Indian hedgehog signaling promotes chondrocyte differentiation in enchondral ossification in human cervical ossification of the posterior longitudinal ligament

Tenascin-C promotes endochondral ossification and fracture healing through Hedgehog and Hippo signaling

Fractures are frequent and severe musculoskeletal injuries. This study aimed to investigate the function of tenascin-C (TNC) in regulating chondrogenic during fracture healing and elucidate the underlying molecular mechanisms. A well-established femur fracture model in male C57BL/6J mice was used to transect the middle diaphysis of the femur. To identify the essential role of TNC, shTNC lentiviruses or TNC protein were administered in the animal model. Micro-CT analysis, histologic analysis, immunostaining assays, and gene expression analysis were employed to investigate the effect of TNC during fracture healing. An in vitro mesenchymal stem cell culture system was developed to investigate the role and molecular mechanism of TNC in regulating chondrogenesis. TNC expression was induced at the inflammatory phase and peaked at the cartilaginous callus phase during fracture healing. Knockdown of TNC expression in callus results in decreased callus formation and impaired fracture healing. Conversely, administration of exogenous TNC promoted chondrogenic differentiation, cartilage template formation and ultimately improved fracture healing. Both the Hedgehog and Hippo signaling pathways were found to be involved in the pro-chondrogenic function of TNC. Our observations demonstrate that TNC is a crucial factor responsible for endochondral ossification in fracture healing and provide a potential therapeutic strategy for promoting fracture healing.

Genetic insights into ossification of the posterior longitudinal ligament of the spine

Ossification of the posterior longitudinal ligament of the spine (OPLL) is an intractable disease leading to severe neurological deficits. Its etiology and pathogenesis are primarily unknown. The relationship between OPLL and comorbidities, especially type 2 diabetes (T2D) and high body mass index (BMI), has been the focus of attention; however, no trait has been proven to have a causal relationship. We conducted a meta-analysis of genome-wide association studies (GWASs) using 22,016 Japanese individuals and identified 14 significant loci, 8 of which were previously unreported. We then conducted a gene-based association analysis and a transcriptome-wide Mendelian randomization approach and identified three candidate genes for each. Partitioning heritability enrichment analyses observed significant enrichment of the polygenic signals in the active enhancers of the connective/bone cell group, especially H3K27ac in chondrogenic differentiation cells, as well as the immune/hematopoietic cell group. Single-cell RNA sequencing of Achilles tendon cells from a mouse Achilles tendon ossification model confirmed the expression of genes in GWAS and post-GWAS analyses in mesenchymal and immune cells. Genetic correlations with 96 complex traits showed positive correlations with T2D and BMI and a negative correlation with cerebral aneurysm. Mendelian randomization analysis demonstrated a significant causal effect of increased BMI and high bone mineral density on OPLL. We evaluated the clinical images in detail and classified OPLL into cervical, thoracic, and the other types. GWAS subanalyses identified subtype-specific signals. A polygenic risk score for BMI demonstrated that the effect of BMI was particularly strong in thoracic OPLL. Our study provides genetic insight into the etiology and pathogenesis of OPLL and is expected to serve as a basis for future treatment development.

Increased Cellular Expression of Interleukin-6 in Patients With Ossification of the Posterior Longitudinal Ligament

STUDY DESIGN

We performed histologic, immunohistochemical, immunoblot examination and suspension array analyses of cytokine expression in cultured cells derived from human cervical ossification of the posterior longitudinal ligament (OPLL).

OBJECTIVE

To determine the roles of interleukin-6 (IL-6) during the maturation of osteoblasts and chondrocytes associated with the development of OPLL.

SUMMARY OF BACKGROUND DATA

Ectopic OPLL affects ~3% of the general population, with a higher incidence in Asian ethnic groups. Alterations in cytokine profiles may influence osteoblast differentiation, but the mechanisms and signaling pathways associated with the ossification process remain unclear.

METHODS

Samples were collected from 14 patients with OPLL who had undergone spinal surgery and seven with cervical spondylotic myelopathy without OPLL. Tissue sections were used for histologic and immunohistochemical studies, and primary cells from ligamentum samples were used for cytokine array and immunoblotting. A suspension array was used to measure the concentrations of 27 inflammatory cytokines or growth factors.

RESULTS

Suspension array and immunoblot analyses revealed significantly elevated levels of IL-6 in OPLL patients. Alterations in IL-6 concentrations were found to alter the expression of the genes Sox9 , Runx2 , and SIRT1 . In addition, immunohistochemical analysis revealed that these factors are present in mesenchymal cells within the degenerative portion of the ligament matrix that is adjacent to the ossification front.

CONCLUSIONS

IL-6 plays a profound role in the osteoblast differentiation process along with the induction of chondrocyte hypertrophy and cell apoptosis in the early stages of ossification in OPLL. These changes in cytokine profiles are essential factors for regulation of the ectopic ossified plaque in OPLL.

Review of Basic Research about Ossification of the Spinal Ligaments Focusing on Animal Models

Ossification of the posterior longitudinal ligament (OPLL) is a heterotopic ossification that may cause spinal cord compression. With the recent development of computed tomography (CT) imaging, it is known that patients with OPLL often have complications related to ossification of other spinal ligaments, and OPLL is now considered part of ossification of the spinal ligaments (OSL). OSL is known to be a multifactorial disease with associated genetic and environmental factors, but its pathophysiology has not been clearly elucidated. To elucidate the pathophysiology of OSL and develop novel therapeutic strategies, clinically relevant and validated animal models are needed. In this review, we focus on animal models that have been reported to date and discuss their pathophysiology and clinical relevance. The purpose of this review is to summarize the usefulness and problems of existing animal models and to help further the development of basic research on OSL.

Cytokine Profile From the Ligamentum Flavum in Patients with Ossification of the Posterior Longitudinal Ligament in the Cervical Spine

STUDY DESIGN

Histological, immunohistochemical, and suspension array analyses of cytokine expression in human cervical ossification of the posterior longitudinal ligament (OPLL).

OBJECTIVES

The aim of this study was to determine whether changes in the cytokine profile reflect the maturation of chondrocytes and osteoblasts are associated with OPLL development.

SUMMARY OF BACKGROUND DATA

OPLL progresses gradually over a prolonged period and may lead to serious spinal cord complications. However, treatment methods only include conservative therapy for neurological symptoms or surgical decompression, whereas preventive therapy for OPLL remains nonexistent.

METHODS

Ligamentous samples were harvested from 24 patients with OPLL who underwent spinal surgery, and five control samples from cervical spondylotic myelo/radiculopathy patients without OPLL. Tissue sections were used for immunohistochemical studies and primary cells were cultured from the ligamentous samples for cytokine profiling. Using a suspension array system, concentrations of 27 inflammatory cytokines or growth factors were measured to generate the cytokine profiles.

RESULTS

Suspension array and immunoblot analysis revealed significant increments in the levels of interleukin (IL)-6, IL-1α, basic fibroblast growth factor, and RANTES in patients with OPLL. Immunohistochemical analysis further revealed that these factors were present in mesenchymal cells within the degenerative portion of the ligamentous matrix.

CONCLUSION

Our findings suggest that specific changes in the cytokine profile during ossification promote osteoblast differentiation, thereby providing new insights into OPLL pathogenesis. Moreover, this work supports the development of a new therapeutic method for preventing OPLL progression by regulating the cytokine profiles.Level of Evidence: 3.

KIF26B Silencing Prevents Osseous Transdifferentiation of Progenitor/Stem Cells and Attenuates Ectopic Calcification in a Murine Model

Ectopic calcification is an osteogenic process that leads to the formation of inappropriate bone within intra-articular soft tissues, often in response to injury or surgery. The molecular mechanisms governing this phenotype have yet to be determined. Using a population genetics approach, we identified an association of the kinesin superfamily member 26b (Kif26b) with injury-induced ectopic calcification through quantitative trait locus analysis of recombinant inbred mouse strains, consistent with a genomewide association study that identified KIF26B as a severity locus for ectopic calcification in patients with hip osteoarthritis. Despite these associations of KIF26B with ectopic calcification, its mechanistic role and functional implications have not yet been fully elucidated. Here, we aim to decipher the functional role of KIF26B in osseous and chondrogenic transdifferentiation of human and murine progenitor/stem cells and in a murine model of non-invasive injury-induced intra-articular ectopic calcification. We found that KIF26B ablation via lentivirus-mediated shRNA significantly arrested osteogenesis of progenitor/stem cells and suppressed the expression of typical osteogenic marker genes. Conversely, KIF26B loss-of-function increased chondrogenesis as demonstrated by enhanced Safranin-O staining and by the elevated expression of chondrogenic marker genes. Furthermore, cell function analysis revealed that KIF26B knockdown significantly decreased cell viability and proliferation and induced cellular apoptosis. Mechanistically, loss of osteogenesis was reverted by the addition of a Wnt agonist, SKL2001, demonstrating a role of KIF26B in canonical Wnt/β-catenin signaling. Finally, intra-articular delivery of Kif26b shRNA in B6-129SF2/J mice significantly hampered the development of intra-articular ectopic calcification at 8 weeks after injury compared with mice treated with non-target scrambled shRNA. In summary, these observations highlight that KIF26B plays a crucial role in ectopic bone formation by repressing osteogenesis, but not chondrogenesis, potentially via modulating Wnt/β-catenin signaling. These findings establish KIF26B as a critical determinant of the osteogenic process in pathologic endochondral bone formation and an actionable target for pharmacotherapy to mitigate ectopic calcification (and heterotopic ossification). © 2021 American Society for Bone and Mineral Research (ASBMR).

Progression of Ossification of Posterior Longitudinal Ligament After Anterior Cervical Discectomy and Fusion in Military Patients Exposed to Minor Trauma

Objective

Ossification of the posterior longitudinal ligament (OPLL) can progress even after cervical spine surgery and may cause neurological injury as a result of minor trauma. The purpose of this study was to investigate the preventive factors associated with OPLL progression after anterior cervical discectomy and fusion (ACDF), a procedure commonly performed in clinical practice.

Methods

We retrospectively investigated 295 male soldiers who underwent ACDF surgery between 2012 and 2017. Patients who were followed up for >12 months using dynamic radiography and computed tomography (CT) were included in the study. Radiological parameters investigated included OPLL progression, C2-C7 angles on dynamic radiography, segmental angles, C2-C7 cervical sagittal vertical axis (C2-C7 SVA), and the T1 slope. These parameters were measured preoperatively and 1 year postoperatively.

Results

A total of 49 patients were enrolled, and 10 patients were confirmed to have OPLL progression. Comparison between the OPLL progression and non-progression groups showed no statistically significant differences in pre- and postoperative cervical range of motion. However, statistically significant differences were observed in the postoperative neutral C2-C7 angle (progression -3.9°±6.4° vs. non-progression -13.4°±7.9°, p=0.001) and the SVA change (progression 5.8±7.9 mm vs. non-progression -3.7±6.3 mm, p=0.00). The cutoff values were -8.01° for the postoperative neutral C2-C7 angle and 1.4 mm for SVA changes.

Conclusion

Increased SVA (>1.4 mm) and a small postoperative neutral C2-C7 angle (>-8.01°) 1 year after ACDF were associated with OPLL progression. It is important to be mindful of these factors during follow-up after ACDF, because additional surgical treatment may be necessary for OPLL progression due to neurological injury caused by minor trauma.

Disruption of morphogenic and growth pathways in lysosomal storage diseases

The lysosome achieved a new protagonism that highlights its multiple cellular functions, such as in the catabolism of complex substrates, nutrient sensing, and signaling pathways implicated in cell metabolism and growth. Lysosomal storage diseases (LSDs) cause lysosomal accumulation of substrates and deficiency in trafficking of macromolecules. The substrate accumulation can impact one or several pathways which contribute to cell damage. Autophagy impairment and immune response are widely studied, but less attention is paid to morphogenic and growth pathways and its impact on the pathophysiology of LSDs. Hedgehog pathway is affected with abnormal expression and changes in distribution of protein levels, and a reduced number and length of primary cilia. Moreover, growth pathways are identified with delay in reactivation of mTOR that deregulate termination of autophagy and reformation of lysosomes. Insulin resistance caused by changes in lipids rafts has been described in different LSDs. While the genetic and biochemical bases of deficient proteins in LSDs are well understood, the secondary molecular mechanisms that disrupt wider biological processes associated with LSDs are only now becoming clearer. Therefore, we explored how specific signaling pathways can be related to specific LSDs, showing that a system medicine approach could be a valuable tool for the better understanding of LSD pathogenesis. This article is categorized under: Metabolic Diseases > Molecular and Cellular Physiology.

CDC5L promotes early chondrocyte differentiation and proliferation by modulating pre-mRNA splicing of SOX9, COL2A1, and WEE1

Ossification of the posterior longitudinal ligament (OPLL) of the spine is a common pathological condition that causes intractable myelopathy and radiculopathy, mainly the result of an endochondral ossification-like process. Our previous genome-wide association study identified six susceptibility loci for OPLL, including the cell division cycle 5-like (CDC5L) gene region. Here, we found CDC5L to be expressed in type II collagen-producing chondrocyte-like fibroblasts in human OPLL specimens, as well as in differentiating ATDC5 chondrocytes. Cdc5l siRNA transfection in murine chondrocytes decreased the expression of the early chondrogenic genes Sox9 and Col2a1, diminished the cartilage matrix production, and enhanced the expression of parathyroid-hormone-related protein (a resting chondrocyte marker). We also showed that Cdc5l shRNA suppressed the growth of cultured murine embryonal metatarsal cartilage rudiments and that Cdc5l knockdown suppressed the growth of ATDC5 cells. Fluorescence-activated cell sorting analysis revealed that the G2/M cell cycle transition was blocked; our data showed that Cdc5l siRNA transfection enhanced expression of Wee1, an inhibitor of the G2/M transition. Cdc5l siRNA also decreased the pre-mRNA splicing efficiency of Sox9 and Col2a1 genes in both ATDC5 cells and primary chondrocytes; conversely, loss of Cdc5l resulted in enhanced splicing of Wee1 pre-mRNA. Finally, an RNA-binding protein immunoprecipitation assay revealed that Cdc5l bound directly to these target gene transcripts. Overall, we conclude that Cdc5l promotes both early chondrogenesis and cartilage growth and may play a role in the etiology of OPLL, at least in part by fine-tuning the pre-mRNA splicing of chondrogenic genes and Wee1, thus initiating the endochondral ossification process.

Tendon-derived cathepsin K-expressing progenitor cells activate Hedgehog signaling to drive heterotopic ossification

Heterotopic ossification (HO) is pathological bone formation characterized by ossification within muscle, tendons, or other soft tissues. However, the cells of origin and mechanisms involved in the pathogenesis of HO remain elusive. Here we show that deletion of suppressor of fused (Sufu) in cathepsin K-Cre-expressing (Ctsk-Cre-expressing) cells resulted in spontaneous and progressive ligament, tendon, and periarticular ossification. Lineage tracing studies and cell functional analysis demonstrated that Ctsk-Cre could label a subpopulation of tendon-derived progenitor cells (TDPCs) marked by the tendon marker Scleraxis (Scx). Ctsk+Scx+ TDPCs are enriched for tendon stem cell markers and show the highest self-renewal capacity and differentiation potential. Sufu deficiency caused enhanced chondrogenic and osteogenic differentiation of Ctsk-Cre-expressing tendon-derived cells via upregulation of Hedgehog (Hh) signaling. Furthermore, pharmacological intervention in Hh signaling using JQ1 suppressed the development of HO. Thus, our results show that Ctsk-Cre labels a subpopulation of TDPCs contributing to HO and that their cell-fate changes are driven by activation of Hh signaling.

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.

Meniscal and ligament modifications in spontaneous and post-traumatic mouse models of osteoarthritis

BACKGROUND

Osteoarthritis (OA) is a whole joint disease that affects all joint tissues, with changes in the articular cartilage (AC), subchondral bone and synovium. Pathologies in menisci and ligaments, however, are rarely analysed, although both are known to play vital roles in the mechanical stability of the joint. The aim of our study was to describe the pathological changes in menisci and ligament during disease development in murine spontaneous and post-traumatic surgically induced OA and to quantify tissue mineralisation in the joint space using micro-computed tomography (μCT) imaging during OA progression.

METHODS

Knees of Str/ort mice (spontaneous OA model; 26-40 weeks) and C57CBA F1 mice following destabilisation of medial meniscus (DMM) surgery (post-traumatic OA model; 8 weeks after DMM), were used to assess histological meniscal and ligament pathologies. Joint space mineralised tissue volume was quantified by μCT.

RESULTS

Meniscal pathological changes in Str/ort mouse knees were associated with articular cartilage lesion severity. These meniscal changes included ossification, hyperplasia, cell hypertrophy, collagen type II deposition and Sox9 expression in the fibrous region near the attachment to the knee joint capsule. Anterior cruciate ligaments exhibited extracellular matrix changes and chondrogenesis particularly at the tibial attachment site, and ossification was seen in collateral ligaments. Similar changes were confirmed in the post-traumatic DMM model. μCT analysis showed increased joint space mineralised tissue volume with OA progression in both the post-traumatic and spontaneous OA models.

CONCLUSIONS

Modifications in meniscal and ligament mineralisation and chondrogenesis are seen with overt AC degeneration in murine OA. Although the aetiology and the consequences of such changes remain unknown, they will influence stability and load transmission of the joint and may therefore contribute to OA progression. In addition, these changes may have important roles in movement restriction and pain, which represent major human clinical symptoms of OA. Description of such soft tissue changes, in addition to AC degradation, should be an important aspect of future studies in mouse models in order to furnish a more complete understanding of OA pathogenesis.

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.

Heterotopic ossification of tendon and ligament

Much of the similarities of the tissue characteristics, pathologies and mechanisms of heterotopic ossification (HO) formation are shared between HO of tendon and ligament (HOTL). Unmet need and no effective treatment has been developed for HOTL, primarily attributable to poor understanding of cellular and molecular mechanisms. HOTL forms via endochondral ossification, a common process of most kinds of HO. HOTL is a dynamic pathologic process that includes trauma/injury, inflammation, mesenchymal stromal cell (MSC) recruitment, chondrogenic differentiation and, finally, ossification. A variety of signal pathways involve HOTL with multiple roles in different stages of HO formation, and here in this review, we summarize the progress and provide an up‐to‐date understanding of HOTL.

Long-term Outcome of Anterior Cervical Decompression With Fusion for Cervical Ossification of Posterior Longitudinal Ligament Including Postsurgical Remnant Ossified Spinal Lesion

STUDY DESIGN

A retrospective study.

OBJECTIVE

The aim of this study was to assess the outcome, perioperative complications, and prognostic factors of anterior cervical decompression and fusion (ACDF) in patients with cervical ossification of posterior longitudinal ligament (OPLL).

SUMMARY OF BACKGROUND DATA

There is little information on the long-term surgical outcome of ACDF including postsurgical remnant ossified spinal lesion.

METHODS

Between 1993 and 2013, 80 patients with cervical myelopathy towing to OPLL underwent ACDF at our hospital. Among these, 42 patients were followed-up for at least 5 years and their data were analyzed.

RESULTS

The mean follow-up period was 7.9 ± 2.8 years, and the overall improvement rate was 59.2% ± 15.0%. Although 12 (15.0%) perioperative complications were observed in 6 patients, accompanied by neurological deterioration, none of the patients had chronic complications. Multivariate logistic regression analysis that included the preoperative Japanese Orthopaedic Association (JOA) score, type of OPLL, occupying ratio of OPLL, and number of fused segments and increase in the transverse area of the cord identified the latter parameter as the only independent and significant determinant of radiological and clinical improvement of >50%. Among the patients with remaining ossified spinal lesions out of the decompressed range (16 patients), postoperative progression was observed in 6 cases (14.3%) who were all of the mixed type; floated lesions within the decompressed range did not show progression during the follow-up. Adjacent segment degeneration was seen in nine (21.4%) patients, and neurological signs and symptoms were seen in only three of the nine patients and only one patient required revision surgery.

CONCLUSION

The long-term clinical outcome of patients with cervical OPLL after ACDF is considered satisfactory. Surgery-related complications and adjacent segment diseases should not be reasons to avoid ACDF. Care should be taken in selecting ACDF with postsurgical remnant ossified spinal lesion, as it could progress postoperatively especially in the mixed type OPLL.

LEVEL OF EVIDENCE

4.

Molecular Pathophysiology of Ossification of the Posterior Longitudinal Ligament (OPLL)

Ossification of the posterior longitudinal ligament (OPLL) can be defined as an ectopic ossification in the tissues of spinal ligament showing a hyperostotic condition. OPLL is developed mostly in the cervical spine and clinical presentations of OPLL are majorly myelopathy and/or radiculopathy, with serious neurological pathology resulting in paralysis of extremities and disturbances of motility lowering the quality of life. OPLL is known to be an idiopathic and multifactorial disease, which genetic factors and non-genetic factors including diet, obesity, physical strain on the posterior longitudinal ligament, age, and diabetes mellitus, are involved into the pathogenesis. Up to now, surgical management by decompressing the spinal cord is regarded as standard treatment for OPLL, although there might be the risk of development of reprogression of ossification. The molecular pathogenesis and efficient therapeutic strategy, especially pharmacotherapy and/or preventive intervention, of OPLL has not been clearly elucidated and suggested. Therefore, in this review, we tried to give an overview to the present research results on OPLL, in order to shed light on the potential pharmacotherapy based on molecular pathophysiologic aspect of OPLL, especially on the genetic/genomic factors involved into the etiology of OPLL.

Molecular analysis of enthesopathy in a mouse model of hypophosphatemic rickets

The bone tendon attachment site known as the enthesis comprises a transitional zone between bone and tendon, and plays an important role in enabling movement at this site. X-linked hypophosphatemia (XLH) is characterized by impaired activation of vitamin D, elevated serum FGF23 levels and low serum phosphate levels, which impair bone mineralization. Paradoxically, an important complication of XLH is mineralization of the enthesis (enthesopathy). Studies were undertaken to identify the cellular and molecular pathways important for normal post-natal enthesis maturation and to examine their role during the development of enthesopathy in mice with XLH (Hyp). The Achilles tendon entheses of Hyp mice demonstrate an expansion of hypertrophic-appearing chondrogenic cells by P14. Post-natally, cells in wild-type and Hyp entheses similarly descend from scleraxis- and Sox9-expressing progenitors; however, Hyp entheses exhibit an expansion of Sox9-expressing cells, and enhanced BMP and IHH signaling. These results support a role for enhanced BMP and IHH signaling in the development of enthesopathy in XLH.

Risk Factors for Significant Intraoperative Blood Loss During Unilateral Expansive Open-Door Cervical Laminoplasty for Cervical Compressive Myelopathy

OBJECTIVE

Unilateral expansive open-door cervical laminoplasty is an effective and safe procedure for cervical compressive myelopathy but occasionally is accompanied with significant intraoperative blood loss (SIBL). The aim of the present study was to elucidate the risk factors for SIBL in patients undergoing cervical laminoplasty for multilevel cervical compressive myelopathy and to identify the effect of SIBL on neurologic complications.

METHODS

A total of 215 patients who received cervical laminoplasty between January 2010 and August 2016 were enrolled in the study. SIBL was defined as 500 mL or more of blood volume during surgery. Patients were subdivided into 2 groups according to whether they suffered from SIBL (n = 26) or not (n = 189). Patient characteristics and clinical data were collected. Univariate and multivariable analysis were performed to identify independent risk factors for SIBL.

RESULTS

The incidence of SIBL during unilateral expansive open-door cervical laminoplasty was 12.1%. The multivariable logistic regression analysis showed Pavlov ratio (odds ratio [OR] 0.002, P = 0.009), ossification of the posterior longitudinal ligament (OR 2.677, P = 0.038), and number of complete hinge fractures (OR 1.842, P = 0.015) were independent risk factors for SIBL during cervical laminoplasty. Patients with SIBL during cervical laminoplasty had a greater rate of neurologic complications during hospitalization (P = 0.012), worse neurologic recovery rate at discharge (P = 0.01), and longer postoperative length of stay (P = 0.003).

CONCLUSIONS

This study suggested that Pavlov ratio, ossification of the posterior longitudinal ligament, and number of complete hinge fractures were independent risk factors for SBIL during cervical laminoplasty. SIBL in cervical laminoplasty was associated with greater neurologic complications and worse neurologic functional recovery.

Conserved signaling pathways underlying heterotopic ossification

Heterotopic ossification (HO), a serious disorder of extra-skeletal bone formation, occurs as a common complication of trauma or in rare genetic disorders. Many conserved signaling pathways have been implicated in HO; however, the exact underlying molecular mechanisms for many forms of HO are still unclear. The emerging picture is that dysregulation of bone morphogenetic protein (BMP) signaling plays a central role in the process, but that other conserved signaling pathways, such as Hedgehog (HH), Wnt/β-catenin and Fibroblast growth factors (FGF), are also involved, either through cross-talk with BMP signaling or through other independent mechanisms. Deep understanding of the conserved signaling pathways is necessary for the effective prevention and treatment of HO. In this review, we update and integrate recent progress in this area. Hopefully, our discussion will point to novel promising, druggable loci for further translational research and successful clinical applications.

Wnt signaling pathway correlates with ossification of the spinal ligament: A microRNA array and immunohistochemical study

BACKGROUND

Ossification of the posterior longitudinal ligament or the ligamentum flavum parallels endochondral ossification. Cell differentiation at the ossification front is known to be important during this process, although the factors regulating its initiation and progression are still unclear. The purpose of this study was to identify factors important for the regulation of chondrocyte/osteoblast differentiation during spinal ossification.

METHODS

Ligamentum flavum tissue was isolated from 25 patients who underwent decompressive surgery for cervical ossification of the posterior longitudinal ligament. Tissue sections were used for in vitro culture to obtain primary cells through migration methods. To identify microRNAs associated with ossification of the posterior longitudinal ligament, cultured cells were prepared from the ligamentous tissue (n = 4; continuous type) or from control ligamentous samples harvested from patients with cervical spondylosis without spinal ossification, and analyzed using a microRNA array. The ligamentous sections were also examined by immunohistochemistry for the expression of candidate microRNA target genes.

RESULTS

The microRNA array identified 177 factors; 12 of which were expressed at significantly different levels in patients with ossification of the posterior longitudinal ligament compared to those in control patients. The hsa-miR-487b-3p was down-regulated in patients with ossification of the posterior longitudinal ligament, which met the false discovery rate of <0.05. This microRNA was predicted to regulate the expression of genes involved in Wnt signaling. Furthermore, immunohistochemistry of Wnt signaling proteins, including Wnt 3a, LRP5/6, and beta-catenin, revealed positive expression in mesenchymal cells and/or premature chondrocytes at the ossification front.

CONCLUSION

Our results suggested that down-regulation of miR-487b-3p plays an important role in the initiation of Wnt signaling during the ossification process. Wnt signaling may regulate both chondrocyte and osteoblast differentiation and the specification of endochondral ossification in the pathogenesis of ossification of the posterior longitudinal ligament or the ligamentum flavum.

Neural EGFL-Like 1 Regulates Cartilage Maturation through Runt-Related Transcription Factor 3-Mediated Indian Hedgehog Signaling

The pro-chondrogenic function of runt-related transcription factor 2 (Runx2) was previously considered to be dependent on direct binding with the promoter of Indian hedgehog (Ihh)-the major regulator of chondrocyte differentiation, proliferation, and maturation. The authors' previous studies identified neural EGFL like 1 (Nell-1) as a Runx2-responsive growth factor for chondrogenic differentiation and maturation. In this study, it was further revealed that the pro-chondrogenic activities of Nell-1 also rely on Ihh signaling, by showing: i) Nell-1 significantly elevated Ihh signal transduction; ii) Nell-1 deficiency markedly reduced Ihh activation in chondrocytes; and iii) Nell-1-stimulated chondrogenesis was significantly reduced by the specific hedgehog inhibitor cyclopamine. Importantly, the authors demonstrated that Nell-1-responsive Ihh signaling and chondrogenic differentiation extended to Runx2^-/- models in vitro and in vivo. In Runx2^-/- chondrocytes, Nell-1 stimulated the expression and signal transduction of Runx3, another transcription factor required for complete chondrogenic differentiation and maturation. Furthermore, knocking down Runx3 in Runx2^-/- chondrocytes abolished Nell-1's stimulation of Ihh-associated molecule expression, which validates Runx3 as a major mediator of Nell-1-stimulated Ihh activation. For the first time, the Runx2→Nell-1→Runx3→Ihh signaling cascade during chondrogenic differentiation and maturation has been identified as an alternative, but critical, pathway for Runx2 to function as a pro-chondrogenic molecule via Nell-1.

The inhibitory roles of Ihh downregulation on chondrocyte growth and differentiation

The proliferative rate of chondrocytes affects bone elongation. Chondrocyte hypertrophy is required for endochondral bone formation as chondrocytes secrete factors required for osteoblast differentiation and maturation. Previous studies have demonstrated that the Indian hedgehog (Ihh) signaling pathway is a key regulator of skeletal development and homeostasis. The aim of the present study was to investigate the function of Ihh in chondrocyte proliferation and differentiation, as well as the underlying mechanisms. Ihh was knocked down in mouse chondrocyte cells using short hairpin RNA. Chondrocyte apoptosis and cell cycle arrest were assessed using flow cytometry and the results indicated that knockdown of Ihh significantly inhibited cell growth (P<0.05) and increased apoptosis (P<0.001) compared with negative control cells. Downregulation of Ihh also resulted in cell cycle arrest at G1 to S phase in chondrocytes. It was also observed that knockdown of Ihh decreased alkaline phosphatase activity and mineral deposition of chondrocytes. The inhibitory roles of Ihh downregulation on chondrocyte growth and differentiation may be associated with the transforming growth factor-β/mothers against decapentaplegic and osteoprotegerin/receptor activator of nuclear factor κB ligand signaling pathway. The results of the present study suggest that chondrocyte-derived Ihh is essential for maintaining bone growth plates and that manipulation of Ihh expression or its signaling components may be a novel therapeutic technique for the treatment of skeletal diseases, including achondroplasia.

The Pathogenesis of Ossification of the Posterior Longitudinal Ligament

Ossification of the posterior longitudinal ligament (OPLL) is a multi-factorial disease involving an ectopic bone formation of spinal ligaments. It affects 0.8-3.0% aging Asian and 0.1-1.7% aging European Caucasian. The ossified ligament compresses nerve roots in the spinal cord and causes serious neurological problems such as myelopathy and radiculopathy. Research in understanding pathogenesis of OPLL over the past several decades have revealed many genetic and non-genetic factors contributing to the development and progress of OPLL. The characterizations of aberrant signaling of bone morphogenetic protein (BMP) and mitogen-activated protein kinases (MAPK), and the pathological phenotypes of OPLL-derived mesenchymal stem cells (MSCs) have provided new insights on the molecular mechanisms underlying OPLL. This paper reviews the recent progress in understanding the pathophysiology of OPLL and proposes future research directions on OPLL.

Multiplex analysis of serum hormone and cytokine in patients with cervical cOPLL: towards understanding the potential pathogenic mechanisms

Cervical ossification of the posterior longitudinal ligament (cOPLL) is one of the major causes of myelopathy. However, the mechanism underlying remains elusive. In the present study, using MILLIPLEX magnetic bead panel, we investigated four serum hormones and six serum cytokines in cOPLL patients and healthy subjects. The results showed that tumor necrosis factore-α (TNF-α) were significantly increased, and DDK-1 was significantly decreased in the serum from male and female cOPLL patients compared with those from healthy controls, respectively. Osteopontin (OPN) and fibroblast growth factor-23 (FGF-23) were significantly increased in male cOPLL patients compared with that in healthy male controls. Further analysis showed that FGF-23 and OPN significantly increased, dickkopf-1 (DKK-1) decreased in the extensive cOPLL group. In addition, a significant positive correlation between the OPN and FGF-23 was observed in male cOPLL patients. The results are useful for understanding the mechanism underlying cOPLL.

MiR-199b-5p inhibits osteogenic differentiation in ligamentum flavum cells by targeting JAG1 and modulating the Notch signalling pathway

Ossification of the ligamentum flavum (OLF) is a pathology almost only reported in East Asian countries. The leading cause of OLF is thoracic spinal canal stenosis and myelopathy. In this study, the role of miR-199b-5p and jagged 1 (JAG1) in primary ligamentum flavum cell osteogenesis was examined. MiR-199b-5p was found to be down-regulated during osteogenic differentiation in ligamentum flavum cells, while miR-199b-5p overexpression inhibited osteogenic differentiation. In addition, JAG1 was found to be up-regulated during osteogenic differentiation in ligamentum flavum cells, while JAG1 knockdown via RNA interference caused an inhibition of Notch signalling and osteogenic differentiation. Moreover, target prediction analysis and dual luciferase reporter assays supported the notion that JAG1 was a direct target of miR-199b-5p, with miR-199b-5p found to down-regulate both JAG1 and Notch. Further, JAG1 knockdown was demonstrated to block the effect of miR-199b-5p inhibition. These findings imply that miR-199b-5p performs an inhibitory role in osteogenic differentiation in ligamentum flavum cells by potentially targeting JAG1 and influencing the Notch signalling pathway.

Notch signaling pathways in human thoracic ossification of the ligamentum flavum

This study investigated the pathological process of Notch signaling in the osteogenesis of ligamentum flavum tissues and cells, and the associated regulatory mechanisms. Notch receptors, ligands, and target genes were identified by quantitative polymerase chain reaction (qPCR) in ligamentum flavum cells and immunohistochemistry in ligamentum flavum sections from ossification of the ligamentum flavum (OLF) patients and controls. The temporospatial expression patterns of JAG1/Notch2/HES1 in human ligamentum flavum cells during osteogenic differentiation were determined by qPCR. Lentiviral vectors for Notch2 overexpression and knockdown were constructed and transfected into ligamentum flavum cells before osteogenic differentiation to examine the function of Notch signaling pathways in the osteogenic differentiation of ligamentum flavum cells. Alkaline phosphatase, Runx2, Osterix, osteocalcin, and osteopontin mRNA levels, alkaline phosphatase activity, and Alizarin Red staining were used as indicators of osteogenic differentiation. JAG1/Notch2/HES1 mRNA levels were up-regulated in ligamentum flavum cells from OLF patients, which increased during osteogenic differentiation. Immunohistochemical analysis suggested positive Notch2 expression at the ossification front. Down-regulation of Notch2 expression decelerated osteogenic differentiation of ligamentum flavum cells, and Notch2 overexpression promoted osteogenic differentiation of ligamentum flavum cells. Expression of Runx2 and Osterix increased in a manner similar to that of Notch2 during osteogenic differentiation of ligamentum flavum cells, and Notch2 knockdown and overexpression influenced their expression levels. Notch signaling plays an important role in OLF, and Notch may affect the osteogenic differentiation of ligamentum flavum cells via interactions with Runx2 and Osterix.© 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1481-1491, 2016.

Avaliação dos ligamentos longitudinais da coluna de ratos Wistar em modelo experimental da terapia Suit

RESUMO Ligamentos adaptam-se de acordo com a intensidade da atividade física e carga mecânica a que são submetidos. Na última década, na área da fisioterapia neurofuncional infantil, têm surgido métodos e protocolos que possuem em comum o termo suit para caracterizar a existência de vestimentas com bandas elásticas ajustáveis e a possibilidade da aplicação de carga sobre o esqueleto humano. Visto que a carga mecânica pode produzir alterações fibrocartilaginosas sobre os ligamentos e que não foram encontrados estudos avaliando o efeito da terapia suit sobre os ligamentos da coluna, justificam-se pesquisas com métodos experimentais de carga. O objetivo deste trabalho foi analisar as espessuras e morfologia dos ligamentos longitudinais da coluna de ratos Wistar quando submetidos à carga mecânica por compressão vertebral. Trinta animais foram separados em cinco grupos (G1 - controle; G2 - simulação do uso de suit; G3, G4 e G5 - manutenção da vestimenta). Ao modelo experimental do suit, em G4 e G5, foram adaptados pesos ou elásticos dispostos em "X" para sobrecarga vertebral de 50% do peso do animal, que permaneceram com a vestimenta por 40 horas ao longo de 4 semanas de experimento, 5 dias por semana. Não houve diferenças significativas para a espessura, assim como não foram observadas mudanças morfológicas nos ligamentos longitudinais. Conclui-se que não houve alterações nos ligamentos longitudinais da coluna em animais submetidos ao modelo experimental de suit terapia.

The H2 blocker famotidine suppresses progression of ossification of the posterior longitudinal ligament in a mouse model

Background

Ossification of the posterior longitudinal ligament (OPLL) of the spine is a common human myelopathy that leads to spinal cord compression. No disease-modifying drug for OPLL has been identified, whereas surgery and conservative management have been established.

Objectives

To evaluate the therapeutic potential of the H2 blocker famotidine for ectopic ossification in the cervical spine in an OPLL mouse model.

Methods

The H2 blocker famotidine was orally administered to Enpp1^ttw/ttw mice, a model of OPLL, at either 4 or 15 weeks of age. Radiological and survival rate analyses were performed to assess the effects of famotidine on OPLL-like lesions and mortality in Enpp1^ttw/ttw mice.

Results

Oral administration of famotidine suppressed the progression of OPLL-like ectopic ossification and reduced mortality in Enpp1^ttw/ttw mice when administration began at 4 weeks of age, early in the development of ossification.

Conclusions

This study points to the use of famotidine as a disease-modifying drug for ectopic ossification of spinal soft tissue, including OPLL.

Anti-TNFα treatment decreases the previously increased serum Indian Hedgehog levels in patients with ankylosing spondylitis and affects the expression of functional Hedgehog pathway target genes

OBJECTIVE

Indian Hedgehog (Ihh) is the ligand that activates the Hedgehog pathway (HH) in the skeleton-the main controller of endochondral ossification. We aimed at assessing serum levels of Ihh in patients with ankylosing spondylitis (AS) and the effect of serum from patients with AS on HH pathway activation.

METHODS

Serum Ihh levels were measured in 59 patients with AS, 70 patients with rheumatoid arthritis (RA), and 53 healthy subjects. The effect of serum from patients with AS on HH pathway activation was evaluated using an osteoblast-like cell line model.

RESULTS

Patients with AS not on anti-TNFα treatment had significantly higher Ihh levels compared to patients with RA not on anti-TNFα treatment (mean ± SEM of OD: 0.370 ± 0.025 vs. 0.279 ± 0.026 for patients with AS and RA, respectively, p = 0.027) and healthy subjects (p = 0.031). Patients with AS on anti-TNFα treatment had significantly lower Ihh levels compared to patients with AS not on such treatment (p = 0.028). Patients with RA on anti-TNF treatment had higher levels of Ihh compared to patients not on such treatment (p = 0.013). PTHrP levels were similar in patients with RA, AS, and healthy subjects and were not affected by anti-TNFα treatment. We next assessed HH pathway activation in Saos2 cells following incubation with serum from AS patients prior to and following anti-TNF treatment. The HH pathway was downregulated following treatment.

CONCLUSIONS

Ihh levels are increased in patients with AS and decrease following anti-TNFα treatment; this finding may have pathogenic and clinical implications.