tissues, but also for ongoing maintenance of articular cartilage after birth. Genetic variation in the strength of BMP receptor signaling may be an important risk factor in human osteoarthritis, and treatments that mimic or augment BMP receptor signaling should be investigated as a possible therapeutic strategy for maintaining the health of joint linings.

Extracellular matrix gene regulation

Extracellular matrix metabolism plays a central role in development of skeletal tissues and in most orthopaedic diseases and trauma such as fracture or osteotomy repair, arthritis, cartilage repair, and congenital skeletal deformity. During development or disease, specific genes must be expressed in order to make or repair appropriate extracellular matrix. For example, specific gene expression patterns are characteristic of bone and cartilage. The precise expression pattern depends on a balance of positive and negative transcription factors, proteins that control the synthesis of mRNA from the specific gene. In cartilage, a number of studies indicate that Sox transcription factors are critical positive regulators in genes such as COL2A1, COL9A2, COL11A2, aggrecan, and CD-RAP. In addition, negative regulators are also essential to fine tune gene regulation in chondrocytes and to turn off gene expression in noncartilaginous tissues. Negative transcription factors in cartilage include partial differentialEF-1, snail/slug, CYRBP1, NT2, and C/EBP. Runx2 and osterix are critical transcription factors for osteogenesis but also have some influence on chondrogenesis. The availability of cis-regulatory sites in specific genes combined with the availability of transcription factors in the nucleus determines the level of gene expression.

Sox6 regulation of cardiac myocyte development

A mouse mutation (p100H/p100H) has been identified that is associated with cardioskeletal myopathy, heart block, delayed growth and early postnatal death. The gene that is disrupted in this mutation encodes the transcription factor Sox6. P19CL6 cells were used as an in vitro cardiomyocyte differentiation system and revealed that Sox6 is expressed exclusively when the cells are committed to differentiate to beating cardiac myocytes. We used the yeast two-hybrid system to identify the Prtb (Proline-rich transcript of the brain) protein as a Sox6 interactor, and subsequently confirmed the interaction by co-immunoprecipitation. Prtb expression in P19CL6 cells increased with differentiation to beating cardiomyocytes. Using the P19CL6 cells stably transfected with noggin, an antagonist of BMP (Bone Morphogenic Protein), we found that BMP expression is required for Sox6 expression in cardiomyocyte differentiation. Surprisingly, the expression of the alpha1c-subunit gene of the L-type Ca2+ channel decreased in P19CL6 cells as they differentiated to beating cardiac cells. Ectopic expression of Sox6 or Prtb alone in P19CL6 cells caused down-regulation of L-type Ca2+ alpha1c expression, but when Sox6 and Prtb were co-transfected to the cells, L-type Ca2+ alpha1c remained at basal levels. A similar relationship of Sox6 and L-type Ca2+ alpha1c expression was seen in vivo (comparing wild-type and p(100H)/p(100H) mutant mice). Thus, Sox6 is within the BMP pathway in cardiac differentiation, interacts with Prtb and may play a critical role in the regulation of a cardiac L-type Ca2+ channel.