Inhibiting calcineurin activity under physiologic tonicity elevates anabolic but suppresses catabolic chondrocyte markers

Yes-associated protein nuclear translocation promotes anabolic activity in human articular chondrocytes

OBJECTIVE

Yes-associated protein (YAP) has been widely studied as a mechanotransducer in many cell types, but its function in cartilage is controversial. The aim of this study was to identify the effect of YAP phosphorylation and nuclear translocation on the chondrocyte response to stimuli relevant to osteoarthritis (OA).

DESIGN

Cultured normal human articular chondrocytes from 81 donors were treated with increased osmolarity media as an in vitro model of mechanical stimulation, fibronectin fragments (FN-f) or IL-1β as catabolic stimuli, and IGF-1 as an anabolic stimulus. YAP function was assessed with gene knockdown and inhibition by verteporfin. Nuclear translocation of YAP and its transcriptional co-activator TAZ and site-specific YAP phosphorylation were determined by immunoblotting. Immunohistochemistry and immunofluorescence to detect YAP were performed on normal and OA human cartilage with different degrees of damage.

RESULTS

Chondrocyte YAP/TAZ nuclear translocation increased under physiological osmolarity (400 mOsm) and IGF-1 stimulation, which was associated with YAP phosphorylation at Ser128. In contrast, catabolic stimulation decreased the levels of nuclear YAP/TAZ through YAP phosphorylation at Ser127. Following YAP inhibition, anabolic gene expression and transcriptional activity decreased. Additionally, YAP knockdown reduced proteoglycan staining and levels of type II collagen. Total YAP immunostaining was greater in OA cartilage, but YAP was sequestered in the cytosol in cartilage areas with more severe damage.

CONCLUSIONS

YAP chondrocyte nuclear translocation is regulated by differential phosphorylation in response to anabolic and catabolic stimuli. Decreased nuclear YAP in OA chondrocytes may contribute to reduced anabolic activity and promotion of further cartilage loss.

Physosmotic Induction of Chondrogenic Maturation Is TGF-β Dependent and Enhanced by Calcineurin Inhibitor FK506

Increasing extracellular osmolarity 100 mOsm/kg above plasma level to the physiological levels for cartilage induces chondrogenic marker expression and the differentiation of chondroprogenitor cells. The calcineurin inhibitor FK506 has been reported to modulate the hypertrophic differentiation of primary chondrocytes under such conditions, but the molecular mechanism has remained unclear. We aimed at clarifying its role. Chondrocyte cell lines and primary cells were cultured under plasma osmolarity and chondrocyte-specific in situ osmolarity (+100 mOsm, physosmolarity) was increased to compare the activation of nuclear factor of activated T-cells 5 (NFAT5). The effects of osmolarity and FK506 on calcineurin activity, cell proliferation, extracellular matrix quality, and BMP- and TGF-β signaling were analyzed using biochemical, gene, and protein expression, as well as reporter and bio-assays. NFAT5 translocation was similar in chondrocyte cell lines and primary cells. High supraphysiological osmolarity compromised cell proliferation, while physosmolarity or FK506 did not, but in combination increased proteoglycan and collagen expression in chondrocytes in vitro and in situ. The expression of the TGF-β-inducible protein TGFBI, as well as chondrogenic (SOX9, Col2) and terminal differentiation markers (e.g., Col10) were affected by osmolarity. Particularly, the expression of minor collagens (e.g., Col9, Col11) was affected. The inhibition of the FK506-binding protein suggests modulation at the TGF-β receptor level, rather than calcineurin-mediated signaling, as a cause. Physiological osmolarity promotes terminal chondrogenic differentiation of progenitor cells through the sensitization of the TGF-β superfamily signaling at the type I receptor. While hyperosmolarity alone facilitates TGF-β superfamily signaling, FK506 further enhances signaling by releasing the FKBP12 break from the type I receptor to improve collagenous marker expression. Our results help explain earlier findings and potentially benefit future cell-based cartilage repair strategies.

Biomimetic and mechanically supportive 3D printed scaffolds for cartilage and osteochondral tissue engineering using photopolymers and digital light processing

Successful 3D scaffold designs for musculoskeletal tissue engineering necessitate full consideration of the form and function of the tissues of interest. When designing structures for engineering cartilage and osteochondral tissues, one must reconcile the need to develop a mechanically robust system that maintains the health of cells embedded in the scaffold. In this work, we present an approach that decouples the mechanical and biochemical needs and allows for the independent development of the structural and cellular niches in a scaffold. Using the highly tuned capabilities of digital light processing-based stereolithography, structures with complex architectures are achieved over a range of effective porosities and moduli. The 3D printed structure is infilled with mesenchymal stem cells and soft biomimetic hydrogels, which are specifically formulated with extracellular matrix analogs and tethered growth factors to provide selected biochemical cues for the guided differentiation towards chondrogenesis and osteogenesis. We demonstrate the ability to utilize these structures to (a) infill a focal chondral defect and mitigate macroscopic and cellular level changes in the cartilage surrounding the defect, and (b) support the development of a stratified multi-tissue scaffold for osteochondral tissue engineering.

Sustained Intra-Articular Release and Biocompatibility of Tacrolimus (FK506) Loaded Monospheres Composed of [PDLA-PEG1000]-b-[PLLA] Multi-Block Copolymers in Healthy Horse Joints

There is an increasing interest in controlled release systems for local therapy in the treatment of human and equine joint diseases, aiming for optimal intra-articular concentrations with no systemic side effects. In this study, the intra-articular tolerability and suitability for local and sustained release of tacrolimus (FK506) from monospheres composed of [PDLA-PEG₁₀₀₀]-b-PLLA multiblock copolymers were investigated. Unloaded and tacrolimus-loaded (18.4 mg tacrolimus/joint) monospheres were injected into the joints of six healthy horses, with saline and hyaluronic acid (HA) in the contralateral joints as controls. Blood and synovial fluid were analysed for the tacrolimus concentration and biomarkers for inflammation and cartilage metabolism. After an initial burst release, sustained intra-articular tacrolimus concentrations (>20 ng/mL) were observed during the 42 days follow-up. Whole-blood tacrolimus levels were below the detectable level (<0.5 ng/mL). A transient inflammatory reaction was observed for all substances, evidenced by increases of the synovial fluid white blood cell count and total protein. Prostaglandin and glycosaminoglycan release were increased in joints injected with unloaded monospheres, which was mitigated by tacrolimus. Both tacrolimus-loaded monospheres and HA transiently increased the concentration of collagen II cleavage products (C2C). A histologic evaluation of the joints at the endpoint showed no pathological changes in any of the conditions. Together, these results indicate the good biocompatibility of intra-articular applied tacrolimus-loaded monospheres combined with prolonged local drug release while minimising the risk of systemic side effects. Further evaluation in a clinical setting is needed to determine if tacrolimus-loaded monospheres can be beneficial in the treatment of inflammatory joint diseases in humans and animals.

The TLR-2/TonEBP signaling pathway regulates 29-kDa fibronectin fragment-dependent expression of matrix metalloproteinases

Tonicity-responsive enhancer-binding protein (TonEBP; nuclear factor of activated T cells 5) is a transcription factor that responds to changes in osmolality. However, recent studies have shown that it also modulates immune responses under inflammatory conditions independently of hyperosmolality. Fibronectin fragments (FN-fs), which are abundant in the synovial fluid of patients with osteoarthritis (OA), induce expression of matrix metalloproteinases (MMPs) via the toll-like receptor-2 (TLR-2) signaling pathway. In this study we examined whether TonEBP is involved in 29-kDa FN-f-induced expression of MMPs. The expression of TonEBP was significantly higher in human osteoarthritis compared with normal cartilage samples. 29-kDa FN-f affected the expression of MMPs 1, 3, and 13 via TonEBP, and expression and nuclear accumulation of TonEBP were induced by activation of the phospholipase C/NF-κB/MAPK signaling pathway and, in particular, modulated by TLR-2. In addition, 29-kDa FN-f induced the expression of osmoregulatory genes, including Tau-T, SMIT, and AR, as well as voltage-dependent calcium channels via the TonEBP/TLR-2 signaling pathway. These results show that 29-kDa FN-f upregulates MMPs in chondrocytes via the TLR-2/TonEBP signaling pathway.

Importance of Osmolarity and Oxygen Tension for Cartilage Tissue Engineering

For cartilage repair in vivo or evaluation of new therapeutic approaches in vitro, the generation of functional cartilage tissue is of crucial importance and can only be achieved if the phenotype of the chondrocytes is preserved. Three-dimensional (3D) cell culture is broadly used for this purpose. However, adapting culture parameters like the oxygen tension or the osmolarity to their physiological values is often omitted. Indeed, articular cartilage is an avascular tissue subjected to reduced oxygen tension and presenting and increased osmolarity compared with most other tissues. In this study, we aimed at evaluating the effect of a physiological oxygen tension (3% instead of 21%) and physiological osmolarity (430 vs. 330 mOsm in nonadjusted DMEM) and the combination of both on the cell proliferation, matrix production, and the phenotype of porcine chondrocytes in a scaffold-free 3D culture system. We observed that a physiological osmolarity had no effect on cell proliferation and matrix production but positively influences the chondrocyte phenotype. A physiological oxygen level prevented cell proliferation but resulted in an increased matrix content/million cells and had a positive influence on the chondrocyte phenotype as well. The strongest benefit was reached with the combination of both physiological osmolarity and oxygen levels; with these conditions, type I collagen expression became undetectable. In addition, at 3% O₂ the chondrocytes-matrix constructs were found to more closely resemble native cartilage regarding the matrix-to-cell ratio. In conclusion, this study clearly demonstrates the benefit of using physiological oxygen tension and osmolarity in cartilage tissue engineering with the combination of both showing the strongest benefit on the chondrocyte phenotype.

Increasing the Medium Osmolarity Reduces the Inflammatory Status of Human OA Chondrocytes and Increases Their Responsiveness to GDF-5

The environment surrounding chondrocytes changes drastically in osteoarthritis (OA). For instance, the osmolarity in cartilage (ranging from 350 to 460 mOsm in healthy tissue) decreases during the progression of OA, reaching 270 mOsm. The objective of this study was to evaluate how osmolarity influences human OA chondrocytes. For this purpose, the osmolarity of the culture medium (340 mOsm) was increased to 380, 420 or 460 mOsm and its effect on the phenotype, matrix production, protease expression, cytokine release and growth and differentiation factor-5 (GDF-5) receptor expression in human OA chondrocytes was evaluated in a monolayer. Afterwards, the same parameters, as well as the responsiveness to GDF-5, were evaluated in 3D culture at 340 and 380 mOsm. Our results revealed that increasing the medium osmolarity increased matrix production but also reduced cytokine release, type I collagen and protease expression. It was also demonstrated that at 380 mOsm, the response to GDF-5 in 3D culture was more robust than at 340 mOsm. For the first time, it was established that a decreased osmolarity plays a role in sustaining inflammation and catabolic activities in OA chondrocytes and decreases their responsiveness to GDF-5. This indicates that osmolarity is a critical aspect of OA pathobiology.

Bioreactor-Controlled Physoxia Regulates TGF-β Signaling to Alter Extracellular Matrix Synthesis by Human Chondrocytes

Culturing articular chondrocytes under physiological oxygen tension exerts positive effects on their extracellular matrix synthesis. The underlying molecular mechanisms which enhance the chondrocytic phenotype are, however, still insufficiently elucidated. The TGF-β superfamily of growth factors, and the prototypic TGF-β isoforms in particular, are crucial in maintaining matrix homeostasis of these cells. We employed a feedback-controlled table-top bioreactor to investigate the role of TGF-β in microtissues of human chondrocytes over a wider range of physiological oxygen tensions (i.e., physoxia). We compared 1%, 2.5%, and 5% of partial oxygen pressure (pO₂) to the ‘normoxic’ 20%. We confirmed physoxic conditions through the induction of marker genes (PHD3, VEGF) and oxygen tension-dependent chondrocytic markers (SOX9, COL2A1). We identified 2.5% pO₂ as an oxygen tension optimally improving chondrocytic marker expression (ACAN, COL2A1), while suppressing de-differentiation markers (COL1A1,COL3A1). Expression of TGF-β isoform 2 (TGFB2) was, relatively, most responsive to 2.5% pO₂, while all three isoforms were induced by physoxia. We found TGF-β receptors ALK1 and ALK5 to be regulated by oxygen tension on the mRNA and protein level. In addition, expression of type III co-receptors betaglycan and endoglin appeared to be regulated by oxygen tension as well. R-Smad signaling confirmed that physoxia divergently regulated phosphorylation of Smad1/5/8 and Smad2/3. Pharmacological inhibition of canonical ALK5-mediated signaling abrogated physoxia-induced COL2A1 and PAI-1 expression. Physoxia altered expression of hypertrophy markers and that of matrix metalloproteases and their activity, as well as expression ratios of specific proteins (Sp)/Krüppel-like transcription factor family members SP1 and SP3, proving a molecular concept of ECM marker regulation. Keeping oxygen levels tightly balanced within a physiological range is important for optimal chondrocytic marker expression. Our study provides novel insights into transcriptional regulations in chondrocytes under physoxic in vitro conditions and may contribute to improving future cell-based articular cartilage repair strategies.

Increased TGF-β and BMP Levels and Improved Chondrocyte-Specific Marker Expression In Vitro under Cartilage-Specific Physiological Osmolarity

During standard expansion culture (i.e., plasma osmolarity, 280 mOsm) human articular chondrocytes dedifferentiate, making them inappropriate for autologous chondrocyte implantation to treat cartilage defects. Increasing the osmolarity of culture media to physiological osmolarity levels of cartilage (i.e., 380 mOsm), increases collagen type II (COL2A1) expression of human articular chondrocytes in vitro, but the underlying molecular mechanism is not fully understood. We hypothesized that TGF-β superfamily signaling may drive expression of COL2A1 under physiological osmolarity culture conditions. Human articular chondrocytes were cultured in cytokine-free medium of 280 or 380 mOsm with or without siRNA mediated TGF-β2 knockdown (RNAi). Expression of TGF-β isoforms, and collagen type II was evaluated by RT-qPCR and immunoblotting. TGF-β2 protein secretion was evaluated using ELISA and TGF-β bioactivity was determined using an established reporter assay. Involvement of BMP signaling was investigated by culturing human articular chondrocytes in the presence or absence of BMP inhibitor dorsomorphin and BMP bioactivity was determined using an established reporter assay. Physiological cartilage osmolarity (i.e., physosmolarity) most prominently increased TGF-β2 mRNA expression and protein secretion as well as TGF-β bioactivity. Upon TGF-β2 isoform-specific knockdown, gene expression of chondrocyte marker COL2A1 was induced. TGF-β2 RNAi under physosmolarity enhanced TGF-β bioactivity. BMP bioactivity increased upon physosmotic treatment, but was not related to TGF-β2 RNAi. In contrast, dorsomorphin inhibited COL2A1 mRNA expression in human articular chondrocytes independent of the osmotic condition. Our data suggest a role for TGF-β superfamily member signaling in physosmolarity-induced mRNA expression of collagen type II. As physosmotic conditions favor the expression of COL2A1 independent of our manipulations, contribution of other metabolic, post-transcriptional or epigenetic factors cannot be excluded in the underlying complex and interdependent regulation of marker gene expression. Dissecting these molecular mechanisms holds potential to further improve future cell-based chondral repair strategies.

Ropivacaine relieves pain and prevents chondrocyte degradation probably through Calcineurin/NFAT1 signaling pathway in osteoarthritis rats

Calcineurin/NFAT1 signaling pathway plays critical roles in maintaining the homeostasis of articular chondrocytes and in regulating the pathogenesis of osteoarthritis (OA). A few studies demonstrate therapeutic values of ropivacaine (Rop) in OA, but the underlying mechanisms have not been defined. Here, we determined whether Calcineurin/NFAT1 signaling pathway mediates the benefits of Rop to OA. OA rat models were established by a single intra-articular injection of monosodium iodoacetate. The pathophysiology of OA was evaluated by measuring hyperalgesia behavior and the expression of NFAT1, calcineurin, catabolic enzymes in chondrocytes, and chondrogenic markers in affected articular cartilage and primary chondrocyte cultures treated with IL-1β. ROP was applied both in vivo and in vitro to examine its effects on the pathophysiology of OA. Hyperalgesia in OA rats was improved by intra-articular injection of Rop. Moreover, Rop suppressed the overexpression of NFAT1, calcineurin, TNF-α, IL-6, MMP1 and MMP3, and reversed the diminution of collagen II and aggrecan, in affected cartilage of OA rats. Similar effects of Rop were also observed in mouse chondrocyte cultures treated with IL-1β. In in vitro preparations, either activation (by increasing extracellular Ca²⁺) or inhibition (by cyclosporin A) of calcineurin blocked the effects of Rop. These results suggest that Rop may have therapeutic potential for OA in three aspects: analgesia, anti-inflammation, and anti-degradation of articular cartilage, probably via down-regulating calcineurin/NFAT1 signaling pathway.

Fibroblast Growth Factor 23 drives MMP13 expression in human osteoarthritic chondrocytes in a Klotho-independent manner

OBJECTIVE

Fibroblast Growth Factor 23 (FGF23) may represent an attractive candidate that could participate to the osteoarthritic (OA)-induced phenotype switch of chondrocytes. To address this hypothesis, we investigated the expression of FGF23, its receptors (FGFRs) and co-receptor (Klotho) in human cartilage and studied the effects of rhFGF23 on OA chondrocytes.

METHOD

Gene expression or protein levels were analysed by RT-PCR and immunohistochemistry. Collagenase 3 (MMP13) activity was measured by a fluorescent assay. MAPK signalling pathways were investigated by phosphoprotein array, immunoblotting and the use of selective inhibitors. RNA silencing was performed to confirm the respective contribution of FGFR1 and Klotho.

RESULTS

We showed that the expression of FGF23, FGFR1 and Klotho was up-regulated at both mRNA and protein levels in OA chondrocytes when compared to healthy ones. These overexpressions were markedly elevated in the damaged regions of OA cartilage. When stimulated with rhFGF23, OA chondrocytes displayed an extended expression of FGF23 and of markers of hypertrophy such as MMP13, COL10A1, and VEGF. We demonstrated that FGF23 auto-stimulation was both FGFR1-and Klotho-dependent, whereas the expression of markers of hypertrophy was mainly dependent on FGFR1 alone. Finally, we showed that FGF23-induced MMP13 expression was strongly regulated by the MEK/ERK cascade and to a lesser extent, by the PI-3K/AKT pathway.

CONCLUSION

These results demonstrate that FGF23 sustains differentiation of OA chondrocytes in a Klotho-independent manner.

Strategies to minimize hypertrophy in cartilage engineering and regeneration

Due to a blood supply shortage, articular cartilage has a limited capacity for self-healing once damaged. Articular chondrocytes, cartilage progenitor cells, embryonic stem cells, and mesenchymal stem cells are candidate cells for cartilage regeneration. Significant current attention is paid to improving chondrogenic differentiation capacity; unfortunately, the potential chondrogenic hypertrophy of differentiated cells is largely overlooked. Consequently, the engineered tissue is actually a transient cartilage rather than a permanent one. The development of hypertrophic cartilage ends with the onset of endochondral bone formation which has inferior mechanical properties. In this review, current strategies for inhibition of chondrogenic hypertrophy are comprehensively summarized; the impact of cell source options is discussed; and potential mechanisms underlying these strategies are also categorized. This paper aims to provide guidelines for the prevention of hypertrophy in the regeneration of cartilage tissue. This knowledge may also facilitate the retardation of osteophytes in the treatment of osteoarthritis.

Physicochemical and biomechanical stimuli in cell-based articular cartilage repair

Articular cartilage is a unique load-bearing connective tissue with a low intrinsic capacity for repair and regeneration. Its avascularity makes it relatively hypoxic and its unique extracellular matrix is enriched with cations, which increases the interstitial fluid osmolarity. Several physicochemical and biomechanical stimuli are reported to influence chondrocyte metabolism and may be utilized for regenerative medical approaches. In this review article, we summarize the most relevant stimuli and describe how ion channels may contribute to cartilage homeostasis, with special emphasis on intracellular signaling pathways. We specifically focus on the role of calcium signaling as an essential mechanotransduction component and highlight the role of phosphatase signaling in this context.

Ser/Thr-phosphoprotein phosphatases in chondrogenesis: neglected components of a two-player game

Protein phosphorylation plays a determining role in the regulation of chondrogenesis in vitro. While signalling pathways governed by protein kinases including PKA, PKC, and mitogen-activated protein kinases (MAPK) have been mapped in great details, published data relating to the specific role of phosphoprotein phosphatases (PPs) in differentiating chondroprogenitor cells or in mature chondrocytes is relatively sparse. This review discusses the known functions of Ser/Thr-specific PPs in the molecular signalling pathways of chondrogenesis. PPs are clearly equally important as protein kinases to counterbalance the effect of reversible protein phosphorylation. Of the main Ser/Thr PPs, some of the functions of PP1, PP2A and PP2B have been characterised in the context of chondrogenesis. While PP1 and PP2A appear to negatively regulate chondrogenic differentiation and maintenance of chondrocyte phenotype, calcineurin is an important stimulatory mediator during chondrogenesis but becomes inhibitory in mature chondrocytes. Furthermore, PPs are implicated to be mediators during the pathogenesis of osteoarthritis that makes them potential therapeutic targets to be exploited in the close future. Among the many yet unexplored targets of PPs, modulation of plasma membrane ion channel function and participation in mechanotransduction pathways are emerging novel aspects of signalling during chondrogenesis that should be further elucidated. Besides the regulation of cellular ion homeostasis, other potentially significant novel roles for PPs during the regulation of in vitro chondrogenesis are discussed.

FK506 protects against articular cartilage collagenous extra-cellular matrix degradation

OBJECTIVE

Osteoarthritis (OA) is a non-rheumatologic joint disease characterized by progressive degeneration of the cartilage extra-cellular matrix (ECM), enhanced subchondral bone remodeling, activation of synovial macrophages and osteophyte growth. Inhibition of calcineurin (Cn) activity through tacrolimus (FK506) in in vitro monolayer chondrocytes exerts positive effects on ECM marker expression. This study therefore investigated the effects of FK506 on anabolic and catabolic markers of osteoarthritic chondrocytes in 2D and 3D in vitro cultures, and its therapeutic effects in an in vivo rat model of OA.

METHODS

Effects of high and low doses of FK506 on anabolic (QPCR/histochemistry) and catabolic (QPCR) markers were evaluated in vitro on isolated (2D) and ECM-embedded chondrocytes (explants, 3D pellets). Severe cartilage damage was induced unilaterally in rat knees using papain injections in combination with a moderate running protocol. Twenty rats were treated with FK506 orally and compared to twenty untreated controls. Subchondral cortical and trabecular bone changes (longitudinal microCT) and macrophage activation (SPECT/CT) were measured. Articular cartilage was analyzed ex vivo using contrast enhanced microCT and histology.

RESULTS

FK506 treatment of osteoarthritic chondrocytes in vitro induced anabolic (mainly collagens) and reduced catabolic ECM marker expression. In line with this, FK506 treatment clearly protected ECM integrity in vivo by markedly decreasing subchondral sclerosis, less development of subchondral pores, depletion of synovial macrophage activation and lower osteophyte growth.

CONCLUSION

FK506 protected cartilage matrix integrity in vitro and in vivo. Additionally, FK506 treatment in vivo reduced OA-like responses in different articular joint tissues and thereby makes Cn an interesting target for therapeutic intervention of OA.

NFATc1 and NFATc2 repress spontaneous osteoarthritis

Osteoarthritis (OA) was once viewed originally as a mechanical disease of "wear and tear," but advances made during the past two decades suggest that abnormal biomechanics contribute to active dysregulation of chondrocyte biology, leading to catabolism of the cartilage matrix. A number of signaling and transcriptional mechanisms have been studied in relation to the regulation of this catabolic program, but how they specifically regulate the initiation or progression of the disease is poorly understood. Here, we demonstrate that cartilage-specific ablation of Nuclear factor of activated T cells c1 (Nfatc1) in Nfatc2(-/-) mice leads to early onset, aggressive OA affecting multiple joints. This model recapitulates features of human OA, including loss of proteoglycans, collagen and aggrecan degradation, osteophyte formation, changes to subchondral bone architecture, and eventual progression to cartilage effacement and joint instability. Consistent with the notion that NFATC1 is an OA-suppressor gene, NFATC1 expression was significantly down-regulated in paired lesional vs. macroscopically normal cartilage samples from OA patients. The highly penetrant, early onset, and severe nature of this model make it an attractive platform for the preclinical development of treatments to alter the course of OA. Furthermore, these findings indicate that NFATs are key suppressors of OA, and regulating NFATs or their transcriptional targets in chondrocytes may lead to novel disease-modifying OA therapies.

A universal method for sequential immunofluorescent analysis of chromatin and chromatin-associated proteins on chromosome spreads

Immunofluorescence has been widely used to study histone modification dynamics and chromosome-associated proteins that regulate the segregation of chromosomes during cell divisions. Since many of these regulatory proteins interact (in)directly to exert their proper function, it is of interest to detect these proteins simultaneously, to establish their spatiotemporal relation. However, the detection of multiple epitopes on the same material is limited by the availability of antibodies derived from different host species. For Western blot membranes, buffers were developed to remove antibodies after the first round of detection and enable a second round of detection. In this study, we establish that this "stripping" principle can also be applied for sequential immunofluorescence on chromosome preparations. We first adapted a drying down fixation technique for the use on cultured cells from different primary cells and cell lines. These chromosome spreads were subsequently used to optimize the stripping procedure for this application. We investigated feasibility and reliability of detection of histones and their posttranslational modifications as well as chromatin interacting proteins in two subsequent rounds of immunofluorescence. We conclude that this method is a reliable option when spatial resolution and co-expression need to be investigated and the material or the choice of antibodies is limited.

Osmolarity determines the in vitro chondrogenic differentiation capacity of progenitor cells via nuclear factor of activated T-cells 5

INTRODUCTION

Previous studies have shown that human articular chondrocytes in vitro are osmolarity-responsive and increase matrix synthesis under cartilage-specific physiological osmolarity. The effects of increased osmolarity on chondrogenesis of progenitor cells in vitro are largely unknown. We therefore aimed to elucidate whether hyperosmolarity facilitates their chondrogenic differentiation and whether Nfat5 is involved.

MATERIALS AND METHODS

ATDC5 cells and human bone marrow stem cells (hBMSCs) were differentiated in the chondrogenic lineage in control and increased osmolarity conditions. Chondrogenic outcome was measured by gene- and protein expression analysis. RNAi was used to determine the role of Nfat5 in chondrogenic differentiation under normal and increased osmolarity.

RESULTS

Increasing the osmolarity of differentiation medium with 100mOsm resulted in significantly increased chondrogenic marker expression (Col2a1, Col10a1, Acan, Sox9, Runx2 and GAGs) during chondrogenic differentiation of the two chondroprogenitors, ATDC5 and hBMSCs. Nfat5 knockdown under both control and increased osmolarity affected chondrogenic differentiation and suppressed the osmolarity-induced chondrogenic induction. Knockdown of Nfat5 in early differentiation significantly decreased early Sox9 expression, whereas knockdown of Sox9 in early differentiation did not affect early Nfat5 expression.

CONCLUSIONS

Increasing the osmolarity of chondrogenic culture media by 100mOsm significantly increased chondrogenic gene expression during the course of chondrogenic differentiation of progenitor cells. Nfat5 may be involved in regulating chondrogenic differentiation of these cells under both normal and increased osmolarities and might regulate chondrogenic differentiation through influencing early Sox9 expression.