Clearance of Senescent Cells From Injured Muscle Abrogates Heterotopic Ossification in Mouse Models of Fibrodysplasia Ossificans Progressiva

Disturbed glycolipid metabolism activates CXCL13-CXCR5 axis in senescent TSCs to promote heterotopic ossification

Heterotopic ossification (HO) occurs as a common complication after injury, while its risk factor and mechanism remain unclear, which restricts the development of pharmacological treatment. Clinical research suggests that diabetes mellitus (DM) patients are prone to developing HO in the tendon, but solid evidence and mechanical research are still needed. Here, we combined the clinical samples and the DM mice model to identify that disordered glycolipid metabolism aggravates the senescence of tendon-derived stem cells (TSCs) and promotes osteogenic differentiation. Then, combining the RNA-seq results of the aging tendon, we detected the abnormally activated autocrine CXCL13-CXCR5 axis in TSCs cultured in a high fat, high glucose (HFHG) environment and also in the aged tendon. Genetic inhibition of CXCL13 successfully alleviated HO formation in DM mice, providing a potential therapeutic target for suppressing HO formation in DM patients after trauma or surgery.

Cell Senescence in Heterotopic Ossification

The formation of bone outside the normal skeleton, or heterotopic ossification (HO), occurs through genetic and acquired mechanisms. Fibrodysplasia ossificans progressiva (FOP), the most devastating genetic condition of HO, is due to mutations in the ACVR1/ALK2 gene and is relentlessly progressive. Acquired HO is mostly precipitated by injury or orthopedic surgical procedures but can also be associated with certain conditions related to aging. Cellular senescence is a hallmark of aging and thought to be a tumor-suppressive mechanism with characteristic features such as irreversible growth arrest, apoptosis resistance, and an inflammatory senescence-associated secretory phenotype (SASP). Here, we review possible roles for cellular senescence in HO and how targeting senescent cells may provide new therapeutic approaches to both FOP and acquired forms of HO.

Inhibition of NF-κB Signaling-Mediated Crosstalk Between Macrophages and Preosteoblasts by Metformin Alleviates Trauma-Induced Heterotopic Ossification

Heterotopic ossification (HO) is a pathological condition that occurs in soft tissues following severe trauma. The exact pathogenesis of HO remains unclear. Studies have shown that inflammation predisposes patients to the development of HO and triggers ectopic bone formation. Macrophages are crucial mediators of inflammation and are involved in HO development. The present study investigated the inhibitory effect and underlying mechanism of metformin on macrophage infiltration and traumatic HO in mice. Our results found that abundant levels of macrophages were recruited to the injury site during early HO progression and that early administration of metformin prevented traumatic HO in mice. Furthermore, we found that metformin attenuated macrophage infiltration and the NF-κB signaling pathway in injured tissue. The monocyte-to-macrophage transition in vitro was suppressed by metformin and this event was mediated by AMPK. Finally, we showed that inflammatory mediator's regulation by macrophages targeted preosteoblasts, leading to elevated BMP signaling, and osteogenic differentiation and driving HO formation, and this effect was blocked after the activation of AMPK in macrophages. Collectively, our study suggests that metformin prevents traumatic HO by inhibiting of NF-κB signaling in macrophages and subsequently attenuating BMP signaling and osteogenic differentiation in preosteoblasts. Therefore, metformin may serve as a therapeutic drug for traumatic HO by targeting NF-κB signaling in macrophages.

Pathogenesis of acquired heterotopic ossification: Risk factors, cellular mechanisms, and therapeutic implications

Heterotopic ossification (HO), including hereditary and acquired HO, is the formation of extraskeletal bone in skeletal muscle and surrounding soft tissues. Acquired HO is often caused by range of motion, explosion injury, nerve injury or burns. Severe HO can lead to pain and limited joint activity, affecting functional rehabilitation and quality of life. Increasing evidence shows that inflammatory processes and mesenchymal stem cells (MSCs) can drive HO. However, explicit knowledge about the specific mechanisms that result in HO and related cell precursors is still limited. Moreover, there are no effective methods to prevent or reduce HO formation. In this review, we provide an update of known risk factors and relevant cellular origins for HO. In particular, we focus on the underlying mechanisms of MSCs in acquired HO, which follow the osteogenic program. We also discuss the latest therapeutic value and implications for acquired HO. Our review highlights the current gaps in knowledge regarding the pathogenesis of acquired HO and identifies potential targets for the prevention and treatment of HO.

Single-Cell Analyses of Heterotopic Ossification: Characteristics of Injury-Related Senescent Fibroblasts

Introduction

Injury-related cellular senescence may be involved in heterotopic ossification, and no research has been performed about this before.

Methods and Results

The study utilized integrated single-cell RNA-sequencing (scRNA-seq) data from heterotopic ossification samples. The number of senescent cells increased from day 3 and reached the highest level at day 21. However, the expression level of Cyclin Dependent Kinase Inhibitor 2A (Cdkn2a) has no such tendency as the change of cell amount, indicating that the expression level of Cdkn2a may be different in different types of senescent cells or the same time of senescent cell at different time points. The expression level of SASPs (senescence associated secret phenotypes) was also different in different types of senescent cells or at different time points. The GO (gene ontology) analysis revealed that the senescent cells were significantly correlated with the ossification processes, like ECM organization, cell adhesion, ossification, cartilage development, etc. Trajectory analysis showed that injury-related senescent fibroblasts (day 7 and 21) and age-related senescent fibroblasts (day 0 and 42) were in different branches. GO analysis demonstrated that injury-related senescent fibroblasts were mainly related to ossification and ECM remodeling. The KEGG (Kyoto Encyclopedia of Genes and Genomes) results revealed that the ossification was significantly corrected with protein processing in PI3K-Akt signaling, MAPK signaling, focal adhesion, etc.

Conclusion

Consequently, we demonstrated that, unlike age-related senescence, the injury-related senescence demonstrated significantly different SASP phenotypes. The injury-related senescence of fibroblasts is associated with heterotopic ossification formation and may act through PI3K/Akt-induced SASPs.

Cellular senescence and senolytics: the path to the clinic

Interlinked and fundamental aging processes appear to be a root-cause contributor to many disorders and diseases. One such process is cellular senescence, which entails a state of cell cycle arrest in response to damaging stimuli. Senescent cells can arise throughout the lifespan and, if persistent, can have deleterious effects on tissue function due to the many proteins they secrete. In preclinical models, interventions targeting those senescent cells that are persistent and cause tissue damage have been shown to delay, prevent or alleviate multiple disorders. In line with this, the discovery of small-molecule senolytic drugs that selectively clear senescent cells has led to promising strategies for preventing or treating multiple diseases and age-related conditions in humans. In this Review, we outline the rationale for senescent cells as a therapeutic target for disorders across the lifespan and discuss the most promising strategies-including recent and ongoing clinical trials-for translating small-molecule senolytics and other senescence-targeting interventions into clinical use.