RSPO2 defines a distinct undifferentiated progenitor in the tendon/ligament and suppresses ectopic ossification

RAC1 inhibition ameliorates IBSP-induced bone metastasis in lung adenocarcinoma

Macrophage-to-osteoclast differentiation (osteoclastogenesis) plays an essential role in tumor osteolytic bone metastasis (BM), while its specific mechanisms remain largely uncertain in lung adenocarcinoma BM. In this study, we demonstrate that integrin-binding sialoprotein (IBSP), which is highly expressed in the cancer cells from bone metastatic and primary lesions of patients with lung adenocarcinoma, can facilitate BM and directly promote macrophage-to-osteoclast differentiation independent of RANKL/M-CSF. In vivo results further suggest that osteolytic BM in lung cancer specifically relies on IBSP-induced macrophage-to-osteoclast differentiation. Mechanistically, IBSP regulates the Rac family small GTPase 1 (Rac1)-NFAT signaling pathway and mediates the forward shift of macrophage-to-osteoclast differentiation, thereby leading to early osteolysis. Moreover, inhibition of Rac1 by EHT-1864 or azathioprine in mice models can remarkably alleviate IBSP-induced BM of lung cancer. Overall, our study suggests that tumor-secreted IBSP promotes BM by inducing macrophage-to-osteoclast differentiation, with potential as an early diagnostic maker for BM, and Rac1 can be the therapeutic target for IBSP-promoted BM in lung cancer.

The roles and mechanisms of the NF-κB signaling pathway in tendon disorders

Both acute and chronic tendon injuries are the most frequently occurring musculoskeletal diseases in human and veterinary medicine, with a limited repertoire of successful and evidenced-based therapeutic strategies. Inflammation has been suggested as a key driver for the formation of scar and adhesion tissue following tendon acute injury, as well as pathological alternations of degenerative tendinopathy. However, prior efforts to completely block this inflammatory process have yet to be largely successful. Recent investigations have indicated that a more precise targeted approach for modulating inflammation is critical to improve outcomes. The nuclear factor-kappaB (NF-κB) is a typical proinflammatory signal transduction pathway identified as a key factor leading to tendon disorders. Therefore, a comprehensive understanding of the mechanism or regulation of NF-κB in tendon disorders will aid in developing targeted therapeutic strategies for human and veterinary tendon disorders. In this review, we discuss what is currently known about molecular components and structures of basal NF-κB proteins and two activation pathways: the canonical activation pathway and the non-canonical activation pathway. Furthermore, we summarize the underlying mechanisms of the NF-κB signaling pathway in fibrosis and adhesion after acute tendon injury, as well as pathological changes of degenerative tendinopathy in all species and highlight the effect of targeting this signaling pathway in tendon disorders. However, to gain a comprehensive understanding of its mechanisms underlying tendon disorders, further investigations are required. In the future, extensive scientific examinations are warranted to full characterize the NF-κB, the exact mechanisms of action, and translate findings into clinical human and veterinary practice.

Leveraging single cell multiomic analyses to identify factors that drive human chondrocyte cell fate

Cartilage plays a crucial role in skeletal development and function, and abnormal development contributes to genetic and age-related skeletal disease. To better understand how human cartilage develops in vivo , we jointly profiled the transcriptome and open chromatin regions in individual nuclei recovered from distal femurs at 2 fetal timepoints. We used these multiomic data to identify transcription factors expressed in distinct chondrocyte subtypes, link accessible regulatory elements with gene expression, and predict transcription factor-based regulatory networks that are important for growth plate or epiphyseal chondrocyte differentiation. We developed a human pluripotent stem cell platform for interrogating the function of predicted transcription factors during chondrocyte differentiation and used it to test NFATC2 . We expect new regulatory networks we uncovered using multiomic data to be important for promoting cartilage health and treating disease, and our platform to be a useful tool for studying cartilage development in vitro .

Statement of Significance

The identity and integrity of the articular cartilage lining our joints are crucial to pain-free activities of daily living. Here we identified a gene regulatory landscape of human chondrogenesis at single cell resolution, which is expected to open new avenues of research aimed at mitigating cartilage diseases that affect hundreds of millions of individuals world-wide.

Growth deficiency in a mouse model of Kabuki syndrome 2 bears mechanistic similarities to Kabuki syndrome 1

Growth deficiency is a characteristic feature of both Kabuki syndrome 1 (KS1) and Kabuki syndrome 2 (KS2), Mendelian disorders of the epigenetic machinery with similar phenotypes but distinct genetic etiologies. We previously described skeletal growth deficiency in a mouse model of KS1 and further established that a Kmt2d-/- chondrocyte model of KS1 exhibits precocious differentiation. Here we characterized growth deficiency in a mouse model of KS2, Kdm6atm1d/+. We show that Kdm6atm1d/+ mice have decreased femur and tibia length compared to controls and exhibit abnormalities in cortical and trabecular bone structure. Kdm6atm1d/+ growth plates are also shorter, due to decreases in hypertrophic chondrocyte size and hypertrophic zone height. Given these disturbances in the growth plate, we generated Kdm6a-/- chondrogenic cell lines. Similar to our prior in vitro model of KS1, we found that Kdm6a-/- cells undergo premature, enhanced differentiation towards chondrocytes compared to Kdm6a+/+ controls. RNA-seq showed that Kdm6a-/- cells have a distinct transcriptomic profile that indicates dysregulation of cartilage development. Finally, we performed RNA-seq simultaneously on Kmt2d-/-, Kdm6a-/-, and control lines at Days 7 and 14 of differentiation. This revealed surprising resemblance in gene expression between Kmt2d-/- and Kdm6a-/- at both time points and indicates that the similarity in phenotype between KS1 and KS2 also exists at the transcriptional level.

Single-cell profiling transcriptomic reveals cellular heterogeneity and cellular crosstalk in choroidal neovascularization model

Choroidal neovascularization (CNV) is a hallmark of neovascular age-related macular degeneration (nAMD) and a major contributor to vision loss in nAMD cases. However, the identification of specific cell types associated with nAMD remains challenging. Herein, we performed single-cell sequencing to comprehensively explore the cellular diversity and understand the foundational components of the retinal pigment epithelium (RPE)/choroid complex. We unveiled 10 distinct cell types within the RPE/choroid complex. Notably, we observed significant heterogeneity within endothelial cells (ECs), fibroblasts, and macrophages, underscoring the intricate nature of the cellular composition in the RPE/choroid complex. Within the EC category, four distinct clusters were identified and EC cluster 0 was tightly associated with choroidal neovascularization. We identified five clusters of fibroblasts actively involved in the pathogenesis of nAMD, influencing fibrotic responses, angiogenic effects, and photoreceptor function. Additionally, three clusters of macrophages were identified, suggesting their potential roles in regulating the progression of nAMD through immunomodulation and inflammation regulation. Through CellChat analysis, we constructed a complex cell-cell communication network, revealing the role of EC clusters in interacting with fibroblasts and macrophages in the context of nAMD. These interactions were found to govern angiogenic effects, fibrotic responses, and inflammatory processes. In summary, this study reveals noteworthy cellular heterogeneity in the RPE/choroid complex and provides valuable insights into the pathogenesis of CNV. These findings will open up potential avenues for deep understanding and targeted therapeutic interventions in nAMD.

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.

Role of the NF-kB signalling pathway in heterotopic ossification: biological and therapeutic significance

Heterotopic ossification (HO) is a pathological process in which ectopic bone develops in soft tissues within the skeletal system. Endochondral ossification can be divided into the following types of acquired and inherited ossification: traumatic HO (tHO) and fibrodysplasia ossificans progressiva (FOP). Nuclear transcription factor kappa B (NF-κB) signalling is essential during HO. NF-κB signalling can drive initial inflammation through interactions with the NOD-like receptor protein 3 (NLRP3) inflammasome, Sirtuin 1 (SIRT1) and AMP-activated protein kinase (AMPK). In the chondrogenesis stage, NF-κB signalling can promote chondrogenesis through interactions with mechanistic target of rapamycin (mTOR), phosphatidylinositol-3-kinase (PI3K)/AKT (protein kinase B, PKB) and other molecules, including R-spondin 2 (Rspo2) and SRY-box 9 (Sox9). NF-κB expression can modulate osteoblast differentiation by upregulating secreted protein acidic and rich in cysteine (SPARC) and interacting with mTOR signalling, bone morphogenetic protein (BMP) signalling or integrin-mediated signalling under stretch stimulation in the final osteogenic stage. In FOP, mutated ACVR1-induced NF-κB signalling exacerbates inflammation in macrophages and can promote chondrogenesis and osteogenesis in mesenchymal stem cells (MSCs) through interactions with smad signalling and mTOR signalling. This review summarizes the molecular mechanism of NF-κB signalling during HO and highlights potential therapeutics for treating HO.

The biological functions and related signaling pathways of SPON2

Spondin-2 (SPON2), also referred to as M-spondin or DIL-1, is a member of the extracellular matrix protein family known as Mindin-F-spondin (FS). SPON2 can be used as a broad-spectrum tumor marker for more than a dozen tumors, mainly prostate cancer. Meanwhile, SPON2 is also a potential biomarker for the diagnosis of certain non-tumor diseases. Additionally, SPON2 plays a pivotal role in regulating tumor metastasis and progression. In normal tissues, SPON2 has a variety of biological functions represented by promoting growth and development and cell proliferation. This paper presents a comprehensive overview of the regulatory mechanisms, diagnostic potential as a broad-spectrum biomarker, diverse biological functions, involvement in various signaling pathways, and clinical applications of SPON2.

Practical Compass of Single-Cell RNA-Seq Analysis

PURPOSE OF REVIEW

This review paper provides step-by-step instructions on the fundamental process, from handling fastq datasets to illustrating plots and drawing trajectories.

RECENT FINDINGS

The number of studies using single-cell RNA-seq (scRNA-seq) is increasing. scRNA-seq revealed the heterogeneity or diversity of the cellular populations. scRNA-seq also provides insight into the interactions between different cell types. User-friendly scRNA-seq packages for ligand-receptor interactions and trajectory analyses are available. In skeletal biology, osteoclast differentiation, fracture healing, ectopic ossification, human bone development, and the bone marrow niche have been examined using scRNA-seq. scRNA-seq data analysis tools are still being developed, even at the fundamental step of dataset integration. However, updating the latest information is difficult for many researchers. Investigators and reviewers must share their knowledge of in silico scRNA-seq for better biological interpretation. This review article aims to provide a useful guide for complex analytical processes in single-cell RNA-seq data analysis.

Ablation of collagen XII disturbs joint extracellular matrix organization and causes patellar subluxation

Collagen XII, belonging to the fibril-associated collagens, is a homotrimeric secreted extracellular matrix (ECM) protein encoded by the COL12A1 gene. Mutations in the human COL12A1 gene cause an Ehlers-Danlos/myopathy overlap syndrome leading to skeletal abnormalities and muscle weakness. Here, we studied the role of collagen XII in joint pathophysiology by analyzing collagen XII deficient mice and human patients. We found that collagen XII is widely expressed across multiple connective tissue of the developing joint. Lack of collagen XII in mice destabilizes tendons and the femoral trochlear groove to induce patellar subluxation in the patellofemoral joint. These changes are associated with an ECM damage response in tendon and secondary quadriceps muscle degeneration. Moreover, patellar subluxation was also identified as a clinical feature of human patients with collagen XII deficiency. The results provide an explanation for joint hyperlaxity in mice and human patients with collagen XII deficiency.

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.

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.

A single-cell transcriptional atlas reveals resident progenitor cell niche functions in TMJ disc development and injury

The biological characteristics of the temporomandibular joint disc involve complex cellular network in cell identity and extracellular matrix composition to modulate jaw function. The lack of a detailed characterization of the network severely limits the development of targeted therapies for temporomandibular joint-related diseases. Here we profiled single-cell transcriptomes of disc cells from mice at different postnatal stages, finding that the fibroblast population could be divided into chondrogenic and non-chondrogenic clusters. We also find that the resident mural cell population is the source of disc progenitors, characterized by ubiquitously active expression of the NOTCH3 and THY1 pathways. Lineage tracing reveals that Myh11⁺ mural cells coordinate angiogenesis during disc injury but lost their progenitor characteristics and ultimately become Sfrp2⁺ non-chondrogenic fibroblasts instead of Chad⁺ chondrogenic fibroblasts. Overall, we reveal multiple insights into the coordinated development of disc cells and are the first to describe the resident mural cell progenitor during disc injury.

Application of Single-Cell and Spatial Omics in Musculoskeletal Disorder Research

Musculoskeletal disorders, including fractures, scoliosis, heterotopic ossification, osteoporosis, osteoarthritis, disc degeneration, and muscular injury, etc., can occur at any stage of human life. Understanding the occurrence and development mechanism of musculoskeletal disorders, as well as the changes in tissues and cells during therapy, might help us find targeted treatment methods. Single-cell techniques provide excellent tools for studying alterations at the cellular level of disorders. However, the application of these techniques in research on musculoskeletal disorders is still limited. This review summarizes the current single-cell and spatial omics used in musculoskeletal disorders. Cell isolation, experimental methods, and feasible experimental designs for single-cell studies of musculoskeletal system diseases have been reviewed based on tissue characteristics. Then, the paper summarizes the latest findings of single-cell studies in musculoskeletal disorders from three aspects: bone and ossification, joint, and muscle and tendon disorders. Recent discoveries about the cell populations involved in these diseases are highlighted. Furthermore, the therapeutic responses of musculoskeletal disorders, especially single-cell changes after the treatments of implants, stem cell therapies, and drugs are described. Finally, the application potential and future development directions of single-cell and spatial omics in research on musculoskeletal diseases are discussed.