Control of chondrocyte gene expression by actin dynamics: a novel role of cholesterol/Ror-alpha signalling in endochondral bone growth

The role of retinoic acid receptor-related orphan receptors in skeletal diseases

Bone homeostasis, depending on the balance between bone formation and bone resorption, is responsible for maintaining the proper structure and function of the skeletal system. As an important group of transcription factors, retinoic acid receptor-related orphan receptors (RORs) have been reported to play important roles in bone homeostasis by regulating the transcription of target genes in skeletal cells. On the other hand, the dysregulation of RORs often leads to various skeletal diseases such as osteoporosis, rheumatoid arthritis (RA), and osteoarthritis (OA). Herein, we summarized the roles and mechanisms of RORs in skeletal diseases, aiming to provide evidence for potential therapeutic strategies.

Inhibition of nuclear receptor RORα attenuates cartilage damage in osteoarthritis by modulating IL-6/STAT3 pathway

Osteoarthritis (OA) is characterized by cartilage destruction, chronic inflammation, and local pain. Evidence showed that retinoic acid receptor-related orphan receptor-α (RORα) is crucial in cartilage development and OA pathogenesis. Here, we investigated the role and molecular mechanism of RORα, an important member of the nuclear receptor family, in regulating the development of OA pathologic features. Investigation into clinical cartilage specimens showed that RORα expression level is positively correlated with the severity of OA and cartilage damage. In an in vivo OA model induced by anterior crucial ligament transaction, intra-articular injection of si-Rora adenovirus reversed the cartilage damage. The expression of cartilage matrix components type II collagen and aggrecan were elevated upon RORα blockade. RNA-seq data suggested that the IL-6/STAT3 pathway is significantly downregulated, manifesting the reduced expression level of both IL-6 and phosphorylated STAT3. RORα exerted its effect on IL-6/STAT3 signaling in two different ways, including interaction with STAT3 and IL-6 promoter. Taken together, our findings indicated the pivotal role of the RORα/IL-6/STAT3 axis in OA progression and confirmed that RORα blockade improved the matrix catabolism in OA chondrocytes. These results may provide a potential treatment target in OA therapy.

Inverse Agonist of Retinoid-Related Orphan Receptor-Alpha Prevents Apoptosis and Degeneration in Nucleus Pulposus Cells via Upregulation of YAP

Intervertebral disc degenerative disease (IDD) is the most common degenerative spine disease, which leads to chronic low back pain and symptoms in the lower extremities. In this study, we found that RORα, a member of the retinoid-related orphan receptor family, is significantly elevated in nucleus pulposus tissue in IDD patients. The elevation of RORα is associated with increased apoptosis of nucleus pulposus (NP) cells. Therefore, we applicated a well-established inverse agonist of RORα, SR3335, to investigate its role in regulating NP cell metabolism and apoptosis. To further investigate the mechanism that SR3335 regulates the pathogenesis of IDD in vitro, tumor necrosis factor alpha (TNF-α) stimulation was used in human NP cells to mimic the hostile environment that leads to degeneration. We found that SR3335 treatment reversed the trend of increased apoptosis in NP cells induced by TNF-α treatment. Next, TNF-α treatment upregulated the expression of type II collagen and aggrecan and downregulated MMP13 (matrix-degrading enzyme matrix metalloproteinase 13) and ADAMTS4 (a disintegrin and metalloproteinase with thrombospondin motifs 4). However, these effects were reversed after SR3335 treatment. Furthermore, we find that SR3335 mediated the effect in NP cells by regulating the YAP signaling pathway, especially by affecting the phosphorylation state of YAP. In conclusion, the reduction of matrix degradation enzymes and apoptosis upon SR3335 treatment suggests that SR3335 is a promising drug in reversing the deleterious microenvironment in IDD patients.

Effects of ultrasound therapy on the synovial fluid proteome in a rabbit surgery-induced model of knee osteoarthritis

Background

Ultrasound (US) therapy may improve osteoarthritis symptoms. We investigated the effects of US on the synovial fluid (SF) proteome in a rabbit knee osteoarthritis (KOA) model to explore its therapeutic mechanisms.

Methods

Sixteen healthy 6-month-old New Zealand white rabbits (eight male, eight female), weighing 2.5–3.0 kg, were randomly divided into groups A and B with eight rabbits per group. Both groups were subjected to right anterior cruciate ligament transaction. Six weeks after surgery, we treated the operated knee joint of group A rabbits with US and of group B rabbits with sham US for 2 weeks. The proteomes of knee joint SF from groups A and B rabbits were then analyzed using a label-free mass spectrometry (MS) quantification method.

Results

We identified 19 protein sequences annotated by 361 Gene Ontology (GO) items. According to the Kyoto Encyclopedia of Genes and Genomes (KEGG) database of rabbit protein sequences, we then annotated the KO numbers of homologous/similar proteins to 32 relevant KEGG pathways. We extracted 10 significantly differentially expressed proteins among the 32 relevant KEGG messages/metabolism pathways. The proteins whose levels were decreased were apolipoprotein A-I (AopA-1), transferrin (TF), carboxypeptidase B2 (CBP2), arylesterase/paraoxonase (PON), fibrinogen alpha chain, and alpha-2-macroglobulin (A2M). The proteins whose levels were increased were molecular chaperone HtpG/heat shock proteins (htpG, HSP90A), decorin (DCN), pyruvate kinase (PK, pyk), and fatty acid-binding protein 4/adipocyte (FABP4, aP2).

Conclusions

US therapy can alter protein levels in SF, which can decrease AopA-1, TF, CBP2, PON, fibrinogen alpha chain and A2M protein levels, and increase HtpG/HSP90A, DCN, PK/PKY, and FABP4/aP2 protein levels in SF of KOA, suggesting that the therapeutic mechanisms of US therapy on KOA may occur through changes in the SF proteome.

Synergistic co-regulation and competition by a SOX9-GLI-FOXA phasic transcriptional network coordinate chondrocyte differentiation transitions

The growth plate mediates bone growth where SOX9 and GLI factors control chondrocyte proliferation, differentiation and entry into hypertrophy. FOXA factors regulate hypertrophic chondrocyte maturation. How these factors integrate into a Gene Regulatory Network (GRN) controlling these differentiation transitions is incompletely understood. We adopted a genome-wide whole tissue approach to establish a Growth Plate Differential Gene Expression Library (GP-DGEL) for fractionated proliferating, pre-hypertrophic, early and late hypertrophic chondrocytes, as an overarching resource for discovery of pathways and disease candidates. De novo motif discovery revealed the enrichment of SOX9 and GLI binding sites in the genes preferentially expressed in proliferating and prehypertrophic chondrocytes, suggesting the potential cooperation between SOX9 and GLI proteins. We integrated the analyses of the transcriptome, SOX9, GLI1 and GLI3 ChIP-seq datasets, with functional validation by transactivation assays and mouse mutants. We identified new SOX9 targets and showed SOX9-GLI directly and cooperatively regulate many genes such as Trps1, Sox9, Sox5, Sox6, Col2a1, Ptch1, Gli1 and Gli2. Further, FOXA2 competes with SOX9 for the transactivation of target genes. The data support a model of SOX9-GLI-FOXA phasic GRN in chondrocyte development. Together, SOX9-GLI auto-regulate and cooperate to activate and repress genes in proliferating chondrocytes. Upon hypertrophy, FOXA competes with SOX9, and control toward terminal differentiation passes to FOXA, RUNX, AP1 and MEF2 factors.

In the development of the mammalian growth plate, while several transcription factors are individually well known for their key roles in regulating phases of chondrocyte differentiation, there is little information on how they interact and cooperate with each other. We took an unbiased genome wide approach to identify the transcription factors and signaling pathways that play dominant roles in the chondrocyte differentiation cascade. We developed a searchable library of differentially expressed genes, GP-DGEL, which has fine spatial resolution and global transcriptomic coverage for discovery of processes, pathways and disease candidates. Our work identifies a novel regulatory mechanism that integrates the action of three transcription factors, SOX9, GLI and FOXA. SOX9-GLI auto-regulate and cooperate to activate and repress genes in proliferating chondrocytes. Upon entry into prehypertrophy, FOXA competes with SOX9, and control of hypertrophy passes to FOXA, RUNX, AP1 and MEF2 factors.

Achondroplasia: Development, pathogenesis, and therapy

Autosomal dominant mutations in fibroblast growth factor receptor 3 (FGFR3) cause achondroplasia (Ach), the most common form of dwarfism in humans, and related chondrodysplasia syndromes that include hypochondroplasia (Hch), severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN), and thanatophoric dysplasia (TD). FGFR3 is expressed in chondrocytes and mature osteoblasts where it functions to regulate bone growth. Analysis of the mutations in FGFR3 revealed increased signaling through a combination of mechanisms that include stabilization of the receptor, enhanced dimerization, and enhanced tyrosine kinase activity. Paradoxically, increased FGFR3 signaling profoundly suppresses proliferation and maturation of growth plate chondrocytes resulting in decreased growth plate size, reduced trabecular bone volume, and resulting decreased bone elongation. In this review, we discuss the molecular mechanisms that regulate growth plate chondrocytes, the pathogenesis of Ach, and therapeutic approaches that are being evaluated to improve endochondral bone growth in people with Ach and related conditions. Developmental Dynamics 246:291-309, 2017. © 2016 Wiley Periodicals, Inc.

Regulation of Cholesterol Homeostasis by Hedgehog Signaling in Osteoarthritic Cartilage

Objective

With no effective therapies to attenuate cartilage degeneration in osteoarthritis (OA), the result is pain and disability. Activation of hedgehog (HH) signaling causes changes related to the progression of OA, with higher levels of Gli‐mediated transcriptional activation associated with increased disease severity. To elucidate the mechanism through which this occurs, this study sought to identify genes regulated by HH signaling in human OA chondrocytes.

Methods

Using human OA cartilage samples, microarray analyses were performed to detect changes in gene expression when the HH pathway was modulated. Results were analyzed for differentially expressed genes, grouped into functional networks, and validated in independent samples. To investigate the effects of chondrocyte‐specific sterol accumulation, we generated mice lacking Insig1 and Insig2, which are major negative regulators of cholesterol homeostasis, under Col2a1 regulatory elements.

Results

HH signaling was found to regulate genes that govern cholesterol homeostasis, and this led to alterations in cholesterol accumulation in chondrocytes. A higher level of Gli‐mediated transcription resulted in accumulation of intracellular cholesterol. In genetically modified mice, chondrocyte‐specific cholesterol accumulation was associated with an OA phenotype. Reducing cholesterol accumulation attenuated the severity of OA in mice in vivo and decreased the expression of proteases in human OA cartilage in vitro.

Conclusion

HH signaling regulates cholesterol homeostasis in chondrocytes, and intracellular cholesterol accumulation contributes to the severity of OA. Our findings have therapeutic implications, since reduction of HH signaling reversed cholesterol accumulation and statin treatment attenuated cartilage degeneration.

Identification of novel regulators of osteoblast matrix mineralization by time series transcriptional profiling

Bone mineralization is a carefully orchestrated process, regulated by a number of promoters and inhibitors that function to ensure effective hydroxyapatite formation. Here we sought to identify new regulators of this process through a time series microarray analysis of mineralising primary osteoblast cultures over a 27 day culture period. To our knowledge this is the first microarray study investigating murine calvarial osteoblasts cultured under conditions that permit both physiological extracellular matrix mineralization through the formation of discrete nodules and the terminal differentiation of osteoblasts into osteocytes. RT-qPCR was used to validate and expand the microarray findings. We demonstrate the significant up-regulation of >6,000 genes during the osteoblast mineralization process, the highest-ranked differentially expressed genes of which were those dominated by members of the PPAR-γ signalling pathway, namely Adipoq, Cd36 and Fabp4. Furthermore, we show that the inhibition of this signalling pathway promotes matrix mineralisation in these primary osteoblast cultures. We also identify Cilp, Phex, Trb3, Sox11, and Psat1 as novel regulators of matrix mineralization. Further studies examining the precise function of the identified genes and their interactions will advance our understanding of the mechanisms underpinning biomineralization.

Lipid transport and metabolism in healthy and osteoarthritic cartilage

Cartilage is an avascular tissue and cartilage metabolism depends on molecule diffusion from synovial fluid and subchondral bone. Thus, nutrient availability is limited by matrix permeability according to the size and charge of the molecules. Matrix composition limits the access of molecules to chondrocytes, determining cell metabolism and cartilage maintenance. Lipids are important nutrients in chondrocyte metabolism and are available for these cells through de novo synthesis but also through diffusion from surrounding tissues. Cartilage status and osteoarthritis development depend on lipid availability. This paper reviews lipid transport and metabolism in cartilage. We also analyze signalling pathways directly mediated by lipids and those that involve mTOR pathways, both in normal and osteoarthritic cartilage.

The Fox and the Rabbits-Environmental Variables and Population Genetics (1) Replication Problems in Association Studies and the Untapped Power of GWAS (2) Vitamin A Deficiency, Herpes Simplex Reactivation and Other Causes of Alzheimer's Disease

Classical population genetics shows that varying permutations of genes and risk factors permit or disallow the effects of causative agents, depending on circumstance. For example, genes and environment determine whether a fox kills black or white rabbits on snow or black ash covered islands. Risk promoting effects are different on each island, but obscured by meta-analysis or GWAS data from both islands, unless partitioned by different contributory factors. In Alzheimer's disease, the foxes appear to be herpes, borrelia or chlamydial infection, hypercholesterolemia, hyperhomocysteinaemia, diabetes, cerebral hypoperfusion, oestrogen depletion, or vitamin A deficiency, all of which promote beta-amyloid deposition in animal models—without the aid of gene variants. All relate to risk factors and subsets of susceptibility genes, which condition their effects. All are less prevalent in convents, where nuns appear less susceptible to the ravages of ageing. Antagonism of the antimicrobial properties of beta-amyloid by Abeta autoantibodies in the ageing population, likely generated by antibodies raised to beta-amyloid/pathogen protein homologues, may play a role in this scenario. These agents are treatable by diet and drugs, vitamin supplementation, pathogen detection and elimination, and autoantibody removal, although again, the beneficial effects of individual treatments may be tempered by genes and environment.

Transcriptional profiling reveals a role for RORalpha in regulating gene expression in obesity-associated inflammation and hepatic steatosis

Retinoid-related orphan receptor (ROR)α4 is the major RORα isoform expressed in adipose tissues and liver. In this study we demonstrate that RORα-deficient staggerer mice (RORα(sg/sg)) fed with a high-fat diet (HFD) exhibited reduced adiposity and hepatic triglyceride levels compared with wild-type (WT) littermates and were resistant to the development of hepatic steatosis, adipose-associated inflammation, and insulin resistance. Gene expression profiling showed that many genes involved in triglyceride synthesis and storage, including Cidec, Cidea, and Mogat1, were expressed at much lower levels in liver of RORα(sg/sg) mice. In contrast, overexpression of RORα in mouse hepatoma Hepa1-6 cells significantly increased the expression of genes that were repressed in RORα(sg/sg) liver, including Sult1b1, Adfp, Cidea, and ApoA4. ChIP and promoter analysis suggested that several of these genes were regulated directly by RORα. In addition to reduced lipid accumulation, inflammation was greatly diminished in white adipose tissue (WAT) of RORα(sg/sg) mice fed with an HFD. The infiltration of macrophages and the expression of many immune response and proinflammatory genes, including those encoding various chemo/cytokines, Toll-like receptors, and TNF signaling proteins, were significantly reduced in RORα(sg/sg) WAT. Moreover, RORα(sg/sg) mice fed with an HFD were protected from the development of insulin resistance. RORα(sg/sg) mice consumed more oxygen and produced more carbon dioxide, suggesting increased energy expenditure in this genotype. Our study indicates that RORα plays a critical role in the regulation of several aspects of metabolic syndrome. Therefore, RORα may provide a novel therapeutic target in the management of obesity and associated metabolic diseases.

Regulation of gene expression by PI3K in mouse growth plate chondrocytes

Background

Endochondral ossification, the process through which long bones are formed, involves chondrocyte proliferation and hypertrophic differentiation in the cartilage growth plate. In a previous publication we showed that pharmacological inhibition of the PI3K signaling pathway results in reduced endochondral bone growth, and in particular, shortening of the hypertrophic zone in a tibia organ culture system. In this current study we aimed to investigate targets of the PI3K signaling pathway in hypertrophic chondrocytes.

Methodology/Principal Findings

Through the intersection of two different microarray analyses methods (classical single gene analysis and GSEA) and two different chondrocyte differentiation systems (primary chondrocytes treated with a pharmacological inhibitor of PI3K and microdissected growth plates), we were able to identify a high number of genes grouped in GSEA functional categories regulated by the PI3K signaling pathway. Genes such as Phlda2 and F13a1 were down-regulated upon PI3K inhibition and showed increased expression in the hypertrophic zone compared to the proliferative/resting zone of the growth plate. In contrast, other genes including Nr4a1 and Adamts5 were up-regulated upon PI3K inhibition and showed reduced expression in the hypertrophic zone. Regulation of these genes by PI3K signaling was confirmed by quantitative RT-PCR. We focused on F13a1 as an interesting target because of its known role in chondrocyte hypertrophy and osteoarthritis. Mouse E15.5 tibiae cultured with LY294002 (PI3K inhibitor) for 6 days showed decreased expression of factor XIIIa in the hypertrophic zone compared to control cultures.

Conclusions/Significance

Discovering targets of signaling pathways in hypertrophic chondrocytes could lead to targeted therapy in osteoarthritis and a better understanding of the cartilage environment for tissue engineering.

New insight on FGFR3-related chondrodysplasias molecular physiopathology revealed by human chondrocyte gene expression profiling

Endochondral ossification is the process by which the appendicular skeleton, facial bones, vertebrae and medial clavicles are formed and relies on the tight control of chondrocyte maturation. Fibroblast growth factor receptor (FGFR)3 plays a role in bone development and maintenance and belongs to a family of proteins which differ in their ligand affinities and tissue distribution. Activating mutations of the FGFR3 gene lead to craniosynostosis and multiple types of skeletal dysplasia with varying degrees of severity: thanatophoric dysplasia (TD), achondroplasia and hypochondroplasia. Despite progress in the characterization of FGFR3-mediated regulation of cartilage development, many aspects remain unclear. The aim and the novelty of our study was to examine whole gene expression differences occurring in primary human chondrocytes isolated from normal cartilage or pathological cartilage from TD-affected fetuses, using Affymetrix technology. The phenotype of the primary cells was confirmed by the high expression of chondrocytic markers. Altered expression of genes associated with many cellular processes was observed, including cell growth and proliferation, cell cycle, cell adhesion, cell motility, metabolic pathways, signal transduction, cell cycle process and cell signaling. Most of the cell cycle process genes were down-regulated and consisted of genes involved in cell cycle progression, DNA biosynthesis, spindle dynamics and cytokinesis. About eight percent of all modulated genes were found to impact extracellular matrix (ECM) structure and turnover, especially glycosaminoglycan (GAG) and proteoglycan biosynthesis and sulfation. Altogether, the gene expression analyses provide new insight into the consequences of FGFR3 mutations in cell cycle regulation, onset of pre-hypertrophic differentiation and concomitant metabolism changes. Moreover, impaired motility and ECM properties may also provide clues about growth plate disorganization. These results also suggest that many signaling pathways may be directly or indirectly altered by FGFR3 and confirm the crucial role of FGFR3 in the control of growth plate development.

Genetic loci that regulate healing and regeneration in LG/J and SM/J mice

MRL mice display unusual healing properties. When MRL ear pinnae are hole punched, the holes close completely without scarring, with regrowth of cartilage and reappearance of both hair follicles and sebaceous glands. Studies using (MRL/lpr x C57BL/6)F(2) and backcross mice first showed that this phenomenon was genetically determined and that multiple loci contributed to this quantitative trait. The lpr mutation itself, however, was not one of them. In the present study we examined the genetic basis of healing in the Large (LG/J) mouse strain, a parent of the MRL mouse and a strain that shows the same healing phenotype. LG/J mice were crossed with Small (SM/J) mice and the F(2) population was scored for healing and their genotypes determined at more than 200 polymorphic markers. As we previously observed for MRL and (MRL x B6)F(2) mice, the wound-healing phenotype was sexually dimorphic, with female mice healing more quickly and more completely than male mice. We found quantitative trait loci (QTLs) on chromosomes (Chrs) 9, 10, 11, and 15. The heal QTLs on Chrs 11 and 15 were linked to differential healing primarily in male animals, whereas QTLs on Chrs 9 and 10 were not sexually dimorphic. A comparison of loci identified in previous crosses with those in the present report using LG/J x SM/J showed that loci on Chrs 9, 11, and 15 colocalized with those seen in previous MRL crosses, whereas the locus on Chr 10 was not seen before and is contributed by SM/J.