Transcription factor Hes1 modulates osteoarthritis development in cooperation with calcium/calmodulin-dependent protein kinase 2

Altered expression of the core circadian clock component PERIOD2 contributes to osteoarthritis-like changes in chondrocyte activity

In osteoarthritis, chondrocytes undergo a phenotype shift characterised by reduced expression of SOX9 (sry-box 9) and increased production of cartilage-degrading enzymes, e.g. MMP13 (matrix metalloproteinase 13) and ADAMTS5 (a disintegrin and metalloproteinase with thrombospondin motifs 5). The chondrocyte clock is also altered. Specifically, the peak level of PER2 is elevated, but peak level of BMAL1 reduced in osteoarthritic chondrocytes. The purpose of this study was to determine whether increased PER2 expression causes disease-associated changes in chondrocyte activity and to identify whether known risk factors for osteoarthritis induce changes in PER2 and BMAL1 expression. Primary human chondrocytes isolated from macroscopically normal cartilage were serum-starved overnight then re-fed with serum-replete media with/without interleukin 1β (IL-1β) (10 ng/mL), hydrogen peroxide (100 µM) or basic calcium phosphate (BCP) crystals (50 µg/mL). Peak level of BMAL1 was lower, whereas PER2 levels remained elevated for longer, in chondrocytes treated with IL-1β, hydrogen peroxide or BCP crystals compared to untreated cells. Levels of SOX9 were lower, whereas levels of ADAMTS5 and MMP13 were higher, in chondrocytes exposed to any of the three treatments compared to untreated cells. Knockdown of PER2 using siRNA partially abrogated the effects of each treatment on chondrocyte phenotype marker expression. Similarly, in chondrocytes isolated from osteoarthritic cartilage PER2 knockdown was associated with increased SOX9, reduced ADAMTS5 and reduced RNA and protein levels of MMP13 indicating partial mitigation of the osteoarthritic phenotype. Conversely, further ablation of BMAL1 expression in osteoarthritic chondrocytes resulted in a further reduction in SOX9 and increase in MMP13 expression. Overexpression of PER2 in the H5 chondrocyte cell line led to increased ADAMTS5 and MMP13 and decreased SOX9 expression. Localised inflammation, oxidative stress and BCP crystal deposition in osteoarthritic joints may contribute to disease pathology by inducing changes in the chondrocyte circadian clock.

Mice harboring a Hajdu Cheney Syndrome mutation are sensitized to osteoarthritis

Osteoarthritis is a joint disease characterized by cartilage degradation, altered gene expression and inflammation. NOTCH1 and NOTCH2 receptors and the JAGGED1 ligand regulate chondrocyte biology; however, the contribution of Notch signaling to osteoarthritis is controversial. Hajdu Cheney Syndrome (HCS) is a rare genetic disorder affecting the skeleton and associated with NOTCH2 mutations that lead to NOTCH2 gain-of-function. A murine model of the disease (Notch2^tm1.1Ecan) was used to test whether the HCS mutation increases the susceptibility to osteoarthritis. The knee of three-month-old Notch2^tm1.1Ecan male mice and control sex-matched littermates was destabilized by resection of the medial meniscotibial ligament, and changes in the joint analyzed two months thereafter. Expression of Notch target genes was increased in the femoral heads of Notch2^tm1.1Ecan mice, documenting Notch signal activation. Periarticular bone and cartilage structures were unaffected in Notch2^tm1.1Ecan mutants subjected to sham surgery, indicating that NOTCH2 gain-of-function had no discernible impact on joint structure under basal conditions. However, destabilization of the medial meniscus increased osteophyte volume and thickened subchondral bone in Notch2^tm1.1Ecan mice compared to wild type littermates. Moreover, destabilized Notch2^tm1.1Ecan mutants exhibited histological signs of moderate to severe cartilage degeneration, demonstrating joint sensitization to the development of osteoarthritis. Chondrocyte cultures from Notch2^tm1.1Ecan mutants expressed increased Il6 mRNA levels following exposure to JAGGED1, possibly explaining the susceptibility of Notch2^tm1.1Ecan mice to osteoarthritis. In conclusion, Notch2^tm1.1Ecan mutants are sensitized to the development of osteoarthritis in destabilized joints and NOTCH2 activation may play a role in the pathogenesis of the disease.

Genetic ablation of adenosine receptor A3 results in articular cartilage degeneration

Osteoarthritis (OA), the most common form of arthritis, is characterized by inflammation of joints and cartilage degradation leading to disability, discomfort, severe pain, inflammation, and stiffness of the joint. It has been shown that adenosine, a purine nucleoside composed of adenine attached to ribofuranose, is enzymatically produced by the human synovium. However, the functional significance of adenosine signaling in homeostasis and pathology of synovial joints remains unclear. Adenosine acts through four cell surface receptors, i.e., A1, A2A, A2B, and A3, and here, we have systematically analyzed mice with a deficiency for A3 receptor as well as pharmacological modulations of this receptor with specific analogs. The data show that adenosine receptor signaling plays an essential role in downregulating catabolic mechanisms resulting in prevention of cartilage degeneration. Ablation of A3 resulted in development of OA in aged mice. Mechanistically, A3 signaling inhibited cellular catabolic processes in chondrocytes including downregulation of Ca²⁺/calmodulin-dependent protein kinase (CaMKII), an enzyme that promotes matrix degradation and inflammation, as well as Runt-related transcription factor 2 (RUNX2). Additionally, selective A3 agonists protected chondrocytes from cell apoptosis caused by pro-inflammatory cytokines or hypo-osmotic stress. These novel data illuminate the protective role of A3, which is mediated via inhibition of intracellular CaMKII kinase and RUNX2 transcription factor, the two major pro-catabolic regulators in articular cartilage.

KEY MESSAGES

Adenosine receptor A3 (A3) knockout results in progressive loss of articular cartilage in vivo. Ablation of A3 results in activation of matrix degradation and cartilage hypertrophy. A3 agonists downregulate RUNX2 and CaMKII expression in osteoarthritic human articular chondrocytes. A3 prevents articular cartilage matrix degradation induced by inflammation and osmotic fluctuations. A3 agonist inhibits proteolytic activity of cartilage-degrading enzymes.

The family of the interleukin-1 receptors

The extracellular forms of the IL-1 cytokines are active through binding to specific receptors on the surface of target cells. IL-1 ligands bind to the extracellular portion of their ligand-binding receptor chain. For signaling to take place, a non-binding accessory chain is recruited into a heterotrimeric complex. The intracellular approximation of the Toll-IL-1-receptor (TIR) domains of the 2 receptor chains is the event that initiates signaling. The family of IL-1 receptors (IL-1R) includes 10 structurally related members, and the distantly related soluble protein IL-18BP that acts as inhibitor of the cytokine IL-18. Over the years the receptors of the IL-1 family have been known with many different names, with significant confusion. Thus, we will use here a recently proposed unifying nomenclature. The family includes several ligand-binding chains (IL-1R1, IL-1R2, IL-1R4, IL-1R5, and IL-1R6), 2 types of accessory chains (IL-1R3, IL-1R7), molecules that act as inhibitors of signaling (IL-1R2, IL-1R8, IL-18BP), and 2 orphan receptors (IL-1R9, IL-1R10). In this review, we will examine how the receptors of the IL-1 family regulate the inflammatory and anti-inflammatory functions of the IL-1 cytokines and are, more at large, involved in modulating defensive and pathological innate immunity and inflammation. Regulation of the IL-1/IL-1R system in the brain will be also described, as an example of the peculiarities of organ-specific modulation of inflammation.

Osteoarthritis-Like Changes in Bardet-Biedl Syndrome Mutant Ciliopathy Mice (Bbs1M390R/M390R): Evidence for a Role of Primary Cilia in Cartilage Homeostasis and Regulation of Inflammation

Osteoarthritis (OA) is a debilitating inflammation related disease characterized by joint pain and effusion, loss of mobility, and deformity that may result in functional joint failure and significant impact on quality of life. Once thought of as a simple “wear and tear” disease, it is now widely recognized that OA has a considerable metabolic component and is related to chronic inflammation. Defects associated with primary cilia have been shown to be cause OA-like changes in Bardet–Biedl mice. We examined the role of dysfunctional primary cilia in OA in mice through the regulation of the previously identified degradative and pro-inflammatory molecular pathways common to OA. We observed an increase in the presence of pro-inflammatory markers TGFβ-1 and HTRA1 as well as cartilage destructive protease MMP-13 but a decrease in DDR-2. We observed a morphological difference in cartilage thickness in Bbs1^M390R/M390R mice compared to wild type (WT). We did not observe any difference in OARSI or Mankin scores between WT and Bbs1^M390R/M390R mice. Primary cilia appear to be involved in the upregulation of biomarkers, including pro-inflammatory markers common to OA.

A Mouse Intervertebral Disc Degeneration Model by Surgically Induced Instability

STUDY DESIGN

An experimental study to develop a mouse model of lumbar intervertebral disc degeneration (IDD).

OBJECTIVE

The aim of this study was to develop a mouse lumbar IDD model using surgically induced instability and to compare the findings of this model to those in human IDD.

SUMMARY OF BACKGROUND DATA

Previously, various kinds of inducers have been used to reproduce IDD in experimental animals; however, there is yet no standard mouse lumbar IDD model without direct injury to intervertebral disc.

METHODS

A total number of 59 C57BL/6J male mice at 8 weeks old were used. Instability of lumbar spine was induced by surgical resection of posterior elements, including facet joints, supra- and interspinous ligaments. We then analyzed time course changes in radiographical (n = 17) and histological analyses (n = 42), and compared these findings with those in human IDD.

RESULTS

Radiographical analyses showed that the disc height began to decrease in the first 2 weeks after the surgery, and the decrease continued throughout 12 weeks. Bone spurs at the vertebral rims were observed in the late stage of 8 and 12 weeks after the surgery. Histological analyses showed that the disorder of the anterior anulus fibrosus (AF) was initially obvious, followed by posterior shift and degeneration of the nucleus pulposus (NP). Proteoglycan detected in inner layer of AF and periphery of NP was decreased after 8 weeks. Immunohistochemistry displayed the increase of type I and X collagen, and matrix metalloproteinase 13 in the anterior AF.

CONCLUSION

Surgical resection of posterior elements of mouse lumbar spine resulted in reproducible IDD. Because the present procedure does not employ direct injury to intervertebral disc and the radiological and histological findings are compatible with those in human IDD, it may contribute to further understanding of the native pathophysiology of IDD in future.

LEVEL OF EVIDENCE

N/A.

Loss of Fgfr1 in chondrocytes inhibits osteoarthritis by promoting autophagic activity in temporomandibular joint

Temporomandibular joint osteoarthritis (TMJ OA) is a common degenerative disease with few effective disease-modifying treatments in the clinic. Fibroblast growth factor (FGF) signaling is implicated in articular cartilage homeostasis, but the functional roles of FGFR1 in TMJ OA remain largely unknown. In this study, we report that deletion of Fgfr1 in TMJ chondrocytes delayed TMJ OA progression in the age-associated spontaneous OA model and the abnormal dental occlusion OA model. Immunohistochemical staining revealed that Fgfr1 deficiency decreased the expressions of MMP13 (matrix metalloproteinase-13), ADAMTS5 (a disintegrin and metalloproteinase with thrombospondin motifs 5), and COL10A1 but increased aggrecan expression level in two TMJ OA models. Furthermore, our data show that inactivation of FGFR1 signaling may promote autophagic activity in TMJ. FGFR1 inhibitor decreased the expressions of Mmp13, Adamts5, and Runx2 in IL-1β-stimulated condylar chondrocytes, whereas autophagy inhibitors abrogated the protective effects of the FGFR1 inhibitor. Thus, our study indicates inactivated FGFR1 signaling ameliorates TMJ OA progression partially by promoting autophagic activity. Manipulation of this signaling may be a potential therapeutic approach to modify TMJ OA.

Therapeutics in Osteoarthritis Based on an Understanding of Its Molecular Pathogenesis

Osteoarthritis (OA) is the most prevalent joint disease in older people and is characterized by the progressive destruction of articular cartilage, synovial inflammation, changes in subchondral bone and peri-articular muscle, and pain. Because our understanding of the aetiopathogenesis of OA remains incomplete, we haven’t discovered a cure for OA yet. This review appraises novel therapeutics based on recent progress in our understanding of the molecular pathogenesis of OA, including pro-inflammatory and pro-catabolic mediators and the relevant signalling mechanisms. The changes in subchondral bone and peri-articular muscle accompanying cartilage damage are also reviewed.

Transcriptional network systems in cartilage development and disease

Transcription factors play important roles in the regulation of cartilage development by controlling the expression of chondrogenic genes. Genetic studies have revealed that Sox9/Sox5/Sox6, Runx2/Runx3 and Osterix in particular are essential for the sequential steps of cartilage development. Importantly, these transcription factors form network systems that are also required for appropriate cartilage development. Molecular cloning approaches have largely contributed to the identification of several transcriptional partners for Sox9 and Runx2 during cartilage development. Although the importance of a negative-feedback loop between Indian hedgehog (Ihh) and parathyroid hormone-related protein (PTHrP) in chondrocyte hypertrophy has been well established, recent studies indicate that several transcription factors interact with the Ihh-PTHrP loop and demonstrated that Ihh has multiple functions in the regulation of cartilage development. The most common cartilage disorder, osteoarthritis, has been reported to result from the pathological action of several transcription factors, including Runx2, C/EBPβ and HIF-2α. On the other hand, NFAT family members appear to play roles in the protection of cartilage from osteoarthritis. It is also becoming important to understand the homeostasis and regulation of articular chondrocytes, because they have different cellular and molecular features from chondrocytes of the growth plate. This review summarizes the regulation and roles of transcriptional network systems in cartilage development and their pathological roles in osteoarthritis.

OCT4 impedes cell fate redirection by the melanocyte lineage master regulator MITF in mouse ESCs

Ectopic expression of lineage master regulators induces transdifferentiation. Whether cell fate transitions can be induced during various developmental stages has not been systemically examined. Here we discover that amongst different developmental stages, mouse embryonic stem cells (mESCs) are resistant to cell fate conversion induced by the melanocyte lineage master regulator MITF. By generating a transgenic system we exhibit that in mESCs, the pluripotency master regulator Oct4, counteracts pro-differentiation induced by Mitf by physical interference with MITF transcriptional activity. We further demonstrate that mESCs must be released from Oct4-maintained pluripotency prior to ectopically induced differentiation. Moreover, Oct4 induction in various differentiated cells represses their lineage identity in vivo. Alongside, chromatin architecture combined with ChIP-seq analysis suggest that Oct4 competes with various lineage master regulators for binding promoters and enhancers. Our analysis reveals pluripotency and transdifferentiation regulatory principles and could open new opportunities in the field of regenerative medicine.

Regulation of Cartilage Development and Diseases by Transcription Factors

Genetic studies and molecular cloning approaches have been successfully used to identify several transcription factors that regulate the numerous stages of cartilage development. Sex-determining region Y (SRY)-box 9 (Sox9) is an essential transcription factor for the initial stage of cartilage development. Sox5 and Sox6 play an important role in the chondrogenic action of Sox9, presumably by defining its cartilage specificity. Several transcription factors have been identified as transcriptional partners for Sox9 during cartilage development. Runt-related transcription factor 2 (Runx2) and Runx3 are necessary for hypertrophy of chondrocytes. CCAAT/enhancer-binding protein β (C/EBPβ) and activating transcription factor 4 (ATF4) function as co-activators for Runx2 during hypertrophy of chondrocytes. In addition, myocyte-enhancer factor 2C (Mef2C) is required for initiation of chondrocyte hypertrophy, presumably by functioning upstream of Runx2. Importantly, the pathogenic roles of several transcription factors in osteoarthritis have been demonstrated based on the similarity of pathological phenomena seen in osteoarthritis with chondrocyte hypertrophy. We discuss the importance of investigating cellular and molecular properties of articular chondrocytes and degradation mechanisms in osteoarthritis, one of the most common cartilage diseases.

Molecular mechanisms underlying osteoarthritis development: Notch and NF-κB

Osteoarthritis (OA) is a multi-factorial and highly prevalent joint disorder worldwide. Since the establishment of murine surgical knee OA models in 2005, many of the key molecules and signalling pathways responsible for OA development have been identified. Here we review the roles of two multi-functional signalling pathways in OA development: Notch and nuclear factor kappa-light-chain-enhancer of activated B cells. Previous studies have identified various aspects of articular chondrocyte regulation by these pathways. However, comprehensive understanding of the molecular networks regulating articular cartilage homeostasis and OA pathogenesis is needed.

ChIP-seq analysis of genomic binding regions of five major transcription factors highlights a central role for ZIC2 in the mouse epiblast stem cell gene regulatory network

To obtain insight into the transcription factor (TF)-dependent regulation of epiblast stem cells (EpiSCs), we performed ChIP-seq analysis of the genomic binding regions of five major TFs. Analysis of in vivo biotinylated ZIC2, OTX2, SOX2, POU5F1 and POU3F1 binding in EpiSCs identified several new features. (1) Megabase-scale genomic domains rich in ZIC2 peaks and genes alternate with those rich in POU3F1 but sparse in genes, reflecting the clustering of regulatory regions that act at short and long-range, which involve binding of ZIC2 and POU3F1, respectively. (2) The enhancers bound by ZIC2 and OTX2 prominently regulate TF genes in EpiSCs. (3) The binding sites for SOX2 and POU5F1 in mouse embryonic stem cells (ESCs) and EpiSCs are divergent, reflecting the shift in the major acting TFs from SOX2/POU5F1 in ESCs to OTX2/ZIC2 in EpiSCs. (4) This shift in the major acting TFs appears to be primed by binding of ZIC2 in ESCs at relevant genomic positions that later function as enhancers following the disengagement of SOX2/POU5F1 from major regulatory functions and subsequent binding by OTX2. These new insights into EpiSC gene regulatory networks gained from this study are highly relevant to early stage embryogenesis.

The noncoding RNA linc-ADAMTS5 cooperates with RREB1 to protect from intervertebral disc degeneration through inhibiting ADAMTS5 expression

Previous studies have indicated the important roles of ADAMTS5 in intervertebral disc degeneration (IDD). However, the mechanisms that regulate ADAMTS5 expression in nuclear pulposus (NP) cells remain largely unknown. Evidence suggests that intergenic transcription may be associated with genes that encode transcriptional regulators. Here, we identified a long intergenic noncoding RNA, linc-ADAMTS5, which was transcribed in the opposite direction to ADAMTS5. In the present study, through mining computational algorithm programs, and publicly available data sets, we identified Ras-responsive element-binding protein 1 (RREB1) as a crucial transcription factor regulating the expression of ADAMTS5 in NP cells. RNA pull-down, RNA immunoprecipitation (RIP), in vitro binding assays, and gain- and loss-of-function studies indicated that a physical interaction between linc-ADAMTS5 and splicing factor proline/glutamine-rich (SFPQ) facilitated the recruitment of RREB1 to binding sites within the ADAMTS5 promoter to induce chromatin remodeling. This resulted in subdued ADAMTS5 levels in cultured NP cells involving histone deacetylases (HDACs). In clinical NP tissues, linc-ADAMTS5 and RREB1 were correlated negatively with ADAMTS5 expression. Taken together, these results demonstrate that RREB1 cooperates with noncoding RNA linc-ADAMTS5 to inhibit ADAMTS5 expression, thereby affecting degeneration of the extracellular matrix (ECM) of the intervertebral disc (IVD).

CpG site DNA methylation patterns reveal a novel regulatory element in the mouse prion protein gene

The cellular isoform of the prion protein (PrP^C) plays critical roles in the development of prion disorders. Although PrP mRNA is ubiquitously present in a tissue-specific manner, the DNA methylation of PrP gene (Prnp) is still unknown. In this study, we demonstrated that the CpG island (CGI, positioned at −218 to +152 bp from the transcriptional start site) including the Prnp core promoter region was completely unmethylated in all tested tissues. On the other hand, CpG methylation in the CGI shore region (positioned at −599 to −238 bp) occurred in various tissue- and site-specific proportions. Interestingly, the correlation analysis between CpG methylation status and PrP mRNA levels showed that one CpG site methylation at −576 was negatively correlated with the PrP mRNA level (Pearson’s r = −0.374, P=0.035). Taken together, our results suggest that Prnp is a typical housekeeping gene and various methylation frequencies of the CGI shore region are likely to affect Prnp expression in a tissue-specific manner.

Systemic inhibition of IL-6/Stat3 signalling protects against experimental osteoarthritis

Objective

To investigate the impact of systemic inhibition of interleukin 6 (IL-6) or signal transducer and activator of transcription (Stat3) in an experimental model of osteoarthritis (OA).

Methods

Expression of major catabolic and anabolic factors of cartilage was determined in IL-6-treated mouse chondrocytes and cartilage explants. The anti-IL-6-receptor neutralising antibody MR16-1 was used in the destabilisation of the medial meniscus (DMM) mouse model of OA. Stat3 blockade was investigated by the small molecule Stattic ex vivo and in the DMM model.

Results

In chondrocytes and cartilage explants, IL-6 treatment reduced proteoglycan content with increased production of matrix metalloproteinase (MMP-3 and MMP-13) and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS-4 and ADAMTS-5). IL-6 induced Stat3 and extracellular signal-regulated kinase (ERK) 1/2 signalling but not p38, c-Jun N-terminal kinase or Akt. In the DMM model, Stat3 was activated in cartilage, but neither in the synovium nor in the subchondral bone. Systemic blockade of IL-6 by MR16-1 alleviated DMM-induced OA cartilage lesions, impaired the osteophyte formation and the extent of synovitis. In the same model, Stattic had similar beneficial effects on cartilage and osteophyte formation. Stattic, but not an ERK1/2 inhibitor, significantly counteracted the catabolic effects of IL-6 on cartilage explants and suppressed the IL-6-induced chondrocytes apoptosis.

Conclusion

IL-6 induces chondrocyte catabolism mainly via Stat3 signalling, a pathway activated in cartilage from joint subjected to DMM. Systemic blockade of IL-6 or STAT-3 can alleviate DMM-induced OA in mice.

A New Splice Variant of Large Conductance Ca2+-activated K+ (BK) Channel α Subunit Alters Human Chondrocyte Function

Large conductance Ca²⁺-activated K⁺ (BK) channels play essential roles in both excitable and non-excitable cells. For example, in chondrocytes, agonist-induced Ca²⁺ release from intracellular store activates BK channels, and this hyperpolarizes these cells, augments Ca²⁺ entry, and forms a positive feed-back mechanism for Ca²⁺ signaling and stimulation-secretion coupling. In the present study, functional roles of a newly identified splice variant in the BK channel α subunit (BKαΔe2) were examined in a human chondrocyte cell line, OUMS-27, and in a HEK293 expression system. Although BKαΔe2 lacks exon2, which codes the intracellular S0-S1 linker (Glu-127-Leu-180), significant expression was detected in several tissues from humans and mice. Molecular image analyses revealed that BKαΔe2 channels are not expressed on plasma membrane but can traffic to the plasma membrane after forming hetero-tetramer units with wild-type BKα (BKαWT). Single-channel current analyses demonstrated that BKα hetero-tetramers containing one, two, or three BKαΔe2 subunits are functional. These hetero-tetramers have a smaller single channel conductance and exhibit lower trafficking efficiency than BKαWT homo-tetramers in a stoichiometry-dependent manner. Site-directed mutagenesis of residues in exon2 identified Helix2 and the linker to S1 (Trp-158-Leu-180, particularly Arg-178) as an essential segment for channel function including voltage dependence and trafficking. BKαΔe2 knockdown in OUMS-27 chondrocytes increased BK current density and augmented the responsiveness to histamine assayed as cyclooxygenase-2 gene expression. These findings provide significant new evidence that BKαΔe2 can modulate cellular responses to physiological stimuli in human chondrocyte and contribute under pathophysiological conditions, such as osteoarthritis.

Notch-Hes-1 axis controls TLR7-mediated autophagic death of macrophage via induction of P62 in mice with lupus

The increased death of macrophages has been considered as a pathogenic factor for systemic lupus erythematosus (SLE), and dysfunction of autophagy may contribute to improper cell death. However, the effect of autophagy on macrophage during the pathogenesis of SLE is still unclear. Here we found that the death rate and autophagy level of macrophages significantly increased in MRL/lpr lupus-prone mice. Activation of toll-like receptor 7 (TLR7) triggered macrophage death in an autophagy-dependent but caspase-independent way in vitro. Moreover, P62/SQSTM1 is thought to have an essential role in selective autophagy. We also demonstrated that P62/SQSTM1 was required for TLR7-induced autophagy, and knockdown of P62 suppressed R848-induced cell death and LC3II protein accumulation. As an important mediator for cell-cell communication, Notch signaling is responsible for cell-fate decisions. Our results showed that activation of TLR7 also upregulated the expression of Notch1, especially its downstream target gene Hairy and enhancer of split 1 (Hes-1) in macrophages. Of note, we found that Hes-1, as a transcriptional factor, controlled TLR7-induced autophagy by regulating P62 expression. Furthermore, to confirm the above results in vivo, TLR7 agonist imiquimod (IMQ)-induced lupus mouse model was prepared. Splenic macrophages from IMQ-treated mice exhibited increased autophagy and cell death as well as enhanced expressions of Notch1 and Hes-1. Our results indicate that Notch1-Hes-1 signaling controls TLR7-induced autophagic death of macrophage via regulation of P62 in mice with lupus.

A Tale of Two Joints: The Role of Matrix Metalloproteases in Cartilage Biology

Matrix metalloproteinases are a class of enzymes involved in the degradation of extracellular matrix molecules. While these molecules are exceptionally effective mediators of physiological tissue remodeling, as occurs in wound healing and during embryonic development, pathological upregulation has been implicated in many disease processes. As effectors and indicators of pathological states, matrix metalloproteinases are excellent candidates in the diagnosis and assessment of these diseases. The purpose of this review is to discuss matrix metalloproteinases as they pertain to cartilage health, both under physiological circumstances and in the instances of osteoarthritis and rheumatoid arthritis, and to discuss their utility as biomarkers in instances of the latter.

Notch Signaling and the Skeleton

Notch 1 to 4 receptors are important determinants of cell fate and function, and Notch signaling plays an important role in skeletal development and bone remodeling. After direct interactions with ligands of the Jagged and Delta-like families, a series of cleavages release the Notch intracellular domain (NICD), which translocates to the nucleus where it induces transcription of Notch target genes. Classic gene targets of Notch are hairy and enhancer of split (Hes) and Hes-related with YRPW motif (Hey). In cells of the osteoblastic lineage, Notch activation inhibits cell differentiation and causes cancellous bone osteopenia because of impaired bone formation. In osteocytes, Notch1 has distinct effects that result in an inhibition of bone resorption secondary to an induction of osteoprotegerin and suppression of sclerostin with a consequent enhancement of Wnt signaling. Notch1 inhibits, whereas Notch2 enhances, osteoclastogenesis and bone resorption. Congenital disorders of loss- and gain-of-Notch function present with severe clinical manifestations, often affecting the skeleton. Enhanced Notch signaling is associated with osteosarcoma, and Notch can influence the invasive potential of carcinoma of the breast and prostate. Notch signaling can be controlled by the use of inhibitors of Notch activation, small peptides that interfere with the formation of a transcriptional complex, or antibodies to the extracellular domain of specific Notch receptors or to Notch ligands. In conclusion, Notch plays a critical role in skeletal development and homeostasis, and serious skeletal disorders can be attributed to alterations in Notch signaling.

HES factors regulate specific aspects of chondrogenesis and chondrocyte hypertrophy during cartilage development

RBPjκ-dependent Notch signaling regulates multiple processes during cartilage development, including chondrogenesis, chondrocyte hypertrophy and cartilage matrix catabolism. Select members of the HES- and HEY-families of transcription factors are recognized Notch signaling targets that mediate specific aspects of Notch function during development. However, whether particular HES and HEY factors play any role(s) in the processes during cartilage development is unknown. Here, for the first time, we have developed unique in vivo genetic models and in vitro approaches demonstrating that the RBPjκ-dependent Notch targets HES1 and HES5 suppress chondrogenesis and promote the onset of chondrocyte hypertrophy. HES1 and HES5 might have some overlapping function in these processes, although only HES5 directly regulates Sox9 transcription to coordinate cartilage development. HEY1 and HEYL play no discernable role in regulating chondrogenesis or chondrocyte hypertrophy, whereas none of the HES or HEY factors appear to mediate Notch regulation of cartilage matrix catabolism. This work identifies important candidates that might function as downstream mediators of Notch signaling both during normal skeletal development and in Notch-related skeletal disorders.

Notch signaling in postnatal joint chondrocytes, but not subchondral osteoblasts, is required for articular cartilage and joint maintenance

OBJECTIVE

Notch signaling has been identified as a critical regulator in cartilage development and joint maintenance, and loss of Notch signaling in all joint tissues results in an early and progressive osteoarthritis (OA)-like pathology. This study investigated the targeted cell population within the knee joint in which Notch signaling is required for normal cartilage and joint integrity.

METHODS

Two loss-of-function mouse models were generated with tissue-specific knockout of the core Notch signaling component, RBPjκ. The AcanCre(ERT2) transgene specifically removed Rbpjκ floxed alleles in postnatal joint chondrocytes, while the Col1Cre(2.3kb) transgene deleted Rbpjκ in osteoblast populations, including subchondral osteoblasts. Mutant and control mice were analyzed via histology, immunohistochemistry (IHC), real-time quantitative polymerase chain reaction (qPCR), X-ray, and microCT imaging at multiple time-points.

RESULTS

Loss of Notch signaling in postnatal joint chondrocytes results in a progressive OA-like pathology, and triggered the recruitment of non-targeted fibrotic cells into the articular cartilage potentially due to mis-regulated chemokine expression from within the cartilage. Upon recruitment, these fibrotic cells produced degenerative enzymes that may lead to the observed cartilage degradation and contribute to a significant portion of the age-related OA-like pathology. On the contrary, loss of Notch signaling in subchondral osteoblasts did not affect normal cartilage development or joint maintenance.

CONCLUSIONS

RBPjκ-dependent Notch signaling in postnatal joint chondrocytes, but not subchondral osteoblasts, is required for articular cartilage and joint maintenance.

Inhibition of Notch1 promotes hedgehog signalling in a HES1-dependent manner in chondrocytes and exacerbates experimental osteoarthritis

OBJECTIVES

Notch ligands and receptors have recently been shown to be differentially expressed in osteoarthritis (OA). We aim to further elucidate the functional role of Notch signalling in OA using Notch1 antisense transgenic (Notch1 AS) mice.

METHODS

Notch and hedgehog signalling were analysed by real-time PCR and immunohistochemistry. Notch-1 AS mice were employed as a model of impaired Notch signalling in vivo. Experimental OA was induced by destabilisation of the medial meniscus (DMM). The extent of cartilage destruction and osteophyte formation was analysed by safranin-O staining with subsequent assessment of the Osteoarthritis Research Society International (OARSI) and Mankin scores and µCT scanning. Collagen X staining was used as a marker of chondrocyte hypertrophy. The role of hairy/enhancer of split 1 (Hes-1) was investigated with knockdown and overexpression experiments.

RESULTS

Notch signalling was activated in human and murine OA with increased expression of Jagged1, Notch-1, accumulation of the Notch intracellular domain 1 and increased transcription of Hes-1. Notch1 AS mice showed exacerbated OA with increases in OARSI scores, osteophyte formation, increased subchondral bone plate density, collagen X and osteocalcin expression and elevated levels of Epas1 and ADAM-TS5 mRNA. Inhibition of the Notch pathway induced activation of hedgehog signalling with induction of Gli-1 and Gli-2 and increased transcription of hedgehog target genes. The regulatory effects of Notch signalling on Gli-expression were mimicked by Hes-1.

CONCLUSIONS

Inhibition of Notch signalling activates hedgehog signalling, enhances chondrocyte hypertrophy and exacerbates experimental OA including osteophyte formation. These data suggest that the activation of the Notch pathway may limit aberrant hedgehog signalling in OA.

Col2-Cre and tamoxifen-inducible Col2-CreER target different cell populations in the knee joint

OBJECTIVE

Collagen type 2 (Col2)-Cre or tamoxifen-inducible Col2-CreER transgenic mouse lines have been used for studies to explore the cellular and molecular pathogenesis of osteoarthritis (OA). The purpose of this study is to investigate whether the targeted cells are the same or different in the two mouse lines.

METHODS

We crossed tamoxifen inducible Col2-CreER and Col2-Cre mice with Rosa tdTomato reporter mice and analyzed the labeling patterns at different time points.

RESULTS

In the Col2-CreER mice, 90.8 [95% confidence interval (CI) (88.3, 93.2)] and 82.8 (77.4, 88.3) % of the articular surface cells are Tomato positive when tamoxifen was administered at 2 and 2.5 weeks of age and strong activity was observed even 4.5 months after injection. However, 46.0 (32.8, 59.1) and 22.2 (11.7, 32.6) % of the surface cells were Tomato positive when tamoxifen was administered at 3 and 4 weeks of age, respectively. Little to no Tomato activity in the articular surface cells was observed when tamoxifen was administered at 8 weeks of age. At any stage of tamoxifen injection, the Tomato activity was detected in growth plate and epiphyseal bone in addition to articular chondrocytes, but little in endosteum and not in the synovium and ligament. In contrast, the targeted tissues in the Col2-Cre mouse line were articular cartilage, growth plate, meniscus, endosteum, ligament, bone and synovium.

CONCLUSIONS

This study demonstrates that the pattern of targeted cells in the inducible Col2-CreER mice are partially overlapping with but different from that of targeted cells in Col2-Cre mice and the pattern varies dependent on when tamoxifen is administered.

A top-notch dilemma: The complex role of NOTCH signaling in osteoarthritis

A study by Liu et al. in the current issue of Science Signaling explores the complex dual role of NOTCH in the etiology of osteoarthritis by comparing gain-of-function mouse models representing aberrant pathological signaling and transient physiological signaling.