Initiation and progression of ossification of the posterior longitudinal ligament of the cervical spine in the hereditary spinal hyperostotic mouse (twy/twy)

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.

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.

Hao1 Is Not a Pathogenic Factor for Ectopic Ossifications but Functions to Regulate the TCA Cycle In Vivo

Ossification of the posterior longitudinal ligament (OPLL), a disease characterized by the ectopic ossification of a spinal ligament, promotes neurological disorders associated with spinal canal stenosis. While blocking ectopic ossification is mandatory to prevent OPLL development and progression, the mechanisms underlying the condition remain unknown. Here we show that expression of hydroxyacid oxidase 1 (Hao1), a gene identified in a previous genome-wide association study (GWAS) as an OPLL-associated candidate gene, specifically and significantly decreased in fibroblasts during osteoblast differentiation. We then newly established Hao1-deficient mice by generating Hao1-flox mice and crossing them with CAG-Cre mice to yield global Hao1-knockout (CAG-Cre/Hao1flox/flox; Hao1 KO) animals. Hao1 KO mice were born normally and exhibited no obvious phenotypes, including growth retardation. Moreover, Hao1 KO mice did not exhibit ectopic ossification or calcification. However, urinary levels of some metabolites of the tricarboxylic acid (TCA) cycle were significantly lower in Hao1 KO compared to control mice based on comprehensive metabolomic analysis. Our data indicate that Hao1 loss does not promote ectopic ossification, but rather that Hao1 functions to regulate the TCA cycle in vivo.

Coagulation, Vascular Morphology, and Vasculogenesis in Spinal Ligament Ossification Model Mice

STUDY DESIGN

In vivo studies of the vascular system in ossification of the posterior longitudinal ligament (OPLL) model mice.

OBJECTIVE

The aim of this study was to investigate blood coagulability, vascular morphology, and vasculogenesis capability, known as venous thromboembolism (VTE) risk factors in the ossification model, tiptoe walking (ttw) mice.

SUMMARY OF BACKGROUND DATA

Patients with OPLL are more likely to develop VTE after spinal cord injury. Capillary mesh invasion of spinal ligaments precedes spinal ligament ossification in ttw mice. Investigation on vascular systems of ttw mice may contribute to clarifying its pathology.

METHODS

Coagulability of blood samples from ttw and C57BL/6 (WT) mice were evaluated at 8, 16, and 24 weeks of age. Vascular morphology was assessed from a Hematoxylin-Eosin stained section by measuring vessel area. A tube formation assay was performed with endothelial cells isolated from the aorta to assess vasculogenesis.

RESULTS

Prothrombin time was significantly shorter in ttw mice than in WT at 8 and 16 weeks. Fibrinogen had a greater increase in ttw mice than in WT at 16 weeks. The vascular area and vascular wall area were significantly smaller in ttw mice than in WT at all timepoints. The ratio of vascular wall area to vascular area was significantly smaller in ttw mice than in WT at 24 weeks. The endothelial cells from ttw mice formed significantly higher numbers of total branching points than WT cells.

CONCLUSION

Ossification model mice had impaired blood coagulation and vascular morphology and high capacity for vasculogenesis. With regard to the pathogenesis of VTE, ttw mice harbor an environment that promotes the development of VTE.Level of Evidence: N/A.

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.

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.

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.

Histomorphometry of ectopic mineralization using undecalcified frozen bone sections

To investigate the correlation between mineral formation and enhanced expressions of some proteins using undecalcified frozen bone sections. Histological studies have revealed that some proteins, such as BMP2, BMPR1A, and Connexin 43, are expressed in and around sites of ectopic ossification. However, the relationship between the expressed proteins considered to be associated with the ossification and mineral formation in vivo is not clear. Ectonucleotide pyrophosphatase phosphodiesterase 1 (ENPP1)-mutant spinal hyperostotic TWY mice and ICR mice as controls were euthanized after calcein labeling, and undecalcified frozen sections were obtained from the middle thoracic spine. Intervertebral disc areas were examined histologically and by measuring calcein-labeled areas and areas showing immunoreactivity for BMP2, BMPR1A, and Connexin 43. Calcein-labeled areas, indicating mineralization in the ectopic mineralization sites, were significantly larger in the mutant mice than in controls. The expression of Connexin 43 was elevated in the annulus fibrosus. Increases in the calcein-labeled areas was not correlated with increases in the areas showing immunoreactivity for Connexin 43 in the annulus fibrosus. There was no statistical correlation between enhanced immunohistochemical expression and elevated calcein-labeled areas. By applying the morphometrical analysis method using undecalcified frozen sections to ENPP1-mutant mice, quantitative evaluation of the mineralization and proteins expressed in the surrounding area in the same animal became possible.

Drug therapy targeting pyrophosphate slows the ossification of spinal ligaments in twy mice

The lack of an effective drug therapy against ossification of spinal ligament (OSL) warrants investigation into the therapeutic target of this disease. An endogenous inhibitor of biomineralization, pyrophosphate (PPi) is a potential therapy for ectopic ossification; however, exogenous PPi is rapidly hydrolyzed by tissue non-specific alkaline phosphatase (TNAP) present in body fluids. In this study, we examined whether a drug therapy targeting PPi is efficacious for the treatment of OSL using the Enpp1^ttw/ttw (twy) mouse model. Twenty male twy mice were randomized into four groups: (i) vehicle (Control); (ii) alkaline phosphatase inhibitor levamisole (5 mg/kg/day sc continuously); (iii) levamisole + exogenous PPi (160 µmol/kg/day sc continuously); and (iv) nuclear retinoic acid receptor-γ (RARγ) agonist (6 µg/kg sc daily). The RARγ agonist, which is a proven inhibitor of ectopic endochondral ossification, was used as a positive control. Treatments commenced when the mice were 5 weeks of age and continued for 4 weeks. Longitudinal micro-computed tomography and postmortem histological analysis were performed. Administration of levamisole alone and in combination with PPi increased serum PPi concentration by 17% and 52%, respectively, compared to that in vehicle-treated mice. The development of OSL in twy mice was suppressed by levamisole + PPi and RARγ agonist treatments, but not by levamisole alone. The levamisole + PPi therapy did not cause osteoporosis, whereas RARγ agonist-treated mice developed osteoporosis. Treatment of twy mice with levamisole in combination with exogenous PPi increased serum PPi level, which slowed the progression of OSL without producing adverse effect on bone. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1256-1261, 2018.

High Osteogenic Potential of Adipose- and Muscle-derived Mesenchymal Stem Cells in Spinal-Ossification Model Mice

STUDY DESIGN

Basic experiments in a mouse model of ossification of the posterior longitudinal ligament (OPLL).

OBJECTIVE

To assess the osteogenic potential of mesenchymal stem cells (MSCs) obtained from muscle and adipose tissue in Tiptoe-walking (ttw) mice, in which cervical OPLL compresses the spinal cord and causes motor and sensory dysfunction.

SUMMARY OF BACKGROUND DATA

In humans, MSCs have been implicated in the pathogenesis of cervical OPLL. Cervical OPLL in ttw mice causes chronic compression of the spinal cord. Few studies have compared the MSC osteogenic potential with behavioral changes in an OPLL animal model.

METHODS

We compared the osteogenic potential and behavioral characteristics of MSCs from ttw mice (4 to 20 weeks old) with those from control wild-type mice (without hyperostosis). Ligament ossification was monitored by micro-computed tomography and pathology; tissues were double stained with fluorescent antibodies against markers for MSCs (CD45 and CD105), at 8 weeks. The Basso Mouse Scale was used to assess motor function, and heat and mechanical tests to assess sensory function. The osteogenic potential of adipose and muscle MSCs was assessed by Alizarin Red S absorbance, staining for osteogenic mineralization, and real-time quantitative polymerase chain reaction for osteogenesis-related genes.

RESULTS

Spinal-ligament ossification began in ttw mice at 8 weeks of age, and the ossified area increased with age. Immunofluorescence staining identified MSCs in the ossification area. The ttw mice became hyposensitive at 8 weeks of age, and Basso Mouse Scale scores showed motor-function deficits starting at 12 weeks of age. Alizarin Red S staining for mineralization showed a higher osteogenic potential in the adipose- and muscle-derived MSCs from ttw mice than from wild-type mice at 4, 8, and 20 weeks of age. Real-time quantitative polymerase chain reaction showed that ttw MSCs strongly expressed osteogenesis-related genes.

CONCLUSION

MSCs derived from muscle and adipose tissue in ttw mice had a high osteogenic potential.

LEVEL OF EVIDENCE

N/A.

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.

Serum biomarkers in patients with ossification of the posterior longitudinal ligament (OPLL): Inflammation in OPLL

Backgroud

Ossification of the posterior longitudinal ligament (OPLL) is characterized by replacement of ligamentous tissue by ectopic new bone formation. OPLL causes narrowing of the spinal canal, resulting in neurological impairment. However, the pathogenesis of OPLL has not been fully elucidated. We investigated whether inflammation occurs in OPLL or not using high-sensitivity CRP (hs-CRP) in a case-control study.

Methods and findings

This study included 103 patients with OPLL in the patient group and 95 age- and sex-matched volunteers with degenerative spinal disease in the control group. Of the 103 OPLL patients, 88 patients who were available for more than 2 years follow-up were checked for OPLL progression. A blood sample was obtained and Hs-CRP, and other routine data, including total protein (TP), albumin (ALB), lactate dehydrogenase (LDH), alkaline phosphatase (ALP), glucose (Glu), calcium (Ca), inorganic phosphate (Pi), white blood cell count (WBC), hemoglobin (Hb) and platelet (PLT), were analyzed. The data were compared between the patients with OPLL and the controls. The severity of the ossified lesions in the whole spine were evaluated by the ossification index (OS index) in patients with OPLL. The data were also compared between the patients with OPLL progression (the progression group) and the patients without OPLL progression (the non-progression group). In the results, the mean hs-CRP in the OPLL group was higher than that in the controls. The Pi in the OPLL group was lower than that in the control group. A negative correlation was found between the Pi and the OS index. The mean hs-CRP in the progression group was higher than that in the non-progression group. There was a positive correlation between the average length of the OPLL progression per year and the hs-CRP.

Conclusions

The results may suggest the occurrence of local inflammation in OPLL and the inflammation might cause OPLL progression. These facts are important for understanding the pathology of OPLL.

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.

Teriparatide Improves Trabecular Osteoporosis but Simultaneously Promotes Ankylosis of the Spine in the Twy Mouse Model for Diffuse Idiopathic Skeletal Hyperostosis

Diffuse idiopathic skeletal hyperostosis (DISH) is a common skeletal disorder in the elderly, which can develop into periosteal hyperostosis and paradoxically into immobilization-associated trabecular osteoporosis. The bone anabolic agent, teriparatide (TPD), seems to be a rational treatment for the immobilization-associated osteoporosis. However, it can lead to development of hyperostosis lesions in DISH patients. Here, we demonstrate TPD effectively treats trabecular osteoporosis while simultaneously promoting ankylosis of the spine in DISH model tiptoe-walking Yoshimura (twy) mice, compared with the ICR mice. Eighteen male twy mice were divided into three groups, and ICR mice were used as a normal control. Subcutaneous injections of TPD or phosphate-buffered saline (PBS) were performed according to three dosing regimens; 40 µg/kg once daily (TPD × 1 group), 40 µg/kg three times daily (TPD × 3 group), and PBS (control; Ctl group). Treatment was commenced at the age of 7 weeks and continued for 5 weeks. Micro-computed tomography (µCT) and histological analysis were performed. Longitudinal µCT study revealed that trabecular bone volume in both the vertebral body and distal femur decreased with time in the Ctl group, but increased dramatically in the TPD × 3 group. The twy mice developed ankylosis of the spine, the progression of which was accelerated with TPD therapy. We also confirmed that TPD therapy promoted ossification of spinal ligaments. Histomorphometrical study revealed that TPD treatment increased bone formation at the vertebrae enthesis region and in the trabecular bone. TPD therapy effectively treats trabecular osteoporosis, but potentially promotes ankylosis of the spine in patients with DISH.

Histological evidence for a supraspinous ligament in sauropod dinosaurs

Supraspinous ossified rods have been reported in the sacra of some derived sauropod dinosaurs. Although different hypotheses have been proposed to explain the origin of this structure, histological evidence has never been provided to support or reject any of them. In order to establish its origin, we analyse and characterize the microstructure of the supraspinous rod of two sauropod dinosaurs from the Upper Cretaceous of Argentina. The supraspinous ossified rod is almost entirely formed by dense Haversian bone. Remains of primary bone consist entirely of an avascular tissue composed of two types of fibre-like structures, which are coarse and longitudinally (parallel to the main axis of the element) oriented. These structures are differentiated on the basis of their optical properties under polarized light. Very thin fibrous strands are also observed in some regions. These small fibres are all oriented parallel to one another but perpendicular to the element main axis. Histological features of the primary bone tissue indicate that the sacral supraspinous rod corresponds to an ossified supraspinous ligament. The formation of this structure appears to have been a non-pathological metaplastic ossification, possibly induced by the continuous tensile forces applied to the element.

Alteration in chondroitin sulfate proteoglycan expression at the epicenter of spinal cord is associated with the loss of behavioral function in Tiptoe walking Yoshimura mice

The objective of this study was to explore the correlation between the alteration in chondroitin sulfate proteoglycan (CSPG) expression at the epicenter of spinal cord and the loss of behavioral function in tiptoe walking Yoshimura mice. The tiptoe walking Yoshimura mice (twy) and Institute of Cancer Research (ICR) mice, aged 20 and 26 weeks, were used in the present study. The behavior assessment, micro-computed tomography and immunofluorescent staining were performed. The compressed spinal cord was histologically analyzed. The results showed that the expression of CSPG was statistically higher at the compressed spinal cord for twy mice compared with that at the normal spinal cord for ICR mice. At the 26th week, a large ossification block at the posterior longitudinal ligament of C1-3 was obviously observed at the micro-CT image We observed the BMS Score was significantly correlated with the expression of glial fibrillary acidic protein, CSPG and hyaluronan (P < 0.05). These findings suggest that compression injury induces the higher CSPG expression at the compressed spinal cord in the twy mice. Furthermore, the alteration in CSPG expression at the epicenter of spinal cord is associated with the loss of behavioral function in twy mice.