Conditional Deletion of the Phd2 Gene in Articular Chondrocytes Accelerates Differentiation and Reduces Articular Cartilage Thickness

PHD1-3 oxygen sensors in vivo-lessons learned from gene deletions

Oxygen sensors enable cells to adapt to limited oxygen availability (hypoxia), affecting various cellular and tissue responses. Prolyl-4-hydroxylase domain 1-3 (PHD1-3; also called Egln1-3, HIF-P4H 1-3, HIF-PH 1-3) proteins belong to the Fe2+- and 2-oxoglutarate-dependent dioxygenase superfamily and utilise molecular oxygen (O2) alongside 2-oxoglutarate as co-substrate to hydroxylate two proline residues of α subunits of the dimeric hypoxia inducible factor (HIF) transcription factor. PHD1-3-mediated hydroxylation of HIF-α leads to its degradation and inactivation. Recently, various PHD inhibitors (PHI) have entered the clinics for treatment of renal anaemia. Pre-clinical analyses indicate that PHI treatment may also be beneficial in numerous other hypoxia-associated diseases. Nonetheless, the underlying molecular mechanisms of the observed protective effects of PHIs are only partly understood, currently hindering their translation into the clinics. Moreover, the PHI-mediated increase of Epo levels is not beneficial in all hypoxia-associated diseases and PHD-selective inhibition may be advantageous. Here, we summarise the current knowledge about the relevance and function of each of the three PHD isoforms in vivo, based on the deletion or RNA interference-mediated knockdown of each single corresponding gene in rodents. This information is crucial for our understanding of the physiological relevance and function of the PHDs as well as for elucidating their individual impact on hypoxia-associated diseases. Furthermore, this knowledge highlights which diseases may best be targeted by PHD isoform-selective inhibitors in case such pharmacologic substances become available.

Sex-Specific Effects of THRβ Signaling on Metabolic Responses to High Fat Diet in Mice

Thyroid hormone (TH) plays a crucial role in regulating the functions of both bone and adipose tissue. Given that TH exerts its cholesterol-lowering effects in hepatic tissue through the TH receptor-β (TRβ), we hypothesized that TRβ agonist therapy using MGL3196 (MGL) would be effective in treating increased adiposity and bone loss in response to a 12-week high-fat diet (HFD) in adult C57BL/6J mice. Transcriptional and serum profiling revealed that HFD-induced leptin promoted weight gain in both males and females, but MGL only suppressed leptin induction and weight gain in males. In vitro studies suggest that estrogen suppresses MGL activity in adipocytes, indicating that estrogen might interfere with MGL-TRβ function. Compared to systemic adiposity, HFD reduced bone mass in male but not female mice. Paradoxically, MGL treatment reversed macroscopic bone mineral density loss in appendicular bones, but micro-CT revealed that MGL exacerbated HFD-induced trabecular bone loss, and reduced bone strength. In studies on the mechanisms for HFD effects on bone, we found that HFD induced Rankl expression in male femurs that was blocked by MGL. By ex vivo assays, we found that RANKL indirectly represses osteoblast lineage allocation of osteoprogenitors by induction of inflammatory cytokines TNFα, IL-1β, and CCL2. Finally, we found that MGL functions in both systemic adiposity and bone by nongenomic TRβ signaling, as HFD-mediated phenotypes were not rescued in TRβ147F knockout mice with normal genomic but defective nongenomic TRβ signaling. Our findings demonstrate that the negative effects of HFD on body fat and bone phenotypes are impacted by MGL in a gender-specific manner.

Metabolic rewiring controlled by c-Fos governs cartilage integrity in osteoarthritis

OBJECTIVES

The activator protein-1 (AP-1) transcription factor component c-Fos regulates chondrocyte proliferation and differentiation, but its involvement in osteoarthritis (OA) has not been functionally assessed.

METHODS

c-Fos expression was evaluated by immunohistochemistry on articular cartilage sections from patients with OA and mice subjected to the destabilisation of the medial meniscus (DMM) model of OA. Cartilage-specific c-Fos knockout (c-FosΔCh) mice were generated by crossing c-fosfl/fl to Col2a1-CreERT mice. Articular cartilage was evaluated by histology, immunohistochemistry, RNA sequencing (RNA-seq), quantitative reverse transcription PCR (qRT-PCR) and in situ metabolic enzyme assays. The effect of dichloroacetic acid (DCA), an inhibitor of pyruvate dehydrogenase kinase (Pdk), was assessed in c-FosΔCh mice subjected to DMM.

RESULTS

FOS-positive chondrocytes were increased in human and murine OA cartilage during disease progression. Compared with c-FosWT mice, c-FosΔCh mice exhibited exacerbated DMM-induced cartilage destruction. Chondrocytes lacking c-Fos proliferate less, have shorter collagen fibres and reduced cartilage matrix. Comparative RNA-seq revealed a prominent anaerobic glycolysis gene expression signature. Consistently decreased pyruvate dehydrogenase (Pdh) and elevated lactate dehydrogenase (Ldh) enzymatic activities were measured in situ, which are likely due to higher expression of hypoxia-inducible factor-1α, Ldha, and Pdk1 in chondrocytes. In vivo treatment of c-FosΔCh mice with DCA restored Pdh/Ldh activity, chondrocyte proliferation, collagen biosynthesis and decreased cartilage damage after DMM, thereby reverting the deleterious effects of c-Fos inactivation.

CONCLUSIONS

c-Fos modulates cellular bioenergetics in chondrocytes by balancing pyruvate flux between anaerobic glycolysis and the tricarboxylic acid cycle in response to OA signals. We identify a novel metabolic adaptation of chondrocytes controlled by c-Fos-containing AP-1 dimers that could be therapeutically relevant.

WNT16 Regulation of the Articular Chondrocyte Phenotype in Mice

The anabolic effects of WNT16 on osteoblasts are well established, however, little is known regarding the role of WNT16 in chondrocytes. In this study, we evaluated Wnt16 expression and its biological effects on mouse articular chondrocytes (ACs), since these cells are key to the development of osteoarthritis. While ACs derived from the long bone epiphysis of 7-day old C57BL/6J mice express multiple Wnts, Wnt5b and Wnt16 represent the two most highly expressed Wnts (expressed at several-fold higher levels than other Wnts). Treatment of serum-free AC cultures, with 100 ng/mL of recombinant human (rh) WNT16 for 24 h (hrs), increased proliferation (20%, p < 0.05) and expression levels of makers (Sox9 and Col2) of immature chondrocytes at both 24 h and 72 h, while Acan increased at 72 h. Expression of Mmp9, a marker of mature chondrocytes was decreased at 24 h. Additionally, WNT16 treatment regulated expression levels of Wnt ligands in a biphasic manner, inhibiting its expression at 24 h, while stimulating expression at 72 h. To determine whether WNT16 exerted anabolic effects on the AC phenotype, ex vivo cultures of tibial epiphyses were treated with rhWNT16 or vehicle for 9 days, and the articular cartilage phenotype was evaluated by safranin O cartilage staining and expression of articular cartilage marker genes. Both articular cartilage area and expression levels of AC markers were increased after rhWNT16 treatment. Our data suggest that Wnt16 expressed in ACs may play a role in regulating joint cartilage homeostasis via its direct effect, as well as through modulating the expression of other Wnt ligands.

Development of an Animal Model for Traumatic Brain Injury Augmentation of Heterotopic Ossification in Response to Local Injury

Heterotopic ossification (HO) is the abnormal growth of bone in soft connective tissues that occurs as a frequent complication in individuals with traumatic brain injury (TBI) and in rare genetic disorders. Therefore, understanding the mechanisms behind ectopic bone formation in response to TBI is likely to have a significant impact on identification of novel therapeutic targets for HO treatment. In this study, we induced repetitive mild TBI (mTBI) using a weight drop model in mice and then stimulated HO formation via a local injury to the Achilles tendon or fibula. The amount of ectopic bone, as evaluated by micro-CT analyses, was increased by four-fold in the injured leg of mTBI mice compared to control mice. However, there was no evidence of HO formation in the uninjured leg of mTBI mice. Since tissue injury leads to the activation of hypoxia signaling, which is known to promote endochondral ossification, we evaluated the effect of IOX2, a chemical inhibitor of PHD2 and a known inducer of hypoxia signaling on HO development in response to fibular injury. IOX2 treatment increased HO volume by five-fold compared to vehicle. Since pericytes located in the endothelium of microvascular capillaries are known to function as multipotent tissue-resident progenitors, we determined if activation of hypoxia signaling promotes pericyte recruitment at the injury site. We found that markers of pericytes, NG2 and PDGFRβ, were abundantly expressed at the site of injury in IOX2 treated mice. Treatment of pericytes with IOX2 for 72 h stimulated expression of targets of hypoxia signaling (Vegf and Epo), as well as markers of chondrocyte differentiation (Col2α1 and Col10α1). Furthermore, serum collected from TBI mice was more effective in promoting the proliferation and differentiation of pericytes than control mouse serum. In conclusion, our data show that the hypoxic state at the injury site in soft tissues of TBI mice provides an environment leading to increased accumulation and activation of pericytes to form endochondral bone.

Lack of Skeletal Effects in Mice with Targeted Disruptionof Prolyl Hydroxylase Domain 1 (Phd1) Gene Expressed in Chondrocytes

The critical importance of hypoxia-inducible factor (HIF)s in the regulation of endochondral bone formation is now well established. HIF protein levels are closely regulated by the prolyl hydroxylase domain-containing protein (PHD) mediated ubiquitin-proteasomal degradation pathway. Of the three PHD family members expressed in bone, we previously showed that mice with conditional disruption of the Phd2 gene in chondrocytes led to a massive increase in the trabecular bone mass of the long bones. By contrast, loss of Phd3 expression in chondrocytes had no skeletal effects. To investigate the role of Phd1 expressed in chondrocytes on skeletal development, we conditionally disrupted the Phd1 gene in chondrocytes by crossing Phd1 floxed mice with Collagen 2α1-Cre mice for evaluation of a skeletal phenotype. At 12 weeks of age, neither body weight nor body length was significantly different in the Cre+; Phd1flox/flox conditional knockout (cKO) mice compared to Cre−; Phd1flox/flox wild-type (WT) control mice. Micro-CT measurements revealed significant gender differences in the trabecular bone volume adjusted for tissue volume at the secondary spongiosa of the femur and the tibia for both genotypes, but no genotype differences were found for any of the trabecular bone measurements of either femur or tibia. Similarly, cortical bone parameters were not affected in the Phd1 cKO mice compared to control mice. Histomorphometric analyses revealed no significant differences in bone area, bone formation rate or mineral apposition rate in the secondary spongiosa of femurs between cKO and WT control mice. Loss of Phd1 expression in chondrocytes did not affect the expression of markers of chondrocytes (collage 2, collagen 10) or osteoblasts (alkaline phosphatase, bone sialoprotein) in the bones of cKO mice. Based on these and our published data, we conclude that of the three PHD family members, only Phd2 expressed in chondrocytes regulates endochondral bone formation and development of peak bone mass in mice.

Primary Cilia: A Cellular Regulator of Articular Cartilage Degeneration

Osteoarthritis (OA) is the most common joint disease that can cause pain and disability in adults. The main pathological characteristic of OA is cartilage degeneration, which is caused by chondrocyte apoptosis, cartilage matrix degradation, and inflammatory factor destruction. The current treatment for patients with OA focuses on delaying its progression, such as oral anti-inflammatory analgesics or injection of sodium gluconate into the joint cavity. Primary cilia are an important structure involved in cellular signal transduction. Thus, they are very sensitive to mechanical and physicochemical stimuli. It is reported that the primary cilia may play an important role in the development of OA. Here, we review the correlation between the morphology (location, length, incidence, and orientation) of chondrocyte primary cilia and OA and summarize the relevant signaling pathways in chondrocytes that could regulate the OA process through primary cilia, including Hedgehog, Wnt, and inflammation-related signaling pathways. These data provide new ideas for OA treatment.

Role of prolyl hydroxylase domain proteins in bone metabolism

Cellular metabolism requires dissolved oxygen gas. Because evolutionary refinements have constrained mammalian dissolved oxygen levels, intracellular oxygen sensors are vital for optimizing the bioenergetic and biosynthetic use of dissolved oxygen. Prolyl hydroxylase domain (PHD) homologs 1-3 (PHD1/2/3) are molecular oxygen dependent non-heme dioxygenases whose enzymatic activity is regulated by the concentration of dissolved oxygen. PHD oxygen dependency has evolved into an important intracellular oxygen sensor. The most well studied mechanism of PHD oxygen-sensing is its regulation of the hypoxia-inducible factor (HIF) hypoxia signaling pathway. Heterodimeric HIF transcription factor activity is regulated post-translationally by selective PHD proline hydroxylation of its HIF1α subunit, accelerating HIF1α ubiquitination and proteasomal degradation, preventing HIF heterodimer assembly, nuclear accumulation, and activation of its target oxygen homeostasis genes. Phd2 has been shown to be the key isoform responsible for HIF1α subunit regulation in many cell types and accordingly disruption of the Phd2 gene results in embryonic lethality. In bone cells Phd2 is expressed in high abundance and tightly regulated. Conditional disruption of the Phd1, Phd2 and/or Phd3 gene in various bone cell types using different Cre drivers reveals a major role for PHD2 in skeletal growth and development. In this review, we will summarize the state of current knowledge on the role and mechanism of action of PHD2 as oxygen sensor in regulating bone metabolism.

Differences in pathways contributing to thyroid hormone effects on postnatal cartilage calcification versus secondary ossification center development

The proximal and distal femur epiphyses of mice are both weight-bearing structures derived from chondrocytes but differ in development. Mineralization at the distal epiphysis occurs in an osteoblast-rich secondary ossification center (SOC), while the chondrocytes of the proximal femur head (FH), in particular, are directly mineralized. Thyroid hormone (TH) plays important roles in distal knee SOC formation, but whether TH also affects proximal FH development remains unexplored. Here, we found that TH controls chondrocyte maturation and mineralization at the FH in vivo through studies in thyroid stimulating hormone receptor (Tshr^-/-) hypothyroid mice by X-ray, histology, transcriptional profiling, and immunofluorescence staining. Both in vivo and in vitro studies conducted in ATDC5 chondrocyte progenitors concur that TH regulates expression of genes that modulate mineralization (Ibsp, Bglap2, Dmp1, Spp1, and Alpl). Our work also delineates differences in prominent transcription factor regulation of genes involved in the different mechanisms leading to proximal FH cartilage calcification and endochondral ossification at the distal femur. The information on the molecular pathways contributing to postnatal cartilage calcification can provide insights on therapeutic strategies to treat pathological calcification that occurs in soft tissues such as aorta, kidney, and articular cartilage.

Targeted Deletion of the Claudin12 Gene in Mice Increases Articular Cartilage and Inhibits Chondrocyte Differentiation

To study the role of Claudin (CLDN)12 in bone, we developed mice with a targeted deletion of exon2 in the Cldn12 gene for skeletal phenotype analysis. Micro-CT analysis of the secondary spongiosa of distal femurs of mice with targeted disruption of the Cldn12 gene and control littermates showed no significant genotype-specific differences in either cortical or trabecular bone parameters for either gender in 13-week-old mice. Immunohistochemistry revealed that while CLDN12 was expressed in both differentiating chondrocytes and osteoblasts of the secondary spongiosa of 3-week-old wild-type mice, its expression was restricted to differentiating chondrocytes in the articular cartilage and growth plate in adult mice. Articular cartilage area at the knee were increased by 47% in Cldn12 knockout (KO) mice compared to control littermates. Micro-CT analyses found that while the trabecular number was increased by 9% and the trabecular spacing was reduced by 9% in the femoral epiphysis of Cldn12 KO mice, neither bone volume nor bone volume adjusted for tissue volume was different between the two genotypes. The expression levels of Clusterin, Lubricin and Mmp13 were increased by 56%, 46%, and 129%, respectively, in primary articular chondrocytes derived from KO compared to control mice. Our data indicate that targeted deletion of the Cldn12 gene in mice increases articular cartilage, in part, by promoting articular chondrocyte phenotype.

Bioactive Inks Development for Osteochondral Tissue Engineering: A Mini-Review

Nowadays, a prevalent joint disease affecting both cartilage and subchondral bone is osteoarthritis. Osteochondral tissue, a complex tissue unit, exhibited limited self-renewal potential. Furthermore, its gradient properties, including mechanical property, bio-compositions, and cellular behaviors, present a challenge in repairing and regenerating damaged osteochondral tissues. Here, tissue engineering and translational medicine development using bioprinting technology provided a promising strategy for osteochondral tissue repair. In this regard, personalized stratified scaffolds, which play an influential role in osteochondral regeneration, can provide potential treatment options in early-stage osteoarthritis to delay or avoid the use of joint replacements. Accordingly, bioactive scaffolds with possible integration with surrounding tissue and controlling inflammatory responses have promising future tissue engineering perspectives. This minireview focuses on introducing biologically active inks for bioprinting the hierarchical scaffolds, containing growth factors and bioactive materials for 3D printing of regenerative osteochondral substitutes.

Research on quantitative measurement method of articular cartilage thickness change based on MR image

In order to study the quantitative measurement method of articular cartilage thickness change based on MR image, 140 patients with no knee joint symptoms and 17 patients with knee joint damage were studied. FLASH, SE and FISP 3 sequences were scanned for all normal knee joints to determine the articular cartilage thickness. 17 patients with knee joint damage were followed up for six months. The thickness of their femoral condyle and femoral trochlea cartilage was measured by FLASH sequence and thickness changes are recorded. The results show that the thickness distribution of normal knee articular cartilage in different parts is not equal, and the thickness of articular cartilage will be gradually thinner in different ages; MR image technique can observe the change of articular cartilage thickness in patients with knee joint damage, reflecting the recovery status of the patient's condition. The results of quantitative measurement of changes in articular cartilage thickness based on MR images are presented herein, and the results are as expected. Experimental data were provided for the clinical treatment of acute knee injury and osteoarthritis. Although there are still some shortcomings in the research process, the research results still provide some reference and guidance for the future exploration of the use of MR images to monitor the condition of arthritis, so this study is a significant research topic.

Vitamin C effects on 5-hydroxymethylcytosine and gene expression in osteoblasts and chondrocytes: Potential involvement of PHD2

Vitamin C (ascorbic acid, AA) is a well-known regulator of bone and cartilage metabolism. However, the mechanisms of AA’s action in these tissues are only partly understood. In this study, we confirmed that AA contributes to bone and cartilage metabolism by showing decreased articular cartilage and trabecular bone in AA-deficient spontaneous fracture (sfx) mutant mice. In vitro, we found that AA exerts differential effects on chondrocyte and osteoblast differentiation. Since AA is known to increase levels of 5-hydroxymethylcytosine (5-hmC) and induce DNA demethylation via the ten-eleven translocases (TETs), and since prolyl hydroxylase domain-containing protein 2 (PHD2), a known mediator of AA’s effects in these tissues, is part of the same enzyme family as the TETs, we next investigated whether increases in 5-hmC might mediate some of these effects. All TETs and PHDs are expressed in chondrocytes and osteoblasts, and PHD2 is localized in both the cytoplasm and nucleus of the cell, lending plausibility to the hypothesis of altered 5-hmC content in these cells. We found that AA treatment increased levels of 5-hmC in both cell types globally, notably including promoter regions of osteoblast differentiation genes. Furthermore, inhibition of PHD2 decreased 5-hmC levels in chondrocyte differentiation gene promoters, and knockdown of Phd2 in chondrocytes reduced global 5-hmC levels, suggesting for the first time that PHD2 may itself directly mediate increases in 5-hmC in chondrocyte and osteoblast genes. Further investigation of this mechanism could lead to novel therapeutic approaches to treat debilitating diseases such as osteoarthritis and osteoporosis.

Up-regulated HIF-2α contributes to the Osteoarthritis development through mediating the primary cilia loss

BACKGROUNDS

Up-regulated HIF-2α (hypoxia induced factor 2) had been demonstrated to contribute to Osteoarthritis (OA) development via inducing the expression of matrix-degrading enzymes. However, the HIF-2α also could promote primary cilia loss through HIF-2α/AURKA (Aurora kinase A)/NEDD9 pathway. And the primary cilia dysfunction is another characteristic of the OA. Thus, we investigated here whether the HIF-2α also contributes the OA development through mediating the primary cilia loss.

METHODS

The primary chondrocytes were isolated from the experimental OA mice induced by destabilization of the medial meniscus (DMM). Chondrocytes were cultured under normoxia (21% O₂) or hypoxia (2% O₂) conditions. The HIF-1α and HIF-2α expressions were assessed by western blot. The cilia formation was counted by immuno-staining the acetylated tubulin. The contribution of HIF-1α or HIF-2α to the primary cilia loss was assessed by knocking-down the HIF-1α or HIF-2α individually. The HIF-2α/AURKA/NEDD9 pathway was validated through over-expressing or knocking-down specific components of the pathway and then counting the primary cilia number. Finally, the pathway was further confirmed in the OA mice.

RESULTS

Hypoxia could induce the expression of both HIF-1α and HIF-2α, and also reduce the number of primary cilia on the chondrocytes isolated from the experimental OA mice. Knocking-down or over-expressing HIF-1α or HIF-2α individually showed that the HIF-2α could induce the primary cilia reduction rather than the HIF-1α. Manipulating the HIF-2α expression could positively affect the AURKA and NEDD9 expression. Manipulating the AURKA and NEDD9 expressions could reverse the function of HIF-2α on primary cilia. In the mice, knocking-down both AURKA and NEDD9 could alleviate the OA development significantly.

CONCLUSION

Up-regulated HIF-2α contributes to the Osteoarthritis development through mediating the primary cilia loss, which might be developed as therapeutic targets for OA treatment.

Long noncoding RNA expression profiles in chondrogenic and hypertrophic differentiation of mouse mesenchymal stem cells

Long noncoding RNAs (lncRNAs) are important regulators for a variety of biological processes. Chondrogenic differentiation of mesenchymal stem cells (MSCs) is a crucial stage in chondrogenesis while chondrocyte hypertrophy is related to endochondral ossification and osteoarthritis. However, the effects of lncRNAs on chondrogenic and hypertrophic differentiation of mouse MSCs are unclear. To explore the potential mechanisms of lncRNAs during chondrogenesis and chondrocyte hypertrophy, microarray was performed to investigate the expression profiles of lncRNA and mRNA in MSCs, pre-chondrocytes, and hypertrophic chondrocytes. Then, we validated microarray data by RT-PCR and screened three lncRNAs from upregulating groups during chondrogenesis and chondrocyte hypertrophy respectively. After downregulating any of the above lncRNAs, we found that the expression of chondrogenesis-related genes such as Sox9 and Col2a1 and hypertrophy-related genes including Runx2 and Col10a1 was inhibited, respectively. Furthermore, the target genes of above lncRNAs were predicted by bioinformatics approaches. Gene ontology and Kyoto encyclopedia of genes and genome biological pathway analysis were also made to speculate the functions of above lncRNAs. In conclusion, the study first revealed the expression profile of lncRNAs in chondrogenic and hypertrophic differentiations of mouse MSCs and presented a new prospect for the underlying mechanisms of chondrogenesis and endochondral ossification.