Evolutionary Selection and Constraint on Human Knee Chondrocyte Regulation Impacts Osteoarthritis Risk.

During human evolution, the knee adapted to the biomechanical demands of bipedalism by altering chondrocyte developmental programs. This adaptive process was likely not without deleterious consequences to health. Today, osteoarthritis occurs in 250 million people, with risk variants enriched in non-coding sequences near chondrocyte genes, loci that likely became optimized during knee evolution. We explore this relationship by epigenetically profiling joint chondrocytes, revealing ancient selection and recent constraint and drift on knee regulatory elements, which also overlap osteoarthritis variants that contribute to disease heritability by tending to modify constrained functional sequence. We propose a model whereby genetic violations to regulatory constraint, tolerated during knee development, lead to adult pathology. In support, we discover a causal enhancer variant (rs6060369) present in billions of people at a risk locus (GDF5-UQCC1), showing how it impacts mouse knee-shape and osteoarthritis. Overall, our methods link an evolutionarily novel aspect of human anatomy to its pathogenesis.

The BMP Pathway and Its Inhibitors in the Skeleton.

Bone morphogenetic proteins (BMPs) constitute the largest subdivision of the transforming growth factor-ß family of ligands. BMPs exhibit widespread utility and pleiotropic, context-dependent effects, and the strength and duration of BMP pathway signaling is tightly regulated at numerous levels via mechanisms operating both inside and outside the cell. Defects in the BMP pathway or its regulation underlie multiple human diseases of different organ systems. Yet much remains to be discovered about the BMP pathway in its original context, i.e., the skeleton. In this review, we provide a comprehensive overview of the intricacies of the BMP pathway and its inhibitors in bone development, homeostasis, and disease. We frame the content of the review around major unanswered questions for which incomplete evidence is available. First, we consider the gene regulatory network downstream of BMP signaling in osteoblastogenesis. Next, we examine why some BMP ligands are more osteogenic than others and what factors limit BMP signaling during osteoblastogenesis. Then we consider whether specific BMP pathway components are required for normal skeletal development, and if the pathway exerts endogenous effects in the aging skeleton. Finally, we propose two major areas of need of future study by the field: greater resolution of the gene regulatory network downstream of BMP signaling in the skeleton, and an expanded repertoire of reagents to reliably and specifically inhibit individual BMP pathway components.

The secreted tyrosine kinase VLK is essential for normal platelet activation and thrombus formation.

Tyrosine phosphorylation of extracellular proteins is observed in cell cultures and in vivo, but little is known about the functional roles of tyrosine phosphorylation of extracellular proteins. Vertebrate lonesome kinase (VLK) is a broadly expressed secretory pathway tyrosine kinase present in platelet α-granules. It is released from platelets upon activation and phosphorylates substrates extracellularly. Its role in platelet function, however, has not been previously studied. In human platelets, we identified phosphorylated tyrosines mapped to luminal or extracellular domains of transmembrane and secreted proteins implicated in the regulation of platelet activation. To determine the role of VLK in extracellular tyrosine phosphorylation and platelet function, we generated mice with a megakaryocyte/platelet-specific deficiency of VLK. Platelets from these mice are normal in abundance and morphology but have significant changes in function both in vitro and in vivo. Resting and thrombin-stimulated VLK-deficient platelets exhibit a significant decrease in several tyrosine phosphobands. Results of functional testing of VLK-deficient platelets show decreased protease-activated receptor 4-mediated and collagen-mediated platelet aggregation but normal responses to adenosine 5'-diphosphate. Dense granule and α-granule release are reduced in these platelets. Furthermore, VLK-deficient platelets exhibit decreased protease-activated receptor 4-mediated Akt (S473) and Erk1/2 (T202/Y204) phosphorylation, indicating altered proximal signaling. In vivo, mice lacking VLK in megakaryocytes/platelets display strongly reduced platelet accumulation and fibrin formation after laser-induced injury of cremaster arterioles compared with control mice but with normal bleeding times. These studies show that the secretory pathway tyrosine kinase VLK is critical for stimulus-dependent platelet activation and thrombus formation, providing the first evidence that a secreted protein kinase is required for normal platelet function.

Protein Cytl1: its role in chondrogenesis, cartilage homeostasis, and disease.

Cytokine-like protein 1 (Cytl1), also named Protein C17 or C4orf4 is located on human chromosome 4p15-p16 and encodes a polypeptide of 126 amino acid residues that displays characteristics of a secretory protein. Cytl1 is expressed by a sub-population of CD34+ human mononuclear cells from bone marrow and cord blood, and by chondrocytes (cartilage-forming cells). In this review, we explore evidence suggesting that Cytl1 may be involved in the regulation of chondrogenesis, cartilage homeostasis and osteoarthritis progression, accompanied by the modulation of Sox9 and insulin-like growth factor 1 expression. In addition, Cytl1 exhibits chemotactic and pro-angiogenic biological effects. Interestingly, CCR2 (C-C chemokine receptor type 2) has been identified as a likely receptor for Cytl1, which mediates the ERK signalling pathway. Cytl1 also appears to mediate the TGF-beta-Smad signalling pathway, which is hypothetically independent of the CCR2 receptor. More recently, studies have also potentially linked Cytl1 with a variety of conditions including cardiac fibrosis, smoking, alcohol dependence risk, and tumours such as benign prostatic hypertrophy, lung squamous cell carcinoma, neuroblastoma and familial colorectal cancer. Defining the molecular structure of Cytl1 and its role in disease pathogenesis will help us to design therapeutic approaches for Cytl1-associated pathological conditions.

Chondromodulin-1 in health, osteoarthritis, cancer, and heart disease.

The human chondromodulin-1 (Chm-1, Chm-I, CNMD, or Lect1) gene encodes a 334 amino acid type II transmembrane glycoprotein protein with characteristics of a furin cleavage site and a putative glycosylation site. Chm-1 is expressed most predominantly in healthy and developing avascular cartilage, and healthy cardiac valves. Chm-1 plays a vital role during endochondral ossification by the regulation of angiogenesis. The anti-angiogenic and chondrogenic properties of Chm-1 are attributed to its role in tissue development, homeostasis, repair and regeneration, and disease prevention. Chm-1 promotes chondrocyte differentiation, and is regulated by versatile transcription factors, such as Sox9, Sp3, YY1, p300, Pax1, and Nkx3.2. Decreased expression of Chm-1 is implicated in the onset and progression of osteoarthritis and infective endocarditis. Chm-1 appears to attenuate osteoarthritis progression by inhibiting catabolic activity, and to mediate anti-inflammatory effects. In this review, we present the molecular structure and expression profiling of Chm-1. In addition, we bring a summary to the potential role of Chm-1 in cartilage development and homeostasis, osteoarthritis onset and progression, and to the pathogenic role of Chm-1 in infective endocarditis and cancers. To date, knowledge of the Chm-1 receptor, cellular signalling, and the molecular mechanisms of Chm-1 is rudimentary. Advancing our understanding the role of Chm-1 and its mechanisms of action will pave the way for the development of Chm-1 as a therapeutic target for the treatment of diseases, such as osteoarthritis, infective endocarditis, and cancer, and for potential tissue regenerative bioengineering applications.

Specification of osteoblast cell fate by canonical Wnt signaling requires Bmp2.

Enhanced BMP or canonical Wnt (cWnt) signaling are therapeutic strategies employed to enhance bone formation and fracture repair, but the mechanisms each pathway utilizes to specify cell fate of bone-forming osteoblasts remain poorly understood. Among all BMPs expressed in bone, we find that singular deficiency of Bmp2 blocks the ability of cWnt signaling to specify osteoblasts from limb bud or bone marrow progenitors. When exposed to cWnts, Bmp2-deficient cells fail to progress through the Runx2/Osx1 checkpoint and thus do not upregulate multiple genes controlling mineral metabolism in osteoblasts. Cells lacking Bmp2 after induction of Osx1 differentiate normally in response to cWnts, suggesting that pre-Osx1+ osteoprogenitors are an essential source and a target of BMP2. Our analysis furthermore reveals Grainyhead-like 3 (Grhl3) as a transcription factor in the osteoblast gene regulatory network induced during bone development and bone repair, which acts upstream of Osx1 in a BMP2-dependent manner. The Runx2/Osx1 transition therefore receives crucial regulatory inputs from BMP2 that are not compensated for by cWnt signaling, and this is mediated at least in part by induction and activation of Grhl3.

The transcriptional co-repressor TLE3 regulates myogenic differentiation by repressing the activity of the MyoD transcription factor.

Satellite cells are skeletal muscle stem cells that provide myonuclei for postnatal muscle growth, maintenance, and repair/regeneration in adults. Normally, satellite cells are mitotically quiescent, but they are activated in response to muscle injury, in which case they proliferate extensively and exhibit up-regulated expression of the transcription factor MyoD, a master regulator of myogenesis. MyoD forms a heterodimer with E proteins through their basic helix-loop-helix domain, binds to E boxes in the genome and thereby activates transcription at muscle-specific promoters. The central role of MyoD in muscle differentiation has increased interest in finding potential MyoD regulators. Here we identified transducin-like enhancer of split (TLE3), one of the Groucho/TLE family members, as a regulator of MyoD function during myogenesis. TLE3 was expressed in activated and proliferative satellite cells in which increased TLE3 levels suppressed myogenic differentiation, and, conversely, reduced TLE3 levels promoted myogenesis with a concomitant increase in proliferation. We found that, via its glutamine- and serine/proline-rich domains, TLE3 interferes with MyoD function by disrupting the association between the basic helix-loop-helix domain of MyoD and E proteins. Our findings indicate that TLE3 participates in skeletal muscle homeostasis by dampening satellite cell differentiation via repression of MyoD transcriptional activity.

Impact of broad regulatory regions on Gdf5 expression and function in knee development and susceptibility to osteoarthritis.

OBJECTIVES: Given the role of growth and differentiation factor 5 (GDF5) in knee development and osteoarthritis risk, we sought to characterise knee defects resulting from Gdf5 loss of function and how its regulatory regions control knee formation and morphology. METHODS: The brachypodism (bp) mouse line, which harbours an inactivating mutation in Gdf5, was used to survey how Gdf5 loss of function impacts knee morphology, while two transgenic Gdf5 reporter bacterial artificial chromosome mouse lines were used to assess the spatiotemporal activity and function of Gdf5 regulatory sequences in the context of clinically relevant knee anatomical features. RESULTS: Knees from homozygous bp mice (bp/bp) exhibit underdeveloped femoral condyles and tibial plateaus, no cruciate ligaments, and poorly developed menisci. Secondary ossification is also delayed in the distal femur and proximal tibia. bp/bp mice have significantly narrower femoral condyles, femoral notches and tibial plateaus, and curvier medial femoral condyles, shallower trochlea, steeper lateral tibial slopes and smaller tibial spines. Regulatory sequences upstream from Gdf5 were weakly active in the prenatal knee, while downstream regulatory sequences were active throughout life. Importantly, downstream but not upstream Gdf5 regulatory sequences fully restored all the key morphological features disrupted in the bp/bp mice. CONCLUSIONS: Knee morphology is profoundly affected by Gdf5 absence, and downstream regulatory sequences mediate its effects by controlling Gdf5 expression in knee tissues. This downstream region contains numerous enhancers harbouring human variants that span the osteoarthritis association interval. We posit that subtle alterations to morphology driven by changes in downstream regulatory sequence underlie this locus' role in osteoarthritis risk.

Loss of BMPR2 leads to high bone mass due to increased osteoblast activity.

Imbalances in the ratio of bone morphogenetic protein (BMP) versus activin and TGFß signaling are increasingly associated with human diseases yet the mechanisms mediating this relationship remain unclear. The type 2 receptors ACVR2A and ACVR2B bind BMPs and activins but the type 2 receptor BMPR2 only binds BMPs, suggesting that type 2 receptor utilization might play a role in mediating the interaction of these pathways. We tested this hypothesis in the mouse skeleton, where bone mass is reciprocally regulated by BMP signaling and activin and TGFß signaling. We found that deleting Bmpr2 in mouse skeletal progenitor cells (Bmpr2-cKO mice) selectively impaired activin signaling but had no effect on BMP signaling, resulting in an increased bone formation rate and high bone mass. Additionally, activin sequestration had no effect on bone mass in Bmpr2-cKO mice but increased bone mass in wild-type mice. Our findings suggest a novel model whereby BMPR2 availability alleviates receptor-level competition between BMPs and activins and where utilization of ACVR2A and ACVR2B by BMPs comes at the expense of activins. As BMP and activin pathway modulation are of current therapeutic interest, our findings provide important mechanistic insight into the relationship between these pathways in human health.

Identification and characterization of adult mouse meniscus stem/progenitor cells.

Meniscal damage is a common problem that accelerates the onset of knee osteoarthritis. Stem cell-based tissue engineering treatment approaches have shown promise in preserving meniscal tissue and restoring meniscal function. The purpose of our study was to identify meniscus-derived stem/progenitor cells (MSPCs) from mouse, a model system that allows for in vivo analysis of the mechanisms underlying meniscal injury and healing. MSPCs were isolated from murine menisci grown in explant culture and characterized for stem cell properties. Flow cytometry was used to detect the presence of surface antigens related to stem cells, and qRT-PCR was used to examine the gene expression profile of MSPCs. Major proteins associated with MSPCs were localized in the adult mouse knee using immunohistochemistry. Our data show that MSPCs have universal stem cell-like properties including clonogenicity and multi-potentiality. MSPCs expressed the mesenchymal stem cell markers CD44, Sca-1, CD90, and CD73 and when cultured had elevated levels of biglycan and collagen type I, important extracellular matrix components of adult meniscus. MSPC also expressed significant levels of Lox and Igf-1, genes associated with the embryonic meniscus. Localization studies showed staining for these same proteins in the superficial and outer zones of the adult mouse meniscus, regions thought to harbor endogenous repair cells. MSPCs represent a novel resident stem cell population in the murine meniscus. Analysis of MSPCs in mice will allow for a greater understanding of the cell biology of the meniscus, essential information for enhancing therapeutic strategies for treating knee joint injury and disease.

EGFL7 is expressed in bone microenvironment and promotes angiogenesis via ERK, STAT3, and integrin signaling cascades.

Angiogenesis plays a pivotal role in bone formation, remodeling, and fracture healing. The regulation of angiogenesis in the bone microenvironment is highly complex and orchestrated by intercellular communication between bone cells and endothelial cells. Here, we report that EGF-like domain 7 (EGFL7), a member of the epidermal growth factor (EGF) repeat protein superfamily is expressed in both the osteoclast and osteoblast lineages, and promotes endothelial cell activities. Addition of exogenous recombinant EGFL7 potentiates SVEC (simian virus 40-transformed mouse microvascular endothelial cell line) cell migration and tube-like structure formation in vitro. Moreover, recombinant EGFL7 promotes angiogenesis featuring web-like structures in ex vivo fetal mouse metatarsal angiogenesis assay. We show that recombinant EGFL7 induces phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), signal transducer and activator of transcription 3 (STAT3), and focal adhesion kinase (FAK) in SVEC cells. Inhibition of ERK1/2 and STAT3 signaling impairs EGFL7-induced endothelial cell migration, and angiogenesis in fetal mouse metatarsal explants. Bioinformatic analyses indicate that EGFL7 contains a conserved RGD/QGD motif and EGFL7-induced endothelial cell migration is significantly reduced in the presence of RGD peptides. Moreover, EGFL7 gene expression is significantly upregulated during growth plate injury repair. Together, these results demonstrate that EGFL7 expressed by bone cells regulates endothelial cell activities through integrin-mediated signaling. This study highlights the important role that EGFL7, like EGFL6, expressed in bone microenvironment plays in the regulation of angiogenesis in bone.

N-linked glycosylation of the bone morphogenetic protein receptor type 2 (BMPR2) enhances ligand binding.

The bone morphogenetic protein (BMP) signaling pathway is essential for normal development and tissue homeostasis. BMP signal transduction occurs when ligands interact with a complex of type 1 and type 2 receptors to activate downstream transcription factors. It is well established that a single BMP receptor may bind multiple BMP ligands with varying affinity, and this has been largely attributed to conformation at the amino acid level. However, all three type 2 BMP receptors (BMPR2, ACVR2A/B) contain consensus N-glycosylation sites in their extracellular domains (ECDs), which could play a role in modulating interaction with ligand. Here, we show a differential pattern of N-glycosylation between BMPR2 and ACVR2A/B. Site-directed mutagenesis reveals that BMPR2 is uniquely glycosylated near its ligand binding domain and at a position that is mutated in patients with heritable pulmonary arterial hypertension. We further demonstrate using a cell-free pulldown assay that N-glycosylation of the BMPR2-ECD enhances its ability to bind BMP2 ligand but has no impact on binding by the closely-related ACVR2B. Our results illuminate a novel aspect of BMP signaling pathway mechanics and demonstrate a functional difference resulting from post-translational modification of type 2 BMP receptors. Additionally, since BMPR2 is required for several aspects of normal development and defects in its function are strongly implicated in human disease, our findings are likely to be relevant in several biological contexts in normal and abnormal human physiology.

A discoidin domain receptor 1 knock-out mouse as a novel model for osteoarthritis of the temporomandibular joint.

Discoidin domain receptor 1 (DDR-1)-deficient mice exhibited a high incidence of osteoarthritis (OA) in the temporomandibular joint (TMJ) as early as 9 weeks of age. They showed typical histological signs of OA, including surface fissures, loss of proteoglycans, chondrocyte cluster formation, collagen type I upregulation, and atypical collagen fibril arrangements. Chondrocytes isolated from the TMJs of DDR-1-deficient mice maintained their osteoarthritic characteristics when placed in culture. They expressed high levels of runx-2 and collagen type I, as well as low levels of sox-9 and aggrecan. The expression of DDR-2, a key factor in OA, was increased. DDR-1-deficient chondrocytes from the TMJ were positively influenced towards chondrogenesis by a three-dimensional matrix combined with a runx-2 knockdown or stimulation with extracellular matrix components, such as nidogen-2. Therefore, the DDR-1 knock-out mouse can serve as a novel model for temporomandibular disorders, such as OA of the TMJ, and will help to develop new treatment options, particularly those involving tissue regeneration.

BMP2 activity, although dispensable for bone formation, is required for the initiation of fracture healing.

Adult bones have a notable regenerative capacity. Over 40 years ago, an intrinsic activity capable of initiating this reparative response was found to reside within bone itself, and the term bone morphogenetic protein (BMP) was coined to describe the molecules responsible for it. A family of BMP proteins was subsequently identified, but no individual BMP has been shown to be the initiator of the endogenous bone repair response. Here we demonstrate that BMP2 is a necessary component of the signaling cascade that governs fracture repair. Mice lacking the ability to produce BMP2 in their limb bones have spontaneous fractures that do not resolve with time. In fact, in bones lacking BMP2, the earliest steps of fracture healing seem to be blocked. Although other osteogenic stimuli are still present in the limb skeleton of BMP2-deficient mice, they cannot compensate for the absence of BMP2. Collectively, our results identify BMP2 as an endogenous mediator necessary for fracture repair.

The role of GDF5 in regulating enthesopathy development in the Hyp mouse model of XLH.

X-linked hypophosphatemia (XLH) is caused by mutations in PHEX, leading to rickets and osteomalacia. Adults affected with XLH develop a mineralization of the bone-tendon attachment site (enthesis), called enthesopathy, which causes significant pain and impaired movement. Entheses in mice with XLH (Hyp) have enhanced Bone Morphogenetic Protein (BMP) and Indian hedgehog (IHH) signaling. Treatment of Hyp mice with the BMP signaling blocker palovarotene attenuated BMP/IHH signaling in Hyp entheses, thus indicating that BMP signaling plays a pathogenic role in enthesopathy development and that IHH signaling is activated by BMP signaling in entheses. It was previously shown that mRNA expression of Gdf5 is enhanced in Hyp entheses at P14. Thus, to determine a role for GDF5 in enthesopathy development, Gdf5 was deleted globally in Hyp mice and conditionally in Scx + cells of Hyp mice. In both murine models, BMP/IHH signaling was similarly decreased in Hyp entheses, leading to decreased enthesopathy. BMP/IHH signaling remained unaffected in WT entheses with decreased Gdf5 expression. Moreover, deletion of Gdf5 in Hyp entheses starting at P30, after enthesopathy has developed, partially reversed enthesopathy. Taken together, these results demonstrate that while GDF5 is not essential for modulating BMP/IHH signaling in WT entheses, inappropriate GDF5 activity in Scx + cells contributes to XLH enthesopathy development. As such, inhibition of GDF5 signaling may be beneficial for the treatment of XLH enthesopathy.

X-linked hypophosphatemia (XLH) is a rare bone disorder that leads to short stature and poorly mineralized bones. As adults, patients with XLH often develop a mineralization of the bone-tendon attachment site, called enthesopathy, that results in significant pain. We previously showed that Achilles bone-tendon attachment sites (entheses) in mice with XLH (Hyp) have an enthesopathy characterized by increased Bone Morphogenetic Protein (BMP) signaling. In the current studies, we show that treating Hyp mice with the BMP signaling inhibitor palovarotene prevents enthesopathy, demonstrating that the increased BMP signaling in Hyp entheses leads to enthesopathy development. We also reported that expression of Gdf5, which activates BMP signaling, is enhanced in Hyp entheses. Therefore, to determine if the enhanced Gdf5 expression leads to the increased BMP signaling seen Hyp entheses, Gdf5 was deleted from Hyp mice and also deleted specifically in the entheses of Hyp mice. In both mouse models, enthesopathy development was attenuated, demonstrating that the increased Gdf5 expression in Hyp entheses plays a role in enthesopathy development. These data indicate that blocking GDF5 and BMP signaling may prevent enthesopathy in patients with XLH.

A novel methodology utilizing microchip implants to monitor individual activity and body temperature for assessing knee pain in group-housed rats.

The pain assessment in animals is challenging as they cannot verbally express the site and severity of pain. In this study, we tried a small implantable actimeter, "Nanotag", to monitor spontaneous locomotor activity and body temperature in animals suffering from a chemical-induced rat knee arthritis as compared to naïve and steroid-treated rats. Nanotag could detect the decrease in locomotor activity quickly after the arthritis induction and anti-inflammation analgesic treatment by intra-articular injection of steroid significantly improved locomotor activity. These changes were in the same line with those of a conventional knee pain evaluation method (incapacitance test). Nanotag can be utilized as the non-interventional, continuous, and completely objective monitoring the amount of pain in rat knee arthritis model. This traditional yet innovative method may be universally applicable to various pain models and species, making it a worthwhile device for research across diverse fields.

TLE3, transducing-like enhancer of split 3, suppresses osteoblast differentiation of bone marrow stromal cells.

In senile osteoporosis the balance of adipogenesis and osteoblastogenesis in bone marrow stromal cells (BMSCs) is disrupted so that adipogenesis is increased with respect to osteoblastogenesis, and as a result, bone mass is decreased. While the molecular mechanisms controlling the balance between osteoblastogenesis and adipogenesis are of great interest, the exact nature of the signals regulating this process remains to be determined. In general, adipogenesis is a reciprocal relationship with osteoblastogenesis in BMSCs. Recently transducin-like enhancer of split 3 (TLE3), was reported to enhance adipogenesis in pre adipocytes. However, the effect of TLE3 on osteoblast differentiation of BMSCs is completely unknown. Here we report that TLE3 not only enhances adipocyte differentiation in BMSCs but also suppresses osteoblast differentiation. Firstly we examined the expression and localization of TLE3. We found that TLE3 is expressed in the nucleus of bone marrow stromal cells and that over-expression of TLE3 induced adipocyte differentiation and suppressed ALP activity induced by treatment with BMP2 in these cells. In contrast, adipocyte differentiation was decreased and ALP activity increased when endogenous TLE3 was knocked down by shRNA in BMSCs. To examine the mechanism by which TLE3 is able to suppress osteoblast differentiation, we focused on Runx2, a transcription factor essential for osteoblast differentiation. We found that TLE3 strongly suppressed ALP activity and OSE2-luciferase activity induced by Runx2 and this repression of Runx2 by TLE3 occurs via HDACs because treatment with TSA, a class I and II HDAC inhibitor, rescued this repression. In conclusion, we identify TLE3 as a suppressor of BMSC differentiation in osteoblast lineage cells in vitro. Our data suggest that TLE3 activity may be a key in balancing adipocyte and osteoblast differentiation in the adult bone marrow microenvironment.

Molecular profiling of synovial joints: use of microarray analysis to identify factors that direct the development of the knee and elbow.

BACKGROUND: Synovial joints develop from the interzone, a dense layer of mesenchymal progenitor cells that marks the site of the future joint. During the morphogenic events that follow, joints attain their distinct shape and organization. The molecular mechanisms controlling the initial specification of synovial joints has been studied, but the question of how individual joints attain the specific structure required for their unique functions remains largely unresolved. Here, we use microarray analysis to compare knee and elbow formation to identify factors involved in the development of specific joints. RESULTS: The knee is enriched for the hindlimb patterning genes Hoxc9, Hoxc10, and Tbx4 and for Tgfbi, Rspo2, and Sfrp2, factors involved in transforming growth factor-beta/bone morphogenetic protein (TGFß/BMP) and Wnt signaling. Consistent with these findings, we show that TGFß signaling directs knee morphogenesis, and is necessary for meniscus development. The tissue surrounding the elbow is highly enriched for genes involved in muscle specification and differentiation, and in splotch-delayed muscleless mutants, elbow, but not knee morphogenesis is disrupted. CONCLUSIONS: Our results suggest there are fundamental differences in how individual joints develop after interzone formation. Our microarray analyses provides a new resource for further investigation of the pathways involved in the morphogenesis of specific synovial joints.

Chondrogenic differentiation of mouse CD105+ stem/progenitor cells on amino-group-functionalized biosilica-hydrogel scaffolds.

OBJECTIVES: Mesenchymal stem/progenitor cells (MSPCs) are critical for tissue regeneration. Moreover, the CD105 antigen identifies early MSPCs with increased chondrogenic differentiation ability. We hypothesized that amine-(NH2)-functionalized biosilica incorporating hydrogel scaffolds, seeded with mCoSPCs105+ would contribute to creating tissue-engineered scaffolds, capable of de novo cartilage synthesis. MATERIALS AND METHODS: Scaffolds were characterized by water uptake, lysozyme degradation, axial compression, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Differentiation stimulus of scaffold functionalization was evaluated using Alcian blue staining. Cartilage-forming abilities of mCoSPCs105+ were evaluated using Quantitative reverse transcription polymerase chain reaction (qRT-PCR) and immunohistochemistry. RESULTS: Biosilica particle incorporation into scaffolds resulted in increased water uptake capacity and compression force withstanding abilities. Amine-(NH2)-group functionalization of biosilica led to significantly increased stem cell differentiation potential, by Alcian blue staining, in the first 3 weeks. Scaffold attachment and viable cell proliferation were observed for 6 weeks under chondrogenic differentiation. Downregulation of Runx2, an increase of Col10a1, Ihh, and maintenance of Sox9, was seen under these culture conditions. mCoSPCs105+ gene expression pattern was defined by the significant upregulation of Col1a1, Col2a1, Prg4, and Agc-1 over 6 weeks of incubation compared to the unsorted control. Immunostaining of cell-seeded scaffolds revealed significantly higher secretion of proteins relevant to cartilage extracellular matrix. CONCLUSION: The preselecting of CD105+ phenotype in MSPCs may enhance tissue regeneration of fibrocartilage and biosilica nanoparticles may be a beneficial additive in tissue engineering of scaffolds.