CTSD upregulation as a key driver of spinal ligament abnormalities in spinal stenosis.

Spinal stenosis (SS) is frequently caused by spinal ligament abnormalities, such as ossification and hypertrophy, which narrow the spinal canal and compress the spinal cord or nerve roots, leading to myelopathy or sciatic symptoms; however, the underlying pathological mechanism is poorly understood, hampering the development of effective nonsurgical treatments. Our study aims to investigate the role of co-expression hub genes in patients with spinal ligament ossification and hypertrophy. To achieve this, we conducted an integrated analysis by combining RNA-seq data of ossification of the posterior longitudinal ligament (OPLL) and microarray profiles of hypertrophy of the ligamentum flavum (HLF), consistently pinpointing CTSD as an upregulated hub gene in both OPLL and HLF. Subsequent RT-qPCR and IHC assessments confirmed the heightened expression of CTSD in human OPLL, ossification of the ligamentum flavum (OLF), and HLF samples. We observed an increase in CTSD expression in human PLL and LF primary cells during osteogenic differentiation, as indicated by western blotting (WB). To assess CTSD's impact on osteogenic differentiation, we manipulated its expression levels in human PLL and LF primary cells using siRNAs and lentivirus, as demonstrated by WB, ALP staining, and ARS. Our findings showed that suppressing CTSD hindered the osteogenic differentiation potential of PLL and LF cells, while overexpressing CTSD activated osteogenic differentiation. These findings identify CTSD as a potential therapeutic target for treating spinal stenosis associated with spinal ligament abnormalities.

Sorafenib inhibits ossification of the posterior longitudinal ligament by blocking LOXL2-mediated vascularization.

Ossification of the Posterior Longitudinal Ligament (OPLL) is a degenerative hyperostosis disease characterized by the transformation of the soft and elastic vertebral ligament into bone, resulting in limited spinal mobility and nerve compression. Employing both bulk and single-cell RNA sequencing, we elucidate the molecular characteristics, cellular components, and their evolution during the OPLL process at a single-cell resolution, and validate these findings in clinical samples. This study also uncovers the capability of ligament stem cells to exhibit endothelial cell-like phenotypes in vitro and in vivo. Notably, our study identifies LOXL2 as a key regulator in this process. Through gain-and loss-of-function studies, we elucidate the role of LOXL2 in the endothelial-like differentiation of ligament cells. It acts via the HIF1A pathway, promoting the secretion of downstream VEGFA and PDGF-BB. This function is not related to the enzymatic activity of LOXL2. Furthermore, we identify sorafenib, a broad-spectrum tyrosine kinase inhibitor, as an effective suppressor of LOXL2-mediated vascular morphogenesis. By disrupting the coupling between vascularization and osteogenesis, sorafenib demonstrates significant inhibition of OPLL progression in both BMP-induced and enpp1 deficiency-induced animal models while having no discernible effect on normal bone mass. These findings underscore the potential of sorafenib as a therapeutic intervention for OPLL.

Regulatory Mechanism between Ferritin and Mitochondrial Reactive Oxygen Species in Spinal Ligament-Derived Cells from Ossification of Posterior Longitudinal Ligament Patient.

Primary spinal ligament-derived cells (SLDCs) from cervical herniated nucleus pulposus tissue (control, Ctrl) and ossification of the posterior longitudinal ligament (OPLL) tissue of surgical patients were analyzed for pathogenesis elucidation. Here, we found that decreased levels of ferritin and increased levels of alkaline phosphatase (ALP), a bone formation marker, provoked osteogenesis in SLDCs in OPLL. SLDCs from the Ctrl and OPLL groups satisfied the definition of mesenchymal stem/stromal cells. RNA sequencing revealed that oxidative phosphorylation and the citric acid cycle pathway were upregulated in the OPLL group. SLDCs in the OPLL group showed increased mitochondrial mass, increased mitochondrial reactive oxygen species (ROS) production, decreased levels of ROS scavengers including ferritin. ROS and ferritin levels were upregulated and downregulated in a time-dependent manner, and both types of molecules repressed ALP. Osteogenesis was mitigated by apoferritin addition. We propose that enhancing ferritin levels might alleviate osteogenesis in OPLL.

Exosomal miR-140-5p inhibits osteogenesis by targeting IGF1R and regulating the mTOR pathway in ossification of the posterior longitudinal ligament.

BACKGROUND: Ossification of the posterior longitudinal ligament (OPLL) is a disabling disease whose pathogenesis is still unclear, and there are no effective cures or prevention methods. Exosomal miRNA plays an important role in the osteogenesis of ectopic bone. Therefore, we focused on the downregulation of miR-140-5p in OPLL cell-derived exosomes to explore the mechanism by which exosomal miR-140-5p inhibits osteogenesis in OPLL. RESULTS: Exosomes were isolated by differential centrifugation and identified by transmission electron microscopy, nanoparticle tracking analysis, and exosomal markers. Exosomal RNA was extracted to perform miRNA sequencing and disclose the differentially expressed miRNAs, among which miR-140-5p was significantly downregulated. Confocal microscopy was used to trace the exosomal miR-140-5p delivered from OPLL cells to human mesenchymal stem cells (hMSCs). In vitro, we verified that exosomal miR-140-5p inhibited the osteoblast differentiation of hMSCs by targeting IGF1R and suppressing the phosphorylation of the IRS1/PI3K/Akt/mTOR pathway. In vivo, we verified that exosomal miR-140-5p inhibited ectopic bone formation in mice as assessed by micro-CT and immunohistochemistry. CONCLUSIONS: We found that exosomal miR-140-5p could inhibit the osteogenic differentiation of hMSCs by targeting IGF1R and regulating the mTOR pathway, prompting a further potential means of drug treatment and a possible target for molecular therapy of OPLL.

The Effect of the NFκB-USP9X-Cx43 Axis on the Dynamic Balance of Bone Formation/Degradation during Ossification of the Posterior Longitudinal Ligament of the Cervical Spine.

Connexin 43- (Cx43-) mediated nuclear factor kappa-light-chain-enhancer of activated B cell (NF-κB) signaling has been found involved in the ossification of the posterior longitudinal ligament (OPLL). However, the underlying mechanism how OPLL is regulated has not been elucidated. In the present study, primary ligament fibroblast were isolated; immunoprecipitation (IP) and liquid chromatography-mass spectrometry (LC-MS) assays were applied to identify potential binding proteins of Cx43. Protein interaction was then confirmed by co-IP assay. Alkaline phosphatase (ALP) activity and alizarin red staining were used to evaluate ossification. Luciferase reporter assay and chromatin immunoprecipitation (ChIP) assay were employed to assess the binding between NF-κB p65 and target gene. Lipoxygenase inhibitor (5,8,11-eicosatriynoic acid, EPA) was applied to induce endoplasmic reticulum (ER) stress, and 4-phenylbutyrate (4-PBA) was used as an ER-stress inhibitor. Expression of USP9X, Cx43, and nuclei p65 in ligaments from patients and controls was detected by Western blotting. The results showed that ubiquitin-specific protease 9 X-chromosome (USP9X), a deubiquitylating enzyme, was a candidate of Cx43 binding proteins, and USP9X inhibited Cx43 ubiquitination. In vitro experiments showed that USP9X promoted ossification of primary ligament fibroblasts and nuclear translocation of NF-κB p65 by regulating Cx43 expression. Moreover, NF-κB can bind to the USP9X promoter to promote its transcription. When ER stress was inhibited by 4-PBA, USP9X levels, NF-κB nuclei translocation, and ALP activity were decreased. Reverse results were obtained when ER stress was induced by EPA. PDTC, an NF-κB inhibitor, could abolish the effects of EPA. Furthermore, USP9X, Cx43, and nuclei p65 were significantly upregulated in ligaments from OPLL patients than non-OPLL controls. USP9X was positively correlated with CX43 and nuclei p65 in OPLL samples. Overall, the findings suggest that the ER stress-NFκB-USP9X-Cx43 signaling pathway plays an important role in OPLL progression.

Hsa-circ-0007292 promotes the osteogenic differentiation of posterior longitudinal ligament cells via regulating SATB2 by sponging miR-508-3p.

Ossification of the posterior longitudinal ligament (OPLL) is a disorder with multiple pathogenic mechanisms and leads to different degrees of neurological symptoms. Recent studies have revealed that non-coding RNA (ncRNA), including long non-coding RNAs (lncRNAs) and microRNAs (miRNAs), could influence the development of OPLL. Nevertheless, the molecular mechanisms linking circular RNAs (circRNAs) and the progression of OPLL is still unknown. The current research explored the expression profiles of OPLL-related circRNAs by microarray analysis, and applied qRT-PCR to validate the results. Subsequently, we confirmed the upregulation of hsa_circ_0007292 in OPLL cells by qRT-PCR and validated the circular characteristic of hsa_circ_0007292 by Sanger sequencing. Fluorescence in situ hybridization (FISH) unveiled that hsa_circ_0007292 was predominantly located in the cytoplasm. Functionally, gain-of-function and loss-of-function experiments showed that hsa_circ_0007292 promoted the osteogenic differentiation of OPLL cells. Mechanistically, the interaction of hsa_circ_0007292 and miR-508-3p was predicted and validated by bioinformatics analysis, dual-luciferase reporter assays, and Ago2 RNA immunoprecipitation (RIP). Similarly, we validated the correlation between miR-508-3p and SATB2. Furthermore, rescue experiments were performed to prove that hsa_circ_0007292 acted as a sponge for miR-508-3p, and SATB2 was revealed to be the target gene of miR-508-3p. In conclusion, our research shows that hsa_circ_0007292 regulates OPLL progression by the miR-508-3p/SATB2 pathway. Our results indicate that hsa_circ_0007292 can be used as a promising therapeutic target for patients with OPLL.

Dkk1 acts as a negative regulator in the osteogenic differentiation of the posterior longitudinal ligament cells.

Ossification of the posterior longitudinal ligament (OPLL) is a spinal disorder characterized by progressive ectopic bone formation in the PLL of the spine. Dickkopf-1 (Dkk1) is a secreted inhibitor of the Wnt pathway that negatively regulates bone formation during skeletal development. However, whether Dkk1 impacts the pathogenesis of OPLL has not been reported. This study is to investigate the role of Dkk1 in the development of OPLL. Our results show that the serum levels of Dkk1 are decreased in OPLL patients compared with non-OPLL controls. The expression of Dkk1 is also reduced in OPLL ligament cells. Downregulation of Dkk1 in ligament cells is associated with activation of the Wnt/ß-catenin signaling, as indicated by stabilized ß-catenin and increased T-cell factor-dependent transcriptional activity. Functionally, Dkk1 exerts a growth-inhibitory effect by repressing proliferation but promoting apoptosis of ligament cells. Dkk1 also suppresses bone morphogenetic protein 2-induced entire osteogenic differentiation of ligament cells, and this suppression is mediated via its inhibition of the Wnt pathway. Our results demonstrate for the first time that Dkk1 acts as an important negative regulator in the ossification of the PLL. Targeting the Wnt pathway using Dkk1 may represent a potential therapeutic strategy for the treatment of OPLL.

Expression Analysis of Susceptibility Genes for Ossification of the Posterior Longitudinal Ligament of the Cervical Spine in Human OPLL-related Tissues and a Spinal Hyperostotic Mouse (ttw/ttw).

STUDY DESIGN: Immunohistochemical and real-time reverse transcription-polymerase chain reaction (RT-PCR) analysis. OBJECTIVE: The aim of this study was to analyze the expression of five susceptibility genes (RSPO2, HAO1, CCDC91, RHPH9, and STK38L) for human ossification of the posterior longitudinal ligaments (OPLL) identified in a genome-wide association study. SUMMARY OF BACKGROUND DATA: Detailed expression and functional studies for the five susceptibility genes are needed to aid in clarification of the etiology and pathogenesis of OPLL. METHODS: Immunostaining, cell culture, and real-time RT-PCR were performed on ossified ligament samples collected during anterior cervical decompression for symptomatic OPLL (n = 39 patients) and on control non-OPLL samples (n = 8 patients). Immunohistochemical analysis in spinal hyperostotic mice (ttw/ttw) (n = 25) was also performed. The sample sections were stained for RSPO2, HAO1, CCDC91, RHPH9, STK38L, Runx2, Sox9, and CD90. The mRNA expression levels of the five susceptibility genes were also analyzed in cultured human OPLL and non-OPLL cells subjected to cyclic tensile strain. RESULTS: Immunoreactivity for RSPO2 and Sox9 was evident in proliferating chondrocytes in human OPLL tissues and ttw/ttw mice. Application of cyclic tensile strain to cultured human OPLL cells resulted in increases in mRNA levels for RSPO2, HAO1, and CCDC91. However, individual differences in expression in human OPLL-related samples were seen. HAO1-positive cells were detected only in 3- to 6-week-old ttw/ttw mice that did not simultaneously express RSPO2-positive samples. CONCLUSION: Among the five susceptibility genes, RSPO2, HAO1, and CCDC91 might be contributory factors in progression of OPLL. RSPO2 may be involved in endochondral ossification, especially in mixed or continuous type OPLL, HAO1 may be an initiation factor for OPLL that is rarely seen in mature human OPLL samples, and CCDC91 may be associated with progression of ossification caused by mechanical stress. These findings provide important insights into the pathogenesis and therapeutic targets for OPLL. LEVEL OF EVIDENCE: N/A.

The roles of autophagy in osteogenic differentiation in rat ligamentum fibroblasts: Evidence and possible implications.

Autophagy, a macromolecular degradation process, plays a pivotal role in cell differentiation and survival. This study was designed to investigate the role of autophagy in the osteogenic differentiation in ligamentum fibroblasts. Rat ligamentum fibroblasts were isolated from the posterior longitudinal ligament and cultured in osteogenic induction medium. Ultrastructural analysis, immunofluorescence assay, western blot, flow cytometry, and lysosomal activity assessment were performed to determine the presence and activity of autophagy in the cells. The mineralization deposit and osteogenic gene expressions were evaluated to classify the association between autophagy activity and the bone formation ability of the spinal ligament cells. The influence of leptin and endothelin-1 on the autophagy activity was also evaluated. Our study demonstrated that autophagy was present and increased in the ligament cells under osteogenic induction. Inhibition of autophagy with either pharmacologic inhibitors (Bafilomycin A and 3-methyladenine) or Belcin1 (BECN1) knocking down weakened the mineralization capacity, decreased the gene expressions of COL1A1, osteocalcin (Ocn), and runt-related transcription factor 2 (Runx2) in the ligamentum fibroblasts and increased cell apoptosis. The Adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK)-BECN1 autophagic pathway was activated in the osteogenic differentiating ligamentum fibroblasts. Leptin significantly increased the autophagy activity in the ligament cells under osteogenic induction. These discoveries might improve our understanding for the mechanism of ossification of the posterior longitudinal ligament (OPLL) and provide new approaches on the prevention and treatment of this not uncommon disease.

Cyclic tensile strain facilitates ossification of the cervical posterior longitudinal ligament via increased Indian hedgehog signaling.

The pathomechanisms of initiation and progression of ossification of the posterior longitudinal ligament (OPLL) are unclear. Indian hedgehog (Ihh) and related signaling molecules are key factors in normal enchondral ossification. The purpose of this study is to investigate the contribution of mechanical strain to OPLL and the relationship of Ihh with OPLL. Sections of the posterior longitudinal ligament (PLL) were obtained from 49 patients with OPLL and from 7 patients without OPLL. Cultured PLL cells were subjected to 24 hours of cyclic tensile strain. To identify differentially expressed genes associated with cyclic tensile strain, microarray analysis was performed. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis identified upregulation of various genes, particularly of the Hedgehog signaling pathway; Ihh and related genes had increased expression compared with controls after 24-hour cyclic tensile strain. In immunoblotting analysis, Ihh, Runx2, Sox9, Gli2, Gli3, and smoothened (SMO) had significantly increased expression after 6- or 12-hour cyclic tensile strain. OPLL samples were strongly immunopositive for Ihh, Sox9, Runx2, Gli2, Gli3, and SMO in the ossification front of OPLL. These results suggest that cyclic tensile strain induces abnormal activation of Ihh and related signaling molecules, and this might be important in the ossification process in OPLL.

The COL6A1 rs201153092 single nucleotide polymorphism, associates with thoracic ossification of the posterior longitudinal ligament.

Thoracic ossification of the posterior longitudinal ligament (T­OPLL) is one of the most common factors that causes thoracic spinal stenosis, resulting in intractable myelopathy and radiculopathy. Our previous study reported that the rs201153092 polymorphism present in the collagen 6A1 (COL6A1) gene was a potentially pathogenic locus for the development of T­OPLL. The present study aimed to determine whether the rs201153092 mutation causes abnormal expression of COL6A1 in Han Chinese patients with T­OPLL, and to examine the effects of this mutation on osteogenesis by establishing a model of osteogenic differentiation. COL6A1 gene mutant and wild­type mouse 3T3­E1 embryonic osteoblast models were constructed to induce the differentiation of these cells into osteoblasts. The potential of the mutation site to induce abnormal expression of the COL6A1 gene and osteogenic markers was assessed via reverse transcription­quantitative PCR and western blot analyses. The results demonstrated that the rs201153092A mutation site resulted in significantly increased COL6A1 gene expression levels in the OPLL tissues obtained following clinical surgery. This mutation was shown to play an important role in the development of T­OPLL by regulating the overexpression of the COL6A1 gene and significantly increasing the expression levels of osteogenic markers. The findings of the present study suggested that the rs201153092A mutant variant could increase the expression levels of COL6A1 and consequently play a role in the pathogenesis of T­OPLL.

Roles and mechanisms of leptin in osteogenic stimulation in cervical ossification of the posterior longitudinal ligament.

BACKGROUND: Hyperleptinemia is a common feature of obese people, and leptin, an adipocyte-derived cytokine, is believed to be an important factor in the pathogenesis of cervical ossification of the posterior longitudinal ligament(C-OPLL). So this research was to identify the relation between the serum leptin and bone metabolic markers and how the leptin induced osteogenic effect in C-OPLL. METHODS: Sixty-four samples were selected to determine the concentration of leptin, insulin, and alkaline phosphatase. And the association of leptin with these factors was also examined. We also evaluate the effect of leptin on the development of C-OPLL and further explored the possible underlying mechanism in vitro. RESULTS: We found that serum leptin concentrations were higher in females than in males. Serum leptin and ALP concentrations were increased significantly in C-OPLL females compared to non-OPLL females. In OPLL subjects, the serum leptin concentration corrected for body mass index correlated negatively with the ALP concentrations. In C-OPLL cells, leptin treatment led to a significant increase in mRNA expressions of ALP and OCN and formation of mineralized nodule. Our experiments reported here that osteogenic effect of leptin in C-OPLL cells could be mediated via ERK1/2, p38 MAPK, and/or JNK signaling pathways. CONCLUSIONS: From this research, we got that leptin treatment led to a significant increase in mRNA expressions of ALP and OCN and formation of mineralized nodule. And the osteogenic effect of leptin in C-OPLL cells could be mediated via ERK1/2, p38 MAPK, and/or JNK signaling pathways.

Quantitative T2-Mapping and T2⁎-Mapping Evaluation of Changes in Cartilage Matrix after Acute Anterior Cruciate Ligament Rupture and the Correlation between the Results of Both Methods.

OBJECTIVES: To quantitatively assess changes in cartilage matrix after acute anterior cruciate ligament (ACL) rupture using T2- and T2⁎-mapping and analyze the correlation between the results of both methods. METHODS: Twenty-three patients and 23 healthy controls were enrolled and underwent quantitative MRI examination. The knee cartilage was segmented into six compartments, including lateral femur (LF), lateral tibia (LT), medial femur (MF), medial tibia (MT), trochlea (Tr), and patella (Pa). T2 and T2⁎ values were measured in full-thickness as well as superficial and deep layers of each cartilage compartment. Differences of T2 and T2⁎ values between patients and controls were compared using unpaired Student's t-test, and the correlation between their reciprocals was analyzed using Pearson's correlation coefficient. RESULTS: ACL-ruptured patients showed higher T2 and T2⁎ values in full-thickness and superficial layers of medial and lateral tibiofemoral joint. Meanwhile, patients exhibited higher T2⁎ values in deep layers of lateral tibiofemoral joint. The elevated percentages of T2 and T2⁎ value in superficial LT were most significant (20.738%, 17.525%). The reciprocal of T2⁎ value was correlated with that of T2 value (r = 0.886, P < 0.001). CONCLUSION: The early degeneration could occur in various knee cartilage compartments after acute ACL rupture, especially in the superficial layer of LT. T2⁎-mapping might be more sensitive in detecting deep layer of cartilage than T2-mapping.

Combined use of leptin and mechanical stress has osteogenic effects on ossification of the posterior longitudinal ligament.

PURPOSE: To evaluate the effects of leptin/leptin receptor (LepR) combined with mechanical stress on the development of ossification of the posterior longitudinal ligament (OPLL), which is a disease characterized by ectopic bone formation of the posterior longitudinal ligament (PLL) and can lead to radiculopathy and myelopathy. METHODS: Six human samples of the PLL were analyzed for the expression of leptin and LepR by RT-PCR and western blotting. PLL cells were stimulated with leptin and mechanical stress delivered via a Flexcell tension system, and osteogenic differentiation was evaluated by RT-PCR and western blotting analysis of osteogenic marker expression as well as by alkaline phosphatase (ALP) staining and alizarin red S staining. Activation of mitogen-activated protein kinase (MAPK), Janus kinase (JAK) 2-signal transducer, activator of transcription (STAT) 3 and phosphatidylinositol 3-kinase (PI3K)-Akt was evaluated by western blotting. RESULTS: Samples from the OPLL group had higher LepR mRNA and protein levels and lower leptin levels than those from healthy controls. Exposure to leptin and Flexcell increased the number of ALP-positive cells and calcium nodules in a dose-dependent manner; this effect was accompanied by upregulation of the osteogenic markers osteocalcin, runt-related transcription factor 2 (RUNX2) and osteopontin. Extracellular signal-regulated kinase, P38 MAPK, JAK2, STAT3, PI3K and Akt signaling, was also activated by the combined effects of leptin and mechanical stress. CONCLUSIONS: Leptin and LepR are differentially expressed in OPLL tissues, and the combined use of leptin/LepR and mechanical stress promotes osteogenic differentiation of PLL cells via MAPK, JAK2-STAT3 and PI3K/Akt signaling. These slides can be retrieved under Electronic Supplementary Material.

The microRNA-10a/ID3/RUNX2 axis modulates the development of Ossification of Posterior Longitudinal Ligament.

Ossification of the posterior longitudinal ligament (OPLL) presents as pathological heterotopic ossification of the spinal ligaments. However, its underlying molecular mechanism is still unclear. Our previous findings suggested that altered microRNA regulatory network are critical for the development of OPLL. Here, we set out to unveiling the detailed mechanism of those altered OPLL-specific microRNAs. We screened a set of differentially expressed OPLL-specific microRNAs from the previous sequencing data and showed that microRNA-10a actively modulates the ossification of posterior ligament cells in vitro. Using a tissue-engineered scaffold grown from 4-week-old BALB/c homozygous nude mice, we found that altered microRNA-10a expression in posterior ligament cells indeed affected the heterotopic bone formation in vivo. Furthermore, computational analysis showed that the negative ossification regulator ID3 is a functional target gene of microRNA-10a, and its expression was also significantly altered during microRNA-10a modulation both in vitro and in vivo. Also, we have demonstrated that the ossification promoting function of microRNA-10a requires ID3, as ID3 actively inhibits RUNX2. Thus, we identified a critical role for highly altered OPLL-specific microRNA-10a in regulating the development of OPLL by modulating the ID3/RUNX2 axis.

Identification of susceptibility loci for thoracic ossification of the posterior longitudinal ligament by whole-genome sequencing.

Ossification of the posterior longitudinal ligament (OPLL) is a myelopathy commonly observed in the cervical spine. By contrast, thoracic OPLL (T­OPLL) is rare but more severe. Previous studies have identified several polymorphisms in osteogenic genes that are associated with the occurrence and development of cervical OPLL. However, few genetic studies have evaluated T­OPLL. The present study aimed to identify the genetic factors for OPLL by performing whole­genome sequencing (WGS) in 30 unrelated northern Chinese Han patients with T­OPLL. Using bioinformatics analyses and damaging­variant prediction algorithms, two deleterious variants [c.1534G>A(p.Gly512Ser)/collagen, type VI, α1 (COL6A1)] and [c.2275C>A(p.Leu759Ile)/inteleukin-17 receptor C (IL17RC)] were identified in seven unrelated patients. These two mutations resulted in markedly increased gene expression levels in peripheral blood samples. To the best of our knowledge, this is the first report to describe the use of WGS analysis of T­OPLL in the northern Chinese Han population. The results revealed two novel potentially pathogenic mutations in patients with T­OPLL.

Ossification of the posterior ligament is mediated by osterix via inhibition of the ß-catenin signaling pathway.

Ossification of the posterior longitudinal ligament (OPLL) involves ectopic calcification of the spinal ligament preferentially at the cervical spine. OPLL is associated with different diseases and occurs by endochondral ossification, which is associated with the activity of different transcription factors. However, the pathogenesis of OPLL remains unclear. Here, we investigated the role of osterix (Osx), a transcription factor that functions downstream of Runx2 and is an important regulator of osteogenesis, in the process of OPLL in a dexamethasone (Dex)-induced model of spinal ligament ossification. Our results showed that Osx is upregulated in patients with OPLL and during the ossification of ligament cells in parallel with the upregulation of osteogenic markers including osteocalcin (OCN), alkaline phosphatase (ALP) and collagen-1 (Col-1). Dex-induced ossification of ligament cells was associated with the downregulation and inactivation of ß-catenin, and these effects were offset by Osx knockdown. Activation of ß-catenin signaling abolished the effect of Dex on ossification and the upregulation of osteogenic markers. Taken together, our results suggest that OPLL is mediated by Osx via a mechanism involving the Wnt/ß-catenin signaling pathway, providing a basis for further research to identify potential targets for the treatment of OPLL.

Targeted next-generation sequencing reveals multiple deleterious variants in OPLL-associated genes.

Ossification of the posterior longitudinal ligament of the spine (OPLL), which is characterized by ectopic bone formation in the spinal ligaments, can cause spinal-cord compression. To date, at least 11 susceptibility genes have been genetically linked to OPLL. In order to identify potential deleterious alleles in these OPLL-associated genes, we designed a capture array encompassing all coding regions of the target genes for next-generation sequencing (NGS) in a cohort of 55 unrelated patients with OPLL. By bioinformatics analyses, we successfully identified three novel and five extremely rare variants (MAF < 0.005). These variants were predicted to be deleterious by commonly used various algorithms, thereby resulting in missense mutations in four OPLL-associated genes (i.e., COL6A1, COL11A2, FGFR1, and BMP2). Furthermore, potential effects of the patient with p.Q89E of BMP2 were confirmed by a markedly increased BMP2 level in peripheral blood samples. Notably, seven of the variants were found to be associated with the patients with continuous subtype changes by cervical spinal radiological analyses. Taken together, our findings revealed for the first time that deleterious coding variants of the four OPLL-associated genes are potentially pathogenic in the patients with OPLL.

Integrated microRNA-mRNA analyses reveal OPLL specific microRNA regulatory network using high-throughput sequencing.

Ossification of the posterior longitudinal ligament (OPLL) is a genetic disorder which involves pathological heterotopic ossification of the spinal ligaments. Although studies have identified several genes that correlated with OPLL, the underlying regulation network is far from clear. Through small RNA sequencing, we compared the microRNA expressions of primary posterior longitudinal ligament cells form OPLL patients with normal patients (PLL) and identified 218 dysregulated miRNAs (FDR < 0.01). Furthermore, assessing the miRNA profiling data of multiple cell types, we found these dysregulated miRNAs were mostly OPLL specific. In order to decipher the regulation network of these OPLL specific miRNAs, we integrated mRNA expression profiling data with miRNA sequencing data. Through computational approaches, we showed the pivotal roles of these OPLL specific miRNAs in heterotopic ossification of longitudinal ligament by discovering highly correlated miRNA/mRNA pairs that associated with skeletal system development, collagen fibril organization, and extracellular matrix organization. The results of which provide strong evidence that the miRNA regulatory networks we established may indeed play vital roles in OPLL onset and progression. To date, this is the first systematic analysis of the micronome in OPLL, and thus may provide valuable resources in finding novel treatment and diagnostic targets of OPLL.

Connexin 43 Affects Osteogenic Differentiation of the Posterior Longitudinal Ligament Cells via Regulation of ERK Activity by Stabilizing Runx2 in Ossification.

AIMS: Connexin 43 is one of the most potent gap junction proteins related to osteoblast differentiation and bone formation. We hypothesized that Connexin 43 is a significant factor in osteogenic differentiation in the posterior longitudinal ligament through the regulation of extracellular signal-regulated kinases (ERK) activity by converging on Runt-related transcription factor 2 (Runx2) activity. In this study, we mapped the activity of Connexin 43 to ERK and Runx2 by extracting longitudinal ligament cell for culture and silencing Connexin expression in addition to dexamethasone treatment in vitro. METHODS: qRT-PCR, Western Blot, and Runx2-responsive Luciferase Reporter Assay were performed to detect the activity of ERK, Runx2 and the expression levels of osseous genes under Connexin 43 modification. RESULTS: Downregulation of Connexin 43 resulted in suppression of dexamethasone-induced osteogenic differentiation, inhibition of the ERK and Runx2 activity, and reduction of osseous gene expression. CONCLUSION: these data support that Connexin 43 significantly regulates osteogenic differentiation in the cells from posterior longitudinal ligament by altering the activity of ERK, and subsequently causing the modification of Runx2.